Wood-plastic Composite Surface Research Articles

Improving interfacial bonding strength between epoxy and PE-based wood plastic composites by micro-riveting

In this paper, we proposed a micro-riveting method using the wood fibers in the wood plastic composites (WPC) to effectively enhance the interfacial bonding strength between the epoxy coating and polyethylene (PE) -based WPC. The WPC surface with wood fibers partially embedded in the PE matrix was prepared using a simple solvent swelling and polishing method. Epoxidized Cardol having a long and hydrophobic chain was modified on the surface of the exposed wood fibers to improve the water-resistance and chemical reactivity of the wood fiber surface. Fourier transforms infrared (FT-IR) spectra, X-ray photoelectron spectroscopy (XPS), contact angle, confocal microscopy, and scanning electron microscope (SEM) have been used to characterize the composition and morphology of the surface structures. A pull-off test has been used to evaluate the interface bonding strength, which increased from 1.88 to 5.27 MPa. This constitutes the highest performance among all the non-plasma surface treatment reports (30 % higher than the highest bonding strength reported in the literature and comparable to plasma-based surface treatments). The epoxy-WPC interface showed excellent water resistance after immersing in sodium chloride (NaCl) solution (3.5 wt%) for 60 days. The proposed micro-riveting method is of high significance for WPC and may contribute to the interface adhesion enhancement of other polymer composites.

Improvement of weathering and thermal resistance of wood-plastic composites with iron oxide nanoparticles

This study comprehensively investigated the effect of iron-oxide nanoparticles (NPs) on weathering durability and thermal resistance of wood-plastic composites (WPCs). The hydrophobic nature of NPs improved the dimensional stability of WPCs. The small size of NPs deposits the voids in the matrix, which helps to increase the mechanical properties, even after weathering test. The decrease of modulus of rupture (MOR) reached up to 16% for control samples, while it was 2% for IO40-1. Despite the intensive weathering conditions, NPs cover the surface of materials like a UV shield, improving WPCs' UV resistance. Moreover, the increase in the wood and NPs content limited the UV degradation, which resulted in lower color changes. The attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) analysis also proved that there were nearly no changes in the characteristic bands of polymer (2916 and 2846 cm−1) and wood (1512 cm−1). The microscopic investigation, however, revealed the deterioration on the surface of WPCs after weathering exposure. Even though UV degradation is inevitable, iron oxide NPs significantly preserved the WPCs surface. However, there were also crack formations, which were also inhibited. On the other hand, NPs decelerated the thermal degradation by acting as a heat barrier. Thermogravimetric analysis (TGA) analysis revealed an increase in the degradation temperature. Limit oxygen index (LOI) values also increased up to 27.6%, which demonstrated an improvement against flammability.

(No. 24) Chalking and fungal contamination on the surface of wood plastic composites under outdoor exposure

(No. 24) Chalking and fungal contamination on the surface of wood plastic composites under outdoor exposure

The effect of zinc oxide nanoparticles on the weathering performance of wood‐plastic composites

AbstractIn recent years, wood‐plastic composites (WPCs) have become among the most popular engineering materials. Most of their usage areas are outdoors, where they encounter various damaging factors. The weathering conditions cause significant deterioration to WPC surfaces, which negatively influences their service life. In this study, zinc oxide nanoparticles at different concentrations (1%, 3%, 5%, 10%) were added to a high‐density polyethylene‐based WPC matrix. The effect of zinc oxide nanoparticles on the weathering performance of WPC was evaluated after 840 hours of an artificial weathering test. The highest colour changes (∆E*) were monitored with control samples exposed for 840 hours. Adding zinc oxide nanoparticles improved the ultraviolet (UV) resistance and decreased the colour changes. The wood flour content also affected the colour changes on the WPC surface. A combination of 10% zinc oxide nanoparticles and 50% wood flour content provided the lowest colour changes. The barrier effect of nanoparticles protected the WPC surfaces from UV light. Zinc oxide nanoparticles also positively affected the load transfer, which restricted the reduction in mechanical properties after the weathering test. The degradation on the surface of WPCs was also investigated using attenuated total reflectance‐Fourier Transform–infrared analysis. The changes in the characteristic bands of polymer and wood indicated that surface degradation was inevitable. Light and scanning electron microscopy images also demonstrated micro‐cracks and roughness on the surface of WPCs. It is concluded that UV degradation is unavoidable, but zinc oxide nanoparticles can improve surface resistance against weathering conditions.

