ACRYLIC HOT MELT PSAs

Transcription

1 ACRYLIC HOT MELT PSAs Charles W. Paul, Principal Business Scientist, National Adhesives, Bridgewater, NJ Cynthia L. Meisner, Senior Chemist, National Adhesives, Bridgewater, NJ Abstract Styrenic block copolymers (SBCs) are the base polymers for the vast majority of hot melt PSAs. The phase separation of the high Tg styrene blocks upon cooling allows these materials to exhibit low melt viscosity yet high adhesive shear strength. Recently, new polymerization technologies have been developed to provide acrylics with the same type of blocky structure. The hard phase is ordinarily methyl methacrylate and the soft phase is composed of low Tg monomers with lower polarity, such as butyl acrylate. These new polymers combine the traditional property advantages of acrylics (such as UV and thermo-oxidative resistance, and high moisture vapor transmission rate (MVTR)) with the processing advantages of hot melts (low capital and operating costs, and fast line speeds). Utilizing this new block acrylic technology, adhesives have been developed for medical and industrial tapes. For medical tapes, formulas with exceptional softness and fast wet-out have been obtained, which provide excellent skin grab and long term wear with painless removal. For industrial tapes, block acrylic formulas exhibit improved heat resistance and thermo-oxidative stability vs. those based on SBCs. Introduction Hot melt adhesives require physical crosslinking upon cooling if they are to exhibit low melt viscosity yet high heat resistance. Simple physical entanglement of polymer chains is inadequate to provide the right balance (1). For this reason, random acrylic copolymers have had little success as hot melt adhesives. Numerous labs have explored methods to synthesize block acrylics by free radical and ionic processes. Early free radical syntheses incorporated polystyrene (2) or polyacrylate macromonomers (3) to form comb-type block copolymers. These were necessarily limited in the molecular weight of the hard teeth blocks to a few thousand Da. More recent work has involved multivalent chain transfer agents (4) and living radical polymerizations (5, 9). Anionic methods (6), analogous to what is used in styrenic block copolymers, have the advantages of better control of polymer architecture and tacticity (7, 8). In addition, very low levels of residual monomer can be obtained without exhaustive purification procedures. In this study, block copolymers with methyl methacrylate (MMA) as the hard endblocks and butyl acrylate (BA) as the soft mid block were studied. Block copolymers polymerized anionically were used to formulate low viscosity adhesives for a variety of PSA applications. Even after high levels of formulation, the improvements expected from an acrylic block copolymer over an analogous styrenic block copolymer were realized. These include: softness at high polymer loading, high moisture vapor transmission rate (MVTR), improved melt stability, greater resistance to UV exposure, and higher heat resistance (SAFT).

