Behavior of volatile depletion for LaRC™‐PETI‐5 amide acid/N‐methylpyrrolidinone solution

The fabrication of composites from polyimide precursors (polyamide acids) that involve condensation reactions requires the removal of volatiles (solvent and reaction by‐products) prior to consolidation in order to achieve a void‐free laminate. Volatile removal is commonly accomplished with a B‐stage processing step. In this study, a PETAA/NMP (p henyle thynyl t erminated a mide a cid in N‐methylpyrrolidinone) solution and prepreg were characterized using therogravimetric analysis (TGA) and microdielectrometry. A master weight loss profile was constructed by superimposing measurements at various heating rates. The TGA results correlated well with the dielectric ionic conductivities. Under the same thermal conditions, volatiles were depleted at slower rates from the wet prepreg than the neat resin solution. Dielectric properties were more sensitive to the residual volatile contents than the TGA measurements in both the neat resin and wet prepreg. Dielectric sensing technology was demonstrated to be a feasible tool for future volatile management in the fabrication of PETI composites. This study demonstrates that the combination of TGA and dielectrometry provides useful information to develop proper processing conditions for composite fabrication from prepregs containing volatiles. © Published 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1906–1916, 2003     

Vacuum‐assisted resin transfer molding using tackified fiber preforms

A low cost composite fabrication process—tackified SCRIMP—is described for fabricating aerospace‐grade composites based on tackification and vacuum‐assisted resin transfer molding (VARTM). Tackification based on a commercial tackifier (FT 500 from 3M) was used to make the net‐shape fiber preform. It was found that tackifier concentration and application conditions play important roles in governing the moldability of tackified fiber preforms. An epoxy resin (PR 500 from 3M) was used in the VARTM process‐SCRIMP at high temperatures. Experimental results show that composites with high fiber content (> 60% by volume) can be manufactured at low cost using tackification. Effects of tackification methods on composite dimension control, void content and mechanical properties were investigated and compared in both RTM and SCRIMP.     

FTIR and SEM analysis of polyester‐ and epoxy‐based composites manufactured by VARTM process

Polyester‐ and epoxy‐based composites containing glass and carbon fibers were manufactured using a vacuum‐assisted resin transfer molding (VARTM) process. Fourier transform infrared (FTIR) spectroscopy analyses were conducted to determine the interaction between fibers and matrix material. The results indicate that strong interaction was observed between carbon fiber and epoxy resin. However, weak interactions between remaining fiber‐matrix occur. Scanning electron microscopy (SEM) analysis was also performed to take some information about strength of interaction between fibers and matrix material. From SEM micrographs, it is concluded that the findings in SEM analysis support to that obtained in FTIR analysis. Another aim of the present work was to investigate the influence of matrix on composite properties. Hence, the strengths of composites having same reinforcement but different matrix systems in axial tension and transverse tension were compared. Short beam shear test has been conducted to characterize the interfacial strength in the composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Numerical investigation on flow through porous media in the post‐infusion process

During the vacuum‐assisted resin transfer molding (VARTM) processing, the post‐infusion behavior after complete wet‐out and before gelation of the resin is critical for the development of the thickness and fiber volume fraction distribution in the cured composite part. The pressure gradient developed during infusion results in a thickness gradient due to the flexible nature of the bagging approach. After full infusion, the resin typically bleeds into a vacuum trap, allowing redistribution of pressure and preform thickness. In this study, a non‐rigid control volume is used to formulate a set of governing equations for analysis of the post‐infusion process. The model is used to investigate the effects of processing parameters and different processing scenarios on resin flow, resin pressure, and thickness variation of the composite laminate. This work provides a tool for optimization of the VARTM process to reduce final part variability. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

In situ polymerization and nano‐templating phenomenon in nylon fiber/PMMA composite laminates

Supercritical Carbon Dioxide (SC CO₂) is used as a reaction/processing medium in the fabrication of fiber‐reinforced composite materials. SC CO₂ allows resin (reactive monomer), to penetrate inside the fibers themselves, partitioning into the amorphous regions of the fiber. The crystal structure then templates polymerization of matrix within the fiber. This process produces a composite that exhibits ultralong‐range order from the nanoscale reinforcement of crystals to the macroscale fiber reinforcement of matrix. In addition, SC CO₂ lowers resin viscosity and aids in wetting out Nylon 6,6 fiber reinforcement in a process similar to reaction injection molding (RIM) or resin transfer molding (RTM). This article will discuss the fabrication technique in detail, including process parameters and the structure of resulting composites and morphology of modified fibers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1600–1607, 2003     

