Durability of wood flour‐plastic composites exposed to accelerated freeze–thaw cycling. Part I. Rigid PVC matrix

This study examined the effects of accelerated freeze–thaw actions on the durability of wood fiber‐plastic composites. Rigid PVC formulations filled with various concentrations of wood flour (both pine and maple) were processed in a counterrotating twin‐screw extruder and exposed to cyclic freeze–thaw actions according to ASTM Standard D6662. Freeze–thaw cycling was also modified by omitting portions of the test (either the water or freezing) to verify whether or not moisture was the primary cause for property loss. The durability of exposed samples was assessed in terms of flexural properties, density, and dimensional stability. Scanning electron micrographs of unexposed and freeze–thaw‐exposed samples were taken to qualitatively evaluate the interfacial adhesion between the wood flour and PVC matrix. The experimental results indicated that the density was not affected by freeze–thaw cycling. The dimensional stability was also relatively unaffected, although greater wood flour content exhibited greater dimensional change. The loss in stiffness of the composites was statistically significant after only two freeze–thaw cycles, regardless of both the wood species and content. Conversely, the strength of the composites was not significantly affected by five freeze–thaw cycles at lower wood flour contents (50 and 75 phr). The deleterious effects of the freeze–thaw actions on the strength of the composites became apparent at higher wood flour content (100 phr) after only two freeze–thaw cycles for maple flour and five freeze–thaw cycles for pine flour. The property loss was attributed primarily to the water portion of the cycling, which appears to have led to the decreased interfacial adhesion between the wood flour and the rigid PVC matrix. J. VINYL. ADDIT. TECHNOL. 11:1–8, 2005. © 2005 Society of Plastics Engineers.     

Coextruded PVC/wood‐flour composites with WPC cap layers

Wood‐plastic composites (WPCs) can absorb moisture in a humid environment owing to the hydrophilic nature of the wood, thereby making the products susceptible to microbial growth and loss of mechanical properties. In this study, rigid poly(vinyl chloride) (PVC)/wood‐flour composites (core layer) were coextruded with either unfilled rigid PVC (cap layer) or rigid PVC filled with a small amount (5–27.5%) of wood flour (composite cap layers) in order to decrease or delay the moisture uptake. The thickness of the cap layer and its composition in terms of wood flour content were the variables examined during coextrusion. Surface color, moisture absorption, and flexural properties of both coextruded and noncoextruded (control) composite samples were characterized. The experimental results indicated that both unfilled PVC and composite cap layers can be encapsulated over rigid PVC/wood‐flour composites in a coextrusion process. The moisture uptake rate was lower when a cap layer was applied in the composites, and the extent of the decrease was a strong function of the amount of wood flour in the cap layer but insensitive to cap layer thickness. Overall, coextruding PVC surface‐rich cap layers on WPCs significantly increased the flexural strength but decreased the flexural modulus as compared with those of control samples. The changes in bending properties were sensitive to both cap layer thickness and wood flour content. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers     

Durability of wood flour‐plastic composites exposed to accelerated freeze–thaw cycling. II. High density polyethylene matrix

This study examined the durability of extruded HDPE/wood‐flour composites exposed to 15 accelerated cycles of water submersion, freezing, and thawing, according to ASTM standard D6662. The durability of both maple and pine composites was assessed by testing the flexural properties and density. Mercury intrusion porosimetry and scanning electron microscopy were also used to evaluate the interfacial adhesion between the matrix and wood flour before and after exposure to accelerated freeze–thaw cycling. Freeze–thaw actions had no apparent effect on the density of the composites after exposure, regardless of the wood species. However, these actions led to moisture uptake, which decreased the interfacial adhesion and increased the pore size and quantity in the composites, which resulted in a significant loss in flexural properties. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 100: 35–39, 2006     

Manufacture of rigid PVC/wood‐flour composite foams using moisture contained in wood as foaming agent

Relatioships between the density of foamed rigid PVC/wood‐flour composites and the moisture content of the wood flour, the chemical foaming agent (CFA) content, the content of all‐acrylic foam modifier, and the extruder die temperature were determined by using a response surface model based on a four‐factor central composite design. The experimental results indicated that there is no synergistic effect between teh CFA content and the moisture content of the wood flour. Wood flour moisture could be used effectively as foaming agent in the production of rigid PVC/wood‐flour composite foams. Foam density as low as 0.4 g/cm³ was produced without the use of chemical foaming agents. However, successful foaming of rigid PVC/wood‐flour composite with moisture contained in wood flour strongly depends upon the presence of all‐acrylic foam modifier in the formulation and the extrusion die temperature. The lowest densities were achieved when the all‐acrylic foam modifier concentration was between 7 phr and 10 phr and extruder die temperature was as low as 170°C.     

