The effect of temperature and strain rate on the impact performance of recyclable all-polypropylene composites

Abstract

Highly oriented polypropylene (PP) tapes, with high tensile strength and stiffness achieved by molecular orientation during solid state drawing, are consolidated to create fully recyclable, high performance “all-polypropylene” (all-PP) composites. These composites possess a large processing temperature window (>30 °C) and a high volume fraction of highly oriented PP reinforcement phase (>90%). This large processing window is achieved by using co-extruded, highly drawn PP tapes. This paper investigates the relationship between the impact resistance of all-PP composite laminates based on these highly oriented co-extruded PP tapes, and the temperature and velocity of impact. Unlike isotropic PP, the highly oriented nature of all-PP composites means that a significant influence of glass transition temperature is not observed and so all-PP composites retain high impact energy absorption even at low temperatures. Finally, the ballistic impact resistance of all-PP composites is investigated and compared with current commercial anti-ballistic materials.

Introduction

A series of recent publications by the same authors describe the creation and mechanical properties of composite materials in which the reinforcement and matrix phase are both polypropylene [1], [2], [3], [4], [5], [6], [7]. The creation of these ‘all-polypropylene’ (all-PP) composites is motivated by the desire to enhance recyclability of composite materials. Conventional composites employ very different materials for the matrix and reinforcement phase and this complicates recycling. At the end of the life of a conventional composite component, recycling essentially requires separation of fibre and matrix, since these typically possess very different recycling requirements. All-PP composites overcome this problem since at the end of the life of an all-PP component, the entirely polypropylene composite can simply be melted down for reuse in a polypropylene (PP) feedstock or even into a subsequent generation of all-PP composite. The development of high modulus, high strength PP tapes allows the creation of high performance all-PP composite laminates which possess a wide range of interesting mechanical properties [3], [5], particularly impact resistance [7]. The influence of velocity and temperature on the impact behaviour of all-PP composites is described in this paper, together with a comparison to conventional composite materials commonly used in impact applications.

The response of a material to impact loading will depend on various factors such as the geometry of the structure and striker, the mass and velocity of the striker, and frequency of impacts. Due to their high strength and stiffness, and good energy absorption due to delaminating failure modes, composite materials generally perform well in impact applications. Carbon and glass fibres suffer from a lack of plasticity which means that non-penetrative impact loads can lead to (often invisible, subsurface) fibre damage, which can drastically reduce the residual mechanical properties of the composite. Thermoplastic fibre composites typically possess sufficient elastic limits to make them less sensitive to damage from lower energy impacts (see Table 1). Thermoplastic fibres such as UHMW-PE have specific applications as impact defence materials, such as personal protection for military or police personnel from direct projectile impact [8], or as spall liners behind ceramic/metallic armour in armoured vehicles to limit proliferation of shrapnel inside a vehicle following impact [9]. Composite ballistic protection can also provide significant weight savings for automotive defence, compared to steel armour [10], [11] and has been assessed as fragment barriers for commercial aircraft [12]. The ballistic impact performance of composites has been modelled with some success to determine the methods of predict deformation [13] and model energy absorption [14].

Falling weight impact testing can provide analytical information about the mechanism on impact such as specimen displacement, duration of impact and energy absorption, but are limited to lower velocities, <10 m s⁻¹. The main difference between falling weight impact and ballistic impact is the velocity of testing, and this can result in a different response by the material. In composite systems, ballistic impacts (typically >250 m s⁻¹) involve the propagation of transverse and longitudinal waves through the specimen, which are not seen in lower velocity impacts (typically <15 m s⁻¹). These transverse waves propagate through the thickness of the specimen, while the longitudinal waves propagate along the fibres at the sonic velocity of the reinforcement, V_s

$$V_{\text{s}}=\sqrt{\left(\frac{E}{\rho }\right)}$$

where E is the tensile modulus of the reinforcement and ρ is the density of the reinforcement. A large sonic velocity will allow the dispersion of energy to as large an area as possible, before local strain at impact site leads to failure. The specific energy absorption capability, e_sp, has also been proposed as a comparative tool for ballistic fibres [8]:

$$e_{\text{sp}}=\frac{\sigma \epsilon }{2\rho }$$

where e_sp is the specific energy absorption capability, σ is the tensile strength of fibre, and ε is the percentage strain to failure of the fibre.

