Thermal and Mechanical Properties of Epoxy/Carbon Fiber Composites Reinforced with Multi-walled Carbon Nanotubes

Multi-scale hybrid composite laminates of epoxy/carbon fiber (CF) reinforced with multi-walled carbon nanotubes (MWCNTs) were fabricated in an autoclave. For laminate fabrication, 0.5 wt% of pristine MWCNTs or silane-functionalized MWNCTs (f-MWCNTs) were dispersed into a diglycidyl ether of bisphenol-A epoxy system and applied on the woven carbon fabric. The neat epoxy/CF composite and the MWCNTs-reinforced epoxy/CF hybrid composites were characterized by thermogravimetric analysis (TGA), thermomechanical analysis (TMA), tensile testing, and field emission scanning electron microscopy (FE-SEM). A significant improvement in initial decomposition temperature and glass transition temperature of epoxy/CF composite was observed when reinforced with 0.5 wt% of f-MWCNTs. The coefficient of thermal expansion (CTE), measured by TMA, diminished by 22% compared to the epoxy/CF composite, indicating an improvement in dimensional stability of the hybrid composite. No significant improvement in tensile properties of either MWCNTs/epoxy/CF composites was observed compared to those of the neat epoxy/CF composite.     

Relative Entropy Method Applied for the Fatigue Life Distribution of Carbon Fiber/Epoxy Composites

This paper verifies the feasibility of the relative entropy method in selecting the most suitable statistical distribution for the experimental data, which do not follow an exponential distribution. The efficiency of the relative entropy method is tested through the fractional order moment and the logarithmic moment in terms of the experimental data of carbon fiber/epoxy composites with different stress amplitudes. For better usage of the relative entropy method, the efficient range of its application is also studied. The application results show that the relative entropy method is not very fit for choosing the proper distribution for non-exponential random data when the heavy tail trait of the experimental data is emphasized. It is not consistent with the Kolmogorov–Smirnov test but is consistent with the residual sum of squares in the least squares method whenever it is calculated by the fractional moment or the logarithmic moment. Under different stress amplitudes, the relative entropy method has different performances.     

Effects of Aramid Fiber and Polytetrafluoroethylene on the Mechanical and Tribological Properties of Polyimide Composites in a Vacuum

Polyimide composites filled with aramid fiber (AF) and polytetrafluoroethylene (PTFE) were prepared by hot press molding. The thermal, mechanical, and tribological properties of the composites were studied systematically. The friction and wear behavior, sliding against GCr15 steel balls, were evaluated in a ground-based wear in space simulation facility using a ball-on-disk tribosystem. The morphologies of the worn surfaces during the sliding process of the composites were analyzed by scanning electron microscopy to reveal the wear mechanism. It was found that the heat-resisting performance and the hardness of the composites were minimally affected by the additives. The flexural strength of polyimide/AF/PTFE (PI-3) decreased when PTFE was added. The wear resistance increased and the coefficient of friction decreased due to the effect of both fillers. In vacuum, the friction coefficients of polyimide (PI-1), polyimide/AF (PI-2), and PI-3 increased slightly with sliding velocity, while the opposite results were obtained in air. With the increase of air pressure the friction coefficients of the samples increased.     

Effect of Processing Parameters on the Interlaminar Shear Strength of Carbon Fiber/Epoxy Composites

The objective of this work was to improve the interlaminar shear strength of carbon fiber/epoxy composites by determining the effect on it of the processing parameters of the cured composites system, i.e., temperature, content of curing agent, and heating rate. Taguchi methodology and analysis of variance were applied for optimizing and statistically determining the significant factors that influenced the mechanical properties of the composites. It was found that the temperature and content of curing agent were equally the primary significant factors in controlling the interlaminar shear strength of the composites. Also, the correlation between water absorption and mechanical properties of the composites was investigated.     

Effects of plasma surface modification on the mechanical properties of carbon fiber and carbon fiber/epoxy composite

_The effect of surface treatment on mechanical properties of carbon fibers has been investigated by application of plasma polymerization of selected monomers in the vapor phase. The role of the fiber-matrix interface on carbon fiber-reinforced epoxy resin composites has also been studied. Composites have been prepared separately by the use of plasma-modified and unmodified carbon fibers in the epoxy resin matrix. The mechanical properties of carbon fibers (Hercules and Grafil) as well as of fiber/epoxy composites were examined by using single filament and three-point bending tests, respectively. It was observed that plasma polymerization treatment at selected plasma conditions led to significant improvement of interlaminar shear and flexural strength values of composites.     

