Particle board reactions

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. 

Milled wood lignin was mixed with the crude enzyme solution of Tram-ties versicolor extracellular phenoloxidases produced on spent sulfite liquor in a ratio of approximately 2 1. This comprised the main part of the two-component bio-adhesive. Industrial particles were bonded with 15% bioadhesive under conventional pressing conditions to have 19 mm particle boards (40 x 50 cm) of the properties described in Table IV. The bonding reaction (crosslinking) took place in aqueous solution at room temperature. If conventional pressing technology is applied, the temperature should be elevated in order to maintain water evaporation within a reasonable press time. 

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.)... 

Preparation of Hot-Melt and Self-Bonded Boards. Reaction conditions for benzylation are dependent on wood species and specimen size. For Akamatsu particles, pretreatments and reaction conditions are as follows wood is first oven-dried at 105°C for 24 h and then immersed in 40% sodium hydroxide solution for 1-2 h at room temperature. These particles are then squeezed to remove excess sodium hydroxide and reacted with benzyl chloride at 120°C for 1-2 h. Benzylated particles are then washed in water and any unreacted reagent is removed by squeezing. Particles are then washed further in a mixture of water-methanol (1 2 v/v) and air-dried for 48 h at 20 C. In general, benzylation of softwood species is more difficult than for hardwoods, and the reactivities of wood from fast-growing trees, i.e., willow or monocotyledons,... 

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). 

Surface Activation. Acid Activation. Acid treatment of cellulose and hemicelluloses generally leads to hydrolysis to monosaccharides, which can subsequently dehydrate and condense to form furan-type compounds such as furfural and its 5-hydroxymethyl adduct. Further reactions lead to polymeric materials of dark color as well as to monomers such as levulinic acid, formic acid, and angelica lactones. Various condensation and solvolysis reactions also accompany the acid treatment of lignin 123). The hydrolysis, dehydration, and condensation reactions have been used to explain formation of covalent bonds between surfaces (85), increase in water resistance (85, 124), and weakening of wood (75) in nonconventional plywood or particle board production. However, very little factual information is available on how far, in terms of the consecutive reactions mentioned, and in what direction, in terms of the parallel reactions mentioned, does the surface of lignocellulosic materials actually change... 

Chronic skin exposure to turpentine may produce a hypersensitivity reaction, with bullous dermatitis and/or eczema. A case-control study of workers in particle-board, plywood, sawmills, and formaldehyde glue factories demonstrated a statistically significant association between chronic exposure (longer than 5 years) to terpenes (the principal component of turpentine) and the development of respiratory tract cancers. 

Formaldehyde and other aldehydes are receiving increasing attention both as toxic substances and as promoters in the photochemical formation of ozone in the atmosphere. They are released into residential buildings from plywood and particle board, insulation, combustion appliances, tobacco smoke, and various consumer products. Aldehydes are released into the atmosphere in the exhaust of motor vehicles and other equipment in which hydrocarbon fuels are incompletely burned. A sensitive method for analyzing aldehydes and ketones is based on the sorption of these compounds to an SPE sorbent and their subsequent reaction with 2,4-dinitrophenylhydrazine (DNPH) on the sorbent. They are then analyzed as their hydrazones by HPLC (Fig. 7.9). A gradient analysis by HPLC may separate as many as 17 components with detection by ultraviolet (UV) light. 

Diisocyanates are an important class of chemicals of commercial interest, which are frequently used in the manufacture of indoor materials. such as adhesives, coatings, foams and rubbers (Ulrich, 1989). In some types of particle board, the diisocyanates have replaced formaldehyde. Isocyanates are characterized by the electrophilic -N=C=0 group, which can easily react with molecules containing hydroxy groups, such as water or alcohols. On hydrolysis with water, primary amines are formed, while a reaction with alcohols leads to carbamates (urethanes). Polyurethane (PUR) products are then obtained from a polyaddition of diisocyanate and diol components. Compounds commonly used in industrial surface technology are 4,4 -diphenylmethane diisocyanate (MDI) and hexamethylene diisocyanate (HDI). The diisocyanate monomers are known as respiratory sensitizers and cause irritation of eyes, skin and mucous membrane. Therefore, polyisocyanates such as HDI-biuret and HDI-isocyanurate with a monomer content <0.5 % are used for industrial applications, and isocyanate monomers will not achieve high concentrations in ambient air. Nevertheless, it is desirable to measure even trace emissions from materials in private dwellings. 

Usually no hardeners are added during the production of wood-based panels (particle board, MDF, OSB, engineered wood products) using PMDI as adhesive. With special additives a distinct acceleration of the hardening reaction and hence shorter press times or lower press temperatures can be achieved [209]. This fact is especially interesting for coldsetting systems as well as for the production of particleboards. Possible catalysts are tertiary amines (e.g., triethanol amine, triethylamine, A,A-dimethylcyclohexylamine) and metal catalysts, based on organic compounds of tin, lead, cobalt, and mercury [208,215-218],... 

For this processing, the waste particle board was chipped but not ground to a fine meal. Milling of the particle board is not necessary because the cellulosic material is rapidly decomposed in the acid-catalyzed reaction, which releases the resin into the solvent, resulting in a more economical process. In the Carves patent 10 the wood and waste paper were first converted to a pulp and to a fluff, respectively. The moisture content of the as-received particle board was 3 wt%. Waste materials such as this with a low moisture content provide a good substrate for the alcohol reaction, since no drying is required. 