2 Neat Polymer Properties 1. Pellets - An Easy to Compound Product Form Anionically polymerized MMA/BA/MMA triblock polymers were obtained in the form of extruded free-flowing pellets. This form is identical to that of SBCs. Other hot melt acrylics are available in release-lined boxes which make adding them to a hot melt mixer cumbersome and time consuming. Pellets can be poured in from a box. 2. Ultra Low Levels of Residual Monomer Due to the high conversion in anionic reactions, and also by virtue of being finished by extrusion (which applies high temperature, vacuum, and creates a large surface to volume ratio) the level of residual monomer is extremely low. Only 12 ppm of butyl acrylate was found in prototype adhesive formulas (K and L). This compares to typical levels of ppm in solution acrylics. Only with more extreme measures can values below 100 ppm can be obtained. (It should be noted, however, that under careful coating and drying conditions, residual monomer levels in the final adhesive films obtained from some solution acrylics can be further reduced, even down to this 10 ppm range.) Consequently, these new block acrylic base polymers are particularly suited to consumer and medical applications where very low residual monomer is paramount. 3. Softer than Styrenic Block Copolymers One disadvantage of styrenic block copolymers is that the entanglement molecular weight (Me) of the end block (styrene) is quite high (18k Da). Thus to obtain the full Tg of the end block polymer, requires either higher overall molecular weight and/or high styrene content. The low Me of MMA (4.7k Da) loosens this constraint. In addition, the mid-block of BA has a much higher Me (17k) vs. isoprene (7k) or butadiene (1.7k) or their hydrogenated counterparts (also about 1.7k). Thus higher levels of polymer can be employed while maintaining stiffness below the Dahlquist criterion of 3 x 10 6 dynes/cm 2 - which is needed to maintain pressure sensitive properties. Figure 1 compares the dynamic mechanical properties of SIS and MMA/BA/MMA, each containing 30% hard phase. The acrylic block copolymer is much softer (lower plateau modulus) and more heat resistant (higher hard block Tg). The SIS shows a Tg of about 100 C followed by an order-disorder transition at about 140 C, whereas the acrylic Tg, while harder to distinguish, appears to be in the range of C for this sample. These base polymer advantages of softness and heat resistance are translated into adhesive formulas as discussed below. Formulation Properties Like styrenic block copolymers, these block acrylics can be highly diluted with suitable tackifiers and diluents. Polymer level in an adhesive formula ranges from 10-45%. By contrast, traditional solution or water-based acrylic PSAs are typically used in their neat form, or with 10-30% tackifier at most. The question arises then, will adhesive formulas where block acrylic polymer is the minor component exhibit the high UV stability of neat acrylic PSAs? And, will the improved melt stability over SBC-based formulas be

3 realized? A series of experiments were conducted to answer these basic questions. In each comparison, formulas based on block acrylics were compared to commercial adhesives. 1. Melt Stability Generally the upper limit for applying SIS-based HMPSAs is 350 F. For this experiment we compared a high temperature SIS-based PSA with two acrylic based formulas. One hundred grams of adhesive was placed in an 8 ounce glass jar, covered with foil, and placed in an oven at 350 F for 24 hrs. As shown in Table 1, with the proper choice of tackifier the viscosity is unchanged for the acrylic formula, whereas the SIS-based formula drops some 61% due to chain scission, and is also prone to gel at the surface. The latter tendency leads to the build up of char in hot melt tanks over time. Thus the stability of the acrylic polymer backbone is translated into improved stability of the formulated adhesive. The more stable the adhesive, the more trouble-free the coating process, and the more consistent the adhesive performance. 2. UV Stability For exterior applications, UV stability is critical, and solution acrylics are the standard. In this test we compare a commercial SIS-based HMPSA (D) with one based on block acrylics (F) and an exterior-grade commercial solution acrylic (E). Two mil films were coated onto PET. Part of the adhesive was covered with clear OPP film (2 mil), part with clear PET film (2 mil), and the remainder was left uncovered and exposed directly to the UV light. The coatings were placed in a QUV chamber with UVA-340 bulbs. This bulb closely simulates the UV spectrum of sunlight, but at much higher intensity. The chamber warmed to about 45 C during the test. Samples were removed and examined after 70, 208, 355, and 707 hrs. Tack of the exposed adhesive, and color were assessed. As can be seen in Table 2, the SIS-based adhesive (D) was slightly yellow and became yellower during exposure. Tack was lost during the first 70 hrs. By contrast the solution acrylic adhesive (E) began and remained clear and tacky throughout. The block acrylic adhesive also remained colorless, but it did lose tack somewhere between 355 and 707 hrs. Attempts to remove the covered adhesive sections from the PET backing were unsuccessful as the OPP and PET coverings had embrittled and cracked during the severe test. This embrittlement was noticeable after 355 hrs. Thus the block acrylic adhesive outlasted these backing films. Based on these results, the block acrylics show promise as a base polymer for exterior-use tapes and labels. 3. Application-Specific Formula Properties a. Medical Tapes Skin is a very rough surface with a hydrophobic exterior. In addition, sweat and natural oils are exuded from its pores which can lead to gradual debonding. The ideal adhesive is one which is very soft, high in peel, permeable to moisture, and capable of slight creep at body T to permit rebonding. While this combination is difficult to attain in one single

