Resealable Pressure Sensitive Adhesives and Products

This invention relates generally to pressure sensitive adhesive compositions, more particularly to compositions having a block copolymer, a plasticizing process oil, and substantial quantities of end block or thermoplastic block associating, non-elastomeric resin, the composition being free of or including low quantities of mid block or elastomeric block modifying resin.

Pressure sensitive adhesives are understood in the art to be compositions that will form a bond upon contacting the material to be adhered thereto, without requiring the application of heat, moisture, radiation, or the like. Adhesive compositions of this general type are considered to be resealable and removable when the bond that they form on contact is readily broken, as desired, without significant damage to the substrate to which the resealable pressure sensitive contact adhesive had been applied.

Pressure sensitive adhesives, as described for example in U.S. Pat. Nos. 3,239,478 and 3,917,607, often include block copolymers having at least two "A" blocks of a non-elastomeric or a thermoplastic compound and at least one "B" block of an elastomeric compound, together with substantial quantities of a tackifying or mid block modifying resin that is compatible with the elastomeric blocks but substantially incompatible with the non-elastomeric blocks. Such adhesive compositions may also include a thermoplastic or end block modifying resin that is compatible with the end or non-elastomeric blocks and incompatible with the mid or elastomeric blocks, which compositions also often include extending or plasticizing process oils. As far as known, compositions of this general type heretofore have not been suitable as a resealable and removable pressure sensitive adhesive for use on generally non-porous materials such as polyolefin surfaces, and especially on thin polyolefin films including polyethylene films of the type utilized in storage or sandwich bags or the like.

Publications such as Canadian Pat. No. 1,083,745 and the published article "Evaluation of Resins In Kraton.RTM.1107 Rubber Based Adhesives" teach that the tackifying or mid block associating resins yield soft, tacky films that have potential use in pressure sensitive adhesives, while the incorporation of a thermoplastic non-elastomeric or end block associating resin within a contact adhesive would be expected to provide an undesirable stiff, non-tacky film, thereby teaching one to expect that the pressure sensitive nature of an adhesive depends to a large extent on the mid block associating or tackifying elastomeric resins within the formulation. Attempts have been made consistent with these teachings to formulate a resealable pressure sensitive adhesive composition suitable for use in connection with polyethylene films and the like, especially those including so-called slip agents, resulting in compositions having poor adhesion to the polyethylene, or having a bond that is too strong, thereby forming a contact bond that is not truly removable because the polyethylene film distorts or tears when attempting to release the contact bond. Some such formulations which initially seem to be acceptable in removal and resealing properties have been observed to change their bonding properties after a short storage time or after exposure to slightly elevated temperatures, as would typically be encountered during transport or storage of various products, such change of bonding properties being detrimental to the ready releasability thereof.

There is, accordingly, a need for an adhesive composition that has pressure sensitive adhesive properties which is suitable for use as a contact adhesive and that is both removable and resealable when used in conjunction with polyolefin surfaces, such as polypropylene, polyvinyl chloride and mylar polyester surfaces, particularly when such surfaces are those of a thin polyethylene film. It has now been discovered that such a composition, which is especially suitable for use in conjunction with low density polyethylene films, is provided by omitting completely or significantly reducing the relative amount of mid block or tackifying elastomeric resin that is included within the modified block copolymer compositions known heretofore. In addition, if the particular block copolymer included within the composition does not have a non-elastomeric block or end block content that is great enough to provide the required removability and resealability properties, the compositions according to this invention further include end block or thermoplastic, non-elastomeric block associating resins that are compatible with the end blocks or thermoplastic, non-elastomeric blocks and incompatible with the mid blocks or elastomeric blocks. Also included is a plasticizing or extending process oil that modifies the mid blocks or elastomeric blocks of the copolymer for achieving suitable removability and resealability.

It is, therefore, a general object of this invention to provide an improved resealable pressure sensitive adhesive.

Another object of the present invention is an improved pressure sensitive adhesive that provides a contact bond which is removable and resealable after the applied adhesive has aged and/or been subjected to moderately elevated temperatures, which conditions would be encountered during normal retail sales channels.