The Mechanical and Tribological Properties of Aramid (Twaron) Fabric/Polytetrafluoroethylene Composites

A series of composites with Twaron fabric as reinforcement and polytetrafluoroethylene (PTFE) as matrix were fabricated with various contents of PTFE, viz. 30, 40, 50, 60, and 70 vol%. The Rockwell hardness and tensile strength of the composites were tested according to the corresponding standards. The composites were also evaluated for their tribological behaviors on an MPX-2000A friction and wear tester. The worn surface and wear debris of the composites were observed by scanning electron microscopy (SEM) and the mechanism is discussed. The PTFE content in the composites had a great influence on both the mechanical and tribological properties. The composite with 40 vol% PTFE provided the proper wetting of the fibers and the best load transfer efficiency and, hence, showed the best mechanical properties and tribological behaviors.     

Fabrication and Flexural Properties of Bagasse Fiber Reinforced Biodegradable Composites

Biodegradable composites made from bagasse fiber and biodegradable resin were fabricated and the flexural properties of the composites investigated in terms of the effects of fiber length, fiber volume fraction, and different alkali treatments of the bagasse fibers. The flexural properties of the composites increased with the increase in fiber length but decreased below the critical fiber length. The flexural properties increased with the increase in fiber volume fraction. The scanning electron microscope (SEM) micrographs showed that compression of the cellulose structure of bagasse fiber after preparation could have caused enhancement in the flexural properties. Furthermore, when comparing the effects of different alkali treatments of the bagasse fibers, maximum improvement in the flexural properties was observed for the 1% NaOH solution treated fiber composites. After alkali treatment, fibrillation occurred and the surface of the treated fibers became finer; this could contribute to improvement in the fiber‐matrix adhesion and result in enhancing the flexural properties.     

Surface and core modification of carbon fibers

Thermal and thermochemical treatments with amines have been shown to improve both the mechanical strengths of carbon fibers and their interfacial bonding forces with polymer matrix. In this report, the wettabilities of the surface-treated carbon fibers were studied by dynamic contact angles with epoxy resin. Two mechanisms occur during the fiber modification processes, namely, the fiber core restructuring and surface etching. The former occurs at elevated temperature and increases the fiber strength significantly. When surface reaction occurs, the number of surface oxides increases to improve the surface wettabilities, which may be observed by the decreased dynamic contact angles. Accompanying the etch-cleaning effects, the fiber strength would also increase initially. Deeper penetration of amine would destroy the graphite crystallinity on fiber surfaces and lower the fiber strength.     

Study on the Structure and Properties of UHMWPE/Epoxy Resin Composite Fiber

The preparation, crystallization behavior, and fiber structure and properties of ultrahigh molecular weight polyethylene (UHMWPE) epoxy resin composite fiber were studied by means of differential scanning calorimeter (DSC), X‐ray diffraction (XRD), Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and tensile testing. The morphology showed a different behavior from pure polyethylene (PE) fiber. The fiber mechanical properties, creep behavior, and thermal properties of UHMWPE fiber can be improved by adding epoxy resin. It's believed that the epoxy can serve as a physical cross‐linking agent to limit the motion or migration of PE molecules and consequently improve the fiber creep property. However, when the content of epoxy resin is higher than 5 wt%, all of the behavior and properties deteriorate.     

光谱法研究MC尼龙6/芳纶复合材料的结构

将经甲苯二异氰酸酯(TDI)和己内酰胺改性的Kevlar纤维作为MC尼龙6的增强体,X射线光电子能谱观察到改性后纤维表面C,N和O元素的含量发生了改变,谱峰也相应发生变化。从红外光谱分析发现,Kevlar纤维和MC尼龙6相混所得谱图只是二者红外谱图的简单叠加,而改性Kevlar纤维可作为己内酰胺阴离子开环聚合的活性中心,且接枝链上的酰胺基可以与基体尼龙形成较强的氢键,有利于提高界面结合。XRD测试表明Kevlar纤维的引入并没有明显改变MC尼龙6的晶型,但其晶粒将具有更严格的三维周期性结构。在相同纤维用量时,改性Kevlar纤维增强的MC尼龙6的晶粒较未改性纤维的完善。当纤维含量小于2%时,纤维的加入有利于生成完善的α球晶,纤维含量大于2%时,α球晶结构含量随着纤维用量的增多反而下降。     

Laser-based Surface Modifications of Aluminum and its Alloys

Aluminum (Al) and its alloys have widespread engineering applications because of their higher strength to weight ratio, ductility, and formability. However, in various applications, mechanical properties such as hardness, corrosion, wear, and fatigue resistance are prerequisite at near surface regions. Such localized modification without affecting the bulk phase can be performed by various surface-engineering approaches including electro-deposition, physical and chemical vapor depositions, thermal spraying, plasma spraying, and organic polymeric coatings. Delamination failure of such coatings from the substrate is often inevitable due to the difference in film-to-substrate elastic modulus associated with the aforementioned processes. Recently, researchers have adopted a new approach of laser surface engineering to modify the near surface regions of metallic substrate by laser beams resulting in superior mechanical properties with the formation of novel microstructures. In this article, the recent developments in the surface modification of Al and its alloy by laser treatment are reviewed. Processing parameters and resulting microstructures of Al and its alloys are briefly summarized, along with their impact on mechanical properties. Finally, this article concludes future research directions.     

