Natural Rubber and Synthetic Rubber - Practice Questions & MCQ

It is a polymer which is capable of returning to its original length, shape or size after being stretched or deformed. Rubber is a common example of an elastomer. The rubber obtained from natural sources is called natural rubber and polymers prepared in the laboratory which are similar to natural rubber are known as synthetic rubbers.

Natural rubber: Natural rubber is a polymer which is obtained from rubber trees in the form of a milky sap known as latex. The latex is coagulated with acetic acid or formic acid. The coagulated mass is then squeezed. The raw natural rubber is a soft, gummy and sticky mass. It is insoluble in water, dilute acids and alkalies but soluble in benzene, chloroform, ether, petrol and carbon disulphide. It absorbs a large amount of water. It has low elasticity and low tensile strength. It breaks when too much stretched. Natural rubber is a hydrocarbon polymer. It has the composition(C₅H₈)_n. Destructive distillation of natural rubber gives mainly isoprene.

                    Isoprene

Vulcanisation of rubber
Natural rubber is soft and sticky and therefore, in order to give strength and elasticity natural rubber is vulcanized. Vulcanization is a process of treating natural rubber with sulphur or some compounds of sulphur under heat as to modify its properties, i.e., to render it non-plastic and to give greater elasticity and ductility. The sulphur reacts with the polymer molecules forming a cross-linked network. This cross-linking gives mechanical strength to the rubber. In addition, fillers, such as carbon black and zinc oxide are usually added to the crude rubber before vulcanization in order to improve its wearing characteristics.        

Synthetic rubber is any vulcanisable rubber like polymer, which is capable of getting stretched to twice its length. However, it returns to its original shape and size as soon as the external stretching force is released. Thus, synthetic rubbers are either homopolymers of 1, 3 - butadiene derivatives or copolymers of 1, 3 - butadiene or its derivatives with another unsaturated monomer. Neoprene, thiokol, and buna-S are some of the examples of synthetic rubber.

1. Neoprene: It was the first synthetic rubber manufactured on large scale. It is also called dieprene. Its monomer, chloroprene is prepared from acetylene. Chloroprene undergoes free radical polymerization to form neoprene. It polymerizes very rapidly and the reaction occurs by 1.4 addition of one chloroprene molecule to the other.
    
   Many of the properties of neoprene are similar to natural rubber, but neoprene is more resistant to action of oils, gasoline and other hydrocarbons. It is also resistant to sunlight, oxygen, ozone and heat. It is non-inflammable. Thus, it is used for making automobile and refrigerator parts, insulation of electric wires and conveyor belts, etc.
2. Buna-S: Buna-S rubber is a co-polymer of three moles of butadiene and one mole of styrene. It is an elastomer.
   
   Buna-S is generally compounded with carbon black and vulcanized with sulphur. It is extremely resistant to wear and tear and therefore, used in the manufacture of tyres and other mechanical rubber goods. It is obtained as a result of free radical co-polymerization of its monomers.
3. Buna-N: It is obtained by co-polymerization of butadiene ad acrylonitrile.
   
   It is very rigid and is very resistant to action of petrol, lubricating oil and many organic solvents. It is mainly used for making fuel tanks.