Background Art [002] It is known to prepare adhesives using block copolymers. For example, US 5292819 (DOW CHEMICAL CO & EXXON CHEMICAL PATENTS INC). discloses that hot melt adhesives can be prepared using radial block copolymers of styrene and isoprene. These polymers are reported to have an overall molecular weight of from 90,000 to 380,000 and an average polystyrene molecular weight of from 10,000 to 25,000.

[003] A more recent patent also discusses preparing adhesives using block copolymers.

US 6232391 (NAT STARCH CHEM INVEST).-. discloses a multipuipose hot melt adhesive comprising an endblock resin in combination with a radial or linear block copolymer of styrene and butadien having a solution viscosity of greater than 1000 cPs at 25% in toluene. The hot melt adhesive is disclosed as being suitable for both construction and elastic attachment in disposable products.

[004] Disposable products, a major application for adhesives, are reported in US 5057571 (FULLER H B LICENSING FINANC).-.

. Therein, it is disclosed that low levels of a radial block copolymer having a molecular weight of greater than about 140,000 can be used to impart a high level of creep resistance, bond strength, and a low viscosity profile in an adhesive. This adhesive is also reported to be useful for both construction and elastic attachment in the preparation of disposable products.

[005] Of course, radial block copolymers are not the only type of copolymers available today to those preparing adhesives.

EP 1348737 (KRATON POLYMERS RES BV).--. discloses preparing bituminous compositions using an A1-B 1-A2-B2 block copolymer.

It is suggested that the polymers disclosed therein may be of use in applications including sound and vibration abatement, adhesives, sealants, and coatings.

[006] Resently, there exists a need for adhesive compositions useful in tapes, labels and disposable articles which have higher service temperatures than can be achieved with conventional polymers. Sulprisingly, it has been found that the linear tetrablock copolymers of the present invention serve as essential components of pressure sensitive adhesive compositions which have service temperatures as much as 20 °C higher than can be achieved with conventional polymers at a given polymer content.

Disclosure of Invention [007] In one aspect, the present invention is a pressure sensitive adhesive comprising a tackifying resin and a block copolymer, the block copolymer having the general formula: S1-B1-S2-B2 wherein S 1, B 1, S2, and B2 are polymer blocks, and B 1 is a block of polymerized conjugated diene comprising at least 50 mole percent isoprene having an apparent molecular weight of from 150,000 to 400,000 ; S 1 and S2 are blocks of polymerized mono-vinylaromatic hydrocarbon having a weight average molecular weight of from 12,000 to 40,000 ; and B2 is a block of polymerized conjugated diene comprising at least 50 mole percent isoprene having an apparent molecular weight of from 15,000 to 60,000 ; and wherein: the adhesive has a shear adhesion failure temperature of at least 75°C ; the weight ratio of B 1 over B2 Zi's from 3.0 to 12.0 ; and the content of polymerized mono-vinylaromatic hydrocarbon is from about 10 to about 35 weight percent.

[008] In another aspect, the present invention is a composition comprising a substrate and adherent thereto the aforementioned pressure sensitive adhesive.

[009] In still another aspect, the present invention is an adhesive tape comprising a tape and adherent to at least one side of the tape, the aforementioned pressure sensitive adhesive.

Mode for the Invention [010] In one embodiment, the present invention is a pressure sensitive adhesive comprising a tackifying resin and a block copolymer, the block copolymer having the general formula : [011] S1-B1-S2-B2 [012] wherein S 1, B 1, S2, and B2 are polymer blocks, and B 1 is a block of polymerized conjugated diene comprising at least 50 mole percent isoprene having an apparent molecular weight of from 150,000 to 400,000 ; S 1 and S2 are blocks of polymerized mono-vinylaromatic hydrocarbon having a weight average molecular weight of from 12,000 to 40,000 ; and B2 is a block of polymerized conjugated diene comprising at least 50 mole percent isoprene having an apparent molecular weight of from 15,000 to 60,000.

[013] For the purposes of the present invention, weight average molecular weights were measured with gel permeation chromatography (GPC). The term"apparent molecular weight", as used throughout the present application, means the molecular weight as measured by GPC relative to commercially available polystyrene calibration standards (according to ASTM D6474-99). Thus, the molecular weights of the S 1 and S2 blocks are the real or true molecular weights of those blocks. On the other hand, the "apparent"molecular weights of the B 1 and B2 blocks are the molecular weights of the polystyrene standard which would elute at the same time. One skilled in the art can readily convert the apparent molecular weights of B 1 and B2 to real or true molecular weights using compositionally dependent correction factors.

