Study Notes: Synthetic Fibres and Plastics

Overview

Synthetic fibres and plastics form a critical topic in the General Science (Chemistry) section of Railway Group D examinations. This topic tests your understanding of man-made materials that have revolutionized modern life. Questions typically appear as 2–3 marks asking about types of synthetic fibres, characteristics of plastics, their uses, or environmental impact.

Mastery of this topic requires clear knowledge of how polymers work, the distinction between natural and synthetic materials, and practical applications of different plastics and fibres. The exam focuses on factual recall rather than complex chemistry, so memorizing classification, properties, and uses is essential. Understanding the environmental aspects (biodegradability, recycling) has become increasingly important in recent exam patterns.

Students must be able to identify synthetic fibres by their trade names, distinguish between thermoplastics and thermosetting plastics, and recall real-world applications. This topic connects well with environmental awareness questions that appear in General Awareness sections.

Key Concepts

• **Polymers** are large molecules formed by joining many small repeating units called monomers through a chemical process called polymerization. Natural polymers include cellulose, starch and proteins, while synthetic polymers include plastics, synthetic fibres and synthetic rubber.

• **Synthetic fibres** are man-made fibres produced from chemical substances (petroleum products) rather than natural sources. They generally have higher strength, elasticity, and durability compared to natural fibres, and dry quickly with minimal wrinkling.

• **Plastics** are polymers that can be moulded into various shapes when heated. They are poor conductors of heat and electricity, lightweight, resistant to corrosion, and generally cheaper than metals. However, they are non-biodegradable, causing serious environmental concerns.

• The two major categories of plastics are **thermoplastics** (soften on heating and can be remoulded) and **thermosetting plastics** (once moulded with heat, they cannot be softened again). This fundamental difference determines their applications.

• **Rayon** was the first synthetic fibre developed, obtained from natural cellulose but processed chemically. It is sometimes called "artificial silk" because it resembles silk in appearance and feel.

• Synthetic materials have disadvantages: they can melt or catch fire easily (lower melting point than natural fibres), do not absorb sweat well, and persist in the environment for hundreds of years causing pollution. Burning synthetic fibres releases toxic gases.

Formulas / Key Facts

**Major Synthetic Fibres:**

• Nylon — First fully synthetic fibre (1931), made from coal, water and air; used in ropes, parachutes, fishing nets, socks, toothbrush bristles
• Polyester (Terylene) — Made from petroleum products; used in clothing, bottles, sails, fabric blends with cotton
• Acrylic — Synthetic wool substitute; used in sweaters, blankets, shawls
• Rayon — Semi-synthetic fibre from wood pulp; used in textile industry, tire cords

**Types of Plastics:**

• Thermoplastics — Polyethylene (polythene), PVC, polystyrene, nylon
• Thermosetting plastics — Bakelite, melamine, formica

**Properties Comparison:**

• Natural fibres are biodegradable; synthetic fibres are non-biodegradable
• Synthetic fibres are stronger and more durable than most natural fibres
• Plastics generally have density less than 2 g/cm³, making them lightweight

**Environmental Facts:**

• Plastic bags take 500–1000 years to decompose in soil
• 4R principle: Reduce, Reuse, Recycle, Recover
• PET (Polyethylene terephthalate) code on bottles indicates recyclable plastic

Worked Examples

**Example 1: Identifying Plastic Type** Question: Electrical switches, handles of cooking utensils, and phone cases are made of Bakelite. Which type of plastic is Bakelite and why is it suitable for these applications?

Solution: Step 1: Bakelite is a thermosetting plastic. Step 2: Thermosetting plastics cannot be softened by heating once moulded. Step 3: Bakelite is a poor conductor of heat and electricity. Step 4: These properties make it ideal for electrical switches (prevents electric shocks) and utensil handles (heat resistance prevents burns). Answer: Bakelite is a thermosetting plastic suitable because it is heat-resistant and a poor conductor of electricity.

**Example 2: Comparing Fibres** Question: Why are synthetic fibres preferred over natural cotton for making sports jerseys and outdoor clothing?

Solution: Step 1: Synthetic fibres (polyester, nylon) dry quickly after washing. Step 2: They are wrinkle-resistant and require less ironing. Step 3: They are more durable and last longer. Step 4: However, they absorb less sweat compared to cotton. Answer: Synthetic fibres are preferred for sports wear because they are quick-drying, durable, and wrinkle-free, though cotton blends are often used to improve sweat absorption.

**Example 3: Environmental Impact** Question: A shopkeeper offers you the choice between a plastic bag and a jute bag. Which should you choose and why?

Solution: Step 1: Plastic bags are non-biodegradable and persist in the environment for centuries. Step 2: They clog drains, harm marine life, and release toxins when burned. Step 3: Jute bags are biodegradable, reusable, and environment-friendly. Answer: Jute bag should be chosen as it is biodegradable and can be reused multiple times, reducing environmental pollution.

Common Mistakes

**Mistake 1: Confusing rayon with fully synthetic fibres** Wrong thinking: Rayon is a completely synthetic fibre like nylon. Correct fix: Rayon is semi-synthetic or regenerated fibre made from natural cellulose (wood pulp) that undergoes chemical processing. It's not made from petroleum products like true synthetic fibres.

**Mistake 2: Thinking all plastics can be remoulded** Wrong thinking: All plastics become soft when heated and can be reshaped. Correct fix: Only thermoplastics soften on heating and can be remoulded. Thermosetting plastics undergo permanent chemical change during moulding and cannot be softened again by heating.

**Mistake 3: Assuming synthetic means better in all respects** Wrong thinking: Synthetic fibres are superior to natural fibres in every way. Correct fix: While synthetic fibres are stronger and more durable, natural fibres like cotton are better at absorbing sweat and are more comfortable in hot weather. Synthetic fibres can cause skin irritation and don't allow skin to breathe.

**Mistake 4: Mixing up Bakelite and plastic bottles** Wrong thinking: Bakelite is used to make water bottles because it's lightweight. Correct fix: PET (a thermoplastic) is used for bottles because it can be moulded easily. Bakelite (thermosetting) cannot be remoulded and is used for fixed-shape items like switches and handles.

**Mistake 5: Believing biodegradable plastics are widely available** Wrong thinking: Most plastics sold today are biodegradable and eco-friendly. Correct fix: The vast majority of commercial plastics are non-biodegradable. True biodegradable plastics are still limited in use and more expensive. Regular plastic waste remains a major environmental problem.

Quick Reference

• Nylon = First fully synthetic fibre; used in ropes, parachutes, fishing nets • Polyester (Terylene) = Common clothing fabric; often blended with cotton • Rayon = Semi-synthetic from wood pulp; called artificial silk • Thermoplastics = Can be remoulded (polythene, PVC, polystyrene) • Thermosetting plastics = Cannot be remoulded once set (Bakelite, melamine) • 4R Principle = Reduce, Reuse, Recycle, Recover plastic waste • PET plastic = Used for bottles; has recycling code and is recyclable • Main disadvantage of synthetic fibres = Non-biodegradable; cause environmental pollution