Thermoplastic Composites with High Mechanical Properties and a Low Processing Temperature

Chapter 1: Introduction

1.1 Background

Fibre reinforced polymer composites have been widely used in aerospace, automobile,  military, and energy applications because of their superior performance combined with a high strength, stiffness, and light weight compared to steel and aluminium [1][2][3]. Moreover, the thermal expansion in fibre reinforced polymer composites is lower than in metals. It is easy to mould or form fibre reinforced polymer composites into a variety of complicated parts. Therefore, they represent important engineering materials that are widely used in global industries [1][3].

The ability of fibre reinforcements (low ductility) to improve the stiffness and strength of fibre reinforced polymer composites is extremely important. The strength and stiffness of fibre reinforced polymer composites increase with increasing fibre volume fraction.  Fibre reinforcements control behaviour under tension and compression of unidirectional fibre reinforced polymer composites, while the polymer matrix part controls shear and transverse strength [4]. Furthermore, polymer matrices (high ductility) also play important roles in supporting and connecting the fibres and transferring the applied stresses on composites to fibres, as well as protecting rigid and brittle fibres from abrasion and corrosion [5]. In other words, the ability of polymers to be reinforced by fibres makes them more attractive for engineering applications [6].

Glass, carbon, polyester and Kevlar are used to reinforce thermoplastic or thermoset polymer matrices [7][8][9][10][11]. Although carbon fibres have a greater Youngʼs modulus than glass fibres, they are more expensive [12]. As such, glass fibres are the most popular fibres for reinforcing polymer matrix composites (PMC) [13][14]. Composite properties are affected by the interfacial adhesion between fibres and a matrix. Strong interfacial adhesion allows the applied stresses on composites to be transferred from matrix to fibres, resulting in promoting the mechanical properties of composites. Silane coupling agents can be used to improve the interfacial adhesion between glass fibres and a polymer matrix [15][5]. Laminated composites based on brittle matrices have low interlaminar fracture toughness, damage tolerance and impact damage resistance, which can be enhanced using matrix toughening agents [17][18][19][22][23][24][145].

Most fibre reinforced polymer composites used in industry depend on thermoset matrices such as unsaturated polyester and epoxy. Nevertheless, fibre reinforced thermoplastic composites have rapidly grown in recent years, and polyamide (PA) and polypropylene (PP) are now particularly common as matrices [15]. This is due to the fact that thermoplastic composites have higher fracture toughness than thermoset composites, as a result of the higher fracture toughness of thermoplastic matrices [4]. Another reason is that the manufacturing cost and time of thermoplastic composites are less than those of thermoset composites [16][17]. Continuous glass fibre reinforced thermoplastic composites with high performance have been widely used in construction, automotive and energy applications such as in car crash beams, in windows, and in some parts of solar cells (see Figure 1.1).

For example, continuous glass fibre reinforced polyamide 6 composites with high fibre volume fraction were used to produce the front crash beam for a HYUNDAI sedan vehicle, as seen in Figure 1.1(A) [21]. Although PA composites have high mechanical properties, the manufacturing cost of these composites is high. This can be attributed to the high temperature required to process PA composites, due to the high melting point of the PA matrix [18][20][22]. PP composites can be a good alternative to PA composites, if manufacturers are searching for a more cost-effective product. This is because the cost of producing PP composites is lower than for PA composites, as a result of the low processing temperature of PP composites (again, due to the low melting point of the PP matrix) [23]. However, there is a compromise, as PP composites display inferior mechanical properties to PA composites.

Thermoplastic polyurethane (TPU) composites can be a good choice instead of PA composites or PP composites due to their good mechanical properties along with a low processing temperature (resulting from the low melting point of the TPU matrix). Glass fibre reinforced thermoplastic polyurethane composites have been used in many applications, such as in the automotive industry and in the construction of energy-efficient buildings [8]. Many researchers [24][25][26][27][28][29][30] have designed different types of segmented TPUs (with soft and hard segments). Hard-segment (HS) content affects the thermal and mechanical properties of TPUs.

For instance, the tensile strength and tensile modulus of TPUs are improved with increasing the HS content [26][54][55][56][57][58][59][60]. As such, annealing treatment enhances micro-phase segregation between soft and hard segments in TPUs [17][32]. This process promotes the crystallinity of TPUs [51][53][66][67][68], which could improve their mechanical properties. On the other hand, a few studies focused on the mechanical properties of glass fibre reinforced thermoplastic polyurethane composites alone [7][31][8][32][8].

Figure ‎1.1(A, B, C, D and E): Applications of high-performance continuous glass fibre reinforced thermoplastic composites [21].

