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     

High temperature vacuum assisted resin transfer molding of phenylethynyl terminated imide composites

Vacuum Assisted Resin Transfer Molding (VARTM) has proven to be a cost effective process for manufacturing composite structures compared with prepreg/autoclave and traditional resin transfer molding (RTM) processes. However, VARTM has not been accomplished with high temperature resins (such as polyimides) until recently, primarily because no resins had low melt viscosity and long melt stability that are required by VARTM. With the recent invention of phenylethynyl terminated imides (PETIs), high temperature VARTM has been achieved. Two processing methods, in‐plane and through‐thickness resin flow, were proposed and tested. Both methods are capable of fabricating polyimide matrix composites; and the carbon fiber laminates yield good fiber‐resin interfacial bonding and comparable mechanical properties to those laminates fabricated using RTM. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Resin infusion of triaxially braided preforms with through‐the‐thickness reinforcement

Vacuum assisted resin transfer molding (VARTM) has shown potential to significantly reduce the manufacturing cost of high‐performance aerospace composite structures. In this investigation, high fiber volume fraction, triaxially braided preforms with through‐the‐thickness stitching were successfully resin infiltrated by the VARTM process. The preforms, resin infiltrated with three different resin systems, produced cured composites that were fully wet‐out and void free. A three‐dimensional finite element model was used to simulate resin infusion into the preforms. The predicted flow patterns agreed well with the flow patterns observed during the infiltration process. The total infiltration times calculated using the model compared well with the measured times.     

SCRIMP用原位分相增韧中温环氧树脂体系制备及复合材料性能

针对复合材料真空辅助树脂浸渍模塑技术(SCRIMP)对树脂体系要求进行配方设计及性能测试。采用动态差示扫描量热(DSC)法,运用Kissinger动力学方程研究了基础树脂的固化反应动力学。通过添加QS-VA-3型原位分相型增韧剂,改善了基础树脂脆性,依据力学性能测试结果确定最佳添加量。对增韧环氧树脂体系进行等温/非等温黏度特性研究,确定了SCRIM P灌注成型过程的适用期,并使用该技术成型了增韧环氧树脂体系/单向玻璃纤维布复合材料试样,发现与玻璃纤维复合后具有强度高、界面性能好及低电压绝缘性能良好特点,但牺牲了高电压环境下绝缘性能。     

Use of eco‐friendly epoxy resins from renewable resources as potential substitutes of petrochemical epoxy resins for ambient cured composites with flax reinforcements

In the last years, some high renewable content epoxy resins, derived from vegetable oils, have been developed at industrial level and are now commercially available; these can compete with petroleum‐based resins as thermoset matrices for composite materials. Nevertheless, due to the relatively high cost in comparison to petroleum‐based resins, their use is still restricted to applications with relatively low volume consumption such as model making, tuning components, nautical parts, special effects, outdoor sculptures, etc. in which, the use of composite laminates with carbon, aramid and, mainly, glass fibers is generalized by using hand layup and vacuum assisted resin transfer molding (VARTM) techniques due to low manufacturing costs and easy implementation. In this work, we study the behavior of two high renewable content epoxy resins derived from vegetable oils as potential substitutes of petroleum‐based epoxies in composite laminates with flax reinforcements by using the VARTM technique. The curing behavior of the different epoxy resins is compared in terms of the gel point and exothermicity profile by differential scanning calorimetry (DSC). In addition, overall performance of flax‐epoxy composites is compared with standardized mechanical (tensile, flexural and impact) and thermal (Vicat softening temperature, heat deflection temperature, thermo‐mechanical analysis) tests. The curing DSC profiles of the two eco‐friendly epoxy resins are similar to a conventional epoxy resin. They can be easily handled and processed by conventional VARTM process thus leading to composite laminates with flax with balanced mechanical and thermal properties, similar or even higher to a multipurpose epoxy resin. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Control of shrinkage and final conversion of vinyl ester resins cured in low‐temperature molding processes

