Properties of phenolic mouldings

Since the polymer in phenolic mouldings is cross-linked and highly interlocked, phenolic mouldings are hard, heat-resistant insoluble materials. 

The chemical resistance of the mouldings depends on the type of filler and resin used. Simple phenol-formaldehyde materials are readily attacked by aqueous sodium hydroxide solution but cresol- and xylenol-based resins are more resistant. Provided the filler used is also resistant, phenolic mouldings are resistant to acids except 50% sulphuric acid, formic acid and oxidising acids. The resins are stable up to 200°C. Some recently developed grades of moulding compounds are claimed to be capable of exposure to 300°C for short periods. 

The mechanical properties are strongly dependent on the type of filler used and typical figures are given in Table 23.2 

As the mouldings are polar, the electrical insulation properties are not outstanding but are adequate for many purposes. At 100°C a typical woodflour-phenolic moulding has a dielectric constant of 18 and a power factor of 0.7 at 800 Hz. 

The newer improved heat-resistant grades are finding use in saucepan handles, saucepan lid knobs, lamp housings, cooker handles, welding tongs and electric iron parts. 

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Table 2.3 Mechanical properties of phenolic moulding compounds [14] ...

Table 9. Typical properties of phenolic moulding powders ...

DAP compounds can be processed by compression and transfer moulding at temperatures in the range 135-165°C and pressures of 14-70 MN/m. Their hot strength is inferior to that of phenolic moulding powders and care must be taken when extracting these materials from a mould cavity. Typical physical and electrical properties are given in Table 14. 

The use of stabilisers (antioxidants) may, however, have adverse effects in that they inhibit cross-linking of the rubber. The influence of phenolic antioxidants on polystyrene-SBR alloys blended in an internal mixer at 180°C has been studied. It was found that alloys containing 1% of certain phenolic antioxidants were gel-deficient in the rubber phase.The gel-deficient blends were blotchy in appearance, and had lower flow rates compared with the normal materials, and mouldings were somewhat brittle. Substantial improvements in the impact properties were achieved when the antioxidant was added later in the mixing cycle after the rubber had reached a moderate degree of cross-linking. 

Moulding powders based on melamine-phenol-formaldehyde resins were introduced by Bakelite Ltd, in the early 1960s. Some of the principal physical properties of mouldings from these materials are given in Table 24.1. 

The application of the moulding powders is limited by their cost, which is greater than that of general purpose phenolics. Main end uses have been for electronic applications, where good electrical properties and heat resistance are required, particularly in mouldings containing inserts. 

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. 

Properties and Application Phenolic resins find numerous application. The main use of phenolic resin is in moulding applications. They are used for automotive, radio and television and electrical appliance parts. 

In Britain, similar experiments were being carried out by a British inventor. Sir James Swinburne, whose search for a material with good electrical properties led him to develop similar materials to Bakelite. His experiments were less complete than those of Baekeland but the two collaborated in the 1920s to develop the Bakelite business in Britain. The success of phenol-formaldehyde mouldings stimulated research into other polymers. 

More important polymers of furfural are based on its ability to react with phenols to form thermosetting resins. The most common resins of this kind are of the novolak type made by reaction of a slight molar excess of phenol with furfural in the presence of an alkaline catalyst (cf. Chapter 12). Usually, blends of phenol-furfural novolaks and phenol-formaldehyde novolaks are used in the production of moulding powders with enhanced flow properties. 

Manley [28] examined a cured phenolic formaldehyde resin (PF) by means of TG-DTA-MS observing a lower sensitivity of TG relative to DTA. (However, new TGA instrumental developments have been reported since.) The TG curve shows loss of phenol m/z 94) DTA observed water m/z 18), ammonia m/z 17) and formaldehyde m/z 29), indicating disrupture of cross-links greatly affecting the mechanical properties of moulded PF compounds. The MS traces show catastrophic deterioration of PF resins at 200°C. The DTA trace also signals a change around 200°C. DTA is thus a useful indicator of temperatures at which engineering... 

Previously phenolic mouldings were used for a similar application in a much simpler unit, but were not considered suitable in the present context because of the detail required. Moulding grades of PF have poorer flow properties than thermoplastics and, as a result, are more difficult to form into deep thin sections or items with many complex features. 

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