Temperature dependence of the interfacial shear strength in glass reinforced polypropylene and epoxy composites

We have recently reported the development of a method which allows the measurement of IFSS over a wide temperature range [6,7]. In this paper we present data obtained using the microbond test in the temperature controlled environment of a thermo-mechanical analyser (TMA). IFSS in glass fibre–polypropylene and glass fibre-epoxy systems in the temperature range -40°C to 150°C are presented and discussed

Effect of Silane coupling agent on mechanical performance of glass fibre

Mechanical performance of commercially manufactured unsized and γ-APS sized boron-free E-glass fibres has been characterised using single fibre tensile test. Both apparent fibre modulus and fibre strength were found to strongly depend on fibre gauge length. The average strength of sized fibres was found 40%-80% higher than unsized fibres at different gauge lengths. Weibull analysis suggested that the failure mode of unsized fibres could be described by unimodal Weibull distribution, whereas the strength distribution of sized fibres appeared to be controlled by two exclusive types of flaw population, type A and B. Comparison of the Weibull plots between unsized and sized fibres revealed that the strength of unsized fibres was likely to be dominated by type A flaws existing on the bare glass surface and type B flaws may be related to the defects on the glass surface coated with silane. This was partially supported by the observation of fractured cross-sectional area using SEM. It was, therefore, proposed that the strength difference between unsized and sized glass fibres may be more reasonably interpreted from the surface protection standpoint as opposed to the flaw healing effect. The results obtained from this work showed that silane coupling agent plays a critical role in the strength retention of commercially manufactured E-glass fibres and the silane effect on the fibre strength is also affected by the change in gauge length of the sample

Development and application of micromechanical techniques for characterising interfacial shear strength in fibre-thermoplastic composites

The development of single fibre pull-out and microbond tests for characterising interfacial strength in thermoplastic composites is reviewed in detail. Manufacture of an experimental jig and sample preparation regimes for both tests are described. The challenges addressed in the sample preparation include the measurement of embedded fibre length for pull-out samples and the low yield rate of axisymmetric resin droplets obtained during sample preparation under nitrogen. The applications of these laboratory developed techniques are demonstrated by characterisation of the interfacial shear strength (IFSS) of glass fibre-polypropylene (GF-PP) and natural fibre-polylactic acid (NF-PLA). The comparison of the IFSS between neat and modified GF-PP showed that both methods were sensitive to the interfacial performance change despite the poor agreement between them for the absolute IFSS values from the same composite. The effect of the material modification was also reflected in load-displacement curves with different behaviour of the frictional motion after complete debonding. When a high level of fibre-matrix adhesion was realised in the composites with weak fibres, the microbond test showed higher feasibility for characterising the IFSS. This was clearly shown in its application to NF-PLA

The role of residual thermal stress in interfacial strength of polymer composites by a novel single fibre technique

The temperature dependence of the interfacial properties of glass fibre reinforced polypropylene and epoxy composites was investigated using a novel microbond test in the temperature controlled environment of a thermo-mechanical analyser. Highly significant inverse dependence of IFSS on testing temperature was observed in both systems. The temperature dependence of the GF-PP IFSS was accounted for by the variation of residual radial compressive stresses at the interface with the test temperature. On the other hand, it was found that the residual thermal stress did not seem to fully account for the temperature dependence of IFSS in GF-Epoxy. Nevertheless, the results clearly showed that GF-Epoxy IFSS had a strong correlation with the modulus of the epoxy matrix

The investigation of fibre reinforcement effects in thermoplastic materials: interfacial bond strength and fibre end parameter

Glass fibres used in the manufacture of fibre reinforced thermoplastic composites (FRTP) are normally sized with a film former which includes a silane coupling agent to improve the interfacial bond strength between glass fibre and matrix . However, during composite failure even an optimized interface cannot stop the initia tion of cracks at the fibre ends, which can lead to large transverse cracks in the matrix or failure by fibre pull-out. In order to help better understand the failure mechanisms of FRTP, thermoplastic microbond tests and photoelasticity experiments have been used to study the interface in model single fibre composites

Investigation of strength recovery of recycled heat treated glass fibres through chemical treatments

The strength loss of thermally treated glass fibre (GF) at elevated temperature is well reported in literature. This phenomenon even occurs at short period of time such as 25 minutes. In the recycling technologies for composites, GFs are usually recovered by degradation of polymeric matrix with thermal and/or chemical treatments. Therefore thermal effect on the strength of GF is a significant factor when restricting the possibilities of recycling this material for a second life. This study reports on the strength of thermally treated commercial GF after acid treatment and silanization of the fibre surface to achieve a proper combination of treatments which may provide us with the ability to recover the mechanical properties of the heat treated GFs. It is thought that silane coupling agents can directly increase and recover the strength of GFs. Two factors associated with this recovery are the possibility of the sizing repairing the damage on the surface of the heat treated GFs and the reduction of the fibre-fibre friction in the bundle through lubricating effect. GF samples were heat treated at 4500C for 25 minutes and coated with silanes, applying different combinations of hydrochloric acid (HCl) and the two silanes used in this study, γ-Aminopropyltrimethoxy Silane (APS) and γ- Methacryloxypropyltrimethoxy Silane (MPS); these fibres were characterized by single fibre testing for strength. The results obtained demonstrated that the fibre strength improves slightly after combination of HCl and MPS treatment, and has a negative effect when the combination of HCl and APS was used. The surface deposition of silane on the surface of the fibre is also discussed using a Scanning Electron Microscope (SEM)

