Volume 36, Issue 2

The objectives of this study were to examine the effects of fiber content and extrusion parameters on polybenzimidazole (PBI) fiber-reinforced polyethylene composites and to determine the optimum values for the tensile strength. The PBI fiber was physically mixed with high density polyethylene (HDPE) and then extruded through a twin screw extruder. The extrusion parameters were studied at different levels, barrel temperatures at 240°C, 250°C and 260°C and screw speeds at 12 rpm, 15 rpm and 18 rpm. The tensile strength was measured using a universal testing machine. A response surface experimental design using Design-Expert was applied to investigate the effect of fiber loading and extrusion parameters (barrel temperature, screw speed) on tensile properties of the resulting composite and consequently analyzing the optimized value for these parameters to yield maximum tensile strength. The analysis predicted a linear model which suggests that in order to achieve maximum tensile strength the screw speed should be 18 rpm, the barrel temperature at 240°C and at a fiber loading of 2%.

In this study, we investigated the use of ultrasonic technology in assisted injection molding and mold designs. We used an ultrasonic device installed in a mold to vibrate a melt directly, thereby converting kinetic energy into thermal energy. In addition, we developed three flat specimens of different thicknesses (3, 1, and 3-1-3 mm) produced by ultrasonic-assisted injection molds. An ultrasonic oscillation device 45 mm in diameter was placed in the cavity and used to vibrate a polycarbonate or a polycarbonate with 30% glass fiber melt at a frequency of 20 kHz. Furthermore, cavity pressure sensors were positioned at the front and rear of the vibration region for analyzing the melt flow behavior under ultrasonic-assisted injection molding conditions. Because of the absorption of ultrasonic energy, local heat was generated inside the resin, thus forming an oscillatory flow during the packing and holding stages, improving the flow characteristics of the melt, and changing the melt flow behavior around the skin layer to reduce the molecular orientation and high shear effect. The freezing rate of the melt was also reduced to eliminate the glass fiber streaks, floating fibers, and fiber orientation, particularly for thinner parts; the hesitation phenomena were then improved to increase the weld line strength.

In this study, the influence of die opening gap on foam attributes during a microcellular extrusion foaming process was investigated. Lower die openings developed higher pressure drops on the foams, as a result of which greater thermodynamic instability was stimulated and, consequently, higher cell density foams along with enhanced expansion ratios were achieved. Further investigations were performed to study the synergistic influence of altering die opening with critical process parameters, namely, screw rotational speed and die temperature, on the foam expansion ratio and morphological transformations. Higher screw rotational speed induced shear nucleation phenomenon, which further enhanced the foaming process significantly. Also, an optimum die temperature was observed, which developed maximum expansion ratio at the lowest die opening gap. This study intends to enhance the understanding of extrusion foam processing among academia as well as among industries.

The fluid-assisted co-injection molding (FACIM) process can be used to produce hollow plastic products with outer and inner layers. It can be divided into two categories: water-assisted co-injection molding (WACIM) and gas-assisted co-injection molding (GACIM). An experimental study of penetration interfaces in overflow FACIM was carried out based on a lab-developed FACIM system. High-density polyethylene and polypropylene were used as the outer layer and inner layer plastics, respectively, in the experiments and the injection sequence was reversible. Six cross-section cavities were investigated in the experiments. The penetration behaviors of water and gas in different sequences and cavities were compared and analyzed. The penetration interfaces were characterized by the residual wall thickness (RWT). The experimental results showed that the RWT of the inner layer in WACIM fluctuated along the flow direction, while that in GACIM was more even. The difference of viscosity between the outer and inner layer melts affected the stability of the interface between them. The penetration sections of the inner layer and the gas were closer to the cavity sections in GACIM, while the penetration sections of the inner layer and the water were closer to the circular forms in WACIM.

A series of latex interpenetrating polymer networks (LIPNs) with various cross-linking levels were prepared with polystyrene-polybutadiene-polystyrene (SBS) and butyl methacrylate (BMA) by emulsion polymerization. The gel content of the resultant latex particles ranged from 13.2% to 90.8%, with the amount of cross-linking agent increased. The structural, thermal and rheological properties of the latex particles were investigated by laser particle analysis, transmission electron microscopy, Fourier transform infrared spectroscopy, gel permeation chromatography, thermogravimetric analysis, differential scanning calorimetry and torque rheometry. Furthermore, in order to study the effect of gel content on the flow properties, a model is shown to predict the existence of a flexibility-rigidity transition of SBS/PBMA LIPNs as the amount of cross-linking agent increased. Torque data were interpreted in terms of shear rate, shear stress and viscosity. The relationship between gel content of SBS/PBMA LIPNs and polyvinyl chloride compositions and their effect on plasticization performance were determined, and the predictions of the model have good agreement with their flexibility-rigidity transition.

The present study describes the preparation of aligned unidirectional bagasse fiber-reinforced vinyl ester (BFRVE) composites and their mechanical properties such as tensile, flexural, shear and impact strength. Composites were prepared by a hand lay-up technique developed in our laboratory with the help of a hot press. Mechanical properties were obtained for different fiber contents by varying the number of layers. The obtained tensile property values were compared with the theoretical results. The results show that the tensile strength increased linearly up to 44 wt% and then dropped. However, the tensile modulus increased linearly from 17 wt% to 60 wt%. In the case of flexural properties, the flexural strength increased up to 53 wt% and started to decrease. However, the flexural modulus also increased linearly up to 60 wt%. The impact strength values were higher than the matrix materials for all the specimens. The short beam shear strength values were also increased up to 53 wt% and then dropped. The modified Bowyer and Bader (MBB) model followed by the Hirsch model shows a very good agreement with experimental results in both tensile strength and modulus.

