Preparation And Characterization Of Carbon Fibrous Material From Liquefied Wood In Phenol

In order to release the shortage of fossil resource, realize the sustainable development of carbon fiber, change the process defects of tradition biomass-based carbon fiber and efficiently improve the utilization of wood-based resource, we carry out the research on preparation of carbon fibers from liquefied wood in phenol. In this paper, factors about synthesis of spinning solution, preparation of precursors and carbon fibers prepared from liquefied wood in phenol were systemically studied, and the optimum processing parameters were defined by orthogonal test; morphological characteristics, changes of molecular structure, thermal properties of spinning solution and precursors were discussed; changes of microstructure, formation and growth of graphite-like microcrystal, graphitization extent of carbon fibers were investigated under different carbonization temperature; carbonization reaction course of precursors were preliminary analyzed by using FTIR, TG-MS and element analysis in order to clarify the carbonization mechanism.The results are as follows:1 )The tensile strength/modulus and breaking elongation of precursors prepared from liquefied wood in phenol increased obviously with increasing the phenol/wood ratio during wood liquefaction. The mechanical properties of carbon fiber precursors dropper with increasing synthetics agent content. The peak decreasing amplitude of tensile strength/modulus of precursors occured at adding synthetics agent content 6%, but the breaking elongation reached the maximum at synthetics agent content 4%. The tensile strength/modulus showed no obvious increase with increasing the synthesis temperature, whereas the breaking elongation decreased and showed obvious decreasing amplitude from 110℃ to 115℃. The mechanical properties of precursors were improved with increasing temperature rise time and then drop. Carbon fiber precursors showed optimum mechanical properties at temperature rise time 40 min.2) The mechanical properties of precursors prepared from liquefied wood in phenol were improved obviously with increasing spinning speed, the tensile strength/modulus increased 138% and 113% respectively when doubled the spinning speed. When concentration of HC1 was below 15%, tensile strength/modulus linearly increased with increasing concentration of HC1. The tensile strength/modulus decreased with increasing concentration of HC1 when concentration of HC1 was above 15%. But the change trend of breaking elongation was converse to that of tesile strength/modulus. During curing time below 2h, tensile strength of precursors remarkably increased with prolonging curing time , but whencuring time was above 2h it showed no obvious increase. Both of modulus and the breaking elongation reached the maximum at curing time 3h. With increasing heating rate, tensile strength/modulus of precursors reached 113MPa and 20GPa at 15℃/hr respectively then decreased, but the breaking elongation kept increasing and reached the maximum 2.31% at 25℃/h.3) FTIR absording band of liquefied wood, spinning solution and precursors visibly changed at 1610cm^-1, 1475 cm^-1, 1454 cm^-1, 1227～1099cm^-1 and 900～650cm^-1 band. It is concluded that preparation of the precursors could be divided into two phrases. In the first stage, formaldehyde (HCHO) from hexamethylenetetramine (HMTA) combined with unreacted phenol during wood liquefaction to produce new hydroxymethyl. At the same time, hydroxymethyl or hydroxymethyl reacting with alive hydrogen of aromatic ring formed diphenyl ether linkage and carbonyl brigdes, so that spinning solution with the low degree of crosslinkage and linearity structure was prepared. In the following stage, many aromatic ring of fiber reacted with ⁺CH₂OH in curing solution during curing reaction; the relative intensity of the out-of-plane CH deformation band decreased; the crosslinkage of fibers were improved; the precursors preliminary had net-crosslinking structure and good mechanical properties.4) The rate of weight loss of liquefied wood, spinning solution, precursors were all relatively slow at rang 0～150℃. Temperature rang 150～600℃ is the main weight loss stage for three materials, which were 36.9%, 41.3% and 43.3% respectively. Thermal degradation of three materials were slow again above 600℃ and the precursors caused the second pyrolysis reaction and arrange. Weight retention ratio illustrated that thermal stability of precursors and spinning solution were better than that of liquefied wood.5) There were two exothermic decomposition peaks on DSC curves of precursors from 0℃ to 600℃. The location of these