10.3.1 Thermosetting Phenol Formaldehyde Resin

Thermosetting and thermoplastic polymers are usually used for making natural fibre composites. These both types of polymers have different applications for their versatile properties. Both polymers have structural similarity as they con­tain long chains. But the major difference is that thermosetting polymers possess

cross-linking chain which gives higher tensile strength and rigidity. That is why they are usually used in structural application and thermoplastics are considered for nonstructural products (Ganga Rao et al. 2007). The application of thermosetting polymers in natural fibre composites is shown in Table 10.2. Thermoset polymers, those that are used as matrices in composites, have sufficient viscosity to flow at some point during the cure process. They can be cast into plate forms to provide blanks. The finished specimens can be machined or moulded into even more complex geometries if necessary to create net-dimension specimens (Fig. 10.3).

The properties and structure of thermosetting resins mostly depends on polycon­densation condition. A number of polycondensation resins such as polyester, polyurethane, melamine formaldehyde, urea formaldehyde (UF) and phenol

formaldehyde (PF) are commercially available today (Pizzi et al. 1999). Among them, phenolic resins are well known for their tremendous mechanical properties, chemical resistance, thermal stability and strong adhesive capacity (Satapathy and Bijwe 2006; Park et al. 2006). This resin has been keeping its outstanding perfor­mance in synthetic polymer industry from the last 90 years (Shafizadeh et al. 1999). Resol is the commercial name of PF resin, which is produced by polycon­densation reaction of formaldehyde and phenol under alkaline medium. A wide range of uses in wood industry, impregnation, thermal insulation and moulding is found for its highly cross-linked, versatile, cure capability and stable nature (Holopainen et al. 1997).