1) Definition of the term composites?

Composites are combination of two or more materials providing synergy that give properties superior to the individual components materials themselves.

For Example: glass fibre alone cannot be shaped into useful structural products except few applications like sound and thermal insulation and in similar manner unsaturated polyester resin (UPR) alone cannot be used to make many useful products except few applications like buttons, putty resin, however when the unsaturated polyester resin and glass fibres are combined, we get composite materials with superior mechanical properties that can be used for wide variety of applications like, boats, wind mill components, doors, electrical insulation components, etc.

2) Why composites are being used in various high-performance applications?

Composites offer very high strength, high stiffness yet with light weight (high specific strength and stifness), and also excellent corrosion resistance, part consolidation (many smaller components that can be combined into a large single component where ever possible and applicable) and properties that can be tailored to meet the requirements including orientation of reinforcement fibres in the required directions.

3)  What are the key characteristics of reinforcement fibres?

Reinforcement fibres are the load carrying members in the composites and they are one dimensional materials and have high stiffness, strength, thermal stability and ability to withstand process conditions and low specific gravity, etc. The reinforcement fibres are available in various physical forms like continuous fibres, woven mat, and non-woven mat, chopped fibres, milled fibres and etc to suit the manufacturing conditions and end user requirements.

4) What are the common reinforcement fibres?

Glass fibre, carbon fibre, aramid fibre, basalt fibre, natural fibres like flax, etc. There are also ceramic fibres such as silicon carbide, alumina, and these high temperature resistance fibres are used in ceramic matrix composites.

5) What is glass fibre?

Glass is combination of oxides of metals and silicates that are fused by melting at a high temperature in a furnace.  It is an amorphous material and the glass is drawn from the molten glass in the form of a fibre/ filament and it is used as a reinforcement materials. The most common form glass fibre used in the industry is E-glass and E stands for electrical and it is basically a calcium alumino boro silicate glass.

6) Why carbon fibre is used for high performance applications?

Currently among the commercially available continuous reinforcement fibres, carbon fibre is the preferred choice for very high-performance applications because of its very high specific strength and stiffness coupled with high fatigue resistance, and chemical resistance.

7) Why carbon fibre usage is less compared to glass fibre?

High cost and its highly complex manufacturing process and is high energy intensive process and it involves much more precise process-controlled technology.

8) How the carbon fibre and glass fibre production technology differ?

 In carbon fibre, the production process involves conversion of an organic fibre into a ceramic fibre materials compared to glass fibre where there is no compositional changes in glass fibre technology during manufacturing process, only a physical change as fibres are drawn from the melt. The composition of glass (for example E Glass), is controlled through the batch mix and the mix is melted and drawn into fibre. Although both fibre manufacturing technologies involves a highly controlled and adoption of precise process parameter conditions, in carbon fibre in addition, it has to be processed in various controlled environmental conditions including conversion process which is done in inert gas atmosphere. In carbon fibre manufacturing, an organic polymeric fibre example polyacrylonitrile -PAN is converted into an inorganic carbon fibre.

9) What is aramid fibre?

The term ARAMID refers to Aromatic Amide, it is basically polyparaphenylene terephtalamide and it has an excellent impact resistance property and it is a preferred materials for bullet proof applications. It has fibrillated structure, internally in each fibre structure.

10) What are the materials used for bullet proof applications other than ARAMID?

The Ultra High Molecular Weight High Density Polyethylene (UHMHDPE) is one of the other fibres widely used for bullet proof applications and it has the molecular weight in the million range which give these materials excellent toughness. It has the specific gravity less than 1 (Its specific gravity is 0.97 g/cc). It is specific gravity is lower than Aramid which is 1.45 g/cc

11) What is basalt fibre?

Basalt fibre is a fibre manufactured from naturally occurring volcanic lava rocks. The rock is melted and drawn into fibre similar to glass fibre forming technology.

12) How basalt fibre and glass fibre are compared in term of performance?

Basalt fibre has better high temperature resistance compared to glass fibre and has less electromagnetic interference.

Basalt fibre cost is higher and also as it is derived from naturally occurring silicate minerals and there may be some minor variations in compositions of basalt fibre compared to glass fibre where its composition is relatively controlled through batch composition/ constituents.

As a rule of thumb, the basalt fibre performance is between glass fibre and carbon fibre.

13) Why ceramic fibres such as silicon carbide, alumina, have limited applications in Fibre Reinforced Polymers (FRP)?

Ceramic fibre such as silicon carbide, find limited application in FRP because of high cost and are mainly used in ceramic matrix composites. These fibres have very high temperature stability compared other fibres and also ceramic matrices are processed at very high temperatures so they are best suited for high temperature applications, whereas the other conventional fibre like glass or natural fibre cannot be subjected to such high process temperatures. The fibres like glass or natural fibres when subjected higher temperatures these materials will lose their characteristics, due to thermal degradation.

14) What are the driving factors for natural fibre composites?

Natural fibres are relatively environment friendly, biodegradable in nature and their specific gravity is relatively less compared to other fibres for example like glass fibre. The natural fibres are of low cost. Natural fibres have good vibration damping properties.

15) What are the natural fibres that are being used for composites applications?

Flax fibre is the one of the natural fibres that is widely being tried for various composite product applications. However, there are lot of product development and applications are being done with other natural fibres such as bamboo, hemp, sisal, jute, coir etc as well.

16) What are the limiting factors for natural fibre composites?

Natural fibres tend to absorb moisture and degrade over a period of time due to their inherent porosity and moisture absorption characteristics especially through cut edges and holes drilled in the composites.

For further reading references:

Handbook of Composites by George Lubin

An Introduction to Composite Materials by D.Hull

Composite Materials: Science and Engineering by Krishnan K Chawla

Published in the January 2025 issue of Advanced Materials Magazine