NO141019B - SPERREBUKK. - Google Patents

Abstract

Barrier buck.

Description

Heat-sensitive thermographic paper sheet that is protected from tearing and soiling by heat-sealable protective cover.

The present invention relates to a heat-sensitive thermographic paper sheet which is

protected against tearing and soiling by wood

using heat sealable, adhesive, protective flexible transparent tear resistant

film.

The invention is particularly applicable to

to produce strong protective cover layers

1 form of a film of dirt-resistant and tear-resistant, transparent material

which should protect heat-sensitive thermographic paper of the type such as e.g. is discussed in US patents 2,663,654, 2,663,655,

2 633 656 and 2 663 657. These thermographic

copy sheets are specially adapted to

undergo a color change when exposed to heat and have been used extensively for copying original documents, e.g. typed letters according to it

method described in US patents 2,740,895 and 2,740,896, and US patent 2,891,165.

Due to the great heat sensitivity

these thermographic copying papers exhibit, transparent protective film layers cannot be expected to be able to

attached or laminated to these by means of

a thermoplastic binder where heat is used to activate the binder,

without the heat-sensitive copy paper undergoing a color change. However, the present invention provides the solution to this problem. Further produces

the invention a heat-adhesive, through-

visible protective film and a method of laminating such a film to underlying paper sheets of a heat-sensitive thermographic copying paper.

The protective film layer according to the invention is firmly connected to the underlying base layer of paper so that it resists delamination at temperatures that are as high or even slightly higher than those originally used to activate the thermoplastic binder. The laminates are flexible and can be treated like ordinary sheets of paper, and as a further advantage the protective transparent film layer prevents the paper from being torn to pieces and/or soiled without covering any marks or signs of this. In the event that the underlying paper is an original document with marks to be preserved without any changes, the protective film, which is permanently attached to the underlying document, serves the additional purpose of making it impossible to change this by using a pencil , pen or other writing implement. When the underlying base paper is a heat-sensitive thermographic paper as indicated above, marks can of course be transferred to this according to the method described in the above-mentioned patents, or marks that have already been applied can be erased by putting in times a heat-induced color change over the entire heat-sensitive paper sheet. This is sometimes an advantage when you want an authorized change without having to draw up a completely new document.

The heat-sealable, tear-resistant transparent film according to the invention is characterized in that the protective film is permanently and transparently laminated to the heat-sensitive thermographic paper sheet by means of a non-rubbery, thermoplastic binder which is not -: sticky up to 50° C, but which quickly becomes sticky at a temperature between 50 and' 85° C.

As far as is known, no one has so far used such an inexpensive, highly resistant protective layer of heat-adhesive film with such properties that, with suitable treatment, it will produce evenly and smoothly laminated, firmly connected protective layers - cover - on the surface of thermographic paper, e.g. by means of radiant energy or by means of a suitable method. The transparent one! protective film according to the invention can, however, suitably be laminated to heat-sensitive thermographic copying paper by using radiation energy as primary. more heat source without the heat-sensitive paper undergoing any color change during the lamination process.

Heat-sensitive thermographic paper sheet which is protected against frosting and soiling by means of the protective cover as indicated above is particularly advantageously produced by a non-stick laminate consisting of the paper backing sheet covered by the protective film that is equipped with a non-sticky, heat-sealable double layer that can be activated between 50 and 85°C, in the heat joint with a radiant energy absorber, is exposed to intense radiation energy that is sufficient to develop such a high temperature in the heat-sealable layer that it is activated to an adhesive state.

Although transparent hot glue.T

The films or foils are well known, so those that have been available until now have either been deformed to a significant extent by attempts to yar-melamine them into heat-sensitive thermographic paper, or the papers have been discolored or poor binding performance has been achieved . Cellulose acetate covered with a highly adhesive rubber-wax binder is a known commercial product, but attempts to adhere this film to graphic originals or thermographic paper by changing a source of thermal radiation result in a poor bond between film and paper, Reforming and curling of the acetater film .or of the underlying paper sheet. Films consisting of polyester covered with a layer of heat-adhesive polyethylene is also known, but these commercially available films have likewise been impossible to use, as either a poor bonding effect or complete destruction of the heat-sensitive thermographic paper is achieved. Thus, to the best of our knowledge, there has not yet been available any transparent heat-adhesive protective film that has produced usable laminates with heat-sensitive thermographic paper with a lamination method that uses a source of heat radiation that is remote from and out of contact with the sheet material to be laminated. The protective arm-sealing films according to the invention are so unusually thin and without mass that, for practical reasons, they are provided with disposable flexible carrier or separation layers which, among other things, should give the film increased mass as well as make it easy .tter ;to [remove even single .coated film from a .stack ay these. The preferred protective film is actually so thin that it is quickly folded by air currents when one lifts the film alone from a table and holds one of its end edges between two fingers. Despite this, the film still lies flat - on the run layer when - it is to be laminated for this - and the lamination - is carried out with the help of radiation energy and a very light pressure, which is achieved, for example, by feeding the film and underlaying ^ together -with an absorption component between a belt and

