SynopsisSeveral solvent systems are presently used to characterize the molecular weight of phenol-formaldehyde resins. However, results reported in the literature for different molecular weights may not be representative of true molecular weight, but rather may be distorted by aggregation and solvation. In this report an effort to clarify this situation was conducted, first by using a suitable calibration with poly(ethy1ene glycol) and then using this polymer as a molecular size standard to determine the size of phenol-formaldehyde oligomers in solution. In the calculation of the molecular sizes of phenol-formaldehyde resols, proper accounting of the variation of the Mark-Houwink parameters with molecular weight for low degrees of polymerization must be made for poly(ethy1ene glycol). The Mark-Houwink constants for poly(ethy1ene glycol) are very similar in presence or absence of salts, and are considered to be unaffected by solution ionic strength. It is not the case for phenol-formaldehyde, w h a e apparent molecular size varies with the nature of the solvent. The actual molecular weight and molecular dimension distribution are discussed for different type A resols used as adhesives in the wood composite industry.

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Wattle tannin-based urea formaldehyde (UF) adhesives for exterior grade plywood panels were formulated and tested. Laboratory work was conducted in Canada and plant trials in Tanzania. Tests showed that tannin-based adhesives gave quite good plywood which met the Canadian Standard Association (CSA) exterior grade requirements under press conditions similar to those currently employed for UF (control) adhesives. Formulations containing formaldehyde and oil instead of paraformaldehyde were found to be slightly less reactive under mill conditions. For particleboard, hydrolysis of the tannin was required for producing adhesive with acceptable viscosity and "pot life" for commercial applications. The hydrolysis of tannin also improved mechanical properties of particleboard. Relatively long press cycles or high press temperatures were required to produce particleboard panels with aged modulus of rupture (MOR) strength above the CSA exterior grade requirements. Prior to particleboard making, size exclusion chromatography (GPC) and differential scanning calorimetry (DSC) were performed on tannin, hydrolysed tannin, tannin-UF adhesive and commercial PF and UF adhesives. GPC indicated, possibly due to aggregation, that the tannin average molecular weight increased upon heating in the presence of alkali. The thermal or kinetic cure characteristics of the tannin-based adhesives, which are activation energy, enthalpy of cure and reaction order, were determined to be comparable with those of UF adhesives. Tannin hydrolysis lowered the activation energy for cure and this resulted in lower press times, as shown in test results. IntroductionThe demand for imported wood adhesives in Tanzania is estimated at 1000 metric tonnes per annum. While Tanzania is importing tonnes of urea-formaldyde (UF) adhesives at high costs (US $ 0.75/kg), it is exporting at relatively low cost (US $ 0.45/kg) highly reactive tannin extract from the bark of the black wattle tree (Acacia mearnsii) which could be used as the main ingredient in the formulation of exterior grade wood composite adhesives (Dalton and many others).

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SynopsisThe influence of carbohydrates on the thermosetting properties of ammonium-spent sulfite liquor (NHBSL) was studied using various fractions of the liquor obtained by ultrafiltration. While a low molecular weight carbohydrate-rich fraction thermoset readily, a higher molecular weight sugar-free fraction failed to thermoset. Optimum results in the use of NH4SSL as a binder for wood particles were obtained by adjusting the carbohydrate to lignin ratio through addition of carbohydrates to the liquor. Similarly, several sugars used alone as binders without addition of lignin gave satisfactory results at slightly higher press temperatures. A study of various sugars and similar molecules in the binding of wood particles suggested that a combination of ene-aldehyde or ketoalcohol functionalities was likely to produce good results. Condensations involving sugars were affected by such variables as carbohydrate chain length, moisture content, acid catalysis, and press temperature.