Introduction to polymer matrix composites (PMC)

1.3.1 Polymer matrix composites

It is well known that, in the past several decades, polymer matrix composites (PMC) have become advanced materials, and can be applied as engineering structural materials for aircrafts or vehicles, as well as biomedical materials for medical uses [58]. Polymer matrix composites are conventionally classified into two groups: thermoset matrix composites (TSC) and thermoplastic matrix composites (TPC). As shown in Table 1-6, thermoset composites have slightly different properties from the thermoplastic counterparts; the former ones usually exhibit much lower strains to failure.

Composite technology is based on taking advantages of the stiffness and strength of high-performance fibers by dispersing them in a matrix, which acts as a binder and transfers the acting load to the fibers across the fiber-matrix interface. To understand how the properties of a composite originate, it is necessary to know the properties of constituents form a composite system. The mechanical properties of a composite are determined by a number of factors, including the moduli and strengths of the fiber and matrix; aspect ratio, length distribution, volume fraction, uniformity and orientation of the fibers, as well as the integrity of the fiber-matrix interface and the interfacial bond strength [59].

The first generation of composite materials based on the more brittle thermoset matrix offers fracture toughness as low as 100 J/m². The development of toughened thermosets and a wide range of high performance thermoplastics have increased this value up to 2000 J/m² [60]. Advanced thermoset epoxy composites are now the most often used in high performance applications due to their unique performance-to-cost ratio. They generally possess excellent properties and are suitable for a large number of processing techniques.

However, thermoset epoxy composites have been found that the properties of toughness and dimensional stability will decrease as the glass transition temperature Tg of the resin used increases.

In addition, a change in temperature and moisture content could result in moisture-induced stress as well as dimensional change in composite body [61-64].

Furthermore, the recursive changes of internal stresses due to water absorption-desorption processes may induce fatigue damage, and in turn influence long-term durability and performance of composite [65].

Thermoplastic matrix composites present a number of advantages over thermoset composites, including increased fracture toughness, lower moisture absorption, potential for reduced life-cycle cost, good welding property, and recyclability [66,67].

1.3.2 High performance carbon-fiber/PEEK (CF/PEEK) composite

Due to the high fracture toughness, high temperature resistance, repairability and ease of manufacture, thermoplastic matrix composites have been studied extensively [68-73]. Among these, the carbon-fiber/PEEK(CF/PEEK) composite is one of candidates to replace conventional epoxy-based composites for aerospace applications. Because of the short processing time needed, the CF/PEEK composite provides flexibility in adapting various manufacturing technologies to improve the production efficiency. However, the recommended processing condition for the CF/PEEK composite requires a forming temperature of 400^oC and a pressure of 1.4 MPa for 15 min [38], which are much higher than those for the epoxy-based composites. The higher requirement of processing conditions might therefore limit the potential to make use of cost-effective manufacturing technologies for fabricating components from the CF/PPEK composite.

During the past decades, many researches have been conducted to study the processing conditions in order to search for the opportunities of broadening the processing window for

the CF/PPEK composites. An inevitable variation of the processing condition is the cooling rate. Gao and Kim [38] found that the cooling rate controlled the degree of crystallinity which in turn was correlated to the interface adhesion, the crystalline morphology, and the bulk mechanical properties of neat PEEK resin. As a result, the interface bond strength, as well as the tensile strength and elastic modulus, decreased with increasing cooling rate.

However, the ductility increased with increasing cooling rate due to its effect on crystallinity and spherullite size. In addition, the interface failure was recognized as brittle debonding in slow-cooled composites. In contrast, the amorphous PEEK-rich interface introduced in fast cooled specimens failed in a ductile manner with extensive plastic yielding.

Morphologically, it was shown that the presence of carbon fibers within the matrix would induce nucleation and growth of crystallites perpendicular to the fiber surface, i.e., transcrystallization, which might impose considerable influence on the fiber/matrix interfacial interaction and the failure behavior in both the matrix and the interface region [70].

In view of the effect of residence time in the molten state of the PEEK reinforced with carbon fibers (APC-2 prepreg by ICI/Fiberite Company, USA) on the number of spherulites present in the bulk matrix, it was found that increasing the residence time would result in a decrease in the number of spherulties, and a well-defined transcrystalline region was subsequently developed on the carbon fiber surface [74]. Consequently, the unidirectional CF/PEEK composite containing a transcrystalline phase showed a higher transverse tensile strength than that of the matrix, owing to a strong interfacial bond between the carbon fiber and the PEEK matrix.

Gao and Kim [75] also conducted a study on the effect of cooling rate on interlaminar fracture toughness of unidirectional CF/PEEK matrix composites. It was shown that the PEEK resin displayed a remarkable 230% improvement in fracture toughness when the

cooling rate was changed from 1 to 80 ^oC/min. Furthermore, they also conducted the study on the effect of cooling rate on impact damage performance of CF/PEEK laminates, and compared with CF/epoxy laminates [76]. They concluded that the ability to resist damage initiation upon impact was higher in the order of fast-cooled CF/PEEK, slow-cooled CF/PEEK, and CF/epoxy laminates. Meanwhile, they showed that the threshold impact energy was higher and the compression-after-impact (CAI) strength reduction rate was lower for the fast-cooled laminates than the slow-cooled counterparts, strongly indicating the higher impact tolerance of the former system.

The CF/PEEK composites possess extraordinary strength-to-weight and stiffness-to-weight ratios along the longitudinal (or fiber reinforced) direction, as compared with steel, Al or Ti alloys in Table 1-6 [77]. For this very reason, the CF/PEEK composites can be applied on high-requirement rigid aerospace or aircraft turbomachinery components, such as centrifugal impellers.

In terms of the biomedical applications, the CF/epoxy composite materials can be applied on the external fixation for bone fracture repair because of their lightweight and sufficient strength and stiffness [78]. On the other hand, the CF/PEEK composite materials have been applied on the internal fixation for bone fracture repair by different ways using implants such as wires, pins, screws, plates, and intramedullary nails [78]. Among various materials studied, CF/PEEK composite materials are reported to be biocompatible [79] and have good resistance to hydrolysis and radiation degradation. Except for their high strength and fatigue resistance, the CF/PEEK composite materials have been shown to be biological inertness with no mutagenicity or carcinogenicity. Moreover, the tissue response to CF/PEEK has been described as minimal. In view of the effect of exposure to saline solution (0.9% NaCl) on the flexural and fracture toughness properties of short carbon fiber reinforced PS (polysulfone),

PBT (polybutylene terephalate) and PEEK composites, CF/PS and CF/PBT composites showed significant degradation of mechanical properties following exposure to saline solution [80]. But there was no such reduction for the CF/PEEK composites, due to good bonding between the carbon fibers and PEEK matrix [81]. Animal studies showed that the CF/PEEK composite elicits minimal response from muscular tissue. Both the in vivo and in vitro aging studies confirmed the mechanical stability of CF/PEEK up to 6 months.