Another object of this invention is an improved pressure sensitive adhesive which, when applied to polyethylene films, provides a contact seal that is removable without distortion or damage to the polyethylene film while still being resealable.

Another object of the present invention is to provide an improved resealable pressure sensitive adhesive suitable for providing means to open and sealingly close the open end of a polyolefin storage bag.

Another object of this invention is to provide a pressure sensitive adhesive composition that incorporates no mid block associating resin, or only very low quantities thereof.

Another object of the present invention is to provide an improved resealable pressure sensitive adhesive and a storage bag having a strip of such adhesive applied near the mouth thereof, which applied adhesive has a bond strength with the opposing polyolefin surface of the bag mouth that is on the order of about 80 to 300 grams per inch of width of adhesive, which bond strength is relatively constant over time and/or upon exposure to temperatures as high as about 140.degree. F.

These and other objects of the present invention will be apparent from the following detailed description thereof.

Compositions according to this invention include a blend of a block copolymer having at least two end or thermoplastic polymer blocks and at least one mid polymer block, said end block being a non-elastomeric polymer block and said mid block being an elastomeric polymer block, in admixture with an end block modifying resin that is incompatible with the mid block, and a plasticizing process oil compatible predominantly with the mid block. The composition further includes up to 14 weight percent of a mid block modifying resin that is compatible predominantly with the mid block and substantially incompatible with the end block, which resin can be omitted altogether from the composition which nevertheless retains its desired properties.

Any one of a variety of well known block copolymers can be incuded within the composition, including those discussed in some detail within U.S. Pat. Nos. 3,239,478 and 3,917,607, the disclosure thereof being incorporated hereinto. These particular block copolymers typically take on the general configuration A-B-A or A-B-A-B, wherein each "A" block, which is generally characterized as an end block, is a thermoplastic polymer block prepared by block polymerization of a monoalkenyl arene such as styrene, alphamethylstyrene, tert-butyl styrene, and vinyl toluene. The elastomeric "B" blocks, which are characteristically identified as mid blocks, are prepared by block polymerizing a conjugated diene such as butadiene or isoprene and thereafter hydrogenating the polymer block to prepare block polymers such as polybutadiene, polyisoprene and saturated polyolefin blocks, or poly(alpha mono olefin) blocks. Such block copolymers, which are prepared by known procedures, include polystyrene-polybutadiene-polystyrene, or styrene-butadiene-styrene block copolymer (SBS), polystyrenehydrogenated polyisoprene-polystyrene, polystyrene-isoprenestyrene block copolymer (SIS), poly(alphamethylstyrene)hydrogenated polyisoprene-poly(alphamethylstyrene), and styrene-ethylene-butylene-styrene block copolymer (SEBS). A typical molecular weight range for such block copolymers is between about 10,000 and 500,000 expressed as average molecular weights.

Another general class of block copolymers included within compositions according to this invention are the radial block copolymers which have radially coupled blocks of elastomeric polymer and non-elastomeric isolated polymer domains surrounded by a continuous phase of elastomeric polymer, with the thermoplastic or non-elastomeric polymer domains being in the nature of end blocks that tie the radial polymers together, thereby reinforcing the radial network of the polymer. Suitable thermoplastic or non-elastomeric blocks include monoalkenyl arenes such as styrene, alphamethylstyrene, tert-butyl styrene, and vinyl toluene and polymers thereof, while the elastomeric blocks include hydrogenated conjugated diene and poly(alpha mono olefin) blocks such as polybutadiene, polyisoprene and saturated polyolefins. A typical radial block polymer of this type is a styreneisoprene copolymer, and a general average molecular weight range of this type of polymer is between about 50,000 and 500,000.

Such block copolymers are present within the compositions according to this invention at a concentration of between about 20 and about 60 weight percent, preferably between about 25 and about 45 weight percent, most preferably between about 30 and 40 weight percent, based on the total weight of the formulation.

