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CHAPTER ONE
INTRODUCTION
1.1 Natural Rubber.
The natural rubber (NR) presently in use by the industry is
obtained by tapping the sap known as latex, from the large forest tree
‘Hevea brasiliensis’, which occur in the equatorial region of America.
(Uptal, 2007).
Rubber is a polymeric substance obtained from the sap of the tree
‘Hevea brasiliensis’. Crude natural rubber is obtained by coagulating
and drying the sap (latex), and is then modified by compounding and
vulcanisation with fillers. It is a polymer of isoprene units.
(Fig 1.1): poly isoprene
Various synthetic rubber can also be made. (Clark et al, 2007)
Natural rubber has a structure that resembles synthetic polymer
of dienes. We could consider it to be a polymer of the conjugated diene,
2-methyl 1, 3-butadiene i.e. isoprene.
Isoprene polyisoprene
The double bond in the rubber molecule are highly important, since
apparently by providing reactive allylic hydrogens – they permit
vulcanisation, the formation of sulphur bridges between different chains
(Morrison et al, 2002).
Rubber or elastomers can be obtained from two sources, which may be
natural or artificial. Consequently, we have natural rubber and synthetic
rubber. The source of natural rubber is a group of plants comprising nearly
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500 different species which yield a white substance, known as latex (which
is a dispersion of polyisoprene) when the surface is cut or wounded. The latex
contains about 25-40% of rubber hydrocarbons dispersed in water in
presence of stabilizer proteins and some fatty acid. (Sharma, 2007).
1.2 Natural rubber plant
Natural rubber of nearly same characteristics occurs in the inner
bark and to a lesser extent in the leaves and roots of more than 500
tropical plants (for example Dandelions, Gauyule, Golden rod, Osage
orange etc.). But none has been proven to be as successful as the latex
from the Hevea brasiliensis, a native of America. More than 90% of the
supply is derived from the tree, “Hevea Brasiliences”. Now rubber is
obtained from variety of trees, shrubs and vines. The plants yield milky
suspension of crude rubber called ‘latex’. The bulky tree of America
gives Belata. The Gutta Percha of south east Asia yields Gutta Percha
and the Gauyule tree of Mexico and California form Gauyule rubber.
In Russia, Kok-Saghir, a tiny wild dendation was discovered to contain
rubber in the form of filaments in its roots in 1931. The improved plants
have been found to contain about 12% rubber and yield is
450kg/hectare only. (Sharma, 2007).
1.3 Latex
Polymer latex is a colloidal dispersion of a rubber or plastic
material in an aqueous medium. The polymer material may be a
polymer of a single, small and ethylenically unsaturated organic
compound or copolymer of two or more of such compounds. The
stability of a polymer latex is due basically to the presence of surface
active material at the interface between the polymer particle and the
aqueous phase. The majority of the lattices are anionic in character
because their polymer particles carry a negative charge. (Imanah, 2001)
The latex is a stable dispersion of a polymer substance in an
aqueous medium with an essentially two-phase system i.e. a dispersed
(discreet, discontinuous or internal) phase or serum and a dispersion
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medium (aqueous, continuous or external) phase or serum The
dispersed phase is made up of isoprene units or particles which are
rubbery in nature while the dispersion phase is the aqueous phase,
which is similar to solvent or serum of the blood in which the dispersed
phase are randomly kept. (Akinlabi 1992)
Natural rubber latex can be obtained from several different
species of plant, the most outstanding source of natural rubber latex
being the tree of “Hevea brasiliensis” from which coins the name
Hevea rubber. Natural Rubber is obtained from latex that exudes from
the bark of the Hevea tree when it is cut by a method known as
“tapping”. Natural rubber latex is a high molecular weight ‘polymer of
natural source consisting of isoprene unit with cis- 1-4 configuration
predominant over Trans 1, 4 configuration as shown below;
Cis-1, 4 (98.8%) Trans – 1, 4 (2.2%)
The sources of natural rubber latex occur widely in nature. There
are over 500 species but only few are of economic importance. The
most common and principal natural rubber producing tree is Hevea
Brasiliences of the family Euphorbiaceae. It contribute almost 95%of
the world’s production. Others are Ficus Elastica from India and
Palaquim from Malaysia produces Gutta parcha rubber. The plant is
grown as shrub and it is the leave which are harvested as they contain
the rubber. The Hevea Brasiliences grow to a height of approximately
60ft on suitable soil, the tap root goes deep into the soil. This ensures
firm support and resistance to drought. The bark is smooth and of
variable colour (a light brown shade predominates). The tree loses its
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foliage once a year. It requires tropical conditions i.e. 80m (230cm) of
rainfall and an average temperature of 800F (26.70C). (Imanah, 2001)
Latex is natural rubber as it is obtained from a rubber tree or any
stable suspension in water of a similar synthetic polymer. Synthetic
latexes are used to make articles from rubber or plastics by such
techniques as dipping (rubber gloves), spreading (water proof cloth)
and electro disposition (plastic coated metal). They are also employed
in paints and adhesives. (Clark et al, 2007).
Freshly tapped Hevea tree latex has a pH of 6.5 to 7.1 and density
0.98g/cm3. The total solids of fresh field latex vary typically from 30
to 40 wt% depending on clone, weather, stimulation, age of the tree,
method of tapping, tapping frequency and other factors. The dry rubber
content is primarily cis 1, 4-poly isoprene,
cis 1, 4-poly isoprene
(Uptal, 2007)
The non-rubber portion is made up of various substances such as sugar,
proteins, lipids, amino acids and soluble salts of calcium, magnesium,
potassium and copper. The solid phase typically contains 96% rubber
hydrocarbon with traces of metal salts. (Uptal, 2007).
Latex is an emulsion of poly hydrocarbon droplets in an aqueous
solution, i.e. it is a colloidal dispersion of negatively charged particles of
rubber about 1000mm (1.2u) in diameter and looks like milk. The charge on
the rubber particles stabilises the emulsion. The percentage of rubber in it is
25-35%. Latex is collected by tapping the tree in such a manner as to allow
the liquid to be accumulated in small cups, which must be collected
frequently to avoid putrefaction and contamination. This latex has the
following average composition: water=60%, rubber hydrocarbon=35%,
proteins enzymes and nucleic acid=3%, fatty acids and esters=1%, inorganic
salts=1%. (Sharma, 2007).
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The composition of the latex varies with the different parts of the tree.
The percentage of the latex decreases from the trunk to the branches and the
type of the soil and the type of the rubber tree. In the tree, rubber is formed
from isoprene (C5H8) by a biochemical reaction in which a particular type of
enzyme acts as a catalyst. The more the latex is removed, the more the plant
regenerates it. (Sharma, 2007).
The fresh latex is sieved to eliminate impurities such as leaves, bark
and dirt present in it. It is then diluted from 25-35% dry rubber content of the
latex to about 15-20% of rubber by using a hydrometer known as metrolac.
After dilution, the latex is allowed to stand for some time as a result of which
sand and sludge etc., in it get settled down. It is then coagulated by adding
acetic acid or formic acid Potassium or ammonium alum are also used as
coagulants.
1.3.1 Improvement on yield (Imanah, 2001)
The yield of NR latex can be improved by two main methods. These
are:
selective Breeding
yield stimulation
The former involved the selection of very good disease-resistant clones
and planting their vessels and or bud-grafting on another young plant.
The latter make use of substance that stimulate the flow of latex. Some
of such substance in use are:
a. Copper II tetraoxosulphate (VI), (CuSO4). Few grammes of
CuSO4 are injected into the tree as aqueous solution. This is
usually done in every 6 months. There is the danger of increased
copper content in the rubber latex and this has precluded the
commercial application/exploration of the procedure.
b. Certain derivatives of phenoxy acetic acid; 2, 4 dichlorophenoxy
acetic acid and 2, 4, 5 trichlorophenoxy acetic acid. These
substance increase the initial rate of latex exudation but reduce
the latex viscosity to encourage flow, thereby affecting the latex
produced.
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1.3.2 Constitution of latex.
The rubber particles will coalesce, of course, were it not for a
layer or sheath of non-rubber constituents, principally proteins, which
is adsorbed on their surfaces and functions as a protective colloid. From
this latex the solid rubber may be obtained either by drying off the
water or by precipitation with acid. The latter treatment yields the purer
rubber, since it leaves most of the non-rubber constituents in the serum
(Trelaor, 1975)
NR latex consist essentially of two phases; dispersed (discrete,
discontinuous or internal) phase, and a dispersion (aqueous, continuous
or external) phrase or serum. The dispersed phase is made up of
isoprene unit or particles which are rubbery in nature.
Poly isoprene unit (cis-form)
Chemical composition: freshly tapped NR latex is a whitish
fluid. The chemical composition is as shown in the table 1 below
Table 1: chemical compositions of natural rubber latex.
Chemical constituents %composition
Total Solid content (TSC) 37.0 ± 2.0
Dry Rubber Content (DRC) 33 ± 1.0
Protenous substance 1.6 ± 0.5
Resinous Substance(Soap MVA) 2.0 ± 0.5
Ash content 0.8 ± 0.2
Sugar 0.8 ± 0.2
Water 63.0 ± 2.0
(Imanah, 2001)
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Total solid content: this is the percentage by weight of the whole latex
which is non – volatile at a definite temperature in an open atmosphere.
This parameter is useful mainly as a measure of the percentage of
volatile, the principal volatile being water.
Dry Rubber Content: this is the percentage by weight of the whole
latex (usually in grams) which is precipitated by acetic acid under
closely defined conditions. It is usually the proportion of the
precipitated or coagulated insoluble material obtained under
standardised set of coagulating conditions.
Alkalinity: alkalinity in latex chemistry means the free alkali content
of a latex. The importance of such a test is explicit in the fact that the
alkalinity of a particular latex can and does affect the pH value of the
latex must be adjusted to suit the particular product under manufacture.
