Heat stability of cured urea-formaldehyde resins by measuring formaldehyde emission
A test method for measuring formaldehyde from urea-formaldehyde (UF) resins at high tempera: tures was developed and used to assess the influence of the reaction pH at synthesis on the formaldehyde emission during cure and heat stability of the cured resins without water. Additionally, 1 3C-CP/MAS solid-state nuclear magnetic resonance (NMR) techniques were used to investigate the structure of cured UF resins before and after high-temperature heating. Formaldehyde emissions during cure were related to the UF resins’methylol group content. Heat stability of cured UF resin increased as the reaction pH at resin synthesis increased. Solid-state NMR spectra show formaldehyde emission from cured UF resins after heating is mainly ascribable to decreases of methyl01 group Tohmura: Senior Researcher, Forestry and Forest Prod. Res. Inst., Tsukuba, Japan Hse: Principal Wood Scientist, USDA Forest Sew., Southern Res. Sta., Pineville. La. Higuchi! Professor, Dept. ofForest Prod., Kyushu Univ., Fukuoka, lapn and dimethylene ether linkage. Significantly, it was revealed that the uron structurecharacteristitally found in the cured UF resin synthesized under strong acid media indicated high heat stability. Introduction . Low cost and proven performance have made urea-formaldehyde resins (UF) the most important wood adhesive for interior applications. However, the formaldehyde emission from ‘UF-bonded wood products has been recognized as a potential source of indoor air pollution leading to inhabitant discomfort and possibly to health problems. The practical concern over the formaldehyde emission has elicited a great deal of research since the 1970s. Recent studies ( 12.30) indicate these major sources of formaldehyde emission from UFbonded wood products are: l unreacted formaldehyde in the UF resins; l released formaldehyde during the condensation reaction between methyl01 groups; and l emitted formaldehyde from the hydrolytic deg radation of the cured resin. Over the past two decades or so, great progress has been made ( 19,26)‘in improving the formaldehyde emission from wood products such as particleboard, hardwood plywood, and medium density fiTohmum, Hse. and Hh (2 l), and treating boards with formaldehyde scavengers and/or a barrier coating after manufacture (23). Despite this practical progress, the effect of the UF resin hydrolysis on formaldehyde emission from boards is not a trivial one. Over the past 40 years, investigators have extensively examined the structure of low molecular weight UF compounds and the physical chemistry of their formation and degradation in aqueous systems. Earlier work by de Jong and de Jonge (4-6), Lanqvist ( 15,16), and others (3,8,27) used classical solution techniques to study the kinetics and equilibria of formation and hydrolysis. Subsequently, chromatographic (7,14,17) and nuclear magnetic resonance (9,13, 22,24,25,28) techniques were applied. It is now generally agreed that the hydrolysis of cured UF resins is responsible for the major portion of liberated formaldehyde from those materials (3 1). Thus, in principle, the board would retain the potential to emit formaldehyde.during its useful life, and the efforts to minimize emission must be directed toward resin stabilization. Nevertheless, there is little reported in the literature on the hydrolytic stability of the cured wood adhesive, probably because most investigators have focused .on the hydrolytic effects on UF-bonded products rather than on cured neat resin. Lack of a database in the literature on the hydrolytic stability of cured, neat resin undoubtedly hampered development of more stable UF wood adhesives to minimize formaldehyde emission. This study developed a new method for the measurement of formaldehyde emissions in neat resins and investigated the heat stability of the cured UF resin in relation to the structure of UF resin and curing conditions.
Characterization of urea—formaldehyde and melamine—formaldehyde adducts and resins by 15N n.m.r. spectroscopy
Abstract 15 N n.m.r. spectra have been recorded for some urea—formaldehyde and some melamine—formaldehyde adducts and resins. For the urea—formaldehyde system, the technique can be used to distinguish between unsubstituted amino groups, secondary and tertiary amino groups carrying methyol substituents, and secondary and tertiary amino groups involved in methylene and methylene ether linkages. For the melamine—formaldehyde system, additional information is available from the aromatic azine nitrogen signals. The information available from 15 N n.m.r. spectra supports and augments that from 13 C and 1 H spectra.
The analysis of group contingency data.
The data analyses utilized in group contingency projects are reviewed. Previous studies are cited to emphasize advantages of nonconsolidated ("individual") over consolidated analyses. Several procedures are described that enable applied researchers to incorporate nonconsolidated data analyses in group contingency studies.