Pressure-sensitive adhesive having improved adhesion to plasticized vinyl substrates

1 338367 PRESSURE-SENSITIVE ADHESIVE HAVING IMPROVED ADHESION TO PLASTICIZED VINYL SUBSTRATES Background of the Invention This invention relates to normally tacky and pressure-sensitive adhesive (PSA) compositions and to PSA tapes made therewith. The invention is particularly concerned with PSA products capable of adhering firmly to plasticized vinyl s~bstrates, both initially and after contact over an extended period of time.

Polyvinyl chloride (or, as it is more commonly referred to, "vinyl") is one of the most versatile and widely used synthetic polymers available today. In its unmodified form, it is rigid and capable of being fabricated into containers, structural components, tubing, etc. When compounded with a plasticizer, it can be calendered into flexible sheet material that can be used as upholstery fabric, floor covering, PSA tape backing, rainwear, etc., or extruded as electrical insulation, auto body side moulding, etc.

Plasticized vinyls typically contain 15-50% by weight of either a mononeric or a polymeric plasticizer; because the monomeric plasticizers are less expensive, they are more commonly employed.

Unfortunately, however, the monomeric plasticizers are of relatively low molecular weight, tending not only to migrate to the surface of the plasticized vinyls but also to volatilize therefrom, gradually embrittling the originally flexible product.

The tendency of plasticizer to migrate into materials with which it comes in contact is also well known; see, e.g., U.S.

Pat.

No. 4,284,681.

Thus, when a PSA is applied to a plasticized vinyl substrate and allowed to remain in contact with it for an extended period of time, - 2 - 1338367 plasticlzer from the vlnyl tends to migrate lnto the PSA, softenlng lt and causlng a decrease ln adheslon; see, e.g., U.K.

Pat.

No. 1,051,125.

Attempts have been made to solve the plastlclzer mlgratlon problem by lnterposlng an lmpenetrable barrler between the vlnyl and the PSA; see, e.g., U.S.

Pats.

No.

4,045,600 and 4,605,592.

Others have developed PSAs that are sald to be less susceptlble to weakening by plastlclzer; see, e.g., Grazlano et al, Development of Acryllc Latex Pressure Sensltlve Adheslve for Plastlclzed PVC fllms, Journal of Plastlc Fllm & Sheetlng, Vol. 2, Aprll 1986, pp. 95-110. A commerclally avallable tape that ls wldely used for adherlng to plastlclzed vlnyl utlllzes a PSA that ls a terpolymer of 2-ethyl hexyl acrylate, vlnyl acetate, and ethyl acrylate.

Another attempt to prevent mlgratlon of plastlclzer from a plastlclzed vlnyl substrate lnto a confrontlng PSA has been to lncorporate plastlclzer lnto the PSA, thereby mlnimizing the plasticizer gradient between the contacting layers; see, e.g., US.

Pat.

No. 4,272,573, U.K.

Pat.

App.

2,162,191-A published January 29, 1966, and European Pat.

App.

150,978 published August 7, 1985.

Despite the llmlted success achleved by products of the type dlscussed ln precedlng paragraphs, there has remalned a strong commerclal deslre for a PSA that was more effectlve, particularly one that not only initially adhered strongly to plasticized vinyl but which also retalned a high degree of adhesion after remainlng ln contact wlth lt for extended perlods of tlme.

In partlcular, none of the prior art products have been able to provide excellent resistance to shear after prolonged contact with plasticlzed vlnyl.

Brlef SummarY The present lnventlon provldes a PSA havlng excellent lnltlal adheslon to plastlclzed vlnyl substrates.

Even more slgnlflcant, however, is the fact that excellent r 60557-3621 - 3 - 1~338367 adhesion ls maintalned after the PSA has been in contact with the vinyl for extended perlods of tlme. In accordance wlth the lnventlon, it ls also posslble to provlde a PSA that retalns excellent shear adheslon after prolonged contact with a plastlclzed vinyl substrate.

The lnventlon comprlses a polymer of 100 parts of monomers conslstlng essentlally of (1) 60-88 parts by welght of acrylic acid ester of nontertiary alkyl alcohol containing 4-14 carbon atoms, such as butyl acrylate, or especially lsooctyl acrylate or isononyl acrylate, (2) 2-30 parts by welght of polar nltrogen-contalnlng vlnyl monomer, such as N-vinyl pyrrolldone, N-vinyl caprolactone, N,N-pentaethylene acrylamide, or N,N-dlmethyl acrylamide, and ~3) 0-12 parts of vlnyl acid such as acryllc acid, methacryllc acid, or itaconic acid. Intimately blended with the polymer, ln an amount sultable to achleve the deslred degree of pressure-sensltive adheslon, ls a plastlclzer such as dioctyl phthalate (DOP), 2-ethyl hexyl phosphate (2EHP), or tricresyl phosphate (TCP).

