Prepreg Processing Research Articles

High-temperature wave-transparent and intrinsically flame-retardant phthalonitrile/quartz composite based on a straightforward hot-melt prepreg process

Wave-transparent composite materials are becoming increasingly critical for advancing robust human-machine interaction systems. A pivotal component of these systems is an organic resin matrix, which is currently the focus of intensive research. Herein, to develop a phthalonitrile resin capable of withstanding temperatures exceeding 500 °C, two types of Si/P-containing alkynyl curing agents (Si-ALK and P-ALK) were synthesized and integrated with sulfur-enriched phthalonitrile monomers (SSS-PN) to fabricate an organic-inorganic hybrid phthalonitrile（PN）resin. The resulting hybrid resin demonstrated exceptional thermal stability (594 °C for 10 % Si-PN-450 °C; 626 °C for 50 % P-PN-450 °C) and thermal oxygen stability (569 °C for 10 % Si-PN-450 °C; 594 °C for 30 % P-PN-450 °C). The Si-ALK/P-ALK and SSS-PN blends maintained stable low-viscosity properties for an extended period of up to 180 min, which is advantageous for the facile production of quartz fiber-infused composites (Si-PN/QF and P-PN/QF) through a straightforward and eco-friendly melt processing approach. The cured PN composite (30 % Si-PN-450 °C/QF vs. 10 % P-PN-450 °C/QF) exhibited significantly improved glass transition temperatures (506 °C vs. 553 °C), acceptable flexural strength (268 MPa vs. 275 MPa), and excellent flame retardancy (less than 8.0 % weight loss exposed at 1300 °C for 200 s). Additionally, the composites demonstrated commendable dielectric properties, with a dielectric constant (Dk) of approximately 4.0 and a dissipation factor (Df) below 0.01 from 25 °C to 600 °C. Collectively, the obtained composites exhibited a synergistic combination of thermal, fire-resistant, and dielectric properties, positioning them as leading candidates for the next generation of wave-transparent materials.

Impact of Process Technology on Properties of Large-Scale Wind Turbine Blade Composite Spar Cap

As wind turbine blade length increases, reconciling lightweight design with strength necessitates continuous advancements in process technology. The impact of three different process technologies–vacuum-assisted resin transfer moulding (VARTM), prepreg, and pultrusion–on the properties of wind turbine blade composite spar caps was investigated using scanning electron microscopy, dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis, and static and fatigue testing. The results demonstrated that the fibre weight content and 0° tensile modulus of the VARTM and pultrusion composites increased as compared to those of the prepreg samples. Subsequently, the properties of a 94-m blade were analysed using the Ansys Composite PrepPost (ACP) and static structure modules in Ansys simulations, and the weights of the spar cap were compared with test data of materials under different process technologies. The results showed that the masses of the spar cap of a 94-m blade in the pultrusion, VARTM, and prepreg processes were 7965, 9170, and 9942 kg, respectively. The quantitative influence rules on the weight of the wind turbine blade spar cap prepared through different process technologies were formulated. The findings of this study are promising and are expected to aid the development of wind turbine blade process technologies.

Stretch broken carbon fiber for primary wind turbine structure

Stretch broken carbon fiber (SBCF) composites offer significant benefits over conventional continuous fiber composites. These include the greatly enhanced formability of unidirectional tape form products prior to cure without any compromise on the mechanical performance after curing. The recent work performed under the US Army funded Stretch Broken Carbon Fiber for Primary Aircraft Structure program by Montana State University (MSU) is reviewed, along with the results achieved by previous efforts to develop this technology. The achievement of shorter stretch break lengths to improve formability is a principal objective of the work and a successful achievement of the recent efforts. In addition, alternate means of manufacturing SBCF prepregs are discussed, including conventional spooled fiber presentation suitable for running on current prepregging equipment, as well as a newly developed direct to prepreg approach, where the stretch breaking of the fibers is done in-line with the prepregging process. While aerospace primary structure has been the focus of current research efforts, applications in other industries such as wind turbine blades are also obvious areas of applications for SBCF technology.

