WO2022130081A1

WO2022130081A1 - Self-adhesive hot-melt formulations with improved characteristics and high stability of adhesive properties on long times

Info

Publication number: WO2022130081A1
Application number: PCT/IB2021/061095
Authority: WO
Inventors: Stefania ASSONI; Alberto BUGANA; Italo Corzani; Matteo GORNATI
Original assignee: Savare' I.C. S.R.L.
Priority date: 2020-12-16
Filing date: 2021-11-30
Publication date: 2022-06-23

Abstract

The present invention discloses self-adhesive hot-melt formulations that exhibit an excellent and novel combination between a high adhesiveness and a high cohesion, even more combined with an exceptional stability on long times of said adhesive properties and with an unusually low migration and bleeding of low molecular weight ingredients, like plasticisers and tackifiers. Said self-adhesive hot-melt formulations comprise a polymer composition formed by a linear, unsaturated styrenic block copolymer, that is asymmetrical i.e. that has poly-styrenic blocks with different molecular weights, and by a fully hydrogenated styrenic block copolymer, linear or radial, that is symmetrical i.e. that has poly-styrenic blocks all with the same molecular weight.

Description

SELF-ADHESIVE HOT-MELT FORMULATIONS WITH IMPROVED CHARACTERIS-

TICS AND HIGH STABILITY OF ADHESIVE PROPERTIES ON LONG TIMES

DESCRIPTION

TECHNICAL FIELD OF THE INVENTION

The present invention concerns new self-adhesive hot- melt formulations, that show an excellent combination between strong adhesiveness and strong cohesion, together with an excep- tionally good stability on long times of said adhesive proper- ties, as well as with an unusually low migration of low molecu- lar weight ingredients, such as plasticisers and tackifiers.

Said self-adhesive hot-melt formulations comprise a polymer com- position, formed by an unsaturated styrenic block copolymer that is linear and asymmetrical, i.e. that has styrenic blocks with different molecular weights, and by a fully hydrogenated styrenic block copolymer, linear or radial, that is symmetrical, i.e. that has all its styrenic blocks of the same molecular weight.

DEFINITIONS

The expressions "comprising" or "that comprise(s)" are used herein as open-ended terms, that specify the presence of what in the text follows said terms, but that does not preclude the presence of other ingredients or features, e.g. elements, steps, components, either known in the art or disclosed herein.

The expression "copolymer(s) " is used herein to mean a polymer in the chemical composition of which at least two mono- mers or more than two monomers are present. Therefore, the term

"copolymer(s) " means herein, unless the contrary is specifically stated, not only polymers in the chemical composition of which two different monomers are present, but also polymers in the chemical composition three, four, five or more different mono- mers are present.

The adjectives "symmetrical" or "asymmetrical", re- ferred to a styrenic block copolymer, mean that its triblock molecules (for linear copolymers) or its poly-block molecules

(for radial copolymers) have respectively styrenic blocks that have all the same molecular weight or that have different molec- ular weights.

The adjectives "self-adhesive" means here the ability of the formulations disclosed by the present invention, to strongly and steadily adhere on a substrate by simple contact under a light pressure, like the one applied by the hand or finger of a user; and the substantive "self-adhesion " means such property.

These expressions are therefore equivalent to similar terms, sometimes used in the technical jargon for defining the fully similar behaviours of certain adhesives, like the term "pressure sensitive" that is referred to a class of adhesives that are able to strongly adhere on a substrate for a simple contact un- der just a light pressure.

"Room temperature", unless specifically defined in a different way, means a temperature equal to 23 °C; and "room conditions" mean the conditions of an environment at a con- trolled temperature and relative humidity, at 23 °C and 50% rel- ative humidity.

The adverb "substantially", referred e.g. to a property, to the value of a parameter or to a definition, means that such property, value or definition represent the property, value or definition of the considered system in its overwhelming majority of cases, apart from small fluctuations or deviations, provided that said fluctuations and deviations, both in their value and number, are irrelevant for the scope and results of the inven- tion.

"Absorbent hygienic articles" refer to devices and/or methods concerning disposable absorbing and non-absorbing arti- ti es, that comprise diapers and undergarments for incontinent adults, baby diapers and bibs, training pants, infant and tod- dler care wipes, feminine catamenial pads, interlabial pads, panty liners, pessaries, sanitary napkins, tampons and tampon applicators, wound dressing products, absorbent care mats, de- tergent wipes, and the like.

"Fibrous substrates" refers to products having an essen- tially planar structure, formed by natural or synthetic fibres or their blends, both in the form of woven and of nonwoven fab- rics, equally used as components in absorbent hygienic articles.

"Polydispersity Index" or "Molecular Weights Distribu- tion Index" or "PDI" refers to a measure of the distribution of the molecular weight in a certain polymer. It is defined as the ratio between the Mass average Molecular Weight Mw, and the Num- ber average Molecular Weight Mn: PDI = Mw / Mn. Greater values of PDI correspond to broader distribution curves of molecular weights and vice versa. Mw, Mn and therefore their ratio Mw / Mn

= PDI, are measurable e.g. by Gel Permeation Chromatography

(GPC).

The expression "initial conditions" or the adjective

"initial", referred to the conditions in which certain function- al or chemical-physical parameters of an adhesive were measured, mean that said measurement has been carried out within 24 hours since the moment when the adhesive has been applied on a certain substrate from the molten state, and it has solidified there for spontaneous cooling; after the complete solidification the sam- pie of adhesive has been kept, until the moment of the test, at

23°C and 50% Relative Humidity.

The expression "aged conditions" or the adjective

"aged", again referred to the conditions in which certain func- tional or chemical-physical parameters of an adhesive were meas- ured, mean that said measurement has been performed after that the adhesive, coated on a certain substrate, has been aged for seven days into an oven kept at 60°C and 75% Relative Humidity.

After this aging the samples of adhesives and substrates are conditioned again at room temperature, by keeping them for 24 hours at 23°C and 50% Relative Humidity. After such condition- ing, they are tested according to the desired test.

BACKGROUND OF THE INVENTION

The elastomeric styrenic block copolymers have been widely used as the main polymers in many formulations of hot- melt adhesive since 1966, the year in which Shell launched on the market the first copolymers Styrene Butadiene Styrene (SBS) and Styrene - Isoprene - Styrene (SIS), that in 1972 were followed by the corresponding fully hydrogenated block copoly- mers, Styrene - Ethylene / Butylene - Styrene (SEBS) and Styrene

- Ethylene / Propylene - Styrene (SEPS), in which the residual unsaturations in the central elastomeric poly-dienic blocks, that are present in SBS and SIS, are fully saturated through a selective hydrogenation.

In the most common and commercially widespread version, all the above mentioned styrenic block copolymers have a very simple molecular structure that essentially consists in two basic structures: a linear structure, typically represented by a struc- ture with three polymeric blocks like S D S in which (S) represents two blocks of poly-styrene, that generally have the same molecular weight, and (D) rep- resents a central elastomeric block, typically a rub- ber like poly-isoprene, or poly-butadiene, or poly-co-

(isoprene-butadiene) or also, in the selectively hy- drogenated polymers, an elastomeric block of poly- ethylene-butylene rubber or of poly-ethylene-propylene rubber etc. a radial structure, typically represented by a struc- ture with N polymeric blocks, that are centrally linked in a "star-shaped" structure like (S-D)_N X, in which "N" is the number of arms and "X" is a multi- functional central chemical group that chemically links the various arms.

According to the polymerisation conditions, all these styrenic block copolymers may also contain, besides linear

"triblock" molecules or "poly-block" radial molecules, variable quantities of "diblock" molecules, i.e. molecules that have the structure S D. As it will be better illustrated later, said content of diblock molecules in the main polymers of a hot-melt adhesive have a significant influence on the main properties of said adhesive, especially on its adhesiveness and cohesion.

Owed to the complete incompatibility between poly- styrene and all the elastomers that form the elastomeric blocks of these block polymers, all the above mentioned copolymers are always formed, in their solid state, by two incompatible phases that are completely separated at a microscopic level. Said phas- es are a continuous elastomeric phase (that is generally quanti- tatively predominant) inside which a phase of pure poly-styrene is dispersed, that is concentrated in multiple microscopic re- gions (domains), separated one from the other and that can as- sume different geometrical shapes, according to the overall quantity of poly-styrene (see later for more details). However, it can be anticipated that the "ideal" morphological structure at the microscopic level for an adhesive based on styrenic block copolymers, in order to achieve sufficiently high adhesiveness, is a structure formed by a continuous elastomeric phase (soft and tacky) that gives good adhesiveness, inside which microscop- ic spherical poly-styrene domains (hard and mechanically re- sistant) are homogeneously dispersed for giving cohesion. In fact, inside these multiple microscopic styrenic domains the styrenic blocks of different molecules are gathered, so that these domains work as "link-points" among different copolymer molecules, in a way that is conceptually equal to what happens for the chemical crosslinking bonds in traditional crosslinked rubbers.

Because of this peculiar poly-phase structure, that physically links among them many different molecules of the same copolymer through the styrenic domains, the styrenic block co- polymers behave, at room temperature, like crosslinked elasto- mers, therefore with excellent stretchability and cohesion and very good elastic properties and elastic recovery. However, dif- ferently from chemically crosslinked elastomers, which, when heated at high temperatures, decompose without melting, the styrenic block copolymers are still fully thermoplastic poly- mers. In fact, when they are heated at sufficiently high temper- atures, their styrenic domains melt, the block copolymer mole- cules are again fully "free", one versus the other, and the co- polymer shows an overall behaviour like the one of any other molten thermoplastic polymer.

This fully peculiar behaviour of said styrenic block co- polymers, that combine the typical properties of crosslinked rubbers, i.e. especially excellent cohesion and elasticity, with the values of the thermoplastic processing, has caused since the beginning the strong commercial success of these copolymers in many fields and applications, included the one in hot-melt adhe- sives. In this field, the hot-melts containing, as their basic polymers, one or more styrenic block copolymer(s) show(s) an ex- cellent combination between good adhesiveness, given by the cen- tral elastomeric blocks, and a strong cohesion at room tempera- ture, cohesion that is caused by the "physical crosslinking ef- feet" given by the styrenic domains. What said about the strong cohesion, is obviously valid for those copolymer molecules in the structure of which at least two styrenic blocks per each molecule are present, i.e. for the triblock linear copolymers and for the radial copolymers; on the contrary, this is not val- id for diblock molecules, that cannot give any contribution to the "physical crosslinking" and therefore to the polymer's cohe- sion. However, even diblock molecules can contribute, through their elastomeric blocks and their lower percent content of poly-styrene (that forms the hardest and not tacky phase) to the overall softness and adhesiveness of the formulation.

Since the '60's of the past century, i.e. the period of their market launch, a vast literature in this field, both tech- nical and concerning patents, has been generated. However, even if these copolymers, have, as pure polymers, an exceptionally good balance between a high softness (and therefore high adhe- siveness of the derived adhesives) and high cohesion, compared to other polymers that are also used as basic polymers for hot- melt adhesives (like for example polyolefins), such optimum bal- ance between adhesion and cohesion can be impaired or even de- stroyed when a styrenic block copolymer is formulated into an adhesive, and therefore it is diluted with high quantities of tackifying resins and/or plasticisers, that are needed, the first ones for further increasing the tackiness and the second ones for optimising the processability of the hot-melt, by sig- nificantly lowering its melt viscosity.

Even when a formulator is choosing a styrenic block co- polymer for using it inside a certain hot-melt adhesive, he fac- es at least two opposite and somehow contrasting needs, between a good adhesiveness and a good cohesion. In fact e.g., if one formulates into a hot-melt a block copolymer with a high styrene content (that forms, as said, the "hard phase" of the system), the final adhesive shows a much increased cohesion; but simulta- neously its adhesiveness (given by the elastomeric phase) is de- creased, because the adhesive tends to become too hard; on the contrary one can risk the opposite effect if e.g., for optimis- ing the adhesiveness, he uses a copolymer with a low styrene content and/or with a high level of diblock, achieving good ad- hesiveness, but with the risk of impairing too much the cohe- sion.

Therefore, we can say that the problem of finding an op- timum balance between a high adhesiveness and a high cohesion in formulating a hot-melt adhesive based on styrenic block copoly- mers, is still an unsolved problem that is still waiting for ad- ditional improved solutions.

For example, a technique largely used in the prior art for improving the poor cohesion of some adhesives, that were formulated in a way to contain low quantities of block copoly- mers (with the aim e.g. of enhancing their adhesiveness and also for avoiding too high melt viscosities), especially when fur- thermore the used copolymers had low levels of poly-styrene, was to add pure aromatic resins (often called "endblock resin" or

"reinforcing resin"), that preferably have very high softening temperatures, like between 120°C and 160°C. These aromatic res- ins, compatible only with the styrenic domains, somehow strengthen their thermal and mechanical resistance, in this way significantly improving the cohesion of the hot-melt adhesive.

