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The specifics of the polyurethane long fiber injection (LFI) process and two technologies that can be used to achieve Class A surfaces were explored in detail at the recent Composites+Polycon 2009.
Usama Younes, principal scientist, polyurethanes, Bayer MaterialScience LLC, presented a technical paper, Development of Class A Polyurethane LFI Composites, during a session on Materials: Resins. According to the author, because of the entrapment of air bubbles and glass read through, it is difficult to produce high quality, Class A surfaces using the LFI process. Bayer MaterialScience scientists have, however, developed two technologies that make it possible to obtain a Class A surfaces using LFI.
The first is use of an in-mould, hybrid polyester gel coat, which acts both as the glossy surface and the barrier layer to the glass read through.
The second is use of an in-mould polyurethane paint, followed by a unique polyurethane barrier coat spray designed to resist both thermal and mechanical deformations, and finally, the addition of the long fibre polyurethane material. Use of the polyurethane barrier spray in conjunction with LFI shortens demould time and also improves the surface quality by preventing the glass from showing through the surface. Younes points out that development of the proper barrier coat material is critical, because the barrier coat itself can cause surface defects. The new class of barrier coat developed by Bayer MaterialScience has a high glass transition temperature and a low roughness value, which was determined through atomic force microscopy (AFM).
The paper also discusses LFI technology and gel time, including a test conducted using dissolved carbon dioxide as a blowing agent. Because a poor surface results when a material starts to gel prior to the mould closing during the LFI process, an extended gel time on a hot mould is essential to achieving a defect-free surface. Water is an environmentally friendly blowing agent, but it also decreases gel time, making it problematic in developing Class A surface composites. Dissolved carbon dioxide achieved both a longer gel time and, with careful control, a high-quality surface.
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