Adhesives wood-based, preparation

There are many potential applications for the liquefaction and dissolution of chemically modified wood. Examples include the fractionation of modified wood components, the preparation of solvent-sensitive and/or reaction-sensitive wood-based adhesives [4,9,11,19], the preparation of resinified wood-based moldings such as the foam type [4], and the preparation of wood-based fibers and their conversion to carbon fibers [20]. 

Only a small amount of work has been done up to now concerning the prediction of bond strengths and other properties based on the results of the analysis of the resin. Ferg et al. [59] worked out correlation equations evaluating the chemical structures in various UF-resins with different F/U molar ratios and different types of preparation on the one hand and the achievable internal bond as well as the subsequent formaldehyde emission on the other hand. These equations are valid only for well defined series of resins. The basic aim of such experiments is the prediction of the properties of the wood-based panels based on the composition and the properties of the resins used. For this purpose various structural components are determined by means of - C NMR and their ratios related to board results. Various papers in the chemical literature describe examples of such correlations, in particular for UF, MF, MUF and PF resins [59-62]. For example one type of equation correlating the dry internal bond (IB) strength (tensile strength perpendicular to the plane of the panel) of a particleboard bonded with PF adhesive resins is as follows [17]... 

Phenol-formaldehyde resins are among the most important polymeric adhesives used in the wood based composite panel manufacturing industries [1]. Phenolic resins are prepared by the reaction of phenol or any substituted phenol with formaldehyde or other aldehydes, in the presence of acidic or basic catalyst. The price of phenol depends on the oil price and is likely to ever increase due to shortage of fossil resources. Hence, several lignin substitute products based on renewable materials derived from annual plants such as flax [2, 3] or kenaf [4], agricultural waste such as sugar cane bagasse [5] and wheat straw [6] or by-products from the... 

The use of synthetic resins Umits somewhat the environmental attractiveness of such adhesives based on purely natural materials, while the use of tannins alone is limited at present by the relatively Umited supply of these materials [1]. Thus, the aim is to prepare an adhesive based on materials of natural origin, satisfying international standards for both performance and emission, which does not emit or even better does not contain any formaldehyde, the composition of which does not include any synthetic resins, and that is less costly and uses widely available materials. This will render wood panel adhesives based on natural materials much more acceptable both economically and environmentally. 

This paper, thus, deals with the preparation of lower cost wood panel adhesives with good performance based on natural materials and needing no fortification by any synthetic resins. 

Cellulose is another natural polymer used to prepare adhesives by chemical modification of the alcohol functional groups in its structure. The source for cellulose can be cotton or wastes from wood processing and related industries. They are thermoplastic, solvent-based adhesives. 

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. 

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