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This study investigated the influence of carboxyl-terminated butadiene acrylonitrile (CTBN) liquid rubbers on the microcracking response of polymeric composite materials to cryogenic cycling. Matrices of carbon fiber/epoxy prepregs were modified with different concentrations of two CTBN liquid rubbers. The glass transition temperature and the interlaminar shear strength of the laminate systems were depressed as a result of the presence of CTBN in the epoxy phase. An increase in total rubber concentration with the continuous phase was found to decrease and in some cases eliminate microcracking in laminates exposed to cryogenic cycling.

The impact of fiber-matrix adhesion on the transverse microcracking of fiber reinforced polymeric materials thermally cycled at cryogenic temperatures was investigated using symmetric cross-ply carbon fiber/epoxy laminates containing fibers with different surface treatments. Past research explored the role of fiber-matrix adhesion in determining the room temperature properties of composite materials, but this work is original in that it examined how fiber-matrix adhesion affected the behavior of composite materials at cryogenic temperatures. Three fiber surfaces were used: Unsized but exposed to an oxidative surface treatment, epoxy sized, and surfactant sized. Modifications of the fiber surfaces changed the adhesion of the matrix to the fibers as determined by interlaminar shear strength and dynamic mechanical analysis. The extent of microcracking in the laminates exhibited a dependence on fiber-matrix adhesion, with high levels of adhesion corresponding to decreased microcracking.

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.

A model prepreg system was used to evaluate the effect of cure temperature on microcracking in polymeric composite materials exposed to cryogenic cycling. Symmetric and unsymmetric carbon fiber/epoxy laminates were fabricated to examine the development of thermal stresses and microcracks at cryogenic temperatures. The residual strains and theoretical curvatures of the laminates were calculated from the composite properties and correlated with the microcrack density and experimentally observed curvatures. Higher cure temperatures resulted in higher stress free temperatures and residual strains in the laminates, which corresponded directly to increased levels of microcracking.

In an effort to understand the implications of the fiber/matrix interphase on the performance of aerospace grade composite materials, a study was performed to examine the properties of the interphase region on the micro-scale. An epoxy resin was cured with several curing agent systems to evaluate the material based variations in experimental detection of glass transitions with scanning thermal microscopy (SThM). After an appropriate material was selected, glass fibers with different finishes were impregnated with an epoxy/amine resin system, and the properties of the interphase regions were examined using scanning thermal microscopy.

Recently, new product designs have been suggested where it is necessary to vary the density of a composite part. In an earlier work, it was shown that the specific gravity of a cured laminate based on carbon fiber prepreg could be tailored to these applications by modifying the polymeric matrix with fine metal powders of various densities. This follow up study focused on altering the density of composites by changing the concentration of iron particles as a matrix modifier in prepreg materials. Experiments were performed to determine prepreg tack, laminate morphology and laminate fracture toughness. It was found that increasing the density of the materials with different concentrations of iron particles did not adversely effect toughness or interlaminar shear strength, although prepreg handling characteristics suffered slightly.

The matrices of carbon fiber/epoxy composites were modified with layered inorganic clays and a traditional filler to determine the effects of particle reinforcement, both micro and nano scale, on the response of these materials to cryogenic cycling. The mechanical properties of the laminates studied were not significantly altered through nanoclay modification of the matrix. The incorporation of nanoclay reinforcement in the proper concentration resulted in laminates with microcrack densities lower than those seen in the unmodified or macro-reinforced materials as a response to cryogenic cycling. Lower nanoclay concentrations resulted in a relatively insignificant reduction in microcracking and higher concentrations displayed a traditional filler effect.

A new family of particulate modifiers was incorporated into an epoxy-based model film adhesive system and the performance was evaluated. The particulate modifiers were selected to include a range of particle sizes, chemistry, and functionality. Thermal analysis, lap shear, and fracture energy tests were performed to characterize the performance of the adhesives. The mechanisms of failure for the adhesives were analyzed in relation to the particle modifier characteristics. Significant differences were found for mode I fracture energy when comparing adhesively joined composite specimens in cocured and bonded situations. Large preformed particle modified adhesives had nearly the same G IC values for both cocured and bonded applications, while the G IC values for the much smaller core-shell particle modified adhesives differed significantly. All particle modified adhesives provided an improvement in mode II fracture toughness over that of the control such that the laminates failed either in compression (through-thickness direction) or through delamination of the prepreg plies.