End block modifying resins according to this invention are generally non-elastomeric resins that are compatible primarily with the end or thermoplastic, non-elastomeric block and substantially incompatible with the mid or elastomeric block, these end block modifying resins typically belonging to the general chemical class of aromatic, thermoplastic hydrocarbons. More particularly, incompatibility in this context refers to general incompatibility, indicated by a cloudy film being formed when the blend is cast from toluene solution, between about 50 parts by weight of the end block resin and 100 parts of the mid block of the block copolymer. Compatibility, when used in this context, is indicated by a clear film being formed upon casting from toluene solution of a blend of between at least about 50 parts of the end block associating resin and 100 parts of the end block of the block copolymer. Such end block associating resins can be aromatic compounds having an unsaturated side chain, including alkyl-aromatic thermoplastic hydrocarbon resins, polystyrene resins, alpha-methylstyrene homopolyers, poly(alpha-methylstyrene-vinyl toluene) copolymers, polyindene resins, metal resinates, coumarone-indene resins, phenolics, mixed aromatics, and the like. Such resins are generally present within the composition according to the invention in order to give a total end block associating resin content of generally between about 5 and about 50 weight percent, preferably between about 10 and about 40 weight percent, most preferably between about 15 and about 35 weight percent based on the total weight of the composition.

Plasticizing oils or extending process oils are incorporated into the compositions according to this invention to the extent that they are not included within the particular block copolymer formulation utilized, many of such formulations including substantial quantities of oil. Such oils are present in compositions according to this invention in order to soften the set adhesive strip and avoid toughness thereof to enhance the agressiveness of the tack at room temperature and in order to lower the viscosity for enhancing flowability of the compositions during application thereof so that they will readily and adequately wet the surface to which they are being applied. These oils should be those that are compatible with and that will swell the elastomeric blocks or mid blocks and that are substantially incompatible with the non-elastomeric or end blocks; oils compatible with the end blocks, such as highly aromatic oils, will render the compositions gummy and soft and thus unsuitable according to this invention. It is thus an aspect of this invention to combine an end block modifying resin with a mid block compatible oil that is incompatible with such end block modifying resin that does not significantly modify the end blocks of the copolymer. Suitable oils include petroleum fractions, alkylaryl resins, vegetable oils, animal oils, synthetic low molecular weight polymers or oligomers such as mixed olefin oligomers and polyterpene oligomers, mineral oils, naphthenic oils and paraffinic oils, preferably any such oils in which less than 5 percent, preferably substantially none, of the carbon atoms are aromatic. Most suitable oils will be of relatively low molecular weight, typically between about 200 and 800 average molecular weight. These extending oils will, whether entirely added to the composition or where the block copolymer itself includes extending oils, be included within the composition in an amount on the order of about a 1:1 weight ratio with the block copolymer, the weight ratio ranging between about 3:1 and about 1:3, with the overall total composition including between about 20 and about 60, preferably between about 25 and about 45 most preferably between about 30 and about 40 weight percent, based on the total weight percent of the composition.

The mid block associating resins, or tackifying resins as they are sometimes referred to, are compatible predominantly with the elastomeric block or mid block and are substantially incompatible with non-elastomeric or end block. These mid block associating resins are compatible with the mid block in that between about 100 and 200 or more parts by weight of the mid block associating resin show a clear film when the particular mid block associating resin is combined with 100 parts of the mid block of the copolymer and cast from solution in toluene. Incompatibility with the end block is indicated by the fact that between about 25 and 50 parts of the mid block associating resin, when cast from solution in toluene with 100 parts by weight of the end block of the copolymer does not form a clear film. Typical mid block associating resins include polyhydric esters of rosin or hydrogenated rosin esters, such as glycerol and pentaerythritol esters of hydrogenated rosins and of highly stabilized rosins, esters of polyhydric alcohols, synthetic polyterpenes, terpene-olefin copolymers, terpene-phenols, tall oil rosin, synthetic saturated hydrocarbon resins, such as saturated alicyclic hydrocarbons, olefinic resins, phenol-aldehyde resins, alpha-pinene resins, beta-pinene resins, terpene-phenolic resins, and copolymers such as of 1,3-pentadiene and 2-methyl-2-butene.

