Investigation of Transcrystalline Interphases in Polypropylene/Glass Fiber Composites Using Micromechanical Tests

A Finite Element Investigation into the Cohesive Properties of Glass-Fiber-Reinforced Polymers with Nanostructured Interphases

Glass-fiber-reinforced polymer (GFRP) composites represent one of the most exploited composites due to their outstanding mechanical properties, light weight and ease of manufacture. However, one of the main limitations of GFRP composites is their weak inter-laminar properties. This leads to resin delamination and loss of mechanical properties. Here, a model based on finite element analysis (FEA) is introduced to predict the collective advantage that a GF surface modification has on the inter-laminar properties in GFRP composites. The developed model is validated with experimental pull-out tests performed on different samples. As such, modifications were introduced using different surface coatings. Interfacial shear stress (IFSS) for each sample as a function of the GF to polymer interphase was evaluated. Adhesion energy was found by assimilating the collected data into the model. The FE model reported here is a time-efficient and low-cost tool for the precise design of novel filler interphases in GFRP composites. This enables the further development of novel composites addressing delamination issues and the extension of their use in novel applications.

Impact of Thermal Properties on Crystalline Structure, Polymorphism and Morphology of Polymer Matrices in Composites

Morphological analysis at different levels is fundamental to understand properties of materials, as these latter are dictated not only by the chemical composition but also by the shape. Solid structures arise from a balance between thermodynamic and kinetic factors, which, especially for polymer composites, depend also on interactions amongst components. In particular, morphology is strongly affected by the heat transfer pattern during crystallization and by the difference in thermal behavior between polymer matrix and filler. Polymers show a spherulitic structure, arising from the start of crystallization in several points of the liquid phase. Within a general rounded shape, spherulites show variability in growth patterns, morphology, and geometry of boundaries. The appearance and the number of spherulites, as well as their growth mechanism, may vary not only in dependence of the chemical composition and the crystalline structures but also, for a same polymer, in consequence of experimental conditions and incorporation of fillers. This article reviews the crystallization process of polymer matrices in the framework of crystal growth and heat transport theories, and explains microstructural differences between composites and neat matrices on the basis of the differences in thermal capacity and conductivity between polymers and additives.

Enhanced Interfacial Shear Strength and Critical Energy Release Rate in Single Glass Fiber-Crosslinked Polypropylene Model Microcomposites

Continuous glass fiber-reinforced polypropylene composites produced by using hybrid yarns show reduced fiber-to-matrix adhesion in comparison to their thermosetting counterparts. Their consolidation involves no curing, and the chemical reactions are limited to the glass fiber surface, the silane coupling agent, and the maleic anhydride-grafted polypropylene. This paper investigates the impact of electron beam crosslinkable toughened polypropylene, alkylene-functionalized single glass fibers, and electron-induced grafting and crosslinking on the local interfacial shear strength and critical energy release rate in single glass fiber polypropylene model microcomposites. A systematic comparison of non-, amino-, alkyl-, and alkylene-functionalized single fibers in virgin, crosslinkable toughened and electron beam crosslinked toughened polypropylene was done in order to study their influence on the local interfacial strength parameters. In comparison to amino-functionalized single glass fibers in polypropylene/maleic anhydride-grafted polypropylene, an enhanced local interfacial shear strength (+20%) and critical energy release rate (+80%) were observed for alkylene-functionalized single glass fibers in electron beam crosslinked toughened polypropylene.

Utilization of Recycled Material Sources for Wood-Polypropylene Composites: Effect on Internal Composite Structure, Particle Characteristics and Physico-Mechanical Properties

In this study, various wood material sources were used for the manufacture of wood-polymer composites (WPC). The materials were categorised as virgin wood particles (VWP), reprocessed WPC particles (RWP) and recycled thermoset composite particles (RCP) and derived from two virgin wood sources, three-layer particleboards, medium-density fibreboards (MDF) boards, or two different wood/polypropylene composites. All produced wood-polypropylene compounds contained 60% wood material and were manufactured using a co-rotating extruder. Malleated polypropylene was used as a coupling agent. Specimens were injection moulded and subsequently tested for their physico-mechanical properties. To characterize particles before and after processing, dynamic image analysis (DIA) measurement were performed. Additionally, X-ray micro-computed tomography (XµCT) was used to characterize the internal structure of the composites and to verify the obtained particle’s characteristics. It was found that length and aspect ratio of particles were remarkably different before and after processing (loss in length of 15–70% and aspect ratio of 10–40%). Moreover, there were notably differences between the particle sources (RCP retained the highest length and aspect ratio values, followed by VWP and RWP). The results suggest that increased aspect ratios can indeed significantly improve mechanical properties (up to 300% increase in impact bending strength and 75% increase in tensile strength, comparing WPC based either on virgin spruce or MDF material). This phenomenon is suggested to be partially superimposed by improved dispersion of particles, which is expected due to lower variance and increased mechanical properties of RWP composites. However, no notable alterations were observed for composite density. Reprocessed WPC and, particularly, RCP material have proved to be an appealing raw material substitute for the manufacturing of wood–polymer composites.