Self-reinforced poly(ethylene terephthalate) composites by hot consolidation of Bi-component PET yarns

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Abstract

Self-reinforced polymer composites or all-polymer composites have been developed to replace traditional glass-fibre-reinforced plastics (GFRP) with good lightweight, mechanical and interfacial properties and enhanced recyclability. Poly(ethylene terephthalate) (PET) is one of the most attractive polymers to be used in these fully recyclable all-polymer composites, in terms of cost and properties. In this work, unidirectional all-PET composites were prepared from skin–core structured bi-component PET multifilament yarns by a combined process of filament winding and hot-pressing. During hot-pressing, the thermoplastic copolyester skin or sheath layers were selectively melted to weld high-strength polyester cores together creating an all-PET composite. Physical properties of the resulting composites including thickness, density and void content were reported. The effect of processing parameters, i.e. consolidation temperature and pressure on mechanical properties and morphology was investigated in order to balance good interfacial adhesion with residual tensile properties of the composite.

Introduction

The concept of self-reinforced polymer composite was first introduced by Capiati and Porter [1] to prepare polyethylene/polyethylene composites using polymers with different melting points. Following this pioneering work, numerous studies have been carried out on these so-called ‘self-reinforced’ or ‘all-polymer’ composites which are based on similar or identical materials for both the matrix and reinforcement. A number of techniques have been used or developed to prepare these composites, including film stacking [2], [3], powder or solution impregnation [4], [5], hot compaction [6], [7], co-extrusion [8] and selective surface dissolution [9], [10], [11].

One of the main advantages of these all-polymer composites over traditional thermoplastic composites such as glass–fibre-reinforced plastics (GFRP), is their enhanced recyclability [8]. For example, glass–fibre-reinforced polymers can only be recycled into new fibre reinforced grades because of the difficulty in separating glass fibres from polymer matrix. Unlike GFRP, all-polymer composites can be melted entirely at the end of the product life for recycling into polymer feedstock which can be used for a wide range of future applications [8]. This enhanced recyclability is desirable to satisfy new environmental legislation which is currently targeting high volume industries such as the automotive and electrical and electronic industry [12].

Bulk polymers, such as polyethylene (PE) and polypropylene (PP), have been particularly good candidates because of their large number of industrial and domestic applications. However, PE possesses a lower glass transition and melting temperature than PP, which means that creep and usage at elevated temperatures can be problematic [13], [14], [15]. Therefore, in recent years more and more effort has been given to PP and its use in self-reinforced polymer composites.

There are a number of successful techniques to prepare all-PP composites. A ‘hot compaction’ technique for preparing self-reinforced polymer composites was initially developed at the University of Leeds, UK [6], [7]. By carefully controlling the compaction temperature, here a fraction of the oriented polymer fibre skins is melted. With the simultaneous application of pressure, molten polymer flows through the remaining fibre cores to consolidate the composite structure. Upon cooling, the recrystallized polymer forms the matrix phase of the composite to bind the fibres together. Following an initial study on all-PE composites, this hot compaction technique was successfully applied to PP fibres [16], [17], [18], [19], [20] and is now commercially available from Propex Fabrics under the trade name Curv® [21]. Although the hot compaction technique is a very attractive method for the preparation of all-polymer composites, its main challenge is the narrow optimum processing temperature window of only a few degrees. The technique requires a careful temperature control during process, since underheating will lead to insufficient wetting and interfacial adhesion, while overheating will destroy the properties of the oriented fibres [22]. Recently, a new route of combining the processes of hot compaction and film stacking was proposed [23] to enlarge the processing temperature window to around 15 °C for hot compaction of woven PP cloth layers.

In order to overcome the processing temperature window challenge for hot compacted composites, an alternative processing route for all-PP composites was proposed by Peijs and co-workers [24], [25], [26], [27], [28], [29], [30], [31]. In this method, high modulus PP homopolymer tapes are co-extruded with a thin coating of a PP copolymer. These co-extruded PP tapes are then constrained in a mould and hot-pressed at a temperature above the melting temperature of the PP copolymer skins but below that of the oriented homopolymer cores [24]. Constraining [30] plus co-extrusion technique has led to a large processing temperature window which can be tailored by the selection of the copolymer, but is typically in the order of 20–40 °C for PP based systems [8]. Processing can therefore occur at temperatures well below the melting temperature of the fibre and the risk of loss of fibre orientation is reduced. In terms of manufacturing, the large processing window also allows the all-PP products to be manufactured directly from woven fabrics or from tapes through a large number of thermoplastic composite processing methods [32], [33], [34], [35]. A range of all-PP composite products based on this co-extrusion method for tapes has been successfully commercialized by Lankhorst-Pure composites in the Netherlands under the trade name PURE® [36] and also by its licensee Milliken in the USA under the trade name Tegris®.

Recently, all-PP composites based on α and β polymorphic forms have been successfully prepared with a large processing window of >25 °C and the advantage of one-component system as the matrix and the reinforcement only differ in their crystalline modifications [37].

