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Composite materials derived from renewable resources have received considerable interest in recent years due to the ever-increasing concern about the depletion of the earth’s fossil resources. The combination of natural fibres and bio-derived resin systems is a potential route to manufacturing bio-composite materials for mass transport, automotive and construction applications. This article describes the development of a new biomass-based prepreg system based on furan resins and natural fibres.
(Published on January-February 2009 – JEC Magazine #46)
1- HANS E. HOYDONCKX, BUSINESS DEVELOPMENT MANAGER, TRANSFURANS CHEMICALS
2- GUNTHER SWITSERS, PROCESS AND PRODUCT DEVELOPMENT ENGINEER, TRANSFURANS CHEMICALS
3- BRENDON M. WEAGER, TECHNICAL MANAGER, NETCOMPOSITES LTD
4- ELAINE L. ARNOLD, BIOMATERIALS R&D ENGINEER, NETCOMPOSITES LTD
Furan resin systems have long been used as thermoset binders for aggregates in the foundry and refractory industries. The success of furan resins in these industries has been sustained by their high temperature resistance in such demanding applications. For example, furan resins are used in a variety of binder systems in the foundry industry, including “furan no-bake” resins, heat-curing and gas-hardened systems.
TransFurans Chemicals recently developed a new range of furan resin systems under the trade name BioRez and Furolite. These resins originate from furan chemical technology, though they are based on prepolymer molecules of furfuryl alcohol. Furfuryl alcohol is a renewable alcohol produced from furfural, which is formed by the acid-catalyzed digestion of hemicellulosic sugars in biomass. With biomass as the sole raw material, it is a renewable and CO2-neutral chemical. The use of this prepolymer matrix structure provides the backbone for formulating a wide variety of cold or hot curing thermosets for a range of applications. BioRez and Furolite resin systems can be applied in different composite manufacturing processes including hot compression moulding (spray-up, BMC, SMC, etc.), vacuum infusion, resin transfer moulding (RTM), hand lay-up and filament winding. Furan resins offer a number of interesting properties such as high stiffness, fire resistance and chemical resistance.
Furan-natural fibre prepreg
The applicability of biomass-based furan resins in composite manufacturing was evaluated in the recent BIOCOMP project, an integrated project for SMEs supported by the European Commission’s Sixth Framework Programme. A primary objective of the BIOCOMP project was to develop high-performance composite materials entirely derived from renewable sources using a new family of furan-based resins reinforced with continuous, aligned natural fibres. This goal was reached by the development of furan-natural fibre prepreg technology.
The possibility of exact fibre placement in the direction of applied loads combined with a high fibre volume fraction is the main engineering characteristic of existing prepreg materials. Natural fibre prepreg materials with fossil-based resins like epoxy have been reported in commercial praxis. With minor adjustment of the curing system and characteristics, it is possible to formulate a furan resin for use as a matrix material in natural fibre prepregs.
Furan resins are generally cured by acid hardeners which trigger a polycondensation reaction, leading to a highly cross-linked polyfurfuryl alcohol network. It is well known that natural fibres – being lignocellosic materials – can be degraded in hydrolytic conditions, catalyzed by both acid and alkaline chemicals through various pulping reactions. In order to compatibilize the acid curing mechanism of a furan resin with natural fibres, a novel catalytic system has been developed which is able to cure the resin whilst protecting the natural fibre from any degradation reaction.
The resulting Furolite furan resin is a ready-mixed resin which is curable at temperatures above 100°C (Fig. 1).
Prepreg production technology
Standard textile impregnation equipment was used to produce this furan prepreg. In the first step, the natural fibre textile material is impregnated with (water diluted) furan resin and the
excess resin is pressed out by a calendar press (Fig. 2). In the second step, the resin is dried, thickened and B-staged in a hot air tunnel oven to yield a prepreg material. A variety of woven and knitted fabrics can be impregnated with this equipment.
Production and performance
The prepregs were moulded into test panels and model products using the vacuum consolidation process. The prepregs were placed on an aluminium plate covered with PTFE-glass cloth and vacuum-bagged using release film, breather cloth, a vacuum bag and sealant tape. A vacuum of 0.95 bar was applied and the part was vacuum-cured in an oven. A range of curing parameters were assessed, including high temperatures for short times and low temperatures for longer periods. The standard curing process was 150°C for 15 minutes.
NetComposites is a UK-based applied R&D and consulting company specializing in fibre-reinforced polymer composites.
The prepreg panels were tested in three-point flexure according to the ISO 14125 test standard. Selected results are summarised in Table 1.
Prepregs based on a unidirectional flax fabric in a 0/90 cross-ply lay-up showed the highest modulus and strength of the natural fibre fabrics. In fact, these flax prepregs had the same mechanical properties as woven glass prepregs. As expected, carbon fibre-furan prepregs had the highest mechanical properties overall, providing a high-performance, partly biobased system.
The prepreg material was used to manufacture a variety of products including sandwich panels with balsa wood and cork cores, high-performance tubes, and structural automotive parts. These products were also moulded under vacuum on both metallic and composite moulds. In order to achieve a good finish on the A surface of the parts, natural fibre veils were used to eliminate voids and fibre print-through (Fig. 3).
This newly developed natural fibre-furan prepreg material is an exciting intermediate material for the construction of 3D designed structural products. The renewable backbone of its two main constituents – natural fibres and furan resin – creates a fully biomass-based material with high strength and stiffness. Its ease of fabrication opens up routes for large volume applications in the mass transport, automotive and construction industries.