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The present study examined the effect of a thin adhesive layer on the modulus of an elastic rubber block bonded between two plates. The plates were assumed to be rigid, both in extension and flexure, and subjected to vertical compression loading. The Gent's approach was used to obtain the analytic deformations of the rubber and adhesive. The analytic deformations were then validated with the finite element model. There was a good agreement between both methods. The modulus of the bonded rubber block, defined as effective modulus, was then studied. The effective modulus was increased by the factor (1+ (a/2t)²(6Grh/Gat+1)-1), which is composed of the shape factor of the rubber block (a/2t, ratio of the bonded and unbonded areas), and the shear stiffness factor (Grh/Gat, ratio of modulus and thickness of rubber and adhesive). The effective modulus does not depend on either factors, when the shear stiffness of the joint is high or Grh/Gat ≥ 10. [ABSTRACT FROM AUTHOR]

Conductive polymer composites are ubiquitous in technological applications and constitute an ongoing topic of tremendous commercial interest. Strategies developed to improve the level of electrical conductivity achieved at a given filler concentration have relied on double-percolated networks induced by immiscible polymer blends, as well as mixtures of fillers in a single polymer matrix, to enhance interparticle connectivity. In this work, we combine these two strategies by examining quaternary composites consisting of high-density polyethylene (HDPE), ultrahigh molecular weight polyethylene (UHMWPE), graphite (G) and carbon fiber (CF). On the basis of our previous findings, we examine the electrical conductivity, morphology, thermal signature and mechanical properties of HDPE/UHMWPE/G systems that show evidence of double percolation. Upon addition of CF, tremendous increases in conductivity are realized. The mechanism by which this increase occurs is termed bridged double percolation to reflect the role of CF in spanning non-conductive regions and enhancing the continuity of conductive pathways. At CF concentrations above the percolation threshold concentration, addition of G promotes increases in conductivity and dynamic storage modulus in which the conductivity increases exponentially with increasing modulus. [ABSTRACT FROM AUTHOR]

The development of conductive polymer composites remains an important endeavor in light of growing energy concerns. In the present work, graphite (G), carbon fiber (CF) and G/CF mixtures are added to high-density polyethylene (HDPE) to discern if mixed fillers afford appreciable advantages over single fillers. The effects of filler type and composition on electrical conductivity, composite morphology and mechanical properties have been examined and correlated to establish structure–property relationships. The threshold loading levels required for G and CF to achieve measurable conductivity in HDPE have been identified. Addition of CF to HDPE/G composites is found to increase the conductivity relative to that of HDPE/G composites at the same filler concentration. This observed increase depends on CF length and becomes more pronounced at and beyond the threshold loading of the HDPE/G composite. Scanning electron microscopy is employed to elucidate the morphology of these multicomponent composites, whereas dynamic mechanical analysis reveals that filler concentration, composition and CF length impact both the magnitude and temperature dependence of the dynamic storage modulus. [Copyright &y& Elsevier]

The present study aims to investigate the moisture absorption of polypropylene (PP)/rubberwood flour (RWF) composites and its effects on dimensional stability. The compositions included different grades of plastic, and the amounts of wood flour, maleic anhydride-grafted polypropylene (MAPP), and ultraviolet (UV) stabilizer were varied. The composite materials were manufactured into panels by a twin-screw extruder. Long-term water absorption (WA), long-term thickness swelling (TS), and degradation of flexural properties of the composites were studied for a range of water immersion times. The WA and TS of the samples increased with RWF content and immersion time. Recycled PP gave higher WA and TS than virgin PP, for the composites with 45 wt% RWF. Increasing MAPP content from 3 to 5 wt% had no significant effect on WA and TS, whereas the addition of 1 wt% UV stabilizer increased them. A MAPP content of 3 wt% is recommended for moisture resistance, while the amount of UV stabilizer should be kept as low as possible. Flexural strength and modulus of composites also decreased with moisture uptake; however, <3% WA did not significantly affect the flexural strength. In contrast, the maximum strain of composites consistently increased with WA. [ABSTRACT FROM AUTHOR]