Vinyl ester resin is a major thermoset polymer used in low‐temperature composite manufacturing processes such as the Seemann composite resin infusion‐molding process (SCRIMP). Volume shrinkage and residual styrene are important concerns for composites produced in such processes. A low‐shrinkage additive (LSA) is a typical agent added to control the volume shrinkage of vinyl ester resins during molding. In this study, the effects of LSA content and the temperature profile (the temperature gradient and peak temperature) on the volume shrinkage control of a vinyl ester resin were investigated. The reaction kinetics of the resin system were also studied. We achieved good volume shrinkage control if we raised the curing temperature slowly to allow sufficient time for phase separation and if the curing temperature reached a high value after phase separation to allow microvoid formation. On the basis of experimental results, we designed an improved SCRIMP to increase resin conversion, reduce resin shrinkage, and produce composites with better properties. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1486–1496, 2003     

Effects of inhibitors and retarders on low temperature free radical crosslinking polymerization between styrene and vinyl ester resin

Many composite manufacturing methods are room temperature processes, which require the resins to be cured at low temperatures. Examples are Seemann Composite Resin Infusion Molding Process (SCRIMP)—a vacuum assisted resin transfer molding (VARTM) process, hand lay‐up and spray‐up. Vinyl ester resins have been widely used in this type of processes because of their versatility as a composite matrix. Low temperature polymerization between styrene and vinyl ester tends to be complex because of the presence of different curing agents. This paper reports on (1) the effects of a promoter (cobalt naphthenate) on the initiatin at low temperatures, and (2) the effects fo a retarder (2,4‐pentanedione) and an inhibitor (1,4‐benzoquinone) on the pot life, gel time, and cure kinetics. A differntial scanning calorimeter (DSC) and a Fourier transform infrared (FTIR) spectrometer were used to investigate the reaction kinetics of vinyl ester resins. The influence of the retarders and inhibitors on the rheological changes of the resin during curing was also studied using a Rheometris Dynamic Analyzer (RDA).     

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     

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     

VARTM工艺用乙烯基酯树脂基体配方及性能研究

以溴化改性乙烯基酯树脂、氨基甲酸酯改性乙烯基酯树脂为混合基体,以E-玻璃纤维(GF)为增强材料,通过真空辅助树脂传递模塑(VARTM)工艺制备乙烯基酯树脂/GF复合材料,对其力学性能和阻燃性能进行了研究.结果表明,在两种树脂质量比为1:1时该复合材料的拉伸强度与弯曲强度达到最大值,且综合性能最优.氧指数随溴化改性乙烯基...     

A kinetic model for free‐radical crosslinking co‐polymerization of styrene/vinylester resin

Vinylester resin is a major thermoset polymer used In low temperature composite manufacturing processes, such as the Seemann Composite Resin Infusion Molding Process (SCRIMP). In this study, the effect of temperature on the cure kinetics of vinylester resin in the range of 35 to 90°C was investigated using a differential scanning calorimeter (DSC) and a Fourier transform infrared spectrometer (FTIR). A mechanistic kinetic model was developed to simulate the reaction rate and conversion profiles of vinylester vinylene and styrene vinyl groups, as well as the total reaction action rate and conversion. Experimental results from DSC and FUR at different temperatures were compared with model predictions. The glass transition temperature of the vinylester resin cured at different temperatures was identified and used to monitor the final conversion change. A series of SCRIMP molding experiments were conducted. The developed kinetic model in conjunction with a heat transfer model was used to simulate the temperature and conversion changes inside the SCRIMP‐molded composites.     

Composites from northern red oak (Quercus robur) leaves and plant oil‐based resins

