Engineering Thermoplastics Guide. Chapter 5. Thermoplastic Composites and Their Applications

This is an excerpt from Chapter 5 of The Engineering Thermoplastics Guide. Download the full guide below for the full text, including technical diagrams and application notes. 

Introduction

Composites are mixtures of two distinct components: the matrix and the reinforcement. The resulting properties of the composite combine the properties of both components, resulting in unique advantages compared to unreinforced materials. Typically, when we speak of composites, we refer to Polymer Matrix Composites (PMCs). These are the most common and widely used composites today. Since their first invention in the 1940s, their use has rapidly increased. Today, the production of these materials amounts to several tens of Mtonnes worldwide. ^[19]

Typically, composites can be classified based on the matrix. In this context, we can distinguish between thermoset matrix composites and thermoplastic matrix composites. While thermoset matrices are initially liquid before they undergo irreversible curing processes, thermoplastic matrices can be melted and solidified repeatedly.

Although both types of composites are suitable for specific applications, thermoplastic composites present unique advantages. They possess improved toughness, are faster and easier to process, and can be recycled. Composites with engineering and advanced thermoplastic matrices result in mechanical and resistance properties that can easily surpass traditional engineering materials in many demanding applications.

In this chapter, the properties of thermoplastic matrix composites will be discussed. You will also learn about their advantages compared to metals and thermoset matrix composites, their processing methods, and the most common applications across different industries.

General Properties of Thermoplastic Composites

The behavior of thermoplastic composites can be understood based on the different characteristics of the fillers. In general, we can distinguish between particulate fillers, typically constituted by small mineral particles, short fiber reinforcements, and long or continuous fiber reinforcements. In the realm of thermoplastic composites, the choice of reinforcing filler plays a pivotal role in determining material performance. Fibers stand out as the most prevalent reinforcement method for thermoplastics due to their unique efficiency in load-bearing. Fiber-reinforced composites can be classified by distinguishing between continuous and discontinuous fibers.

Continuous fiber-reinforced thermoplastic composites possess fibers extending uninterrupted from end to end. This results in superior load-bearing capacity compared to their discontinuous counterparts, which employ shorter, randomly distributed fibers. In continuous fiber-reinforced thermoplastic composites, different arrangements of fibers are possible, each resulting in unique advantages.

For example, unidirectional (UD) fibers provide outstanding mechanical properties in the longitudinal direction due to fiber alignment. On the other hand, the transverse direction (90° with respect to the fiber alignment axis) typically possesses much weaker mechanical properties. Woven fiber reinforcements, where fibers are bundled in tows and interlaced in specific patterns, yield a balance between longitudinal and transverse properties. Braided fabrics, with their variable interlacement angles, are used for enhanced impact resistance and torsional load-bearing applications. Longer fibers, while enhancing mechanical properties, result in longer processing time.

In contrast, discontinuous fiber-reinforced composites possess fibers characterized by lower aspect ratios. While these composites possess more modest mechanical properties compared to continuous fiber-reinforced composites, they exhibit much easier processability. In fact, they can typically be processed with the same methods used for unfilled thermoplastics.

To understand the properties of discontinuous fiber-reinforced thermoplastics, the most fundamental property is critical fiber length. This parameter is determined by different factors, such as the intrinsic tensile strength of the fiber and its bond with the matrix. Critical fiber length governs the load transfer from the matrix to the fiber. In general, low critical fiber length is beneficial. This is because when the fiber length in the composite is greater than the critical fiber length, the maximum load-bearing capacity of the fiber is exploited. ^[20]

Filler Types and Properties

Carbon fiber

Carbon fiber is one of the most widely used reinforcements for composites. Its market is expected to grow significantly from 3.7 billion USD in 2020 to 8.9 billion by 2031, reflecting the increasing popularity of this material. When combined with thermoplastic polymer matrices, from commodity plastics to advanced thermoplastic materials like PEEK, carbon fiber yields composite materials with exceptional properties.

