CH 1105
ENGINEERING CHEMISTRY
Akib Jabed Shovon
Department of Mechanical Engineering
Khulna University of Engineering & Technology
Khulna 9203
Roll: 1705006
Rubber

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Rubber:
A tough elastic polymeric substance made from the latex of a tropical plant or synthetically.
Important Properties of Rubber:
 Elasticity
 Flexibility
 Mechanical strength & toughness
 Impermeable to gas & liquid
 Resistance to chemical action environment
 Resistance to tearing & abrasion
 Tackiness
 Plastic property
Classifications of Rubber:
Depending upon the source the rubbers may be classified into two classes:
1. Natural Rubber (Poly-isoprene).
2. Synthetic Rubber/Elastomer (Butyl rubber, styrene rubber, neoprene, SBR etc.)
Sources of Rubber:
Natural rubber obtained from the plants & synthetic rubber is synthesized in the laboratory by chemical
reaction. Hevea Brasiliensis plant produce best quality of rubber. Rubber obtained from tree in the form
of milky juice known as latex which contains 30-40% rubber. The rest of the latex is mainly water.
Ammonia is added to latex as preservative to avoid putrefaction & contamination. Chemically latex is a
colloidal dispersion of negatively charged particles of rubber. The charges on the rubber stabilize the
emulsion & on removal of charged particles destabilize the emulsion. Therefore, coagulation of latex is
carried out by removing the negative charges adding an electrolyte, usually 1-2% acetic acid. The
coagulation is very much sensitive to the pH of the solution. The maximum yield is obtained at a pH range
(4.77-5.05). On coagulation the solid rubber is separated out from the liquid part of the latex. The
coagulated rubber is then passed through a roller to make sheet & squeeze out serum. It is then dried in
open sky. The dried rubber obtained in this way is called pale crepe rubber. The thickness of this rubber
is not uniform as the surface of the rubber is not smooth & it rotates at a variable speed. Alternatively,

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crude rubber is passed through a smooth roller rotating at a uniform speed makes relatively more uniform
& thicker rubber sheet. In this process rubber is dried in a smoked house. The crude rubber obtained in
this process is called smoke rubber. The smoke makes the rubber brown & serves as a preservative by the
formation of mould.
Chemical Composition of Rubber:
Chemically natural rubber is cis-poly isoprene. The molecular weight of natural rubber varies from
130,000-240,000.
Isoprene Poly-isoprene
Natural rubber decolorizes bromine water, forms ozonide with ozone & reacts with hydrochloric acid. This
indicates natural rubber contains unsaturated hydrocarbon.
Chemical structure of rubber is-
cis-poly isoprene trans-poly isoprene (gutta percha)
Vulcanization:
Vulcanization is the process if incorporating Sulphur into crude rubber under the influence of heat &
pressure in order to change rubber into an elastic, strong, non-plastic, non-tacky material. The extent of
vulcanization depends upon-
1. The amount if vulcanizing agent.
2. Duration of vulcanization (curing time).
3. Temperature of vulcanization.
In the vulcanization process Sulphur atoms reacts with poly-isoprene chains & produces links between
different parts of the same chain as well as it links between two or more chains of the crude rubber. These
cross-links reduce the extensibility of rubber & makes it harder. The hardness of vulcanized rubber
Polymerization
Ziegler-Natta
catalyst

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depends upon the percentage of incorporated Sulphur. The amount of incorporated Sulphur varies
from 1-5% ordinary soft rubber & 40-50% for hard rubber. In such cases rubber loses its elastic
property. An important example of vulcanized hard rubber is ebonite, which contains certain
percentage of carbon in addition to high percentage of Sulphur.
The time of vulcanization is known as curing time. It affects the properties of vulcanized rubber.
Under vulcanized rubber is soft, has also excessive stretching effect & low tensile strength. On the
other hand, over vulcanized rubber has high tensile strength & decreased stretching effect.
Compounding:
The act of mixing other ingredients with rubber & processing the mixed compound is called compounding.
The substances, which are usually used in compounding & their effect on the properties of rubber, are
described below:
1. Accelerators:
These are chemical substances, which increase the rate of curing & thereby shorten the time
required for vulcanization. These improve the physical & chemical properties of rubber
compounds. The commonly used accelerators are lime, magnesia, shite lead, aldehydes, amines,
thiazoles, thiocarbamates, guanidine etc.
2. Accelerator Activators:
The substances that are used to increase the accelerating effects of activators in the vulcanization
process are called accelerator activators. These substances however don’t have any action if they
are used alone. The most commonly used accelerator activator is zinc oxide (ZnO).
3. Antioxidants:
Because of the presence of double bond in all monomeric units, rubber has very low resistance to
the action of atmospheric oxygen, ozone etc. To protect rubber goods from the attack of
atmospheric oxygen & ozone antioxidants are used. They essentially negative catalyst & are used
in small in amount (1%). The most commonly used antioxidants are complex organic compounds
particularly secondary amine, phenolic substances, phosphites etc.
4. Reinforcing Agents:
These substances are added in the compounding to increase the mechanical strength, rigidity &
toughness of the rubber. The commonly used reinforcing agents are carbon black, ZnO, MgCO3,
BaSO4, CaCO3, some clays etc.

