Effect of Heat on Fibres MCQs Quiz | Class 9

Understanding the Effect of Heat on Fibres

Understanding how different fibres react to heat is a fundamental aspect of fabric care and home science. Proper knowledge of heat sensitivity helps in maintaining the life, appearance, and texture of our clothes. Applying the wrong temperature during ironing can cause irreversible damage like scorching, melting, or shrinking.

Key Concepts: Heat Sensitivity and Fibre Types

Fibres are broadly categorized based on their reaction to heat. This is crucial for determining the correct ironing temperature and washing method.

1. Natural Fibres

• Cotton and Linen (Plant-based): These fibres have high heat resistance. They can withstand high ironing temperatures, especially when damp. However, excessive, prolonged heat will cause them to scorch (turn yellow or brown) and eventually burn, leaving a grey ash. They smell like burning paper.
• Wool and Silk (Animal-based): These are protein fibres and are more sensitive to heat than cotton. High heat can make them brittle and yellow. They burn slowly and self-extinguish when removed from a flame. Wool smells like burning hair, while silk has a similar but less distinct odor. They should be ironed at a medium to low temperature, often with steam or a pressing cloth.

2. Man-made Fibres

• Rayon (Regenerated Cellulosic): Rayon is more sensitive to heat than cotton. It should be ironed at a medium temperature, preferably while damp, to avoid damage. It burns similarly to cotton.
• Nylon and Polyester (Synthetic/Thermoplastic): These fibres are thermoplastic, meaning they soften and melt when heated. They are very sensitive to high heat. If an iron is too hot, these fabrics will melt, shrink, and form a hard, black bead. They must be ironed at a very low temperature.

Ironing and Fabric Care Guide

Most modern irons use a dot system to indicate the appropriate temperature for different fabrics. Using this guide helps prevent damage.

Quick Revision Points

• Thermoplastic fibres like nylon and polyester melt when heated.
• Natural fibres like cotton and wool scorch or burn but do not melt.
• Always check the garment’s care label before ironing.
• Ironing fabrics when they are slightly damp can produce better results, especially for cotton and linen.
• Use a pressing cloth for delicate fabrics like silk and wool to provide an extra layer of protection from direct heat.
• The smell of a burning fibre can help identify it: paper for cotton, hair for wool.

Extra Practice Questions

1. Why is it recommended to iron cotton when it is damp?
   Answer: Ironing damp cotton helps the heat to penetrate the fibres more effectively, easily removing wrinkles and resulting in a crisp finish.
2. What is the main difference between how natural and synthetic fibres react to heat?
   Answer: Natural fibres (cotton, wool) tend to scorch and burn at high temperatures, while synthetic thermoplastic fibres (polyester, nylon) melt and shrink.
3. Which fibre is known to be very strong but is sensitive to high heat?
   Answer: Nylon is a very strong synthetic fibre, but it has a low melting point and is sensitive to high heat during ironing.
4. What does the term ‘thermoplastic’ mean in the context of fibres?
   Answer: A thermoplastic fibre is one that becomes soft and moldable when heated and hardens upon cooling. This property allows it to be heat-set into permanent pleats but also makes it susceptible to melting.
5. A garment label says “Cool iron”. What type of fibre is it likely made from?
   Answer: It is likely made from a synthetic fibre like polyester, nylon, or a delicate fibre like silk, which are all sensitive to high heat.