The mechanical properties of composites from recycled waste plastic and waste sawdust are of interest in trying to convert these waste streams to useful products. The development of these composites from natural fiber is therefore receiving widespread attention due to the growing environmental awareness. The effects of compositions were investigated including different grades of plastic (virgin and recycled) and amounts of wood flour, coupling agent, and ultraviolet (UV) stabilizer on mechanical and physical properties of polypropylene/rubberwood flour (RWF) composites. Virgin polypropylene gave better mechanical properties than recycled (recycled polypropylene (rPP)), both in composites and as unfilled plastic. RWF content exceeding 25 wt% enhanced the strength of RWF-reinforced rPP composites. The modulus and hardness of composites increased linearly with wood flour loadings. Maleic anhydride-grafted polypropylene (MAPP) as a coupling agent increased the strength, modulus, and hardness of the composites. However, addition of 1 wt% UV stabilizer degraded the mechanical properties. Therefore, 4 wt% MAPP content is recommended to achieve good mechanical properties of rPP/RWF composites, while the amount of UV stabilizer should be as small as possible to avoid its negative influence. [ABSTRACT FROM AUTHOR]

Composites of rubberwood flour (RWF) and recycled polypropylene (rPP) were produced into panel samples by using a twin-screw extruder. The effects on creep behavior of mixture fractions of rPP, RWF, maleic anhydride-grafted polypropylene (MAPP), and ultraviolet (UV) stabilizer were studied in a D-optimal mixture design. Creep was significantly affected by the composition. Increasing the fraction of RWF decreased creep, while MAPP and UV stabilizer increased it. The models fitted were used to optimize a desirability score that balanced multiple creep characteristics. The model-based optimal formulation 50.5 wt% rPP, 44.9 wt% RWF, 3.5 wt% MAPP, 0.1 wt% UV stabilizer, and 1.0 wt% lubricant was experimentally validated to have low creep closely matching the model predictions. [ABSTRACT FROM PUBLISHER]

The effects of time, temperature, and stress on the flexural creep of composites from recycled polypropylene (rPP) and rubberwood flour (RWF) were experimentally investigated and numerically modeled. Creep of rPP/RWF composites increased with an increase of time, temperature, and stress. A critical temperature of rPP composites containing 44.5wt% RWF is 65°C. Burger, Power law, and HRZ models fit the creep profiles well in general, but at high temperature and stress levels the Power law and HRZ models performed poorly. However, the HRZ model interpolated creep well across the applied stresses, or across the temperatures. The time-temperature superposition (TTS) and time-stress superposition (TSS) principles were used to model long-term creep. The master curves from TTS and TSS principles were in good agreement with each other. They predicted that the lifetime limitation by long-term creep exceeds 10years for 15MPa stress at 25°C. All these results pertain to a specific formulation of rPP/RWF composites. [ABSTRACT FROM AUTHOR]

Highlights: [•] Composites based virgin PP had smaller loss of MOR and MOE than based recycled PP. [•] Composites with 1wt% UV stabilizer had smaller loss of MOR and MOE than without. [•] Increase of rubberwood flour content in composites increased loss of MOR and MOE. [Copyright &y& Elsevier]

D-optimal mixture experimental design was used to determine the optimal mixture of composites from rubberwood (Hevea brasiliensis) flour and recycled polypropylene and to systematically analyze the effects of composition, namely recycled polypropylene, rubberwood flour, maleic anhydride-grafted polypropylene, and ultraviolet stabilizer fractions. Panel samples were extruded, and their properties were characterized. The overall compositions significantly affected water absorption, thickness swelling, flexural strength and modulus, and maximum strain. Water absorption and thickness swelling increased with the fraction of rubberwood flour. At long immersion times, flexural strength and modulus decreased, but maximum strain increased with high fraction of rubberwood flour. The fraction of maleic anhydride-grafted polypropylene only slightly affected water absorption and flexural properties, while the ultraviolet stabilizer fraction had a clear negative effect increasing water absorption and decreasing flexural properties. The models fitted were used for optimization of a desirability score, substituting for the multiple objectives modeled. The optimal formulation found was 68.9 wt% recycled polypropylene, 25.0 wt% rubberwood flour, 5.0 wt% maleic anhydride-grafted polypropylene, 0.1 wt% ultraviolet stabilizer, and 1.0 wt% lubricant. This formulation of the composites can be used for most suitable applications based on the moisture resistance. [ABSTRACT FROM PUBLISHER]

Abstract: A mixture design was used in experiments, to determine the optimal mixture for composites of rubberwood flour (RWF) and reinforced recycled polypropylene (rPP). The mixed materials were extruded into panels. Effects were determined of the mixture components rPP, RWF, maleic anhydride-grafted polypropylene (MAPP), and ultraviolet (UV) stabilizer, on the mechanical properties. The overall composition significantly affected flexural, compressive, and tensile properties. The fractions of recycled polypropylene and rubberwood flour increased all the mechanical material properties; however, increasing one fraction must be balanced by decreasing the other, and the rubberwood flour fraction had a higher effect size. The fraction of MAPP was best kept in mid-range of the fractions tested, while the UV stabilizer fraction overall degraded the mechanical properties. Our results suggest that the fraction of UV stabilizer should be as small as possible to minimize its negative influences. The models fitted were used for optimization of a desirability score, substituting for the multiple objectives modeled. The optimal formulation found was 50.3wt% rPP, 44.5wt% RWF, 3.9wt% MAPP, 0.2wt% UV stabilizer, and 1.0wt% lubricant; the composite made with this formulation had good mechanical properties that closely matched the model predictions. [Copyright &y& Elsevier]

