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In the last few decades, fiber reinforced composites have been established as the materials of choice for lightweight applications in a large spectrum of applications ranging from aerospace, defense, and marine industries to automotive products and consumer goods. With the growing shift to sustainable resources, natural fibers, especially plant fibers, received increased interest throughout the years. Among these natural fibers, silks stand out with low stiffness and a high failure strain, unlike conventional fibers such as carbon or glass. Although gaining traction as a natural alternative reinforcement, silk still has little to no commercial uses despite its higher performance. Besides its higher mechanical properties and lightweight, silk exhibits other attractive properties such as improved flame retardancy and biodegradability. To take advantage of these features, proper fiber/matrix adhesion must be achieved. Such silk/matrix bonding can be inferred from the silk/resin affinity during composite manufacturing. In this study, the affinity/wettability of several silk/resin systems were analyzed via static contact angles using imageJ software to determine candidates for silk reinforced composite laminates with better adhesion. To this end, a combination of four silk fibers and three resin systems were investigated. The investigated silk fibers were Ahimsa, Charmeuse, Habotai, and Tussah; and the resins included a vinyl ester (Hydrex) and two epoxies (INF114 and INR). For Tussah fibers, initial contact angles were consistently one of the lowest. However, these fibers exhibited a higher contact angle over time compared to the other silk fibers studied. Conversely, Ahimsa silk fibers showed the highest initial contact angle, then quickly dropped to complete wetting. Habotai fibers dropped towards complete wetting quickly, however, consistently slowed considerably shortly after. Charmeuse fibers performed similarly to Ahimsa fibers with Hydrex, however was considerably slower to wetting with the other resins. Among the investigated resins, Hydrex showed the best affinity to silk fibers with the majority of the lowest initial contact angles and the fastest to complete wetting. INF114 consistently receded at a slower, albeit steady, rate until reaching complete wetting apart from Tussah. INR showed the highest initial contact angles and never reached complete wetting after an hour for two of the four silks investigated. Therefore, the best silk/resin affinity was observed for the Ahimsa and Charmeuse silk fibers and the Hydrex vinyl ester resin. In future work, silk composites with these constituents would be investigated.

Thanks to its comparable specific mechanical properties to glass fibers, silk is a natural fiber that can be used as an eco-friendly alternative to synthetic reinforcing fibers in composite materials. Compared to natural fibers, especially plant fibers, silk enjoys higher mechanical performances, lower density, and higher elongation even at low temperatures, silk also exhibits other attractive qualities like flame resistance and being naturally continuous. However, silk is known to be prone to moisture absorption from surrounding humid environments. Moisture absorption may alter the silk/resin dynamics during composite manufacturing, and later lead to premature degradation in the composite thermomechanical properties. This study investigates the effect of humidity on silk/resin wettability using two different resins (one epoxy and one vinyl ester) and three different silk architectures. Silk fibers are first exposed to different relative humidity environments. Subsequently, the affinity of the conditioned silk to a set of resins is assessed through measurements of silk/resin contact angle over time. Different silk/resin systems were observed to have contrasting responses to humidity exposure. While some silk/resin systems, such as Ahimsa/epoxy, did not show any change after humidity exposure. Other combinations showed tremendous susceptibility of silk/resin affinity to prior exposure of silk to humidity. For instance, although starting at virtually the same initial hydrophobic contact angle of ~123 degrees, Habotai silk/epoxy samples had contrasting wetting times. While the dried Habotai silk reached full wetting after around 5 minutes, the silk samples exposed to humidity took around 1 hour to reach full impregnation. These findings demonstrate the importance of humidity exposure control in silk reinforced composites.

Since the mass outbreak of COVID-19 globally, the unique challenges of the pandemic have demanded the global economy, governments, and scientific community adapt in unprecedented ways. Despite pre-existing federal stockpiles of personal protective equipment (PPE) in countries such as the United States, rising shortages and resource constraints have compounded the complexity of curbing the spread of the pandemic and treating patients. To face such shortages, healthcare workers in different parts of the world have been reusing PPE, especially personal protective gloves, and possibly sanitizing them through more cost-effective means such as a simple bleach and water solution. To preserve current and future PPE resources, this study investigates the effect of repeated diluted bleach treatment on mechanical properties of representative gloves to determine if reuse is an acceptable practice. This study aims to determine how bleach sanitization may affect material degradation of gloves when used in working environments. To this end, tensile tests were performed on elastomeric exam gloves with and without bleach sanitization treatments. Control data were prepared for both non-conditioned and humidity conditioned glove samples. Additional glove samples were subjected to ten repetitions of bleach exposure as outlined by Centers for Disease Control and Prevention US (CDC US) guidelines. Subsequently, all glove samples were tensile tested, and mechanical properties were determined. A statistically significant (p ≤ 0.05) loss of stiffness was observed for some of the tested samples, ranging as high as a 90% loss in stiffness. This research could serve to inform medical professionals as to whether sanitization through bleach treatments is acceptable and if so, at how many repetitions this treatment could potentially compromise the glove’s ability to function as intended.