It is a principal feature of this invention that these mid block associating resins are either completely omitted from the compositions or they are included at only very low levels, the maximum concentration being no more than 14 weight percent, based upon the total weight of the composition, preferably no more than 7 weight percent, and most preferably no more than 5 weight percent. When it is desired to include a mid block associating resin in the compositions according to this invention, they are typically included at concentrations of between about 1 and 7 weight percent, preferably between about 3 and 5 weight percent, based upon the total weight of the composition. Formulations otherwise in accordance with this invention but including over 14 percent by weight of mid block associating resin result in bond strengths that are unacceptable for use as removable and resealable pressure sensitive adhesives on polyethylene surfaces, especially when such surfaces are relatively thin films that are susceptible to distortion and destruction when making and breaking a seal made with compositions including such greater quantities of mid block associating or elastomeric resins.

Most often, it is desirable to include composition-stabilizing additives within the compositions of this invention. Such additives include heat stabilizers such as butyl zimate or butylated hydroxy toluene to retard polymer breakdown at elevated temperatures and inhibitors, antiozonants, ultra violet inhibitors, and/or antioxidants such as mixed phenyl phosphites, tri(nonylated phenyl)phosphite, butylated hydroxy toluene, stearyl thiodipropionate, 2,2'-methylene-bis(4-methyl-6-tertbutylphenol), octadecyl beta-(3,5-tertbutyl-4-hydroxyphenyl)propionate, tetra-bis-methylene-3-(3',5-tert-butyl-4'-hydroxyphenyl propionate)methane, modified hindered phenols, 1,3,5-trimethyl-2-4,6-tris(3,5-ditert-butyl)-4-hydroxy-benzyl)benzene, 1,2-dihydro-2,2,4-trimethylquinoline polymer, dibetanaphthyl-p-phenylene diamine, 2,4-dihydroxybenzophenone, substituted hydroxyphenyl benzotriazoles, substituted benzotriazoles, 2-hydroxy-4-(2-hydroxy-3-methacrylyloxy) propiobenzophenone, actylphenyl salicylate, and resorcinol monobenzoate. Additives of these general types are included in concentrations not greater than about 5 weight percent, based on the total weight of the composition, usually between about 0.5 and about 3 weight percent, most typically between about 1 and about 2.5 weight percent of total additives within the composition.

In proceeding with the method according to this invention, between about 20 and about 60 weight percent of a block copolymer is blended with between about 20 and about 60 weight percent of an extending oil and between about 5 and about 50 weight percent of an end block associating non-elastomeric resin, all based upon the total weight of the composition, and the blend is heated until the components are dissolved together. It is often desirable to also include up to about 5 weight percent of one or more preservatives or stabilizers. Mid block associating or elastomeric tackifying resins may be included if desired, provided it is at not more than about 14 weight percent of the total composition. Such formulations are blended and cast or thermally applied to a desired configuration, such as an adhesive strip, in accordance with generally accepted application procedures, such adhesive strip being a contact or pressure-sensitive strip that is readily released while also being resealable.

Products in accordance with this invention include an ahesive strip applied to a polymer film such as a polyolefin film taking the form of a storage bag having a closure portion that can be reopened and resealed as desired without damaging or severely distorting the film. More particularly, the preferred product is a reclosable polyolefin film container having opposing panels and a strip of pressure sensitive adhesive having the composition according to this invention which had been applied by solution casting or by hot melt procedures. Acceptable sealability and unsealability are exhibited by these products even when the film substrate incorporates slip agents for improving the handling properties of the completed product especially reduction of static cling of the panels together, even when the film is a low density polyethylene.

The following specific examples will more precisely illustrate the invention and teach the procedures presently preferred for practicing the same, as well as the improvements and advantages realized thereby.

Blended together within a conventional mixing apparatus were about 35 weight percent of an A-B-A block copolymer, about 35 weight percent of a process oil and about 30 weight percent of an end block associating resin, this formulation including no mid block associating resin. The blend was applied by standard hot melt application techniques onto a polyolefin film and allowed to set for about 20 minutes, after which the set adhesive was contact adhered to another portion of the polyolefin film and found to provide an adhesion bond suitable that readily peels away without damaging the film and that is easily resealed by contact.

