Processability,morphology and mechanical properties of wood flour reinforced high density polyethylene composites

Abstract

Wood flour reinforced high density polyethylene (HDPE) composites have been prepared and their rheological properties measured. The melt viscosity decreased as the processing temperature increased and the wood flour content decreased. A power law model was used to describe the pseudoplasticity of these melts. Adding wood flour to HDPE produced an increase in tensile strength and modulus. Composites compounded in a twin screw extruder and treated with a coupling agent (vinyltrimethoxysilane) or a compatibliser (HDPE grafted with maleic anhydride) exhibited better mechanical properties than the corresponding unmodified composites because of improved dispersion and good adhesion between the wood fibre and the polyalkene matrix. Scanning electron microscopy of the fracture surfaces of these composites showed that both the coupling agent and compatibiliser gave superior interfacial strength between the wood fibre and the polyalkene matrix.     

HDPE/竹粉复合材料的界面演变及流变特性

采用扫描电子显微镜-X射线能谱仪跟踪硅烷偶联剂中Si元素在高密度聚乙烯(HDPE)基竹塑共混体系界面处的分布,并结合旋转流变仪研究了共混体系加工过程中的界面演变过程。发现随着加工时间的延长,界面处Si元素的含量相对增加,体系的动态模量、黏度升高。表明偶联剂连接的竹粉与HDPE分子链间的相互作用增强,界面层厚度增加。共混体系的动态流变测试结果表明,竹粉填充体系在低频末端区的线性黏弹行为显著不同于HDPE基体,表现出"类固体"特性,动态流变测试对偶联剂的加入所引起的体系的黏弹行为及结构变化响应较敏感。Cole-Cole曲线可反映竹粉粒子网络结构及竹粉与HDPE基体界面相关的松弛信息,体现体系界面性质及竹粉与分子链间网络结构的变化。     

Dynamic mechanical properties of polystyrene/low density polyethylene blends

The dynamic mechanical properties of polystyrene/low density polyethylene blends and of polystyrene/polyethylene/di-block polystyrene-polyethylene copolymer blends have been investigated in the temperature range −160°C to +100°C. It is shown that anomalies in the low temperature shear modulus data of polystyrene-polyethylene blends are a consequence of non-adhesion between the components. From similar data of blends containing a partial di-block PS-PE copolymer it appears that only very small amounts of copolymer are needed to ensure adhesion between the polystyrene and polyethylene phase. Further it is shown that for modulus considerations of the blends, LDPE together with partial PS-PE copolymer can be treated as a single phase. In some cases the presence of copolymer causes formation of a continuous network throughout the polystyrene matrix, as reflected by a low value for the shear modulus of these blends. Phase reversal of polystyrene-polyethylene blends results in an increase of the loss modulus at 40°C which is ascribed to an increased friction caused by phase entanglements. This increase is more pronounced if an excess of polyethylene is present which is again a consequence of non-adhesion between the components.     

振动挤出HDPE管材的结构与力学性能

使用自制的电磁动态塑化挤出机和螺旋芯棒式机头挤出高密度聚乙烯(HDPE)管材.采用爆破压力测试,拉伸性能测试,差示扫描量热法分析等研究振动频率和振幅对HDPE管材结构与力学性能的影响.振动挤出的HDPE管材周向强度显著提高,实现了管材的双向自增强.与稳态相比,振动挤出的HDPE管材结晶度提高,熔点升高,结晶完善;爆破压力最大提高了34.2%,轴向拉伸屈服应力最大提高了5.3%.     

Dynamic mechanical evidence for the glass transition of high density polyethylene

The effects of shock cooling and swelling with carbon tetrachloride on the dynamic mechanical spectrum of high density polyethylene have been studied with a torsional pendulum. The resulting spectra have a well-defined loss peak at ?65°C which is associated with the glass transition of high density polyethylene. Similar experiments on polypropylene and low density polyethylene demonstrate the similar effects of these treatments on their glass transitions.     

