Water bonded wood products, formaldehyde

Even though formaldehyde release from UF-bonded wood products has been studied for more than 25 years, only very little Is known about how formaldehyde Is stored In UF-bonded wood products. In fact. It is not even known whether storage of formaldehyde is a physical or a chemical process. Formaldehyde is gaseous at room temperature, but it can polymerize forming para-formaldehyde, and it readily dissolves In water forming methyIenegIycoI (2). The most likely physical storage process is absorption by moisture. Water is present in wood in two forms free water In the cell cavities In form of liquid... 

The structure of cellobiose and Its 13C-NMR spectrum are shown in Figure 3a. The spectra have been identified (18-20). Cellobiose is water soluble. Figure 3b shows the spectrum of reaction products with formaldehyde at different molar ratios obtained by 15 min reaction at 150°C, i.e. under conditions that correspond to those during the manufacture of UF-bonded wood products. As expected, formaldehyde can react with several different functional groups. Therefore, complex mixtures of products are formed. 

Due to its affinity for water, formaldehyde will concentrate in wood products in their water reservoirs. Since wood collects water in Its S-2 secondary wall on the surface of wood cellulose, formaldehyde will come into contact with wood cellulose. This work shows that formaldehyde can be expected to react with wood cellulose forming hemiacetals. Since this reaction is reversible, these hemiacetaIs constitute a temporary reservoir for formaldehyde within wood. This fact may explain the complex formaldehyde release and absorption properties of UF-bonded wood products. 

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. 

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. 

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... 

A second type of urethane adhesive is currently being introduced to the U.S. construction industry. The adhesive was originally developed in Japan. The system is two component. The isocyanate component is a modified polymeric MDI. The cross-linkable component contains water with water-soluble or water-emulsifiable resins. The adhesive produces strong, water-resistant bonds and has shown promise as a possible lower cost substitute for resorcinol formaldehyde resins in the adhesion of laminated beams. The product is now being used in Japan to bond wood and various other porous substrates at the rate of about 10 million pounds per year. The first U.S. application of the product has been the bonding of wood doors. The opportunities for products of this type should be excellent in view of the ever-increasing governmental pressures on solvents. 

Chemical modification with dopamine was also used as strength and water-resistance aids for SPI adhesives (Liu et al., 2002). Dopamine is an amino acid with two adjacent phenolic hydroxyl groups, and is the primary component responsible for marine adhesive properties. The Liu modification scheme creates an SPI that is similar to mussel proteins used for surface adhesion. Increased water-resistance compared to other stand-alone SPI adhesives was achieved. Bond strength depends on the phenolic functionality in the synthesized compounds (Liu, 2002). Much interest in this adhesive has developed because it is a strong and resilient adhesive, which is formaldehyde-free, making it suitable for interior wood products. 

The most common resin for lignocellulosic composites is urea formaldehyde. About 90% of all lignocellulosic composite panel products are bonded with UF [12]. UF is inexpensive, reacts quickly when the composite is hot-pressed, and is easy to use. UF is water-resistant, but not waterproof. As such, its use is limited to interior applications unless special treatments or coatings are applied. UF resins are typically used in the manufacture of products where dimensional uniformity and surface smoothness are of primary concern, for example, particleboard and medium density fibreboard (MDF). Products manufactured with UF resins are designed for interior applications. They can be formulated to cure anywhere from room temperature to 150 °C press times and temperatures can be moderated accordingly. UF resins (often referred to as urea resins) are more economical than PF resins and are the most widely used adhesive for composite wood products. The inherently light colour of UF resins make them quite suitable for the manufacture of decorative products. 

Performance and durability of wood products bonded with isocyanate and urethane adhesives have been the subject of several investigations. The strength of composition board is generally determined by its modulus of rupture (MOR), which is a flexure to break test its modulus of elasticity (MOE), or stiffness and its internal bond strength (IB), or tensile strength. These tests are run on dry and on water-soaked or wet samples, and are described in ASTM test method D 1037. As compared to phenol-formaldehyde adhesives, composition boards prepared with isocyanate or urethane adhesives are reported to display similar or superior initial strengths. Moisture resistance of isocyanate-bound par-... 