BN&Al2O3/环氧树脂复合材料粘接层对LED灯结温的影响

自制BN/EP(环氧树脂)复合材料和Al₂O₃/EP复合材料作为LED灯PCB板和散热铝块之间的粘接层材料,采用精密钻孔的方法用高精度测温仪测量LED灯正常工作时的温度分布,讨论粘接层对结温的影响,并与COMSOL Multiphysics软件模拟结果进行对比分析。实验测量LED结温与模拟结温变化趋势基本一致,结温会随着粘接层厚度的增加而上升、随着粘接层复合材料热导率的增加先快速降低而后趋于平缓。最终得到PCB板和散热铝块间最佳粘接层厚度和粘接层复合材料配比,当BN的质量分数为60%时,BN/EP复合材料粘接层的热导率最高,此时LED结温为75.2 ℃,比纯环氧树脂粘接层LED的结温降低了27.6 ℃。而Al₂O₃/EP复合材料粘接层LED的最低结温为78.2 ℃,此时Al₂O₃的质量分数为50%。     

Effect of Residual Stresses on Delamination Cracks in Fiber Reinforced Composites

In the processing of cross-ply fiber reinforced materials, residual stresses, as well as possible transverse cracking may arise. These affect the stress field about a delamination between two layers. In this investigation, the effect of residual stresses resulting from curing and transverse cracks is examined. A 0°/90°/0° ply system is considered with a delamination assumed between one of the 0° and 90° layers. The residual stresses along the interface without the delamination are calculated. First, this analysis is done neglecting the transverse cracks in the 90° layer. Then, the transverse cracks are included and several methods are employed to calculate the residual stresses. These include the shear lag method, a semi-analytic method and the finite element method. It is seen that the latter two methods produce similar results. By means of the superposition principle, the stress intensity factors resulting from the residual stresses are obtained for the delamination. Use is made of the conservative M-integral with tractions along the crack faces.     

聚四氟乙烯材料表面激光改性与刻蚀

利用波长为248 nm的准分子激光束在不同激光能量密度下照射聚四氟乙烯(PTFE)材料的表面,并用扫描电镜(SEM)、X射线光电子能谱(XPS)、拉曼(Raman)光谱等手段对激光处理前后样品的表面形貌、化学成分和结构进行测量和分析,进而对激光与聚四氟乙烯相互作用的机理进行了研究。实验结果表明,激光辐照使聚四氟乙烯表面产生去氟效应,导致表面碳化、分子链的交联以及含氧基团的产生,随着激光能量密度的增加,C=C双键逐渐形成。这些结构的变化可以导致表面硬度和粘结性增强。激光能量密度的大小对照射后样品表面的物理性质和化学结构有着重要的影响,它是聚合物表面激光改性和烧蚀的关键因素。     

Influence of Fiber Surface Treatment on Mechanical Properties of CF/PET Composites

Carbon fiber (CF) / poly (ethylene terephthalate) (PET) composites were prepared with various contents (2–15wt%) of short carbon fibers. To investigate the effect of surface treatment of the CF on the mechanical properties of the composites, three specimens were prepared; those with short carbon fibers (called SCF), short carbon fibers oxidized with nitric acid (called NASCF) and the fibers oxidized with nitric acid and treated with silane coupling agent (called SCSCF). Flexural, tensile and impact tests were performed to observe mechanical behavior of the specimens. The morphology of the specimens was also studied with a scanning electron microscope (SEM). SCSCF composite had better mechanical properties than the other composites with the same content of carbon fibers since the coupling agent resulted in better interfacial adhesion between the fiber and the matrix.     

Studies on the Thermal Properties of Epoxy Resins Modified with Two Kinds of Silanes

Dimethyldiethoxysilane (DMDES) and diphenyldimethoxysilane (DPDMS)-containing epoxy resins were synthesized by dehydration polycondensation. The chemical structures were determined by FT-IR, ¹H NMR, and ¹³C NMR. The cured samples, with 4, 4′-diaminodiphenylmethane (DDM) as curing agent, were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and tensile and impact testing. Results showed that DMDES and DPDMS-modified epoxy resins possess higher glass transition temperatures, better thermal stability, and better fracture toughness than the neat epoxy resin.     