Rheological and mechanical properties of composites made from wood flour and recycled LDPE/HDPE blend

The effect of compounding method is studied with respect to the rheological behavior and mechanical properties of composites made of wood flour and a blend of two main components of plastics waste in municipal solid waste, low-density polyethylene (LDPE) and high-density polyethylene (HDPE). The effects of recycling process on the rheological behavior of LDPE and HDPE blends were investigated. Initially, samples of virgin LDPE and HDPE were thermo-mechanically degraded twice under controlled conditions in an extruder. The recycled materials and wood flour were then compounded by two different mixing methods: simultaneous mixing of all components and pre-mixing, including the blending of polymers in molten state, grinding and subsequent compounding with wood flour. The rheological and mechanical properties of the LDPE/HDPE blend and resultant composites were determined. The results showed that recycling increased the complex viscosity of the LDPE/HDPE blend and it exhibited miscible behavior in a molten state. Rheological testing indicated that the complex viscosity and storage modulus of the composites made by pre-mixing method were higher than that made by the simultaneous method. The results also showed that melt pre-mixing of the polymeric matrix (recycled LDPE and HDPE) improved the mechanical properties of the wood–plastic composites.     

Effectiveness of functionalized polyolefins as compatibilizers for polyethylene/wood flour composites

The effects of various types of compatibilizers on the mechanical properties of high‐density polyethylene/wood flour (HDPE/WF) composite were investigated. Functionalized polyolefins such as maleated and acrylic acid grafted polyethylenes, maleated polypropylene (PPgMA) and styrene‐ethylene/butylene‐styrene triblock copolymer (SEBSgMA) were incorporated to reduce the interfacial tension between the polyethylene matrix and the wood filler. Among them, it was found that maleated linear low‐density polyethylene (LLDPEgMA) gave maximum tensile and impact strength of the composites, presumably because of better compatibility with the HDPE matrix. Similar but less enhanced improvements in the mechanical properties, depending on the compatibilizer loading, were seen for the SEBSgMA system. Whereas acrylic acid grafted high‐density polyethylene (HDPEgAA) and maleated polypropylene (PPgMA) only slightly improved tensile modulus and tensile strength; and they both increased with increasing loadings of compatibilizers. A scanning electron microscopic study was employed to reveal the interfacial region and confirm these findings. In addition, dynamical mechanical thermal measurements also revealed the interaction between filler and matrix, and FTIR spectroscopy was used to assign the chemical fixation and the various chemical species involved at the surface of the wood fillers before and after surface treatment.     

Effects of crystallinity of polypropylene (PP) on the mechanical properties of PP/styrene-ethylene-butylene-styrene-g-maleic anhydride (SEBS-g-MA)/teak wood flour (TWF) composites

Polymer Bulletin - Tensile and impact properties of PP/SEBS-g-MA/WF composites up to wood flour volume fraction 0.31 are evaluated. Tensile modulus and strength increased while elongation-at-break...     

Melt rheological properties of polypropylene–wood flour composites

This article presents study of melt rheological properties of composites of polypropylene (i-PP) filled with wood flour (WF), at filler concentrations of 3–20 wt%. Results illustrate the effects of (i) filler concentration and (ii) shear stress or shear rates on melt viscosity and melt elasticity properties of the composites. Incorporation of WF into i-PP results in an increase of its melt viscosity and a decrease of melt elasticity such as die swell and first normal stress differences; these properties, however, depend on filler concentration. Processing temperature of the filled i-PP increases as compared to the nonfilled polymer.     

Preparation and properties studies of paraffin/high density polyethylene composites and phase-change coatings

The form-stable paraffin/high density polyethylene (HDPE) composites and phase-change coatings were prepared and characterized in this study. The paraffin acts as thermal absorbing material and HDPE serves as the supporting material, which provides structural strength and prevents the leakage of melted paraffin. Scanning electronic microscope (SEM) showed that the paraffin is dispersed uniformly into porous network of HDPE. Differential scanning calorimeter (DSC) determined the melting temperature and heat storage capacity of the composite piece to be 49.6 °C and 150.88 kJ/kg, respectively. Moreover, results indicated that the composite pieces showed better thermal stability than composite powders after 6 temperature cycling experiments. The phase-change coatings with 40 wt% composites as functional filler showed good adhesion strength and shock resistance, and could decrease the surface temperature obviously comparing with nude Al alloy plates.     