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5. Inert Fillers:
These are added to lower the cost of the product by increasing the amount without
deteriorating the quality of the product. These are generally low cost substances such as
chalk, BaSO4, barytes, lithophane, low cost oils, resins, tars, pitches etc.
6. Coloring Agents:
These substances are generally known as pigments. These are used to impact desired color
to the rubber.
 TiO2, Lithophane for white color.
 Lead chromate & Potassium chromate for yellow color.
 Ferric oxide for brown color.
 Antimony sulfide for crimson color.
 Ultramarine blue for blue color.
 Chromium oxide for blue color etc.
7. Plasticizers & Softeners:
These substances are added to increase tenacity & adhesive properties, of the rubber. The
most commonly used plasticizers are oils, waxes, stearic acid, resin etc. Plasticizers also
function as softeners. Softeners include quite good number of materials, which are used for
a number of purposes such as lubricants, extenders etc. The most commonly used softeners
are napthalic oil, aromatic oil, coal tar, fats, different vegetable oils, esters etc.
8. Processing Aids for Uncured & Under Cured Stock:
Under cured stock are usually sticky. Thus for their processing lubricants like substances are
added. Lubricants are the substances, which prevent the sticking of rubber to the mill,
calendar, extruder, roller, moulding machine etc. These substances are more or less insoluble
in the rubber stock. Petroleum, waxes, oils, asphalts, pitch, xylene etc. are used commonly.
9. Miscellaneous Agents:
In addition to the above described substances some other substances are occasionally added
to impart specific properties to the vulcanized rubber. For example, sand or silica is added to
the rubber, which is used as eraser.

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Techniques of Vulcanization:
1. Mould Cure method:
In this method crude rubber is mixed with requisite amount of vulcanizing agents. The mixture
is taken in a mould, which is closed under hydrostatic pressure. Vulcanization is then carried
out by heating the mould placing it between steam hot plates.
2. Steam Cure Method:
In this method also the crude rubber is mixed with requisite amount of vulcanizing agents.
The mixture is then heated by steam under pressure.
3. Hydraulic Cure Method:
In this method vulcanization is carried out by taking a mixture of crude rubber & vulcanizing
agents in a container. The mixture is then heated by using boiling water under some definite
temperature & pressure.
4. Electrical Curing Method:
This is newer technique of vulcanization. In this technique the vulcanization is carried out in
a high frequency electric field.
Properties of Vulcanized Rubber:
 It has good tensile strength & extensibility.
 Resiliency of vulcanized rubber gets much improved.
 Water absorption capacity of vulcanized rubber is considerably lowered.
 Vulcanized rubber is resistant to organic solvents such as petrol, CCl4, benzene, fats & oils.
 It is better electrical insulator. Ebonite is excellent insulator.
 It has much lower tackiness.
 It is very easy to manipulate the vulcanized rubber to produce desired shape.
 Working temperature range following vulcanization gradually increases from (10 to 60)°C
(raw rubber) to (-40 to 100)°C.
 Vulcanized rubber has much lower elasticity. The elasticity depends upon the extent of
vulcanization.

