Accurate characterization of moisture absorption in polymeric materials

The importance of using the exact solution of the hindered diffusion model is demonstrated on experimental data from a nanoclay/epoxy composite.Ye

Modeling and Experimental Validation of Anomalous Moisture Absorption of Micro and Nanocomposite Laminates

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.YesPolymer Processing Society Asia/Australia Conference PPS-2016, October 11-14, 2016, Chengdu, Chin

Manufacturing silk/epoxy composite laminates : challenges and opportunities

Presented at the 34th International Conference of the Polymer Processing Society, May 24, 2018.Application of natural fibers in polymer composites has been gaining popularity in several industries pursuing environmentally friendly products. Among the natural fibers with proven potential applications, silk fibers have recently received considerable attention from researchers. Silk fibers provide higher mechanical properties compared to other commonly used natural fibers such as sisal, jute, and hemp. Silk may also exhibit comparable specific mechanical properties to glass fibers. However, silk composite laminates are rarely used in commercial products due to a number of fabrication challenges. This paper investigates such challenges for silk/epoxy laminates, especially issues related to manufacturing and preform architecture. First, challenges arising from preform architecture (i.e., random and woven preforms) are presented. Unlike glass fibers for which random mats are easier to manipulate, handling random silk preform proves to be more challenging, particularly compared to woven silk fabrics. The random silk/epoxy laminates show higher thickness variation and lower compaction, yielding lower fiber content. Second, fabrication of laminates by vacuum bag/wet lay-up and vacuum assisted resin transfer molding (VARTM) processes are presented. VARTM is found to be more appropriate for silk/epoxy laminate fabrication, as it allows a uniform impregnation of the silk preform, yielding higher part quality and limited void formation. Moreover, applying 0.21 MPa (30 psi) external pressure to the VARTM laminates allows to increase the fiber content of both random and woven silk/epoxy laminates from ~17 and ~30% to ~21 and ~33%, respectively. In contrast, wetting of silk preform during wet lay-up process, which is operator dependent, is difficult to achieve; and the produced laminates have high void content. Furthermore, SEM images show a weak silk/epoxy adhesion in laminates fabricated without external pressure. Finally, the mechanical performance of these laminates is assessed. The woven silk/epoxy laminates fabricated by pressurized VARTM exhibits the highest improvement in the specific flexural strength and modulus over pristine epoxy with 30 and 65% increase, respectively.YesPeer reviewed for the Proceedings of the 34th International Conference of the Polymer Processing Society, Taipei, Taiwan, May 21st-25th 2018

Moisture Absorption of Carbon/Epoxy Nanocomposites

Moisture absorption of composites with nanoscale carbon additives such as carbon nanotubes, carbon nanofibers, graphite nanoplatelets, and carbon black is investigated using thermogravimetric data and a non-Fickian hindered diffusion (Langmuir-type) model. The moisture absorption parameters are determined using this model for six different types of carbon/epoxy nanocomposites. The absorption behaviors obtained at different humidity levels and thermal environments are recovered by minimizing the error between the experimental data and model predictions, thus enabling the accurate determination of the moisture equilibrium level. The absorption behavior and the weight gain of all nanocomposites are shown to be accurately represented by this model over the entire absorption period. The presence of carbon nanomaterials is found to induce varying levels of non-Fickian behavior, governed by the nondimensional hindrance coefficient. This behavior is enhanced with the nanomaterial content and separate from the slight non-Fickian behavior of all neat epoxy samples. The molecular bonding during diffusion, as well as the interfacial moisture storage, could be among the reasons for non-Fickian behavior and should be included in the absorption models for accurate characterization of carbon/epoxy nanocomposites

Silk as a Natural Reinforcement: Processing and Properties of Silk/Epoxy Composite Laminates

With growing environmental awareness, natural fibers have recently received significant interest as reinforcement in polymer composites. Among natural fibers, silk can potentially be a natural alternative to glass fibers, as it possesses comparable specific mechanical properties. In order to investigate the processability and properties of silk reinforced composites, vacuum assisted resin transfer molding (VARTM) was used to manufacture composite laminates reinforced with woven silk preforms. Specific mechanical properties of silk/epoxy laminates were found to be anisotropic and comparable to those of glass/epoxy. Silk composites even exhibited a 23% improvement of specific flexural strength along the principal weave direction over the glass/epoxy laminate. Applying 300 kPa external pressure after resin infusion was found to improve the silk/epoxy interface, leading to a discernible increase in breaking energy and interlaminar shear strength. Moreover, the effect of fabric moisture on the laminate properties was investigated. Unlike glass mats, silk fabric was found to be prone to moisture absorption from the environment. Moisture presence in silk fabric prior to laminate fabrication yielded slower fill times and reduced mechanical properties. On average, 10% fabric moisture induced a 25% and 20% reduction in specific flexural strength and modulus, respectively