COMPARATIVE PROPERTIES OF SILYLATED POLYURETHANE, SILICONE AND NON-SILICONE PRESSURE SENSITIVE ADHESIVES Roy M. Griswold and Robert L. Frye, Momentive Performance Materials, Wilton, CT Introduction SPUR + * pressure sensitive adhesives based on silylated polyurethane resins form the basis of a new PSA technology platform 1. Since introduction SPUR + PSAs have proven to be excellent candidates for medical laboratory labeling. Medical protocols expose label and adhesive to such solvents as xylene, aqueous alcohol, formaldehyde, antibody and staining solutions for several hours at room to 100 o C temperatures. SPUR + adhesive performance under these stingent conditions ensures accurate tracking of a patient s sample while improving laboratory processing efficiency. Since introduction, additional developmental work in health care, aerospace, automotive and electronics applications continues. The critical factor in these applications is resistance to chemicals such as jet fuels, hydraulic fluids, lubricants, and solvents such as DMSO, DOP, and DMF for periods up to several weeks. This paper summarizes SPUR + PSAs resistance to such chemicals and makes comparisons to a high performance acrylic, and silicone adhesives. Technical Approach Silylated polyurethane resin synthesis involves well established polyurethane prepolymer methodology resulting in either terminal hydroxyl or isocyanate functionality which can be further reacted with a properly selected silane. Silylated Urethane (SPUR + ) (R O) 3 Si-R-X-C-NH O NH-C-X-R-Si(OR ) 3 O In the presence of moisture, hydrolysis of the alkoxy (OR) groups occurs followed by condensation to form a stable Si-O-Si crosslinked network 2 during cure. The same type of catalysts used in the polyurethane synthesis, such as tin and amines also facilitates the condensation reaction. Designing SPUR + pressure sensitive adhesives requires proper selection of polyols, an NCO index yielding high molecular weight, choice of solvent(s) and silane concentration. Additionally the adhesive coating and curing process can influence final properties. As in the case for polyurethane synthesis selection of polyols is varied and can include polybutadiene, polyester, polyether, polyether-polyester, polyester-polyamide, and combinations thereof. Incorporating a degree of branching into the polyol blend was found to enhance the
adhesive cohesive strength, while the silane concentration influences adhesive tack, peel adhesion and chemical resistance properties 1. When incompatible polyol blends are selected the choice of solvent(s) along with modified processing variables are key in the adhesive synthesis and resulting properties. Properties can be further altered by blending with tackifier resins and conventional adhesives, as might be desired for drug delivery applications. Furthermore, it was found that blending the non-silyated polyurethane prepolymer with the silylated polyurethane adhesive gives unique adhesive properties. In a typical reaction, ethyl acetate or ethyl acetate/toluene mixture was used to dissolve and reflux dry the starting polyols at approximately 40 wt% solids concentration. The starting NCO/OH index was selected to yield a high molecular weight hydroxy terminal prepolymer. Organotin catalyst and polyisocyanate were added and reacted at approximately 75 o C until the wt% NCO was below 0.01% by standard titration method. Further reaction with isocyanatopropyltrimethoxysilane yielded the SPUR + PSAs of this paper. Typical coating and curing of SPUR + adhesives involves preparing a coating formulation by reducing the solids to 30 wt%, addition of water and a condensation catalyst with thorough mixing. Formulated baths have approximately 8-12 hour usefulness, after which the increased bath viscosity may decrease coating quality. Adhesive coatings of this paper were made using a flexbar coater head with drying/curing in a three zoned oven. Standard exit web temperature in the range of 80-135 o C with oven dwell time of 1.5 minutes is recommended. Conventional silicone release liners were used for the laminate construction. A 2 mil PET facestock and release liner were used, the dried/cured adhesive thickness being 1 mil. A post cure time of 3-7 days has been observed 1 so all coated samples were tested after one week room temperature aging. For the purposes of this paper the silylated polyurethane adhesives were prepared from two different types of polyols ( SPUR + A and SPUR + B) along with a copolymer (SPUR + C) of the two.the polyols from adhesives A and B to make adhesive C were not compatible and the previously procedure was modified in the synthesis. All adhesive coatings were clear after cure with no visible evidence for incompatibility indicative of macro domains of IPN for the compositions. The comparitive adhesives included a commercially available solvent acrylic, a methyl- and a phenyl silicone. The acrylic adhesive is recommended for high temperature and chemical resistant applications. This adhesive was coated using a standard lab drawdown procedure, air dried 10 minutes then cured at 130 o C for 5 minutes in a lab oven. All silicone adhesives were formulated using 2 wt% benzoyl peroxide then similarly coated, dried and cured at 150 o C for 5 minutes. Facestock and adhesive thicknesses were the same as for the SPUR + adhesives. Adhesive properties are summarized in Table 1 below. Peel adhesion results for the SPUR + adhesives were consistent with the comparitive adhesives on higher energy substrates while higher on lower energy substrates. Adhesive C notibly stands out among the SPUR+ adhesives for peel adhesion having a sensitivity to higher energy substrates while a very consistent peel on the plastic substrates. The differences between the copolymer and homopolymer SPUR + s is attributed to development of micro domains. Neither increased dwell times after application of tapes, nor varied temperatures after tape application altered the results indicating a lack of a
