| With the increasing scarcity of forest resources in the world, the wood-based panel industry is developing rapidly. As a following, the problems of formaldehyde brought a serious threat to room environment and human health. In recently years, people actively promote green environmental protection concept and pursue the harmonious development between human and environment. The effective way to solve these problems is to research and development of the environment-friendly bio-composites.This research is aimed at the preparation of environmental friendly wood fiber/polylactic acid (PLA) wood composite materials. The techniques of "High speed mixing-Flat hot press" was applied, taking wood fiber which comes from wood residues as matrix components, oxidation modified ammonium lignosulfonate which comes from pulping byproducts as a bonding component and the biodegradable green plastics (PLA) as a reinforcing component.This research mainly includes four aspects:(1) based on the single-factor test of oxidative modification conditions, instrumental analysis and performance analysis of physics and chemistry, the optimization conditions of oxidation modified ammonium lignosulfonate were determined;(2) By means of the orthogonal test design, range analysis and variance analysis. The preparation technology for wood fiber/oxidation modified ammonium lignosulfonate composite materials was optimized, and the performance characterization of composite materials was discussed by the application of modern instrument analysis technology;(3) Based on single-factor test of technology factors to explore the influence law of the technological factors on the performance of the wood fiber/polylactic acid composite material;(4) Process optimization for composite material was determined by Response Surface Methodology. The bonding mechanism of the composites was explored by FTIR, XRD, TG-DTG, DSC and SEM. The conclusions of this research are as follows.(1) Molecular structure changes of oxidation modified ammonium lignosulfonate and the optimized conditions of oxidation modificationIn both oxidation conditions, the chemical structure and reactivity of ammonium lignosulfonate are certain to change and improve. Under acidic conditions, some aromatic ring was destroyed and methoxy was reduced along with the aromatic ring hydroxylation in the molecular structure of ammonium lignosulphonate, while under basicity conditions mainly dominated by oxidative degradation. Such as hydroxy were oxy-genated, carbonyl were increased and methoxy were shed. The oxidizing conditions and amount of oxidants had a clear influence on physical and chemical properties of the materials. The optimization of the process conditions for oxidation modification was as follow:under acidic conditions, pH value was 3-4, oxidant (H2O2) amount was10%and the ferrous sulfate dosage was1%.(2) Process optimization of Wood fiber/oxidation modified ammonium lignosulfonate composite material.The composite material was made by the wood fiber as matrix, the oxidation modified ammonium lignosulfonate as the adhesive oxide. The optimization of the process was as follow: additive amount of oxidation modified ammonium lignin sulfonate was25%, the slab moisture was20%, the pressing time was7min, the hot-pressing temperature was170℃, the preliminary pressure was2MPa and the hot-pressing pressure was lOMPa. There is a gap between the indicators of physical and chemical performances of the composites and the requirements of the national standard of medium density fiberboard GB/T11718-2009except24h thickness swelling. The material has broad application prospects in building shelves, wooden decorative plates, product packaging and other areas.(3) Combining performances of wood fiber/oxidation modified ammonium lignosulfonate composite materialThe wood fiber and the oxidation modified ammonium lignosulfonate in composite materials have been formed a good crosslinking of combination through hydrogen bonding, esterification reaction or condensation polymerization and condensation reaction. The thermal degradation of chemical compositions in the wood was the main reason to increasing relatively crystallinity of the composite materials. The composite materials owned good stiffness and impact resistance properties, but the thermal stability was decreased.(4) Optimization on level range of technological factors of wood fiber/polylactic acid composite material.The additive amount of oxidation modified ammonium lignin sulfonate as well as the addition of PLA component has a high degree of significant or notable impact on physical and chemical properties of composite material. It is also has a facilitating role for increasing physical and chemical properties of composite material. The level range of additive amount of oxidation modified ammonium lignin sulfonate was20%-30%, while the mass fraction of PLA components was20%～40%in the sum quality of wood fiber and PLA.(5) Optimization technology of wood fiber/poly lactic acid composite materialsBuild four groups of response surface regression models respectively regarding Static bending strength, Elastic modulus, Internal bond strength and24h thickness swelling as objective function by Response Surface Methodology. Variance analysis showed that four groups of mathematical regression model both were accurate and reliable, which can be used to analyze and forecast the response value. The optimization technology parameters of composite materials were as follow:the addition of PLA was33%, the additive amount of oxidation modified ammonium lignin sulfonate was25%, the hot-pressing time was7.5min, the hot- pressing temperature was170℃, the preliminary pressure was2MPa, the hot-pressing pressure was10MPa and the slab moisture was25%. The addition of PLA, the additive amount of oxidation modified ammonium lignin sulfonate and the hot-pressing time as well as their interaction had a significant effect on the physical and chemical properties of composite material.(6) Bonding mechanism of the wood fiber/poly lactic acid composite materialThe characteristics of cross-sectional shape of composite materials showed that the wood fiber/poly lactic acid composite material owning the properties of ductile materials. The wood fiber, the oxidation modified ammonium lignosulfonate and the PLA in composite materials have been formed a good crosslinking of combination through hydrogen bonding, esterification reaction and condensation polymerization, so as to improve the interface compatibility of composite materials. Under the surface activity role of oxidation modified ammonium lignosulfonate, on the one hand, part of molecular chain and paragraph structure of the PLA were damaged, which reducing the overall hot stability of composite materials; On the other hand, it promoted the molecular chain and paragraph moving to a certain extent in PLA increasing activities of molecular chain and paragraph, which was conducive to nucleation and crystallization of poly lactic acid and improved the relatively crystallinity of composite materials. As a result of the physical and chemical performance of composite materials were improved. Moreover, the glass transition temperature (Tg), cold crystal temperature (Tc) and the melting temperature (Tm) of composite materials were all moving to a low temperature, resulting in the heat resistance reduced and plasticity enhanced of composites, which were benefit for processing. |
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