Geometrical Effects in Determination of Fickian Mass Diffusivity of Polymers

Hydrophilicity of polymers makes them prone to moisture absorption that leads to degradation of mechanical properties. Kinetics of moisture ingress needs to be fully characterized to perform reliable designs with polymeric materials. The rate of diffusion is the essential parameter in determining the time scale of the moisture uptake in polymeric materials. The model from which the diffusion coefficient is to be determined can be mathematically complex when the viscoelastic relaxation and diffusion time scales are comparable (i.e. Deborah Number ~ l). However, Fickian type of diffusion is shown to be adequate in modeling the moisture absorption into a broad range of polymers. Most methods for determining the diffusion coefficient are based on the solution of Fick's second law in semi-infinite and slab domains from which, a closed form solution has been adapted by the American Society of Testing and Materials (ASTM). However, those techniques either do not consider the errors due to finite sample dimension or the correction factors provided are not precise enough. In addition, fabrication of samples conforming ASTM standard (i.e. length or width to thickness ratio of 100 or greater) may not be practical due to difficulties in producing and testing very thin coupons. In this study, the solution of the Fickian diffusion equation for a three-dimensional rectangular domain is utilized to generate mass gain data for geometries with length to thickness ratios ranging from l to I 00. These data are then used to demonstrate the errors introduced by the two conventional methods used to determine the diffusion coefficient for sample dimensions deviating from an infinitely wide slab. After applying the correction factor suggested by ASTM, up to 13% error is observed in the diffusion coefficient. In order to improve the prediction of diffusion coefficient, a least square curve fit method, which yields accurate predictions regardless of the sample geometry, is proposed.YesPeer reviewed and presented at the 23rd Oklahoma AIAA/ASME Symposium

Process Induced Defects in Resin Transfer Molded Composites

PreviewResin transfer molding (RTM) is an attractive, versatile and cost-effective alternative to autoclave processing for manufacturing geometrically complex, structural polymer matrix composites (PMC). However, process induced defects such as microvoids or unwetted, dry spots often limit wider usage of RTM parts in high performance, mission critical applications. Understanding morphology of these defects, in addition to their formation mechanisms and removal techniques is an important step towards developing improved RTM processes. In this work, process induced defects in RTM parts are presented and contrasted to other defects encountered in PMCs. Defects in PMCs, which are classified as design induced and process induced, are both reviewed. Thereafter, more attention is drawn on voids and dry spots since they are known to be the most significant defects in RTM PMCs. Hence, dry spot formation mechanisms in RTM and available prevention techniques are summarized. In addition to adverse effects, formation mechanisms, and characterization methods of voids as well as their removal techniques are presented.Ye

Nano-scale Flexible Interphase in a Glass Fiber/Epoxy Resin System Obtained by Admicellar Polymerization

Organosilane coupling agents are widely used in the composites industry to improve the wetting of inorganic reinforcements by low surface energy resins. An increased wettability is often a harbinger of better mechanical properties in a structural composite. Silane coatings effectively increase the spreading of liquid matrixes over glass reinforcement by altering the surface energetics of glass, not by extensive coverage, but by eradication of the high-energy sites present in the oxide surface. Commercial sizings often applied to glass fibers contain up to 10% of the active silane agent, while the remaining 90% is a mixture of lubricants, surfactants, anti-stats, and film formers. Recent investigations have demonstrated that non-reactive components tend to remain in high concentrations within the interphase, thus weakening the resin network crosslink density and increasing the potential for water ingress. Further, sizing formulations are proprietary and designed for specific resin system, which make them expensive, consequently limiting their widespread use. In this paper, admicellar polymerization, a versatile technique to prepare elastomeric thin films of styrene-isoprene copolymer and polystyrene on the surface of random glass-fiber mats is presented. This hydrophobic coating of monolayer thickness applied to the glass fibers is not expected to disrupt the matrix cross-linking reaction; and due to its higher elastic modulus, is believed to cause a change in the stress distribution along the fiber length. Admicellar-modified reinforcements were impregnated with an epoxy resin system: EPON 815C/EPICURE 3232, and molded by Resin Transfer Molding (RTM) into disk shaped parts. Tensile strength, stiffness and interlaminar shear strength (ILSS) were measured for the flexible interphase composites, and compared to parts containing commercially sized and bare fibers. Void fraction, void size and shape distributions, as well as water diffusivity were investigated for each system.YesPeer reviewed and presented at the 18th International Conference of the Polymer processing Society

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

Nanoclay has been gaining acceptance as a nano-meter scale reinforcement for polymers during the last two decades [1]. It has proven to be successful for reinforcing thermoplastics [2], however, its utilization in thermosetting resins has been problematic. To improve dispersion of nanoclay into thermosetting resins, a number of companies developed surface modifications, which replace the sodium ions with larger organic molecules. In the current study, we are investigating the dispersion characteristics of three commercially available nanoclays from Southern Clay Products Inc., by combining microscopic image analysis and wavelength dispersive spectrometry.YesPeer reviewed and presented at the 22nd International Conference of the Polymer Processing Society

