Лако-красочные материалы — производство

A. Phenol-Formaldehyde Wood Binders

Phenolic resins are used as binders for exterior-grade plywood and particleboard, which need the superior water resistance provided by these resins. In the manufacture of ply­wood, the phenolic resin adhesive is usually applied to the wood veneers by roller or extrusion coating. The coated veneer is then cross-grained, stacked, and cured in a multi­daylight press for 5 to 10 min at 120 to 130°C and at 11 to 16kg/cm2. In the manufacture of particleboard, the phenolic resin adhesives are sprayed onto the wood chips, or sprayed plus spread by continuous blenders. The glued wood chips are formed into a mat and then pressed for 5 to 12 s/mm, according to thickness, press temperature and moisture content, at 190 to 230° C and 25 to 35kg/cm2. The only type of phenolic resins used commercially for this application are resol-type resins, which have the following structure:

These are hardened by heating after the addition of small amounts of wax emulsion and insecticide solution in the case of particleboard, and of vegetable or mineral fillers and tackifiers in the case of plywood. Accelerators are sometimes added in both types of glue mixes. The pH of these resins varies between 10 and 13.5 and is generally between 12 and 12.5.

In dealing with wood related factors that affect glue bonds, it is important to remember that adhesion is at least 95% physicochemical in nature. The mechanical aspects of bond formation (such as keying cured adhesive solid in the wood surface) contribute negligibly to the bond strength or wood failure. The main chemical forces in thermosetting resin adhesion are primary valence bonds, covalent bonds, and hydrogen bonds, plus secondary forces such as van der Waals and London forces and any other types of electrostatic, dipolar, and associative forces. It is therefore essential that the resin contains a significant number of functional groups and that the wood surface presents a significant
number of reactive sites to enable the resin to bond. Any factors that limit resin function­ality or block reactive sites on the wood structure necessarily impede adhesion.