| The use of agricultural biomass straw to resolve the shortage of timber resources and lowering the release of formaldehyde during man-made board fabrication and application have drawn great concern of the general public. In this study, with cotton stalks as a base material and glucomannan/ chitosan and polyvinyl(KCP) blending adhesive as binder, environmentally friendly cotton stalks oriented recombinant board(CSOB) was fabricated by hot-pressing techonology. Hot pressing parameters of CSOB and the ratio of elements in blending adhesives were optimized. Formaldehyde-free CSOB which was in line with national standards was fabricated through optimized pressing parameters and blending adhesive ratio. Molding mechanisms of CSOB were explored by scanning electron microscopy and fluorescence microscopy. Heat transfer and temperature distribution of CSOB were determined during fabrication. Based on infinite flat plate heat transfer theory and assumptions, the heat transfer equation of CSOB was established, and the equation predictive accuracy was verified. And it provided theoretical and technical basis for the preparation of formaldehyde-free cotton stalk recombinant materials. The main results in this study are as follows:(1) Chemical constituent and thermal properties of straws were tested. Chemical composition of different sources straw, thermal characteristics and specific heat capacity curves of cotton stalk and ternary blend adhesives were analyzed by cellulose analyzers, thermal properties analyzer and differential scanning calorimetry(DSC). The results found that cellulose hemi cellulose and lignin contents of cotton stalks in this study were 402.79 g/kg, 118.09 g/kg and 151.01 g/kg, respectively. Thermal conductivity and specific heat of cotton stalks in this study were 0.13 W/m·℃ and 1363 J/kg·℃ which were determined by thermal properties analyzer. Specific heat capacity curves of cotton stalks and KCP blending adhesive were also obtained by DSC. Through analysis of specific heat capacity curves, it was found that specific heat of cotton stalk first increased and then decreased at a temperature from room temperature to 150 ℃. Maximum value was obtained for 3150 J/kg·℃ at about 90℃. Specific heat of blending adhesive rised between room temperature to 105 ℃ and decline between 105 ℃ to 160 ℃. Micromorphology of ternary blend adhesive film were observed by scanning electron microscope. The tesults showed that ternary blend adhesive films were uniform and close texture, indicating KGM/ CA and PVA were found compatible with each other.(2) Hot pressing parameters of CSOB were optimized. With modulus of rupture( MOR) modulus of elsticity(MOE) and internal bonding(IB) as index, hot pressing parameters of CSOB were optimized through single factor experiment and orthogonal test. The optimal hot pressing parameters were hot pressing time of 15 min, board target density of 0.8 g/cm3 and hot pressing temperature of 150 ℃.(3) Elements ratios of KCP blending adhesive were optimized special for CSOB. The effects of glucomannan content/ chitosan content and PVA content on blending adhesive properties were tested. The total solid content was also analyzed as a factor affecting the properties of blending adhesive. Comparison between blending adhesive in this study and the sold adhesives was carried out. Through bonding strength and viscosity of the blending adhesive tested, the optimal ratio was obtained: glucomannan concentration of 2.0%, chitosan concentration of 2.0 % and polyvinyl alcohol concentration of 0.6%. By this ratio of blending adhesive, the MOR/ MOE and IB of CSOB were 42.91 MPa, 8.84 GPa and 0.44 Mpa, which were copatible with the precious bending adhesive ratio. The viscosity of blending adhesive was 73.2 m Pa·s, which significantly decreased comparied with bending adheisves without optimization.(4) Penetration form of blending adhesives and molding mechanisms of CSOB were investigated. Penetration form of blending adhesives in the CSOB and deformation of cotton stalks were observed and analyzed by scanning electron microscopy(SEM) and fluorescence microscopy. Adhesive penetration of blending adhesibe in the CSOB was analyzed by fluorescence microscopy and image-J software. In the hot pressing progress, cotton stalk bundles got plastic deformation. With the pressing pressure increasing from 2 MPa to 5 MPa, the aspect ratio of sieve tube in cotton stalks increased from 1.7:1 to 3.4:1. While the adhesive contents increasing from 100 g/m2 to 300 g/m2, the adhesive layer to which the surface of cotton stalk bundles attached increased from 0.3 μm to 0.9 μm. Penetration form of KCP blending adhesive in cotton stalks was mainly a cell cavity penetration. Binding mode of blending adhesive and cotton stalk fiber included mechanical bonding, chemical bonding, diffusion binding and adsorption bonding.(5) Temperature distribution and its effecting factors of CSOB mat during hot-pressing progress were investigated. Temperature distribution of CSOB mat during hot-pressing progress was tested using K-type thermocouple and 32-channel data acquisition card. Influencing factors including board density/ board thickness and moisture contents on the heat transfer progress were analyzed. Thermal characteristics containing specific heat thermal conductivity and thermal diffusion coefficient of KCP boards with different board density and different adhesive contents were analyzed by thermal characteristic analyzer. The results showed when the mat moisture content increased from 5% to 20%, and hot pressing temperature increased from 110 ℃ to 190 ℃, heating time of cotton stalk recombinant material core slab decreased. With the sheet density increasing from 0.4 g/cm3 to 0.8 g/cm3, thermal conductivity and specific heat of CSOB increased, Effects of resin content and board thickness on thermal conductivity and specific heat of CSOB was not obvious.(6) Differential equation of heat transfer in the CSOB with blending adhesive was built and verified. With infinite plate heat transfer theory and assumptions, heat transfer differential equations of CSOB mat were obtained. The heat transfer differential equation was solved using Matlab software and numerical solution of differential equations was solved and verified by the experiments. The results showed that differential equation could better predict the internal heat distribution and changing trend of CSOB. This study provided a data and theoretical reference for fabrication and research of formaldehyde-free agro-based panels. |
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