Wood Thermoplastic Composites Research Articles

A Branched Polyelectrolyte Complex Enables Efficient Flame Retardant and Excellent Robustness for Wood/Polymer Composites.

Wood/thermoplastic composites (WPCs) have been restricted in some fields of building construction and electrical equipment because of their inherent high flammability and lower toughness. In this work, a branched crosslinking network polyelectrolyte complex (PEC) has been designed by incorporation of polyethyleneimine (PEI), a cation polyelectrolyte end capped amine groups, into cellulose nanocrystals (CNC), and ammonium polyphosphate (APP) via self-assembling. The hydrogen bonding interactions, penetration, and mechanical interlock provided by PEC effectively enhance the interfacial bonding within matrix, wood fibers, and flame retardant. Interestingly, it generates abundant micropores on the inner structure of WPC. The excellent interfacial bonding performance and easy-to-move molecular chain successfully transfer the stress and induce energy dissipation, simultaneously giving rise to higher strength and toughness for WPC. As well as the PEC endows WPC with a promotion in both smoke suppression and UL-94 V-0 rate. Additionally, the peak heat release rate and total smoke release for WPC obviously reduce by 36.9% and 50.0% respectively in presence of 25% PEC. A simple, eco-friendly, and concise strategy exhibits prospects for fiber-reinforced polymer composites with effective flame retardancy and mechanical robust properties.

Effects of water soaking-drying cycles on thermally modified spruce wood-plastic composites

The overall aim of this work was to gain more insight on the potential of modified wood (TMW) components for use in wood–thermoplastic composites (WPCs). Laboratory-scale TMWPCs were produced, and the effects of severe water soaking–drying cycles on the samples were studied. Water sorption behavior and resulting dimensional and micromorphological changes were also studied, and the results were compared with those of unmodified wood–plastic composites (UWPCs) used as control. The TMW was prepared by cutting a spruce board into half and subjecting one-half to an atmosphere of superheated steam at atmospheric pressure with a peak temperature of 210°C, with the other unmodified wood (UW) half as a control. The TMW and UW components were then prepared by a Wiley mill and thereafter sifted into smaller (mesh 0.20-0.40 mm) and larger (mesh 0.40-0.63 mm) size fractions. A portion of the wood components were also subjected to thermal extraction (HE). Composite samples with these different wood components, polypropylene (PP) matrix, and maleated PP (MAPP) as coupling agent (50/48/2 wood/PP/MAPP ratio by weight) were then prepared by using a Brabender mixer followed by hot pressing. The matching micromorphology of the composites before and after the soaking–drying cycles was analyzed using a surface preparation technique based on ultraviolet-laser ablation combined with scanning electron microscopy. The results of the water absorption tests showed, as hypothesized, a significantly reduced water absorption and resulting thickness swelling at the end of a soaking cycle for the TMWPCs compared with the controls (UWPCs). The water absorption was reduced with about 50-70% for TMWPC and 60-75% for HE-TMWPC. The thickness swelling for TMWPCs was reduced with about 40-70% compared with the controls. Similarly, the WPCs with HE-UW components absorbed about 20-45% less moisture and showed a reduced thickness swelling of about 25-40% compared with the controls. These observations also were in agreement with the micromorphology analysis of the composites before and after the moisture cycling which showed a more pronounced wood–plastic interfacial cracking (de-bonding) as well as other microstructure changes in the controls compared with those prepared with TMW and HE-UW components. Based on these observations, it is suggested that these potential bio-based building materials show increased potential durability for applications in harsh outdoor environments, in particular TMWPCs with a well-defined and comparably small size fractions of TMW components.

