Moulding Polyurethane

RECYCLING OF INJECTION-MOULDED POLYURETHANE SCRAP IN THE PRODUCTION OF SATURATED POLYESTER RESINS... 

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... 

Cannon has developed complete systems for the manufacture of moulded polyurethane automotive seating elements made with varying combinations of several raw materials. The resultant cushions - although produced in a random sequence on the same moulding line - are characterised by mechanical properties tailored to the specific application of each part [1]. 

Glass flakes are used in resin-based coatings, to reduce permeability to moisture, vapours, and solvents they have also been used in reaction moulded polyurethanes to improve surface finish. 

HIGH RESILIENCE MOULDED POLYURETHANE FOAM (50g/L DENSITY)... 

Fig. 5. Dynamic compression characteristics of cored rubber latex, conventional moulded polyurethane and high resilience moulded polyurethane foam.

The most commonly used group of plastics in this class are polyurethanes which are resistant to most solvents, with the exception of n-methyl pyrollidone which swells the substrate but improves adhesion. RIM (reaction injection moulded) polyurethane finds use in the automotive sector for bumpers, spoilers and external trims. Polyurethanes are affected by heat because of their cellular structure, often craters or pimples are formed in the coating as a result of the substrate s porosity. Hence components are often manually prepared with primer filler, pore filler and several de-nib stages prior to the application of a topcoat. In recent years this high labour requirement has resulted in the replacement of polyurethane by alternative plastics. 

Ideally the flexibility of the substrate and polymer should be matched e.g. RIM (reaction injection moulded) polyurethane requires a more flexible system than ABS (acrylonitrile-butadiene-styrene). 

This includes wire enamels on a base of polyvinyl formal, polyurethane or epoxy resins as well as moulding powder plastics on phenol-formaldehyde and similar binders, with cellulose fillers, laminated plastics on paper and cotton cloth base, triacetate cellulose films, films and fibres of polyethylene terephthalate. 

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. 

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. 

There is persisting interest in nylon-RIM materials as alternatives to polyurethane-RIM. Advantages of the nylon materials are the better shelf life and lower viscosity of the reaction components, ability to mould thick-walled articles, absence of a need for mould lubrication and the ability to avoid using isocyanates with their associated hazards. The main disadvantages of nylon-RIM are the need to have heated storage tanks and elevated temperature reactions, difficulties in catalyst handling and the high water absorption of the product. Possible markets include exterior car body components and appliance and business machine components. 

For some years there has been concern at the amount of styrene vapour in workshops preparing reinforced polyester laminates. More recently this has increased interest in polyester-polyurethane hybrids and in the further development of closed moulding and resin transfer moulding techniques as well as greater use of lower styrene levels. 

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. 

The properties of the polyurethane moulding compositions are also very similar to nylon 66. The greatest difference in properties is in water absorption, the 6,4-polyurethane absorbing only about of that of nylon 66 under comparable conditions. This results in better dimensional stability and a good retention of electrical insulation properties in conditions of high humidity. Resistance to sulphuric acid is somewhat bettter than with nylon 66 but both types of polymer are dissolved by phenols and formic acid. 

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. 

At present there are five types of thermoplastic rubber (TPR). Three of these, the polyurethane, the styrenic and the polyester are termed segmented block copolymers in that they consist of thermoplastic molecules grafted to the rubbery molecules. At room temperature it is the thermoplastic molecules which clump together to anchor the rubbery molecules. When heat is applied the thermoplastic molecules are capable of movement so that the material may be shaped using conventional thermoplastic moulding equipment. 

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. 

Rosato, D.V. ed. Rosato, D.V. (ed.) Blow Moulding Handbook, Hanser, Munich (1989). Hepburn, C. Polyurethane Elastomers (ch 6-RIM) Applied Science Publishers, London (1982). Martin, L, Pultrusion Ch 3 in Plastics Product Design Handbook - B ed by E. Miller, Dekker Inc, New York (1983). 

Pipe fittings such as bends, tees and crosses, unless large enough to enable spray application of the lining, are usually lined by casting using internal mould formers. The pipe fitting is assembled with the internal mould and preheated, the polyurethane mixture injected and the whole is oven cured. 

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