Objective surface evaluation of fiber reinforced polymer composites

The mechanical properties of advanced composites are essential for their structural performance, but the surface finish on exterior composite panels is of critical importance for customer satisfaction. This paper describes the application of wavelet texture analysis (WTA) to the task of automatically classifying the surface finish properties of two fiber reinforced polymer (FRP) composite construction types (clear resin and gel-coat) into three quality grades. Samples were imaged and wavelet multi-scale decomposition was used to create a visual texture representation of the sample, capturing image features at different scales and orientations. Principal components analysis was used to reduce the dimensionality of the texture feature vector, permitting successful classification of the samples using only the first principal component. This work extends and further validates the feasibility of this approach as the basis for automated non-contact classification of composite surface finish using image analysis.

Optimizing oxygen transport through La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fiber by microstructure modification and Ag/Pt catalyst deposition

This work compares the oxygen permeation fluxes of five different La0.6Sr0.4Co0.2Fe0.8O3−δ membranes (e.g. disk, conventional hollow fiber, modified hollow fiber, Ag- or Pt-deposited hollow fiber membranes) to elucidate the dominance of a particular oxygen transport limiting step (e.g., bulk-diffusion or surface reaction) within each of these membranes. At 900 °C and 100 mL min–1 helium gas sweep rate, the oxygen fluxes for disk, conventional hollow fiber, modified hollow fiber, Ag-deposited modified hollow fiber, and Pt-deposited modified hollow fiber membranes are 0.10, 0.33, 0.84, 1.42, and 2.62 mL min–1 cm–2, respectively, denoting enhanced performance in this sequential order. More than 300% enhancement of fluxes is evidenced by modifying the geometry from disk to conventional hollow fiber. This is attributed to the thickness reduction from 1 mm to 0.3 mm, thus implying bulk-diffusion and surface reaction as the jointly limiting transport step for this disk membrane. In contrast to a conventional hollow fiber that has a sandwich cross-sectional structure (e.g. dense center layer sandwiched by two finger-like layers) as well as dense outer and inner circumference surfaces, the modified hollow fiber has only one dense layer in its outer circumference surface with finger-like porous layer extending all the way from outer cross-sectional part to the inner cross-sectional part. This microstructural difference, in turn, provides substantial reduction of membrane thickness and enlarges surface reaction area for modified hollow fiber (relative to conventional hollow fiber), both of which contributes to the reduced bulk-diffusion and surface reaction resistance; evidenced by almost 250% oxygen flux enhancement. To enhance the performance even further, catalyst (e.g., Ag or Pt) deposition on the outer circumference surface of modified hollow fiber can be utilized to reduce its dominating surface reaction resistance. While both catalysts increase the oxygen fluxes, Pt reveals itself as the better candidate relative to Ag due to melting-induced aggregation and growth of Ag at 950 °C.

A theoretical analysis for fiber contacts in multilayer nanofibrous assemblies

A theoretical analysis is presented for the estimation of the number of contacts between fibers in random multilayer nanofibrous assemblies with arbitrary fiber diameter and orientation. The statistics of fiber contacts for single-layer nanofiber mats were considered first, and the equations were developed for three-dimensional multilayer nanofibrous assemblies by considering the superposition of the single-layer assemblies. Based on the theoretical approach presented here for multilayer nanofibrous networks, the network porosity, mean fiber diameter and a function of fiber aspect ratio contribute to a model to determine the average number of fiber contacts per unit fiber length in multilayer nanofibrous mats. The theory is studied parametrically and results compared with the work of a model presented by Samson. It is shown that the presented model compared to the existing models is more sensitive with the fiber diameter in the nano-scale. It is also believed that the presented theory for fiber-to-fiber contacts is more realistic and useful for further studies of multilayer nanofibrous assemblies.