Said pure aromatic resins, based e.g. on alpha-methyl-styrene and para-methyl-styrene, are described for example, together with their possible uses in formulating hot-melt adhesives, in the patent Application EP 0 157293 A1; and adhesive formula- tions, based on styrenic block copolymers whose cohesion is en- hanced through the use of large quantities of such reinforcing endblock resins (even up to 30% by weight), are for example de- scribed and claimed in patent US 6391 960. However, as illus- trated e.g. in the 16^th Chapter of the book "Handbook of Pressure

Sensitive Adhesive Technology" by D. Satas, book the contents of which are herein incorporated as a reference, the addition of said aromatic resins, especially if they are added in high per- centages like the ones claimed by US 6 391 960, has an effect that is substantially equivalent to the one of increasing the overall level of styrene in the basic copolymer. In fact such resins, by solubilising only inside the poly-styrene domains, increase the volume of said domains. Similarly to what it would happen with an increase in the level of the total poly-styrene in the basic copolymer, this addition of aromatic resins leads to a series of effects that, even if cohesion is improved, cause a significant deterioration of the adhesiveness, by making the formulation harder and less tacky. Moreover, such addition, by increasing the volume of the styrenic domains, increases also the melt viscosity of the adhesive and favours the possible change from a microscopic morphology characterised by spherical styrenic domains, that are dispersed into a continuous elasto- meric phase (the best morphology for an optimum adhesiveness), to a morphology characterised by "cylinders " of poly-styrene, a morphology that is disastrous for a good tackiness and adhesive- ness (see later for more details). Furthermore, said pure aro- matic resins are often much more expensive than the majority of other resins commonly used as additives in hot-melt adhesives and so their use significantly worsens the ratio between achieved benefits and the adhesive's cost.

A further serious problem of hot-melt adhesive formula- tions based on styrenic block copolymers that has not yet found satisfactory definitive solutions and that is still waiting for significant improvements, is the problem of the insufficient thermal and oxidative stability shown by most of such formula- tions. As well known, the hot-melt adhesives are processed in the molten state at high temperatures, generally between 130°C and 180°C, and more frequently between 150°C and 170°C. Accord- ing to the application process and to the structure of the in- dustrial lines on which they are used, these adhesives can re- main molten at these high temperatures even for a few days, gen- erally in the presence of oxygen; ideally, even in these very challenging conditions, they should degrade as little as possi- ble. In particular, it is especially difficult to avoid these thermal and oxidative degradations at high temperature (or at least limit them to acceptable levels) for those hot-melt adhe- sives that contain styrenic block copolymers having unsaturated elastomeric blocks, like the SIS copolymers (Styrene - Isoprene

Styrene) and the SBS copolymers (Styrene Butadiene Sty- rene). Such unsaturated poly-diene elastomers have an extremely strong and quick ability to undergo oxidation at high tempera- ture. But even when oxygen is absent, they are anyhow easily de- graded because of an intrinsic "chain weaknes"s on their double bonds. In fact, even just by the action of just high tempera- ture, and even more easily if oxygen is also present, their chains are subjected to radical chain scission reactions that, in some cases, can lead to an excessive and deleterious decrease of molecular weights. This happens for examples with SIS copoly- mers, the formulations of which therefore show, after thermal degradation, dramatic decreases of their melt viscosities.

On the contrary, in the case of SBS copolymers, said radical reactions mostly lead to subsequent re-linking reactions of the radical segment generated by the thermal degradation, with equally excessive and deleterious increases of their molec- ular weights. As a consequence, the formulations based on SBS show, after thermal degradation, a significant and equally dele- terious increase of their melt viscosities.

The massive thermal degradation in the molten state of such copolymers and of the adhesive formulations containing them, both during their industrial manufacturing and during their subsequent application in the molten state, is the cause of unacceptable variations not only in their viscosities and processability, but also in their adhesiveness and cohesion.

Moreover, these phenomena anyhow cause also the generation of a certain quantity of degradation products with a low or very low molecular weight, that can e.g. be the cause of bad odours or produce skin rashes or allergic reactions in possible users. It is clear that similar problems are particularly critical if said adhesives, as it often occurs, are used inside absorbent hygien- ic articles.

In the technical and patent literature, said thermal degradations are frequently quantified by e.g. measuring the change of melt viscosity that an adhesive formulation, based on the mentioned styrenic block copolymers, shows after having been kept for a certain time in the molten state at a fixed tempera- ture. Typically, most users of hot-melt adhesives require that the change in the viscosity of the molten adhesive, kept for ex- ample at a typical average processing temperature of about 325°F

= 163°C, does not exceed in 24 hours a maximum value between

20% and 30%; while actually, in the industrial practice, for many of said formulations based on SIS or SBS, one can observe variations in the melt viscosity that can be as high as 20% in just two hours and that can exceed 30% just after four hours of aging.

The most obvious solution for this problem is to add some stabilising and anti-oxidant additives to said hot-melt ad- hesives; such additives, widely known from the Prior Art, are for example, sterically hindered phenols, secondary aromatic amines, organic phosphites or phosphonites, thio-esters etc. In any case, all these compounds are relatively toxic substances; and, even if all hot-melt adhesives always comprise, e.g. at a level of some percent, at least one of these anti-oxidants, their use must be limited, especially in adhesives destined to particularly critical uses, like in adhesives that contact skin or food, in absorbent hygienic articles, in medical articles, and so on. However, it is widely known that, in the field of in- dustrial hot-melt adhesives, many formulations based on SIS and SBS, having anyhow good general characteristics, are anyhow on the edge of unacceptability for their poor resistance to degra- dation, when one tries to stabilise them just through the addi- tion of these anti-oxidant additives, that are added up to the maximum allowed level, due to toxicological reasons.

For removing such severe drawbacks, for example the pa- tent US 6 162859 shows formulations that are formed by blend- ing SIS and SBS, even if, as recognised by the inventors them- selves, these two types of polymers are mutually incompatible and, for being somehow made compatible in a sufficient way, they need the addition of an unusually complex and expensive blend of as many as three different tackifiers.

The use of such polymeric blend SIS + SBS, with the aim of removing the effects ooff tthhee thermal degradations of these polymers and the consequent excessive variation in their melt viscosity, apparently seems to work. But actually this happens just because, as said, while the SIS copolymers thermally de- grade by decreasing their average molecular weight and conse- quently their melt viscosity, on the contrary SBS copolymers tend, in the same conditions, to crosslink and so to increase their average molecular weight and consequently their melt vis- cosity.

In this way, said patent can sshhooww,, in Example 2-A, an adhesive formulation the polymeric system of which is formed by

5% by weight of a SIS plus 15% by weight of a SBS. Such adhe- sive, after 24 hours at 163°C, shows a viscosity change by just

22%; while a Comparative Example, whose polymer consists only in a SIS, in the same aging conditions, shows a viscosity change by

50%. However, it is well clear that the solution claimed by US 6

162 859 cannot be accepted, because it shows just a purely ap- parent "thermal stability". Actually, the adhesive is anyhow se- verely degraded by heat and, after 24 hours, it is in reality a blend of many degraded chemical species, all of them strongly impaired, even if the fact that some of these species have in- creased their viscosity compared to the initial one, while on the contrary other species have decreased their viscosity, cre- ates the illusory result of having just a relatively small change in the apparent overall viscosity.

Just for remedying this poor thermal stability of the unsaturated SBS and SIS styrenic block copolymers, later Shell synthesised and marketed the corresponding copolymers with their fully hydrogenated elastomeric blocks, i.e. SEBS and SEPS; and, as a consequence, in the field of adhesives, also an equally vast technical and patent literature concerning hot-melt adhe- sive formulations based on such hydrogenated copolymers has started since then to flourish. While in general the thermal stability of these adhesive formulations is certainly improved, compared with the previous formulations of unsaturated copoly- mers, however these formulations based on SEBS and SEPS have poor adhesive properties, significantly worse than the older ad- hesives based on SIS and SBS. This derives from the fact that as well known in the technology and science of adhesives- the central elastomeric block of an SEBS or of an SEPS, i.e. a poly ethylene - butylene rubber (EBR) or a poly ethylene pro- pylene rubber (EPR) give, for a whole series of reasons that is useless to discuss here in details, adhesives of poor quality, having especially a low adhesiveness. This fact is indirectly shown also by the fact that, as well known, the correspondent pure elastomers, i.e. poly ethylene butylene rubber (EBR) and poly ethylene - propylene rubber (EPR), are almost never used as basic polymers in hot - melt adhesives.

Because of their intrinsic poor adhesiveness, these self-adhesive formulations based on fully hydrogenated styrenic block copolymers, like SEBS and SEPS, are therefore mainly used for applications that request an adhesive strength, expressed e.g. as "Peel Strength", that is not high, and in which, on the contrary, the improved stability to high temperature degrada- tions is particularly important. Examples of similar application fields are e.g. medical plasters and more generally several oth- er applications in the medical field; the so-called "removable adhesives" with a low peel strength, largely used in electron- ics, optics and other various industrial fields, and so on. For example, the patent US 6197 845 discloses hot-melt adhesive formulations, based on SEBS, that have a sufficiently low adhesiveness for allowing an article, such as a plaster, to directly adhere on the user's skin and then to be peeled off without causing any significant pain.

In a similar way, the patent US 8 721 832 discloses pressure sensitive hot-melt adhesives, suitable for forming ad- hesive tapes or strips that must have, in some peculiar uses, a sufficiently low adhesive strength, such that they are "rede- tachable without residue or destruction". The inventors high- light that the previous vast technical and patent literature, concerning adhesives based on SIS and SBS, discloses adhesive formulations that are very aggressive and which have an adhesive strength that is too high for the uses targeted by their inven- tion. Therefore, they assert to prefer hot-melt adhesive formu- lations based on partially or totally hydrogenated block copoly- mers; and actually, their disclosed Examples show formulations based only on hydrogenated styrenic block copolymers, like SEBS and SEPS.

Beyond these solutions, disclosed by the Prior Art for solving the previously mentioned problems, other more general problems remain still unsolved; problems that concern all hot- melt adhesive formulations based on styrenic block copolymers, both in their not-hydrogenated version (SIS and SBS) as well as in their fully hydrogenated one (SEBS or SEPS and similar satu- rated polymers).

Such more general problems are e.g. illustrated in the patent US 3 935 338 to Shell, i.e. the chemical company that first has industrially manufactured such polymers. It is neces- sary to also tell beforehand that, as for example explained in

Chapter 5 of the book "Handbook of Thermoplastic Elastome"rs by

J. G. Drobny, book that is herein incorporated as a reference, the styrenic block copolymers have significantly higher melt viscosities, if compared, at equal average molecular weights, to other polymers that do not have a multi-phase structure, for ex- ample poly-olefins,. This phenomenon has been explained with the fact that, even inside the molten polymer, at least up to about

180°C, such multi-phase structure, with fully incompatible and distinct styrenic and elastomeric regions, continues to exist.

Such "separated" morphology, even in the molten state, generates a much higher viscosity, even at equal average molecular weights, owing to the so called "filler effect", an effect that is widely known by all persons with an average skill in the sci- ence of polymers.

About this, just the patent US 3935338 highlights that all adhesive formulations based on styrenic block copolymers suffer from a series of general problems. In particular, their basic block copolymer(s), that as said has/have a very high mol- ten viscosity on its own, must be necessarily formulated (for developing its/their adhesiveness and for making it/them easily processable as hot-melt adhesives) first of all with a large quantity of one or more tackifiers, such tackifiers being com- pounds that can have various chemical compositions, but always with a low molecular weight. Typically, the Mass average Molecu- lar Weight Mw of industrially used tackifiers varies between

1,000 and 3,500 atomic mass units; only in rare cases their Mw can be as low as about 600 to 800, or higher than 3,500, up to about 5,000. Such binary blends, made by a block copolymer + a tackifier, even if they are significantly less viscous than the pure polymers, however still show viscosities that are still too high and that cannot be used in the field of hot-melt adhesives, like e.g. 15,000 mPa.s at 150°C or even greater.

Therefore, it is always necessary, for further lowering the viscosities to easily processable values, to add also some quantity of a liquid plasticiser at low viscosity, e.g. a miner- al oil. Said plasticising oils have very low average molecular weights, even lower than the ones of tackifiers, that are typi- cally centered around 500 units of atomic mass and generally variable between about 300 and 700.

However, the above mentioned patent US 3 935 338 high- lights also that too large quantities of these plasticising oils may significantly impair both the cohesion and the adhesive strength of a formulation. But said patent doesn't take into ac- count a further and very important negative effect, that is shown by adhesive formulations, based on styrenic block copoly- mers and that contain large quantities of low molecular weight additives. Such critical negative effect consists in the fact that said adhesives, formulated with high levels of plasticising oils in order to have acceptably low viscosities and containing also high levels of tackifiers, in order to develop a sufficient adhesiveness, are not fully stable during suitably long times.

