Volume 38, Issue 4

This study focus on fabricating a Cissus Quadrangularis Fiber (CQF) reinforced epoxy hybrid composite with the addition of titanium diboride (TiB 2 ) as filler. A compression molding technique was employed to fabricate the composite samples. The volume of the CQF was maintained at 30 wt%, and TiB 2 was added with various weight proportions ranging from 0 % to 10 %. The mechanical, thermal, viscoelastic, and dielectric properties of the hybrid composite samples were evaluated. Scanning electron microscopy (SEM) was used to examine the impact of filler addition on the matrix-fiber bonding of the tensile fractured test specimens. The results revealed that the composite with 8 wt% filler produced high mechanical properties and comparable dielectric properties. Based on these findings, the fabricated composites are recommended for suitable applications in the automotive, electrical, and construction industries.

In capillary rheometry of a polymer melt, the total pressure drop consists of three primary components: shear viscosity, extensional viscosity, and normal stress differences. Traditionally, viscoelastic constitutive equations have been used to produce accurate predictions regarding the pressure drop. It is difficult to investigate the primary components in past experimental measurements and numerical computations. The useful improvement of the classic White-Metzner viscoelastic constitutive equation coupled with the weighted shear/extension viscosity is performed in isothermal capillary flow simulations. It is significant to analyze the extension-induced pressure drop for a short die and the shear-induced pressure drop for a long die, along with a small contribution of normal stress differences.

The famous informed isotropic (IISO) viscosity successfully dominates the broader core structure of fiber orientation distributions for injection-molded long fiber composites, due to the flow-fiber coupling effect of stretching deformation. Recently, the GNF-X (Generalized Newtonian Fluid eXtended) constitutive equation of weighted shear/extensional viscosity has been shown to possess the potential to demonstrate the extension-induced corner vortex in the entry flow of a polymer melt. Using GNF-X, three-dimensional injection-molding flow simulations of a center-gated disk are, therefore, performed for verifying the effect of extensional viscosity on the core structure of fiber orientation, namely, the extension-induced fiber orientation. The equivalent of GNF-X and IISO is demonstrated for predicting fiber orientation distribution, while their primary parameter relationship is particularly found herein. It is significant to visualize dramatic patterns of extensional rate occurring in the center-gated disk simulation.

The rapid development of polymer blends and nanocomposites has put forward new requirements for the mixing performance of extruders. In comparison with shear flow field, extensional flow field has been shown to have unique advantages in improving dispersive mixing performance and reducing energy consumption. However, building an extensional flow field in a conventional single-screw extruder remains challenging. In this work, a new mound-shaped extensional mixing element (M-EME) was designed based on the geometric characteristics of a sine curve. To investigate the effect of this M-EME on the mixing properties of a conventional single-screw extruder, a numerical simulation analysis of this M-EME was performed. The results showed that the proportion of the region with a mixing index greater than 0.55 in the M-EME was higher than 43 %, and that the highest mixing index of the M-EME reached around 0.9, confirming the dominance of the extensional flow field in the M-EME. Moreover, it was observed that the changes in amplitude and period of the sine function have no significant effect on the distribution of the mixing index. The findings from this work provide a viable way to generate extensional flow fields in conventional screw extruders.

Three different microcapsules, namely dicyclopentadiene (DCPD)-urea formaldehyde (UF) based single-walled microcapsules, DCPD-UF-Siloxane (DCPD-UF-Si) based double-walled microcapsules and DCPD-Carbon nanotubes-UF based dual-core microcapsules were synthesized, and their corresponding self-healing composites were prepared. This paper mainly focuses on the synthesis procedure of various microcapsules and a comparative study on the effect of microcapsules over the final composite properties. The core content of the microcapsules was measured and compared with theoretical calculations. DSC & TGA analyses have shown that the novel microcapsules (DCPD-UF-Si, DCPD-CNT-UF) and their composites have better thermal stability compared to DCPD-UF microcapsules. Epoxy-carbon fiber (2 wt.%) composite specimens with three different microcapsules were tested for surface morphology, mechanical, thermal and electrical properties. SEM analysis has shown that the microcapsules have a rough outer surface and smooth inner surface. The average diameter and shell thickness of the microcapsules were measured for all types of microcapsules. Addition of double-walled and dual-core microcapsules has reduced the glass transition temperature of the composites by 10 °C. Also, SHC with DCPD-UF-Si and DCPD-CNT-UF microcapsules have shown better thermal stability (300 °C) compared to DCPD-UF microcapsules (220 °C). The incorporation of CNT based microcapsules inside the composite has also improved the electrical conductivity by 2.2 times, without compromising on self-healing efficiency (78 %). Therefore, these novel microcapsules can be potential candidates for making multifunctional polymer composites for aerospace, windmills and automotive applications.