Susceptibility of polymeric composites to moisture has been well known for several decades. Most high performance epoxy or bismaleimide (BMI) resins and their fiber-reinforced composites may absorb up to 5 wt% moisture which could lead to 10-30% reduction in various mechanical properties, including flexural strength, stiffness, impact resistance, and interlaminar shear strength (ILSS). In particular, fiber-matrix interface and process-induced defects such as microvoids often act as moisture storage sites, thus increasing the maximum intake level. It has been common practice to use a Fickian model to characterize the diffusion of moisture into polymeric composites. However, in several high-performance and mission critical applications, more sophisticated models accounting for the edge effects, anisotropy of absorption, molecular interactions, and interfacial storage are required to fully describe the long-and short-term moisture absorption dynamics. In this article, a model that combines the classical Fickian behavior and diffusion hindrance due to molecular bonding is used to explain anomalous absorption. The hindered diffusion model (HDM) is shown to predict both short-term Fickian and long-term anomalous, non-Fickian absorption behavior often observed in structural composites. The total amount of absorption is shown to be the sum of bound and unbound liquids, which are coupled through a differential diffusion and a temporal storage model. The accuracy of the model predictions is discussed by comparing the model predictions with the experimentally measured mass gain of graphite/epoxy laminates and clay/epoxy nanocomposites. It is shown that the anomalous moisture absorption dynamics observed in these laminates can be accurately predicted by the hindered diffusion model.

In recent years, fiber-reinforced composites integrated key technological areas such as aerospace, defense and automotive industries [1-5]. The attractiveness of these man-made materials is mainly due to their lower costs and lower weight to strength ratios when compared to conventional materials. Another attractive feature that certain manufacturing techniques can generate near net shape, complex geometry composites [1-3]. However, composite materials suffer from a number of drawbacks that limit their usage in load-bearing components. Some of these drawbacks are manufacturing-related such as voids and dry areas as well as poor fiber-matrix adhesion that severely affect thermomechanical properties of composite parts [5-9]. Other drawbacks are inherent to composites such as moisture absorption or even nanofillers dispersion [10-12]. As the demand for fuel efficiency becomes a must in today's reality, composite materials are more than ever coveted to play a greater part in the transportation field. Consequently, there is a pronounced need for developing enhanced composite materials with better mechanical properties, lower costs and longer lifespan. Since mechanical properties and lifespan of composite parts have been shown to depend heavily on processing parameters [1-14], optimizing the latter can play a key role in this endeavor. During the last five decades, a myriad of research articles have described composites mechanical properties and lifespans dependence on processing parameters. Moreover, aerospace and automotive companies have carried out countless experiments to optimize their manufacturing costs. These data can be congregated and used to better understand their effect on the manufactured part final properties as well as these properties variation during its lifespan. Using big data analysis techniques to analyze this massive amounts of data and extract the best processing parameters for fabricating composite parts for specific applications has the potential of revolutionizing composite materials manufacturing. The final drive being a wider usage of such materials in load bearing component for aerospace and automotive industries, and thus higher overall fuel efficiency for all transportation modes.

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 Epicure 3282 curing agent. Reinforcement is provided by six layers of randomly oriented planar glass fiber performs with 0.459 kg/m 2 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%.

