Types of Polymers Binders

The term binder is used to mean the matrix in which the fibres are embedded or treated. The binders, sometimes called adhesives or matrixes, in this chapter refer to polymeric materials having reasonable mechanical properties. The binder may be thermosetting or thermoplastic polymers. 

Thermosets are based on crosslinked polymers. They harden permanently with the aid of catalysts and/or heat, and cannot be remelted without degrading their polymeric structure. The thermosetting family includes phenolics, epoxides, alkyds, polyurethanes, melamine, urea-formaldehyde and unsaturated polyesters. 

The most common resin for lignocellulosic composites is urea formaldehyde. About 90% of all lignocellulosic composite panel products are bonded with UF [12]. UF is inexpensive, reacts quickly when the composite is hot-pressed, and is easy to use. UF is water-resistant, but not waterproof. As such, its use is limited to interior applications unless special treatments or coatings are applied. UF resins are typically used in the manufacture of products where dimensional uniformity and surface smoothness are of primary concern, for example, particleboard and medium density fibreboard (MDF). Products manufactured with UF resins are designed for interior applications. They can be formulated to cure anywhere from room temperature to 150 °C press times and temperatures can be moderated accordingly. UF resins (often referred to as urea resins) are more economical than PF resins and are the most widely used adhesive for composite wood products. The inherently light colour of UF resins make them quite suitable for the manufacture of decorative products. 

Melamine-formaldehyde (MF) resins are used primarily for decorative laminates, treating paper, and paper coating. They are typically more expensive than PF resins. MF resins may be blended with UF resins for certain applications (melamine urea). 

Isocyanate as diphenylmethane di-isocyanate (MDI) is commonly used in the manufacture of composite wood products. MDI is used primarily in the manufacture of OSB. Facilities that use MDI are required to take special precautionary protective measures due to its toxicity. These adhesives have been chosen based upon their suitability for the particular product under consideration. Factors that must be taken into account include the materials to be bonded together, moisture content at time of bonding, mechanical property and durability requirements of the resultant composite products, and of course, resin system costs. 

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The amount and types of solvent will depend on the requirements to VOC and the type of polymer (binder) to be dissolved in the paint. Another important parameter is the sprayability of the paint which also depends on the type and amount of solvents. 

Table 1.2. Types of polymer binders for inhibited CM and their chief processing and service characteristics... 

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