IMPROVING THE PERFORMANCE OF HOT MELT PSAs USING EXTRUSION OR CROSSLINKING TECHNOLOGIES

IMPROVING THE PERFORMANCE OF HOT MELT PSAs USING EXTRUSION OR CROSSLINKING TECHNOLOGIES Janelle Cameron Senior Chemist H.B. Fuller Company 1200 Willow Lake Blvd. St. Paul, MN 55110 Mark Kroll Research Chemist H.B. Fuller Company 1200 Willow Lake Blvd. St. Paul, MN 55110 ABSTRACT The traditional deficiencies of hot melt pressure sensitive adhesives (PSA) are due to the use of relatively low molecular weight polymers, resulting in limited heat and plasticizer resistance. Two methods of overcoming this deficiency are the development of high molecular weight, extrudable PSAs, and post-crosslinking the hot melt after application. Higher molecular weight raw materials can be used to produce high viscosity PSAs with improved performance. These PSAs are processed using extrusion. This adhesive technology provides fast slot die processing speeds, very high cohesive strength, and relatively high heat resistance. Some grades offer improved plasticizer resistance over conventional hot melt PSA. The materials are now available in a convenient pellet form. Crosslinkable hot melt PSAs that are designed to be crosslinked by UV light or electron beam, are also now available. These products offer high processing speeds, relatively low application viscosity, high resistance to plasticizers and solvents, aggressive tack, and very high heat resistance. Performance data compared with alternative adhesive technologies will be provided along with performance on a variety of release liners. INTRODUCTION The tape and label market is always under pressure to provide products that can perform under extreme conditions. High performance applications stretch the ability of the adhesive to resist heat, plasticizers and chemical exposure. Historically, the only practical answer has been to resort to solvent borne crosslinkable PSAs. However, when using solvents, special solvent recovery and handling equipment is needed. Due to anticipated regulatory changes, there is a need for alternatives to solvent-based adhesives in the tape and labeling industry. Upcoming MACT regulation increases the pressure for upgrading equipment and HAP release reporting where solvent based PSAs are in use. The EPA is expected to issue the MACT (maximum achievable control technology) standard in November of 2000, which will require manufacturer compliance within three years of the regulation's issue. The MACT is an EPA standard that requires implementation of the best technology available to minimize hazardous air pollutants from certain processes. This will most likely require a capital equipment upgrade of emission control systems or a conversion to a non-polluting product for many converters now using solvent based adhesives (1). Therefore, alternatives to solvent-based adhesives are a definite necessity in the industry. Hot melt adhesives, being 100% solids are one clear alternative. 93

