Pultrusion process resin system

The characteristics of the three most common thermoset resin systems used in pultrusion are compiled in Table 11.2 [3]. It is noteworthy that unreinforced polyesters and vinylesters shrink 7-9% upon crosslinking, whereas epoxies shrink much less and tend to adhere to the die. These epoxy characteristics translate into processing difficulties, reduced processing speed, and inferior component surface finish. It is normal practice to use resin additives to improve processability, mechanical properties, electrical properties, shrinkage, environmental resistance, temperature tolerance, fire tolerance, color, cost, and volatile evaporation. It is normally the resin, or rather its reactivity, that determines the pulling speed. Typical pulling speeds for polyesters tend to be on the order of 10-20 mm/s, whereas speeds may exceed lOOmm/s under certain circumstances. Apart from the resins characterized in Table 11.2, several other thermosets, such as phenolics, acrylics, and polyurethanes, have been tried, as have several thermoplastics (as will be discussed in Sec. 11.2.6). 

This is a marriage of two processes pultrusion and compression moulding. A pultrusion in-feed system is employed for management of the reinforcement and to apply the resin mix but the conventional pultrusion die is replaced with one which will open and close. This allows variations in the cross section and linearity of the component which are not possible with pultrusion. 

Pultrusion is a modern technique used for producing continuous fiber-reinforced profile in which the orientation of the fiber is kept constant during cure. Although this process is utilized for both thermoplastic and thermoset resins, it is mainly suitable for thermosetting resins like polyester, epoxy and phenolic resin systems. Jute, available in continuous forms such as mat, roving, tapes, yarn, etc., is impregnated with... 

Figure 6.22 Schematic illustration of pultrusion process, using liquid polyester or other reactive thermosetting resin systems.

The chemical system impacts on gel conversion, and the gel point will determine any increases in modulus. For the case of the pultrusion process, the maximiun pressure value is obtained at the moment when the resin layer close to the wall die is almost fully reacted. From there on, pressure begins to decrease until all tiie material is totally cured and with a high modulus value. Cure or gelation under pressure is desirable in the pultrusion process because it provides better surface of the composite and less void content. 

Pultrusion is a continuous process involving pulling a collection of fibres on a creel system in the form of a roving, tow, mat or fabric through a resin bath (for impregnation) and then through a heated die to cure the resin and impart a constant cross-section to the product. Figure 6.7. 

Three-dimensional analyses of heat transfer and cure in pultrusion of epoxy-resin composites have been examined by Chachad et al. (1995, 1996) and Liu et al. (2000). Carlone et al. (2006) review finite-difference and finite-element process models used for predicting heat transfer and cure in pultrusion. In this work they recommend the following empirical nth-order cure model for predicting cure kinetics of epoxy-resin composites, which is then coupled to the system s energy balance to predict thermal properties and cure conversion ... 

In essence, pultrusion is an apparently simple process. However, the production of complex and consistently high quality profiles requires a high degree of technical resource and experience. For example, the precise positioning of the reinforcement materials, in front of the die entrance, is of paramount importance. It is achieved by a carefully engineered in-feed system. This guides, locates, folds and tensions the various layers of reinforcing mat and uni directional fibre. It also wipes excess resin from the reinforcement. 

Pultrusion is a versatile process that allows the use of selective type of reinforcement made of different materials. Resin formulation and reinforcement selection determine the properties of the final product. A trade-off is mostly required to accomplish desired properties while keeping the product cost low. However, to have the trade-off done optimally, some groundwork and preliminary studies are required. The material composition also decides the thermal mass in the system that plays a part in its rate of conversion to the final form. 

CompositGS. There has been substantial progress in improving the processability of phenolics in composite materials (104-106) to the point where a variety of materials are available for pultrusion, filament winding, and resin-transfer molding (see Composites, Fabrication). More complicated composite systems are based on aramid and graphite fibers. 

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