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.
Size exclusion chromatography of urea formaldehyde resins in dimethylformamide containing lithium chloride
A rapid, reproducible method for investigating the molecular mas distribution of urea formaldehyde resins by size exclusion chromatography has been developed. By using concentrated lithium chloride solution to prepare the sample, materials of high viscosity and high molecular mass can be easily dissolved. Chromatography in dimethyl formamide containing lithium chloride eliminates hydrogen bonding and ensures that realistic values for molecular mass averages are obtained.
Engineering bio-inks for 3D bioprinting cell mechanical microenvironment
Three-dimensional (3D) bioprinting has become a promising approach to construct­ing functional biomimetic tissues for tissue engineering and regenerative medicine. In 3D bioprinting, bio-inks are essential for the construction of cell microenviron­ment, thus affecting the biomimetic design and regenerative efficiency. Mechanical properties are one of the essential aspects of microenvironment, which can be char­acterized by matrix stiffness, viscoelasticity, topography, and dynamic mechanical stimulation. With the recent advances in functional biomaterials, various engineered bio-inks have realized the possibility of engineering cell mechanical microenviron­ment in vivo. In this review, we summarize the critical mechanical cues of cell micro­environments, review the engineered bio-inks while focusing on the selection princi­ples for constructing cell mechanical microenvironments, and discuss the challenges facing this field and the possible solutions for them.