Linear viscoelastic and morphological description of multiphase systems affected by processing parameters

Influence of processing methods, in terms of comparing compression and injection moldings, on the rheological behavior of polycarbonate (PC)/acrylonitrile‐butadiene‐styrene (ABS) blends and PC/ABS/glass fibers composites is presented. Blend compositions and fiber content are considered as material variables. For blends, the effect of the processing route on the viscoelastic functions is evident only for low shearing frequencies. Injection molding created morphology with cocontinuous character, while compression molded blends have “relaxed” structure, where dispersed phase domains are several times larger than in injection molded ones. The glass fiber reinforcement led to the significant differences in viscoelastic properties of composites processed by injection and compression molding. Injected composites have both moduli always higher than compression molded. Also, fiber lengths are reduced more for compressing molding. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Monitoring of nylon 6,6/carbon fiber composites processing by X‐ray diffraction and thermal analysis

This work reports a study made to obtain carbon fiber/nylon 6,6 prepreg composites by hot‐compression molding. Thermogravimetric analysis (TG) and crystallinity degree determination were carried out to monitor the nylon 6,6 behavior during the different steps of the composite processing. The homogeneity of the carbon fiber/polymer matrix distribution was verified using microscopic analyses and the fiber content was determined by the acid‐digestion method. The results show that the processing parameters employed were adequate, allowing the manufacture of laminates with good texture and an adequate reinforcement/matrix relation (60/40). However, improvements need be done to minimize the pullout effect observed in the tensile specimens. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3114–3119, 2002     

Predicting molding forces during sheet molding compounds (SMC) compression molding. II: Effect of SMC composition

One of the fastest‐growing applications of SMC compression molding is the manufacture of truck body panels. Because of their large size, the molding forces required are substantial and have a major influence on the molding cycle. Also, as SMC moves towards parts requiring higher strength, the fiber length and percentage by weight of fibers must increase. This will also contribute to larger molding forces. In this paper, a procedure is presented to evaluate the SMC rheological parameters needed to predict molding forces. In addition, the effect of SMC composition on the molding forces is investigated. In particular, we evaluate the effect of reinforcement type (glass versus carbon) and level, filler level and thickener level. It was found that the factors most affecting molding forces are the reinforcement length and level; and the filler level. In addition, it was discovered that for SMC thickened with magnesium oxide, the level of thickener does not affect the molding force.     

Structural Carbon/Epoxy Prepregs Properties Comparison by Thermal and Rheological Analyses

Two different carbon/epoxy prepreg materials were characterized and compared using thermal (DSC, TGA, and DMA) and rheological analyses. A prepreg system (carbon fiber preimpregnated with epoxy resin F584) that is currently used in the commercial airplane industry was compared with a prepreg system that is a prospective candidate for the same applications (carbon fiber prepreg/epoxy resin 8552). The differences in the curing kinetics mechanisms of both prepreg systems were identified through the DSC, TGA, DMA, and rheological analyses. Based on these thermal analysis techniques, it was verified that the curing of both epoxy resin systems follow a cure kinetic of n order. Even though their reaction heats were found to be slightly different, the kinetics of these systems were nevertheless very similar. The activation energies for both prepreg systems were determined by DSC analysis, using Arrhenius's method, and were found to be quite similar. DMA measurements of the cured prepregs demonstrated that they exhibited similar degrees of cure and different glass transition temperatures. Furthermore, the use of the rheological analysis revealed small differences in the gel temperatures of the two prepreg systems that were examined.     