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.

A key aspect of composite technology lies in the ability to design a part to a specific strength and thickness. Recently, new applications were identified where it was desirable to vary the density in specific areas of a part. In this work, a model prepreg system was modified with fine metal and thermoplastic particles for specific weight tailoring. These particles were placed on the prepreg surface, forming a partially interlayer modified composite when laid up and cured. Experiments were performed on the specific gravity and fracture toughness of laminates made from the prepregs. The results showed that it is possible to tailor the density of a composite system without changing the manufacturing process or fracture toughness requirements.

Four different size distributions of preformed rubber particles were used to interlayer toughening carbon fiber composites. Prepregs were made using single pass impregnation with model epoxy resins containing the different preformed particle distributions at two concentrations. The particle distributions and average particle sizes were selected to investigate the effect on laminate structure and fracture toughness. It was found that the average particle size affected the interlayer formation in the cured laminate. A significant difference in the fracture toughness of the laminates was found due to a change in the particle size distribution, average particle size, concentration, and resultant laminate structures. In this study, it is shown that the particle size distribution must be taken into account when designing interlayer toughened composites so that mode I and mode II toughness can be optimized.

High-performance film adhesives are often employed in aerospace composite structures to provide adequate adhesion between substrates. This study was performed to evaluate the fracture properties of a commercial film adhesive and a carbon fiber prepreg material in both co-cured and bonded applications. In addition, the fundamental reasons for macroscopic fracture properties were evaluated on the microscale using interphase analysis and surface analysis techniques. From this information, it was possible to examine the commingling of prepreg and adhesive resins in the bond line interphase region. The information presented in this study points to a critical aspect of material selection and utilization that is often not considered in composite design: resin compatibility. *Dedicated to Prof. Francisco J. Balta Calleja on the occasion of his 65th birthday.

A review is presented that focuses on preformed particle modification of high performance thermosetting resins and composite systems. The modifiers reviewed consist of thermoplastic and rubber preformed particles with no size limitations. Both organic and inorganic preformed polymer particles are considered but not glass or hollow spheres. In this text, preformed particles are defined as those which do not require phase separation and remain in the shape in which they were added to the neat resin or composite. Therefore, these particles may be developed prior to the resin formulation and then added to the thermosetting resin or developed in situ (during resin formulation) before the resin is catalyzed or cured. This technical review of preformed particle modification of thermosetting resins and composite systems summarizes the utilization of these materials and their performance.

Model 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%.

Previous work on the characterization of interphase regions in thermoset- ting composite systems has focused on the inference of an interphase layer from effects noticed through macroscale mechanical and thermal testing. With the development of atomic force microscopy and active thermal probes for this technique, it is now possible to examine material thermal properties on a much smaller scale. Variations in mi- croscale thermal properties of an aerospace-grade thermosetting resin system were evaluated for carbon and glass fiber reinforcement, using the modulated local thermal analysis mode of a TA Instruments 2990 TA. The variations observed clearly demon- strate the presence of a soft interphase layer in the glass material and underline the importance of fiber-matrix interactions during the formation of the interphase. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1643-1649, 2001

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.

Low temperature cure cyanate ester resin systems were developed and modified with epoxy-terminated butadiene acrylonitrile rubber (ETBN) and impregnated into woven glass fabric. Mode I and mode II interlaminar fracture toughness values of the cured laminates were evaluated as a function of rubber concentration. Mode I fracture toughness increased to almost twice that of the unmodified system, while mode II fracture toughness remained essentially unchanged. Composite samples were subjected to aging experiments in water and the absorption/desorption behavior was investigated as was the effect on thermal performance. The presence of rubber was found to reduce the rate of matrix deterioration but also caused a substantial increase in water uptake. It was found that although the addition of rubber to the matrices decreased the unconditioned (dry) T g all specimens showed the same reduction in T g after equilibrium water absorption.