Effects of gamma irradiation with and without compatibilizer on the mechanical properties of polypropylene/wood flour composites

In this study, polypropylene/wood flour (Hevea brasiliensis) composites at 40 wt% filler content were prepared using a twin-screw extruder and an injection moulding machine. The effects of gamma irradiation with and without maleic anhydride graft polypropylene (PP-g-MA) compatibilizer (3% relative to the wood flour content) on the flexural properties, tensile properties, and creep behavior were investigated. The irradiation in nitrogen and air atmospheres was performed at various radiation doses (i.e. 5, 10, 20 and 30 kGy). The results revealed the improvement of mechanical properties and creep behavior was found in the presence of gamma irradiation at low radiation doses (5 and 10 kGy), while the composites irradiated at radiation doses over 10 kGy rendered the decrease of mechanical properties. Furthermore, at the same radiation dose, the composites irradiated in nitrogen atmosphere tended to provide significantly higher mechanical properties than the ones irradiated in air atmosphere. Interestingly, the great enhancement of creep resistance was observed, i.e. the tensile strains (6 h of static loading) of the irradiated composites (at 10 kGy) with and without compatibilizer were approximately 36% and 19% lower than that of the untreated composite, respectively. In addition, the Burger’s creep model is applied in order to determine the creep parameters of the composites.     

Dynamic mechanical properties of particulate-filled composites

The relative shear moduli of composites containing glass spheres in a rubbery matrix obey the Mooney equation, analogous to the relative viscosity of similar suspensions in Newtonian liquids. However, when the matrix is a rigid epoxy, the relative shear moduli are less than what the Mooney equation predicts but greater than what the Kerner equation predicts. Relative moduli are less for rigid matrices than for rubbery matrices because (1) the modulus of the filler is not extremely greater compared to that of the rigid matrix; (2) Poisson's ratio is less than 0.5 for a rigid matrix; (3) thermal stresses in the matrix surrounding the particles reduce the apparent modulus of the polymer matrix because of the nonlinear stress—strain behavior of the matrix. This latter effect gives rise to a temperature dependence of the relative modulus below the glass transition temperature of the polymer matrix. Formation of strong aggregates increases the shear modulus the same as viscosity is increased by aggregation. Torsion or flexure tests on specimens made by casting or by molding give incorrect low values of moduli because of a surface layer containing an excess of matrix material; this gives rise to a fictitious increase in apparent modulus as particle size decreases. The mechanical damping can be markedly changed by surface treatment of the filler particles without noticeable changes in the modulus. The Kerner equation, which is a lower bound to the shear modulus, is modified and brought into closer aggrement with the experimental data by taking into account the maximum packing fraction of the filler particles.     

Dynamic mechanical properties of methacrylic-acid-grafted polyethylene films

A dynamical mechanical relaxation study has been made of low density polyethylene films to which methacrylic acid has been grafted by γ irradiation. The grafted films retain the original degree of crystallinity and show only slight changes in melting points and melt viscosities. This indicates that the grafted methacrylic acid side chains are long, few in number, and completely phase separated from the polyethylene matrix. Three dispersion regions are observed in plots of the loss modulus, E″ vs. temperature at constant frequency and these are labeled γ, β, α′, in order of increasing temperature. The α′ peak, above 215°C was assigned to microbrownian segmental motions accompanying the T_g of polymethacrylic acid. The β peak, at ?20°C, was assigned to motions accompanying the T_g of branched polyethylene, and the γ peak, at ?120°C, was assigned to local motions of a few CH₂ sequences in polyethylene.     

The effect of filler content and size on the mechanical properties of polypropylene/oil palm wood flour composites

The effect of filler content and size on the mechanical properties of a new type of wood-based filler, oil palm wood flour (OPWF), in polypropylene (PP) was investigated. Four sizes of OPWF filler at different filler loadings were compounded using a twin screw compounder. All sizes of filler showed a similar trend of declining mechanical properties with increasing filler content. In terms of size, the composites filled with larger-sized filler showed higher modulus, tensile and impact strengths, particularly at high filler loadings. The OPWF used in this study was not treated with any coupling agent.     

Influence of modified wood fibers on the mechanical properties of wood fiber-reinforced polyethylene

Wood fiber of aspen was used as a reinforced filler in linear low-density polyethylene (LLDPE). To improve the compatibility between the wood fiber and the LLDPE matrix, the wood fiber was treated with titanate coupling agents (i.e., TC-PBT and TC-POT) or grafted by acrylonitrile. Both treatments resulted in an improvement in the mechanical properties of the resultant composites compared with the composites filled with the untreated wood fiber. Moreover, the grafting method displayed a more obvious benefit than that of titanate coupling methods to the mechanical property improvement. This was attributed to the crystalline structure of the wood fiber to be destroyed by grafting acrylonitrile, and the amorphous fiber was easily deformed to enhance fiber adhesion at the LLDPE matrix. In addition, the effect of the concentration of the filled wood fiber and the amount of coupling agent or grafting ratio on the mechanical properties of composites are discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1561–1568, 1997     

