Nam Kyeun Kim

Polymer–Polymer and Single Polymer Composites Involving Nanofibrillar Poly(vinylidene Fluoride): Manufacturing and Mechanical Properties

Nanofibrillar polymer–polymer composites (NFCs) and single polymer composites (SPCs) were produced using linear low density polyethylene (LLDPE) and poly(vinylidene fluoride) (PVDF). The NFCs were fabricated by means of a microfibrillar composite concept comprising melt blending, cold drawing, and compression molding retaining the highly oriented PVDF reinforcing nanofibrils (diameter of approximately 250 nm) dispersed without any agglomeration in the isotropic LLDPE matrix. The SPC films were prepared by partial surface premelting of neat PVDF nanofibrils (diameter of about 130 nm) using hot compaction at 148°C (about 20°C below the complete melting of PVDF), thus preserving the PVDF nanofibrillar identity. Tensile testing of NFCs based on LLDPE and PVDF showed an increase in the tensile modulus by 135% and in the tensile strength at break by 211%, as compared to those of an isotropic LLDPE film. Furthermore, the PVDF SPCs showed an enhancement of tensile modulus of 30% and strength at break of 305% when compared to those of an isotropic PVDF film.

Nanofibrillar Poly(vinylidene fluoride): Preparation and Functional Properties

Poly(vinylidene fluoride) (PVDF) forming β-phase polymorphic modification shows strong ferro- and piezoelectric property as a crucial electroactive polymer and can be obtained by subjecting the highly oriented PVDF fibers to strong electric fields. Two different nanofibrillar materials: PVDF nanofibrillar single polymer composite (NF-SPC) and electrospun PVDF nanofibers were produced for this research. PVDF NF-SPC was prepared by hot compaction of PVDF nanofibrils separated from a PVDF nanofibrillar composite and a high electric field was used to drive out electrospun PVDF fibrils. Polymorphic change and morphology of constituents of the two samples were investigated and measurement of ferro- and piezoelectric behavior was performed after poling treatment. As a result, highly oriented nanofibrils were observed from the two samples and the formation of the β-phase was clearly detected compared to undrawn isotropic PVDF. Higher remnant polarization values and piezoelectric resonance behavior were detected from the samples, in which the β-phase polymorphic modification dominates.

Natural fibers: Their composites and flammability characterizations

The concept of green composites offers challenges to the designers in many aspects, which warrants the identification of more environment-friendly resources. In this regard, replacing synthetic fibers with natural ones in composite materials has to play a major role in manufacturing engineering. The advantages, such as low cost, low density, high toughness, relatively high specific strength properties, low abrasiveness, low energy consumption in fabrication, and CO2 neutrality of some natural fibers, provide the researchers incentives to use these materials in new developments. Most recent research efforts have removed many fiber–matrix compatibility problems; however, poor thermal stability is a major drawback in using these materials, especially in transportation and aerospace applications. Generally, natural fibers are considered as heat sources in composites. If the fiber cellulose content is high, it tends to increase the flammability due to high levels of levoglucosan, but the amount of lignin content in the fibers leads to char formation after initial ignition and provides a thermal barrier. This phenomenon can be used to control the fire growth, by selecting good combination of materials and obtaining effective homogeneous composites under suitable processing conditions. This chapter initially gives a brief account of various types of natural fibers, suitable matrix reinforcements, general methodologies for flammability measurements, and finally describes some specific flame retardance results using two types of natural fibers, namely plant-based kenaf and animal-based wool fibers. The effects of various intumescent ammonium polyphosphate (APP)-based fire retardants are discussed. It is clear that some statistical analysis is necessary to get the necessary fire characteristics without sacrificing too much of mechanical performance. The fire characterization has been carried out through horizontal and vertical burn tests, and by studying peak heat release rate, total heat release, and smoke constituents, using a cone calorimeter.

Multi-Functional Properties of Wool Fibre Composites

Composites sheets based on short wool fibres and polypropylene were fabricated by extrusion process. A three-factor two-level experimental design using Taguchi method was applied in manufacturing the composites to explore the contribution of each parameter on mechanical properties. Fire retardant behaviour of the composites with different fibre weight ratios was investigated by horizontal burning test and cone calorimetric analysis without the addition of any fire retardant agent. Reduction of burning rate with increase in the wool fibre content was observed and suitable formulation of the composites was selected for evaluation of mechanical properties.

Tensile behaviour of open hole flax/epoxy composites: Influence of fibre lay-up and drilling parameters

The present study reports on a systematic experimental test that evaluates the effect of drilling parameters on the open-hole tensile properties of flax fibre epoxy reinforced laminates. Additionally, three lay-up configurations of the flax fibre, namely, [0°/90°]6, [0°]6 and [±45°]6, were investigated and compared. The results demonstrated that the [0°]6 lay-up configuration was superior in terms of mechanical or tensile strength retention of the composites given an 8 mm hole. Conversely, damage due to delamination was not significant due to the changes in feed rate and cutting speed. This suggests that the drilling parameters were less sensitive towards the mechanical strength of the flax fibre composites with the drilled hole. The less severity of the delamination can be attributed to the high fracture toughness (high mode I critical strain energy release) of the flax fibre when compared with that of the synthetic fibre reinforced composite counterparts. The use of the step drill bit design also potentially contributes to the reduced severity of the delamination damage to the flax fibre composites.

An Attempt to Find a Suitable Biomass for Biochar-Based Polypropylene Biocomposites

Four biomass wastes (rice husk, coffee husk, coarse wool, and landfill wood) were added with biochar and polypropylene (PP) to manufacture biocomposites. Individual biomasses were tested for their combustion behavior using cone calorimeter. Biocomposites were analyzed for their fire/thermal, mechanical, and morphological properties. Wood had the most desirable comprehensive effect on both the mechanical and fire properties of composites. In particular, wood and biochar composite exhibited the highest values of tensile/flexural properties with a relatively low peak heat release rate. In general, application of waste derived biochar and biomasses drastically reduced the susceptibility of neat PP towards fire.

The mechanics of biocomposites

Abstract Composite materials having bio-based constituents are strong candidates for biomedical applications due to the possibility of potential biocompatibility. Therefore an overview is presented regarding the theoretical aspects related to the design of polymeric biocomposites. Since, the fundamental mechanical response of most composites is in macro and micro level, the basic concepts for designing composite materials have been discussed. In particular, the mechanics of composite laminates along with prediction models are introduced. Moreover, the mechanics of short fibre and particulate composites are thoroughly examined. Two innovative bio-based reinforcements, namely wool (as short fibre) and biochar (as particulate) are reviewed for their individual mechanical properties simultaneously with their effect on the performance properties of the resulting biocomposites. Finally, an interesting class of biocomposites, called cellulose nanocomposites and its mechanics have been explored which holds immense potential for new age biocomposites in medical applications.