The vulcanization reaction of ethylene vinyl acetate copolymer (EVM) rubber is fast, resulting in poor processing safety. EVM is often filled with flame-retardant fillers as insulating or sheathing material for wires and cables. Herein, the effects of flame-retardant magnesium hydroxide (Mg(OH) 2 ), aluminum hydroxide (Al(OH) 3 ) and of the traditional reinforcing fillers carbon black (CB) and silicon dioxide (SiO 2 ) on the vulcanization kinetics of EVM were investigated. The vulcanization characteristics showed that the scorch time ( T 10 ) of the unfilled EVM (KB), SiO 2 /EVM, Mg(OH) 2 /EVM, and Al(OH) 3 /EVM composites was about 1.75 min. T 10 of the CB/EVM composite was 2.22 min. Compared with KB, the activation energy ( E a ) increased by about 15 kJ/mol for CB/EVM composites and by about 5 kJ/mol for SiO 2 /EVM, Mg(OH) 2 /EVM and Al(OH) 3 /EVM composites. The results indicate that CB delays the vulcanization time of EVM rubber, slows down the rate of vulcanization reaction and improves the safety of vulcanization. The addition of SiO 2 , Mg(OH) 2 and Al(OH) 3 has little effect on the vulcanization reaction. The mechanical properties show that CB/EVM is more uniformly vulcanized and has the best mechanical properties with a tensile strength of 17.61 MPa and elongation at break of 404.58 %. Mg(OH) 2 /EVM and Al(OH) 3 /EVM samples have prominent vulcanization non-uniformity resulting in poor mechanical properties.

Natural fiber-reinforced composites must be hybridized with synthetic fibers to enhance their mechanical strength. They are appropriate for structural applications after hybridizing with more than 40 % of synthetic fibers. The hybrid composite becomes a partly biodegradable material due to the blend of natural fibers. The localized hybridization approach has been used to minimize the quantity of synthetic fiber used in hybridization. The region around the drilled hole of a composite has been identified as stress concentrated region. In that stress-concentrated region, a polyester strip of different widths is deployed to reinforce it to reduce the delamination occurrence. The present work deals with Jute Fiber reinforced unsaturated polyester resin with local hybridization of woven polyester made up of Perma Core T-18 polyester fiber strands. The interfacial shear and tensile strengths are evaluated for the proposed composite. The results revealed that the yield strength of the proposed composite increased from 18.32 MPa to 29.62 MPa due to local hybridization. The increment of 69.24 % in yield strength is more significant for structural applications.

In this study, a noncoplanar diimide diacid monomer (DIDA) was synthesized by direct condensation of trimellitic anhydride (TMA) with m-tolidine. The noncoplanar unit was incorporated into poly(amide-imide)s (PAIs) main chain by Yamazaki-Higashi phosphorylation of DIDA with various aromatic diamines. Encouragingly, all of the PAIs show good solubility in some common solvents such as N,N-Dimethylacetamide (DMAc), N-methyl-2-pyrrolidinone (NMP), and m-cresol. In addition, the soluble PAIs show good optical transmittances of beyond 85 % at 500 nm due to the decreased crystallization ability. On the other hand, PAIs possess good mechanical properties with tensile strengths of 72–90 MPa and tensile moduli beyond 2 GPa. Meanwhile, the designed PAIs also exhibit excellent thermal properties: their glass transition temperatures ( T g ) range from 278 to 314 °C, initial decomposition temperatures (5 % weight loss temperatures, T 5wt% ) are beyond 470 °C, coefficients of thermal expansion (CTE) are below 10 ppm/°C. The excellent mechanical and thermal properties are due to the strengthened hydrogen bonding interaction among the amide groups. Therefore, it is believed that incorporating noncoplanar unit and amide group into the polymer main chain at the same time can simultaneously improve processability, optical transparency, mechanical and thermal properties. Furthermore, it is worth noting that the char of PAI at 800 °C is as high as 72.5 %, which is one of the highest known values. When PAI was incorporated into epoxy resin, the blend passed UL94 V-0 rating due to the high charring capability of PAI.

The surface of an injection mold is prone to erosion and wear under the impact of the filler during plastics injection molding, which leads to premature failure of the mold. The mechanism of erosion and wear of a filler on the mold has not been clarified yet. A numerical technique was employed to simulate the erosion and wear process, and the influence of environmental parameters and particle characteristics, such as erosion velocity, erosion angle, temperature, and particle shape were studied. The results showed that the erosion velocity and erosion angle are important factors that affect erosion and wear. Finally, the relationship between change of particle energy and erosion and wear of the material is studied from the perspective of energy.