[014] While isoprene is the preferred conjugated diene for preparing the block copolymer component of the adhesives of the present invention, other suitable conjugated dienes can also be used. For example, 1, 3-butadiene, 2, 3-dimethyl-1, 3-butadiene, 1, 3-pentadiene, 1,3-hexadiene and others conjugated dienes having from 4 to 8 carbon atomsecan also be used with the present invention. Mixtures of such dienes'with isoprene may also be used. Referably, the blocks of conjugated diene independently comprise at least 50 mole percent, preferable at least about 80 mole percent, and most preferably, at least 99 mole percent isoprene.

[015] Similarly, styrene is the preferred mono-vinylaromatic hydrocarbon to prepare the A1 and A2 blocks of the present invention, but other mono-vinylaromatic hy- drocarbons can also be used. For example, other mono-vinylaromatic hydrocarbons useful with the present invention include o-methylstyrene, pmethylstyrene, p tert-butylstyrene, 2,4-dimethylstyrene, a-methylstyrene, vinyl naphthalene, vinyl toluene and vinyl xylene, or mixtures thereof. Referably the blocks of polymerized mono-vinylaromatic hydrocarbon are made of styrene or a mixture independently comprising at least 80 mole percent styrene, the former being preferred. Most preferably, the blocks of polymerized mono-vinylaromatic hydrocarbon are made of styrene or a mixture independently comprising at least 99 mole percent styrene.

[016] The block copolymers useful with the present invention are preferably poly (styrene-isoprene-styrene-isoprene) polymers. To achieve the properties important to preparing a pressure sensitive adhesive of the present invention, the block copolymer must have the structural and molecular weight limitations as set out above.

Thus, B 1 has an apparent molecular weight of from 150,000 to 400,000. Fteferably, B 1 has an apparent molecular weight of from 250, 000 to 350,000. In like manner, B2 has an apparent molecular weight of from 15,000 to 60,000, preferably from 25,000 to 50,000. S 1 and S2 each independently have a weight average molecular weight of from 12,000 to 40,000, preferably from 20,000 to 35,000.

[017] The block copolymers that are used to prepare the pressure sensitive adhesives of the present invention have a B 1 block with a greater apparent molecular weight than the B2 block. The weight ratio"W'of B 1 over B2 is from 3.0 to 12.0, preferably from 4.0 to 8.0. Most preferably, W is about 6.0.

[018] The block copolymers of the present invention have a greater weight content of conjugated diene than mono-vinylaromatic hydrocarbon. The content of polymerized mono-vinylaromatic hydrocarbon in the block copolymers used to prepare the pressure sensitive adhesives of the present invention is from 15 to 30 weight percent. More preferably, the content of polymerized mono-vinylaromatic hydrocarbon in the block copolymers used to prepare the pressure sensitive adhesives of the present invention is from 20 to 25 weight percent.

[019] The block copolymers useful with the present invention are preferably prepared by anionic polymerization. The preparation ouf these block copolymers is well known to those skilled in the art and has been described in US 3265765 US 3231635 US 3149182 --.

; US 3239478 --.

US 3431323 and US RE27145E-.

. Typically the polymerization is carried out in a hydrocarbon solvent, such as cy- clohexane, using an alkyl lithium initiator, such as sec-butyl lithium. The polymers are preferably prepared by the sequential polymerization method in which each polymer block is prepared by sequential addition of each monomer to the reactor. Any block copolymer having the above described structure and molecular weights can be used with the present invention.

[020] The pressure sensitive adhesives of the present invention additionally include a tackifying resin. The tackifying resins useful in the practice of the present invention include hydrocarbon resins, synthetic polyteipenes, natural polyterpenes and rosin esters. These resins are preferably semi-solid or solid at ambient temperatures and soften or become liquid at temperatures ranging generally from 40 to 135°C, preferably from 70 to 120°C.

