Levent Aktas

Effect of Nanoclay Content on Void Morphology in Resin Transfer Molded Composites

Effects of nanoclay content on morphology and spatial distribution of voids in resin transfer molded nanoclay/E-glass/epoxy composite disks are investigated. Closite®25A nanoclay loads of 2, 5, and 10wt% are mixed by sonication with a low-viscosity epoxy resin prior to filling the mold cavity containing 13.6% E-glass preform by volume. A disk without nanoclay is also molded. Once the molded composites are cured, voids on radial composite samples are evaluated via microscopic image analysis. The addition of nanoclay is found to result in a significant increase in the apparent viscosity of the clay-epoxy mixture, thus increasing the molding pressure. Void occurrence is observed to increase considerably with increasing nanoclay content, from 2.1% in the composite without nanoclay to 5.1 and 8.3% in the composites molded with 5 and 10wt% nanoclay, respectively. However, the composite with 2wt% nanoclay yields the lowest void content of 0.7%. Voids are observed to be, in average, smaller after the addition of nanoclay at all nanoclay concentrations. Presence of nanoclay in the impregnating resin induces at least 60% reduction in voids located inside fiber tows, which are trapped by the fluid front motion during impregnation. Irregularly shaped voids are also observed to decrease with increasing nanoclay content. A nonuniform void content and morphology is observed radially, which seems to be affected by the flow kinematics as well as possible breakdown and filtration of clay clusters.

Moisture absorption and wet-adhesion properties of resin transfer molded (RTM) composites containing elastomer-coated glass fibers

Transient water sorption studies were carried out at constant temperature (45 °C) to assess the hydrolytic stability and wet-adhesion properties of glass fiber/epoxy composites having different sizings. Lower effective diffusivity values correlated with improved overall mechanical performance in relation to the control (unsized) samples, and revealed the importance of changing the surface energy characteristics of glass fibers by using distinctively hydrophobic pure polymers. Admicellar polystyrene and styrene-isoprene coatings formed over the inorganic reinforcement appear to create an interface with much higher resistance to moisture attack than the organosilane/matrix interface in composites with commercial sizing. This fact was corroborated by comparing their effectiveness in property retention, which showed the mechanical property (e.g. ultimate tensile strength, stiffness and interlaminar shear strength) increased with respect to the uncoated composites in the dry state as well as after water saturation. Poor wet-adhesion properties of commercial sizings in humid conditions could perhaps be attributed to higher contents of inert material present in these coatings. Fractography analysis was consistent with the previous observations regarding catastrophic failure in composites without coating, and suggested that interfacial debonding, extensive fiber pullout and matrix crazing were the major contributors to the overall failure mechanism. Failed surfaces of both commercial and elastomer-coated composites also showed areas with fiber pullout, but in this case, matrix residues remained on the fiber surfaces, yielding a much rougher appearance. Good fiber-matrix adhesion, particularly in admicellar-coated composites, was also revealed by the presence of hackles and more tortuous failure paths.

Calorimetric and Rheological Measurements of Three Commercial Thermosetting Prepreg Epoxies

The cure kinetics of three different thermosetting resins are investigated using differential scanning calorimetry and oscillatory shear rheometry. For the latter, two different types of plates are used, smooth plates and grooved plates; the latter are used to improve sample–plate contact. In addition, oscillatory compression rheology is used; however, machine compliance prevents accurate measurements at high conversions. A fractional conversion is defined based on the maximum storage modulus achieved at a given temperature, and is compared to the fractional conversion calculated from enthalpy measurements. As expected, the rates of reaction derived from these fractional conversions are very different for calorimetry and rheometry. However, the rates of reaction using the two types of plates are identical, although the grooved plates give much more reproducible storage moduli. A number of previously used mathematical expressions are employed to fit the calorimetric and rheological data, and the activation energies calculated from these fits are compared.

Effect of Preform Thickness and Volume Fraction on Injection Pressure and Mechanical Properties of Resin Transfer Molded Composites

An experimental study is performed to characterize the effect of the thickness of random preforms on injection pressure and mechanical properties of resin transfer molded (RTM) parts. Center-gated, disk-shaped parts are molded using two different chopped-strand glass fiber preforms. Both preforms have random microstructure but different planar densities (i.e., different uncompressed layer thicknesses). Tensile strength, short-beam shear strength, and elastic modulus are measured for parts molded with each preform type at three different fiber volume fractions of 6.84, 15.55, and 24.83%. Although mechanical properties are found to increase linearly with volume fraction, significant difference is not observed between disks containing thick and thin mats at equivalent fiber volume fraction. However at the same fiber content, parts molded with thin mats require significantly lower injection pressures compared to parts containing thick mats. To characterize this phenomenon, a pressure-matching method to determine planar permeability is presented. Permeability values for each preform would provide a quantitative description of the required injection pressure due to changes in preform thickness, with lower permeabilities resulting in higher injection pressure. Transient pressure data is collected at a fixed radial location within the mold cavity during filling for both preforms at three different volume fractions. Permeability is obtained by fitting the theoretical pressure equation derived from Darcy’s law to the transient pressure data. Permeability values are found to be as much as 227% higher for thin mats compared with thick mats at the same fiber content. The results demonstrate that equivalent mechanical properties can be obtained at lower injection pressures by using thinner random mats.

