Effect Of Compression Combined With Steam Treatment On The Structure And Properties Of Cell Walls Of Cunninghamia Lanceolata

Plantation wood resources have become an important component of strategic reserve of wood resources. However, due to loose structure and poor durability of low-quality plantation wood inherent short comings hindered it to be used efficiently. Compression combined with steam treatment(CS-treatment) is a technology at certain humidity, temperature and mechanical stress to implement a stable radial compression wood. CS-treatment can improve and enhance the physical and mechanical properties of wood, while maintaining the natural properties of wood materials. It’s of importance to investigate the effect of CS-treatment process on the wood cell wall structure and properties. It’s significant to understand the influence of CS-treatment conditions to the relationships between cell wall structure and chemical composition changes and physical and mechanical properties. This can provide practical guidance to the modification and manufacture of low-quality plantation wood utilized for environment-friendly functional biomaterials.In this work, the effect of CS-treatment to Chinese fir(Cunninghamia lanceolata Hook.) on its chemical and physical changes was studied. Radial compression(compression ratio of 25% and 50%) in combination with steam treatment(140℃, 160 ℃and 180℃) was investigated. The variation of cell wall ultrastructure, mesopore structure, chemical composition, crystal structure and cell wall moisture absorption and micro-mechanical properties were characterized. This would facilitate the understanding of mechanism involved in the CS-treatment process.Due to CS-treatment bordered pits and parietal wall showed significant changes. When the steam treatment temperature was over 160℃, both 25% and 50% compression ratio resulted in seriously distorted "Z" type cell wall in earlywood and the transition zone cell wall was observed to be polygonal. Whereas the latewood cell lumen showed closure phenomenon, especially small-diameter wood being treated by 50% compression ratio that cell lumen has been completely closed. Microfibrils of the bordered pit margo of tracheid wall were destroyed, resulting in 0.09-0.2 μm pore structure in the pit membrane. 25% and 50% compression ratio at 160℃ can result in a stable compression.During CS-treatment process a large number of slit-like mesopores(? = 3.7 nm) were formed. With both 25% and 50% compression ratio the BET specific surface area(SBET) was gradually increased with treatment temperature was increased. When the temperature reached to 180 ℃ of 50% compression the SBET reached to the maximum. The values of SBET were 7.169 m2 g-1 for earlywood and 11.480 m2 g-1 for latewood, indicating a increase of 2.4 times and 8.1 times compared with untreated wood cell wall, respectively. More apparent increase was observed in latewood in comparison to earlywood when the compression ratio was 50%.During the CS-treatment process the hemicellulose degraded first. Under the 25% compression ratio, O=C-OH in the glucuronic acid unit of xylan content decreased sharply with the increase of steam treatment temperature, and the maximum decrease of 33% and 54% were observed at steam treatment temperature of 180°C. The threshold of xylan content changes was recognized as the 50% compression ratio and steam treatment temperature of 160°C, where 42% and 41% xylan contents were observed in earlywood and latewood, respectively. Lignin undergoes both degradation and cross-link reactions. Under the 25% compression ratio, the cross-link reaction was dominant at the steam treatment temperature of 160°C and the degradation reaction was dominant at the steam treatment temperature of 180°C. Under the 50% compression ratio and the steam treatment temperature over 160°C, the thermal-degradation of lignin becomes the dominant. Cellulose has better thermo stability and no changes were observed at steam treatment temperature of 160°C, however, the cellulose chain of the amorphous region started to degrade at the steam treatment temperature of 180°C.The CS-treated Chinese fir latewood cell wall cellulose has increases crystallinity and crystallite width. About 28-38% increase of degree of crystallinity was observed. The statistical results showed that the steam treatment temperatures and compression ratios did not affect the degree of crystallinity of latewood cell wall cellulose significantly. The crystallization of cell wall cytoplasm and cellulose increased the crystallite width. The stream treatment temperatures did not affect the crystallite width significantly under the 25% compression ratio, while significant increase of 26% was observed at the conditions of 50% compression ratio and steam treatment temperature of 180°C. In addition, both the compression ratio and steam treatment temperature did not affect the cell wall cellulose microfibril arrangement.With the increases of steam treatment temperature and compression ratio, the moisture absorption of CS-treated wood cell walls decreased. The maximum decrease of moisture absorption of CS-treated wood cell walls was observed at the conditions of 50% compression ratio and 180°C steam treatment, in which 30% and 31% decreases were observed in the earlywood and latewood, respectively. The statistical results showed that under the 25% compression ratio, the changes of moisture absorption of CS-treated wood cell walls were not significant at the steam treatment temperatures of 140°C and 160°C, while the significant differences were observed at the steam treatment temperatures of 160°C and 180°C. In addition, under the 50% compression ratio, significant differences were observed at the steam treatment temperatures of 140°C and 160°C and no significant differences were observed at the steam treatment temperatures of 160°C and 180°C(P<0.05). Based on the changes of chemical composition and cell wall porosity, we postulated the moisture absorption of CS-treated wood cell walls was mainly derived from the-OH of cell wall cellulose amorphous region, and the-OH and –COOH of hemicellulose and lignin amorphous region, not the capillary condensed water. This speculation was also supported by the Hailwod-Horrobin adsorption model results.The CS-treatment increased the hardness and elastic modulus of latewood cell walls. The hardness of latewood cell wall increased with the steam treatment temperature and compression ratio, which reached the highest of 25% increase at the conditions of 50% compression ratio and 160°C steam treatment. In addition, the elastic modulus reached the highest of 37% increase, and lowest of 14% increase(at the condition of 50% compression ratio and 180°C steam treatment). The statistical results showed the change of steam treatment temperature and compression ratio did not affect the elastic modulus significantly(exception of the conditions of 50% compression ratio and 180°C steam treatment).