CA1280527C - Emulsion polymerized sec-butyl acrylate latexes suitable for use aspressure sensitive adhesives - Google Patents

Abstract

ABSTRACT

An emulsion polymerized latex composition suitable for use as a pressure sensitive adhesive having from 82 to 99 percent, based on total polymer weight of sec-butyl acrylate and from 1 to 18 weight percent, based on total polymer weight, of a polymer prepared from monomers selected from the group consisting of alkyl esters of acrylic acid and methacrylic acids wherein the alkyl chain has up to 20 carbon atoms, monovinylidene aromatics and ethylenically unsaturated carboxylic acids and mixtures thereof. Additionally, the latex composition can include from 1 to 90 weight percent of a tackifying resin, based on total solids weight. Adhesives prepared with the subject latex compositions provide an excellent balance of adhesive properties, i.e., peel and shear adhesion.

Description

lZ80~;Z7 EMULSION POLYMERIZED SEC-BUTYL ACRYLATE LATEXES
SUITABLE FOR USE AS PRESSURE SENSITIVE
ADHESIVES AND PROCESS FOR PREPARING THE SAME

The present invention relates to aqueous emulsion polymerized latexes having a predominant proportion of sec-butyl acrylate suitable for use as pressure sensitive adhesives and to articles comprising ~urfaces coated with such pressure sensitive adhesive polymers.
As used in the art, the term "pressure sensitive adhesive" designates a polymeric material which, in solvent-free form, is aggre~sively and permanently tacky at room temperature, and will firmly adhere to a variety of di~similar surfaces upon mere contact without the need of more than finger or hand pressure. It is generally desirable that the critical surface energy of the polymeric material be as low as possible in order that the pressure sensitive adhesive adhere well. Typically, low critical surface energies are associated with highly nonpolar polymers.

, .

28,632-F -1--2- 1280~;~7 Pressure sensitive adhesives are conventionally used in the form of films which provide adhesive bonds between normally nonadhering surfaces. The films have been prepared by casting from organic solvent solutions pressure sensitive interpolymers and evaporating the - solvent. A disadvantage of such organic solutions arises from large quantities of organic solvent effluent whi~h is discharged into the atmosphere.
Manufacturers of pressure sensitive articles are, 0 therefore, switching increasingly from organic solutions to aqueous emulsions of pressure sensitive interpolymers to minimize atmospheric pollution.
Aqueous emulsions of essentially water-insoluble polymers are applied to surfaces to provide pressure ~ensitive adhesive coatings.
The most important properties recognized by the pressure sensitive adhesive industry are adhesion or peel strength, "tack", and resistance to shear at elevated temperatures. The addition of a tackifier can improve the peel strength and adhesion exhibited by a pressure qensitive adhesive at room temperature.
Unfortunately, at higher temperatures, most tackifiers ~erve as plasticizers and promote shear failures.
A balance of desirable properties can be achieved by preparing a copolymer comprising varying amounts of "soft" monomer (i.e., having a glass transi-tion temperature below 25C) and "hard" monomer (i.e.,having a glass transition temperature above 25C). For example, large amounts of "soft" monomers such as butadiene, i-butyl acrylate, n-butyl acrylate, t-butyl acrylate or 2-ethylhexyl acrylate in a copolymer improve the adhesive strengths of the resulting pressure sensltive adhesives, but at the expense of 28,632-F -2--3- lZ805~7 thermal shear endurance. Conversely, the polymers comprising large amounts of "hard" monomers will exhibit good thermal shear endurance at the expense of tack and adhesive properties.

