U-F moulding materials

Thermosetting compositions based on urea-formaldehyde are widely employed because of their low cost, wide colour range, rigidity and good electrical properties. 

A moulding powder based on urea-formaldehyde will contain a number of ingredients. Those most commonly employed include the following  

In order to obtain a sufficient rate of cure at moulding temperatures it is usual to add about 0.2-2.0% of a hardener (accelerator). This functions by decomposing at moulding temperatures to give an acidic body that will accelerate the cure rate. A very large number of such latent acid catalysts have been described in the literature, of which some of the more prominent are ammonium sulphamate, ammonium phenoxyacetate, ethylene sulphite and trimethyl phosphate. 

Urea-formaldehyde powders have a limited shelf-life but some improvement is made by incorporating a stabiliser such as hexamine into the moulding power. In some formulations the cure rate and the related time for flow are controlled by keeping the latent acid catalyst fixed and adjusting the stabiliser. 

Metal stearates such as zinc, magnesium or aluminium stearates are commonly used as lubricants at about 1% concentration. Other materials that have been used successfully include oxidised paraffin wax and sulphonated castor oil. 

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U-F moulding materials are relatively much less important than they were 30 years ago. Western European production in 1991 was approximately 70000 tonnes, slightly greater than P-F moulding material production. U-F resin used for this application is, however, probably less than 5% of total U-F resin production. 

Conventional U-F moulding powders originally formulated for compression moulding are often satisfactory but special injection grades have been developed. Ideally the moulding material should have good flow characteristics between 70... 

When they were first introduced, the value of U-F moulding powders lay in their availability in a wide range of colours, at that time a novelty amongst thermosetting moulding composition. The wide colour range possible continues to be a reason for the widespread use of the material but other useful features have also become manifest. 

As with the U-F moulding powders the relative importance of M-F moulding powders for other plastics materials and also in other uses for melamine-formaldehyde resins has declined. 

Whilst the injection moulding process has now been widely accepted for phenolics the transition from compression moulding has been less extensive with U-F materials. The basic reason for this is that the U-F materials are more difficult to mould. This has been associated with filler orientation during moulding, which can lead to stress peaks in the finished product which the somewhat brittle resin in less able to withstand than can a phenolic resin. 

Currently, one of the most important commercially available materials today are the urea-formaldehyde (U-F) resins. Their applications include coatings, adhesives, castings, moulding compounds and textiles. Maciel et al. have produced a series of extensive papers in this area concentrating on both and CP/MAS [13-16]. 

Breitenfellner, F. Kainmuelle, T. (Ciba Geigy Corp.). Flame-retarding, reinforced moulding material based on thermoplastic polyesters and the use thereof. U.S. Patent 4546126, Oct. 8, 1985. 

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