Design and Fabrication of a Customized and Fully‐Recyclable Carbon Fibers/Epoxy Composites Prostheses via a Functional Additive Manufacturing Approach

AbstractThis study presents an innovative approach based on the combination of two different techniques for fiber‐reinforced epoxy composites manufacturing, i.e., additive manufacturing (AM) and hand lay‐up‐assisted vacuum bagging. It represents a proof of concepts for the manufacturing of a customized and lightweight lower‐limb prostheses having built‐in functional elements, such as sensors. In particular, a soluble tool for fabricating a complex‐shaped carbon reinforced epoxy composite is produced by AM. The 3D printed model is properly designed to integrate a functional element, in a customized way, within the composite structure itself. Moreover, a fully‐recyclable epoxy resin system is used as matrix for the fabricated composite part to recover the expensive functional part and carbon fibers as soon as the prostheses has reached the end of its life (EoL). The prostheses recycle is carried out through a chemical recycling process that relies on the presence of acid‐cleavable groups within the epoxy‐cured network that allows for the cleavage of the crosslink under certain conditions. A similar approach is proposed in the state of art, but by using the traditional autoclave prepreg process and without considering the handling of the part at its EoL.

Preparation and Validation of a Longitudinally and Transversely Stiffened Panel Based on Hybrid RTM Composite Materials.

In the face of the difficulty in achieving high-quality integrated molding of longitudinally and transversely stiffened panels for helicopters by resin-matrix composite materials, we combine the prepreg process and the resin transfer molding (RTM) process to propose a hybrid resin transfer molding (HRTM) for composite stiffened panel structures. The HRTM process uses a mixture of prepreg and dry fabric to lay up a hybrid fiber preform, and involves injecting liquid resin technology. Using this process, a longitudinally and transversely stiffened panel structure is prepared, and the failure modes under compressive load are explored. The results show that at the injection temperature of the RTM resin, the prepreg resin dissolves slightly and has little effect on the viscosity of the RTM resin. Both resins have good miscibility at the curing temperature, which allows for the overall curing of the resin. A removable box core mold for the HRTM molding is designed, which makes it convenient for the mold to be removed after molding and is suitable for the overall molding of the composite stiffened panel. Ultrasonic C-scan results show that the internal quality of the composite laminates prepared using the HRTM process is good. A compression test proves that the composite stiffened panel undergoes sequential buckling deformation in different areas under compressive load, followed by localized debonding and delamination of the skin, and finally failure due to the fracture of the longitudinal reinforcement ribs on both sides. The compressive performance of the test specimen is in good agreement with the finite element simulation results. The verification results show that the HRTM process can achieve high-quality integrated molding of the composite longitudinally and transversely stiffened panel structure.

Mitigating void growth in out-of-autoclave prepreg processing using a semi-permeable membrane to maintain resin pressure

In this work, we investigate the use of discontinuous resin films in prepregs (semipregs) combined with a semi-permeable (air-permeable, resin-impermeable) release film intended to allow through-thickness air evacuation while simultaneously restricting resin loss. In situ measurements of resin pressure were deployed to test the hypothesis that resin pressure was maintained during prepreg cure when using a semi-permeable release film. Concurrently, visualization of the tool-side surface during cure revealed efficient evacuation of entrapped air. Porosity in laminates formed at high temperatures when using resin-permeable consumables, but did not form when using resin-impermeable (semi-permeable) consumables. To confirm that the observed void growth behavior was due to a loss in resin pressure, experiments were conducted to measure resin pressure during cure with both resin-permeable and resin-impermeable (semi-permeable) consumables. In both cases, resin pressure peaked before decreasing, a finding attributed to resin flowing to fill dry regions in the fabric, present by design. The drop in resin pressure, however, was greater in magnitude and longer in duration when using resin-permeable edge and bag-side surface boundaries, indicating that the observed void growth at elevated temperature was caused by a loss in resin pressure. Use of a semi-permeable membrane was effective in retaining resin content and mitigating such porosity.

In-situ X-ray computed tomography characterization of compaction-creep-recovery response and statistical void analysis of carbon/epoxy prepregs

Vacuum bag only (VBO) prepreg processing requires several time-consuming experimental trials in order to achieve a void volume fraction (Vv) of less than 2%. Alternatively, a limited number of in-situ X-ray computed tomography (XCT) compaction-creep experiments on prepregs under pressures that are equivalent to the VBO process can potentially reduce both time and cost whilst assisting in identifying and understanding the mechanisms associated with void formation during processing. In this study, compaction-creep tests at the first dwell temperature during the cure cycle were conducted at 0.1 MPa for different dwell periods (1, 2, and 4 h) using a specially-designed compression stage installed inside an XCT machine. Based on the thickness values measured at different stages of the test, the percentage of compaction-creep and permanent deformations were determined. Three-dimensional (3D) XCT-aided geometrical models were developed both before and after the compaction-creep tests, followed by Vv calculations at each stage. The results showed that higher levels of permanent deformation were recorded in samples tested using a dwell time of 4 h, due to a lower level of recovery compared to other dwell periods. Furthermore, the most suitable condition, in terms of the lowest Vv, was found in samples manufactured with a dwell time of 2 h, where relatively small voids, with a diameter less than 100 μm and a Vv of 0.57% were observed. These values are significantly lower than those observed in uncompressed samples which have many larger voids with a diameter greater than 200 μm and a Vv of approximately 15%.