An economical way of using carbon fibers in sheet molding compound compression molding for automotive applications

The automotive industry is extremely cost sensitive. This is one of the main reasons why sheet molding compound (SMC) compression molding is the most popular fiber‐reinforced polymeric composites manufacturing method in this industry. SMC compression molding economics are better suited for the automotive industry than processes such as resin transfer molding or any of its variations. For automotive SMC molders to take advantage of the added stiffness provided by carbon fibers, as an alternative to the widely used glass fibers, the manufacturing process needs to be simplified as much as possible. In actual manufacturing of SMC it is not easy to combine glass fibers with carbon fibers; it will be a lot more convenient to combine SMC plies only with carbon fibers together with SMC plies with only glass fibers. This will also allow molders that will normally use only glass fibers‐based SMC to secure carbon fiber SMC from a separate supplier and use it only where it makes economic sense. The main purpose of this study is to investigate the relative improvement in physical properties that can be achieved by substituting glass fibers by carbon fibers in a per ply basis. POLYM. COMPOS. 27:718–722, 2006. © 2006 Society of Plastics Engineers     

Dynamic viscoelasticity of hybrid kevlar and glass fiber reinforced LLDPE in the molten state

Kevlar and glass fibers were used to reinforce linear low density polyethylene (LLDPE), and composite sheets of 0.8, 1.5 and 2.5 mm thicknesses were obtained by using a compression molding technique. Dynamic viscoelastic properties of non‐hybrid and hybrid composites of various compositions at 200°C are evaluated. Storage modulus (G′) and loss modulus (G″) increase with angular frequency (ω) and reinforcement. Replacement of glass fiber by Kevlar at constant loading of fibers in LLDPE increases the value of G′, G″ and η′. The fractured surface of composite shows the gradient orientation of fibers particularly in 2.5 mm thick sheet. Top and bottom layers show relatively two‐dimensional orientation as compared to the middle layer, which shows random orientation. The orientation of fibers decreases G′ and η′ of Kevlar fiber and hybrid fiber hybrid fiber reinforced LLDPE composites. The effect of change in distance between parallel plate of rheometer (change in strain amplitude) on dynamic rheological properties is studied and reported here.     

碳纤维布表面处理对酚醛树脂复合材料力学性能的影响

以碳纤维布(CB)为增强相,丁苯橡胶为增韧剂,酚醛树脂(PF)为基体,通过模压成型工艺制得了PF/CB复合材料,研究了CB表面处理方式、丁苯橡胶含量及加工成型温度对PF/CB复合材料的界面结合及力学性能的影响。结果表明,丙酮处理CB、氧化处理CB及加工成型温度的提高都能改善纤维与基体的结合程度,提高界面结合力。但氧化处理CB随着加工成型温度的提高,易断裂,对复合材料的增强作用有所减弱。丁苯橡胶加入量为12%时PF的加工及冲击性能为最佳。     

模压及其增强材料发展动向

综合分析了模压技术及模压材料的国内外发展情况,文中认为,SMC、BMC用玻璃纤维正向环保型方向发展,热塑性复合材料模压成型已成为当今发展热点之一,适合高强度、大型制件成型的高压树脂传递模塑(RTM)正逐步应用于汽车制造等产业。     

Stress relaxation behavior of glass fiber‐reinforced polyester composites prepared by the newly proposed rubber pressure molding

Stress‐relaxation behavior of glass fiber‐reinforced polyester composites, prepared by a recently developed manufacturing method called rubber pressure molding (RPM), is investigated with special reference to the effect of environmental temperature (−70°C to +100°C), fiber volume fraction (30–60%), and initial load level (1–5 kN). It is found that the stress‐relaxation rate decreases with an increase in the applied load of composites and a decrease in temperature. Below glass transition temperature, the rate of stress relaxation increases with an increase in volume fraction of fibers in the composites, whereas above glass transition temperature, it increases with a decrease in the volume fraction of fibers. The experimental results for a given composites are summarized by four values, the slopes of the two straight lines (two separate relaxation processes), and their intercepts upon the stress axis. Both the slopes are dependent upon the applied load, temperature, and volume fraction of fibers in the composites. Relaxation times in both primary and secondary are calculated over the wide range of temperatures, loads, and volume fraction of fibers in the composites. It depends strongly on the temperature, but does not depend strongly on the applied load and volume fraction of fibers. The performances of the composites are also evaluated through conventional compression‐molding process. The rate of stress relaxation is small when the composites are made of newly proposed RPM technique when compared with the conventional process. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