a guide roller. There is no wrinkling or deformation of the film - during this process, the cover film does not just lie

flat, but is also stable -.dimensionally. It can be easily handled so that improved laminates are obtained with the protective film firmly anchored as part of this.

Furthermore, the binder-provided cover film is not sticky at temperatures

up to 5.0° C and could thus, -when one disregards the handling difficulties, be used without a separation layer. In addition to this non-adhesive property, the film is quickly acfied to a highly adhesive state at temperatures from .5.0° C^85° £?•

The invention will now be described in connection with the accompanying drawings, top view: Fig. 1 is a schematic enlarged cross-section of a laminate produced according to the invention. Fig. 2 is a schematic enlarged cross-section of the essential features of a stacking method for the heat-adhesive, transparent protective film. Fig. 3 is a schematic view in the form of a material flow diagram and shows the production of the transparent film, and

fig. 4 is a schematic section which shows the essential parts of the apparatus and the matrix of the method in connection with the laminating methods according to the invention.

In order to facilitate the overview, many elements and parts in the drawings have been given simple descriptive names so that the invention can essentially be derived from the drawings without reference to the description. However, it should be specified that this understanding of the invention from a glance at the drawings cannot be taken as a substitute for the complete description.

The laminate" on' fig". 1 f: ex. may not include an additional layer or layer, and preferably will include an additional, almost imperceptible layer, one primer between the film and the binder. This primer may be a partially discontinuous one coating and serves the purpose of improving therein permanent biridevifknirig of the biridemidlayer of the transparent film.

The base sheet of heat-sensitive<*> thermographic' paper' on' fig.' 1 includes" fléré lág, where only' some are constituted by' cellulose, rriehs" the others are" other organic or' inorganic material'; This' also satisfies" beteg-riélséri"<<<p>a<p>'if> '-år<k:> Many sheet- or" plåte-måtérlål" which" ef" known as lysøfrif ihtligé also satisfy déririe påpirbétegnélsé bå'd'e föf 'o'g'et'te'r''exp"bnerihgeri . One "characteristic" career feature is "the basic or" base sheet is<*> åEven* such a nature<*> of it's career is<*>so<*>easily that this becomes"a"problem when it- ef in 'brUk'ellér' ri'år"that' is* used often", f.' ex', reri^ofdrebb^fbr^sélgéré: Where d'é'våf"me'f'øls'omme"p'åp'ir"to be protected, lamination, å* et' gjérinbm-sightig*be'protected Elsé"sark' elléf' -sj ikt" is carried out before the p'åpir'ét<;> skonéfes' to "to" perform heat release- of the fafgéfbråirdrihgén, provided that the binder midelel layer' and' other" deléf of'therein g'jerinomsiktigébé The 'shuttleséséfilrii' does not contain' the hoen' kéfriikåliéf' soiri kåri' réagéré iri^d^ét heat-sensitive sheet and"riØyt'r"alise"ré its' vårmeff ølsbmirie" property: As' shown' on'fi' fig : 2' and of food vålseii 40 p"å~ fig": 4; ef the'transparent" våfriief01-sbmirié' vafmékléberide protectionsésf ilfh' 20 (ie:. dferi' gjeririorirsiktigé" film" soiri ér' bé-lågFirietféftbihderiilddél) f bftf iirnsvislévéft f bfbrukef rie' with' one separating or"' one" öfmririgs sheet*" 15 i'ké" kléberide and adskilibaf f bf*iri'd"elséJ rti ed' riirfte. An expedient åniiefifigs sheet is"one with' tykélse ' and" properties" such as" et<f> duplisérifigs-gjénbbfrislågs-papirv (onibh skir in typical papéf), although thicker sheets may also do good service. Utéri-årfriéririgsårk^ilié^det^fa from a practical point of view' ve"fé" megéf väisible to" distinguish simple coated- filmafk" ffå'restéri"of"'stab"e- len. Because it is required to add a substance that absorbs heat radiation in connection with the lamination, it is sometimes desirable to use separating or reinforcing sheets which in themselves have some ability to absorb heat radiation. Thin kraft paper or gray paper can serve this purpose effectively, but coulis impregnated paper sheets or sheets colored or tinted with infrared absorbing substances are also useful.

