Particleboard binder

Adhesive. Urea-formaldehyde water-based dispersions are the most widely used particleboard binders. The low-cost, rapid curing, and colorless properties of urea-formaldehyde adhesives make them the adhesive of choice for most interior particleboard. These adhesives have been continuously improved by the resin manufacturers, resulting in reduced press times without detrimental effects on their storage life or handling characteristics. 

The initial development of polymeric MDI as a particleboard binder predates the above work but it is, we believe, based upon the same chemistry. Exterior or structural particleboard has been manufactured in North America and in Europe for years with predominantly phenol-formaldehyde binders and, in a few cases, with melamine-modified urea-formaldehyde binders. The familiar urea-formaldehyde resins are subject to hydrolysis and are thus suitable for interior board only. 

The West German parent of Mobay Chemical Corporation was very active, along with Deutsche Novopan, in the development of isocyanate as a particleboard binder in Europe. The work of Sachs (12, 13, lL) and of Deppe (L, 5.), along with the aforementioned commercial developments, has provided a background for interest in isocyanate binders in North America. This interest is beginning to be reflected in the literature. For example, Hse (15.) has published papers on the development of both plywood and flakeboard adhesives which combine isocyanate and phenolic resins. In the former case, he claims adequate bonding with wetter veneers than can be used with conventional phenolic adhesive. In the case of... 

Combinations of isocyanates with other resins to be used as particleboard binders have been reported by several research groups. Mixtures with formaldehyde were claimed to give strong, moisture-resistant bonds. Increased and very good moisture resistance were reported... 

For VOCs, control options are multiple. Source reduction or removal includes product substitution or reformulation. Particleboard or pressed w ood has been developed and used extensively in building materials for cabinet bases and subflooring and in furniture manufacturing for frames. If the product is not properly manufactured and cured prior to use as a building material, VOCs can outgas into the interior of the residence or building. Other sources of VOCs may be paints, cleaning solutions, fabrics, binders, and adhesives. Proper use of household products will lower volatile emissions. 

Several large applications for one-component moisture-cure urethane adhesives are available. Polymeric MDI is an exceptional binder for wood products, such as oriented strand board and particleboard. One-component urethane windshield adhesives are used almost exclusively in both the OEM and automotive aftermarket. One-part urethane adhesives are used to assemble the sidewalls for recreational vehicles (RV s), manufactured housing, and mobile homes. In construction applications, one-part urethanes are used to bond metal doors, hardwood flooring, panels, and partitions. 

The production of moisture resistant particleboard by treatment with a maleic anhydride -glycerol mixture and using phenol formaldehyde as the matrix material has been investigated (Fujimoto etal., 1987). Boards prepared from modified wood showed considerable improvements in modulus of elasticity and internal bond strength when compared to control boards. Composites made from aspen fibres modified with SA, MA or acetic anhydride using phenol-formaldehyde (PF) or polypropylene as binder have also been studied (Clemons etal., 1992 Rowell etal., 1993b). The reaction of wood with MA was found to proceed at a slower rate than with SA. The volume increase due to modification... 

Kiguchi, M. and Suzuki, M. (1985). Physical properties of particleboard with acetylated chips and binder. Mokuzai Gakkaishi, 31(3), 200-208. 

Phenolic resins produced by the reaction of phenol and formaldehyde were used as adhesives by Leo Baekeland in the early 1900 s. This inexpensive resin is still used for binding thin sheets of wood to produce plywood. Urea resins produced by the reactions of urea and formaldehyde have been used since 1930 as binders for wood chips in particleboard. 

One technique being used is to bond solid wood with monomeric materials that are highly reactive with hydroxyl groups such as those in the wood fiber wall. For example, Schoring et al. (65) synthesized particleboards using as a binder hexamethylene diamine and polyvinyl chloride at 140°C. They postulated that the polyfunctional amine covalently linked the wood components, especially lignin, to the polyvinyl chloride. 

Properties of the HDA boards prepared from the activated substrate may be compared to the phenol-formaldehyde (PF) boards prepared using 6 resin solids. Comparison may also be made to the property requirements from commercial standard CS 236-66, type 2 medium density (below 0.8 g/cc), Class 1 particleboard fabricated using durable and highly moisture resistant and heat resistant binders suitable for interior and certain exterior applications. Specifications of this class of particleboard, generally prepared from PF resins, are MOR, minimum = 2,500 psi M0E, minimum =... 

Another report came from Canada in 1959 when Bender published research results of utilizing eastern Canadian barks as furnish for wet-process insulation board and hardboard (33). Bark species included in the study were black spruce and balsam fir each contained 25-35% wood. A Sprout-Waldron disk refiner was used to prepare the bark fiber, and boards were made with lh% wax emulsion but contained no added binder. Physical tests indicated the boards met some commercial specifications the author believed that addition of more woody fiber would improve the properties. In addition, a few experimental dry-process particleboards were made with addition of some unnamed binder that was a byproduct material. 

One year later, Burrows published results from a comprehensive series of experiments using 100% Douglas-fir bark as furnish for particleboard (35). No binder was used he relied upon the "self-bonding" properties of Douglas-fir bark. Variables included bark particle size, mat moisture content, pressing pressure, and use of various overlays. Additional boards were made from ponderosa pine, western hemlock, and white fir barks. A pilot-plant-size run was made using results gathered from the study. 

Biblos and Coleman investigated another type of potential structural composite product (53). They made and tested panels consisting of a particleboard core from sawdust and bark and faces of veneer. All material was southern pine, and 9% urea formaldehyde served as binder. Strength tests indicated the composite panels were superior to conventional two-layer floor systems of 1/2-inch plywood plus 5/8-inch particleboard underlayment. 

