Prepreg Processing Research Articles

Manufacturing and characterization of carbon fibre/epoxy composite prepregs containing carbon nanotubes

Prepregs of carbon fibre-reinforced polymer (CFRP) composites containing carbon nanotubes (CNTs) have been successfully produced based on a solventless prepregging process. This work provides one of the most comprehensive evaluation to date of the processing and cure parameters affected by the incorporation of CNTs in CFRP prepreg manufacturing. The results confirmed that high-speed shear mixing and functionalisation of CNT are effective in lowering the viscosity of the CNT-epoxy nanocomposites. Based on the findings obtained from the rheological studies, suitable processing parameters are chosen for lab-scale production of CFRP composite prepregs. The effects of surfactant-treated CNTs on cure behaviour of epoxy and CFRP prepreg are evaluated. The catalytic activity of CNTs is negligible for a 0.5 wt% filler content whereas it becomes prominent when the CNT content is increased to 1.0 wt%. The degree of cure is lower for the prepregs containing 0.5–1.0 wt% CNTs than those without or containing a lower CNT content.

New manufacturing process for thermally sprayed prepregs with local UD fiber reinforcement for semi solid forming of light metal MMC

Thermal spraying and thixoforging technologies can be combined in a new manufacturing method for the production of light metal matrix composites. This paper describes the production of unidirectional (UD) and tailored local fiber reinforced MMC based on this new method. For manufacturing of UD fiber reinforced metals, fiber prepregs are produced by coating of fiber bundles with light metal matrix material. For further processing to the final MMC, these prepregs are laminated, heated up and densified by thixoforging to near net-shape components. Compared to conventional technologies for the integration of fibers in light metal matrices, like squeeze casting, hot pressing and diffusion bonding, this method offers the possibility to realize complex component geometries with short cycle times. However, the focus of this paper lies on the flexible processing of fiber prepregs with local, already near net-shape fiber reinforcement by continuous fiber deposition and simultaneous coating with the matrix alloy by thermal spraying. As a case study, the manufacturing of a wheel rim with local reinforcement in the central area will be discussed.

E-Glass/Phenolic Prepreg Processing by Solvent Impregnation

The increased interest in polymer matrix composites is partly due to their vast application in aeronautical and aerospace engineering. The preparation of prepreg is frequently the first move in the manufacture of parts. Although industry has developed reliable and successful processes yet there is a need for further scientific investigation because most existing methods have been devised from practices based on trial and error. This paper reports a new approach to the design and operational features of prepregging machines. Our understanding of the subject is then utilized for the production of E-glass/phenolic prepregs. Characterization of manufactured prepregs by resin content, solvent contents, extent of impregnation and tack and drape properties permits comparison of different operational cycles and design parameters. The results have confirmed the success of our approach of successive decreases in the pressure of the squeezing roller in the soaking zone and successive increases in the chamber temperatures in the drying zone. This idea can be utilized in standard engineering operations to describe the operation protocol for the equipment and to give criteria for setting up the impregnation process, along with the design conditions for prepreg manufacturing.

20401 低圧成形ガラス繊維強化ポリイミドの力学特性に及ぼすプリプレグ作製条件の影響(材料加工(1))

20401 低圧成形ガラス繊維強化ポリイミドの力学特性に及ぼすプリプレグ作製条件の影響(材料加工(1))

Curing of composite components by ultraviolet radiation: A review

AbstractCuring of polymer matrices by ultraviolet (UV) irradiation can be applied to a variety of processes in the production of composite components, as long as the component can be directly irradiated. Wet lay‐up techniques, vacuum infusion type processes with UV‐transparent membranes, filament winding, and prepreg processes have been adapted to UV curing. Unlike in thermal curing, the curing time is in the order of magnitude of minutes rather than hours, which means a significant reduction in cycle time. The radiation can be generated by a variety of sources suitable for various specific applications and different curing strategies. The most frequently used radiation sources are mercury arc lamps. Because of the absorption of radiation passing through matter, the thickness of laminates for efficient application of UV curing is limited. The curing mechanism is either radical polymerization for acrylate‐based resins or cationic polymerization for epoxies and vinyl ethers. The properties of the UV‐cured polymer matrix are determined by the cross‐linking density. This depends on the type and concentration of the photoinitiator and of the (optional) diluents, the intensity and the duration of the irradiation, and the temperature at which the curing process takes place. POLYM. COMPOS., 27:119–128, 2006. © 2006 Society of Plastics Engineers.

Influence of aromatic amine hardeners in the cure kinetics of an epoxy resin used in advanced composites

Composite structures for aerospace applications are mainly made by the well-known prepreg technology. In order to achieve adequate prepreg processing schedules, and consequently maximum fiber strength utilization, one has to know in deep the cure kinetics of matrix, which held the fibers together. This work describes a procedure to study the cure kinetic and has as example how aromatic amine hardeners influence the cure kinetics of an epoxy resin used in advanced composites. The investigation was carried out by using the DSC technique and it was found that depending on the system used the cure kinetics of the formulation obeys order n or autocatalytic order.

