UPSC Prelims 2023 Q53: Carbon Fibres, Uses and Recycling — Explained

Consider the following statements:

1. Carbon fibres are used in the manufacture of components used in automobiles and aircrafts.

2. Carbon fibres once used cannot be recycled.

Which of the statements given above is/are correct?

  1. 1 only
  2. 2 only
  3. Both 1 and 2
  4. Neither 1 nor 2

Answer: (a) 1 only

Statement-wise explanation

  • 1: correct — Carbon fibre is usually used as carbon-fibre-reinforced plastic (CFRP), where the fibres are set in a resin. It is very light and very stiff, it is strong, and it does not corrode. Weight saving matters most in aircraft and cars, so CFRP is used for wing and fuselage parts, drone frames, chassis and structural panels, seat backs and oil pans. Boeing's 787 is about 50% composites by weight, most of it carbon-fibre composite.
  • 2: incorrect — Carbon fibre can be recovered from used or scrap composites. In pyrolysis, the composite is heated to about 400–700 °C without oxygen, which burns off the resin and leaves the fibres. Pyrolysed fibres typically keep at least 90% of their tensile strength. Solvolysis (dissolving the resin) can give fibres close to virgin quality. Recycled carbon fibre is already sold commercially, for example as nonwoven mats, so 'cannot be recycled' is false.

How to eliminate

Statement 1 is close to common knowledge: carbon fibre is known as the light, strong material of race cars, aircraft and sports goods. That rules out the two options that leave Statement 1 out. Statement 2 uses an absolute claim: once used, carbon fibre 'cannot be recycled'. In science and technology questions, a flat 'cannot' about a material is usually false, because recycling routes almost always exist even when they are costly. Choose the option with Statement 1 alone. If you are not sure about the absolute-word rule, remember this fact: carbon fibre is recycled mainly by pyrolysis, which burns off the resin and keeps the fibre. The real problem is that recycling is expensive and the recovered fibres come out shorter, not that recycling is impossible.

Know more

What carbon fibre is. Carbon fibres are very thin filaments made mostly of carbon. They are made by heating a precursor fibre to high temperature without oxygen. The precursor is usually polyacrylonitrile (PAN), and sometimes pitch or rayon. The fibres are rarely used alone. They are woven or laid into a polymer resin such as epoxy to make carbon-fibre-reinforced plastic (CFRP), a composite material.

Why it is used. It is light and very stiff, strong under tension, and does not corrode. It also keeps its shape well.

Sector Typical uses
Aerospace & defence Wings, fuselage panels, UAV frames, rocket motor casings
Automobiles Chassis and structural panels, body parts of high-end and electric vehicles
Clean energy Type-IV CNG and 700-bar hydrogen tanks (carbon-fibre-wrapped), wind-turbine blades
Others Sports goods, prosthetics, bridge and building retrofits

How it is recycled.

Route How it works Result
Mechanical Grinding or chopping the composite Short fibres plus filler; lowest value
Pyrolysis (thermal) Heating to about 400–700 °C without oxygen to burn off the resin Clean fibres keeping at least 90% of tensile strength; the main commercial route
Solvolysis (chemical) Dissolving the resin with solvents Fibres close to virgin quality

Recycling is hard because the resin, usually a thermoset, cannot be melted and reshaped. It is not impossible. The drawbacks are cost, shorter fibres and loss of fibre alignment.

India angle. Carbon fibre is a priority 'speciality fibre' under the National Technical Textiles Mission. Indian carbon fibre goes into defence production and into indigenous composite gas cylinders.

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