CHAPTER ONE

1.0 INTRODUCTION 1.1 Resins Resins are low molecular weight amorphous polymers. They normally have a melting or softening range, and are brittle at the solid state. Their main applications are in adhesives, inks, and chewing gum. Mostly, they are used together with larger polymers, which form the backbone of the adhesive and thus generate cohesion. Formulators use resins to create the best balance between adhesion and cohesion. Resin can be divided as synthetic and natural resins (Chretien , 1999). Synthetic resins systems that are commonly used in advanced fibre-reinforced composites are namely, thermosets and thermoplastics. The difference between the two categories is that a thermosetting material undergoes an irreversible reaction during processing, where by it is permanently hardened due to the formation of cross-links between the monomer units. Thermosetting resins generally degrade if heating is continued . Major thermosetting resins for adhesive are phenol-formaldehyde (PF) resins, and melamine formaldehyde. Others are epoxy and polyester resins. Thermoplastic resins are polymers that soften and flow at elevated temperature and solidity at low temperatures. They are also called hot melt adhesives. Adhesives strengths with these thermoplastics resins are mainly from mechanical interlocking. Polypropylene and polyethene are examples of thermoplastics ( Knop and Pilato 1985; Martin, 2013). Rosin, a natural resin, is one of the oldest raw materials for the adhesives industry. There are three types of resin manufactured, gum rosin, wood rosin and tall oil rosin, all generated from the pine tree. Rosin resins, unlike hydrocarbon resins are not polymers. In fact, they are a blend of different molecules. They contain abietic acid and its double bond isomers as main 2 components. Their molecules have poor stability caused by unsaturation. Stability can be improved by either disproportionation or hydrogenation. Phenolic resins are therefore often classified as “water-soluble”, “alcohol-soluble”, “oil-soluble”, etc ((Chretien, 1999; Pizzi, 2003).

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