CHAPTER

2 Materials a v ~ d Methods

2.1 Materials used

2.1.1 Styrene-butadiene rubber

C t y r e n e Butadiene Rubber (SBR) marketed under the trade name

J s y n a p r e n e (SBR-1502) was obtained from Synthetics and Chemicals Ltd.,

Bareilley, situated at the state of Uttar Pradesh. 'l'he rubber used for the

present study was the technically specified form of the rubber. It is a

copolymer of styrene and butadiene, manufactured by cold emulsion

polymerisation system using fatty acid and rosin soap emulsifier. It is a

non-staining and non-discolouring cold SBR grade. l h e rubber from the

same lot has been used in all investigations. Table 2.1 compares the physical

and thermal specifications of the unvulcanized and pure gum vulcanizate.

Table 2.1. Cor~rpnrisor~ ofp11.ysicaI arid thermal spec~~cnfic~rrs of SBR-1502 - ~ ..~ .~ ~

1 -~ ~~~

IJure G u n ~ Properties Unvulcanized

vulcanizate

1 Dissipation factot* 1 0.0009 1 0.0009 1

Density1-2(gcm~3)

Dielectric constanth at 1 kHz

1 TR 1°I.KI Source: Ref. 1-8

0.933

2.5

Heat Capacity35 kJ Kg 1 K 1

1 Thermal cortductivity7 "[Wm 1K '1

Chapter 2 Materials and Methods I t 7

0.980

2.66

1.89 - --

0.190

1.83

0.250

The Table 2.2 gives the chemical constituents, processing and technical

properties of SRR-1502 (Synaprene).

Table 2.2. Properties of Synaprene (SBR-1502)

1 Chemical cor~stituents(%) I Minimum I Maximum 1 1 Styrene content 1 21.5 1 25.5 1

1 Ash I I 1.5 1 Anti oxidant

- -~ - -~ - . .

I'rocessing and technical properties

- - - ~ ~ - ~~. ~. . ~ .~ ~- - . ... - -

Mooney Viscosity[ML(1+4)] at 100°C 46 58

Tensile strength (Kg/cm*)

Elongation at break(%)

1 Modulus at 300%(Kg/cm2) I 70 1 170 1 Cure time(Min.)

-.-- . - . - Source: Ref. 9-11

2.1.2 Sisal fibre

Sisal fibre of good grade in the form of filaments was supplied by the

local processing unit at Marthandom situated in the state of Tamil Nadu,

India. These fibres are extensively grown in the southern most parts of

Kerala, Tamil Nadu and Orissa.

Sisal fibre is made up of a large number of ultimate cells, cemented

together and these cells are much shorter than those of cotton or flax are. The

Chapter 2 Mater~als and Methods 64

fibre is coarse, stiff and rather brittle with high values for moisture absorption

(- 14%). The physical properties of the sisal fibre are given in Table 2.3.

Table 2.3. Plt.ysicnl chnrncterisrics of sisaljibre

I Water absorption I 150 (%) (24hrs.) 1

r-- Properties

Diameter

Density

Value(Unit)

0.1212 (mm)

1.45 (g/cm3)

~ ..... ~~ .. . ~

Cell Wall ttiicknrss 12.5 (pm) ... .

Micro fibrillar angle 20-25 (Degrees)

~ . ~ . ~

Tensile strength

Youngs Modulus ~~ ~-

Size of cells (long) ~ ~ ~

Cell length(L)/Cell breadth (8)

Source: Ref. 12

-- - . . 17-22 (GN/m2) p-~-~~ ---- ~ . .

0.5-4 (mni)

The main chemical constituents of sisal fibres are cellulose (78%),

hen~icellulose and pectins (lo%), lignin (8%), waxes 2% and ashes 1%.12.

Cellulose is matlr up of a large number of conclensrcl glucose units, and

hemicellulose are various types of sugars allied to glucose such as xylose,

arabinose and uronic acids. Lignin, a different type of material, is highly

complex in nature and is composed of a group of closely related substances

having phenolic properties. Much of the reactivity of the jute depends on the

uronic acid groups of the hemicellulose and phenolic hydroxyl groups of

lignin.

