| Lignocellulosic materials have attained much attention for their wide range of sources,light weight,high specific strength and specific modulus.The main components of the lignocellulosic material include cellulose,lignin and hemicellulose,etc.,which is an ideal filling material.It is expected to partly replace synthetic fiber materials to enhance polymers to compensate for their lower mechanical properties.However,lignocellulosic materials themselves also have some shortcomings,such as easy moisture absorption,large performance differences,and worse durability than synthetic fiber materials.Therefore,it is necessary to study the mechanical properties and durability of lignocellulosic materials reinforced polymer composites,which can provide reference for the engineering design and application field expansion of this kind of materials.This research took the wood/polypropylene(PP)composite as the research object,and conducted a systematic study of its basic mechanical properties,thermal stability and dynamic mechanical properties,and analyzed the mechanism of reinforcement effect of wood short fibers on PP matrix,and studied the degradation degree of the mechanical properties of materials under accelerated material aging conditions such as temperature,force speed,and moisture,and established the master curve of bending strength and storage modulus with time to predict the long-term mechanical properties of composite materials.The specific research content is as follows:(1)Preparation and characterization of wood/PP composite materials.In view of the poor dispersibility of wood short fibers in PP and the poor compatibility of wood short fibers with PP,this research adopted a two-step method(extrusion-injection)and used maleic anhydride-PP copolymer(MAPP)as a compatibilizer to prepare the composite materials: firstly,the mixed pellets including wood short fibers and PP were prepared by extrusion molding method with higher content of wood short fibers(70 wt.%),then the mixed pellets including wood short fibers and PP,PP masterbatches and MAPP pellets were mixed uniformly according to a certain mass ratio and then were fed into the injection molding machine for molding.Finally,the characterization of the basic mechanical properties(tensile,bending,impact),thermal stability(DSC,TG)and dynamic mechanical properties(DMA)of the prepared composite materials was carried out.It was found that wood short fibers can significantly improve the strength and stiffness of the PP matrix,the strength and modulus of the composite material increased linearly as the content of wood short fibers increasing.Specifically,the tensile strength and tensile modulus of the composites were up to 58.75% and 197.47% higher than that of PP respectively,and the flexural strength and flexural modulus were up to 134.46% and 258.25% higher than that of PP respectively,which showed that the quality of the prepared composite materials was relatively stable.However,as the content of wood short fibers increased,the impact resistance of composite materials decreased,and the failure mode of the material changed from toughness to brittleness.Integrating the melting temperature,crystallinity,initial thermal degradation temperature and other indicators of PP and wood short fiber/PP composite materials,it was found that the thermal stability of composite materials was not much different.When the content of wood short fibers was too high,the dispersion uniformity of wood short fibers in the PP matrix was relatively poor,resulting in the weakening of the effective stress transfer at the interface,and the thermal stability of the composites was slightly reduced.Dynamic mechanical analysis showed that the storage modulus of the material increased with the increase of the content of wood short fibers,which was consistent with the static mechanical test results.(2)The effect of temperature and bending strain rate on the flexural behavior and long-term mechanical properties of composite materials.To study the long-term performance of materials,artificially accelerated material aging methods are generally used.As a polymer-based material,lignocellulosic material reinforced polymer composites have typical viscoelastic mechanical properties,which are mainly affected by the external environment temperature,humidity,and force speed.Therefore,the long-term mechanical properties of composite materials can be studied by changing the conditions of temperature and moisture.Taking the three-point bending experiment of composite materials as an example and exploring the effect of temperature and bending strain rate on the bending behavior and mechanism analysis for composite materials.It was found that the bending strength and bending modulus increased with the increase of bending strain rate,and decreased with the increase of temperature.However,the higher the temperature,the more significant the reinforcement of wood short fibers to the PP matrix;it was proved that the applicability of the time-temperature superposition principle to the bending behavior of composite materials in the nonlinear viscoelastic region,and the horizontal shift factor was used to construct the master curve of flexural strength and storage modulus,which can be used to predict the long-term static and dynamic mechanical properties of the material;with the aid of Eyring's general theory of velocity process and the Arrhenius equation,the activation volume and activation energy of plastic deformation for the material were calculated,among them,the calculated activation energy of plastic deformation for PP was close to the bond-breaking activation energy of PP.Based on the DMA test results in multi-frequency mode,the apparent activation energy of the material was calculated to evaluate the durability of the material.(3)The water absorption behavior and the degradation of mechanical properties of the material under humid and hot conditions.The study of the water absorption behavior for PP and composite materials with different content of wood short fibers at 23 °C,60 °C,and 80 °C was carried out.The diffusion type and mathematical methods proved that the water absorption process of the material conformed to the Fick diffusion,or the water absorption process was dominated by Fick diffusion;the diffusion constant was acquired according to the curve of water uptake and the effective equilibrium moisture content of the material,in view of the water absorption curve and diffusion constant,it was discovered that the rate of water absorption increased as the content of wood short fibers and temperature increasing.Compared with the reported diffusion constant during the water absorption process of wood-reinforced PP composites,the diffusion constant during the water absorption process of the composites prepared in this research was smaller,indicating that the preparation method adopted in this research was successful;based on the existing model for predicting water absorption for a long time,the theoretical water absorption curve of the material can be obtained.It was found that the first six series of the theoretical model can better fit the experimental results.The parameters of exponential Arrhenius-type relation for diffusion constant can be obtained based on the relationship of the diffusion constant and temperature,which can be used to predict the diffusion constant at different temperatures.The mechanical properties of the composites would decrease significantly,and the degradation degree of its mechanical properties would increase with the increase of the equilibrium water content after the material reached the effective water absorption balance under humid and hot conditions.Moreover,water molecules had obvious plasticizing effect on the material.It was found that the loss factor of the material increased,and the effective transfer of stress at the interface decreased after absorbing water through DMA test.Based on the above research,this paper revealed the reinforcement mechanism of wood short fibers on the PP matrix,and the mechanism of temperature and bending strain rate on the mechanical properties of wood short fiber/PP composites.It was proved that the applicability of the time-temperature superposition principle to the bending behavior of composite materials in the nonlinear viscoelastic region.A master curve of the bending strength and storage modulus with time that can predict the longterm mechanical properties of the material was established.The law of water absorption of wood short fiber/PP composites was discovered,and the water absorption model and diffusion constant model of the material was obtained,and the relationship between the degradation degree of mechanical properties and the equilibrium moisture content of the materials under humid and hot conditions was studied.The above studies are of benefit for the engineering design and application of wood short fiber/PP composites,and provided reference for the research of other natural fiber reinforced polymer composites. |
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