Ask Dr. Dave
QUESTION: We are a custom thermoforming company that produces a large volume of vacuum-formed parts. We use a solvent-based polyurethane adhesive (solvent is MEK) to wrap vinyl around the edges of the finished parts, adhering to the backside of an ABS plastic part. The vinyl has a thin cloth back. The adhesive has a viscosity of 4000 cP, and we apply it with a brush as recommended by the manufacturer. We have a new application that we are trying to perfect and are trying to use this same adhesive. This application is also vinyl-to-ABS, but in a larger area that makes brushing undesirable. We have tried to thin the adhesive and spray it on both surfaces, but it dries out too quickly and we do not get a good bond. Would you have any suggestions on how we could make this adhesive work for us, or would you be able to recommend another type?
ANSWER: One possibility would be to apply the adhesive with a paint roller-type applicator. This should allow you to do larger areas quickly and achieve similar coating thickness as with the brush. You might also consider thinning the adhesive with a higher boiling solvent and trying the spraying again. A final alternative might be to try a high-performance acrylic latex or ethylene-vinyl chloride latex. If successful, this would also remove the flammable and toxic solvents from your operation.
QUESTION: We are a European engine manufacturer looking for a more cost-efficient way to apply gaskets for a range of applications. We are very interested in liquid RTV silicones, but the common understanding here is that they don't have enough resistance to oil. Any ideas?
ANSWER: Conventional wisdom has always suggested poor oil resistance in silicones; unfortunately, this generalization has prevented the more widespread use of these materials in engine applications. In the late 1980s, there were considerable improvements in silicone formulations that allowed both good adhesion to oily surfaces and outstanding long-term resistance to oil immersion. A few perceptive engineers saw the tremendous cost benefits to applying these gasketing materials robotically - early pioneers were the Pontiac Division of General Motors and Hyundai of Korea. Later, there was more widespread use in the U.S. auto industry. Unfortunately, these developments were never fully transferred to Europe where more conventional molded gaskets - notably acrylic (the so called ACM rubbers) - gained much more importance. Some European engineers have the mistaken impression that Americans leave huge amounts of oil in their parking lots because of the use of silicone gaskets. This is just not true; the fact is U.S. silicones were better formulated than their European counterparts. You should still be cautious when using a silicone if it can be exposed to a lot of gasoline. Normally, a fluorosilicone or fluoroelastomer is necessary for these types of applications.
Questions for publication should be directed to Dr. Dave Dunn at 242 Trails End, Aurora OH 44202; phone (330) 562-2930; fax (253) 681-8460; e-mail DrDave242@att.net ; or visit http://www.fldenterprises.com .