The use of alkyl acrylic esters in preparing polymers and copolymers which are then employed as pressure sensitive adhesives is disclosed in U.S.
Patent 3,691,140. However, U.S. Patent 3,691,140 discloses only acrylic ester polymers prepared by aqueous suspension polymerization as being suitable for tacky, infusible microspheres. It has not been documented that sec-butyl acrylate has superior properties when employed in preparing pressure sensi-tive adhesives with respect to other alkyl acrylicesters.
In view of the fact that the pressure sensitive adhesives which have been documented to date have bee~
prepared through balancing properties such as cohesion, adhesion and high temperature shear resistance, it would be highly desirable to provide a polymer which can be employed aq an aqueous emulsion which has a low critical surface energy and which exhibits extremely hLgh cohesion and adhesion without the loss of high temperature shear resistance.
The present invention is an emulsion polymerized latex suitable for use as a pressure ~ensitive adhe ive (PSA) compricing from 82 to 99.5 percent by weight of total polymer of sec-butyl acrylate and from 18 to 0.5 percent by weight of total polymer of monomers selected from the group consisting of alkyl esters of acrylic and methacrylic acids, wherein the alkyl chain has up to 20 carbon atoms, 28,632-F -3-4 ~L~80S~7 monovinylidene aromatics and ethylenically unsaturated carboxylic ~cids.
In one embodiment, the minor portion is ethylenically unsaturated carboxylic acids such as itaconic or acrylic acid. In another embodiment, the sec-butyl acrylate polymer portion comprises from 95 to 97 percent by weight of total polymer. In yet another embodiment, the composition additionally comprises from 1 to 90 percent by weight of t!otal solids of a tackifying resin, more preferably from 30 to 50 percent by weight on a total solids basis.
The excellent adhesive characteristics of the present emulsion polymerized latex composition make it partlcularly ~uitable for use as a pressure sensitive adhesive (PSA). In particular, an excellent balance of shear bond and peel adhesion is provided by polymers comprising predominantly sec-butyl acrylate.
The critical monomer which is employed to prepare the pressure sensitive adhesives of this inven-tion is sec-butyl acrylate. Typically, the monomer is prepared by reacting sec-butyl alcohol with acrylic acid.

Other monomers which are commonly employed in preparing pressure sen~itive adhesives are also suitable in preparing the pressure sensitive adhesives 3 of this invention. Typical are those alkyl esters of acrylic and methacrylic acid~ wherein the alkyl chains contain up to about 20 carbon atoms. Common examples include iso-butyl acrylate, 2-ethylhexyl acrylate, iso-octyl acrylate, n-dodecyl acrylate, 2-methylbutyl acrylate, methyl methacrylate and ethyl acrylate.

28,632-F -4--5- ~ 2805;~

Other monomers which are commonly employed in preparing pressure sensitive adhesives, and which are useful herein include, for example, the monovinylidene aromatics such as styrene, a-methylstyrene and vinyl toluene; the a-olefins and diolefins such as ethylene, propylene, butene-~, hexene-l, 1,3-butadiene and~
isoprene; and other vinyl compounds such as àcrylonitrile,.viny.l acetate, ~inyl propiona~e, vinyl chloride and vinylidene chloride.
1'0 Still other monomers which are commonly employed in preparing pressure sensitive adhesives include the ethylenically unsaturated carboxylic acids quch aq acrylic acid, methacrylic acid, itaconic acid, fumaric acid and maleic acid. Also useful are the ethylenically unsaturated sulfonic acids such as 2-acrylamide-2-methylpropane sulfonic acid.
The polymers of this invention most advanta-geously have a minimum of 82 percent, preferably 95 percent and more preferably 97 percent by weight of total polymer of sec-butyl acrylate and a maximum of 99.5 percent, preferably 99 percent and more preferably 97 percent by weight of total polymer of sec-butyl acrylate.
The remaining portion of said polymer can comprise other monomers such as those listed above and more preferably the remaining portion is made up of the ethylenically unsaturated carboxylic acids. Even more preferred are monomers of itaconic and acrylic acid present in an amount of from 0.5 to 5 percent by weight of total polymer.

