Formaldehyde-based thermosetting adhesives

The use of formaldehyde-free adhesive resins from whey and whey byproducts for manufacturing construction-quality boards could resolve these problems simultaneously. The demand for formaldehyde-based thermosetting adhesive resins in the United States was estimated to be 1.9 billion pounds in 1983 (5). The anticipated requirement for resins and the potential availability of raw materials from whey are a fortuitous combination. 

A wood-based composite can be defined as a composite material mainly composed of wood elements. These wood elements are usually bonded together by a thermosetting adhesive (wood truss products could also be regarded as wood-based composites, but connected by metal connectors). The commonly used adhesives include urea-based adhesive (such as urea formaldehyde resin), phenolic-based adhesive (including phenol resorcinol adhesives), isocyanate-based adhesive, and adhesives from renewable resources (like soybean, lignin etc). The wood elements in wood composites can be in many different forms such as ... 

So, in this chapter the interest and importance of the polymer gel theory approach to the formulation of adhesives will be briefly shown and, in particular, we will limit this to thermosetting wood adhesives obtained by polycondensation such as phenol-formaldehyde-, urea-formaldehyde-, melamine-formaldehyde- and resorcinol-formaldehyde-based adhesives. The same approach is, however, very valid for other polycondensation resins, and also for adhesives obtained by routes other than polycondensation. 

Phenol-formaldehyde (PF) resins are widely used as thermosetting adhesives for exterior-grade wood composites. Since phenol is traditionally derived from petroleum-based products, the production of PF resins from such phenol is relatively expensive and subject to changes in the price of petroleiun. One approach for reducing the cost of PF resins is to replace the petroleum-based phenol with phenols derived from renewable materials. Numerous studies... 

Chapter 5 Thermosetting formaldehyde based adhesives, by Dr A. Pizzi, a detailed study of UF, PF, RF and MF adhesives and glues, their chemistry and their uses in woodworking, panels and furniture (this chapter will be followed in later volumes by a chapter on Wood Based Panels and a chapter on Isocyanate Based Adhesives ). 

Chapter 4 Construction sealants, by Philippe Cognard Chapter 5 Silicone sealants, by Andreas Wolf, Dow Corning Europe Chapter 6 Thermosetting formaldehyde based adhesives, by Dr A. Pizzi Volume 4 Adhesives for large volume applications and industries (Packaging, Graphic Arts, Hot melts, etc.)... 

Phenolic Resins. PhenoHc resins [9003-35 ] (qv) are thermosets prepared by the reaction of phenol with formaldehyde, through either the base-cataly2ed one-stage or the acid-cataly2ed two-stage process. The Hquid intermediate may be used as an adhesive and bonding resin for plywood, particle board, ftberboard, insulation, and cores for laminates. The physical properties for typical phenoHc laminates made with wood are Hsted in Table 1. 

U.S. Pat. No. 6,291,558 [26] discloses a composite material prepared by spraying a urea-formaldehyde resin mixed with a thermosetting lignosulfonate-based resin material to wood particles, to assist in the adhesion of the wood particles. 

The first synthetic thermosets used as adhesives were phenol-formaldehyde resins produced at the end of the nineteenth eentury, historically linked to Baekeland s process which attained industrial status at the beginning of the twentieth century [4], Furanic condensates appeared mueh later as a result of the marketing of 2. They were first used as foundry binders by Quaker Oats in 1960. The use of furanic resins in the aerospace industry began ten years later. Although furanic resins represent a mere 1 % of the total thermoset produetion, the high added-value of these materials amply justifies their use. In fact, furan-based adhesives and binders are fire-, solvent-, and acid- or alkali-resistant. They are known, however, to display two main drawbacks related to their sensitivity to shrinkage and oxidation. 

Resins based on para-substituted phenols can be either one-step or two-step, but they cannot cure to a thermoset state. In the manufacture of phenolic resins, smaller quantities of acetaldehyde and furfuraldehyde are used in addition to formaldehyde. Furthermore, resorcinol, bisphenol A, and p-alkylphenols are employed, in addition to phenol, when special properties are desired. Formaldehyde concentrations of 37-50 weight % in aqueous solutions are most commonly employed. The catalysts most frequently used are acids such as oxalic, hydrochloric, sulfuric, p-toluenesul-fonic, and phosphoric and bases such as sodium, calcium, and barium hydroxide. In the weakly acidic range metal carboxylates are employed. Thermoset phenolic resins are employed as structural adhesives for laminating and bonding applications. Para-alkyl-substituted resins are employed as tackifiers in contact adhesives, pressure-sensitive adhesives, and hot-melt adhesives. 

Phenolics based on phenol formaldehyde thermosetting resins, two-part cold or heat and pressure cured. More expensive than most adhesives, but gives strong bonds for structural applications and good environmental resistance. 

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