In fact, as clearly explained e.g. in Chapter 3 of the book "Technology of Pressure-Sensitive Adhesives and Products" by I. Benedek and M. Feldstein, book that is herein incorporated as a reference, hot-melt adhesives, based on styrenic block co- polymers are necessarily formulated by using plasticisers and tackifiers that are compatible only with the central elastomeric block of the polymer molecule; on the contrary, it is demanded that such additives are totally incompatible with the styrenic blocks, in order to avoid a disastrous loss in the adhesive's cohesion. Therefore, it's extremely frequent that, in this class of formulations, the quantities of plasticisers and tackifiers that one must add exceed the "limit-levels" that can be held by the central elastomeric block.

This phenomenon can quite often cause -over time- a par- tial demixing from the adhesive formulation of plasticising oils and of tackifiers, that, because of their very low molecular weight that favours a quick molecular mobility, migrate inside the adhesive itself, eventually blooming on its surface in sig- nificant quantities and separating from the adhesive formula- tion. This is a quite negative phenomenon, that is known in the technology of hot-melt adhesives with the word "bleeding" and that causes various deleterious problems in this family of adhe- sives. In particular, the separation, even a partial one, of plasticisers and tackifiers, even on short times and at room temperature, causes a significant worsening of all the main ad- hesive's properties, starting from an unacceptable decay of its adhesiveness, and an equally unacceptable increase in its molten viscosity. Moreover, these additives, that migrate and bleed on the adhesive's surface in the form of very thin liquid films, can cause the detachment of the adhesive itself from the surfac- es on which it had initially adhered.

Finally, it's important to recall that, especially for applications in the field of absorbent hygienic articles, said low molecular weight additives that bleed on adhesive's surface, can generate bad odours, can stain the user's underwear and even can, in a possible contact with the skin, cause skin rashes or dangerous allergic effects.

It is therefore very important, both for keeping during time the initially good adhesive and cohesive properties and for avoiding harmful and potentially dangerous effects in use, to avoid or limit to the utmost this deleterious phenomenon of sep- aration on long times, from the formulation, of the low molecu- lar weight additives, such as tackifiers and plasticisers.

Thus, we can conclude that, in spite of the vast exist- ing technical and patent literature over more than fifty years after the market introduction of styrenic block copolymers in the field of hot-melt adhesives, both in the unsaturated and saturated forms, adhesive formulations that can offer at once a satisfactory overall solution for the main weakness of these ad- hesives are still missing; i.e. that may in particular show high adhesiveness and cohesion, combined with an excellent thermal stability; moreover, further having also a low melt viscosity and a substantial stability on long times of adhesive proper- ties. This latest characteristic can also be expressed as a sub- stantial absence of internal migrations / separations of the low molecular weight additives, like plasticisers and tackifiers, that it is always necessary to add, even at high levels, in for- mulating these block copolymers.

SUMMARY OF THE INVENTION

The problem that the present invention intends to solve is to formulate self-adhesive hot-melt formulations, that show significantly improved characteristics. In particular: an unexpectedly novel and improved combination between an excellent adhesiveness and an equally excellent cohe- sion, two properties that are generally antithetical in a hot-melt adhesive. This allows to surprisingly achieve adhesive properties (e.g. Peel Strength) that are sig- nificantly improved, even when compared with similar formulation, for example based only on other SIS and SBS copolymers, or even more based only on SEBS or SEPS co- polymers. Furthermore, the present novel hot-melt formu- lations are able to combine said excellent and improved adhesiveness with an unexpectedly good cohesion, at a level normally not achievable in adhesives based only on

SIS or SBS block copolymers. This unexpectedly improved combination of properties makes the present hot-melt ad- hesive formulations especially suitable for being used as self-adhesive / pressure sensitive glues in the field of absorbent hygienic articles. In fact, from one side their significantly improved adhesiveness allows the ab- sorbent hygienic articles, containing the present adhe- sives, not only to strongly adhere on the user's under- wear, but also to avoid all possible displacements and detachments of the article during its use, in spite of the most various movements of the user's body. Besides this, their exceptional cohesion allows e.g. to practi- cally get rid of any possible adhesive's residues that might remain on the underwear or on the user's skin, during the article's removal. a significantly improved stability on long times of the initial and optimal adhesive and cohesive properties of the present adhesives, with obvious advantages both for the user and for the manufacturer of articles that con- tain the present self-adhesive formulations; a surprisingly improved retention, inside the present self-adhesive formulations, of plasticisers and of other low molecular weight ingredients, present in such formu- lations, especially of tackifiers, blocking their sepa- ration from the formula and their migration / bleeding outside the adhesive itself; a low molten viscosity, in spite of their mentioned op- timum cohesion, which two properties are again generally antithetical in a hot-melt adhesive. Such particularly low viscosity of the present formulations, not only al- lows their excellent processing and application even e.g. on industrial lines working at high or very high speed ( for example 200 m / minute and even more) but it allows also to apply the present adhesives in the molten state, at low temperatures, significantly lower than the ones of similar previous formulations. This fact further allows to use them also on thermo-sensitive substrates or on unusually thin plastic films, that would be de- formed or destroyed by the contact with too hot adhe- sives; a significantly increased stability at high tempera- tures, which allows their processing and application in the molten state without substantial degradations, caused by the combined action of heat and of atmospheric oxygen.

All the above mentioned characteristics, that are highly and unexpectedly improved compared to the prior art, are at once achieved by an adhesive formulation having the characteristics of claim 1) and of claims from 2) to 29); by a bonded structure showing the characteristics of claim 30); by an absorbent hy- gienic article showing the characteristics of claims from 31) to

33); and by other articles showing the characteristics of claims 34) and 35). The other sub-claims disclose preferred em- bodiments.

DETAILED DESCRIPTION OF THE INVENTION

In general terms, the present invention concerns novel self-adhesive hot-melt formulations that comprise a polymer com- position formed by:

A) from 5% to 95% by weight of at least one elastomeric styrenic block copolymer, linear, non-hydrogenated / unsaturated and with an asymmetrical structure, that i.e. in its triblock fraction, has a structure repre- sented by the following formula: S1 D S2 where S1 and S2 represent blocks of poly-styrene having differ- ent molecular weights; and D represents a central elastomeric unsaturated / non-hydrogenated block of a homopolymer of a con- jugated diene, for example poly-isoprene or poly-butadiene, or of a copolymer of two conjugated dienes, e.g. a poly-isoprene- butadiene copolymer. This linear and unsaturated styrenic block copolymer, with an asymmetrical structure or the blend of said copolymers, may contain up to 75% by weight of diblock.

B) from 95% to 5% by weight of at least one elastomeric styrenic block copolymer, having a linear or radial structure, that has a fully hydrogenated central elastomeric block or blocks and that has also a symmetrical structure, i.e. that, in its triblock or poly-block fraction, has a linear or radial structure represented by the following formula:

S3 C - S3 or (S3 - C)n - X wherein S3 represents two or more blocks of poly-styrene having the same molecular weight and C represents one or more central elastomeric blocks, fully hydrogenated / saturated, of a poly- ethylene-butylene copolymer or of a poly-ethylene-propylene co- polymer or of a poly-ethylene- co (ethylene / propylene) copoly- mer. Moreover, in the radial copolymers, X represents a poly- functional group that assures the chemical bond among the dif- ferent arms of the copolymer and where n is an integer, greater than 2. This styrenic block copolymer, linear or radial, fully hydrogenated and with a symmetrical structure or the blend of said copolymers, may contain up to 35% by weight of diblock.

In a first embodiment of the present invention, the Mass average

Molecular Weights Mw of the two different blocks of poly-styrene S1 and S2 of polymer A) range from 8,000 and 40,000 atomic mass units and differ one from the other for at least 5,000 atomic mass units.

In a second embodiment of the present invention, when polymer A) contains a diblock level different from zero, the diblock struc- ture may be represented by the formula S1 - D or by the formula

S2 - D or by blends of these two structures. It is herein pre- ferred the case in which the diblock fraction is substantially formed by molecules S1 - D, in the hypothesis that S1 represents the poly-styrenic block with the lowest molecular weight between S1 and S2.

In a third embodiment of the present invention, the above disclosed polymer composition preferably comprises from

10% by weight and 90% by weight, more preferably from 15% by weight and 85% by weight of copolymer A); and preferably from

90% by weight and 10% by weight, more preferably from 85% by weight and 15% by weight of copolymer B), referred to the total weight of A) plus B).

In a fourth embodiment of the present invention, the above disclosed polymer composition builds up from 10% by weight to 50% by weight of the total hot-melt adhesive formulation, and preferably from 15% by weight to 40% by weight.

In a fifth embodiment of the present invention, the herein disclosed self-adhesive hot-melt formulations comprise also, in addition to the above disclosed polymer composition, at least a further styrenic block copolymer, non-hydrogenated and with a symmetrical structure, with i.e. poly-styrenic block with equal molecular weights, that is also linear or radial, and the central elastomeric block (or blocks) of which is / are identi- cal, for what concerns its / their chemical nature, to the cen- tral elastomeric blocks of the non-hydrogenated asymmetrical co- polymer (SIS or SBS or S - I/B - S) comprised in the basic poly- mer composition. Moreover, in this embodiment, the self-adhesive formulations, disclosed herein may also comprise a polyolefin or a polyolefin copolymer, both preferably amorphous, or their blends.

In a sixth embodiment of the present invention, the self-adhesive hot-melt formulations, disclosed herein, also com- prise from 30% by weight to 80% by weight of at least one tacki- fier or of a blend of tackifiers; and from 5% by weight to 50% by weight of at least one plasticiser or of a blend of plasti- cisers, plasticizers that are liquid at room temperature.

In a seventh embodiment of the present invention, said plasticizer or blend of plasticisers comprises at least one syn- thetic poly-alpha-olefin (PAO), that is liquid at room tempera- ture.

The self-adhesive hot-melt formulations according to the present invention surprisingly show a combination of highly val- ued and positive properties for an adhesive, that are signifi- cantly improved compared to similar pressure sensitive adhe- sives, based on styrenic block copolymers, both non-hydrogenated / unsaturated or fully hydrogenated / saturated, but that do not comprise the above disclosed novel polymer composition.

These properties, that are significantly and surprising- ly improved compared to the usual self-adhesive formulations disclosed by the previous Art, allow the use of the present ad- hesives mainly in the field of absorbent hygienic articles, e.g. for feminine hygiene or for incontinent adults.

In particular, such highly positive and surprisingly im- proved properties, compared to what disclosed in the previous

Art, may be summarised as follows: a greatly improved adhesiveness that for example allows the absorbent hygienic articles, comprising the present self-adhesive formulations, not only to strongly adhere on the user's underwear, but also to stay.in-place with- out moving or detaching during use, even during a pro- longed wearing lasting several hours, in spite of all the most various movements of the user: himself / her- self. an optimal cohesion, coupled with the above mentioned strong adhesion, which fact is an especially important achievement for practical uses, because often the two properties of high adhesion and high cohesion are mutu- ally in contrast (see also later). Said high cohesion of the adhesive formulations disclosed herein, allows to practically get rid of any possible adhesive's residues that might remain on the underwear or on the user's skin, during the article's removal. a surprisingly good stability on long times of the adhe- sive properties of the present hot-melt formulations; especially of their Peel Strength that remains practi- cally constant from the moment of the glue's manufactur- ing to its final use, with obvious advantages both for the user as well as for the manufacturer of articles that comprise the present adhesives, e.g. during the im- mediate quality control of articles, just after the man- ufacturing on industrial lines. an exceptionally good retention inside the formulation of plasticizers and of all the other possible components at low molecular weight (especially the tackifiers), blocking their separation / de-mixing and their migra- tion / bleeding outside the adhesive itself. a low melt viscosity, despite the previously mentioned high cohesion (low molten viscosity and high cohesion in the solid state are generally two antithetical proper- ties in a hot-melt adhesive, as it is well known by any person averagely expert in this field). a significantly increased stability at high temperatures against thermal and oxidative degradations in hot condi- tions, which fact allows their application at high tem- peratures, e.g. between 130°C and 180°C, without any substantial degradation, caused by the combined action of heat and of atmospheric Oxygen. an excellent processability, eevveenn in processes at high line speeds, like 200 m / min and more.

BEST WAY FOR PRACTICING THE PRESENT INVENTION

It is possible to highlight that two of the above men- tioned, substantially improved characteristics, i.e. the high consistency in time of the adhesive properties and the improved retention of low molecular weight components, are most likely somehow interrelated, because an undesired separation / migra- tion on long times of any component out of the formulation can only contribute to modify its adhesive characteristics. Further- more, it is the case to also highlight that a substantially de- creased migration / separation of low molecular components out of the adhesive is a particularly important and welcome improve- ment, especially in the field of absorbent hygienic articles, because in this way both possible contaminations of the under- wear and of the user's body itself are avoided, and possible al- lergic or skin-irritating reactions that might be caused by low molecular weight ingredients, like plasticising oils and tacki- fiers, are also avoided.