The reinforcing sheet is also a useful aid for producing the heat-adhesive protective film. It forms a carrier that frees the film from deforming tensile stresses when the binder-coated film structure is to be pushed forward. - As shown in fig. 3, the transparent film 10' as a bowl is coated with a heat-adhesive binder fed from a roller over guide rollers 11 and. from there passed between coating rollers 12 and 13" which are placed over a biride medium bath which is heated in the tub or trough 14'. The coating rollers rotate as shown in the drawing and are pressed tightly together so that they also pull the film between them to some extent. Binder is simultaneously taken from the tub by the roller 12, which rotates in the tub, and a very thin layer of binder is applied to the film 10. While this is taking place, a reinforcing sheet or carrier in the form of a sheet of paper 15 with the same width as the film drawn from a feed roller and after passing guide rollers 16 placed in non-adhesive connection with the film 10 on the back side of this. Although Fig. 3 shows the union of the paper 15 - in the film after the coating process, it must be specified in this connection that the formation of the loose laminate can be carried out before adhesive is added to the transparent film: The loose laminate is pulled inside the apparatus along a path which is fixed by guide rollers 17 and cooling drums oni rotates above r the binder coating-and-arriating sheet or -banner relieves the no-non-risky film from-the-essential-of the draft bracing. After the binder-coated side of the film is cooled, see the following description, the loose laminate is wound up in a suitable manner directly on the eri trohrmel so that it darines- a suitable-roll as soiri'Shown in detail in Fig. 4. In which force used to wind up the roll is also conveniently used to pull the laminate through the apparatus as shown in Fig. 3. The material can be unwound from the coated storage roll and cut into shapes dark for stacking

as shown in fig. 2, or the roll itself can

also used directly as material for automatic lamination of transparent film for1 printed documents, thermographic

paper and the like as shown in fig. 4. From what has been said above, it is obvious that it is important to use carrier or reinforcing sheets of non-stretchable material, especially in cases where the transparent film itself is of such a nature that it will be stretched when it is passed through parts of the device.

[ The preferred heat-adhesive, transparent protective films according to the invention are those which consist of a polyester, or equivalent, film substrate (backing) with a thickness of up to 0.0254 mm. Film with a thickness down to 0.00762 mm and even down to 0.00508 mm are usable, and film with a thickness up to 0.0508 mm or 0.0762 mm will give usable results, but not without loss of some of the flexibility desired for the coated papers. Film with a thickness of up to approx. 0.0254 mm gives little change in the flexibility or pliability of the final product and at the same time gives the necessary tear strength and dirt resistance that is required. The thinner films are also advantageous because they have little mass or weight which allows a faster completed lamination process, as heat is lost in the film itself.

When a thermographic coating is to be applied to a transparent film using a heat melting method, it is of significant importance that the film used does not melt during application. Polyester film such as polyethylene terephthalate (under the trade name Mylar) satisfies this requirement and also has several other advantageous properties. It does not discolour or become brittle during storage and can thus provide protection for as long as required. It is resistant to tearing and creasing, resistant to oil and grease as well as to most of the most common organic solvents such as acetone, carbon tetrachloride and heptane. It is also resistant to degradation under the influence of water and is very durable in the laminates according to the invention when these undergo several treatment steps where skin fat, moisture and other contaminants are applied. Even acid solutions such as sulfuric acid and alkalis such as sodium hydroxide cause mostly no detectable degradation provided that the film is dried relatively shortly after contamination.

