Bayer MaterialScience scientists have developed two technologies to obtain a Class A surfaces using long fiber injection (LFI) composites, which till now, was a major challenge. The specifics of the LFI process and two technologies that can be used to achieve Class A surfaces were explored in detail at the recent COMPOSITES+POLYCON 2009 (Jan. 15 - 17, 2009) - the annual conference, sponsored by the American Composites Manufacturers Association, took place in Tampa, Fla. 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.
Out of the two technologies for the purpose, the first one is to use of an inmold, hybrid polyester gel coat, which acts both as the glossy surface and the barrier layer to the glass read through. The second one is the use of an inmold 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 fiber polyurethane material. As per the company release, use of the polyurethane barrier spray in conjunction with LFI shortens demold 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 discussed 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 mold closing during the LFI process, an extended gel time on a hot mold 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.