The amount of plasticizer generally constitutes 2-35% (preferably 2-10%) by weight of the PSA. In most cases, the PSA will be in the form of a thin layer carried by a backing to which lt may be either permanently or temporarlly adhered.

Where the PSA ls permanently adhered, the resultant product ls a normally tacky and pressure-sensltlve adheslve tape. Where the backlng ls provlded wlth a release coatlng, the PSA ls in the form of a so-called transfer tape.

Where a PSA havlng excellent shear adheslon to plastlclzed vinyl ls deslred, an effectlve amount of crossllnklng agent (e.g., 0.15 part of a photoreactlve s-trlazine) may be added to the PSA. Presently preferred crosslinking agents include s2,4-blstrichloromethyl-6(4-methoxyphenyl)-strlazlne~, and 2,4-bls trichloromethyl-6(3,4-dimethoxyphenyl)-s-triazine~.

It ls surprlslng that an extremely small amount of plastlclzer (as low as 2%) ln the PSA ls effectlve to 60557-3621 ~4~ 1 338367 impart excellent adhesion to vinyl containing as much as 50% monomeric plasticizer. It is also surprising that the plasticizer in the PSA need not be the same as the plasticizer in the vinyl.

It has been found empirically that the unpredictably good adhesion, even after extended heat aging, flows from the incorporation of the polar N-containing vinyl monomer, although the reason is not fully understood.

Detailed Description In the examples that will appear below, certain test procedures were used to evaluate the ability of a given PSA to adhere to plasticized vinyl substrates.

Peel Adhesion. This test is conducted in accordance with a modified version of PS~C-14, a procedure specified in "Test Methods for Pressure-Sensitive Tapes," 8th edition, available from the Pressure-Sensitive Tape Council, 1800 Pickwick Ave., Glenview, IL 60025-1377, U.S.A. A strip of the tape to be tested, 2.54 cm wide x 30 cm. long, is adhered to a test panel, which in turn is mounted on a sled and clamped horizontally in the lower jaw of a tensile testing machine. One end of the tape is then stripped away and clamped in the upper jaw of the tensile tester. The jaws are then separated at approximately 30.5 cm/minute while measuring the force required to remove the tape at an angle of 90. Results are measured in oz/inch of width and converted to N/dm.

180 Peel Adhesion. This test is conducted in accordance with PSTC-7, a procedure specified in the same publication referred to above. A strip of tape 12.7 mm wide and about 30 cm long is adhered to a panel about 5 cm x 12.5 cm. One end of the tape is then doubled back on itself to expose about 2.5 cm at the end ~f the panel.

This exposed end is then clamped in the upper jaws of a tensile testing machine and the doubled-back free end of the tape clamped in the lower jaws. The jaws are then separated at about 30 cm/minute. The force required to -5- ~ 338367 strip the tape from the panel is measured in oz/l/2 inch and reported as N/dm.

T-Peel Adhesion. A 25.4-cm x 10-cm strip of the tape to be tested is laminated to a flexible strip of the substrate to which, adhesion is to be determined. The two strips of the 4-cm length adjacent one end are separated and clamped in the upper and lower jaws of a tensile tester respectively. The jaws are then moved apart at approximately 30 cm/minute while measuring the force required to separate the two strips.

Shear Adhesion.

This test is conducted in accordance with PSTC-7, a procedure specified in the same publication referred to above. A 12.7-mm x 10-cm strip of the tape to be tested is applied to a vertical test panel so that a terminal 25.4-mm x 12.7-mm area is in contact. A weight of either 250 or 500 grams is then applied to the free end of the tape and the time to failure noted.

If no failure has occurred in 10!000 minutes, the test is discontinued.

Understanding of the invention will be enhanced by referring to the following examples, in which all parts and percentages are by weight unless otherwise noted.

Example 1 A pressure-sensitive adhesive tape was made by partially photopolymerizing a mixture of 60 parts isooctyl acrylate (IOA) and 24 parts N-vinyl pyrrolidone (NVP) in the presence of 0.04 part 2,2-dimethoxy-2-phenyl acetophenone (IRG 651) photocatalyst.

The partial photopolymerizing was accomplished in an inert (nitrogen) atmosphere using a bank of 40-watt fluorescent black lights (Sylvania model no. F48T12/22011/40) to provide a coatable syrup having a viscosity of about 1.5 Pa.s.

