Thermosets pultrusion

Matrix flow relative to the reinforcing fibers is caused by thermal expansion of the fiber-matrix mass within the confines of the die and by the geometrical constriction of the die taper. Once the matrix flow distribution is known, the matrix pressure distribution may be determined using a flow rate-pressure drop relationship. One-dimensional flow models of thermoset pultrusion have been reasonably well verified qualitatively [15-17]. A onedimensional flow model of thermoplastic pultrusion [14,18] has similarly been compared with experimental data and the correlation found to be encouraging [19]. 

Figure 11.7 Dimensionless matrix flow relative to the fibers in a linearly tapered die as a function of initial fiber volume fraction (thermoset pultrusion)...

Ma et aL (1986) designed unique thermosetting pultrusion equipment that consisted of a fibre drying chamber, a preheated resin tank, a die with multiple heating zones and a post-cure chamber. The equipment reportedly improved the pultrusion rate and the properties of the pultruded part. 

Composite processing uses specialized methods. For thermoplastics, glass mat thermoplastic (GMT) is the main method but prepreg, lay-up, tape winding, pultrusion are also used to produce composites. SMC/BMC are the main processes for thermosets but lay-up is currently used for large parts and prepregs are used for high-tech parts. 

A thermoplastic is extruded onto the reinforcements impregnated with a thermoset resin before passing through the pultrusion die. 

In most applications, polyester and vinyl ester resins are used as the matrix materials. Epoxies are also used, although they require longer cure times and do not release easily from the pultrusion dies. Hence, thermosetting resins are most commonly used with pultrusion, although some high-performance thermoplastics such as PEEK and polysulfone can also be accommodated. In addition to the resin, the resin bath may contain a curing agent (initiator, cf. Section 3.3.1.2), colorants, ultraviolet stabilizer, and fire retardant. 

Figure 7.88 Viscosity change of a thermosetting resin in a pultrusion die. Reprinted, by permission, from P. K. Malhck, Fiber-Reinforced Composites, p. 349. Copyright 1988 by Marcel-Dekker, Inc.

Many different thermosetting polymers are used in pultrusion, eg, polyester, vinyl ester, epoxy, and urethane. Reinforcements must be in a continuous form such as rovings, tows, mats, fabrics, and tapes. Glass fibers are the low cost, dominant composition, but a ram id and carbon fibers are also used. 

Figure 11.1 Schematic of basic pultrusion operation for thermosets...

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). 

The predominant advantage of pultrusion as a manufacturing method for thermoset composites is the low production cost, which is closely related to the reasonably high... 

As pointed out in the beginning of the chapter—and perhaps also noticeable from the process description earlier—the pultrusion process at first appears rather simple and straightforward however, several intricacies arise on closer inspection. The key technology issues of pultrusion of thermoset-matrix composites are usually considered to be resin formulation, temperature control, material guidance, and die design (not in order of importance). It is typically the skill in these areas that distinguishes a successful pultruder from his hapless competitor. 

Thermoset polymeric esters consisting of neopentyl glycol, propylene glycol, trimethylol propane, adipic acid, maleic anhydride, and 2-ethyl hexanol were prepared by McAlvin et al. (3) and were used in blends containing styrene monomer. The mixture was cured by pultrusion and had improved weatherabil-ity characteristics. 

Recendy, storm doors have been constmcted of advanced thermoplastic composites. Stampable, glass-mat reinforced polypropylene sheet is used to create a high strength outer skin. These compression molded skins are welded together using a friction or ultrasonic process then injection molded with a polyurethane foam core to produce an insulated structure. New technology for window frames incorporate the pultrusion of frame channels to produce a thermoset composite channel that can be filled with fiber glass for further insulation enhancement (12,31,33,34,48,49,54—56,60—67). 

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