.I BACKGROUND Conventional hot melt adhesives are based on low molecular weight thermoplastic polymers. These thermoplastic polymers soften and melt when heated. This results in the adhesive becoming fluid, and therefore provides excellent coating characteristics. This phenomenon is precisely why hot melt pressure sensitive adhesives typically have low heat resistance. In order for the viscosity of the HMPSA formulation to be low enough at application temperature, low molecular weight polymers and/or plasticizing oils have to be utilized. The use of these low molecular weight polymers and plasticizing oils also impair the performance of the adhesive in its upper service temperature range. Therefore, hot melt pressure sensitive adhesives have not had the performance requirements necessary to displace solvent-based adhesives. H.B. Fuller Company is introducing two high performance 100% solid alternatives to solvent-based adhesives. A family of high performance, extrusion grade, hot melt, pressure sensitive adhesives. These adhesives are offered in a pellet form: The pelletized hot melt pressure sensitive adhesives (HMPSAs) offer improved heat resistance and shear properties over conventional HMPSAs. The pelletized HMPSAs have been designed for use in a single screw extruder that can handle extremely high viscosity materials. An extruder can handle materials of extremely high viscosity, reportedly in the range of 0.02 to 2 kpa*s at 177 C (2). Pelletized HMPSA will offer increased production rates over current solvent and water-based systems, and cost savings can also be realized from the fact that coating equipment requires minimal floor space. There are four products available in this family of pelletized HMPSAs with various levels of tack, viscosity, and heat resistance. Another technology being introduced is UV-curable HMPSAs. The industry has recognized UV curable PSAs as potential alternatives to solvent-based acrylics. However, they have made limited inroads due to high costs and slower line speeds. Recent work by H.B. Fuller has resulted in a more attractive answer to the market demands. A line of UV curable hot melt adhesives that run at high line speeds and offer step-out improvement in high temperature shear has been developed. This technology can also provide plasticizer and chemical resistance not previously available through conventional HMPSAs. TAPE BENCHMARK STUDY The ideal PSA would have excellent coating characteristics, give outstanding adhesion values, and perform over a very wide temperature range. 180 peel data gives information on the adhesion performance of an adhesive. Shear adhesion failure temperature (SAFT) data is one measure of the upper service temperature range of a PSA. In order to gauge where our two technologies were in comparison to other tapes, a benchmark study was completed for several commercially available tapes. The physical properties of these tapes were compared to both the pelletized HMPSA technology and the new UV curable technology. The polymer base of each commercial tape was determined. The technologies represented in the study include natural rubber, un-crosslinked solvent-based acrylics, styrenic block copolymers, pelletized HMPSA and the UV-curable HMPSA. We found the heat resistance of the pelletized HMPSAs to be comparable to the high performance commercial tapes. The heat resistance of the UV curable adhesives was found to be significantly higher (>100 F) than the other 94

technologies tested (see Figure 1). We also found the UV-curable technology to have comparable adhesion characteristics to the other technologies tested...,'ii Figure 3: Plasticizer Resistance of Pelletized HMPSA's by Type of Plasticizer m HL-2081 4 Mil 2 m HL-2688 4 Mil ii HL-2697 4 Mil 1 El HL-2807 1 Mil 0 Polymeric Linear plasticizer @26 plasticizer S711 pph resin @ 21 pph and adipate plasticizer @ 8 pph DOP DOP plasticized @ plasticized @ 30 pph 50 pph EIHM-1597 1 Mil 1 - Gummy, transfers 2 - Soft on edges, slight tr. 3 = No affect, clean peel EXTRUSION GRADE HMPSAs Extrusion grade hot melt pressure sensitive adhesives in pellet form can achieve the required performance needed for a great number of the applications now served by solvent-based adhesives. The typical performance properties of conventional hot melt pressure sensitive adhesives and those for extrusion grade hot melt pressure sensitive adhesives are compared in Table 1. Pelletized hot melt pressure sensitive adhesives have shear adhesion failure temperatures up to 125 C and softening points up to 140 C. When compared to conventional hot melt pressure sensitive adhesives this is a significant improvement. The adhesion properties of pelletized hot melt pressure sensitive adhesives can be tailored to the required application; they can be removable, repositionable or permanent in adhesion. 95

TABLE 1 CONVENTIONAL PELLETIZED 180 Peel Values to SS* _< 8 pounds/inch _< 6 pounds/inch Loop Tack Values to SS* _< 10 pounds/sq, inch _< 8 pounds/sq, inch Polyken Tack* _< 1500 grams _< 1500 grams Shear Adhesion Failure Temperature* _< 95 C _< 130 C Viscosity at 175 C _< 20,000 cps > 50,000 cps Mettler Softening Points _< 110 C _< 160oC *All tests were run on a film of 2 mil PET with 1 mil of adhesive. Although conventional hot melt pressure sensitive adhesives have provided an alternative method of using adhesives without organic solvents, their main limitation is lack of high temperature shear resistance. Conventional hot melt reservoirs pump the molten adhesive to the hot melt coater. This system has viscosity limitations and accordingly the adhesives suitable for this method of hot melt coating are restricted in molecular weight. Extrusion coating therefore permits the use of high molecular weight polymers in formulating an adhesive composition (3). The use of higher molecular weight polymers in extrusion grade HMPSA formulations offers improved heat resistance and shear properties over conventional hot melt pressure sensitive adhesives (see Figure 2). Figure 2" Static Shear Data for Pelletized HMPSAs 1400 1200 ~:::.-:-::::::.-::= 1000 800 600 % ~i~!-... N-! [] @ St. Shear @6ooc! St. Shear @80oc rq St. Shear @1oooc 400 200 ~:.~ @i,m / H M-1597 H L-2081 H L-2697 H L-2807 H L-2688 N N 96