A thermoviscoelastic analysis of process‐induced internal stresses in thermoplastic matrix composites

A numerical tool for predicting the evolution of internal and residual stresses during the processing of thermoplastic matrix composites has been developed. Based on a finite element formulation, the model accounts for the anisotropy, vis‐coelasticity and heterogeneity of the materials and represents mechanisms of both stress generation and stress relaxation. The viscoelastic properties are described by a linear thermoviscoelastic formulation. The model allows the buildup of stresses during processing to be monitored, in particular when the material is cooling through its transition temperatures, and enables the prediction of stress release and the resulting part waipage on demolding. Its use is demonstrated for unidirectional and crossply polyetherimide/glass fiber (PEI/GF) laminates processed by compression molding, and the influence of cooling conditions on stress levels is shown.     

Gamma Radiation Effects on the Glass Transition Temperature and Mechanical Properties of PMMA/soot nanocomposites

Summary Poly(methyl methacrylate) (PMMA) was produced via free radical polymerization. Polymer samples were sonicated and melt compounded to form 1%, 0.5%, and 0.25% PMMA/soot samples. Soot containing unpurified carbon nanotubes was provided by NASA Ames Research Center. The composites were compression molded and exposed to ionizing radiation in air from a Cesium-137 source. The PMMA/soot samples were characterized before and after exposure to radiation. Differential Scanning Calorimetry (DSC) was used to determine glass transition temperatures. Mechanical properties of composites were characterized via Dynamic Mechanical Analysis (DMA) and microhardness measurements. The glass transition temperatures from this study, when compared to similar studies conducted by Harmon et. al on purified single-wall and multi-wall carbon nanotube composites indicate that soot composites possess lower radiation resistance.     

Anomalies,influencing factors,and guidelines for DMA testing of fiber reinforced composites

This study systematically assessed the measurement of dynamic properties of a range of fiber reinforced composite materials using dynamic mechanical analysis (DMA) instrument. The discrepancy in the moduli from DMA to ASTM tests was investigated. The study showed that proper specimen preparation, maintaining appropriate aspect ratio (span to thickness ratio) to reduce the transverse shear deformation, and sufficient loading are critical to measure correct properties from DMA test. The guidelines on aspect ratio and loading for plastics to high-modulus carbon fiber composites are presented as a design chart and equations, respectively. The study also found that the glass transition temperature (T_g) was independent of specimen aspect ratio and T_g is lower for multidirectional composites when compared with its unidirectional composites. The particle interleaved T800H/3900-2 composite showed two glass transition temperatures (140 and 198°C), the lower value is due to the effect of interleaving by thermoplastic particles, and the higher value is the T_g of its base matrix. This lowering of T_g would have significant effect on the application temperature of the material. This phenomenon was not observed here to fore in the literature. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Pultruded fiber reinforced thermoplastic poly(methyl methacrylate) composites. Part II: Mechanical and thermal properties

A novel process has been developed to manufacture poly(methyl methacrylate) (PMMA) pultruded parts. The mechanical and dynamic mechanical properties, environmental effects, postformability of pultruded composites and properties of various fiber (glass, carbon and Kevlar 49 aramid fiber) reinforced PMMA composites have been studied. Results show that the mechanical and thermal properties (i.e. tensile strength, flexural strength and modulus, impact strength and HDT) increase with fiber content. Kevlar fiber/PMMA composites possess the highest impact strength and HDT, while carbon fiber/PMMA composites show the highest tensile strength, tensile and flexural modulus, and glass fiber/PMMA composites show the highest flexural strength. Experimental tensile strengths of all composites except carbon fiber/PMMA composites follow the rule of mixtures. The deviation of carbon fiber/PMMA composite is due to the fiber breakage during processing. Pultruded glass fiber reinforced PMMA composites exhibit good weather resistance. They can be postformed by thermoforming, and mechanical properties can be improved by postforming. The dynamic shear storage modulus (G′) of pultruded glass fiber reinforced PMMA composites increased with decreasing pulling rate, and G′ was higher than that of pultruded Nylon 6 and polyester composites.     

