4.2Natural Rubber
Natural rubber is a naturally occurring polymer obtained from the latex of rubber trees (Hevea brasiliensis). It plays an essential role in various industrial and commercial applications, such as gloves, tyres, and earthquake isolators.
Learning Objectives
- Explain natural rubber in terms of naming, structural formula, and its properties.
- Experiment on latex coagulation and explain the mechanism behind latex coagulation and coagulation prevention.
- Explain the vulcanisation process using sulphur and alternative vulcanisation methods.
- Compare and contrast the properties and elasticity of vulcanised rubber and unvulcanised rubber.
Mechanism of Latex Coagulation and Rubber Vulcanisation
How charge neutralisation leads to coagulation and how sulphur cross-links prevent polymer chains from sliding.
Latex Coagulation
Negatively charged protein membranes repel each other. Acid supplies H+ ions that neutralise the charges. Particles collide, membranes break, and rubber polymers entangle into a solid mass.
Rubber Vulcanisation
Sulphur reacts with C=C double bonds in polyisoprene chains, forming sulphur cross-links between adjacent polymer chains. These cross-links act as molecular springs, returning chains to their original positions after stretching.
Brief Explanation
Understand the structure, coagulation and vulcanisation of natural rubber.
Structure of Natural Rubber
Structural formula of isoprene: \(\text{CH}_2=\text{C}(\text{CH}_3)-\text{CH}=\text{CH}_2\).
Repeating unit of polyisoprene: \(\text{[-CH}_2-\text{CH}=\text{C}(\text{CH}_3)-\text{CH}_2\text{-]}_n\).
The rubber particle consists of rubber polymers enclosed by a protein membrane that is negatively charged on the outer surface.
Characteristics of Natural Rubber
- Soft white solid at room temperature.
- Elastic — can be stretched and returns to its original shape.
- Low heat resistance — becomes soft and sticky at high temperatures.
- Electrical insulator.
- Easily oxidised at C=C double bonds.
- Reactive to acids, alkalis and organic solvents.
- Waterproof.
| Process | Key agent | Mechanism |
|---|---|---|
| Coagulation by acid | H+ ions from acid | Neutralise negative charges on protein membranes → particles collide → membranes break → polymers entangle. |
| Natural coagulation | Lactic acid from bacteria | Bacteria in air secrete lactic acid; H+ ions neutralise charges slowly. |
| Prevention of coagulation | OH− ions from ammonia solution | Neutralise H+ ions produced by bacteria, preserving negative charges and keeping latex liquid. |
| Vulcanisation | Sulphur cross-links | Sulphur reacts with C=C double bonds, forming cross-links between polymer chains. |
Try These Questions First
Answer mentally, then check the explanation.
What is the IUPAC name and homologous series for the monomer of natural rubber?
Why does latex coagulate after being exposed to air for several hours without adding acid?
Why is vulcanised rubber more resistant to oxidation by air compared to unvulcanised rubber?
Common Mistakes
Avoid these errors when explaining natural rubber chemistry.
Common Misconceptions
Compare each misconception with the correct fact.
Latex is a pure polymer solution.
Latex is a colloid, not a solution; rubber particles are dispersed in water and do not dissolve.
Vulcanised rubber has no double bonds at all.
Vulcanisation reduces the number of double bonds where cross-linking occurs, but it does not necessarily eliminate every single double bond.
Vulcanisation can only ever be carried out using sulphur.
Sulphur is the primary method, but alternative vulcanisation methods exist using peroxides, metal oxides, or irradiation.
Summary
Key facts to remember.
- Polymer & Monomer: Natural rubber is polyisoprene; its monomer is isoprene (2-methylbut-1,3-diene).
- Latex Stability: Negatively charged protein membranes keep rubber particles apart by electrostatic repulsion.
- Coagulation: Acids supply H+ to neutralise negative charges → membranes rupture upon collision → polymers entangle to form solid rubber.
- Coagulation Prevention: Alkalis (e.g., NH3) provide OH− to neutralise acids produced by bacteria.
- Vulcanisation: Cross-linking polymer chains using sulphur (or peroxide/irradiation/metal oxide) across C=C bonds.
- Enhanced Properties: Vulcanised rubber is harder, stronger, more elastic, more heat-resistant, and more oxidation-resistant than unvulcanised rubber.
Quick Activity
Complete all parts before checking your score.
A. Objective Quiz
What causes latex to remain in a dispersed liquid state under normal fresh conditions?
Which of the following is an alternative vulcanisation method that does NOT require sulphur?
B. Fill in the Blanks
The IUPAC name for the monomer of natural rubber is ______.
In vulcanised rubber, adjacent polymer chains are held together by ______ cross-links.
C. Match the Substance or Process with Its Role
Drag each choice on desktop. On mobile, tap a choice and then tap the matching box.
Answer Choices
Ethanoic acid Ammonia solution Vulcanisation Unvulcanised rubberRole / Characteristic
Keywords
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