As already said, the hot-melt self-adhesive formula- tions, according to the present invention, comprise a polymer composition that, per se, comprises:

A) from 5% by weight to 95% by weight of at least one elas- tomeric styrenic block copolymer, linear, non- hydrogenated / unsaturated and with an asymmetrical structure, i.e. in its triblock fraction, has a struc- ture represented by the following formula: S1 D S2 where S1 and S2 represent blocks of poly-styrene having different molecular weights; and D represents a central elastomeric block unsaturated / non-hydrogenated of a homopolymer of a conjugated diene, for example poly- isoprene or poly-butadiene, or of a copolymer of two conjugated dienes, e.g. a poly-isoprene-butadiene copol- ymer. This linear and unsaturated styrenic block copoly- mer, with an asymmetrical structure or the blend of said copolymers, may contain up to 75% by weight of diblock.

B) from 95% to 5% by weight of at least one elastomeric styrenic block copolymer, having a linear or radial structure, that has a fully hydrogenated central elas- tomeric block and that has also a symmetrical struc- ture, i.e. that, in its triblock or poly-block frac- tion, has a linear or radial structure represented by the following formula: S3 C - S3 or (S3 - C)n - X where S3 represents two or more blocks of poly-styrene having the same molecular weight and C represents one or more central elastomeric blocks, fully hydrogenated / saturated, of a poly-ethylene-butylene copolymer or of a poly-ethylene-propylene copolymer or of a poly- ethylene- co ethylene / propylene) copolymer. Moreo- ver, in the radial copolymers, X represents a poly- functional group that assures the chemical bond among the different arms of the copolymer and where n is an integer, greater than 2. This styrenic block copoly- mer, linear or radial, fully hydrogenated and with a symmetrical structure or the blend of said copolymers, may contain up to 35% by weight of diblock.

We have already mentioned the peculiar bi-phasic micro- scopic structure that all styrenic block copolymers have in their solid state; and we have also anticipated that the quanti- ty and morphology of the poly-styrenic regions (domains), that are present together with the elastomeric phase, strongly affect most of the chemical-physical parameters of the block copolymer and hence of the adhesives in which it is comprised. For exam- ple, this morphology influences the softness of the adhesive (a property given by the elastomeric phase) and, as a consequence, the tackiness and adhesiveness of the derived adhesives; or also it influences the cohesion, essentially given by the styrenic domains ("hard" phase) and by their number, volume, geometrical shape and so on.

At the same time, said microscopic bi-phasic morphology substantially affects also other essential parameters of a hot- melt adhesive, besides its balance between a good adhesiveness and a good cohesion. In particular, this bi-phasic morphology affectsi in a particularly significant way the adhesive's melt viscosity, even at equal level of copolymers contained in the adhesive and at equal average molecular weight of said copoly- mers, and consequently it strongly affects also the hot-melt's processability. As e.g. thoroughly explained in Chapters 3 and 11 of the book "Thermoplastic Elastomers" by N.R. Legge, G.Holden and H.E.

Schroeder, book the contents of which are herein incorporated as a reference, at equal chemical nature and average molecular weight of the central elastomeric block, as the weight fraction

(and therefore also the volume fraction) of poly-styrene inside the copolymer increases, the dispersed poly-styrenic phase takes essentially three different morphologies: at relatively low lev- el of poly-styrene, the poly-styrenic domains have a morphology formed by many microscopic spheres, separated one from the other and that are homogeneously dispersed inside the continuous and incompatible elastomeric phase. As the quantity of poly-styrene in the copolymer increases, and therefore also the number and volume of these poly-styrenic domains increase, the spherical morphology of these isolated regions of poly-styrene disappears, and the styrenic phase takes the shape of long cylinders, which cylinders, being bulkier and even more being "more continuous in space" on significantly larger dimensions, substantially stiffen the copolymer, therefore affecting in a very negative way the tackiness and adhesiveness of a possible derived adhesive formu- lation. If the level of poly-styrene further increases, there is an additional change in the microscopic biphasic morphology: the elastomeric phase is no more the "continuous phase and the co- polymer's microscopic morphology becomes a "lamellar structure" with alternated lamellae, made of pure poly-styrene and of pure elastomer. It is evident that an adhesive with a morphology like this latest one, is completely useless because of its excessive hardness. The levels of poly-styrene at which these changes be- tween the different morphologies occur depend on the level of incompatibility between the poly-styrene and the elastomeric phase; and therefore these changes depend on the chemical nature of the elastomeric block, as well as on the average molecular weights of the various blocks.

Again from the mentioned reference, we can add , as an example, that a Styrene - Isoprene - Styrene copolymer typical- ly shows a morphology with spherical styrenic domains, homoge- neously dispersed inside the continuous poly-isoprene phase, up to a level of poly-styrene of about 20 to 25 % by weight. If the poly-styrene level increases over this limit, the SIS copolymer passes to a cylindrical morphology of its styrenic domains, a morphology that has an increased cohesion, but that has also a much lower softness and adhesiveness; and furthermore this cy- lindrical structure transforms into an "alternated lamellar" structure, with alternated lamellae of poly-styrene and poly- isoprene, when the overall content of styrene reaches about 35 to 40% by weight of the copolymer.

For the sake of a complete information, one can add that, if the styrene's content in the copolymer continues to in- crease, the poly-styrenic phase becomes the continuous phase, while the elastomeric phase becomes the dispersed phase, first in the shape of cylinders and then, if the poly-styrene's level becomes greater and greater, in the shape of dispersed spheres, in this way retracing, with inverted phases, the already de- scribed morphologies.

It is clear that, for having a sufficient softness and so a sufficient tackiness and adhesiveness, only those morpholo- gies that have a continuous elastomeric phase and a dispersed poly-styrenic phase, with a spherical geometry, can give good adhesives.

It is furthermore also clear that, on the basis of what said, the number and strength of said spherical styrenic domains are the main cause of the excellent cohesion shown by these co- polymers. However, it is also true that, even if a spherical morphology of styrenic domains is kept, an increase in their number and volume leads to at least two major negative effects: an impairment of the adhesive properties, caused in any case by the hardening of the adhesive; and a negative increase of the melt viscosity, due to the well known and already mentioned

"filler effect".

As again thoroughly explained in the above mentioned reference, and for making even more complex the discussion about such microscopic morphologies of styrenic block copolymers and of adhesives based on them, it is necessary to add that the lev- el and the "sharpness" in the separation between the two dis- tinct phases, the elastomeric phase and the styrenic domains, are greatly affected by the incompatibility between poly-styrene and the specific type of elastomer used in a certain block co- polymer. As widely known by any person averagely skilled in the science of polymers, such incompatibility, at equal chemical na- ture of the elastomer, depends on the average molecular weights of the elastomeric and of the styrenic blocks; and it increases with the increase of all these molecular weights. For example, all other conditions being equal, in order to have a sufficient- ly sharp phase-separation, that can give the typical bi-phasic behaviours of these copolymers, it is necessary that the various blocks in a certain copolymer, both the elastomeric blocks and even more the poly-styrenic ones, have average molecular weights that are not lower than about 5,000 atomic mass units.

About the influence of the chemical nature on the extent and morphology of this phase-separation, all the fully hydrogen- ated elastomers, like poly-ethylene-butylene and poly-ethylene- propylene, show the strongest incompatibility with poly-styrene.

Therefore, the derived block copolymers SEBS and SEPS are the ones that show the maximum phase-separation of styrenic domains; hence, they show also the maximum cohesion, but at the same time also the maximum hardness (and therefore the lowest adhesive- ness) and even the maximum viscosity in the molten phase, vis- cosity that is often so high to be "at the upper limit" for a not too difficult processability.

On the contrary, the incompatibility between poly- styrene and poly-butadiene, even if it is anyhow significant, is substantially lower than the one of the previous copolymers.

Therefore, at equal level of poly-styrene, the SBS block copoly- mers show a "less sharp" phase separation and hence a better softness and adhesiveness, and a relatively lower melt viscosi- ty; however, for the same reasons, they are showing also a slightly worse cohesion.

Finally, the SIS block copolymers, that contain poly- isoprene as their elastomeric block, show a good adhesiveness, but suffer again from a relatively insufficient cohesion, espe- cially when SIS copolymers that contain high levels -e.g. 20% by weight and more- of diblock molecules are formulated in an adhesive (as it often happens for increasing to the utmost the adhesiveness), that are able to substantially improve the adhe- siveness, but that at the same time substantially impair the co- hesion.

For concluding, it is therefore clear that the micro- scopic morphology and geometric structure of the dispersed poly- styrenic phase are essential for determining an optimum balance between the various adhesive's properties that are equally im- portant for an adhesive; especially between its adhesiveness, cohesion and low melt viscosity (and consequently its good pro- cessability).

The inventors of the present invention have surprisingly discovered that, by using peculiar styrenic block copolymers, linear and non-hydrogenated, that therefore can formally belong

(considering just their overall chemical composition) to the al- ready well known families identified with the acronyms SIS or

SBS or S I/B - S, but that anyway possess as their fully novel and peculiar characteristic, compared to all the previous simi- lar polymers, an asymmetrical structure of their styrenic blocks, that i.e. exhibit two poly-styrenic blocks having suffi- ciently different molecular weights in the same triblock mole- cules; and then by combining, in a novel polymer composition, said peculiar asymmetric and linear SIS or SBS or S I/B S with at least one fully hydrogenated, linear or radial, styrenic block copolymer, having on the contrary a symmetrical structure, i.e. belonging, as chemical composition, to the families known with the acronyms SEBS or SEPS or SEEPS, the hot-melt adhesives formulated from said peculiar and novel polymer composition ex- hibit significantly improved properties, compared both to the correspondent adhesives that are formulated by separately using the two types of copolymers comprised in the present polymer composition, and also compared to analogous formulations that, together with the same SEBS or SEPS, comprise a chemically simi- lar SIS or SBS (e.g. with the same level of styrene and of iso- prene or butadiene), but that on a contrary has a "symmetrical " structure, i.e. that contains two styrenic blocks with the same molecular weight in the same triblock molecule, as it happens in most commercial SIS and SBS copolymers presently manufactured and traded.

Said surprisingly improved properties, compared to the previous Art, can be summarised as the contemporary presence of at least four essentially important characteristics for a hot- melt adhesive:

1) an exceptionally good combination between good adhesive- ness and good cohesion. Said optimised combination at a contemporary high level, of these two generally anti- thetical properties in a hot-melt adhesive, is revealed in an especially apparent and unexpected way, for exam- ple, by the fact shown e.g. in the below Examples that it is possible to keep a very god cohesion (that is typical of pure SEBS and SEPS copolymers) even blending them with major quantities of an SIS copolymer, even more containing (for developing the maximum adhesive- ness) unusually large quantities of diblock, equal to

50% by weight or even greater, aatt least up ttoo 75% by weight. AAss it is well known by any person, averagely skilled in the field of hot-melt adhesives, formulations based on symmetrical SIS or SBS and containing high or very high levels of diblock (e.g. greater than about 40% by weight) exhibit excellent adhesiveness, however com- bined with a poor cohesion, a cohesion that gets worse and worse the more the percent content of diblock is in- creased. In fact, symmetrical SIS (or SBS) with a high level of diblock are generally used only in minor quan- titles as "enhancers of adhesiveness", together with other SIS (or SBS) at low diblock. They are practically never used as main polymeric components of an adhesive, which would have a very poor cohesion; while on the con- trary the adhesive formulations according to the present invention may surprisingly use similar high-diblock SIS copolymers as their main polymeric ingredient in the formulation, without losing the optimal cohesion;

2) an unexpectedly good consistency and maintenance in time of the excellent initial adhesive and cohesive proper- ties, a characteristic that is revealed also by an out- standing ability in retaining inside the formulation, all the low molecular weight ingredients, like tackifi- ers and plasticisers, blocking or decreasing to minimal negligible levels their migration and separation from the formulation;

3) a very significantly improved resistance to thermal and oxidative degradations at high temperatures;

4) a positively low melt viscosity, even when one uses in formulating copolymers at high molecular weight (with the aim e.g. of further improving the already good cohe- sion). This low viscosity allows an excellent processa- bility and an easy application of the present adhesive formulations, even on lines operated at high or very high speed (e.g. 200 m/min and even higher) even at rel- atively low temperatures, with the possibility therefore to apply them even on thermo-sensitive substrates, that would be distorted or destroyed by adhesive applied at too high temperatures.

The combination, at an optimal level, of these four im- portant characteristics, makes the herein disclosed novel self- adhesive hot-melt formulations especially suitable for being used as pressure-sensitive adhesives in absorbent hygienic arti- ti es. In fact, for example, the high adhesiveness, combined with a high cohesion, allows an easy application in use by bonding the absorbent article on the user's underwear. The article

"stays in place", i.e. does not move or detaches during its use even when, on the contrary, the user has sudden and forceful movements. And at the end of its use, at the moment of the arti- cle's detachment from the user's underwear, the article can be removed without leaving any adhesive residues on the underwear or on the skin.

Moreover, the excellent stability, both against thermal and oxidative degradations as well as against the migration and separation of tackifiers and plasticisers, avoids the generation of malodorous substances. It avoids also that such low molecular weight compounds may contact the user's skin, causing possible rashes, and in some more severe cases, even severe allergic re- actions.