28,632-F -5-~2~305Z7 The polymers are prepared using conventional emulsion polymerization techniques. For example, conventional initiators, solvents, emulsifiers and chain transfer agents, are introduced along with the desired monomers into a temperature controlled reactor and agitator. The reaction is carried out in an oxygen-free environment for sufficient time to convert the monomers to pol.ymer. The molecular weight of the polymer can vary. Generally, low molecular weight polymers having molecular weights of less than approximately 25,000 exhibit properties more similar to tackifiers than adhesives, i.e., high peel adhesion and low shear adhesion. Therefore, medium molecular weight polymers having molecular weights of approximately 25,000 to 500,000 and high molecular weight polymers having molecular weights of approximately l,000,000 are preferred for preparing the present adhesives.
However, low molecular weight polymers of sec-butyl acrylate can be advantageously admixed in place of tackifier or as a partial substitution therefor.
Whereas the polymeric composition as described above is suitable for use as an adhesive material, it is generally desirable to incorporate a tackifier resin. Thus, the pressure-sensitive adhesive of this invention can comprise the latex composition described above and a tackifying resin emulsion or tackifier.
Tackifying resins u~eful in the preparation of the 3 pre~sure sensitive adhesive are commercially available and are generally disclosed in U.S. Patent 4,189,419.
Typi¢ally the resins comprise emulsified resin, partially decarboxylated rosin, glycerol ester of polymerized rosin, partially dimerized rosin, natural resin, hydrogenated wood rosin, plasticized 28,632-F -6--7 1280~

hydrogenated rosin, aliphatic hydrocarbon resins derived from petroleum, aromatic resins derived from petroleum, terpene resins, coal tar polyindene resins, ethylene vinyl acetate copolymer resins, terpene phenolics, coumarone-indenes, rosin esters, pentaerythritol esters, and polydicyclobutadiene resins. The properties of the adhesive composition can be varied for particular applications by the selection of an appropriate tackifying resin.
Naturally the particular tackifiers employed can contain conventional additives such as, for example, softeners, plasticizers, antioxidants and inert fillers which can be emulsified along with the tackifying resin or emulsified separately and mixed with the tacki~ying resin emulsion.
Tackifiers are desirably added in an amount such that the latex is given additional tack (quick stick and peel adhesion) without detracting unduly from the shear adhesion. Suitable formulations will typically have from 1 to 90 percent, preferably from 20 to 80 percent, and most preferably from 30 to 50 percent tackifer by weight of total solids in the formulation.
Typically, the pressure~sensitive adhesive composition can be prepared by blending the desired 3~ amount of latex and tackifier in any conventional manner. It is understood that no requirement or limitation to the scope of the invention is intended as to how the latex and tackifier components are combined.
The compositions of the present invention may be used as the adhesive component in pressure sensitive 28,632-F -7--8- ~80S27 tapes, films and foams. They adhere well to resin surfaces such as plasticiz.ed poly(vinyl chloride), Mylar~ brand polyester film, cellulose acetate, nylon, polyethylene and polypropylene, ~s well as to paper, metal and painted surfaces. They are especially useful as the adhesive component of decorativ~ uinyl sheets, decals, vinyl foam and tiles.
Articles of manufacture such as tapes, decals, decorative vinyl sheets and transfer films containing the pressure sensitive resin composition of the present invention are prepared by coating the resin on the appropriate substrate by conventional coating methods.
Such articles conventionally include a release paper ~or temporary protection of the adhesive film until the ~inal adhesive bond is made. The thickness of the adhe~ive fllm is generally in the range of from 5 to 125 micrometers.
Application of the film to the substrate is conventionally carried out on roll coaters such as rever~e roll and gravure roll coaters. The resin emulsion viscosity is adjusted to between 25 and 5,000 centipoises (0.025 and 5 pascal-seconds) with higher visco~lties within the range preferred for reverse roll coating and lower viscosities within the range preferred for gravure coating. Typically, coatings are applled to the substrate moving through the rolls at a rate of between 3.05 meters per minute and 305 meters per minute.
- While th`e present invention has been described with particular reference to certain specific embodi-ments thereof, it will be understood that certainchanges, substitutions and modifications may be made 28,632-F -8-g ~Z805~7 therein without departing from the scope thereof. This invention also contemplates the use of fillers, extenders, stabilizers, antioxidants, plasticizers, tackifiers, flow control agents, adhesion promoters and dyes, in the pressure sensitive resin emulsions and the pressure sensitive resins of this invention.
As mentioned earlier, the use of sec-butyl acrylate as the major component in a PSA provides e~ceptional adhesive properties when compared to other alkyl acrylate esters or conventional PSAs. In particular, sec-butyl acrylate PSAs characteristically have very good shear bond and good peel adhesion.
Because shear and peel strengths are antagonistic properties, it is very desirable to formulate PSAs having peel qtrengths comparable to conventional PSAs while increasing shear strength.
ExamPle I
The adhesive properties of isomeric butyl acrylates including sec-butyl acrylate were measured at various tackifier level~. The basic polymer formulation consisted of 97 percent by weight of total polymer of the isomeric-butyl acrylate and 3 percent by weight of total polymer of acrylic.acid. The tackifier employed was Hercuies Picconal' A600 E (a 55 percent non-volatile emulsion of hydrocarbon resins in water) and is shown in percent by weight.on a total solids basi~. The tests were conducted by coating adhesive paper with the formulated PSAs. PTSC Test Method No. 1 was employed to measure peel adhesion and PTSC Test Method No. 7 for shear adhesion. The results of the tests are shown in Table I.