Thermal History Effects on Moisture Absorption of Fiber-Reinforced Polymer Composites

Fiber-reinforced polymer composites may offer numerous attractive features such as low cost, high specific performance, and ease of production. However, there are concerns about the overall durability of these materials, especially for sustained performance under severe and changing environmental conditions. There also is a general lack of data on the effects of life-cycle thermal history and temporal changes on moisture absorption dynamics in polymer composites. This work investigates the effects of previous life-cycle thermal history on moisture absorption in resin transfer molded glass/epoxy composites. Disk-shaped parts were fabricated using EPON 815C resin and EPICURE 3282 curing agent. Reinforcement was provided by six layers of randomly-oriented planar glass fiber preforms, yielding approximately 32% fiber volume fraction. Samples cut from the molded disks were initially immersed in water at room temperature for 48 hours. The samples were then divided into three groups and subjected to 0ºC, -25ºC, or liquid nitrogen for another 48 hours to impose various levels of life-cycle damage. A set of specimens was subjected to the same absorption cycle at room temperature in order to characterize the baseline behavior of the composite samples. Afterwards, all samples went through a desorption cycle to remove the absorbed moisture. The specimens were then immersed in water at room temperature for 18 months. Their masses were measured at periodic intervals to quantify the amount of water absorbed. Moisture intake during the 18-month period was found to increase considerably in composite samples subjected to harsher environmental/thermal conditions. In addition, short beam shear strength and stiffness reductions after freezing and after moisture absorption were measured. SBSS and stiffness dropped considerably for all composite parts after thermal conditioning. Property drop was observed to be much more significant for the samples that were exposed to lower freezing temperatures before moisture absorption. As much as 28 and 17% reduction in SBSS and stiffness, respectively, was observed for samples subjected to liquid nitrogen. After saturation with moisture, additional property drops were observed in all composite samples.YesPeer reviewed and accepted for presentation at the 32nd International Conference of the Polymer processing Society

Microscopic Void Analysis in Resin Transfer Molded Composites

Composites are being increasingly used in various applications because of their light weight and high performance. One of the common disadvantages of composite materials is the formation of defects during manufacturing such as voids and dry spots that adversely affect mechanical and physical properties. Although void formation mechanisms and analytical prediction of performance degradation due to voidage have been studied in more detail, knowledge of morphology and spatial variation of voids in resin transfer molded composites is limited. In this work, the radial and through-the-thickness variation of void morphology in a resin transfer molded composite disk is investigated. A resin transfer molded composite disk is fabricated using a mixture of epoxy resin EPON 815C and curing agent EPICURE 3282. The molding is achieved by injecting the mixture into a center-gated mold cavity preloaded with randomly-oriented glass preform, yielding a fiber volume fraction of 18%. After demolding, a radial sample is cut out of the disk and embedded into a quick cure acrylic resin for subsequent polishing. Spatial void distribution as well as the shape and size of voids are investigated along the radial axis and through the thickness of the disk. The studied cross-section is divided into three radial regions. Each region is subsequently divided into three thickness layers. Using 200X magnification, each layer and region is scanned, and the images of identified voids are captured. Image Tool® software is utilized to determine the area and maximum length of each void. These data are processed to establish the void morphology of each spatial location. Significant spatial variation in void content, size and shape are found to exist in the molded composite. These variations seem to correlate well with the local velocity of the fluid front during filling, which can be analyzed through the capillary number. The slower moving fluid front traps fewer and more circular voids, whereas the voids might be sheared into more elliptical and irregular shapes near the parts’ surface and inlet gate. These findings are believed to be applicable to other liquid molding processes for composite materials where similar flow kinematics exists during mold filling.YesPeer reviewed and presented at the 23rd Oklahoma AIAA/ASME Symposium

