Flexural creep behavior of discontinuous thermoplastic composites: Non-linear viscoelastic modeling and time–temperature–stress superposition

Flexural creep behavior of nylon 6/6, polypropylene and high-density polyethylene long fiber thermoplastic (LFT) composites was studied according to ASTM D-2990. Neat polymers were tested for baseline data and compared with the 40 wt.% E-glass reinforced LFTs, all processed by compression molding. All materials exhibited non-linear viscoelasticity and showed a succession in creep resistance consistent with static flexural yield strength. A four parameter empirical model used for short fiber thermoplastics (SFT), proposed by Hadid et al., was found to provide an excellent fit to the experimental data. Time-compliance data from flexural creep and dynamic mechanical analysis (DMA) were combined to utilize short-term flexural creep tests to predict lifetime of the composites. A time–temperature–stress superposition (TTSSP) procedure was used, where stress-based vertical shifts were applied in addition to horizontal shifts used in a traditional time–temperature superposition (TTSP). Master curves obtained by this method projected the long-term creep properties, the order of creep resistance being consistent with the flexural creep data.     

Application of Kramers–Kronig relations to time–temperature superposition for viscoelastic materials

Dynamical mechanical analysis (DMA) is an experimental technique commonly used to study the frequency and temperature dependence of the mechanical properties of viscoelastic materials. The measured data are traditionally shifted by application of the time–temperature superposition principle to obtain the master curves of the viscoelastic material. The goal of this work is to present a methodology to determine the horizontal and vertical shift coefficients to be applied to the isotherms of storage and loss moduli measured. The originality lies in the calculation of the shift coefficients by a method requiring fulfilment of the Kramers–Kronig relations conveying the causality condition. The computed vertical shift coefficients are compared to the coefficients predicted by the Bueche–Rouse theory.     

Comparison of two micromechanical models for predicting elasto-plastic response of wood–plastic composites

The mechanical response of wood- and cellulose-filled polymers and its comparison to analytical models is studied in this article. To model the elasto-plastic response of the wood–plastic composite (WPC), two explicit semi-analytical micromechanical methods were used: Mori–Tanaka Method (MTM) and Generalised Method of Cells (GMC). For experimental purpose, several test specimens composed of matrix polypropylene (PP) or polystyrene (PS) and filled with wood or cellulose short fibres of different length to width aspect ratio and various volume fractions were injection moulded. Tensile testing was then used to gain experimental data, which were then compared to the calculated prediction of proposed micromechanical models to test their applicability. The comparison of results show that both methods can accurately predict the response of the composite in the elastic area; however Mori–Tanaka Method can achieve better results when forecasting plastic deformations of wood–plastic composites.     

Influence of refiner fibre quality and fibre modification treatments on properties of injection-moulded beech wood–plastic composites

Thermo-mechanical pulp (TMP) fibres made from beech wood were produced using increasing refiner gap widths and thus with increasing fibre length and coarseness. Fibres (60% by weight) were compounded in an internal kneading mixer using high-density polyethylene as the matrix and injection-moulded. Fibre lengths and length/width ratios were determined (a) before processing and (b) after injection-moulding and Soxhlet extraction using the optical FibreShape system. An increase in fibre length resulted in a decrease in water absorption and an improvement in flexural strength and modulus of elasticity of the wood–plastic composites (WPC). However, flexural strength of the WPC with TMP fibres was not improved compared to WPC with wood flour when maleic anhydride-grafted polyethylene (MAPE) was used as a coupling agent. After injection-moulding, differences in length of the various TMP fibre types were minor. Fibre geometry before processing strongly influences the water absorption and flexural properties of the composite. Fibre treatment with emulsified methylene diphenyl diisocyanate (EMDI) resin before compounding was shown to be equally efficient in reducing water absorption and improving flexural strength as the addition of MAPE during the compounding step.     