Efficient flame-retardant hybrid coatings on wood plastic composites by layer-by-layer assembly

Fire retardant treatments that reduce the heat release rate of wood plastic composites (WPCs) have been intensively studied. However, few studies have reported on increasing the ignition time using precise control of nanostructured hybrids coating on WPC surfaces. Here, a nanocoating layer, composed of polyethyleneimine, Na-montmorillonite (MMT) and graphitized multiwalled carbon nanotubes, is deposited on the WPC surface via a layer-by-layer assembly (LbL). The nanocoating layer has favorable pull-off adhesion performance in dry and wet conditions. The protective layer has a “brick-mortar-rebar” nanostructure generating a thermal and gas barrier. This nanostructure prevents oxygen permeation and volatile gases release during material combustion. The thermal kinetics and decomposition reactions are restricted, increasing the ignition time from 49 s to 237–286 s, and 30% char formation due to the thermal shielding nature of MMT and the high charring capability of the coating nanostructure. The coating layer does not affect the WPC mechanical properties negatively. We foresee that this efficient multilayer hybrid coating method for improving fire-retardancy and smoke suppression could be applied to other materials, such as wood, foams, aerogels and polymer substrates.

Surface Activation of Polylactic Acid-Based Wood-Plastic Composite by Atmospheric Pressure Plasma Treatment.

Wood-plastic composite (WPC) based on a polylactic acid (PLA) matrix is a promising material since it is biobased, degradable, sustainable, and 3D printable. However, due to its coloring, visible layers after 3D-printing, and small build volumes of these printers, a coating or gluing of parts might be required. This study investigates the influence of a dielectric barrier discharge (DBD) plasma treatment of PLA-based WPC to activate the surface and improve, e.g., coating capabilities. X-ray photoelectron spectroscopy (XPS) measurements showed the oxidation of the surface due to the formation of carbonyl and carboxyl groups. Laser scanning microscopy revealed a surface roughening after the treatment. Contact angles of water and diiodomethane decreased significantly after the plasma treatment and the consecutively calculated surface free energy increased. Finally, two practical adhesion tests revealed an improvement of the applied acrylic dispersion coating’s adhesion to the WPC surface: The assigned cross-cut class improved, and the pull-off strength increased from 1.4 to 2.3 N/mm2.

Accuracy of the pneumatic follower for the wooden surface quality assessment

Wooden or wood-plastic composite surfaces are often valued for their aesthetic appearance and the quality of the surface. Independent from the technology, the surface features may be assessed using typical roughness parameters. In this paper, a pneumatic non-contact method is proposed. Despite certain limitations in surface characteristics measurement, air gauges proved to be a good tool for wooden surfaces, much cheaper than the laser profilometers. In the current research, a novel non-contact air gauge was combined with a slider to eliminate the influence of the asperities of higher order (waviness) and to protect the wooden surface from being damaged by the measuring nozzle. The measurement signal was used to control a follower that kept constant distance between the surface and the nozzle edge. The measuring speed was set at 0.8 mm/s, and the back-pressure was measured with accuracy of 0.01 kPa with real time acquisition. A series of measurements was taken, and the results were compared with the ones obtained from Perthen S8P profilometer (contact measurement) and WYKO NT 1100 interferometer microscope.

