O’zbekiston respublikasi oliy va o’rta maxsus ta’lim vazirligi mirzo ulug’bek nomidagi o’zbekiston milliy universiteti

No t e s

* at least — juda bo’lmasa
XIV. Answer the following questions after reading the text given below

1. 
   Which use of rubber is familiar to you?
   
2. 
   Where can rubber be found?
   
3. 
   What is done to rubber to protect it from microorganisms?
   
4. 
   Who discovered the process of vulcanization?
   
5. 
   What properties of natural rubber were responsible for many disappointing results in the manufacture of waterproof textiles prior to 1839?
   
6. 
   How is the life of rubber products prolonged now?
   
7. 
   How can we obtain pure cellulose?
   

RUBBER

Apparently rubber was first discovered by natives in Central and South America. They used it as waterproofing material and as elastic balls in their games. Samples of this material were carried back to Europe by early explorers. It was soon found that small pieces of the elastic substance were handy for erasing pencil marks and thus it was given the name "rubber". This one use alone is painfully familiar to students working problems or changing compositions.

Although rubber can be found in the milky fluid of certain plants such as the goldenrod or dandelion, its chief source is latex obtained from the rubber tree. This tree originated in Brazil, but since about 1900 it has been extensively cultivated on plantations in the East Indies. The latex is a colloidal suspension of rubber in water collected from the bottom of V-shaped cuts made through the outer bark of the tree. The rubber in the latex is coagulated by acidification and the precipitated material is rolled into balls or sheets which are smoked to protect them from microorganisms.

Natural rubber is a thermoplastic material that becomes soft and sticky when heated and hard and brittle when cooled. Prior to 1839, these properties were responsible for many disappointing results in the manufacture of raincoats and other waterproof textiles. In experiments designed to improve the properties of rubbers Charles Goodyear, in 1839 accidentally spilled a mixture of rubber and sulfur on a hot stove and discovered the process he later called vulcanization.

When properly applied, this process not only prevented softening and brittleness at extremes of temperature, but also increased the elasticity and toughness of the product.

The modern rubber industry has developed many special types of rubber for various purposes and has improved the quality by continuous research. Antioxidants are used to prevent the reaction of the double bonds with oxygen of the air. This treatment prolongs the life of rubber tubing, gloves, hot water bottles*, and other articles made of rubber.

Carbon black and a fine silica powder called carbon white are used as agents to increase the tensile strength, stiffness, abrasion and tear resistance of tires made of rubber. More recent research by the rubber industry involves vulcanization by radiation with gamma rays. Not only smaller articles, but also full-sized automobiles tires have been vulcanized without the use of heat or curing agents.

Another important naturally occurring polymer is cellulose. It is a condensation polymer consisting of recurring glucose molecules joined together by the splitting out of a molecule of water. Cellulose is a derivative of a six-carbon polyhydric alcohol. Pure cellulose may be obtained from cotton by removing the wax with organic solvents, and pectic, substances with hot 1 per cent sodium hydroxide. Cellulose is not as important a polymer as the various esters and derivatives that can prepared from it by several chemical treatments.

Starch and glycogan, like cellulose are naturally occurring polymers of glycose. Cellulose occurs in the framework or supporting structure of plants. It is insoluble in water and in general has properties that fit its role as a protective rigid external covering of plants. Starch, on the other hand, is the storage form of carbohydrates in plants, is soluble in hot water and used as a source of food. Glycogen is similar to starch in properties and is the storage form of carbohydrates in animals.