Лако-красочные материалы — производство

A PF resol is prepared which constitutes the resin. Resorcinol chemicals or RF novolaks, in aqueous or water-alcohol solutions, are used as hardeners according to the scheme in Formula 8.

A. Adhesive 5: Phenol-Resorcinol-Formaldehyde and Liquid Hardener

A PRF similar to adhesive 2 is prepared and a PF resol of the same type as adhesive 4 is used as a hardener. The schematic curing reaction is shown in the Formula 9.

The PRF adhesives are always delivered as a liquid and must be blended with a hardener before use. PRF adhesives of type 2 in which a liquid PRF adhesive is mixed with a powder hardener have been the most commonly used industrial systems, although in

Europe the use of sludge hardeners of type 2 above is fairly common too. When using a powder hardener in adhesives of types 1, 2, and 3, they are mixed before use in a mass/ mass ratio of liquid adhesive resin (50-60% solid content) to powder hardener of 5:1. The powder hardener is generally a mixture of 10 parts paraformaldehyde and 10% fillers. It is comprised of 200-mesh wood flour or a mixture of wood flour and nutshell flour, also 200 mesh. Adhesives of types 4 and 5 have a liquid resin to liquid hardener ratio of 1:1 by mass. This is so because the hardener is also a resin. Adhesives of types 4 and 5 have been used quite extensively in the past in certain markets but have now been superseded by adhesives of type 2 which have several handling advantages.

Adhesives of type 3, although good, have not really caught on commercially and were developed as an alternative to PRFs of type 2 due to the ever increasing price of oil-derived phenol. In this regard adhesives of type 2 in which the phenol has been completely substituted by a flavonoid tannin have also been developed and have been used commercially in a few southern hemisphere countries for more than 30 years now. Their preparation and performance is detailed in the tannin adhesives chapter (Chap. 27). Pure RF adhesives (type 1) were used extensively earlier (until some 25-35 years ago). They fell into disfavor because of the high price of the resorcinol chemical needed to make them and also due to the shortage of resorcinol supply in 1960 and 1970 (oil crises). They are still used in some industrial applications, particularly at low tempera­ture curing and when difficult wood-gluing problems arise, but they constitute less than 1% of the total market by volume, except in tire cord adhesives which consume in general between 1 and 12% of the resorcinol produced in the world (hence about one sixth of the amount used for wood adhesives). PRF adhesives together with liquid hardeners are used quite extensively in Europe, as they have several handling advantages for this market. These adhesives are mixed before use in a mass/mass ratio of adhesive to liquid hardener from 5:1 to 2.5: 1 of even 1: 1 (depending on the type). The powder hardener is generally a mixture of equal parts paraformaldehyde fine powder and fillers, these latter comprising 200-mesh wood flour or a mixture of wood flour and nutshell flour, 200-300 mesh. Adhesive types presenting a liquid hardener use as hardeners formaldehyde, paraformaldehyde, formurea, and, when necessary to avoid undue for­maldehyde smell, oxazolidine. In addition these hardeners also contain fillers and thick­ening agents.

All properly formulated resorcinol-based adhesives must have a viscosity low enough in aqueous alcoholic solutions to flow with ease into all the interstices of the wood surface. Wetting ability is promoted by the alcohol. The paraformaldehyde used as a hardener in powder hardener types is an addition polymer composed of a few to over 100 formaldehyde monomers. It dissolves slowly in water, by depolymerization to for­maldehyde monomers. The rate of depolymerization depends on the degree of polymer­ization of the paraformaldehyde, the size of the particles, and the pH. Therefore, the working life or pot life of a glue mix can be adjusted by selecting the type of paraformal­dehyde and the pH correctly, but the ratio of resorcinol to phenol will also have an influence on the pH value to be chosen. Fillers are added to give consistency to the glue mix, to control viscosity and thixotropic characteristics, to form a fibrous reinforcement of the adhesive film, and to lessen the cost. Wood flour is used as a filler to obtain better gap filling properties where rough or uneven surfaces must be bonded, or where low bonding pressures must be used. Nutshell flours, such as coconut shell flour, walnut shell flour, peach pip shell flour, macadamia nut shell flour or even olive stone flour are used as fillers to provide smooth-flowing powder mixtures. Inorganic fillers are also used, especially in Europe, although this is a practice to avoid as it is very heavy on saws afterwards. Clays and fumed silica can also be used in very small amounts to control the thixotropic con­sistency of the glue mix.

As the formaldehyde reacts with the resorcinol-based resin, condensation occurs, with the formation of high-molecular-weight polymers. There is considerable secondary force interaction between the growing resorcinol polymers and the noncrystalline hemi — cellulosic and lignocellulosic molecules of the substrate. The highly polar methylol groups and the phenolic hydroxy groups link to cellulose and lignin groups by van der Waals, hydrogen, and electrostatic bonds. The growing adhesive polymers continue to interact to form colloidal particles and then a gelatinous film. This mechanism depends

strongly on the moisture content of the wood, which determines the rate of water and solvent absorption.

The advantage of ambient-temperature curing is that the moisture escapes gradually from the hard film formed on curing, inducing a minimum of residual stresses on the joint and allowing the glue line to assume the aspect of a molecularly porous solid. As a consequence, the hard film is able to transpire in the same way as wood, which minimizes checking or crazing and allows the glued joint to survive exposure to the extremes of humidity cycles. To shorten the curing time heating chambers (40-60°C) are often used in the manufacturing of glulam, and an even faster curing can be achieved with the use of radiofrequency curing presses. Typical levels of strength and wood failure results obtained in specific standard tests are shown in Table 1.