Effects of copper amine treatment on mechanical properties of PVC/wood‐flour composites

Copper amine–treated wood flour was added to PVC [poly(vinyl chloride)] matrix in order to manufacture PVC/wood‐flour composites. Effects of copper treatments on the mechanical properties of PVC‐wood composites were evaluated. Unnotched impact strength, flexural strength, and flexural toughness of the composites were significantly improved by the wood‐flour copper treatment. The optimum copper concentration range was 0.2 to 0.6 wt% of wood flour. Fractured surfaces were examined by using scanning electron microscopy (SEM) combined with energy‐dispersive spectroscopy (EDS). PVC/wood interfacial debonding was the main fracture mode of untreated wood‐flour composites, whereas wood‐particle pullout and breakage dominating the fractured surfaces of copper‐treated wood‐flour composites. On the fractured surfaces, more PVC could be found on the exposed copper‐treated wood particles than on untreated wood, a result suggesting improved PVC‐wood interfacial adhesion after copper treatments. J. Vinyl Addit. Technol. 10:70–78, 2004. © 2004 Society of Plastics Engineers.     

Tensile properties,morphology, and thermal behavior of PVC composites containing pine flour and bamboo flour

Composites of unplasticized poly(vinyl chloride) (PVC) blended with bamboo flour and pine flour, respectively, were prepared in a batch mixer followed by compression molding. The effects of wood flour fillers on the morphology, static mechanical properties, and thermal properties of the composites were investigated. Compared with neat PVC resin, the introduction of both bamboo flour and pine flour significantly improved the stiffness of the composites, while decreasing the tensile strength to some extent. Tensile tests showed that pine flour–filled composites exhibited better mechanical properties than those filled with bamboo flour with the same particle size at the same loading level. Scanning electron microscopic examination revealed good dispersion and alignment tendency of short pine fiber within the composites at a lower loading level. Moreover, experimental results indicated that both bamboo flour and pine flour additions showed no obviously adverse effect on the thermal stabilities of these composites. Based on the comprehensive properties, these composites meet the need of woodlike material for use as wood structures. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1804–1811, 2004     

Durability of Wood Flour-Thermoplastic Composites under Extreme Environmental Conditions and Fungal Exposure

Abstract

The main purpose of this research work was to investigate the response of wood flour reinforced polyethylene composites to 2 hours water boiling, five complete boiling and freezing cycles and fungal (Gloeophyllumm Traebum/Brown-rot fungus) exposure. Five composite formulations were manufactured and analyzed (0, 50, 60, 70 and copper carbonate treated 60% wood flour/polyethylene composites). The results showed that wood flour loading decreased the resistance of the composites to moisture and fungal environment. The exposure of the composites to 2 hours water boiling and five complete boiling and freezing cycles caused serious damage to the interfacial adhesion between wood flour and polyethylene matrix due to contraction and swelling stresses developed during cyclic exposure. The addition of 1 percent copper carbonate salt during compounding of wood flour and polyethylene prevented the colonization and proliferation of fungus on the surface of the composites, but had a negative effect on the water uptake and flexural properties of the composites.     

Foaming of rigid PVC/wood‐flour composites through a continuous extrusion process

The effects of chemical foaming agent (CFA) types (endothermic versus exothermic) and concentrations as well as the influence of all‐acrylic processing aid on the density and cell morphology of extrusion‐foamed neat rigid PVC and rigid PVC/wood‐flour composites were studied. Regardless of the CFA type, the density reduction of foamed rigid PVC/wood‐flour composites was not influenced by the CFA content. The cell size, however, was affected by the CFA type, independent of CFA content. Exothermic foaming agent produced foamed samples with smaller average cell sizes compared to those of endothermic counterparts. The experimental results indicate that the addition of an all‐acrylic processing aid in the formulation of rigid PVC/wood‐flour composite foams provides not only the ability to achieve density comparable to that achieved in the neat rigid PVC foams, but also the potential of producing rigid PVC/wood‐flour composite foams without using any chemical foaming agents.     

