Particle board, formaldehyde resins

The single largest use for the phenol-formaldehyde resins is in adhesive applications for the production of plywood, chipboard, and particle board. The resin can comprise as much as one-third of the weight of the board, particularly of particle boards, which contributes to a total demand for phenolics in the U.S.A. of over half a million metric tonnes per year. They are also used as the matrix adhesives for the production of several types of grindstones. In combination with paper, woven cotton, glass fiber, etc., components, phenolics contribute to the production of engineering and decorative laminates in the form of rods, tubes, and sheets. The sheet products Arborite and Formica are familiar as the finished surfaces of furniture, bathroom, and kitchen counter tops and other areas where attractive patterns and water resistance are important characteristics. Molded products from phenolics are also important where heat or electrical resistance is required, such as saucepan and toaster handles, switches, and the printed circuit boards used in computers. Recent phenolics production in the U.S.A. has totaled over 500,000 metric tonnes per year, not including fillers [38]. 

Urea, formaldehyde Stepwise polycondensation Particle-board binder resin (60%), ... 

Amino and Phenolic Resins. The largest use of formaldehyde is in the manufacture of urea—formaldehyde, phenol—formaldehyde, and melamine—formaldehyde resins, accounting for over one-half (51%) of the total demand (115). These resins find use as adhesives for binding wood products that comprise particle board, fiber board, and plywood. Plywood is the largest market for phenol—formaldehyde resins particle board is the largest for urea—formaldehyde resins. Under certain conditions, urea—formaldehyde resins may release formaldehyde that has been alleged to create health or environmental problems (see Amino RESINS AND PLASTICS). 

Formaldehyde. Worldwide, the largest amount of formaldehyde (qv) is consumed in the production of urea—formaldehyde resins, the primary end use of which is found in building products such as plywood and particle board (see Amino resins and plastics). The demand for these resins, and consequently methanol, is greatly influenced by housing demand. In the United States, the greatest market share for formaldehyde is again in the constmction industry. However, a fast-growing market for formaldehyde can be found in the production of acetylenic chemicals, which is driven by the demand for 1,4-butanediol and its subsequent downstream product, spandex fibers (see Fibers, elastomeric). 

Wood Bonding. This appHcation requires large volumes of phenoHc resins (5—25% by weight) for plywood, particle board, waferboard, and fiberboard. Initially, phenoHc resins were used mainly for exterior appHcations, whereas urea—formaldehyde (UF) was used for interiors. However, the concern over formaldehyde emission has caused the replacement of UF by phenol-formaldehyde adhesives. 

Particle board and wood chip products have evolved from efforts to make profitable use of the large volumes of sawdust generated aimually. These products are used for floor undedayment and decorative laminates. Most particle board had been produced with urea—formaldehyde adhesive for interior use resin demand per board is high due to the high surface area requiring bonding. Nevertheless, substantial quantities of phenol—formaldehyde-bonded particle board are produced for water-resistant and low formaldehyde appHcations. 

Poly(vinyl alcohol) is employed as a modifier of thermosetting resins used as adhesives in plywood and particle board manufacture (314,315). The polymer is added to urea-formaldehyde or urea—melamine—formaldehyde resins to improve initial grab, to increase viscosity, and, in general, to improve the characteristics of the board. 

Amino Resins. Amino resins (qv) include both urea- and melamine—formaldehyde condensation products. They are thermosets prepared similarly by the reaction of the amino groups in urea [57-13-6] or melamine [108-78-1] with formaldehyde to form the corresponding methylol derivatives, which are soluble in water or ethanol. To form plywood, particle board, and other wood products for adhesive or bonding purposes, a Hquid resin is mixed with some acid catalyst and sprayed on the boards or granules, then cured and cross-linked under heat and pressure. 

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. 

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). 

Formaldehyde from particle board, pressed wood, urea-formaldehyde foam insulation, plywood resins, hardwood paneling, carpeting, upholstery ... 

Uses. Manufacture of formaldehyde resins, which are used as adhesives in particle board, plywood, and insulating materials countertops and wall paneling coating to fabrics to impart permanent press characteristics manufacture of rubber, photographic film, leather, cosmetics, embalming fluids, insulation disinfectants and fumigants... 

In contrast to particle boards bonded with synthetic resins, the biobonded boards do not emit any harmful vapors as do, for example, formaldehyde-bonded boards. 

The second report on MDF in 1976 came from Chow at the University of Illinois (62). He, too, worked with hardwood barks including cottonwood, red oak, white oak, and walnut. Furnish was prepared in pressurized refiners and by hammermilling. Urea formaldehyde resin percentages were 5.0, 7.5, and 10.0%, plus 1% wax. He concluded that the fiber from the pressurized refiners was superior to hammermilled particles. Cottonwood and white oak furnish gave better boards, exceeding or approaching requirements of present standards for type 1-B-l commercial particleboard. 

