Phenol-carbohydrate-urea-formaldehyde resins

Phenol/carbohydrate/urea/formaldehyde resin with molar ratios of 1 1 0.125 2, 2.09 Pa-s viscosity, bonded between two parallel-laminated yellow birch veneers for 5 minutes. 

This chapter reports work on two aspects of this adhesive system 1) tests on the strength of panels bonded with phenol/carbohydrate/urea/formaldehyde (P/C/U/F) adhesive compositions outside the ranges previously reported (9,10) and 2) analysis of chemical reactions in this resin system. 

The amounts of lignins, tannins, and carbohydrates available as residues from processing of forest trees dwarf the commodity adhesive market. At the same time, the forest products industry is especially reliant on adhesives, since over 70% of all wood products are bonded, and their production consumes about 45% of all phenolic and 85% of all urea-formaldehyde resins produced in the United States. 

The author has found the carbohydrate binders (e g. starch, dextrin) superior to phenol formaldehyde resins, maleic and phthalate type resins, polyester, glyptal resins, shellac, vegetable rosins, urea formaldehyde resin and melamine resin in all experiments to date. Dextrins and starches combine good binding properties and low cost with simple handling requirements. 

UF, urea-formaldehyde resin MUF, melamine fortified UF resin MF/MUF, melamine and melamine-urea resins (MF resins are only used mixed/coreacted with UF resins MUPF, melamine-urea-phenol-formaldehyde resin PF/PUF, phenol and phenol-urea-formaldehyde resin (P)RF, resoreinol-(phenol-)formaldehyde resin PMDI, polymeric methylenediisocyanate PVAc, polyvinylacetate adhesive old nat.adhesives, old (historic) natural adhesives (e.g., starch, glutin, casein adhesives) nat.adhesives, natural adhesives (e.g., tannins, lignins, carbohydrates) inorg.adhesives, inorganic adhesives (e.g., cement, gypsum) activation activation constituents of wood to function as adhesives (i.e., lignin). 

Modified Synthetic Adhesives. Phenol-formaldehyde (68) and urea-formaldehyde (69) are important synthetic adhesives. Phenol-formaldehyde adhesives (PF) find a variety of applications including bonded abrasives, foundry applications, fiber bonding, and wood bonding. Urea-formaldehyde adhesive resins (UF) are used generally to bond wood products. I will illustrate the modification of synthetic adhesives with carbohydrates using both these general types of adhesives. 

The presence of numerous hydroxyl groups able to react with formaldehyde makes starch-derived products suitable chemicals for formaldehyde-based resins. Research on this subject started many years ago and showed that in a number of applications it is possible to partially replace or extend urea formaldehyde, phenol formaldehyde and melamine formaldehyde resins without significantly affecting the finished product s performance. In many applications, adhesive systems based on formaldehyde resins incorporate a polysaccharide component. More than 4.5 Mio mto of formaldehyde-based resins have been produced in Western Europe alone. The use of carbohydrates allows lower consumption of oil-based resins and, consequently, reduced release of formaldehyde in the environment. 

Figure 1. Resins can be classified as being formed from combinations of a phenolic compound (P typically phenol), an aldehyde (A typically formaldehyde), a nitrogenous compound (N typically urea), and a carbohydrate (C).

Phenol-formaldehyde type polymers had been the only exterior-durable adhesives for wood bonding, until the recent limited use of isocyanates. Both systems are petrochemical-based. Several researchers substituted carbohydrates for part of phenolic adhesives (1-4) > producing solid, fusible novolak resins. Recently, reaction of carbohydrate acid-degradation products with phenol and formaldehyde has produced liquid resols (5). Gibbons and Wondolowski (6,7) replaced a considerable amount of phenol with carbohydrate and urea to pro-... 

Carbohydrate-phenolic-based resins have shown promise for partial replacement of phenol and formaldehyde in exterior plywood adhesives (7,2). Such resins are produced in a two-stage reaction sequence. First, the carbohydrate is reacted with phenol, and sometimes urea, under acid catalysis at elevated temperatures (up to 150 °C), to produce an acid-stage resin. The acid-stage resin is then made basic, formaldehyde added, and the reaction continued at lower temperatures to produce a resol-type resin. Adhesives formulated from these resins have curing speeds consistent with present-day plywood production needs in the western United States, veneers are typically dried to 0 to 7% moisture content and the adhesive cured by hot pressing the panels at approximately 140 to 150 °C and 1.2 MPa. 

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