Composites Part A: Applied Science and Manufacturing
The influence of fibre length, diameter and concentration on the impact performance of long glass-fibre reinforced polyamide 6,6
Introduction
In recent years there has been strong growth in the use of long glass-fibre thermoplastic composite systems in semi-structural and engineering applications. These thermoplastic matrix composite systems combine ease of processing with property advantages such as enhanced toughness and an unlimited shelf life. Furthermore, their intrinsic recyclability is rapidly being recognised as a strong driving force for their further application. Their potential for high-volume processing combined with high levels of end use performance and associated lower manufacturing costs has spurred this expansion of research and development activities on thermoplastic matrix composites. Glass-fibre reinforced polyamides are excellent composite materials in terms of their high levels of mechanical performance and temperature resistance. The mechanical performance of these composites results from a combination of the fibre and matrix properties and the ability to transfer stresses across the fibre–matrix interface. Variables such as the fibre content, diameter, orientation and the interfacial strength are of prime importance to the final balance of properties exhibited by these injection moulded thermoplastic composites [1], [2], [3], [4], [5], [6], [7].
Short fibre reinforced thermoplastics have been used in the automotive industry for many years and there has recently been a strong growth in the use of polyamide based materials in under-the-hood applications [8]. More recently, there has been an increasing growth in the use of long fibre thermoplastic composite systems in semi-structural and engineering applications. It is interesting to note that the growth rates for polypropylene based long fibre compounds has far exceeded that of other long fibre thermoplastic systems over the last decade. This has occurred despite the fact that many of the early developments and long fibre thermoplastic products were based on polyamide polymers [9], [10], [11], [12]. It may well be that part of the background to this phenomenon lies in the excellent levels of profitability, processibility and performance of these materials. Achieving the correct balance of these “3P’s” is critical to the success of any product in its appropriate market. Notwithstanding these facts there has been considerable discussion recently that the next major long fibre development may be in thermoplastic systems based on higher performance resins than polypropylene. Glass-fibre reinforced polyamides are excellent composite materials, however, the mechanical properties of polyamide based composites decrease markedly upon the absorption of water and other polar fluids [13], [14], [15]. There also exist a number of well documented differences in the structure–performance relationships of short fibre reinforced polyamide and polypropylene composites and it can be expected that there will also be differences when comparing these resins reinforced with long fibres.
In this report data are presented on the mechanical performance of long fibre reinforced polyamide 6,6 which may be relevant to the above discussion. Injection moulded long fibre reinforced polyamide 6,6 samples have been prepared with a range of fibre contents (0–50% weight) and a sizing chemistry optimized for polyamide reinforcement. These long fibre compounds have been produced with glass fibres having average fibre diameters of 10, 14 and 17 μm. Mechanical performance has been determined for both the “dry-as-moulded” state (DaM) and after hydrolytic and temperature conditioning and compared with reference short fibre composites based on 10 μm diameter fibre in the same polymer matrix. Data on the influence of the above variables on the residual fibre length and fibre orientation distribution in the moulded composites and the composite modulus and strength have been presented previously [16], [17]. In this paper data on the influence of the above conditioning environments and micromechanical parameters on the composite notched and unnotched impact performance are presented and discussed.
Section snippets
Experimental
The glass samples used for the production of the long glass-fibre pellets were continuous Advantex® glass (boron free E-glass) Type 30® packages produced on a single production bushing. The glass was coated with sizing formulation R43S, which is a polyamide compatible sizing optimized for continuous glass products. Samples (LF10, LF14, LF17) were produced with a range of fibre contents using nominal fibre diameters of 10, 14, 17 μm and linear density (tex) of 1200, 2400, 3500 g/km as previously
Results
The results for the weight average residual fibre length in the moulded composites are presented in Fig. 1, where the error bars indicate the 95% confidence limits on the averages. It is clear that pultruded ‘long fibre’ (LF) compounds deliver significantly longer fibres to the moulded composite in comparison to the extruded short fibre (SF) compound. It is also clear that there exist significant trends for fibre length versus fibre content in Fig. 1. The residual fibre length decreases
Discussion
The inverse relationship between residual fibre length and fibre diameter has been previously noted in injection moulded SFPA composites [6], [14] and it was further shown that the fibre aspect ratio (length/diameter) was approximately constant for samples with different fibre diameters at the same fibre content. In Fig. 11 the length data from Fig. 1 are considered in terms of residual fibre aspect ratio. It can be seen that the LF results all appear to collapse onto a single trend-line.
Conclusions
This study of injection moulded long glass-fibre reinforced polyamide 6,6 composites has revealed that the average residual fibre length of all LFPA samples exhibited an inverse dependence on fibre content at fixed fibre diameter and a strong dependence on the average fibre diameter at fixed fibre content. These dependencies resulted in an invariable residual fibre aspect ratio for long fibre samples prepared with the same fibre content and different fibre diameters. The product of the residual
Acknowledgement
The author gratefully acknowledges the support of Owens Corning Science and Technology with the preparation and testing of the materials used in this study.
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