CA1083745A - Hot-melt pressure sensitive adhesive - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A hot-melt pressure sensitive adhesive, pourable at 150-200°C, comprises 10-30% by weight of a rubbery copolymer; 15-40% by weight of a naphthenic or paraffinic oil, or a mixture thereof, having an initial boiling point above 200°C; and 40-65% by weight of a terpene tackifier resin. The rubbery copolymer may be a teleblock copolymer comprising molecules having at least three branches radially branching out from a central hub, each branch having polystyrene terminal blocks and a butadiene segment in the center; or a three-block A-B-A block copolymer, the A blocks being polystyrene and the B block being a rubbery ethylene-butylene polymer; or a mixture thereof.
The adhesive has an advantageous combination of properties for use in adhesive-coated absorbent articles, such as sanitary napkins.

Description

~ 374S :-This application is a division of application Serial No. 255,563, which issued on 12 February, 1980, as Canadian Patent No. 1,071,519, entitled "Absorbent Article With Adhesive Strip".
FIELD OF THE INVENTION
This invention relates to articles Eor absorbing fluids. An aspect of this invention relates to articles for absorbing body fluids such as blood, urine, menses, and other exudates, secretions, and excreta (including fecal matter).
Still another aspect of this invention relates to a sanitary napkin provided with a pressure-sensitive adhesive means for attaching the napkin to a supporting garment.
DESCRIPTION OF THE PRIOR ART
Absorbent structures have been developed which are particularly well-suited for use with a supporting garment or body-encircling member, wherein the absorbent structure is temporarily attached to the member or garment with a pressure-sensitive adhesive means. Typical examples of such absorbent structures include sanitary napkins and diaper-like inserts for attachment to the inside of rubber panties. The inserts for rubber panties which serve, in combination with the panties, as diapers, are similar in structure to the sanitary napkins, except that they are wider and are designed to receive and absorb urine and fecal matter. The sanitary napkins are generally of two types--a highly absorbent type for periods of heavy menstrual flow and a far-less absorbtive type for the so-called light flow (which typically occurs near the end of a menstrual period). An example of the highly absorbtive type of sanitary napkin is described in U.S.
Patent 3,672,371 ~Roeder) issued June 27, 1972~ `
All of the aforementioned absorbent structures generally comprise: (1) an elongated absorbent pad having essentially two major surfaces, (2) an outer covering layer -adhered to one or both of the two major surfaces, (3) an

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adhesive layer on the outer layer which is intended for contact with the garment or body-encircling member, and (4) a release liner means for covering and protecting the adhesive layer until the absorbent structure is put to use.
Several types of pressure-sensitive adhesive systems have been suggested for use as the aforementioned pressure-sensitive adhesive layer on the outwardly-facing surface of the outer layer of the absorbent structure. For example, some work has been done with solvent-based pressure-sensitive adhesive (PSA) systems. However, the generally low-viscosity of such PSA systems can result in a rapid penetration or impregnation of the absorbent pad, leading to poorly controlled results with respect to coating of the PSA system. Furthermore, any solvent-based system can have the usual problems of health hazards from solvent vapors, solvent recovery to reduce costs, etc., associated with it.
Water-based adhesive systems have also been suggested for providing the PSA layer on the outer surface of the absor-bent pad structure. It has been generally found, however, that the water-based adhesives tend to "set" or dry down rather slowly, particularly as compared to the solvent-based type.
Still another approach which has been attempted is the use of transfer tapes to transfer a coherent layer of PSA
onto the desired portion of the pad structure. One difficulty with this approach is that the resulting coherent layer of adhesive can be used only as the PSA means for adhering the pad to the garment. Since there is no impregnation of the absorbent pad structure with the transferred adhesive, the adhesive does not ordinarily add to or reinforce the integrity of the absorbent pad structure. With a more fluid adhesive, ~ 5137~9~S