This means an adequate estimation of the free alkali content in the latex
under study. This test is particularly attractive for latex foam rubber
production. Standard procedures are mainly for natural rubber latex, or
synthetic lattices, pH values are often used as an indirect estimation of
alkalinity.
1.3.3 Latex preservation
Latex preservation is usually by ammonisation in the gaseous form
from pressurised cylinders, for bulk latex or liquid or small laboratory
quantity. This is usually applied almost after tapping. The long term
preservation involves preservation sufficient to ensure that latex remain
a liquid for a few hours or a few days before being processed into
various form of dry rubber. About 0.2 – 0.5% W/W of ammonia is used.
Other short term preservatives, known as anticoagulants which can also
be used are sodium sulphite, formalin (10% solution of formaldehyde
in water), KOH, boric acid (ammonia). Ammonia preservative level
required are recommended
Specific end products
Table 2: percentage of ammonia for latex preservation
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End products % Ammonia
preservative level
1. Ribbed smoke sheet 0.01
2. Pale crepe 0.01
3. Crumb rubber block 0.01
4. Latex Concentrate
a. Immediate processing whole
field latex
0.01
b. Same day 0.15
c. Next morning 0.30
5. Field latex for
a. Two day storage 0.60
b. Three day storage 0.75
c. Indefinite period storage 1.00
1.3.4.Tapping of latex
The rubber plant produces a milk-white latex that contains the
natural rubber hydrocarbon in a fine emulsion form in an aqueous
serum. After a thin shaving of bark of the Hevea tree has been cut, the
latex that comes out is allowed to flow into a cup through a spirit that
is stuck into the bark below the bottom end of the cut. A little of sodium
sulphite solution put into the empty cup before tapping helps prevent
some darkening or discoloration of the latex which may otherwise
develop as a consequence of an enzymatic reaction in the latex
involving its phenolic constituents producing the dark coloured
pigment melanin. (Premamoy, 1992)
Ethylene stimulant tapping system enhanced latex production in
gram per tapping of young-tapping of rubber trees. However, there was
no significantly different of cumulative latex production compared
with the conventional tapping system. In addition, dry rubber content
(%DRC) and girth increment of ethylene stimulation tapping system
tended to decrease from regular. Bark consumption was significantly
reduced under the stimulant application treatments. However, latex
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physiology tended to be affected under the stimulated treatments; this
may lead to negative impact in the long term. (Thongchai and Sayan,
2012)
1.3.5.Concentration of rubber latex (Uptal, 2007)
The ammonia preserved latex which is known as normal (un-
concentrated) latex is not suitable for commercial use as it contains
considerable amount of non-rubber constituents which are detrimental
to the quality of products and also contains too much water which is
costly for transportation. The latex, is therefore, concentrated to about
60% rubber solids before leaving the plantation. This concentration
process is carried out either by centrifuging creaming, electro
decantation or evaporation.
The first two processes make use of increasing the gravitational
force of the rubber particles, by applying centrifugal force on the
former by adding a creaming agent like sodium alginate, gum
tragacanth etc. in the latter process. Both these processes of
concentration result in a decrease of non-rubber content, the
centrifuging process being superior in this respect.
The concentrated latex obtained by electro decantation process
which utilises the negative charge on the tiny rubber particles, is similar
in composition to the centrifuged latex. However cost economics does
not favour this process to be exploited on commercial scale.
The evaporated latex contains all the non-rubber constituents
present in the original normal latex. It contains a small amount of
ammonia. Because of its high stability, evaporated latex is useful in
compounding heavily loaded mixes, hydraulic cement etc.
The centrifuged latex is most widely used in industry. Latex
concentrate constitutes slightly more than 8% of the global natural
supply of rubber, and about 90% of this is centrifuged.
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1.3.6. Coagulation of rubber latex (Sharma, 2007)
The latex collected from the tree is strained and a preservative
(NH3) may be added. The rubber is then separated from the latex by a
process known as coagulation, which is effected by adding acetic acid
to the diluted latex. As a result of addition of acetic acid, the rubber
hydrocarbon, which is the main disperse phase, gets coagulated in the
solid form known as crude rubber. The crude rubber is composed of
90-95% rubber hydrocarbon (C5H8), 2.4% proteins and 1.2% resins.
The latex is coagulated to about 15% rubber content and the
coagulation is carried out by removing the negative charge on the
emulsion particles by an electrolyte, usually 1-2% acetic acid, alcohol
or formic acid.
The amount of formic acid is generally such that it brings about
the pH range of 5.05-4.77. Generally 40 c.c of formic acid of 90%
strength is needed for 100 litres of latex having 12 % solids. The acid
is added in excess where rubber is coagulated to soft white mass, called
coagulum is washed. It is then treated for preparing smoked rubber,
crepe rubber, gutta percha etc.
The rubber hydrocarbon form a sheet on the surface of serum and
are separated out in the form of sheet. The sheet is squeezed by the
sweet rolls to expel the absorbed serum. The bulk of the rubber which
is sent to the market is in the form of the smoke sheets. It is
accomplished by sheeting the coagulated rubber on even shaped rollers,
with light washings and drying from 7-11 days at 40-500C in an
atmosphere of smoke produced by burning coconut husks or hard
wood. The smoke makes the rubber brown and serve as a preservative
by preventing the formation of moulds in the serum substance in the
rubber sheets. The smoked sheets are pressed and marketed. The pale
crepe rubber is made by adding NaHSO3 to the latex before
coagulation. The coagulated mass is washed and passed through speed
rollers to remove serum and the sheets are dried without smoking.
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1.4. Crude natural rubber (Sharma, 2007)
a. CREPE RUBER: it is a crude form of natural rubber and is
prepared by adding a retarder such as NaHSO3 to the coagulum.
The function of the retarder is to retard the action of oxidases
(which oxidises the rubber). The retarder also prevents the
discolouration and softening of the rubber. 0.5lbs of NaHSO3 is
sufficient for 100lbs of dry natural rubber in the latex. Now a
coagulum is allowed to drain for about 2 hours and then passed
through a creeping machine, which consists essentially of two
rollers (about 3mm apart and 50cm wide.) with longitudinal
grooves, upon which water is sprayed. The spongy coagulum
when passed through such rollers with different rotational speeds,
is converted into a sheet by undergoing shearing and masticating
action. The sheet thus obtained, possess an uneven rough surfaces
resembling crepe paper. The sheet passes through a number of
many creping machines one after the other.
b. SMOKED SHEET: this is also a variety of crude natural rubber
and can be prepared by carrying out the coagulation of latex in
long tanks 30cm deep and 1m width. The tanks are provided with
vertical grooves (about 4cm apart) on sides and fitted with the
metal plates which run across the width of the tank. Diluted latex
is poured into the tanks with plates removed and the coagulating
agent, formic acid or acetic acid is added and the mixture is stirred
thoroughly. The partition plates are then inserted into the grooves
and tanks are allowed 16 hours to stand undisturbed. As a result,
a tough slabs of coagulum are passed through a series of smooth
rollers moving at the same speed and water is sprayed
simultaneously at the centre of the roller. Slabs are allowed to
pass through 3 or 4 roller machines one after another and
clearance between the rollers is decreased from one machine to
another and the final roller has a clearance of such a design so as
to give ribbed-pattern to the final rubber sheet. Rubber patterns
on the surface of the rubber sheets exposes greater surface and
hence facilitates drying and prevents the sheets from adhering
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together on stacking. The sheets so obtained are hung in open for
hours and then hung in a smoke house at a temperature of about
40-50% for about 4 days. The amber coloured crude rubber thus
obtained is translucent because of removal of water.
1.5 Draw backs of raw rubber (Sharma, 2007)
Pure rubber is as useless as pure gold. There are a number of draw
backs of raw rubber for example:
a. Rubber is brittle at low temperature and soft at high temperature.
Thus it can be used only in limited temperature range of 10-600C
b. It is too weak to be used in heavy duty operation. Its tensile
strength is only about 200kg/cm2
c. On stretching, it undergoes permanent deformation.
d. Non-resistance to mineral oils, organic solvents and even action
of water. It has large water absorption capacity.
e. Readily attacked by strong oxidising agents such as conc H2SO4,
conc HNO3, chlorine, sodium hypochlorite, chromic acid etc.
f. On exposure to atmospheric air, it undergoes peroxidation. As a
result, it’s durability is considered decreased.
g. It is also non-resistance to non-polar solvents such as gasoline,
benzene, CCl4, vegetable oils etc.
1.6 Latex technology
Latex technology is highly specialised field that is not too familiar
to most chemist and even many rubber compounders. The art and
science of handling rubber problem is more intricate than regular
rubber compounding and requires a good background in colloidal
solution systems. While latex differs in physical form from dry rubber,
the properties of latex polymer differs only slightly from its dry rubber
counterpart. Unlike the dry rubber, which must be masticated,
(mechanically sheared) before use, the latex polymer need not be
broken down for application, thus retaining its original high molecular
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weight which results in higher modulus products. Other advantages
enjoyed by applications involving latex are lower machinery costs and
lower power consumption, since the latex does not have to be further
processed into dry form and compounding materials may simply stirred
into the latex using conventional liquid mixing equipment. (Uptal,
2007)
1.7. Characterisation of natural rubber crumb (Imanah, 2001)
Plasticity and plasticity retention index: the plasticity of the rubber
is generally related to the molecular weight of the raw rubber (or
polymer). The level of plasticity is indicative of the processabilty of
natural rubber. The plasticity retention index, PRI, is an indication of
the susceptibility of natural rubber to thermal oxidation. A high index
indicates good resistance of the material to thermal oxidation.
Dirt content: dirt content is a major criterion in the grading of natural
rubber during quality control or its conformance to the technical
specification. The emphasis placed on this criterion is warranted insular
as the existence of foreign bodies in rubber directly affects the life span
of finished articles, particularly those subjected to dynamic stress. The
purpose of this operating procedure is to describe a method of
determining the dirt content of raw natural rubber. It is not applicable
to superficial dirt resulting from contamination. It consists of the total
dissolution of the rubber in an appropriate solvent. The resulting
solution is then filtered through a gauze sieve with 45µm square
openings, and the residues are dried and weighed.