Following the success of all-PP composites, researchers have focused on the study of all-polymer composites based on other polymers for more potential properties and applications. Although better than PE, PP has still a relatively low glass transition temperature (Tg  −15 °C) and melting temperature (Tm  165 °C), limiting the use of all-PP composites in the field of high temperature applications. Therefore, polymers with higher Tg and Tm have become interesting candidates for developing a new range of all-polymer composites. Poly(ethylene terephthalate) (PET) has a Tg of 67–90 °C and a Tm of 260–290 °C [38], which makes it attractive for high temperature applications not attainable for all-PP composites [39]. While most thermoplastics can be recycled, PET recycling is also more practical than for most other plastics because of the existence of well developed recycling schemes based on abundant and well identifiable post-consumer waste streams such as soft drink bottles. Moreover, the low cost of raw PET materials, which after upgrading [40], [41], [42] could be based on recyclate, make all-PET composites a promising cost-effective replacement for glass–fibre-reinforced PET or PBT composites. One of the current uses for recycled PET bottles is already for the manufacture of PET fibres for polar fleece materials or carpets. The use of recycled PET grades in combination with upgrading technologies such as chain extenders [40] or solid state polymerization [41], [42] could potentially lead to future high-strength fibre applications and all-PET composites based on PET waste streams, creating truly sustainable materials for high-end applications.

Previously, the hot compaction technique has already been applied to prepare all-PET composites by Ward and co-workers [43], [44]. Although this neat method promotes adhesive bonding between PET fibres, again its relatively narrow processing temperature window (∼6 °C) adds complexity to the composite manufacturing process. Moreover, it was found that during hot compaction, certain areas in the interior of the PET fibres began to melt at the same time as the exterior of the fibres [43].

All-PET composites based on oriented tapes were prepared in a previous study [3] by a classic thermoplastic composite film stacking manufacturing technique of high-strength PET tapes with copolymer PET films. The large processing temperature window originating from the use of thermoplastic copolyester films with a lower melting temperature than oriented PET tapes, allows all-PET composites to be prepared using low cost manufacturing techniques such as hot-pressing, filament winding, direct stamping, vacuum bag moulding, etc. In this work, all-PET composites are prepared from bi-component PET multifilament yarns by a combined process of filament winding and hot consolidation. Bi-component PET yarns were co-extruded using two different PET grades in a skin or sheath–core construction. The low melting temperature thermoplastic copolyester sheaths were melted into matrix phase during hot-pressing while the high melting temperature polyester cores remain as reinforcement phase. Two processing variables, i.e. consolidation temperature and pressure, were varied independently from 200 to 240 °C and from 1.3 to 4.0 MPa, respectively. The purpose of heating is to melt the copolyester fibre skins to produce the matrix phase and bond PET fibre cores together, while the purpose of pressure is to laterally constrain fibres and to facilitate the matrix to wet-out the remaining fibre cores and promote densification.

One of the main advantages of all-PET composites prepared with these bi-component yarns is a uniform distribution of matrix. Conventionally, resin is added to wet fibres in a separate impregnation step, which can be done by a variety of methods such as passing the fibres through a resin bath or injecting the resin into the fibre preform. Having the matrix (sheath) of given quantity already been positioned around the fibre (core) means the resulting composite will have uniform properties due to excellent wetting and the uniform distribution of fibres in the matrix. Moreover, unidirectional composites prepared with bi-component yarns have an added advantage of being able to achieve high levels of fibre packing and orientation since fibres are less likely to be distorted during the manufacturing process, compared to traditional resin impregnation methods.

Section snippets

Materials

All-PET composites were prepared using bi-component multifilament PET yarns (1000 filaments, dtex 2277), kindly provided by Teijin Fibre, Japan. A sheath–core structure was specially designed for these bi-component yarns, which consists of a high melting temperature thermoplastic polyester core and a low melting temperature thermoplastic copolyester sheath. Table 1 shows details of these co-extruded bi-component PET yarns as given by the producer.

Composite manufacturing

Unidirectional composite laminates were prepared

Melting behaviour of bi-component PET yarn

The DSC curves of bi-component PET yarn are depicted in Fig. 5. There are two endothermal signals in the first heating step, which represent sheath and core polyester phases, respectively. The broad endothermal signal has a peak at 209.6 °C. The sharp signal has two individual close peaks at 250.3 °C and 251.9 °C, respectively, which correspond to the multiple melting peaks for PET reported in literature [46]. The temperature window of about 40 °C exists between the endothermal peaks of the two

Conclusions

All-PET composites were prepared from bi-component PET multifilament yarns by a combined process of filament winding and hot-pressing. Composites with a high fibre volume fraction (∼70%) and a large processing window were achieved due to the large melting temperature difference between the thermoplastic copolyester sheath and thermoplastic polyester core. A good balance between longitudinal and transverse mechanical properties was obtained when PET yarns were consolidated in a hot-press at ∼220 

Acknowledgements

The authors would like to thank Teijin Fiber, Japan for kindly supplying bi-component PET multifilament yarns.

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