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. c ~ 1~ ~ C ~ s ~C . I I o 2 8, 6 32-F -10-~805i~7 The adhesive measurements from Table I
demonstrate that adhesives prepared with sec-butyl acrylate exhibit the best combination of adhesive properties either neat or blended with tackifier. More pronounced is the excellent maintenance of shear adhesion at increasing levels of tackifier with respect to the comparative n-butyl and iso-butyl acrylates.
Example II
The adhesive properties of a sec-butyl acrylate PSA were measured against commercially available a¢rylic emulsion polymers. The results are tabulated below in Table II. The PSAs were coated onto 2 mil (0.05 mm) Mylar~ with a #30 wire wound rod and tested according to PSTC Test Method No. 1 for peel adhesion and PSTC Test Method No. 7 for shear adhesion. No tackifier was added to the subject PSA which was a 97 percent sec-butyl acrylate based on the total weight of polymer and 3 percent acrylic acid based on the total welght of polymer. The commercial PSAs were tested without modification. Peel adhe~ion is shown in pounds per linear inch (pli) and shear adhesion in hours (hr).

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_ 28, 632-F -12-, ~, '- ' .

1 3 ~8~ 7 Table II shows that the 97 percent sec-butyl acrylate and 3 percent acrylic acid PSA was far superior in shear adhesion than the commercial acrylic emul~ion PSAs and comparable with respect to peel adhesion.
Example III
The following adhesive co~positions were prepared and their adhesive properties measured. All percentage~ are by weight of total polymer. The subject adhesive contained 97 percent sec-butyl acrylate and 3 percent acrylic acid having a Tg of -7.5C and a high molecular weight (Sample 1).
Comparative adhesives containling 97 percent n-butyl acrylate and 3 percent acrylic acid having a Tg of -44.4C and a high molecular weight (Sample 2); 96 percent n-butyl acrylate and 4 percent itaconic acid having a Tg of -44C and moderate molecular weight (Sample 3); and 62 percent n-butyl acrylate, 35 percent styrene and 3 percent acrylic acid having a Tg of 10C
and high molecular weight (Sample 4) were prepared.
The adhesive properties were measured as before and are recorded in Table III.

28,632-F -13-1 4 ~80~;~7 ~ABLE III
Peel Shear Adhesion,Adhesion ~Dl~ PSA pli (N/M) (hr) 1~7 sec-butyl acrylate/
3 acrylic acid 3.1 (543~ 1000.0 297 n-butyl acrylate~
3 acrylic acid 1.4 (245) 3 396 ~-butyl ac~ylate/
4 itaconic acid 1.8 (315) 64 462 n-butyl acrylate/
35 styrene/3 acrylic acid0.2 (35)~ 0 jumpy (discontinuous brittle failure of adhesive bond).

In this comparison, it was queried whether the high Tg or high molecular weight of sec-butyl acrylate contributed to its good adhesive properties.
Aocordingly, Samples 2 and 3 which have high and moderate moleqular weight respectively and similar Tg were prepared. Neither sample outperformed the sec-butyl acrylate even though the Tg was lower andmolecular weight was approximately equal in one case and lower in the other. Next, Sample 4 was prepared to have a Tg much higher than Samples 2 and 3 and close to the subject PSA (Sample 1) to see if this made it more 3 competitive. Again, the results showed that sec-butyl acrylate remained superior. This data indicates that the excellent adhecive properties of sec-butyl acrylate are not attributable to molecular weight or its high Tg. Therefore, sec-butyl acrylate is itself believed to be responsible for good adhesive properties.

