Synthetic phenolic resin-based composites

The loose term renewable resources adhesives has been used to identify polymerie eom-pounds of natural, vegetable origin that have been modified and/or adapted to the same use as some classes of purely synthetic adhesives [1]. At present two classes of these adhesives exist one already extensively commercialized in the southern hemisphere and the other on the slow way to commercialization. These two types of resins are tannin-based adhesives [2] and lignin adhesives [3 ]. Both types are aimed primarily at substituting synthetic phenolic resins. In some aspects, such as performance, they closely mimic, or are even superior to, synthetic phenolic adhesives, while in others they behave in a vastly different manner from their synthetic counterparts. In this chapter we focus primarily on tannin-based adhesives because they have already been in extensive industrial use in the southern hemisphere, in certain fields of application, for the past 20 years. These adhesives are of some interest not only for their excellent performance in some applications but also for their mostly environmentally friendly composition. Lignin adhesives are treated briefly here and in detail in Chap. 28. 

In contrast to expensive synthetic fibers, the use of natural fibers is a very cheap means of reinforcement, especially in countries where these natural products are readily available. Natural fibers, when compared to synthetic fibers, have a lower density and a lower tensile modulus. However, a combination of a sisal fiber with a resole type phenolic resin based on cashew nut shell liquid, a naturally occurring monomer, produces a very cost effective composite with a thermal stability greater than 200°C. The cashew nut shell liquid is a mixture of phenol, 3-(8 -pentadecenyl)phenol, 3-(8 -pentadecenyl)-5-(hydroxy)phenol, 3-(8 -pentadecenyl)-5-(hydroxy) -6-methylphenol, and2-(8-pentadecenyl-6-(hydroxy)benzoicacid [195]. 

It was not until the commercialization of synthetic plastics resins in the 1930s that an almost unlimited variety of base materials became available for compounding into adhesives and sealants. Most of the thermoplastic resins were soluble in organic solvents and were used as solvent adhesives for molded plastic articles of the same base composition and sometimes for other materials. Poly(vinyl chloride) (PVC), a thermoplastic developed in 1927, is used today in solvent formulations to bond PVC articles such as coated fabrics, films, foams, and pipe. In the early 1930s, phenolics came into importance as adhesive resins. Before that time they were used as coating varnishes [9, p. 239). About 1931 development of the use of a new phenolic resin for plywoods and veneers began [9, p. 239]. 

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]. 

Most amino and phenolic thermosets are used in the production of composites, mainly prepared with wood (wood-plastic composites, WPC) [68]. Particleboard, or the chipboard is a kind of WPC, which is produced from wood particles, such as wood chips or saw dusts, through their glueing together with synthetic resins or other suitable binders, by pressing and extruding afterwards. In most particleboards, the resin used is formaldehyde-based. 

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