Volatile matter: in the case of natural rubber, the determination of
volatile matter is taken as a measure of moisture content, which is one
of the specifications. The principal involved in this experiment is that;
the test piece, taken from a homogenised sample, is weighed and thinly
sheeted on a laboratory mill, then dried in an oven until constant mass
is attained. The volatile matter content is calculated as being the loss of
mass during oven drying.
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1.8. Refining of crude rubber
The following processes are performed to obtain a rubber
products:
1.8.1. Break down
The polymeric chain of the rubber are broken by mastication or
kneading the warm rubber between warm rollers. During breakdown,
the rubber loses its reversibility gradually and turns plastic i.e.
Mouldable. (Sharma, 2007)
1.8.2. Compounding
This means mixing of the raw rubber with other ingredients so as to
impart the desired properties to the products suitable for particular use.
This is done during breakdown. In compounding, it is necessary to
know the service conditions to which the rubber products is to be
exposed and the ease in manufacturing the items from the rubber
compound. (Sharma, 2007)
The two major types of crumb rubber compounding methods are:
continuous and discontinuous. The discontinuous system is fairly old
and in most cases refers to Banbury mixers or roll mills. Capacities for
discontinuous system range from 250 to 5,000 kg/hr and a sizeable
investment are required. However an efficient processing system will
allow the compounder to operate economically at high volume. On the
other hand a continuous compounding system has capacities of up to
2,500 kg/hr. For quality and economics, continuous systems offer
better uniformity of products with less batch-to-batch variation than a
discontinuous system. (Brydson, 1978)
In latex technology, concentrated latex is first blended with the
various additives as required for different applications. The blending of
different additives is known as compounding. Latex compounding
involves not only the addition of the proper chemicals to obtain
optimum physical properties in the finished products but also the proper
control of the colloidal properties which enables the latex to be
transformed from the liquid state into the finished product. Viscosity
control in the latex is very important. The particle size of the latex has
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a great effect on viscosity. Large particles generally results in low
viscosity. Dilution with water is the most common way to reduce
viscosity. Certain chemicals such as trisodium phosphate, sodium
dinaphthyl methane disulfinate are effective viscosity reducers. (Uptal,
2007)
In addition to rubber, the major part of the compounded rubber,
the following raw materials are also mixed, depending upon the service
conditions of the item to be made from it, in the processing methods
like mixing, calendaring, extruding etc. (Sharma, 2007)
1.8.2.1. Reinforcing agents
Inorganic substances such as zinc oxide, carbon black, magnesium
carbonate or clay are added to improve the tensile strength of the
rubber. They provide strength and rigidity to the rubber products.
Carbon black is the most important reinforcing agent. It increases the
strength of the rubber to such an extent that it can withstand a pull of
4,000 pounds to a square inch without breaking. It also improves
resistance to abrasion and is exclusively used in tyre industry.
Acetylene black produces electrically conducting rubber. “Soot of ethyl
silicate”, which is a white powder and resembles carbon black particles,
has now been found to be a substitute for carbon black (Sharma, 2007)
1.8.2.2. Inert fillers
Fillers improve the hardness and serve as diluents. Manufacturers
investigated the possibility of diluting rubber at a time when the cost
was comparatively high. They surprisingly observed that hardness,
strength and resistance to abrasion may be greatly improved by adding
fillers, such as limestone, talc, soft carbon black, barytes, zinc oxide
etc. (Sharma, 2007).
1.7.2.3. Softeners and Extenders
These helps in mastication and include petroleum oils, pine tars, coal
tar fractions, palm oil etc. they also serve as lubricants and plasticizers.
In general, softeners and plasticisers are added to give the rubber
greater tenacity and adhesion. The term softener includes a large
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number of substances which have been used for a number of purposes.
For example, processing aids for uncured stock are used to lubricate the
stock (to impart oiliness to the stock to prevent excessive sticking to
mill or calendar roll). In other to produce tack in the stock, softeners
are added to cured stock. The tack is the property which makes possible
the assembling of different rubber pieces into a composite article.
Softeners also help in dispersing the pigments in the stock. In other to
make the product more elastic, elasticizers are added to cured stock
where they improve flexibility, rebound, hysteresis and compression on
set. (Sharma, 2007)
In application like toy, balloons, softeners are added to soften them so
that they may be inflated. Softening agents in general used are liquid
paraffin, paraffin wax and steric acid. (Uptal, 2007).
1.8.2.4. Antioxidants or Age resistors
Rubber decomposes when exposed to air, heat, light or oxygen.
Antioxidants protect the rubber goods from attack by air, heat, light and
even ozone in the atmosphere. Commercial antioxidants are generally
either of the amine type or of the phenolic type. Example are Nphenyl-
2-naphthylamine, alkylated diphenylamine, p-p-diamino biphenyl
methane etc. The amines are strong protective agents and are widely
used in tyres and other dark coloured goods.in the case of light coloured
goods, alkylated phenols and their derivatives are used to give
moderate oxidant protection with minimum discolouration. (Sharma,
2007).
Because of the great surface area exposure of most latex products,
protection against oxidation is very important. Many applications
involve light coloured products, which must not darken with age or on
exposure to light. Non-staining antioxidants such as hindered phenols
(styrenated phenols) must be used. Where staining can be tolerated,
amine derivatives such as phenylene diamino, phenyl beta-
naphthylamine, ketone-amine condensates may be used. These have
good heat stability and also effective against copper contamination,
which cause rapid degradation of rubber. (Uptal, 2007).
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Antioxidants are substances that slow the rate of reactions.
Various antioxidants are used to prevent the deterioration of rubber,
synthetic plastics, and many other materials (Clark etal, 2007).
1.8.2.5. Colour and pigments
Inorganic substances example, iron oxide, titanium dioxide and other
coloured pigments or organic dyes are used for colouring purposes.
These are added to give the rubber product the desired colour. TiO2 is
the usual pigment for white products. Chromium oxide, ferric oxide,
antimony sulphide, lead chromate are added to get green, red, crimson
and yellow colour products respectively. Organic lake colours, carbon
black, zinc oxide, calcium carbonate etc. are also used in finely divided
form. For example. The manufacture of sponge rubber with non-
interconnecting air sacks, backing powder material such as NaHCO3 is
added. (Sharma, 2007).
1.8.2.6 Vulcanising agents
The most important vulcanising or curing agent is still sulphur. The
other vulcanising agents are sulphur mono chloride, selenium,
tellurium, thiuram sulphides, polysulphite polymers etc. (Sharma,
2007).
Curing or vulcanisation which involves the chemical reaction of the
rubber with sulphur in presence of an activator such as (zinc oxide) and
accelerator, manifests itself in an increase in strength and elasticity of
the rubber and an enhancement of its resistance to ageing.
Vulcanisation of latex may be effected by either of the two ways;
A. The rubber may be vulcanised after it has been shaped and dried,
or
B. The latex may be completely vulcanised in the fluid state so that
it deposits elastic films of vulcanised rubber on drying. The latter
process, however does not yield products of high quality and is
resorted to only in the production of cheaper articles, example toy
balloons. (Uptal, 2007).
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1.8.2.6. Accelerators
Organic compounds having nitrogen or sulphur or both are used to
increase the rate of vulcanisation of rubber from several hours to few
minutes. In addition, less sulphur is required and more uniform product
is obtained. The addition of accelerators has also been found to
decrease vulcanisation temperatures. Examples of accelerators are 2-
mercaptobenzothiozole, benzothiozolyl disulphide, zinc di ethyl
dithiocarbomate, tetramethyl thiuram disulphide, 1, 3-diphenyl-
guanidine etc.structures of some accelerators are given below:
Diphenyl guanidine
N
C.SH
S
Mercaptobenzene thiazol
N N
C S S C
S S
Benzothiazyl disulphide
The problem of scorching or premature vulcanisation is rarely
encountered in practical latex work and hence ultra-accelerators such
as zinc diethyl dithiocarbamate (ZDC) alone or in combination with
zinc salts of mercaptobenzothiazole (ZMBT), tetra methyl thiuram
disulphide (TMTD), polyamines and guanidines are used. The latter
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two also function as gel sensitizers or secondary gelling agents, in the
preparation of foam rubber. The doses of the vulcanising ingredients
are adjusted according to the requirements of the end products. (Uptal,
2007).
Guanidnines, when used alone, have a long scorch time and a long
vulcanisation time required. Generally, when used as the sole
accelerator, the properties of the compound, especially resistance to
ageing, are poor. However, in combination with accelerators such as
mercapto accelerators, they have significant effect on the behaviour of
the secondary accelerator increasing crosslink density and
vulcanisation rate. They behave synergistically with thiurams,
dithiocarbamates and to a lesser extent with sulphonamides giving
considerable activation effects. Di-o-tolylguanidine can be used as a
plasticizer for polychloroprene. The particle size of the guanidines can
be critical; above 200mesh they will not disperse readily.
1.9. Vulcanisation
Vulcanization is the conversion of rubber molecules into a
network by formation of crosslinks. Vulcanizing agents are necessary
for the crosslink formation. These vulcanizing agents are mostly
sulphur or peroxide and sometimes other special vulcanizing agents or
high energy radiation. Since vulcanization is the process of converting
the gum-elastic raw material into the rubber-elastic end product, the
ultimate properties like hardness and elasticity depend on the course of
the vulcanization. (Zorge, 2011)
Vulcanisation is a process for which rubber is hardened with
sulphur or sulphur compounds (Clark et al, 2007).
Vulcanisation is carried out by heating crude rubber in presence of
sulphur or dipping it in solution of S2Cl2 in CS2. Vulcanisation depends
upon:
1. Amount of sulphur used: by increasing the amount of sulphur, the
rubber can be hardened
2. Temperature
20
3. Duration of heating.
(Sharma, 2007).