it is at least possible to use the PSA system for a double purpose; as the PSA layer on the outer surface and as a means for cementing together seams (or the like) in the structure.
So-called hot-melt PSA's are known. However, these PSA systems typically lack one or more performance requirements for a sanitary napkin or diaper pad PSA. ~-,' ,' SUMMARY OF THE INVENTION
A hot-melt PSA system has now been discovered which can be used as the PSA for adhesive-coated absorbent structures.
Like many hot-melt systems, the hot-melt PSA used in this invention comprises a tackifier, a rubbery-base material, and a plasticizer or diluent, e.g. a hydrocarbon process oil, for adjusting the tack, the viscosity, and the like. It has been found that the rubbery base used in the hot-melt PSA should comprise a rubbery block copolymer which imparts a variety of desired characteristics. Two different types of block copolymers `
have been found to meet the requirements of a PSA for sanitary napkins and, further, to provide some surprising advantages.
These two types of block copolymers both contain rubbery mid-blocks and relatively stiffer polystyrene terminal blocks. In one preferred embodiment of the invention, the block copolymer is of the A-B-A type wherein the A blocks comprise polystyrene and the B block is a rubbery poly (ethylene-butylene) center block.
In a second preferred embodiment, the polymer is a teleblock copolymer comprising molecules having at least three branches radially branching out from a central hub, -each of the branches having polystyrene blocks and a butadiene segment in the center.

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7~5 A tacky, hot-melt pressure sensitive adhesive, pourable at 150-200C, according to the present invention, comprises:
(a) 10-30~ by weight of a rubbery copolymer selected from the group consisting of a radial block-copolymer, an A-B-A block copolymer, and mixtures thereof, the radial block copolymer being a teleblock copolymer comprising molecules having at least three branches radially branching out from a central hub, each branch having polystyrene terminal blocks and a butadiene segment in the center, the A-B-A block copolymer being a three-block A-B-A block copolymer, the A blocks being polystyrene and the B block being a rubbery ethylene-butylene polymer;
(b) more than 15 but less than 40% by weight of an essentially hydrocarbon oil selected from the group consisting of naphthenic oils, paraffinic oils, and mixtures thereof, the essentially hydrocarbon oil having an initial boiling point above 200C, and (c) 40-65~ by weight of a terpene tackifier resin.
DETAILED DESCRIPTION
The following detailed description is directed not only to the hot-melt pressure sensitive adhesive, which is the subject of the present divisional application, but also describes absorbent structures in accordance with Application Serial No. 255,563, now issued as Canadian Patent No. 1,071,519, of which the presen-t application is a division.

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, )837~5 As pointed out previously, a typical absorbent structure of this invention is a sanitary napkin of the t~pe disclosed in U.S. Patent 3,672,371 (Roeder), issued June 27, 1972. A structure of this general type can include an absor-bent pad enclosed in a fluid-pervious wrapper which is over-lapped to provide a seam, the seam typically running lengthwise ;
through the middle of a major surface of the pad. The layer (or layers) of PSA (pressure-sensitive adhesive) can be applied to this seam (overlapped) area. If the PSA has sufficient fluidity when applied, it can permeate the seam, binding together the overlapped area and sealing the absorbent pad inside the fluid-pervious wrapper. A release liner over the PSA layer protects it until the napkin is to be used, at which time the liner is peeled off and the PSA layer is pressed against the inside of an underwear garment or the like. The major surface ~ `

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on which the PSA is coated does not actually have to be fluid-pervious, since the body fluids enter the absorbent structure from the opposite surface. For this reason, the absorbent structure can contain other elements such as a fluid-impervious film integral with the PSA-coated surface and the absorbent pad. Fur-thermore, the liner or envelope enclosing the absorbent pad can comprise a plurality of layers of material such as waterlaid sheets, nonwoven webs, and the like. The absorbent pad itself can be a batt or frictionally entangled mass of fibers with properties similar to an absorbent cotton mass. Absorbent cotton itself can also be used as the absor-bent pad material.
When the purpose of the absorbent structure is to absorb a very light flow of blood and menses, the total structure can be simpler. In this case the absorbent pad can be much thinner and need not be surrounded or sealed in completely by a fluid-pervious layer, due to the inherently greater integrity of the thinner pad. For example, the entire structure can consist essentially of the thin pad (totally exposed on one major surface), a fluid-impervious film in adherent contact with the other major surface of the pad (this film serves as the outer covering layer), the PSA layer coated on the exposed surface of the impervious film, and the release liner pro-tecting the PSA layer.
In any of these structures, the PSA layer need not be coextensive in area with the major surface on which it is coated. On the contrary, the typical PSA layer is very much narrower in width and can be somewhat shorter in length.
When the absorbent structure is an insert for rubber panties and is intended to serve as a diaper, it is typically ~3374S :-wider than a sanitary napkin and may be somewhat longer.
Otherwise, the structure is similar. On the other hand, if the absorbent structure is intended for receiving wound exudates, a different arrangement of elements is generally desirable.
The properties of the PSA layer are peculiar to this art. For example, the PSA, when applied, should have a low enough viscosity to flow into place readily and pre-ferably also to permeate the aforementioned seam area or overlap area. However, the viscosity must not be so low that the PSA composition will permeate through the outer covering layer and into the absorbent pad itself. Furthermore, the ;
viscosity should be high enough to permit some degree of a property similar to "hold-out"--that is, a significant amount of the PSA composition should remain on the exposed surface of the covering layer and form a coherent PSA layer. This "hold-out" effect can be most effectively obtained if the PSA, during application to the outer covering layer, has a signi-ficant amount of thixotropy. For a hot-melt PSA, it is ordinarily preferred that the composition have a viscosity ranging from about 500 to 10,000 centipose (cps) in the pour temperature range, which would typically be 100 - 250C., more typically 150 - 200C. (e.g. 300 - ~00F.).
Another important property is the ability to solidify or congeal or "set" very rapidly--preferably almost instantan-eously. Hot-melt PSA's are particularly desirable in this regard, since they can cool down from the application temperature to a solidification temperature almost instantly under normal ambient conditions.
Among the most peculiar properties needed for the PSA are the adhesive bond characteristics. These characteristics :