Carbon fiber possesses an outstanding strength-to-weight ratio, making it one of the most  desirable reinforcements in weight-sensitive applications such as aerospace. In addition to its exceptional mechanical properties, carbon fiber also provides excellent corrosion, fire, and chemical resistance. Its low thermal expansion makes it ideal for high-temperature applications. Carbon fiber also possesses good electrical conductivity, making it an excellent choice for ESD-sensitive applications in electronics. However, in continuously electrified applications, carbon fibers should not be the only ESD-supporting medium in reinforced thermoplastics. Additional ESD-supporting additives are recommended to prevent electrical arcing. ^[21]

It’s important to consider the environmental impact of carbon fiber. The manufacturing process for virgin carbon fiber (vCF) is energy-intensive and can generate hazardous compounds. In contrast, recycled carbon fiber (rCF) is a sustainable option, with much lower energy requirements and reduced environmental impact.

Mitsubishi Chemical Group’s portfolio stands out for its selection of carbon fiber-reinforced thermoplastic composites. KyronMAX® is a range of short fiber-reinforced composites suitable for injection molding or 3D printing. The KyronTEX® family of pre-impregnated materials offers both random fiber-reinforced composites, ideal for high-impact resistance, and continuous fiber-reinforced composites, optimal for high-strength requirements. KyronMAX® and KyronTEX® feature matrix materials from PP to advanced thermoplastics like PEI and PEEK, with exceptional mechanical performance.

Glass fiber

In the context of thermoplastic composites, glass fibers are the predominant choice, making up 95% of the reinforcement fibers utilized in plastics. This material provides a combination of high strength, low weight, and low cost, which makes it suitable for replacing metals in various structural and semi-structural applications.

Fiberglass also demonstrates high tensile strength and is highly resistant to corrosion. It excels in electrical insulation, is non-flammable, and maintains dimensional stability without warping or degrading over time. Moreover, the low thermal conductivity of glass fiber can be exploited in construction and building materials.

Among the innovations in glass fiber composite technology, MCG developed Glass Mat Thermoplastics or GMT. This range of materials features continuous glass fiber mats impregnated with a thermoplastic matrix. The long fiber structure provides GMT with exceptional impact resistance and energy absorption, even at low temperatures, making it less prone to brittleness and shattering compared to traditional glass fiber-reinforced materials.  

Particulate fillers

The use of particulate fillers is a common practice in the plastics industry, aimed at enhancing the properties of thermoplastics. The impact of these inorganic fillers on physical, mechanical, and other properties is closely linked to their inherent characteristics, depending on their nature, shape, particle size, state of matter, surface features, and dispersion within the polymer matrix.

A range of particulate fillers, such as calcium carbonate, ceramics lay, silica, graphite, carbon black, and more can be employed in thermoplastic materials. These fillers can improve both the mechanical properties of thermoplastics and other characteristics, such as thermal and electrical conductivity. ^[22]

Advantages of Thermoplastic Composites

Thermoplastic matrix composites offer numerous advantages over both metals and thermoset matrix composites. In comparison to metals, they provide optimal mechanical properties, such as high specific strength and modulus. In general, metals are much denser than plastics, resulting in an exceptional strength-to-weight ratio in thermoplastic composites. This property makes these materials especially suitable for weight-sensitive and energy-saving applications, including automotive and aerospace.

Many thermoplastics provide impressive resistance to corrosion and chemicals compared to metals. Thermoplastic composites also feature a low coefficient of thermal expansion, making them well-suited for applications where temperature variations are common. Additionally, thermoplastic composites offer remarkable design flexibility and ease of processing. They can be used for overmolding applications, which allow for the creation of complex, multi-material parts or products with enhanced features and performance. They also require minimal secondary processing, streamlining the manufacturing process and reducing production costs.

When compared to thermoset matrix composites, thermoplastic composites possess several key advantages. These composites excel in toughness and impact resistance, making them reliable choices for demanding applications. Because no curing steps are required in the manufacturing of thermoplastic composites, these materials are quicker to process, resulting in shorter lead times and higher production rates compared to thermosets. ^[20]

Thermoplastic composites also provide longer shelf life and are less prone to degradation over time. Notably, thermoplastic composites are easily recyclable compared to thermosets. This contributes to sustainable practices and circular design, while also emitting fewer volatile organic compounds during production. Because thermoplastics are versatile, encompassing a broad spectrum of physical mechanical, thermal, and chemical properties, the characteristics of the final composite can be optimized for each desired application.

