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

Glass fibre sizings and the composites industry : the current state of play

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

The influence of fibre cross section shape on critical fibre length and composite strength

The development of the micromechanical theories of composite materials has played an important role in studying and understanding the performance of fibre reinforced composites. Many of the concepts and equations developed to predict composite micromechanical performance were developed early in the history of the composite materials and are unquestioningly embedded in the collective psyche of the composites community. One such concept is the role of the fibre length to diameter ratio (L/D), which is often found in micromechanical discussions of composite performance parameters such as modulus, strength and impact resistance [1-3]. This leads on to the concept of critical fibre length (Lc), which is of particular use when considering the strength of composites [2]. Interestingly many of the considerations of composite micromechanics make use of the assumption, or approximation, that the fibres have a circular cross section. This is understandable since at the time of the development of these concepts most reinforcement fibres, such as glass fibres, often did have a circular cross section. However, recently there have been a number of developments from glass fibre manufacturers in the production of non-circular cross section glass fibres [4-6]. Furthermore, it is well known that many of the carbon fibres on the market do not have circular cross sections [7] and the more recent upsurge in interest in the use of natural fibres as a composite reinforcement also brings many different, and variable, fibre cross sections into the mix [8,9]. Hence, non-circular cross section fibres are now widely available and in use in many composite applications. This prompts the question whether all of the micromechanical analysis that we know and love is fully compatible with the reinforcement potential of these non-circular fibres. In a series of recent papers we noted how the commonly used assumption of circularity in determining the cross sectional area of natural fibres could lead to large errors in the values obtained for fibre strength [8,9]. In this paper we consider the effect of non-circularity on the micromechanical stress transfer capability of the fibre-matrix interface and its potential effects on composite performance

The role of the epoxy resin : curing agent ratio on composite interfacial strength and thermal performance

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

Recover : regenerating the strength and value of thermally recycled glass fibres

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

Investigation of the effect of hot water and water vapour treatments on the strength of thermally conditioned E-glass fibres

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

XPS and AFM study of interaction of organosilane and sizing with e-glass fibre surface

Organosilanes are often used in commercial sizings for glass fibres to provide wettability with the resin and promote strong interfacial adhesion to the matrix in a fibre reinforced polymer composite. The silane treatment is introduced as part of a complex deposition from an aqueous emulsion immediately at the spinaret and determines the optimum properties of the cured composite. To understand the interaction of organosilanes contained in sizings for glass surfaces, XPS was used to investigate the adsorption of γ-aminopropyltriethoxysilane (APS) from a simple sizing system containing a polyurethane (PU) film former. It has been found that both APS and the sizing (containing APS and PU) deposits on E-glass fibre surfaces contained components of differing hydrolytic stability. The differences observed in the AFM images of APS coated E-glass fibres before and after water extraction also confirmed that the APS deposit contained components with different water solubility

A study of the thermal degradation of glass fibre sizings at composite processing temperatures

Although not fully understood, it is well recognized that optimal working of glass fibre sizings is necessary to maximize the performance of glass fibre reinforced polymer composites. It is important that the organic components in such sizings continue to function after exposure to the high temperatures often experienced during composite processing. This study presents the results on the thermal stability of polypropylene and epoxy compatible glass fibre sizings obtained using TGA, microbond adhesion measurement and composite mechanical testing. Test results indicate that the performance of commercial polypropylene compatible glass fibre sizings can be significantly compromised by thermo-oxidative degradation at normal composite processing temperatures. A significant reduction in composite performance is directly related to a loss of fibre-matrix adhesion caused by thermal degradation of some of the principal sizing components

An investigation of glass-fibre-reinforced polyamide 66 during conditioning in various automotive fluids

Injection moulded glass-fiber reinforced polyamide 66 composites and unreinforced polymer samples have been characterised during conditioning up to 900 hours in water, ethylene glycol and water-glycol mixture at 50°C and 70°C. All materials showed significant fluid and temperature dependent weight and volume increase. Glass reinforcement significantly reduced the polymer fluid uptake. The absorption of the antifreeze mixture initially follows a simple rule of mixtures of the absorption of the two individual components. However, after absorption of approximately 5% a significantly higher than predicted level of antifreeze absorption was observed. This coincided with a significant increase in the volumetric swelling coefficient. Dynamic mechanical analysis and unnotched impact testing indicated significant changes in composite mechanical performance dependent on conditioning fluid and temperature