Composites: Types, Processing and Application

What is Composites?
Composites are suitable substitutes of aluminium, titanium and steel in certain applications, because they are lightweight, have good performance properties, low carbon and low energy footprint. Composites are categorized into textile composites, green composites, biocomposites and hybrid composites. Among all types of composites, green composites attracted considerable interest due to environmental friendliness, sustainability and completely biodegradable in different environments, without leaving any toxic residues. In addition, regulatory agencies have stipulated stringent guidelines and legislations to stop production of materials that are hazardous to the environment. There are several global players operating in the industry of composites using different processing technologies. These key players are collaborating with researchers to find new ways to improve the quality of the material and production capacity while reducing the price of the product. The market of composite is growing rapidly, and it is expected to grow by 10% from 2017 to 2025. The leaders in composite market are America, Asia Pacific, Europe, Middle East and Africa.

Polymer composites have been used in various applications, such as automotive, aerospace, construction and packaging; and their market is growing immensely. Man made fibers such as glass and carbon have been used as reinforcing agents in polymer composites to enhance their performance. However, the combination of one or two fiber-reinforced polymer composites, also known as ‘hybrid composites’.

Classification of Composites:
Generally composites are four types. These are-

Textile composites
Biocomposites
Green composites
Hybrid composites

Above composites are described below:

1. Textile composites:
Textile composites (or fiber-reinforced composites) have received considerable interest in various applications over the last few decades due to their unique properties. Various types of reinforcements in polymer composites are textile materials, especially if the polymer composite is reinforced with fibrous reinforcement. Fibrous reinforcements have been explored since the inception of composites. These reinforcements include fibers (short and long), yarn and fabrics. Fabrics are classified into woven, non-woven and knitted structures. Moreover, textile composites can be manufactured according to their desired end application. Among all types of textile reinforcement, woven fabric is the most preferred, as it is easier to handle and has good tensile strength in both warp and weft directions.

The natural fibers have been used as a composite material by the ancient Egyptian. They mixed Nile mud with straw for the manufacturing of the bricks and producing stronger bricks after baking them in sun. Bast fibers, such as hemp, flax, ramie, bamboo, sisal, leaf fibers, seed fibers, grass fiber, or wood fibers are suitable to be used for manufacturing of the composite materials.

Textile composites are used typically because of their high strength-to-weight and stiffness-to-weight ratios.

2. Biocomposites:
Biocomposite materials are produced by combining polymer matrix and natural fibers that have their own distinguish properties. However, after combining them, the resultant material possesses superior properties in comparison to those of individual polymer matrix and natural fibers, and they are suitable for various technical applications. Polymer matrix provides the structure and shape of the material, whereas natural fibers improve the performance (tensile, flexural, impact etc.) of the resultant biocomposites. Biocomposite is a new emerging field. Range of polymers has been studied as matrices reinforced with natural fibers. Polymers are synthesized from fossil fuels, bio-based resources and a combination of both.

Synthetic polymers include PP, polyethylene, polyvinyls, phenolics and polystyrene. The majority of biocomposite materials to date are made from synthetic polymers for a wide range of applications due to their low production cost, easy processability, lightweight and mouldability into different shapes. Synthetic polymers reinforced with natural fibers have been extensively used in packaging and automotive applications.

3. Green composites:
The development of green composites made from 100% bio-based materials has been a research hotspot. These materials offer several advantages such as low cost, acceptable biodegradation, low density, high aspect ratio and high specific strength, which place them among highperforming materials. The driving force behind the development of 100% green composites is the growing attention to reduce the negative environmental impact caused by the synthetic polymers and synthetic polymer-based composites, limited fossil fuel resources and the lack of knowledge about natural fibers properties that will enhance the performance of composites. Various natural fibers which have been used to produce green composites including flax, sisal, kenaf, cotton, hemp and agave. They are abundantly available and renewable. Agricultural by-products such as sugar cane bagasse, maize stalks have also been used as reinforcements.

4. Hybrid composites:
Hybrid composite is when two or more fibers or fillers are used to reinforce a single polymer or one or more fibers or fillers used to reinforce a polymer blend. The hybrid composites have better tensile properties in comparison to individual reinforced polymer composite. In the case of different fillers reinforced polymer matrix, one filler complement disadvantages of the other filler, i.e. one type of filler in the hybrid composite can be expensive and possess high tensile modulus, whilst the other type can be inexpensive having low tensile modulus.

However, in the case of synthetic and natural fibers reinforced polymer composites, the incorporation of synthetic fibers helps to reduce the moisture absorption and enhance the properties, whereas natural fibers reduce carbon footprint and the price of the end product. The properties of hybrid composites depend on various factors; these factors include fiber loading, alignment and orientation of fibers, dispersion of fibers, fiber dimensions and interfacial adhesion between fibers and polymer matrix or matrices. Hybridization can be performed by combining synthetic and synthetic fibers, synthetic and natural fibers, natural and natural fibers and incorporation of nanofillers such as nanoclays, carbon nanotubes, graphite sheets and metal oxide nanoparticles in reinforced polymer composites.

Processing of Composites:
There are many processing techniques for polymer composites. These methods include solvent casting, melt compounding, compression moulding, injection moulding, extrusion process, etc. The selection of a particular processing method is dependent on the desired application, type of polymer and reinforcement to be used.