Wood/plastic composites co‐extruded with multi‐walled carbon nanotube‐filled rigid poly(vinyl chloride) cap layer

Wood/plastic composites (WPCs) can absorb moisture in a humid environment due to the hydrophilic nature of the wood in the composites, making products susceptible to microbial growth and loss of mechanical properties. Co‐extruding a poly(vinyl chloride) (PVC)‐rich cap layer on a WPC significantly reduces the moisture uptake rate, increases the flexural strength but, most importantly, decreases the flexural modulus compared to uncapped WPCs. A two‐level factorial design was used to develop regression models evaluating the statistical effects of material compositions and a processing condition on the flexural properties of co‐extruded rigid PVC/wood flour composites with the ultimate goal of producing co‐extruded composites with better flexural properties than uncapped WPCs. Material composition variables included wood flour content in the core layer and carbon nanotube (CNT) content in the cap layer of the co‐extruded composites, with the processing temperature profile for the core layer as the only processing condition variable. Fusion tests were carried out to understand the effects of the material compositions and processing condition on the flexural properties. Regression models indicated all main effects and two powerful interaction effects (processing temperature/wood flour content and wood flour content/CNT content interactions) as statistically significant. Factors leading to a fast fusion of the PVC/wood flour composites in the core layer, i.e. low wood flour content and high processing temperature, were effective material composition and processing condition parameters for improving the flexural properties of co‐extruded composites. Reinforcing the cap layer with CNTs also produced a significant improvement in the flexural properties of the co‐extruded composites, insensitive to the core layer composition and the processing temperature condition. Copyright © 2009 Society of Chemical Industry     

Novel coupling agents for PVC/wood‐flour composites *

Effective interfacial adhesion between wood fibers and plastics is crucial for both the processing and ultimate performance of wood–plastic composites. Coupling agents are added to wood–plastic composites to promote adhesion between the hydrophilic wood surface and hydrophobic polymer matrix, but to date no coupling agent has been reported for PVC/wood‐fiber composites that significantly improved their performance and was also cost‐effective. This article presents the results of a study using chitin and chitosan, two natural polymers, as novel coupling agents for PVC/wood‐flour composites. Addition of chitin and chitosan coupling agents to PVC/wood‐flour composites increased their flexural strength by ～20%, their flexural modulus by ～16%, and their storage modulus by ～33–74% compared to PVC/wood‐flour composite without the coupling agent. Significant improvement in composite performance was attained with 0.5 wt% of chitosan and when 6.67 wt% of chitin was used. J. VINYL ADDIT. TECHNOL., 11:160–165, 2005. © 2005 Society of Plastics Engineers     

Online measurement of rheological properties of PVC/wood‐flour composites

By using a factorial design approach, this study examined the effect of the component materials on the viscoelastic properties of PVC/wood‐flour composites. Statistical analysis was performed to determine the effects of wood‐flour content, acrylic modifier content, and plasticizer content on the die swell ratio and viscosity of the composites measured online on a conical twin‐screw extrusion capillary rheometer. The viscoelastic properties of the samples also were measured using dynamic mechanical analyzer (DMA). Wood‐flour content and acrylic modifier content were the two important variables affecting the die swell ratio, whereas the addition of a low level of plasticizer did not affect this ratio. The die swell increased with the increased acrylic modifier content, but it was reduced considerably by adding wood flour into the PVC matrix. The true viscosity of neat PVC and PVC/wood‐flour composites decreased with the plasticizer content, irrespective of the acrylic modifier content. However, the addition of acrylic modifier significantly increased the viscosity of unfilled PVC, while an opposite trend was observed for the composites, owing to the differing effect of acrylic modifier on the melt elasticity and viscosity of these materials. J. Vinyl Addit. Technol. 10:121–128, 2004. © 2004 Society of Plastics Engineers.     

Development of poly(vinyl chloride)/wood composites. A literature review

Poly(vinyl chloride)/wood fiber (flour) composites are currently experiencing a dramatic increase in use. Most of them are used to produce window/door profiles, decking, railing, and siding by using conical counterrotating intermeshing twin‐screw extruders. Heat stabilizers, processing aids, impact modifiers, lubricants, and pigments are still important for PVC/wood composite formulations. Poly[methylene(polyphenyl isocyanate)] (PMPPIC), γ‐aminopropyltriethoxysilane, maleated polypropylene (MAPP), and copper metallic complex have proved to be effective coupling agents for this composite system. Mechanical properties of PVC/wood composites can be enhanced by combining wood with mica or glass fibers to form hybrid reinforcements. Ultraviolet light resistance and weathering dimensional stabilities of PVC/wood composites are superior to those of natural wood. Density reduction can be achieved through the microcellular foaming technique by using chemical blowing agents, such as azodicarbonamide and sodium bicarbonate, or physical blowing agents, such as carbon dioxide. J. Vinyl Addit. Technol. 10:59–69, 2004. © 2004 Society of Plastics Engineers.     