Surface Activation of Wood Plastic Composites (WPC) for Enhanced Adhesion with Epoxy Coating

Abstract This work aims to improve the adhesive force of epoxy coating with a wood-plastic composite (WPC) surface. First, WPC was formulated using the extrusion technique through different percentages of wood flour (WF) with virgin polyethylene (PE) in the presence of a coupling agent. Dynamic mechanical analysis and water percentage gain (WPG%) were performed to find the optimized WF/PE mixing ratio. The optimized formula was subjected to surface treatment by different methods, including chromic acid oxidation, ultraviolet irradiation, and flame and heat treatment. Then the treated surface of WPC was coated with nano-modified epoxy resin using carboxylated carbon nanotube, organically modified nanoclay (Cloisite-30B), and reactive rubber nanoparticles. The surface treatment of the WPC was characterized with contact angle measurement and Fourier Transform Infrared (FTIR), while the interaction between the WPC and epoxy coat was investigated by attenuated total reflection FTIR microscopy analysis. The adhesion strength of the nano-modified epoxy showed significant enhancement when investigated by pull-off method and cross-cut test. Nano-scratching was utilized in exploring the effect of different surface treatments on enhancing adhesion to epoxy. The interfacial morphological behavior of epoxy coating on the WPC surface was investigated by Scanning Electron Microscope. The results indicated that chemical treatments helped for the functionalization of the WPC surface, leading to stronger adhesion with epoxy. These epoxy-coated WPC composites recorded enhanced mechanical properties because of the chemical reaction between the WPC surface and epoxy.

Fungal contamination on the surface of wood plastic composites under outdoor exposure

Fungal contamination on the surface of wood plastic composites under outdoor exposure

Termite and decay resistance of bioplast-spruce green wood-plastic composites

Wood-plastic composites (WPCs) are very promising and sustainable green materials to achieve durability without using toxic chemicals. These materials, produced by blending biopolymers and natural fillers, permit us not only to tailor the desired properties of materials according to the characteristics and ratios of wood and polymers but are also the solution to meet environmental and sustainability requirements. This study focuses on the durability evaluation of green WPCs made from a blend of an entirely biodegradable biopolymer (BIOPLAST GS2189 supplied by Biotec-Germany) and spruce wood sawdust. The spruce sawdust with different amounts (from 0 to 30% weight) was introduced into Bioplast and the obtained blends were injected into injection molds in order to manufacture the samples, which are eco-friendly materials and biodegradable in specific conditions. To determine the biological resistance of the produced WPCs, the decay and termite resistance tests, conducted according to screening tests adapted from European Standards, were carried out in relation to the wood content. The results showed an increase in fungal and termite degradation levels with increasing amounts of wood in Bioplast. It also showed a relationship between the water uptake due to fungi growth and a decrease in the resistance against fungal and termites. The optical microscopy observations performed on WPC specimen surfaces highlighted the presence of microcracks on the surface of WPCs after their decay exposure, resulting in an embrittlement of the composite containing high wood content. These observations were discussed in order to understand the pathways of degradation mechanisms in these WPCs. Although Bioplast partial substitution by wood decreased the resistance of WPCs to fungal and termite attacks, the elaborated WPCs in this study were still in the class of strongly durable material.

Comparative surface and adhesion properties of mechanical abrasion, flame and radiation-based surface treated wood plastic composites made with different polymers

Wood plastic composites (WPCs) show low surface energy and poor adhesion properties, and they can be improved by applying surface treatment. In this study three WPCs made with polyethylene, polypropylene and poly(vinyl chloride) have been treated by mechanical abrasion, flame and radiation-based (UV/ozone, atmospheric pressure plasma torch, Ar:O2 low pressure plasma) treatments. The influence of the wood content and polymer nature in the WPC on the surface modifications and adhesion properties produced by the surface treatments has been assessed. The surface treatments by mechanical abrasion and flame produced slight changes in the WPC surface, mainly in roughness, irrespective of the wood content and polymer nature in the WPC. The radiation-based surface treatments were the most efficient in increasing the surface energy (particularly the polar component) and produced ablation and surface oxidation of the WPC, the most noticeable modifications were obtained in the WPCs made with polyolefin. The adhesion (180° peel strength and cross-hatch adhesion) of the WPCs made with polyolefin was enhanced by surface treatment with both plasmas, irrespective of the adhesive or coating nature, and a relationship between the surface modifications produced by the plasma treatments and the adhesion was found. The surface modifications by the different treatments were less marked for the WPC made with poly(vinyl chloride) due to its smooth surface, low wood content and the existence of carbonate filler in its composition; furthermore, lower adhesion than for the surface treated WPCs made with polyolefin was obtained, and the highest 180° peel strength and cross-hatch adhesion were obtained with different surface treatments, the adhesion also depended on the coating nature. The effectiveness of the surface treatment in improving the adhesion of the WPCs depended on their wood content, polymer nature and surface roughness.