Filtration and Breakdown of Clay Clusters during Resin Transfer Molding of Nanoclay/Glass/Epoxy Composites

Dispersion of nanoclay clusters during resin transfer molding of nanoclay/glass/epoxy disks is investigated. In addition to a center-gated disk containing only 14% glass fibers, three nanocomposite disks are fabricated with the addition of 2, 5 or 10 wt% Cloisite® 25A nanoclay. The spatial distribution of nanoclay clusters along the radial axis of the nanocomposite disks are characterized at two length scales. Clusters larger than 1.5 μm are characterized by performing image analysis on the SEM micrographs whereas smaller nanoclay clusters are identified by wavelength dispersive spectrometry. Results obtained from image analysis indicate that nanoclay clusters are filtered out by as much as 50% in the flow direction by the glass fiber preforms. In addition, increasing nanoclay content led to higher filtration, suggesting that cluster formation is more prominent at higher nanoclay loadings. Cluster size distribution analyses revealed that the outer edges of the disks, on average, contain finer nanoclay particles. For instance, the outer edge of the nanocomposite with 2% clay contains 22% more small nanoclay clusters compared to center of the disk. Glass transition temperature, Tg, of four specimens obtained from each molded disks is characterized under oscillatory shear. Glass transition temperature of the samples are shown to increase with the nanoclay content, yielding a 40% higher Tg at 10% nanoclay loading compared to glass/epoxy composite without clay. Increasing glass transition temperature with increasing nanoclay content may be an indication of intercalation of nanoclay within the epoxy matrix.

Effect of Fiber Content on Void Morphology in Resin Transfer Molded E-Glass/Epoxy Composites

Effect of fiber volume fraction on occurrence, morphology, and spatial distribution of microvoids in resin transfer molded E-glass/epoxy composites is investigated. Three disk-shaped center-gated composite parts containing 8, 12, and 16 layers of randomly-oriented, E-glass fiber perform are molded, yielding 13.5%, 20.5%, and 27.5% fiber volume fractions. Voids are evaluated by microscopic image analysis of the samples obtained along the radius of these disk-shaped composites. The number of voids is found to decrease moderately with increasing fiber content. Void areal density decreased from 10.5 voids/mm 2 to 9.5 voids/mm 2 as fiber content is increased from 13.5% to 27.5%. Similarly, void volume fraction decreased from 3.1% to 2.5%. Increasing fiber volume fraction from 13.5% to 27.5% is found to lower the contribution of irregularly-shaped voids from 40% of total voids down to 22.4%. Along the radial direction, combined effects of void formation by mechanical entrapment and void mobility are shown to yield a spatially complex void distribution. However, increasing fiber content is observed to affect the void formation mechanisms as more voids are able to move toward the exit vents during molding. These findings are believed to be applicable not only to resin transfer molding but generally to liquid composite molding processes.

Dispersion Characterization of Nanoclay in Molded Epoxy Disks by Combined Image Analysis and Wavelength Dispersive Spectrometry

The state of nanoclay dispersion in a molded epoxy disk and its effects on the thermomechanical properties of the resulting nanocomposite are analyzed. A commercially available nanoclay, Cloisite® 25A, is mechanically mixed at 2 wt % with EPON 815C epoxy resin. The epoxy/clay compound is then mixed with EPI-CURE 3282 curing agent by a custom made molding setup and injected into a disk shaped mold cavity. Upon completion of curing, nanoclay dispersion is quantified on a sample cut along the radius of the composite disk. Dispersion of nanoclay clusters larger than 1.5 μm are analyzed by digital image processing of scanning electron micrographs taken radially along the sample, whereas dispersion at smaller scales is quantified by compositional analysis of clay via wavelength dispersive spectrometry (WDS). Digital images of the microstructure indicate that amount of nanoclay clusters that are larger than 1.5 μm remain approximately constant along the radius. However, size analysis of nanoclay clusters revealed that they are broken down into finer clusters along the radius, possibly due to the high shear deformation induced through the thickness during mold filling. Compositional analysis by WDS signified that approximately 0.4 wt % of the nanoclay is dispersed to particles smaller than 1.5 μm, which are not visible in micrographs. Tensile and three-point bending tests are conducted on additional samples cut from the molded disks. Except for slight reduction in flexural strength, up to 9.5% increase in tensile strength, stiffness, and flexural modulus are observed. Glass transition temperature is determined under oscillatory torsion and observed to increase by 4.5% by the addition of nanoclay.

