Prepreg Tack Research Articles

Void formation model and measuring method of void formation condition during hot pressing process

AbstractFor resin matrix composites, voids are common defects that can seriously deteriorate the properties of the composite parts. Thus, the elimination of voids is a crucial element in controlling the manufacturing process of composite parts. This article focuses on void formation originating from hygroscopic water for resin matrix composite laminates prepared with hot pressing process. The Kardos void formation model was developed to analyze the critical resin pressure for the initiation of voids, and the influencing factors were investigated experimentally to validate the modified model. It is found that resin pressure and gel temperature are the two key parameters to control void defects and that entrapped air in prepreg stacks must be considered in the void formation model. Furthermore, a simple method was established to measure the relationship between porosity and the processing parameters, and the void formation conditions of the resin and the prepreg stack were also studied. The theoretically predicted void formation conditions and the experimental results were compatible for the studied cases. These results are valuable for eliminating void defects, optimizing processing parameters, and enhancing the performance of composite parts. POLYM. COMPOS., 31:1562–1571, 2010. © 2009 Society of Plastics Engineers

Experimental Analysis of Prepreg Tack

A probe tack test apparatus is designed to characterize the tack of carbon-epoxy prepreg. Tests are performed on both pure resin and prepreg. The maximum debonding force seems to be a relevant measure of tack. First, results show that the response of pure resin is similar to that of viscous silicon oil. Second, the shape of the response curve obtained for prepreg beyond the maximum value of the debonding force is mainly due to structural effects. Third, the influence of contact force, contact time, debonding rate, probe temperature and ageing conditions on the prepreg tack is investigated in relation with physical phenomena involved in the debonding phase.

High temperature organic/inorganic addition cure polyimide composites, part 1: Matrix thermal properties

AbstractStructure‐thermal property interrelationships are characterized and reported for organic/inorganic addition cure polyimide composite matrices based on 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride, the reactive terminal group 4‐phenylethynyl phthalic anhydride, and stoichiometric controlled diamine ratios of 1,3‐phenylenediamine, 1,4‐phenylenediamine, or 4,4′‐(1,3‐phenylenediisopropylidene) bisaniline, combined with bis(p‐aminophenoxy) dimethyl silane or an α, ω‐bis(3‐aminopropyl) polydimethylsiloxane oligomer. Polymerization of monomer reactants resin solutions, carbon fiber prepregs and composites, and imidized oligomers are characterized to relate molecular chemical structure and morphology to glass transition temperature, processing characteristics, thermodynamic properties, and thermal stability. Glass transition temperature, thermal decomposition temperature, and char yield were found to increase with increasing siloxane block length in the imide backbone. As the concentration of inorganic component in the imide oligomer backbone increased, the cured glass transition temperature decreased. Char yield and thermal decomposition temperature were observed to decrease as the inorganic component concentration increased. Incorporation of bis(p‐aminophenoxy) dimethyl silane into the imide oligomer structure did not provide any significant advantages over traditional polyimides relative to thermal properties or composite processing, but aminosiloxanes improved composite toughness, prepreg tack, and composite processability. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Simultaneous measurement of load and position with an embedded chirped sampled fibre grating

In this paper we present the use of a chirped sampled grating for distributed sensing of pressure over a small area (e.g. a few centimetres). The grating is embedded in fibre-reinforced composite. It is found that the shift in the central wavelength of each sub-grating in the chirped sampled grating increases monotonically with an increase in the applied force. The central wavelength of the sub-grating gives the location of the applied pressure. When a surface comes into contact with the sensor, the distribution of the pressure determines the shift in central wavelength of the various sub-gratings. The sub-grating that experiences the maximum pressure will show maximum wavelength shift whereas adjacent sub-gratings will show less shift. This method gives the location and magnitude of the load even when the load is not directly applied to any of the sub-gratings. The sensor design comprises of a sampled chirped grating embedded in unidirectional fibre-reinforced composite prepreg. The prepreg enhances the mechanical strength and the unidirectional embedding reduces birefringence. The number of layers in the prepreg stack varies the sensitivity.