The main requirements for the transparent film are that it remains pliable or flexible, that it does not break down or become brittle so that it can easily tear to pieces or be destroyed after a short period of exposure to sunlight or the like. The material should also have a very low absorption of radiation energy or not absorb such energy at all. Instead of polyethylene terephthalate film, film of cellulose derivatives, for example cellulose triacetate, can of course be used, when there are not too strict requirements for durability without degradation. Oriented polypropylene film can be used. Even transparent polyvinyl chloride film can be satisfactorily used when the thermoplastic properties and the extra stretchability associated with polyvinyl chloride are compensated for by using non-stretchable carrier or reinforcing sheets and binders which are activated at very low temperatures, preferably below the heat deformation point of polyvinyl chloride film. When using a protective film with a heat deformation point close to the activation temperature for the binder coating, it is required to be extra careful to avoid deformation or wrinkling during lamination, therefore it is absolutely preferred to use films with a deformation point above 90° C. Polyethylene terephthalate film types , or transparent film of fluorinated ethylene (for example, film produced with the trade name "Teflon" and with the trade name "Kel-F") which is appropriately primed

(pre-frosted) is used when maximum strength, thermal stability, resistance to degradation and long service life are to be achieved.

The transparent protective film according to the invention resists that materials

adheres to it, but still adheres itself to an underlying substrate by gluing as part of the lye laminate.

The thermoplastic binder quickly becomes adhesive within a limited, relatively low temperature range, but remains non-adhesive at temperatures up to 50° C. Activation should take place at a well-defined temperature point or within a small temperature range whose widest limits are from 50° C to 85° C, preferably 50°—70° C so that the described lamination process can be carried out quickly and efficiently and at the same time a firmly connected laminate should be obtained as soon as it leaves the laminating apparatus.

A wide variety of different types of binders can be used to achieve this. Preferred binder types, however, are those consisting of mixtures of waxy or crystalline organic materials with well-defined melting points together with one or more plasticized non-rubbery, film-forming thermoplastic resins. The binder is applied in the form of a very thin coating. The application quantity of from 675 g on 120 m<2> and Up to 135 and on 120 m<2> is preferred, and one cannot apply more than 2250 g on 120 m<2> t and achieve a satisfactory result. Larger coating quantities result in lamination difficulties such as wrinkling or wrinkling and binder oozing at the edges and further entail the use of unnecessarily large quantities of material. Larger coating amounts also result in increased weight or mass that must be heated and which causes poorer character sharpness during heat copying when the heat copying is performed after lamination of the protective film to an underlying thermographic paper.

The binder components are optically compatible with each other in that, after heat activation to an adhesive or sticky state and union with a paper according to the said lamination process, they form a mass which is transparent under the conditions in which it is to be used. But the binder layer can, under other viewing conditions than those used here, be non-transparent and even opaque.

Plasticizers in liquid form in the binder layer are appropriate to give the laminated product according to the invention sufficient flexibility, especially in cases where polymeric resin materials with a high melting point are used which are strong but somewhat brittle. These also serve, together with the waxy or crystalline component, to change the final character of the binder so that it has high adhesiveness within the low limited temperature range of from 50° to 80° C, although the non-rubbery polymerized resin alone at this temperature is not even softened. When a crystalline component has been chosen which melts below 85° C and preferably below 70° C, due to its high plasticizing effect on a particularly polymerized resin, (for example when diphenyl phthalate is used) or when the resin is particularly de- seen to include a plasticizing component within its molecular structure and in reality is internally plasticized, the liquid plasticizing agent can be substantially reduced or even omitted. But when crystalline plasticizers with a high melting point are used, a plasticizer in liquid form can advantageously be added to keep down recrystallization of the crystalline plasticizer.

It should be specified that the waxy or crystalline component as such does not need to melt at temperatures below 70° C and even below 85° C to be useful in the binder composition. It can melt at temperatures as high as approx. 90° C and even higher, and still becomes sticky at temperatures below 85° C. But crystalline additives such as diphenyl phthalate (melting point approx. 68° C), or waxy materials such as paraffins which melt at even lower temperatures, completely to approx. 50° C, are applicable and usable. Waxes with a melting point above 85° C should preferably be added to a plasticizer that is liquid at room temperature or a plasticizer that melts at least below 85° C. Eutectic mixtures are possible, but a pure eutectic mixture should not be fully relied upon have the necessary adhesive properties.

The preferred film-forming polymeric resin has rather bulky molecules which apparently improve the bond strength of the finished laminate. Typical resins include polyvinyl butyral, polyvinyl acetate and ethyl cellulose. They have characteristic melting points or liquid condensation temperatures well above 70° C, or even above 85° C.