In a typical reaction, a mixture of 16 g of chipped laminated particle board, 150 mL of ethanol, and 3 g of concentrated sulfuric acid was heated at 200 C for 30 minutes in a 300-mL Parr pressure reactor. The mixture was allowed to cool to room temperature. The product mixture was filtered with a Biichner funnel, and the charcoal residue was washed with ethanol and dried to give 5.4 g of charcoal. The filtrate was neutralized with sodium bicarbonate and distilled to recover the excess ethanol and the water, ethyl formate, and trace amounts of diethyl ether produced in the reaction. To extract the levulinate ester, diethyl ether was added to the residue from the distillation, which contained a small amount of ethanol. The insoluble portion of the distillation bottoms gave the resin product (3.3 g). The ether-soluble portion contained ethyl levulinate and a small amount of furfural. After the ether was stripped on a rotary evaporator, the residue (4.5 g) was vacuum-distilled to give ethyl levulinate (3.8 g) and impure furfural (0.4 g). 

A larger-scale reaction was conducted with 240 g of chipped laminated particle board in 2.2 1 of ethanol in a 1-gallon autoclave at 200 C for 40 min. The heatup and cooldown times for the reaction were much longer, but similar wt% yields of charcoal, esters, and resin were obtained. 

A limited matrix of tests was performed to determine effects of process variables, including the type of alcohol used in the processing of particle board. The acid-catalyzed reaction of particle board in ethanol was carried out at several temperatures and reaction times to determine optimum conditions. The concentration of sulfuric acid in ethanol was 2% in all tests. The product yields given in Table I are based on as-received particle board. 

Yields for several alcohols were determined for the reaction of particle board at 190°C for 40 min reaction time (2% sulfuric acid catalyst). The results for the various alcohols are given in Table II. Methanol and ethanol are essentially equivalent in forming the alkyl levulinate on a weight percent basis. For longer-chain alcohols, the yields drop off substantially with size. In the case of the aminoethanol, methoxyethanol, and hexanol, the solid material is recovered as brown granules and cannot be dried easily. No alkyl levulinate is found in the solvent phase. By comparison with water as the solvent, the charcoal yield was 46 wt% and the levulinic acid was 18% as determined by gas chromatography. 

Table II. Formation of Alkyl levulinates in Reactions of Particle Board in...

Levulinic acid is known to react with aldehydes at carbon atoms adjacent to both the acid (a) and keto (P and 8) groups to form a mixture of the a-, P-, and 8- alkylidene or arylidene derivatives 11,12), With excess formaldehyde, levulinic acid is substituted with five or six hydroxymethyl (methylol) groups, and cyclization to a lactone occurred 18), These condensation reactions could be utilized to produce inexpensive resins from the crude ethyl levulinate obtained from waste particle board without distillation. Another reason for studying the reactions of the ester with aldehydes is to determine the potential for similar reactions occurring during the cellulose... 

A similar reaction of ethyl levulinate was conducted with a UF preparation. The acid-catalyzed reaction incorporated the levulinate into the resin structure, forming aii insoluble mass. This reaction does not occur to a large extent during the initial particle board degradation owing to the large volume of ethanol solvent however, it can cause a problem in the workup of the products after the ethanol has been removed. The reaction explains the failure of attempts to distill ethyl levulinate directly from the levulinate-resin product mixture without prior extraction of the ester from the resin. These attempts resulted in low ester yields and the formation of intractable resin. Thus, by extracting the ester we can avoid the inadvertent condensation reaction with the resin. 

In summary, conversion of waste particle board and plywood directly to levulinate and formate esters utilizes waste that represents a disposal problem in a process that generates easily purified products with minimal wastewater. Commercialization of this process depends on market values for levulinate esters that are typical of chemical intermediates for resins and coatings. New challenges are presented in finding ester reactions and subsequent purification steps that lead to low-cost derivatives that will be useful for these industries. The economics may also be favorable for producing ester fuel additives with no purification required. 

Phenol formaldehyde, a synthetic thermosetting resin obtained by the reaction of phenols with aldehyde, is used for making plywood, particle board, medium density fiber boards and other wood- and lignocellulose-based panel and wood joinery. Phenolic... 

For a general introduction, see Wood adhesives - basics. Aminoplastic resins are polycondensation products of the reaction of aldehydes with compounds carrying amine or amide groups. Formaldehyde is by far the primary aldehyde used. The name amino-plastic is traditional, but is a misnomer these are thermosetting, irreversibly hardening resins. Two main classes of these resins exist urea-formaldehyde (UF) resins and melamine-formaldehyde (MF) resins. Today, for wood adhesives, pure MF resins are not used anymore because of cost, and only melamine-urea-formaldehyde (MUF) adhesives are used. The main use of these resins is in composite products such as particle board, plywood, medium density fibreboard (MDF), oriented strand board (OSB), glu-lam/flngerjointing and furniture. 

Urea-formaldehyde These are widely used for particle board and MDF manufacture and, to a smaller extent, for plywood. They are produced by the reaction of formaldehyde with urea, in molar ratios of between 1.2 and 1 and 2.0 and 1. Low molar ratios are preferred to minimize emission of formaldehyde in service although low molar ratio resins have had inferior strength and water resistance and are slower curing, recent formulations have overcome these drawbacks to some extent. Curing occurs at elevated temperatures, with ammonium chloride, a common catalyst. 

Phenolics or phenol-formaldehyde structural adhesives are chemically reactive systems that cure to form thermosets. In one-component systems, meltable powders (resols) are used as binders for particle board or as alloys (including nitrile-phenolics, vinyl - phenolics, and epoxy-phenolics), which are used in the structural bonding of metals. In two-component systems, the resin and catalyst are mixed and then heated to initiate curing. Both systems cure by a condensation reaction that produces a byproduct. 

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