Phenol-carbohydrate , adhesive

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. 

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. 

Several chapters also demonstrate the use of smaller molecular-weight carbohydrates (i.e., monomers) in adhesives. Tony Conner and his colleagues (Chapter 25) explore the partial replacement of phenol-formaldehyde adhesives used to bond wood with various wood-derived carbohydrates. A1 Christiansen (Chapter 26) and Joe Karchesy and his coworkers (Chapter 27) investigate the very complicated chemistry and the practical application of adhesives based on the reaction of a carbohydrate with urea and phenol. Tito Viswanathan (Chapter 28) describes his attempts to utilize a very large carbohydrate waste stream, whey permeates from the processing of cheese, for the production of wood adhesives. 

Why then are these lignin sulfonates not used as a partial replacement for phenol in phenol-formaldehyde-based wood adhesives The first reason is that the presence of the sulfonate groups confers a water sensitivity to the adhesive. This sensitivity is exacerbated by the presence of water-soluble carbohydrates. A second reason is the low reactivity of the lignin sulfonates with formaldehyde and the consequent low level of crosslink density achieved in the final adhesive. A third reason is the molecular size of some of the lignin sulfonates. Large molecular weight material cannot penetrate the cell walls of the wood to form an adhesive continuum between contiguous wood particles. 

Choice of the Lignin Modification Reaction. The phenolysis reaction was selected as a means of modifying the structure and reactivity of the ammonium lignin sulfonate for three main practical reasons. First, because this lignin derivative is soluble in (and will ultimately be used in conjunction with) liquid phenol itself second, because unreacted phenol, unlike other reaction solvents, would not have to be removed from the phenolated product after reaction and before conversion to the adhesive resin and third, because lignins and carbohydrates are known to react with phenols under acidic conditions (6,7). 

Figure 1 illustrates the fact that resins and adhesives formed by the possible combinations of a phenolic compound, a nitrogenous compound, an aldehyde compound, and a carbohydrate have been reported in the literature. The exact conditions used to formulate the resins and adhesives represented in Figure 1 vary considerably. For example, additional circles representing acidic, basic, and neutral reaction conditions could be added. In most instances, the exact chemistry that occurs during the formulation of resins at each intersection is not known. Indeed, in many cases, the component actually reacting into the resin or adhesive system may not be the original carbohydrate added at the start. In this and other respects, these formulations will overlap with those discussed in the next section. 

Table III lists a number of selected references that describe the formulation of resins or adhesives at each intersection in Figure 1. PF, UF, UF modified with phenolics, and PF modified with nitrogenous compounds (e.g., urea) have not been included, because they do not contain carbohydrates and because they are in common use. The resin and adhesive systems that have been investigated most recently are those formed by the combination of carbohydrates with PF, both with and without the addition of a nitrogenous compound. Our attempts at the Forest Products Laboratory to use carbohydrate modified PF to bond wood are discussed in Chapter 25.

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. 

Carbohydrate-phenolic-based resins can be modified to change their physical and chemical properties, and faster curing adhesives can be made from these modified resins. However, the nature of the research presented here is exploratory, and much remains to be done. In particular, the molecular structure of these resins needs to be defined. 

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. 

Lignin is often considered nature s adhesive. It is the least understood and most chemically complex polymer of the wood-structure triad. Its composition is based on highly organized three-dimensional phenolic polymers rather than linear or branched carbohydrate chains. Lignin is the most hydrophobic (water-repelling) component... 

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). 

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. 

The primary chemical classes from which adhesives are made include epoxies, acrylics, phenolics, urethanes, natural and synthetic elastomers, amino resins, silicones, polyesters, polyamides, aromatic polyheterocyclics, and the various natural products such as carbohydrates and their derivatives as well as plant- and animal-based proteins. Chemical class was once a relatively clean differentiator of adhesives, but so many adhesives now are hybrids, designed to take advantage of specific attributes of more than one chemical class or type of material. Hybridization can be accomplished by incorporating into an adhesive a nonreactive resin of a different chemical class adding another type of reactive monomer, oligomer,... 

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