Preparation and characterization of Nextel 720/alumina ceramic matrix composites via an improved prepreg process

AbstractIn this work, the Nextel 720 continuous fiber reinforced alumina ceramic matrix composites (CMCs) were prepared by an improved prepreg process. The alumina matrix was derived from aqueous slurry, which consisted of organic glue, alumina sol, nanometer alumina powders, and micrometer alumina powders. This design provided a densely packed matrix for the CMC, and made the whole process relatively simple. The ratio of different alumina components in aqueous slurry was optimized to obtain good sintering activity, high thermal resistance, and excellent mechanical properties simultaneously. Furthermore, a preceramic polymer of mullite was used to strengthen the ceramic matrix through a multiple infiltration process. The final CMC sample achieved a high flexural strength of 255 MPa and a good high‐temperature stability. After 24 h of heat treatment at 1100°C, 85% of the maximum flexural strength still had been retained.

A thermal latent imidazole complex containing copper (II) as the curing agent for an epoxy-based glass fiber composite

Epoxy-based glass fiber composites are widely used in various fields, such as automotive, marine, and aircraft, owing to their high strength and light weight compared to traditional metal materials. However, the epoxy-based resins or prepregs cannot be stored for a long time at room temperature owing to their high reactivity. In this study, 1-cyanoethyl-2-ethyl-4-methylimidazole (CEMI) is a complex with copper chloride to improve its thermal latency towards epoxy resins, and the epoxy-based glass fiber composite is made by prepreg processing. The composition of the imidazole–copper (II) complex (Complex-S) is characterized by elemental analysis and inductively coupled plasma optical emission spectrometry. Thermal dissociation of Complex-S is conducted by differential scanning calorimetry and Fourier transform infrared spectroscopy. With Complex-S as a latent curing agent, the thermal and mechanical properties of epoxy resins are studied. The epoxy systems show distinctly prolonged shelf life at room temperature. Moreover, the composite with Complex-S has a higher flexure strength and modulus comparable to that with CEMI.

Styrene-free synthesis and curing behavior of vinyl ester resin films for hot-melt prepreg process

Vinyl ester (VE) resin films were developed from epoxy resins via esterification with methacrylic acid without a reactive diluent to fabricate carbon fiber (CF) prepregs via a hot-melt impregnation process. The viscosity of VE resins suitable for the hot-melt prepreg method was optimized by mixing two different bisphenol-A epoxy precursors, with low and high molar masses. Unidirectional CF prepregs were fabricated via the hot-melt impregnation method using as-prepared VE films. Manual layup of as-prepared VE prepregs with subsequent curing in an autoclave led to the successful fabrication of CF-reinforced composites with good tensile and flexural strength. The kinetics and mechanism of the radical crosslinking of VE synthesized using various mixing ratios of two different epoxy precursors were studied using nonisothermal differential scanning calorimetry. Quantitative analyses with the autocatalytic model and model-free methods indicated that the higher viscosity of VE causes a higher energy barrier associated with molecular mobility and diffusion.

Specimens manufacturing by infusion for the ”Outcome Lower Skin” structural tests pyramid (Clean Sky 2)

The aim of this project had been the infusion specimens manufacturing to support design, analysis and qualification for the “Outcome Lower Skin” pyramid structural tests inside the Clean Sky 2 programm. It is the reinforcement lower skin structural tests pyramid for the C295 military aircraft from Airbus Defence & Space. Along the project UD dry fiber manufacturing process had been developed until to obtain an automatic, robust and efficiency process in order to be able to manufacture a lower skin with stringers and spars integrated and that will be able to compete with the typical prepreg process with regard to fabricability, ratio, performance and cost. The manufacturing process includes the dry fiber pre-forms automatic lay-up by AFP (Automated Fiber Placement), a hot-forming process for the stringers and finally a process infusion by VAP (Vacuum Assisted Process). To achive this goal, the AFP has been set up to optimize the laying parameters for the processing of the material to be used (½” Hexcel Hi-Tape fiber) and RTM6 resin. On the other hand, several tests of hot forming had been carried out modifying process variables, and several infusion bags configurations had been used to stringers and skin integration by infusion (VAP) until to find the key process parameters to obtain valid parts for the structural test pyramid.

