1. Introduction

Natural rubber is a biopolymer obtained primarily from the latex of the rubber tree (Hevea brasiliensis). It is an elastomer, capable of large reversible deformations. Its main constituent is poly(cis-1,4-isoprene).

Chemical formula: (C₅H₈)_n

2. Source

Natural rubber is obtained as latex, a milky colloidal suspension, from Hevea brasiliensis by tapping.

• Contains 30–40% rubber particles (polyisoprene)
• Water (60–70%) acts as the medium
• Proteins, resins, and other impurities present

3. Composition of Latex

4. Isolation of Natural Rubber

Steps to obtain dry rubber:

1. Collection: Latex tapped from trees.
2. Coagulation: Addition of acids (formic or acetic acid) coagulates polyisoprene.
   
   Latex (C₅H₈)_n + HCOOH → Coagulated rubber + H₂O
3. Rolling & Washing: Removes water and impurities.
4. Drying: Air or smoke-dried sheets ? commercial rubber.

5. Structure

- Polymer unit: cis-1,4-isoprene
- Structural formula:

6. Properties

• Elasticity: stretches and returns to original shape
• Resilience: high energy recovery
• Toughness: strong when vulcanized
• Solubility: insoluble in water; soluble in benzene, toluene
• Thermal stability: softens above 60–70?°C

7. Vulcanisation

Chemical treatment with sulfur to improve elasticity, strength, and durability. Forms crosslinks (-S_x-) between polyisoprene chains.

• Increases tensile strength
• Improves elastic recovery
• Reduces softening and stickiness

8. Types of Natural Rubber

• RSS (Ribbed Smoked Sheets) — standard commercial sheet
• CV (Centrifuged Latex Rubber) — high purity latex
• Crepe Rubber — rolled and washed sheet for industrial use

9. Uses of Natural Rubber

• Tyres and tubes
• Conveyor belts
• Footwear and gloves
• Adhesives and coatings
• Sports goods, medical devices (catheters, gloves)