Urea-Formaldehyde Adhesives Ureas

These structural adhesive alloys are polyvinyl acetals. 

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Vapors emitted from the materials of closed storage and exhibit cases have been a frequent source of pollution problems. Oak wood, which in the past was often used for the constmction of such cases, emits a significant amount of organic acid vapors, including formic and acetic acids, which have caused corrosion of metal objects, as well as shell and mineral specimens in natural history collections. Plywood and particle board, especially those with a urea—formaldehyde adhesive, similarly often emit appreciable amounts of corrosive vapors. Sealing of these materials has proven to be not sufficiently rehable to prevent the problem, and generally thek use for these purposes is not considered acceptable practice. 

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. 

These adhesives are essentially for gluing wood. Urea—formaldehyde adhesives are used in the manufacture of plywood, in the fortification of starch... 

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. 

Developments in glued laminated structures and panel products such as plywood and chipboard raises the question of the durability of adhesives as well as wood. Urea-formaldehyde adhesives are most commonly used for indoor components. For exterior use, resorcinol adhesives are used for assembly work, whilst phenolic, tannin and melamine/urea adhesives are used for manufactured wood products. Urea and casein adhesives can give good outdoor service if protected with well-maintained surface finishes. Assembly failures of adhesives caused by exudates from some timber species can be avoided by freshly sanding the surfaces before glue application. 

The major disadvantage associated with urea-formaldehyde adhesives as compared with the other thermosetting wood adhesives, such as phenol-formaldehyde and polymeric diisocyanates, is their lack of resistance to moist conditions, especially in combination with heat. These conditions lead to a reversal of the bond-forming reactions and the release of formaldehyde, so these resins are usually used for the manufacture of products intended for interior use only. However, even when used for interior purposes, the slow release of formaldehyde (a suspected carcinogen) from products bonded with urea-formaldehyde adhesives is observed. 

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. 

Conner, A.H. (1996) Urea Formaldehyde Adhesive Resins, in Polymeric Materials Encyclopedia, Vol. 11 (ed. J.C. Salamone), CRC Press, Boca Raton, USA, pp. 8497-501. 

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. 

Another problem of a seemingly unrelated nature is the formaldehyde emission from building boards that are currently manufactured using urea-formaldehyde adhesive resins. Formaldehyde has been implicated as a carcinogen and can also cause severe upper respiratory problems and contact dermatitis in some individuals (3-5). 

Urea-Formaldehyde Adhesives used are epoxies, nitrile-phenolics, phenol-formaldehyde, resorcinol-formaldehyde, furan, polyesters, butadiene-nitrile rubber, neoprene, cyanoacrylate, and phenolic-polyvinyl butyral (1). 

Plyamine [Reichhold]. TM for a group of liquid water-soluble urea-formaldehyde adhesive resins. 

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. 

Because face veneers are decorative and thin, a colorless glue line is desired to prevent discoloration on the face. Urea-formaldehyde adhesives are predominate in the manufacture of hardwood plywood. Well over 95% of all hardwood plywood consumed in the U.S. is made from UF adhesives. 

It would appear that the wall paneling industry, on average, has probably been able to achieve a 70% to 95% reduction in formaldehyde emissions and still maintain the use of urea-formaldehyde adhesives. 

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. 

Furthermore, realistic regulations should make possible product improvement and proper product utilization. In the early 1970s it was unthinkable to manufacture board with urea-formaldehyde adhesives having a F/U ratio of 1.5-1.6. Nowadays, it is possible to manufacture boards of the same quality with glues having a F/U of... 

The first three chapters deal with particleboard, medium density fiberboard, hardwood plywood, and softwood plywood, the four most widely used wood panel products. Chapter four compares these products with other consumer products. Chapters five through seven explain the basic chemistry of formaldehyde with cellulose and wood components and provide a current understanding of the nature of liquid urea-formaldehyde adhesive resins. The next two chapters present new analytical methods that might become useful in the future. Chapters eight and eleven through sixteen explain the complex nature of the latent formaldehyde present in the products and its correlation to formaldehyde emission from wood products. Chapters fifteen and sixteen describe currently popular formaldehyde reduction methods. The last two chapters discuss the problems involved in reducing formaldehyde emission by regulating air levels or source emissions. 

Cured urea-formaldehyde adhesive is characterized by the presence of methylene bridges between strongly hydrogen-bonded urea linkages. Consequently, cured UF adhesives are inherently stiff and brittle. Incorporation of DDDU with its 12 methylene groups into the resin structure results in cured UF adhesive with a more flexible network. The increased flexibility decreases internal stress and the associated flaws, and hence the fracture energy increases. 

M. Chiavarini, N. Del Fanti, R. Bigatto Compositive Characterization of Urea-Formaldehyde Adhesives by NMR Spectroscopy," Angew. Makromol. Chem. 46 (1975) no. 695, 151-162. 

Urea-formaldehyde adhesives were patented in 1920 but were first eommercialized around 1937. During World War II, stareh was modified with urea resins to make both waterproof adhesives and impregnants for paper, which led in the 1940s to phenolic-impregnated paper for the first durable honeyeomb eore for lightweight rigid honeycomb panels. 

Exactly the same type of approach can be used for the formulation of other polycondensation adhesives. For example, the formulation of melamine urea formaldehyde adhesive resins for wood panel products can also be successfully approached in the same way as has been shown above for the cold-setting PRF adhesives. 

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