Urethane processing reaction injection molding

Recent advances in poly (urethane) reinforced reaction injection molded (RRIM) parts has led to use in components such as fenders for small trucks, low-density interior door panels, and higher heat-resistant body panels. Rear fenders on the Chevrolet Silverado and the GMC Sierra trucks (Fig. 2), made of RRIM polyurea, are the first application of a RRIM polymer designed to withstand electrocoat, primer, and topcoat paint line process temperatures. 

The late 1950s saw the emergence of cast elastomers, which led to the development of reaction injection molding (RIM) at Bayer AG in Leverkusen, Germany, in 1964 (see Plastics processing). Also, thermoplastic polyurethane elastomers (TPUs) and Spandex fibers (see Fibers, elastomeric) were introduced during this time. In addition, urethane-based synthetic leather (see LEATHER-LIKE MATERIALS) was introduced by Du Pont under the trade name Corfam in 1963. 

Thermoset polyurethanes are cross-linked polymers, which are produced by casting or reaction injection molding (RIM). For cast elastomers, TDI in combination with 3,3,-dichloro-4,4,-diphen5lmethanediamine (MOCA) are often used. In the RIM technology, aromatic diamine chain extenders, such as diethyltoluenediamine (DETDA), are used to produce poly(urethane ureas) (47), and replacement of the polyether polyols with amine-terminated polyols produces polyureas (48). The aromatic diamines are soluble in the polyol and provide fast reaction rates. In 1985, internal mold release agents based on zinc stearate compatibilized with primary amines were introduced to the RIM process to minimize mold preparation and scrap from parts tom at demold. Some physical properties of RIM systems are listed in Table 7. 

Reaction-injection-molded (RIM) urethane foams are using various tin/amine catalysts, with some special variations developed expressly for this processing technique. Amine/amine and tin/tin combinations are also under development (22). 

Competitive penetration of polypropylene into other applications has primarily taken place in polyethylene, polystyrene, polyvinyl chloride (PVC), thermoplastic polyester, nylon-6 or -6/6, and sometimes directly from metals or thermoset polymers like phenolic or reinforced reaction injection molded (RIM) urethane. The reasons for market penetration by PP replacement vary widely with an assortment of material design options chemical resistance, heat resistance, recycleability, processability, economics, and aesthetics. 

As mentioned previously, the reaction injection-molding process combines two precisely metered and well-mixed reactive streams. An example is the process that reacts catalyzed highly reactive streams of urethane components. One stream contains a polyether backbone, a catalyst, and a cross-linking agent. The other stream has an isocyanate. In addition, a blowing agent is included in one of the streams. 

In fact, RIM was the first plastic to be approved for bumper fascia in North America to meet the low-temperature crashworthiness demanded by some OEMs. Unfortunately, due to process complexity, RIM could not be fabricated at a high enough rate to meet the demands of large car platforms. Faster cycle time for part production is delimited by the speed of curing chemistry and the ability of the resin to flow into molds. Furthermore, the automotive industry wanted to move away from the use of thermoset polymer systems that cannot be remelted. This attribute of thermosets was labeled by the industry as non-recyclable, and the industry made room for other thermoplastics such as TPO that can be remelted. Reaction-injection-molded parts are made from urethane epoxies, polyesters, and polyamides. A study of cost versus performance reported that RIM gives the best cost-performance characteristic for composite materials and is competitive with steel. ... 

The polysiloxane network is formed during part fabrication. In injection molding, the acetylenic alcohol, which acts as a fugitive inhibitor of the vinyl-addition reaction, is volatilized at low temperature as the pellets enter the feed throat. The platinum complex is activated at the process temperature of the urethane (170-185 °C). The vinyl-addition reaction is initiated by the melt state, and the parts generated demonstrate mechanical properties consistent with the formation of a silicone IPN. The fabricated parts are translucent. The physical properties of this formulation (PTUE 205) are given in Table 1. 

Many of the polymers such as caprolactam, silicone, urethane, and polyester are multicomponent reactive liquids that may be molded by the reaction injection process which is particularly suitable for large area pieces such as auto body parts, furniture, and housing sections, because of the fast cycle and low mold clamp pressure requirement. This system greatly diminishes the capital investment requirement for such large parts. A 5000-ton conventional injection press is presently the largest and it costs 1,000,000. An automobile fender would need a 12,000 ton press costing 2,000,000 by the conventional injection process with reaction injection equipment cost might be one tenth of the machine cost for the older process. 

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