Reaction Injection Molding Elastomers
Reaction injection molding (commonly referred to as "RIM") is a processing technique for forming relatively large urethane elastomer structural panels. To provide adequate structural rigidity, it is common to incorporate glass or mineral fiber fillers into the polymer structure [63, 64]. As with all composites, process innovation has created numerous hyphenated forms of RIM. The basic process is superficially similar to that of some cast elastomer processes and thermoplastic injection molding. The components of the urethane matrix are energetically mixed and injected or poured into a mold. In many cases, the mold is prearranged with a glass fiber mat that provides composite reinforcement. Other embodiments may have the glass or other filler applied to the casting polymer solution and the mold subsequently closed. The details of the injection process and the composite phase employed are coded into the prefix used to identify the RDVI species. For instance,
• Reinforced RIM (R-RIM)—This is the process in which short mineral fibers are dispersed into the polyol phase and mixed with the isocyanate phase prior to injecting the reacting mixture into a closed mold.
• Structural-RDVI  (S-RIM)—This is the process in which prior to injection of the reacting mixture, the mold is prelaid with a woven glass mat or prepreg. Usually, a closed mold but not required. This technique may also be termed LD-RIM for low-density RIM.
FIGURE 9.20 Volume production of polyurethane building blocks for RIM applications.
• LFI-PUR —This is the process in which chopped long glass fiber is randomly applied to a mold simultaneously with the reacting polyurethane resin and the mold either left open or closed.
A pultrusion operation is yet another way of making a PU composite . In a pultrusion operation, a fiberglass roving, yarn, or mat is drawn through a molten polymer or reactive system, and cured by continuous draw through an oven. The result is a material of constant cross-section. While often anisotropic in properties, unidirectionality can be offset by the use of off-axis fiber alignments.
Historically, the primary use of RDVI PU products has been for automotive structures such as bumpers, vertical structural panels, and under-the-hood applications that are not in direct contact with the engine. However, the increasing use of formulated thermoplastic olefin elastomers (referred to as TPOs) has seen the utilization of RDVI decrease dramatically (Fig. 9.20). The drop in RDVI consumption reflects a significant drop in automotive production at that time. In the 1990s, there was a similar fall-off in RDM-based PU consumption due to the rapid adoption of the less-expensive, lighter, and less complex TPOs in the automotive industry. Other uses for RIM have remained approximately steady in terms of volumes including continued use in some automotive platforms, tractor panels, snowmobile panels, and furniture .
In terms of a competitive landscape for material solutions, PU composites can be compared with technologies employing epoxy matrices in resin transfer operations, or sheet molding compound (SMC) usually employing an unsaturated polyester resin. A qualitative comparison of properties obtained by these techniques is provided in Table 9.13 .
The processing conditions of a RIM part are as much a part of formulation conditions as final composite properties. The ability to rapidly and reliably produce high-quality parts at competitive costs will be implied in the choice of each component and how the overall system is employed (Table 9.14).
TABLE 9.13 Comparison of properties obtained for various plastic composites
TABLE 9.14 A modified S-RIM formulation that could be scaled for making an automotive door panel
FIGURE 9.21 Geographical segmentation of RIM production in 2012.
The trends in RTM PUs are those mostly expressing the interests of manufacturers of RIM components rather than RDVI consumers (Fig. 9.21). From 2008 through 2013, approximately 35 patents on RDVI technology were filed. The subject matter of these patents varied widely but was primarily on the application of RDVI to specific automotive parts. Although an average of about six patent filings a year is indicative of the general lack of commercial excitement in RDVI technology, the most prevalent focus was on the application of RIM to automotive run-flat tires . In nonautomotive applications, there were several patents filed on the application of RDVI methods to making windmill components . In terms of RIM methods, there is activity protecting methods of fabrication using chopped long fiber to make Class A finished body panels, and techniques to improve PU wetting of glass mat preforms in S-RIM operations . In addition, there was a patent on the use of seed oil-derived polyols in RDVI formulations .