Development of a Numerical Model to Predict In-Plane Heat Generation Patterns During Induction Processing of Carbon Fiber-Reinforced Prepreg Stacks

A numerical model is proposed to describe in-plane heat generation spatial response during induction processing of carbon fiber-reinforced thermoplastics. The model is based on a unified approach that considers three possible heating mechanisms: fiber heating (Joule losses in fiber), noncontact junction heating (dielectric hysteresis), and contact junction heating (Joule losses at junctions). A lumped meshing scheme is used to construct a numerical representation for cross-ply and angle-ply orientations of 2-ply prepreg stacks. Heat generation patterns are calculated based on voltage and current conservation laws and verified with induction heating of AS4 carbon fiber-reinforced polyetherimide (AS4/PEI) prepreg stacks. Excellent agreement is found except at very low angle-ply orientations where the predicted heating patterns show significant deviations from the experiment results. A sensitivity analysis is also performed to assess the relationship between heating patterns and material and process parameters. The results show that the stack angle between plies and intrinsic prepreg microstructure can significantly affect the heating patterns.

Variable Density Composite Systems Constructed by Metal Particle Modified Prepregs

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.

Future requirements in the characterization of continuous fiber-reinforced polymeric composites (IUPAC Technical Report)

Abstract Characterization of continuous fiber-reinforced composites is examined in terms of processing, properties, and structure. Five processing and five property topics are then examined in terms of reviewing some of the historic background in these areas with the aim of identifying current issues and requirements for the future. The topics covered in the processing section are: polymeric matrix, impregnation, interfacial effects, residual stresses, and pre-preg tack. In the mechanical properties section the topics are: choice of standard, recycling and reusability, durability, environmental strength, and toughness. The paper provides a ten-point plan for future requirements.

A study on the induction heating of carbon fiber reinforced thermoplastic composites

Recent work in the literature has identified a new heating mechanism during induction processing of carbon thermoplastic prepreg stacks: contact resistance between fibers of adjacent plies. An experimental methodology has been developed to estimate the contact resistance through heating tests based on the properties of the composite and geometry of the specimen. Measured values indicate comparable resistance values at the contact region, compared to resistance in the fiber direction, for AS-4/PEI prepreg stacks under vacuum pressure. The measured values can serve as inputs for induction heating models and process models of carbon thermoplastic prepreg stacks.

Gas permeation and viscoelastic deformation of prepregs in composite manufacturing processes

AbstractGas permeation and creep deformation of a commercial prepreg, which exhibits viscoelastic characteristics, were investigated as a function of time, temperature, and consolidation pressure. Experiments using a prepreg stack demonstrated that the material exhibited a linear viscoelastic bulk deformation under vacuum/autoclave pressure and furthermore, the in‐plane gas flow exhibited non‐Darcian flow behavior with a permeation hysteresis. This behavior was viewed and analyzed by two viscoelastic relaxation processes: (1) bulk dimensional relaxation, and (2) microscopic pore structure rearrangement. A modified standard linear solid (SLS) viscoelastic model was used to interpret the creep compliance and dynamic gas permeability utilizing two independent relaxation parameters. By visual investigation of pore sizes and their distribution, air permeation was found to take place mostly through the interlaminar porosity network for the prepreg system examined.

Perceptions of Prepreg Tack for Manufacturability in Relation to Experimental Measures

Article Perceptions of Prepreg Tack for Manufacturability in Relation to Experimental Measures was published on September 1, 1995 in the journal Science and Engineering of Composite Materials (volume 4, issue 3).

Effect of contact resistances on the thermal conductivity of an unconsolidated fibre-reinforced thermoplastic prepreg stack

This paper deals with the effect of contact resistances on the effective through-the-thickness thermal conductivity of an unconsolidated thermoplastic composite prepreg stack. The thermal conductivity of a 40-ply unconsolidated APC-2 stack is experimentally determined and the result compared with published values for a consolidated APC-2 laminate. Experimental results show that the effect of increasing the pressure on or temperature of the stack is to reduce the contact resistances between the plies and hence increase the effective thermal conductivity of the stack. In general, the results presented show that contact resistances between the individual plies cause the effective thermal conductivity of the unconsolidated stack to be three times lower than that of a corresponding consolidated laminate.