The crude rubber is intimately mixed with about 3% ground
sulphur, accelerator and activator and then heated to about 1500C (for
tyres it is 1530C). Vulcanisation is a progressed reaction and is only
allowed to a definite stage. The time of vulcanisation and temperature
is reduced by adding accelerators and activators. When the amount of
sulphur is increased, the rubber is hardened and by increasing the
percentage of sulphur to 40-45%, a non-elastic substance, called
ebonite is obtained. Vulcanisation is a free radical initiated chain
reaction. The free radical is furnished by the accelerator by removing
the hydrogen ion from isoprene mesomer forming an active centre.
Rubber on vulcanisation undergoes great reduction in plasticity,
whereas elasticity is largely maintained. During vulcanisation, in many
cases the double bonds break and form chains with sulphur which in
turn links with other chains. The sulphur atoms used in bridge
formations maybe one to eight, but are usually two. Actually, in the
vulcanised process, sulphur add at the unsaturated bonds forming
bridges and crosslinking of the linear molecules into practically infinite
three dimensional structures takes place. (Sharma, 2007).
The double bonds in the rubber molecule are highly important,
since apparently by providing reactive allylic hydrogens – they permit
vulcanisation, the formation of sulphur bridges between different
chains. These cross-links make rubber harder and stronger, and do
away with the tackiness of the untreated rubber. (Morrison etal, 2002)
The extent of stiffness of vulcanised rubber depend on amount of
sulphur added. For example, a battery case rubber may contain even
30% sulphur, but a tyre rubber is a chemical process applied to both
natural and synthetic rubbers in order to improve elasticity and other
mechanical properties. Vulcanisation of raw rubber with sulphur can
be representing as
21
(Sharma, 2007)
The reaction between rubber (usually the diene rubbers- homo
polymers or copolymers) and sulphur is very slow. Until about the first
two decades of the 20th century, some inorganic oxides (of Pb, Ca, Zn,
Mg, etc.) were used to achieve sulphur vulcanisation of natural rubber
at faster rates at high temperatures. Even then, heating for long hours
using at least 8-10 parts of sulphur for 100 parts of rubber was
necessary (Premamoy, 1992).
The technology of sulphur vulcanisation as practiced today is
radically different and far more efficient and economical. This has been
possible through incorporation of small doses of one or more organic
substances commonly known as accelerators. A variety of accelerators
are currently available and they are more approximately classified
according to the speed of curing, they are classified as slow accelerator
and ultra-accelerator. The principal chemical types are guanidines,
thiazoles sulphonamides, dithiocarbamates, thiuram
sulphides,xanthates and aldehydeamines. (Premamoy, 1992).
The elasticity of certain materials including vulcanised natural
rubber at ambient temperatures is well known. When such materials are
22
subjected to a stress, they experience a rapid and clearly defined strain,
and this strain disappears immediately the stress is removed. Moreover
they undergo very large deformation under an applied stress;
vulcanised natural rubber, for example, can be extended to 1500% of
its original length. Such behaviour is the essence of rubber like
elasticity. In reality, the ideal rubber does not exist. Real rubbery
materials do have a small element of viscosity about their mechanical
behaviour, even though their behaviour is dominated by the elastic
element. Even so, real rubbers only demonstrate essentially elastic
behaviour, i.e.instanteneous strain proportional to applied stress, at
small strains. Such rubberlike elasticity is exhibited only by
macromolecular materials, generally those in which there is a small
amount of cross linking. Raw natural rubber, for example, is not elastic
until it has been lightly cross linked. This is achieved by incorporating
a small amount of sulphur of the order of a few percent by weight,
together with an appropriate accelerator, followed by heating. This
treatment with sulphur, so called vulcanisation, links the
macromolecules at particular points to form a high network structure
which gives the material the elasticity generally regarded as
characteristics of rubber. (Nicholson, 1997)
Depending on the sulphur accelerator ratio the sulphur vulcanising
systems can be categorised as: (a) the conventional or high sulphur
vulcanisation system (CV) where sulphur is added in the range of 2-3.5
parts per hundred rubber (phr.) and the accelerator in the range 1- 0.4
phr. [b] the efficient vulcanising (EV) system where sulphur is added
in the range of 0.3-0.8 phr. and accelerator in the range of 6.0-2.5 phr.
and (c) the semi efficient (SEVI system where sulphur is added in the
range 1-1.8 phr. and accelerator in the range 2.5-1 phr", As the CV
system has got greater amount of sulphur compared to the accelerator
the possibility of polysulphidic linkage formation is higher. At higher
temperatures the polysulphidic linkage may break to mono and
disulphidic. This explains the reversion at higher temperatures, which
leads to low strength and modulus. Properties like compression set and
thermal stability are better for EV systems primarily due to the lower
amount of polysulphidic linkages. (Susamma, 2002)
23
1.10. The elastic properties of rubber
The well-known properties natural rubber is displayed only at
temperatures above the glass transition temperature, which is -700C;
whether synthetic or natural, provided it is cross-linked, displays
comparable properties above its glass transition temperature. These
properties are stated briefly:
Extremely high extensibility, frequently up to 10 time, generated
by low mechanical stress; and
Complete recovery after mechanical deformation.
To this should be added a 3rd property, which is not well known.
High extensibility and recovery are due to deformation induced
changes in entropy (McCrum etal, 2003).
An ideal classical, rubbery properties are displayed by polymers
cross-linked by valence bonds (main chain bonds or sulphur bridges)
these are termed chemical cross-links. Physical crosslinks are also very
important in many useful rubbery materials. Physically crosslinking the
chain, the chains are not chemically attached one to another, but are
effectively pinned together in one of 3 ways:
By the relative molecular mass being so high, the chains become
grossly entangled – these entanglements act as physical cross
links.
By the chains entering and leaving crystals – the chains are pinned
together in the crystal or
By the chains entering rubbery domains from glassy domains.
(Buckley et al, 2003)
An ideal rubber consists of flexible cross linked polymer chains
undergoing violent liquid like motions no matter what the type of
cross-linking, physical or chemical, the elastomers all have this
in common: the macro – molecules between cross-linking
undergo extremely rapid molecular movement. (Bucknall et al,
2003)
24
1.11. The role of filler networking in dynamic properties of filled
rubber. (Wang, 1999)
One of the consequences of incorporation of filler into a polymer
is a considerable change in the dynamic properties of rubber, both
modulus and hysteresis. This phenomenon has been investigated in
depth especially in relation to rubber products. It has been recognized
that, for a given polymer and cure system, the filler parameters
influencing dynamic properties in different ways, i.e. multiple
mechanisms may be involved. Among others filler networking, both its
architecture and strength, seems to be the main (though not only)
parameter to govern the dynamic behaviour of the filled rubber.
1.12. Importance of rubber (Sharma, 2007)
Elasticity and rubberiness: this is the most important property of
rubber. On stretching it elongates to nearly 200% of its original form
when the stretching force is removed. For example a rubber band can
be stretched to 5 to 10 times its original length and as soon as the
stretching force is released, it return to its original length.
Flexibility: this is another property of rubber, as a result of which a
rubber sheet is almost as flexible as a piece of cloth.
Strength and toughness. These are two properties due to which the
elastic property may be put to use even under abnormal condition.
Rubber is highly impermeable to water and air. This property is used
to hold water as in hoses and rubber bottles or to keep water out as in
rain coat.
Rubber is highly resistant to cutting, tearing and abrasion over a wide
range of temperatures.
Rubber is not attacked by atmospheric gases and chemicals have no
corrosive effect on it.
25
Uncured rubber has the property of tackiness when two fresh surfaces
of milled rubber are pressed together, they coalesce to form a single
piece. This is called tackiness. For example, the separate components
of green tyre are held together by their tackiness and when cured, they
fuse together to make a single piece.
1.13. Properties of natural rubber vulcanizates
NR vulcanizates combine a range of properties which are of great
interest from a technological point of view.
1.13.1. Mechanical properties
Hardness: natural rubber can be produced in a wide range of hardness,
from very soft up to ebonite hardness. This can be done by changing
the amount of fillers and softeners in the compound on the one hand,
or through the sulphur concentration on the other. Leathery
compositions, obtained by using 10 to 20 phr of sulphur have, however,
poor strength and aging properties. Hardness range is of little interest
technologically, except for a few applications like floor tiles and roll
covers. (Werner, 1989)
Hardness represents the elasticity of the material. The lower the
hardness the more elastic the material is. Two scales are normally used:
Shore-A and micro-IRHD. They are roughly the same. The instruments
used for the measurement are:
Durometer: a pointed conical indentor when pressed against a sample,
is pushed back into the case of the tester against a spring and this
motion is translated into movement of the pointer on the dial. The
harder the sample the farther it will push back the indentor point and
the higher will be the numerical reading on the scale. The unit is Shore-
A.
IRHD tester: a dead-load is applied to the indentor for a specific time
and the hardness is obtained from the depth of the indentation. (Zorge,
2011)
Tensile strength: Natural rubber vulcanisates has a high tensile
strength, even in gum vulcanisates. This property is exploited in the
design of soft, thin walled, and very strong products, such as surgical
26
gloves, prophyl axis, or balloons. By adding reinforcing fillers to
compounds, the tensile strength increases. (Werner, 1989)
Tensile strength is the maximum tensile stress reached in stretching a
test piece, usually a flat dumbbell shape, to its breaking point. By
convention, the force required is expressed as force per unit area
of the original cross section of the test length. Elongation, or strain, is
the extension between bench marks produced by a tensile force applied
to the test piece and is expressed as a percentage of the original distance
between the marks. Elongation at break, or ultimate elongation, is the
elongation at the moment of rupture. (Zorge, 2011)
Elongation at break: the ultimate elongation depends, naturally very
much on the nature and amount of fillers in the compound, and on the
degree of vulcanisation. (Werner, 1989).
Abrassion Resistance: A test piece is pressed against a rotating drum
covered with an abrasive cloth. The loss in weight (volume) is
measured after a certain number of revolutions and gives an indication
of the abrasion resistance.
