Cure-volume-temperature relationships of epoxy resin and graphite/epoxy composites

Taking the material thermal expansion and curing shrinkage in polymer composites into account, this work aims to quantitatively obtain the relationships among the degree of cure, the volumetric change rate and the temperature (CVT) at the atmospheric pressure. According to reaction kinetics and nonlinear deformation theory, the mathematical models are established to implicitly describe the CVT relationships of epoxy resin as well as its unidirectional graphite fiber composites, and then the volumetric change rates of the two types of materials during curing process are obtained via the finite element method. On the basis of the numerical analysis and data fitting, two new explicit CVT expressions are constructed. The numerical analysis of the cure-volume-temperature relationships is helpful not only for the prediction of material warpage and internal stress, but also for the process optimization, such as the shrinkage compensation of polymer composites. By comparing the present results with the results in published articles, the validity of the present results is proven indirectly.     

Physical aging in graphite/epoxy composites

Matrix-dominated mechanical behavior of a graphite/epoxy composite has been found to be affected by sub-T_g annealing. Postcured (±45°)_4S specimens of Thornel 300 graphite/Narmco 5208 epoxy were quenched from above T_g and given a sub-T_g annealing at 140°C for times up to 10⁵ min. The ultimate tensile strength, strain-to-break, and toughness of the composite material were found to decrease as functions of sub-T_g annealing time. No weight loss was observed during the sub-T_g annealing. The time-dependent change in mechanical behavior is explained on the basis of free-volume changes that are related to the physical aging of the nonequilibrium glassy network-epoxy. The results imply possible changes in composite properties with service time.     

Water transport in graphite/epoxy composites

The diffusive and mechanical behavior of tetraglycidyl diaminodiphenyl methane (TGDDM) resin-based composites and diglycidyl ether of bisphenol-A (DGEBA) resin-based graphite/epoxy composites were investigated during water sorption at different temperatures. The water-absorption kinetics in both systems at 50, 70, 90, and 100°C were fitted by a Fickian diffusion model. However, a Langmuir-type, two-step sorption behavior was observed for water transport in DGEBA-based systems at 50 and 70°C. Using scanning electron microscopy, internal cracks due to water absorption were found in the DGEBA-based samples after conditioning at 90 and 100°C in water, whereas no cracks were detected in TGDDM-based samples conditioned in water at 100°C. Ultrasonic testing did not show significant modulus or density change of the TGDDM-based samples conditioned in water at 100°C. No significant changes of dynamic modulus or damping factor were observed for the TGDDM-based samples redried after immersion in 100°C water, whereas slight changes were observed above 120°C for the samples containing absorbed water. However, both water-containing and redried DGEBA-based samples showed a decrease of dynamic modulus and an ω-transition around 120°C. A single-fiber fragment test revealed that the absorbed water at 80°C reduced significantly the interfacial shear strength of DGEBA/DDA resin-AS4 fiber samples and DGEBA/DDA resin-AU4 fiber samples. © 1993 John Wiley & Sons, Inc.     

The nonlinear viscoelastic characterization of graphite/epoxy composites

Fiber reinforced plastics offer many structural advantages for a variety of designs. Before these material systems can achieve more widespread usage, however, some of the unanswered questions regarding the long-term behavior and durability of these materials must be answered. Nonlinear compliance data is presented for T300/934 and T300/5208 graphite/epoxy composites. Several nonlinear viscoelastic techniques are compared for modeling the response. Comments for applying nonlinear models to orthotropic materials are given. A procedure for predicting long term laminate response from short term unidirectional data is reviewed.     