Thermal and mechanical properties of aluminum powder-filled high-density polyethylene composites

Thermal conductivity and mechanical properties such as tensile strength, elongation at break, and modulus of elasticity of aluminum powder-filled high-density polyethylene composites are investigated experimentally in the range of filler content 0–33% by volume for thermal conductivity and 0–50% by volume for mechanical properties. Experimental results from thermal conductivity measurements show a region of low particle content, 0–12% by volume, where the particles are distributed homogeneously in the polymer matrix and are not interacting with each other; in this region most of the thermal conductivity models for two-phase systems are applicable. At higher particle content, the filler tends to form ag-glomerates and conductive chains resulting in a rapid increase in thermal conductivity. The model developed by Agari and Uno estimates the thermal conductivity in this region. Tensile strength and elongation at break decreased with increasing aluminum particles content, which is attributed to the introduction of discontinuities in the structure. Modulus of elasticity increased up to around 12% volume content of aluminum particles. Einstein's equation, which assumes perfect adhesion between the filler particles and the matrix, explains the experimental results in this region quite well. For particle content higher than 30%, a decrease in the modulus of elasticity is observed which may be attributed to the formation of cavities around filler particles during stretching in tensile tests. © 1996 John Wiley & Sons, Inc.     

Electrical properties of natural-fiber-reinforced low density polyethylene composites: A comparison with carbon black and glass-fiber-filled low density polyethylene composites

The electrical properties of sisal fiber-low density polyethylene (LDPE) and coir fiber-LDPE composites have been studied. The dielectric constant progressively increases with increase of fiber loading and decreases with increase of frequency in the case of all composites. The dielectric constant of sisal-LDPE composites has been studied as a function of fiber length. Volume resistivity values decrease with fiber content. The increase of dielectric constant with fiber loading is more predominant at low frequencies in both the sisal fiber-LDPE and coir fiber-LDPE composites. The results of the natural-fiber-filled composites were compared to those of the carbon and glass-fiber-filled LDPE composites. The dielectric constant of carbon-black-loaded LDPE composites increases with carbon content, and the increase is sharper at high carbon content. This is associated with the network formation of carbon black in LDPE matrix. © 1997 John Wiley & Sons, Inc.     

POE-g-Si对PP/木粉复合材料结构和性能的影响

以硅烷接枝乙烯-1-辛烯共聚物(POE-g-Si)为增容剂,制备了聚丙烯(PP)/POE-g-Si/木粉复合材料。研究了POE-g-Si对PP/木粉复合材料的晶态结构、界面相容性、动态力学性能及力学性能的影响。加入POE-g-Si有效改善了复合材料的界面相容性;PP中β晶型消失,结晶度由71.50%降至61.91%,木粉与基体界面处出现横晶;α,β转变峰均增强;复合材料拉伸强度、抗冲击性能均明显提高。     

Functionalization of high density polyethylene in melt state through ultrasonic initiation and its effect on mechanical properties of glass fiber reinforced composites

Summary Functionalization reaction of high density polyethylene (HDPE) with γ-methacryloxy-propyltrimethoxysilane (MAS) or with MAS and MAH performed in melt state through ultrasonic initiation by a laboratory-scale ultrasonic extruding reactor was studied in this paper. The effect of ultrasonic intensity on the percentage of grafting and melt flow rate of the functionalized products was investigated. The results show that by imposing ultrasonic vibration during melt-extruding process, the scission of HDPE chain bonds can be caused to form macroradicals, the functionalization reaction of HDPE with MAS or with MAS and MAH can be realized. The percentage of grafting and the melt flow rate of the functionalized products depend upon the ultrasonic intensity and reaction temperature. The fuctionalization reaction of HDPE with MAS can be promoted by adding a second grafting monomer MAH. The ultrasonic-induced products have a higher reactivity with the coupling agents coated on the surface of glass fibers, the mechanical properties of the composite improved by the ultrasonic induced product are higher than that of by peroxide initiated product and the mechanical properties of HDPE/GF composite modified by HDPE-g-MAH-MAS are higher than that of by HDPE-g-MAH. The SEM experimental results indicate that an oriented crystal layer exists between the interface of glass fiber and the HDPE matrix, the interfacial bonding strength is the determining factor of the formation of the oriented crystal layer.