改性环氧基复合材料工艺性能试验研究

本文通过对环氧树脂增韧改性研究和袋压成型方法研究,制备出一种环氧基复合材料体系,经过工艺试验确定了较为适当的增韧剂添加百分含量、复合材料中基体与增强材料的比例参数以及袋压工艺参数。实验结果表明,袋压成型的改性环氧基复合材料在拉伸强度、拉伸模量、剪切强度、冲击韧性等性能上均有显著的提高。     

Magnetic‐clamping structures for the consolidation of composite laminates

Vacuum bags are used in conjunction with autoclaves to generate the consolidation pressures and temperatures required to manufacture aerospace composites. As the scale of continuous fiber composite structures increases, autoclave processing becomes prohibitively expensive or infeasible. The objective of this study is to develop flexible magnetic clamping structures to increase the consolidation pressure during the conventional vacuum bagging of composite laminates, thereby obviating the need for an autoclave. The ferromagnetic rubber, which consists of rubber filled with iron particles, developed in this study provides a conformable and reusable vacuum bag that provides increased consolidation through attractive forces produced by electromagnets. Experiments and finite‐element modeling indicate that consolidation pressure in the range of 100 kPa can be generated by such a device with realistic power requirements. The effects of the magnetic clamping device process parameters on the consolidation pressure magnitude are modeled theoretically and are characterized experimentally. In addition, a method for the efficient design of the magnetic‐clamping device is developed. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Elevated‐temperature vacuum‐assisted resin transfer molding process for high performance aerospace composites

Vacuum‐assisted resin transfer molding (VARTM) is commonly used for general temperature applications (<150 °C) such as boat hulls and secondary aircraft structures. With growing demands for applications of composites in elevated temperature environments, significant cost savings can be achieved by employing the VARTM process. However, implementation of the VARTM process for fabricating elevated temperature composites presents unique challenges such as high porosity and low fiber volume contents. In the present work, a low cost and reliable VARTM process is developed to manufacture elevated temperature composites for aerospace applications. Modified single vacuum bagging infusion and double vacuum bagging infusion processes were evaluated. Details of the method to obtain high quality composite parts and the challenging issues related to the manufacturing process are presented. Density and fiber volume fraction testing of manufactured panels showed that high quality composite parts with void content less than 1% have been consistently manufactured. A property database of the resin system and the composites was developed. A three‐dimensional mathematical model has also been developed for flow simulation and implemented in the ABAQUS finite element package code to predict the resin flow front during the infusion process and to optimize the flow parameters. The results of the present study indicate that aircraft grade composite parts with high fiber volume fractions can be manufactured using the developed elevated temperature VARTM process. © 2013 Society of Chemical Industry     

Electrical and mechanical properties of multi‐walled carbon nanotubes reinforced PMMA and PS composites

The use of multi‐walled carbon nanotubes (MWCNT) as reinforcing material for thermoplastic polymer matrices, polymethyl methacrylate (PMMA), and polystyrene (PS) has been studied. MWCNT were synthesized by chemical vapor deposition (CVD) technique using ferrocene‐toluene mixture. As‐prepared nanotubes were ultrasonically dispersed in toluene and subsequently dispersed in PMMA and PS. Thin polymer composite films were fabricated by solvent casting. The effect of nanotube content on the electrical and mechanical properties of the nanocomposites was investigated. An improvement in electrical conductivity from insulating to conducting with increasing MWCNT content was observed. The carbon nanotube network showed a classical percolating network behavior with a low percolation threshold. Electromagnetic interference (EMI) shielding effectiveness value of about 18 dB was obtained in the frequency range 8.0–12 GHz (X‐band), for a 10 vol% CNT loading. An improved composite fabrication process using casting followed by compression molding and use of functionalized MWCNT resulted in increased composites strength. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Reduction of void content of vacuum‐assisted resin transfer molded composites by infusion pressure control

Vacuum‐assisted resin transfer molding is a promising technique for making large and complex composite structures. However, void formation remains a problem. Two primary contributors to void formation, non‐uniform resin flow and continuous evaporation of resin under low pressure, were experimentally studied. Improved pressure control at the vent is proposed to reduce the void content of the manufactured composite material: at the start of the resin infusion, the pressure at the vent is set to the full vacuum of the equipment, while after the resin has saturated all of the reinforcements, the pressure at the vent is increased slightly. The full vacuum at the start of infusion avoids air entrapment, and the slightly higher pressure later in the process restrains the resin evaporation. A lower void content is obtained. POLYM. COMPOS., 36:1629–1637, 2015. © 2014 Society of Plastics Engineers     