Tear Resistance: the tear resistance is also influenced by the strain
crystallization, and it is therefore very good. (Werner, 1989)
Elastic rebound. NR vulcanisates have a high rebound. With small or
conventional amounts of zinc oxide in the compound, rebound values
of 70% or more are achievable with NR vulcanisates, but this value
reduces somewhat, if natural rubber is compounded with reinforcing
fillers. (Werner, 1989)
1.13.2. Damping properties, dynamic fatigue resistance
The favourable elastic properties manifest themselves in very low
damping (low hysteresis), and a low heat build-up in dynamic
deformations. This behaviour, combined with very short relaxation
times, qualify natural rubber especially for products which function in
dynamic applications. These include vibration and suspension elements
and tires. The dynamic fatigue resistance of NR vulcanisates is also
excellent. (Werner, 1989)
27
1.14.3. Heat and age resistance
Heat resistance: the heat resistance of NR vulcanisates is insufficient
for many technical applications. According to VDE (Association of
German Engineers) tests, the heat resistance as measured by the
temperature at which a vulcanisates has a % maximum elongation still
in excess of 100% after 20,000 hours, is about 700C. The heat resistance
of many NR vulcanisates are primarily determined by the choice of
vulcanising agents, vulcanisation conditions, fillers, and secondarily,
by the choice of protective agents. (Werner, 1989)
Aging resistance: in order to obtain a good aging resistance of NR
vulcanisates, it is necessary to use protective agents in the
compounding, and to use thiazol accelerators in short cure cycles with
not too high temperatures. However, even under optimum conditions,
the aging resistance of NR vulcanisates does not reach that of most
synthetic rubber vulcanisates. (Werner, 1989)
Weather and ozone resistance: even after vulcanisation, the NR has
double bonds in the polymer chain. Therefore, it has an insufficient
weather and ozone resistance, particularly in light coloured
vulcanisates. This can be improved if carbon black is added to the
compound, or especially if paraffin, and micro crystalline waxes, or
certain enol ethers are added. (Werner, 1989)
1.13.4. Low temperature flexibility
Even without the aid of special softeners, the low temperature
flexibility of NR vulcanisates is better than that of most synthetic
vulcanisates. (Werner, 1989)
1.13.5. Compression set
At ambient temperature and slightly elevated temperature, the
compression set of NR vulcanisates is relatively low. At lower
temperature the compression set appears to be poor due to the tendency
of the rubber to crystallize, while at higher temperatures, the poor heat
resistance of the NR vulcanizate has a detrimental effect on the
compression set. (Werner, 1989)
28
1.14.6. Swelling resistance
Since NR is non-polar, its vulcanisates have little resistance to swelling
in non-polar solvents. When in contact with mineral oils, benzene, and
gasoline, the volume of NR vulcanizates increases several hundred
percent.in alcohols, ketones, and esters, the vulcanizates swell less,
however. (Werner, 1989)
1.13.7. Electrical properties
When properly compounded, the electrical properties of NR
vulcanisates are also unique. Specific resistivities of 1016 Ohm-cm can
be obtained, and thus, NR qualifies readily as electrical insulator.
(Werner, 1989)
1.14. Rubber bullet
Rubber bullets (also called rubber baton rounds) are rubber or rubber-
coated projectiles that can be fired from either standard firearms or
dedicated riot guns. They are intended to be a non-lethal alternative to
metal projectiles. Like other similar projectiles made from plastic, wax,
and wood, rubber bullets may be used for short range practice and
animal control, but are most commonly associated with use in riot
control and to disperse protests. These types of projectiles are
sometimes called baton rounds. Rubber projectiles have largely been
replaced by other materials as rubber tends to bounce uncontrollably.
Such kinetic impact munitions are meant to cause pain but not serious
injury. They are expected to produce contusions, abrasions, and
hematomas. (Wikipedia encyclopaedia)
However, they may cause bone fractures, injuries to internal
organs, or death. In a study of injuries in 90 patients injured by rubber
bullets, one died, suffered permanent disabilities or deformities and
41 required hospital treatment after being fired upon with rubber
bullets. (Wikipedia encyclopaedia).
Rubber bullets are one of the less-lethal (nonlethal) weapons, which are
increasingly used to incapacitate dangerous individuals, avoiding use
29
of firearms. An autopsy examination of a man who was shot with
improved rubber bullets revealed that the bullet caused pulmonary
contusion. The bullet was 30 g in weight and consisted of a sponge
foam nose with 40-mm diameter and a plastic body. He was not
incapacitated and died of suicidal gunshot wound. The case raised a
question as to how severe an injury is necessary to deter a person
without causing death. A variety of rubber bullets have been used in
the world, and they have occasionally produced severe or lethal
injuries. A review of the literature demonstrated that the feature of
injuries appeared to be related to the type of missile. It becomes more
important for a forensic pathologist to be familiar with rubber bullets
and injuries caused by them as the use of less-lethal weapon increases.
(Masahiko et al, 2009)
The first rubber bullet was introduced by British forces in Northern
Ireland in 1970.It resembled a baton made of rubber, being 15 cm in
length and approximately 140 g in weight. The bullet was highly
unstable in flight and lost energy very rapidly because of the trembling
movement and high wind resistance. Over 55,000 rubber bullets were
fired in Northern Ireland between 1970 and 1975.9 The death rate was
estimated at one in 16,000 rounds used, and the rates of serious injury
and disability were one in 800 and 1900 rounds, respectively. (Millar
et al, 1975)
1.15 Ebonite
Ebonite was a brand name for very hard rubber first obtained by
Charles Goodyear by vulcanising rubber for prolonged periods. It is
about 30% to 40% sulphur. Its name come from its intended use as an
artificial substitute for ebony wood. The material is known generically
as hard rubber and has been formerly called “vulcanite”, although that
name now refers to the mineral vulcanite. (Wikipedia encyclopaedia).
When rubber is heated for a prolonged period with a large amount
of sulphur (up to 40 – 45 parts), a stiff, rigid and hard product is formed
and it is commonly called hard rubber or ebonite. Presence of
accelerator make the ebonite compound scorch – prone. Slow or
delayed action accelerators are preferred. Besides rubber, sulphur and
30
accelerator, other ingredients commonly used are fillers in the form of
factice, reclaim, bitumen, finely ground coal dust and ebonite dust. For
full saturation of natural rubber, binding of minimum of 32% sulphur
corresponding to a product of empirical formula (C5H8S) n is to be
expected. But a much higher proportion of combined sulphur (up to 40
– 45%) found in most commercial ebonites from natural rubber points
to the formation of limited polysulphide cross-links and some intra-
molecular cyclic di or polysulphide structures. Polychloroprene is used
to impart toughness to ebonite. (Ghosh, 1992).
1.15.1 Properties of ebonite
Before considering the manufacture of ebonite in detail, it is important
to draw attention to some outstanding properties:
a. Ebonite is so stable chemically that ageing presents no problem
and ample life is normally achieved without the use of
antioxidant.
b. Reinforcement by the use of carbon black or mineral ingredients
is not possible. These materials affect the mechanical strength
adversely, or at least do not improve it.
c. Owing to the dark colour of the basic material when vulcanised,
the possibilities of pigmentations are very limited, and any
departure from black involves the sacrifice in some physical
properties.
d. Most properties of ebonite are comparatively insensitive to the
quality of the rubber hydrocarbon used, provided that it is free
from foreign matter.
e. Times of vulcanisation are normally very long compared with
those of soft rubber.
f. There is a large reduction of about 6% in volume on
vulcanisation, which necessitates large allowance for shrinkage
and special methods in moulding.
g. The chemical combination of rubber and sulphur in proportions
used in ebonite is strongly exothermic. Since all ebonite mixing
contain a high portion sulphur and usually of ebonite dust, the
consistency of the compound during processing prior to
vulcanisation is never that of a “pure gum” stock.
31
h. In spite of the high proportion of the sulphur used, blooming does
not occur. (Naunton, 1961)
Whether used for mechanical or electrical purposes, ebonite
should not soften and yield under pressure at an unduly low
temperature. The extent of deformation at a given elevated
temperature is variously termed yield, plastic yield or cold flow, and
the temperature at which a predetermined yield occurs under a
standard condition of test is referred to as yield temperature, plastic
yield temperature or softening point (Naunton, 1961)
The shape of the load - elongation curve for ebonite resembles
that of metals in that after an initial stage of elastic extension it
shows a fairly definite yield point, after which much of the
elongations takes place rapidly and irreversibly with little further
application of load. Moreover, this irreversible extension takes place
only in a portion of test specimen, so that the tensile breaking
elongation or its equivalent in the other types of test as judged by
relative movements between the points of application of the load are
of little significance. Within the elastic range, Young’s modulus may
be determined, but it is not a value which is often called for, in fact,
it varies with composition and vulcanisation over a small range in
comparison with the great variation which can be produced in the
“modulus” of soft rubber (Naunton, 1961).
1.15.2 Ebonite products
Ebonite products may broadly be classified according to their
primary function as follows:
1. Mechanical, for example water-meter components, pipe stems,
fountain pens, piston rings for hot water pumps, textile machinery
accessories, combs and surgical apparatus.
2. Chemical, for example battery boxes and separators, chemical
pipe work and accessories, pumps for chemicals, tank linings,
buckets and roller coverings.
3. Electrical, for example sheets, rods, tubes and mouldings for the
machinery of electrical insulating components, such as
32
inductance coils, plugs and sockets, x-ray cable terminators and
handle coverings for electricians tools.
There are also various products which do not fall into this
classifications, for example cellular ebonite for thermal insulation of
refrigerators, or for floats.