Chapter 2 Mater~als and Memods 0 5

2.1.3 Rubber chemicals

Crosslinking agent-sulphur (p = 2.05), accelerator-CBS (p = 1.30),

antioxidant-'I'DQ (p = 1.08) were of commercial grade obtained from the

Alkali and Chemicals Corporation of India Ltd., Rishra.

2.1.4 Special chen~icals

Resorcinol (p = 2.36) and hexamethylene tetramine (p = 1.33) were of

chemically pure grade supplied by E.Merck (A.G) M&B, U. K. respectively.

Precipitated silica (Vulcasil-S) (p = 1.95) was supplied by Bata India Ltd.,

Calcutta, India.

2.1.5 Other chemicals

Zinc oxide (p = 5.5) and stearic acid (p= 0.92) were of chemically pure

grade.

2.1.6 Solvents

Benzene, toluene, xylene used were of analytical grade.

2.2 Surface modification of fibres

2.2.1 Pre-treatment of fibres

The treatment of fibres enhances the interfacial adhesion between the

fibre and the rubber matrix and it reduces the fibre-fibre hydrogen bonding

by sufficient wetting. Interface is a region of at least several molecular layers

thick whose properties are intermediary between the fibre and matrix phases

because of the peculiar restrictions in its molecular motion in this zone.

Matrix molecules may be anchored to the fibre surface by chemical reaction or

Chapter 2 Materials and Methods 66

absorption, which determines the extent of interfacial adhesion. The interface

may be composed of an additional constituent that is added to the composite

as a bonding agent or as an interlayer between the two components of the

composite. The interfacial adhesion can be improved by two Cvays. This is

done by the formation of an interlayer pre-treatment or chemical modification

of the fibre surface and the incorporation of a bonding agent.

Here we have included pre-treatments like, hydration, salt water,

benzene-alcohol mixture, SBl7 solution and PSMA coating to enhance the

interfacial interaction.

Secondly, the surface modification of the fibres can be done with

chemical treatments following chemical reactions without destroying the

fibrous nature. ?he different surface modifications carried out in our present

work include mercerisation, acetylation, benzoylation, peroxide treatment,

and permanganate treatment.

Lastly, the incorporation of a bonding system during the mixing

process was also adopted.

(a) Water treatment

Short sisal fibres were immersed in water at 25'C for 1 hour. After

that, these fibres were washed several times with water in order to remove

the easily extractable impurities from the surface of the fibres. ?he fibres

were dried in an air oven at 70°C for two days and kept in polythene bags to

prevent the moisture absorption.

(b) Benzene-alcohol mixture

'I'he alkali treated fibres were refluxed with 1:l benzene-alcohol

mixture for 1 hour by using fractional distillation in soxhlet apparatus. 'I'he

resulting fibres were taken and air-dried and used for compounding.

Chapter 2. Mater~ais and Methods 67

(c) Salt water treatment

A 10% solution of NaCl is prepared and the untreated fibres were

soaked in the salt water for 1 hour. These fibres were taken out and dried in

air oven.

(d) SBR solution lreall~lent

The raw rubber solution (1%) is prepared by boiling SBR in toluene.

The alkali treated fibres were soaked in the resulting rubber solution and is

kept for 30 minutes. Then these fibres were taken out, air-dried and kept in

polythene bags.

(e) PS-MA coating

A 5% solution of polystyrene-maleic anhydride copolymer is prepared

and the untreated fibres were refluxed for 1 hour. 'l'he resulting mixture was

decanted to take out fibres and they were dried.

2.2.2 Methods of chemical modification

(a) Mercerisation

Untrmtcd short fibres of length 6 nlm were imn~ersed in 18% solution

of caustic soda at 28°C for 1 hour. These fibres were washed several times

with cold water and finally with acidified water (ElCl 0.1N). These fibres

were dried in an air oven and then soaked in glacial acetic acid for 1 hour at

the same temperature.