Composite materials were successfully made out of plant oil‐based resin and northern red oak (Quercus robur) leaves collected in the fall. The viscosities of the bio‐based resins (MAESO and MAELO) were suited to high temperature resin transfer molding (RTM) and vacuum‐assisted resin transfer molding (VARTM) processing. The leaves have a waxy epidermal surface layer that needs to be dewaxed prior to use in a composite part. We used different treatments (such as benzene–ethanol, boiling water, and strong detergent) to dewax the leaves; all three methods seem to give good results; but boiling water and strong detergent were considered the best treatments for the dewaxing of leaves and boiling water was considered the greenest dewaxing method. The compatibility between the resin and the leaves was improved with a silane treatment which resulted in composites with higher mechanical stiffness than the resin itself. With 10 wt % leaves, we obtained an improvement in the composite modulus of about 14% from which we could estimate the leaf modulus at about 5.3 GPa. An alternative method to produce biocomposites from leaves without the need for silane treatments consists in carbonizing the leaves first at 215°C for 12 h, and then at 450°C for 1 h. The composites made with leaves and bioresins derived from functionalized triglycerides have the potential for use in high volume applications with low costs such as housing, construction, civil infrastructure, toys, and furniture. The use of leaves as a biocomposite filler has several advantages including (a) reduced cost, (b) improved properties of the resin, (c) composites with high bio content, (d) removal of a waste material with subsequent prevention of burning with attendant health hazards, and (e) interesting design aesthetics for interior and exterior decoration. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2013     

高性能、低成本复合材料用环氧体系的研究

通过凝胶化测试和示差扫描量热分析(DSC)研究了CYD128环氧树脂/部分钝化间苯二甲胺固化体系的反应特性,测试了该体系浇铸体的力学性能和耐热性。设计了一种新型低成本真空辅助传递模塑(VARTM)成型工艺并研究了该成型工艺对复合材料力学性能的影响。结果表明,浇铸体弯曲强度可达131.0 MPa,拉伸强度71.8 MPa,拉伸模量3.1 GPa,冲击强度37.5 kJ/m2,玻璃化温度(Tg)98.1℃。使用新工艺得到的复合材料力学性能优异,其中连续玻璃纤维、连续碳纤维增强复合材料弯曲强度分别达到950.2 MPa、1 097.4 MPa。     

Hygrothermal cycling effects on the durability of phenolic based composites

Work has been performed to investigate the thermal and mechanical properties of carbon fiber/phenolic resin composites as engineering materials for the aerospace industry. These materials are cost effective while displaying excellent temperature and fire resistance as well as good mechanical properties. All phenolic and epoxy composite specimens used here were prepared by resin transfer molding (RTM) to model a cost‐effective process. Hygrothermal cycling effects on the property changes of phenolic composites were evaluated through thermal, mechanical, and morphological tests. The fracture performance of a phenolic composite modified with a silicone‐based additive decreased after fewer hygrothermal cycles than unmodified phenolic and epoxy composites. Results from dynamic mechanical analysis (DMA) experiments showed that the modified phenolic composite was more significantly affected by the hygrothermal cycling than the unmodified phenolic composites. Fatigue tests showed that the phenolic composites that were not exposed to hygrothermal cycling had more resistance to fatigue cycles than the epoxy composites.     

气囊辅助RTM工艺用复合裙模具的设计与制备

针对某型号上用VARTM工艺制造的复合裙纤维体积含量偏低的缺点,提出用气囊辅助RTM工艺制造该复合裙。该工艺通过向气囊内充压,对预成型体施加一定的成型压力,从而提高构件纤维体积含量及力学性能。本文设计制备了气囊辅助RTM工艺用复合裙模具。工艺实验表明,借助该模具能顺利实施气囊辅助RTM工艺,制备出高性能的复合裙。     

Influence of epoxy sizing of carbon‐fiber on the properties of carbon fiber/cyanate ester composites

The high modulus carbon fiber (M40J) sized by epoxy resin E51 and E20 reinforced bisphenol A dicyanate (2,2′‐bis(4‐cyanatophenyl) isopropylidene resin composite was prepared in order to investigate the influence of epoxy sizing of the fiber on the properties of the composite. Differential scanning calorimetry (DSC) and fourier transforms infrared (FTIR) analysis showed that epoxy resin have catalytic effect on cure reaction of cyanate ester. Mechanical properties of the composite revealed that M40J fiber sized by epoxy resin could improve the flexural strength and interlaminar shear strength of M40J/bisphenol A dicyanate composites. The micro‐morphology of the composite fractures was studied by means of scanning electron microscopy (SEM). Reduced flaws were observed in the M40J‐bisphenol A dicyanate interface when the sized fiber was used. Water absorption of the composites was also investigated. It was found that the water absorption descended at the initial boiling stage (12 h). POLYM. COMPOS, 27: 591–598, 2006. © 2006 Society of Plastics Engineers     