About 35.3 weight percent of a styrene-isoprene-styrene block copolymer having a total styrene to isoprene ratio of 14:86, a specific gravity of 0.92, and a Brookfield viscosity of 1600 centipoises at 25.degree. C. for a solution of 25 weight percent of the polymer within toluene, the SIS polymer itself containing no plasticizer oil, such SIS polymer being blended with about 35.3 weight percent of a paraffinic process oil having a flash point of about 245.degree. C. and a molecular weight of about 530, together with about 29.4 weight percent of polyalphamethylstyrene/vinyl toluene copolymer as an end block, or polystyrene, associating resin having a flash point of about 245.degree. C. and a softening point of 75.degree. C. The resulting blend was cast onto a polyolefin surface from a solution of 45 weight percent solids and 55 weight percent of solvent blend of 20 percent toluene and 80 percent heptane. The casting took the form of an adhesive strip which was subsequently subjected to peel adhesion testing and found to possess an adhesive strength of between about 85 and 140 grams per inch of adhesive strip width.

The peel adhesion test utilized was the PSTC-1 test for single coated tapes in which the seal is made under a uniform pressure, one end thereof is doubled back at an angle of 180.degree., the adhesive strip is peeled away from the opposing polyolefin surface at a rate of 12 inches per minute, the peel force is measured, the average pull value is obtained during peeling the next two inches of length after the first inch, and the peel adhesion force is calculated in grams per inch of strip width. The test is run under standard conditions at a temperature of 23.degree. C.+2.degree. C. and a relative humidity at that temperature of 50%+2%. Such testing is carried out promptly after application, except when accelerated or high temperature storage conditions are to be simulated, whereupon the substrate and strip are first held at about 60.degree. C. for 16 to 24 hours prior to testing.

The SIS copolymer of Example 2, to about 37.5 weight percent thereof, was blended with about 37.5 weight percent of the plasticizing process oil of Example 2, together with about 25 weight percent of an end block modifying copolymer of alpha methyl styrene and vinyl toluene, such copolymer having flash point of about 285.degree. C. and a softening point of 120.degree. C., the resulting blend cast onto a polyolefin film from toluene and subjected to the PSTC-1 test, the adhesive strength so determined being between 150 and 200 grams per inch of adhesive strip width, the adhesive strip forming an acceptable seal that was both removable and reclosable with minimal distortion of the polyolefin film, which had a thickness of 2 mils.

The SIS copolymer of Example 2 was combined in a 1:1 weight ratio with 35.3 weight percent of the plasticizing oil of Example 2, 10.3 parts by weight of the end block associating resin of Example 3 and 19.1 weight percent of the end block associating resin of Example 2, after which the blend was cast, via a toluene hexane solution, onto a polyethylene film including a slip agent within its formulation and having a 2 mil thickness. After the strip had set, it was found to provide a good pressure sensitive seal with an opposing layer of the same 2 mil thick polyethylene film, it further being observed that the seal was removable without any significant distortion of the polyethylene film, the adhesive strength being tested out at about 100 grams per inch of adhesive width.

About 33.1 weight percent of a styrene-isoprene-styrene was blended with about 33.1 weight percent of the oil of Example 2, about 9.6 weight percent of the end-block associating resin of Example 3, about 17.9 weight percent of the end block modifying resin of Example 2, about 4.8 weight percent of a rosin ester and mid block associating resin and about 0.75 weight percent butyl zimate, about 0.37 weight percent of tetra-bis methylene 3-(3,5-ditert-butyl-4 hydroxyphenyl)-propionate, and about 0.37 weight percent of octadecyl 3-(3,5-ditert-butyl-4-hydroxyphenyl)propionate, which blend was cast by hot melt techniques as a strip on the inside surface of one panel of a polyethylene sandwich bag and allowed to set, after which it was aged at about 38.degree. C. at about 55 percent relative humidity. The product was allowed to cool for one hour and was tested at about 25.degree. C. to give a PSTC-1 adhesive strength of about 200 grams per inch of adhesive strip width, the bond having an adhesive peel strength of about 250 grams per inch of adhesive width when tested to about 60.degree. C. The sandwich bag was readily sealed and unsealed without any significant distortion or damage thereto.