A simulation study on the effects of particle size on the consolidation of polymer powder impregnated tapes

Abstract The powder prepreg technique offers a low-cost, high-speed method of manufacture of composites. Consolidation of the prepreg tows is a key step in the manufacture of a final `useful' part. The efficiency of the consolidation for any polymer powder prepreg tape produced from powder prepreg processes would conceivably depend on the material properties of the polymer, particle size, temperature, consolidation pressure and volume fraction of the tape. A mathematical process model that identifies and describes a set of parameters to predict the consolidation conditions of a given polymer powder prepreg tape has been developed. A simulation study that identifies the different parameters and compares the advantages and limitations of polymer physical properties and particle size has been performed. An integrated Flow Resistance (FR) index has been developed to mathematically describe the consolidation process. The results show that small powder particle sizes are beneficial to the process.

Power issues in the manufacturing and characterization of polymers and composite materials

AbstractDynamic Mechanical Analysis (DMA), although utilized for more than three decades, has been restricted so far to material characterization rather than processing. Recently the importance of power for describing viscoelasticity in prepreg process operations has been demonstrated. This present work views DMA as an integral part of the power analysis of processes. In this study, four types of prepreg were produced under the same power of impregnation utilizing epoxy resins with various PEI contents. Prepregs were tested for compressive and toughness power as well as loss power. Laminates were developed and analyzed by DMA. DMA data was treated according to the established power methodology and compared with the tack power of the original prepreg. Specific compressive power was generally found to increase with increasing storage DMA power, while specific toughness power was found to show the opposite trend. The inverse of power, defined as tolerance, was also investigated. The out‐of‐phase DMA tolerance was found to correlate to loss prepreg power, bringing a deeper understanding to the fundamentals of viscoelasticity formation. This analytical methodology can be applied beneficially to many topics to provide a better understanding of polymers and composites, as well as smart materials.

Thermal and air permeation properties of a carbon fiber/toughened epoxy based prepreg system

This study addresses thermal and air permeation properties of a new toughened prepreg system. Voids in the uncured prepreg structure can affect the void content in the final composite structure. A new, toughened prepreg system, commercially available for aircraft structural application, was utilized in this study. The prepreg was subjected to thermal and rheological characterization to understand the basic prepreg properties. These experiments were followed by a prepreg air permeation study to investigate prepreg processing and its influence on the prepreg structure. Crosslinking of the resin matrix was monitored with prepreg specimens without extracting resin from the prepreg. Along with thermal property measurements, the air flow rate significantly decreased in initial static experiments, followed by equilibrium permeability values. An air permeation model divided the air permeability into intralaminar and interlaminar permeabilities. Interlaminar air permeation was found to be more pronounced than intralaminar air permeation in this particular prepreg system. These permeation measurement results were explained using optical microscopy, proving that the application of vacuum could eliminate significant porosity in the laminate. Collectively, understanding prepreg thermal and air permeation properties was considered to be important; the voids in uncured prepreg may cause the voids in the final composite structure. Voids in the prepreg can be attributed to the heterogeneity and anisotropy of the toughened prepreg structure, resulting from particular prepreg processing techniques. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 5–16, 1997

Effect of impregnation conditions on prepreg properties and honeycomb core crush

AbstractThe 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 effect of fabric tension and the number of impregnation rollers on woven fabric prepreg quality and cured laminates

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.

Development and hot‐melt impregnation of a model controlled flow prepreg system

AbstractA 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.

Prepreg processing science and engineering

AbstractResin flow and fiber spreading during the prepregging process were investigated experimentally using a scale‐down version of a commercial hot‐melt prepregger with bismaleimides and carbon fibers as model systems. Specifically, several dimensionless parameters such as the Prepreg Flow Number, fractional resin uptake, resin distribution function, fractional width change, and prepregging efficiency were defined to characterize the prepregging process as well as the quality of the prepreg. Fiber spreading during the prepregging process was observed to be a viscoelastic phenomenon depending on the impregnation temperature. For resin impregnation into collimated fiber tows, all the experimental data points obtained at different operating conditions were superimposed onto a single line showing a temperature‐pressure‐velocity superposition for the prepregging process as predicted by the definition of the Prepreg Flow Number (PFN). Finally, three fundamental steps of the prepregging process were identified and confirmed with cross‐sectional micrographs of unaged prepregs produced at different temperatures.

Prepreg process analysis

AbstractA description of the hot‐melt prepreg process was developed by coupling resin matrix flow at the coating and impregnation zones with heat transfer considerations. Both bismaleimide (BMI) and epoxy resin systems were used as matrices. With continuous carbon fiber reinforcement. For the laboratory scale prepregger utilized in this study, the thickness of the resin film produced in a doctor blade was approximately 63% of the blade distance, showing good agreement with experimental data. In addition, the measured heat transfer coefficient between prepreg and atmosphere indicated that surface convective resistance is large compared to the internal conductive resistance and thus the chill plate in the prepregging operation plays on important role in determining prepreg quality. By formulating an analytical methodology in terms of the “Prepreg Flow Number” (PFN) that combined important operating parameter (e.g., temperature, pressure, and production rate), a temperature‐pressure‐velocity superposition was confirmed for the prepregging process. In addition, the model could easily be extended to different scales and types of prepregging processes with the aid of dimension‐less parameters that are quite descriptive of the operating conditions employed in prepreg manufacturing.