Water absorption behavior of heat-treated and untreated red balau saw dust/LDPE composites: Its kinetics and effects on mechanical properties

The hygroscopic nature of wood limits the use of wood thermoplastic composites (WTC) in outdoor industrial and domestic applications. To reduce this tendency, red balau saw dust was heat treated at 180 and 200°C for 1 h and compounded with Low Density Poly(ethylene) (LDPE) into 20 and 37 wt% and then molded into test specimens by injection molding. Samples were immersed in distilled water at room temperature for 4 months. Heat-treated wood composites showed remarkable water resistance relative to untreated ones. Wood composites made from wood treated at 180 and 200°C exhibited almost similar water absorption pattern. Reduced water absorption of heat-treated wood composites relative to untreated ones indicates that heat treatment has resulted in a degree of modification of the wood. Most of the composites displayed the Fickian mode of water absorption with n values close to 0.5. Also, the diffusion coefficient reduced with wood content in untreated wood composites due to interaction of water with the polar groups in wood through hydrogen bonding. Untreated wood composites exhibited poorer mechanical properties with water immersion as a result of degradation due to moisture. The mechanical properties of the heat-treated wood composites were not adversely affected with water absorption. Therefore, heat treatment can reduce the proneness to water absorption in WTCs and alleviate the detrimental effects on mechanical properties.

Effect of birch wood flour on the density, water absorption, thickness swelling, and thermal conductivity of wood thermoplastic polyurethane composites

In this study, some of the important properties of experimentally manufactured wood thermoplastic polyurethane composites were determined. Specimen having 30%, 40%, 50%, and 60% wood flour were mixed with rigid polyurethane foam. Physicalproperties (density, water absorption, and thickness swelling) and thermal conductivity of wood–plasticcomposites (WPC) were investigated. Results indicated that the density of WPCs decreased by increasing wood flour content. The results also revealed that water absorption and thickness swelling of the composites increases with increasing the percentage of wood flour content. It was found thatthermalconductivity of the composites slightly increased with the increasing of wood flour content. Experimental results of thermal conductivity were compared with a theoretical model (Maxwell – Garnett model), the errors associated with the above model, with respect to experimental ones, varies between 40% to 50%.

Pullout Performance of Common Construction Fasteners from Long Fiber Thermoplastic Composites

The recent increase in cyclone activity in the last 20 years has pressured the construction industry to look for alternative materials for roof construction. Although plywood is a cheap material, its mechanical properties are highly dependent on moisture content. Long fiber thermoplastics are an emerging type of plastic materials that feature enhanced mechanical properties due to high aspect ratio discontinuous fibers in a tough polymer matrix. The present study focuses on comparing the pullout strength of commonplace fasteners from plywood and long fiber thermoplastic composite panels. Specimens were tested according to ASTM standards D1761-88 and D6117-97 for mechanical fasteners in wood and long fiber thermoplastic (LFT) composites. Common construction nails like 6D, 10D and 16D and a #8 screw were used to compare the performance in terms of withdrawal. The data showed that LFT composites had withdrawal strength on average two to three times that of plywood for all tested fasteners types. Experimental data was analytically verified by APA TT-109 guidelines.

Effects of chemical modification of wood flour on the rheological properties of high‐density polyethylene blends

ABSTRACTChemical modification of lignocellulosic fibers can improve interfacial adhesion and dimensionally stabilize the resulting plastic composites. This study examined the rheological properties of wood flour/high density polyethylene (HDPE) melts after poplar wood flour was modified with glutaraldehyde (GA, mainly cell wall cross‐linking) and 1,3‐dimethylol‐4,5‐dihydroxyethyleneurea (DMDHEU, mainly poly‐condensation). Results show improvement in both the dispersibility of treated wood flour in the HDPE and its interfacial compatibility. Treatment with GA decreased melt viscosity, moduli, and shear stress as evidenced by rheometry. However, the modifying effects of DMDHEU were not observed, which was mainly due to reduced HDPE content. This study indicates that chemical modification of wood flour is a promising approach to improve the processability of highly filled wood thermoplastic composites via extrusion/injection molding processing. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41200.