To this syrup was then added 2 parts acrylic acid (AA), 14 parts DOP plasticizer, 0.15 part 2,4-bistrichloromethyl6(4-methoxyphenyl)-s-triazinet, a photoreactive s-triazine crosslinking agent (XL-353), and 0.10 part IRG 651 ~ de mark -6- 1 338367 photocatalyst. The mixture was knife coated onto a first biaxially oriented polyethylene terephthalate film and covered by a second such film at a knife setting which was adjusted to squeeze the syrup to provide a uniform coating about 0.127 mm thick, the surfaces of the two films in contact with the adhesive composition having previously been provided with low adhesion release coatings. The composite was exposed to a bank of Sylvania 40-watt ultraviolet fluorescent lamps, providing ultraviolet radiation at a rate of 1 mw/sec/cm2.

Example 2 Same as Example 1 except that the IOA:NVP:AA:DOP ratio was 60:21.8:7.6:10.6.

Example 3 Same as Example 1 except that the IOA:NVP:AA:DOP ratio was 69:17:4:10 and the DOP plasticizer was replaced with TCP.

Example 4 Same as Example 1 except that the IOA:NVP:AA:DOP ratio was 69:17:4:10 and the XL-353 crosslinker was replaced with 2,4-bistrichloromethyl-6(4-methoxyphenyl) s-triazinet.

Example 5 Same as Example 4 except that the IOA:NVP:AA:DOP ratio was 67.6:22.5:2.0:7.9.

Example 6 Same as Example 4 except that the IOA:NVP:AA:DOP ratio was 68:25:2:5.

35Example 7 Same as Example 4 except that the IOA was replaced by isononyl acrylate (INA).

- ~7~ 1 33-8367 Example 8 Same as Example 4 except that the IOA was replaced by butyl acrylate (BA).

5Example 9 Same as Example 4 except that the DOP was replaced by mineral oil.

To obtain a somewhat more viscous syrup for coating purposes, blends of syrups were prepared generally as taught in Examples 1-9 except that copolymers were prepared and blended.

Example 10 An 80:20 IOA:NVP was prepared, as was a 90:10 IOA:AA syr~p. Equal parts of the two syrups and DOP were then blended, after which photopolymerization was completed. The resultant adhesive thus had an IOA:NVP:AA:DOP ratio of 66:6.6:3.3:33.

Example 11 An 80:20 IOA:NVP copolymer was prepared; 67 parts of this copolymer was then blended with 7.5 parts AA and 25.5 parts DOP, after which photopolymerization was completed. The resultant adhesive thus had an IOA:NVP:AA:DOP ratio of 53.6:13.4:7.5:25.5.

Example 12 Same as Example 4 except that the 17 parts of NVP was replaced with 17 parts of N,N-dimethylacrylamide.

Example 13 Same as Example 4 except that the 17 parts of NVP was replaced with 17 parts of N,N-pentamethylene acrylamide.

-8- ~ 338367 Control A Same as Example 4 except that no NVP was included, and the IOA:NVP:AA:DOP ratio was 88:0:7:5.

The tapes of Examples 1-11 and Control A were then evaluated for their adhesion to two types of plasticized vinyl, viz., a black electrical cable jacketing plasticized with 33.8~ 2EHP and a decorative trim strip plasticized with 27.8% 2EHP. D~plicate samples of each tape were adhered to each of the two test panels. One taped panel of each pair was allowed to stand at room temperature (RT) for three days, while the other taped panel of each pair was placed in a 70C (HT) oven for one week. (It is believed that exposure at HT for one week corresponds to exposure at RT for about one year.) As a frame of reference, the best commercially available tape for adhesion to plasticized vinyl was subjected to the same conditions. (The control tape, identified as "Control B" had a 12.5-micrometer polyester film backing coated on each side with a 57-micrometer layer of a PSA believed to be a 2-ethyl hexyl acrylate:ethyl acrylate copolymer blended with a vinyl acetate polymer.) 1800 peel adhesion tests were then performed, with the following results:

~9~ 1 338367 TABLE I -- Adhesion, N/dm, to Substrate after Exposure Indicated Cable Jacketing Vinyl Trim 5 Example No RT HT RT HT 1 59 46 61 44 2 81 61 74 35 3 70 53 79 42 4 61 57 61 46 99 66 77 66 6 79 31 83 42 7 48 35 77 28 8 57 59 59 48 9 66 59 74 66 72 39 11 118 85 12 99 57 110 77 13 88 61 125 83 Control A 42 11 48 20 15 Control B 28 26 The tape of Example 4 was then compared to Control tape B for adhesion to the s~rface of vinyl bars containing varying degrees of plasticizer. In each case, a roll of the tape to be tested was placed in a 70 C oven for 3 days prior to testing. 90 peel adhesion tests were performed after aging the taped substrate one week at 70 C. Shear tests were performed at 70 C. using a stainless steel panel as the substrate and applying either a 250-g or 500-g weight. Results are tabulated below:

TABLE II -- Comparative Adhesion and Shear Tests 90 Peel AdhesionSheaO Adhesion at N/dm70 C., Minutes Substrate Control B Example 4Control B Example 4 UPVC 114 256 Semi-rigid PVC 103 95 Flexible PVC 30 68 Highly plas ticized PVC 19 48 Stainless Steel 1,728 10,000+ (250 g) (500 g) -lo- 1 338367 It will be noted that, in almost every instance, the peel adhesion of the tape made according to the present invention was significantly better than the best prior art tape commercially available. It will also be noted that the shear adhesion of the experimental tape was dramatically superior to that of the prior art product.

