CA1137680A - Pressure sensitive adhesive - Google Patents

Abstract

An optically clear blend of (1) a water-soluble polymer of a vinyl lactam having the structure <IMG> in which Y represents an alkylene bridge having three to five carbon atoms and (2) a tacky water-insoluble copolymer of (A) an ester or a mixture of esters having the structure <IMG> in which R' represents hydrogen or methyl and R1 represents alkyl having from 1 to 14 carbon atoms and (B) from 1 to 12% by weight, based on the copolymer, of an ethylenic monomer containing an acid group, said polymer being present in an amount from 1 to 30% by weight of the blend, said copolymer having a glass transition temperature below 0.degree.C and being present in an amount from 70 to 99% by weight of the blend and having a viscosity less than 50,000 cp at 350.degree.F. The blend is useful as a pressure-sensitive adhesive composition.

Description

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This invention relates to a composition useful as a pressure-sensitive adhesive which ls a blend of a poly(vinyl lactam) and an interpolymer of (l) an acrylic or methacrylic ester or a mixture of esters and (2) an ethylenically unsaturated acid, the blend possessing a unique combination of properties and - I
sharacteristics.
Pressure-sensitive adhesive compositions are commonly applied to the flexible backinq or tape on which they are supported during use by coating them in the form of a solution or dispersion in a suitable vehicle such as an organic solvent or ~ater or by coating them in the form of a hot melt free from vehicle. In order to be useful, pressure-sensitive adhesive compositions must possess not only good tack but also good cohesive strength and the desired high degree of adhesion. All of these properties are generally interdependent, a change in one usually causing a chan~e in the others. Althou~h various low molecular weight homopolymers of alkvl acr~lates have long been known to be tack~v materials, they have possessed insufficient cohesive strength to be useful by therrlselves as pressure-sensitive adhesives, particularly those of the type used on tapes or backings intended to adhere to the skin, and it has been necessary to copolymerize them with selected other monomers to achieve the desired combination of pro~erties, as descrihed for example in Samour U.S. Patent 3,299,010.
It has also long been -the prac-tice to formulate pressure-sensitive adhesive com~ositions bv blendinq toqether compatible components each of which in itself lacks one or more of the required properties but which combine to satisfy the requirements,

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as in the case o~ blends of two different hydrophobic water-in-soluble copolymers as described in Guerin et al. U.S. Patent No. 4,045,517 It has also been proposed to react water-soluble poly(N-vinyl lactams) with polymeric carboxylic acids (including copolymers) to form water-insoluble compositions useful for a variety of purposes, as described in Stoner et al. U.S. Patent 2,901,457. As pointed out by Stoner et al. at column 4, lines 56-73, the reaction product there described always has sub-stantially the same properties and contains the two polymeric components in the same proportions regardless of the proportions of the two used to make the product. The compositions of the present invention, on the other hand, vary in properties and in proportions of components depending upon proportions of starting materials. In Ono et al. U.S. Patent 3,975,570, it has been proposed to improve the moisture permeability of conventional pressure-sensitive adhesives which are copolymers of alkyl acrylates with acrylic or methacrylic acid by blending with them hydroxyethyl cellulose, and it was stated that blends of such adhesives w.ith poly(vinyl pyrrolidone) did not exhibit improved : 20 moisture permeability.
It has now been found that water-insoluble pressure-sensitive adhesive compositions having excellent tack, cohesion, and adhesion and capable of being applied to a backing by con-ventional hot melt coa-ting e~uipment can be made by preparing an ;
optically clear blend comprising a water soluble poly(N-vinyl lactam) and a water-insoluble copolymer of an (~) an ester or a mixture of esters having the structure ~' o 1]

~L376B~

in which R' represents hydrogen or methyl and Rl represents alkyl having 1 ~o 1~ carbon atoms and (B) from 1 to 12%, preferably from 4 to 7% by welght of the copolymer of an ethylenic monomer con-taining an acid group, the copolymer being tacky, having a glass transition temperature below OC., and having a viscosity of less than 50,000 cp at 350~F., and being soluble in organic solvents (i.e., substantially free from cross-linking), so that it possesses by itself insufficient cohesive strength to be useful as a pressure-sensitive adhesive.
The N-vinyl lactams, polymers of which can be used in the present invention include those having the structure X~ i ~ C=O

CH=CH2 '' .

in which X represents an alkylene bridqe having three to _ive carbon atoms, such as l-vinvl-2-pyrrolidone, 1-vinyl-5-methyl-2-pyrrolidone, l-vinyl-2-piperidone, and N-vinyl-~-caprolactam; the polymers mav have molecular weights from 10,000 to 1,000,000 or more, Polymers of l-vinyl-2-pYrrolidone are Preferred. The amount o.f:
polymeric N-vinyl lactam in the blend can vary from 1 to 30% by wei~ht of the blend, depending upon the precise copolymer present and the precise proPerties desired in the blend.