Effect of raw material composition of wood plastic composites on surface roughness parameters evaluated with a robust filtering method

This study extensively investigated the surface roughness of injection molded wood plastic composites (WPCs) produced from different amounts of wood flour, polymer matrix, mineral filler, and other additives. A larger range of roughness parameters that used in the previous literature were obtained from nine different WPC compositions by using a robust filtering method (robust Gaussian regression filter) to have a better understanding of the overlaying quality of the WPC samples. Three melt flow index (MFI) of the polymer were tested (MFI 3.6, 12, and 25). It was found that WPCs produced with the polypropylene having a MFI value of 25 were the smoothest. It was noticed that not only the wood flour percentage matters but the combination of wood flour–mineral filler was also important. The WPC compositions with lower polymer amount in favor of more wood flour and mineral filler led to rougher surfaces. A decrease of wood flour in favor of increasing the mineral filler participation percentage had a surface smoothing effect. At the same wood flour content, a decrease in polymer combined with an increase in mineral filler led to rougher WPC surface. Among the tested WPC compositions, the smoothest surface was obtained in the specimens produced from 50% wood flour, 0% mineral filler, and around 40% polymer by weight. The results should be helpful to anticipate the effect on surface roughness of the percentage participation for each amount of the wood or mineral filler, polymer matrix, and additives in further development of WPC combinations.

The Use of Low-pressure Plasma on Enhancing the Attachment of Al2O3 Nanoparticles to Wood–Plastic Composites

Wood–plastic composites (WPC) are widely used for many purposes due to their interesting properties, but they have poor surface adhesion due to the chemical inertness of the matrix. Thus, in this work, the effect of plasma treatment on the WPC was investigated regarding its influence in the aluminum oxide (Al2O3) nanoparticles attachment to the surface. WPC were prepared in a polypropylene (PP) matrix, plasma-treated at 100 W for 600 s, and then covered with Al2O3 nanoparticles dispersion. The WPC/Al2O3 surfaces have been investigated by means of morphology, surface roughness, chemical structure, wettability, and nanohardness. Plasma treatment improved the attachment of Al2O3 onto WPC, which was confirmed by the higher presence of aluminum and oxygen-containing functional groups and the reduction of the intensity of peaks of methylene (CH2) and methyl (CH3) groups on the WPC surface. The higher surface reactivity of plasma-treated WPC resulted in a better distribution of the nanoparticles over the entire surface. In addition, plasma treatment avoided the formation of coffee-ring phenomenon but it was able to create cone-like structures on the WPC, increasing the surface roughness due to the etching effect and the attachment of Al2O3 nanoparticles. Plasma treatment followed by Al2O3 nanoparticles attachment increased the wettability, hardness, and elastic modulus of WPC at nanometric scale.

