Polyurethanes reaction injection moulding

An important development of polymerisation casting is that of reaction injection moulding. Developed primarily for polyurethanes (and discussed further in Chapter 27), the process has also found some use with polyamides and with epoxide resins. 

Reaction injection moulding techniques, developed primarily for polyurethanes (see Chapter 27), have also been adapted for nylon 6 in what must be considered as a variation of the polymerisation casting technique. 

Not only are these reactions of importance in the development of the cross-linked polyurethane networks which are involved in the manufacture of most polyurethane products but many are now also being used to produce modified isocycuiates. For example, modified TDI types containing allophanate, urethane and urea groups are now being used in flexible foam manufacture. For flexible integral foams and for reaction injection moulding, modified MDIs and carbodi-imide MDI modifications cU"e employed. 

Reaction injection moulding is associated with polyurethanes and is different entirely from moulding techniques used commonly for thermoplastics in that the starting materials usually are liquids at room temperature. As the name implies, a chemical reaction takes place in the course of moulding and complex polyurethane items can be made in one step from mixtures comprising polyol, polyisocyanate, and a suitable promoter of the reaction. 

Formulations for producing polyurethanes (PUs) by reaction injection moulding (RIM) usually contain mixtures of polyols and diols in order to achieve the desired properties in the moulded part. The present work forms part (1) of a systematic investigation into the effects of polyol blends and glass fibres on the physical properties of unfilled and filled PUs formed by RIM. In the case of unfilled PUs, by using a multi-component polyol mixture, it is possible to investigate the effects on properties of (a) polyol structure, molar mass and functionality, (b) the relative proportions of diol-based hard blocks and triol-based soft blocks and (c) polyol blend compatibility. The... 

Injection moulding can be applied to two-component thermosets in a process known as Reaction Injection Moulding. The two liquid components are mixed and then injected into a closed mould. In the case of epoxies and polyurethanes, heat is produced on mixing due to the exothermic reaction between the two components (polyol and isocyanates in the case of polyurethanes), so no external source of heat is required. 

In reaction injection moulding (RIM), two monomers are injected into a mould, where polymerisation and crosslinking occur. It is used mainly with polyurethanes to make large automotive panels. The chemistry of polyurethanes was described in Chapter 4. Other systems used include a block copolymer between a crystalline polyamide (nylon 6) and a rubbery polyether (polypropylene oxide). In principle, any polymerisation reaction that can be substantially completed after about 30 s in the mould is a candidate for RIM. 

Journal of Applied Polymer Science 42,No.8,20th April 199 l,p.2169-90 ANALYSIS OF REACTION-INJECTION-MOULDED POLYURETHANES BY NEAR-INFRARED DIFFUSE REFLECTANCE SPECTROSCOPY Miller C E Eichinger B E WASHINGTON,UNIVERSITY... 

Catalysts are essential components in the liquid injection moulding (LIM) or reaction injection moulding (RIM) processes. Prepolymer systems for solid polyurethane elastomers usually do not require catalysts, and if long-term hydrolytic stability is required in the final polymer their use is undesirable. 

Protection from isocyanate vapours liberated during polyurethane manufacture is usually achieved by installing permanent exhaust ventilation units which either exhaust directly to the atmosphere or pass their exhaust fumes through scrubbers which extract the isocyanate vapour through a sodium carbonate spray tower before atmospheric exhaustion occurs. Continuous vertically positioned exhaust hoods are common where continuous conveyor lines are involved for localized extract situations, vertical down-draught or horizontal extract modes are much safer for operatives, being designed to remove all isocyanate vapour away from an operative s face and body. These latter situations apply particularly in the manufacture of cast-moulded and reaction-injection moulded products. 

The increasing use of reaction injection moulded (RIM) and similar polyurethanes in motor vehicle components has stimulated interest in bonding this polymer. It has not proved particularly easy and so far the best results are from polyurethane adhesives and, to a lesser extent, from toughened acrylics. Often, preparation is required prior to the urethane adhesive being used - regrettably with apparently physiologically active materials. 

RRIM is a development of the RIM process (reaction injection moulding). The essential feature of an RIM system is a 2-component liquid resin system which reacts rapidly when the two components mix, to form a fully developed polymer inside the mould. Usually these systems are polyurethanes, although others, especially nylon-based systems, are under development. 

RRIM polyurethanes are an extension to Reaction Injection Moulding (RIM) which is a relatively recent, but now established, plastics processing technique. Simplified, RRIM is the inclusion of a reinforcement - short glass fibres - into one of the components used in the RIM process. However, the inclusion of glass reinforcement into one of the reactive components, usually the polyol, has meant that certain processing equipment modifications were necessary in order to handle the... 

The modern polyurethane (PU) foam sole may be regarded as evolving from these processes. Both direct and indirect processes are used. The moulding process is an application of reaction injection moulding (RIM), in which the appropriate chemical reagents are metered via a mixing chamber into the mould, where the polyurethane forms. Clarks process produces microcellular foams but, for some products, separate injections of skin and foam (two-shot process) has been adopted more recently. 

A full account of basic polyurethane science will be found in PST 3. The process used is reaction injection moulding, and in the next section we shall see how the chemistry given in PST 3 is adapted to the process for the manufacture of shoe soles. 

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