纳米石墨片/环氧树脂复合材料的制备及性能

分别采用低温固化剂和高温固化剂制备了纳米石墨片/环氧树脂复合材料。通过电阻测试仪和材料试验机研究了纳米石墨片的含量对复合材料导电性能和力学性能的影响规律,并将溶液混合法与直接混合法制备的复合材料的性能进行对比,同时比较了纳米复合材料的性能与微粉石墨/环氧树脂复合材料的性能。结果表明,溶液混合法制备的复合材料逾渗阈值更低,可得到填料质量分数达60%、体积电阻率为0.0085 Ω·cm的纳米复合材料。当填料质量分数高于4%时,纳米复合材料的力学性能低于微粉复合材料。     

Effects of reactive and non-reactive fiber coatings upon performance of graphite/epoxy composites

Interlayers of controlled composition and thickness were applied to commercial graphite fiber bundles by electrochemical copolymerization, and the impact and interlaminar shear strength of composites from these coated fibers were examined. Glycidyl acrylate/methyl acrylate copolymers represented coatings that were reactive to the epoxy matrix during curing; acrylonitrile/methyl acrylate copolymers represented non-reactive systems. The reactive systems showed 10 to 30 percent simultaneous improvement in impact and interlaminar shear strengths, while the non-reactive system failed at the interlayer-epoxy interface and showed no improvement. There is an optimum interlayer thickness of 0.1 to 0.15 micron; the possible reasons are discussed. A detailed scanning electron microscope study illustrates how the structure of the composite fracture surface varies with the systematic changes in interlayer reactivity, composition, and thickness. Determination of the locus of failure is discussed. The observations are consistent with the mechanical property measurements.     

Impact damage resistance of graphite/epoxy laminated composites

Damage in graphite/epoxy laminated composites resulting from low-velocity impact was studied. The impact damage mechanisms and mechanics were first investigated by adopting the impactors with a line-nosed head from which the damage was simplified from a complicated three-dimensional pattern to a two-dimensional one. Based on the results of the line-loading impact study, a model was developed for predicting the impact damage in the materials resulting from point-loading impact. The model consists of a stress analysis for calculating transient dynamic stresses during impact and a failure analysis for predicting the extent of delaminations resulting from the impact. A computer code, “3DIMPACT”, was developed based on the model during the investigation. The predictions from the code agreed fairly well with the test data.     

Effect of aging and moisture on the reactivity of a graphite epoxy prepreg

The effect of moisture on the curing reaction of Hercules 3501-6/AS graphite-epoxy prepreg was investigated. Test samples of prepreg were aged under humid conditions, while control samples were aged under identical temperature conditions but with negligible humidity. Determinations of the amounts of moisture absorbed and desorbed fom the samples were accomplished gravimetrically. Dielectric analysis was used to monitor the relative reaction rates and overall activation energies of the prepreg after various aging periods. Changes taking place in monomer concentrations were observed using high performance liquid chromatography (HPLC). As aging continued, samples aged under humid conditions displayed, significant decrease in the overall activation energy accompanied by increased reaction rates. This change did not occur in the control samples. Samples aged in high humidity showed increased conversion of monomers to polymer when compared to dry aged samples. The aging time required before the above changes became apparent was temperature dependent; the higher the aging temperature, the sooner the changes in reactivity occurred.     

碳纳米管增强环氧／石墨复合材料的制备与性能研究

采用粉末冶金法制备了碳纳米管增强环氧／石墨复合材料,并研究了酸洗处理对复合材料弯曲强度、硬度和导电性能的影响。结果表明：与未处理碳纳米管相比,酸处理的碳纳米管增加了环氧／石墨复合材料的弯曲强度和硬度,降低了电阻率。酸处理的碳纳米管增强环氧／石墨复合材料的弯曲强度达到21．9MPa,比未添加碳纳米管时提高了近22％;同时复合材料的硬度达到最大值21．7HS,比未添加碳纳米管时提高了近10％;复合材料的电阻率达到了最小值45036μΩ·cm,比未添加碳纳米管时复合材料的电阻率降低了近17％。     

环氧树脂/黏土纳米复合材料研究进展

综述了目前环氧树脂/黏土纳米复合材料的制备方法和表征手段,介绍了环氧树脂/黏土纳米复合材料的优异性能,并对其增韧增强改性机理作了探讨,最后展望了环氧树脂/黏土纳米复合材料的应用前景。     