(b) Acetylation

Acetylated fibre was prepared from the chopped raw sisal fibre as per

the methods reported by Chand et al.17. The fibre was first immersed in 18%

Chaoter 2 Mater~als and Methods 68

aqueous NaOH solution at 35°C for one hour, washed with water several

times and then drird. The fibres were soaked in glacial acetic acid for one

hour, decanted and then soaked in acetic anhydride containing two drops of

Con. I12S04 for 5 minutes. The fibres were filtered through a Buchnor

funnel, washed with water and then dried in an oven at 70°C for 24 hours.

(c) Benzoylation

Untreated fibres were soaked in 18% NaOH solution for */2 hour,

filtered and washed with water. Then it was suspended in 10% NaOH

solution and agitated with 50 ml benzoyl chloride. The mixture was kept for

15 minutes, filtered, washed thoroughly with water and dried between filter

paper. These fibres were then soaked in ethanol for 1 hour to remove the

u ~ e a c t e d benzoyl chloride and was finally washed with water and dried in

the air oven at 70°C.

(d) Benzoyl peroxide treatment

The alkali treated fibres (20 g) were soaked with 6% solution of benzoyl

peroxide in acetone for 30 minutes. The solution was decanted and the fibres

were air-dried.

(e) Per~nanganate treatment

The alkali treated fibres (20 g) were soaked with KMnOa solution in

acetone for 1 minute. This was then decanted and the fibres were dried in air.

2.2.3 Incorporation of dry bonding system

A two component dry bonding system consisting of hexamethylene

tetramine and resorcinol are used as the bonding agent. The resorcinol was

Chapter 2: Mater~als and Melhods (19

grounded well and added in the molten state in order to disperse it

homogeneously in the masticated SBIZ matrix'J. ?'he hexa and resorcinol were

added to the mix during the milling process as per the same mixing sequence

given in Table 2.4.

Table 2.1. Formulariorl of Recipe ~ ~ . ~ - ~ - ~ ~

Ingredients phr

.&

100

. .~ ~~~ . .- ~ .~ ~ ... ~~

Variable Variable . -- 1 Variable ~ ~ ~ -- ~ ~ ~

Variable 'N-cyclo hexyl benzathiazole sulphenamide; ",2,4-Trimethyl 1,2-dihydroxy quinoline polymerized, <Hexamethylene Teh.amine; dPrecipitated silica(Vulcasil-S).

2.3 Characterisation of fibre and composites

2.3.1 Infrared spectroscopy

Infrared (I[<) spectra of raw and chemically modified sisal fibres were

obtained with Shinladzu IR 470 lnfrared spectrophoton~eter, Shimadzu,

Japan, by using solid KBr pellet technique. Fibre samples were cut into small

pieces and grounded before mixing it with KBr.

2.3.2 Scanning electron microscopy

The SElvl photographs provided in this investigation were taken using

JEOL 35 C model scanning electron microscope. The principle of which was

detailed by Johari r t n1.15. 'lhe fracture surfaces were carefully cut from the

Chapter 2 Malerlals and Methods 70

failed test specimens without touching the surface and were sputter coated

with gold within 24 hours. ' f ie fractured speci~nens and the gold-coated

samples were stored in a dessicator till the SEM observations were made.Ih

2.3.3 Optical microscopy

An optical stereo microscope was used for calculating fibre length

distribution in SBR matrix and also for observing the die swell behaviour of

extrudates and restricted equilibrium swelling behaviour of sisal fibre

reinforced composites.

2.4 Preparation of composites

2.4.1 Fibre preparation

l'he raw sisal fibres were chopped to different lengths viz., 2, 6 and

10 mm and washed with water to remove the undesirable materials. Then

these fibres were dried in an air oven at 70°C for 5 h. and it was then kept in

polythene bags to prevent moisture absorption before mixing.

2.4.2 Compounding

Mixes were prepared on a laboratory size two roll-mixing mill (300 x

150 mm) at a friction ratio of 1:1.25 in the case of ,SBR. SBR was first

masticated to attain plasticity by careful control of temperature, nip gap, time

of mastication and by uniform cutting operation. The compounding

ingredients were added in the following order as per ASI'M designation

D15-62T, cross-linking agent, activators, silica, accelerators and antioxidants.