thermodynamic driver to disassociate these domains. For tack, all of the chemical resistant adhesives were lower than the less chemical resistant silicone adhesives. Table 1: Comparison of adhesive properties for 25 micron adhesive coated on PET film. SPUR + PSA A SPUR + PSA B SPUR + PSA C Solvent Acrylic Methyl Silicone Low Phenyl Silicone High Phenyl Silicone Probe Tack, 100g/cm 2,(g/cm2) 400 565 386 454 1130 910 1420 Quick Stick, PSTC-5 (g/25mm) 462 318 286 287 458 541 61 Peel Adhesion (PSTC- 101) PVC 1803 1389 996 1306 1043 944 680 PMMA 1861 1548 1009 1582 1006 991 1161 Polycarbonate 1150 1488 930 1345 1252 1102 1341 BOPP 1078 920 948 828 573 1005 635 HDPE 1291 1167 1035 856 494 1077 492 Stainless Steel 1331 1960 1281 1254 1196 1036 1196 Aluminum 1355 1140 906 1213 1071 912 1161 Glass 1256 1118 630 1391 975 1006 1754 As with most adhesives SPUR + adhesive tack and peel adhesion can be modified. Unlike conventional adhesives these can be altered by use of tackifier resins or the polyurethane prepolymer. Figure 1 illustrates this using resin tackifier(s) for SPUR + Adhesive A measuring probe tack. Similar peel adhesion and lap shear responses were observed as for tack. Table 2 illustrates the blending of the polyurethane prepolymer with a SPUR + adhesive. This technique offers repositionability without sacrificing final adhesion. Interestly it was found that this unique property cannot be achieved by the synthesis of the composite composition of the blend. Further this technique offers improved solvent resistance over use of tackified SPUR + adhesive. Lastly SPUR + adhesives can be blended with other adhesives like an acrylic or silicone adhesive as shown in Table 3. These blends offer a new class of adhesives compositions for altering permeability parameters for medical/pharmaceutical applications. Figure 1
Table 2 Table 3 SPUR+ PSA A (I) and PSA B (II), Silicone PSA (III) and Acrylic (IV) Blends Solvent Resistant Testing Solvent resistance was determined by preparing peel adhesion samples per PSTC-101 with the dwell time extended to 24 hours ensuring maximum bonding to the stainless steel panels. A set of panels were immersed into each solvent or fluid. Samples were periodically removed over a four week period and tested for peel adhesion along with observation of general appearance of the adhesive for solvent/fluid incursion around the tape perimeter and for cohesive failure. All testing was in replicated for all points. All adhesives performed well in motor oil (10W-30) and brake fluid. Only the SPUR + PSAs and the acrylic were resistant to DMSO, DOP and power steering fluid while the silicone adhesives failed. Surprisingly only SPUR + adhesive A passed DMF exposure with minimal change in peel adhesion. All others failed within one day immersion. Figures 2 and 3 display results for all adhesives immersed in diesel fuel and jet hydraulic fluid (Skydrol ). SPUR + adhesives B, C and the acrylic adhesives demonstrated excellent diesel fuel resistance with minimal perimeter incursion/adhesive swell. However adhesives A and the silicones deteriated rapidly. In the jet hydraulic fluid only SPUR + adhesives A and C offered acceptable chemical resistance. The acrylic adhesive appears acceptable in Figure 3, however perimeter incursion of the fluid resulted in adhesive being dissolved and squeezed out when samples were removed and excess fluid wiped off. Optimum chemical resistance is achieved using SPUR + adhesive C with the exception in DMF.
Figure 2: Diesel Peel Adhesion versus Immersion Time, hrs. 2500 Peel Adhesion (PSTC-101), g/in 2000 1500 1000 500 0 Adhesive B A cry lic Adhesive C Adhesive A S ilico n e s 0 7 14 21 28 Immersion Time, Days Figure 3: Jet Hydraulic Fluid Peel Adhesion versus Immersion Time, hrs. 2500 Peel Adhesion (PSTC-101), g/in. 2000 1500 1000 500 Silicones Adhesive B Adhesive C Adhesive A Acrylic 0 0 7 14 21 28 Immersion Time, Days Summary A new pressure sensitive adhesive technology platform based on silylated polyurethane resins (SPUR + PSAs) were evaluated for adhesive and chemical resistance properties. Comparisons to conventional acrylic and silicone adhesives demonstrated SPUR + PSAs offer excellent adhesive properies for applications requiring long-term exposure to chemicals. Furthermore the properties can be altered for repositionability, and permeability for such as medical/pharmaceutical, automotive and aerospace markets.
Acknowledgements The authors thank Mark Bisaillon for invaluable assistance in lab syntheses and evaluations of the materials described herein, and Momentive Performance Materials management for support in presenting this worky. * SPUR + is a trademark of Momentive Performance Materials Skydrol is a registered mark of Monsanto Company References 1. R. M. Griswold, R. P. Eckberg, 2006 PSTC Conference Proceedings 2. F. D. Osterholz, E. R. Pohl, Kinetics of the Hydrolysis and Condensation of Organofunctional Alkoxysilanes: A Review, J. Adhesion Sci. Technol., 6 (1992) 127-129.