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60 The PCB Design Magazine • April 2017 RESINS MAINTAIN A "POOL" OF LIGHT DOWN UNDER A couple of pressing problems needed spe- cialist input from our Australasian and UK- based technical teams. Firstly, the lighting unit had to be sealed not just against freshwater, but also saltwater, with saltwater pools being par- ticularly popular in Australia. These pools are not chlorinated in the usual way and swimmers avoid the unpleasant smell of chlorine and its effects on the eyes and skin. Salt resistance was therefore the first imperative when it came to choosing an appropriate resin for this applica- tion. Apart from being able to tolerate a work- ing temperature range of 5° to 40°C, as well as being flame-retardant, the resin colour was another important issue for this project, with a light blue shade preferred for aesthetic rea - sons. Our customer had been using another company's epoxy resin on this application, but due to the highly exothermic reaction associ- ated with epoxies, some slight deformation of the base unit was occurring as a result of the high temperatures generated during resin cure, which was deemed totally unacceptable. So, our choice was narrowed down to polyurethane, but which product from our large portfolio of polyurethane resins was going to work for this customer? Polyurethane resins offer excellent water re- sistance; indeed, some formulated products are designed specifically for marine applications, such as our UR5041, which has proved very ef- fective in applications where the potted unit is likely to be immersed and continually operating in saltwater. Providing exceptional resistance to seawater, this resin has the added benefit of an exceptionally wide operating temperature range (-60°C to 125°C), so it was certainly one to con- sider for the Australian job. Then there's UR5083, one of our particular- ly high-performing resin systems that has the unique ability to "self-heal" if penetrated. This is ideal for applications involving the sealing of submerged units or underwater cabling and wir- ing where connectors or components need to be passed through the resin after application. The resin maintains contact with the potted unit, while sliding off a wire or connector as it is removed and closing up behind it to provide a moisture barrier. Another contender was our UR5528 poly- urethane resin, which features excellent chemi- cal and water resistance. Our customers have used this on many occasions to protect marine electronics, or other applications where mois- ture ingress is a potential issue. We finally decided to offer the customer our UR5097 encapsulation and potting compound. The cured polyurethane has great thermal con- ductivity, an important property as far as LED lighting units are concerned, and it has a wide temperature range. It is also flame retardant to UL94, which was another of our customer's re- quirements. As was the case with all the other polyurethane resins we considered, the ex- tremely low water absorption rate of polyure- thane was considered the most critical property for this application. Indeed, UR5097 met all the requirements of this project, with the exception of the colour, that is. Apart from UR5083, which is a light straw colour, our other polyurethane resin contenders for this project were all black as standard, so we had some reformulation to do in order to meet the customer's requirement for a light blue. The colour of a batch of the chosen UR5097 resin was duly altered to the desired shade, and we made sure it was right prior to shipping by matching it against an RAL standard. With the first production batch of material manufactured to the customer's specification and shipped to Australia, we were confident of a good result. However, during a courtesy visit to the customer by our Australia and New Zealand Manager Mike Woods, accompanied by yours truly, we discovered that there had been a cou- ple of issues with the material. " Salt resistance was therefore the first imperative when it came to choosing an appropriate resin for this application. "

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