Лако-красочные материалы — производство

A series of different novolak-like materials are prepared by copolymerization of resorcinol with resorcinolic A rings of polyflavonoids, such as condensed tannins [62-64]. The copolymers formed have been used as cold-setting exterior-grade wood adhesives, com­plying with the relevant international specifications. Several formulations are used. The system most commonly used commercially relies on the simultaneous copolymerization of resorcinol and of the resorcinolic A rings of the tannin, due to their comparable reactivities toward formaldehyde.

The final mixture of the products of this system is an adhesive that can be set and cured at ambient temperature by the addition of paraformaldehyde. Other cold-set sys­tems exist and are described in the more specialized literature [2,62-64]. The typical results obtainable with these adhesives are indicated in Table 3.

A particularly interesting system now used extensively in several southern hemi­sphere countries is the so-called ‘‘honeymoon’’ fast-setting, separate-application system [66,67]. In this system one of the surfaces to be mated in the joint is spread with a standard synthetic phenol-resorcinol-formaldehyde adhesive plus paraformaldehyde hardener. The second surface is spread with a 50% tannin solution at pH 12. When the two surfaces are

jointed, fingerjoints develop enough strength to be installed within 30 min and laminated beams (glulam) need to be clamped for only 2.5 to 3 h instead of the traditional 16 to 24 h, with a consequent considerable increase of factory productivity. This adhesive system also provides full weather — and boil-proof capabilities.