The amount of copolymer present in the blcnd can be varied from 70 -to 99% by weight of the blend. Preferably, the pressure-sensitive adhesive composition contains only the blend of polym~ric N-vinyl lactam and the specified copolymer, although small amounts of conventional additives such as stabilizing agents, may also be presen-t to prevent 376~3~

deterioration of the blend during processing at elevated temperatures. Other conventional additives such as pigments, coloring agents, etc. may also be present.

The ethylenic monomer containing an acid group which is S copolymerizable with the ester can contain a carboxvlic acid ~roup or sulfonic or phosphonic, such as acrvlic or methacrylic ~ acid, crotonic acid, maleic acid, 2-sulfoethyl methacrylate, and ; l-phenyl vinyl phosphonic acid. Preferably the acid group in the ethylenic monomer is carboxylic, acrylic acid being the monomer of cholce. The ester with which the ethylenically unsaturated monomeric acid is copolymeFized may be for example butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate and the corresponding methacrylates or mixtures thereof. Esters of acrylic acid with mixtures of different alcohols having from 1 to 14 carbon atoms are preferred; particularly preferred are alkyl acrylates in which the alkyl groups have from 4 to 8 carbon atoms.
The copolymer need not be one consisting solely of the specified ester monomer and ethylenic monomer containing an acid group but may contain up to 20~, k~y weight of the total copolymer, of another copolymerizable ethylenically unsaturated monomer such as vinyl acetate, styrene, acrylonitrile, e-tc. In general, the presence of such a third comonomer in the copolymer does not adversely affect the properties of the pressure-sensitive adhesive and may be desirable in some cases to reduce cost or to increase cohesive strength.
The blend can be made by mixing together solutions or dispersions of the polymeric N-vinyl lactam and of the copolymer ,,~ . ~_, `~J

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in any desired vehicles or solvents which are miscible with each othcr, then removing -the vehicle or solvent, as by ev~poration. It is also possible to blend the polymer or copolymer on conventional mixing equipment such as a two-roll mill or Sigma blade mixer.
In the case of those blends containing amounts of polymeric N-vinyl lactam near the upper end of the specified range of propor-tions, i.e., from 10-30% by weight of the blend, the pressure-sensitive adhesive composition displays increased moisture vapor permea~ility as compared to compositions containing smaller amo~nts of the polymer. This is an advantageous feature in the case of adhesive tapes or sheets having porous backing or reinforcement which are intended to be applied to the skin.
Although different polymers and copolymers are normally considered to be incompatible with one another when mixed, and incapable of forming a homogeneous blend havin~ properties different from either of the components, the blends of the present invention are optically clear or at worst slightly hazy in appearance, evidence that the blends are homogeneous at least to the extent that no discrete particles of either component greater than 4000 A in diameter are present. secause of this homogeneity, the low cohesive s-trength of the tacky copolymer is increased and the high fluidity is decreased by the presence in the blend of the polymeric N-vinyl lactam which acts as a reinforcing agent.
The blends of the present invention can be applied to any of -the usual flexible backings or reinforcements employed in manufacturing adhesive tapes and sheets either by spreading, ~3768~

coating or casting a mixed solution or dispersion of the blend on the backing, then removing solvent or liquid vehicle, for example by evaporation; however, the blends possess the unique capability of being applied in molten condition using conventional hot melt coating equipment because their hot melt viscosities are less than 100,000 cps at 350F.; no curing or cross-linking of the blend is required. Conven-tional pressure-sensitive adhesives containing copolymers of alkyl acrylates and acrylic or methacrylic acid of sufficiently high viscosity to have adequate cohesive streng-th, have a melt viscosity above 100,000 cps at 350F. and cannot be applied to a backing in molten condition using conventional equipment.
In order to achieve maximum stability and service life of the blend, it may be desirable to include in the pressure-sensitive adhesive composition, in addition to the polymericN-vinyl lactam and the copolymer, a small amount of conventional stabilizer such as 1~ by weight of tetrakis-[2,4-di-tert-butyl-phenyl]-4,4'-biphenylylenediphosphonite.
The following specific examples are intended to illustrate more fully the nature of the present invention without acting as a limitation upon its scope. The molecular weights referred to herein are the peak molecular weights as determined by gel permeation chromatography.