Epoxy-based film adhesive formulations were developed with 10 wt % solid carboxyl functional rubber. Due to the high rubber content and resulting viscosity restrictions, the rubber could not be prereacted with the epoxy before hot-melt filming. Therefore, an esterification catalyst was used to perform this reaction in situ before the epoxy curing reactions. The performance of this adhesive system is compared to that of one without the esterification catalyst. A significant difference in the flow characteristics was observed with incorporation of the esterification catalyst, but only small variations in mechanical performance were found.

Glass transitions of amorphous polystyrenes with low polydispersity were evaluated using the modulated Local Thermal Analysis mode of the TA Instruments 2990 µ TA and evaluating the thermomechanical signal. Transition temperature variance and fraction of transitions measured were compared for high molecular mass thermosetting materials and the melt of Nylon 6.6. The transition reproducibility was found to decrease as the molecular size of the polymer samples increased. Reproducibility also decreased for thermosetting materials when the experimental ramp rate was decreased. Heat transfer within the specimen was evaluated using finite element analysis, allowing scaling of microscale experimental results for comparison to bulk transitions.

Model 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.

This work identifies the necessary attributes of variable temperature cure epoxy-based 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 G IC and G IIC 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 G IIC for the commercial prepreg was observed for the two cure temperatures.

Porosity 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.

The effects of fiber tension, line speed, and impregnation temperature and pressure in the prepregging process used to manufacture a commercial high temperature toughened epoxy prepreg were investigated in a Design of Experiments (DOE) to understand core crush in honeycomb composite structures. The prepregs developed in this DOE were characterized by tack, permeation, optical microscopy, and frictional resistance. Of these methods, frictional resistance was found to correlate with core crush. Tack, permeation, and optical microscopy provided a basis for understanding this relationship through impregnation and morphology. Prepregs manufactured with high fiber tension showed greater crush and less frictional resistance than prepregs with manufactured low fiber tension. These low tension prepregs were found to have more fibers at the prepreg surface, allowing them to grip the adjacent surface and resist slippage. By identifying the key factors influencing honeycomb core crush, the prepregging process was modified, producing a crush-resistant prepreg for end use manufacture.

The effects of fabric tension and number of impregnation rollers used during hot-melt prepreg processing were investigated as they relate to woven fabric prepreg and final composite part quality. Specifically, the processing parameters varied in a design of experiments (DOE) were applied fabric tension and the number of impregnation rollers, while maintaining the same impregnation force. Six experimental prepregs were characterized in terms of prepreg thickness, resin content, tack, and morphology. The results show that fabric tension had a large influence on the prepreg characteristics due to more cylindrical tow shapes and a ridged fiber bed. The number of impregnation rollers was found to affect only the characteristics of the prepregs manufactured with no tension. To investigate the effects of tension on cured composites, six ply laminates were made with each experimental prepreg. Two cure cycles, differing only in consolidation pressures, were used to examine the void content and morphology in the cured laminates. The laminates made with prepregs manufactured with high tension and cured only under vacuum had a greater void content than laminates made with prepregs manufactured with no tension. In both cure cycles, the tow shape in the laminates made from the prepreg manufactured with high tension remained in almost the same shape as in the uncured prepreg. The void content, however, was negligible for all laminates cured at higher consolidation pressure.

A controlled-flow epoxy-based model prepreg resin system was developed. The formulation of the model controlled-flow resin was designed from performance information obtained from a commercially available controlled-flow resin, presently used in the aircraft industry. Thermoanalytical techniques including rheometry were used to provide the necessary information to develop the model system along with a formulation methodology developed by Seferis and co-workers. The model resin formulation, which was a combination of tetraglycidyl ether of methylenedianiline (TGMDA), diglycidyl ether of bisphenol-A (DGEBA), carboxyl-modified butadiene/acrylonitrile rubber (CMBN), carboxyl-terminated butadiene / acrylonitrile rubber (CTBN), bisphenol-A (BPA), diaminodiphenyl sulfone (DDS), and dicyandiamide (DICY), was hot-melt impregnated into unidirectional carbon fibers on a laboratory scale hot-melt prepreg machine. A two-parameter, three-level design of experiments was performed on the prepreg processing parameters in which impregnation temperature and pressure were varied. Thus, a total of nine different experimental prepregs were produced and characterized by resin content, extent of impregnation, and tack. The results from the characterization of the nine experimental prepregs are compared with the effects of the prepreg processing conditions. These results are also compared with the results generated for the commercial controlled-flow resin. Collectively, this work provides a fundamental basis by which the analysis and rational utilization of controlled-flow matrix prepregs can be effected.

