Influence of weathering on visual and surface aspect of wood plastic composites: Correlation approach with mechanical properties and microstructure

Abstract

This study aims to assess and understand simultaneously the changes in visual aspect, surface topography and mechanical properties of a wood flour reinforced polypropylene (PP) composite under natural and artificial weathering conditions. Compounds were produced through twin-screw extrusion followed by injection molding in order to obtain dog bone samples. A one-year natural outdoor exposure and a xenon-arc accelerated weathering were studied. As expected, mechanical performances are decreased under UV exposure. This performance degradation goes along with important changes in visual aspect. In addition to commonly used microscopy observations, other methods such as colorimetry, spectrogoniometry and confocal rugosimetry are used to characterize the surface aspect through color, gloss and roughness properties. These analyses revealed that ageing induces a surface bleaching, a wood particle protrusion and numerous micro-cracks. Moreover the chain scission attested by Size Exclusion Chromatography (SEC) and lixiviation on the surface induces roughness and gloss loss. Otherwise biocomposites were more degraded under natural weathering. This paper gives new insights in understanding how weathering affects physico-mechanical properties of wood plastic composites (WPC). For that, the Principal Component Analysis (PCA) gives global overview about effects of type of weathering and wood rate on physical, mechanical and chemical properties by calculating their contributions to the drawing of axes (Principal Components) during PCA.

Introduction

Wood plastic composites (WPC) are spreading all around the world and are mainly used in construction applications (decking, fencing …). Indeed the total production of WPC in Europe was of 305 000 metric tons (MT) in 2012 and is expected to rise to 780 000 MT or even 1 250 000 MT in 2020 [1]. The widest used matrices are polyethylene (PE), polyvinyl chloride (PVC) and polypropylene (PP). Otherwise, the success of wood-based materials can be explained by their aesthetic appeal [2].

However, the hydrophilic character and solar radiation sensibility of wood flour limit the WPC durability under outdoor exposition [3]. Indeed cellulose and hemicelluloses contained in vegetable fibers and flours absorb water and cause swelling of composites under high humidity rate or rain.

Many studies investigated on the accelerated weathering of WPC. They examined the influence of the time of exposure on the mechanical properties, the chemical composition and the visual properties [3], [4], [5], [6], [7], [8]. All researchers agreed that oxidation reactions due to high temperatures or UV radiation, particles swelling with high humidity in the environment and weak adhesion between polymer and fibers were the main causes of alteration of mechanical and visual properties. In fact UV radiation induces photo-oxidation especially of lignin, which is the more sensitive to UV light component present in flours because of presence of chromophoric groups in its structure [4] linked to the surface bleaching. Two reactions may occur. Firstly a chain scission of lignin macromolecules can lead to the formation of paraquinonic structures causing a yellowing of WPC. Then paraquinones can reduce to hydroquinones which give a white color to the composites (photo-bleaching). This discoloration of WPC from brown to chalky white has been observed for wood flour polypropylene composites by Peng et al. [4]. The comparison between WPC and pure lignin reinforced polypropylene composites showed that the discoloration was more pronounced for the last composite and confirms that lignin is more sensitive to UV weathering. Another composite pure cellulose/PP was tested and the bleaching of this composite was slower: the material was more stable. Also, due to the sensitivity of lignin face to environmental conditions, WPC sometimes require the addition of color stabilizers [9], [10], [11], [12].

Fewer studies investigated the effects of natural weathering on WPC. For example, Fabiyi et al. compared the effects of exterior and accelerated weathering on pine wood flour-high density polyethylene and polypropylene composites [13]. Infrared characterization revealed the formation of oxygenated molecules such as carbonyl groups, carboxylic acids and esters. It implied a surface oxidation of WPC. They also noted a bleaching of WPC by spectrocolorimetric analysis, suggesting that lignin has been degraded during the two types of weathering. Homkhiew et al. worked on the influence of plastic grades (virgin and recycled polypropylene), the influence of the loading of wood flour and the addition of UV stabilizers on the discoloration after natural exposition [14]. As well as for Fabiyi's studies, a color fading of the composite was observed and increased with the wood content even if stabilizers retarded a little the rate of discoloration. Butylina et al. noted two stages of surface lightness reduction after the bleaching of WPC during an exterior weathering: the first was due to dusty deposits on the surface and the second was the result of the decrease of solar radiation in autumn [10].

The polymer matrix and wood flour are mainly deteriorated on composites surface during weathering which results in a reduction of the molecular weight of the polymer [5] and an evolution of surface roughness [6], [15]. Peng et al. measured the surface roughness of WPC before and after accelerated weathering by AFM analysis [6]. They noted an increase of the roughness parameter due to the degradation of polypropylene and wood loss on weathered surfaces. According to Muasher and Shain, the photo-degradation mechanism of the polymer occurs via Norrish type I and results in formation of free radicals [12]. These highly reactive molecules affect the polymer and lead to termination via crosslinking or polymer chain scission. Then the roughness caused either by the formation of cracks after photo-oxidation [8], [16], or a wood particles loss or finally a decrease in the interfacial bonding between the wood flour and the polymer matrix [17] induce a loss of gloss for the biocomposites.

Even if lots of studies investigated the evolution of color of WPC during weathering, just a few ones dealt with the gloss and roughness thanks to spectral measurements. Until today, the techniques used to evaluate the surface state of WPC are based on atomic and electronic microscopic measurements (AFM, SEM). Although they present high resolution, these characterization methods sometimes imply sample destruction or damage and results can be dependent on measurement conditions (for instance, heights measured during the AFM analysis can be dependent on the dot's type used and the analysis under air can interfere results). Here optical measurements will be used and involve spectral reflection measurements. The advantages of these rugosimeters are double. It consists in non-destructive measurements and shorter analysis times are required.

In regards to this state of the art, the novelty of this paper consists in a simultaneous analysis of visual aspect changes, surface topography and mechanical properties of a non-stabilized wood flour reinforced polypropylene (PP) composite under natural and artificial weathering conditions. Firstly, the weathering influence on WPC is studied by every characteristic assessment one by one. Then, a Principal Component Analysis (PCA) approach clearly shows which properties gather or differentiate samples and the links between these properties.