Without for this binding to any specific theory, one might e.g. suppose that the "more disordered" structure of an asymmetrical SIS copolymer (compared to a similar symmetrical copolymer), i.e. with two styrenic blocks having significantly different molecular weights, makes somehow slower and more dif- ficult the "aggregation / gathering" of the styrenic blocks of the various molecules, into poly-styrenic spherical domains dur- ing the polymer's solidification from the melt.

In particular, one might suppose that, in comparing the morphologies of the formed poly-styrenic spherical domains, com- pared to a symmetrical SIS with an equal overall molecular weight and an equal level of styrene, an asymmetrical SIS exhib- its a different morphology of the styrenic domains, with more numerous and on average smaller spheres. This because, in con- sidering the two poly-styrenic block of each polymeric chain, one block having a higher molecular weight than the other, the shorter blocks may more easily work as multiple "formation nu- clei" for more numerous new distinct styrenic domains; while the block at heavier molecular weight have a much higher steric dif- ficulty in diffusing inside the mass and in reaching the far- thest, already formed styrenic domains for merging with them.

This change in the morphology of the styrenic domains

(in spite of still keeping a spherical geometry for them) sig- nificantly affects a few properties of the block copolymers and hence of the adhesives containing such polymers.

In particular, by comparing an asymmetrical SIS with a symmetrical SIS, both having the same poly-styrene content and similar molecular weight, one can infer that, on first approxi- mation, an SIS with asymmetrical styrenic blocks should be sig- nificantly softer (the hardness of a block copolymers depends both on the percent of poly-styrene and on its morphology, like the average volume of the poly-styrene domains), but that it is also less mechanically strong.

This is exactly what is observed for example in two lin- ear and similar SIS copolymers, an asymmetrical one Quintac

3190 and a symmetrical one Kraton D 1165 that have the same overall content of styrene (30% by weight) and similar mo- lecular weights, copolymers that are used in the below Examples

(see later for more information on these two copolymers). For example, while the asymmetrical SIS Quintac 3190 has a Shore A hardness (measured according to ASTM D2240) equal to just 42, the analogous symmetrical SIS Kraton D 1165 has a Shore A hard- ness equal to 59, i.e. greater for more than 40%. In a similar way, while the asymmetrical Kraton SIS has a Tensile Strength at

Break (measured according to ASTM D412-06) equal to 21 MPa, the asymmetrical Quintac 3190 copolymers has a Tensile Strength at

Break of about 8 MPa, therefore lower for about 62%.

These properties, that significantly vary only for the influence of the asymmetry iinn the molecular weights of the styrenic blocks, however affect also two other important proper- ties of the derived adhesives, one of these two properties being modified in a very positive way, and the second one being modi- fied in a negative way. I.e., in comparing two hot-melt adhe- sives, formulated with equal contents of the basic copolymer and of the other ingredients, and separately using in two parallel formulations, an asymmetrical SIS and a symmetrical one, both with the same overall level of styrene, it is reasonable to ex- pect that the adhesive comprising just the asymmetrical SIS has a significantly stronger adhesiveness than the adhesive that is based on just the similar symmetrical SIS, given its much stronger softness.

However, one also expects that, owed to the lower "ro- bustness" of its styrenic domains, that in an asymmetrical SIS are on average smaller, an adhesive formulated just with the co- polymer Quintac 3190 has also a cohesion that is lower than the cohesion exhibited by a similar formulation based just on the similar symmetrical copolymer Kraton D 1165.

This fact has as a consequence that adhesive formula- tions containing only an asymmetrical SIS, like Quintac 3190, are adhesive with excellent, very high adhesiveness that however suffer from overall unsatisfactory performances, mainly because of their very poor cohesion, as shown e.g. in the below Compara- tive Example 1.

What the inventors of the present inventions have sur- prisingly found is that, by blending an asymmetrical SIS, like

Quintac 3190, with a fully hydrogenated symmetrical copolymer, i.e. of the type SEBS, SEBS etc. (being these latest copolymers typically much harder and less adhesive than SIS), it is not possible to notice, as one could expect, any significant harden- ing of the blend. The high softness and hence adhesiveness of the asymmetrical SIS is substantially fully retained even in the blend, while it is significantly increased, to optimum levels, the adhesive's cohesion.

On the contrary, if an equal quantity of the same SEBS is for example blended with an analogous symmetrical SIS, the increased level of poly-styrene hardens the adhesive in an ex- cessive way, causing a sharp and severe further decrease in the already lower adhesiveness, as demonstrated by the below Compar- ative Example 3.

Without being again bound by any theory, one can for ex- ample suppose that the already discussed phenomena about the morphology of styrenic domains, and the differences between an asymmetrical and a symmetrical SIS, even at equal overall con- tent of styrene, remain valid even when one blends such SIS co- polymers with an SEBS. Even more, one could suppose that the fact of adding, to an already "disordered" asymmetrical SIS, an SEBS that contains a third type of styrenic blocks, different for length and molecular weight from both the two styrenic blocks of the asymmetrical SIS, creates a further increase in the "molecular disorder" that makes even more difficult and slower the growth of the spherical styrenic domains. Therefore, in spite of an increase in the overall content of poly-styrene in the formulation, given by the addition of SEBS, the poly- styrenic domains remain numerous and relatively small, in this way neutralising the significant hardening that might derive from the addition of the SEBS' poly-styrene, and in this way keeping an excellent adhesiveness together with the new and sub- stantially increased cohesion.

On the contrary, in a similar formulation, based instead on a symmetrical SIS, one can think that the "new disorder", generated by the introduction of the SEBS' styrenic block with a different molecular weight, is not sufficient for balancing the prevailing hardening effect, due to the massive additional level of poly-styrene. Therefore, the formulations becomes globally harder and tends to lose adhesiveness, as e.g. shown by the be- low Comparative Example 3, that has a Loop Tack of 8.8 N, i.e. lower by about 34% than the Loop Tack of the Example 1 according to the present invention; this just by simply substituting the linear and asymmetrical SIS Quintac 3190, having 30% by weight of poly-styrene (used in the below Example 1 according to the present invention), with an equal quantity of the linear but symmetrical SIS, still at 30% by weight of poly-styrene, Kraton

Dll65 (used in the Comparative Example 3).

Moreover, from the same Comparative Example 3, when com- pared with Example 1 according to the invention, it is possible to notice two further negative effects, due to blending with the same SEBS, not a linear and asymmetrical SIS, as taught by the present invention, but on the contrary a similar SIS, with the same content of styrene, but having however a symmetrical struc- ture. Such two additional negative effects are both a poor sta- bility over time of the formulation, revealed by its very bad

Index of Oil Migration, equal to as much as 0.40, and the unac- ceptably high change of its Peel Strength after aging. Even the melt viscosity of the comparative formulation, containing the symmetrical SIS, is significantly increased, which is a further negative effect. Also in this case, one can suppose that said increase in viscosity, caused e.g. by the additional incompati- bility between the two different elastomers contained therein, poly-isoprene and poly-ethylene-butylene, is severely revealed in the blend of SEBS with the symmetrical SIS, while, on the contrary, this negative effect is also better balanced and miti- gated in the blend of SEBS with the asymmetrical SIS, most like- ly again for the peculiar differences in the microscopic mor- phologies of these two polymeric systems.

For achieving the unexpected and very positive and use- ful novel effects and properties, outlined above, the inventors found that, in the triblock molecules of the styrenic block co- polymer, linear, non-hydrogenated and with an asymmetrical structure, the Mass average Molecular Weights of the two poly- styrenic blocks, present in the same triblock molecule, must differ by at least 5,000 atomic mass units, preferably they must differ for at least 8,000 atomic mass units, and even more pref- erably they must differ by at least 10,000 atomic mass units, in any case being equal to at least 5,000 atomic mass units the minimum Mass average Molecular Weight of all the poly-styrenic block present in the polymer composition, that is comprised in the self-adhesive hot-melt formulations according to the present invention.

In the case when said styrenic block copolymer, linear, non-hydrogenated and with an asymmetrical structure, comprises also diblock molecules, in such diblock molecules the poly- elastomeric block (poly-isoprene or poly-butadiene or poly- isoprene-butadiene) has a Mass average Molecular Weight that is substantially equal to the weight of the analogous elastomeric block present in the triblock molecules, and said Mass average Molecular Weight is in any case equal to at least 5,000 atomic mass units. On the contrary, the Mass average Molecular Weight of the styrenic blocks in the diblock molecules, may be equal to the molecular weight of the shorter styrenic block, present in the triblock molecules; or, vice versa, it may be equal to the molecular weight of the longer styrenic block, present in the triblock molecules; or, even, the diblock fraction of the copol- ymer may be a random blend of diblock molecules, some with the shorter styrenic blocks, and other ones with the longer styrenic blocks.

It has been observed that, all other boundary conditions being equal, the case in which the diblock molecules of the co- polymer SIS or SBS or S-I/B-S, have a poly-styrenic block, the

Mass average Molecular Weight of which is substantially equal to the lowest average molecular weight, between the weights of the two different poly-styrenic blocks, present in the triblock mol- ecules, seems to generate the above mentioned beneficial effects to a greater extent, and therefore this option is the preferred one. Without being again bound by any theory, one can suppose that a larger quantity of poly-styrenic blocks, having a rela- tively lower molecular weight, may favour the already mentioned phenomenon of a more frequent nucleation of poly-styrenic do- mains, that are then even more numerous and on average with smaller volumes.

The self-adhesive hot-melt formulations, disclosed in the present invention, and that comprise the above described polymer composition, comprise said polymer composition in a quantity that may range from 10% by weight to 50% by weight, preferably from 15% by weight and 40% by weight.

OTHER COMPONENTS OF THE HOT-MELT ADHESIVES ACCORDING TO THE PRE-

SENT INVENTION

Additional polymers

In a further embodiment of the present invention, the herein disclosed self-adhesive hot-melt formulations comprise also further optional polymeric ingredients, in addition to the above disclosed polymer composition. In particular, said option- al polymeric ingredients can belong to three different chemical families: i) elastomeric styrenic block copolymers, linear or radial, non-hydrogenated and with a symmetrical structure, i.e. the triblock molecules of which have two poly-styrenic blocks of the same length and same average molecular weight; ii) poly- olefins and their copolymers; iii) their blends.

The addition of such further polymeric ingredients is generally aimed at improving even more the tackiness and the ad- hesive properties of the herein disclosed hot-melt formulations.

Then, those polymers, homopolymers or copolymers, that are per se softer and tackier, are preferred.

Therefore, in the case when are added, as optional addi- tional ingredients, an elastomeric styrenic block copolymer (or a blend of elastomeric styrenic block copolymers) that is / are linear or radial, non-hydrogenated and with a symmetrical struc- ture, this copolymer or blend of copolymers has first of all an elastomeric block that is equal, in its chemical nature, to the one of the elastomeric styrenic block copolymer, linear, non- hydrogenated and with aann asymmetrical structure, comprised in the basic polymer composition. This is order to favour the com- patibility of the resulting blend. Furthermore, for influencing in a positive way the overall adhesiveness, the optionally added copolymer or blend of copolymers, is particularly soft, i.e. it contains no more than 30% by weight of poly-styrene and prefera- bly it contains at least the 15% by weight of diblock.

On the contrary, in the case when said optional polymer- ic additional ingredients are a poly-olefin, a poly-olefin co- polymer or a blend thereof, said olefin polymers have preferably a rather low level of crystallinity, so as to be again particu- larly soft and tacky. Then, we prefer, as additional poly-olefin ingredients in the present formulations, amorphous-alpha-poly- olefins, known also with the acronym APAO, and their blends.

Said additional polymeric ingredients and their blends are comprised in the self-adhesive hot-melt formulations accord- ing to the present invention, in an overall quantity that can vary between zero and 20% by weight, and preferably between zero and 15% by weight.

Tackifiers

The self-adhesive hot-melt formulations, comprising the above described polymer composition, comprise also at least one tackifier or a blend of tackifiers. Said tackifier or tackifiers has/have a Ring & Ball softening point, measured according to

ASTM D 36-95, that is comprised between 5°C and 160°C.

Among all the possible families of tackifiers, well known in the field of hot-melt adhesives, the preferred ones, as components of the formulations according to the present inven- tion, belong to those chemical families that are the most com- patible with the various elastomeric blocks, contained in the herein used block copolymers, i.e. poly-isoprene, poly- butadiene, poly-ethylene-butylene etc. In fact, as widely known to every person with an average expertise in the field of hot- melt adhesives, the hot-melts based on styrenic block copoly- mers, can develop satisfactory adhesive properties through in- teractions between tackifiers and only their elastomeric blocks, interactions between tackifiers and styrenic domains being ex- eluded, as much as possible, which interactions might lead to a disastrous loss of cohesion.