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Differences between Plastic & Rubber:
No. Plastic Rubber
1. It is defined as organic material which have a
polymeric structure & are derived from oil &
petroleum.
It is defined as organic material which have a
polymeric structure & are derived from the
latex of rubber trees.
2. It is a by-product. It is a natural product.
3. It mainly consists of plasticizer, stabilizer, filler,
pigments etc.
It mainly consist of iso-propene, other organic
compounds, water etc.
4. It is non deformable. It is deformable.
5. They are less elastic in nature. They are naturally more elastic.
6. They have toxicity because of their insolubility
in water.
They have high toxicity because of water
content.
7. Synthetic plastic is made from natural gas or
petroleum.
Synthetic rubber is made from crude oil.
Raw Materials for Synthetic Rubber:
 Coke
 Limestone
 Petroleum
 Natural Gas
 Alcohol
 Ammonia
 Coal Tar etc.
Neoprene:
Synthetic rubber obtained by the polymerization of chloroprene is called neoprene. This is the first
successfully synthesized elastomer.
 Synthesis of Neoprene:

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 Properties of Neoprene:
1. Closely related to natural rubber.
2. Resistant to oxidizing agent (such as oxygen, ozone), ageing & high temperature.
3. Well resistant to vegetable & mineral oil.
4. Soluble is polar solvents.
5. Not attacked by alkalis below 50% strength.
6. Higher elongation percentage but lower tensile strength.
7. Good weathering resistance.
8. Superior resistance to flame sunlight & air etc.
9. Can be vulcanized to be a considerable extent by heat along properly vulcanized neoprene
is extremely resistant to oxidative degradation.
10. The general physical properties can be enhanced by compounding it with metallic oxides
such as MgO or ZnO.
 Uses of Neoprene:
1. Excellent for manufacturing good quality of tyre.
2. Normally used as standard engineering materials in oil refining plants, gaskets etc.
3. Used for making conveyor belts, printing rollers, rubber cements etc.
4. During world war it was used for making container for transportation of gasoline.
5. Used in wire, cable coating, industrial hoses, shoe heels etc.
6. Gloves, coated fabrics, synthetic leather can also be prepared from neoprene rubber.
Styrene Butadiene Rubber (SBR):
This is generally composed of about 3 parts of butadiene & 1 part of styrene. During World war II, it was
the most important synthetic rubber, which met up high demand of natural rubber.
 Synthesis of SBR:

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 Properties of SBR:
1. High resistance to abrasion, ozone & weathering action.
2. Gets readily oxidized, swells in oils & solvents.
3. High load bearing capacity & good resilience.
4. Difficult to mill, mix & calender this rubber.
5. Poor tack forming properties.
6. Can be vulcanized in a similar manner as natural rubber by Sulphur, Sulphur monochloride
etc.
 Uses of SBR:
Usually used for manufacturing lighter duty tyres, hoses, belts, moulded goods, unvulcanized
sheet, gum, floor mat, shoe’s soles, electrical insulators, adhesives, tank lining, carpet backing,
coated fabrics etc.
Butyl Rubber:
Like SBR, another co-polymeric elastomer is butyl rubber. It is a co-polymer of isobutylene & isoprene or
butadiene.
 Synthesis of Butyl Rubber:
isobutylene (97%) isoprene (3%) butyl rubber
 Properties of Butyl Rubber:
1. Interchain force is very high as a result it possesses high tensile strength.
2. Impermeable to gas including air.
3. Excellent resistant to heat, abrasion, ageing, different chemicals like inorganic acids (e.g.
HF, HCl, HNO3, H2SO4 etc.) & organic polar solvents (e.g. alcohol, acetone etc.).
4. Like natural rubber, it is soluble in organic solvents (e.g. benzene, petrol etc.).
5. Very high resistance to ozone.
6. Can be vulcanized to a limited extent.
7. It has insulating properties too.

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 Uses of Butyl Rubber:
Suitable for making tube, gloves, tank lining, insulation of high voltage wire & cables etc.
Silicon Rubber:
Like carbon, the tetra-valent nature of silicon made the possibility of formation of long chain with silicon
& oxygen atoms without having carbon atom in it. Silicon rubbers are prepared by forming such chain of
silicon & oxygen. The other valencies of silicon atom are filled up by alkyl groups & some cases by phenyl
or aryl group.
 Synthesis of Silicon Rubber:
 Properties of Silicon Rubber:
1. Remarkable stability at high temperature & low temperature even at -90°C.
2. Highly water repellent & have high resistance to corona discharge.
3. Good electrical properties but lower tensile strength elongation than hydrocarbon
rubbers at normal temperature.
4. Have comparatively low swelling effect in oil & not affected by many chemicals.
 Uses of Silicon Rubber:
1. Mainly used as good lubricant, paint & protective coating because of its high temperature
resistance.
2. Used as protective coating for fabric finishing & waterproofing.
3. Used as sealing material in searchlight & aircraft engines.
4. Used as adhesives for making insulation for washing machines.
5. Used for making blankets for iron board covers.