37~5 must be obtained wi-thout sacrificing room temperature tack ("probe tack" or "wet grab") and without creating a situation where the adhesive will offset or transfer to the garment.
The requirements for the adhesive bond are that the PSA should have adequate peel strength and yet have the ability to release itself from the undergarment without pulling of fibers or tearing the garment. There are presently no adequate standardized tests for measuring these adhesive bond properties;
however, the PSTC-l test is helpful to give some indication of the required amount of peel strength. See U.S. Patent

3,672,371, column 3, line 51 et seq. Standard 180 peel and shear tests are also helpful. For example, a PSA of the invention has been found to have a 180 peel strength ("Mylar"*
substrate, 12 in./min) ranging from about 4 to about 5 or 6 pounds per inch width (piw) and a tensile shear strength (0.1 in./min) of about 15 to about 35 pounds per square inch (psi).
The hot-melt PSA compositions which have been found to have the best combination of properties for use in sanitary napkins and diaper inserts comprise a rubbery or elastomeric block copolymer, an essentially hydrocarbon oil, and a tacki-fier resin, all of which are described in more detail subsequently.

The Rubbery Copolymer -The two basic types of rubbery or elastomeric block copolymers particularly well-suited for use in this invention are:
(a) an A-B-A block copolymer having polystyrene end blocks and a rubbery polyolefin center or mid block, the polyolefin being a rubbery poly(ethylene-butylene) block, and *Trade Mark (b) a teleblock copolymer comprising molecules having at least three branches radially branching out from a central hub, each said branch having polystyrene terminal blocks and a butadiene segment in the center.
The A-B-A block copolymer is considered a two-phase polymer comprising polystyrene domains in a rubbery poly (ethylene-butylene) matrix. In order to obtain desired levels of tack, it (and this is also true of the teleblock copolymer) is tackified with a resin and an oil. The A-B-A polymer is a true elastomer (according to the ASTM definition of "elastomer") and has an elongation at break well in excess of 200%, e.g. 500%. The elongation at break for the teleblock (radial) copolymer is in approximately the same range or slightly higher, e.g. 590%.
To ensure rubberiness or elastomeric behavior in the radial copoLymer, the number of butadiene units should be greater than the number of styrene units. Shore A hardness for the teleblock (radial) copolymers can be in the range typical of true rubbers, e.g. above 60 or 70.
The ultimate tensile strength of the rubbery copolymer (whether of the A-B-A type or the radial type) is moderately high, e.g. above 2000 p.s.i., more typically above 3000 psi.
.. .. .
Tensile strengths above 5000 psi are possible with the A-B-A
structure.

