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.
The degradation behaviors of hemp fibers reinforced PP biocomposites under outdoor and artificial weathering were compared to establish a correlation. For this purpose, several measurements were performed throughout the expositions. Mechanical performance was tested by three-point bending test. Microstructure and chemical composition changes were also assessed. Otherwise, visual aspect and topography were determined. The artificial weathering effectively accelerated the degradation mechanisms. Oxidation pathways and surface aspect alteration of both polymer and biocomposites occurred faster. However, whereas biocomposites were mainly subjected to outdoor conditions due to high sensitivity of hemp fibers, neat PP was globally mostly affected by laboratory chamber conditions. Its oxidation rate largely outstripped reinforced materials ones. Principal Component Analysis was used for verifying the differences of variables correlations profiles between artificial and exterior ageing dataset in order to compare the degradation mechanisms. Through the statistical analysis, some attempts were made to find equivalence between artificial and outdoor weathering times thanks to properties degradation rate similarities.
This work aims to investigate exterior and under glass weathering representing decking and car interior end uses of hemp fibers reinforced polypropylene (PP) biocomposites. For this reason, mechanical flexural tests were firstly performed. Then, the evolution of the PP matrix microstructure was determined through Differential Scanning Calorimetry (DSC). The chemical composition was studied by infrared spectroscopy to understand the photo-and thermo-chemical mechanisms. CIELab system-based colorimetric measurements were carried out to determine the evolution of the chromaticity and lightness. Through a new approach, gloss was obtained by determining the type of reflection of materials as either specular or diffuse, and the surface aspect was characterized by rugosimetry. The influence of the rate of hemp fibers (from 0 to 30 wt%) was studied. Results showed that biocomposites were generally more sensitive than neat PP whatever the weathering conditions. However, each type of weathering assessment allowed understanding the contribution of each degradation factor. Indeed, rainfall or UV-A rays induced an increase in the vinyl concentration and the formation of cracks on the surface whereas the carbonyl functional groups rate was not influenced by the type of weathering. The high temperatures under windshield glass favored a chemicrystallization and biocomposites yellowing at the first period of exposition whereas outdoor exposure induced red color loss.
Two natural weathering of neat polypropylene (PP) and hemp fibers reinforced PP biocomposites were investigated. The objective was to emphasize the relationships between the properties of materials according to the fiber loading and the weathering time in order to bring new insights of degradation mechanisms understanding. For this purpose, a Principal Component Analysis (PCA) method was applied to the dataset. The treatment carried out by isolating materials loaded by the three different hemp fiber rates particularly outstand the link between mechanical properties and products issued from oxidation. The close correlation between whitening parameter and C]C bond level also confirmed the lignin degradation way. Finally, depending on the weathering duration, the properties characterized either fiber loading or weathering state (unweathered or weathered) whereas mechanical performance differentiated the three different non-weathered samples whatever the exposure time.
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