[021] Tackifying resins that can be used with the present invention include, but are not limited to: (1) natural and modified rosins such, for example, as gum rosin, wood rosin, tall oil rosin, distilled rosin, hydrogenated rosin, dimerized rosin, and polymerized rosin; (2) glycerol and pentaerythritol esters of natural and modified rosins such, for example, as the glycerol ester of pale, wood rosin, the glycerol ester of hydrogenated rosin, the glycerol ester of polymerized rosin, the pentaerythritol ester of hydrogenated rosin, and the phenolic-modified pentaerythritol ester of rosin; and (3) copolymers and teipolymers of natural tetpenes, e. g., styrene/teipene and al- phamethylstyrene/teipene. Also useful are (4) polyteipene resins resulting from the polymerization of teipene hydrocarbons, such as the bicyclic monoteipene known as pinene. Other resins that can be used are the hydrogenated polyteipene resins; (5) t phenolic modified teipene resins and hydrogenated derivatives thereof such, for example, as the resin product resulting from the condensation, in an acidic medium, of a bicyclic teipene and a phenol; (6) aliphatic petroleum hydrocarbon resins resulting from the polymerization of monomers consisting primarily of olefins and diolefins; also included are the hydrogenated aliphatic petroleum hydrocarbon resins; (7) aromatic petroleum hydrocarbon resins, and mixed aromatic and aliphatic hydrocarbon resins, and the hydrogenated derivatives thereof ; (8) aromatic modified alicyclic petroleum hydrocarbon resins and the hydrogenated derivatives thereof ; and (9) alicyclic petroleum hydrocarbon resins and the hydrogenated derivatives thereof.

[022] Preferably, the tackifying resin is selected from the group consisting of C5 hy- drocarbon resins, hydrogenated C5 hydrocarbon resins, styrenated C5 resins, C5/C9 resins, styrenated terpene resins, fully hydrogenated or partially hydrogenated C9 hy- drocarbon resins, rosin esters, rosin derivatives, polyteipene resins and mixtures thereof.

[023] While the aliphatic hydrocarbon resins and hydrogenated hydrocarbon resins are preferred, any tackifying resin known to those of ordinary skill in the art of preparing pressure sensitive adhesives to be useful can be used with the present invention. The amount of tackifying resin used in the formulation is usually from 25 parts to 300 parts by weight per 100 parts of polymer.

[024] The pressure sensitive adhesives of the present invention may additionally include a plasticizing oil, normally one which is preferentially compatible with the B blocks of the block copolymer. Various plasticizing oils are useful in the practice of this invention. The preferred plasticizing oils useful with present invention are the petroleum derived oils which are relatively high boiling materials containing less than 50 percent and in some embodiments only a minor proportion of aromatic hy- drocarbons. Alternately, the oil may be totally non-aromatic. Oligomers such as polypropylenes, polybutenes, hydrogenated polyisoprene, hydrogenated polybutadiene, polypiperylene and copolymers of piperylene and isoprene, or the like, having average molecular weights between 350 and 10,000 may also be used.

Vegetable and animal oils including glyceryl esters of fatty acids and polymerization products thereof may also be used. Any oil known to be useful to those of ordinary skill in the art of preparing pressure sensitive adhesives can be used with the present invention. If used, the amount of plasticizer is from zero to 200 parts by weight per 100 parts of polymer.

[025] Optionally, the adhesive formulation may also contain an arene-block-compatible resin. Normally, the resin should have a softening point above about 100°C as determined by ASTM E28 using the ring and ball apparatus. Mixtures of arene- block-compatible resins having high and low softening point may also be used. Useful resins include coumarone-indene resins, polystyrene resins, vinyl toluene- alphamethylstyrene copolymers, alphamethylstyrene resins, and polyindene resins. If used, the amount of arene-block-compatible resin varies from 0 to 200 parts by weight per 100 parts of polymer.

[026] The pressure sensitive adhesives of the present invention may also include other materials. For example, the pressure sensitive adhesives of the present invention can include an antioxidant. Exemplary antioxidants include high molecular weight hindered phenols and multifunctional phenols such as sulfur and phosphorous- containing phenols. As another example, the pressure sensitive adhesives of the present invention can include fillers. The type and amount of filler can vary depending upon the use to which the pressure sensitive adhesive will be put. Still another example of material useful as a component of the pressure sensitive adhesives of the present invention is a pigment. Where color is an issue, the adhesives of the present invention can be prepared using a pigment that will either mask or allow the pressure sensitive adhesive to stand out in contrast to the substrate upon which it is adhered.

[027] If the adhesive is to be applied from solvent solution, the organic portion of the formulation will be dissolved in a solvent or blend of solvents. Aromatic hydrocarbon solvents such as toluene, xylene or Cyclo Sol 100 (Shell), also available as aromatic 100 (ExxonMobil), are suitable. Aliphatic hydrocarbon solvents such as hexane, naphtha or mineral spirits may also be used. If desired, a solvent blend consisting of a hydrocarbon solvent with a polar solvent can be used. Suitable polar solvents include esters such as isopropyl acetate, ketones such as methyl isobutyl ketone, and alcohols such as isopropyl alcohol. The amount of polar solvent used depends on the particular polar solvent chosen and on the structure of the particular polymer used in the formulation. Usually, the amount of polar solvent used is between 0 and 50 weight percent in the solvent blend.