Before the addition of fibres, the batch was tliorougldy cooled. In the case of

sisal-SBR compounds, in order to ensure better dispersion, sisal fibre was

mixed at the end of the sequence with the rubber compound after the

Chapter 2. Matertals and Methods 71

addition of all ingredients. Stearic acid was mixed in its molten state after the

addition of cross-linking agent.

After the complete mixing, the stock was sheeted out and passed

through the tight nip six times endwise and finally sheeted out at tight nip

gap to ensure maximum fibre orientation in the grain direction'7. The

compounded stock was kept overnight before vulcanization. ?he total time

of mixing and roll temperature (70-80°C) were kept constant throughout the

study. When sulphur was incorporated at the initial time of mixing, the

temperature of the mix was kept at 35 - 40°C since it is the cross-linking agent

used in the system.

All ingredients were added in the masterbatch form. The quantity of

ingredients in phr (parts per hundred rubber) and the sequence of addition of

ingredients are given in Table 2.4.

2.4.3 Vulcanization

Vulcanization was carried out in an electrically heated press having

(30 x 30) cm platens at 150°C and at a pressure of 45 Kg/cm? on the mould at

respective optimum cure times as obtained from the Monsanto Rheometer

R-100. Moulds were cooled quickly in water at the end of curing cycle and

stored in a cold and dark place for 24 hours and were used for subsequent

physical tests and chemical analysis. For thicker sanlples having thickness

more than 6mrn(like heat build-up, compression set and abrasion) additional

time based on the sample thickness were used to obtain satisfactory

mouldings.

In order to preparc samplcs with fibres oriented longitudinally, the

compounded sheet was folded along the grain direction and placed in the

mould. Similarlv, for samples w+th fibres oriented transversely, the sheet was

folded across the grain direction.

Chapter 2 Mater~als and MeLhods 72

2.4.4 Fibre breakage and fibre length distribution

Shear forces occurred during milling operations have oriented most of

the fibres along the grain direction, but this also caused fibre breakage. In

order to study the extent of fibre breakage, the fibres were extracted from the

green compound by dissolving the rubber compound in toluene and their

length and diameter were measured by using a travelling microscope. 'I'he

distribution of fibre lengths in the mixes is calculated based on 100 fibres from

the chopped raw fibres before mixing and after extraction from the milled

compound.

The distribution of fibre lengths can be represented in terms of

moments of the distributionlR~'9.

The number and weight average fibre lengths can be defined as

where L, is the number average fibre length, i,, the weight average fibre

length, and Ni, the number of fibres having length Li. ?he value of L,/L,,, the

polydispersity index, can be taken as a measure of fibre length distribution. -

The values of L,, L, and L,/ in are calculated based on 100 fibres for the

chopped sisal fibres and fibres extracted from the mix.

2.4.5 Time of optimum cure

Optimuni cure times at 150°C were detrrmined with the help of

Monsanto Ilheometer (11-100). 'l'he optimum cure time corresponds to the

time to achieve 909'0 (tw) of the cure calculated from the formula,

Chapter 2: Materials and Methods 73

where MH and ML are the maximum and minimum torques respectively

expressed in dNm.

2.5 Analysis of composite properties

'I'he mechanical properties of the untreated, treated and bonding agent

incorporated short sisal fibre reinforced SBR composites were evaluated here.

2.5.1 Physical and mechanical analysis

At least 4 specimens per sample were tested for each property and the

mean of these values was reported. Except hardness and resilience, the tests

were carried out both along and across the grain direction. The fibre

orientation was maximum along the grain direction. In the case of hardness

and resilience, the direction of fibre alignment is normal to the direction of

application of the load.

(a) Green strength

Green strength was determined by using a method developed by

Foldi.20. The green strength of the uncured 2mm thick composites, i.e., tensile

properties of uncured 'green' samples were measured in Zwick Tensile

Testing Machine at a strain rate of 50 cm/min. 'Ihe surface tack was

eliminated by 'surface precuring' (pressing the sample at 120°C for two

minutes between two sheets of aluminium film in an electrically heated

hydraulic press). For practical reasons 'green strength' was defined as stress

at the yield poult.

