Wood adhesives formaldehyde release

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. 

Formaldehyde Release from Wood Panel Products Bonded with Phenol Formaldehyde Adhesives... 

Formaldehyde is also released from aminoplasts and their derivatives, such as urea-formaldehyde foam insulation (UFFI), wood adhesives, and textile finishing agents. It is this supplemental, industrial source of formaldehyde that has become the subject of risk analysis. Should we allow products that serve our daily comfort to alter our environment by releasing an irritating vapor with a pungent odor ... 

HIS BOOK SUMMARIZES OUR CURRENT UNDERSTANDING of many problems related to measuring, abating, and understanding formaldehyde emission from wood products bonded with formaldehyde-based adhesive resins. It contains expanded and updated versions of selected papers presented at an ACS symposium, Formaldehyde Release from Cellulose in Wood Products and Textiles. In addition, three chapters from participants who could not attend the meeting were added. 

R. Marutzky, Release of formaldehyde by wood products, in Wood Adhesives Chemistry and Technology, Vol. 2, A. Pizzi, Ed., Dekker, New York, 1989, Chap 10. 

Another class of thermosetting resins used as adhesives involves the polymerization of formaldehyde with urea or phenols. These materials are very cheap and find extensive use in binding wood, e.g., plywood, chipboard, and particle board. Foamed urea-formaldehyde has also been used as insulation (UFFl) in homes because of its ability to be injected into the walls of older homes. Unfortunately, the foam is not very stable and shrinks and hydrolyzes to liberate formaldehyde and possibly other toxic vapors. Recent work has, however, shown that the addition (and subsequent removal) of ammonia (NH3) to the foam can reduce the level of formaldehyde released by the polymer. This release of formaldehyde also occurs in plywood and particle board, and these materials must be sealed to avoid the formation of toxic levels (TLV = 0.10 ppm or 120 pg/m ) of formaldehyde. 

Amino and Phenolic Resins. The largest use of formaldehyde is in the manufacture of urea—formaldehyde, phenol—formaldehyde, and melamine—formaldehyde resins, accounting for over one-half (51%) of the total demand (115). These resins find use as adhesives for binding wood products that comprise particle board, fiber board, and plywood. Plywood is the largest market for phenol—formaldehyde resins particle board is the largest for urea—formaldehyde resins. Under certain conditions, urea—formaldehyde resins may release formaldehyde that has been alleged to create health or environmental problems (see Amino RESINS AND PLASTICS). 

In 1993, worldwide consumption of phenoHc resins exceeded 3 x 10 t slightly less than half of the total volume was produced in the United States (73). The largest-volume appHcation is in plywood adhesives, an area that accounts for ca 49% of U.S. consumption (Table 11). During the early 1980s, the volume of this apphcation more than doubled as mills converted from urea—formaldehyde (UF) to phenol—formaldehyde adhesives because of the release of formaldehyde from UF products. Other wood bonding applications account for another 15% of the volume. The next largest-volume application is insulation material at 12%. 

In the home, fonnaldehyde sources include household chemicals, pressed wood products (especially when new) (EPA 1996), combustion sources (NRC 1986), and some new fabrics (Schorr et al. 1974) and garments. A number of common household products may release formaldehyde to indoor air, including antiseptics, medicines, dish-washing liquids, fabric softeners, shoe-care agents, carpet cleaners, glues, adhesives, and lacquers (Kelly et al. 1996). If children use or play with some of these products, or are present when they are used, additional exposure to formaldehyde may occur. Many cosmetic products contain formaldehyde and some, such as nail polish and nail hardeners, contain high levels (Kelly et al. 1996). If children place these products in their mouth or on their skin, or sniff them, they will be exposed to elevated levels of formaldehyde. 

It is important to note here that higher temperatures probably increase emissions from phenolic panels simply by accelerating the release of that small amount of residual formaldehyde that originates from the adhesive and subsequently becomes adsorbed to the wood substance and water in the wood. Because phenolic resins are very stable chemically, any temperature-related increase in emissions would not be expected to be associated with resin degradation. Consequently, temperature would be expected to exert much less influence on emissions from panels which have been aired out than from fresh panels. Indeed, this trend is shown by the data, as discussed below. 

The lack of reaction between methylolated phenol and cellulose reported by Allan and Neogi seems to contradict the findings of Chow and coworkers. One possible explanation for this disparity could be the difference In available free formaldehyde In their systems. Allan s model phenolic adhesive would have the equivalent of only one mole of formaldehyde per mole of phenol and would not be expected to have significant quantities of free formaldehyde. The resins used by Chow and coworkers had about 2 moles of combined formaldehyde per mole of phenol. Such resins are able to release formaldehyde during cure idien condensation occurs between two methylol groups. This formaldehyde ml t then add at the aliphatic hydroxyls on cellulose or lignin resulting In condensation, as proposed by Chow, between the methylolated wood components and the phenolic resins. 

Tests with composite wood products have also yielded certain volatiles of different preservatives, in addition to formaldehyde. These can be different volatile solvents or free monomers and plasticisers or from coatings applied to them, as well as from the adhesives. The latter are commonly phenol- or formaldehyde-based and they are used in the manufacture of compressed fibre, composite board and plywood materials. As far as the coatings are concerned, some products that are sealed with a polymeric film or coating that can trap residual volatiles and allow a slow gradual release of VOC over a period of time). The binding agents used in particle board and plywood can contain volatile phenols and traces of residual solvents. 

In 1977 the Canadian goverrmient subsidized the introduction of UFFT in older homes to conserve energy. The UFFI proved to be unstable in some cases due to improper installation, and as a result formaldehyde levels in some homes exceeded the threshold limit value (TLV) of 0.10 ppm (120 (ig/m ). Ammonia was able to neutralize the acid, and it was also shown that the water-soluble polymeric amine, polyethyleneimine, could remove the liberated formaldehyde. Nmietheless, the Canadian government then paid the homeowners an estimated 272 million ( 5,000 to 57,700 homes) to remove the UFFI. The urea formaldehyde resin is commonly used as the adhesive resins in plywood and particle board and will initially release formaldehyde if not sealed. As more composite wood products find their way into buildings, greater concern about indoor air is warranted. 

The major application of UF resins, which nearly correspond to 70% of the utilization of these materials, is in the field of adhesives and, more particularly, in the industry of wood and its derivatives (particle boards, plywood...). Compared to the other adhesives used in the same field, UF resins exhibit a moisture sensitivity that restricts their use as materials for interior applications owing to a tendency to slowly release formaldehyde. 

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