The composite methodology, developed by Seferis and coworkers, was used to describe the kinetics for both a commercial and a model controlled-flow epoxy-based resin system throughout cure and degradation. By utilizing this previously developed generalized methodology, capable of describing two or more kinetic mechanisms acting in series or parallel, a fundamental understanding of the kinetic behavior of a prepreg system from cure through degradation was established. Differential scanning calorimetry (DSC), and simultaneous differential thermal analysis-thermogravimetric analysis (SDT) were utilized to provide the experimental kinetic information. Two approaches were used to determine the activation energies for each of the resin systems, and a comparison is made between these approaches and the two thermoanalytical techniques. Using the determined kinetic parameters, the kinetic model was compared with experimental kinetics throughout cure and degradation at heating rates from 2-20°C/min. The results show that the kinetic model fits the experimental data well. In addition, the results demonstrate that the same weighting factors are applicable to both the model and commercial controlled-flow resins.

Lightning damage in polymer composites is manifested by damage at both the macroscopic or visual level and within the material microstructure. This article illustrates the effects of the laboratory-generated lightning strikes on polymeric composites.

This article focuses on the sample preparation methods for titanium honeycomb composites, boron fiber composites, and titanium/polymeric composite hybrids. These include mounting, sectioning, grinding, and polishing. The article also provides information on the sample preparation of unstaged and staged prepreg materials for optical analysis.

Microstructural analysis of the composite matrix is necessary to understand the performance of the part and its long-term durability. This article focuses on the microstructural analysis of engineering thermoplastic-matrix composites and the influence of cooling rate and nucleation on the formation of spherulites in high-temperature thermoplastic-matrix carbon-fiber-reinforced composites. It also describes the microstructural analysis of a bio-based thermosetting-matrix natural fiber composite system.

This article describes how composite specimens are sectioned, documented, and labeled during sample preparation. The mounting procedures for the specimen are summarized. The article explains sample clamping, which involves not mounting the specimens using an adhesive or casting resin and corresponds to clamped samples used in automated polishing heads. It details that cavity molds involve mounting the composite specimens using a casting resin in a preset mold. The article also discusses the mounting of composite materials for hand polishing.

This article illustrates the polymer matrices used for composite materials. It describes the use of prepeg materials in manufacturing high-performance composites. The article discusses the various infusion processes for the development of fiber-reinforced composites, namely, resin transfer molding, vacuum-assisted resin transfer molding, and resin film infusion. It explains the composite- and matrix-toughening methods for fiber-reinforced composites, such as dispersed-phase toughening and interlayer toughening. The article concludes with information on optical microscopy, which provides an insight into the micro- and macrostructure of fiber-reinforced composites.

Analyzing the structure of composite materials is essential for understanding how the part will perform in service. Assessing fiber volume variations, void content, ply orientation variability, and foreign object inclusions helps in preventing degradation of composite performance. This article describes the optical microscopy and bright-field illumination techniques involved in analyzing ply terminations, prepreg plies, splices, and fiber orientation to provide the insight necessary for optimizing composite structure and performance.

This article describes the dispersed-phase toughening of thermoset matrices by the development of multiphase-structure thermosetting matrices using rubber and/or thermoplastic materials. It discusses two main methods for manufacturing prepregs, namely, single-pass impregnation and double-pass impregnation. The article illustrates reflected-light optical microscopy techniques to evaluate the morphology of thermoplastic materials for determining the material quality and correlating key microstructural features with material performance.

Rough grinding and polishing of specimens are required to prepare fiber-reinforced composite samples for optical analysis. This article discusses the consumables, process variables, and the equipment that influence the sample preparation procedure. It describes the hand and automated grinding methods. The article summarizes the rough and final polishing steps for both hand and automated techniques. Common artifacts that may be created during grinding and polishing steps of composite samples are reviewed. These include scratches, fiber pull-out, matrix smears, streaks, erosion of different phases, and fiber and sample edge rounding and relief.