(b) Mill shrinkage

The mill shrinkage was determined by ASTM D 1917-891' method. The

compounded stock was milled for 1 min. The mill roll was opened slowly

Chapter 2: Materials and Methods 74

and evenly until the band has just disappeared. 'I'he three specimens of

approximately 2 cm. width, one from the centre and one half way between

the centre and each edge were cut while mill was running. Care was taken

not to stretch the sample. Four specimens were placed on a smooth, well-

dusted surface with the inside or smooth surface down. The specimens were

then placed in oven at 100°C for an hour. Final length was measured after

cooling at 25°C for an hour.

The mill shrinkage, reported in percentage was calculated according to

the following equation:

- C - L Shrinkage (%) - - x 1 0 0

"

where, C, the c i~~cun~fe re~~ce of the mill rolls (47.88 c ~ n ) and I., the average

length of three specimens.

(d) Fibre orientation

Fibre orientation was evaluated from the scanning electron

microscopic technique. The photographs of the fractured ends clearly

identified the fibre orientation in the composite whether it is longitudinal or

transverse.

(e) Tensile strength and elongation at break

In this investigation, these tests were carried out according to ASTM

Designation 0 412-51'1' using dumbbell specimens. All the above tests were

carried out at 26 k 2°C. Samples were punched from vulcanized sheets both

along and across the grain direction using a dumbbell die (C-type). The

thickness of the narrow portion was measured by bench thickness gauge.

Two marks were made, one inch apart, in the middle of the narrow portion.

The sample was held tight by the two grips in a 'Zwick (1475)' Universal

'resting Machine, the upper grip of which being fixed. 'I'he rate of separation

of the power-actuated grip was 50cms per minute. The load at break was

read from the dial. The elongation at break was measured with the help of a

scale. From the recorded loads, the stress was calculated on the basis of

original cross-sectional area.

The tensile strength is reported in MPa. (Conversion factor: 1 MIJa =

10.197 Kgf/cm* = 1 N/mm2 = 1 M N/m2 and the Elongation at break (%) is

reported in percentage.

Stress-strain curves were obtained using Zwick-1475 (UTM) at a

crosshead speed of 500 nini/min.

(f) Tear resistance

The tear strength was calculated as per the ASIM method D 624 - 48.

The test pieces were cut from the vulcanized sheets both along and across the

grain direction unnicked at 90° angle. The test was carried out on a 'Zwick-

1475' (UTM). 'I'he speed of extension was 50cnis per minute and the

temperature was kept 26 f 2°C. 'The tear strength can also be calculated

according to the following equation:

Ultimate Load (N) Tear Strength (kN/rn) = -- P 5 )

'1 hickness (nin~)

The tear strength has been reported in kN/m. ' (Conversion factor:

Ilkg/cm = 0.98 kN/m).

(g) Hardness

Shore A type Durometer was eniployed to find out the hardness of the

vulcanizates. The instrument uses a calibrated spring to provide the indenting

force. The load inlposed by the spring varies with indentation. Readings

-- Chapter 2: Mater~ais and Methods 76

were taken after 15seconds of the indentation when firm contact has been

established with thc specinlens. 'lhc method employed is the same as that in

ASTM D 676 - 52 T. In this case, the direction of application of load is normal

to the fibre orientation.

(h) Rebound resilience

Dunlop Tripsometer (BS 903, Pt.22,1950) was used to measure rebound

resilience. The sample was held in position by suction. It was conditioned by

striking with the indentor six times. The temperature of the specimen holder

and sample was kept constant at 35'C. Rebound resilience was calculated as

follows:

1 - Cos 0 2 Rebound resilience (%) = x 100

where 02 and 01 are the final and initial rebound angles respectively. 81 was

fixed at 45' in all tests.

