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Natural Rubber
Natural Rubber
Rubber and History
Rubber trees or the scientific name
Hevea brasiliensis
were brought to Malaysia in 1877 and has continued to be developed as an important commodity in our country until the present day.
Among Malaysia’s major exports are rubber gloves, condoms and catheters.
Latex is a white fluid obtained from rubber trees when the tree bark is tapped.
Natural rubber that is found in latex is a natural polymer known as polyisoprene.
Latex is a colloid and not a solution.
Rubber particles do not dissolve in water but disperse separately in water.
Monomer for polyisoprene is isoprene or with the IUPAC name 2-methylbut-1,3-diene.
The protein membrane of rubber particles is negatively charged on the outer surface.
This causes rubber particles to repel each other, preventing coagulation.
Structural formula of isoprene or 2-methylbut-1,3-diene
Cross-section of rubber particle
Characteristics of Natural Rubber
The characteristics of natural rubber depend on rubber polymers that are natural elastomeric polymers.
The presence of double bond C=C in the rubber polymer structure affects the resistance of natural rubber towards oxidation by air.
Characteristics
Remarks
Soft
Soft white solid at room temperature.
Elastic
Natural rubber can be stretched and can return to its original shape when released.
Low heat resistance
At high temperature, natural rubber will become soft and sticky.
Electrical insulator
Natural rubber cannot conduct electricity and is a good electrical insulator.
Easily oxidised
Oxygen in the air can react with the double bond between carbons and cause natural rubber to be oxidised.
Reactive to chemicals
Reacts easily with acids, alkalis and organic solvents.
Waterproof
Natural rubber is water impermeable.
Extra info
The Malaysian Rubber Board is an agency responsible for the rubber industry in Malaysia that includes conducting studies on the latest technology involving natural rubber.
Products from natural rubber
Tyre
Rubber boots
Rubber band
Gloves
Soles of shoes
Coagulation of latex
Latex is collected in liquid form for processing or it can be left to coagulate and to be collected a day after it was tapped.
The collection of latex in liquid form should be done quickly because once exposed to the air for a few hours, the latex will start to coagulate and turn solid.
Coagulation process of latex
Step
Explanation
1
The negatively charged protein membrane causes rubber particles to repel each other.
2
Hydrogen ions,
\(H^+\)
from acid neutralise the negatively charged protein membrane.
3
Rubber particles collide with one another that causes the protein membrane to break.
4
Rubber polymers combine with one another that causes latex to coagulate.
Acid can coagulate the latex by neutralising the negatively charged protein membrane.
Coagulation can take place by adding a weak acid into the latex, or it can occur naturally by leaving the latex exposed to the air.
Bacteria in the air enter the latex and secrete lactic acid, which eventually causes the latex to coagulate.
Ways to process latex before taken to factory
Sheet rubber
Block rubber
Cup lump rubber
Latex Coagulation Prevention
Latex is also needed in liquid form to produce specific products, such as gloves and rubber tubes.
Alkaline solutions such as ammonia,
\(NH_3\)
is added into the latex to ensure that coagulation does not take place.
Alkaline solutions consist of hydroxide ions,
\(OH^−\)
that can neutralise the acid produced by the bacteria.
The protein membrane of rubber particles remains negatively charged and rubber particles will continue to repel when they draw near one another.
Vulcanisation of Rubber
Natural rubber is soft and easily oxidised when exposed to the air for a long period of time.
This condition renders natural rubber unsuitable to be used for certain conditions or applications.
The characteristics of natural rubber can be improved by undergoing the vulcanisation process.
Definition of vulcanisation of rubber
A process of producing rubber that is more elastic and with better quality through the production of cross-links between polymer chains
During a vulcanisation process, the double bond between carbons found in rubber molecules will react with sulphur or other substances to produce sulphur cross-links.
Vulcanised rubber
Alternative Vulcanisation Method
Vulcanisation by using sulphur is the main method used to produce vulcanised rubber from natural rubber.
Nonetheless, this vulcanisation method cannot be used for certain types of rubber, particularly synthetic rubber which does not contain C=C.
There are a few alternative vulcanisation methods that can be utilised to produce vulcanised rubber without using sulphur.
Vulcanised rubber produced is free from sulphur and more environmentally friendly.
Alternative vulcanisation method
Peroxide
Irradiation
Metal oxide
Properties of Vulcanised Rubber
Vulcanised rubber has different characteristics from unvulcanised rubber due to the formation of cross-links during the vulcanisation process.
The formation of sulphur cross-links reduces the double bonds between two carbon atoms in vulcanised rubber that makes it harder to be oxidised.
Strong sulphur cross-links prevents the rubber polymer from sliding when it is stretched and can return to its original shape when released.
High heat energy is needed to break the linkage.
Unvulcanised rubber
Vulcanised rubber
This condition makes vulcanised rubber more elastic and has high heat resistance.
These unique characteristics of vulcanised rubber allow various items to be produced compared to unvulcanised rubber.
Vulcanised rubber
Unvulcanised rubber
Elasticity
More elastic
Less elastic
Hardness
Hard
Soft
Strength
High
Low
Resistance towards heat
Resistant to high heat
Less resistant to high heat
Resistance towards oxidation
More resistant towards oxidation
Easier to be oxidised
Chapter : Polymer
Topic : Natural Rubber
Form 5
Chemistry
View all notes for Chemistry Form 5
Related notes
Polymer
Synthetic Rubber
Redox Reaction
Standard Electrode Potential
Voltaic Cell
Electrolytic Cell
Extraction of Metals from its Ore
Rusting
Types of Carbon Compounds
Alkane
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