~xamples 1 - 15 I
A series of copolymers of alkyl acrylates with varying proportions of acrylic a^id was prepared by conventional solution polymerization procedures by dissolving the desired proportions of ~
!
i 768al monomers in a suitable solvent such as benzene or ethyl acetate and by employing as the initiator of polymerization a small amount (O. 1o by weight of the monomers) of a free radical generator such as benzoyl peroxide or 2-t-butylazo-2-cyanopropane. Polymerization was carried out at 85-95C. to a high degree of conversion, of the;
order of 97Q, to provide copolymers having insufficien-t cohesive strength to be useful by themselves as pressure sensitive adhesives having molecular weights from 62,000 to 566,000. These copolymers ater removal oE the solvent by heating in vacuum were aggressively tacky in nature but lacked sufficient cohesive strength to be satisfactory pressure-sensitive adhesives by themselves. A
tabulation of -the copolymers and of their molecular weights follows:

Wt. Ratio of ~crylic Acrylic Ester ~.ol. ~t., No. Ester ` to Acid Thousands . . _ _ (1) n-Bu-tyl 98:2 82 (2) " 97:3 127

(3) " 97;3 200 20 (4) " 95 5 58 (5)2-Ethylhexyl 98 2 566 (6)2-Ethylhexyl 97 3 128 (7) " 97:3 207 (8) " 96.5:3.5 304 25 (9) " 96:4 62 (10) " 96:4 ~84 (11) " 96:4 112 (12) " a5.3:4 7 105 (13) " 94.5:5.5 85 30 (1~) " 9~.5:5.5 105 (15) " 93:7 105 ~3~68~

A series of l-vinyl 2-pyrrolidone homopolymers, all water-soluble but having differing reported molecular weights, was obtained from commercial sources, as follows:
Polyvinyl Pyrrolidone : No. Mol Wt, Thousands (1) 10 (2) ~0 (3) 360 Blends of the foregoing two series in varying proportions by weight were then prepared as shown in Table I. In some cases, the acrylate copolymer, still in solution in the solvent in which it was polymerized, was mixed with a :~
solution of the poly~vinyl pyrrolidone) in a compatible solvent such as chloroform and the two solvents were -then removed by volatilization at reduced pressure. In other cases, the solvent was first removed from the solution of acrylate copolymer by heating at reduced pressure and the solvent-free copolymer was then blended with solvent free poly~vinyl pyrrolidone) by milling on a heated 2-roll mill. The blend was then applied to a standard backing or support consisting of a 1.5 mil polyethylene~ :
terephthalate film (Mylar)*, the blends which were formed in solution being applied by spreading the solution on the film before evaporating the solvent and the blends formed by milling being applied by calendering. The product in each case was a pressure-sensitive adhesive tape having an adhesive layer from 1-3 mils in thickness.

*Trademark - 9 -~9~37~

The adhesive properties of the tape were then determined with the results set forth in the following table. The probe tack was determined by means of a Polyken* probe tack tester as described in United States Patent 3,21~,971 having the following 4 functional parts: (1) a cylindrical steel probe attached to the compression loaded spring of (2) a series L Hunter mechanical force gauge ~Hunter Spring Company, Brochure 750/FG, revised February 1961), (3) an annulus having an opening slightly larger than the diameter of the probe and (~) a carrier for the annulus which moves down to bring the annulus around the probe and then up to remove the annulus there-from. The carrier moves at a speed of 0.1 inch per second. At the beginning of the test, the carrier is at its uppermost point of travel and the annulus rests upon the carrier so that the opening in the annulus is in line with the probe positioned beneath it. In carrying out the test, a strip of tape is placed upon the annulus, adhesive side down, and spanning the annulus, open-ing. As the carrier is driven downwardly by the synchronous motor, the ad-hesive surface exposed through the opening is brought into contact with the flat surface of the probe so that the tape and the annulus attached thereto are suspended on the probe as the carrier continues farther on its downward path. The carrier then reverses its movement returning to pick up the annu-lus, thereby separating the tape from the probe surface. Separation begins after one second cOntact between the probe and the adhesive. The force re-quired to separate the tape from the probe is recorded on a gauge. The re-corded value is the probe tack value. Measurements were made employing a loading of 100 grams/cm .
The peel adhesion values represent the forces required to remove a one-inch wide adhesive tape from a stainless steel surface after contact therewith for 2 minutes at a temperature of about 75F. The tape is stripped from the surface at an angle of 180 at a rate of 12 inches per minute.