Therefore the tackifiers comprised in the herein dis- closed, self-adhesive hot-melts, can be selected among those tackifiers that are the most compatible with the elastomeric blocks, like aliphatic and cyclo-aliphatic hydrocarbon tackifi- ers, and their partially or fully hydrogenated derivatives; ali- phatic/aromatic and cyclo-aliphatic / aromatic tackifiers, and their partially or fully hydrogenated derivatives; poly-terpenic and modified-terpenic tackifiers, and their partially or fully hydrogenated derivatives; rosins and Tail-Oil resins, their es- ters and their partially or fully hydrogenated derivatives. Hy- drocarbon tackifiers, both aliphatic, cyclo-aliphatic, aliphat- ic/aromatic and cyclo-aliphatic/aromatic, rosins and Tail-Oil resins and their esters and derivatives are particularly pre- ferred.

Said tackifiers and their blends, compatible essentially only with the poly-elastomeric blocks, make up from 30% by weight to 80% by weight, preferably from 40% by weight and 70% by weight and more preferably from 45% by weight and 65% by weight of the hot-melt adhesive formulation. As already discussed, pure aromatic resins generally need to be avoided, because being substantially compatible only with poly-styrenic domains, they may impair the hot-melt's cohe- sion, without being able to develop any tackiness or adhesive- ness. However, it is possible to add minor quantities of pure aromatic resins that in any case must have a particularly high Ring & Ball softening point, not lower than 100°C and preferably not lower than 110°C. In fact, it is know that such high sof- tening point aromatic resins may, on the contrary, positively contribute to an increase of the hot-melt's cohesion, because their addition to the poly-styrenic domains increases the do- mains' melting point, and their volume and strength. However, for not causing at the same time, also an unacceptable increase of the overall hardness, it is necessary that such pure aromatic resins with high softening points aarree added in suitably small quantities, not greater than 10% by weight of the global hot- melt formulation.

Plasticisers

The self-adhesive hot-melt formulations, according to the present invention, comprise also at least one plasticiser, liquid at room temperature, or a blend, liquid at room tempera- ture, of two or more plasticisers.

The plasticiser or the blend of two or more plasticis- ers, liquid at room temperature, are selected for example among paraffinic mineral oils and naphthenic mineral oils and their blends; paraffinic and naphthenic hydrocarbons, liquid at room temperature, and their blends; oligomers of poly-olefins and of their copolymers, liquid at room temperature, and their blends, like oligomers derived from ethylene, propylene, butenes, iso- butylene, their copolymers and their blends; plasticizers liquid at room temperature formed by esters, like phthalates, benzo- ates, sebacates etc.; vegetable oils; and blends thereof.

A particular preference is given to mineral oils, both paraffinic and naphthenic, and to their blends, and even more to the synthetic oligomers of poly-alpha-olefins, liquid at room temperature, known with the acronym of "PAO" (poly-alpha- olefin). Said synthetic oligomeric plasticisers, liquid at room temperature and suitable for the present invention, are synthe- sised from olefins from C2 to C20 and their blends and have a

Number Average Molecular Weight from 150 to 15,000 atomic mass units, preferably from 200 to 10,000 atomic mass units, and even more preferably from 400 to 6,000. Said PAO plasticisers are fully saturated and can have a substantially paraffinic struc- ture, both linear and branched. Among these liquid synthetic poly-alpha-olefins those that show a very narrow distribution of their molecular weights, are particularly preferred as expressed by a Polydispersity Index not greater than 2.5 and preferably not greater than 2.0 and are especially preferred the ones syn- thesised by using metallocenic catalysts.

The use, as plasticisers in the present self-adhesive hot-melt formulations, of these liquid synthetic poly-alpha- olefins, surprisingly further improves a few important proper- ties, that, while already present at an excellent level by using more traditional plasticisers, like e.g. mineral oils, show a further and unexpected improvement by using these liquid syn- thetic poly-alpha-olefins as plasticisers. Such further improved properties are in particular: an unexpectedly further improved cohesion of the adhe- sive; a further decrease of the separation and migration from the adhesive formulation of low molecular weight compo- nents, like the plasticisers themselves and tackifiers, decrease that gives also a further improvement of the stability on long times of the adhesive properties;

Liquid synthetic poly-alpha-olefins especially suitable as plasticisers in the self-adhesive hot-met formulations ac- cording to the present invention, are manufactured and traded e.g. by ExxonMobil with the trade marks SpectraSyn and Elevast; by Ineos with the trade mark Durasyn; by Chevron Phillips with the trade mark Synfluid etc.

The self-adhesive hot-met formulations according to the present invention comprise a plasticizer or a blend of plasti- cizers, liquid at room temperature, according to what described above, at a level between 5% by weight and 50% by weight, pref- erably between 10% by weight and 40% by weight, and more prefer- ably between 15% by weight and 35% by weight, referred to the global hot-melt formulation.

The hot-melt self-adhesive formulations according to the present invention may further comprise up to 10% by weight of at least one stabilizer, like anti-oxidants, anti-UV photo- stabilizers and their blends.

Moreover, they can further comprise up to a maximum quantity of 15% by weight of other optional additional compo- nents, like mineral fillers, pigments, dyes, perfumes, surfac- tants, antistatic agents.

Here below the methods used for measuring the most im- portant functional and chemical-physical parameters of the adhe- sive formulations according to the present invention and/or of their components are now illustrated.

Melt Viscosity

The dynamic melt viscosity of a molten material at a certain temperature is expressed in mPA.s and is herein measured according to the Method ASTM d 3236-88.

Ring & Ball Softening Point

The Ring & Ball Softening Point of the hot-melt adhe- sives disclosed in the present invention and of their components is herein measured according to the Method ASTM D 36-95.

Needle Penetration

The Needle Penetration of the self-adhesive formula- tions, disclosed herein, i.e. a measurement of their softness

(and indirectly also of their adhesiveness) is herein measured at 23°C according to the Method ASTM D 1321 - 04.

Tack

The tack of the present self-adhesive formulations is herein measured on stainless steel according to the Method ASTM

D 6195-03 for Loop Tack. In this test, the tested adhesive is coated at a temperature of 160°C as a continuous strip at the basis-weight of 50 g/m² on a poly-ethylene terephthalate film, having a thickness of about 50 microns, like the Cavilen films, sold by the Company Effegidi International (Italy).

Peel Strength

The Peel Strength, is defined, in the field of adhe- sives, as the average force per unit of length / width that is necessary for separating two substrates, that have been glued by using the tested adhesive formulation. This separation is per- formed at a fixed and constant speed and under a fixed and con- stant detachment angle. Said force is herein measured according to the method ASTM D 3330-02, under an angle of 180 degrees and at a separation speed of the two substrates equal to 1,000 mm / minute.

For said Peel Strength tests, the tested adhesive is coated, at the temperature of 160°C, as a continuous strip at the basis weight of 20 g/m², on a poly-ethylene film, with a thickness of about 22 microns, supplied by Poligof (Italy). Each sample, is cut at a width of 50 mm, and then it is glued, on its outside face of the plastic film, with a double sided adhesive tape like 710 TNT, supplied by Sicad Group (Italy), on a stain- less steel panel, the coating of the tested adhesive being turned upwards. On the adhesive coating, a strip of a standard cotton fabric, like Style 460-60 supplied by Testfabrics (USA) is then put, so that the part glued by the tested adhesive has a width of 50 mm and a length of 250 mm. On this glued area a steel weight of 2,000 g is then put for 30 seconds. After this, the Peel Strength test is carried out. For this, one blocks in the clamps of a dynamometer, from one side in a vertical posi- tion the steel panel (on which the plastic film is glued), and from the other side, the non-adhered part of the cotton strip.

For each type of tested adhesive, six tests are carried out and the average value of the six measured strengths is taken as the

Peel Strength of the adhesive under test.

If such test is performed within 24 hours from the coat- ing from the molten state of the adhesive on the poly-ethylene film, the measured Peel Strength is considered as the Peel

Strength under initial conditions.

For checking the change on aging of such adhesive param- eter, the samples of the various tested adhesives, coated on plastic films, are aged for seven days in an oven kept at 60°C and 75% Relative Humidity. After such period of aging, the sam- ples are taken out from the oven, conditioned for 24 hours at room conditions, and then they are tested according to the same above described procedure for measuring Peel Strength. In such a way the Peel Strengths in aged conditions are measured.

Internal Cohesion / Index of Residues

The internal cohesion of an adhesive formulation is measured herein according to the following method: a Peel

Strength test on stainless steel is carried out according to method ASTM D 3330-02, under an angle of 180 degrees and a speed of 300 mm / min for the separation of the two substrates. The samples of adhesive for this type of test are prepared by coat- ing, at the temperature of 160°C and at the basis weight of 50 g/m², a continuous strip of the tested adhesive on a 50 microns thick poly-ethylene terephthalate (PET) film, like Cavilen films, supplied by Effegidi Intenational (Italy). Each sample is cut at a width of 25.4 mm. Then each adhesive sample is adhered on a stainless steel panel for an overall glued area equal to

45.7 cm² . This test is performed at "initial conditions" i.e. within 24 hours since the coating from the molten state of the tested adhesive on the PET film. After having detached the adhe- sive strip at the speed of 300 mm / min, like in a Peel Strength test according to the above mentioned ASTM method, one visually examines the smooth, mirror-like surface of the AISI 304 steel panel. In case one detects areas of the panel in which the steel surface is clearly stained with residues of adhesive, left dur- ing the detachment, these areas are highlighted by marking their contours with a black felt-tip pen. A translucent graph-paper sheet is put in contact with the steel panel surface and the overall area, that is stained for the presence of adhesive's residues, is measured with a precision of 1 mm² . The Index of Residues, that is inversely proportional to the internal cohe- sion of the tested adhesive, is expressed as the percent value of the stained area, that shows adhesive's residues, compared to the global area on which the adhesive strip has been adhered.

Index of Oil Migration

The Index of Oil Migration measures the quantity of low molecular weight ingredients in the adhesive formulation (like plasticisers, tackifiers etc.) that, after aging, slowly demix and separate from the formulation and then bleed on the outside surface of the adhesive, causing in this way possible severe changes during time of the main properties of the adhesive. This

Index is measured according to the following method: for each tested formulation, six adhesive strips, coated on a substrate of the already described PET film, at a basis weight of 25 g/m² and having a width of 65 mm, are adhered on a sheet of white pa- per so that the total contact area between the adhesive and the paper is 193 cm² . The paper used for this test is white paper, at a basis weight of 75 g/m², supplied by Paper do Brasil (Bra- zil) under the trade marlk REY Light. Each adhesive strip is ad- hered on a single sheet of paper and it is compressed by rolling on it, twice in opposite directions, a roll having a weight of

2,000 g.

The strips, each adhered on a separate single sheet of paper, are aged in an oven at the temperature of 80°C for 72 hours. After such period of aging, the paper sheets undergo a measurement of their Luminosity parameter "L". measured through a colorimeter as Spectrophotometer CM-5 supplied Konica Minolta.

Such measurement of L is done on the sheet of paper's side oppo- site to the one where the adhesive coating resides. For each sheet of white paper, that has been possibly stained by the mi- gration and bleeding of low molecular weight ingredients, three measurement of luminosity are made, in three different positions of the sheet. The average value of the three measurements, L, is compared with the luminosity value of the original clean paper,

L*, through the formula: (L* - L) / L* Said calculation give the Index of Oil Migration, that is proportional to the quantity of low molecular weight compo- nents that, on long times, are separated from the formulation and bleed on the adhesive's surface. In a formulation that is ideally stable to aging, such Index has a value of zero. On the contrary, with formulations having increasing instability to ag- ing and increasing migrations in time, such parameter shows greater and greater values, that theoretically tend to a limit value equal to 1. For the paper used in the present method, the luminosity value for the clean paper L* has numerical value equal to 90.

Thermal and oxidative stability in the molten state

The stability of the herein disclosed self-adhesive for- mulations to thermal and oxidative degradations in the molten state, is measured by aging the tested adhesive for 72 hours in an oven kept at the average processing temperature (for this kind of adhesives) of 160°C, and then measuring the dynamic vis- cosity at 170°C of the aged adhesive. The percent change, in the viscosity at 170°C, between the adhesive in initial conditions and after such aging in the molten state, for 72 hours at 160°C, gives a measurement of the hot-melt's resistance to the thermal and oxidative degradation in the molten state at high tempera- ture.

EXAMPLES

The present invention is better illustrated by the fol- lowing examples, which are given herein merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. Un- less specifically indicated otherwise, parts and percentages are given by weight.

EXAMPLES ACCORDING TO THE INVENTION

EXAMPLE 1

The following self-adhesive hot-melt formulation, ac- cording to the present invention, is prepared by mixing its com- ponents in the molten state at 170°C.

The self-adhesive hot-melt formulation of Example 1, according to the present invention, comprises a polymer composition made of two styrenic block copolymers, one of them being a non- hydrogenated styrenic copolymer, that has a linear and asymmet- rical structure, that i.e. has, in its triblock molecules, two poly-styrenic blocks with a different molecular weight; while the other copolymer, forming the polymer composition, is a fully hydrogenated styrenic copolymer, that has a "traditional" sym- metrical structure, i.e. with two poly-styrenic blocks with identical molecular weight in its triblock molecules.