Although there is a variety of formulations, most ebonites fall into one
or other of the following types:
a. Unloading ebonite: this may be of high grade containing
nothing but rubber, sulphur and high grade ebonite dust, or in
cheaper grades many contain materials such as reclaim, factice
and bitumen, but with no deliberate addition of mineral fillers.
b. Loaded ebonite: in this class, mineral fillers are added to
produce a harder materials or to reduce deformation at elevated
temperature.
c. Flexible ebonite: these are usually unloaded, but modified
either by the use of a lower proportion of sulphur or by
inclusion of modifying ingredients, such as Polychloroprene,
polyisobutylene or butyl rubber. The object may be either to
make the material softer at normal temperatures or to increase
the resistance to impact.
d. Special products: for example cellular and micro porous
ebonites. (Naunton, 1961)
1.16.3 Vulcanisation to achieve ebonite
Vulcanisation with sulphur present takes place when 0.5 – 5 parts
(by weight) of sulphur is combined with 100 parts of rubber. If the
reaction are allowed to continue until considerably more sulphur has
combined (say 30 – 50 parts per hundred parts of rubber), a rigid, non
– elastomeric plastic known as hard rubber or ebonite is formed.
Tensile strength reaches a maximum with elongation remaining high,
in the early soft cure region, then both fall to much lower values with
increasing amount of combined sulphur. (Billmeyer, 1984).
If rubber is heated for lengthy periods of time with larger amounts
of sulphur than are used for vulcanisation, the product is, at first, very
33
cheesy and weak – the so-called rotten rubber stage. On further heating,
however, more sulphur combines and a hard rubber product is formed,
termed variously “ebonite”, “vulcanite”, or “hard rubber”. Work, has
shown, however, that as much as 43% sulphur can be combined with
rubber, from which it must be assumed that it is attached in both
polysulphide crosslinks and some intramolecular cyclic structures.
(Blow, 1971).
Heating requires to achieve ebonite varies up to 10hours at 1500C;
some accelerators in particular diphenylguanidine are effective in
reducing the cure time. The reaction is strongly exothermic, and this
fact must be taken into account in moulding and forming operations.
The volume loss on vulcanisation is very high, approximately 6%.
Unlike their behaviour in soft rubber, carbon black and mineral filler,
reduce the strength of the material and the only purpose in their
incorporation is to minimise softening, or more exactly the deformation
under load at elevated temperatures. Reclaim, factice, bitumen, and
especially finely ground ebonite dust are, however, used as fillers to aid
processing and to vary final properties. Polychloroprene imparts
flexibility to ebonite. (Blow, 1971).
During the cure of a sample ebonite of a given rubber-sulphur
ratio, the best properties in most respects are obtained by given a full
cure. If the temperature of cure is changed, the rate of combination of
sulphur increases with temperature by a factor of about 2.3 per 100C.
Rate of cure: vulcanisation of ebonite normally occupies some hours,
and it is necessary for economic reasons to reduce this time as much as
possible by the use of high temperatures and of accelerators.
Accelerators are useful both in causing a quick set up in the early stage
of cure and also in completing the combination of sulphur in the later
stage; but the range of activity is less than in soft rubber, and the order
of activity of various types of accelerator is not the same. Zinc oxide is
not effective in activating organic accelerators, but acts as a retardant
under some conditions. Diphenylguanidine (DPG), ethylenediene
aniline and tetramethylthiuram disulphide has been used with little
effect on tensile strength. The rate of set up, the acceleration of sulphur
34
combination in the later stages of cure, the rate of attainment of good
physical properties and the maximum safe rate of cure without damage
by the exothermic reaction have been studied. Considering all these
aspects, butyraldehyde-amine or DPG in combination with magnesium
oxide were selected as the preferred accelerator systems. Although not
acting chemically as accelerators, mineral rubber, ebonite dust and
fillers also assisted in setting up quickly in moulds. The effect of
ebonite dust on the course of vulcanisation is complicated. It absorbs
and reacts with some of the elemental sulphur in the mixing, reduces
the available for combination with the new rubber and so delays the
attainment of a given state of vulcanisation. Unless additional sulphur
is added, the development of the full physical properties may be
prevented, however long the cure. The dust does, however increase the
rate at which the stock sets up to a hardness sufficient for removal from
a mould. (Naunton, 1961).
In the Rubber Industry the effects of compound variations on
curing characteristics are important in compound development studies
or production control. In compound development, the composition of
the ingredients can be varied until the desired vulcanization
characteristics are achieved. For all this, the Computerized Rheometer
with Micro-processor temperature controls is an equipment of vital
importance. The Rubber Compounder feels handicapped without a
Rheometer. The inventions of new Polymers & Rubber Chemicals
leads the compounder to an embarrassing position regarding their
choice & use. The Rheometer is an only equipment in the Rubber
Industry which helps the Compounder to choose the right material and
its appropriate dose to meet the end requirements of the product. The
Rheometer not only exhibits the curing characteristics of the Rubber
Compound but it also monitors the processing characteristics as well as
the physical properties of the material. The “Cure Curve” obtained with
a Rheometer is a finger print of the compound’s vulcanization and
processing character. (Singhal, 2003)
35
CHAPTER TWO
APPRATUS MATERIALS AND METHODS
2.1. APPARATUS
Every apparatus used for this work were from Rubber Research
Institute Iyanomo (Imanah, 2001)
1. Compression moulding machine: TECHNO LOIRE PLC 50T-3P.
Made in England.
2. Banbury Pullen 2-Roll Mill. BR-1600. Made in England.
3. Wallace Abrader. Model-Wallace Ref A2. Made in England.
4. Pocket Durometer Hardness Tester. ZHT 2093. Made in England
5. Rheometer: ALPHA TECHNOLOGIES. Oscillating Disc
Rheometer ODR 2000 MODEL. Made in France.
6. Oven: GRIFFEN 300FC. Isuzu Seisakusho Co. LTD Tokyo
Japan.
7. Metrolac – zeal. SPGRT6628. Made in England
8. Zwick/Roell tensometer: 300 series electromechanical test
machines. Made in England.
9. Metal Metrolac jar: made in Nigeria.
10. Measuring cylinder: SPG1000 mL graduated. Made in England
11. Plastic bowl: Made in Nigeria
12. Weighing balance: Sartorius Ag. Gottingen.BP 1215. Made in
England
13. Flat bottom glass dish
14. Stop watch: 31305 model. Made in China
15. Wallace Plastimeter: P12E. S/no C97008/28. Made in England.
16. Grinder – Apex grinder. Mesh 100.
17. Micrometre Screw gauge. Model 196A6Z. Made in Germany.
18. Microtome (Punch): model dumbbell punch C88036.
19. Scissors
20. Unglazed non-gummed acid free cigarette paper: King size
Rizla, made in England
21. Desiccator. Product number-Z553808. Made in England.
22. Petri dish
36
23. Conical flask: Pyrex SPG 1000ml graduated. Made in England
24. Hot plate. Clifton hot plate. HP1.30E. made in China.
2.2. MATERIALS USED
1. Ammonia (NH3): obtained from rubber research institute
Iyanomo, and was used as obtained.
2. Natural rubber Latex: obtained from rubber research institute
Iyanomo and was used as obtained.
3. Distilled Water (H2O): obtained from rubber research institute
Iyanomo and was used as obtained.
4. Crumb rubber: obtained from the coagulation of latex,
followed by other procedures in rubber research institute
Iyanomo
5. High aromatic hydrocarbon white spirit: obtained from rubber
research institute Iyanomo and was used as obtained. Used to
dissolve the peptizer during preparation of peptizer solution.
6. Xylyl mercaptan (XM): obtained from rubber research institute
Iyanomo and was used as obtained. It was used as peptizer.
XM
7. Steric acid (crystalline shining powder) CH3(CH2)16CO2H:
37
8. Magnesium oxide (MgO) (fine powder) – as activator –
Obtained from rubber research institute Iyanomo and was used
as obtained.
9. Di phenyl guanidine (DPG) – as an accelerator - obtained from
rubber research institute Iyanomo and was used as obtained.
Diphenylguanidine
10. Sulphur (powder): as a cross-linking agent. Obtained from
rubber research institute Iyanomo and was used as obtained.
11. Polymerised 2, 2, 4-trimethyl-1, 2-dihydroquinoline (TMQ):
as an anti-oxidant. Obtained from Rubber Research Institute
Iyanomo and was used as obtained.
TMQ
12. Ebonite dust: produced and was used as produced. As a
setting agent.
38
2.3. METHOD
2.3.1 Latex characterisation.
These tests were carried out in Quality Control Laboratory,
Rubber Research Institute of Nigeria Iyanomo (RRIN)
2.3.1.1. Dry rubber content (metrolac method)
1. One litre of latex was mixed with two litres of water in a plastic
bowl and was stirred thoroughly.
2. One litre of the above mixture was measured out into a metrolac
jar.
3. The metrolac was slowly dipped into the above one litre of the
mixture.
4. The value at the mark which the upper part of the mixture made
on the metrolac was taken and was used to calculate the dry
rubber content of the rubber in percentage. (Reji, 2009)
2.3.1.2. Total solid content (TSC).
1. The weighing balance was set/standardised.
2. The flat bottomed glass dish was weighed and recorded.
3. 2.53g of latex was weighed and was introduced into the weighed
flat bottom glass dish.
4. The latex above was dried in an oven for 80 minutes at 1000C
while covered with a glass thimble.
5. The above sample was weighed and placed back into the oven
6. The drying and weighing was continued until a constant weight
was obtained. (Imanah, 2001)
2.3.2. Crumb Characterisation.
These tests were carried out in Quality Control Laboratory, Rubber
Research Institute of Nigeria Iyanomo (RRIN)
2.3.2.1 Plasticity (P0) and plasticity retention index (PRI)
The test was carried out as shown in the stages below
A. Setting/adjusting the instruments
39
- The plastimeter was set by following the instruction on the
plastimeter screen.
- The oven was switched on 4 hours before the experiment
commenced, so that it can be heated up to 1400C.
B. Preparation of test sample
- 360g of crumb rubber was cut out from the rubber crumb. To get
a good sample, the rubber was cut out from the four corners of
the crumb rubber.
