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Glass fibre reinforcements form the backbone of the composites industry. Possibly the most critical component involved in the manufacture of glass fibres and their composites is the fibre sizing. The formulation and application of a glass fibre size to reinforcement fibres is a key contributor to the cost-effective production of glass fibres and their processability into composite materials with optimized short and long term performance. However, due to the lack of reliable and verifiable information on the physical and chemical nature of sizings the generally available understanding of these complex chemical mixtures does not reflect the level of importance that they have in determining the success of any glass reinforced composite material in a specific application. A number of recent reviews have highlighted some serious issues for the composites industry related to the state of play in glass fibre sizing technology [1-4]. It has been suggested that the nature of all the complex interactions involved in size formulations, size application, fibre drying, fibre wetting impregnation and composite performance are not at all fully understood, even by those with inside knowledge of size formulations. It has also been highlighted that there is practically no information available in the open literature on what methods, if any, the manufacturers of glass fibres use to characterise their sizings and control the quality and consistency of their sized products. This problem extends to their composite industry customers who consequently have little, or no, real guidance on tools to employ for quality control of sized glass fibres, other than monitoring issues during full production processing. Even then, quality issues with sizings may only become apparent after many years in the long term performance of the final composite parts. Because of the lack of any comprehensive or reliable database on the science and technology of glass fibre sizings and the lack of accepted analysis and characterisation techniques and standards for sizings the number of knowledgeable researchers outside of the glass fibre producers is extremely small. This is compounded by the secrecy surrounding sizing formulations, the unique conditions of sizing application during glass fibre manufacture [2], and the very small concentration of the complex size mixtures in a final composite part. This situation makes it challenging for those without insider knowledge to draw any significant conclusions about the state of play and whether real progress is being made in this important area of composites science. However, there is little evidence of any recent real innovation in commercial glass fibre sizing development and it seems likely that the area is in near stagnation as many glass fibre products and their sizes are high up on the development S-curve, resulting in rapidly diminishing returns on effort. Indeed the glass fibre industry is on record as saying that product (i.e. sizing) development across the glass fibre industry is moving at a slower pace than in the past [5]. It is well established that the pace of technology development in any scientific field is directly proportional to the level of information sharing and to the number of well-informed researchers active in the field [6-8]. While rapid and expanding progress is made in developing areas of materials science where background information is more openly available, new size development continues to be carried out by an exceedingly small number of researchers. These very limited numbers of researchers operate in an isolated and restricted environment with little or no open exchange of information. This situation seriously reduces the probability of an innovation in the field leading to a jump to a new S-curve which could bring rapid acceleration in the performance of resultant new composite products. Consequently, it seems that it is highly likely that the current overall state of size development is a serious barrier to the innovation of improved glass fibre reinforced composite materials and is something that urgently needs to be addressed on an industry-wide level.

Presentation given at the 5th International Conference on Mechanics of Composites, Lisbon, Portugal A summary of some of the issues encountered in application of the microbond test to vinyl ester resins, and strategies developed to overcome these.

The tensile strength of single water-sized E-glass fibres that were thermally conditioned, either in air or under vacuum, was investigated. The vacuum removed water from the conditioning atmosphere, as well as the fibre surfaces, at room temperature but retained tensile strength of fibres treated in the absence of water were not significantly different from those thermally conditioned in a standard air furnace. The results suggest that water, either in the treatment atmosphere or on the surface of the fibres, is not a significant factor in fundamental glass fibre strength loss at an elevated temperature. It may, therefore, be necessary to consider alternative theories to explain this strength loss.

It is probably not an overstatement to say that organosilanes are the most important chemicals used in the glass fibre, and consequently the composites, industry. One of the best-known assertions about silanes is that they promote chemical bonding across the fibre-matrix interface. This concept was fixed in the collective consciousness of the composites community early in its history when developments were focussed strongly on reactive matrices. Indeed, the chemical bridging mindset is strongly entrenched in the interface research community and extends to most other fibre-matrix combinations. However, the development of thermoplastic matrix composites raises questions about the simplistic chemical bridging model of silanes at the interface. A growing number of researchers have also commented on residual stress contributing to the stress transfer capability at the fibre-matrix interface. We will review experimental data on the temperature dependence of the apparent interfacial shear strength (IFSS) in glass fibre-polypropylene and of glass fibre-epoxy composites. This phenomenon is characterised by a large drop in IFSS when the test temperature is raised above the matrix glass transition temperature. These results can be shown to support the hypothesis that the apparent IFSS in composites can be largely explained by residual thermal stresses in the system