Effect of Distribution Media Length and Multiwalled Carbon Nanotubes on the Formation of Voids in VARTM Composites

The first part of this paper characterizes the effect of tooling and process parameters such as the length of distribution media used in vacuum assisted resin transfer molding (VARTM) of composite laminates. To achieve this goal, a number of 6-ply, woven carbon fiber=epoxy laminates are fabricated by using various lengths of distribution media. The spatial variations of mechanical properties of these laminates are characterized using a three-point bending fixture. It is shown that for relatively thinner laminates, extending the distribution media degrades the flexural properties by as much as 14%, possibly due to air pockets entrapped during through-the-thickness impregnation of the fibrous fabric. In the second part, a minimum distribution media length is used to investigate the mechanical property and microstructure changes due to multiwalled carbon nanotubes (MWNTs) dispersed in the composite laminates. In addition, effects of different nanotube functionalization and morphology are characterized via scanning electron microscopy and optical microscopy. To achieve adequate nanotube dispersion in the epoxy resin, both tip sonication and mechanical mixing have been used. The effect of sonication time on the dispersion of nanotubes is reported by monitoring the temporal changes in the nanotube cluster size. Even at volume fractions less than 1%, almost 10% improvements in flexural properties is observed. Extensive void formations are reported for laminates containing MWNTs, possibly preventing greater improvements in mechanical properties. [DOI: 10.1115/1.4004700]

Wetting of Compacted Nanoclay Powder by Epoxy Resin

Spreading behavior and advancing contact angle of a low viscosity epoxy resin on three commercially available nanoclays — Cloisite® Na+ , Cloisite® 15A and Cloisite® 25A — at 52°C is investigated. In addition, effect of temperature on spreading dynamics of epoxy on Cloisite® Na+ is analyzed at 33, 52 and 77°C. For wetting experiments, nanoclay powder is compressed into 12.7mm diameter disk shaped compacts under 20MPa pressure. The surface topologies of the compacts are analyzed by scanning electron microscopy (SEM), where as energy dispersive x-ray analysis (EDXA) is utilized to quantify the chemical composition of the surface. An epoxy drop is placed on each of the compacts and spreading is monitored via a CCD camera equipped with a high magnification lens. Temporal evolution of the advancing contact angle as well as drop penetration into the nanoclay compact is determined using the drop profiles extracted from the sessile drop images. Spreading of epoxy on Cloisite® Na+ is observed to be 12-fold faster at 77°C compared to 33°C. Analogous to its spreading speed, rate of penetration of resin into the nanoclay compact increased 20-fold in the same temperature range. Behavior of different nanoclay types are assessed by repeating the wetting experiments on Cloisite® 15A and Cloisite® 25A compacts. Unlike Cloisite® Na+ , which did not have a finite static contact angle, Cloisite® 15A and Cloisite® 25A yielded static contact angles of 59.2°and 40.1°, respectively. These differences are attributed to different surface energies as a result of different chemical compositions of the surfaces and dissimilar surface topologies.Copyright

Performance of glass woven fabric composites with admicellar-coated thin elastomeric interphase

Abstract Adequate stress transfer between the inorganic reinforcement and surrounding polymeric matrix is essential for achieving enhanced structural integrity and extended lifetime performance of fiber-reinforced composites. The insertion of an elastomeric interlayer helps increase the stress-transfer capabilities across the fiber/matrix interface and considerably reduces crack initiation phenomena at the fiber ends. In this study, admicellar polymerization is used to modify the fiber/matrix interface in glass woven fabric composites by forming thickness-controlled poly(styrene-co-isoprene) coatings. These admicellar interphases have distinct characteristics (e.g. topology and surface coverage) depending on the surfactant/monomer ratios used during the polymerization reaction. Overall, the admicellar coatings have a positive effect on the mechanical response of resin transfer molded, E-glass/epoxy parts. For instance, ultimate tensile strength of composites with admicellar sizings improved 50–55% over the control-desized samples. Interlaminar shear strength also showed increases ranging from 18 to 38% over the same control group. Interestingly, the flexural properties of these composites proved sensitive to the type of interphase formed for various admicellar polymerization conditions. Higher surface coverage and film connectedness in admicellar polymeric sizings are observed to enhance stress transfer at the interfacial region.