A report from West Virginia University by Koch and Hall (57) discussed the use of hardwood bark as particleboard furnish, with and without added binders. Species included red oak, soft maple, and black birch. Initial studies indicated no-binder boards had to be compressed to 70 pounds per cubic foot to maintain their integrity. Both strength and dimensional stability were enhanced by pressing boards at 400°F instead of 300°F. Longer press times (15, rather than 10 minutes) also helped. Later, boards were made with 5% added starch powder. One potential use of this product was for expandable horticultural planting containers. Both raw and composted barks were tried, with promising results. 

J. T. White reported at the Washington State University Symposium on Particleboard in 1973 that 18.7 percent of the formaldehyde manufactured in the U. S. A. went into wood binders likewise, 8.9 percent of the urea chemical produced in the U. S. A. went into wood binders. 

Well over 95 percent of the hardwood plywood production in the United States uses urea-formaldehyde as the adhesive bonding agent between veneers for reasons of quality, intended use and economics. In this industry, formaldehyde evolution is a subject of concern because normally all of the adhesive formaldehyde released under elevated temperatures is not tied up in the set glue line. This is an area that is of real concern to the particleboard industry which uses urea-formaldehyde as its binder. 

There have been many attempts to replace these resins with lignin derivatives for wood composite adhesives suitable for plywood, particleboard and waferboard. Most of these studies have been empirical in nature, and few have achieved further consideration for industrial application. As wood binders, technical lignins are variable in quality and poorly reactive in comparison to conventional resin systems such as phenol-formaldehyde (PF) resins. Consequently, they are not utilized on their own. Indeed, if they were, this would adversely affect production quality and times, and necessitate equipment changes. In the wood composite industry, resins having such deleterious effects are not likely to be used even if savings could be made in terms of material costs. 

Wood-based panel products are usually bonded with synthetic adhesives based on condensates of phenol, resorcinol, urea, or melamine with formaldehyde. Particleboards and fiberboards can also be bonded with mineral binders like cement or gypsum. Wood adhesives derived from natural products have more... 

Wattle tannin resins are also used to manufacture other resins, such as foams comparable to phenolics, as waterproofing additives, and binders for corrugated cardboard or charcoal briquettes. This discussion, however, deals only with particleboard, plywood, glulam, and finger-jointing exterior-grade wood adhesives. Formulations of the adhesives will be mentioned ad hoc, if at all necessary, as they have already been extensively discussed in articles and reviews in the relevant literature.(7)... 

Furfural 69, which can be manufactured from hardwood waste, is used for the manufacturing of a furfural-acetone monomer <2000MI15>. This monomer is then used to obtain furfural resins which are suitable as binders for the production of particleboards. In a further application, these furan resins of low toxicity have been used as adhesives in the manufacture of plywood <1999MI24>. The use of such resins is recommended as it leads to products with advantageous economic and ecological indicators. The result is plywood with good physiomechanical properties when the Monomer furfuryl alcohol (FA) was used as the adhesive. 

Figure 21 Modulus of rupture in bending of surface-benzylated and self-bonded boards. BE21% (O) benzylated board with a 21 WPG BE38% (A) benzylated board with a 38 WPG BE51% ( ) benzylated board with a 51 WPG. PF ( ), conventional particleboard bonds using phenol-formaldehyde resin binder acted as a control. (From Ref. 32.)...

Figure 25 Modulus of rupture of particleboards combined with chemically modified particles. U, Untreated particles A, acetylated particles B, benzylated particles SG, specific gravity of board. Note isocyanate resin was used as a binder for particles U and A. Benzylated particles were used on the surfaces of the boards. Willow Salix Arakiana koidz.) particles were used to manufacture boards.

Both particleboard and MDF core are characteristically 3-ply and have two potential sources of formaldehyde the adhesive used to adhere the hardwood face and back to the core, and the adhesive binder used in the manufacture of the particleboard or MDF. Hardwood plywood manufacturers are typically not vertically integrated and do not produce composition board cores, thus are dependent on other companies or plants for particleboard and MDF. 

During the past decade, urea formaldehyde and phenol formaldehyde resin binders have contributed greatly to the progress of wood industries. Formaldehyde is widely used as a major component in the production of building materials, such as particleboard and plywood, and in urea formaldehyde foam insulation. However, the emissions of formaldehyde from these products create considerable concerns not only in the working environments but also in residences, mobile homes, and office buildings. These concerns have also been stimulated by reports on the health effects and carcinogenicity associated with formaldehyde exposure. 

Formaldehyde is liberated during the condensation reactions that take place when the urea formaldehyde resin binder in particleboard is cured by hot pressing. Some of this formaldehyde is retained in the board and is available for subsequent emission to the surroundings. 

During the manufacture (hot pressing) of the particleboard the formaldehyde is concentrated in the core of the board. Tests run on laboratory made particleboard with the same binder level throughout the board, have shown about 75% higher content of extract-able formaldehyde in the core than in the face ( ). Emission tests indicate an even greater difference between the two layers of the board. 

The concentration of formaldehyde in the air of a room containing particleboards, will depend on the content of formaldehyde in the boards and on the rate of its release. The formaldehyde content of a particleboard is determined by the binder used to manufacture the board and a number of production parameters. The release rate is affected by the temperature and the relative humidity of the surrounding air, but also by some of the physical properties of the board. The most important one probably is the diffusion resistance of the surface layer, which may be expressed by means of a mass transfer coefficient. 

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