In the specific case, the asymmetric copolymer Quin- tac 3190 is an S-I-S copolymer supplied by Zeon (Japan). It is believed that Quintac 3190, in its triblock molecules, has two blocks of poly-styrene in the same molecule that have different length and molecular weight; more in details, one block having a Mass average Molecular Weight Mw equal to

47,000 atomic mass units, and the other block has Mw equal to 15,800 atomic mass units. Therefore, the difference in molecular weight between the two different styrenic blocks is 31,200 atomic mass units.

Moreover, it is believed that Quintac 3190 has, still in its triblock molecules, a global Mass average Mo- lecular Weight Mw equal to 159,500 atomic mass units; that it has an overall content of poly-styrene of 30% by weight and that it contains 60% by weight of diblock molecules (S -

I) in which the poly-styrenic block has a Mw equal to 15,800 atomic mass units.

Taipol 6014 is, on the contrary, a fully hydrogenat- ed S-EB-S copolymer, that is linear and that has a more tra- ditional, symmetrical structure. It has an overall content of poly-styrene equal to 18% by weight and it does not con- tain any diblock.

The presence of this polymer composition, having such novel combination between a non-hydrogenated asymmet- rical copolymer and a fully hydrogenated symmetrical copoly- mer, gives the self-adhesive formulation shown in Example 1 a full series of various and unexpectedly positive new prop- erties. In particular: in spite of its rather great content, i.e. 22% by weight, of two copolymers having significantly high molecular weights, the formulation of Example 1 ex- hibits a very low melt viscosity, equal to 1,690 mPa.s at 170°C, a low value that ensures an optimal processability, even on industrial lines operated at very high speeds, like 200 m/min and even more. Fur- thermore, despite also the great content in polymers even with a high content of poly-styrene, the formu- lation is very soft and strongly tacky and adhesive, as e.g, shown by its high softness, measured by Nee- dle Penetration as great as 99 dmm at 23°C, and even more by a Loop Tack as high as 13,0 N and by an ini- tial Peel Strength equal to 2.66 N per 50 mm width ( see the following Table 1 and Table 2 for the values of Loop Tack and of Peel Strength of the herein de- scribed formulations). said high adhesiveness is also unexpectedly combined with an excellent internal cohesion, despite the un- usually high diblock content of the main asymmet- rical S-I-S copolymer. In fact, the formulation of

Example 1, when it is tested for its residues ac- cording to the previously described test, gives an

Index of Residues equal to zero, as shown in the be- low Table 3. the peculiar polymer composition, used herein, shows also an unexpectedly high ability in blocking the separation and migration of low molecular weight in- gredients, like plasticisers and tackifiers. This positive behaviour ensures a substantial constancy, even after aging on long times, of its main adhesive properties, with e.g. a substantially constant value of the Peel Strength, even after a forced, acceler- ated aging, for as much as seven days, at the tem- perature of 60°C, according to what previously de- scribed. Moreover, its minimal separation and migra- tion of low molecular weight ingredients, avoid pos- sible severe negative effects, like the generation of bad odours or the production of skin-rashes or of allergic reactions, in those cases when these adhe- sives may contact the skin of a user. Said unexpect- edly low migration of low molecular weight ingredi- ents is clearly expressed by a low Index of Oil Mi- gration, as low as 0,20; as well as by an excellent consistency on long times of the Peel Strength in aged conditions, that still has the very good value of 2.18 N per 50 mm width. Therefore, after aging, the formulation of Example 1 loses as little as

18.0% of its initial Peel Strength. The Indexes of

Oil Migration and the aged Peel Strength, for all the adhesive formulations discussed herein, are shown in the below Table 4 and Table 2.

Even the resistance of the herein disclosed formula- tion to thermal and oxidative degradations, for 72 hours in the molten state and at the typical processing temperature of 160°C, is excellent because after thermal and oxidative aging, the formulation of Example 1 shows a decrease in its melt viscosity at 170°C as little as 21.2%. For complete- ness of information, the above self-adhesive formulation has a Ring & Ball Softening Point equal to 89.6°C.

EXAMPLE 2

The following self-adhesive hot-meIt formulation, according to the present invention, is prepared by mixing its components in the molten state at 170°C.

The self-adhesive hot-melt formulation of Example 2 is different than the one of Example 1 just for the identity of the second fully hydrogenated copolymer in the basic pol- ymer composition. Here in fact a different SEBS polymer that contains a much higher level of poly-styrene is used. Said copolymer, commercially supplied by Kraton with the trade mark Kraton G 1652, has a poly-styrene content of 30% by weight and a diblock content equal to zero.

Despite the much greater level of poly-styrene in the herein used polymer composition, compared to the previ- ous Example 1, the self-adhesive formulation of Example 2 is still very soft and tacky, as shown by Needle Penetration at

23°C equal to 77 dmm. Moreover, the excellent properties, highlighted in Example 1, are here confirmed and even in some cases further improved. The self-adhesive formulation of Example 2, in fact, has a melt viscosity even lower than the one of the previous Example, being equal to just 1,430 mP.s at 170°C. Its adhesiveness is excellent, as shown by its Loop Tack equal to 10.8 N and by its initial Peel

Strength equal to 2.35 N per 50 mm width.

This formulation continues to have an optimum inter- nal cohesion, as shown by its Index of Residues that is equal to zero and an excellent stability against aging, as shown by the negligible value of its Index of Oil Migration, as little as 0.17, and by a Peel Strength under aged condi- tions that, with a very good value of 1.9 N per 50 mm width, differs for as little as 19.3% from its initial Peel

Strength. The self-adhesive formulation of Example 2 shows also an excellent resistance to thermal and oxidative degra- dations during aging in the molten state. In fact, after having been aged for 72 hours at 160°C, its viscosity at

170°C decreases for only 22%. The formulation of Example 2 has a Ring & Ball Softening Point equal to 90.3°C.

EXAMPLE 3

The self-adhesive hot-melt formulation of Example 1 is identical to the formulation of Example 1, apart from the fact that the paraffinic mineral oil Primol 352 is substituted as plasticiser with an equal quantity of SpectraSyn 10, a synthetic liquid poly-alpha-olefin (PAO) supplied by ExxonMobil. This Ex- ample shows in particular that, in the adhesive formulations ac- cording to the present invention, by using PAO's as plasticis- ers, in the place of more traditional plasticisers as mineral oils, the already very good low Index of Oil Migration of Exam- pie 1 can be surprisingly further lowered to the exceptionally low value of 0.08, that indicates a practically null separation and migration of the low molecular weight ingredients.

This outstandingly good stability against separation and migration of light ingredients, causes also an extraordinary consistency over time of adhesive properties, even after accel- erated aging, as e.g. shown by an aged Peel Strength as high as

2.43 N per 50 mm width, that differs for as little as 12.6% than the initial Peel Strength, the value of which is 2.78 N per 50 mm width.

The internal cohesion of this formulation is again ex- cellent as shown by an Index of Residues still equal to zero.

The exceptional stability in time of this self-adhesive formula- tion is confirmed also for what regards its optimal resistance to thermal and oxidative degradations during aging in the molten state, at 160°C for 72 hours. In fact, after such aging, the present formulation has a viscosity at 170°C that is equal to

1,585 mPa.s, that i.e. differs only by 17.9% from the analogous value of viscosity at initial conditions.

The self-adhesive formulation of Example 3 has moreover an initial viscosity at 170°C equal to 1,930 mPa.s; an optimal

Loop Tack equal to 11.7 N; Needle Penetration at 23°C of 92 dmm, and a Ring & Ball Softening Point of 96.0°C.

COMPARATIVE EXAMPLES

COMPARATIVE EXAMPLE 1

The self-adhesive hot-melt formulation of Comparative

Example 1 has been prepared by mixing its components in the mol- ten state at 170°C. It differs from the previous examples ac- cording to the invention for the presence of just one of the two styrenic block copolymers that form the polymer composition com- prised in the formulations of the Examples according to the in- vention. In particular Comparative Example 1 comprises only the linear and asymmetrical unsaturated styrenic block copolymers

(S-I-S) Quintac 3190.

The self-adhesive hot-melt formulation of Compara- tive Example 1, even if it keeps a few good properties al- ready present in the previous Examples, shows how basic is the presence also of the second, fully hydrogenated and sym- metrical, styrenic block copolymer (S-EB-S), is in order to especially keep the optimum balance between adhesiveness and cohesion, that characterises the previous Examples according to the present invention. In fact, the above comparative formulation has e.g. a melt viscosity at 170°C equal to 1,120 mPa.s, and its ad- hesiveness is very good as shown by its Loop Tack, equal to as much as 14.0 N and by its initial Peel Strength equal to

2.53 N per 50 mm width. However, the absence of the second polymer, i.e. the fully hydrogenated and symmetrical S-EB-S, leads to an unacceptable worsening of its internal cohesion, as shown by an Index of Residues that has the inappropriate- ly too high value of 27%. The stability to aging becomes poor, as shown by a value of the index of Oil Migration equal to 0.30, and by a Peel Strength under aged conditions that, with a value of 1.8 N per 50 mm width, differs from the initial Peel Strength by as much as 28.8%. Moreover, the resistance in aging to thermal and oxidative degradations, after 72 hours in the molten state at 160°C, is very poor, as one can expect from a formulation exclusively based on an unsaturated and easily degradable S-I-S copolymer. In fact, after such aging in the molten state at 160°C, this compara- tive formulation shows a loss of as much as 44.9% in its viscosity, showing an aged viscosity at 170°C equal as low as 617 mPa.s.

The formulation of Comparative Example 1 has a Nee- die Penetration at 23°C equal to 122 dmm and shows a Ring & Ball Softening Point equal to 95.2°C.

COMPARATIVE EXAMPLE 2

The following self-adhesive hot-melt comparative formulation, is prepared by mixing its components in the molten state at 170°C. It differs from the previous examples according to the invention for the presence of just one of the two styrenic block copolymers that form the polymer com- position comprised in the formulations of the Examples ac- cording to the invention. In particular, Comparative Example

2 comprises only the linear and symmetrical, fully hydrogen- ated styrenic block copolymers (S-EB-S) Taipol 6014.

The self-adhesive hot-melt formulation of Comparative

Example 2, although able to keep a few good properties already present in the previous Examples, shows, like the formulation of

Comparative Example 1, how basic is the presence of both the two copolymers that form the polymer composition herein disclosed is for achieving an optimum balance between good adhesiveness and good cohesion, that characterises the formulations according to the present invention. This is true also for as several other properties of the adhesives of the present invention that, on the contrary, in this Comparative Example assume fully unac- ceptable values.

In fact, the self-adhesive hot-melt formulation of Com- parative Example 2 has e.g. a much higher melt viscosity, equal to as much as 4,270 mPa.s at 170°C. And in spite of that this high viscosity gives an excellent cohesion, as shown by an Index of Residues equal to zero, however its adhesiveness is extremely poor and unacceptably low, as it is evident from its Loop Tack equal to as little as 5.4 N and from its initial Peel Strength equal to just 0.96 N per 50 mm width. This very bad adhesiveness is also indirectly expressed through its too low value for the

Needle Penetration at 23°C, that with a value of as little as 49 dmm, well shows the fact that this self-adhesive formulation is excessively hard and so very little tacky. Also the stability to aging is unacceptably low, as demonstrated by the too high value of its Index of Oil Migration, equal to as much as 0.38, and by a Peel Strength in aged conditions that, with a value of 0.58 N per 50 mm width, differs for as much as 39.1% less, from its in- itial Peel Strength.

On the contrary, as it can be expected from a formu- lation comprising just a fully saturated copolymer like Tai- pol 6014, its resistance to thermal and oxidative degrada- tions after aging for 72 hours in the molten state at 160°C, ls excellent, because this formulation shows a viscosity variation at 170°C of just 13.6%. However, it is obvious that such positive property of strong resistance to degrada- tions at high temperatures, is practically useless, given the bad adhesive properties of this comparative formulation, both in initial and in aged conditions. The formulation of

Comparative Example 2 has a Ring & Ball Softening Point equal to 72.9°C.

COMPARATIVE EXAMPLE 3

The self-adhesive hot-melt formulation of Compara- tive Example 3 is prepared by mixing its components in the molten state at 170°C. Even if this Comparative Example keeps the presence of a polymer composition formed by a lin- ear and unsaturated styrenic block copolymer S-I-S and by a fully hydrogenated S-EB-S, the below disclosed comparative formulation uses, in opposition with the teachings of the present invention, as the first unsaturated copolymer, an S-

I-S copolymer, that is linear and that has the same styrene content (30% by weight) of the Quintac 3190 S-I-S copolymer, used in the previous Examples according to the invention, but that however has a symmetrical structure of its poly- styrenic blocks. Such linear and symmetrical S-I-S copolymer with 30% by weight of poly-styrene, is traded as Kraton D

1165 by Kraton Company (USA). Kraton D 1165 has a diblock content equal to 20% by weight and it is believed that, in its triblock molecules, the two identical poly-styrenic block in each molecule have the same Mass average Molecular

Weight Mw equal to 19,200 atomic mass units each, while the overall Mass average Molecular Weight of the copolymer is believed to be equal to 141,000 atomic mass units.

