Curable Film Forming Compositions

This invention is directed to reducing the energy requirements to improve the properties of multicomponent polymer systems which form pressure-sensitive adhesives.

Properties of unsaturated pressure sensitive adhesive compositions can be improved by use of actinic radiation, such as ultraviolet (UV) radiation; electron beam (EB) radiation; and chemical or thermal cure. Normally, the adhesive is applied to facestock and/or a release liner and subjected to a suitable curing action to improve such properties as elevated temperature shear.

One means of cure is electron beam (EB) radiation. While the facestock and/or release liner can sustain electron beam dosages up to a certain level, e.g., 80 to 100 kiloGray (kGy), going beyond that level can result in degradation of components of adhesive label and tape constructions such as the face stock and/or release liner and/or adverse reactions between the adhesive and the silicon release agent of the release liner.

Increased radiation requirements, whether EB or actinic, will reduce the speed at which an adhesive coated substrate can pass under the radiating surface or increase the number of radiating surfaces. Both are costly.

The present invention is directed to reducing the energy required to achieve a positive modification in a pressure sensitive adhesive formed of at least two components, one an unsaturated elastomeric polymer component, the other an organic additive component which is one or more organic additives which are at least dispersable in the elastomeric component. Typically, the organic additive component is provided to tackify or plasticize the elastomeric component.

According to the present invention, there are provided pressure-sensitive adhesive compositions which comprise in combination at least one unsaturated elastomeric polymer capable of undergoing gel forming reactions in the presence of free radicals preferably generated by actinic radiation or electron beam radiation, and at least one organic additive which is substantially nonresponsive to free radicals and which is at least dispersable and preferably soluble in the elastomeric polymer. The elastomeric polymer preferably has a glass transition temperature of from about -20.degree. to about -100.degree. C. The organic additive affects a property of the pressure-sensitive adhesive composition, such as tack or other properties. The improvement resides in the use of a organic additive which is substantially nonresponsive to the action of free radicals, as defined herein, while providing, upon cure, a pressure-sensitive adhesive composition having a glass transition temperature of at least 10.degree. C., preferably at least 20.degree. C. below use temperature.

After cure, with the increase of gel content, the combination exhibits a positive change in properties such as elevated temperature shear. This occurs at substantially lower levels of free radical generation that would have been required were the organic additive to consume a significant amount of free radicals.

FIG. 1 illustrates the EB dosage requirements to achieve incipient gel formation using unsaturated tackifiers in unsaturated elastomers as a function of tackifier concentration;

FIGS. 2 and 3 establish as compared to a free radical consuming tackifier, that use of dosage for tackifying organic additives employed in accordance with this invention will not change the incipient gel dosage of unsaturated elastomeric copolymer to which they are added in varying concentrations;

FIG. 4 illustrates the effect of unsaturation on relative incipient gel dosage as a function of tackifier concentration for aromatic tackifiers at different levels of hydrogenation; and

FIG. 5 compares the relative incipient gel dosage as function of tackifier concentration for Excorez.TM. 1310, Foral.TM. 85 and Regalrez.TM. 1033.

The present invention is directed to reducing the energy requirements of free radical cross-linking of unsaturated elastomeric polymers which are part of multi-component compositions such as pressure-sensitive adhesives in which unsaturated elastomeric polymers are tackified or otherwise modified as to physical properties by addition of at least one organic additive.

Properties of the pressure-sensitive adhesive composition are, in accordance with the present invention, enhanced by free radical cure, with free radicals preferably generated by electron beam (EB) radiation, or actinic radiation, such as ultraviolet (UV) curing, with or without photoinitiators and/or photosensitizers. The invention also contemplates thermal curing with or without thermal initiators. The improvement resides in utilizing as a property modifying additive, an organic additive which is substantially nonresponsive to the action of free radicals.

By the term "organic additive which is substantially nonresponsive to the action of free radicals" (also organic additive herein) there is meant saturated organic compounds and organic compounds which, when blended with an unsaturated elastomeric polymer in proportions of about 40 parts by weight of the unsaturated elastomeric polymer and 60 parts by weight organic compound, will form a blend having a relative incipient gel dosage, i.e. the ratio of incipient gel dosage of the blend to the incipient gel dosage of the unsaturated elastomeric polymer, of no more than about 1.85, preferably 1.65. The measurement is made for a blend which is free of external cross-linking agents and for an EB radiation at 200 KV.

