A Review of Wood Polymer Composites Rheology and Its Implications for Processing

Effect of Agricultural Biomass Residues on the Properties of Recycled Polypropylene/Polyethylene Composites

The aim of the study was to assess the usefulness of agricultural biomass residues as reinforcement in recycled polymer matrices. In this study, recycled polypropylene and high-density polyethylene composites (rPPPE) filled with three types of biomass residues, sweet clover straws (SCS), buckwheat straws (BS) and rapeseed straws (RS), are presented. The effects of the fiber type and the fibers content on the rheological behavior, mechanical properties (including tensile, flexural and impact strength), thermal stability and moisture absorbance were determined, in addition to morphological analysis. It was revealed that the addition of SCS, BS or RS increased the materials' stiffness and strength. The reinforcement effect increased as the loading of the fibers was increased, especially for BS composites in the flexural test. After the moisture absorbance test, it was found that the reinforcement effect slightly increased for the composites with 10% fibers but decreases with 40% fibers. The results highlight that the selected fibers are a feasible reinforcement for recycled polyolefin blend matrices.

Evaluation of the Mechanical, Thermal and Rheological Properties of Hop, Hemp and Wood Fiber Plastic Composites

The aim of this study was to evaluate the use of waste natural fibers from milled hop bines and hemp stalks, without chemical treatment, and compare them to a commercial wood fiber for use in wood-plastic composite (WPC) materials. The fibers were characterized (density, fiber size and chemical composition). WPCs were produced by the extrusion of a blend of fibers (50%), high-density polyethylene (HDPE) and coupling agent (2%). The WPCs were characterized for their mechanical, rheological, thermal, viscoelastic and water resistance properties. Pine fiber was about half the size of hemp and hop fibers and thus had a higher surface area. The pine WPC melts had a higher viscosity than the other two WPCs. Additionally, the tensile and flexural strengths of the pine WPC were higher than those of hop and hemp WPCs. The pine WPC was also shown to have the least water absorption followed by hop and hemp WPCs. This study highlights that different lignocellulosic fibers influence their WPC properties. The properties of the hop- and hemp-based WPCs were comparable to commercial WPCs and can be improved by further milling/screening the fibers to a smaller particle size (volumetric mean of ~88 μm) to increase their surface area, fiber-matrix interactions and improve stress-transfer.

Tensile Behavior of 3D Printed Polylactic Acid (PLA) Based Composites Reinforced with Natural Fiber

Natural fiber-reinforced composite (NFRC) filaments for 3D printing were fabricated using polylactic acid (PLA) reinforced with 1–5 wt% henequen flour comprising particles with sizes between 90–250 μm. The flour was obtained from natural henequen fibers. NFRCs and pristine PLA specimens were printed with a 0° raster angle for tension tests. The results showed that the NFRCs’ measured density, porosity, and degree of crystallinity increased with flour content. The tensile tests showed that the NFRC Young’s modulus was lower than that of the printed pristine PLA. For 1 wt% flour content, the NFRCs’ maximum stress and strain to failure were higher than those of the printed PLA, which was attributed to the henequen fibers acting as reinforcement and delaying crack growth. However, for 2 wt% and higher flour contents, the NFRCs’ maximum stress was lower than that of the printed PLA. Microscopic characterization after testing showed an increase in voids and defects, with the increase in flour content attributed to particle agglomeration. For 1 wt% flour content, the NFRCs were also printed with raster angles of ±45° and 90° for comparison; the highest tensile properties were obtained with a 0° raster angle. Finally, adding 3 wt% content of maleic anhydride to the NFRC with 1 wt% flour content slightly increased the maximum stress. The results presented herein warrant further research to fully understand the mechanical properties of printed NFRCs made of PLA reinforced with natural henequen fibers.

Comparison of Melting Processes for WPC and the Resulting Differences in Thermal Damage, Emissions and Mechanics

The necessity for resource-efficient manufacturing technologies requires new developments within the field of plastic processing. Lightweight design using wood fibers as sustainable reinforcement for thermoplastics might be one solution. The processing of wood fibers requires special attention to the applied thermal load. Even at low processing temperatures, the influence of the dwell time, temperature and shear force is critical to ensure the structural integrity of fibers. Therefore, this article compares different compounding rates for polypropylene with wood fibers and highlights their effects on the olfactory, visual and mechanical properties of the injection-molded part. The study compares one-step processing, using an injection-molding compounder (IMC), with two-step processing, using a twin-scew-extruder (TSE), a heating/cooling mixer (HCM) and an internal mixer (IM) with subsequent injection molding. Although the highest fiber length was achieved by using the IMC, the best mechanical properties were achieved by the HCM and IM. The measured oxidation induction time and volatile organic compound content indicate that the lowest amount of thermal damage occurred when using the HCM and IM. The advantage of one-time melting was evened out by the dwell time. The reinforcement of thermoplastics by wood fibers depends more strongly on the structural integrity of the fibers compared to their length and homogeneity.