peaks move to high temperature with increasing heating rate and the shape of peaks became larger and wider. According to Kissinger and Crane formula, the activation energy of two peaks were 69.36kJ·mol^-1 and 59.02kJ·mor^-1; the reaction order were 0.862 and 0.734, respectively.6) Tensile strength and modulus of carbon fibers increased with increasing carbonization temperature and time, and decreased with the rise of heating rate.The trend of the breaking elongation was opposite to that of tensile strength and modulus.7) By orthogonal design analysis, satisfactory results of synthesis of spinning solution were obtained: phenol/wood ratio 6/1, synthesis temperature 120℃, synthetics agent content 4% and time for temperature rising 40min. Optimum parameters of spinning and curing process of fibers were spinning speed 72r/min, concentration of HCl 18.5%, heating rate 10℃/h, curing time 4h. Then the precursors prepared by above methods were carbonized for 2 hours by heating from a room temperature to 1000℃ at a heating rate 2℃/min in a stream of nitrogen. Tensile strength, Modulus and yield of these carbon fibers were 1.7GPa, 159GPa and 60% respectively.8) Surface of both the precursors and carbon fibers were smooth, which had no obviousdisfigurements such as pores, grooves, sediments and so on. They had an ellipse cross-section and the border of both is the more compact than the center. Many fine pores were mainly observed around the center of the cross section of carbon fibers.9) Above carbonization temperature 400℃, the (002) crystal plane diffraction peak of the precursors at were obviously heighten with the rise of carbonization temperature, and gradually approached to the (002) plane peak of graphitoidal. Carbone fibers showed obvious (100) crystal plane diffraction peak, which illustrated that this material changed from non-crystal to crystal structure. With temperature rising, the value of d₀₀₂> and d₁₀₀ decreasd; Values of the crystallite sizes La and Lc, the Lc/d₀₀₂ rate reduced then increased. The g value corresponding to the change degree of graphitoidal structure increased. These resluts implied that the amount of graphitoidal crystal of carbon fibers increased with increasing carbonization temperature, arrange among layers of graphite crystal was regularly piled up, and both the carbon net structure and the graphitoidal degree of material were developped and improved remarkably .10) There were no bands in the Raman spectral of the precursors between 1200 cm^-1 and 1700 cm^-1, which showed the precursors had no graphitoidal structure. Above 500℃, the Raman spectra of carbon fibers appeared the known D band which is related to disorder carbon and G band which corresponds to graphite with varying characteristics that are similar to other graphitoidal material. With carbonization temperature rising, intensity of the D band decreased and that of G band increased; the integrated intensity ratio R=I_D/I_G (R-value) which is the degree of disorder of material gradually decreased with increasing carbonization temperature. It is illustrated that structure of carbon fibers prepared from liquefied wood in phenol transformed from two dimension carbon layers to the ordered hexagonal graphitoidal crystal structure with the rise of carbonization temperature.11) With increasing heat-treatment temperature, specific surface area, BET surface area, micropore area, total pore volume and micropore volume of carbon fibers increased, but pore radius decline. Temperature rang 600～800℃ was the critical stage at which the pore structure of carbon fiber changed. Adsorption isotherms of carbon fibers transform type I-A to type I-B during carbonization, and desorption isotherms convert from type A with tubular capillary to type B with capillary.12) From results of FTIR, TG-MS, element analysis under different carbonization conditions, it is concluded that the complex carbonization of the precursors could be divided into three phases. In the first stage, the carbonziation temperature was lower than 350℃, the cleavage of intramolecular ether linkage and the removal of end hydroxymethyl mainly occured, and some molecules such as H₂O, CO₂, CH₂OH and free phenol were found. In the second stage, 300 ℃～600 ℃ was the main rang of carbonization; the bond cleavage occured at different positions of the most molecule chain; many substance were observed at around 500℃, such as phenol, methyl-phenol, dimethyl-phenol, CO, CO₂,CH₄, benzene, methylbenzene and so on. In the third stage, the carbon net structure continuallydevelopped above 700℃; molecule structure took place a further reaction at around 800℃ accompanying the production of CO₂ and methylbenzene.