Fatigue delamination behaviour of carbon fibre/epoxy composites interleaved with thermoplastic veils

Interleaving thermoplastic veils has proved to enhance the interlaminar fracture toughness of carbon fibre/epoxy composites under static loading conditions. However, the fatigue delamination behaviour has yet to be investigated. Herein, meltable Polyamide-12 (PA) veils and non-meltable Polyphenylene-sulphide (PPS) veils were used for interlay toughening of unidirectional (UD) and non-crimp fabric (NCF) laminates that were manufactured using a prepreg process and resin transfer moulding process, respectively. The results of Mode-I fatigue delamination tests demonstrated a significant improvement in the fatigue life of the laminates due to interleaving. Additionally, the fatigue resistance energy has been maximumly increased by 143% and 190% for the UD and NCF laminates, respectively. The microscopy analysis revealed that the toughening mechanisms of thermoplastic veils were affected by the form of the thermoplastic veils in the laminates (melted or non-melted), the fracture mechanisms of the reference laminates and the adhesion/miscibility between the thermoplastic veils and the epoxy.

Process Chain Development for the Fabrication of Three-Dimensional Braided Oxide Ceramic Matrix Composites.

Fiber composites with a three-dimensional braided reinforcement architecture have higher fiber volume content and Z-fiber content compared to a two-dimensional braided reinforcement architecture; as a result, the shear strength increases. Porous oxide fiber composites (OFCs) have the inherent weakness of a low interlaminar shear strength, which can be specifically increased by the use of a three-dimensional fiber reinforcement. In this work, the braiding process chain for processing highly brittle oxide ceramic fibers is modified; as a consequence, a bobbin, which protects the filament, is developed and quantitatively evaluated on a test rig with regard to tension and filament breakage. Subsequently, a braiding process is designed which takes into account fiber-protecting aspects, and a three-dimensional reinforced demonstrator is produced and tested. After impregnation with an Al2O3-ZrO2 slurry, by either a prepreg process or a vacuum-assisted process, as well as subsequent sintering, the three-dimensional braid-reinforced OFC exhibits an interlaminar shear strength (ILSS) which is higher than that of two-dimensional braid- or fabric-reinforced samples by 64–95%. The influence of the manufacturing process on the relative macropore content is investigated and correlated with the mechanical properties.

Effects of environmental conditions on uncured prepreg characteristics and their effects on defect generation during autoclave processing

Primary load-bearing composite structures require high laminate quality. This is achieved by autoclave prepreg processing where high external pressures enhance consolidation. As a result, parts of simple geometry can be manufactured with void contents less than 1%. However, defects such as wrinkles and voids are typically found within more complex structures. This research investigates defect formation within autoclave prepreg processing stages and is focused on concave female corners with high laminate thickness to corner radius ratios. Experimental studies were carried out to investigate the effect of humidity exposure to uncured prepreg on defect formation during the steps of autoclave processing. It is hypothesized that moisture uptake influences uncured prepreg characteristics such as compaction behaviour, interply friction and prepreg tack, and therefore achieved quality following consolidation. Results have shown that defects already occur during the layup and debulking process and that humidity affects the uncured prepreg characteristics and thereby the consolidation quality.

Effect of rheological transitions in matrix resin on flow mechanism of carbon Fiber/Epoxy prepreg

There are two types of flow mechanism in prepreg processing: shear flow, in which resin and fibers flow together, and percolation flow, in which resin flows out of the gaps between fibers. In this study, we experimentally evaluated how these flow mechanisms change during cure including the gelation stage. Results of compression tests at various initial degree of cure at the start of compression showed that the maximum amount of shear flow is almost constant before matrix gelation. In the gelation region of matrix resin, shear flow still occurs at the gel point whilst percolation flow has completely stopped. Furthermore, it was suggested that the limit of the shear flow in the gelation stage can be described by the combined effects of the shear modulus of the fiber-bed and the shear storage modulus of matrix resin.

Enabling pseudo-ductility in geopolymer based glass-ceramics matrix composites by slurry dilution

Carbon fibre reinforced composites with matrix consisting of porous glass-ceramics obtained by high temperature heat-treatment of geopolymer based material are studied. The matrix is obtained from a slurry. Several composites have been manufactured by in-laboratory stratification of pre-impregnated plies with slurries diluted at various rates. Slurry dilution changes the porosity and leads to pseudo-ductile behaviour of the composite. Dilution implies a drop of non-linearity stress threshold together with a slower decrease of the strength. An optimal dilution rate is evidenced, which exhibits high strength but lower non-linearity stress threshold, and then, a significant failure strain. The optimized prepreg material is compared with a reference composite processed by an industrial prepreg process. The origin of the pseudo-ductility is related to decohesion at the interface between tows and inter-tow matrix. RTM also provides a pseudo-ductile composite but with very low non-linearity stress threshold, in relation to poorly impregnated tows.

