2.9 Natural Rubber

Natural polymer
■ Natural polymers

	►	A polymer that exists naturally.
	►	Example of natural polymers: protein, starch and natural rubber.

■ Formation of natural polymer

	►	Proteins are natural polymers composed of long chain molecules known as polypeptides. Each molecule consists of a polypeptide amino acids monomer linked by a peptide chain.
	►	Starch is a plant storage material produced by the process of photosynthesis. When the monomers undergo condensation polymerization of glucose, starch polymers formed together with the removal of water molecules: ○ $\begin{array}{c}{\mathit{nC}}_6{H}_{12}{O}_6\\ \text{glucose}\end{array}\to \begin{array}{c}\text{~}{(C_6{H}_{10}{O}_5)}_n\text{~}\\ \text{starch}\end{array}+n{H}_{2}O$
	►	Natural rubber is a biosynthesis polymer formed in the rubber tree. The monomer of natural rubber is methylbuta-1,3-dienes or isoprene. Natural rubber polymer molecules also known as poliisoprena and is formed by the addition polymerization process. ○ $\begin{array}{c}{\mathit{CH}}_{2}C({\mathit{CH}}_3){\mathit{CHCH}}_2\\ \text{isoprene}\end{array}\to \begin{array}{c}{\mathrm{[}{\mathit{CH}}_{2}C({\mathit{CH}}_3){\mathit{CHCH}}_2\mathrm{]}}_n\\ \text{rubber polymer}\end{array}$

Properties of natural rubber
■ Properties of natural rubber

	►	Soft
	►	Elastic
	►	Does not conduct electricity.
	►	Non-heat resistant. If a natural rubber is heated to temperatures above 50°C, it will melt and sticky. When cooled, the rubber becomes hard and brittle, like plastic. Natural rubber will decompose when heated to temperatures above 200°C.
	►	Insoluble in water, dilute alkali or acid but soluble in organic solvents such as benzene, gasoline, carbon disulphide, and chlorinated hydrocarbons.
	►	The presence of the double bond in the carbon chain causing latex rubber easily oxidized by chemicals, which are strong oxidizing agents.

The uses of natural rubber
■ Original natural rubber, which is not chemically treated have limited use. Natural rubber is used as:

	►	Rubberized bitumen roads (mixture of cement, bitumen and latex rubber to the road surface paving)
	►	Elastic
	►	The insulating material like blankets and rubber foam.
	►	The material to absorb vibration such as rubber block and rubber bearings.
	►	The material to increase friction, such as footwear, sandals, rubber or door.

■ Natural rubber will be chemically treated before use.

	►	Most of the natural rubber latexes are vulcanized to modify the original properties. Vulcanized natural rubber is stronger, flexible and resistant to heat.
	►	Filler materials such as carbon powder, zinc oxide, magnesium carbonate and clay-resistant rubber is added to the original to make it more compact and powerful. This property is important in the manufacture of vehicle tires.
	►	Antioxidants is added into natural rubber to make items such as tires and rubber tubes.

Structure of the rubber particles
■ Latex

	►	Milky white fluid.
	►	A colloidal solution containing about 30% natural rubber and 70% water.

■ Structure of the rubber particle

	►	In the rubber latex, rubber molecules in small groups encompassed by the negatively charged cell membrane proteins.
	►	The protein membrane colloid particles will repel each other to prevent the rubber polymer molecules come together to become one big lump.

■ Coagulation Process of Latex

	►	Coagulation of latex can be done by adding acid to it or leaving it in the air for 1 to 2 days.
	►	This is due to the presence of acid produced by the action of bacteria in the rubber latex.
	►	Positively charged ion, H+ ions from the acid will neutralize the negative charges on the membrane of colloidal particles in natural rubber latex.
	►	Collisions between colloidal particles will break the membrane.
	►	When membranes break, polymer molecules of rubber released and combine with one another.
	►	This is known as the latex coagulation process.
	►	The latex coagulation process can be accelerated by the addition of dilute acid solutions as a supplier of H+ ions into rubber latex. Acid commonly used is methanoic acid (HCOOH) where coagulation will occur within a few minutes.
	►	The following animation shows the coagulation process of latex.

■ Preventing coagulation of latex by alkali

	►	The addition of an alkaline solution such as ammonium hydroxide (, ammonia water) solution will prevent the coagulation of rubber latex.
	►	The alkaline solution is added to neutralize the acid produced by bacteria in rubber latex.
	►	The negative charges on the membrane of colloidal particles of latex rubber can be maintained.
	►	Thus latex rubber remains as a colloidal solution.

■ This video contains information on the coagulation of rubber.

Laboratory Activity 2.9.1 : Coagulation of Latex

Vulcanisation of rubber
■ Vulcanized rubber

	►	Rubber that has been mixed and heated with sulphur.
	►	The weakness of properties such as soft, non-heat resistant in natural rubber can be overcome through a vulcanized process.
	►	Vulcanisation process can be carried out by: ○ Heat natural rubber with sulphur ○ Mix natural rubber with a solution of sulphur monochloride in methylbenzene.

■ Vulcanisation of Rubber

	►
	►	After polymerization, the natural rubber polymer chain still has a double bond which allows it to be vulcanized.
	►	In vulcanisation process, sulphur atoms diffuse into the structure of natural rubber to form cross-links between neighbour polymer chains of natural rubber.
	►	With the formation of sulphur atom cross-links, the polymer molecules become more compact.
	►	A more compact polymer chain arrangement make vulcanized rubber become less elastic and stronger.
	►	Elasticity and strength of vulcanized rubber depend on the amount of sulphur cross-links formed between rubber polymer molecules.

■ Comparison of vulcanised and unvulcanised rubber

►

Differences Natural rubber Vulcanised rubber Tensile strength Low High Hardness Soft Hard Melting point / Heat resistant Lower Higher. The addition of sulphur to the rubber polymer chains has increased the relative molecular mass. Thus, the melting point of vulcanized rubber is also higher, and it is more resistant to the action of heat. Elasticity Low High Resistant to oxidation Low High. As the number of double bonds decreased due to the formation of the double bond cross-sulphur clusters, the vulcanized rubber are more resistant to oxidation.

Laboratory Activity 2.9.2: Characteristic of vulcanised and unvulcanised rubber