Optimization of Typha Fibre Extraction and Properties for Bio-Composite Applications Using Desirability Function Analysis

The effect of extraction time, temperature, and alkali concentration on the physical and mechanical properties of cattail (Typha latifolia L.) fibres were investigated using five levels of time (4, 6, 8, 10, and 12 h), four levels of temperature (70, 80, 90, and 95 °C), and three levels of NaOH concentration (4, 7, 10%, w/v) in a 3 × 4 × 5 factorial experimental design. The extraction parameters were optimized for bio-composite application using a desirability function analysis (DFA), which determined that the optimum extraction time, temperature and NaOH concentration were 10 h, 90 °C, and 7%, respectively. A sensitivity analysis for optimal treatment conditions confirmed that the higher overall desirability does not necessarily mean a better solution. However, the analysis showed that the majority of optimum settings for time, temperature, and concentration of NaOH found in the sensitivity analysis matched with the optimum conditions determined by DFA, which confirmed the validity of the optimum treatment conditions.

Effect of thermal annealing and filler ball-milling on the properties of highly filled polylactic acid/pecan nutshell biocomposites

Biodegradable polymer composites reinforced with agri-food lignocellulosic biowaste represent cost-effective and sustainable materials potentially able to replace traditional composites for structural, household, and packaging applications. Herein, the preparation of polylactic acid (PLA)/pecan (Carya illinoinensis) nutshell (PNS) biocomposites at high filler loading (50 wt%) is reported, alongside the effect of two environmentally friendly physical treatments, namely ball-milling of the filler and thermal annealing on biocomposites. PNS enhanced the thermal stability, the viscoelastic response, and the crystallinity of the polymer. Furthermore, filler ball-milling also increased the melt fluidity of the biocomposites, potentially improving melt processing. Finally, the presence of PNS remarkably enhanced the effect of thermal annealing in the compounds. In particular, heat deflection temperature of the biocomposites dramatically increased, up to 60 °C with respect to the non-annealed samples. Overall, these results emphasize the potential of combining natural fillers and environmentally benign physical treatments to tailor the properties of PLA biocomposites, especially for those applications which require a stiff and lightweight material with low deformability.

A Critical Review on Wood-Based Polymer Composites: Processing, Properties, and Prospects

Waste recycling is one of the key aspects in current day studies to boost the country's circular economy. Recycling wood from construction and demolished structures and combining it with plastics forms wood-polymer composites (WPC) which have a very wide scope of usage. Such recycled composites have very low environmental impact in terms of abiotic potential, global warming potential, and greenhouse potential. Processing of WPCs can be easily done with predetermined strength values that correspond to its end application. Yet, the usage of conventional polymer composite manufacturing techniques such as injection molding and extrusion has very limited scope. Many rheological characterization techniques are being followed to evaluate the influence of formulation and process parameters over the quality of final WPCs. It will be very much interesting to carry out a review on the material formulation of WPCs and additives used. Manufacturing of wood composites can also be made by using bio-based adhesives such as lignin, tannin, and so on. Nuances in complete replacement of synthetic adhesives as bio-based adhesives are also discussed by various researchers which can be done only by complete understanding of formulating factors of bio-based adhesives. Wood composites play a significant role in many non-structural and structural applications such as construction, floorings, windows, and door panels. The current review focuses on the processing of WPCs along with additives such as wood flour and various properties of WPCs such as mechanical, structural, and morphological properties. Applications of wood-based composites in various sectors such as automotive, marine, defense, and structural applications are also highlighted in this review.

Computational Scale-Up for Flood Fed/Starve Fed Single Screw Extrusion of Polymers

A novel scaling-up computer system for single screw extrusion of polymers has been developed. This system makes it possible to scale-up extrusion process with both starve feeding and flood feeding. Each of the scale-up criteria can be an objective function to be minimized, represented by single values or functional dependencies over the screw length. The basis of scaling-up is process simulation made with the use of the GSEM program (Global Screw Extrusion Model). Scaling-up is performed using the GASES program (Genetic Algorithms Screw Extrusion Scaling) based on Genetic Algorithms. Scaling-up the extrusion process has been performed to increase extrusion output according to the scaling-up criteria defined by the single parameters of unit energy consumption, polymer plasticating rate and polymer temperature, as well as by the process parameters profiles of the temperature and plasticating. The global objective function reached the lowest value for the selected process parameters, and extrusion throughput was significantly increased.

