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What is rubber?

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Rubber is an elastomerthat is, a polymer that

has the ability to regain its original shape after

being deformed. Rubber is also tough and resistant to weathering

and chemical attack.

Elastomers can be naturally occurring polymers,such as natural rubber, or they can be

synthetically produced substances, such as butyl

rubber, Thiokol, or neoprene.

For a substance to be a useful elastomer it must

possess a high molecular weight and a flexible

polymer chain.

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Composition and Structure

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Crude rubber is primarily hydrocarbon in

nature.

In 1826 English chemist Michael Faraday

(17911867) analyzed natural rubber and

found it to have the empirical (simplest)

formula C5H8, along with 2 to 4 percentprotein and 1 to 4 percent acetone-soluble

materials (resins, fatty acids, and sterols).

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The molecular weights of rubber molecules range

from 50,000 to 3,000,000.

Sixty percent of the molecules have molecular

weights of greater than 1,300,000.

The repeating unit in natural rubber has

the cis configuration (with chain extensions onthe same side of the ethylene double bond),

which is essential for elasticity.

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If the configuration is trans (with chain

extensions on opposite sides of the ethylene

double bond), the polymer is either a hardplastic (naturally occurring gutta-percha,

obtained from the leaves of Palaquium, a

species of sapotaceous Malaysian and EastIndies trees) that was used for wire and cable

coating during the nineteenth century; or a

substance like gutta-percha (balata, obtained

from Mimusops globosa, trees native to

Panama and South America), used for modern

golf ball covers.

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Rubber latex.harvesting rubber. Latex being

collected from a

tapped rubber tree

NEXT>>>tapping knife

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Methods Of atex Rubber Tapping

1. All natural rubber originates in the

Hevea tree. The trees are rarelytapped once every two days.

2. A tapper starts the trek around the

plantation before dawn. At each

tree a sharp knife is used to shave

off the thinnest possible layer from

the intact section of bark.

3. The cut must be neither too deep,

nor too thick.

4. This starts the latex flowing, and the

tapper leaves leaves a little cupunderneath the cut, latex will

coagulate into a 'cup lump.'

5. The tapper returns a few hours later

and collects the stuff in the cup --

either cup lump or latex. NEXT>>>

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The latex obtain from the rubber trees is preservedwith ammonia to prevent coagulation. It is then takento the collections centre.

At the collection centre, latex is poured into a big tank.Methanoic acid is added to coagulate the latex.

The coagulated latex is pressed into rubber sheetsusing rollers. The sheets are cut into suitable sizes andsent to smoke house, which is usually a rubber factory,for processing.

The rubber sheets are washed with machine using soapand water to remove external dirt.

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The washed rubber sheets are hanged on bamboostems for drying in an oven of about 8 metres long, 4metres wide and 6 metres high. Drying takes about 3-5

days depending on the thickness of the rubber sheets.

The oven is heated by burning wood from older rubbertrees. The temperature inside the oven is maintained

at 70 degree celcius. This is called smoking.

After smoking, the rubber sheets become moretransparent, and impurities can be more easily seen

and removed. These smoked sheets are called RibbedSmoked Sheets or RSS. They are graded depending ontheir purity, elasticity and colour.

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The RSS sheets are packing according tointernational shipping requirements. A hydraulic

machine is used to press the sheets into cubes.Rubber packed in this form is called bulk rubber.

The RSS bulk rubber is coated with a whitepowder which is a mixture of kerosine andcalcium carbonate to protect the bales fromsticking together.

Bulk rubber is mainly used for tyre manufacturingbut it has to be vulcanized first.

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Coagulation and how to prevent it Alkali such as ammonia solution are added to latex to prevent coagulation. This

is due to the hydroxide ions from alkali neutralise hydrogen ions produced by

lactic acid as a result of bacteria attack on protein.

Because there are no hydrogen ions to neutralise the negative charges on the

rubber particles, they remain negatively charged, hence cannot combine and

coagulate.

The negatively charged rubber particles repel each other, preventing themselves

from combining and coagulating.

__

_

_

__

_

_

_

__

_ ___

_

_ __

___

_

_

_

__

_

_

_

_

Negatively ChargeRepulsion

Rubber Polymer

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Acid such as methanoic acid are added to make the

latex coagulate.

This is due to hydrogen ions from the acid neutralisethe negatively charges on the surface of the

membrane. A neutral rubber particle is formed.

When these neutral particles collide with each other,their outer membrane layers break up. The rubber

polymers are set free.

The rubber polymers start to coagulate by combiningtogether to form large lumps of rubber polymers

which then precipitate out of the latex solution.

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H+

H+

H+H+

H+H+H+

H+

H+

H+

H+ H+

H+

H+

H+H+H+

H+

H+

H+

Neutralised Rubber Particle

Collide

Rubber Polymer

Coagulate

Coagulation Process of Latex

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Latex can still coagulate if acid are not added.

Normally, the latex will coagulate of left

overnight.

This is due to bacteria from the air slowly

attack the protein on the membrane to

produce lactic acid. Ionisation of the lactic

acid produces hydrogen ions. The hydrogen

ions neutralise the negative charges to formneutral rubber particles, allowing coagulation

to occur.

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Bulk rubber or raw natural rubber is not an

elastomer yet. It is a thermoplastic and it

becomes soft and sticky on hot days and hard

when it is cooled to lower temperatures. This

is because the polymeric molecules in raw

rubber have no cross-linking bonds and are

free to slide past one another.

