RIM process

Dicyclopentadiene is also polymerized with tungsten-based catalysts. Because the polymerization reaction produces heavily cross-Unked resins, the polymers are manufactured in a reaction injection mol ding (RIM) process, in which all catalyst components and resin modifiers are slurried in two batches of the monomer. The first batch contains the catalyst (a mixture of WCl and WOCl, nonylphenol, acetylacetone, additives, and fillers the second batch contains the co-catalyst (a combination of an alkyl aluminum compound and a Lewis base such as ether), antioxidants, and elastomeric fillers (qv) for better moldabihty (50). Mixing two Uquids in a mold results in a rapid polymerization reaction. Its rate is controlled by the ratio between the co-catalyst and the Lewis base. Depending on the catalyst composition, solidification time of the reaction mixture can vary from two seconds to an hour. Similar catalyst systems are used for polymerization of norbomene and for norbomene copolymerization with ethyhdenenorbomene. 

Unlike polyurethane-RIM processes, nylon-RIM reactions are endothermic and require temperatures of 130-140°C. In contrast to the polyurethane-RIM systems, this enables thick wall parts to be made. Cycle times of 2-3 minutes are comparable to those for polyurethane-RIM. In the development stage, current work is concerned with reducing moulding times and optimising moulding conditions. 

Advantages of the RIM process over conventional injection moulding include ... 

The RIM process was originally developed for the car industry for the production of bumpers, front ends, rear ends, fascia panels and instrument housings. At least one mass-produced American car has RIM body panels. For many of these products, however, a number of injection moulding products are competitive, including such diverse materials as polycarbonate/PBT blends and polypropylene/EPDM blends. In the shoe industry the RIM process has been used to make soling materials from semi-flexible polyurethane foams. 

Interest in the RIM process appears to have abated somewhat in the 1990s. Nevertheless, nearly 100000 tonnes of polyol and polyisocyanate were consumed for this application in the USA alone in 1993. 

The basic RIM process is illustrated in Fig. 4.47. A range of plastics lend themselves to the type of fast polymerisation reaction which is required in this process - polyesters, epoxies, nylons and vinyl monomers. However, by far the most commonly used material is polyurethane. The components A and B are an isocyanate and a polyol and these are kept circulating in their separate systems until an injection shot is required. At this point the two reactants are brought together in the mixing head and injected into the mould. 

Since the reactants have a low viscosity, the injection pressures are relatively low in the RIM process. Thus, comparing a conventional injection moulding machine with a RIM machine having the same clamp force, the RIM machine could produce a moulding with a much greater projected area (typically about 10 times greater). Therefore the RIM process is particularly suitable for large... 

The RIM process involves the high-pressure impingement mixing of two or more reactive liquid components and injection of the mixture into a closed mold at low pressures. Large and thick products can be molded using fast cycles with relatively low-cost materials. Its low energy requirements with relatively low investment costs make RIM attractive (9). 

Figure 7.85 Schematic diagram of the reaction injection molding (RIM) process.

In this review of LCM we will try to be general when possible, but we are forced to specialize to the RTM process in many cases. In particular the special features of the S-RIM process (high-speed mixing, materials, etc.) are almost completely left out of the discussion. For readers with a special interest in this topic we recommend the book by Macosko [3]. 

In RIM processes, two or more reactive components are mixed together, starting the reaction between the components before the mixture is dispensed into the mold. This tends to increase the viscosity of the liquid that is dispensed due to an increase in molecular weight of the polymers or pre-polymers formed in the initial reaction. An increased viscosity can prohibit complete filling of the mold and permeation of the preform. This tends to decrease the adhesion between the matrix and the fibers. Poor interfacial adhesion between the reinforcement and matrix phase can cause a material to have less than desirable stiffness and strength. 

Epoxy resins are an important class of polymers used for reactive processing. However, it is rather difficult to find a formulation which provides sufficiently high process rates to be useful in the modern processing equipment used for the RIM-process. 

The so-called RIM-process (reactive injection molding) is a current realization of the reactive molding process. The heart of the process is the shock mixing of the reactive ingredients, which is achieved by collision of two jets injected at a pressure of 10 - 20 MPa. The reactive mixture is injected into the mold in a laminar flow regime the pressure at this stage does not exceed 0.1- 0.4 MPa.259 The practical development of this method relies on automatic control systems and modem high quality equipment. 

The most successful application of the RIM-process is in the production of polyurethane-based materials. Other systems, such as composites based on polycaproamide, epoxy resins, and unsaturated polyesters can also be processed by reactive injection molding. New reactive systems have also been specially created for the RIM-process260 because of the exceptional opportunities it offers for manufacture of finished articles from engineering plastics with a high modulus of elasticity and impact strength. The automotive industry, which is the main customer for RIM-articles, can utilize this technology to manufacture of massive parts such as body panels, covers, wings, bumpers and other made of newly developed plastics. 

The general requirements for compositions, that can be effectively used in the RIM process depend on the conditions of the main process operations, each of which imposes some requirements on the reagents in the reactive mix. The following list is an overview of these requirements ... 

It is clear that polymers that are synthesized in solutions, emulsions or suspensions are not appropriate for RIM technology. The use of monomers or oligomers that are polymerized above their melt temperature is also unlikely, because the a mold needs initial heating in order to initiate the reaction and then must be cooled to solidify the article. This is not convenient for industrial operations. There are some additional special requirements. For example, the formation of volatiles is generally undesirable but may be useful if the RIM-process is used for manufacturing porous articles.23... 

Some typical operating parameters for processing various polymers by the RIM method are listed in Table 4.1. Table 4.2 contains the main performance characteristics of polymers produced by the RIM process. 

The molds in a RIM-process plant are located in a separate assembly, which has devices for opening and clamping a mold and, in some cases, for changing it. The pressure in the RIM-process is considerably lower than in traditional injection molding of thermoplastics thus, the requirements for the mold material are less rigid. Molds may be made from aluminum or reinforced plastics, since high-strength steel is not necessary. 

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