Fabrication of long glass fiber reinforced polyacetal composites: Mechanical performance,microstructures, and isothermal crystallization kinetics

A thermoplastic pultrusion was carried out to prepare the long fiber reinforced thermoplastic (LFT) composites based on polyacetal (POM) matrix on the custom‐designed pultrusion equipment. The investigation on mechanical performance revealed that the POM‐based LFT composites achieved much higher tensile, flexural, and impact strength than the short glass fiber reinforced ones at the same fiber loadings. Such a promising reinforcement effect is attributed to the feature that the residual fiber length in the injection‐molded LFT products is greatly superior to that in short fiber reinforced ones. This takes full advantage of the strength of the reinforcing fiber itself. The scanning electronic microscopy demonstrated that the fiber fracture and fiber pull‐out concurred on the tensile and impact fracture surfaces, and the former preceded the latter. The isothermal crystallization kinetics of the POM‐based LFT composites was also intensively studied, and the results indicated that the crystallinity of POM domain was enhanced by the heterogeneous nucleation of glass fiber, but the crystallization rate was postponed due to the interspace restriction toward crystalline growth caused by long glass fiber. These kinetic parameters provided information on the processing conditions of POM‐based LFT composites for the injection and compression molding. POLYM. COMPOS., 36:1826–1839, 2015. © 2014 Society of Plastics Engineers     

Processing and properties of new high-temperature,lightweight composites based on foam polyimide binder

We report a method for making novel, lightweight (ρ = 0.3–1.1 kg/dm³) polymer composites based on high-temperature foam polyimide binder, carbon fibers, and organic fibers. The density and mechanical properties of the foam composite can be varied over a relatively wide range, depending on the volume contents of the fiber and air pores. The resin's high glass transition temperature of 260°C, coupled with the high thermal stability of carbon or polyimide fibers, contributes to its excellent retention of mechanical properties at elevated temperatures. The temperature at the beginning of weight loss is not lower than 570°C and depends on the kind of fiber felt. The combination of excellent thermal and specific mechanical properties of foam composites together with exceptional thermal stability and processability on conventional molding equipment can provide unusual performance for the new design of advanced materials and structures.     

碳纤维布/玻纤布增强聚氨酯复合材料的研究

以聚氨酯为基体树脂,分别以碳纤维布、玻璃纤维布和这两种纤维布交替铺叠作为增强材料,采用真空辅助灌注成型工艺制备了4种复合材料.考察了纤维布的铺层结构对复合材料的弯曲、拉伸和冲击性能的影响.结果显示,复合材料的拉伸模量和弯曲模量随碳纤维含量增加而增加,冲击强度则降低.分别采用TGA、DMA和SEM对复合材料的热性能、界面...     

Preparation and characterization of nylon‐6/PPy/MMT composite of nanocomposite

Series of composites consisting of polypyrrole/montmorillonite nanocomposites in the matrix of Nylon6 has been synthesized and characterized in this work. The composites were processable, so that test samples were prepared by compression‐molding of the materials for electrical property measurements. Intercalated structures were confirmed by wide‐angle X‐ray diffraction and TEM studies for PPy/MMT nanocomposites. A two‐phase structure was determined for the fused samples consisting of two separated N6 and PPy phases by using scanning electron microscopy analyses. A conductivity threshold was measured at 15%(w/w) loading level of PPy in the composites. Electrical resistivity–temperature behavior of the samples was investigated and a resistivity peak was observed at 100°C for the samples. It was proved that the glass transition temperature of PPy around 100°C should be the responsible factor for the observed resistivity peak, as studied by thermogravimetic analysis and differential scanning calorimetry thermal methods. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Effect of thermal history on properties of block-copolyetheresters with poly(tetramethylene 2,6-naphthalenedicarboxylate) segments

