Prepreg Systems Research Articles

Prepreg age monitoring via differential scanning calorimetry

Fabrication of composite parts from prepregs often requires layup and preparation times of days and even weeks, during which prepregs undergo room-temperature aging. The aging process can compromise compaction, tack, and overall quality of composite parts, and thus a need exists for an accurate and convenient method to monitor the extent of prepreg aging as a function of out-time. Here, we report a method to monitor prepreg age, which involves measurement of changes in glass transition temperature as a function of room-temperature aging time. Samples from three out-of-autoclave prepreg systems were aged in ambient conditions and tested periodically using modulated differential scanning calorimetry. A linear increase in glass transition temperature with prepreg age was noted. Results are discussed in the context of monitoring the chemical aging of epoxy resins that occurs at ambient temperature.

On the interply friction of different generations of carbon/epoxy prepreg systems

Using pre-stacked material that is formed as a first step in the manufacturing process offers reduced process cycle time for production of complex structural components. The forming is achieved by forcing layers to deform by e.g. intraply deformation and interply slippage, where the latter is the scope of this study. The prepreg–prepreg friction is experimentally determined for four unidirectional carbon/epoxy prepreg systems. The materials differ considering volume fraction of fibres, fibre stiffness and phase of thermoplastic toughener (solved or particles). The study shows large individual differences between the tested materials, where the material systems with particle tougheners seem to obey a boundary lubrication friction, while the other materials show hydrodynamically dominated friction. A large difference between the high and low friction materials, almost a factor of 10, correlates to trends seen in the herein performed surface roughness measurements. Vacuum as well as autoclave consolidated materials are tested.

Structural and material properties of a rapidly cured thermoplastic‐toughened epoxy system

AbstractThermoplastic‐toughened epoxy resins are widely used as matrices in modern composite prepreg systems. Rapid curing of thermoplastic‐toughened epoxy matrix composites results in different mechanical properties. To investigate the structure–property relationship, we investigated a poly(ether sulfone)‐modified triglycidylaminophenol/4,4′‐diamino diphenyl sulfone system that was cured at different heating rates. An intermediate dwell was also applied during the rapid heating of the thermoplastic‐modified epoxy system. We found that a higher heating rate led to a larger domain size of the phase‐separated macrostructure and also facilitated more complete phase separation. The intermediate dwell helped phase separation to proceed even further, leading to an even larger domain size of the macrostructure. A carbon‐fiber‐reinforced polymer matrix composite prepreg based on the poly(ether sulfone)‐modified multifunctional epoxy system was cured with the same schedule. The rapidly heated composite laminates exhibited higher mode I delamination fracture toughness than the slowly heated material. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Improvement in Fiber-Reinforced Polymer–Concrete Bond by External Pressure

An experimental study evaluated the improvement in the bond of fiber-reinforced polymer (FRP) and concrete to dry and wet substrates from applied pressure. In the study, 12 full-sized pile specimens, including four controls, were wrapped by using two different glass FRP systems–- a prepreg and a wet layup–-and two different layouts typically used for unidirectional and bidirectional fibers. Wrapping was conducted inside a partially filled tank to ensure that the bonded areas of the dry and wet regions were identical. Sustained pressure was maintained during curing by using pressure or vacuum bagging. Bond improvement was evaluated from more than 400 pull out tests. Results showed that external pressure led to improved bond in both the dry and submerged regions. However, vacuum bagging was better for prepreg systems, whereas pressure bagging was better for wet layups. Transverse fiber layout typically used with bidirectional fibers gave better bond in controls where no external pressure was applied.