For electron beam radiation, incipient gel dosage is determined as a minimum amount of electron beam (EB) dosage in kiloGray required to form a toluene insoluble gel. Insoluble gel is measured by placing 200 milligrams of a directly irradiated sample into 10 grams of toluene checking for the presence of insoluble material after 24 hours standing. Radiation is increased (or decreased) in increments of 10 kGy. If the solution appears clear upon visual inspection it is filtered through qualitative filter paper to check for the presence of gel. The procedure is repeated for each 10 kGy dosage level increment and the dosage at which gelation occurs is interpolated from the data. If a gel was not observed, for example, at 60 kGy, but was at 70 kGy, the value of incipient gel dosage is reported as 65 kGy. In each instance, the sample is coated from a toluene solution onto a release paper (50-75g/m.sup.2) and dried in an oven. The sample surface was directly exposed to EB radiation at 200 KV with inerting to 400 ppm O.sub.2 or less. For values reported herein, the EB unit used was manufactured by Energy Sciences.

By the preferred use of organic additives which are substantially nonresponsive to the action of free radicals, the energy required to achieve a level of cross-linking within the unsaturated elastomeric copolymer can remain unchanged and independent of organic additive concentration.

For unsaturated compounds, saturation to a level sufficient to meet the definition can be achieved by hydrogenation or otherwise eliminating aromatic or aliphatic unsaturation with addition of groups which do not consume free radicals. In the alternative, saturated or partially hydrogenated organic compounds may be blended with saturated and/or highly saturated organic additives to achieve the desired result, namely a reduction of incipient dosage requirements to acceptable levels.

While not limiting, the invention will, for simplicity, be described in terms of improving the properties of pressure-sensitive adhesives based on unsaturated natural and synthetic elastomeric polymers including, but not limited to, block, random or multiarmed copolymers and mixtures thereof. Among the useful unsaturated elastomeric polymers there may, however, be mentioned natural rubber, polybutadiene, polyisoprene, butyl rubber, ethylene propylene diene rubbers, styrene-butadiene block copolymers, styrene-butadiene-styrene block copolymers, styrene-isoprene block copolymers, styrene-isoprene-styrene block copolymers, multiarmed styrene-isoprene block copolymers and the like. Useful unsaturated elastomeric polymers are also disclosed in U.S. Pat. No. 4,556,464 to St. Clair incorporated herein by reference.

Preferably, the elastomeric block polymers to which the invention is directed are ABA block or multiarmed (AB).sub.x block copolymers, wherein x has a value of 2 or more and mixtures thereof and wherein A is a block comprising at least one monoalkenyl arene, preferably styrene, alpha methyl styrene, vinyl toluene and the like, and B is an elastomeric conjugated diene block such as a polybutadiene or a polyisoprene block with polyisoprene blocks preferred.

More preferably, the elastomeric copolymers are formed of styrene-butadiene-styrene block copolymers and/or styrene-isoprene-styrene block, multiarmed styreneisoprene block copolymers, polybutadiene and polyisoprene. Mixtures of elastomers may be employed.

Unsaturated elastomeric polymers forming the base resin of the invention are or may be adapted to hot melt, solvent or emulsion coating. They are preferably free radical cross-linked using actinic radiation, with or without a photoinitiator, or by electron beam (EB) radiation. Cure is to overcome the major deficiency of pressure-sensitive adhesives based on unsaturated elastomeric polymers, namely, to have acceptable elevated temperature cohesive strength. Crosslinking of the base polymer has been used to enhance cohesive properties of the adhesives especially to improve elevated temperature shear performance. Thermal crosslinking has been the most commonly used method limited to solution or emulsion polymers.

Thermal crosslinking during hot melt processing of unsaturated elastomeric polymers has been the most difficult to achieve due to undesirable and premature crosslinking during compounding, storage and coating operations. As a consequence in-line mixing of crosslinking agents with a melt has been required to avoid premature cure.

Actinic and Electron Beam radiation can overcome this difficulty; however, if external crosslinkers are included, thermal instability may become a problem.

All systems can benefit by the use of external crosslinkers. Crosslinkers which are multifunctional monomers such as acrylates and methacrylates are thermally reactive and the process to achieve cure also requires in-line mixing to avoid premature crosslinking independent of the ultimate means of cure. The same is true of other crosslinking agents which are functional at much lower concentrations. Polythiol crosslinkers, for instance, are functional at concentrations of about 10% or less by weight of the total composition. The polythiol cross-linkers include for instance, pentaerythritoltetrathioglycolate, pentaerythritol-tetra(3-mercaptopropionate), trimethylolethanetrimercaptopropionate, trimethylolpropanetrithioglycolate, trimethylolpropane tri(3-mercaptopropionate) and the like.

