Polyurethane moulding compounds

E Polyurethane Roving for thermoplastics - short-fibre moulding compounds (e.g. nylon)... 

The increasing use of polyester sheet moulding compounds, other composites and light metals in automotive panels will increase demand for polyurethanes and epoxies. 

Polyurethane adhesives currently used to bond most thermosets, including glass fibre reinforced plastics (GRP), sheet moulding compound (SMC), glass fibre reinforced epoxy (GRE), and thermoplastic composites including ABS, PVC, acrylics, polycarbonates, etc., are pre sented in Table 20. 

Hexamethylene diisocyanate is a liquid with a volatility of the same order as that of tolylene diisocyanate. It is respiratory irritant and also has powerful effects on the skin and eyes. Hexamethylene diisocyanate was one of the first diisocyanates utilized for making polyurethanes, being used to prepare fibres and moulding compounds. These applications are no longer of importance but hexamethylene diisocyanate now finds use mainly in coatings which are more light stable than those based on aromatic isocyanates. 

A method of manufacturing rubber articles by pouring a compounded latex into an absorbent hollow mould the skin of rubber thus formed is removed, dried and vulcanised. The term is also apphed to the pouring into moulds of liquid polymer systems based on silicone or polyurethane elastomers. 

A practical route to reduce the effects of photo-degradation in PUs (as in the case of other polymers) is the addition of suitable light stabilizers by different ways. The most used procedure is the additive stabilization method. A more effective method to increase the maintenance of the stabilizer in the polymer is the chemical bonding of these compounds to the macromolecular chain even in the course of polyurethane synthesis. Some attempts to introduce in such way the stabilizers in polymers are known. Thus, a benzotriazole UV absorber was mixed with a prepolymer containing isocyanate groups and the chain extender directly into moulding machine (cone. 0.5%) [64, 323]. The stabilizer can be also mixed into polyol component before its reaction with the isocyanate component. The stabilizer that usually contains OH groups could form chemical bonds with the macromolecular chain in some extent [324]. 

It is necessary for the development of suitable methods of product fabrication to have some types of polyurethane elastomer which can be processed as solids using similar methods to those employed for conventional natural and synthetic rubbers, namely two-roll mill or internal mixing and compounding procedures, calendering, extrusion, moulding, fabrication of complex rubber-textile laminates and the like. 

Materials/characteristics Physical blowing agents low-boil hydrocarbons. Chemical blowing agents fluorocarbons (for polyurethane and PS foams) sodium bicarbonate/citric acid or azo compounds, etc., for moulded structural foams. 

A trend of recent years has been to supply additives processed into safe forms, in pellets, capsules, liquids and masterbatch concentrates and so, for the compounder and processor, the hazards have largely been taken out, but it is still important that they should be aware of them. For most compounders and processors of thermoplastics, the hazard from additives can now be safely contained upstream, during manufacture of the additives. In moulding and fabricating wet polymer systems, such as polyesters and polyurethanes, however, it is essential to take precautions. 

Polyurethanes available for electronic components include Baydur CSP from Bayer which is claimed to offer high dimensional accuracy and the ability to provide complex geometries, variable wall thicknesses and excellent surface reproduction when moulded. PU and epoxy resin potting compounds are used to encapsulate electronic devices and their components to give them enhanced mechanical and electrical stability as well as providing protection from moisture, thermal or physical shock and vibration. 

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