Recent progress in the field of nanotechnology has increased the curiosity in new polymers reinforced with nanosized particles. These new polymers, called nanocomposites, are reported to exhibit strikingly improved mechanical, thermal, optical and physico-chemical properties, when compared to the pristine polymers or conventional (microscale) composites. Other exciting features of nanocomposites include increased moduli, strength and heat resistance, decreased gas permeability and improved flammability. While most of the work to date in literature has focused on nanocomposite property evaluation and synthesis, a full realization of these advantages critically relies on complete dispersion of nanosized particles in the polymer, which is still one of the most important and not fully resolved issues in the manufacturing of nanocomposites.Depending on how many dimensions of the dispersed particles are in the nanometer range, nanocomposites could be in various forms of which only two; nanoclays and carbon nanotubes, have been widely used. The latter has been recorded to possess remarkable thermal and electrical properties, exceptional stiffness and strength, extraordinary resilience and low density, in many studies. As a result of their exceptionally high aspect ratio, combined with specific stiffness and strength, carbon nanotubes are indicated as one of the most ideal candidates for reinforcing composites in many advanced technology applications, such as; computing and data storage technology, electronics devices, sensors, nanoelectromechanical systems and tips in scanning probe microscopy for imaging.Another promising nanoscale filler is the abundantly available layered silicate, a.k.a. nanoclay. Individual nanoclay particles are silicate platelets that typically range from 1 to 10 nm in thickness and 1 to 13 μm in length. Although nanoclay was first introduced to reinforce thermoplastics, its utilization has been expanded to reinforce thermosetting resins due to their wider industrial usage, higher strength, and lower viscosity. Nanoscale fillers have been also added to fiber-reinforced composites with wellestablished manufacturing processes in order to improve their mechanical and thermal properties, flame retardancy, as well as water absorption and gas permeability resistance. These improved properties are obtained only when nanoclay is intercalated and/or exfoliated into the polymeric matrix.In light of these promising results, nanocomposites can play a special role in vitalizing the economies of emerging countries by closing the technological gap with the West. Governments can invest in, or at least encourage, businesses and ideas that exploit these new materials in essential sectors of their economy. In the case of the kingdom of Morocco, the case of three industrial sectors inscribed in the Emergence Plan of the government are discussed: the Aerospace industry, the automotive industry and water management. Applications of nanocomposites can advance the added value of all three sectors, which are vital to the sustained development of the country.

Inclusion of nanoclay in various polymer systems is being studied as a viable improvement method for thermo-mechanical properties for more than a decade. It has also been shown that addition of nanoclay into polymers significantly enhances thermal durability of the part. It is possible that the addition of nanoclay to conventional fiber-reinforced composites would provide enhanced mechanical and thermal performance. However, the interaction of nano-scale clay platelets and somewhat larger clay clusters with micro-scale fibers might lead to non-uniform clay content throughout a molded composite part. In this study we investigate the dispersion of nanoclay in resin transfer molded clay/glass/epoxy disks. For this purpose 4 composite disks are fabricated by resin transfer molding. In addition to 14% glass fiber reinforcement, disks contained either 0, 2, 5 or 10wt.% Cloisite® 25A nanoclay. The spatial distribution of nanoclay clusters in the radial axis of the nanocomposite disks are characterized at two length scales. Clusters larger than 1.5µm are characterized by performing image analysis on the SEM micrographs obtained at 50X, whereas smaller nanoclay clusters are identified by wavelength dispersive spectrometry. In addition, to characterize the interaction of nanoclay with the polymer at the molecular level, glass transition temperature of each disk is measured under oscillatory shear.

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 Epicure 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.

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.

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 100. 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.

The type and volume fraction of fibrous preform used in resin transfer molding has significant influence on the properties of the molded composite parts. In this study, effect of preform architecture and fiber volume fraction on the mechanical properties of resin transfer molded composites is experimentally investigated. A molding setup is constructed to fabricate center-gated, disk-shaped parts at constant flow rate. Two types of chopped, random glass-fiber preforms having different planar densities but otherwise similar structure are used. Molding is performed at three different fiber volume fractions, 9.8, 21.2, and 33.1 %, using both mat types. EPON 815C epoxy resin, mixed at 4.7:1 by volume with EPICURE 3282 curing agent, is used in order to obtain parts for mechanical characterization. Elastic modulus, tensile strength, and short beam shear strength are obtained by testing specimens cut from three locations within the molded parts. Both stiffness and ultimate tensile strength are found to increase linearly with volume fraction. Experimental data show little change in the average tensile properties of disks having different mats at the same fiber volume fraction. However, slightly more variation in tensile properties is observed in individual specimens containing the higher density mats. Similarly, short-beam shear strength is found to be unaffected by the choice of thick or thin preforms at the volume fractions tested.

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.