Prediction of color changes in acetaminophen solution using the time–temperature superposition principle

A prediction method for color changes based on the time–temperature superposition principle (TTSP) was developed for acetaminophen solution. Color changes of acetaminophen solution are caused by the degradation of acetaminophen, such as hydrolysis and oxidation. In principle, the TTSP can be applied to only thermal aging. Therefore, the impact of oxidation on the color changes of acetaminophen solution was verified. The results of our experiment suggested that the oxidation products enhanced the color changes in acetaminophen solution. Next, the color changes of acetaminophen solution samples of the same head space volume after accelerated aging at various temperatures were investigated using the Commission Internationale de l’Eclairage (CIE) LAB color space (a*, b*, L* and ΔE*ab), following which the TTSP was adopted to kinetic analysis of the color changes. The apparent activation energies using the time–temperature shift factor of a*, b*, L* and ΔE*ab were calculated as 72.4, 69.2, 72.3 and 70.9 (kJ/mol), respectively, which are similar to the values for acetaminophen hydrolysis reported in the literature. The predicted values of a*, b*, L* and ΔE*ab at 40?°C were obtained by calculation using Arrhenius plots. A comparison between the experimental and predicted values for each color parameter revealed sufficiently high R² values (>0.98), suggesting the high reliability of the prediction. The kinetic analysis using TTSP was successfully applied to predicting the color changes under the controlled oxygen amount at any temperature and for any length of time.     

Validation of the time–temperature superposition principle for crack propagation in bituminous mixtures

The time–temperature superposition principle (TTSP) is known to be valid in the small strain domain where the behaviour of bituminous mixtures is linear viscoelastic (LVE). The behaviour is then called thermorheologically simple. In this work, an experimental campaign was performed at University of Lyon/ENTPE (France) to check the validity of the TTSP in the linear domain in the tridimensional case and also when cracks occur and propagate in bituminous mixture. A four-point bending test, which has been designed at University of Lyon/ENTPE, was used as crack propagation test. First, a complex modulus test is performed on cylindrical specimen in the LVE domain. Then, a series of crack propagation tests are carried out at different temperatures and different imposed displacement rates. The same shift factors obtained for master curve of complex modulus is also applied for the crack propagation tests analysis. The results allow obtaining a unique curve, for identical loadings when plotting as a function of reduced time. This result confirms that the TTSP is also valid for crack propagation in bituminous mixtures.     

Effect of a novel coupling agent,alkyl ketene dimer,on the mechanical properties of wood–plastic composites

The objective of this study was to investigate the incorporation of poplar wood fibers both with and without a novel coupling agent, alkyl ketene dimer (AKD), on the mechanical properties of wood fiber/polypropylene (PP) composites. The resulting properties were compared to those obtained with the most commonly used coupling agent, maleic anhydride grafted PP (MAPP). Tensile and impact strengths of the composites decreased with increasing poplar wood fibers content. Tensile modulus of the composites increased by the incorporation of the wood fibers content up to 70 wt% but further increment in the wood fibers decreased the tensile modulus. At the constant content of poplar wood fibers (70 wt%), the tensile strength determined for the coupled composites with 5% AKD increased by 41% in comparison with the non-coupled composites while the tensile modulus increased by 45%, the impact strength of the coupled composites increased by 38%. The performance of 5% AKD on the mechanical properties of the composites is a little better than 3% MAPP. The good performance of 5% AKD is attributed to the enhanced compatibility between the poplar wood fibers and the polymer matrix. The increase in mechanical properties of the composites demonstrated that AKD is an effective coupling agent for wood fiber/PP composites.     

Applicability of empirical models for evaluation of stress ratio effect on the durability of fiber-reinforced creep rupture–susceptible composites

Fiber-reinforced polymer–matrix composites are known to exhibit loading rate- and time-dependent mechanical response. Their fatigue strength is determined by a complex interaction of damage processes governed by loading duration and cycle number. Apart from mechanistic approaches, a number of empirical models of various sophistication have been proposed to predict the durability of composites, differing in the amount of experimental data needed for their application. The accuracy of several such models is evaluated by comparing the prediction to the experimentally determined stress ratio effect on fatigue life of glass fiber-reinforced polyester–matrix composite. It is found that the accuracy of prediction generally improves with increasing the amount of test data needed for model calibration. However, the most accurate method of fatigue life estimation, among the selected ones, is by the modified Goodman diagram.     

Elastomer modified polypropylene–polyethylene blends as matrices for wood flour–plastic composites

Blends of polyethylene (PE) and polypropylene (PP) could potentially be used as matrices for wood–plastic composites (WPCs). The mechanical performance and morphology of both the unfilled blends and wood-filled composites with various elastomers and coupling agents were investigated. Blending of the plastics resulted in either small domains of the minor phase in a matrix of major phase or a co-continuous morphology if equal amounts of HDPE and PP were added. The tensile moduli and yield properties of the blends were clearly proportional to the relative amounts of HDPE and PP in the blends. However, the nominal strain at break and the notched Izod impact energies of HDPE were greatly reduced by adding as little as 25% of the PP. Adding an ethylene–propylene–diene (EPDM) elastomer to the blends, reduced moduli and strength but increased elongational properties and impact energies, especially in HDPE-rich blends. Adding wood flour to the blends stiffened but embrittled them, especially the tougher, HDPE-rich blends, though the reductions in performance could be offset somewhat by adding elastomers and coupling agents or a combination of both.     