The self-adhesive hot-melt formulation of Compara- tive Example 3 shows the essential importance of having the asymmetrical structure for the unsaturated copolymer in the polymer composition comprised in the self-adhesive formula- tions disclosed by the present invention. In fact, the for- mulation of Comparative Example 3 shows first of all a very bad stability in aging. This is highlighted by a too high value of its Index of Oil Migration, as great as 0.40, and also by an excessive decrease on long time aging of its Peel

Strength. In fact, while the Peel Strength in initial condi- tions has a value of 1.43 N per 50 mm width, after aging such strength drops to 0.81 N per 50 mm width, with a de- crease of as much as 43.7%. Therefore, the self-adhesive formulation of the present Comparative Example is a very poor adhesive. In fact, even if the values of its adhesive properties in initial conditions are on average good, as e.g. shown by a Loop Tack of 8.8 N, the dropping to too low levels during aging of such properties, makes this adhesive practically unusable.

This negative result is not balanced by the fact that the same formulation shows an average resistance to thermal and oxidative degradations in the molten state for

72 hours at 160°C. In fact, after such aging, this adhesive shows a decrease in its viscosity at 170°C equal to 33.5%, compared to its initial viscosity still at 170°C.

On the contrary the cohesion is good, as shown by a value of the Index of Residues equal to 6%. For completeness of information, the viscosity of the present formulation at

170°C in initial conditions is 1,760 mPa.s; its Needle Pene- tration at 23°C is equal to 77 dmm; and its Ring & Ball Sof- tening Point is 88.7°C.

COMPARATIVE EXAMPLE 4

The self-adhesive hot-melt formulation of Compara- tive Example 4 is prepared by mixing its components in the molten state at 170°C. In the present Comparative Example the fully hydrogenated styrenic block copolymer S-EB-S, com- prised in the basic polymer composition, contains an exces- sive level of diblock molecules, contrary to the teachings of the present invention. In fact, the S-EB-S copolymer used herein, Kraton G 1726, is a linear and symmetrical copoly- mer, with 30% poly-styrene, that however contains an exces- sive level of diblock, equal to 70% by weight.

The self-adhesive hot-melt formulation of Compara- tive Example 4 hence shows a general worsening of all the main properties, especially of cohesion. The initial viscos- ity at 170°C is 760 mPa.s; the needle Penetration at 23°C is equal to 111 dmm and its Ring & Ball Softening Point is

85.8°C.

The unacceptable worsening of all its characteris- tics is even more evident in its very poor internal cohe- sion, as shown by the disastrous value of 83% of its Index of Residues, and by the poor stability to aging, as shown by a value of its Index of Oil Migration equal to 0.31. Also the aged Peel Strength, with a value of 1.51 N per 50 mm width is lower for 34.6% than its initial Peel Strength, equal to 2.31 N per 50 mm width.

The initial Loop tack on stainless steel, with a value of 12.5 N, shows that this self-adhesive formulation is very tacky, even if its very poor cohesion makes this formulation practically unusable. The same can be said for its average resistance to thermal and oxidative degrada- tions, for aging in the molten state for 72 hours at 160°C.

After such aging, indeed, the self-adhesive formulation of

Comparative Example 4 is equal to 481 mPa.s, i.e. it is low- er by 37.1% than its initial viscosity at the same tempera- ture.

TABLE 1

TABLE 2

TABLE 3

TABLE 4

TABLE 5

Claims

1) Self-adhesive hot-melt formulation, characterised in that it comprises a polymer composition formed by:

A) from 5% to 95% by weight of at least one elasto- meric styrenic block copolymer, linear, non- hydrogenated / unsaturated and with an asymmetrical structure, that in its triblock fraction, has a struc- ture represented by the following formula:

S1 D S2 where S1 and S2 represent two blocks of poly-styrene having different molecular weights; and D represents a central elastomeric block unsaturated / non- hydrogenated of a homopolymer of a conjugated diene, for example poly-isoprene or poly-butadiene, or of a copolymer of two conjugated dienes, e.g. a poly- isoprene-butadiene copolymer; and where both S1 and S2 and D have a Mass average Molecular Weight Mw greater than 5,000 atomic mass units;

B) from 95% to 5% by weight of at least one elasto- meric styrenic block copolymer, that has a fully hy- drogenated central elastomeric block and a symmet- rical, linear or radial structure, represented by the following formulas:

S3 C - S3 or (S3 - C)n - X where S3 represents two or more blocks of poly-styrene having the same molecular weight and C represents one or more central elastomeric blocks, fully hydrogenated

/ saturated, of a poly-ethylene-butylene copolymer or of a poly-ethylene-propylene copolymer or of a poly- ethylene- co (ethylene / propylene) copolymer; where- in, in the radial copolymer, X is a poly-functional group that ensures the chemical bond among the differ- ent arms of the copolymer and where n is an integer, greater than 2; and wherein both S3 and C have a Mass average Molecular Weight Mw greater than 5,000 atomic mass units. 2)Self-adhesive hot-melt formulation as in 1), wherein the

Mass average Molecular Weights Mw of the two poly-styrenic blocks S1 and S2 of the unsaturated, linear and asymmetrical copolymer A), differ one from the other by at least 5,000 atomic mass units, preferably by at least 8,000 atomic mass units and more preferably by at least 10,000 atomic mass units.

3)Self-adhesive hot-melt formulation as in 2), wherein said

Mass average Molecular Weights Mw of the two poly-styrenic blocks S1 and S2 of the unsaturated, linear and asymmetrical copolymer A) are between 8,000 and 20,000 atomic mass units for the block S1 having the lower molecular weight; and are between 18,000 and 40,000 atomic mass units for the block S2 having the higher molecular weight.

4)Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 3), wherein the unsaturated, linear and asymmetrical copolymer A) comprises from zero to

75% by weight of diblock molecules.

5)Self-adhesive hot-melt formulation as in 4), wherein the diblock molecules have substantially the same structure S1 -

D, where S1 is the poly-styrenic block with the lowest Mass average Molecular Weight Mw, between S1 and S2.

6)Self-adhesive hot-melt formulation as in 1), wherein the fully hydrogenated, linear or radial, symmetrical copolymer

B) comprises from zero to 35% by weight of diblock molecules.

7)Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 6), wherein the polymer com- position comprises from 10% by weight to 90% by weight, and preferably from 15% by weight to 85% by weight of copolymer

A), and from 90% by weight to 10% by weight, and preferably from 85% by weight to 15% by weight of copolymer B), refer- ring to the total weight of A) plus B).

8)Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 7), wherein the polymer com- position makes up from 10% by weight to 50% by weight of said global hot-melt self-adhesive formulation, and preferably from 15% by weight to 40% by weight.

9)Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 8), comprising from zero to

20% by weight, and preferably from zero to 15% by weight, of at least one additional polymer, being such polymer selected among: i) linear or radial, symmetrical and non-hydrogenated styrenic block copolymers, the elastomeric blocks of which are chemically identical to the elastomeric block D of copol- ymer A); ii) or poly-olefins and their copolymers, especially amorphous poly-alpha olefins (APAO) and their copolymers; iii) or their blends.

10) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 9), comprising from

30% by weight to 80% by weight, preferably from 40% by weight to 70% by weight, and more preferably from 45% by weight to

65% by weight, referring to the total weight of the adhesive formulation, of at least one tackifier or of a blend of tackifiers, having a Ring & Ball Softening Point between 5°C and 160°C.

11) Self-adhesive hot-melt formulation as in 10), wherein the tackifier or the blend of tackifiers is selected among aliphatic and cyclo-aliphatic hydrocarbon tackifiers, and their partially or fully hydrogenated derivatives; aliphat- ic/aromatic and cyclo-aliphatic / aromatic tackifiers, and their partially or fully hydrogenated derivatives; poly- terpenic and modified-terpenic tackifiers, and their partial- ly or fully hydrogenated derivatives; rosins and Tail-Oil resins, their esters and their partially or fully hydrogenat- ed derivatives.

12) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 11), comprising from zero to 10% by weight of at least one purely aromatic tacki- fying resin, derived from alpha-methyl-styrene or para- methyl-styrene or indene or benzofuran or their blends.

13) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 12), comprising from 5% by weight to 50% by weight, preferably from 10% by weight to 40% by weight, and more preferably from 15% by weight to

35% by weight, referring to the total weight of the adhesive formulation, of at least one plasticiser that is liquid at room temperature, or of a blend of two or more plasticisers that are liquid at room temperature.

14) Self-adhesive hot-melt formulation as in 13), wherein the plasticiser or the blend of plasticisers, liquid at room temperature, is selected among paraffinic and naphthenic min- eral oils, and their blends; paraffinic and naphthenic hydro- carbons, liquid at room temperature, and their blends; plas- ticisers liquid at room temperature formed by esters, like phthalates, benzoates, sebacates; vegetable oils; and blends thereof.

15) Self-adhesive hot-melt formulation as in 13) and 14), wherein the plasticiser or the blend of plasticisers, liquid at room temperature, is selected among oligomers, liquid at room temperature, of poly-olefins from C2 to C20 and their copolymers, having a Number average Molecular Weight Mn from

150 to 15,000 atomic mass units, preferably from 200 to

10,000 and even more preferably from 400 to 6,000.

16) Self-adhesive hot-melt formulation as in 15), wherein the oligomeric poly-olefin plasticiser, liquid at room tem- perature or the oligomeric poly-olefin plasticisers, liquid at room temperature, forming the comprised blend, have a Pol- ydispersity Index Mw / Mn not greater than 2.5 and preferably not greater than 2.

17) Self-adhesive hot-melt formulation as in 16), wherein the oligomeric poly-olefin plasticiser, liquid at room tem- perature or the oligomeric poly-olefin plasticisers, liquid at room temperature, forming the comprised blend, are synthe- sised by using metallocene catalysts.

18) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 17), comprising from zero to 5% by weight, preferably from zero to 3% by weight, of a wax or of a blend of waxes. 19) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 18), comprising up to

10% by weight of at least one stabiliser, like anti-oxidants, anti-UV photo-stabilisers and their blends.

20) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 19), comprising from zero to 10% by weight of at least one additional component, selected among mineral fillers, pigments, dyes, perfumes, surfactants, anti-static agents, and their blends.

21) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 20), having a

Brookfield viscosity, measured according to the test method

ASTM D 3236-88, at the temperature of 170°C, that is not higher than 10,000 mPa.s, preferably not higher than 8,000 mPa.s, more preferably not higher than 5,000 mPa.s and even more preferably not higher than 3,500 mPa.s.

22) Self-adhesive hot-melt formulation as in 21), that, after aging for 72 hours at the temperature of 160°C, has a

Brookfield viscosity at 170°C that is not different for more than 30% from the initial value of viscosity at the same tem- perature.

23) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 22), having a Ring & Ball Softening Point, measured according to the test method

ASTM D 36-95, not higher than 135°C and preferably not higher than 120°C.

24) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 23), having a Needle

Penetration at 23°C, measured according to the test method

ASTM D 1321-04, not lower than 55 dmm and preferably not low- er than 60 dmm.

25) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 24), having an initial

Peel Strength, measured according to the test method ASTM D 3330-02, at the basis weight of 20 g/m², not lower than 1.5 N per 50 mm width, and preferably not lower than 1.2 N per 50 mm width.

26) Self-adhesive hot-melt formulation as in 25), having an aged Peel Strength that is not different by more than 25% from the value of its initial Peel Strength.

27) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 26), having an initial Loop-Tack, measured on stainless steel according to the test method ASTM D 3330-02, not lower than 6 N.

28) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 27), having an Index of Oil Migration not higher than 0.28.

29) Self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 27), having an Index of residues not higher than 10%, and preferably not higher than 5%.

30) A bonded structure comprising i) a first substrate; ii) a second substrate iii) a self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 29), bonding the first substrate to the second substrate, which, when applied to a basis weight between 0.5 g/m 2 and 50 g/m² , gives the bonded structure a Peel Strength, greater than 0.50 N per 100 mm width.

31) An absorbent hygienic article, comprising the self- adhesive hot-melt formulation according to any one of the preceding claims from 1) to 29) or comprising the bonded structure as in 30).

32) An article as in 31), wherein said article is an ab- sorbent article for feminine hygiene, a baby-diaper, a train- ing pants diaper, a diaper for incontinent adults.

33) An absorbent hygienic article as in 31) and 32), wherein the self-adhesive hot-melt formulation according to any one of the preceding claims from 1) to 29) is used as a pressure sensitive adhesive for adhering said article on a garment or on the skin of a user. 34) An article comprising the self-adhesive hot-melt for- mulation according to any one of the preceding claims from 1) to 29), wherein said article is an adhesive tape, an adhesive label, an adhesive plastic film or a plaster for skin.

35) An article comprising the self-adhesive hot-melt for- mulation according to any one of the preceding claims from 1) to 29), wherein said article is a packaging.