Bonding to wood

A separate mention is merited for a special molded hardboard product. These are made by a process in which either a fiber mat or hardboard panel is placed between two shaped platens and press-molded to a three-dimensional configuration. The most common resulting shape is a doorskin which resembles a wood panel door. The doorskins are bonded to wood frames to make an excellent, attractive, and relatively inexpensive door. This ftber/panel mol ding process is also used to make a wide variety of molded interior linings used in automobile manufacture. 

Plastic laminated sheets produced in 1913 led to the formation of the Formica Products Company and the commercial introduction, in 1931, of decorative laminates consisting of a urea—formaldehyde surface on an unrefined (kraft) paper core impregnated with phenoHc resin and compressed and heated between poHshed steel platens (8,10). The decorative surface laminates are usually about 1.6 mm thick and bonded to wood (a natural composite), plywood (another laminate), or particle board (a particulate composite). Since 1937, the surface layer of most decorative laminates has been fabricated with melamine—formaldehyde, which can be prepared with mineral fiUers, thus offering improved heat and moisture resistance and allowing a wide range of decorative effects (10,11). 

Polymerization kinetics have been used to question whether or not phenolic adhesives covalently bond to wood. Chow (23)... 

The fact that covalent bonding can be an important, and possibly necessary, contribution to water-proof adhesive bonds to wood has convinced many scientists to study methods of enhancing adhesion by increasing the probability of covalent bonding between wood and adhesive, or directly between wood particles. This subject is still in its infancy with solid wood, although pulp and textile fiber scientists have produced an enormous volume of literature from which wood scientists can draw. 

The new technical developments have made possible quick bonding to woods, papers, and porous surfaces. Polyolefins, which comprise approximately 50% of the U.S. thermoplastic production, are now bondable with the cyanoacrylic ester adhesives. These new capabilities are sure to provide for continued market growth in the years ahead. 

In a study concerned with the decay resistance provided by isocyanate bonding to wood, the distribution of the methyl isocyanate reaction in southern pine showed that 60% of the lignin hydroxyls and 12% of the holocellulose hydroxyls are substituted at the point where resistance to biological attack occurs. Therefore, it can be surmised that the chemical bonding of wood by isocyanates through the urethane link can contribute significantly to the excellent performance of diphenylmethane-diisocyanate X or polymeric isocyanates XI as adhesive binder in particle boards (24, 25). 

There have been many attempts to investigate the effect of extractives on cure chemistry and bonding to wood (33-35). For example, the effect of extractives from pressure-refined hardwood fiber on urea-formaldehyde resin was studied (34, 35) and it was found that the ethanol-soluble extractives decreased the gel time as much as 41%, and the sequentially extracted water-soluble extractives increased the gel time in excess of 65%. There was little correlation between the extractive content and gel time however, an empirical relation between the pH of the extractives and the gel time was observed (35). The effect of several species of wood on the gel time of urea-formaldehyde resin have also been studied (36). In these studies the gel time was correlated with the pH and acid buffering capacity of the extract. 

The chemistry of the bonding processes must be understood better for further progress to be made. Although there is no lack of hypotheses explaining the chemical transformations taking place, only a limited amount of work has been done to allow an insight into what actually happens. In many cases even such items as the chemical nature of surface activation and existence of covalent bonds to wood surface are not understood. 

There have been contradictory reports about the reaction of wood with formaldehyde from UF-resins. At room temperature, and up to the boiling point of water, wood absorbs only very little formaldehyde. Thus, gine chips treated with 35 wt% formaldehyde solution for 30 min at 160°C retain less than 0.01 wt% formaldehyde (3). Forest products scientists generally assume that UF resins do not bond to wood (4). However, at higher temperatures, wood absorbs formaldehyde and irreversibly changes its physical properties. Thus, after 15 hrs of exposure at 120 C, 7 wt% formaldehyde is retained by solid oak and causes a 50% reduction in swelling (5-8). Since wood cellulose is... 

Epoxy Room temp Excellent with most substrates Excellent Excellent Used for high-performance lamination and for bonding to wood and aluminum... 

These are used in bonding metal or reinforced plastic facings to paper (resin impregnated) honeycomb structures, cork and rubber compositions, cyclized and unvulcanized rubbers, steel to vulcanized rubber, and electrical applications. They are also used as primer for metals to be bonded to wood with phenolics. Polyvinyl butyral-phenolics lack the shear strength and toughness of the polyvinyl formal-phenolic type. ... 

Another approach to improve adhesive bonding to wood is to graft acid anhydrides to its surface, and then react the resulting carboxylic acid group with the adhesive. Mallon and Hill [22] used NMR and FTIR to show that succinic anhydride reacts with hydroxyl groups on wood, and the acid groups can subsequently be reacted with hexamethylene diamine. The reaction scheme is shown in Fig. 4. 

D. M. Brewis, J. Comyn, and C. Phanopoulos, Durability and surface analytical studies of adhesive bonding to wood, J. Adhes. 21, 303 (1987). 

Adhesives are also used in the manufacturing of prefabricated sections or assemblies. Examples include complete wall sections with the sheathing bonded to wood frames roof sections with sheathing bonded to j oists and rafters to provide T-beam action and box beams and other types of composite beams for joists or rafters. ] Phenolic adhesives are used in the fabrication of laminated beams and trusses. Epoxies are also widely used for bonding and the repair of concrete. ]... 

Applications where acrylics see use include the heat sealing of cellophane to metal foil and metallized polyester film or polypropylene film. These films are used primarily in the food packaging area. Other applications are in the overlay area, where clear or photoembossed vinyl films are bonded to wood substrates to produce a simulated expensive wood grain finish. 

VAR 5898 Higher melting version of 5125. Outstanding bonds to wood, metal (preheated) and most thermoplastics. Excellent general purpose high performance adhesive. 123 218 185 Mod- erate 225°F... 

Polyurethane Best for window and door use, as it bonds to wood. Unlike silicone, polyurethane caulk dries hard. 

Plywood, particle board and timber laminations - Interior grade plywood and particle board are made using urea formaldehyde adhesives due to their good bonding to wood and low cost. Their disadvantage is poor water resistance. Exterior grades of plywood use phenol formaldehyde adhesives due to their good water resistance and low cost. 

University of Wales Bangor, Department of Agricultural and Forest Sciences (C Hill -bonding to wood and wood composites)... 

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