Urea-formaldehyde particleboard

Figure 3. Urea-formaldehyde particleboard elution by nitrogen relative humidity (RH) effects. (0.4 NCM. P as in Figure 1.) (ML85 5430)...

Figure 8. Formaldehyde liberation from particleboards and CH20-sorbed wood at 27°C and 80 percent relative humidity (RH) weighing bottle test with -80 mesh materials. (o Southern pine impregnated with pH 2 tartaric acid and vapor-equilibrated with CH20/salt solution at 15 pet RH as before except pH 3 tartaric acid 0 urea-formaldehyde particleboard b phenol-formaldehyde particleboard, parentheses indicating approximate values P and MC as in Figure 7.) (ML85 5435)...

Figure 10. Formaldehyde loss ratios at 20 percent relative humidity for various materials. (Formaldehyde removed from a material divided by that removed from urea-formaldehyde particleboard. Board elution by nitrogen. Resin liberation by weighing bottle test. PF = phenol-formaldehyde) (ML85 5437)...

Figure 11. Formaldehyde loss ratios at 80 percent relative humidity (RH) and in water. (Loss ratio = CH2O liberated relative to that from urea-formaldehyde particleboard in same test.

Tannin-Induced Formaldehyde Release Depression in Urea-Formaldehyde Particleboard... 

Approximately one million metric tons of urea-formaldehyde resin are produced annually all over the world. More than 70% of this urea-formaldehyde resin is consumed by the forest products industry. The resin is used in the production of an adhesive for bonding particleboard (61% of the urea-formaldehyde used in the industry), medium-density fiberboard (27%), hardwood plywood (5%), and as a laminating adhesive (7%) for bonding furniture case goods, overlays to panels, and interior flush doors, for example. 

In similar bending-creep tests, both control and acetylated pine particleboards made using melamine-urea-formaldehyde adhesive failed because T. palustris attacked the adhesive in the glueline ( ). Mycelium invaded the inner part of all boards, colonizing in both wood and glueline in control boards but only in the glueline in acetylated boards. 

Table 12.3 summarizes the uses of formaldehyde. Two important thermosetting plastics, urea- and phenol-copolymers, take nearly one half the formaldehyde manufactured. Urea-formaldehyde resins are used in particleboard, phenol-formaldehyde resins in plywood. 1,4-Butanediol is made for some polyesters and is an example of acetylene chemistry that has not yet been replaced. Tetrahydrofiiran (THF) is a common solvent that is made by dehydration of 1,4-butanediol. 

Urea-formaldehyde and melamine-formaldehyde adhesives are resins in particleboard. 

Another use of urea is for resins, which are used in numerous applications including plastics, adhesives, moldings, laminates, plywood, particleboard, textiles, and coatings. Resins are organic liquid substances exuded from plants that harden on exposure to air. The term now includes numerous synthetically produced resins. Urea resins are thermosetting, which means they harden when heated, often with the aid of a catalyst. The polymerization of urea and formaldehyde produces urea-formaldehyde resins, which is the second most abundant use of urea. Urea is dehydrated to melamine, which, when combined with formaldehyde, produces melamine-formaldehyde resins (Figure 96.2). Melamine resins tend to be harder and more heat-resistant than urea-formaldehyde resins. Melamine received widespread attention as the primary pet food and animal feed contaminant causing numerous cat and dog deaths in early... 

Brown (1999b) reported formaldehyde and VOC emissions from new, unfinished particleboard and MDF (both using urea formaldehyde resins) in Ausbalia. Formaldehyde emissions over the first three weeks exhibited first-order decay behavior that predicted little to no formaldehyde emission after 6 months. However, further emission measurements at 8 months showed the products sbll emitted formaldehyde at approximately one-half the new product rate (also further unpublished measurement at 2 years showed the same emission rate as at 8 months). It was concluded that the wood-based panels emitted formaldehyde by a double-exponen-ttal model, the early- to late-term emissions including the free formaldehyde in the products but the long-term emissions consisbng of only the formaldehyde... 

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 disadvantages of the urea-formaldehyde adhesives lie in their lack of durability and in their characteristic pungent formaldehyde odor. For particleboard applications subject to high temperature and moisture exposure, phenol-formaldehyde adhesives are required, since the urea-formaldehyde polymer is hydrolyzable and hydrolysis is enhanced with moisture and heat. 