1. Solvent casting method:
This method is widely used for the preparation of biocomposites, and it requires small amounts of both polymer matrix and reinforcements. In this method, a polymer is dissolved in a suitable solvent system. After dissolution, the reinforcement is added to prepare a homogeneous mixture. When homogeneity is attained, the solvent is removed either by vaporization or precipitation to form a film. This method achieves uniform distribution and good dispersion of reinforcement in a polymer matrix.

2. Melt compounding method:
So far, this is the most prevalent method for industrial processing of biocomposites. This process involves direct mixing of the polymer matrix and reinforcement. In this method, both the dry polymer and reinforcement are mixed, compressed and heated and passed through a high-pressure system. It uses a top-down approach to breakdown the aggregated reinforcement during mixing. This process is more applicable in the development of biocomposites reinforced with nano and submicron-sized reinforcements. The dispersion of the reinforcement is attained by employing high shear stresses combined with diffusion at high temperatures.

3. Compression moulding:
This technique involves the moulding of the semi-finished material by applying heat under compressive pressure until the final product is produced. It is normally carried out at different pressures and temperatures depending on the polymer used. This technique offers good advantages, such as moulding of a large amount of semi-finished material, high reproducibility, low cycle time and production cost.

4. Injection moulding:
This technique produces a material by injecting a molten polymer into a mould to form a desired material with required structural design. The sample (resin granules and short fibers) is placed in a heated barrel, and when the granules are melted, the screw injects the melt and transports it into a tool cavity chamber, where it cools and solidifies to form a desired material.

This technique renders numerous advantages, such as manufacturing a complex geometry of the structure, low labour cost, possibility to use recycled material and industrial scale.

5. Extrusion process:
Extrusion process is widely used in the plastic industries, and it is considered as a high-temperature and short-time process to produce granules, semi-finished and finished material. It consists of a single and/or twin screw extruders. Single screw extruders are used to melt, mix and homogenize the material.

It is normally preferred for blending pure polymers at moderate operational speed in a short period of time and when pre-pelletizing is not necessary. The disadvantages of single screw extruders are poor performance under high pressure and lower production compared to twin screw extruders. Twin screw extruders, on the other hand, are capable of producing pellets by mixing two or more materials that are heat and shear sensitive.

6. Liquid composites moulding:
Liquid composite moulding such as resin transfer moulding and vacuum-assisted resin infusion moulding have been used to fabricate composite laminates over several decades and it is still being used. These techniques offer combined benefits of high quality, reproducibility and repeatability ease and clean handling, scalability, flexibility as well as reducing volatile organic compounds. These techniques utilize the pressure difference between vacuum and environmental pressures to compress and secure preform against the mould and drawsresin into the preform. Composites produced by these techniques have applications in marine, aerospace, defense and automotive.

Application of Textile Composites:
Composite materials have long been utilized in various fields including automotive, aerospace, civil infrastructure, durable goods, and defense owing to their improved properties. Listed below are some of the applications of fiber-reinforced composites.

Aerospace:
Composites were first used in military aircraft during World War II, and since then, several types of composites have been developed and have been used in the aerospace industry. Owing to their various improved properties such as light weight, high strength, ability to withstand high temperatures, and thermal expansion, fiber-reinforced polymer composite materials have become popular materials for the construction of aircraft and spacecraft.

Civil construction:
Fiber-reinforced polymer composites are marketed as the materials of the 21st century because of their superior properties such as high thermomechanical properties, excellent corrosion resistance, and high strength-to-weight ratio. In the last few decades, applications of advanced fiber-reinforced composites have been studied for soundproof and insulated buildings, highways and bridges, decking for marine and naval structures, utility poles, renewable energy harvesting, pipelines, natural composites for green buildings, and others.

Sports:
The improved properties of composite materials render their application not only in automotive, aerospace, and other industrial fields but also in sports equipment. In order to obtain sports items with improved properties, glass fibers and carbon fibers are mainly being reinforced with polymeric matrix composites.

Biomedical:
In the last few years, the medical sector has gained interest in fiberreinforced polymer composites for various applications such as components of magnetic resonance imaging scanners and C-armed X-ray couches, scanners, surgical target tools, and devices. They can also be used in prosthetics such as springs and orthotics like anterior foot, podiatriccorrecting insoles, and braces. The radiolucent property of carbon-fiberreinforced polyetheretherketone implants permits improved, artifact-free radiographic imaging compared to traditional metal implants. Fiberreinforced composite resin (FRC) prostheses also offer advantages such as good minimal invasive treatment, aesthetics, and an ability to bond to the abutment teeth.

Defense:
Modern warfare across air, ground, and sea demands weight-saving, protection, and high-strength materials. Fiber-reinforced polymer composites are the best-suited materials for these purposes owing to their high strength-to-weight ratio, stiffness, toughness, and design flexibility as compared to other engineering materials. Fiber-reinforced polymer composites in defense sectors are used in military aircraft, land systems, UAVs, naval vessels, and weapons. The most common reinforcement material in marine applications is the E-glass fiber.

References:

Fibers to Smart Textiles: Advances in Manufacturing, Technologies, and Applications Edited by Asis Patnaik and Sweta Patnaik
Natural Fiber Textile Composite Engineering By Magdi El Messiry
Advanced Textile Engineering Materials Edited by Shahid-ul-Islam and B.S. Butola
Design and Manufacture of Textile Composites Edited by A. C. Long

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