[028] In one embodiment, the present invention is a removable tape. Tapes are dis- tinguished from labels in that tapes are load bearing while labels are not, the adhesive on a label being required to merely hold the label in place. Masking tapes provide one example of a removable tape. Masking tapes are often used, for example, to mask surfaces during painting. Typically, a masking tape is applied to a surface, a task such as painting is performed, and then the masking tape is removed when the tape user has finished the task. The tape should be easy to apply, stay in place without lifting or curling under task conditions, and remove cleanly and easily without breaking, damaging the surface, or leaving adhesive residue. In the practice of the present invention, the adhesive can be on at least one side of the tape with a double sided tape being within the scope of the present invention.

[029] One method of preparing adhesives is solvent coating. In solvent coating, the adhesive components are dissolved in a hydrocarbon solvent, the solution is coated onto a backing, and the coated product is dried to remove the solvent. The solvent coating process is particularly applicable to tapes and also labels.

[030] In another embodiment, the pressure sensitive adhesives of the present invention are applied to a substrate as a hot melt adhesive. In a hot melt adhesive process, the adhesive is heated to a point where it can be applied directly to a substrate. Usually the substrate is a tape, label, or a disposable article. Occasionally, the substrate is a transfer sheet and the adhesive is then brought into contact with a second substrate with a greater affinity for the adhesive than the first substrate. The first substrate can be removed leaving the adhesive on the second substrate.

[031] For tack properties of the adhesives, Rolling Ball Tack (in units of cm) was measured by ASTM D3121 and Polyen Robe Tack (in units of kg) was measured by ASTM D2979. Adhesion to stainless steel (in units of pounds force per inch of tape width) was measured by the 180 Peel test, ASTM D903. Shear strength (in units of hours) to stainless steel was measured by the Holding Power test, ASTM D6463, using a 1.27 cm by 1.27 cm (0.5 inch by 0.5 inch) contact area and a 2 kg load. Upper service temperature was measured by the Shear Adhesion Failure Temperature (SAFT) test, ASTM D4498. In the SAFT test, bonds 2.54 cm by 2.54 cm (1 inch by 1 inch) were formed of adhesive, on a Mylar (polyester) backing, to adhesive on another strip of Mylar, using a 2 kg (4.5 pound) rubber roller. The assembly was suspended vertically in an oven at 38°C, and allowed to come to equilibrium. A 0.5 kg weight was suspended from the free end of the adhesive tape, and the temperature was raised at 22°C/hr. The temperature at which the bond softened and the weight fell was recorded.

SAFT was reported as the average of two such determinations. Adhesive melt viscosity (in units of Pas) was measured with a Brookfield Thermocell viscometer according to ASTM D3236.

[032] The adhesives of the present invention are particularly useful in construction of disposable articles such as disposable diapers, sanitary pads, and the like. The disposable articles are most frequently prepared using a hot melt construction adhesive. Suitable adhesives must possess a low melt viscosity so they are easy to apply but yet a high enough service temperature that the adhesive will perform satis- factorily when worn at body temperature. The adhesives of the present invention have a surprisingly high Shear Adhesion Failure Temperature in formulations where the viscosity is low enough for easy application. This provides a significant advantage over the use of other block copolymers in preparing hot melt construction adhesives.

Another advantage is that the polymers of this invention give adequate performance in formulations containing lesser concentrations of polymer as compared to conventional block copolymers. In nearly all commercial hot melt construction adhesives employing block copolymers, the block copolymer is one of the most expensive components in the formulation. Thus, the block copolymers of the present invention give lower cost adhesives as compared to conventional block copolymers.

[033] EXAMPLES AND COMPARATIVE EXAMPLES [034] The following examples are provided to illustrate the present invention. The examples are not intended to limit the scope of the present invention and they should not be so interpreted. Amounts are in parts by weight or weight percentages unless otherwise indicated.

[035] Examples 1-7 [036] Ressure sensitive adhesives were prepared using block copolymer A. Block copolymer A was a styrene-isoprene-styrene-isoprene block copolymer having the following characteristics: molecular weight (kg/mole): S1 = 28, B1 = 247, S2 = 28, B2 = 40, polystyrene content = 22 percent, W = 6.2. The block copolymer was admixed with an aliphatic hydrocarbon tackifying resin (PICCOTACTM 95 by EASTMAN) and a petroleum plasticizing oil sold (SHELLFLEXI'M 371 by SHELL). The adhesive components in the quantities shown in Table 1 along with 0.1 percent antioxidant (IRGANOXTM 1010 by Ciba) were mixed at 40 percent solids in toluene and cast upon a Mylar film. After evaporation of the toluene, the adhesive properties were tested and the results reported in Table 1.