Approximately 40 weight percent of a styrene-isoprene-styrene resin, about 40 weight percent of the paraffinic plasticizing process oil of Example 2, about 12 weight percent of the end block associating resin of Example 3, about 6 weight percent of a mid block associating rosin ester, and about 2 total weight percent of heat and ultra-violet stabilizers are blended together and cast as an adhesive strip onto a polyethylene sandwich bag having a slip agent within the polyethylene formulation, such possessing a bond peel strength of between 250 and 300 grams per inch of adhesive strip width.

About 35 weight percent of a styrene-isoprene-styrene block copolymer was blended with about 35 weight percent of the extending oil in Example 2, about 10 weight percent of the end block associating resin of Example 3, about 18.5 weight percent of the end block associating resin of Example 2, about 1 weight percent of butyl zimate stabilizer and 0.5 weight percent of butylated hydroxy toluene antioxidant. This composition was cast in toluene to prepare a resealable pressure sensitive adhesive strip near the opening of a polyethylene storage bag, the adhesive strip, when set, providing a reclosable sealing means that does not damage or distort the bag when it is closed and reopened.

Approximately 31 weight percent of the SIS block copolymer of Example 2, about 31 weight percent of a paraffinic process oil, about 10.5 weight percent of a poly-alpha-methylstyrene/vinyl toluene copolymer end block modified having a ball-and-ring softening point of about 120.degree. C., about 19 weight percent of a poly-alpha-methylstyrene/vinyl toluene copolymer having 75.degree. C. softening point, about 7 weight percent of a hydrogenated resin ester having a softening point of about 105.degree. C., about 1.1 weight percent butyl zimate, and about 0.40 weight percent butylated hydroxy toluene. This composition was applied as an adhesive strip to one panel of a polyethylene film sandwich bag near the opening thereof. One such adhesive strip was applied by hot melt application, and another such adhesive strip was applied by solvent from a casting formula having about 45 weight percent of composition solids, about 44 weight percent hexane and about 11 weight percent toluene. Both adhesive strips had satisfactory peel strengths to provide a resealable seal for polyethylene storage bags.

About 33.5 weight percent of a radial block copolymer having a styrene to isoprene ratio of 16 to 84 and a molecular weight of about 275,000 was blended with about 33.5 weight percent of a paraffinic process oil, 21.7 weight percent of an end block or thermoplastic modifying or tackifying resin comprised primarily of alpha methyl styrenes and having a softening point of about 84.degree. C., together with 9.1 weight percent of a synthetic polyterpene mid block or elastomeric component and 2.2 weight percent butyl zimate. Peel adhesion tests (PSTC-1) on polyethylene gave 110 grams per inch of adhesive strip width and 120 grams after storage at 140.degree. F. for one hour, which values are indicative of good pressure sensitive adhesive properties.

34.8 weight percent of a styrene-isoprene-styrene block copolymer was blended with 34.8 weight percent of a paraffinic process oil, about 1.1 weight percent butyl zimate, about 1.1 weight percent of octadecyl 3-(3,5-ditert-butyl-4-hydroxyphenyl)propionate uv inhibitor, and about 28.2 weight percent of an end block associating resin that contains polyindene structures, substituted styrenes and substituted indenes and that has a melting point of 100.degree. C. and a flash point of about 250.degree. C. The resulting blended composition exhibited an adhesion to polyethylene film of 210 grams per inch of adhesive strip width.

A blended composition containing 33.5 weight percent of each of the SIS polymer and the paraffinic oil of Example 2, 2.2 weight percent of stabilizer, 21.7 weight percent of an end block modifier that is largely alpha methyl styrene and that has an 84.degree. C. softening point and 7 weight percent of a synthetic polyterpene mid block associating resin.

While in the foregoing specification, certain embodiments and examples of this invention have been described in detail, it will be appreciated that modifications and variations therefrom will be apparent to those skilled in the art; accordingly, this invention is to be limited only by the scope of the appended claims.