Wood disintegration in cells or cell walls facilitates the maximum compression in hot press. Under the alkaline condition, cellulose, hemicellulose and lignin all would be swelled or hydrolyzed at different levels. On the other hand, alkaline solution is normally used in the pulping industry. Alkaline solution, such as sodium hydrate or potassium hydroxide (10вЂ“20%), is the mostly chemical pretreatment method used for the wood densification manufacturing. Wood polymers always degraded and extracted in the chemical pretreatment. However, combination of chemical pretreatment and mechanical compression maybe an effective method to solve the recovery in densified wood. Some methods, such as high-temperature steam, resin impregnation, and high-frequency microwave heating, are mostly used for deformation fixation of densified wood in the wood industry. Previous reports stated that water absorption expansion ratio in the compression direction reached up to 50% if the deformation in densified wood was not fixed. Therefore, the recovery of deformation under humidity condition is an inevitable problem. However, partly elastic deformation is reversible in this densified wood, especially in earlywood. As a result, the pre-treated wood allows low compressive force while without breaking wood cell walls during the next hot-pressing. Then, the glass-transition temperature of amorphous polymers (hemicellulose and lignin) decreases under the hydrothermal condition. The cell walls of wood would be swelled by water although it cannot penetrate into crystalline region of cellulose. The thermo-mechanical densification, thermal-hydro-mechanical, hygro-mechanical steam treatment, mechanical-microwave treatment, and high-pressure treatment were developed to manufacture densified wood, in which the thermal-hydro is the most common and simplest pretreatment method currently. The process of softening is known as a pretreatment and combined with following mechanical pressing, achieving better compressing result. Whatever the compression methods, softening of wood material before mechanical pressing is required. In recent decades, many approaches were completed for manufacturing densified wood, such as mechanical thermal pressing, viscoelastic thermal compression (VTC). As early as 1930s, the densified wood was used replacing metal materials in military aircraft in USA and Germany. Wood densification is one of the effective techniques for improving the strength of low density wood species, which can effectively extend wood product application due to the enhanced density of surfaces or entirety of wood. All the results suggested that the densified softwood can be easily fabricated using the proposed method and the new densified softwood can be appropriately used as interior decoration materials. Degradation of polymers in wood cell walls also was reconfirmed by the difference of fracture interface. Modulus of rupture (MOR) and modulus of elasticity (MOE) in the thickness direction of densified wood also markedly enhanced. The density of densified wood increased with the wood thickness up to 1.227В gВ cm в€’1, accounting for a 169% increase compared to that of the pristine wood. Sufficient removal of wood polymers resulted in the average compression ratio of about 80% in the radial direction of the natural wood.

FTIR results confirmed that the color changes can be explained by the degradation of hemicellulose and lignin in wood cell walls and migration of extractives during the densification process. The values of brightness L* and b* decreased but the value of a* showed a slight increase in the densified wood. Surface color of Abies wood was changed obviously after the densification. The appearance, color, chemical composition, and physiology and mechanical properties before and after the densification treatment were examined by the colorimeter, FTIR and mechanical testing machine, respectively. Densified Abies wood was fabricated by combining NaOH/Na 2SO 3 solution treatment, hot-pressing and post mechanical fixation. To enhance the wood compressive quality, chemical pretreatments for pristine wood have widely been used. Wood densification can improve the strength of low density wood species and extend wood product applications.

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