Formaldehyde release curing conditions

Formaldehyde release from UF-bonded wood products depends on the resin formulation and on curing conditions. The basic chemistry of... 

Formaldehyde is generally the aldehyde used in the preparation, setting, and curing of tannin adhesives. It is normally added to the tannin extract solution at the required pH, preferably in its polymeric form of paraformaldehyde, which is capable of fairly rapid depolymerization under alkaline conditions, and as urea-formalin concentrates. Hexamethylenetetramine (hexamine) may also be added to resins due to its potential formaldehyde releasing action under heat. Hexamine is, however, unstable in acid media [24] but becomes more stable with increased pH values. Hence under alkaline conditions the liberation of formaldehyde might not be as rapid and as efficient as wanted. Also, it has been fairly widely reported, with a few notable exceptions [25], that bonds formed with hexamine as hardener are not as boil resistant [26] as those formed by paraformaldehyde. The reaction of formaldehyde with tannins may be controlled by the... 

Novolac resins are produced when formaldehyde reacts with a molecular excess of phenol under acidic conditions. As there are few or no methylol groups in the molecules, these resins show very low reactivity and more aldehyde must be added in the process before curing. This is often done by the addition of a formaldehyde releaser, such as paraformaldehyde or hexamethylenetetraamine. 

The type of polymer obtained depends on factors such as the pH and temperature of reaction, the ratio of melamine to formaldehyde, and the type of catalyst employed. For decorative laminates, melamine-formaldehyde is prepared by reacting melamine in stainless steel kettles under reflux, alkaline conditions with 37% to 46% formaldehyde in aqueous solution. The reaction temperatures used vary from 80 to 100°C and are maintained until the condensation has reached the desired end point—that is, reacted sufficiently but still water-soluble. The end point is checked by measurements of viscosity, cure time, and water tolerance. Depending on the type of laminate to be produced, other constituents (surfactants, plasticizers, release and anti-foam agents) normally are added to the base resin before impregnation of the surface papers. It is common practice also at this stage to adjust the pH by adding acid catalysts. 

A third reason for predicting very low emissions of formaldehyde from phenolic panels is that the cured resin is extremely stable and does not break down to release additional formaldehyde, even under extremely harsh environmental conditions ( ). The high resistance of phenolic resins to deterioration under severe service conditions is, of course, a principal reason they are used so widely in making exterior types of wood panel products. Because of their chemical stability the U.S. Environmental Protection Agency has declared that phenol formaldehyde resins represent a consumptive use of formaldehyde, meaning that formaldehyde is irreversibly consumed in its reaction with phenol so that the formaldehyde loses its chemical identity (3). 

Environmental Protection. The potential release of formaldehyde during application and cure is a serious problem in the handling of amino resins. Depending on the structure and the conditions of manufacture a resin can contain 0.25-3% free... 

Phenolic. Phenolic adhesives are based on phenol-formaldehyde resins and are one of the earliest groups of structural adhesives to be developed. They are available in a variety of forms but usually suffer the disadvantage that they often release significant quantities of water on curing. For this reason heated presses are usually required and their employment almost inevitably involves the use of capital equipment and some degree of procedural complexity. Despite this, they and the related resorcinol-formaldehyde resins remain popular wherever structural integrity must be maintained under severe environmental conditions. 

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