Accelerated ultraviolet weathering of PVC/wood‐flour composites

Ultraviolet weathering performance of polyvinyl chloride (PVC) filled with different concentrations of wood flour was studied. Extruded PVC/wood‐flour composite samples were subjected to cyclic ultraviolet lamps/condensation exposures and assessed over a total of 400 and 2600 hours. Each assessment consisted of DRIFTFTIR and XPS collections, contact angle measurement, color measurement, and tensile property testing. The experimental results indicated that wood flours are effective chromophore materials since their incorporation into a rigid PVC matrix accelerated the degradation of the polymeric matrix. Photodegradation converted unfilled PVC samples to a colored material of lower extensibility. Although composite samples exhibited greater discoloration than unfilled PVC samples, they retained all their original strength and stiffness properties even after 2600 hours of cyclic UV irradiation/condensation exposures.     

Thermoplastic polymers as modifiers for urea–formaldehyde wood adhesives. III. In situ thermoplastic‐modified wood composites

Acrylic monomers and free‐radical initiators were dispersed in an aqueous urea–formaldehyde (UF) suspension and polymerized in situ to afford a suspension containing 5 wt % thermoplastic (5 g of thermoplastic/100 mL of suspension). The viscosity of the thermoplastic‐modified UF suspension (65 wt % solids at 25°C) ranged from 240 to 437 cP versus 121 cP for the unmodified UF control. Wood‐flour composites (sugar maple and 50 wt % adhesive) were prepared with thermoplastic‐modified UF suspensions and cured with the same cycle used for the composites prepared with the unmodified UF adhesive (control). The effect of the thermoplastic‐modified UF adhesive was evaluated on the notched Izod impact strength and equilibrium moisture uptake of the wood‐flour composites. The notched Izod impact strength of the composites prepared with modified UF adhesives increased by as much as 94% above that of the control. The increase depended on the initiator and the monomer composition. The modification affected the equilibrium moisture uptake and rate of moisture uptake in the wood‐flour composites. Preliminary results for particleboard prepared with 10 wt % modified UF adhesive (5% thermoplastic in the UF resin) and unoptimized cure conditions confirmed a significant effect of the thermoplastic modification on both the internal‐bond strength and thickness swelling of the particleboard. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Reinforcement of rigid PVC/wood‐flour composites with multi‐walled carbon nanotubes

Multi‐walled carbon nanotubes (CNT) were compounded with PVC by a melt blending process based on fusion behaviors of PVC. The effects of CNT content on the flexural and tensile properies of the PVC/CNT composites were evaluated in order to optimize the CNT content. The optimized CNT‐reinforced PVC was used as a matrix in the manufacture of wood‐plastic composites. Flexural, electrical, and thermal properties of the PVC/wood‐flour composites were evaluated as a function of matrix type (nonreinforced vs. CNT‐reinforced). The experimental results indicated that rigid PVC/wood‐flour composites with properties similar to those of solid wood can be made by using CNT‐reinforced PVC as a matrix. The CNT‐reinforced PVC did not influence the electrical and thermal conductivity of the PVC/wood‐flour composites. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers.     

Expansion mechanisms of plastic/wood‐flour composite foams with moisture,dissolved gaseous volatiles,and undissolved gas bubbles

The large quantity of moisture in wood‐flour may lead to the deterioration of the cell structure of foamed plastic wood‐flour composites in terms of cell size, non‐uniformity, and poor surface quality. Since these anomalies can cause poor mechanical properties of the foamed composites, the removal of the moisture from wood‐flour becomes a critical issue with respect to the improvement of these properties. The wood‐flour in this experimental study was first oven‐dried at different temperatures and then subjected to acetone extraction and thermogravimetric analysis (TGA). The oven‐dried wood‐flour was blended with plastic and then subjected to extrusion foaming. The results obtained from the TGA studies indicate that most volatiles were released from the extractives. Conversely, a comparative experimental study of the foaming behavior of these plastic/wood‐flour composites versus that of undried wood‐flour composites confirms that removal of the adsorbed moisture from wood‐flour results in a better cell morphology. However, it seems that some gaseous emissions released from wood‐flour are soluble in plastic and thereby favorably contribute to the development of the cell morphology. This paper describes the expansion mechanisms of wood‐flour composite foams resulting from the adsorbed moisture and dissolved gaseous emissions as well as resulting from the finely dispersed undissolved gas bubbles released from a chemical blowing agent.     

