Comparative Study On The Moisture State And Dynamical Mechanical Properties Between Poplar Normal Wood And Tension Wood

Both moisture content and moisture state of wood have a profound influence on the evaluation and processing of wood material.It determines that the related research work in this field is importance of wood utilization.Broad-leaved trees are tend to form tension wood in the upper side of inclining trunk when its upright growth is affected.Tension wood contributes to restore their normal growth state.Therefore,difference of moisture content and moisture state between normal wood and tension wood was compared,and further the influence of different drying methods on them was analyzed.Besides that,dynamic mechanical properties of normal wood and tension wood was also analyzed.It hopes that works stated above are useful to guide the production,processing and utilization of wood materials.In this paper,poplar normal wood and tension wood were selected as objects.Firstly,changes of wood moisture content,cell wall water retention capacity and dynamic mechanical behavior after different time of air dried were compared and analyzed,and the changes between normal wood and tension wood were deliberated expounded.Secondly,the effects of different drying methods(room temperature drying,freeze drying and 105 ℃ drying)on the crystal structure and dynamic mechanical behavior of wood cell wall were analyzed later then,and the differences between normal wood and tension wood were also compared.Finally,effects of different drying treatments on the water retention capacity of wood cell wall were studied,and the effects in normal and tension wood were further analyzed.The main conclusions are listed as follows:There are two exothermic peaks both existed at normal and tension wood in the cooling period of DSC test.It is suggested that the exothermic I peak of DSC close related to the heterogeneous crystallization of free water in wood and the exothermic II peak was allocated to the homogenous crystallization of a small amount of water in wood.The water content of phase transformation in normal wood is greater than that in tension wood,that is to say,the amount of freezable water in normal wood is higher than that of tension wood.Since the non-freezable water mainly consists of bound water,it is suggested that the bound water content in cell wall tension wood is higher than that in cell wall normal wood when wood is at a higher water content(water content is higher than fiber saturation point).In the temperature range from-150 ℃ to 100 ℃,two obvious mechanical relaxation peaks appear in both normal and tension wood.The higher amount of lignin and hemicellulose in normal wood cell wall resulted in the larger area of α relaxation peak.There were more sites in the tension wood gelatinous layer are able to adsorb water molecules,resulting into a larger temperature range of tension wood β relaxation peak greater than normal wood.In addition,it is found that the energy storage modulus(E’)of sample tension wood with different water content is always less than that of sample normal wood.This may be because the gelatinous layer in the cell wall of tension wood fiber is weakly connected with the adjacent wall layers.Upon dehydration,the gelatinous layer is very easy to fall off or shrink in the cell lumen.In order to further accurately compare the dynamic mechanical properties of normal wood and tension wood,the porosity both of normal wood and tension wood samples were obtained by analyzing the cross section images of wood,and then the DMA test data were adjusted accordingly.The results showed that porosity could achieve data correction to a certain extent.Compared with the other two drying methods,105 ℃ drying can promote the rearrangement of cellulose molecular chain to form a larger crystalline area.Conversely,freeze drying has the weakest promotion effect on co-crystallization of molecular chain of cellulose,and the smallest size are formed.At the same time,the process of drying and rapid dehydration at 105 ℃ will reduce the regularity of microfiber arrangement and increase the microfiber angle accordingly.The energy storage modulus(E’)of tension wood dried at 105 ℃ was the highest,and the energy storage modulus(E’)of freeze-dried tension wood was the lowest.This is because there is a certain degree of crystallization in the amorphous region of cellulose during the drying process at 105 ℃,which increases the stiffness of the sample.The sublimation of ice crystals in freeze-drying process makes the cell wall of wood shrink,which may cause certain damage to the cell wall,and finally reduces the stiffness of wood samples.After room temperature drying and freeze-drying,the original spatial sites in normal wood cell walls that provide homogeneous nucleation for water are transformed into pores that contain non-freezable water.The adsorption sites,pore structure and crystal structure of tension wood cell wall all changed,the changes were greater than what happened in normal wood.Along with reducing the capacity of absorption water in cell wall,the origin area contained non-freezing free water in tension wood cell wall also changed into an isolated and sealed space which allow homogeneous nucleation of water.In the drying process of 105 ℃,liquid water in the sample vaporizes and evaporates rapidly in the drying process,which will form a large vapor pressure in local areas,thus damaging the structure of cell wall.It may provide space for water homogeneous crystallization nucleation of normal wood,but at the same time destroy the original pore structure of tension wood,resulting in tension wood cell wall cannot store more water.