' ~ ' The Essentially Hydrocarbon Oil .
The oily liquids used in compounding PSA's of this invention are essentially hydrocarbon process oils which are preferably low in aromatic content. For example, an analysis ' ', _ g _ ' ~

?3~7~5 of the types of carbon atoms in oils used in this invention indicate that aromatic carbons comprise less than 5% of the oil, while naphthenic carbons (i.e. carbons of cycloaliphatic compounds and the like) can range from about 25 to 60% and paraffinic carbons can range from about 35 to 75%. Accord-ingly, these oils are typically referred to as "naphthenic"
or "paraffinic" process oils.
These oils are preferably very low in volatility.
Initial boiling points, under normal ambient pressure con-ditions, can range from well over 400F. (i.e. above about 200C.) to almost 800F. (almost 430C.). The least volatile fraction in the process oils can have a boiling point in excess of about 1000F. (about 535C.), so that the entire boiling range can cover several hundred degrees, e.g. 600-1100F. (315 - 600C.).
The aromatic content of the oils as determined by clay gel analysis (in weight percent) can range from less than 1% up to 15~ or more; however, aromatic content should be low and should not exceed about 20% by weight. The molecular weight of the oil is typically above 200 and can be above 600. Most naphthenic and paraffinic process oils, however, tend to have a molecular weight within the range of 250 - 600.

The Tackifier Resin Although various types of tackifier resins are known, such as hydrogenated rosin esters, esters of polyhydric alcohols, phenol-aldehyde resins, and the like, the preferred tackifiers are of the type known as "hydrocarbon resins".
In industrial practice "hydrocarbon resin" is a term of art relating to resins in the molecular weight range of a few ' ~413~37~5 hundred up to about 6,000 or 8,000 which are obtained or synthesized from rather basic hydrocarbonaceous materials such as petroleum, coal tar, turpentine, and the like. A
good description of "hydrocarbon resins" can be found in Kirk-Othmer, Encyclopedia of Chemical Technology, Second Edition, Volume 11, Interscience, New York, 1966, pp242. Many of the so-called "hydrocarbon resins" commercially available today are "terpene" resins, i.e. polymers with repeating terpene units. These polymers can be homopolymers, or copoly- ~ -mers, (including terpolymers, etc.), since terpene is an olefin which can be copolymerized with other olefins. Terpene-phenols are also produced.
All hydrocarbon resins do not work with equal effectiveness, and a commercially available resin known as "Wingtack" 95 (trade mark of Goodyear Chemical Company) is especially suited for use in this invention. This hydrocarbon `
resin is compatible with other components of the hot-melt PSA
and has a softening point (ball and ring method) of 100C. +

5. "Wingtack" 95 is believed to be a by-product of isoprene or polyisoprene production and is also believed to be a synthetic terpene resin derived from a mixed olefin feed stock.
The hot-melt PSA compositions of this invention can be formulated with techniques known in the art using heated mixers and the like. The rubbery copolymer and the oil can be blended together readily at moderately elevated temperatures (e.g. 150 - 300F.). The tackifier resin can be added to the copolymer oil mixture. If a pigment is included in the PSA

composition, it should be added to the copolymer/oil blend before the tackifier resin is introduced into the composition.

lo~a74s The resulting hot-melt PSA, once it is heated to the temperature where it will flow readily, can be applied to the outer covering layer of the absorbent structure or article by any of the techniques known in the art, including flow coating, roller coating, knife coating, or the like. The PSA
can also be extruded into place by using a hot extruder or die face.

The Resulting Hot-Melt PSA Compositions As is known in the art, various other components can be added to modify the tack, rheology characteristics (including melt viscosity, thixotropy), adhesive bond strength character-istics, rate of "set", low temperature flexibility, etc., color, odor, etc., of a hot-melt PSA. For example, liquid resins are sometimes used as a partial or total replacement or substitution for process oils, although such a substitution is not preferred in the context of this invention.
The proportions of components in a hot-melt PSA of this invention are preferably selected to provide the adhesive bond characteristics described previously. It is also important that these adhesive characteristics remain reasonably constant during storage or non-use in a hot-melt applicator and during normal storage of the article (e.g. a sanitary napkin) prior to use. Still further, it is desirable that adhesive properties remain generally within the prescribed ranges from one production batch to another. "Aging" tests of the A-B-A block copolymer embodiment of the PSA indicate generally stable adhesive properties at hot-melt applicator temperatures (e.g.
with the range of 100-250C., more typically 150 - 200C.) and, after coating onto the article, with the range of temperatures typically associated with storage and shipping 37~i (e.g. from about -20C. to about ~50C.).
With respect to this embodiment, the most noticeable downward trend in adhesive properties detectable upon storage of coated samples in 1 to 24-week aging tests appears to be in the peel strength o~ samples stored at 120F. (48.9C.). Even this trend does not indicate a significant deterioration in peel strength.
In short, these data generally indicate a reasonable degree of stability in the internal structure of the hot-melt PSA. For example, the process oil is reasonably well retained within a matrix or the like provided by the tackifier resin and/or the rubbery block copolymer.
The tackifier resin (e.g. "Wingtack" 95, trade mark of Goodyear Chemical Company), provides a number of desired properties (e.g. increased shear strength of the adhesive bond) particularly at levels in excess of 30 weight-% (e.g. 40 weight -% or more). However, if the resin is increased at the expense of the process oil content, there can be a significant loss of tack and undesirable increases in viscosity. Accordingly, the amounts of tackifier resin is preferably less than 70 weight-%
of the composition (e.g. 65 weight-% or less, more typically 50-60 weight-%).
The process oil content is preferably above 10% by weight (e.g. at least 15 weight-~) to provide sufficient tack and a low enough viscosity at the pour temperature or temperature of application (e.g. 150 - 200C.). However, excessive amounts of oil (for a given content of rubbery base) can reduce shear strength and cohesive strength and possibly lead to transference of PSA from the article to the undergarment. Accordingly, it is ordinarily preferred that the amount of process oil in the PSA be less than 40 welght-%.