1.2 Research Aims and Objectives

The main aim of the current project is to design thermoplastic composites with high mechanical properties and a low processing temperature that could be used in the construction, automotive and aircraft industries. As a result of previous work carried out in this area, two TPU resins with high HS content are used as matrices (ductile TPU matrix and brittle TPU matrix). It is aimed to use plain woven E-glass fabric (GF) as fibre reinforcements with low and high fibre volume fractions (V_f).

To broaden the scope of the project, it is hypothesised that annealing treatment for the TPU matrices and GF-TPU composites may promote their thermal, dynamic mechanical and mechanical properties. The interfacial adhesion between the glass fibres and the ductile TPU matrix could be improved by sizing the glass fibres (after desizing) with a low concentration of aminosilane sizing. This might increase the mechanical properties of (low V_f) GF-TPU composites based on the ductile TPU matrix.

It is also aimed to produce (high V_f) GF-TPU composites based on a brittle TPU matrix, and these composites may have low fracture toughness, damage tolerance and impact-damage resistance. A ductile TPU matrix could be used as a matrix toughening agent to improve the fracture toughness, damage tolerance and impact-damage resistance of (high V_f) GF-TPU composites based on the brittle TPU matrix. In order to achieve these aims, the following research objectives were identified:

• To synthesise TPUs with 70% and 90%wt. HS (TPU/70 and TPU/90).
• To find the best procedure to cast the TPU solutions.
• To produce GF-TPU/70 composites with 25% V_f.
• To produce GF-TPU/90 composites with 25% and 50% V_f.
• To find the best compression-moulding conditions for TPU matrices and GF-TPU composites.
• To find the best annealing time for TPU matrices and GF-TPU composites with 25% V_f.
• To study the effect of HS content and annealing treatment on the thermal, dynamic mechanical and mechanical properties of TPU matrices and GF-TPU composites with 25% V_f.
• To find the best fibre-desizing treatment method for GFs.
• To find the best silane sizing concentration and sizing procedure for GFs.
• To investigate the effect of fibre surface treatments on the mechanical properties and morphology of GF and GF-TPU/70 composites with 25% V_f.
• To find the best technique with which to manufacture the GF-TPU/90 composites with 50% V_f.
• To modify the TPU/90 matrix with 25% and 50%wt. of TPU/70.
• To study the influence of adding TPU/70 to a TPU/90 matrix on the thermal, dynamic mechanical and mechanical properties and morphology of GF-TPU/90 composites with 50% V_f.
• To use a PP matrix and GF-PP composites with 25% and 50% V_f as a benchmark.

1.3 Thesis Structure

This thesis consists of seven chapters. The content of each chapter in this thesis can be summarised as follows:

Chapter 1 gives a summarised introduction of the background and the aims of this project. The structure of this thesis is also presented here.

Chapter 2 provides general information about thermoplastics, TPUs, glass fibres, fibre-desizing treatment, silane coupling agents and fibre reinforced polymer composites and includes a literature review of annealing treatments and hard-segment content of TPUs, fibre surface treatments and matrix toughening agents for composite laminates. The literature review includes the processing temperature of some continuous glass fibre reinforced thermoplastic composite laminates.

Chapter 3 describes the raw materials used in the experiments carried out for this work, as well as the steps required for TPU synthesis and TPUs casting, glass fibre surface treatments, TPU blending and the fabrication of TPUs and GF-TPU composites. This chapter also presents the thermal, physical and mechanical test methods and equipment used to characterise the glass fabrics, TPUs and GF-TPU composites.

Chapter 4 (paper 1) includes the results and discussion of the effects of hard-segment content and annealing treatment on the thermal, dynamic mechanical and mechanical properties of TPU matrices and GF-TPU composites with 25% V_f. A PP matrix and GF-PP composites with 25% V_f are used as a benchmark in this chapter.

Chapter 5 (paper 2) presents the results and discussion related to the effect of fibre surface treatments on the mechanical properties and morphology of GF and GF-TPU/70 composites with 25% V_f. GF-PP composites with 25% V_f are also used as a benchmark in this chapter.

Chapter 6 (paper 3) includes the results and discussion of the effect of using TPU/70 as a matrix toughening agent on the thermal, dynamic mechanical and mechanical properties of GF-TPU/90 composites with 50% V_f. The results and discussion of the effect of V_f on the mechanical properties of GF-TPU/90 composites are also presented in this chapter, in comparison with GF-PP composites with 25% and 50% V_f as a benchmark.

Chapter 7 provides the conclusions of this work and suggestions for future work.