Mechanical and thermal properties of graphite platelet/epoxy composites

Anhydride-cured diglycidyl ether of bisphenol A (DGEBA) reinforced with 2.5-5% by weight graphite platelets was fabricated. The structural, mechanical, viscoelastic and thermal properties of these composites were studied and compared. XRD studies indicated that the processing of composites did not change the original d-spacing of pure graphite. Tensile property measurements of composites indicated higher elastic modulus and tensile strength with increasing concentration of graphite platelets. The storage modulus and glass transition temperatures (T_g) of the composites also increased with increasing platelet concentration, however, the coefficient of thermal expansion decreased with the addition of graphite platelets. The thermal stability was determined using thermogravimetric analysis. The composites showed higher thermal stability in comparison with pure epoxy and increased char concentration for higher graphite concentration. The effects of reinforcement on the damage mechanisms of these composites were investigated by scanning electron microscopy.     

改性石墨/丁腈橡胶复合材料的性能

采用十六烷基三甲基溴化铵、硅烷偶联剂KH 570以及聚乙二醇分别对石墨表面进行了有机改性,通过机械共混法制备了改性石墨/丁腈橡胶复合材料,考察了改性剂种类以及改性石墨用量对复合材料物理机械性能及摩擦性能的影响,并用扫描电子显微镜表征了石墨在橡胶基体中的分散情况及复合材料磨损表面情况。结果表明,随着改性石墨用量的增加(20份以内),复合材料的物理机械性能有所上升,摩擦系数不断下降;3种改性剂中,KH 570改性石墨所制备复合材料的物理机械性能及摩擦性能较优,当添加20份KH 570改性石墨时,其在橡胶基体中分散较好,磨损表面最为光滑、平整,复合材料的物理机械性能最佳,摩擦系数达到最低值(0.7)。     

Interfacial micromechanics and effect of moisture on fluorinated epoxy carbon fiber composites

Carbon fiber composites have witnessed an increased application in aerospace and other civil structures due to their excellent structural properties such as specific strength and stiffness. However, unlike other structural materials, carbon fiber composites have not been as widely studied. Hence, their increased application is also accompanied with a serious concern about their long‐term durability. Many of these applications are exposed to multiple environments such as moisture, temperature, and UV radiation. Composites based on conventional epoxies readily absorb moisture. However, recently synthesized fluorinated epoxies show reduced moisture absorption and hence potentially better long‐term durability. The aim of this project is to study the effect of moisture absorption on fluorinated‐epoxy‐based carbon fiber composites and their comparison with conventional epoxy carbon fiber‐based composites. Microbond tests are performed on fluorinated and nonfluorinated epoxy‐based single fiber samples before and after boiling water degradation. It is found that fluorinated epoxy‐based single fiber coupons showed relatively reduced degradation of interface when compared with the nonfluorinated epoxy single fiber coupons. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Stress measurement in graphite/epoxy uniaxial composites by X-rays

X-rays can be used to measure residual as well as applied stresses in polymers and reinforced polymeric composites by diffracting from filler particles that are embedded before curing. We have investigated various fillers that exhibit suitable stress-induced shifts of diffraction angle when embedded in uniaxial graphite fiber/epoxy composites. Stresses in the particles determined by X-rays are proportional to the corresponding composite stresses, in agreement with the model of H. T. Hahn. Results indicate that the stress sensitivity (change in X-ray strain per MPa applied to the composite) increases in the order W, CdO, Ni, Ag, Nb, Al. The elastic range terminates at a yield point beyond which the stress sensitivity is markedly reduced for metallic fillers, but no such yield point has appeared with CdO particles. Oppositely directed particle strains are seen after unloading from stress levels that have exceeded the yield point. Theoretical predictions for these findings are discussed. Residual stresses in the particles were determined after curing and storage under ambient conditions. In the fiber direction the residual stresses were always found to be positive and larger than the principal stresses orthogonal to these.     

