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.
Formation and cure of novolacs: NMR study of transient molecules
High-resolution magnetic resonance spectroscopy has been used to investigate three areas of novolac resin chemistry: first, the lifetimes of glycol species in formalin under acid and base conditions and the reactivity of methylene glycol towards alcohols; second, the in situ identification of novolac resin intermediates; third, the novolac–hexamethylenetetramine cure mechanisms. It is shown that the mean lifetime of a given glycol species is an order of magnitude shorter under basic conditions than under an acid of equal normality. The ratio of the equilibrium constants for the addition of methylene glycol to methanol, benzyl alcohol, and phenol was measured as 70:20:1. New reactive intermediates were discovered while monitoring the reaction of phenol and formalin. The reactions of various phenols with hexamethylenetetramine were studied, and short-lived benzoxazines were detected at 100–120°C whenever ortho hydrogen was available on the phenolic nucleus. The NMR peak assignments in the reaction sequence: benzoxazine, dibenzylamine, tribenzylamine, diphenylmethane are presented. Spectra of the very elusive p-benzylamines of 2,6-xylenol were recorded at 100–155°C. At 170°C, the lifetimes of these p-benzylamines are too short to afford proton resonance spectra. No evidence of hexahydrotriazine or (CH2 = N-CH2)3N type structures was found in 100 MHz NMR spectra of reaction mixtures of a phenol and hexamethylene-tetramine.
Chapter 1 – Nanotechnology: A Toolkit for Cell Behavior
Tissue engineering seeks to effectively manipulate cellular populations to improve the function of damaged or diseased tissues and organs. Many techniques can affect cell behavior; however, in order to maintain desirable function, scientists and engineers design solutions that are as biomimetic as possible. True biomimeticity is very challenging without the incorporation of nanotechnology. Since cells in human tissues are surrounded by a 3D hierarchical tissue extracellular matrix containing numerous nano components, a revolutionary change in tissue engineering is to explore biomimetic nanobiomaterials and advanced 3D nano/microfabrication techniques for creating novel tissue constructs and regulating cell behavior. This chapter will provide an overview of recent nanotechnology in tissue engineering applications. We will put special emphasis on integrating cutting-edge 3D nano/microfabrication techniques with nanobiomaterials for complex tissue and organ regeneration.