body of the project. editeddocx

1

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

2

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

3

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

4

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).

5

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.

6

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)

7

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

8

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

9

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.

10

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.

11

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

12

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

13

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.

14

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

15

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

16

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).

17

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).

18

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

19

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

http://en.wikipedia.org/wiki/Rubber

http://en.wikipedia.org/wiki/Projectile

http://en.wikipedia.org/wiki/Firearm

http://en.wikipedia.org/wiki/Riot_gun

http://en.wikipedia.org/wiki/Non-lethal_weapon

http://en.wikipedia.org/wiki/Plastic_bullet

http://en.wikipedia.org/wiki/Wax_bullet

http://en.wikipedia.org/wiki/Wood

http://en.wikipedia.org/wiki/Riot_control

http://en.wikipedia.org/wiki/Riot_control

http://en.wikipedia.org/wiki/Protest

http://en.wikipedia.org/wiki/Baton_round

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


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


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.

body of the project. editeddocx

Documents