Fabrication Of Orinted Biomass-high Density Polyethytlene Composites Using Hot Pressing Process And Its Molding Mechanism

China has very abundant biomass resources, and the total production of biomass stalks isover0.7billion ton in2012and is expected to increase in the future. It is a big challenge toutilize those abundant biomass stalks and increase their economy values. This study usedcotton stalk and sweet sorghum stalk as raw materials to fabricate composites with high-density polyethylene （HDPE） using hot pressing process. The fabrication process ofcomposites was investigated, besides the thermal stability of biomass stalks, heat transfer andthermal degradation of mat during hot press were also analyzed. The results are concluded asfollowing:1. High performance liquid chromatography （HPLC） was used to analyze the chemicalcomponents of sweet sorghum stalk, and the results showed that cellulose, hemicellulose,lignin and water-soluble sugar content of sweet sorghum were24.58%,21.07%,19.08%and10.38%, respectively.2. Thermal stability of cotton stalk, sweet sorghum stalk and sweet sorghum rind and pithwas analyzed by thermogravimetric analyze （TGA）, and showed that their degradationtemperatures were236℃,185.8℃,199.3℃and179.2℃at mass loss of2%, respectively.3. Cotton stalk-HDPE oriented composites （CSHC） and sweet sorghum stalk-HDPEoriented composites （SSHC） were manufactured using powdered HDPE and film-formedHDPE as adhesive. The effects of stalk length-to-diameter ratio, thermal treatmenttemperature and HDPE content on the physical and mechanical properties of composites wereinvestigated. The results showed that the CSHC had better mechanical performance at greaterlength-to-diameter ratio of cotton stalk and had optimal mechanical properties at thermaltreatment temperatures of103℃and140℃with best thickness swelling property at thethermal treatment temperature of170℃. The SSHC with10%HDPE content had bettermechanical properties, water resistance property and vertical density distribution than thosewithout HDPE content. The mechanical properties of SSHC had a downward trend whenHDPE content increased from10%to40%. The addition of MAPE, PF and pMDI couplingagents in the composites increased the interfacial adhesion between the biomass stalks andHDPE, which resulted in physical and mechanical increase of composites.4. The molding mechanisms of biomass stalk-HDPE composites were investigated using pressure testing duing hot pressing process, infrared spectroscopy, microstructure andcomposite failure observation. The results showed that the higher the HDPE content ofcomposite mat, the smaller the pressure of hot press to obtain the same mat density and thegreater the variation of vertical density profile of resulted composites. Mechanicalinterlocking and chemical bonding that produced by HDPE and coupling agents improved theinterfacial adhesion between biomass stalk and HDPE and contributed to the increase ofphysical and mechanical properties. The composites failure occurred at stalk pith, interfacebetween stalks and between stalks and HDPE.5. Independent parallel reaction model was employed to predict the thermal degradationof biomass stalks during hot pressing process. The reaction energies of water-soluble water,hemicelluloses, cellulose and lignin of sweet sorghum stalk were101，110，202and26kJ/mol respectively, and their pre-exponential factors were1.2×10¹¹，5.0×10⁹，3.0×10¹⁷and20min¹, respectively. Thermal degradation of sweet sorghum stalk at composites with10%HDPE content under the hot-press temperature of160℃for10minutes was predicted andfound to be5.4%,2.8%and2.0%at surface, one quarter thickness position and core of mat.6. Mathematical models for predicting specific heat capacity of sweet sorghum andHDPE were created based on experimental testing data. The specific heat capacity of sweetsorghum and HDPE before melting increased with temperature. Gauss equation and Lorentzequation were used to fit the apparent specific heat capacity of HDPE caused by heatabsorption during melting.7. Mathematical model including HDPE content, density and temperature for predictingthermal conductivity of biomass stalk-HDPE oriented composites was established based ontesting data at steady-state condition. The results showed that thermal conductivity of SSHClinearly increased with composite density and temperature, and non-linearly increase withHDPE content.8. One-dimensional heat conduction model including heat absorption of HDPE duringmelting was created and numerically solved using Matlab. Simulation results of the modelwere compared with experimental testing data and found had good consistency. Thesimulation and testing results showed that the heat absorption caused by HDPE meltingincreased with HDPE content when the mat temperature reach the melting temperature ofHDPE, and resulted in lower temperature of mat during final stage of hot press processing athigher HDPE content.