2.5.2 Melt flow studies

The rheological studies of untreated and treated short sisal fibre

reinforced SBR conlposites were carried out using an Instron Capillary

Rheometer Model 32112'. It is an extrusion barrel assembly, consisting of a

hardened steel barrel enclosed in an aluminium jacket to which electrical

heating elements are clamped. The system has a tungsten carbide capillary

inserted in the bottom end. A plunger is driven into this barrel at a constant

speed and the force needed to drive the plunger is measured and registered

on the front panel of the rheometer; or recorded on a chart recorder, which is

an optional accessory. 'l'he barrel heating elements are supplied with A/c

mains through thyristors. 'lhe temperature control is achieved by comparing

the barrel temperature virith that set on digital thumbwheels, the difference

.- Chapter 2 Mater~als and Memads 77

level being used to create pulse trains which trigger the thyristors to reduce

the error. A capillary of l /d ratio 20 and an angle of entry 90" were used in all

experiments. All studies were done in the shear range of 3.68 - 1226.9 Sec-',

due to the instrument limitations.

A typical capillary is outlined in

the Figure 2.1, wluch is self-explanatory.

While carrying out the studies on

rheological bel~aviour of short sisal

fibre-SBR composites, cross-linking

agent was not added for compounding

and the extent of mastication of raw

SUR was kept constant durisg mixing

procedure. Small pieces of

unvulcanized sisal-SBR mixes were

Barrel

Material

Shear stresses

pushed into the

rheo~neter from

The plunger was then inserted and ( I forced down with the moving cross head until a small amount of material

appears at the exit of the capillary.

barrel of the capillary

the top of the barrel.

A warming period of 150 scconds was given so that including the

travel time of the plunger the sample gets warmed up for five minutes before

Figure 2.1. Terminology diagrnm of capillary tube sltowirz~ rlte t*elocity proJile of melt pow

actual testing. After the warm up period, the samples .were forced down to

the capillary by the plunger. Thus, the melt was extruded through the

capillary at predetermined plunger speeds. The initial position of the

plunger was kept constant in all experiments and melt viscosities at different

shear rates were obtained from a single charge of the material. l'he plunger

speeds varied from the lowest speed to higher speeds. Each plunger speed

was continued until the recorded force plot stabilises and then the crosshead

Chapter 2: Mater~als and Methods 78

was stepped to the next speeds. Six speeds, generally 0.06, 0.2, 0.6, 2, 6 and

20 cni/min rcspcctively, of the crosshead were repeated and it was observed

in almost all the cases that reproducibility is acceptable.

The rheological analysis was done at different temperatures such as 90,

100, 110, 120 and 130°C. The temperature controllers were arranged to

maintain a gradation in barrel temperature, the highest being the lower zone

where the capillary was placed. The difference between the successive

temperature zones in the barrel was kept at 5OC, i.e., for a test temperature of

90°C, the middle zone was kept at 85OC and the upper zone at 85°C. The

temperature of the lower zone is reported. The temperature inside the barrel

and capillary was varied between 90 - 130°C, with an accuracy of 1°C. The

experiments were carried out at six different shear rates.

(a) Treatment of data

The pressure drop due to the flow within the barrel may be

determined by measuring the load required to extrude the material with no

capillary fitted into the barrel. The shear stress, T ~ , increases linearly with

distances, r, from the centre line and is given by:

where A P is the pressure drop across the length, I= , of capillary tube.

I h e forces and the cross-head speed are converted to apparent shear

stress (rw) and shear rate (ywa) at the wall respectively22.

The true wall shear stress was calculated as,

- -. . . - - Chapter 2 Mater~als and Mettrods 7')

where F is the force on the plunger (N), A cross sectional area of the P

plunger (mm2), and lc(mmj and dc(mrn) the length and diameter of the

capillary respectively.

?'he apparent shear rate was calculated by using the equation:

VXH xdh2 But, Q = -- -

60 4

where V,,, cross head speed in cm/min., dl,, diameter of the barrel, d,,

diameter of the capillary. But in the case of non-Newtonian liquids, the

velocity profile deviates from parabolic depending on pseudoplastic

behaviour, i.e., the wall shear rate will be different from that given above.

-['he correct wall shear rate may be fclund by using the Rabinowicli

correction":

The factor (3n'+1)/4n1 is the Rabinowich correction applied to calculate the

true shear rate.

d(lnr,,,) But, n' =

d(lny,,.,,l

i.e. , n' is the slope of the graph of In r vs in y n!which is obtained by the

regression analysis of the said plot.