[037] Table 1 1 2 3 4 5 6 7 Block Copolymer 15 20 25 30 30 40 50 A (Wt %) Tackifying Resin 60 60 60 60 50 50 50 (Wt %) Plasticizing Oil 25 20 15 10 20 10 0 (Wt %) Rolling Ball Tack 10 17 2 16 1 3 13 Polyken lSobe Tack 0. 51 0.66 0.93 1.11 0.62 0.73 0. 80 Tack 180° Peel 3.1* 2.2* 3. 6* 4. 6** 5. 9** 6.5** 6.8 Holding Power 7 >100 >100 SAFT 87 96 102 111 111 139 149 [038] * Cohesive failure wherein Mylar and steel plate have adhesive residue.

[039] ** Partial cohesive failure.

[040] Comparative Examples A-G [041] Ressure sensitive adhesives were prepared using block copolymer B. Block copolymer B was a conventional styrene-isoprene-styrene polymer which was made by coupling a styrene-isoprene diblock polymer. Block copolymer B had the following characteristics: molecular weight (kg/mole): S 1 = 15.5, B 1 = 180, S2 = 15.5, polystyrene content = 22 percent, uncoupled diblock content = 18 percent. Block copolymer B was used to prepare pressure sensitive adhesives substantially identically to Examples 1-7 and the results reported in Table 2.

[042] Table 2 A B C D E F G Block Copolymer 15 20 25 30 30 40 50 B (Wt %) Tackifying Resin 60 60 60 60 50 50 50 (Wt %) Plasticizing Oil 25 20 15 10 20 10 0 (Wt %) Rolling Ball Tack 12 >30 >30 >30 1 10 >30 Polyken lSobe 0. 70 0.22 0.48 0.20 0. 69 0.79 0.90 Tack 180° Peel 2.8* 3.1* 4.1* 11* 4. 6** 5.0** 5.7 Holding Power 8 >100 >100 SAFT 82 90 94 102 101 118 132 [043] * Cohesive failure wherein Mylar and steel plate have adhesive residue.

[0441 ** Partial cohesive failure.

[045] Comparison of results in Tables 1 and 2 shows that, at a given polymer content, block copolymer A gives the highly desirable result that it gives about a 5°C to about a 20°C higher SAFT than the conventional block copolymer B.

[046] Examples 8-10 [047] Hot melt construction adhesives were prepared using block copolymer A. Block copolymer A was admixed with an aromatic modified aliphatic hydrocarbon tackifying resin (ESCOREXTM 5600 by ExxonMobil) and a plasticizing oil (TUFFLOTM 6056 by CITGO). The adhesive components in the quantities shown in Table 3 along with 0.25 percent IRGANOX 1010 antioxidant were mixed as hot melts in a sigma blade mixer for about 45 minutes at 175°C. Adhesive melt viscosity was measured at 150°C. The hot melt adhesives were then dissolved at 50 percent in toluene and cast on Mylar.

After evaporation of the toluene, 180°peel and SAFT were measured. The results are shown below in Table 3.

[048] Comparative Examples H-J [049] Hot melt construction adhesives were prepared and tested substantially identically to Examples 8-10 except that the conventional block copolymer B was used as the block copolymer. The results are shown below in Table 3.

[050] Table 3 8 9 10 H I J Block Copolymer 11 15 20 A (Wt %) Block Copolymer 11 15 20 B (Wt %) Tackifying Resin 64 64 64 64 64 64 (Wt %) Plasticizing Oil 25 21 16 25 21 16 (Wt %) 180° Peel 5.2 5.9 7.9 4.8 6.0 8.4 SAFT 79 82 91 66 72 73 Melt Viscosity 5.7 19.5 106 1.2 2.9 9.2 [051] Results in Table 3 show again that, at a given polymer content, block copolymer A gives SAFT values about 10°C to about 18°C higher than the conventional block copolymer B. Results also show that an SAFT value of 75°C can be achieved using only about 10 percent of block copolymer A in the formulation whereas about 20 percent of the conventional block copolymer B is required to achieve an SAFT value of 75°C. Since the block copolymer is usually more expensive than the resin and oil, block copolymer A will give lower cost adhesives than the conventional block copolymer B.