Special optical additives with special adhesion properties can also be included in the binder. The use of chlorinated terphenyl resin to improve anchoring to the primed polyethylene substrate is illustrative of this. This also serves advantageously to lower the softening point of the polyvinyl butyral resin and constitutes an additional adhesive additive as well as a reinforcing agent, but the small improvements achieved with the binder in this respect are not significant.

As a particular example of a heat-adhesive, transparent film adapted for lamination according to the invention, the following serves: A 0.01524 mm thick biaxially oriented polyethylene terephthalate film ("Mylar") is passed together with a carrier sheet of thin kraft paper of the duplicating transparent envelope type through the apparatus which is shown in fig. 3. The mylar film has previously been coated on one side with a firmly anchored binder layer 1-5 microns thick, which can for example be a layer of a copolymer of vinylidene chloride, an acrylic ester and itaconic acid, or polyisocyanate or polyisothiocyanate and any of the commonly known methods and materials can be used for priming. The primed film is of course placed in the coating device in such a way that the primed side is applied with primer from the vessel 14.

On the primed film surface is applied a water-insoluble binder consisting of the following parts by weight: 4 parts polyvinyl butural resin (manufactured commercially by Shawinigan Products' Corp: under the trademark "Butvar B-76^1")-, 6 parts aryl phosphate which is in liquid form - at room temperature, especially 2-biphenyl-diphenyl phase as a plasticizer for the resin, 6 parts of a synthetic wax-like substance with a sharp melting point of approx. 85" G which is produced by the hydrogenation of castor oil ■ which has as its main constituent - 12-hydroxystearic acid glycer-rhr with less than 1 amount of mixed glycerinef of this acid and other stearic acids (manufactured commercially" under the trade mark "Cåstbfwax*)-, and two parts yellowish; transparent chlorinated' 60'%' by weight terphenyl resin with softening point at approx. 100°— 105° G (ASTM' E> — 28)'; (manufactured commercially by Monsanto-Chemical Co: under the trade name "Aroclor 5460"). These components are preliminarily mixed by stirring or the like at -130° C so that they are mixed into a free-flowing, even mass; bg' the water-insoluble-binder mixture is kept for coating in the form of a hot melt at 130° G in the trough. A coating of approx. 900g per 120 m2.

The polyvinyl butyral riarpix starts to soften - at a temperature - of approx. 200° G (cold air). This material seems to have its softening point completely: hidden or modified by the other components of the binder so that the whole mixture gets a rather sharp 'adhesive activation' at approx. 60°C.

After the binder has been applied, it is cooled and the product is wound up into rolls as previously described.

The resulting product can be suitably used for lamination according to the method illustrated in fig: 4'. Further details of suitable laminating apparatus are discussed in US patent 2""85ri65' During lamination, the protective film is fed from a roll and together with the carrier or separation sheet which absorbs radiation energy and a heat-sensitive thermographic copying paper such as that ~which is discussed in US patent 2 663 654 (color change at approx. 80° C) guided between, a guide roller 41 and a guide belt 42 which lets radiant energy pass through, past an intense Jstfålings energy source-in a reflector 44- which' shown in fig. 4.- The feeding speed is dependent on the ambient "temperature" in the immediate vicinity of the laminate. It is possible to achieve >high lamination speed.

The following - special conditions are found favorable - for the lamination: The radiant energy source 43 is an infrared emitting tubular, quartz lamp which functions with a color temperature of 2800° K at the inner focus of an elliptical reflector and are

jplaced at' a distance' of from' 2-5'—5' cm from"

• the laminate which' is to be varreklebés-' (see'- US'

(patent no: 2'891 1*65)'. The 'Ambient' temperature' of the belt and guide Vålseri<1> in' aridfe1 lareas than" where" the laminate passes, jer" ffa1 26°<:>—32° C: A bévegélses Velocity |for the laminate through* the device on' about<J>. 7'6S icm per: sec. has 'provided' full activation' of-the in-heat-adhesive' binder-on-part-transparent' protective' film' and firm' fbran-k-rihg of the-activated-binder' to the' heat-sensitive' thermographic' copy paper of the laminate without arising "color change in this. Such lamination results with the aid of this heat bonding method have not, as far as we know, been previously

