The Chinese fir wood was impregnated using a cyclic increasingpressure method (CIPM) with phenolic prepolymers as the impregnating modifier. Unmodified Chinese fir and progressive increasing-pressure method (PIPM) impregnated Chinese fir were used as reference samples and were compared and analyzed. The product’s chemical structure, internal morphology, crystal structure, and heat resistance were characterized. The transversal and longitudinal sections showed better filling effects, so that it bore greater external loading and reduced the water storage space. CIPM infused more phenolic prepolymer into the Chinese fir. Not only producing more physical filling but also forming more hydrogen bond associations and chemical bond combinations. Compared with PIPM and unmodi- fied Chinese fir, the CIPM impregnated Chinese fir had better mechanical strength and water resistance. The cellulose chains in CIPM impregnated Chinese fir were more closely linked and their crystallinity were clearly improved. Changes in internal morphology and crystal structure explained the reason why the mechanical properties and water resistance of CIPM impregnated Chinese fir were improved significantly. This Chinese fir had lower thermal decomposition rates, higher decomposition residual rates, and smaller combustion flames, which confirmed that it possessed improved heat and fire resistance.

Chinese fir; phenolic prepolymer; cyclic increasing pressure method; chemical structure; crystalline structure; heat resistance