用于低表面能材料的UV胶粘剂.doc

RadTech Asia 2001, Kunming, ChinaUV Curable silicone adhesive for difficult to bond materialsHAO Jianqiang and HARA Osamu AbstractA UV curable silicone composition was developed which was comprised of an addition reaction product of amino-terminated organopolysiloxane and a vinyl group-containing isocyanate, an acrylic ester monomer and a photoinitiator. The composition can be cured by UV to tough elastomers and showed excellent adhesion to difficult to bond materials such as PE, PP, PET, polyolefin, etc. without need of any pretreatment of the surfaces. The compositions were transparent and found useful as elastomeric adhesives, potting and sealing agents.Ultraviolet curing (UV), Difficult to bond, Polypropylene (PP), Polyethylene (PE), Poly(ethylene terephthalate) (PET), Silicone, Acrylic ester, Sealing, Potting, Adhesive, Adhesion1. INTRODUCTIONPolyethylene (PE), polypropylene (PP), and poly (ethylene terephthalate) (PET) are most widely used plastics in modern society and received much more attention today due to their harmony with the environment and ease of recycle. The unit assembly or adhesion of these plastics such as laminating are often encountered, however, it is known in the art they are difficult to bond with each other by adhesives because of their low surface energy, low polarity, and little or no porosity. To improve the adhesion of these materials, surface treatments, such as plasma treatment, corona discharge, chemical oxidation, and primer coating have been used to alter the surface. But these treatments are expensive and time consuming, and are not always possible. In applications concerned with these difficult to bond materials, hot melt adhesive, cyanoacrylate with primer system, two-component epoxy resin, and two-component acrylate adhesive have been developed. But these systems are still not satisfactory to meet the needs of high speed line requirement. Primer treatment or premixing handling for two-component systems is also negative factors in practical applications. UV curable resins are one-component and can be cured in seconds make it a good choice preferable in high speed line production. However, there are few reports of UV curing type resins focusing on these difficult to bond materials. Here, we concentrate the study on UV curable silicone to search for the possibility of applications into difficult to bond materials. UV curable silicones are well known in the art, some of them are commercialized as surface-release coating, potting and sealing agents (1-6). They have found wide applications in industry mainly attributed to their excellent low and high temperature properties, biological inertness, and also electrical properties. In spite of the excellent properties, they are not suitable for adhesive applications due to their weak adhesion strength. Room temperature vulcanization (RTV) silicone is often reinforced by fumed silica to achieve desirable rubber strength, but the content of fumed silica is usually constrained to below 20 wt% due to dramatic build-up of viscosity, thus constraining the further improvement of rubber strength. In the case of UV curable silicone, the addition of large amount of fumed silica leads to a reduction of UV light transmittance and therefore decreases the depth-through curability. Moreover, fumed silica is only prominent in increasing the rubber strength, it is not effective to improve the adhesion to nonpolar plastics such as PE, PP, and PET. To improve the tensile property of UV curable silicone, combination of UV curable acrylic ester monomers with silicone has been proposed and proven an efficient and versatile alternative technique (2, 5). In this paper, a new composition is developed which is comprised of an addition reaction product of an organopolysiloxane having amino groups and/or at least one hydrolyzable group at both ends of its molecule and a vinyl group-containing isocyanate, an acrylic ester monomer and a photoinitiator. The composition is used to study the adhesion properties to those difficult to bond materials such as PE, PP, PET, poly (cyclo-olefin), etc. Figure 1. Preparation chemistry of vinyl-terminated polydimethylsiloxane (VTPDMS).2. EXPERIMENTAL2.1 Preparation of polydimethylsiloxane oligomerSilanol-terminated polydimethylsiloxane (viscosity of 1000 centistokes, molecular weight of 26000) 1 mole was reacted with 2.2 mole R1-substituted aminopropyltrimethoxysilane (R1 refers to hydrogen, methyl, ethyl, propyl, butyl and/or phenyl) under bismuth octoate condensation catalyst (0.05 wt %) at room temperature (RT). The reaction was carried out in vacuum for 5 hours to remove the by-product of methanol. Then