Filled fibres

Core sheath type (C/S) In this type of fibre one of the components is completely surrounded by another component. Core sheath type fibres can be self-bonding (e.g. two components with varying melting points with the lower melting point polymer on the sheath), surface tailored fibres (e.g. sheath containing expensive additives) or filled fibres (e.g. a core of recycled material covered by a sheath with the desired properties). 

There are less exotic ways of increasing the strength of cement and concrete. One is to impregnate it with a polymer, which fills the pores and increases the fracture toughness a little. Another is by fibre reinforcement (Chapter 25). Steel-reinforced concrete is a sort of fibre-reinforced composite the reinforcement carries tensile loads and, if prestressed, keeps the concrete in compression. Cement can be reinforced with fine steel wire, or with glass fibres. But these refinements, though simple, greatly increase the cost and mean that they are only viable in special applications. Plain Portland cement is probably the world s cheapest and most successful material. 

Whereas Tefzel is said to be an internally stablised copolymer of TFE and ethylene, other copolymers that are compounds of similar copolymers with stabilisers of antioxidants are now also available (Hostaflon ET by Hoechst and Aflon by Asahi Glass Co.). Glass-fibre-filled grades are also available. 

The glass-fibre-filled types can be obtained in two ways. 

The glass-fibre nylons have a resistance to creep at least three times as great as unfilled polymers. In the case of impact strength the situation is complex since unfilled nylons tend to break showing tough fracture whereas the filled polymers break with a brittle fracture. On the other hand the glass-filled polymers are less notch sensitive and in some tests and service conditions the glass-filled nylons may prove the more satisfactory. 

Table 18.7 Comparison of glass-fibre-filled and unfilled nylon 66...

Glass-fibre-filled grades of these toughened polymers are also available but these do not show the same improvement in toughness over normal glass-fibre-filled nylons. 

As with the aliphatic polyamides, the heat deflection temperature (under 1.82 MPa load) of about 96°C is similar to the figure for the Tg. As a result there is little demand for unfilled polymer, and commercial polymers are normally filled. The inclusion of 30-50% glass fibre brings the heat deflection temperature under load into the range 217-231°C, which is very close to the crystalline melting point. This is in accord with the common observation that with many crystalline polymers the deflection temperature (1.82 MPa load) of unfilled material is close to the Tg and that of glass-filled material is close to the T. ... 

A low moulding shrinkage (0.005-0.007 cm/cm) in unfilled grades down to about 0.002 cm/cm in 30% glass-fibre-filled grades. 

Glass-fibre/mineral-filled colour compounds. 

Glass-fibre-filled polysulphones are also available. These show significantly increased creep resistance and lower coefficients of thermal expansion (Table 21.4). 

In the late 1970s several developments occurred causing renewed interest in poly(ethylene terephthalate) as a plastics material. These included the development of a new mouldable grade by ICI (Melinar) and the development of a blow moulding technique to produce biaxially oriented PET bottles. In addition there appeared a glass-fibre filled, ionomer nucleated, dibenzoate plasticised material by Du Pont (Rynite) (see Chapter 26). 

Industrial grade materials employ fillers such as asbestos, silica and glass fibre. These are incorporated by dry-blending methods similar to those used with woodflour-filled phenolic compositions. 

As with poly(ethylene terephthalate) there is particular interest in glass-fibre-filled grades. As seen from Table 25.8, the glass has a profound effect on such properties as flexural modulus and impact strength whilst creep resistance is also markedly improved. 

High softening temperatures (glass-fibre-filled grades are better than polycarbonates and modified PPOs). 

Carbon-fibre-filled grades exhibit interesting tribological properties and useful antistatic behaviour. 

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