As with the multifunctional acrylates and methacrylates, the polythiol cross-linkers are preferably added to the composition by in line mixing. The purpose is to avoid premature crosslinking before cure the pressure sensitive adhesive composition.

The unsaturated elastomeric base copolymers are not normally pressure sensitive adhesives and pressure-sensitive adhesive properties are induced by the addition of other hydrocarbon materials known as tackifiers. One use of the organic additives of this invention is as a tackifier.

To avoid excess consumption of free radicals and the introduction of excess energy into the system, any significant amount of hydrocarbon added to modify the properties of the elastomeric resin is in the form of organic additives which are substantially nonresponsive to free radicals. Their inclusion does not substantially affect the ability to achieve improved elevated temperatures properties such as shear by the action of the free radicals and add their beneficial properties, e.g., tack to the cured product.

As examples of organic additives which are substantially nonresponsive to free radicals there may be mentioned hydrogenated organic compounds, such as hydrogenated aromatic resins including hydrogenated polystyrene, polyalpha-methyl styrene, polyvinyl toluene, copolymers of styrene with other monomers and the like; hydrogenated aliphatic resins derived from petroleum based products; highly hydrogenated rosins and rosin esters: hydrogenated white oil, mineral oil and the like. As specific tackifiers employed in the practice of the invention there may be mentioned hydrogenated styrene based resins such as Regalrez.TM. resins designated as 1018, 1033, 1065, 1078, 1094 and 1126 manufactured and sold by Hercules, Inc.; Regalrez.TM. 6108 a 60% hydrogenated aromatic resin, also manufactured by Hercules; hydrogenated C5 and/or C.sub.9 hydrocarbon feed stocks such as Arkon.TM. P-70, P-90, P-100, P-125, P115, M-90, M-100, M-110 and M-120 resins manufactured and sold by Arakawa Chemical and Regalite.TM. R-100, MGB-63, MGB-67, MGB-70, resins manufactured and sold by Hercules, Inc.; hydrogenated Polycyclo-pentadienes such as Escorez.TM. 5320, 5300 and 5380 resins manufactured and sold by Exxon Chemical, hydrogenated polyterpene and other naturally occurring resins such as Clearon.TM. P-105, P-115, P-125, M-105, M-115 manufactured and sold by Yasuhara Yushi Kogyo Co. Ltd. of Japan and Eastotack.TM. H-100, H-115 and H-130 resins manufactured and sold by Eastman chemical and the like; Kaydol.TM. hydrogenated mineral oil manufactured and sold by Witco Chemical and the like.

To be generally useful, organic aromatic additives should effectively be at least 50% preferably at least 60% saturated and for the aliphatic hydrocarbon at least 65% preferably 80% of unsaturated groups in the product as formed should be saturated or otherwise rendered nonresponsive to the action of free radicals. Complete saturation is preferred or at least saturation to a level where upon inclusion into the elastomer there will be a negligible additional consumption of free radicals over that required to achieve the same level of cure by crosslinking of the unsaturated elastomeric polymer. Some aromatic unsaturation is necessary for compatibility with butadiene containing elastomers.

Organic additives which serve a tackifying function normally present in a concentration ranging from about 40% to about 90% by weight, of mixture of total, preferably from about 45% to about 85% by weight of the mixture of unsaturated elastomeric polymers and tackifying organic additives. Compositions containing less than about 40% by weight of an organic additive typically do not have sufficient "quickstick" or initial grab and compositions having too high a tackifying organic additive have too low a cohesive strength even when cross-linked.

The compositions of the instant invention may be and normally are made up of components (unsaturated elastomeric polymer and organic additives) having multiple glass transition temperatures. To be functional as a pressure sensitive adhesive the composition must have at least one glass transition temperature at least about 10.degree. C. below use temperatures, preferably at least 20.degree. C. below use temperatures.

As compared to ABA block copolymers, the presently preferred unsaturated elastomeric polymers are multi-armed styrene isoprene block copolymers having the formulas (SI).sub.x where x generally has a value greater than 2.

Linear styrene-isoprene (SI) and styrene-isoprene-styrene (SIS) block copolymers do not perform as well as multi-armed (SI) polymers in forming radiation-cured pressure sensitive adhesives at lower curing doses. This problem may be solved by the use of organic additives of this invention which, unlike most conventional tackifiers, do not consume an excessive amount of radical during crosslinking thereby conserving cost and product quality.