The utilization of biochemically modified microfibers from grain by-products as reinforcement for polypropylene biocomposite

The presented research study aims to evaluate microfibers from grain by-products as a substitute for wood flour in wood-thermoplastic composites. Grain husks are an abundant and cheap source of annual, renewable raw material, which besides lignocellulose, may also contain substantial amounts of starch, proteins and fats. These grain residues may nega- tively affect the mechanical properties of their composites, and generate an odor when decomposition occurs at higher tem- peratures during plastics processing. Such odors may also be present in the end-product. In order to overcome this drawback, in this research study, a simple and effective enzymatic treatment is proposed. This environmental friendly process removed protein, starch and fats in selective manner. Treated microfibers have shown enhanced thermal stability for ca. 20°C at 1% of weight loss. This correlates with lower amount of odor emission during plastics processing as well as in the final, injection molded parts (25-65% decrease). The mechanical properties of composites were either preserved, or slightly improved. All results were compared to standard injection molded softwood WPC.

Isothermal crystallization kinetics of Kevlar fiber‐reinforced wood flour/high‐density polyethylene composites

AbstractKevlar fiber [KF; poly(p‐phenylene terephthalamide)] can be used as a reinforcing element to improve the mechanical properties of the resulting wood thermoplastic composites. This study was devised to investigate the effects of incorporation of KF on the isothermal crystallization kinetics of high‐density polyethylene (HDPE) in the resulting composites using differential scanning calorimetry. Avrami model was applied to describe the isothermal crystallization process, and the fold surface‐free energy was calculated according to the Hoffman–Lauritzen theory. Comparative study of neat HDPE, wood flour/HDPE composite (WPC), virgin KF‐reinforced WPC (KFWPC), and grafted Kevlar fiber (GKF)‐reinforced WPC (GKFWPC) showed that the overall crystallization rate, the activation energy, the equilibrium melting temperature, and the fold surface‐free energy of the WPC were apparently changed due to the addition of KF; the crystallizability exhibited an order of KFWPC > GKFWPC > HDPE > WPC. The incorporation of virgin KF may cause the heterogeneous nucleation to induce a change in the crystal growth of HDPE from the tridimensional to bidimensional or to the mixed patterns. Avrami exponent values of the composites decreased with time, confirming the change of crystallization behavior. This study demonstrates that both the KF and GKF can act as nucleating agents to improve the crystallization rate of HDPE. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Effects of the Size of Wood Flour on Mechanical Properties of Wood-Plastic Composites

In this article, wood-plastic composites(WPCs) were manufactured with wood flour(80～120mesh、40～80mesh、20～40mesh、10～20mesh) combing with high density polyethylene(HDPE). Effects of the size of wood flour on mechanical properies and density of composites were investigated. Results showed that particle size of wood flour had an important effect on properitiesof WPCs. Change of mesh number had a outstanding effect on flexural modulus, tensile modulus and impact strength, howere, little effect on flexural strength and tensile strength. When mesh number of wood flour changed from 80～120mesh to 10～20mesh,flexural modulus and tensile modulus were respectively enhanced by 42.4% and 28.4%, respectively, and impact strength was decreased by 35.5%.Size of wood flour basically had no effect on density of composite within 10～120mesh. The use of wood flour or fiber as fillers and reinforcements in thermoplastics has been gaining acceptance in commodity plastics applications in the past few years. WPCs are currently experiencing a dramatic increase in use. Most of them are used to produce window/door profiles,decking,railing,ang siding. Wood thermoplastic composites are manufactured by dispering wood fiber or wood flour(WF) into molten plastics to form composite materials by processing techniques such as extrusion,themoforming, and compression or injection molding[1]. WPCs have such advantages[2]：(1)With wood as filler can improve heat resistance and strength of plastic, and wood has a low cost, comparing with inorganic filler, wood has a low density. Wood as strengthen material has a great potential in improving tensile strength and flexural modulus[3]；(2) For composite of same volume, composites with wood as filler have a little abrasion for equipment and can be regenerated；(3)They have a low water absorption and low hygroscopic property, They are not in need of protective waterproof paint, at the same time, composite can be dyed and painted for them own needs；(4)They are superior to wood in resistantnce to crack、leaf mold and termite aspects, composites are the same biodegradation as wood；(5)They can be processed or connected like wood；（6）They can be processed into a lots of complicated shape product by means of extrusion or molding and so on, meanwhile, they have high-efficiency raw material conversion and itself recycle utilization[4]. While there are many sucesses to report in WPCs, there are still some issues that need to be addressed before this technology will reach its full potential. This technology involves two different types of materials: one hygroscopic(biomass) and one hydrophobic(plastic), so there are issues of phase separation and compatibilization[5]. In this paper, Effects of the size of wood powder on mechanical properties of WPCs were studied.