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Vulcanization treatment of rubber is a reaction by which

polymer chains of rubber are linked together to prevent

chains from slipping past one another.

Vulcanization is used to improve the physical properties

of the natural rubber. When rubber is heated with

sulphur, a chemical reaction occur to produce a rubber

which is stronger and more resistance towardsoxidation.

The process of hardening rubber by heating it with

sulphur of treating it with sulphur compound such as

sulphur dichloride (S2Cl2) is called vulcanization. Zincoxide (ZnO) is used as a catalyst in the vulcanization

process.

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The presence of the double bond in rubber

allows vulcanization to take place. During

vulcanization, sulphur reacts by replacinghydrogen bonds, either attached to a carbon

atom in the C=C bond, or a carbon atom next

to a C=C bond. This causes cross-linking of

polymer chains through disulphide (-S-S-)

bonds.

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Sulfur cross-links the rubber molecules in the

vulcanization process.

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To acquire rubber elastic properties , rubber for allapplications are vulcanized, exceptions are rubber cementand adhesion types. Vulcanisation transforms raw naturalrubber into an elastomer by giving it resilience, strength

and durability.

Rubber with wide range of physical properties can beprepared by controlling the amount of sulphur used in the

vulcanization. Hard rubber is vulcanized rubber in whichabout 30% of sulphur has been added before heating, softrubber has less than 5% of sulphur.

After the sulphur and rubber are mixed, the compound isusually placed into moulds and subjected to heat andpressure. The vulcanised rubber is not sticky like rawrubber, and neither hardens when cooled nor softens muchexcept with great heat.

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The three-dimensional cross-linkind structure makes

vulcanised rubber a harder, stronger subatances that is

suitable for automobile tyres. Cross-linking also improve

the elsaticity of the rubber.

Natural rubber is not suitable for uses that requires

strength or durability. When natural rubber is stretched,

the polymer chains slide pass each other and the rubberpulls apart.

In contrast, the vulcanized rubber chains cannot slide

over one another when stretched because the polymersare linked together by disulphide linkage. The vulcanized

rubber will return to its original shape when the

stretching force is removed.

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Furthermore, when the vulcanized rubber is

pulled, the polymer chains will stretch but

cross-linking between polymer chains

prevents tearing. Thus, vulcanized rubberbecomes harder, more elastic and less sticky

when hot. The durability and tensile strength

can be increased by adding and reinforcingfiller.

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Properties of Natural

Rubber

P i f N l R bb

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Properties of Natural Rubber

Crude rubber is a tough and an elastic solid. It becomes

soft and sticky as the temperature rises.

Its specific gravity is 0.915.

The most important property of natural rubber is its

elasticity. When stretched, it expands and attains its

original state, when released. This is due to its coil-likestructure. The molecules straighten out when stretched

and when released, they coil up again. Therefore

applying a stress can easily deform rubber. Note that

when this stress is removed, it retains its original shape.

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Raw natural rubber has elasticity over a narrow range

of temperature from 10 to 60 degrees centigrade.

Because of this, articles made of raw natural rubberdon't work well in hot weather.

Raw natural rubber has low tensile strength and

abrasion resistant.

It absorbs large quantities of water.

It is insoluble in water, alcohol, acetone, dilute acids

and alkalis.

It is soluble in ether, carbon disulphide, carbontetrachloride, petrol and turpentine.

Pure rubber is a transparent, amorphous solid, which

on stretching or prolonged cooling becomes crystalline.

f l bb

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Properties of Vulcanisation Rubber

If sulphur is added to rubber and heated, its shape can be

maintained and its hardness increased.

The process of heating rubber with sulphur for the

betterment of its properties, is called vulcanisation.

Vulcanisation enhances the following properties of

rubber.

tensile strength

Hardness

Elasticity

ability to withstand heat changes

Similarities: Elastic, heat and electrical insulator

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The differences between natural rubber and

synthetic rubber.

Natural Rubber Synthetic Rubber

Low hardness High hardness

Easily flammable Not easily flammable

Dissolves in organic solvents Does not react with organic solvents

Loses stability at high temperature Keeps stability in high temperature

Less elastic More elastic

Softer Harder

Easily oxidized Less easily oxidized

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Uses of rubber in our daily life The use of rubber is widespread, ranging from

household to industrial products, entering theproduction stream at the intermediate stage or as final

products.

Tires and tubes are the largest consumers of rubber. The

remaining 44% are taken up by the general rubber goods

(GRG) sector, which includes all products except tires

and tubes.

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Other significant uses of rubber are door and window

profiles, hoses, belts, and dampeners for the automotive

industry in what is known as the "under the bonnet"

products.

Because of its elasticity, resilience, and toughness,

natural rubber (NR) is the basic constituent of many

products used in the transportation, industrial,

consumer, hygienic and medical sectors.

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Gloves and footwear(medical, household and industrial) are

also large consumers of rubber and toy balloons, although

the type of rubber used is that of the concentrated latex.

Significant tonnage of rubber is used as adhesives in manymanufacturing industries and products, although the two

most noticeable are the paper and the carpet industry.

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Rubber is also commonly used to make rubber bands

and pencil erasers.

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Rubber is use for medical and health sector such as

condom and syringes.