I have found that high molecular weight styrene-isoprene multi-armed block copolymers, alone or together with other elastomers, such as SB, SBS and SIS when formulated with organic additives which are saturated, hydrogenated tackifying resins give pressure sensitive adhesive formulae which show superior ease of cure and when cured superior elevated temperature shear properties. Properties after cure, match or surpass the elevated temperature shear performance of any hot melt adhesives commercially available.

Presently preferred formulation ranges for high performance pressure-sensitive adhesives containing multiarmed (or radial) styrene-isoprene block copolymers are on a by weight basis as follows:

15-60 parts unsaturated elastomeric polymer(s)

85-40 parts tackifying organic additives

0-10 parts crosslinking agents

Antioxidants are added as required. In low concentrations, the multiarmed block copolymers may be considered as an elastomeric crosslinking additive.

The following Examples and Controls are designed to illustrate the instant invention.

Base unsaturated elastomeric polymers used were a mixture of linear styrene-isoprene-styrene (SIS) and styrene-isoprene (SI) block copolymers known as Kraton.TM. D-1107 and D-1111; styrene-butadiene-styrene (SBS) block copolymers known as Kraton.TM. D-1101 and D-1102 and DX-1300 and multi-armed (SI).sub.x block copolymer known as Kraton.TM. D-1320X.sup.1 all manufactured and sold by Shell Chemical Company and styrene-butadiene block copolymer known as Solprene.TM. 1205 manufactured and sold by Housemex, Inc. As representative cf an unsatisfactory tackifying unsaturated alphatic resin there was used Escorez.TM. 1310, a petroleum based hydrocarbon resin manufactured and sold by Exxon Chemical Company, and Piccolite.TM. A-115, an alpha-pinene resin manufactured and sold by Hercules, Inc. Foral-85, a well known hydrogenated rosin ester manufactured and sold by Hercules, Inc. may be functional or nonfunctional as an organic additive depending on the elastomer(s) it is combined with. The organic additives which are used to illustrate the instant invention are of the Escorez.TM. 5000 series. Also used to illustrate the practice of the invention are Regalrez.TM. 6108, 1078 and 1000 series of resins. Kaydol.TM. mineral oil was as a representation of hydrogenated oil. The invention is primarily illustrated in respect of effect of unsaturation of the organic compound, namely, a tackifier, on incipient or relative incipient gel dosage (the incipient gel dosage cf a mixture divided by the incipient gel dosage of the elastomer) required to initiate gel formation versus resin concentration in percent by weight. Incipient gel dosage is measured as defined above. To establish product properties, the formulation was coated from toluene onto a release paper (50g/m.sup.2), dried in an oven and laminated to a 50 micron thick polyester film. EB radiation was through the polyester film. 180.degree. Peel in Newtons per Meter (N/M) were determined using PSTC-1 at 20 minute dwell. Loop tack was measured by forming a loop from a 1 inch by 8 inch strip, adhesive face out, inserted in the jaws of an Instron tester and moving the loop at the rate of 12 inches per minute onto a stainless steel panel, then removing the strip at the rate of 12 inches per minute as soon as one square inch of contact is made. The highest force required to remove the loop is reported in N/M. Shear reported in Kiloseconds (K. S.) was for 0.5.times.0.5 inch overlap on a stainless steel panel at a 500 gram force load. Also known as TRW-6-1523 and DX-XL.

While the invention has been described in terms of pressure sensitive adhesives, it embraces other cured compositions comprising a free radical cured mixture of at least one unsaturated elastomeric polymer and at least one organic additive which is substantially nonresponsive to the action of free radicals and present in an amount sufficient to modify a physical property of the elastomeric polymer.

The study was performed to establish the effect of using conventional tackifying resins to tackify elastomers on the electron beam (EB) dosage required to achieve incipient gel formation as a function of tackifier (resin) content. There was employed SIS elastomeric polymers known as Kraton.TM. D-1107 (Control 1), SBS block copolymers Kraton.TM. D-1101 (Control 2) and DX 1300 (Control 3) and Kraton.TM. D-1320X, a multi-armed styrene-isoprene block copolymers (Control 4).

The study established, first of all, the base level of EB dosage necessary to achieve a cohesive strength improvement as evidenced by gel formation in the base elastomer, more particularly, where at least 10% of the elastomer of adhesive composition formed an insoluble gel. FIG. 1 shows that incipient gel dosage increases dramatically as the elastomer is diluted with conventional tackifiers. Since conventional elastomer based pressure-sensitive adhesives contain about 40-90% by weight added tackifying resins, this means that the typical adhesive formulations required an EB dosage of 2-5 times higher than the dosage required for the elastomer itself to achieve an incipient gel formation when an unsaturated C-5 hydrocarbon Escorez.TM. 1310, (Controls 1 and 4) and Piccolite A-115 alpha-pinene (Controls 2 and 3) tackifiers are used as the tackifying resins. The elastomer requiring the lowest dosage was the multiarmed (SI).sub.x copolymer.