Processing of Thermoplastic Composites

The properties of thermoplastic composites, including matrix viscosity, reinforcement volume percentage, and the form of the reinforcement, significantly impact their processability. In general, unlike thermoset composites, which are liquid during the processing stage, thermoplastic composites are made of already polymerized thermoplastics. These materials have therefore much higher viscosity and present unique challenges during processing.

Especially for thermoplastic composites reinforced with discontinuous fibers or particulate fillers, viscosity is a critical variable. The addition of discontinuous fibers or particulate fillers further increases the material’s viscosity, impacting flow and filling during processing. For this reason, specific precautions should be adopted. 

Short fiber-reinforced composites can be processed with many of the approaches used for unfilled thermoplastics, e.g. injection molding or extrusion. However, care is needed to ensure the uniform dispersion of the fibers. Tools and machines used for processing should be sufficiently resistant to the abrasive effect of the fibers. ^[6]

In the pre-impregnation step, the thermoplastic matrix is combined with reinforcement to create composite preforms. This includes techniques such as melt impregnation, powder impregnation, commingling, film stacking, and solution impregnation. These processes ensure that the fibers are fully enveloped by the thermoplastic matrix before forming the composite into the desired components.

These pre-impregnated materials are transformed into composite parts and products in the consolidation step. Different methods, such as thermoforming and filament winding, can be used for the consolidation of thermoplastic composites.

Thermoforming of thermoplastic composites involves using heat and pressure to shape flat sheet preforms into a desired three-dimensional part. The process begins by preheating the sheet using either conduction, convection, or radiant heating. The preheated sheet is then placed onto a temperature-controlled, preheated mold, where it conforms to the mold's surface as it cools. The excess material is trimmed and can be reprocessed, reducing waste. Thermoforming encompasses various techniques, ranging from simple sheet bending to more complex methods like vacuum forming and pressure forming, which utilize negative pressure (vacuum) or positive pressure (compressed air). ^[20]

Filament winding of thermoplastic composites consists of winding pre-impregnated filaments onto a rotating mandrel. This process involves several steps. First, pre-impregnated filaments are pre-heated. Then, a machine winds these materials onto a mandrel, which rotates on an axis. Post-consolidation is finally achieved through heat and pressure. Depending on the context, the mandrel can be recovered or integrated into the finished part. The main advantage of filament winding is that it is suitable for high reinforcement volumes, reaching up to 80%. This results in outstanding mechanical properties for the most demanding applications.  

In recent years, 3D printing of thermoplastic composites has emerged. Additive manufacturing enables design freedom with complex geometries and can contribute to waste and cost reduction. Fusion deposition modeling (FDM), a typical extrusion-based 3D printing process, is commonly used for printing short fiber-reinforced thermoplastic composites. Matrix materials ranging from standard plastics to advanced, high-temperature thermoplastics can be used, offering a wide range of properties and expanding the possibilities for additive manufacturing with thermoplastic composites. ^[20]

 Applications of Thermoplastic Composites

 Thanks to their outstanding mechanical properties, low weight, and excellent chemical and thermal resistance, thermoplastic composites can find applications in several industrial sectors, from aerospace and automotive to sporting goods. In these fields, thermoplastic composites can substitute metals and thermoset composites, with significant effects in weight reduction, sustainability, performance, and energy savings.

 Carbon fiber-reinforced thermoplastic composites are becoming increasingly significant in the  aerospace sector and MCG is playing a leading role in this transition. MCG is collaborating with Boeing to explore the potential of KyronTEX® thermoplastic composites for aircraft sidewall panels. KyronTEX® thermoplastic composites employ recycled carbon fiber and are easier to recycle compared to the thermoset composites employed in the past. This innovation will lead to improved sustainability, reduced emissions, and circularity without compromising performance.

In the sporting goods industry, weight reduction and outstanding mechanical performance are critical for athletes. For example, MCG assisted a bow manufacturer experiencing a 14% failure rate in idler wheels. The solution involved implementing KyronMAX® S-2220, a 20% short carbon fiber-reinforced product. This new material led to a greatly improved failure rate, with increased toughness and strength critical in professional archery. For the manufacturing process, existing tooling was adapted to work with KyronMAX®, eliminating the need for new investments.

You have just been reading an excerpt from Chapter 5 of The Engineering Thermoplastics Guide. Download the full guide below for the full text, including technical diagrams and application notes.