Plasticizer Resistance One very difficult application for an adhesive is adhering to PVC films. The plasticizers commonly used to soften the PVC films typically destroy the cohesive strength of the adhesive. Utilizing high molecular weight polymers in the adhesive formulation enables one to overcome the plasticizer migration (2). Figure 3 shows the performance of pelletized HMPSAs on 25-mil thick vinyl film (Wiman Corporation, Plastics Division, P.O. Drawer 1494, St. Cloud, MN 56302). The adhesive was coated on 2 mil PET at a coating thickness of 1 mil. Strips of the coated adhesive were laminated to the vinyl substrate using a 2-kg roller. The lamination was placed in a 70 C oven for 1 week. The lamination was then removed from the oven and the bonds were evaluated for plasticizer migration. A qualitative scale was generated to define the effect of the plasticizer on the adhesive. A value of 1 indicates that the plasticizer had attacked the polymer of the adhesive and totally destroyed the cohesive strength of the polymer. A value of 2 indicates that the plasticizer has begun to affect the polymer system. The adhesive is softened slightly and there is some transfer. A value of 3 indicates that the adhesive is not affected by the plasticizer, and it peels cleanly away from the vinyl. Rgure 3: Rasticizer Resistance of Pelletized ~ ' s Type of Rasticizer by 2 1-!--- I I-L-2081 4 IVil II H_-2688 4 IVil II I-L-2697 4 MI I~ I-L-2807 1 MI O... Unear plastidzer ~ plastidz~" $711 pph resin @21 pph a'~ adipate ~asud~@8 pph D(~ D(~ ~~dzed @ ~~dz~ @ 30pph 50pph 97 13!-M1597 1 MI 1 - Gurrr~, transfers 2- Soft on edges, slight tr. 3- No affect, dean peet

Advantage of pellets The key to the pelletized hot melt pressure sensitive technology is the ability to provide the pelletized adhesive in a free-flowing form. Through the use of a two step coating system, it is possible to obtain a pellet that is non-blocking in nature. U.S. Patent # 5,688,449 describes a method that comprises first contacting a polymeric resin in particulate form with a thermoplastic adhesive binder to form a coating of the binder directly adhered onto the resin particles (4). Then, the method comprises contacting the binder-coated resin particles with an additive agent to form a uniform coating of the additive agent directly adhered onto the binder coating of the binder-coated resin particles. It is a feature of this invention that the resultant resin particles coated with a binder and an additive agent are typically dry and non-tacky, and thus, typically exhibit dust-free and free-flowing behavior. The results of the blocking tests show that the use of the two-step process to produce the pelletized hot melt pressure sensitive adhesives is essential in order to provide free flowing pellets. Blocking tests were run on pellets produced with a conventional onestep process and pellets produced with the two-step process described in patent # 5,688,449. Results after seven days at 120 F show the pellets produced by the conventional one-step method did not remain block free for any equivalent container height, while the pellets produced with the two-step process remain block-free at 120 F at a container height equivalent to 38". As mentioned earlier, the intended method of application for the extrusion grade hot melt ' pressure sensitive adhesives in pellet form is utilizing a single-screw extruder. This pelletized form makes it possible to easily feed the extruder and there is a potential that these pellets can be vacuum fed. This is one benefit of pelletized hot melt pressure sensitive adhesives, which can be realized through an ease of use for the customer, and a potential cost savings. UV-CURABLE HMPSAs UV-curable HMPSAs are a second technology platform being offered by H.B. Fuller Company for solvent replacement. This method of improving the physical properties of the HMPSA is to introduce a cross-linked network into the polymer system. Crosslinking the polymer yields a HMPSA with excellent high temperature shear resistance, excellent heat resistance, and excellent plasticizer resistance. Effect of Cure on UV-Curable HMPSA Bulk Properties DMA is an excellent tool to examine the effect of the curing on the bulk properties of the adhesive. Fig. 4 shows plots of modulus G' (G'/(A) [dyn/cm2]) versus temperature for HM-9001 before and after UV-curing. The curve for uncured adhesive shows a typical performance window for a thermoplastic adhesive. It shows a plateau modulus up to about 100 C (212 F). At that temperature the G' drops showing the melt region which gives good processing properties with hot melt coating equipment. Upon exposure to UV light, the adhesive undergoes cross-linking and this plateau is extended. Thus the upper service temperature of the adhesive is dramatically improved. Melt flow does not occur upon increasing temperature. 98

