Design and manufacture of all-PP sandwich panels based on co-extruded polypropylene tapes
Introduction
Sandwich panels are relatively common composite structural products and are employed in areas such as sports equipment, and in automotive and aeronautical applications. A typical sandwich panel is composed of three layers, in which two thin sheets (faces) of a stiff and strong material are separated by a thick core of low-density material [1]. Composite laminates are often employed as the faces of a sandwich panel due to their high mechanical performance and low density. Sandwich structures have optimised specific flexural stiffness (flexural rigidity) because the separation of the two faces by a low density core increases the moment of inertia and hence contributes to the flexural stiffness of the panel, with only a small increase in panel mass. In addition to the high stiffness and low density of sandwich panels, an extra advantage of sandwich panels is that the designer can tailor the properties of the sandwich by adjusting the geometrical parameters of the panels (thickness of the core and the faces) independently of the material used.
The different material characteristics required of the core and faces of a sandwich panel often demands the use of different materials. This complicates recycling, since recycling can necessitate costly material separation at the end of life of the product, if the constituent materials of the product have different recycling processes. Therefore, the concept of creating a sandwich panel in which the faces and the core are made from the same material is attractive since it would remove this separation step in the recycling process. However, the desire to make a sandwich panel composed only of one material must be balanced with the need for adequate mechanical properties from the final sandwich panel.
A candidate material for the construction of sandwich panels is polypropylene (PP). PP is commonly used for a range of applications due its low cost, environmental stability, ease of processing and because it is relatively easy to recycle [2], [3], [4]. However, due its modest mechanical properties, PP often needs to be reinforced to satisfy load-bearing applications. Glass fibres are popular reinforcing elements for PP due to their relatively low cost and high mechanical performance. Alternative composite systems reinforced with natural fibres such as flax, hemp and sisal have also been reported [5], [6] and may appear to be more environmentally friendly [7]. However, although these natural fibres are renewable and can be incinerated, degradation and heterogeneity of fibres can cause performance issues. Clearly, the introduction of any foreign filler whether a glass or a natural fibre, complicates recycling [8]. However, composite materials based entirely on polypropylene have been developed and reported in literature [9], [10], [11], [12], [13]. These “all-PP” composites show potential for use as faces of sandwich panels. all-PP composites consist purely of a propylene copolymer matrix reinforced with a very high volume fraction of high-performance PP tapes. Since these composites do not contain any foreign reinforcements, recycling can be achieved by simple thermal processing at the end of life of the structure [14], and recycled in a subsequent generation of all-PP composites or simply used as a feedstock for less demanding applications. Because these all-PP composites are entirely thermoplastic, the performance of these composites can be expected to vary at different strain rates and temperatures, and this is the focus of a separate publication [12].
This paper describes the development of recyclable sandwich panels based on all-PP composite faces and PP based cores. Flexural testing is used to compare all-PP faced sandwich panels with sandwich panels having glass reinforced PP faces. The bond strength between the face plates and the sandwich core is also assessed. Sandwich panels constructed using the same PP based cores but commercial glass reinforced polypropylene face panels will be presented for comparison. Finally the minimum weight criterion of a sandwich panel with respect to a given flexural stiffness developed by Kuenzi [15] was found appropriate to compare the performance of the different face materials.
Section snippets
Designing with all-PP laminates
The mechanical properties of all-PP laminates are presented in Table 1, compared to glass fibre reinforced polypropylene (GFRPP). The mechanical properties of GFRPP depend strongly on the architecture of the reinforcement and the volume of glass fibres present. Woven glass fibre reinforced PP (woven glass/PP), such as Twintex®, with a glass content of 60 wt% (percentage by weight) woven fabrics of continuous fibre, has considerably higher mechanical properties than glass mat thermoplastics (GMT)
all-PP sandwich face panels
It has been widely reported in literature that it is possible to create highly oriented PP fibres or tapes, with high tensile strength and stiffness, by molecular orientation achieved during solid state drawing [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], and thus it is conceivable to use such tapes as a reinforcement for a composite material. Much has been published on alternative processing routes to achieve single polymer composites such as combining high performance
Specimen production
Sandwich panels were produced consisting of all-PP, GMT or woven glass/PP face panels, and a core composed of either a PP honeycomb or a PP foam. The stacking sequence used to create the sandwich panels was: face panel-adhesive film-core-adhesive film-face panel, as illustrated in Fig. 6. The stacked collection of layers is then subjected to heat and pressure to bond the sandwich faces to the core material, by melting the adhesive film. The method used to apply this heat and pressure depends on
Differential scanning calorimetry
To determine the melting behaviours of both of the copolymer adhesive films investigated in this paper, differential scanning calorimetry (DSC) was performed on 5 mg samples of polymer taken from the pellet form, using a TA Instruments DSC Q1000 differential scanning calorimeter. To remove the effect of thermal history of the DSC results, samples were heated in the DSC from ambient temperature to 180 °C at 10 °C min−1 and then cooled to 20 °C, also at 10 °C min−1. Immediately, the samples were
Differential scanning calorimetry
Fig. 11 shows the DSC data obtained for copolymer LS and HS melt adhesives. Also indicated on Fig. 11 are the manufacturers recommended sealing temperatures. It is clear that copolymer LS melt adhesive has a lower temperature endothermic event associated with the manufacturers melt sealing temperature, which is absent from copolymer HS melt adhesive. The lower melting temperature of copolymer LS melt adhesive allows adhesion of the sandwich faces to the sandwich cores at lower temperatures,
Conclusions
The use of material selection tools to assess the performance of all-PP composites in structural sandwich applications reveals that there is a potential to reduce the weight of applications where GFRPP is currently used. This is particularly true in the case of stiffness limited design. It was, however, stressed that the low compressive strength of all-PP can be a limiting factor for the use of all-PP structures loaded in flexure or compression.
It has been shown that 100% PP sandwich panels can
Acknowledgement
This work was sponsored by the Dutch Government’s Economy, Ecology and Technology (EET) programme for sustainable development, under Grant No. EETK97104.
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