Phenol–formaldehyde resin curing and bonding in steam‐injection pressing. I. Resin synthesis, characterization, and cure behavior

Abstract

SYNOPSISTwo different phenol-formaldehyde ( P F ) resole resins are serving as models in a study aimed a t establishing the effects of moisture, temperature, pressure, and time on resin cure and bonding during the pressing of wood flakeboard. This phase of the program had two goals: first, to characterize the two resins in terms of their structure and chemistry during synthesis, aging, and cure-using viscosity measurement, gel permeation chromatography (GPC), nuclear magnetic resonance (NMR) , differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) , and dynamic mechanical analysis (DMA); second, to make a preliminary evaluation of the utility of DSC, FTIR, and DMA for measuring the degree of resin cure. The two resins differed significantly in relative amounts of hydroxymethyl groups and methylene linkages (NMR) , in molecular weight and its distribution (GPC), and in reaction rate (as measured by viscosity, DSC, FTIR, or DMA). The degree of cure developed during constant heating rate DSC scans was calculated for a series of maximum DSC temperatures from both the loss in hydroxymethyl groups (FTIR) and the decrease in available exothermic heat (DSC) . Agreement between the two methods was quite good, considering the inherent difficulties in quantifying infrared data. For comparison, the degree of cure developed during constant heating rate DMA scans was calculated for a series of maximum DMA temperatures from both the increase in storage modulus (DMA) and the decrease in exothermic heat (DSC after rewetting) . Samples that apparently achieved complete cure in the DMA still exhibited significant residual cure potential in the DSC. We attribute the lower apparent cure in the DMA to loss of moisture from samples during the DMA scan, with consequent loss in plasticization and molecular mobility. INTRODUCTIONSteam-injection pressing is a recent development for manufacturing reconstituted wood panel products. Compared with conventional panel pressing, steam-injection pressing permits more rapid cure of thicker panels and yields more uniform density cross sections.' However, the very different time-temper-* To whom correspondence should be addressed. ature-moisture regime during steam injection may cause profound differences in the chemistry and physics of resin cure and resin-wood bonding. Unfortunately, the present understanding of the chemistry and physics of resin cure and bonding is limited, even for conventional pressing of reconstituted wood products. As a result, the industry optimizes adhesives and the bonding process primarily on an empirical basis.An investigation is underway at the Forest Products Laboratory in cooperation with the University of Wisconsin-Madison to establish how the cure and wood bonding of different phenol-formaldehyde Table I ) for our initial investigations, primarily on the basis of the significant differences between their compositions and molecular weight distributions. Formula 2 is a conventional alkaline resole PF flakeboard adhesive, whereas...

Effects of Pinus pinaster bark extracts content on the cure properties of tannin-modified adhesives and on bonding of exterior grade MDF

“…As it can be observed, the E 0 curves of the PUFT adhesives show two activity regions, unlike the commercial PF and the PUF prepolymer. In the rst region, between 30 and 110 ± C, the storage modulus initially increases, so it could be assumed that a hardening process had taken place [4]. The initial increase of the rigidity modulus observed in the case of tannin-modi ed adhesives can be understood, because they do undergo a pre-cure process when the PUF prepolymer and the very reactive Pinus pinaster bark tannins aqueous solutions are mixed and left overnight to copolymerise, which is re ected in their viscosity increase.…”

“…The presumably lower reactivity of the commercial PF and the PUF prepolymer compared to the tanninmodi ed adhesives at this stage could be a possible explanation for their lack of stiffness. Beyond 120 ± C the E 0 modulus drastically rises in all adhesives and reaches a maximum around 160 ± C. During this period, the increase in the E 0 re ects multiple polymerization and crosslinking reactions [4] and it ends with a complete mechanical curing of the adhesives [14], i.e. the adhesives reach their maximum rigidity or stiffness.…”

The effects of temperature and humidity on phenol-formaldehyde resin bonding

“…A number of indirect analytical techniques have been used to characterize the cure of PF resins by responding to either the chemistry or physics of the curing process. Such techniques include: (1) nuclear magnetic resonance (NMR) (Woodbrey et al 1965;Maciel et al 1984); (2) Fourier transform infrared (FTIR) spectroscopy (Myers et al 1991); (3) ultraviolet spectroscopy (Chow 1969;Chow and Hancock 1969;Chow and Mukai 1972); (4) differential thermal analysis (DTA) or differential scanning calorimetry (DSC) (White and Rust 1965;Burns and Orrell 1967;Kurachenkov and Igonin 1971;Chow 1972;Chow et al 1975;Kay and Westwood 1975;Christiansen and Gollob 1985); (5) torsional braid analysis (TBA) (Steiner and Warren 1981;Kelley et al 1986); and (6) dynamic mechanical analysis (DMA) (Young et al 1981;Young 1986a;Young 1986b;Geimer et al 1990;Kim et al 1991;Follensbee et al 1993;Christiansen et al 1993). The resin samples measured by these indirect methods are usually in the forms of pure liquid or solids, mixtures with various portions of wood powders, and resin-impregnated glass cloth or wood wafers.…”

Exterior OSB preparation technology at high moisture content - Part 1: Transfer mechanisms and pressing parameters

“…The consequence of this is an increase in thickness swelling (Hse 1975). Palardy et al (1989Palardy et al ( , 1991 have for instance demonstrated that it is possible to reduce structural/damage and residual/stress in wood at higher moisture contents and lower temperature when using a diphenylmethane diisocyanate adhesive. But pressing at higher moisture content is never an easy task, as independently of steam pressure, side problems associated with excessive adhesive migration from the glue-line and retardation of resin hardening occur (Myers et al 1991, Hse et al 1994.…”

“…this would allow the reduction of the press speci®c pressure (iii) pressing at higher M.C. would make it possible to reduce board thickness swelling (Palardy et al 1989(Palardy et al , 1991 and improve the dimensional stability of the panel as well as decrease the water absorption sensitivity of phenolic adhesives in OSB panels.…”