4 adhesive, it was thought that block acrylic formulas provide a new approach with high potential. As mentioned previously, these adhesives can be formulated to be extremely soft yet heat resistant. Figure 2 compares the dynamic mechanical properties of two block acrylic adhesives formulated for use in medical tapes with solution rubber and solution acrylic formulas sold for the same purpose. With the block acrylics, extremely soft adhesives can be formulated which still exhibit high softening points (note the PMMA block Tg at about 110C). To obtain these very soft adhesives requires a high level of dilution (low polymer content). By choosing appropriate tackifiers and diluents, high values of moisture vapor transmission were obtained from these formulas. Figure 3 compares MVTR values of several block acrylic formulas against medical grades of a solution acrylic and an SISbased HMA. All of the acrylic-based formulas are between (g/m 2 -day), while the SIS-based hot melt has an MVTR of <100. A high MVTR promotes healing and long term adhesion. Lab tests of adhesive tapes applied to the skin of volunteers indicate that 24 hr adhesion levels far superior to SISbased adhesives and comparable to solution acrylics can be obtained, with no mass transfer. Adhesion to a permeable medical backing, Medifilm 390, was excellent. (An SIS-based medical adhesive adhered poorly to this particular backing.) Cytotoxicity of block acrylic-based adhesives K and L, and also the block acrylic base polymer, was tested by the iso-elution method (International Organization for Standardization Method 10993, part 5). All results were negative (non-cytotoxic). b. Industrial Tapes and Labels The softness, clarity, and UV stability of the block acrylic formulas described above bode well for their application in graphic labels. Peel properties of a prototype (A) were compared to a commercial graphics-grade solution acrylic adhesive (G). As shown in Table 3, peel on steel is comparable. The block acrylic prototype is superior on HDPE (2.5 lb/in vs. 1.5 lb/in.) and higher in tack. With vinyl backings, shrinkage is too high and will need to be improved for commercial acceptance using this type of backing. Figure 4 demonstrates the much softer nature of the HMPSA based on block acrylic. For industrial tapes, the high Tg of the syndiotactic PMMA end blocks should permit higher SAFT values to be obtained. A prototype block acrylic formula (B) for tapes was compared to commercial tape adhesives; one based on SIS (C) and the other a solution acrylic (H). As shown in Table 4, the block acrylic adhesive is lower in adhesion to S.S., but provides good adhesion to HDPE and higher SAFT than the SIS-based C. Figure 5 shows the much higher softening point of B vs. C. Even higher SAFT and better peel is expected from block acrylic formulas upon increasing the stiffness of the adhesive by addition of more polymer.

5 Conclusions Initial experiments indicate many of the expected benefits of block acrylic-based HMPSAs over conventional SIS-based materials can be realized. These benefits include improved thermal and UV stability, extraordinary softness, high MVTR, and increased SAFT. These new materials should thus provide further momentum to the long term growth of hot melt adhesives, as they continue to capture market share from solution and water-based alternatives in medical, graphics, and tape markets. References 1. C.W. Paul, Hot Melt Adhesives, in Adhesion Science and Engineering Vol. 2 - Surfaces, Chemistry and Applications, M. Chaudhury and A.V. Pocius editors, New York, 2002, p J.A. Schlademan, U.S. 4,551,388, 11/5/ J.R. Husman et al., U.S. 4, , 11/19/ M. Yoshida et al., U.S. 5,679,762, 10/21/ M.B. Ali et al., EP , 8/24/ B.C. Anderson, G.D. Andrews, P. Arthur, Jr., H.W. Jacobson, L.R. Melby, A.J. Playtis, and W.H. Sharkey, Macromolecules, 14 (1981), p T. Kakehi, M. Yamashita, and H. Yasuda, Reactive & Functional Polymers, 46 (2000), p N. Utsumi et al., Japan Kokai , 11/2/ Y.L. Dar et al. U.S Acknowledgements The authors are grateful to National Starch and Chemical Company for permission to publish this work. Table 1. Thermal Stability of HMPSAs Block-Acrylic Based Adhesives A (tackifier x) B (tackifiers y and z) SIS-Based Adhesive C Viscosity at 350 F (cp) Initial After 24 hrs at 350 F % Change Char Ring at Surface? none none light