- The 360g of the crumb rubber was passed through a two roll mill
six times at a nip setting of 1.65mm to homogenise the rubber
crumb. The homogenised rubber was the test sample.
C. Preparation of the test piece
- From the test sample, 25 gram was cut out and was passed
through the two roll mills 3 times. The test piece was folded into
half on each pass, such that the final sheet measures 0.36mm.
- From the test portion, 2 sets of 3 circular disc were punched out
using the punch. These circular discs were the test piece. One set
was used to determine the original plasticity, while the other was
for the plasticity retention index.
D. Determination of original plasticity.
- The test piece was wrapped and sandwiched between cigarette
paper and was introduced into the plate in the plastimeter.
- A force of 100N was applied to compress the disc. A force of
100N was applied when the lever of the plastimeter was moved
from the rear to the front.
- The plasticity was read in the dial micrometre and the reading was
recorded
- The above initial plasticity measurement was carried out on the
remaining two test piece of the first set, and the result was
recorded.
E. Determination of plasticity retention index (PRI)
- Three of the second set of the test piece were placed in the oven
for 30 minutes at a temperature of 1400C
- After the 30 minutes, the three test pieces were brought out and
cooled in a desiccator for another 30 minutes.
40
- The plasticity retention index of each of the test piece was
measured as was the original plasticity in step D above. (SAR,
1998)
2.3.2.2. The volatile matter content
1. Test piece of 10g mass was cut out from the homogenised sample.
2. The test piece was passed once between the rolls of a laboratory
mill (nip setting at 0.55mm)
3. The test piece above was reweighed and was placed on a weighed
petri dish in such a way that the maximum surface was exposed.
4. The test piece was dried in an oven operated at 1800C for three
hours; after which it was removed and cooled in a desiccator and
was reweighed. The process was repeated until a constant weight
was obtained. (SAR, 1998)
2.3.2.3. Dirt content
The dirt content of the crumb rubber was determined as explained
below:
A. Pre-experimental preparation
- 200g of natural rubber crumb was cut out from the four corners
of cuboid shaped natural rubber crumb.
- The rubber, which was cut out, was passed through the two roll
mill to homogenise the rubber.
- The solvent was filtered with a filter paper.
- The conical flask was warmed in a hot plate.
- The sieve was washed with the filtered solvent, dried in an oven
at 1000C, and then was allowed to cool in a desiccator; and was
weighed.
B. Preparation of the peptizer solution.
- One litre of white spirit (i.e. the solvent), was measured using the
measuring cylinder and was added into a 4 litres conical flask.
- 4g of xylyl mercaptan (XM) – the peptizer – was weighed and
was dissolved in the above solvent.
- The above solution was stirred gently.
41
C. Preparation of the test piece.
- 30g of the homogenised sample was passed between the rolls of
the laboratory mill twice. (The nip setting of the roll mill was set
at 0.6mm).
- Approximately 12g of the above sample was measured out.
D. Dissolving the test piece.
- 250ml of the peptizer solution was measured out into a conical
flask.
- The 12g test piece was cut out into several small pieces and was
dropped into the flask containing the 250ml of the solution of
peptizer.
- The beaker containing the solution was covered and kept on the
laboratory table for 5 minutes.
- The above mixture was heated to 1250C for 2 hours 30 minutes
for proper dissolution of the rubber. The beaker was stirred
occasionally during the heating periods.
E. Filtration
- The dissolved rubber solution was poured, while hot, through the
prepared sieve.
- The dirt retained in the conical flask was washed with the solvent
and was poured also through the sieve.
- The sieve was washed again with the solvent, high aromatic
hydrocarbon white spirit, and was dried for one hour at 1000C
- After the drying, the sieve and the content was cooled in a
desiccator, and was then weighed to the nearest 0.1mg. (SAR,
1998)
2.4 PRODUCTION OF THE EBONITE MATERIALS.
Three major raw material for the production of ebonite material are raw
natural rubber, ebonite dust and ebonite powder. Five different ebonite
materials were produced. The ebonite material was produced in majorly
42
two steps – production of ebonite dust and the production of the ebonite
material.
2.4.1 Production of ebonite dust.
1. The compound ingredients was weighed for the different five
mixes. The formulation for the different ebonites are shown in
table 2.0 below, the difference being on their sulphur content.
Table 2.0: formulation for the production of ebonite dust for the five
different samples.
Ingredients (in phr) A B C D E
Raw Natural rubber 100 100 100 100 100
steric acid (Hst) 2 2 2 2 2
Magnesium oxide (MgO) 5 5 5 5 5
Diphenyl guanidine (DPG) 6 6 6 6 6
TMQ 0.67 0.67 0.67 0.67 0.67
Sulphur 32 34 36 38 40
Note: phr means parts per hundred gram of rubber.
2. The weighed raw natural rubber was masticated in a laboratory
two roll mill, to reduce the molecular weight to a level that the
compound ingredients can be incorporated easily into the rubber
3. The compound ingredients were added to the masticated raw
rubber in the two roll mill. The ingredients were added according
to the following order – steric acid, MgO, TMQ, DPG, and
sulphur.
4. Procedure 2 and 3 was repeated for the remaining four mixes.
5. The compounded rubber was cured one after the other in a
compression moulding machine for 9hours each to produce a hard
rubber (ebonite) which was crushed in a grinding machine to
produce a dust. The machine was set at 1500C during the curing.
The dust produced was the ebonite dust, which was used as filler
for the ebonite.
6. The ebonite was crushed in Rubber Research Institute of Nigeria
using a grinding machine.
43
2.4.2 Production of ebonite materials
1. The compounding ingredients were weighed for the
different five mixes. The formulation for the different
ebonites are shown below, the difference being in their
sulphur content. The formulation is as shown in table 2.1
below.
Table 2.1: formulation for the production of ebonite for the five
different samples.
Ingredients (in phr) A B C D E
Raw Natural rubber 100 100 100 100 100
steric acid (Hst) 2 2 2 2 2
Magnesium oxide (MgO) 5 5 5 5 5
Diphenyl guanidine (DPG) 6 6 6 6 6
TMQ 0.67 0.67 0.67 0.67 0.67
Sulphur 32 34 36 38 40
Ebonite dust 32 34 36 38 40
2. The weighed raw natural rubber was masticated in a
laboratory two roll mill, to produce a flat sheet so that the
compound ingredients can be incorporated easily into the
rubber
3. The compounding ingredients were added to the raw
rubber in the two roll mill. The ingredients were added
according to the following order – steric acid, MgO,
TMQ, DPG, ebonite dust and sulphur.
4. Procedure 2 and 3 were repeated for the remaining four
mixes.
5. Cure characteristic test was carried out in an oscillating
Disc Rheometer (ODR) machine for each of the four
mixes. The machine was set at one hour curing time.
6. The compounded rubber was cured one after the other in
a compression moulding machine for 1 hour each. The
machine was set at 1500C.
44
7. After the 1 hour curing, the compression moulding
machine was switched off and the mould was removed.
8. The cured material was removed from the mould. The
cured material was the ebonite material.
9. Abrasion resistance, hardness and tensile strength test
was carried out on the five different ebonites produced.
45
CHAPTER THREE
RESULTS AND DISCUSSIONS
3.0 The results for the characterisation of natural rubber latex and
natural rubber crumb are summarised in the table below
Table 3.1: results of characterisation of latex and crumb rubber.
Property Values
Dry Rubber Content (DRC) (%) –
Latex
32.60
Total Solid Content (TSC) (%) –
Latex
39.66
Original Plasticity(P0 ) (mm) –
Crumb
45.00
Plasticity Retention Index(PRI)
(%) – Crumb
92.20
Dirt Content (%) – Crumb 0.17
Volatile matter (%) – Crumb 0.81
Ash content (%) – Crumb 0.36
3.1 Discussion on latex characterisation
The results show that the total solid content of the rubber [TSC] is 39.66%,
which is within the range in table 1 of the literature review. This means
that the latex used has less volatile matter (mainly water). (Imanah, 2001)
46
The dry rubber content, (DRC), a parameter which determines the
rubber content of the rubber latex and marketability was found to be
32.6%. This is fairly high and makes the rubber suitable for commercial
applications.
The overall assessment of the rubber latex indicates that the TSC and
DRC are remarkably good for commercial utilization. (SAR, 1998).
3.2 discussion on crumb characterisation.
The crumb rubber was characterized for parameters which
will determine its suitability for solid rubber application.
The parameters determined include, dirt content, ash content
volatile matter, and initial plasticity, others were plasticity after
heating for 30mins at 1400C and plasticity retention index (PRI).
The properties which could reduce the quality of the crumb rubber
such as dirt content, and volatile matter were found to be very low
and insignificant in polymer destabilization. These results are
shown on table 3.1 (SAR, 1998)
The dirt content was found to be only 0.17% of the rubber
crumb while the ash content is 0.32% in the crumb rubber. Volatile
matter including principally water was found to be 0.81%.
A very major quality control parameter used in solid rubber
processing is plasticity. The initial plasticity of rubber sample, the
plasticity after ageing and plasticity retention index are indices
used here. The results for these parameters as shown in table
3.1were found to conform to those Standards of African Rubber
which is suitable for compounding. (SAR, 1998)
47
3.3. Results and discussions on the physico-chemical properties of the
ebonite materials.
3.3.1 The results of the abrasion resistance is as shown in table 3.2
below:
Table 3.3: abrasion resistance test result for the five samples.
Samples W0(g) WF(g) ΔW(g) NOR % ΔW
A(32% S) 37.00 33.86 3.14 1000 8.49
B(34% S) 37.00 35.43 1.57 1000 4.24
C(36% S) 37.00 36.04 0.96 1000 2.59
D(38% S) 37.00 35.51 0.40 1000 1.08
E(40% S) 37.00 36.08 0.38 1000 1.05
NOTE: W0 = initial weight in gram, WF=final weight in gram,
ΔW=weight loss after abrasion. % ΔW= percentage of the material lost
after abrasion which is a measure of abrasion resistance.