Influence of alcohol pre‐infusion on the quality of VARTM composites

In the vacuum assisted resin transfer molding (VARTM) process, part‐to‐part variations such as the uncertainty in the permeability and race tracking phenomenon make it difficult to achieve consistent mold filling and ensure part quality of composites. Alcohol pre‐infusion was presented in this study as a novel real‐time monitoring and control approach for the flow process in the VARTM process, alcohol test fluid is infused before the actual resin infusion to locate the potential dry spots without using the large quantity of sensors. Then corresponding process control strategy is designed, such as opening the auxiliary gate at specific moment on those predicted dry spot locations to compensate flow defects. Moreover, alcohol can be easily removed by heat without changing the local permeability. The influence of alcohol pre‐infusion on the quality of VARTM composites were investigated in this study. The mechanical tests were conducted to verify that the alcohol pre‐infusion approach has no significant effect on composite properties because alcohol can be removed from fiber by heat and air flow. Specifically, DMA, TGA, and FTIR spectrum proved that negligible difference existed on the resin–fiber interface between the composites with or without alcohol pre‐infusion. Finally, the microscopy results revealed a similar failure path in a resin matrix. TMA results also demonstrated similar dimension stability. This alcohol pre‐infusion approach was effective when compared with computer simulation and could eliminate the occurrence of dry spots and voids without using sensors or data‐acquisition system. The control schemes were shown in a case study to be capable of compensating the flow defects and achieving desired fill patterns in the face of permeability uncertainty. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Effect of packing pressure on fiber orientation in injection molding of fiber‐reinforced thermoplastics

Injection molding of fiber‐reinforced polymeric composites is increasing with demands of geometrically complex products possessing superior mechanical properties of high specific strength, high specific stiffness, and high impact resistance. Complex state of fiber orientation exists in injection molding of short fiber reinforced polymers. The orientation of fibers vary significantly across the thickness of injection‐molded part and can become a key feature of the finished product. Improving the mechanical properties of molded parts by managing the orientation of fibers during the process of injection molding is the basic motivation of this study. As a first step in this direction, the present results reveal the importance of packing pressure in orienting the fibers. In this study, the effects of pressure distribution and viscosity of a compressible polymeric composite melt on the state of fiber orientation after complete filling of a cavity is considered experimentally and compared with the simulation results of Moldflow analysis. POLYM. COMPOS. 28:214–223, 2007. © 2007 Society of Plastics Engineers     

Investigation of unsaturated polyester composites reinforced by aspen high‐yield pulp fibers

Aspen chemithermomechanical pulp fiber‐reinforced unsaturated polyester (UPE) composites were fabricated using premade paper handsheets. The effects of handsheet wet‐pressing pressure, grammage, and subsequent fabrication methods on the composite properties were evaluated. The composites obtained using the optimum process parameters had tensile moduli and tensile strengths comparable with those of traditional glass fiber‐reinforced UPE composites. The pressed composites had very consistent tensile moduli that were well fitted by the Halpin–Tsai and Tsai–Pagano models. The classical Kelly‐Tyson and Bowyer‐Bader models significantly underestimated the composite tensile strengths and the potential reasons for this discrepancy are discussed. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

Use of wet‐laid techniques to form flax‐polypropylene nonwovens as base substrates for eco‐friendly composites by using hot‐press molding

The wet‐laid process with flax (base) and polypropylene (binder) fibers has been used to obtain nonwovens for further processing by hot‐press molding. Mechanical characterization of nonwovens has revealed that slight anisotropy is obtained with the wet‐laid process as better tensile strength is obtained in the preferential deposition direction. The thermo‐bonding process provides good cohesion to nonwovens, which is critical for further handling/shaping by hot‐press molding. Flax:PP composites have been processed by stacking eight individual flax:PP nonwoven sheets and applying moderate temperature and pressure. As the amount of binder fiber is relatively low (<30 wt%) if compared with similar systems processed by extrusion and injection molding, it is possible to obtain eco‐friendly composites as the total content on natural fiber (flax) is higher than 70 wt%. Mechanical characterization of hot‐pressed flax:PP composites has revealed high dependency of tensile and flexural strength on the total amount of binder fiber as this component is responsible for flax fiber embedment which is a critical parameter to ensure good fiber–matrix interaction. Combination of wet‐laid techniques with hot‐press molding processes is interesting from both technical and environmental points of view as high natural fiber content composites with balanced properties can be obtained. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

Self‐healing and interfacially toughened carbon fibre‐epoxy composites based on electrospun core–shell nanofibres

Dual components of a self‐healing epoxy system comprising a low viscosity epoxy resin, along with its amine based curing agent, were separately encapsulated in a polyacrylonitrile shell via coaxial electrospinning. These nanofiber layers were then incorporated between sheets of carbon fiber fabric during the wet layup process followed by vacuum‐assisted resin transfer molding to fabricate self‐healing carbon fiber composites. Mechanical analysis of the nanofiber toughened composites demonstrated an 11% improvement in tensile strength, 19% increase in short beam shear strength, 14% greater flexural strength, and a 4% gain in impact energy absorption compared to the control composite without nanofibers. Three point bending tests affirmed the spontaneous, room temperature healing characteristics of the nanofiber containing composites, with a 96% recovery in flexural strength observed 24 h after the initial bending fracture, and a 102% recovery recorded 24 h after the successive bending fracture. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44956.     