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i~?37~5 The rubbery copolymer base assists in providing many, if not most, of the key properties contemplated for hot-melt PSA's of this invention. However, minor amounts of these rubbery materials are effective, and excessive amounts can increase the viscosity drastically, interfering with flow properties in the 100 - 250C. range discussed previously.
At amounts below 5% by weight, the contribution of the rubbery base to the PSA is ordinarily insufficient, and amounts of 10 weight-% or more are preferred. At rubbery copolymer amounts above 30% (e.g. 35%), the increase in viscosity (even at elevated pour temperatures such as 350F.) rapidly approaches inconvenient levels. Accordingly, the optimum amount of rubbery copolymer appears to be within the range of 10 - 20 weight-%.
The principle and practice of this invention is illustrated in the following nonlimiting examples.

Example 1 In this Example, a block copolymer with the A-B-A
structure was used. The commercial embodiment of the copolymer is sold by Shell chemical Company under the trade mark "KRATON"
GX 6500 (more recently referred to as "KRATON" 1650). This commercially available material is a three block copolymer with polystyrene endblocks and a rubbery poly(ethylene-butylene) midblock. It is considered a two-phase polymer consisting of polystyrene domains in a rubbery poly(ethylene-butylene)matrix. Typical properties of the copolymer are as follows:
Tensile strength psi: 6000 Elongation at break : 500%
Modulus at 300% extension, psi: 950 3~S
. ~ '' ' Solution viscosity (measured with a Brookfield Model RVT Viscometer): 20% by weight in toluene, centipoise: 2000 25% by weight in toluene, centipoise: 12000 The product is available in particulate form, e.g.
in crumbs about 1/8 inch in size.
The formula for the hot-melt PSA of this Example, was as follows:

10Parts by Weight Ingredient 30.0 Naphthenic process oil*
15.0 "KRATON" 1650 (trade mark;
formerly sold under the trade mark "KRATON" GX 6500) 1.0 Pigment dispersion (43 parts by weight of pigment dispersed in 57 parts by weight of the naphthenic process oil) 54.0 "Wingtack" 95 (trade mark of Goodyear Chemical Co. for hydrocarbon tackifier resin) *See detailed description which follows.
After the "KRATON" was completely blended into the process oil, the pigment dispersion was added under high shear mixing. The mixing temperature was maintained below 300F.
After the pigment was dispersed in the "KRATON"/oil composition, the "Wingtack" 95 was added, and mixing was continued until a uniform composition was obtained.
The naphthenic process oil used in the composition was "Tufflo 6204" (trade mark). This process oil has a ~ ~374~i viscosity (SUS/100F.) of 1965, a specific gravity (60/60) of 9.9206, a flash point (COC) of 440F., a pour point of 0F., and a sulfur content which is less than 0.001%. The molecular weight of the oil is 440, and clay gel analysis indicates 0% asphaltenes, 9% polar compounds, or resins, 14.8% aromatics, and 85.2% saturates. Carbon type analysis indicates 2% aromatic carbons, 52% naphthenic carbons, and ..
46% paraffinic carbons. The boiling range is 630 - 1018F., the initial boiling point being 630F., and 95% of the oil being distilled at 1018F. Only 5% of the oil distills at 630F. and 50% distills at 710F.
Ten pounds of the resulting hot-melt PSA was tested for peel strength and shear strength. The 180 peel strength was conducted at 12 inches per minute and the tensile shear test was conducted at 0.1 inches per minute. Results were :
as follows:
Aging Time* Peel (piw) Shear (psi) Zero 4.9 29.1 8 hours 5.2 30.7 24 hours4.6 29.8 48 hours4.8 32.3 *Length of time adhesive was held in the hot-melt applicator pot at 193C. (380F.) prior to application to the test specimens.
Three samples were tested and averaged to arrive a-t the values given in the above table.
Properties of the resulting hot-melt PSA were as follows:
Ring and ball softening point: 202 - 240F.
Specific gravity: 0.922 .