.l'hus apparent viscosity, qa is calculated by,

Chapter 2. Mater~als and Methods 80

(b) Die swell and extrudate morphology

'She extrudates were collected on a glass plate as they emerge from the

capillary die, care being taken not to stretch them. The extrudate diameter

was measured at different times and it was noticed that there was no further

change in the diameter after 24 hours. Thus all the samples were kept for 24

hours to attain equilibrium before final readings were recorded, using a

travelling microscope. The surface morphology of the extrudates obtained

after the capillary extrusion was analysed under optical and SEM techniques.

2.5.3 Diffusion and transport phenomena

The vulcanized sisal-SBR composite samples were cut circularly

(diameter = 1.94 cm) for swelling measurements. ?he thickness of the

composite was measured using a micrometer screw gauge. Ury weights of

the cut samples were taken before immersion in the liquid contained in

airtight weighing bottles. The samples were removed from the bottles at

periodic intervals, the wet surface was quickly dried using a piece of blotting

paper and weighed immediately in airtight weighing bottles. During swelling

any change in the diameter and thickness of the san~yle .was determined by

means of vernier callipers and a micrometer, respectively.

The uptake of the liquid by the polymer during swelling was expressed

as moles of liquid sorbed by lOOg of the polymer. This method was found to

be more convenient for comparison of sorption data and was adopted by

earlier researchers" 25.

Chapter 2 Mater~ais and Methods 81

(a) Swelling data analysis

(i) Mensureinerzt of adhesion

Compared to silica and silicate fillers, when carbon black is used with

short fibres and bonding agents, the carbon blacks appear to show a

preferential effect in promoting adhesion between fibers and rubbers. In

order to determine the volume fraction of rubber in the unswollen

vulcanizate, the test specimen was weighed both in air and water. The

difference between the two weights gave the volunle of the samples. Using

the base formulation, the amount of rubber present in the weighed sample of

each specimen and its volume were calculated. From these data the volume

fraction of rubber present in dry specimen was calculated and it is denoted as

VI. The improved adhesion between short fiber and rubber can be evaluated

from a relation,

The dry specimens were then swollen in solvents up to equilibrium

swelling volume. The weight of swollen samples was determined by sorption

gravimetric method. 'l'he imbibed solvent in the specimen was dried off by

placing it in an air oven. The resulting weight of the specimen is noted. From

this the volume fraction of rubber in the swollen sample.was calculated using

the following relation to establish the extent of crosslinking,

- (D-fT) p,~' Vc -

(D-fT)pr l+Ao ps'

where D, the weight after drying out, f, the fraction of insoluble components,

'I; the weight of the sample, pr, the density of rubber ps, the density of

solvent (I'oluerie = 0.866 g/cc), Ao, the weight of the imbibed solvent.

Chapter 2: Materials and Methods 82

Anisotropic swelling studies provided information on the strength of

interface, degree of dispersion of fibers and their alignment in the elastomer

matrix. In order to assess the extent of swelling behavior of composites, gum

and fiber filled vulcanizates were swollen in toluene at room temperature,

and their swelling parameters were evaluated.

(a) Su~ell ing index

Swelling index is calculated by the following equation,

- - w2-w1

Swelling Index(%) x 100 (2.16) W I

where W1, the initial weight of the sample, W2, the final or swollen weight of

the sample.

(b) Swell ir~g Coefficient

The swelling behavior of composites can also be analysed from the

swelling coefficient values. It is an index of the ability, with which the

samples swells and is determined by the equation,

As 1 Swelling coefficient, a = - x -

m s

where A, , denotes the weight of the solvent sorbed at the equilibrium

swelling, m, the weight of the sample before swelling, s, the density of the

solvent used.

The results of sorption experiments were obtained by plotting the mole

percentage uptake (Q,) of the solvent by 100 gms of the SHlZ gum vulcanizate

Chapter 2 Mater~als and Methods 83

and fiber composites versus square root of time for different solvents. The

mole percent uptake Qt for the composite samples were evaluated using the

following equation,

where W2 is weight of the sample after swelling, W1 is the weight of the

sample before swelling and M, is the molecular mass of the solvent.

