Natural Rubber

Rubber-- an alkene polymer (number of small monomer units), also known as elastomers which have high elastic properties due to weak Vander Waal force of attractions. Rubber can be obtained from 400 different plants. The major source, however, is the rubber tree Hevea Brasiliensis.

The term rubber was coined by Joseph Priestley. 

    

Polyisoprene (a natural rubber) can be prepared by polymerisation of isoprene in the presence of Ziegler- Natta catalyst.                                                                                             

Process to obtain natural rubber: 
Rubber is harvested from the bark of tree in the form of latex (milky fluid in rubber tree).            
It includes following steps :  

1) Collection of latex-- Latex is collected in  some bowls fixed on the bottom of trees. Thin  shaving of bark to collect latex is done by tapping knife. Then large amount of latex is collected in plastic or aluminium vessels.                                    

2) Coagulation-- Collected latex is kept for 9-10 hours to coagulate. Coagulation of latex is done by adding formic acid to latex.                              
Pre- coagulation highly affects the quality of rubber sheet.                                                              

3)Removal of excess water-- Coagulated latex also contains water in high proportions, and    excess of which is squeezed using rollers, which  also results into thin sheets of rubber.                      

4) Drying of rubber sheet-- After that, rubber  sheets are dried up and then sent to factories. 

Vulcanisation of rubber : 

In this process, a certain amount of molten sulfur is added to molten rubber at a temperature ranging between 373 to 415k. This was first performed by Charles Goodyear in 1839. 

The purpose of vulcanisation is to introduce stiffness to the rubber. 

The extent of stiffness depends upon amount of sulfur added. For example, rubber containing about 5% sulfur is used in making rubber tyres while rubber containing 30% sulfur is employed for making cases of batteries. 

Vulcanised rubber shows many improved properties over natural rubber: 

a) It is tough and non- sticky. 

b) It is non- oxidized.

c) It shows resistance to attack by organic- acids.

d) It is more elastic.