From table 3.3, it is seen that the abrasion resistance, increases with
increasing level of sulphur in the different sulphur mixes. The 40%
sulphur content ebonite has the highest abrasion resistance while the 32%
sulphur ebonite has the least abrasion resistance. Also the rate at which
the abrasion resistance increases, decreases with the increasing level of
sulphur. As seen in the table, the difference between abrasion resistance
for the sample A and B is 4.25%; while that between sample D and E is
0.03%. All the information is explained well in figure 3 below. (Zorge,
2011)
48
3.3.2. The result for the hardness test is as shown below in table 3.3
Table 3.4. Hardness test for the five samples
Sample %IRHD Increase in hardness (%)
A(32% S) 60 – 65 -
B(34% S) 62 – 65 -
C(36% S) 62 – 68 3
D(38% S) 66 – 73 5
E(40% S) 75 – 88 15
Note: IRHD means “International Rubber Hardness Degree”
From the result in table 3.4 above, the hardness of the ebonite increases
with increasing level of sulphur. The rate of the increase, increases also
with the increasing level of sulphur in the material, for example, the
difference between the maximum hardness of sample B and C is 3 while
that of D and E is 15, and that of sample A and B is below detection
level. This is due to increase in crosslinking of the rubber chains by the
sulphur atoms as the vulcanisation proceeds. The more the sulphur and
ebonite dust, the higher the crosslink density and the harder the
material; consequently the higher the abrasion resistance. (Zorge, 2011)
3.4 tensile strength test result and discussion.
The result of the tensile strength test for the five ebonite material are
shown in the graphs in the appendix – figure F to J. Table 3.5 below shows
the tensile strength, elongation at break, tear force and tear strength of the
five samples as obtained from figure F to J in the appendix
49
Table 3.5 Tensile strength, elongation at break, tear force and tear
strength of the five samples as obtained from figure 1-5 above in the
appendix.
Table 3.5. Tensile strength, elongation at break, tear force and tear
strength of the five samples
Parameters A B C D E
Tensile
strength(MPa)
5.6 3.5 4.7 1.7 2.0
Elongation at
break (%)
375 128 1187 198 142
Tear force (N) 44.40 28.20 37.80 12.70 16.3
Tear
strength(MPa)
4.8 3.5 4.7 1.6 2.0
From table 3.5 above, it is seen that the tensile strength and elongation at
break drops to a very much lower value as the cross link density increases
from sample A to E, although this does not follow a regular trend. This
conforms with Bill Meyer’s proposal that “the tensile strength reaches a
maximum with elongation remaining very high in the early soft cure
region, then both falls to much lower less values with increasing amount
of combined sulphur”. (Meyer, 1984). Also the drop in elongation at break
from sample A to E also shows that the elasticity drops with increasing
cross link density and also the hardness and brittle nature of the material
increases with increase in cross link density. (Zorge, 2011)
50
It is also seen from table 3.5 that the tear strength and tear forces drops
drastically as we move from sample A to E, although the trend does not
follow a regular trend. This also shows that the material becomes brittle
and less tough with increasing cross link density. (Zorge, 2011)
3.5 Discussion of the tensile strength results.
In figure F, which is for the 32% sulphur content ebonite, the strain
increases linearly with stress up to 5%elongation. Beyond 5% elongation,
little increase in stress, lead to high %elongation until the material break
at 375% elongation. (Billmeyer, 1984)
In figure G, which is for the 34% sulphur content ebonite, the strain
increases with stress up to 10% elongation. Beyond 10%elongation, little
stress leads to very high increase in %elongation until the material breaks
at 128% elongation. This curve shows that sample B has elastic properties
more than samples C to E. But the %elongation at break shows that
sample C is tougher than sample B. (Billmeyer, 1984)
In figure H, which is for the 36% sulphur content ebonite, the strain
increases linearly with the stress only for 25%elongation. Beyond 25%,
little stress leads to a very high %elongation until the material breaks at
1187%elongation. The curved nature of the slope and the high
%elongation shows that the material still has elastic property and that the
material is also tough. The curve also show that sample C is less hard
compared to sample D and E, this also agrees with the hardness test in
table 3.3. (Billmeyer, 1984)
In figure I, which is for the 38% sulphur content ebonite, the strain (i.e.
%elongation) increases linearly with increasing stress until the material
51
break at 192% elongation (as shown in table 3.5). The straight line slope
shows that the material is hard and brittle and breaks in a brittle manner.
The elongation at break shows that the material is less hard and brittle
compared to sample E in figure J. This also agrees with the hardness test
in table 3.3 – that sample E is harder than D. (Billmeyer, 1984)
In figure J, which is for the 40% sulphur content ebonite, the strain (i.e.
%elongation) increases linearly with increasing stress until the material
break at 375% elongation (as shown in table 3.5). The straight line slope
shows that the material is hard and brittle and breaks in a brittle manner.
From figure 3, it is seen clearly that the hardness increases with increasing
level of sulphur. Also the graph shows that the rate of the increase,
increases with the increasing level of sulphur, making the slope to be
inform of a curve rather than straight line. (Billmeyer, 1984)
52
Figure 3: plot of the properties (tensile strength, hardness, t90 and
abrasion) of the ebonite materials against phr of sulphur.
From the plot in the figure 3, the tensile stress dropped with increasing
level of sulphur i.e. increasing crosslink density. This also conforms with
Bill Meyer’s proposal that the tensile strength dropped to a much less
value with increasing crosslink density. Although the trend is not regular
as seen in the increase in tensile strength from 34% sulphur to 36%
sulphur but this dropped further as the sulphur content increased to 38%.
But in the overall sulphur increase, there is a much lower drop in tensile
strength. (Billmeyer, 1984)
0
10
20
30
40
50
60
70
80
90
100
32 34 36 38 40
pro
per
ties
sulphur content (pph)
hardness(%IRHD)
TENSILE STRENGHT(Mpa)
ABRASION(% ΔW)
53
It is also seen from figure 3 that the abrasion resistance in form of %
weight loss (% ΔW) decreases with increasing level of sulphur i.e.
increasing cross link density. (Billmeyer, 1984)
Table 3.6: effect of the increasing level of sulphur on the t90, tensile
strength, hardness and abrasion lost.
Sample Sulphur
content(phr)
Tensile
strength(MPa)
Hardness(IHRD) Abrasion
lost (%)
t90
(min)
A 32 5.6 60 – 65 8.49 52.00
B 34 3.5 62 – 65 4.24 20.50
C 36 4.7 62 – 68 2.59 50.39
D 38 1.7 66 – 73 1.08 45.10
E 40 2.0 75 – 88 1.05 18.30
The t90 was obtained from the graph in figures A – B in the appendix. t90
is the time taken for 90% curing to take place. Increasing level of sulphur
leads to drop in t90 from 52.00(for sample A) to 18.30 for sample E,
although this does not follow a regular trend. (Singhal, 2003)
In figure A for the 32% sulphur, t90 is 52.00 min and so it take longer time
for 90% of the crosslinking to occur, leading to low cross link density. This
lead to high tensile strength compared to other sample, low hardness and
high abrasion lost. It is also seen that beyond 60 min, the curve keep rising,
meaning that the cross linking also continued. (Singhal, 2003)
54
In figure B for the 34% sulphur content, t90 dropped to 20.50 min and the
curve start to reverse – leading to increased crosslink density. This
increased the hardness and reduced the abrasion lost but the tensile
strength dropped due to increased crosslink density. (Singhal, 2003)
In figure C for the 36% sulphur, the t90 is seen to increase to 50.93, leading
to low cross link density as it take a longer time for up to 90% of the curing
to occur. This lead to reduction in abrasion lost, tensile strength increases
but the increase in hardness is small as seen in the table 3.5 above.
(Singhal, 2003)
In table 3.6 for the 38% sulphur, the t90reduced to 45.10 as expected,
leading to increased cross link density. This as seen in table 3.6 above
shows that this leads to decrease tensile strength, although the hardness
and abrasion resistance increased. (Singhal, 2003)
In table E for the 40%sulphut, the t90 reduced drastically to 18.30 min. this
lead to increased cross link density and thus the tensile strength reduces
while the hardness increases. The abrasion lost also decreases. (Singhal,
2003)
3.6 Relating the five samples to rubber bullet production
For rubber bullet production, sample E is too hard and it may kill. Sample
E also has low tensile strength. Although sample E has low t90 that makes
it economically cheap for commercial production, it cannot be used for
rubber bullet production since rubber bullet is expected to be non-lethal;
also the low tensile strength may not withstand the heat of the bullet.
Sample D also has low tensile strength and relatively hard. So it cannot
be used for rubber bullet production because of the reasons given above
55
for sample E. in addition to that, the relatively high t90 makes it
economically expensive for commercial production.
Sample C has moderate hardness, with relatively high tensile strength
and moderate abrasion lost; but the t90 is high making it economically
expensive for rubber bullet production. So sample C can be used for
production of rubber bullet where there is cheap and constant power
supply to compensate for the long period of time the material would be
heated in the moulding machine for curing.
Sample B has relatively high tensile strength, moderate hardness,
moderate abrasion lost and very low t90. The low t90 makes it economically
cheap for commercial production of rubber bullet.
Sample A has very high abrasion lost, with low hardness. Although it has
lowest t90 and the highest tensile strength, its low hardness and high
abrasion resistance makes it non suitable for rubber bullet production.
Also the graph of its tensile strength shows that it has elastic property,
this will make it to bounce uncontrollably when fired at a target.
CONCLUSION: After investigation on the physico-mechanical
properties and the cure time of the five ebonite materials, it was found
out that production of hard rubber vulcanizate suitable for rubber bullet
production is feasible via the ebonite powder. The study also showed
that Sample B (with 34 sulphur phr) is most suitable for rubber bullet
production, but with cheap and constant power supply sample C (with
36 sulphur phr) may be dominant in quality. Since rubber bullet is the
responsibility of the state, sample C is also adopted.