Lumber banding consists of gluing lumber strips, 1/2 to 2 inches in width, on the particleboard edges. These strips are normally used in applications where the particleboard is to be covered with wood veneers. The solid wood strip can be machined to decorative edges and, with the veneer surfaces, the panel is fully as functional and attractive as a solid wood panel, but at a lower cost. The lumber bands are normally bonded to the particleboard with polyvinyl acetate or urea-formaldehyde adhesives, cured rapidly by either contact or high frequency heating. 

Dost in 1971 reported on a study where redwood bark fiber was used in three-layer particleboard (44). Amount of bark in the furnish, by weight, was 0, 10, 20, and 30% hammemilled disk flakes or Pallmann flakes of redwood wood made up the remainder of the furnish. Urea formaldehyde resin was applied at three percentages. Test results showed surface smoothness and strength properties (MOR, MOE, and IB) decreased with increasing bark content in the boards. Water absorption decreased, but thickness swelling and linear expansion increased as the amount of bark increased. 

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. 

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. 

Some report that over 50 percent of the urea-formaldehyde resins consumed went into particleboard. This is brought out because there may be a shift away from urea resin for certain types of oriented particleboard used in structural plywood constructions. Historically, particleboard has been used for inner plies as previously mentioned in some hardwood plywood. There is now one plant in production in Idaho which produces mechanically oriented strand particleboard for use specifically as core for softwood plywood production. It is anticipated that this trend to some degree will increase in the future, and phenolic resins appear to be the mechanism with which this particleboard will be bonded. 

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. 

Particleboards are composed of discrete particles of wood bonded together by a synthetic resin adhesive, most commonly urea-formaldehyde or phenol-formaldehyde. The material is consolidated and the resin cured under heat and pressure. The strength of the product depends mainly upon the adhesive and not upon fiber... 

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... 

Ebewele, R. O., River, B. H., Myers, G. E., and Koutsky, J. A., lYtlyamine-modified urea-formaldehyde resins. II. Resistance to stress induced 1 moisture cycling of solid wood joints and particleboard, J. Appl. Polym. Scl. 43, 1483, 1991. 

It was not until after World War II that particleboard appeared as the first product to use urea formaldehyde (UF) as an adhesive. Particleboard provided a low cost way of converting low grade raw materials into a useful product. Capacity increased rapidly after the World War II to meet the huge demand for building materials, especially in Europe and the technology quickly spread to other countries Experience with this product showed the benefits of controlling the particle size and geometry and lead to improved performance. 

The most widely used wood panel products are particleboard, softwood plywood, hardwood plywood, medium density fiberboard (MDF) and waferboard. The most common adhesive is urea-formaldehyde resin (UFR). Phenol-formaldehyde resins (PFR) are second in volume and melamine-formaldehyde resins (MFR) are a distant third. Recently,... 

Hardwood plywood products are decorative in nature and are designed for interior use. Over 95X of all hardwood plywood is made with urea-formaldehyde adhesives. Responding to concerns about formaldehyde and certain wood products, test methods for measuring surface emissions were developed in the early 1980 s. Emissions from most hardwood plywood and particleboard products have decreased 65% to 95% in recent years primarily by use of low emitting UF adhesives and/or scavengers. Good correlation has been demonstrated between product test methods and indoor levels of formaldehyde in experimental manufactured homes. Decorative surface finishes can act to either increase or decrease surface emissions, depending on the nature of the finish and the substrate. 

Reduction in the emission characteristics of unfinished hardwood plywood is currently being achieved primarily by the use of low formaldehyde to urea molar ratio formulations. For the manufacture of hardwood plywood and particleboard, formaldehyde to urea molar ratios have been reduced to a range of 1.15/1 to 1.3/1. An important caveat low F/U ratios perhaps should be considered a proxy for the potential to reduce emissions through improved urea-formaldehyde adhesive technology rather than the exclusive means for improvement. Reducing the F/U ratio is not always the most effective way of reducing emissions in consideration of the variety of hardwood plywood constructions, products, and thicknesses. ... 

Virtually all wood panel products such as plywood and particleboard are manufactured using either urea formaldehyde or phenol formaldehyde adhesives. Urea formaldehyde adhesives are used in hardwood plywood and in certain types of particleboards. These adhesives are not waterproof, and products made with them are normally used indoors for paneling, furniture, shelving and floor underlayment. 

Although formaldehyde emissions from some products glued with urea formaldehyde adhesives can cause indoor air quality problems under certain conditions, such problems have not been associated with phenol formaldehyde-bonded (phenolic) products. Unfortunately, however the commonplace usage of the generic terms particleboard and plywood has failed to distinguish between product types and has led to a great deal of confusion among consumers. 

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. 

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