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Thermosetting formaldehyde based resins are typically used as the adhesives in MDF manufacture and to some extent act as the matrix in the wood fibre-polymer composite. These have the advantages over epoxy resins, the conventional matrix material used in engineering composites, of being around a quarter the price, with similar properties and easier workability. The adhesive technology used in MDF is inherited from particleboard manufacture where there has been a gradual transition from the almost exclusive use of urea formaldehyde to the use of melamine formaldehyde (MF), phenol formaldehyde (PF) melamine-urea-formaldehyde (MUF), and phenol-urea-formaldehyde (PUF) adhesives. MF and PF resins have been shown to have superior adhesive properties, particularly on exposure to moisture, compared to UF resins [Suzuki et al., 1987].
UF, MF and PF resins are condensation products of the reactions of urea,
melamine and phenol, respectively, with formaldehyde. They are
initially water soluble which is very important to the blowline mixing
of adhesive and fibre during the manufacture of MDF. Some of the
properties of the formaldehyde resins used to manufacture the boards
examined in this study are shown in Table 
. Resin
specifications vary greatly depending on their intended
application. For example, resins used in the manufacture of MDF have a
low degree of polymerisation to prevent them from curing as wet
resinated fibres are dried in the blowline.
Table: Some properties of the
thermosetting resins used in MDF manufacture [Flinn and Trojan, 1990].