The mechanical performance of continuous basalt fibres (BFs) and E-glass fibres (GFs) was investigated using single fibre tensile and microbond interface testing. Fibre surface topography was also studied, using atomic force microscopy. Continuous fibres of identical compositions with various surface sizings were used; interfacial shear strength was investigated using polypropylene (PP) matrix. Data were analysed alongside E-glass fibres with similar surface coatings. The tensile performance of basalt fibres varied significantly depending on the surface sizing applied at point of manufacture but little to no difference was found compared with the similarly sized E-glass fibres. The apparent interfacial shear strength (IFSS) between BFs and PP was significantly improved when sizing was applied. Both unsized BF and GF demonstrated very poor IFSS with PP but the application of silane to GFs produced a significantly greater improvement in the apparent adhesion. Characteristic differences in the surfaces of both unsized and silane-coated BFs and GFs were found by AFM analysis.

This paper focuses on analyzing the interfacial and thermal properties of an epoxy resin glass fibre reinforced composite. The interface was studied using the microbond test to investigate interfacial shear strength values while thermo-mechanical analysis and differential scanning calorimetry were used to find variations in the glass transition temperature and coefficient of thermal expansion. For both, the role of the epoxy resin: curing agent ratio was studied to see if it influenced fibre-matrix adhesion and whether it had similar effects on thermal properties. It was found that the epoxy resin: curing agent ratio did indeed influence both interfacial and thermal properties, with maximum performance occurring around the stoichiometric point.

The processing and reuse of end-of-life composite products in an environmentally friendly manner is an important challenge facing the industry. The development of an economically viable process for regenerating the properties of thermally recycled glass fibres would have significant technological, economic and environmental impacts. Thermal recycling processes for composites are relatively technologically advanced; however, they present a substantial challenge when considering their use for recycling of glass fibre reinforced materials. A combination of exposure to elevated temperatures in the region 450 – 600 °C and to mechanical damage has been shown to cause significant strength loss in glass fibres of up to 90 % of their original value. The recovered fibres are thus unsuitable for use as reinforcement in a second generation composite. Methods of strength recovery that may be applied to such recycled fibres are therefore of interest, particularly if these methods are relatively technologically straightforward. An investigation of possible strength recovery methods using hot water or water vapour was carried out on E-glass fibres. The methods were derived from similar studies on silica in which significant strengthening effects were presented alongside theoretical frameworks to explain the phenomenon [1–3]; a maximum threefold increase in strength following water vapour treatment at 250 °C was demonstrated on silica artificially weakened by abrasion.

Results are presented from the ReCoVeR project on the regeneration of the strength of thermally conditioned glass fibres. Thermal recycling of end-of-life glass fibre reinforced composites or composite manufacturing waste delivers fibres with virtually no residual strength or value. Composites produced from such fibres also have extremely poor mechanical performance. Data is presented showing that a short hot sodium hydroxide solution treatment of such recycled fibres can more than triple their strength and restore their ability to act as an effective reinforcement in second life composite materials. The implications of these results for real materials reuse of recycled glass fibres as replacement for pristine reinforcement fibres are discussed.

The interfacial properties as Interfacial Shear Stress (IFSS) in fibre reinforced polymers are essential for further understanding of the mechanical properties of the composite. In this work a single fibre testing method is used in combination with an epoxy matrix made from Araldite 506 epoxy resin and triethylenetetramine (TETA) hardener. The IFSS was measured by a microbond test developed for a Thermal Mechanical Analyzer. The preliminary results indicate that IFSS has an inverse dependency of both testing temperature and the mixing ratio of hardener and epoxy resin. Especially interesting was the decreasing dependency of mixing ratio at higher temperature.

This paper focuses on an investigation into the role of the epoxy resin: curing agent ratio in composite interfacial shear strength of glass fibre composites. The procedure involved changing the percentage of curing agent (Triethylenetetramine [TETA]) used in the mixture with several different percentages used, ranging from 4% up to 30%, including the stoichiometric ratio. It was found by using the microbond test, that there may exist a relationship between the epoxy resin to curing agent ratio and the level of adhesion between the reinforcing fibre and the polymer matrix of the composite.

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.