Anil N. Netravali

Interfacial and mechanical properties of environment-friendly “green” composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) resin

Physical and tensile properties of pineapple fibers were characterized. Tensile properties of pineapple fibers, like most natural fibers, showed a large variation. The average interfacial shear strength between the pineapple fiber and poly(hydroxybutyrate-co-valerate) (PHBV) was 8.23 MPa as measured by the microbond technique. Scanning electron microscopy (SEM) photomicrographs of the microbond specimens revealed an adhesive failure of the interface. Fully degradable and environment-friendly “green” composites were prepared by combining pineapple fibers and PHBV with 20 and 30% weight content of fibers placed in a 0°/90°/0° fiber arrangement. Tensile and flexural properties of these “green” composites were compared with different types of wood specimens. Even though tensile and flexural strength and moduli of these “green” composites were lower than those of some wood specimens tested in grain direction, they were significantly higher than those of wood specimens tested in perpendicular to grain direction. Compared to PHBV virgin resin, both tensile and flexural strength and moduli of these “green” composites were significantly higher. SEM photomicrographs of the fracture surface of the “green” composites, in tensile mode, showed partial fiber pull-out indicating weak bonding between the fiber and the matrix.

Characterization of interfacial and mechanical properties of “green” composites with soy protein isolate and ramie fiber

Environment-friendly fiber-reinforced composites were fabricated using ramie fibers and soy protein isolate (SPI) and were characterized for their interfacial and mechanical properties. Ramie fibers were characterized for their tensile properties and the parameters for the Weibull distribution were estimated. Effect of glycerol content on the tensile properties of SPI was studied. Interfacial shear strength (IFSS) was determined using the microbond technique. Based on the IFSS results and fiber strength distribution, three different fiber lengths and fiber weight contents (FWC) were chosen to fabricate short fiber-reinforced composites. The results indicate that the fracture stress increases with increase in fiber length and fiber weight content. Glycerol was found to increase the fracture strain and reduce the resin fracture stress and modulus as a result of plasticization. For 10% (w/w) of 5 mm long fibers, no significant reinforcement effect was observed. In fact the short fibers acted as flaws and led to reduction in the tensile properties. On further increasing the fiber length and FWC, a significant increase in the Youngs modulus and fracture stress and decrease in fracture strain was observed as the fibers started to control the tensile properties of the composites. The experimental data were compared to the theoretical predictions made using Zwebens model. The experimental results are lower than the predicted values for a variety of reasons. However, the two values get closer with increasing fiber length and FWC.

‘Green’ composites using cross-linked soy flour and flax yarns

Environment-friendly, fully biodegradable, ‘green’ composites based on plant based fibers and resins are increasingly being developed for various applications as replacements for non-degradable materials derived from petroleum that are currently being used. Unlike petroleum, plant based proteins, starches and fibers are yearly renewable. In addition, these green composites may be easily composted after their life, completing natures carbon cycle. In this study, soy flour (SF) was modified by cross-linking it with glutaraldehyde (GA). The cross-linked soy flour (CSF) polymer was characterized for its tensile and thermal properties. The effect of glycerol on the mechanical properties of the soy flour was characterized and optimized. CSF polymer showed improved tensile properties and thermal stability, compared to unmodified SF resin, for use as a resin to fabricate composites. Unidirectional green composites using flax yarn and CSF resin were fabricated and characterized for their tensile and flexural properties. The composite specimens exhibited fracture stress and Youngs modulus of 259.5 MPa and 3.71 GPa, respectively, and flexural strength of 174 MPa, in the longitudinal direction. These properties seem to be sufficient for considering these green composites for indoor structural applications.

A study of physical and mechanical properties of poly(hydroxybutyrate-co-hydroxyvalerate) during composting

Changes in physical and mechanical properties of poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) during degradation in a composting medium were studied. Effect of composting of up to 50 days was studied. Specimen weight loss, scanning electron microscopy (SEM), capillary viscometry, Fourier-transform infrared spectroscopy with accessory for attenuated total reflectance (FTIR-ATR), differential scanning calorimetry (DSC) and tensile testing were performed to characterize the changes in the physical and mechanical properties of PHBV during its degradation in the composting medium. The results from the analysis of weight loss, SEM, molecular weight, FTIR, DSC and tensile testing, particularly the physical and mechanical properties, suggest that the degradation of PHBV in compost medium is enzymatic rather than hydrolytic and occurs from surface and the degraded material leaches out.