stoichiometric 2-isocyanatoethyl methacrylate was added and the addition reaction was allowed to take place with stirring in a nitrogen atmosphere at room temperature for an additional 30 minutes, affording polydimethylsiloxane oligomer having vinyl end groups and hyrolyzable methoxy groups attached to silicon atoms, here refers to VTPDMS oligomer. The preparation scheme is shown in Figure 1.2.2 FormulationsThe VTPDMS oligomer obtained above was formulated with isobornyl acrylate monomer (IBX-A), a photoinitiator, dibutyltin dilaurate (DBTDL) moisture curing catalyst, methyltrimethoxysilane dehydrating agent and a radical polymerization inhibitor by stirring under nitrogen atmosphere. Formulations are shown in Table 1.2.3 Performance testingTensile properties were evaluated using standard JIS K 6251 (Japan Industrial Standard) specifications. Tensile strength, percent (%) elongation, hardness (Shore A) data were obtained on dumbbell (No. 3) samples cured according to the following schedule: UV radiation of cumulative dose of 6000 mJ/cm2 (1000 mJ/cm2 for 6 times) and with or without moisture cure for 7 days at 23 0C and 50% relative humidity (RH). Moisture cure is always at the same conditions at 23 0C and 50% (RH) for 7 days unless indicated otherwise. Tear strength and peel strength testing were conducted in accordance with JIS K 6850 and 6854 specifications, respectively, and samples were cured under UV radiation dose of 6000 mJ/cm2 and moisture. The substrate coupons were washed with ethanol and allowed to dry prior to preparation of the test specimen. All the substrate coupons were UV transparent or semi-transparent unless indicated otherwise. Poly (cyclo-olefin) (abbreviated to PCO) coupons are supplied by ZEON Corporation, Japan, under trademark of Zeonex 480.3. RESULTS AND DISCUSSION3.1 Surface curabilityVTPDMS oligomer derived from silanol-terminated polydimethylsiloxane with viscosity of 1000 cSt was used in all formulations of UV curable silicones, and the viscosity of VTPDMS oligomer thus obtained was 6100 mPas. Addition of IBX-A monomer dramatically decreased the viscosities of the formulations as is shown in Table 1, this makes it suitable for coating and potting applications.It is known that UV curable silicones have poor surface curability. The surface remains tacky after UV cure because of the strong oxygen inhibition effect attributed to high oxygen permeability of silicone. The surface curability can be improved by increasing the concentration of initiator and increasing light intensity, or by adding some readily oxidizable compounds such as thiols or amines as reported (6). Here it was found that the addition of moisture curing catalyst such as DBTDL also improved surface curability. The surface became tack free upon 800 mJ/cm2 cumulative UV exposure by addition of 0.35wt% DBTDL into the formulations, while 2400 mJ/cm2 cumulative dose was necessary to attain the same level of surface cure without DBTDL. 3.2 Dumbbell properties with UV and moisture curesVTPDMS oligomer has hydrolyzable groups of methoxy at the ends of the molecule and can undergo moisture crosslinking followed by UV cure. Silicones which have UV and moisture dual-cure mechanisms have been found useful in coating or sealing applications in which shaded parts can also be cured by a secondary cure mechanism of moisture (1-4). The formulations listed in Table 1 are curable both to UV and moisture, however, moisture cure of these compositions only leads to soft gelation except for F-1, and do not produce practical adhesion strength because there are large amount of unreacted acrylic ester monomers remain and act as plasticizer. In these formulations, moisture cure behaves as a compensate crosslinking to improve the final strength of elastomer after UV cure. Table 2 shows the dumbbell properties of silicone formulations cured by UV and with or without moisture. It revealed that the additional moisture cure increased tensile strength by 38% - 72% compared to those cured only by UV. As is expected, the additional moisture cure also increased hardness and decreased elongation at break due to an increase in crosslinking density. Moisture curing catalyst like organic tin or organic titan is necessary to accelerate the cure speed in which DBTDL be the most widely accepted one. It is valuable to compare the effect of DBTDL on moisture cure in term of tensile strength. F-3 containing 0.35wt% DBTDL was compared with F-7 containing no DBTDL. Tensile strength of F-3 increased by 65% upon additional moisture cure, while that of F-7 only increased slightly. It suggested that the additional moisture cure using DBTDL as a catalyst is very effective in