Characterization of composites based on expanded polystyrene wastes and wood flour

This paper aims to evaluate the potential for the use of recycled expanded polystyrene and wood flour as materials for the development of wood plastic composites. The effects of wood flour loading and coupling agent addition on the mechanical properties and morphology of wood thermoplastic composites were examined. In addition, a methodology for the thermo-mechanical recycling of expanded polystyrene waste was developed. The results show that the mechanical properties decreased as the wood flour loading increased. On the other hand, the use of poly(styrene-co-maleic anhydride), SMA, as a coupling agent improved the compatibility between the wood flour and polystyrene matrix and the mechanical properties subsequently improved. A morphological study revealed the positive effect of the coupling agent on the interfacial bonding. The density values obtained for the composites were compared with the theoretical values and showed agreement with the rule of mixtures. Based on the findings of this work, it appears that both recycled materials can be used to manufacture composites with high mechanical properties and low density.

Flexure and water sorption properties of wood thermoplastic composites made with polymer blends

AbstractMost WPC extrusion manufacturers utilize single polymer systems, where only one class of polymers is utilized. The concept of utilizing blended polymer systems can potentially provide lowered material costs, while maintaining or improving the composite properties. This cost savings can potentially be obtained in the ability to utilize mixed recycled plastics without the added costs of separation. Composites made of wood flour and binary blends of polypropylene (PP), high density polyethylene (HDPE), and polyvinyl‐chloride (PVC) were extruded and evaluated for their mechanical and physical performance. The WPCs were also exposed to repeated extrusion runs to determine the influence of enhanced melt‐blending of the composite properties. Torque rheometry, flexural, and water sorption tests revealed subtle differences between the blends and single polymer composites. With increasing extrusion runs, the WPCs water diffusion was decreased, whereas the strength and stiffness of the blends and single polymer systems increased or showed no change. These results indicate that polymer blends can be successfully utilized for commercial WPCs. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Processing of wood-thermoplastic composites

Natural and wood fibre plastic composites (WPC) are among the most rapidly growing markets within the plastics industry. They are manufactured by combining either wood or other natural fibers such as flax, hemp, jute or kenaf with polymers. Thermoplastic resins, such as polypropylene, polyethylene or polystyrene, are mixed with other materials (such as wood), and form a composite product. The resulting composite can be easily processed into various shapes and can be recycled. The experiments were made to study the behaviour of the wood flour-polypropylene composite during injection moulding, and to what extent the mechanical properties deteriorate after several processing cycles, i.e. mechanical recycling. Regarding efficiency of energy recovery, the heating value of wood-thermoplastic composite with different contents of wood and thermoplastics was determined as well.

Influence of wood flour moisture content on the degree of silane-crosslinking and its relationship to structure–property relations of wood–thermoplastic composites

The objective of this work was to examine how the moisture content of wood flour affects the degree of crosslinking when producing silane-crosslinked wood–thermoplastic composites. Crosslinked composites were produced by adding a silane solution to the compounding process of wood flour and polyethylene. Crosslinked composites of pre-dried as well as non-dried wood flour were prepared and their degree of crosslinking at various storage conditions was determined. Mechanical properties and the creep response of the crosslinked composites were tested in order to establish structure–properties relations. The results showed that all crosslinked composites displayed higher strengths and lower creep responses compared with non-crosslinked control samples. However, the degree and rate of crosslinking proved to be lower when a larger amount of moisture was present in the compounding process. It was concluded that the silane-grafting yield was lower when wood flour of a higher moisture content was used.

