Research On Natural Plant Fiber Modification Process By Plasticizing And Its Promoting Mechanism

Natural plant fibers are the most abundant renewable resources in the nature world, which are biodegradable and compatible with environment. The major components of natural plant fibers are cellulose, hemicellulose, and lignin. The functional groups of these components are employed in preparation of functional materials with versatile applications, including materials for water absorbing, heavy metal ion absorbing, medical and health, and so on. Among all the fibers, thermoplastic composites are focused currently because of their environmental compatibility. The thermal stability of natural plant fibers are decreased by esterification and etherification thus functional materials with good thermoplasticity can be obtained. Thermoplastic fibers can be made into high-performance and biodegradable composites by thermoforming in replacement of fossil based materials partially. This will reduce the dependency on fossil resources as well as find a new way to applications of natural plant fibers. Furthermore, it will have an important significance on the ease of energy, environment, and ecology issues in China.In this dissertation, natural softwood fibers were selected as resources. The fiber modification processes through cyanoethylation, benzylation, and alkylation were studied as well as the properties of the modified fibers based on previous research.The optimal process condition for softwood fibers modification by acrylonitrile cyanoethylation was given by followings:fibers were immersed in KSCN saturated1mol/L NaOH aqueous solution for2h under the beating degree of40℃SR; the dosage of acrylonitrile was8.5mL per gram of absolute dry fibers; reaction was carried out at50℃for2h. The weight gain rate of modified fibers reached92.31%, the nitrogen content was12.88%, and the corresponding degree of substitution was2.91(elemental analysis).The optimal process condition for softwood fibers modification by benzyl chloride benzylation was given by followings:the dosage of benzyl chloride was5mL per gram of absolute dry fibers; the concentration of NaOH aqueous solution was30wt.%; the dosage of NaOH aqueous solution was5mL per gram of dry fibers; the reaction was carried out at105℃for5h. The weight gain rate of modified fibers was129.47%, the carbon content was67.30%, and the corresponding degree of substitution was1.58. In presence of cetyl trimethyl ammonium bromide catalysts, the weight gain rate of modified fibers was149.53%, the carbon content was increased to71.55%, and corresponding degree of substitution reached2.24.The optimal process condition for softwood fibers modification by epoxy chloropropane alkylation was given by followings:the dosage of epichlorohydrin was5mL per gram of absolute dry fibers; the dosage of6mol/L NaOH aqueous solution was4mL per gram of absolute dry fibers; the dosage of cetyl trimethyl ammonium bromide catalyst was0.01g; the dosage of1,4-dioxane was7.5mL per gram of absolute dry fibers; the reaction was carried out at70℃for6h. The epoxy value of modified fibers was8.39mmol/g, the weight gain rate was88.67%, and the corresponding degree of substitution was2.56.Thermoplastified fibers were analyzed and characterized by IR, XRD, SEM, TGA, and DSC. The characteristic absorption peaks of cyanoethylated, benzylate, and alkylated fibers were observed from IR spectrum (-CN at2257cm-1; new absorption peaks at3062cm-1,3029cm-1,736cm-1,and697cm-1; enhanced absorption peak intensity of ether bonds of the alkylated fibers). The SEM photo exhibited that orignial fibers were coated by modified products. Modified fibers were round and erect with volume expansion. The calculations based on XRD photos showed the crystallinity was reduced to7.26%,23.14%, and21.40%. TGA and DSC analysis indicated that fibers had thermoplasticity after modification. The glass transition temperatrues of cyanoethylated, benzylated, and alkylated fibers were85℃,134℃, and137℃.Comprehensively analysing the difficulty of of plasticizing reaction and the thermoplasticity of modified fiber, it was clear that cyanoethylation modification could give modified fiber better thermoplasticity under some mild conditions. However, the temperature of benzylation was higher and alkylation was time-consuming, and the thermoplasticity of modified fiber was not in the same class as cyanoethyl fiber. So, cyanoethylation was more appropriate for plasticizing softwood fibers. The reaction kinetics of the modification process was analyzed by studying the influences of reaction temperatures and reaction times on the modification effects of softwood fibers. The rate constant and apparent activation energy at different temperatures were determined,In case of excessive modification reagents dosage, all the cyanoethylation, benzylation, and alkylation were first-order reactions. The rate constants of cyanoethylation were1.04and1.62at40℃and50℃, and the apparent activation energy was36.58kJ/mol. The rate constants of benzylation were0.12,0.15, and0.18at95℃,100℃, and105℃, and the apparent activation energy was46.06kJ/mol. The rate constants of alkylation were0.11,0.25, and0.35at60℃,70℃, and80℃, and the apparent activation energy was48.14kJ/mol.The influences of some minor components on the thermoplastification effects of Mason pine wood powders were studied. The benzyl alcohol extractives and the lignin constituents had effects on the chemical modification. After the removal of benzyl alcohol extractives, the weight gain rates of cyanoethylated, benzylated, and alkylated Mason pine wood powder were increased8.27%,5.88%, and7.31%. At the same time, the removal of lignin from raw biomass opened the diffusion tunnels of modification reagents, which reinforced the cyanoethylation, benzylation, and alkylation performance significantly. The weight gain rate of modified products increased with the amount of lignin removed from the resources. TGA curves revealed that the thermal stability of modified wood powder was lower than original Mason pine wood powder. The thermoplasticity of Mason pine wood powder were strengthened with the amount of lignin removed and weight gain rate increasing.Finally, the influences of the physical structure property changes on the effects of thermoplastification were studied. It was observed that there was relationship between the weight gain rates of cyanoethylation, benzylation, and alkylation and the properties of modified products. The results showed that the chemical structures and crystal categories were not changed by mechanical refining. However, the hydrogen bonds of fibers were destroyed, which resulted in external fibrillation, reduction of crystallinity, and increase of water retention value of the fibers. The accessibility and chemical reactivity of cellulose fibers were also enhanced consequently. The weight gain rates of modified fibers increased with the beating degree of pulp fiber increasing. IR spectrum showed that the peak intensities of modified fibers were reinforced with the increase of beating degree and weight gain rate. The TGA and DSC curves of modified fibers exhibited that refining could increase the reactivity, decease the stability, and enhance the thermoplasticity of natural softwood fibers. Compared with the fibers unrefined, the glass transition temperature of cyanoethylated fiber(beating degree49°SR) decreased from135℃to83℃, the glass transition temperature of benzylated fiber(beating degree49°SR) decreased from162℃to119℃, and the glass transition temperature of alkylated fiber(beating degree49°SR) reduced from189℃to132℃.