r FIGURE 4: DMA PROFILE FOR HM-9001 HM-9001 Cured vs Uncured 1010 109 I 10 e z~ % A /N A _ G I 107. 10 s 10 s _ 10 4,, -40.0-20.0 0.0 i i J i J = i = 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 Temp [ C] UV-HMPSA Performance Highlights Table 2 shows a comparison of three UV curable HMPSAs. The table compares basic performance attributes. All line speeds are reported for two-600 Watt lamp curing of a 1- mil thick film. TABLE 2 - HMPSA Performance Attributes Basic Performance Requirements* HM-9001 Application Temperature ( F) 325 HM-9002 325 WM-8001 225 Line Speed** (ft/min.) 500 300 500 180 Peel (15 min.) 4.0 4.4 2.5 Loop Tack (oz/in) 30 45 100 SAFT ( F) 35o+ 350+ 350+ Static Shear 1 "X1" @ 100 C 24 Hrs.+ (2 psi) "1 mil adhesive on 2 mil polyester to SS ** 2 X 600 Watt Lamps, H Bulb 99 24 Hrs.+ (2 psi) 24 Hrs.+ (1 psi)

,-. Figure 5 shows a comparison of the peel performance of the three formulations on different substrates. Two of the three adhesives tested show good adhesion to highdensity polyethylene, and they also show good adhesion to low density polyethylene. This is something that is typically a weakness for acrylic-based PSAs. Rgt~e 5: UV.Curable HIVlPS Adhesion vs,,;,. "~' 5.0 4,0!i! ii i i!i iii!!!i!! ii!!i!i!!i!i!i!i!iiiii!i!iii!! i!! ii!!ii!i! i ii!i ii!iii ii!i!i i iiiiiii ii ii!!ii e 2.O n. 0.0 I ii!i!iiill m SS U~E H~~ PP ~~d Glass PVC @ HM-9001 m HM-9002 D WM.80(X) Thick film curing & Performance UV curing of thick PSA films has traditionally posed a challenge to the adhesive formulator. It is important to design the adhesive as to assure a balanced cure-through at adequate cure-speeds. HM-9002 is the result of an experimental design aimed to meet the challenge. Table 3 summarizes PSA properties on stainless steel at 5 to 10 mils. Keeping line speeds in the range of 40 to 60 fpm, with a single 600W/inch UV unit, results in excellent balance of adhesion/cohesion for the thick-film PSA. Table 3 Thickness (mil) Speed (fpm) 1- F-600 (H bulb) Shear, 2psi (hours) Looptack (# sq. in.) Peel, (#/inch) SAFT ( F) 7.5 10 60 50 40 24+ @ 100 C 24+ @ 70oC 24+ @ 70oC 8.4 9.5 10.0 8.9 10.9 10.0 350+ 300 260 100