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Brookfield Viscosity (Brookfield Thermosel [trademark] speed 20 rpm spindle SC-4-27, 8 gram sample):
1,300-1,650 cps at 350F.
3,800-5,000 cps at 325F.
700-900 cps at 375F.
This hot-melt PSA was found to be particularly well-suited for sanitary napkin structures of the type described in U.S. Patent 3,672,371.

Example 2 ~ -This example illustrates the use of a radial (teleblock) copolymer, the commercial embodiment in this case ;
being "SOLPRENE" 502, a trade mark of Philips Petroleum Company;
typical properties of "SOLPRENE" (trade mark) 502-CX are as followsO
Specific gravity: 0.91 Melt flow, 190C./21.6Kg: 0.2 Molecular weight: 91000 Shore A hardness: 79 -~utadiene/styrene ratio: 70/30 Tensile strength, psi: 3900 Modulus at 300% extension, psi: 540 Elongation at break: 590 Compression set (Method B) 22 hours, 78F.: 35~
The formula used in the case of the "SOLPRENE" polymer was as follows:
Parts by Weight Ingredient -29.0 Naphthenic process oil ("Tufflo"
6204, see Example 1) 15.0 "SOLPRENE" 502 (see preceding description) ,'.

2.0 Pigment dispersion (see Example 1) 54.0 "Wingtack" 95 (trade mark; see Example 1) The procedure ~or preparing the hot-melt PSA was the same as that of Example 1. The specific gravity of the resulting product was 0.922. The ring and ball soften.ing point was 204 - 240E., the viscosity at 350F. was 750 - 940 centipoise (cps), determined on a Brookfield Thermosel at a speed of 20 rpm, spindle SC-4-27, with an 8 gram sample.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A tacky, hot-melt pressure sensitive adhesive, which is pourable at 150-200°C, said adhesive comprising:
(a) 10-30% by weight of a rubbery copolymer selected from the group consisting of a radial block-copolymer, an A-B-A block copolymer, and mixtures thereof, said radial block copolymer being a teleblock copolymer comprising molecules having at least three branches radially branching out from a central hub, each said branch having polystyrene terminal blocks and a butadiene segment in the center, said A-B-A block copolymer being a three-block A-B-A block copolymer, said A blocks being polystyrene and said B block being a rubbery ethylene-butylene polymer;
(b) more than 15 but less than 40% by weight of an essentially hydrocarbon oil selected from the group consisting of naphthenic oils, paraffinic oils, and mixtures thereof, said essentially hydrocarbon oil having an initial boiling point above 200°C, and (c) 40-65% by weight of a terpene tackifier resin.

2. An adhesive according to claim 1 wherein the weight ratio of said essentially hydrocarbon oil to said rubbery copolymer is within the range of 4:3 to 4:1.

3. An adhesive according to claim 1 wherein the ratio of essentially hydrocarbon oil to rubbery copolymer is within the range 1.5:1 to 3.5:1.

4. An adhesive according to claim 1, 2, or 3, wherein said adhesive consists essentially of:
(a) 10-20% by weight of said rubbery copolymer;
(b) 15-35% of said essentially hydrocarbon oil;
(c) 40-65% by weight of said terpene tackifier resin; and (d) essentially the balance, if any, being a pigment dispersion.

5. An adhesive according to claim 1 wherein said adhesive consists essentially of:
(a) 15% by weight of said rubbery copolymer;
(b) 29-30% by weight of said essentially hydrocarbon oil;
(c) 54% by weight of said terpene tackifier resin;
and (d) essentially the balance, if any, being a pigment dispersion.

6. An adhesive according to claim 1, 2, or 3, wherein the viscosity of said adhesive in the temperature range of 150-200° C. ranges from about 500 to about 10,000 centipoise.