ANALYSIS OF CURE REACTION KINETICS OF ARAMID/TOUGHENED-EPOXY PREPREG SYSTEM AIMING AT PROCESSING OF COMPOSITE MATERIAL FOR STRUCTURAL APPLICATIONS

This work is aimed at investigating the cure, kinetics and rheological parameters of aramid/toughened epoxy prepreg. The cure reaction evolves simultaneously with changes in rheology, which is normally referred to as rheo-kinetic behavior. Understanding the cure behavior of a resin system is essential in the development and optimization of composite fabrication processes. The cure kinetics and rheological behavior were evaluated using a differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and a rheometer. Values of the kinetic parameters were obtained from dynamic DSC scans using an nth order reaction model. Rheological measurements as a function of temperature and time were made for the prepreg system. With the determination of the kinetics for the cure and of the rheological parameters of the prepreg systems, cure could be established that lead to manufacturing high-performance composite. The intention is to present a simple methodology to devise cure cycles for the automotive structural parts, therefore a simple model was selected which worked adequately for the desired accuracy. The effect of toughener was not taken into consideration as the scope was to aid the decision about the curing process.

Development of a Representative Unit Cell Model for Bi-axial NCF Composites

Composite materials fabricated from non-crimp fabric (NCF) are showing potential in primary aerospace components and structures. They can be used in common RFI, RIFT, and RTM manufacturing processes, and are showing promise to replace traditional pre-preg systems. The composite designer hence needs a technique, which can be used to predict the elastic properties of the NCF laminate including manufacturing defects from these infusion processes. The present study develops a representative unit cell (RUC) model of the NCF blanket, which can include potential variability in the NCF structure. Representative unit cells have been developed to predict the two- and three-dimensional mechanical properties in the bi-axial NCF composite. The model is based on the two-dimensional methods of Naik and Scida for woven fiber composites, incorporating the elastic anisotropy theory of Lekhnitskii to extend the predictions to three dimensions. Continuous tow layers with uniform thickness and fiber volume fraction are assumed, and the effects of crimp due to stitching and surrounding resin regions are simulated. The input data are based on measurements and observations of a ±45° carbon-epoxy NCF composite having a 63% fiber volume fraction and less than 1% voids. Output from the model is obtained in terms of the stiffness and compliance matrices as well as traditional engineering properties. For the tensile and compressive moduli, comparisons with data from the carbon/epoxy NCF composite show the values are over-predicted by 11.5—11.6% in tension and 13.5—13.9% in compression. Improved predictions are obtained using a simple stiffness model that allows for different behaviors of the resin as continuous layers or gaps between tows. The resin layers, compared to the resin as gaps, significantly decrease the in-plane moduli, and the model is considered to simulate local through-thickness shear deformation in the resin layers.

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.

Long‐term behavior of partially cured epoxy prepreg systems under tensile loading

AbstractFor the determination of the influence of processing conditions on the long‐term behavior of high performance thermoset composites, two different glass fiber laminate systems with varying degrees of cure were examined. The laminates were conditioned in both moist and dry environments, and characterized via thermoanalytical and mechanical investigations. During the progression of moisture conditioning, both systems exhibited a reduction in the measured glass transition temperature as a result of the plasticization of the matrix. Tensile tests revealed that a direct correlation exists between the processing conditions and the long‐term performance. Employing a linear model, the long‐term mechanical behavior as a function of processing and moisture absorption can be described. Thus, the manufacturer has access to a tool for calculating the mechanical and thermomechanical properties depending on the processing and environmental conditions. Polym. Compos. 25:289–297, 2004. © 2004 Society of Plastics Engineers.

Effects of yarn constitutions and fabric specifications on electrical properties of hybrid woven fabrics