Wood‐thermoplastic composites manufactured using beetle‐killed spruce from Alaska

AbstractThe primary objectives of the study were to characterize the critical properties of wood flour produced using highly deteriorated beetle‐killed spruce for wood‐plastic composite (WPC) production and evaluate important mechanical and physical properties of WPC extruded using an industry standard formulation. Chemical composition analysis indicated no significant differences in wood constituents between highly deteriorated and sound wood. Preliminary investigation with Fourier transform infrared spectroscopy (FTIR), however, indicated partial degradation or depolymerization of carbohydrate components in highly deteriorated wood compared to sound wood from green trees; effects of these changes could be seen in cell collapse and poor interaction between thermoplastic matrix and deteriorated wood fiber. Physical and mechanical properties of extruded WPCs manufactured from highly deteriorated material were comparable to WPC properties produced using pine wood flour that served as a control material. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers

Effects of cross-linking on mechanical and physical properties of agricultural residues/recycled thermoplastics composites

Maleated graft polyolefins as cross-linking agents (CAs) are widely used to improve properties of wood thermoplastic composites made by melt extrusion process. In this study, novel CAs, free isocyanate group (NCO)-terminated urethane pre-polymers (UPs) were synthesized and used in manufacturing wheat straw (WS)/recycled polyethylene (PE) composites. The composites using polymeric diphenylmethane diisocyanate (pMDI) as a CA were also made in comparison. The relationship between composite properties and the level of CA and its content as well as the composite density and hot pressing time were investigated based on wood based board processes. The results show that the internal bonding (IB) strength, the IB after soaked in boiling water for 2 h (2hWIB), the modulus of rupture (MOR), the modulus of elasticity (MOE) and the 24 h thickness of swell after absorption of water (24hTS) of the composites are significantly improved with increased CA contents and composite densities. The optimal hot pressing time is 1.1 min/mm at 180°C. The cross-linking function is attributed to the reaction between free NCOs of CA molecules with hydroxyls of WS and the moisture in the raw materials, as well as the interaction between weak polar chain segments in the CA molecules to the non-polar PE. It is highly feasible to manufacture high quality composite using WS and recycled PE as raw materials when cross-linked with just 2.5% of UPs.

Strength analysis of chair base from wood plastic composites by finite element method

The present paper is aimed at solid modelling of an office chair base made up of wood thermoplastic composites by using finite element method. This is an attempt to evaluate the feasibility of substituting conventional materials in furniture production by wood composite materials. As the structural performance of any furniture product is utmost important to the manufacturers, it is essential for the product designers to build a prototype and test it under different loading conditions. Owing to the complexities involved in the deformation process of wood composite products, a powerful analysis technique (finite element analysis) has been applied to validate the product prototype.

Durability and mechanical properties of silane cross-linked wood thermoplastic composites

In this study, silane cross-linked wood–polyethylene composite profiles were manufactured by reactive extrusion. These composites were evaluated regarding their durability and mechanical properties in comparison with two non-cross-linked wood–polyethylene composites. An addition of only 2% w/w of silane solution during manufacturing was enough to achieve almost 60% degree of cross-linking after curing. The cross-linked composites showed flexural toughness superior to the non-cross-linked composites. The cross-linked composites also absorbed less moisture during a boiling test in water and this was an indirect evidence of improved interfacial adhesion. After accelerated weathering for 1000–3000 h the general trend was a decrease in flexural modulus and strength of both the non-cross-linked and cross-linked composites. The decrease in modulus seemed to be lower for the cross-linked composites while the decrease in strength seemed to be higher compared to the non-cross-linked composites. Weathering also resulted in a considerable colour fading of the composites. Water absorption–freeze–thaw cycling decreased the flexural modulus of non-cross-linked composites considerably while there was no statistical decrease in modulus for the cross-linked composites. There was only an insignificant decrease in strength for the composites after the water absorption–freeze–thaw cycling.