Enhanced mechanical properties of bamboo fiber/HDPE composites by grafting poly(amido amine) onto fiber surface

The mechanical properties of bamboo fiber composites depend on the interfacial strength between fiber and high-density polyethylene (HDPE) matrix. Different poly (amido amine) (PAMAM) dendrimers were grafted onto bamboo fiber to improve the interfacial strength of the resulting composites. The surface morphology of the resulting materials was characterized by scanning electron microscopy and atomic force microscope. Surface characteristic the bamboo fiber surface were examined by X-ray photoelectron spectroscopy and Fourier transform infrared (FT-IR). The characterization results revealed that PAMAM were chemically grafted onto the surface of bamboo fiber.     

Surface Analysis of PBO and Modified SPBO Fiber by Contact Angle Measurements and XPS

In order to improve surface properties of poly (p‐phenylene benzoxazole) (PBO) fiber, modified SPBO containing ionic groups (–NaSO₃) was prepared for the first time by polymerization from 1, 3‐diamino‐4, 6‐dihydroxybenzene dihydrochloride (DAR) and terephthalic acid (TPA), with addition of selected amounts of 5‐sulfoisophthalic acid monosodium salt (SIPA) in place of the TPA. The SPBO polymer with a different content of SIPA, 1.5% SPBO (mol ratio), and 3% SPBO was prepared. The SPBO fiber was obtained via liquid crystal spinning through dry‐jet wet‐spinning techniques. The contact angles between fiber and water/ethyl alcohol were measured by an OCA 40 Micro dynamic contact angle analysis system. The contact angles of SPBO to water and alcohol were smaller than those of PBO to either of them and the wetting process of water and ethyl alcohol on SPBO fiber was faster than on PBO fiber. In addition, the results showed that the surface free energy could be increased up to 40.3 mJ/m², i.e. by 13.55%. Through XPS analysis, it was found that the surface nitrogen‐to‐carbon ratio was increased from 0.0157 to 0.0915 and the oxygen‐to‐carbon ratio was increased from 0.2331 to 0.3070 after incorporation of the ‐NaSO₃ ionic groups; in addition, the ionization energy (or binding energy) of C_1s and O_1s decreased.     

Effect of Fiber Surface Modification on the Interfacial and Mechanical Properties of Kenaf Fiber-Reinforced Thermoplastic and Thermosetting Polymer Composites

The surfaces of kenaf fibers were treated with three different silane coupling agents. 3-glycidoxypropyltrimethoxy silane (GPS), 3-aminopropyltriethoxy silane (APS), and 3-methacryloxypropyltrimethoxy silane (MPS). Among them, the most effective one for the property improvement was GPS when it was applied to the kenaf fiber surfaces at 0.5 wt%. Thermoplastic polypropylene (PP) and thermosetting unsaturated polyester (UPE) matrix composites with chopped kenaf fibers untreated and treated at different GPS concentrations from 0.1 wt% to 5 wt% were fabricated using compression molding technique. The present study demonstrates that the interfacial, flexural, tensile, and dynamic mechanical properties of both kenaf/PP and kenaf/UPE composites importantly depend on the GPS treatments done at different concentrations. The greatest property improvement of both thermoplastic and thermosetting polymer composites was obtained with the silane treatment at 0.5 wt% and the mechanical properties were comparable with E-glass composites prepared the same polymer matrix under the corresponding fiber length and fiber loading. The results also agreed with each other with regard to their interfacial shear strength, flexural properties, tensile properties, storage modulus, with support of fracture surfaces of the composites.     

Preparation and Characterization of Nano-Calcium Carbonate/Silane Coupling Agent Master Batch and Its Effect on Epoxy Resin

A nano-calcium carbonate (CaCO₃)/silane coupling agent (NCC/SCA) master batch was prepared by the reaction of SCA (γ-aminopropyl triethoxy silane, trade name KH550) with the hydroxyl groups of nano-CaCO₃. Both Fourier transform infrared spectroscopy and thermal gravimetric analysis indicated that the nanoparticles were grafted by SCA. An epoxy resin was modified by adding the NCC/SCA master batch. A simple dipping test suggested that a better dispersion of the treated NCC in epoxy could be obtained than that of the untreated NCC. Then samples of epoxy nano-composites were prepared by a hot press process. The compressive property of epoxy nano-composites was investigated; the results of these mechanical property tests revealed that the compressive strength, elastic modulus, and the total fracture work of the epoxy matrix filled with the treated NCC were significantly improved relative to that filled with the untreated NCC.