Abstract This paper reports a fabrication method for conductive open-end friction core-spun yarn (OFCY), which, as described, are constructed from open-end friction spun yarn (OFSY) and core stainless steel wire (SW) combined within the yarn twisting zone of the friction drum during the open-end friction spinning process. To facilitate weaving of SW and reduce the material cost, initially, the sheath stainless steel (SS), Kevlar (KS) or rayon (RS) staple fiber and core stainless steel wire are combined to form the OFCY yarn by using a DREF III open-end friction spinning method. The OFCY yarns were successfully woven into a variety of woven structures produced from a semi-automatic loom. The primary objects of this research are to develop new conductive yarns and fabrics, which could be suitable for shielding home electronic devices, electrical appliances from electromagnetic fields. Such fabrics could be incorporated into prepreg systems used for composite manufacture. In these fabrics, SW is used as a core component that is wrapped around a sheath SS and KS or RS to form an OFCY yarn in which OFCY yarn strength remains the same and electrical properties are better than for an equivalent OFSY yarn. Stainless steel staple fibers (SS) and wire (SW) are incorporated as conductive fillers to enhance the electromagnetic shielding properties of the woven fabric. The electromagnetic shielding effectiveness (EMSE) of various woven fabrics was obtained using a coaxial transmission line holder in the frequency range of 300 kHz–3 GHz. The variations of EMSE, the surface and volume resistivities of the woven fabric structure, and blend ratios of the OFCY yarns are also described.

Interlayer toughened unidirectional carbon prepreg systems: effect of preformed particle morphology

Abstract A new production process was used to change the surface morphology and reduce the water sensitivity of a commercial preformed rubber particle used for interlayer composite toughening. These particles were incorporated in a model epoxy resin and impregnated into unidirectional carbon fibers. The mode II fracture toughness of a laminate made with experimental particles was 250% higher than the control system and 100% higher than a laminate made with the commercial preformed rubber particles. The new particles were also found to reduce the damage area resulting from impact; however, the ultimate laminate compression strength after impact was lower for the experimental particle modified composite than the commercial particle modified laminate. The composites were also subjected to hot–wet conditions and the initial water absorption rate was less for the experimental particle modified laminates than the laminate containing the commercial material. Yet, after 6400 h, the laminates made with experimental particles were found to absorb more water than the other materials.

Effect of Prepreg Geometry on the Prepreg and Plain Weave Composite Properties

Studies on the physical and mechanical properties of the different continuous fiber impregnated thermoplastic composite (COFIT) prepreg geometry such as width and thickness were carried out. The effect of using these different types of prepreg geometries in the making of a woven composite system were examined too. As a result of the pultrusion type prepregging system which is a prototype machine, three types of prepregs based on width and thickness can be classified. The correlation of the prepreg geometry with the end properties of these three prepreg systems was analyzed. It was found that the properties of the prepreg are governed by the geometry of the prepreg, which further influences the properties of the woven composite system.

Matrix and fiber influences on the cryogenic microcracking of carbon fiber/epoxy composites

Abstract Cryogenic cycling effects on symmetric carbon fiber/epoxy laminates were examined using model prepreg systems. The properties of the composite materials studied were altered through the introduction of variations in their structure and composition. The curing agent used, matrix backbone flexibility, toughening agents, and longitudinal coefficient of thermal expansion of the reinforcing fibers were changed to investigate their role in cryogenic microcracking. Examination of the laminates after cycling provided insight into the mechanism and origins of thermal stress-induced microcracking. Matrix properties and fiber tensile modulus were shown to have a significant impact on the response of the composite materials to cryogenic cycling. It was found in this study that higher glass transition temperatures of the laminates and the presence of toughening agents in the matrix decreased the microcracking propensity of these laminates. Higher tensile moduli and linear coefficients of thermal expansion of the fibers were found to increase the microcrack density in the laminates studied.

Toughened carbon fiber prepregs using combined liquid and preformed rubber materials

AbstractModel epoxy resin formulations were modified with defferent concentrations of liquid rubber and preformed crosslinked rubber particles and impregnated into unidirectional carbon fibers. The liquid rubber was used to increase the mode I fracture toughness of the interlayer toughened prepreg systems, which already have high mode II fracture properties. In this paper it is shown that the mode II fracture toughness is not sacrificed by the incorporatiom of carboxyl terminated butadiene acrylonitrile rubber (CTBN) in the matrices, while mode I fracture toughness can be increased by as much as 100%.