improving elastomer strength without much sacrifice of elongation. All of the formulations showed good adhesion strength to poly (cyclo-olefin), especially those reinforced by IBX-A. The shear strength to poly (cyclo-olefin) is larger for F-3 with DBTDL than for F-7 without DBTDL indicated that the secondary moisture cure can not only improve the tensile strength, but also improve the adhesion strength.Table 1. Formulations of UV curable silicones.Components by weightF-1F-2F-3F-4F-5F-6F-7VPDMS oligomer 100100100100100100100IBX-A02040608010040Photoinitiator5.464.2Polymerization inhibitor0.1DBTDL0Methyltrimethoxysilane0.5Viscosity (mPas)6000212011407405203801140Table 2. Dumbbell properties of UV curable silicone formulations.CureF-1F-2F-3F-4F-5F-6F-7Hardness (Shore A)UVA23A34A45A70A86A91A45UV+moistureA31A43A64A85A93A97A50Tensile Strength at break (MPa)UV0.4171.003.323.10UV+moisture0.5761.505.497.089.3110.43.51Elongation at break (%)UV88156223227247255254UV+moisture80120175178203220245Shear strength (MPa)Poly(cyclo-olefin)1.935.785.884.02Failure modeCFCFsubstratesubstratesubstratesubstrateCFCF: cohesive failure.3.3 Reinforcement of UV curable silicone by acrylic ester monomerAlthough it is known that UV curable acrylic ester monomers can reinforce silicone elastomer, only limited monomers are found compatible with silicone because of its low cohesive energy. It is expected that incorporation of polar urethane or urea groups into siloxane oligomer backbone may improve miscibility of the silicone with various reactive diluents (5), however, most of such compatible reactive diluents are found to be not readily UV curable. IBX-A belongs to be one of few monomers both compatible to silicone and readily UV curable. Dumbbell properties of UV curable silicones showed remarkable reinforcement by addition of IBX-A (Table 2). Compared with F-1 which contains no acrylic ester monomer, the addition of 20 to 100% phr (parts per hundred) IBX-A into VTPDMS greatly improved the tensile strength and simultaneously increased the elongation at break. Appropriate content of IBX-A (20 - 60% phr) is important to obtain an elastomeric adhesive which is favorable in practice for adhering those of dissimilar substrates, the crosslinked silicone would become hard and glassy beyond this content range. Figure 2. Effect of UV exposure dose on the peel strengths of PET (0.5 mm thick sheet).3.4 Dependence of adhesion strength on UV exposure doseEffect of UV exposure dose on peel strength was studied by using 0.5 mm thick PET sheet. The adhesive layer was controlled at thickness of 0.13 - 0.15 mm. The specimen were exposed to various doses of UV and followed by moisture cure. All of the specimen were found to be cohesive failure indicated the strong adhesion of this adhesive to PET. As shown in Figure 2, the peel strength takes off at around 1000 mJ/cm2 and levels off at around 3000 mJ/cm2. Further UV exposure can only slightly increase the peel strength implying the UV cure almost is completed at about 3000 mJ/cm2 exposure for this system. Post moisture cure increased peel strength by about 14 - 214% depending on the UV exposure dose, in which dramatic increase of peel strength was found in low UV exposure.3.5 Adhesion strengths to difficult to bond materialsShear strengths to various adherends cured by UV and moisture are shown in Table 3 using F-3 formulation as a model. Since PE is not UV transparent, acrylic resin and PCO were used as the opposite coupons to be exposed to UV irradiation. Shear strengths listed in Table 3 indicates that the formulation has excellent adhesion to PE, PP, PET, PCO, PC, and acrylic resin, in which the failure modes are cohesive or substrate. The shear strength was in high level concerned with the elastomeric property of this UV curable silicone. The high adhesion strength to such difficult to bond materials is still not totally understood, it is suggested that insertion of urea groups into silicone backbone and addition of IBX-A monomer may play an important role in improving the surface adhesion between silicone and adherends. The UV curable silicones showed unexpectedly high adhesion strength not only to low surface energy materials but also to polar materials such as PC and acrylic resins, which can not be explained by simple solubility principle. Chemistry or polarity of silicone formulation at the surface adhered to the substrates may rearrange to response to the surface properties of various adherends, as that has been found in dimethoxysilyl-terminated polypropylene oxide/epoxy resin system (7). Otherwise, photoinitiator also played a role in improvement of adhesion to low surface energy. Good adhesion to PE, PP