Mehdi Tajvidi

Water absorption behavior of composites from sawdust and recycled plastics

The water absorption of wood plastic composites (WPCs) prepared from sawdust and virgin and/or recycled plastics (HDPE and PP) was studied. Wood flour was prepared from sawdust and mixed with different virgin or recycled plastics at 50% by weight fiber loading. The mixed materials were compression molded into panels. Long term water absorptions of manufactured WPCs were evaluated by immersing them in water at room temperature for several weeks (up to 1750 h). Water diffusion coefficients were also calculated by evaluating the water absorption isotherms. Results indicated that the maximum water absorption and diffusion coefficients increased by increasing the proportion of recycled HDPE and/or PP. Time to reach saturation also decreased at higher recycled plastic contents. Water absorption of the studied composites was proved to follow the kinetics of a Fickian diffusion process.

Mechanical Performance of Hemp Fiber Polypropylene Composites at Different Operating Temperatures

In order to quantify the effect of temperature on the mechanical properties of hemp fiber polypropylene composites, formulations containing 25% and 40% (by weight) hemp fiber were produced and tested at three representative temperatures of 256, 296, and 336 K. Flexural, tensile, and impact tests, as well as dynamic mechanical analysis, were performed and the reduction in mechanical properties were evaluated. Impact resistance was independent of temperature, whereas flexural and tensile properties were strongly affected. The highest reductions were observed in stiffness (modulus) values and flexural properties were reduced to a higher degree. The reductions in mechanical properties were well explained by a simple quadratic curve-fitting procedure applied to experimental data. Dynamic mechanical analysis revealed no change in glass transition temperature when the fiber content was increased but the composite material had better temperature resistance at higher fiber content. The results of the present study will be helpful in determining the end-use application of these composite materials.

Mechanical and Physical Properties of Wood-Plastic Composite Panels

In this research wood-plastic composite (WPC) panels were produced from high density polyethylene, MDF, and particle-board waste at 60, 70, and 80 wt% fiber loadings using the dry blend/hot press method. Physical and mechanical properties of the panels were studied and compared with conventional MDF and particle-board panels. The results indicated that the studied properties of the composites were strongly affected by the kind and proportion of the wood fiber and polymer. Maximum values of the flexural modulus of the WPC panels were reached at 70% fiber content. The flexural strength and impact strength of the WPC panels declined when fiber content increased from 60 to 80%. The flexural modulus of the WPC panels was lower than that of the virgin MDF panels but the flexural modulus of the composites with 70% fibers was close to that of particle-board panels. Flexural strength of MDF panels was noticeably higher than those of wood-plastic composites whereas the flexural modulus of particle-board panels was comparable to that of the wood-plastic composites at 80% fiber content. Furthermore, water uptake of wood-plastic samples increased with the increase in fiber content; however, it was relatively low compared with virgin MDF and particle-board panels.

Effect of Thermomechanical Degradation of Polypropylene on Mechanical Properties of Wood-Polypropylene Composites

In this research, the influence of thermomechanically degraded polypropylene (PP) on mechanical properties of beech sawdust-PP composites was studied. For this purpose, a virgin PP (VPP) was thermomechanically degraded by two times extrusion under controlled conditions in a twin-screw extruder at a rotor speed of 100 rpm and a temperature of 190°C. The results showed that melt flow index, flexural modulus, and hardness of PP were significantly increased by extrusion and re-extrusion of VPP. The PP (virgin and recycled PP in each stage) and beech sawdust were compounded at 60% weight sawdust loading in a counter-rotating twin-screw extruder in the presence or absence of maleated polypropylene (MAPP) to produce sawdust-PP composites. The nominal cross section and density of the manufactured composites were 70 × 10 mm2 and 1 g/cm3, respectively. From the results, the composites containing recycled PP exhibited higher flexural properties and hardness and lower impact strength. In the presence of MAPP, all mechanical properties increased.

Thermal Degradation of Natural Fiber-reinforced Polypropylene Composites:

The objective of the present article was to study the thermal degradation behavior of natural fiber polypropylene composites. Composite materials composed of 50% various natural fibers (wood flour, rice hulls, newsprint, and kenaf fibers) and polypropylene were studied using thermogravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC). The effect of compatibilzer on the thermal stability of the composites was also evaluated. Contributions of components of one composite formulation to thermal degradation were also evaluated. It was found that among natural fibers, rice hulls were the least thermally stable ones in a polypropylene matrix. The compatibilizer slightly reduced thermal stability while enhancing fiber—matrix interaction and fractional crystallinity. The overall thermal degradation profile of the composite materials was found to be a more or less arithmetic average of those of the components.

