Progress 11/01/01 to 10/31/04
Outputs
FPL: Substitution of resorcinol in HMR with progressively greater amounts of 2-methylresorcinol, to give lower crosslinking, decreased wood-epoxy bond durability. Thus, mechanical aspects of the primer are important. Storage of primed wood for long times or at elevated temperatures, to drive HMR's chemical reactions, had no effect on subsequent wood-epoxy bond durability. This appears to indicate that primer-epoxy chemical bonding is not a factor. Possibly, primer molecules at free, outer surfaces of the primer would be cured, with few active hydroxymethyl groups. Attempts to detect penetration of chlororesorcinol-tagged HMR into wood by electron diffraction have not been fruitful. Chromatography shows that 2-methyl resorcinol and 4-chlororesorcinol react at approximately the same rate as normal resorcinol. Time and money were unfortunately consumed by both a funding delay at the beginning, which forced intermittent hiring, and a several weeks of bonding problems
caused by an unanticipated catalyst neutralization reaction. We did not cast films to assess relative stiffness differences of normal vs 2-methylresorcinol substituted HMR. VPI: Swelling and stress relaxation experiments suggest that HMR operates by altering the properties of the wood cell wall, reducing the initial swelling rate, but increasing the long-term equilibrium swelling. This suggests that crosslinking involving HMR inhibits polymer chain relaxations. Solid-state NMR results suggest that a relatively mobile population of wood polymers is affected, both in lignin and cellulose (the technique is not reliable for hemicellulose). Reactive methylol functionality remains up to 120 days after initial HMR treatment of wood, under very dry conditions. Preliminary NMR results show no evidence of epoxy resin chemically reacting with HMR-treated wood. Maine: Water uptake during 30-minute immersions of maple veneers was dramatically reduced by just 5- to 30-minute treatments in HMR. HMR
treatment improves dimensional stability of wood compared to untreated samples after 24-hour immersion. Three-point bending of wood veneers showed that the stiffness of HMR-treated samples increased with treatment times, whereas those for water-soaked specimens decreased with soaking time. HMR was found to have a solubility parameter closer to that of lignin than to those of hemicellulose or cellulose. The stiffening effect of HMR in lignin is postulated to be due to antiplasticization, an uncommon effect that occurs in some glassy polymers. No AFM work was done, and reduced funding had eliminated most of it anyhow. Attempts to use vinyl ester resin as a thin layer adhesive in a bondline between wood substrates did not work, even with control samples: insufficient exotherm may be the problem. It was found that drying HMR-primed wood surfaces using infrared heat for 20 minutes to expedite subsequent bonding with epoxy adhesives gave at least equally durable bonds in water-soak tests
compared to room-temperature drying requiring many hours.
Impacts
It was found that exterior durability of wood-adhesive bonds in exterior structural members seems to require dimensional stabilization of the wood surface by penetration and reinforcement of the wood cell substance. Manufacturers can now target this phenomenon to improve bonds to wood of many exterior or high moisture use structures. Production line bonding with epoxy (and likely other nonaqueous) adhesives is facilitated by using heat to dry the primed surface within minutes, without decreasing eventual bond durability.
Publications
- Son J., and D.J. Gardner. 2004. Dimensional stability measurements of thin wood veneers using the Wilhelmy plate technique. Wood Fiber Sci. 36 (1):98-106.
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Progress 11/01/01 to 10/31/02
Outputs
FPL: Substitution of progressively greater levels of 2-methylresorcinol for resorcinol in hydroxmethylated resorcinol primer is designed to lead to lower crosslinking densities. Such substitutions progressively decreased the ability of HMR primer to promote epoxy bonds to wood that resist delamination, as shown by stringent accelerated aging tests. Thus the crosslinked nature of the primer is an important aspect of action; that is, enhanced adhesive durability with the primer comes partly from its crosslinked nature, and not from a more linear polymeric structure. Three months of work were lost from unexpected effects of pH change during storage. VPI: Work was delayed for 7 months because of a hiring delay after funding was unexpectedly late. Swelling experiments measured by DMA have started. Swelling behavior of untreated yellow-poplar and most of the treated samples (HMR or individual components) follow the two-stage diffusion model proposed by Bagley and Long for
absorption behavior of polymers. This seems to indicate that wood is following normal polymeric absorption behavior. Secondly, swelling history influences later swelling of samples. Thus, simple treatment of wood with HMR may influence later swelling partly because of simple exposure to water in the HMR, and caution must be used in interpreting results. Thirdly, the thermal history of the sample influences the swelling behavior of the samples. So, using thermal treatment to promote uniform cure of epoxy adhesives may affect the final swelling in itself. This provides another caution in interpreting results. Nuclear magnetic resonance experiments were delayed because of equipment factors but are now starting. U. Maine: The drying time of HMR-primed wood can reduced by changing from a schedule of overnight at room temperature to 20 minutes or less at 60 C, while the subsequent bonds of epoxy to wood were improved. This appears to indicate that an industrial process using HMR priming
could be much more efficient, and therefore economic, than previously thought. Work on dynamic mechanical properties of HMR-treated wood has been progressing. It appears the overall time scale for the project (2 years) can still be adhered to, although some individual parts have been delayed.
Impacts
It seems that heating the HMR-primed wood can greatly accelerate its drying time before the wood can be bonded with epoxy resin. This would greatly speed operations such as making bonds between glass-reinforced tension laminations and glulam beams, which would improve the economics of production.
Publications
- No publications reported this period
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