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This study focuses on the effect of carbon fibers (CFs) addition on the tribological properties of both polyoxymethylene (POM) and POM/polytetrafluoroethylene (PTFE) blends. Two kinds of CFs with different fiber length (i,e., 40 [micro]m and 1 mm) were compared. Results showed that the addition of longer CFs could effectively reduce the wear rate of pure POM and POM/PTFE composites. Both high P * V friction coefficient and mass wear rate of POM/PTFE particle/CF composites were significantly reduced when compared with the POM/PTFE particle counterparts, which is related to the easier formation of transfer film on the sliding countersurface and better load transfer of CFs. This study provides a new perspective for improving the tribological properties of engineering plastics by tailoring the geometry of functional fillers. KEYWORDS carbon fiber, high P * V, polyoxymethylene, tribological properties, 1 | INTRODUCTION There is attractive attention to replace metal or ceramic parts with self-lubricating engineering plastics due to their weight advantage, good processability, and excellent resistance to corrosive conditions, [...]

The influence of different shear stresses induced by changing injection molding speeds on molecular chain orientation and lamellar branching of [beta]-nucleated iPP/poly(ethylene terephthalate) (PET) microparts was investigated using two-dimensional wide-angle X-ray diffraction and 2D-small-angle X-ray scattering. Results indicated that the prevailing shear stress can promote the formation of parent-daughter [alpha]-crystal structure and twisted shish-kebab structure in subsequent microparts. The diffraction of (300) plane of [beta]-crystals was also observed at varying injection speeds. Increasing injection speeds can significantly enhance the content of [beta]-crystals from 24 to 41% for [beta]-nucleated iPP microparts. Additionally, the content of [beta]-crystals was further enhanced in P-nucleated iPP/PET microparts with in situ formation of PET microfibrils under intensive shearing conditions. The addition of both PET and P-nucleation agents coupling with high shearing conditions exerts a synergetic effect on the development of [beta]-crystals. However, the orientation degree of crystal lattice decreased with increasing injection speeds for both [beta]-nucleated iPP and iPP/PET microparts. KEYWORDS crystallization, microinjection molding, microstructure, polypropylene, 1 | INTRODUCTION Recently, the development of miniature devices has attracted attention as more and more functions are being integrated into small systems. [1-3] Also, microinjection molding (MIM) has become [...]

Flexible ablative materials play a key role in thermal protection systems to protect space vehicles during hypersonic flight missions. In this work, epoxy resin modified liquid silicone rubber was adopted as elastomeric matrix. Two inorganic fibers (carbon fibers (CF) and quartz fibers (QF)) and two organic fibers (aramid fibers (AF) and poly (p-phenylene benzobisoxazole) fibers (PBO)) were chosen as functional fillers. The ablation resistance and thermal insulation properties were evaluated using oxyacetylene torch test. Microstructure and phase composition of the char layer was fully characterized to investigate the ablation mechanism. SEM observations revealed that the fibers are well dispersed and integrated in the matrix. TGA analysis indicated that the inorganic fibers possess significantly higher thermal stability than organic fibers. In addition, a molten silica film can be formed on the material surface, which can exert a better thermal protection effect on the matrix. Furthermore, the ablation test shows that the linear ablation rate decreases first and then increases with an increase of fiber content. This work provides basic data to guide optimal selection of fibrous ablatives to enhance ablation performance. KEYWORDS ablation resistance, fibers, liquid silicone rubber, thermal insulation, 1 | INTRODUCTION Ablative materials are the base of thermal protection system (TPS), which supports the rapid development of the entire aerospace technology. These sacrificial materials are extensively employed to [...]

In this study, ablative resistance of single fiber‐ and hybrid fiber‐reinforced epoxy‐modified vinyl silicone rubber (EMVSR) composites was systematically studied. Three types of fibers, namely aramid fiber (AF), basalt fiber (BF), and carbon fiber (CF), were employed as the reinforcements. The results indicated that the incorporation of fiber reinforcements effectively suppressed the expansion and strengthened the char layer. Besides, the quality of char layer was quite different when different types of fibers were used. The char layer of AF‐modified samples had more surface defects, BF‐modified samples were brittle, and CF‐filled samples were dense and flat. The use of hybrid AF/BF fibers improved the ablation and anti‐erosion performance of EMVSR. Meanwhile, the thermal insulation performance was enhanced because the back‐face temperature was the lowest with the addition of 1 phr AF and 5 phr CF. The above results indicated that the ablation and anti‐erosion resistance and the quality of char layer as well as the thermal insulation behavior of EMVSR were significantly improved by incorporating fibrillar fillers, which demonstrate practical applications as flexible anti‐ablation materials for thermal protection systems. Highlights: Epoxy‐modified vinyl silicone rubber was used for preparing ablative composites.The effect of adding different fibers on the ablation properties was studied.The presence of single fibers improved the strength of char layer.The use of hybrid fibers was found effective in enhancing the ablative properties.The intrinsic property and entanglement of fibers affected the ablative properties. [ABSTRACT FROM AUTHOR]

A series of density gradient polyimide (PI) aerogels were prepared using layerby-layer assembly and radial freezing method. Briefly, a layer of poly(amic acid) (PAA) ammonium salt aqueous solution was first radially frozen. Then, the second layer of PAA ammonium salt (PAS) aqueous solution of different concentration was added on the top of the first PAS layer and radially frozen. A multilayer gradient PAS solid sample could therefore be fabricated by repeating this similar procedure. The density gradient PI aerogels were obtained after freezing drying and final thermal imidization treatment. Each layer of gradient PI aerogels had anisotropic pore structure, which consisted of tube-like pores along the radial direction and toward the center axis of the cylindrical samples. The compressive strength of five-layer gradient PI aerogel was higher than that of three-layer gradient and single-layer PI aerogels with the same density. The gradient PI aerogels exhibited anisotropic heat transfer behavior in the direction of density gradient, and heat transfer from the higher density side to the lower density side was faster. KEYWORDS anisotropic, density gradient, layer-by-layer assembly, polyimide aerogels, 1 | INTRODUCTION Materials have always played an irreplaceable role in the development of human society and the advancement of science. Since the beginning of new century, the intersections between [...]