Comparison of Void Reduction Methods for Resin Transfer Molded Composites

Liquid composite molding (LCM) processes such as resin transfer molding (RTM) have been successfully used to manufacture high performance fiber-reinforced composites at a low cost. However, these composites usually suffer from dry spots and voids formed during molding that decrease their load-bearing capability. Void formation mechanisms and detrimental effects of voids on mechanical performance of molded parts have been studied extensively. In contrast, knowledge of voidage reduction strategies is very limited. In addition, although some void reduction methods are proposed in the literature, no systematic comparison between these techniques has been offered. In this study, three voidage reduction methods are compared: Bleeding for 30 seconds (i.e. continuing resin injection after complete preform impregnation), packing at 570 kPa (i.e. applying a post-fill pressure after impregnation), and 6% compression by thickness (i.e. compressing the molded part at high temperatures after cure). For each method, two RTM composite disks are fabricated by injecting an epoxy resin into center-gated mold cavities preloaded with randomly-oriented glass preform. Void contents, shape and size are obtained via microscopic image analysis. Resulting levels of voidage, as well as void morphologies, from the six molded parts are compared in order to assess the best void reduction technique. Considerable variations in voidage reduction, as well as void size are observed between composites fabricated using different void reduction methods. Packing and compression helped reduce void contents within the composite parts, while bleeding -widely used in the industry- resulted in a 34% increase in voidage. Moreover, voidage reduction levels induced by packing were found significantly higher than those experienced after compression. Packing resulted in over 90% decrease in both void content and void areal density, whereas compression only reduced void occurrence by 38%. In terms of void morphology, voids become more elongated for all three studied methods. However, only packing significantly reduces void sizes. Hence, the best voidage reduction method is believed to be packing. In addition, packing is effective, cost-free, straight-forward, and can be applied to most liquid composite molding processes.Ye

Effect of Repeated Loading on Moisture Absorption of Fiber Reinforced Polymer Composites

Although known for their ease of production and high specific properties, fiber reinforced polymer composites are highly susceptible to environmental conditions such as moisture absorption. While the adverse effects of moisture on composite parts are well documented, effects of repeated loading on moisture absorption of composites are not fully known. This work investigates the effect of repeated loading on moisture absorption of resin transfer molded glass/epoxy composites. Disk-shaped parts (D=152.4 mm) are fabricated using Epon 815C resin and Epi-cure 3282 curing agent. Reinforcement is provided by six layers of randomly oriented planar glass fiber performs with 0.459 kg/m2 surface density, yielding approximately 32.4% fiber volume fraction. Three point bending tests are performed on samples cut from the molded disks in order to evaluate their short beam shear strength (SBSS). After SBSS is determined, load levels that correspond to 30, 50, 70 and 90% of the SBSS are applied 1, 2, 5 and 10 times, thus imparting various levels of low-cycle fatigue damage. A set of unloaded specimens is kept to characterize the baseline moisture absorption behavior of the composite samples. The fatigue-loaded specimens are subsequently immersed in water at room temperature. The masses of specimen are measured at periodic intervals to quantify the amount of moisture absorbed. Maximum moisture absorption is found to increase almost linearly with the stress level applied to the different composite parts. Maximum moisture absorption is also found to increase with the increasing number of loading cycles. In addition, stiffness reduction after loading and after moisture absorption is measured. Stiffness reduction, initially correlated to the number of loadings, is observed to substantially increase after moisture absorption for all samples and reach a plateau around 12.85%.YesPeer reviewed and presented at the 27th International Conference of the Polymer Processing Society

Multiscale Dispersion Characterization and Breakdown of Nanoclay Clusters during Molding

Thermo-mechanical properties of polymers can be significantly altered by the addition of nano-scale particulates such as carbon nanotubes and nanofibers. Among the nano-scale particulates, inclusion of nanoclay is proven to improve thermal and mechanical properties of polymers significantly even at small volume fraction levels. In addition, nanoclay is a viable commercial alternative to conventional fillers owing to its low-cost and accessibility. However, akin to various particulates, extensive agglomeration of nanoclay in polymer matrices presents difficulties in its utilization. In this study, we implement a multi-scale approach to characterize the dispersion of three different types of nanoclays. Cloisite® 15A, 25A and 30B are individually mixed with Epon 815C epoxy resin, by the aid of a sonicator. The resin/nanoclay compound is then mixed with Epi-cure 3282 curing agent and injected into center-gated disk shaped molds. The dispersion state of nanoclay is characterized by using samples cut along the radius of the molded composite disks. Nanoclay clusters larger than 1.5µm are characterized by performing digital image analysis on the scanning electron micrographs, whereas smaller clusters are identified by wavelength dispersive spectrometry. In addition, intra-cluster structure is studied by transmission electron microscopy. It is found that the effectiveness of dispersion increases in the order of Cloisite® 15A, 25A and 30B. For instance the average content of clusters larger than 1.5µm is determined as 4.6vol.% for Cloisite® 15A, whereas the same value for 25A and 30B are 3.39vol.% and 3.45vol.%, respectively. The nanoclay clusters are observed to break down into smaller pieces in the flow direction, regardless of the nanoclay type. For example, small Cloisite® 30B clusters (Area<3µm2) make up 37.8% of the nanoclay content at the inlet, whereas the same value is calculated to be 46% at the outer edge of the disk. Several nano-voids are detected in the intra-cluster regions from the TEM images. These nano-voids are suspected to result from insufficient dispersion of nanoclay in epoxy matrix.YesPeer reviewed and presented at the 25th Oklahoma AIAA/ASME Symposium