Natural rubber is a versatile material derived from plant sources, distinguishing it from synthetic rubbers produced from petrochemicals. It is widely used across various industries, appearing in products ranging from vehicle tires to medical devices. It begins its journey as a milky fluid harvested from specific trees, undergoing a series of transformations to become the durable and flexible solid material.
The Living Source
The primary biological source of natural rubber is the Hevea brasiliensis tree, commonly known as the Pará rubber tree. This tree species is native to the Amazon basin, specifically regions spanning Brazil, Bolivia, Peru, and other parts of South America. Indigenous communities in the Amazon first utilized the milky sap from these trees for various purposes, including making balls and waterproofing materials. In the late 19th century, Hevea brasiliensis seeds were successfully cultivated outside their native range, leading to extensive commercial plantations predominantly in Southeast Asia, including countries like Thailand, Indonesia, Malaysia, and India.
The Raw Liquid
The raw material harvested from the rubber tree is a milky, sticky fluid called latex, which is distinct from plant sap. Within the rubber tree, latex serves a protective function, acting as a defense mechanism against herbivores and pathogens. When the tree’s bark is wounded, the latex flows out, sealing the injury and deterring pests with its stickiness and other compounds. To collect this liquid, a traditional method known as tapping is employed. This involves making a thin, diagonal incision into the tree’s bark, allowing the latex to drip into a collecting cup. Tapping is performed every other day once a tree reaches approximately six years of age.
From Sap to Solid
The conversion of liquid latex into solid natural rubber involves a series of processing steps. After collection, the latex is transferred to large tanks. The most common method to transform the liquid into a solid mass is through coagulation, which causes the rubber particles suspended in the latex to curdle. This is achieved by adding a dilute acid, such as formic acid, to the latex. The acid lowers the pH, leading to the formation of a soft, curd-like material called coagulum.
Once coagulated, the rubber undergoes washing to remove excess water, impurities, and non-rubber components. The washed coagulum is then passed through a series of rollers, which press out more water and shape the rubber into thin sheets. These sheets are then subjected to various drying methods to remove remaining moisture. Common drying techniques include air drying, where sheets are hung in well-ventilated areas, or smoke drying, which exposes the sheets to smoke from burning wood. Other methods like oven drying are also used.
Key Characteristics
Natural rubber is primarily composed of polyisoprene, specifically cis-1,4-polyisoprene, which is a large molecule made of repeating isoprene units. While polyisoprene forms the bulk of natural rubber, it also contains small amounts of other naturally occurring substances such as proteins, fatty acids, resins, and inorganic salts.
As an elastomer, natural rubber exhibits key physical characteristics. It possesses high elasticity, allowing it to stretch significantly and return to its original shape. It also has high tensile strength, which refers to its resistance to breaking under tension, and good resilience, indicating its ability to absorb and release energy. Natural rubber offers good resistance to water and performs well at low temperatures.