Green composites. I. physical properties of ramie fibers for environment-friendly green composites

The surface topography, tensile properties, and thermal properties of ramie fibers were investigated as reinforcement for fully biodegradable and environmental-friendly ‘green’ composites. SEM micrographs of a longitudinal and cross-sectional view of a single ramie fiber showed a fibrillar structure and rough surface with irregular cross-section, which is considered to provide good interfacial adhesion with polymer resin in composites. An average tensile strength, Young’s modulus, and fracture strain of ramie fibers were measured to be 627 MPa, 31.8 GPa, and 2.7 %, respectively. The specific tensile properties of the ramie fiber calculated per unit density were found to be comparable to those of E-glass fibers. Ramie fibers exhibited good thermal stability after aging up to 160°C with no decrease in tensile strength or Young’s modulus. However, at temperatures higher than 160°C the tensile strength decreased significantly and its fracture behavior was also affected. The moisture content of the ramie fiber was 9.9%. These properties make ramie fibers suitable as reinforcement for ‘green’ composites. Also, the green composites can be fabricated at temperatures up to 160°C without reducing the fiber properties.

Effects of Strain Rate and Gauge Length on the Failure of Ultra-High Strength Polyethylene Fibers

Ultra-high strength polyethylene filaments taken from a single spool of yarn were examined for strain rate and gauge length effects. As noted in previous research, high strength polyethylene exhibits pronounced strain rate effects that may be seen in strength changes, stress-strain behavior, and and topography of the fracture zones. Unlike most polymeric fibers, ultra-high strength polyethylene seems to exhibit no gauge length effects over the range from 10 to 200 mm, holding the strain rate constant. This latter effect, if true in general, would be a substantial advantage for polyethylene fibers, because scaling effects would be minimized.

An acoustic emission technique for measuring fiber fragment length distributions in the single-fiber-composite test

Abstract This paper describes the application of an acoustic emission (AE) technique for locating the positions of fiber breaks, and thus determining the lenght distribution of the fiber fragments resulting when a composite specimen containing a single fiber is loaded to failure. By means of a micromechanical model, it is possible to determine from the fragmentation lenghts a measure of the interfacial shear strength between the fiber and the matrix. The AE technique also provides a means for characterizing the statistics of fiber strength, particularly for lengths less than 100 fiber diameters. In contrast to existing optical techniques, the acoustic measurements can be automated and are usable with non-transparent matrices. The technique is demonstrated with E-glass fibers embedded in two epoxy blends. A comparison is made of the statistics for the fragment aspect ratios and resulting interfacial shear strength values determined by acoustic emission and optical techniques. Good agreement is found between the results obtained from these two measurement methods.

Excimer laser surface modification of ultra-high-strength polyethylene fibers for enhanced adhesion with epoxy resins. Part 1. Effect of laser operating parameters

The effects of a pulsed XeCl excimer laser (308 nm) on ultra-high-strength polyethylene (UHSPE) fibers and the fiber/epoxy resin interface were studied. SpectraTM 1000 (UHSPE) fibers were treated in air with a pulsed excimer laser with different energy density levels and numbers of pulses. Chemical and topographical changes of the fiber surfaces were characterized using X-ray photoelectron spectroscopy (XPS), dynamic wettability measurements, and scanning electron microscopy (SEM). The fiber/epoxy resin interfacial shear strength was evaluated by the single-fiber pull-out test. The XPS spectra demonstrated that the fiber surface undergoes photodissociation because of the laser treatment, which results in the incorporation of oxygen at the surface. The wettability data showed that the fiber becomes more polar after laser treatment and also more wettable. SEM photomicrographs revealed that the surface roughness of the fibers increases after the laser treatment. The interfacial shear strength (IFSS) results ...

Continuous micro-indenter push-through technique for measuring interfacial shear strength of fiber composites

Abstract This paper describes a continuous push-through, micro-indentation technique for measuring the fiber-matrix interfacial shear strength. E-glass fibers embedded perpendicular to the plane of thin polished specimens of epoxy matrix, with and without coupling agents, were indented with a micro-indenter until failure of the interface occurred and the fibers were pushed through the epoxy. The results show over 60% higher interfacial shear strength for fibers with coupling agent than for fibers without coupling agent. Average shear strength values obtained via the indentation technique are in good agreement with those obtained from the single-fiber-composite test. Absence of acoustic emission signals for debonding of the fibers coupled with no sudden drops in load vs indentation depth suggest that in this geometry the debonding is a slow, continuous process for both fiber surface treatments.

Ammonia plasma treatment of ultra-high strength polyethylene fibres for improved adhesion to epoxy resin

A study of the effect of plasma treatments on the mechanical properties and adhesion of ultra-high strength polyethylene fibres to epoxy resin is reported. Fibres were treated with ammonia plasma under various time and power conditions. The fibre/matrix interfacial shear strength was measured using load and fibre pull-out data obtained in a single-fibre pull-out test. The debonding was optically as well as acoustically monitored. Optical birefringence patterns were visible at the fibre debond region. Acoustic emission signals generated from debonding and stick-slip processes were also detected. A more than four-fold increase in the interfacial shear strength was achieved by plasma treating the fibres at the discharging condition of 30 W and 0.5 torr for 1 min. The birefringence patterns showed, qualitatively, that the shear in the matrix around the fibres increased for treated fibres and extended further into the matrix material. Surface topography of the pulled out fibres showed that the failure mode was unchanged by the treatment.