possible: Eh' urider search-of the first*lamiri'e--calls" that- are- made will- quickly- give- an- answer' to'- whether it is required' with1 a-slightly lower-level-speed-to" get better' anchoring remove the protective film of the underlying paper, or if it is necessary to use a higher speed to avoid discolouration of the heat-sensitive paper: it is necessary to avoid the extremities but the best" results in terms of smooth, tear-resistant ring" are obtained-' at' temperatures which are'- close to' but- some11 below the fewfgef-firing temperature'for-the heat-sensing paper sheet, even' when' the binder on!- the film is of a type which is activated at a lower temperature: the lamination temperature is from 82°—88° C, but below the color deterioration temperatures it consequently gives the best* results even when the activation temperature for the thermoplastic coating ^on^the highly visible protective film tear lief' as low as 60° C, that is, the film becomes sticky at 60°'a Because it in låriiinaststfu there may be a slight temperature difference between the adhesive layer and the thermographic paper's interior, where it is possible that the temperature in the adhesive layer is slightly above that which causes a color change in the underlying thermographic paper; however, such a temperature difference is so small that it is negligible: It is clear that, according to the foregoing, the larrinization rate can be adjusted upwards if the ambient temperature rises; But after the period of operation, the conditions will stabilize so that little or no further adjustments will eventually be necessary: Continuous lamination of protective film for single-feed heat-sensitive paper (or continuous webs of the paper ) is thus carried out quickly and in this way in a timely manner, whereby the laminate product has increased tear resistance and greater resistance to soiling and similar damage.

As an additional aid to avoid

wrinkles in the laminate can a sheet of paper with

smooth surface is fed through the device

located on the underside of the reinforcing or supporting sheet. A suitable smooth surface is the one produced by coating paper with a smooth silicone additive

of well-known type.

The specific thermoplastic binder on a protective film, which is to be laminated to a heat-sensitive paper, must be activated to an adhesive state at a temperature that is not higher than the color change temperature of the underlying heat-sensitive sheet, in other words that temperatures above approx. . 85° C is usually inappropriate because many are heat sensitive

paper begins to undergo color change

at approx. 85° C

If desired, the laminating apparatus can be modified, for example by

Use a self-absorbing belt

sufficient radiation enation for it to be

heated by the source 43 and thus exhibits

a heat concentration in the area where

the laminate is in contact with the belt. One like that

modification can do that. unnecessary to

pass an absorbent carrier or reinforcement sheet through the device. The absorbent used is applied very thinly

so that it has little mass for conserving energy. As a further modification, radiation adsorptivity can be built

into the laminate materials themselves (for example the binder or the underlying paper sheet) and enable heat bonding such as

stated in the foregoing.

If desired, the protective film can be applied to both sides of the paper sheet.

This not only serves to protect documents from dirt and tearing, but

makes the document more attractive. It also strengthens paper that is worn, torn, has

holes, folds or other weak points.

Claims (4)

1. Heat-sensitive thermographic paper sheet which is protected against tearing and soiling by means of a heat-sealable, glued, protective flexible transparent tear-resistant film, characterized in that it protective film is permanently and transparently laminated to the pair sheet using a non-rubbery, thermoplastic binder which is non-tacky up to 50°C, but quickly becomes sticky at a temperature between 50 and 85°C. 2. Protected paper sheet according to claim 1, characterized in that the protective film is polyethylene terephthalate. 3. Process for the production of heat-sensitive thermographic paper sheet which is protected against tearing and soiling by means of a protective cover as stated in one of claims 1 and 2, characterized in that a non-glued laminate consisting of the paper backing sheet covered by the protective film which is provided with a non-sticky, heat-sealable coating which can be activated between 50 and 85°C, in heat-conducting connection with a radiant energy absorber, is exposed to intense radiant energy sufficient to develop such a high temperature in the heat-sealable layer that this is activated to a sticky state. 4. Heat-sealable protective film which is suitable for use in the method according to claim 3, characterized in that it consists of a thin, strong and flexible, transparent film substrate having a non-delaminating layer of non-rubber-like binder, a firmly anchored to one side of said film substrate, and said binder is normally non-adhesive and non-blocking at temperatures up to 50° C, but quickly becomes sticky and adheres to paper surfaces at temperatures between 50 and 85° C, and the binder comprises a polyvinyl butyral resin, a liquid organic plasticizer which is similar to said resin, as well as hydrated castor oil wax.