Analysis and characterization of prepreg tack

AbstractAs part of an engineering analysis and experimental methodology to characterize prepreg tack, a compression‐to‐tension test was optimized to enhance reproducibility and generate intrinsic property data. With the resulting stress‐strain compression and tension data, a theoretical model was developed to describe tack as a bulk viscoelastic property of a prepreg laminate stack. Using the viscoelastic analysis, four intrinsic material parameters to characterize prepreg tack could be defined. These were 1) relaxed modulus, 2) unrelaxed modulus, 3) relaxation time, and 4) initial void content of the prepreg stack. Relaxed and unrelaxed moduli of the prepreg stack were independent of temperature, while the relaxation time was highly dependent on temperature and matrix viscosity. In addition, the relaxation time was found to be influenced by resin/fiber content and prepreg surface characteristics, which also influenced the void content of the prepreg stack. Using these measured parameters, good agreement was observed between theory and experimental data for both the stress‐strain curve of the tack test and the simplified compression tack index (CTI*), defined as the ratio of output energy of the prepreg stack during tensile unloading to input energy during compressive loading.

Prepreg aging in relation to tack

AbstractThermoanalytical measurements and tack tests were both performed using a commercially available carbon fiber/epoxy prepreg system (Hercules 3501–6) to examine changes caused by aging as they affect handling and processability of thermosetting matrix‐based composites. Combining these techniques, a relationship between prepreg bulk and surface characteristics in relation to aging was investigated. Isothermal kinetic studies at low temperatures showed maximum conversions (αm) that increased with increasing cure temperatures. In addition, a linear relationship between glass transition temperatures (Tg) and conversions (α) was observed regardless of aging (or cure) temperatures. Energy of separation of prepreg stacks, which may be viewed as a measure of prepreg tack, showed a maximum value at a specific temperature. The maximum energy of separation was observed in the temperature range of 20–25°C above the glass transition temperature for a given sample. However, the maximum energy of separation values decreased with increasing aging times (or conversions), implying that prepreg tack was a viscoelastic property rather than a viscous property of the resin matrix in the prepreg.

Compression Thermal Analysis of the Consolidation Process for Thermoplastic Matrix Composites

Consolidation of thermoplastic prepregs was measured with an integrally- heated parallel platen apparatus attached to a servo-hydraulic mechanical testing machine. The apparatus was designed as a small-scale, well-instrumented press The lamination or consolidation process was viewed as a superposition of three distinctly occurring events identified as void volume reduction, fiber spreading, and autohesion. Consolidation was measured in relation to the original prepreg thickness and was reported as compressive or consolidation strain as a function of temperature. The derivative of the consolidation strain, the consolidation strain rate, was found to be qualitatively descriptive of vis coelastic phenomena occurring in the prepreg stack during consolidation. The apparatus was sensitive enough to identify glass and melt transitions of the polymer matrix, and to provide a measure of the net consolidation for a given processing cycle. The strain and the strain rate data were compared to thermoanalytical prepreg data obtained by Differential Scanning Calorimetry (DSC), and Dynamic Mechanical Analysis (DMA). Three different thermoplastic matrix composite systems were examined with this apparatus: Poly (etheretherketone) (PEEK), Poly(etherimide) (PEI), and Poly(ethylene terephthalate) (PET). Each of these materials exhibited a distinct consolidation profile. Collectively, the results generated by these diverse systems established the usefulness of this consolidation thermal analysis as a processing screening tool for thermoplastic-based composites.

Determination of prepreg tack

Prepreg materials — fibre-reinforcement preimpregnated with uncured resin — are widely used for the manufacture of large composite material components. Current commercially available prepreg materials often show significant variations in tack strength, from point to point within a sheet and from sheet to sheet. Such inconsistencies can lead to void formation in the final composites laminate. This paper describes the techniques and apparatus developed for the investigation of the surface tack strength of adhesives in general and of prepreg materials in particular, as a function of contact time and pressure and of rate of separation. It is hoped that the more detailed knowledge of prepreg tack will enable the production of more consistent material and hence the manufacture of improved quality composite laminates.