Moulding compositions

Injection moulding - Elastomers Compression moulding - Composites Hand/spray lay-up - Composites... 

Because polycarbonates are good light absorbers, ultraviolet degradation does not occur beyond a depth of 0.030-0.050 in (0.075-0.125 cm). Whilst this is often not serious with moulded and extruded parts, film may become extremely brittle. Improvements in the resistance of cast film may be made by addition of an ultraviolet absorber but common absorbers cannot be used in moulding compositions because they do not withstand the high processing temperatures. 

Between 10 and 15 parts of hexa are used in typical moulding compositions. The mechanism by which it cross-links novolak resins is not fully understood but it appears capable of supplying the requisite methylene bridges required for cross-linking. It also functions as a promoter for the hardening reaction. 

Another commonly employed low-cost organic filler is coconut shell flour. This can be incorporated into the moulding composition in large quantities and this results in cheaper mixes than when woodflour is used. The mouldings also have a good finish. However, coconut shell flour-filled mouldings have poor mechanical properties and hence the filler is generally used in conjunction with woodflour. 

In some countries the extensive use of asbestos as a filler is somewhat discouraged because of the hazards associated with its use. In other parts of the world moulding compositions of enhanced heat resistance have been developed by the use of especially heat-resisting polymers used in conjunction with asbestos and other mineral fillers. 

In order that the rate of cure of phenolic moulding compositions is sufficiently rapid to be economically attractive, curing is carried out at a temperature which leads to the formation of quinone methides and their derivatives which impart a dark colour to the resin. Thus the range of pigments available is limited to blacks, browns and relatively dark blues, greens, reds and oranges. 

In some moulding compositions other special purpose ingredients may be incorporated. For example, naphthalene, furfural and dibutyl phthalate are occasionally used as plasticisers or more strictly as flow promoters. They are particularly useful where powders with a low moulding shrinkage are required. In such formulations a highly condensed resin is used so that there will be less reaction, and hence less shrinkage, during cure. The plasticiser is incorporated to... 

Injection moulding compositions have a number of requirements with regard to granule flow and cure characteristics not always met by conventional formulations. For example, granules should be free-flowing (i.e. of a narrow particle size distribution and not too irregular in shape). There are also certain requirements in terms of viscosity. 

Today the phenol-formaldehyde moulding compositions do not have the eminent position they held until about 1950. In some, important applications they have been replaced by other materials, thermosetting and thermoplastic, whilst they have in the past two decades found use in few new outlets. However, the general increase in standards of living for much of this period has increased the sales of many products which use phenolics and consequently the overall use of phenol-formaldehyde moulding powders has been well maintained. 

As an alternative to the wet process described above, moulding compositions may be made by mixing a powdered resin or a methylol derivative with other ingredients on a two-roll mill or in an internal mixer. The condensation reaction proceeds during this process and when deemed sufficiently advanced, the composition is sheeted off and disintegrated to the desired particle size. This dry process is not known to be used in any current commercial operation. 

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. 

Table 24.1 Properties of mouldings prepared from urea-formaldehyde and melamine-formaldehyde mouldings compositions (Testing according to BS 2782)...

Melamine (I,3,5-triamino-2,4,6-triazine) was first prepared by Liebig in 1835. For a hundred years the material remained no more than a laboratory curiosity until Henkel patented the production of resins by condensation with formaldehyde. Today large quantities of melamine-formaldehyde resins are used in the manufacture of moulding compositions, laminates, adhesives, surface coatings and other applications. Although in many respects superior in properties to the urea-based resins they are also significantly more expensive. 

The principal application of melamine-formaldehyde moulding compositions is for the manufacture of tableware, largely because of their wide colour range, surface hardness and stain resistance. The stain resistance does, however, leave something to be desired and one aim of current research is to discover alternative materials superior in this respect. Cellulose-filled compositions also find a small outlet for trays, clock cases and radio cabinets and other purposes. The mineral-filled powders are used in electrical applications and knobs and handles for kitchen utensils. 

To produce a moulding composition, aniline is first treated with hydrochloric acid to produce water-soluble aniline hydrochloride. The aniline hydrochloride solution is then run into a large wooden vat and formaldehyde solution is run in at a slow but uniform rate, the whole mix being subject to continuous agitation. Reaction occurs immediately to give a deep orange-red product. The resin is still a water-soluble material and so it is fed into a 10% caustic soda solution to react with the hydrochloride, thus releasing the resin as a creamy yellow slurry. The slurry is washed with a counter-current of fresh water, dried and ball-milled. 

Polyesters are eneountered in many forms. They are important as laminating resins, moulding compositions, fibres, films, surface coating resins, rubbers and plasticisers. The common factor in these widely different materials is that they all contain a number of ester linkages in the main chain. (There are also a number of polymers such as poly(vinyl acetate) which contain a number of ester groups in side chains but these are not generally considered within the term polyester resins.)... 

Although phenolic and amino moulding powders remain by far the most important of the thermosetting moulding compositions a number of new materials have been introduced" over the last 30 years based on polyester, epoxide and silicone resins. 

The polyester alkyd moulding compositions are also based on a resin similar to those used for laminating. They are prepared by blending the resin with cellulose pulp, mineral filler, lubricants, pigments and peroxide curing agents on... 

This monomer has been used as the basis of a laminating resin and as a reactive diluent in polyester laminating resins, but at the present time its principal value is in moulding compositions. It is possible to heat the monomer under carefully controlled conditions to give a soluble and stable partial polymer in the form of a white powder. The powder may then be blended with fillers, peroxide catalysts and other ingredients in the same manner as the polyester alkyds to form a moulding powder. Similar materials may be obtained from diallyl isophthalate. 

The alkyd moulding compositions are used almost entirely in electrical applications where the cheaper phenolic and amino-resins are unsuitable. 

One limitation of epoxide moulding compositions is their short shelf life (typically 1-3 months), which necessitates strict stock control. The compounds may be compression, transfer or injection moulded, although compression moulding is preferred for long-fibre grades. 

Table 26.13 Properties of a typical epoxide moulding composition (BS 771 Test Methods where applicable)...

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. 

Some typical properties of a cured silicone moulding composition are given in Table 29.4. 

Chlorinated rubber is extensively employed in industrial corrosion-resistant surface coatings, for which purpose it is marketed by ICI under the trade name Alloprene. Although thermoplastic moulding compositions have been made by plasticising with the common ester plasticisers such as tritolyl phosphate they are of no commercial importance. 

Although generally ignored in plastics literature the bituminous plastics are still of interest for specific applications. The moulding compositions consist of fibrous and mineral fillers held together by a bituminous binder together with a number of minor ingredients. 

The bulk of bituminous materials are used for road making and building applications which are outside the scope of this book. Only a very small percentage is used in moulding compositions and few data have been made publicly available concerning the properties of these compositions. 

Moulding compositions contain a number of ingredients. These may include ... 

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