Rheological investigations on the flow behavior of polymer‐microsized iron powder composites

With respect to the fabrication of metal parts carrying surface relief microstructures using composite reaction molding as a rapid prototyping variant of micrometal injection molding, the influence of microsized iron powder on the viscosity of different unsaturated polyester resins applied as polymer binder has been investigated systematically. The initial binder viscosity determines the viscosity of the composite and hence the accessible critical filler load. A further dilution of the unsaturated polyester resin with styrene allows a significant increase of the filler load at constant shear rate and temperature from 36 up to 57 vol% iron content. A successful estimation of the critical filler load using different established empirical models initially developed for ceramic‐filled solutions or dispersions is possible. A pronounced flow activation energy increase at high iron loads was also observed. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Key factors affecting permeability measurement in the vacuum infusion molding process

The composites industry, under increased environmental constraints, is seeking to shift from existing open mold manufacturing processes for composite parts. A promising manufacturing technology known as the vacuum infusion molding process is gaining acceptance among composite-parts manufacturers since it involves low tooling cost and allows complete elimination of volatile organic compounds (VOC). The process is similar to the resin transfer molding process; however, in the vacuum infusion technique, a polymeric film, often referred to as vacuum bag, replaces the stiff mold cover. The film is sealed against the lower half of the mold, at the periphery. Air expelled from the mold cavity results in the compaction of the reinforcement by the atmospheric pressure present on the outer side of the polymeric film. Finally, resin impregnates the mold cavity, usually through a resin distribution channel. The process is mainly developed for large-scale structures, where material cost is an important parameter and users cannot afford any production pitfalls. Among process parameters that affect resin flow in the vacuum infusion molding process is the permeability of the reinforcement stack, which has to be measured and evaluated taking into consideration the requirements of the process. A possible approach is the definition of a parameter that defines the maximum infused length, and this parameter will take into account the structure of the reinforcement, the resin viscosity, the fiber volume fraction and inlet geometry.     

Analysis of the vacuum infusion molding process

The vacuum infusion molding process is becoming increasingly popular for the production of large composite parts. A comprehensive model of the process has not been proposed yet, making its optimization difficult. The flexible nature of the vacuum bag coupled to the varying pressure inside the mold cavity results in a variation of the cavity thickness during the impregnation. A complete simulation model must incorporate this phenomenon. In this paper, a complete analysis of the vacuum infusion molding process is presented. The analysis is not restricted to the theoretical aspects but also reviews the effect of the main processing parameters. The parameters investigated in this paper are thought to be those of most interest for the process, i.e. the compaction of the reinforcement, the permeability, the infusion strategy and the presence of flow enhancement layers. Following the characterization experiments, a 1‐D model for the vacuum infusion molding process is presented. This model is derived assuming that an elastic equlibrium holds in the mold cavity during mold filling. Even though good agreement was found between simulation results and experiments, it is concluded that additional work is needed on the numerical model to integrate interesting findings from the experimental part.     

不同工艺碳纤维增强复合材料构件的力学性能

针对碳纤维增强复合材料在电动汽车车身上的应用,分别采用手糊、真空袋压和模压三种成型工艺,设计制备了碳纤维增强复合材料层合板和双帽形管件两种实验样件。对样件进行了单向拉伸和三点弯曲试验,对构件的力学性能、表面质量、微观结构和破坏形式进行了比较,并分析了不同工艺性能存在差异的原因。研究表明:采用模压工艺成型的复合材料结构件气泡少,孔隙率小,表面质量最好;模压成型的层合板拉伸强度比手糊成型的提高了14.39%,管件弯曲强度比手糊成型管件提高了一倍,比真空袋压成型管件提高了47.58%。研究证实模压成型相对于手糊和真空袋压工艺,产品具有较优良的力学性能和一致性,较适合于电动汽车等轻量化结构的成型。     

Estimating the residual fatigue lifetimes of impact‐damaged composites using thermoelastic stress analysis

A new experimental method is presented for quantifying impact damage and estimating the remaining fatigue lifetime of impact damaged polymer matrix composite materials. The procedure is demonstrated using composites of glass fiber reinforced polyurethane produced by injection molding and structural reaction injection molding. Thermoelastic stress analysis (TSA) was used to quantify the stress concentration associated with impact‐damage in test samples of each composite. Following impact and TSA imaging, the samples were fatigued to failure over a range of stress amplitudes. The TSA‐derived stress concentration factors were used to determine a modified stress amplitude that collapsed the impact‐fatigue data onto a master stress‐life curve. This approach provides a quantitative measure of impact damage and a practical methodology for estimating the residual fatigue lifetime of impact; damaged composites.