An experimental study of defect evolution in corners by autoclave processing of prepreg material

Autoclave prepreg processing is commonly used to produce high-quality structural components. Unlike in flat geometries, defects such as corner thickening, wrinkling and voids may be generated within complex structures during processing. This research investigates defect formation within the layup, debulking and consolidation stages of processing, focused on female moulded corners with high thickness to radius ratios. The influence of varying corner radii and proportion of 0°-fibres on the formation and size of corner defects was investigated. The wrinkling severity, corner thickening, and material flow behaviour of the samples was quantified through optical microscopy analysis. To characterise laminate corner-thickness variation following each processing step (layup, debulking, consolidation), a non-destructive laser measurement method was developed. It is demonstrated that corner thickening initially occurs during the layup and debulking process. Visualisation of the prepreg mass flow in corners shows that a higher 0°-fibres content increases the shear flow of the material during consolidation.

In situ resin age assessment using dielectric analysis and resin cure map for efficient vacuum infusion

The physical state of epoxy resin designed for vacuum infusion was assessed in situ by immersing a dielectric sensor into the resin pot. The measured ion viscosity of aged resin was directly converted to a degree-of-cure metric using a resin cure map constructed by correlating cure kinetics and dielectric analysis data. Next, an age-adjusted infusion process map was employed to define a nominal infusion window and to identify key process metrics. Finally, process simulations and flow contour maps were used to validate and refine the process map and to guide adjustment of infusion process parameters based on resin age and part size/geometry. The study describes a pathway to more efficient use of aged resin using in situ process diagnostics, cure map design, and process simulation. The methodology employed to evaluate resin age and to adjust process parameters accordingly can be extended to other composite manufacturing processes, including conventional prepreg processing.

Cure cycle modification for efficient vacuum bag only prepreg process

Over the past decade, increasing use of continuous fiber-reinforced polymer composites has created a demand for manufacturing methods with lower costs, higher production rates, and improved processing efficiency. To meet the growing demands, vacuum bag only (VBO) prepreg processing has been proposed and implemented in industrial settings. However, in the absence of high consolidation pressure, VBO prepreg must undergo compaction for longer durations during cure and requires use of more elaborate processing schemes to conform to complex geometries. The main objective of our cure cycle modification was to reduce overall manufacturing time for more efficient processing, while maintaining robust part quality. This study demonstrates the effect of cure cycle on formability and part quality of three complex-shaped composite structures, a bulkhead, fuselage, and I section frame, featuring drop-offs, corners and sandwich areas consisting of less than 10 plies. Three different cure cycles were chosen: Reference cure cycle 1, Modified I and Modified II cure cycle. The reference was modified based on resin cure kinetics/viscosity modeling results and “effective flow number” to shorten the overall cure cycle time while maintaining robust part quality. To compare the quality of manufactured parts, destructive test and digestion method were used. For the bulkhead parts, Modified I was proven to be more effective in meeting the commercially acceptable part criteria (void content, ply wrinkle, resin ridge, and surface resin starvation), whereas Reference failed to meet the requirements, showing pervasive presence of porosity in drop-offs, corners and sandwich areas. The fuselage and I section frame parts produced with Modified I and Modified II were shown to meet the part quality requirements, with slight improvements in surface quality observed with the Modified II method.

Optical measurement of voids in situ during infusion of carbon reinforcements

Liquid composite molding (LCM) is growing in importance as an alternative to traditional prepreg-autoclave methods for manufacture high-performance composites. The most significant roadblock to industry’s implementation of LCM is the usually higher void content compared with prepreg processing. One tool for reducing void levels in LCM involves optimization of flow velocity, which requires models to be developed to describe void formation at a given velocity. To help solve this problem, the following research illustrates the first known method for optical void measurement in situ during infusion in a carbon fiber reinforcement. Similar to previous studies on glass fiber, this work utilizes fluorescent dye and a digital camera to produce sufficient contrast and resolution for image analysis. Visible bubbles are photographed against the opaque carbon fiber background. An automated method of image analysis is outlined, which was used to analyze 230 images for three different flow orientations of a single fabric, producing the highest amount of experimental data seen so far on in situ void measurement. The resulting data identifies a minimum velocity threshold for minimal macro-void formation. The resultant void characterization framework will better enable optimization of LCM processing for high-performance composites based on carbon reinforcements.