The effect of temperature and strain rate on the mechanical properties of highly oriented polypropylene tapes and all-polypropylene composites
Introduction
In a series of recent papers [1], [2], [3], [4], composite materials in which both the fibre and the matrix are based on polypropylene (PP) have been described. These so-called “all-PP” composites are designed to compete with traditional thermoplastic composites such as glass fibre reinforced PP, so must possess comparable or superior mechanical properties. One of the main advantages of all-PP composites is the enhanced recyclability which is achieved by using the same polymer for both fibre and matrix phase of the composite. Unlike glass fibre reinforced PP composites, all-PP composites can be entirely melted down at the end of the product life for recycling into PP feedstock. However, the entirely thermoplastic nature of these composites raises important questions regarding the mechanical performance at elevated temperatures and low strain rates.
The concept of improving the mechanical properties of polymers by molecular orientation, without the addition of foreign reinforcements is not new. So-called “self-reinforced polymers” have been the subject of numerous publications focussing on a range of polymers and processing routes including polypropylene [5], [6], [7], [8], polyethylene [9], [10], [11], [12], [13], [14], polyethylene terephthalate [15], [16], [17], polyethylene naphthalate [18], poly(methyl methacrylate) [19], [20], [21], [22], polyamide [23] and liquid crystal polymers [24], [25]. Numerous studies have also been presented specifically for biomedical applications, where the self-reinforcement of bioresorbable polymers is required for load bearing orthopaedic applications, in which foreign reinforcements may complicate biocompatibility or bioresorption profiles [26], [27].
Like many of the “self-reinforced” polymers described in literature, the all-PP composites presented in this paper are wholly thermoplastic, and they can be expected to show varying mechanical properties with temperature and strain rate. This can limit the application of such materials and so must be understood. In the general case of partially or wholly amorphous polymers, the effect of temperature and time during mechanical loading can be considered equivalent. For example, loading such a polymer at a high strain rate can be considered equivalent to loading at a low temperature and vice versa. With increasing temperature, the modulus of polymers generally decreases, although thermal transitions occur which can impart a dramatic step-change in stiffness over a relatively small range of temperatures. These thermal transitions are caused by conformational changes becoming possible due to increased thermal energy in the system and can greatly affect the practical applications of a polymer under load. Polypropylene is semi-crystalline and exhibits a complex combination of thermal transitions occurring in the crystalline phase as well as in the amorphous phase. The presence of the crystalline phase can also impose restrictions on the mobility of the amorphous phase, which further complicates the prediction of the mechanical performance of such semi-crystalline polymers at elevated or reduced temperatures.
This paper has two aims. The first aim of this paper is to investigate the response of highly oriented polypropylene tapes to dynamic mechanical loading at a range of temperatures. These highly oriented polypropylene tapes are thermally bonded together to form all-PP composite structures, and so the precursor tapes are solely responsible for the mechanical performance of the resulting composites. The second aim of this paper is to investigate the response of all-polypropylene composites to mechanical loading at a range of temperatures and strain rates. By characterising these responses, mastercurves can be created in order to help predict the performance of all-PP composites at strain rates outside those achievable in the laboratory.
Section snippets
All-polypropylene composite manufacture
The all-PP composites described in this paper are composed of highly oriented PP tapes that have been woven into a plain weave fabric and consolidated into laminates. The entire composite fabrication process is described in full detail elsewhere [3], [28], [29], but is summarised here. PP tapes are co-extruded using two single screw extruders to produce a tape with a skin–core–skin (A:B:A) morphology, with a thickness ratio of 5.5:89:5.5. The core layer is a homopolymer blend composed of a
DMTA of single tapes
Dynamic mechanical thermal analyses (DMTA) were performed on co-extruded PP tapes, with draw ratios of λ = 1(undrawn), 4, 6, 12 and 17. Specimens were tested in a TA Instruments DMAQ800 DMTA machine operating in a tensile testing mode. A gauge length of 10 mm was used, and since all tapes were drawn from the same original co-extruded tape, thickness and width of tape decrease with increasing draw ratio. The test specimen was cooled to below −50 °C, allowed to stabilise and then heated at a rate of 1
Dynamic thermal mechanical behaviour of individual tapes
The effect of temperature on the dynamic modulus, E∗, of a range of tapes with increasing draw ratio at a frequency of 1 Hz is shown in Fig. 1. Also shown for comparison in Fig. 1 is the dynamic modulus of an undrawn PP tape. The dynamic moduli of these tapes clearly decreases with increasing temperature as would be expected. Each of the tapes show a curve that is similarly shaped to the undrawn (isotropic) specimen and in the case of drawn tapes, the dynamic modulus increases with increasing
Conclusion
All-PP composites have been created which possess similar mechanical properties to traditional glass fibre reinforced PPs. As a result of the high degree of molecular orientation present in the all-PP precursor tapes, a significant glass transition is not seen in dynamic mechanical testing. This is advantageous as it results in a tough failure mode at sub-zero (sub-Tg) temperatures and hence has positive implications for application of all-PP composites at low temperatures or high strain rates.
Acknowledgements
The authors would like to acknowledge the contribution of Dr. Edwin Klompen at Eindhoven University of Technology, Netherlands, to this project. The co-extruded PP tapes used in this study were kindly supplied by Lankhorst Indutech BV, Netherlands. This work is sponsored by the Dutch Government’s Economy, Ecology and Technology (EET) programme for sustainable development, under Grant Number EETK97104.
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