Accelerated weathering of fire-retarded wood–polypropylene composites

In this study, the influence of fire retardants, namely aluminum trihydrate, zinc borate, melamine, graphite, titanium dioxide on the durability of polypropylene-based co-extruded wood–plastic composites is studied. The composites underwent accelerated weathering under a xenon-arc lamp source during 1000 h. FTIR analysis of the composite surface revealed a degradation process which was accompanied by chemical changes, including vinyl-like and carbonyl groups accumulation; fire retardants did not influence the photo-oxidation mechanism of the composite. Fire retardant-loaded samples had smaller color change compared to the unfilled one. The tensile properties of all composites declined after the weathering. Significant changes in the surface morphology of the weathered composites were observed with a scan electron microscope.     

Applicability condition of time–temperature superposition principle (TTSP) to a multi-phase system

The applicability condition of the time–temperature superposition principle (TTSP) to a multi-phase system is analytically discussed assuming a mixture law. It was concluded that the TTSP does not hold for a multi-phase system in general but does hold for a multi-component system in which some components have the same temperature dependence and the others have no temperature dependence. On the basis of the results, the application of the TTSP to plant materials such as wood and bamboo was examined using a mixture law and a stretched-exponential function having a characteristic relaxation time τ ₀ and a stretching parameter β. Wood can be treated as a multi-phase system consisting of a framework (f) and matrix (m). In this case, it was expected that the TTSP holds for the matrix in the shorter time region t?τ _0f under T<T _gf , while the TTSP holds for the framework in the longer time region t?τ _0m under T>T _gm , where t and T _g is a measurement time and the glass transition temperature, respectively.     

Accelerated weathering of wood–polypropylene composites containing minerals

Accelerated weathering tests were carried out on wood–polypropylene composites containing minerals. Three different mineral fillers were studied: calcium carbonate, wollastonite and talc. Colour changes were evaluated after distinct periods; the total time of exposure of the composites to UV irradiation was 2000 h. The weathering resulted in significant colour fading of the composites. The composites containing mineral fillers had higher changes of colour (lightness) than the reference composite. Scanning electron microscopy analysis revealed deterioration of the polymer surface layer in all weathered composites. Exposure of the reference composite to UV irradiation resulted in the disappearance of the polypropylene surface layer and disclosure of wood fibres, which led to a higher drop in the lignin content of this composite compared to mineral-containing composites. A substitution of part of the wood with mineral fillers resulted in decreased water absorption and thickness swelling of mineral-containing composites, compared to the reference composite. Exposure to water immersion-freeze–thaw cyclic treatment and UV irradiation led to a decrease in the Charpy impact strength of the composites, except for the composite containing talc.     

Effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy

This paper examines the effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy. The processed material with an ultrafine-grained (UFG) structure (d ≈ 150 nm) was prepared by multiaxial forging. Uniaxial constant stress compression and constant load tensile creep tests were performed at 648–698 K and at stresses ranging between 300 and 600 MPa on the UFG processed alloy and, for comparison purposes, on its coarse-grained (CG) state. The values of the stress exponents of the minimum creep rate n and creep activation energy Q _c were determined. Creep behaviour was also investigated by nanoindentation method at room temperature under constant load. The microstructure was examined by transmission electron microscopy and scanning electron microscope equipped with an electron back scatter diffraction unit. The results of the uniaxial creep tests showed that the minimum creep rates of the UFG specimens are significantly higher in comparison with those of the CG state. However, the differences in the minimum creep rates of both states of alloy strongly decrease with increasing values of applied stress. The CG alloy exhibits better creep resistance than the UFG one over the stress range used; the minimum creep rate for the UFG alloy is about one to two orders of magnitude higher than that of the CG alloy. The indentation creep tests showed that annealing had little effect on the creep behaviour in UFG Ti alloy at room temperature.     

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.     

