This is the second report on wood hardening technologies. In this report, the performance for flooring based on wood treatability, chemical retention, dimensional stability, wear resistance, pull-off strength, and hardness were investigated in Douglas fir, western hemlock, hard maple, aspen, and Amabilis fir treated using two different processes. Tests were also conducted on wood modified with nanoparticles with sol-gel method, wood hardened by different methacrylates formulations, and wood impregnated with MUF resin and polymerized with hot press compression.
On the basis of the wood samples treated at Mill A, it was concluded that hard maple sapwood had the best treatability. Western hemlock and Douglas fir had very good treatability. Aspen and Amabilis had poor treatability. Chemical treatments at both Mill A and Mill B increased the density of the five tested wood samples, compared with control samples. Under the same treatment conditions, Amabilis fir and western hemlock had the highest chemical retention. Hard maple had the lowest chemical retention.
Generally, chemical treatments at both Mill A and Mill B improved the dimensional stability and water absorption property of five tested wood samples, compared with control samples. Under the same treatment conditions, western hemlock had the highest anti-swelling efficiency in radial and tangential direction and the greatest improvement in water absorption. Chemical treatments did not improve the wear resistance of all treated wood samples. But both Mill A and Mill B treatments improved the wear resistance of hard maple and western hemlock, compared with untreated controls. Hard maple had lower wear index than western hemlock, but western hemlock had greater wear resistance improvement than hard maple.
Chemical treatments at both Mill A and Mill B improved the hardness of the samples of five tested wood species, compared with untreated controls. Western hemlock treated at Mill B was the hardest and had the greatest improvement in hardness. Compared with untreated hard maple, western hemlock and Douglas fir treated at Mill B were harder. Chemical treatments at both Mill A and Mill B improved the pull-off strength of tested wood species, compared with controls. Of all the samples, hard maple treated at Mill B had the greatest pull-off strength. Western hemlock and Douglas fir both treated at Mill B had greater pull-off strength than untreated hard maple. They also had the greatest pull-off strength improvement.
On the basis of two Chinese wood species tested, the sol-gel method can be selectively applied to wood to improve wood hardness. One of the factors that determined the level of hardness improvement in wood was chemical retention.
Different methacrylates had different impacts on modified hard maple. Monomer retention by volume in treated hard maple was similar regardless of formulation combinations, but monomer retention by weight was different than formulation combinations because of differences in monomer densities. Methacrylates in wood enhanced wood hardness and hardness modulus. The Brinell hardness correlated highly with hardness modulus. Different methacrylates resulted in different wood water absorptions and had different effects on treated wood dimensional stabilities in tangential direction.
Hard maple and poplar impregnated with MUF resin improved wood hardness and dimensional stability in terms of thickness swelling and water absorption, compared with controls. Resin impregnated hybrid poplar was harder than untreated hard maple. Compression had a tendency to decrease chemical retention in poplar. Impregnation together with compression improved hybrid poplar hardness, but had a tendency to decrease the hardness of compressed hard maple. With the experimental parameters used, impregnation of wood with MUF resin caused cracks, especially in compressed wood samples.
Successfully applying wood hardening technologies in the wood flooring industry depends heavily on the processing cost and on marketing. It also relies on empathy for better life, better quality and better environment. Thus, research should be focused on reducing production costs and long-term performance. MUF resin offers an alternative for hardening wood at a relatively low cost. Preliminary test results indicate that impregnating hybrid poplar and hard maple improved the wood hardness and dimensional stability. Impregnation, together with hot press compression, improved the hardness and dimensional stability of poplar. Compression has the potential to reduce chemical retention in wood and reduce process cost without compromising wood hardness and dimensional stability. Thus, impregnation together with hot press compression should be tested in a methacrylates-wood system. A comparison between a methacrylates-wood system and MUF resin-wood system would be very helpful. Further research on MUF resin impregnation and hot press compression should be done on low-density wood species to improve impregnation efficiency and reduce chemical retention. The resin formulation and hot press compression parameters should be optimized. The long term durability of the treated wood should be confirmed.