Environmental Engineering Reference
In-Depth Information
wood adhesives are colorless and provide excellent bonding performance.
They are, however, somewhat chemically unstable, releasing monomeric
HCHO on hydrolysis of methylol end groups and, less commonly, methyl-
ene bridges. Their decomposition is sensitive to product moisture levels as
well as relative humidity. Because of resin sensitivity to moisture, UF-
bonded wood products are intended only for indoor use. Historically UF-
based adhesives were formulated with relatively high HCHO to urea ratios
(F:U 1.5:1) to enhance performance by ensuring that there was sufficient
HCHO present to achieve cross-linking of all primary and secondary amino
groups. Because of this excess HCHO associated with the resin, UF-based
wood adhesives emitted significant levels of free HCHO into indoor envi-
ronments, particularly in the first months or so in the life of a product.
Because of high HCHO emissions, indoor concentrations, and health com-
plaints, UF-bonded wood products are presently manufactured with low
F:U ratios (e.g., 1.05:1) and thus emit much less HCHO. Though HCHO
emissions from UF-bonded wood products are substantially lower than
those of two decades ago (<10%), they continue to be a significant source
of indoor HCHO concentrations. Most emissions are associated with the
hydrolytic decomposition of the resin copolymer.
Phenol-formaldehyde resins receive significant use as exterior-grade
adhesives in the manufacture of softwood plywood and oriented-strand
board (OSB) products that are widely used in new home construction. Phe-
nol-formaldehyde (PF)-bonded wood products have historically had low
HCHO emissions compared to UF-bonded wood products. Emissions from
the latter were once 1000 times greater than from PF-bonded products.
Formaldehyde is produced in the thermal oxidation of a variety of
organic materials. As a consequence, it is found in the emissions of motor
vehicles, combustion appliances, wood fires, and tobacco smoke. It is also
produced in the atmosphere as a consequence of photochemical reactions
and hydrocarbon scavenging processes, and in indoor air as a result of
chemical reactions.
2.
Exposures
Formaldehyde is omnipresent in both ambient and indoor environments.
Ambient concentrations are usually <10 ppbv in urban/suburban loca-
tions but may reach peak levels of 50 ppbv or more in urban areas subject
to significant atmospheric photochemistry (e.g., south coast of California).
Formaldehyde levels in indoor environments are on average significantly
higher (order of magnitude or more) in residential, institutional, and
commercial buildings than background ambient levels. Concentrations
vary from structure to structure, depending on the nature of sources
present and environmental factors which may affect emissions and indoor
concentrations.
Historically, the major sources of HCHO emissions have been wood
products bonded with UF resins, UF-based acid-cured finishes, and in houses
retrofit insulated (in the 1970s and early 1980s) with UFFI. Formaldehyde
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