Лако-красочные материалы — производство

Certain attributes of phenolic resins have been designed to give the strongest and most durable plywood bonds. Laboratory and field experience have demonstrated that certain types of PF plywood resins perform significantly better on veneers than do others. These superior resins have several properties in common:

1. They are relatively low in alkali content, generally about one-third to not more than one-half molar.

2. They have a lower molecular weight for hardwood veneers than do phenolic resins designed for softwood gluing.

3. They are high in methylol group content. Alternatively, they may contain free formaldehyde or require a matching catalyst that contains paraformaldehyde

4. Even in dried adhesive carrier films or powdered resins, phenolic resins for hardwoods shares the B-stage characteristic of reliquefying briefly under heat and pressure to allow transfer and flow on the glue lines of a plywood panel. This liquefaction can occur without water. This is unlike all but the most recently developed softwood phenolic resins.

5. Phenolic resins for hardwoods have higher thermal softening points than those of many other conventionally prepared resins. This indicates a network that has more cross-links after final cure and also greater durability.

6. They have a 40 to 45% solids content and 150 to 600 cP of viscosity at 25°C.

In general, lower resin alkali content and lower molecular weight are associated with slower cure, which explains why softwood plywood adhesive resins are both more alkaline and more condensed. Adequate exterior-grade adhesion can be attained on softwood veneer if these considerably condensed phenolic resins, which also cure more quickly, are used. Conversely, the increased functionality of hardwood adhesive resins partially compensates for their inherently slower cure rate. Hardwood phenolic resins require about 30% longer press times for adequate cure.

Notwithstanding their good adhesive capability, phenolic resins for hardwood gluing carry one distinct disadvantage: they do not prepress as well as softwood phenolic resins. Their lower condensation and longer flow are not assets. Prepressing is important because it is done to minimize face veneer losses and to reduce precure times on hot platens. Therefore, other means can be employed to obtain the required tackiness. Additives and adhesive formulations for this purpose are available. To increase the prepress capacity of low alkali, long-flow phenolic resins suitable for hardwood gluing, small amounts of starch or poly(vinyl alcohol) can be added to the resin glue mix just before use.

The finished resin viscosity is increased to allow for the thickening effect of these additives without reducing the average molecular weight of the phenolic resin. Water — soluble thickeners (such as hydroxyethyl cellulose, polyethylene glycols, and maleic anhy­dride copolymers) contribute to prepress tack. However, they cause a large increase in resin viscosity, and the amount added must be small. Consequently, the benefit to prepressing is limited. Most animal-and vegetable-based thickeners, such as gum arabic, are subject to hydrolysis in alkaline phenolic resins, and lose effectiveness in a matter of hours.