Construction And Properties Of Poplar Fiber-based Self-adhesive Phase Change Energy Storage Materials

With the continuous consumption of fossil fuels and the increase of energy demand,the development of green and renewable materials has become the global energy development trend.As a resource-rich,renewable and degradable natural material,wood fiber is an ideal choice to replace fossil resources.The development of new wood fiber-based composite materials from the macro-structure and micro-nano molecular levels can not only alleviate the energy crisis,and alleviate the ecological and environmental problems caused by the over-exploitation of fossil resources,but also provide theoretical basis and technical support for the development of new materials under the background of"double carbon".Lignocellulosic self-adhesive material,as a new degradable,shapeable and self-adhesive plant fiber material,is expected to replace wood and petroleum-based plastics due to its good mechanical properties and scalability,which has broad development prospects in the fields of packaging,logistics and construction.In this paper,Poplar wood residues are used as raw materials,fibers that meet the performance requirements of poplar fiber-based self-adhesive materials are prepared by combining chemical and physical methods;the fiber is made into a poplar fiber-based self-adhesive material through a hot-press molding process;based on the self-adhesive properties of the material,the porous poplar fiber-based self-adhesive matrix is prepared by adjusting the structure of the material.A simple vacuum impregnation method was used to impregnate the phase change material into the fiber-based self-adhesive matrix to prepare poplar fiber-based self-adhesive phase-change energy storage materials.The morphology,microscopic morphology,crystal structure and chemical composition of fibers were characterized by chemical composition analysis,optical microscope,scanning electron microscope（SEM）and X-ray diffractometer（XRD）.In order to reveal the self-adhesive mechanism of poplar fiber-based self-adhesive materials,the structural changes of lignin,hemicellulose,and cellulose under different hot-pressing conditions were analyzed by two-dimensional nuclear magnetic hydrogen spectroscopy（2D-HSQC）,quantitative phosphorus spectroscopy(³¹P-NMR),and Fourier transform infrared spectroscopy（FTIR）,high performance liquid chromatography（HPLC）,thermogravimetric analysis（TG）.Mechanical properties,hydrophobic properties,thermal stability,and thermal properties are tested by universal mechanical testing machines,contact angle analyzers,simultaneous thermal analyzers,and differential scanning calorimeters（DSC）.The research contents and results of this paper are as follows:（1）A high-efficiency separation method for wood fibers was developed by using atmospheric pressure low-concentration alkali impregnation combined with mechanical grinding.The effects of Na OH concentration,infusion time and grinding method on fiber morphology,chemical composition and mechanical properties of poplar fiber-based self-adhesive materials were studied by single factor experiments,and the dissociation method of fibers was determined.The results showed that when the infusion temperature is 100oC,the solid-to-liquid ratio is 1:4,the Na OH concentration is 8%,and the infusion time is 3 h,the fibers separated by twin-roller milling has good fiber morphology and high lignin content,the crude fiber yield is 81.5%,the average fiber length is 0.86 mm,and the aspect ratio is 47.5.The fiber-based self-adhesive material prepared under the condition has good mechanical properties and can be recycled many times.（2）Single factor test and orthogonal test were used to optimize the hot-pressing process parameters of poplar fiber-based self-adhesive materials.Through the single factor test method,180oC,190oC and 200oC were selected as the three levels of hot-pressing temperature in the orthogonal test,7 MPa,8 MPa and 9 MPa were selected as the three levels of hot-pressing pressure in the orthogonal test,and 65%,70%and 75%were used as the three levels of the initial moisture content of the hot-pressing in the orthogonal test;through the optimization of the orthogonal test method,the best hot-pressing condition parameters were determined:the hot-pressing temperature is 190oC,and the hot-pressing pressure is 8 MPa,the initial moisture content of hot pressing is 75%.After optimization,the tensile strength of the poplar fiber-based self-adhesive material is 84.8 MPa,and the bending strength is 134.6 MPa.Compared with the index before optimization,the tensile strength is increased by 4.8%,the bending strength increased by 2.2%.The influence weight of various factors in the hot-pressing condition on the mechanical properties of fiber-based self-adhesive materials is the initial moisture content of hot-pressing,hot-pressing pressure and hot-pressing temperature.（3）Poplar fiber-based self-adhesive materials with different hot-pressed initial moisture contents were prepared by hot-pressing method.By studying the chemical structure changes of lignin,hemicellulose and cellulose and the properties of materials,it was revealed that poplar fiber-based self-adhesive materials Self-adhesive mechanism of self-adhesive materials.The research results are as follows:With the increase of the initial moisture content of hot pressing,the fiber-based self-adhesive material has a tighter bond between fibers,smaller pores between fibers,and a uniform surface of the material,which enhances the water resistance of the material.The link ratio of ligninβ-O-4?linkages decreased,the proportion ofβ-β?andβ-5?linkages increased,the proportion of aliphatic hydroxyl groups decreased,and the content of phenolic hydroxyl groups increased,at the same time,the molecular weight of lignin increased and the thermal stability was improved.These results show that the depolymerization and condensation reactions of lignin occur simultaneously under the hydrothermal environment of hot pressing,and the condensation reaction is the main one,thereby improving the self-adhesive strength of the material.Hemicellulose was hydrolyzed under the hydrothermal synergistic effect,the degree of branching decreased,the content of sugar groups decreased,and the content of furfural decreased.Lignin and its depolymerization products reacted with furfural to form an adhesive similar to"phenolic resin",which increased the self-adhesive strength of the fiber-based self-adhesive material.The hydrothermal synergistic effect increased the degree of hydrolysis of cellulose,leading to the degradation of the amorphous region,and the hydroxyl groups of the microfibrils on the surface of the crystalline region and the amorphous region were exposed,forming hydrogen bonds with each other,thereby improving the self-adhesive strength of the fibers.Through the integration of research results,the adhesion mechanism of poplar fiber-based self-adhesive materials is summarized.（4）The poplar fiber-based self-adhesive material prepared by adjusting the parameters of hot-pressing conditions can be used as the encapsulation matrix of phase-change materials.The hot-pressing temperature is 190oC,hot-pressing pressure is 2 MPa,and the initial moisture content of hot-pressing is 75%,the poplar fiber-based self-adhesive matrix prepared under the conditions has a porous structure from the micro and macro levels,with a porosity of 36.9%,which provide micro-nano channels for the impregnation of phase change materials,and the encapsulation efficiency of PEG-2000 is 77%,the phase change temperature is 51oC,the melting enthalpy is 118.7 J/g and the crystallization enthalpy is 102 J/g,which has good thermal reliability and mechanical properties.The phase change energy storage composite material has a thermal conductivity of 0.17 W/（m·K）and a thermal diffusivity of 0.47 mm²/s.It has good thermal insulation performance and has excellent potential in transportation,storage,decoration and construction.