Hygroscopic thickness swelling rate of composites from sawdust and recycled plastics

The thickness swelling rates of compression molded wood plastic composites (WPCs) prepared from 50% by weight sawdust and 50% by weight virgin and/or recycled plastics (HDPE and PP) were studied. Thickness swelling rates of the manufactured WPCs were evaluated by immersing them in water at room temperature and monitoring thickness changes for several weeks. A swelling model developed by Shi and Gardner (Composites Part A, 37:1276–1285, 2006) was used to study the thickness swelling process of WPCs. The parameter KSR of the model can be used to quantify the swelling rate. The results indicated that composites containing PP had lower equilibrium thickness swelling and also shorter equilibrium time (time to reach equilibrium thickness swelling). The swelling model developed was a good predictor of the hygroscopic swelling process of wood plastic composites. Composites containing PP (virgin and recycled) had higher KSR than those containing HDPE. The minimum and maximum KSR values were observed in composites made of virgin HDPE and a mixture of recycled plastics, respectively. With increasing recycled plastic content KSR linearly decreased.

Effects of Water Absorption on Creep Behavior of Wood—Plastic Composites

The effects of filler loading and immersion in water on the creep/ recovery behavior of composites made from MDF (medium density fiberboard) flour (as natural fiber) and recycled HDPE (high density polyethylene) (as resin) were studied (at 60, 70, and 80% by weight fiber loadings). Nominal density and dimensions of the manufactured panels were 1 g/cm3 and 35 × 35 × 1 cm3, respectively. Maximum values of flexural modulus and strength of panels were obtained at 70 and 60% fiber content, respectively. The creep strain decreased as the lignocellulosic flour level increased. Water absorption has negative effect on creep behavior of MDF flour/HDPE composites. For all filler contents, it can be seen that the creep strain increases when the immersion time increases. This is believed to occur as a result of the cumulative effect of absorbed water on fiber matrix debonding and easier relaxation of the molecules at higher moisture contents leading to larger deformations at longer times.

Evaluation of Time Dependent Behavior of a Wood Flour/High Density Polyethylene Composite

Short-term flexural creep and stress relaxation tests were conducted on a wood plastic composite containing 30% high density polyethylene (HDPE), 67% fir wood flour, 2% compatibilizer (MAPE), and 1% lubricant. In creep tests, applied stress levels ranged from 30 to 60% of measured flexural strength. The principle of time—stress superposition was applied to form a master curve extending for a maximum of 4 years. The horizontal shift factors conformed to an Arrhenius type equation. Stress relaxation tests were also carried out at strain levels ranging from 30 to 60% of the ultimate strain. The principle of time—strain superposition was applied to form a stress relaxation master curve that extended for 67 days. The horizontal shift factors also conformed to an Arrhenius type equation. The resulting master curves were compared with extrapolated creep and stress relaxation models. To determine whether time—stress superposition is valid for the studied composite material, creep shift factors were applied to stress relaxation data and vice versa. In both creep and stress relaxation tests, it was found that the application of superposition was verified. The results indicated that the studied composite material was rheologically simple, and a single horizontal shifting on time axis was adequate to predict the long term performance of the material.

Tunable self-assembly of cellulose nanowhiskers and polyvinyl alcohol chains induced by surface tension torque.

This article focuses on the formation of the surface tension torque (STT) phenomenon close to the dry-line boundary layer during evaporation of the liquid phase of a solution casted shape-anisotropic nanoparticle suspension (here, cellulose nanowhisker (CNW)) or dissolved polymer (here, polyvinyl alcohol (PVA)) and its effects on self-assembly of the cellulose nanocrystals and polymer chains. The results confirm that the STT tends to align both the CNWs and the PVA chains tangential to the dry-line boundary layer. By careful control of the advancement of the dry-line, achieving special linear and curved patterns of both the CNWs and the PVA chains proportional to the mold position and geometry is possible. The STT phenomenon is explained and simplified in terms of a physical model. Understanding of the STT phenomenon and its effects on the alignment and self-assembly of the CNWs and PVA chains is necessary especially when achieving alignment using a modulated external magnetic or electric field is desired. The STT is safe, inexpensive, easy, and efficient, and can be a good alternative to the magnetic and electric field orientation methods.

Effect of Cellulose Fiber Reinforcement on the Temperature Dependent Mechanical Performance of Nylon 6

In order to quantify the effect of temperature on the mechanical properties of pure nylon 6 and its composite with cellulose fibers (containing 25 wt% cellulose fibers), the materials were sampled and tested at three representative temperatures of 256, 296, and 336 K. Flexural and tensile tests were performed and the reductions in mechanical properties were evaluated. The highest reductions were observed in stiffness (modulus) values and the cellulose fibers remarkably enhanced the high temperature resistance of nylon. The reductions in mechanical properties were well explained by a simple quadratic curve fitting procedure applied to experimental data. Dynamic mechanical analysis (DMA) was also performed to study the effect of temperature on mechanical performance. No shifting in glass transition temperature was observed, but the composite material showed less viscous behavior as seen by its lower mechanical loss factor (tan δ) values in the rubbery state. The results of the present study will be helpful in determining the end-use application of these composite materials.