Experimental verification of theoretical prediction of fiber to fiber contacts in electrospun multilayer nano-microfibrous assemblies: effect of fiber diameter and network porosity

Average number of fiber-to-fiber contacts in a fibrous structure is a prerequisite to investigate the mechanical, optical and transport properties of stochastic nanomicrofibrous networks. In this research work, based on theoretical analysis presented for the estimation of the number of contacts between fibers in electrospun random multilayer nanofibrous assembles, experimental verification for theoretical dependence of fiber diameter and network porosity on the fiber to fiber contacts has been provided. The analytical model formulated is compared with the existing theories to predict the average number of fiber contacts of nanofiber structures. The effect of fiber diameters and network porosities on average number of fiber contacts of nano-microfiber mats has been investigated. A comparison is also made between the experimental and theoretical number of inter-fiber contacts of multilayer electrospun random nanomicrofibrous networks. It has been found that both the fiber diameter and the network porosity have significant effects on the properties of fiber-to-fiber contacts.

Overcoming interfacial affinity issues in natural fiber reinforced polylactide biocomposites by surface adsorption of amphiphilic block copolymers

This work demonstrates that the interfacial properties in a natural fiber reinforced polylactide biocomposite can be tailored through surface adsorption of amphiphilic and biodegradable poly (ethylene glycol)-b-poly-(L-lactide) (PEG-PLLA) block copolymers. The deposition from solvent solution of PEG-PLLA copolymers onto the fibrous substrate induced distinct mechanisms of molecular organization at the cellulosic interface, which are correlated to the hydrophobic/hydrophilic ratios and the type of solvent used. The findings of the study evidenced that the performance of the corresponding biocomposites with polylactide were effectively enhanced by using these copolymers as interfacial coupling agents. During the fabrication stage, diffusion of the polylactide in the melt induced a change in the environment surrounding block copolymers which became hydrophobic. It is proposed that molecular reorganization of the block copolymers at the interface occurred, which favored the interactions with both the hydrophilic fibers and hydrophobic polylactide matrix. The strong interactions such as intra- and intermolecular hydrogen bonds formed across the fiber−matrix interface can be accounted for the enhancement in properties displayed by the biocomposites. Although the results reported here are confined, this concept is unique as it shows that by tuning the amphiphilicity and the type of building blocks, it is possible to control the surface properties of the substrate by self-assembly and disassembly of the amphiphiles for functional materials.

Highly Stretchable Conducting SIBS-P3HT Fibers

Poly(styrene-β-isobutylene-β-styrene)-poly(3-hexylthiophene) (SIBS-P3HT) conducting composite fibers are successfully produced using a continuous flow approach. Composite fibers are stiffer than SIBS fibers and able to withstand strains of up 975% before breaking. These composite fibers exhibit interesting reversible mechanical and electrical characteristics, which are applied to demonstrate their strain gauging capabilities. This will facilitate their potential applications in strain sensing or elastic electrodes. Here, the fabrication and characterization of highly stretchable electrically conducting SIBS-P3HT fibers using a solvent/non-solvent wet-spinning technique is reported. This fabrication method combines the processability of conducting SIBS-P3HT blends with wet-spinning, resulting in fibers that could be easily spun up to several meters long. The resulting composite fiber materials exhibit an increased stiffness (higher Young’s modulus) but lower ductility compared to SIBS fibers. The fibers’ reversible mechanical and electrical characteristics are applied to demonstrate their strain gauging capabilities.

Spinning Carbon Nanotube-Gel Fibers Using Polyelectrolyte Complexation

Biopolymer-single walled carbon nanotube (SWNT)-biopolymer fibers were prepared using a continuous flow spinning approach. Polyelectrolyte complexation was facilitated by injecting a SWNT-biopolymer dispersion into a coagulation bath containing a biopolymer of opposite charge. We showed that the ability to spin fibers and their properties depend on processing conditions such as polyelectrolyte pH, sonolysis regime (conditions employed to disperse SWNT) and the order of adding the anionic and cationic biopolymer solutions. Maximizing the ionic nature through changes in the pH increased spin-ability, while combining a sonicated dispersion with an as-prepared (non-sonicated) polyelectrolyte solution allowed us to optimize sonolysis conditions while retaining spin-ability of fibers with smooth surface morphology. Addition of the cationic biopolymer-SWNT dispersion to the anionic biopolymer solution resulted in mechanical reinforcement with the increase in SWNT loading fraction. All fibers decreased their electrical resistance upon exposure to water vapor.