In this work, the influence of polytetrafluoroethylene (PTFE) filler size and testing conditions (i.e., air, water, and lubricating oil) on the tribological properties of poly(arylene ether nitrile) (PEN) was systematically investigated. The results showed that the addition of PTFE was beneficial to improve the tribological properties of PEN‐based composites which was related to the easier formation of transfer film on the surface of friction pair. Samples which were tested in water demonstrated a relatively higher friction coefficient (μ) and wear loss when compared with those tested in dry air and lubricating oil scenarios, which was attributed to the fact that friction induced heat and wear debris could be timely removed by water. In addition, the infiltration of water further reduced the interaction between PTFE filler and PEN, which aggravated the wear loss of sample blocks. When tested in lubricating oil, pure PEN showed the lowest wear loss when compared with that of PEN/PTFE composites. At a given content (20 wt%) of PTFE fillers, PEN/PTFE1.5μm exhibited the lowest μ in lubricating oil whereas PEN/PTFE5μm demonstrated the lowest specific wear loss in air condition (1.18 × 10−6 mm3/N·m). This work provided some useful information for the design and application of PTFE‐containing polymer composites that can be targeted in different lubrication scenarios in industrial fields. [ABSTRACT FROM AUTHOR]

In this work, transient thermal response and ablation behavior of liquid silicone rubber composites containing fluxing/ceramic forming fillers were investigated under different heat flows using an oxyacetylene flame. The results indicated that the introduction of zinc borate (ZB) and aluminum oxide (Al2O3) effectively reduced the temperature at various depths of the samples, and they improved the thermal insulation properties and lowered pyrolysis rates. The above finding was attributed to the heat absorption arising from water release and melt filling as well as the vitrified reaction of solid melt due to the decomposition of ZB. Besides, the melting and exfoliation of Al2O3 and the formation of aluminum silicate (Al2SiO5) caused heat absorption effect. Additionally, the mass ablation rates and line ablation rates increased with rising heat flows coupling with a decrease of compressive strength of the char layers. In a nutshell, the effect of adding ZB/Al2O3 on the thermal insulation behavior of epoxy‐modified vinyl silicone rubber (EMVSR) composites under different heat flows was elucidtaed. This work served as a reference for the design and preparation of flexible ablative materials for thermal protection applications. [ABSTRACT FROM AUTHOR]

In this study, polypropylene (PP) composites filled with two different types of graphite particles, that Is, flake-shaped synthetic graphite (SG) and low-temperature expandable graphite (LTEG), were prepared by melt blending, followed by microinjection molding ([micro]IM). The microparts had three consecutive zones with decreasing thickness along the flow direction (FD). Results showed that, in addition to the larger particle size, the in situ exfoliation of LTEG during melt processing is crucial to the overall enhancement of electrical conductivity when compared with their SG-containing counterparts, as corroborated by morphology observations. Moreover, the preferential alignment of conductive particles favors the construction of conductive pathways along the FD. The melting and crystallization behavior for PP, PP/LTEG, and PP/SG materials, and samples from each section of the corresponding microparts were evaluated by differential scanning calorimetry. Results indicated that both the addition of graphite particles and the typical thermomechanical history of nlM (i.e., high shearing and cooling rates) experienced in different sections of the three-step microparts influence the melting and crystallization behavior of the composites., INTRODUCTION Recently, there has been growing interest in microparts of functional filler-loaded polymer composites due to their potential applications in electronics and microelectromechanical systems. Therefore, several microprocessing methods, such as [...]

Lightweight and anisotropic porous carbon materials have promising applications in thermal insulation and electromagnetic interference (EMI) shielding fields. Herein, lightweight polyimide foams (PIFs) with anisotropic pore structures were prepared by adopting a microwave-assisted foaming method and the obtained PIFs were employed as derivatives to fabricate carbon foams, i.e., PICFs. The PICFs with aligned ellipsoidal strip-like pore structures presented low volumetric shrinkage (44.87–50.24%) and weight loss (36.33–48.17%), along with favorable compressive strength (102.96 kPa), and high temperature thermal resistance. In addition, the anisotropic pore structure imparted efficient thermal insulation and infrared thermal stealth properties to PICFs, with thermal conductivity as low as 0.0356 W (m K)−1 along the horizontal direction (perpendicular to the pore growth direction). When carbonized at 1400 °C, PICF1400 with a low density (22.9 kg m−3) and thin thickness (1.9 mm) exhibited a high electrical conductivity of 23.91 S m−1 in the horizontal direction. The EMI shielding effectiveness (SE) and specific EMI SE reached as high as 53.2 dB and 12216 dB (g cm−2), respectively. Moreover, PICF800 exhibited a high absorption coefficient of 0.91. Therefore, a facile strategy was proposed for fabricating lightweight PICFs which possess multifunctional properties enabling potential applications in high-end engineering sectors. [ABSTRACT FROM AUTHOR]