Scaling complications of dual temperature cure resin prepreg systems in airplane part manufacture

Carbon and glass fabric epoxy prepreg with a highly accelerated curing system were compared and used to build a Boeing 737 flap hinge fairing production part. The prepreg materials, uncured and cured, were analyzed to determine physical and mechanical properties. Initial evaluations of the carbon prepreg indicated unique and desirable characteristics including dual temperature cure and excellent handling characteristics. Therefore, this material was selected for the use in the development of a production part. However, the part selected for manufacture, a Boeing 737 flap hinge fairing, required the use of both carbon and glass fabric prepregs. Therefore, the resin system, which was developed for carbon prepreg, had to also be impregnated into glass fabric. After cure of the part, it was found that areas on the part were scorched including the bagging material. This scorching led to an investigation of the exotherm produced by the accelerated resin in the cure of the carbon and glass prepreg materials. Two sets of test panels were made to simulate optimal and poor processing conditions for the glass and carbon fiber fabric prepregs. Thermocouples were placed in the layers of prepreg to track the temperature effects during the curing process. It was found that the glass fabric prepreg did not conduct the heat produced by the curing exotherm as well as the carbon material and therefore was the cause of material degradation or charring of the components in the composite system.

Processing of highly elastomeric toughened cyanate esters through a modified resin transfer molding technique

AbstractModel cyanate ester resins containing different quantities of epoxy functional butadiene‐acrylonitrile rubber (ETBN) to improve the fracture performance were developed as matrices for composites. With the elastomeric modification, the resin systems exhibited rheological characteristics inappropriate for laminate fabrication by conventional resin transfer molding (RTM). To fabricate the carbon fiber based laminates in one liquid molding operation successfully, a process named bleed resin transfer molding (BRTM) was established. The BRTM process combines features of RTM and resin film infusion processes (RFI) and was therefore appropriate for processing high viscosity matrix resins. A novel catalyst was selected for the cyanate ester resin that provided enough latency for the impregnation steps in the BRTM process. Through the use of thermal analytical tools, a high degree of phase separation and conversion was obtained. The conversion and the glass transition temperature were found not to decrease with increasing elastomer content, which is in contradiction to most toughening modifications. Mode I and Mode II interlaminar fracture toughness were found to increase significantly with increasing elastomer content. In Mode I, an increase of up to 140% was observed. Collectively, this work showed that through the use of the BRTM technique, matrices with toughness improvements usually only achieved by prepreg systems can be processed in an RTM‐like manner.

Variable temperature cure polyetherimide epoxy‐based prepreg systems

AbstractThis work identifies the necessary attributes of variable temperature cure epoxybased prepreg systems as they relate to high performance prepreg systems capable for composite repair. Model polyetherimide epoxy blend resins were developed and hot‐melt impregnated into woven carbon fabric and compared with a commercial prepreg system. It was found that when the PEI content was increased from 0 to 14 wt% in the base resin of the prepregs, the GIC and GIIC fracture toughness increased by over 70%. The fracture toughness was found to be similar when the model prepreg was cured at either 121°C or 177°C, a result of only a 9% difference in conversion and complete phase separation of the PEI at both cure temperatures. Void content in vacuum cured laminates were found to decrease as the PEI content was increased because of a large quantity of resin in the interstitial areas between the longitudinal and transverse tows. A comparison of the model and commercial prepreg system demonstrated many similarities and some significant differences. For example, the commercial prepreg had a 15% difference in conversion when cured at 121°C versus 177°C and very little PEI phase separation after both cure cycles. As a result, a significant difference in GIIC for the commercial prepreg was observed for the two cure temperatures.