28,632-F -14--15_ ~Z805%7 EXAMPLE IV
The adhesive properties of sec-butyl acrylate at variou~ tackifer levels were measured. The basic polymer formulation consisted of 87 weight percent sec-butyl acrylate, 10 weight percent styrene and 3 weight percent acrylic acid, all based on the total weight of the formulation. The tackifier employed was Hercules Picconal~ A600E (a 55 percent non-volatile emulsion of hydrocarbon resins in water) and is shown in percent by weight on a total solids basis. The tests were conducted by coating adhesive paper with the formulated PSAs. PTSC Test Method No. 1 was employed to measure the peel adhesion. The results of the tests are shown in Table IV.
Table IV
Sec-butyl Tackifier Peel Adhesion, _crYlate (wt. ~) lwt. ~) ~li (N/m) ZO
oo 0 4 ( 70 ) 1 . 2 ( 210 ?1 29 1. 7 ( 297 . 5 s6 44 2 . 8 ( 490 5.0 (875) The above data qhow that the peel adhesion of the PSA
increases as the level of tackifier is increased.
ExamDle V
The adhesive properties of another sec-butyl acrylate pressure sensitive adhesive formulation containing varying levels of a tackifier were measured as in Example IV. The basic polymer formulation consisted of 90 weight percent sec-butyl acrylate and 28,632-F -15-80~;~7 10 weight percent styrene. The tackifier employed as Hercules Picconal~ A600E. The results of the tests are shown in Table V.
Table V

Sec-butyl Tackifier Shear Adhesion, acrYlate (wt. %)(wt. %) (min.) The above data show that the shear adhesion property of the PSA diminishec as the level of tackifier employed i~ increased.

.

.

-28,632-F -16-

Claims (10)

1. An emulsion polymerized latex composition suitable for use as a pressure sensitive adhesive comprising: (a) from 82 to 99 weight percent, based on total polymer weight, of sec-butyl acrylate and (b) from 1 to 18 weight percent, based on total polymer weight, of a polymer prepared from monomers selected from the group consisting of alkyl esters of acrylic acid and methacrylic acids wherein the alkyl chain has up to 20 carbon atoms, monovinylidene aromatics and ethylenically unsaturated carboxylic acids and mixtures thereof.

2. A composition as claimed in Claim 1, wherein said component (b) is an ethylenically unsaturated carboxylic acid.

3. A composition as claimed in Claim 1, which comprises 97 weight percent of sec-butyl acrylate and 3 weight percent of acrylic acid, both based on total polymer weight.

28,632-F -17-

4. A composition as claimed in Claim 1, which comprises 87 weight percent sec-butyl acrylate, 10 weight percent styrene and 3 weight percent acrylic acid, all based on total polymer weight.

5. A composition as claimed in Claim 1, which additionally includes from 1 to 90 weight percent of a tackifying resin, based on total solids weight.

6. A composition as claimed in Claim 5, wherein said tackifying resin is present in an amount of from 30 to 50 weight percent, based on total solids weight.

7. A process for preparing a latex composition suitable for use as a pressure sensitive adhesive which comprises emulsion polymerizing (a) from 82 to 99 weight percent, based on total polymer weight of sec-butyl acrylate and (b) from 1 to 18 weight percent, based on total polymer weight, of a polymer prepared from monomers selected from the group consisting of alkyl esters of acrylic acid and methacrylic acids wherein the alkyl chain has up to 20 carbon atoms, monovinylidene aromatics and ethylenically unsaturated carboxylic acids, and mixtures thereof.

8. A process as claimed in Claim 7, wherein component (a) comprises 97 weight percent of sec-butyl acrylate and component (b) comprises 3 weight percent acrylic acid, both weight percentages based on total polymer weight.

9. A process as claimed in Claim 7, wherein component (a) comprises 87 weight percent sec-butyl acrylate and component (b) comprises 10 weight percent 28,632-F -18-styrene and 3 weight percent acrylic acid, all weight percentages based on total polymer weight.

10. A process as claimed in Claim 7, wherein a tackifying resin is added to the mixture of monomers.
28,632-F -19-