Improving mechanical properties of poplar wood by combining delignification and densification treatment

in the present study a modification method combing Delignification and densification was used to improve the mechanical properties of fast growing poplar wood, aimed to extend their applications into structural and semi-structural purposes. Recent studies showed that partial removal of lignin before densification may increase the compressibility and hydrogen bond formation during densification led to considerable mechanical properties improvement.

poplar wood blocks were delignified with glycerol as an organic solvent. One level of delignified blocks were densified immediately after washing and another level were surface impregnated with glycerol-maleic anhydride and densified to polymerization during densification further increase the mechanical properties of densified wood. At a third level, in order to compare the method of delignification and polymerization with heat treatment, the wood blocks were hydrothermally treated in temperature and pressure conditions similar to delgnification and then densified. The compressing ratio was considered 80% for three levels. In order to investigate the morphological changes in the control and modified samples, scanning electron images were prepared. In order to investigate the effect of modification on mechanical properties, compression and tension parallel to grain, static bending, hardness and impact tests was conducted.

the lignin removal content was 55% and weight gain after impregnation was 7.5%. Electron microscopic images showed filling of wood lumens with cell wall materials in hydrothermally treated samples, and also some cracks were appeared because of increasing brittleness of cell wall. In delignified-densified samples, a lot of lumens were observed probably because of set recovery after press. In delignified-impregnated-densified samples, the lumens were filled by polyesters formed by reaction of glycerol and maleic anhydride. The tensile strength of delignified-densified samples were more the hydrothermally treated ones because of hydrogen bond formation between cellulose unites. Impregnation with glycerol-maleic anhydride led to further increase of tensile strength due to ester bond formation. The highest amount of tension and compression modulus was belonging to hydrothermally treated samples as a function of condensation reaction of lignin and increasing the crystallinity of cellulose. In impregnated-densified samples because of filling the lumens and strong ester bond formation, the tensile and compressive modulus was higher than the delignified-densified samples. The flexural strength of impregnated –densified sampls were considerably increased because of polyester formation in surface. The highest hardness modulus was belonging to delignified-impregnated-densified samples. The highest impact strength was related to the delignified-densified samples and impreganating with glycerol-maleic anhydride led to a little decrease.

Results of mechanical tests showed the effectiveness of the combined delignificaton-impregnation- densification modifying method on improvement of specific tension, compresiive and hardness strength compared to the control ones. Specific impact strength of delignified-densified samples was higher than the control and two other levels attributed to higher ductility. Unlike flexural strength, the specific flexural strength of delignified-impregnated-densified samples was lower than the control, attributing to the strength of sample surface; densification was conducted considerable (80%) in the volume of samples, not only in surface, therefore, despite of higher flexural strength, the specific flexural strength decreased.