*Trademark - 10 -37~

The creep resistance values are determined by providing a polislled s-taillless steel -tube, one inch in outside diameter, mounted horizontally within a constant temperature chamber main-tained at 104F. + 2F. The tube is provided with a slot 1/16 inch wide extending along its upper fàce parallel to the tube axis.-A six inch length of adhesive tape to be tested is draped over the tube with its adhesive surface in contact with the tube and with its free ends extending downwardly approximately the same distance on opposite sides of the tube, the tape extending across the slot perpendicularly thereto. Each length of tape is not over one-inch wide and to each of its lower ends is secured a weight of one pound per inch of tape width. After the weighted tape has been maintained in the chamber for 15 minutes, it is severed transversely along the gap in the slot and the time in hours required for downward movement of each end by a distance of one-half inch is measured. The creep resistance is expressed in hours.
The melt viscosity at 350F. is measured as Brookfield Thermosel (Viscometer ~odel RVT) viscosity using a No. SC4-27 spindle at 2.5 rpm.
The results are as follows:

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Certain compositions of blends of alkyl acrylate col)ol~mers COntaini.ll~ hi~her proportioJls of PVP possess two tim~s or greater wa-ter permeability (Table II) than the conventional non-blended acrylate pressure sensitive adhesives. Water permeabili-ty of an adhesive is a desirable characteristic especially in health care adhesive tape products for application to skin. It is measured by ASTM E96 Procedure E.

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The blend in each of the examples is an op-tically cl~r mi~ur~, an~l, as can be seen from the foregoing, all of the blends form useful pressure-sensitive adhesive tapes. It is also clear from the foregoing tabulation what effect is produced by variations in the amount of acryLic acid present in the copoly~ler, the molecular weight of the copolymer, the relative proportion of copolymer to poly(vinyl pyrrolidone), and the molecular weight of the poly(vinyl pyrrolidone). The properties of other blends of the present invention can readily be determined by extrapolation or interpolation from the results set forth above, bearing in mind that in general lesser amounts of acid comonomers are required, when a sulfonic or phosphonic acid group is present, to achieve the same effect as when a carboxylic acid group is used.
Similar results can be obtained using copolymers of acrylic acid with other alkyl acrylates or mixtures thereof and by using other water-soluble po:Lyvinyl lactams in place of poly (vinyl pyrrolidone).

. . . . . .. .... .. . . .

Claims (14)

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

1. A composition consisting essentially of an optically clear blend of (1) a water-soluble polymer of a vinyl lactam having the structure in which X represents an alkylene bridge having three to five carbon atoms, and (2) a tacky water-insoluble copolymer comprising (A) an ester or a mix-ture of esters having the structure in which R' represents hydrogen or methyl and R1 represents alkyl having from 1 to 14 carbon atoms and (B) from 1 to 12% by weight, based on the co-polymer of an ethylenic monomer containing an acid group, said polymer being present in an amount from l to 30% by weight of the blend, said copolymer having a glass transition temperature below 0°C and being present in an amount from 70 to 99% by weight of the blend and being substantially free from cross-linking having a viscosity less than 50,000 cp at 350°F so that it possesses by itself insufficient cohesive strength to be useful as a pressure-sensitive adhesive.

2. A composition as claimed in claim 1 in which X represents -CH2-CH2-CH2-.

3. A composition as claimed in claim 2 in which R' represents hydrogen and (B) is acrylic acid.

4. A composition as claimed in claim 3 in which R1 represents alkyl having from 4 to 8 carbon atoms.

5. A composition as claimed in claim 4 in which R1 represents n-butyl.

6. A composition as claimed in claim 4 in which R1 represents 2-ethyl-hexyl.

7. A composition as claimed in claim 2 in which the viscosity of the copolymer is less than 50,000 cp at 350°F. and the amount of polymer is from 1 to 8% by weight of the blend.

8. An adhesive sheet material comprising a flexible backing and sup-ported thereon a pressure-sensitive adhesive layer comprising a blend as claimed in claim 1.

9. An adhesive sheet material comprising a flexible backing and sup-ported thereon a pressure-sensitive adhesive layer comprising a blend as claimed in claim 2.

10. An adhesive sheet material comprising a flexible backing and sup-ported thereon a pressure-sensitive adhesive layer comprising a blend as claimed in claim 3.

11. An adhesive sheet material comprising a flexible backing and sup-ported thereon a pressure-sensitive adhesive layer comprising a blend as claimed in claim 4.

12. An adhesive sheet material comprising a flexible backing and sup-ported thereon a pressure-sensitive adhesive layer comprising a blend as claimed in claim 5.

13. An adhesive sheet material comprising a flexible backing and supported thereon a pressure-sensitive adhesive layer comprising a blend as claimed in claim 6.

14. An adhesive sheet material comprising a flexible backing and supported thereon a pressure-sensitive adhesive layer comprising a blend as claimed in claim 7.