Self‐adhesive honeycomb prepreg systems for secondary structural applications

AbstractPorosity in cured honeycomb parts was investigated using model prepreg systems and compared with a commercial solvent produced prepreg. Woven glass fabric was used as the reinforcement in both of the systems. Three main solvents were identified in the commercial prepreg. These solvents were found to cause porosity or reduce thermal properties in model solvent prepreg based laminates when used either separately or in combination. Lowering the solvent content reduced the porosity in the prepreg but was limited by resin staging. The high level of porosity, resulting from solvents, found in these commercial prepreg systems motivated the investigation of the feasibility of changing the method of impregnation from solvent to hot‐melt. Hot‐melt model prepreg produced essentially void‐free laminates as well as honeycomb parts. Model prepreg produced by hot‐melt impregnation was found to be more robust, providing the capability of changing the degree of resin impregnation in the fiber bed. A low level of impregnation resulted in the best fillets to the honeycomb core, while still producing essentially void free parts. The use of both solid and liquid carboxyl‐functional elastomers was incorporated in the epoxy based resin to provide flow control and self‐adhesive characteristics. Mechanical properties of both the model hot‐melt and commercial prepreg based honeycomb structures and laminates were compared. The model hot‐melt prepreg was found to have better cured properties along with the necessary prepreg characteristics providing an optimized self‐adhesive controlled flow prepreg system.

Thermal characterization and comparison of structural prepregs with different cure temperatures

Abstract Two different epoxy based prepreg systems were characterized and compared using thermal analysis. A prepreg system presently used in the commercial airplane industry was compared with a prepreg system that is a prospective candidate for the same applications. The commercial system in use is a controlled flow resin prepreg system which is a 177°C cure system, while the prospective prepreg system is marketed as a dual prepreg system that can be cured at either 121°C or 177°C. Thermal characterization techniques including differential scanning calorimetry (DSC), dielectric analysis (DEA), and dynamic mechanical analysis (DMA) were used to investigate these systems. The difference in the curing mechanisms of both prepreg systems were identified through these thermal analysis techniques. Although, the kinetics of these systems were found to be vastly different their heats of reactions were very similar. The activation energies for the prepreg systems were determined by DSC using Kissinger's method and were found to be quite different. DMA measurements on autoclave cured composites demonstrated that the prepregs obtained a different degree of cure as well as different glass transition temperatures ( T g ). Furthermore, the use of DEA and DMA demonstrated a difference in gelation of the two prepreg systems examined.

Phase Separation in Thermosetting Blends Studied by Dynamic Dielectric Analysis

Phase separation in unreinforced reactive blends has been studied using a variety of experimental techniques. A brief review of such studies is given, followed by a detailed description of the theory and practical applications of the new technique of dynamic dielectric analysis, DDA, as used in this context. Methods of data analysis and the extent of information available from in situ, real-time, measurements by dynamic dielectric analysis are illustrated by examples of results on model blends of simple epoxy/hardener mixtures with CTBN or POP rubber, or thermoplastic PES or PMMA. Deliberate ion doping is shown to enhance the phase separation features in the dielectric signal. Data are also presented on complex commercial thermoset/thermoplastic blends utilized as the resin matrices in carbon fibre pre-preg systems. Phase separation in a blend of epoxy and thermoplastic PEI can be studied by DDA within a glass fibre composite; this presents no additional difficulty compared with the studies of the corresponding unreinforced blend.

Frictional resistance of thermoset prepregs and its influence on honeycomb composite processing

Abstract A testing device was developed to measure the frictional resistance of woven prepregs at temperatures and consolidation pressures typical of those used in honeycomb sandwich processing. To demonstrate the use of this new device, the frictional resistance of three woven carbon fibre/epoxy prepreg systems were measured. It was found that frictional characteristics vary between prepregs and that these differences do not exhibit a one-to-one correlation with the viscosity of the matrix resin or prepreg. Further, prepreg exhibiting lesser frictional resistance showed greater degrees of core crush. These observed differences were shown to be dependent on both resin viscosity and distribution of the resin on the prepreg surface. Finally, integration of these results into the manufacture of test panels suggests that core crush observed in honeycomb production may be strongly influenced by frictional resistance changes during cure.