Simultaneous measurements of permeability and capillary pressure of thermosetting matrices in woven fabric reinforcements

AbstractA simple apparatus was designed and constructed capable of measuring the unsteady‐state permeability and the capillary pressure simultaneously in a simulated composite impregnation experiment. It was found that the Kozeny‐Carman equation used to describe the permeability of composites during impregnation adequately described experimental results for woven fabric preform up to porosity values of 0.5. Above this value, observed deviations were attributed to interfacial effects between adjacent woven fabric layers. For woven fabric preforms made of T‐300 carbon fibers, a maximum capillary pressure of 3.7 × 104 Pa (=5.4 psi) was observed at low porosity values. Thus, the capillary pressure may compete with other pressure sources in low pressure processes, such as the prepregging process. The woven fabric preform used in this study is observed to have a permeability similar to a unidirectional fibrous preform along the transverse direction. Furthermore, an existing modeling methodology capable of predicting permeability and capillary pressure through different preforms was found to be valid for fibrous preforms of complex orientation.

Effect of advancement on epoxy prepreg processing—a dielectric analysis

AbstractWith the relatively recent trend toward adaptive process control during processing of structural composites, there is need for an understanding of the nature of variations during cure that are encountered. Part‐to‐part variations can arise because of differences in thermal exposure arising from part placement or part thickness, or from chemical differences due to formulation or advancement. Microdielectric sensors are particularly well suited for monitoring thermoset cures from initial point of flow, through the liquid region, and on into the solid state. In this study, microdielectric sensors are used to systematically study the influence of resin advancement on cure behavior. Relationships between advancement and critical cure phenomena such as flow temperature, point of viscosity minimum, and reaction end point are discussed.

Solvent‐induced crystallization in polyetherimide thermoplastics and their carbon fiber composites

AbstractSolvent‐induced crystallization (SINC) was observed in a polyetherimide (PEI), a thermoplastic used as a matrix in carbon fiber composites. This observation was made using wide angle X‐ray scattering (WAXS), differential scanning calorimetry (DSC), and optical microscopy. It was discovered that methylene chloride induces crystallization in the PEI by penetrating the surface and swelling the bulk polymer. Prepreg processed using N‐methyl pyrrolidone (NMP) was also crystalline. One processed above the crystalline melting point (Tm), no crystallinity in the sample was found, as the PEI did not crystallize from the melt. The observed crystallization of both the neat polymer and its carbon fiber prepreg was exclusively through a solvent‐induced process, although it is likely that the mechanism through which crystallization occurs during solvent prepreg processing is different than the diffusion‐controlled mechanism demonstrated with methylene chloride. A solvent prepregging process may involve a low molecular weight or monomer solution as well as other polymerization by products. Measurements using WAXS showed a maximum degree of crystallinity of 30%, as induced by methylene chloride. A value of 85 J/g for the heat of solvent‐induced crystallization in the PEI was calculated from the DSC measurements.

Manufacture of Powder-Impregnated Thermoplastic Composites

The use of thermoplastic composites has been limited by the lack of reliable techniques to make prepregs of consistent quality. Dry powder impregnation methods offer a new approach to the solution of this problem. This paper discusses the features that are essential to the development of an acceptable dry powder process The physical principles that govern the manufacture of thermoplastic prepreg tapes of controllable fiber-matrix volume fractions and the consolidation of these tapes to make void-free composite parts are presented A novel powder prepregging process suitable for the manufacture of high-quality powder-impregnated tapes and their subsequent consolidation to make void-free parts is discussed. It is shown that a well-designed dry powder process can achieve fine control over both fiber-matrix composition in the prepreg tape and intimate wetting between in dividual fibers and the thermoplastic matrix Powder prepreg tapes can be consolidated into void-free specimens with simple consolidation cycles developed for a specific fiber- matrix combination

Automation of Thermoplastic Composite Processing

An experimental model of a new automated pre-preg lay-up process is developed to determine the effects of process parameters on the consolidation and quality of parts made of polyetheretherketone (PEEK) reinforced with carbon fibers (APC-2). Temperature, tape velocity, and applied pressure affect the degree of consolidation of the laminates produced using this process. The consolidation's dependence on applied pressure is clearly demonstrated. A threshold pressure of approximately 275 kPa (40 psi) exists for APC-2, below which consolidation does not occur. This threshold is followed by a sharp increase in consolidation up to an applied pressure of 900 kPa (130 psi), and a much slower rate of increase in consolidation for pressures in excess of 900 kPa. The effect of velocity is qualitatively explored. The results of the experiment are discussed with reference to the development of robotic lay-up cells and continuous processes for the production of quasi-isotropic laminated sheet.

The electroretinogram, visual evoked potentials and spike discharges in the visual cortex of the albino rat : M. Ebe, I. Hamma and Y. Ishiyama (Tokyo, Japan)

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.