A new generation of cable grade poly(vinyl chloride) containing heavy metal free modifier

Many additives are used to improve the performance of cables in terms of increasing their flame retardancy, thermal stability, thermal conductivity, and other characteristics. Unfortunately, most of these additives contain heavy metals. Therefore, the main objective of this study is to introduce a material representing a new generation of environmentally friendly heavy metal-free stabilizers for cable grade poly(vinyl chloride) that can compete with traditional materials in terms of performance and distinctive properties. This unique additive is Oxydtron, a synthetic silicate or simply nanocement. The tests performed are rheological properties represented by a capillary rheometry analysis, limiting oxygen index, and volume resistivity. The most significant improvement in Bagley correction measurements was 14.61%; 18.13%; and 27.20% more than poly(vinyl chloride) basic formulation when using 5wt.% Oxydtron at 160 °C, 170 °C, and 180 °C, respectively. Also, the mean increases in relaxation time were 3.200 times, 8.825 times, and 12.458 times more than poly(vinyl chloride) basic formulation with 1wt.%, 3wt.%, and 5wt.% of Oxydtron, respectively. Furthermore, the Oxydtron lowered the value of the accompanying thermal gradient of the L.O.I test, reducing the heat-affected zone. The best result was with the extrusion processing method due to the uniformity of the processing conditions. However, the thermal gradient analysis showed residual heat stress in the test samples after cutting the burning layer and re-testing the samples again; this causes them to burn faster. This situation requires caution for designs that are exposed to high temperatures without burning. The optimum improvement in volume resistivity value was 14.71% and 38.24% more than poly(vinyl chloride) basic formulation after adding 5wt.% and 7wt.% of Oxydtron, respectively.

Rotational Rheology of Wood Flour Composites Based on Recycled Polyethylene

In this paper, we study the effect of the addition of wood flour as a filler in a recycled polyethylene (r-PE) in view of its potential applications in 3D printing. The composites, prepared by melt mixing, are characterized with torque measurements performed during the compounding, dynamic rotational rheology, and infrared spectroscopy. Data show that the introduction of wood results in increased viscosity and in sensible viscous heating during the compounding. The r-PE appear to be stable at temperatures up to 180 °C while at higher temperatures the material shows a rheological response characterized by time-increasing viscoelastic moduli that suggests a thermal degradation governed by crosslinking reactions. The compounds (with wood loading up to 50% in wt.) also shows thermal stability at temperatures up to 180 °C. The viscoelastic behavior and the infrared spectra of the r-PE matrix suggests the presence of branches in the macromolecular structure due to the process. Although the addition of wood particles determines increased viscoelastic moduli, a solid-like viscoelastic response is not shown even for the highest wood concentrations. This behavior, due to a poor compatibility and weak interfacial adhesion between the two phases, is however promising in view of common processing technologies as extrusion or injection molding.

Tribological Behavior of a Rubber-Toughened Wood Polymer Composite

Wood polymer composites or WPCs are increasingly used as substitutes for natural wood in outdoor applications due to their better environmental sustainability and the consequent reduction in carbon footprint. In this paper, the presence of an elastomer used as a toughening agent (Santoprene by Exxon Mobil) in a polypropylene-based WPC containing 50 wt % wood flour was investigated in terms of its tribological behavior by dry sliding wear tests. These were performed after two environmental pre-conditioning treatments, i.e., drying and water soaking. The ball-on-disk configuration under a constant load was chosen along two sliding distances. Dynamic mechanical thermal analyses were used to reveal the effect of the toughening agent on the storage modulus and damping factor of the composites. Results in terms of weight loss measurement and coefficient of friction were obtained, together with surface morphology analysis of the worn surfaces at the scanning electron microscope and 3D profilometer. An abrasive wear mechanism was identified, and it was shown that the toughening agent improved wear resistance after both pre-treatments. This beneficial effect can be explained by the increase in strain at break of the WPC containing the elastomer. On the other hand, the water soaking pre-treatment produced severe damage, and the loss of material cannot be completely compensated by the presence of the toughening agent.

Production of Nanocellulose by Enzymatic Treatment for Application in Polymer Composites

In the last few years, the scientific community around the world has devoted a lot of attention to the search for the best methods of obtaining nanocellulose. In this work, nanocellulose was obtained in enzymatic reactions with strictly defined dispersion and structural parameters in order to use it as a filler for polymers. The controlled enzymatic hydrolysis of the polysaccharide was carried out in the presence of cellulolytic enzymes from microscopic fungi-Trichoderma reesei and Aspergillus sp. It has been shown that the efficiency of bioconversion of cellulose material depends on the type of enzymes used. The use of a complex of cellulases obtained from a fungus of the genus Trichoderma turned out to be an effective method of obtaining cellulose of nanometric dimensions with a very low polydispersity. The effect of cellulose enzymatic reactions was assessed using the technique of high-performance liquid chromatography coupled with a refractometric detector, X-ray diffraction, dynamic light scattering and Fourier transform infrared spectroscopy. In the second stage, polypropylene composites with nanometric cellulose were obtained by extrusion and injection. It was found by means of X-ray diffraction, hot stage optical microscopy and differential scanning calorimetry that nanocellulose had a significant effect on the supermolecular structure, nucleation activity and the course of phase transitions of the obtained polymer nanocomposites. Moreover, the obtained nanocomposites are characterized by very good strength properties. This paper describes for the first time that the obtained cellulose nanofillers with defined parameters can be used for the production of polymer composites with a strictly defined polymorphic structure, which in turn may influence future decision making about obtaining materials with controllable properties, e.g., high flexibility, enabling the thermoforming process of packaging.