Biological performance of wood–plastic composites containing zinc borate: Laboratory and 3-year field test results

Six different formulations of wood–plastic composites (WPC) fabricated from wood and polypropylene (PP) were tested in the laboratory against decay and termites and in a protected above-ground field test in southern Japan. Variables examined included comparisons of untreated and zinc borate (ZnB) incorporated formulations, wood content ratio, wood particle size and increased surface area via surface grooves (channels) to promote moisture infusion. A standard method originally designed to test durability of solid wood was modified for testing WPC. Wood decay fungi and Formosan subterranean termite activity in laboratory and field tests resulted in different mass losses, post-decay moisture contents and field test ratings depending on their wood and ZnB content. The results show that as wood content increased, mass losses also increased. Addition of ZnB at 1% (w/w) retention level significantly decreased mass losses of wood–plastic composite when exposed to laboratory decay and termite tests. The effects of surface grooves and wood particle size were less important, compared to wood particle content. All WPC tested were highly resistant to fungal decay under protected above-ground field conditions during 36 months. Termite attack, on the other hand, started at earlier stage reducing mean ratings 1 year after the installation.     

Probing of wood–cement interactions during hydration of wood–cement composites by proton low-field NMR relaxometry

Proton NMR T ₂ relaxometry has been applied to investigate phenomena involved in wood–cement composites during hydration. The transformation of capillary pore water into hydrates and gel pore water, as well as the microstructural changes occurring in the cement matrix, was continuously monitored during the first 28 days of hydration. Water in wood and its transfer into the matrix as cement hardens were also evidenced with the method. It has been found, for example, that some of the water in the mixture is retained in wood in the form of bound or free water, depending on the initial water content. By measuring the area under the different peaks, the consumption of water during hydration can be measured and the advancement of the hydration process can be evaluated via the hydration advancement coefficient α. The cement hardening within the composite has been also studied in the presence of calcium chloride, an accelerating agent. The acceleration was clearly evidenced at the early stage of the hydration process. The influence of extractives has been evaluated by comparing the hydration behaviour of composites prepared from Eucalyptus saligna (low extractives content) and Afzelia bipendensis (high extractives content), and a new compatibility index based on NMR relaxometry measurements has been proposed.     

Investigation on withdrawal resistance of various screws in face and edge of wood–plastic composite panel

Withdrawal resistances of screws driven into commercial wood–plastic composite (WPC) panels in both face and edge directions have been measured and the results have been compared with those of conventional medium density fiberboard (MDF) and particleboard. Three types of screws namely; sheet metal screw (gauge #4, 8, 10, 14), wood screw (gauge #8) and drywall screw (gauge #8) were used. The results have indicated that withdrawal resistances of screws in WPC panels in both directions increase as screw diameter, loading rate and penetration depth increase. Similar increases were observed when pilot holes diameter were increased close to the root diameter of the screws. Beyond this limit, increasing the pilot hole diameter up to the nominal diameter of the screws, significantly reduced withdrawal resistance. No significant differences were observed between different types of screw. Face and edge withdrawal resistances of screws in WPC panels were higher as compared with those of MDF and particleboard panels.     

Heat treated wood–nylon 6 composites

Heat treatment is a relatively benign modification method that is growing as an industrial process to improve hygroscopicity, dimensional stability and biological resistance of lignocellulosic fillers. There also has been increased interest in the use of lignocellulosic fillers in numerous automotive applications. This study investigated the influence of untreated and heat treated wood fillers on the mechanical and rheological properties of wood filled nylon 6 composites for possible under-the-hood applications in the automobile industry where conditions are too severe for commodity plastics to withstand. In this study, exposure of wood to high temperatures (212 °C for 8 h) improved the thermal stability and crystallinity of wood. Heat treated pine and maple filled nylon 6 composites (at 20 wt.% loading) had higher tensile strengths among all formulations and increased tensile strength by 109% and 106% compared to neat nylon 6, respectively. Flexural modulus of elasticity (FMOE) of the neat nylon 6 was 2.34 GPa. The FMOE increased by 101% and 82% with the addition of 30 wt.% heat treated pine and 20 wt.% heat treated maple, where it reached maximum values of 4.71 GPa and 4.27 GPa, respectively. The rheological properties of the composites correlated with the crystallinity of wood fillers after the heat treatment. Wood fillers with high crystallinity after heat treatment contributed to a higher storage modulus, complex viscosity and steady shear viscosity and low loss factor in the composites. This result suggests that heat treatment substantially affects the mechanical and rheological properties of wood filled nylon 6 composites. The mechanical properties and thermogravimetric analysis indicated that the heat treated wood did not show significant thermal degradation under 250 °C, suggesting that the wood-filled nylon composites could be especially relevant in thermally challenging areas such as the manufacture of under-the-hood automobile components.