Лако-красочные материалы — производство

A number of additives and modifiers contribute useful properties to phenolic resins used for wood gluing. Multipurpose additives are the amino resins and urea-formaldehyde and melamine-formaldehyde polymers. These include not only urea and melamine ply­wood adhesive resins, but also dimethylol urea, trimethylol melamine, and hexamethylol melamine. Added in amounts from 5 to 15% of phenolic resin solids, they improve resin tack and prepressing, increase long-assembly-time tolerance, shorten pressing times, and enhance resin functionality. This results in stronger bonds on wood veneers. As long as they are used in limited quantities, they have a negligible effect on long-term phenolic bond durability. They appear to be well protected from hydrolytic degradation by the cured phenolic polymer network. The dispersion of the amino resin molecules in the alkaline medium of the phenolic resin glue mix inhibits their curing reaction, which is acid catalyzed. It causes them to function as methylolated cross-linking units for the PF polymer.

Urea by itself can be used in great amounts in the system for producing PF resins of very high molecular weight and such a high viscosity as to be almost solid at ambient (but not higher) temperature: such a resin while still hot is ‘‘drowned’’ in up to 40% of urea (based on resin solids) to decrease the viscosity of the resin to a perfectly manage­able level. These PF in urea solutions have high reactivity and fast curing time thanks to the great advancement of the resin [31,50]. They show little copolymerization with the PF in their liquid state and some copolymerization in the hardened network, the major­ity of the urea still remaining free to leach out after curing. For these reasons they must not be confused with copolymerized PUFs of high urea proportion which also are operative in fast hardening time applications and are described more fully elsewhere in this chapter [31,32].

Formaldehyde in liquid solution or solid form and formaldehyde-generating com­pounds are also phenolic resin additives that improve functionality and decrease curing times. Paraformaldehyde is used most frequently, but hexamethylenetetramine, formalde — hyde/sodium bisulfite complexes, tris(hydroxymethyl)nitromethane, and glyoxal are also used. Significant effects are obtained when 3 to 5% is added, based on phenolic resin solids. Further reduction in curing time is possible if 1 to 2% resorcinol is added or resorcinol-acting natural extractives such as condensed (flavonoid) tannin extracts (wattle, mimosa, and others). This can be either mixed or not mixed with paraformalde­hyde and added to the liquid phenolic resin glue mix. Equally ester accelerators such as glycerol acetate (triacetin), guanidine carbonate, and other esters are also used as very effective cure accelerators and to improve the final hardened strength (See Section III. B.1 on a — and p-set acceleration).

Formaldehyde additions overcome the effect of phenolic extractives in certain hard­wood species, which prevent proper cure or adhesion of PF resins. Free formaldehyde appears to react rapidly with these phenolic extractives before they can interfere with the phenolic resin curing mechanism. With certain wood species that are rich in extractives, this technique has been used to increase bond durability from interior-grade to true exterior-grade performance.

Natural phenolic compounds are used as both replacements for substantial portions of synthetic phenol in plywood adhesive resins and as glue mix additives to improve performance; 4 to 6% is added, based on phenolic resin solids. They bring about improve­ments in assembly time tolerance and flow with no significant change in adhesion. Glue mix additions of wattle tannin or other condensed flavonoid tannin extracts with or without additional formaldehyde produce faster hot-pressing cycles. However, some assembly time tolerance and pot life, has to be sacrificed in the process, but full exterior — grade durability is retained.

The lignin residues from wood pulp production are another class of extractives currently receiving attention as phenolic resin additives. Substituted phenols, such as cresols and xylenols, have been used as glue mix additives for phenolic adhesive resins to improve assembly time tolerance. They are also used as solvents to remove oleo — resinous deposits on the surface of pitchy softwood veneers. They can be used as flow promoters in phenolic hardwood adhesives. To avoid interference with the rate of resin curing, the amount added should not exceed 3 to 4% of the phenolic resin solids content.

Complexing additives commonly include the soluble salts of boron, chromium, zinc, cobalt, lead, manganese, magnesium, and others. When added to phenolic resin adhesives, some of these compounds have been successful in reducing pressing times and in improving prepress performance [51]. Borax is widely used in North America to shorten the prepress cycles of phenolic plywood glues for softwoods. However, these compounds tend to increase the molecular weight of a phenolic resin by complexing several molecules together through their phenolic hydroxyl groups. The gain in the resin molecular weight and prepress tack is sometimes accompanied by a reduction in assem­bly time tolerance and the loss of the B-stage melt-flow behavior. In hardwood gluing, this is sometimes not advantageous, and the addition of complexing salts should be approached with caution.

Mixed borate salts are very effective as a treatment for the preservation of wood products against fungi and most insects. However, the boron salts, which become localized in high concentration on the veneer surface, tend to gel the phenol resin before it can reach the wood surface and bond to it. However, very dilute aqueous solutions of borates (i. e.,

0. 25%) applied to softwood veneers in their green state decrease their thermal degradation during high-temperature drying and preserve their reactive sites for bonding with phenolic adhesives.