FIG. 2 shows the relative EB dosage level required to achieve incipient gel formation for the compositions based on Kraton.TM. D-1107. To illustrate the invention there Was used Regalrez.TM. 1033 an 100% hydrogenated (saturated) aromatic resin manufactured by Hercules, Inc. (Example and Escorez.TM. E-5380, a saturated aliphatic hydrocarbon manufactured by Exxon (Example 2). The comparison was Control 1. As can be seen incipient dosage to gel increased in proportion to the amount of unsaturated tackifying resin introduced to the rubber whereas the use of the hydrogenated tackifiers causes the incipient dosage to remain the same or in some instances reduced.

Using the same procedure as the previous Examples and Controls, FIG. 3 shows the effects of a saturated tackifier , Escorez.TM. 5380, (Example 3) and an unsaturated tackifier, Escorez.TM. 1310 (Control 5) have on the relative incipient gel dosage required to achieve gel formation for a multi-armed styrene-isoprene block copolymer (Kraton.TM. -D-1320X).

Table 1 tabulates the improved high temperature properties induced to Kraton.TM. D-1107 and Kraton.TM. D-1320X using as the saturated tackifier Regalrez.TM. 1078. While 180.degree. peel remain essentially unchanged there is dramatic improvement in elevated temperature shear.

FIG. 4 shows the relative incipient gel formation dosage for Kraton.TM. D-1107 using an aromatic tackifier of different levels of unsaturation. They are manufactured and sold by Hercules, Inc., under the designation Piccolastic.TM. A-50 (0% hydrogenated), Regalrez.TM. 3102 (30% hydrogenated); Regalrez.TM. 6108 (60% hydrogenated) and Regalrez.TM. 1033 (100% hydrogenated). As shown in FIG. 4, the higher the degree of hydrogenation the lower the dosage required, for a given level of resin concentration, to form a gel. This is important since a certain amount, usually 30-40% of the base rubber, must be crosslinked to form a network in order to achieve a significant improvement in physical properties.

As recognized, partially hydrogenated rosins have been used as premium tackifier resins for a long time. One of the best is Foral 85 manufactured by Hercules, Inc. According to information available from Hercules, Inc., it is about 60% hydrogenated. FIG. 5 establishes that for Kraton.TM. D-1107, Foral 85 (Control 8) is on a comparative basis significantly better than Escorez.TM. 1310 (Control 1) as a relatively low free radical consumer but not as good as Example 6 where Regalrez.TM. 1033 a saturated hydrocarbon was used as the tackifier.

Endex.TM. 160 is an aromatic end-block reinforcing resin compatible with the polystyrene phase of the block copolymer Kraton D-1107 but not compatible with the elastomeric polyisoprene phase. It does not substantially interfere with crosslinking in presence of an organic additive (Table 2) or a blend thereof (Table 3) but synergistically provided exceptionally high elevated temperature shear strength on EB cure.

The following is to show that a mixture of saturated tackifiers Escorez.TM. 5300 and Regalrez.TM. 1065 can be used to improve high temperature shear performance of a multiarmed styrene-isoprene rubber Kraton.TM. D-1320X.

The formulations in parts by weight is shown in Table 4 and the adhesive properties as a function of EB dosage in Table 5 wherein "ETS" means elevated temperature shear in kiloseconds.

The multiarmed copolymer Kraton.TM. D-1320X has an incipient gel dosage of 25 kGy. A mixture of 30 parts of the multiarmed copolymer, 31 parts Escorez.TM. 5300 and 39 parts Regalrez.TM. 1065 also required 25 kGy to achieve incipient gel formation.

Table 6 establishes that other saturated compounds, namely brominated hydrocarbon fire retardants, can be successfully added to Kraton.TM. 1107 and reduce incipient gel dosage.

Control 20 and Examples 26-29 are for the combination of an SBS copolymer Kraton.TM. D-1102 tackified with a mixture of Regalrez.TM. 6108 and Kadol Oil a hydrogenated mineral oil in the presence of trimethylolpropane tri(3-mercaptopropionate) as a multifunctional polythiol crossliner. As can be seen in Table 7, the combination gives on cure excellent elevated temperature shear.