Effects of moisture on properties of epoxy molding compounds

In this article the effects of moisture on a novel epoxy molding compound, including the mechanical properties, temperature transition, and thermal degradation behavior, are studied. The experimental results reveal that the absorbed water acts predominantly as a crazing agent, continuously decreasing the mechanical strength with the time in water. The glass-transition temperature decreases at the early stage and is finally equilibrated. The thermal degradation behavior of the materials is not greatly influenced by the hydrothermal age. The decomposition of samples in oxygen is composed of two independent steps: the thermal degradation and oxidation at high temperature. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2253–2259, 2001     

Effect of surfactant treatments on interfacial adhesion in single graphite/epoxy composites

Interfacial adhesion between graphite fibers and epoxy matrices is one of the important parameters in controlling composite performance. Experiments were performed to verify the idea that polymerizable and nonpolymerizable nonionic surfactants can promote wetting and increase interfacial shear strength. Using a single-fiber-composite (SFC) technique with a Poisson/Weibull model of fiber strength and flaws, the effective interfacial shear strength (ISS) was determined. The effect of surfactant treatments on interfacial adhesion in single graphite/epoxy composites was also compared with the effects of treatment with deionized water, absolute alcohol, and acetone. The experimental results showed that the fibers treated with the nonpolymerizable surfactant gained 37% in interfacial shear strength relative to those untreated or those treated with deionized water, absolute alcohol, or acetone. The polymerizable surfactants showed a 10% increase in interfacial shear strength. The lognormal distribution provided the best fit for critical aspect ratios obtained from the SFC tests.     

Mode I fracture resistance characteristics of graphite/epoxy laminated composites

Mode I crack resistance behavior of fiber‐reinforced (graphite/epoxy) composites laminated unidirectionally and anti‐symmetrically was studied. Double cantilever beam (DCB) specimens of stacking sequences, [O₁₂//O₁₂] and [(O/90)_3s//(90/O)_3s] were used where // represents the initial crack location. Resistance curves (R‐curves) were constructed for three initial crack lengths in order to determine the effects of initial crack length on the resistance behavior. The resistance force, G_R, for a crack increment was determined from the compliance calibration method. The results show that for the case of [(O/90)_3s//(90/O)_3s], the initial crack deviated from the midplane and propagated in a zigzag fashion within 13^th(90‐deg), while the crack propagated along the midplane for a [O₁₂//O₁₂] case. The results also show that for both cases, G_R was affected by the initial crack length before G_R was stabilized. However, G_R was not affected by initial crack length when G_R was stabilized for each case.     

Ultrasonic evaluation of graphite/epoxy composites with different curing conditions

Ultrasonic evaluation of AS4/3501-6 graphite/epoxy composites and Hercules 3501-6 epoxies with different curing conditions has been carried out. A differential scanning calorimeter and a dynamic mechanical analyzer were used to characterize the cure status of these materials. The anisotropic elastic moduli, through-thickness longitudinal wave dispersion and attenuation were measured by different ultrasonic techniques. Effects of curing conditions on the mechanical properties of the composites and corresponding epoxies were discussed.     

蒙脱土改性环氧树脂及其复合材料性能研究

以蒙脱土(MMT)作为EP(环氧树脂)的改性剂制备EP/MMT纳米复合材料。考察了MMT含量对EP/MMT体系的凝胶时间、黏度和力学性能等影响。结果表明:MMT的加入明显缩短了EP体系的凝胶时间,并显著缩短了EP体系达到高黏度的时间;当w(MMT)=4%时,EP/MMT纳米复合材料的力学性能相对最好,其浇铸体的拉伸强度、弯曲强度和冲击强度分别为85 MPa、140 MPa和35 kJ/m2,其复合材料的拉伸强度和弯曲强度分别为160 MPa和200 MPa。     

酚醛型环氧树脂改性氰酸酯复合材料性能的研究

通过DSC分析,粘度、介电性能、力学性能及耐油性测试对酚醛型环氧树脂改性氰酸酯树脂复合材料的性能进行了研究。结果表明,改性氰酸酯树体系在70～160℃具有较低的粘度,理想工艺是在125～130℃下30～45min后开始加压;改性氰酸酯树脂表观活化能和反应级数分别为60.81kJ/mol和0.8846。改性氰酸酯复合材料具有良好的力学性能、介电性能和耐油性能。