Electrospinning core‐shell nanofibers for interfacial toughening and self‐healing of carbon‐fiber/epoxy composites

This article reports a novel hybrid multiscale carbon‐fiber/epoxy composite reinforced with self‐healing core‐shell nanofibers at interfaces. The ultrathin self‐healing fibers were fabricated by means of coelectrospinning, in which liquid dicyclopentadiene (DCPD) as the healing agent was enwrapped into polyacrylonitrile (PAN) to form core‐shell DCPD/PAN nanofibers. These core‐shell nanofibers were incorporated at interfaces of neighboring carbon‐fiber fabrics prior to resin infusion and formed into ultrathin self‐healing interlayers after resin infusion and curing. The core‐shell DCPD/PAN fibers are expected to function to self‐repair the interfacial damages in composite laminates, e.g., delamination. Wet layup, followed by vacuum‐assisted resin transfer molding (VARTM) technique, was used to process the proof‐of‐concept hybrid multiscale self‐healing composite. Three‐point bending test was utilized to evaluate the self‐healing effect of the core‐shell nanofibers on the flexural stiffness of the composite laminate after predamage failure. Experimental results indicate that the flexural stiffness of such novel self‐healing composite after predamage failure can be completely recovered by the self‐healing nanofiber interlayers. Scanning electron microscope (SEM) was utilized for fractographical analysis of the failed samples. SEM micrographs clearly evidenced the release of healing agent at laminate interfaces and the toughening and self‐healing mechanisms of the core‐shell nanofibers. This study expects a family of novel high‐strength, lightweight structural polymer composites with self‐healing function for potential use in aerospace and aeronautical structures, sports utilities, etc. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

大丝束CF/EP汽车地板VARTM模拟与高温力学性能

汽车轻量化是全球面临的共同问题,采用更具成本优势的大丝束碳纤维（CF）增强复合材料是实现汽车轻量化结构化的重要途径。然而,大丝束碳纤维在液体成型时,单束过多的纤维丝易导致纤维束内微观浸润困难,易产生干斑、气泡等缺陷。同时,传统的汽车电泳烘干工艺对复合材料的高温性能提出了挑战。鉴于此,本文采用0°/90°双轴向缝编大丝束碳纤布和耐高温环氧树脂（EP）,开展了纤维渗透率测试和汽车地板真空辅助树脂传递成型（VARTM）模拟优化研究,设计、制造了成型模具,成功试制出汽车地板样件,超景深显微镜观测显示纤维束内和层间浸润良好,无明显缺陷。高温在线拉伸和应变测试显示,温度对材料拉伸模量影响显著而对强度影响不大,180℃高温下应变恢复能力良好,表明该材料在高温下仍具备较好的强度和抗蠕变性能,该结果对指导复合材料能否通过传统汽车的电泳烘干工艺具有重要意义。     

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     

Control of volume shrinkage and residual styrene of unsaturated polyester resins cured at low temperatures. II. Effect of comonomer

The effect of a comonomer, methyl methacrylate (MMA), on volume shrinkage and residual styrene content of an unsaturated polyester (UP) resin with low profile additives (LPAs) cured at low temperature was investigated by an integrated reaction kinetics-morphology-property analysis. MMA affects the volume shrinkage and residual styrene content differently depending on MMA to styrene (St) CC bond molar ratio. At low MMA/St ratio, residual styrene decreases and the volume shrinkage of the resin system remains unchanged. At high MMA/St ratio, residual styrene can be substantially reduced, but the resin system suffers poor volume shrinkage control. Reactivity of the comonomer MMA and its compatibility to other components in the resin system can explain the observed results. A series of Seemann composites resin infusion molding process (SCRIMP) were conducted to study the relationship among materials, processing, and properties of molded composites in low temperature curing processes.