Pultrusion properties

Lam P.K. and Piggott M.R. (1989b). The durability of controlled matrix shrinkage composites Part 2, Properties of carbon fiber-epoxy copolymer pultrusion, J. Mater. Sci. 24, 4427-4431,... 

Pultrusion is a steady-state process in which the fiber-resin mass changes its properties as it moves from the entrance to the exit of the die. In order to track the temperature, polymer conversion, and other properties of the fiber-resin mass as it moves along the die, it is useful to define a representative volume element (RVE) that rides along the fiber at the line speed of the pultrusion process. An RVE is defined such that it will contain both the solid phase (i.e., fibers and resin), irrespective of its location in the composite. In real-life pultrusion, a thermocouple wire that passes through the pultrusion die tracks the temperature of an RVE in the composite. 

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). 

Pultrusion [PHENOLIC RESINS] (Vol 18) [PLASTIC PROCESSING] (Vol 19) [REINFORCED PLASTICS] (Vol 21) [COMPOSITEMATERIALS - SURVEY] (Vol 7) resin properties required for [COMPOSITE MATERIALS - POLYMER-MATRIX - THERMOSETS] (Vol 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 ... 

D 5028 (1996) Test method for curing properties of pultrusion resins by thermal analysis... 

VE polyurethane resins have mechanical properties similar or superior to those of conventional VE and epoxies. Characteristics include a heat distortion temperature of 120C (248F). Ultimate elongation of an unreinforced molding compound without fillers is 5.5% tensile strength is 80 MPa and flexural strength 150 MPa. The resins can be custom-formulated. Applications include customized automobile parts, recreational vehicles, outdoor equipment, tubs/showers and electrical parts. The resins are suitable for standard molding processes some were specifically developed for pultrusion, RIM, foam, adhesive, and polymer concrete applications. 

The properties of composite materials cannot be predicted adequately by considering the fibre and resin constituents one by one. An important mechanism of composite failure under stress is delamination caused by differences between the engineering properties of successive plies or layers. These differences arise from the fact that successive layers may have different fibre orientations [34] or, occasionally, different fibres. It is a feature of laminates made by stacking pre-impregnated layers of reinforcement and is not an issue with, for example, unidirectional pultrusions. The process of delamination has been reviewed by Davies [35]. The fabrication of three-dimensional composites is an important step towards reducing or eliminating unwanted delaminations. Such materials are at an advanced stage of development. 

Pultruded sections are used for handrails, cable trays, ladders and even bridge construction. The parallel orientation of the continuous fibres ensures that they are protected with a covering of resin, which enhances weathering properties a polyester veil can be used to improve protection. Despite this, some deterioration in surface gloss is experienced fairly rapidly on standard pultrusions if a surface coating is not used, but mechanical properties are retained for many years. 

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