Study on properties of composites reinforced by heat treated glass fibres simulating thermal recycling conditions

In the present study, commercial chopped glass fibres were heat treated at 300°C, 450°C, 500°C and 600°C to imitate a composite thermal recycling process. The heat treated fibres were extrusion compounded and injection moulded with polypropylene to form composites. The heat treatment increased the susceptibility of the fibres to length degradation during the melt processing particularly at higher conditioning temperatures. Comparison with the Cox model revealed that the stiffness of the composite was affected by the reduced fibre length. The reduced fibre length did not significantly contribute to the reduction of the tensile strength and the impact strength. These properties were deteriorated by other factors such as the strength degradation of the glass fibres and the reduced fibre matrix interaction. Thus a post treatment which recovers the fibre strength and optimizes the fibre-matrix interface will be essential to produce thermally recycled glass fibre composites with high mechanical properties

Facing up to the challenges of natural fibres as an engineering composite reinforcement : Abstract and Presentation

The use of glass fibre reinforced polymer materials has increased dramatically during the last half century, becoming the standards of high performance in automotive, aeronautical and innumerable other high performance applications. Glass fibres currently represent more than 95% of the reinforcement fibres used globally in the engineering composites industry. However, the increasing pressure on natural resources and the large amounts of energy required in glass fibre production has led to an upsurge in the research of fibres derived from natural sustainable sources as potential reinforcements for high performance composite materials. It has been claimed that natural fibres show significant potential as environmentally friendly alternatives to conventional reinforcements such as glass fibres (Bledzki 1999, Mohanty 2002, Wambua 2003). Nevertheless, a certain level of reinforcement performance is required from such fibres in order to succeed in engineering applications. In this context many researchers refer to the respectable level of axial modulus of some natural fibres, which can be made to appear even more attractive by comparing modulus/density ratios (see Table 1). However, such claims implicitly depend on the use of simple micromechanical models suitable only for isotropic materials and often ignore the well documented low levels of shear and transverse modulus of these highly anisotropic natural fibres (Cichocki 2002, Baley 2006, Ntenga 2008, Thomason 2009, Gentles 2010, Thomason 2010, Shah 2012). Consequently, an overwhelming number of the published results based on such justifications have failed to fulfil the expectation of matching glass fibre reinforced composite performance. Despite the very large amount of research resource expended in this area, natural fibres continue to exhibit only moderate reinforcement in stiffness and very little positive (and often negative) effects on the strength and impact resistance of composites, often resulting in materials with many characteristics lower than the original base polymer. The majority of the widely available and commercially attractive, low cost, “technical” natural fibres present a number of challenges in terms of their characterization, processing and performance. A list of these challenges would include the very high levels of natural fibre variability, the very high levels of natural fibre anisotropy leading to very poor transverse and shear reinforcement performance, the non-circular natural fibre cross section with “diameters” often orders of magnitude larger than man-made fibres, the introduction of voids in the composites through the natural fibre lumen, the very poor (often negative) reinforcement performance of natural fibres composites, the high moisture content of natural fibres both before and after processing into composites, the health and performance issues related to the bioactive nature of these fibres, the temperature sensitivity during composite processing of natural fibres – in particular odour issues, the very low levels of stress transfer capability at the natural fibre – matrix interface, the unsuitable delivery form of many natural fibres for use in standard composite industry processes, and the need for many high cost treatments and processing steps involved in attempting to overcome these, and many other, challenges in order to produce a reinforcement that has anywhere near the potential to replace glass fibres in any high volume engineering application. This paper will review a number of these challenges and focus in detail on two areas, natural fibre anisotropy and their non-circular cross section, which although almost universally acknowledged, have received little attention in respect of their consequences on the characterization and performance of natural fibres as a composite reinforcement

Effects of hydrolysis ageing on the performance and dimensional stability of glass-fiber reinforced polyamide 66

Results of an in-depth study of hydrolysis testing on the mechanical performance, weight change, and dimensional stability of injection moulded glass-fiber reinforced polyamide 66 automotive composites are presented. Composite and resin samples have been characterised after conditioning in water-glycol mixtures at 70°C, 120°C and 150°C for a range of times up to 1000 hours. The results reveal that hydrothermal ageing results in significant changes in the mechanical performance, weight, and dimensions of these materials. Mechanical performance after conditioning at different temperatures could be superimposed when considered as a function of the level of fluid absorbed by the composite matrix