Profile extrusion and mechanical properties of crosslinked wood–thermoplastic composites

AbstractChallenges for wood‐thermoplastic composites to be utilized in structural applications are to lower product weight and to improve the long‐term load performance. Silane crosslinking of the composites is one way to reduce the creep during long‐term loading and to improve the mechanical properties. In this study, silane crosslinked wood‐polyethylene composites were produced by reactive extrusion and subsequently manufactured into rectangular profiles. The silane crosslinked composites were stored in a sauna at 90 °C to increase the degree of crosslinking. The toughness of the silane crosslinked composites was significantly higher than for the non‐crosslinked composites. Improved adhesion between the wood and polyethylene phases is most likely the reason for the improved toughness of the crosslinked composites. There was no significant difference in flexural modulus between the crosslinked and non‐crosslinked composites. In addition, impact testing showed that the impact strength of the crosslinked composites was considerable higher (at least double) than the non‐crosslinked. The effect of temperature on the impact strength of the composites indicated slightly higher impact strength at −30 °C than at 0° and at 25 °C, and then an incrase in impact strength at 60 °C. Crosslinking also reduced the creep response during short‐term loading. Moreover, scanning electron microscopy on the fracture surface of the crosslinked composites revealed good adhesion between the polyethylene and wood phases. POLYM. COMPOS. 27:184–194, 2006. © 2006 Society of Plastics Engineers

Friction of Nonwoven Wood–Polypropylene Fiber Mats on Heated Steel Platens

Pultrusion is a composites manufacturing process where a polymerimpregnated fiber mat or roving is pulled through a stationary die. The manufacture of wood–thermoplastic composites utilizing pultrusion requires knowledge of the composites' frictional properties. Many processing variables (e.g., normal stress, die temperature, sliding velocity, and moisture content) influence the frictional properties of synthetic and natural polymers. The coefficient of dynamic friction (k) was determined for the contact of wood fiber–polypropylene (PP) composites against a smooth heated steel surface. The k was found at varying PP fiber percentages, applied normal stresses, platen temperatures, and dwell times. A decrease in k was observed with increasing applied normal stress and dwell time, with normal stress having the greatest influence on frictional properties. An increase in k was observed with increasing PP fiber content and surface temperature above the melt temperature (Tm) of PP. Assuming an adhesion-controlled friction mechanism, the contact area and interfacial shear stress played an important role in property development.

A process for coupling wood to thermoplastic using graft copolymers

AbstractA chemical modification method has been developed to convert lignin into lignin–styrene graft copolymers. The graft products are macromolecular surface active agents because, within each molecule, a hydrocarbon sidechain has been grown off of a natural oxyphenylpropyl backbone. Surface activity of the graft copolymers was indicated by their capacity to form stable emulsions between incompatible fluid phases and to adhesively bond to wood surfaces. Dynamic contact angle measurement using the Wilhelmy plate technique showed that the graft copolymers changed the contact angle of water on wood from 50° to 110°. Coating birch wood (Betula papyrifera) with lignin–styrene graft copolymerization product increases the water contact angle and the binding strength of polystyrene on the wood. Lap shear strengths increased 56%, from 1826 to 2840 kPa, when the wood was coated with a graft copolymer containing 51.7% lignin. Binding was tested by coating the birch with a solution of graft copolymer, drying the wood, injection molding polystyrene onto the surface of the wood, and pulling the polystyrene off the wood surface in a lap shear brace. These properties of the copolymers were used to design a process for forming wood‐thermoplastic composites. © 1995 John Wiley & Sons, Inc.