For example, amino-resins are water-based condensation products of formaldehyde in urea or melamine. They are typically used for bonding wood, particle board, and decorative laminates. 

These adhesives, more properly called phenol-formaldehyde adhesives, are condensation products of formaldehyde and a monohydric phenol [31]. They dominate the field of wood adhesives and represent one of the largest volumes of any synthetic adhesive. Phenolics are also among the lowest cost adhesives and may be formulated as water dispersions, to allow penetration into the cell structure of wood which is important for the formation of permanent bonds. Beyond the wood and wood products area, unmodified phenolics are used mainly as primers, to prepare metal surfaces for bonding, and as binders, for such varied products as glass wool insulation mats, foundry sand, abrasive wheels, and brake lining composites. Phenolics are supplied either as one-component, heat-curable liquid solution, as powder, or as liquid solution to which catalysts must be added. The curing mechanisms are different for these two groups [3]. 

Different phenoHc resins are used for different types of wood for example, plywood adhesives contain alkaline-catalyzed Hquid resole resins. Extension with a filler reduces cost, minimizes absorption, and increases bond strength. These resins have an alkaline content of 5—7% and are low in free phenol and formaldehyde. Because many resins have a high water content and limited storage stabiHty, they are frequently made at or near the mill producing the plywood product. The plywood veneers are dried, coated with resin, stacked for pressing, and cured at 140—150°C. 

C-NMR spectra of water soluble cellulose model compounds indicate that formaldehyde is capable of reacting with wood cellulose functions under hot press conditions as well as at room temperature yielding hemiacetals. The formation of hemiacetals is reversible, and thus constitutes a reservoir for formaldehyde storage. Due to its affinity for water, formaldehyde released during the manufacture of UF-resin bonded products will be retained in the aqueous phase of wood. Wood contains about 9 wt% of moisture. Most of this is in the S-2 secondary cell walls that consist mainly of wood cel IuIose. 

Phenol-formaldehyde resin is the most common adhesive for exterior applications due to its water resistance, low initial viscosity and its ability to bond various types of wood substrates [1], Because of its resemblance to phenolic moieties, studies on tannin have been oriented towards an alternative formulation to replace the current synthetic phenol-formaldehyde or phenol-resorcinol-formaldehyde adhesives [2-4], A few suitable alternative natural resources such as oil palm shell, pecan shell nut, lignin, starch, rice bran and tannin are also available for this purpose. Among these materials, tannins represent the best immediate substitute for phenol in wood adhesive production [1],... 

NeoPac E-850 is a formaldehyde-free, water-based aliphatic polyester urethane-acrylic dispersion. This system was developed specifically for use in wood furniture adhesives due to its bond strength and excellent water and heat resistance. As a water-based polymer, it can be highly loaded with mineral fillers to reduce the overall water content in the formulated adhesive without loss of adhesion properties. This is a necessary requirement for most wood laminating production methods in controlling viscosity, wood penetration and faster drying. 

The product can be used in adhesives to form flat or shaped laminated parts. The nature of the resin imparts the necessary performance to withstand bond stresses in the laminate along bends in the formed components. In addition, it exhibits good water and fatigue resistance properties. The product meets the standards of major flat-pack supplier regulations regarding formaldehyde-free and wood furniture construction performance. 

The greatest volume of wood adhesives for structural applications are thermosetting phenol-formaldehyde (PF) or urea-formaldehyde (UF) polymers or their derivatives. PFs are the commodity for exterior applications, while UFs are the commodity for interior applications. Because each composite product and the associated process may require a somewhat different resin, many variations of PF and UF resins are available. Isocyanate crosslinking resins are used to a lesser extent than PFs or UFs in manufactured board. Although these resins impart a durable, water resistant bond suitable for exterior application, the cost per pound of adhesive is greater than for PFs or UFs. 

Applications. UFs are the primary commodity binder in caul and caulless particleboard processes and medium density fiberboard (MDF). They can be formulated to develop tack and generally are used as a neat liquid. Acid catalysts are frequently used unless the wood species is acidic such as southern pine or oak. Products manufactured with UFs are rated for interior application. Use of MUFs or MFs increases bond durability and imparts water resistance to the bond, speeds cure rate, and reduces potential formaldehyde release. 

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