Using azobisisobutyronitrile as a thermal initiator, two different copolymers were prepared as 40% solids solutions in ethyl acetate at 55C., the first having a 69:17:4:10 IOA:NVP:AA:DOP ratio and the second having a 77:19:4 IOA:NVP:AA monomer ratio. Each of the two solutions was then diluted to 25% solids by adding ethyl acetate. Four additional examples and a control were prepared from these copolymers as described below:

Example 14 To the first copolymer described in the preceding paragraph was added 0.5% of a proprietary crosslinking agent, based on the polymer solids. The resulting solution was then coated on a polyethylene-coated paper backing and the solvent evaporated in a 150 C. oven over a ten-minute period to leave a 127-micrometer adhesive coating.

Example 15 Same as Example 14 except that crosslinking was effected under U.V. light with 0.15% of the s-triazine crosslinker of Example 4.

Example 16 To the second copolymer described above was added 0.5% of the proprietary crosslinking agent of Example 8, and 10% DOP.

The resulting solution was then coated on a polyethylene-coated paper backing and the solvent evaporated to leave a 127-micrometer adhesive coating.

-11- 1 338367 Example 17 Same as Example 16 except that crosslinking was effected under U.V. light with 0.15~ of the triazine of Example 4.

"Control C" for Examples 14-17 Same as Example 8 except that for the first copolymer a conventional 90:10 IOA:AA adhesi~-e was substituted.

90 peel adhesion tests were run against vinyl trim and cable jacketing substrates with each of tapes 14-17 inclusive and the control. Duplicate samples were prepared, one being allowed to age 72 hours at room temperature and the other one week at 70 C. Results are tabulated below:

TABLE III -- 90 Peel Adhesion to Indicated Substrates, N/dm Cable Jacketing Vinyl Trim 20 Example RT HT RT HT 14 146 76 120 74 146 78 132 66 16 136 78 108 68 17 144 64 120 72 Control C 38 10 44 14 The tabulated data show that superior adhesion is obtained when the PSA is a copolymer of monomers essentially comprising polar N-containing vinyl monomer, in this case NVP.

As is apparent from the foregoing illustrative examples, numerous variations of the invention will occur to those skilled in the art who read this disclosure. A few general principles might be noted. When it is essential to have a PSA that possesses high shear adhesion (e.g., for attachment to a vertical vinyl surface), crosslinking is desirable. It should be pointed out, however, that adhesives displaying high shear tend to lack -12- 1 338367 ready conformability and "rapid wet out," or quick stick.

When the amount of plasticizer in the adhesive is increased, a greater percentage of NVP or analogous monomers can be used. Increasing the amount of plasticizer, without further modification, tends to soften the adhesive and reduce its holding power.

Glass microbubbles can also be incorporated in adhesives made in accordance with the invention to achieve a foamlike product similar to that disclosed in U.S.

Pat.

No. 4,223,067; similarly, polymeric microbubbles can be incorporated in such products. Transfer tapes made in this manner possess an apparently unique ability to bond firmly to the plasticized vinyl a~tobody moulding strips in widespread use.

Examples 19 and 20 Plasticized adhesive compositions generally similar to those of Example 4 were prepared in essentially the same way and applied to the back surface of hook-and-loop fastener material of the type described in U.S.

Pat.

No. 3,009,235. The backing of the fastener material was a woven nylon fabric, and the adhesive was applied in a layer about 0.25 mm thick. T-peel adhesion tests were then run against a clear 0.375-mm vinyl substrate containing 26~ of a semilinear phthalate (believed to be 2-ethyl hexyl phthalate) plasticizer, after 24 hours dwell at room temperature and after aging at 70 C. for 7 days. Results are tabulated below:

180 Peel adhesion, N/dm Example IOA:NVP:AA:DOP ratio Initial Heat-aged 18 67:28.6:2.2:2.2 224 156 19 65:28:2.7:4.3 219 163 These products were useful as closures for plasticized vinyl notebooks and blanket bags. Compared to the adhesive -13- 1 338367 on conventional hook-and-loop products, which is a ketone-activatable adhesive, the PSA on the products of Examples 18 and 19 were free from air pollution, more convenient, and faster bonding.

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