Wood Composites—these are resin-bonded composite boards where the particles are wood shavings, flakes, chips, or fibers bonded with thermosetting adhesives that can be urea formaldehyde, melamine formaldehyde, phenol formaldehyde, or diisocyanate. In recent years, the markets for OSB and MDF board have been rapidly increasing. Most particle board production uses urea-formaldehyde as a binder that is acid setting. Hence, sodium borates (alkaline) can interfere with the setting. As a result, boric acid has been the major boron compound used as the flame retardant in particle board.28 29 Typically, a loading of 12%-15% of boric acid in MDF is required to meet the ASTM E-84 Class A rating. If sodium borate is used as a flame retardant, phenol-formaldehyde binder, that is compatible with alkaline chemicals, is commonly used. 

Plywood and particle board are often glued with cheap, waterproof urea-formaldehyde resins. Two to three moles of formaldehyde are mixed with one mole of urea and a little ammonia as a basic catalyst. The reaction is allowed to proceed until the mixture becomes sympy, then it is applied to the wood surface. The wood surfaces are held together under heat and pressure, while polymerization continues and cross-linking takes place. Propose a mechanism for the base-catalyzed condensation of urea with formaldehyde to give a linear polymer, then show how further condensation leads to cross-linking. (Hint The carbonyl group lends acidity to the N—H protons of urea. A first condensation with formaldehyde leads to an inline, which is weakly electrophilic and reacts with another deprotonated urea.)... 

Particle board with sizes 600 X 600 X 16 mm was obtained in laboratory conditions with the use of mixtures of OSR and phenol-formaldehyde resin of KF-MT brand. OSR was introduced as 17 mass % water suspension. Physical-mechanical characteristics of these... 

Application Urea-formaldehyde resins are used as adhesives in the wood-working industry and are typically used in the production of plywood and particle board. They are available as concentrated solutions or in powder form as a result of the spray-drying process. 

This process cost-effectively produces high-quality glues at large quantities. The product is a low-formaldehyde resin adhesive, suitable for veneering, plywood and particle board production by the hot pressing process. The quality of the bonding complies with the requirements of DIN 68705, Part 2 respectively to DIN 68763-V20. For particle board, a perforate value according to DIN EN 120 of less 10 mg HCHO/100 g dry board will be maintained. 

In the chemical industry, simple aldehydes and ketones are produced in large quantities for use as solvents and as starting materials to prepare a host of other compounds. For example, more than 1.9 million tons per year of formaldehyde, H2C=0, is produced in tlie United States for use in building insulation materials and in the adhesive resins that bind particle board and plywood. Acetone, (Cll3)2C = 0, is widely used as an industrial solvent approximately 1.2 million tons per year is produced in the United States. Formaldehyde is synthesized industrially by catalytic oxidation of methanol, and one method of acetone preparation iirv olves oxidation of 2-propanol. 

An alkaline catalyst like NaOH is used, with formaldehyde to phenol ratio varied from 1 1 to 3 1 (usually 1.8-2.4 ) for particle boards. Apart from catalyzing the hydroxymethylation of phenol, NaOH also serves to provide the required water solubility of the resin, even at a high molecular weight, through the formation of sodium phenoxides. Furthermore, it accelerates the curing of the resin. 

As indicated previously, a significant feature of these resins is the very low content (<0.1%) of the monomers, phenol and formaldehyde. The molecular weights of resins used in plywood manufacture are higher, and the viscosity is in the range of 700 (wet process) or 450 mPa s (dry process) (9). In the manufacture of particle boards, which consist of —95% by weight of wood chips bonded together by the adhesive, the viscosity of the resin used is much lower, about 130 mPa s (9). In both cases, the resin has a dry solids content of 40-50%. 

Formaldehyde release from pressed wood products is due to latent formaldehyde. During the pressing process, hot steam from moist wood particles transfers heat, formaldehyde, and other volatiles from the surface of the mat to the core of the board where un reacted urea-formaldehyde resin components accumulate. The resulting formaldehyde concentration in the core is approximately twice that of the surface. Release of formaldehyde is diffusion-controlled and gradually decreases over time (Meyer and Hermanns 1985). Formaldehyde can also be produced by hydrolytic cleavage of unreacted hydroxymethyl groups in the formaldehyde resins. Melamine formaldehyde resins generally are more stable, and the amounts of formaldehyde emitted from them are much lower (WHO 1989). 

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