Excessive walking and running can cause foot injuries due to continuous high impact force and pressure on the foot sole, especially in the heel and the forefoot regions. This problem can be relieved with insoles made of various cushioning materials. The cushioning pads of insoles at forefoot and heel regions significantly influence plantar pressure and reduce the forces during walking and running. However, no studies have compared the mechanical properties of different cushioning materials as regards decreasing the plantar pressure and forces during these activities. The effects of different cushioning materials on the reduction of force and pressure at forefoot and heel regions, especially during running, were compared in this study. Mechanical properties of insoles for compression, hardness and cushioning were evaluated and analyzed. Moreover, the plantar pressures measured by the Pedar X system were collected and compared among volunteers. It was found that the EVA additional pad improves the cushioning by the insole. However, an insole made of PE was the best pad for reducing plantar pressure during both walking and running activities. In addition, the relationship between plantar pressure reduction and compressive property of insoles was found. [ABSTRACT FROM AUTHOR]

Flexible thermoplastic elastomers (TPE) were prepared for fused deposition modeling (FDM) or 3D printing. These materials can be used for medical purposes such as disposable soft splints and other flexible devices. Blends of 50% epoxidized natural rubber (ENR-50) and block rubber (Standard Thai Rubber 5L (STR5L)) with polycaprolactone (PCL) were produced and compared. The purpose of this study was to investigate the properties of natural rubber (NR) and PCL in simple blends with PCL contents of 40%, 50%, and 60% by weight (except at 75% for morphology study) in the base mixture (NR/PCL). The significant flow factors for FDM materials, such as melting temperature (Tm) and melt flow rate (MFR), were observed by differential scanning calorimetry (DSC) and via the melt flow index (MFI). In addition, the following mechanical properties were also determined: tensile strength, compression set, and hardness. The results from DSC showed that the melting temperature changed slightly (1–2 °C) with amount of PCL used, and there was a suspicious point in the 50/50 blends with both types of rubber. The lowest melting enthalpy of both blends was found at the 50/50 blended composition. The MFI results showed that PCL significantly affected the melt flow rate of both blends. The ENR-50/PCL blend flowed better than the STR5L/PCL blend. The conclusion was that this was due to the morphology of its phase structure having better uniformity than that of the STR5L/PCL blend. In compression set testing or measuring shape recovery, rubber directly influenced the recovery in all blends. The ENR-50/PCL blend had less recovery than the STR5L/PCL blend, probably due to the functional effects of epoxide groups and polarity mismatch. The hard phase PCL significantly affected the hardness of samples but improved shape recovery of the material. The ENR-50/PCL blend had better tensile properties than the STR5L/PCL blend. The elongation at break of both blends improved with a high rubber content. Hence, the ENR-50/PCL blend was superior to STR5L/PCL for printing purposes due to its better miscibility, uniformity, and flow, which are the keys to success for optimizing the fused deposition modeling conditions as well as the overall mechanical properties of products. Most blends in this study were only slightly different, but the 50/50 blend of ENR-50/PCL seemed to be near optimal for 3D printing. [ABSTRACT FROM AUTHOR]

Impact force remains the primary cause of foot injury and general discomfort with regard to footwear. The footwear industry traditionally relies on modified elastomers (including natural rubber) whose properties can be physically adjusted by varying the constituents in the rubber formulations. This work aims to investigate the effect of filler/plasticizer fractions on shock attenuation of natural rubber soles. The statistical response surface method (RSM) was used to optimize the loading of natural rubber, fillers (carbon black and china clay) and a plasticizer (paraffinic oil). A novel predictive equation addressing the effects of additives on the physical and mechanical properties of the shoe sole was successfully created using the RSM. Our results demonstrate how the concentrations of these components regulate final properties, such as impact force absorption and hardness, in the commercial manufacture of shoe soles. While a higher loading level of plasticizer promotes reductions in hardness and impact force, as well as energy dissipation, in these modified elastomers, these properties were improved by increasing the filler content. [ABSTRACT FROM AUTHOR]