Composite manufacture from thermosetting resins

The combination with fibres has proved difficult however. Often there are issues with compatibility between bio-resins and fibres (both natural and synthetic), which cause defects in the composite structure and ultimately poorer physical properties. Castor-oil polyurethane was compared with phenolic resins when infused over sisal fibres however, the phenolic resins showed better structural performance when compared with the castor oil-based material [52]. This is not always the case, as some improvements have been made. Soybean oil thermoset polymers were used in a glass/flax hybrid composite resulting in improved mechanical performance [73], Thermoset resins were produced from triglyceride oils with a wide range of properties (tensile modulus 1-2 GPa, glass transition temperature Tg 70-120 °C) and glass- and hemp- fibre composites were manufactured [74,75]. 

Investigations have shown that it is possible to manufacture all-natural composites using hemp and flax fibres using soy oil resins [14]. The tensile and flexural properties of the resins were shown to be improved by the addition of the fibres as observed for regular thermoset resins. However, this type of composite gave lower values than polymer-natural fibre-reinforced composites. Wollerdorfer and Bader [15] made similar observations for different all-natural composites using soya resins. Researchers [16] have reported studies on the properties of composites from hemp non-woven mats and polyester resin. The authors observed that mechanical properties were found to reach maximum at a fibre loading of 30%. 

The manufacture of composite wood products requires the use of bonding thermosetting resins mentioned before. In addition, to protect these products from biological degradation, (e.g., fungal induced decay), preservatives (insecticides and/or fungicides) are combined with resins or applied separately to the composite material. Other propertymodifying chemicals such as waxes and fire retardants [84] may also be used. 

Wood composite products are conventionally manufactured from wood materials having various geometries (eg, fibers, particles, strands, flakes, veneers, and lumber) combined with a thermoset resin and bonded in a press (4). The press applies heat (if needed) and pressme to activate (cross-link) the adhesive resin and bond the wood material into a sohd panel, lumber, or beam having good mechanical (strength and stiffness) and physical (form, dimensional stabihty, etc) properties. 

Similar to all commercial plastics, the traditional and more commonly used thermosetting resins are considered as petrochemicals, having been manufactured from petroleum. Some of the primary distillation products of crude oil, which can be classified either as olefins or aromatics, serve as precursors for the synthesis of thermosets. For example, epoxy resins are manufactured by the reaction of epichlorohydrin, a chloro-oxirane, and a derivative of propylene, with bisphenol A, which is a derivative of cumene. Another example would be the unsaturated polyesters (UPs), which are derivatives ultimately originating from ethylene (ethylene glycol) and benzene (maleic acid) [6]. Epoxies and polyesters constitute more than 95% of the thermoset composite market of the two, polyester-based systems predominate in volume by about 10-fold [6, 7]. Other thermoset resins used in reinforced form are phenolics, vinyl esters, and polyimides. Details of the properties and applications of these thermoset systems will be further discussed in the following section. 

Thermoset plastics have also been pyrolysed with a view to obtain chemicals for recycling into the petrochemical industry. Pyrolysis of a polyester/styrene copolymer resin composite produced a wax which consisted of 96 wt% of phthalic anhydride and an oil composed of 26 wt% styrene. The phthalic anhydride is used as a modifying agent in polyester resin manufacture and can also be used as a cross-linking agent for epoxy resins. Phthalic anhydride is a characteristic early degradation product of unsaturated thermoset polyesters derived from orf/io-phthalic acid [56, 57]. Kaminsky et al. [9] investigated the pyrolysis of polyester at 768°C in a fiuidized-bed reactor and reported 18.1 wt% conversion to benzene. 

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