This paper deals with the experimental characterization of a bulk adhesive fracture in the quasi-static range using a conventional machine and at high strain rates using the Hopkinson bar technique. For very high strain rates, the direct impact Hopkinson bar technique is used. The studied adhesive is a toughened epoxy which can be used for impact resistance in bonded assemblies. It is worth studying the adhesive sensitivity to strain rate as a bulk material in order to distinguish between its response within a structure when used to bond substrates and its response as pure material. Experiments were carried out under compression and shear loadings. A special grip was conceived to mount the specimen for shear tests. In both loading types, this adhesive was found to be highly strain-rate-sensitive. The adhesive strength increases with strain rate until reaching a maximum value then it drops for very high strain rates. Although this behavior is uncommon, it has been reported already in three previous works.

Damping and vibration characteristics of basalt-aramid/epoxy hybrid composites with different basalt/aramid fiber mixing ratios were investigated. Unidirectional basalt and twill weave aramid fibers were used as reinforcement. Non-hybrid basalt/epoxy and aramid/epoxy composite laminates were also fabricated for comparison. Dynamic characteristics of the composite laminates were determined experimentally using dynamic modal analysis. Damping properties were calculated with the logarithmic decrement method using a vibration response envelope curve. Loss modulus, storage modulus and damping ratio of the structures were also considered. It was observed that the results of hybrid configurations showed a distribution between non-hybrid basalt/epoxy and aramid/epoxy composites. Furthermore, the employment of aramid fibers in composite laminates enhances the damping properties of laminates, but reduces the strength values.

Poly(lactic acid) (PLA) based composite films with different content of talc (5–15 wt%) were prepared by the solvent casting method. The effect of talc on morphological, structural, thermal, barrier and mechanical properties of neat PLA was investigated. The PLA/talc composites revealed a polymorphic crystalline structure, as demonstrated by X-ray diffraction (XRD) study and differential scanning calorimetry (DSC) analysis. The PLA/talc composites also exhibited significantly improved barrier properties (up to 55% compared to neat PLA), as shown by water vapor permeability (WVP) tests. The puncture measurements showed improved mechanical properties at lower content of talc (up to 5 wt%), and increased brittleness of the PLA/talc composite films at higher talc concentrations.

This work is dedicated to polyester urethane (PEU)-based biocomposites, with special focus placed on techniques for compatibilisation to heighten interfacial adhesion between the PEU matrix and flax fibres. Tests were conducted on the effects of modification so as to increase interfacial adhesion between the flax fibres and the polyester matrix. These tests involved a commercial silane-based compatibilising additive, two experimentally synthesised agents, oleic acid (OA) and di-tert-butyl peroxide (DTBP). Furthermore, the flax fibres underwent acid or alkali treatment. The biocomposites were characterised by gel permeation chromatography, infrared spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). Mechanical properties were investigated through tensile testing. Biocomposites with a commercial silane-based additive and synthesised agent, based on maleic-anhydride, were assessed as the best solution. Nevertheless, all modifications, excluding alkali treatment of fibres, significantly increased the performance of the material.

Ethylene-propylene-diene-monomer (EPDM) is one of the most frequently used outdoor insulating materials. Like other polymeric materials, EPDM and its composites also degrade in outdoor applications. For evaluation of aging of insulating materials, accelerated multistress aging/weathering is an efficient method. In the current paper, comparative multistress aging performance investigation of neat EPDM (NE), EPDM with 20% microsilica (microcomposite, EMC), EPDM with 5% nanosilica (nanocomposite, ENC) and EPDM 20% microsilica 5% nanosilica (hybrid composite, EHC) is presented. Materials prepared according to ASTM D3182-07 were subjected to uniform ultraviolet (UV) radiation, heat, humidity, salt fog and acid rain at 2.5 kV voltage for 5000 h in a specially fabricated weathering chamber. For timely analysis of the aged materials, Fourier transform infrared (FTIR) spectroscopy, Swedish Transmission Research Institute (STRI) hydrophobicity classification and scanning electron microscopy (SEM) were used along with the critical visual inspection. Increasing discoloration was found in all materials which was proportional to% wt of the filler in composites. Improved hydrophobicity and better surface smoothness was recorded in composites as compared to NE, which was higher in the case of the microcomposite and hybrid composite. FTIR results showed least reduction in hydrocarbon bonds and lowest variation in chalking index in the case of the microcomposite.

This study aims to fabricate far-infrared (FIR)/electromagnetic shielding composite fabric and its composite yarn. Five types of composite yarns with different sheath components were made by using bamboo charcoal (BC) fibers, phase change material (PCM) roving and stainless steel (SS) fibers via a ring spinning frame, and then fabricated into five elastic warp-knitted fabrics with different weft yarns using a crochet knitting machine. The mechanical properties of different constituents of composite yarns and their fabrics, as well as FIR emissivity and electromagnetic shielding effectiveness (EMSE) of resulting fabrics were evaluated. The results show that BC/SS composite yarns and their fabricated warp-knitted fabrics display the highest tensile strength. Warp-knitted fabrics containing BC fibers possess the highest FIR emissivity. EMSE of the fabricated warp-knitted fabrics improves proportionally with the number of the lamination layers. The resulting multifunctional elastic knitted fabrics apply as athletic clothing, underwear, socks, protective or healthcare products in the future.