Effect of highly degraded high‐density polyethylene (HDPE) on processing and mechanical properties of wood flour‐HDPE composites

The influence of highly degraded high‐density polyethylene (HDPE) on physical, rheological, and mechanical properties of HDPE‐wood flour composites was studied. For this purpose, the virgin HDPE was subjected to accelerated weathering under controlled conditions for 200 and 400 h. The virgin and exposed HDPE and pine wood flour were compounded to produce wood flour‐HFPE composites. The results showed that the accelerated weathering highly degraded HDPE. Degradation created polar functional carbonyl groups and also produced extensive cross‐linking in HDPE and consequently poor processibility. The interruptions in the flow characteristics of the degraded HDPE potentially caused processing hurdles when using them for extrusion or injection molding manufacturing as only small part (10%) of virgin HDPE could be replaced by highly degraded HDPE for wood flour‐HDPE composite manufacturing. The mechanical properties of composites containing highly degraded HDPEs were similar to the composites with virgin HDPE and in some cases they exhibited superior properties, with the exception being with the impact strength. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Flame retardancy,antifungal efficacies,and physical–mechanical properties for wood/polymer composites containing zinc borate

This work aimed to examine flame retardancy, antifungal performance and physical–mechanical properties for silane‐treated wood–polymer composites (WPCs) containing zinc borate (ZnB). ZnB with content from 0.0 to 7.0 wt% was added to WPCs, and silane‐treated wood contents were varied. The polymers used were poly(vinyl chloride) (PVC) and high‐density polyethylene (HDPE). The decay test was performed according to the European standard EN 113. Loweporus sp., a white‐rot fungus, was used for antifungal performance evaluation. Antifungal performance was observed to decrease with wood content. Incorporation of ZnB at 1.0 wt% significantly increased the antifungal performance of WPCs. ZnB content of greater than 1.0 wt% lowered the antifungal properties of WPCs. The results suggested that the wood/PVC composite exhibited better antifungal performance than the wood/HDPE composite. The addition of wood flour to PVC and HDPE decreased flame retardancy, whereas the incorporation of ZnB retained the flame retardancy. ZnB was found to be more appropriate for wood/PVC than wood/HDPE as a result of hydrogen chloride generated from the dehydrochlorination reaction of PVC. The results indicated that the addition of ZnB did not affect the physical‐mechanical properties of neat polymers and the composites. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of material parameters on the diffusion and sorption properties of wood‐flour/polypropylene composites

Composites of wood in a thermoplastic matrix (wood–plastic composites) are considered a low maintenance solution to using wood in outdoor applications. Knowledge of moisture uptake and transport properties would be useful in estimating moisture‐related effects such as fungal attack and loss of mechanical strength. Our objectives were to determine how material parameters and their interactions affect the moisture uptake and transport properties of injection‐molded composites of wood‐flour and polypropylene and to compare two different methods of measuring moisture uptake and transport. A two‐level, full‐factorial design was used to investigate the effects and interactions of wood‐flour content, wood‐flour particle size, coupling agent, and surface removal on moisture uptake and transport of the composites. Sorption and diffusion experiments were performed at 20°C and 65 or 85% relative humidity as well as in water, and diffusion coefficients were determined. The wood‐flour content had the largest influence of all parameters on moisture uptake and transport properties. Many significant interactions between the variables were also found. The interaction between wood‐flour content and surface treatment was often the largest. The diffusion coefficients derived from the diffusion experiments were different from those derived from the sorption experiments, suggesting that different mechanisms occur. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 752–763, 2007     

表面处理剂对PVC仿木复合材料性能的影响

使用4种不同类型的含高活性反应基团的聚氨酯处理剂对木粉表面进行处理,并制备了聚氯乙烯(PVC)/木粉复合材料,研究了表面处理剂的交联度、不同用量和高活性反应基团—NCO的含量对复合材料性能及结构的影响。结果表明,使用聚氨酯处理剂对木粉表面进行处理可以明显改善复合材料的流变性能,并明显提高复合材料的力学性能;扫描电镜观察表明,木粉经聚氨酯处理剂改性后与PVC的相容性明显得到改善。