Fig. 1 compares sonic velocity and specific energy absorption capability for some common reinforcing fibres. Equally performing composites shall be considered to absorb the same energy upon impact. The energy absorption can be described to affect a circular area of composite laminate of radius, r, with an energy absorption described by the specific energy absorption capability, e_sp. Since the radius of material absorbing impact is due to the transmission of a longitudinal stress wave in the fibres at V_s, the area, a, of material absorbing impact energy could be described by:

$$a=\mathrm{\pi }{r}^2$$

or,

$$a\propto \mathrm{\pi }{V}_{\text{s}}^2$$

Equally performing composites would absorb equal energy, so energy absorbed in this area could be designated, c²:

$$c^2=\mathrm{\pi }{V}_{\text{s}}^2\times e_{\text{sp}}$$

Combining constants, gives:

$$c=V_{\text{s}}\times \sqrt{e_{\text{sp}}}$$

Since graphically in Fig. 1, axes are V_s and e_sp, Eq. (6) represents a curve which describes two materials which have equal performance based solely on these two parameters. However, this curve only accounts for the two criteria of sonic velocity and specific energy absorption capability of the fibres as described in Fig. 1. It can be seen that neither the highest draw ratio PP tapes used in this study [16], nor a highly oriented UHMW-PP fibre [15] feature highly in either axis and so would be unlikely candidates for ballistic applications. In fact, both the PP tape and the UHMW-PP fibre show similar performance on this graph, since UHMW-PP falls near the curve of equivalent performance for PP tapes. Using the design criteria described above, it can be seen that the PP tape used in this research would have a performance just below that of glass fibres. Thus high modulus, high strength, low density and large strain to failure are required to provide good ballistic impact resistance, but this method does not exhaustively determine the suitability of a reinforcing fibre for a ballistic application, due to the range in possible (non-fibre-related) energy absorption methods during ballistic impact, and also the fibre architecture within the composite. This calculation also assumes that the reinforcing elements (fibres or tapes) exhibit constant mechanical performance (i.e. tensile strength, strain to failure and modulus) regardless of the applied strain rate. Previous research has indicated that the mechanical properties of all-PP composites are largely unaffected by strain rate [4], when comparing mechanical properties at strain rates >1 s⁻¹.

One of the main performance indicators for ballistic performance is the V50 number. This merely gives the velocity at which a particular panel will stop 50% of a certain type of projectile fired at it [17]. Thus a material can have a range of V50 values, each referring to a specific projectile or specimen thickness. Another value which is perhaps more useful to compare different materials is the energy per areal density absorbed by a material, which considers specimen dimensions, density and energy of the projectile. This research considers impact from a falling weighted striker and also ballistic impact by 9 mm full metal jacket (FMJ) parabellum projectiles and 1.1 g (17 grain) fragment simulating projectiles (FSPs). FSPs are machined steel cylinders which simulate projectiles from fragmentation grenades or shrapnel from explosions [18]. Fig. 2 shows a 1.1 g FSP, on the left-hand side, and a plastic firing sabot on the right-hand side. Nine-millimeter bullets are considered here because they are typical projectiles from common handguns. There is a range of different international ballistic standards which specify the energy per areal density of a material that is required to stop a given type of projectile. One of the most popular of these is the American National Institute of Justice (NIJ) range of standards, which specify that a minimum number of ballistic projectiles must by stopped by a defined area of specimen.

Despite Fig. 1 indicating that PP is unlikely to be an ideal candidate for highly impact resistant applications, the very high volume fraction of reinforcing phase present in all-PP composites compared to traditional composites, combined with their low density may enhance their competitiveness.

The effect of composite processing conditions on the response of all-PP composites to falling weight impact with a constant velocity has been investigated [16] and is published elsewhere [7]. However, since all-PP composites are wholly thermoplastic, it may be expected that there is a strong relationship between mechanical properties and temperature, which may influence impact resistance. To determine the energy absorbed during impact at different temperatures, falling weight impact tests at temperatures between −50 °C and 120 °C are investigated. In addition, these impact tests were performed at a range of velocities to determine the effect of impact velocity on energy absorption. Finally, some initial ballistic impact tests were performed on all-PP plates to investigate the response of all-PP composites to very high velocity impacts. This ballistic testing also allows a comparison to be made between all-PP and some current ballistic commercial materials.