6 Table 2. Effects of UV Exposure on Adhesives (QUV-UVA-340 bulb) Adhesive Type Initial Color a Time to Lose Tack b Color after 707 hrs D SIS-Based -HM 1 <70 2 E Solution Acrylic 0 >707 0 F Block Acrylic-HM a: 0 = clear, 1 = very pale yellow, 2 = pale yellow b: samples examined after 70, 208, 355, and 707 hrs. Table 3. Adhesive Properties of Graphics Grade Adhesives Block Acrylic-Based Adhesive (A) Graphics-Grade Solution Acrylic (G) Appearance colorless colorless Peel (lb/in) a S.S. 20 min S.S. 1 week 5.6 HDPE, 20 min. 2.5 HDPE, 1 week 2.0 Loop Tack on S.S. (oz./in 2 ) a. 2 mil adhesive coatings on 2 mil PET Table 4. Adhesive Properties of Industrial Tape Grades Peel (lb/in) a S.S. 20 min. S.S. 1 week HDPE, 20 min. HDPE, 1 week Shear (hrs) 4 psi, RT 0.5 in. x 1 in. x 1kg SAFT ( F) 1 psi, 1/min Block Acrylic- Based HMPSA (B) SIS-Based Tape Grade HMPSA (C) Solution Acrylic Tape Grade (H) > in. x 1 in. x 0.5kg Loop Tack (oz./in 2 ) a. 2 mil adhesive coatings on 2 mil PET

7 G' ( ) [dyn/cm 2 ] MMA / BA / MMA SIS tan_delta ( ) [ ] Temp [ C] Figure 1. Dynamic mechanical analysis of SIS vs. MMA/BA/MMA at 30% hard phase Solution Rubber Adh. - I Solution Acrylic Adh. - J Block Acrylic Hot Melt Adh. - K Block Acrylic Hot Melt Adh. - L G' ( ) [dyn/cm 2 ] tan_delta ( ) [ ] Temp [ C] Figure 2. Dynamic mechanical analysis of medical grade adhesives.

8 MVTR (g/m2-day) Block-Acrylic HMA - K Block-Acrylic HMA - L Solution Acrylic - M SIS-Based HMA -N Figure 3. Moisture vapor transmission rate using the inverted cup method. Adhesive films were 1.5 mils thick and backed with a porous film (Medifilm 390) Block Acrylic HMA - A Solution Acrylic Adh. - G G' ( ) [dyn/cm 2 ] tan_delta ( ) [ ] Temp [ C] Figure 4. Dynamic mechanical analysis of graphics grade adhesives.

9 Block Acrylic HMA - B Solution Acrylic Adh. - H SIS- Based HMA - C G' ( ) [dyn/cm 2 ] tan_delta ( ) [ ] Temp [ C] Figure 5. Dynamic mechanical analysis of industrial tape grade adhesives.

10 Charles W. Paul is a Principal Business Scientist in the Adhesives Division of National Starch and Chemical Company in Bridgewater, N.J. He is currently leading efforts in the New Adhesive Technology Group to develop a variety of crosslinkable hot-melt adhesives and novel systems for skin adhesion. Paul received his PhD degree from the University of California at Berkeley in He has been active in polymer science research throughout his career, conducting research in the areas of plasma-initiated polymerization, diffusion in polymer systems, associative networks, thermosetting polyimide oligomers for aerospace, thermoplastic starch-based formulas for plastics and adhesives, starch-based hair fixatives, and thermoplastic and thermosetting hotmelt adhesives. He is an inventor on over 20 patents, author of numerous journal articles, and contributor to several books. Paul can be reached by at charles.paul@nstarch.com.