Honeycomb-cored sandwich panels increase part stiffness at a lower weight than monolithic composite materials. This article illustrates an area of a honeycomb-cored sandwich structure composite cross section that is viewed using transmitted polarized light. This area shows the differences in the constituents and resin intermingling. The article discusses the factors that govern the honeycomb core movement and honeycomb core crush, with illustrations. Some common tests performed on honeycomb composites to characterize the skin-to-core bond strength are the climbing drum peel and flatwise tensile tests. The article concludes with a description on the reasons for core failure, which are analyzed after these tests.

Polymer composite materials are subject to degradation if not appropriately protected from the environment. This article describes the effects of heat and atomic oxygen and ultraviolet-light on composite material surfaces, with illustrations.

This article describes the microcrack analysis of composite materials using bright-field illumination, polarized light, dyes, dark-field illumination, and epi-fluorescence.

This article describes methods for analyzing impact-damaged composites in the aircraft industry. These include C-scan and x-radiography methods and optical microscopy. The article reviews brittle-matrix composite and tough-matrix composite failures. It explains the different types of composite failure mechanisms such as thermoplastic-matrix composite failure mechanisms, untoughened thermoset-matrix composite failure mechanisms, toughened thermoset-matrix composite failure mechanisms, dispersed-phase and rubber-toughened thermoset-matrix composite failure mechanisms, and particle interlayer-toughened composite failure mechanisms.

This article describes the various aspects relating to the selection and preparation of ultrathin-section specimens of fiber-reinforced polymeric composites for examination by transmitted light microscopy. It provides information on the contrast-enhancement methods used by transmitted-light microscopy and optimization of microscope conditions. Examples of composite ultrathin sections analyzed using transmitted-light microscopy contrast methods are also presented.

Voids in fiber-reinforced composite materials are areas that are absent of the composite components: matrix (resin) and fibers. Voids have many causes but generally can be categorized as voids due to volatiles or as voids that result from entrapped air. This article describes the analysis of various types of voids. It reviews techniques for analysis of voids at ply-drops, voids due to high fiber packing, and voids that occur in honeycomb core composites. The final section of the article discusses void documentation through the use of nondestructive inspection techniques and density/specific gravity measurement methods.

API has found that with the use of phosphorous containing compounds, used in the textile industry, substantial reduction in FST properties can be realized for VaRTM resins systems without loss in mechanical and physical properties. With using this new technology, API was able to develop an epoxy based resin system that exceeds the present state of art epoxy VaRTM resin formulations. Collectively, as stated in the objective of this SBIR, API was able to develop a VaRTM process able resin that had equal cone calorimeter results as Derakane 510-A, but had at least a 65% lower smoke value.

Publisher Summary The brittle nature of highly crosslinked epoxy resins has limited their use in many areas. To overcome this drawback, thermally stable thermoplastics have been blended with epoxy resins in an attempt to increase their fracture toughness. One such engineering thermoplastic is polyetherimide (PEI). In this investigation, solvent-free, PEI modified epoxy-based resins capable of variable temperature cure are developed and hot-melt impregnated into woven carbon fabric. The prepreg tack, porosity in vacuum cured parts, and interlaminar fracture toughness of prepreg-based laminates are investigated as a function of PEI content in the base epoxy matrix. In this chapter, solvent free PEI epoxy blend hot-melt prepreg systems are developed that are capable of being cured with both 121°C and 177°C cure cycles while achieving similar final properties. For a prepreg system to be considered variable temperature cure, a final conversion resulting in less than 10% difference is necessary when cured at either cure cycle. Therefore, a high conversion at low temperature cure is required. It is found that relatively “dry” prepreg resin systems resulted in the lowest void content in vacuum cured parts. It is found that the PEI contained in the model resin completely phase separated when cured and formed a discontinuous phase. Through this investigation the resin, prepreg, and laminate characteristics are identified that provide the basis of variable temperature cure prepreg system necessary for high-performance applications and repair purposes.