2.5.4 Analysis of electrical properties

Disc samples of 2 mm-thickness and 10.2 mm diameter were used.

Samples were prepared by cutting from the rectangular composite specimens

using a die. The test samples were coated with conductive silver paint on

either side. Copper wires were fixed on both sides of samples as electrodes.

The capacitance, resistance and dissipation factor were measured directly at

room temperature, using a 4192 LF Impedance Analyser (Hewlett-Packard,

USA) by varying the frequencies (5 Hz - 13 MHz). Two specimens were

tested for each set of samples.

Dielectric Constant (El) was calculated from the capacitance using the

equation.

C x t

where E', the dielectric constant of the material, Ea the permittivity(or

capacitivity) of air, i.e., 8.85 x 10-12 F.m-1, C, the Capacitance, A, Area of cross

section of the sample, t, the thickness of the sample.

The volunie resistivity (p) can be calculated from the resistance using

the equation,

Chapter 2: Mater~als and Methods 84

where 1) is the volume resistivity, Rv is the resistance, A is the area of cross- 8

section of the sample and t is the thickness of the sample.

Dielectric loss (E"), dielectric constant (E'), and dissipation factor (tan6)

are related by the equation,

2.5.5 Dynamic mechanical thermal analysis(0MTA)

The vulcanized rectangular sheets of specimens (5 x 8 cm) having a

thickness of 4mm were used for the dynamic mechanical experiments. The

analysis was done using different saniples having longitudinal and transverse

grain direction. l h e dynamic storage modulus (E'), loss modulus (E") and

loss factor (mechanical damping (tan6)) were measured as a function of

temperature using a dynamic mechanical thermal analyser (DMTA-Polymer

Laboratories, MK-11). The temperature range over which properties were

determined was 20 to 150°C at a heating rate of l0C/min. The samples were

tested at various frequencies such as 0.1, 1, 5, 10 and.50 Hz with strain

amplitude of 4% at a heating rate of l0C/min.

2.5.6 Thermal properties

(a) Thermogravimetric analysis (TGA)

The thermal behaviour of untreated and heated fibres, and sisal/SBR

composites were studied using a delta series TGA-7 system. A small amount

Chapter 2: Materials and Methods 85

(1-4 mg) of the sample was taken for the analysis and the samples are heated

from 35 to 745°C at a rate of 1O0C/min. The TGA and D'I'G thermograms are

drawn for each sample.

(b) Kinetics of degradation

'l'he kinetic parameters of thermal degradation can be evaluated from

isothermal and non-isothermal methods. The integral equation, which is used

to derive the kinetic degradation of composites, is in the form26,

where, g(u) is the kinetic model function, u, the reaction fraction

decomposed, A, pre-exponential factor(Arrhenius parameter) which is

calculated from the intercept by the relation,

AR i.e., Intercept = In -

$1:

where R, the universal gas constant, E, the energy of activation, and T, the

absolute temperature. The entropy of activation, is calculated by using the

relation,

where A, the pre-exponential factor, k, the Uoltzmann constant, h, the

Planck's constant and T , the peak temperahue in the WTG curve. The kinetic

analysis of thermal degradation reaction of the sisal/SUli composites was

done by a multiple linear regression analysis through a computer software

program. In this method, the 'L'G data were analysrd using nine mechanistic

equations and the effect of fibre loading and chemical treatment on kinetic

parameters was studied.

Chapter 2 Materials and Methods 86

(c) Differential scanning calorimetric analysis

The thermal behaviour of sisal/SBR composites was measured using a

mettler differential scanning calorimeter. The samples were inserted into the

apparatus at room temperature and immediately heated to 200°C and kept

for 1 min. at this temperature in order to remove volatile impurities. The

samples are fkst cooled to -80°C using liquid nitrogen and the scan was made

from -80 to 100°C at a heating rate of 10°C min' in helium atmosphere. For

the determination of T, (glass transition temperature), two tangents are

drawn at the baselines of the DSC curve and the perpendicular bisector gives

the T, of the sample.

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Chapter 2 : Materials and Methods 87

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Chapter 2 Mater~als and Methods 88