Source and Chemical Composition
Origin from Latex: Natural rubber, which is also called getah asli in Malay, is obtained from a white, milky liquid called latex. This latex comes from rubber trees, mainly a species called Hevea brasiliensis. The trees are cut or tapped to let the latex flow out, and then it is collected.
Main Monomer – Isoprene: The basic unit (or monomer) that makes up natural rubber is a molecule called isoprene. Its scientific name is 2-methylbuta-1,3-diene, and its chemical formula is C₅H₈. Isoprene is a small molecule that can join with many others to form a long chain.
Double Bonds in Isoprene: Each isoprene molecule contains two double bonds between carbon atoms (C=C). These double bonds are very important because they help the isoprene molecules join together during the polymerisation process.
Polymer Type: When many isoprene molecules join together, they form a large molecule called a polymer. The polymer made from isoprene is called cis-1,4-polyisoprene. This is made through a reaction called addition polymerisation, where the double bonds break and the monomers link up.
Repeating Units: Natural rubber is made of repeating isoprene units linked together in a long chain. These repeating units give rubber its unique properties like elasticity and flexibility.
Properties of Natural Rubber
Elasticity and Flexibility: Natural rubber can stretch a lot and then return to its original shape. This is because the forces between its long chains are weak and allow the chains to slide and move when pulled, then return when released.
Softness in Natural State: In its raw form, without any chemical treatment, natural rubber is soft and not very strong. It can be easily bent or shaped, but it wears out quickly because there are no extra links to make it strong.
Oxidation Vulnerability: When natural rubber is left out in the air, oxygen and ozone in the environment can react with it. This causes the rubber to become hard, brittle, and crack over time.
Low Heat Resistance: Natural rubber is not good at handling heat. When it gets hot, it becomes soft and sticky. This happens because the heat makes the rubber chains move more freely and lose their shape.
Latex and Its Properties
Colloidal Nature: Latex is a mixture called a colloid, which means it has tiny rubber particles spread evenly in water. These particles are so small that they don’t settle down and stay mixed.
Stabilisation by Proteins: The rubber particles in latex are covered with protein layers. These proteins carry negative electrical charges that help keep the particles from sticking together.
Importance of Negative Charge: Because the rubber particles have negative charges, they repel (push away) each other. This stops them from clumping and keeps the latex in a stable, smooth liquid form.
Coagulation of Latex
Charge Neutralisation Triggers Coagulation: If the negative charges on the rubber particles are removed or neutralised, the particles can come together. This process is called coagulation. It often happens when acids are added to the latex.
Formation of Coagulum: Once the particles lose their negative charge, they stick together and form a solid mass called a coagulum. This is how solid rubber is separated from the liquid latex.
Common Coagulants: To make rubber solidify from latex, acids such as ethanoic acid (also called acetic acid) or methanoic acid are added. These acids remove the negative charges, allowing the particles to clump together.
Preventing Coagulation
Use of Alkaline Stabilisers: To keep latex from turning solid too early, alkaline chemicals like ammonia or hydroxides are added. These are basic substances that help keep the rubber particles separated.
Maintaining Stability: These alkaline substances release hydroxide ions, which increase the negative charges on rubber particles. The stronger the repulsion, the more stable the latex remains without forming clumps.
Vulcanisation
Definition and Inventor: Vulcanisation is a process where rubber is heated with sulfur to make it stronger and more useful. This important invention was discovered by Charles Goodyear in the 1800s.
Sulfur Cross-Links: During vulcanisation, sulfur atoms form bridges (called cross-links) between the long chains of rubber molecules. These links are strong chemical bonds that connect one chain to another.
Strength and Hardness Improvement: These sulfur cross-links prevent the chains from sliding past each other too easily, which makes the rubber stronger, tougher, and better at holding its shape.
Enhanced Elasticity: Even though the chains are linked, the rubber can still stretch. After stretching, the cross-links help the chains go back to their original shape, giving rubber excellent elasticity.
Better Heat Resistance: With cross-links in place, the rubber doesn’t become soft or sticky when heated. This makes vulcanised rubber more useful in hot environments.
Oxidation Resistance: Cross-links in vulcanised rubber also help it resist damage from oxygen and ozone. This means it lasts longer and doesn’t break down as quickly as natural rubber.
Prevents Stickiness: Vulcanised rubber doesn’t get sticky when the temperature changes. The sulfur cross-links keep the chains in place, stopping them from moving too much.
Chemical Mechanism: In vulcanisation, sulfur reacts with the double bonds in isoprene units to create the cross-links. This chemical reaction makes the rubber stronger and more stable.
Uses of Natural Rubber
Tyres Manufacturing: Because vulcanised rubber is strong, flexible, and heat-resistant, it is perfect for making vehicle tyres. It can handle pressure, heat, and rough surfaces well.
Gloves Production: Latex from natural rubber is used to make gloves that fit well and stretch easily. These gloves are used in hospitals and factories to protect hands.
Hoses and Seals: Rubber is used in hoses and seals because it can bend and return to shape easily. It also seals gaps tightly, which is useful in engines and machines.
Other Applications: Natural rubber is used in many items like elastic bands, mats, rubber boots, and toys. These products need to be bendy, strong, and long-lasting.