..,~i Plasticizer Mi.qration Resistance Utilizing a cross-linking technology enables one to overcome the plasticizer migration by implementing a chemical crosslink that will maintain its integrity even when exposed to plasticizers or excessive heat. To determine whether the UV-curable HMPSA discussed in this paper has adequate plasticizer resistance, the same test method used for the pelletized HMPSA was employed. After one week at 70 C, the UV-curable hot melt adhesives peel cleanly without leaving any residue on the plasticized PVC substrate. Another application that is usually dominated by a solventbome acrylic psa is vinyl labelstock. Conventional HMPSAs do not have adequate shelf life due to their susceptibility to plasticizers. The method used to determine whether the UV-curable has adequate shelf life is as follows: a sample of the coated substrate (vinyl- adhesive - silicone liner sandwich) is placed in a 70 C oven for one week to accelerate the migration of the plasticizer. After the sample is aged, the coated substrate is applied to a stainless steel panel and a 180 peel value is determined after a 24-hour dwell time. The value obtained is compared to a 24-hour dwell, 180 peel adhesion value of an unaged sample. A minimum of 60% peel retention is considered acceptable (reference). The values for the cross-linked technology and a conventional block copolymer system are given in Table 4. A second factor that becomes important when evaluating an adhesive for use on a vinyl substrate is the amount of shrinkage of the vinyl on the liner. Shrinkage of the vinyl becomes a factor when the coated substrate is to be printed and/or die-cut later. Measuring the amount of shrinkage of the coated vinyl facestock on the liner determines if the adhesive can tolerate the migration of the plasticizer. The manufacturer can tolerate no more than 2% shrinkage of the vinyl. The table below indicates the amount of shrinkage measured for the UV-curable technology versus conventional block copolymer technology. The amount of shrinkage was determined by aging a 3" by 3" sample of a vinyl film coated with 1.0 mil of an adhesive and aging the sample in a 70 C oven for 48 hours. The film was measured after this time and the amount of shrinkage calculated. The vinyl film tested contains 30 pph of an aliphatic phthalate plasticizer. The amount of shrinkage calculated for the UV-curable hot melt was 1.9% vs. 8% for a conventional hot melt PSA (5). i................................. TABLE 4" UV-Curable Plasticizer Tests UV-Curable technology... B!0ck copolymer technol0gy Retention of 180 peel 75% retention 40% retention _ /0 Shrin kag e...... 0. 4% 8.0% 101

CONCLUSION Two technologies have been presented as alternatives to solvent-based adhesives. Both of these technologies are 100% solids and utilize conventional hot melt coating equipment for application. The main advantage of extrusion grade hot melt pressure sensitive adhesives is that they offer high heat resistance and excellent plasticizer resistance. The availability of the free-flowing pellet allows the adhesive to be fed using a single-screw extruder. This offers the possibility of a HMPSA to be vacuum fed, offering a melt-on-demand system. Exciting new developments in the area of UV curable hot melt adhesives have also been made. Information has been given for three formulas developed to meet market needs. These products have been shown to cure at high line speeds and offer stable performance over a wide window for the coating process. All three formulas show excellent high temperature SAFT's and high shear holding power. This technology gives excellent plasticizer migration resistance and excellent adhesion to low energy surfaces, including high density and low density polyethylene. REFERENCES 1. Lipka, Bernard J., Effect of MACT on PSA Tape Manufacturing, PSTC Tech XXI Proceedings, 1998. 2. Satas, D. (Ed.), Handbook Of Pressure Sensitive Adhesive Technology, Second Edition, 1989. 3. Skeist, I. (Ed.), Handbook Of Adhesives, Third Edition, 1990. 4. Fox, S.A., U.S. Pat. 5,688,449 (November 18, 1997). 5. Lombardi R. (1988). Higher Performances Seen for Waterborne Adhesives. Paper, Film & Foil Converter, March, 74-78...' ACKNOWLEDGEMENTS Thanks go to Marga Acevedo, and Les Clapp for all their input on this paper. 102 i