Surface modification and adhesion of wood-plastic composite (WPC) treated with UV/ozone

Because of the surfaces of wood-plastic composite (WPC) materials are enriched in polymers of low surface energy, they exhibit low adhesion properties. UV/ozone is proposed as surface treatment for increasing the surface energy and adhesion of WPC materials made with different polymers (polyethylene, polypropylene and polyvinyl chloride). UV lamp-WPC surface distance and time of UV exposure were varied for optimizing UV/ozone treatment of WPC, and UV dose used ranged between 2.02 × 10−14 and 5.05 × 10−12 J·s/m2. UV/ozone treatment created new carbon-oxygen polar groups in WPC surfaces and increased their surface energy, mainly their polar component. Furthermore, ablation of the outermost WPC surface was produced, more noticeably by reducing the distance between WPC surface and UV lamp and by increasing the duration of the treatment. Noticeable increase in 180° peel adhesion was obtained in the joints made with UV/ozone treated WPC at 10–30 mm distance during 1–5 min (i.e., UV dose between 5.61 × 10−14 and 2.53 × 10−12 J·s/m2). Although 180° peel strength of joints made with acrylic adhesive tape and UV/ozone treated WPC for 10 min and 10 mm distance (UV dose: 5.05 × 10−12 J·s/m2) was not increased because of dominant effect of ablation over creation of polar groups, the cross-hatch adhesion to different coatings was highly improved, irrespective of the polymer used and the wood content of WPC; however, the surface modifications and adhesion of UV/ozone treated WPC were more marked when its wood content was higher and by using UV dose between 0.10 × 10−12 and 2.53 × 10−12 J·s/m2.

Study of Mold Invasion on the Surface of Wood/Polypropylene Composites Produced from Aqueous Pretreated Wood Particles, Part 2: Juniperus procera Wood-Branch

Mold invasion by Trichoderma harzianum T6776 over the surface of wood-plastic composites (WPCs) made from Juniperus procera wood-branch and polypropylene with a melt-blending technique was examined using scanning electron microscopy (SEM) and electron dispersive X-ray spectroscopy (EDX). Before the addition of coupling agent, the WPC samples were made from untreated and pretreated wood-branch particles of J. procera with either cold or hot water and then mixed with polypropylene to produce panels. The surfaces of WPC samples were inoculated by a mold suspension of T. harzianum. SEM-EDX measurements of WPCs made from J. procera particles showed little or no growth of T. harzianum, irrespective of treatment with cold or hot water. The results suggest that WPCs made from the particles of J. procera wood-branch pretreated with either cold or hot water could be useable in wet conditions. In addition, using of J. procera as durable wood for manufacturing of WPCs had good effects on the prevention of the mold infestation over the surfaces of the produced panels.

Mold Invasion on the Surface of Wood/Polypropylene Composites Produced from Aqueous Pretreated Wood Particles, Part 1: Date Palm Midrib

The effect of particle pretreatments on the biodeterioration of wood-plastic composites (WPCs) was investigated. WPCs made from untreated and pretreated frond midrib particles of date palm were used. Before the addition of a coupling agent, the wood particles were pretreated or extracted with either cold or hot water and mixed with polypropylene to produce panels, which were then superficially inoculated with an ascomycete’s fungus Trichoderma harzianum T6776. The WPC surfaces were studied using scanning electron microscopy (SEM) and electron dispersive X-ray spectroscopy (EDX) measurements. In comparison with the control, an intensive growth of T. harzianum hyphae was found over the WPC surface manufactured from untreated date palm midrib particles with the colonies clearly visible. The pretreatments of date palm particles reduced the growth of T. harzianum in comparison with the control and untreated particles. The results suggested that particle pretreatments could be a suitable way to limit the growth of molds over WPC surfaces made from date palm midrib.

Abiotic and biotic degradation of post-consumer polypropylene/ethylene vinyl acetate: Wood flour composites exposed to natural weathering