The effect of compression molding on the thermal transitions and crystalline properties of block-copolyetheresters with hard segments of poly(tetramethylene 2,6-naphthalenedicarboxylate) and soft segments of poly(tetramethylene oxide) were investigated by differential scanning calorimetry (DSC), X-ray diffraction, thermal stimulated current (TSC), and dynamic mechanical analysis (DMA). The X-ray diffraction patterns of compression molded samples of the block-copolymers were considerably different from those of the corresponding samples with slow-cooling history. After compression molding, the diffraction peaks were changed completely indicating a different crystalline structure for the polyester segments, and the diffraction peaks became sharper indicating a higher crystallinity. The DSC results also showed that the melting point and crystallinity of the polyester segments were increased after compression molding. The glass transition temperatures of the polyether soft phase and polyester hard phase also were determined by DSC, TSC, and DMA separately with consistent data and were found to be dependent on the content of polyether segments and the molecular weight of the poly(tetramethylene ether)glycol (PTMEG) used. A γ-transition was observed by TSC and DMA and seemed to be independent of the composition and the thermal history. The glass transition temperatures of the polyether soft phase and the polyester hard phase of the block-copolymers derived from PTMEG 650 and PTMEG 1000 shifted to a lower temperature after compression molding possibly because of the partial miscibility between the comprising segments in these two series. The abrupt drop in log G′ in the temperature range of −10–15°C for the block-copolymers derived from PTMEG 2000 was caused by the melting of the polyether segments and indicated that the crystalline properties of the polyether segments could affect their mechanical properties. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1441–1449, 1999     

Modeling nonisothermal impregnation of fibrous media with reactive polymer resin

The manufacture of polymer composites through resin transfer molding (RTM) or structural reaction injection molding (SRIM) involves the impregnation of a fibrous reinforcement in a mold cavity with a reactive polymer resin. The design of RTM and SRIM operations requires an understanding of the various parameters, such as materials properties, mold geometry, and mold filling conditions, that affect the resin impregnation process. Modeling provides a potential tool for analyzing the relationships among the important parameters. The present work provides the physical model and finite element formulations for simulating the mold filling stage. Resin flow through the fibers is modeled using two-dimensional Darcian flow. Simultaneous resin reaction and heat transfer among resin, mold walls, and fibers are considered in the model. The proposed technique emphasizes the use of the least squares finite element method to solve the convection dominated mass and energy equations for the resin. Excellent numerical stability of the proposed technique provides a powerful numerical method for the modeling of polymer processing systems characterized by convection dominated transport equations. Results from example numerical studies for SRIM of polyurethane/glass fiber composites were presented to illustrate the application of the proposed model and numerical scheme.     

碳玻混编单向复合材料的性能分析

采用碳纤维质量含量分别为7.4%、10.7%、13.8%的三种碳玻层间混编单向织物制备了纤维增强环氧树脂复合材料,分析了该类材料的力学性能与工艺性能。结果表明:碳玻层间混编复合材料的0°拉伸模量和0°压缩模量均随碳纤维含量的提高而升高,掺入碳纤维后碳玻混杂复合材料的0°拉伸强度比纯玻纤复合材料的有所降低,但随碳纤维含量的增加而升高,碳玻层间混编复合材料的0°压缩强度则没有明显的变化规律;掺入碳纤维后,碳玻层间混编复合材料的90°拉伸强度和模量均有所下降;低碳纤维含量的碳玻层间混编单向织物具有良好的Z向渗透性能。该类新材料未来有望在风电叶片结构减重和成本优化上发挥重要作用。