The effects of natural weathering on the visual appearance and chemical changes of wood plastic composite (WPC) formulations based on post-consumer polypropylene/ethylene vinyl acetate (PP-EVA) matrix were investigated. The WPC composition used was 70/30 (w/w) (recycled PP-EVA/wood flour). Besides, the effectiveness of using coupling agent on adhesion of WPC and its influence in degradation were evaluated. Colorimetry, scanning electron microscopy, Fourier transform infrared spectroscopy, mechanical test, and biodegradation in simulated soil after natural weathering were used in this research. The results showed the samples with longer exposure time to natural weathering presented significant color change, increased in carbonyl index, and wood loss on weathered WPC surfaces. Weathered WPC exhibited decreased in mechanical properties, higher mass loss after biodegradation test when compared with no weathering WPC, reaching to 15.0% mass loss against 3.7%, respectively. Climatic conditions directly affect the characteristics of all composites, thus indicating a significant photo-oxidation of the samples with a longer time of exposure to weathering, and this natural ageing has facilitated the WPC biodegradation. POLYM. COMPOS., 2015. © 2015 Society of Plastics Engineers

Effects of volatile chemical components of wood species on mould growth susceptibility and termite attack resistance of wood plastic composites

This study mainly aimed at investigating the effects of volatile chemical components of wood species on mould and termite resistance of wood plastic composites (WPC) using artificial accelerated tests. The morphology characterization of surface and fracture of WPC was estimated by SEM and digital instrument. Volatile chemical components of wood species extractives were analyzed by GC–MS. The results indicated that the sequences of mould resistance of WPC were ranked as: Cunninghamia lanceolata and Melaleuca leucadendra (level 0) > Eucalyptus grandis × Eucalyptus urophylla (level 1) > Pinus massoniana (level 2) > Liquidambar formosana and Ricinus communis (level 4). The sequences of termite resistance of WPC were ranked as: C. lanceolata (level 1, ML = 4.52%) > M. leucadendra (level 1, ML = 5.73%) > E. grandis × E. urophylla (level 1, ML = 6.48%) > L. formosana (level 2, ML = 6.72%) > P. massoniana (level 2, ML = 7.08%) > R. communis (level 3, ML = 10.40%). It was also suggested that 8-propoxy-cedrane, cedrol, α-cedrene and β-cedrene in C. lanceolata, 2,3-dihydro-2,2-dimethyl-3,7-benzofurandiol, 3-demethyl-colchicine and squalene in M. leucadendra; 2,3-dihydro-2,2-dimethyl-3,7-benzofurandiol and stigmast-4-en-3-one in E. grandis × E. urophylla were potentially crucial to provided positive effects on biodegradation resistance. Longifolene, caryophyllene and α-pinene in P. massoniana; 4-hydroxy-3,5-dimethoxy-benzaldehyde, 3,5-dimethoxy-4-hydroxycinnamaldehyde and cinnamyl cinnamate in L. formosana; 5-hydroxymethylfurfural and 1,6-anhydro-β-d-glucopyranose in R. communis led to the opposite results.

A conceptual model to understand the correlation between topography and wetting of polypropylene- and polyethylene-based wood–plastic composites

Microscopic and physico-chemical methods were used for a comprehensive surface characterization of different extruded polypropylene- and polyethylene-based wood–plastic composite (WPC) formulations. The surfaces were analysed using stereophotogrammetry, high-resolution scandisk confocal microscopy and scanning electron microscopy, resulting in detailed information about the topography and surface morphology. In addition, dynamic water contact angle measurements were carried out to characterize the wettability of the samples. The effects of polymer type, polymer content, additive content and processing method on the resulting topography and wetting were investigated. The correlation between topography and wetting resulted in a conceptual model of WPC surface morphology and wetting, which may be applied to optimize adhesion properties of this composite material.

Surface deterioration of wood plastic composites under outdoor exposure

Outdoor exposure tests of wood plastic composites (WPCs) were conducted at seven sites in Japan. The tests examined the process of chalking on the surface of the WPCs and the weather factors affecting chalking. Observations using a scanning electron microscope and a polarization optical microscope (POM), analyses of Fourier transform infrared spectroscopy, as well as differential scanning calorimetry showed that the deterioration of both wood and polyolefin elements occurs on the surface of WPCs. The POM observations revealed that surface sanding of WPCs in the manufacturing process promoted more chalking. Furthermore, it became clear that among the weather conditions studied, temperature and global solar radiation had the greatest influence on chalking, as determined from the results of single regression analysis.