Materials and

&pwimenf;a/ Meuiods

Abstract

The materials used for the study are

given in this chapter. The synthesis

and characterisation of the new

accelerator DTB are also described in

this chapter. The experimental

techniques used for compounding,

curing and the measurement of

physical and mechanical properties

are discussed. The procedures for the

analysis of the various parameters and

properties are given in this chapter.

2.1. Materials

Natural rubber,, (poly cis 2-methyl butadiene) used was ISNR-5

obtained from the Rubber Research Institute, Kotkayan~, Kerala, India. The

characteristics of'1SNR-5 are given in Table 2.1

The structure of natural rubber IS

Table 2.1. Characteristics of ISNR-5

Property Approximate values

Glass transition temperature (OC)

Density (glcc:)

Solubility ( ~ r n . ~ )

Nitrogen content(%)

Dirt(%)

Volatile matter (%)

Ash content (1%)

Plast~city (Po)

Plasticity retention Index (PRI)

Styrene Butacliene Rubber (SBR) marketed under the trade name

Synaprene (SBR-1502) was obtalned from Synthetics and Chemicals Ltd.,

Bareily, Uttar Pradesh, India. The structure of (SBR) is given below.

The chemlcal constituents of SBR-1502 are given in Table 2.2.

Table 2..2. Properties of Synaprene (SBR- i 502)

Chemical constituent (%I)

Minimum Maximum

Styrene content 21.5 25.5

Volatile matter - 0.75

Organic acid 4.75 7

Soap - 0.5

Ash 1.5

The accelerators, DCBS (Vulcacit DZ), TBBS (Vulcacit NZ) and

MBS (Vulcacit MZ) were obtalned from Bayer-AG, Germany. Other

rubber chemicals such as zinc oxide, stearic acid and sulphur were all

commercial grades obtained from Ranbaxy Ltd. Bombay., India. Propane-2-

thiol, I-hexanethiol and piperidine, analytical grade were supplied by E-

hlerck, Gemany. Anilinr:, carbon disulphide, ammonia, thiourea, NaOH,

HCI etc. used were laboratory grade. HAF Black (N330) was obtained from

United C'arbon India Limited. Bombay, India. The structures of the

accelerators used are

IIC'BS (N-d~cyclohexyl benzothaizole 2-sulphenamide)

1 HHS I N-t- butyl benzoth~azole 2-sulphenam~de)

(>I:>- CH3

MB:S (2-morpholinothio benzothiazole )

2.2. Synthesis of' 1.-Phenyl-2,4-Dithiobiuret @TB) [I]

Ammonium phenyl dithiocarbamate was first prepared by the

reaction of carbon disulphide (50ml) with aniline (55ml) in strong

ammoniacal medium. Aniline was added dropwise to a stirred mixture of

carbon disulphide and ammonia (100ml) kept at O'C. The product thus

obtained was stearn distilled with lead nitrate to obtain phenyl

isothiocyanate. It was then added drop-wise to a stirred solution of thiourea

and powdered sodium hydroxide in acetonitrile (15ml) and the reaction

mixture was heated. ,411 the reagents were taken in equimolar compositions.

When a clear solu.tion resulted, it was diluted with water and acidified with

concentrated hydrochloric ac~d. The crude I-phenyl- 2,,4- dithiobiuret obtained

was d~ssolved in a minimum quantity of aqueous sodium hydroxide to remove

any unreacted thiourea. This was then filtered. Ihe alkaline filtrate on

acidificat~on y~elded I-phenyl 2,4 dithlobiuret. The sequence of reactions is

given in Scheme 2 1

i Pb(NSh

NCS

(Q>N*-cs- Nkecs-NH2

NH2csNH2 0 Scheme 2.1. Re:action route for preparation of IITB. [ I ]

2.3. Characterization of DTB

The crude DTB was purified by recrystalising it in ethanol. Pure

samples were analyzed using Infra-red spectra, H-NMR spectra and Mass

spectra.

About 5rng of pure DTB was pelletized with KBr and the IR

spectrum was taken in a Shimadzu 1R 470 Spectrophotorneter. The spectrum

obtained is given in the Figure 2.1. Characteristic absorbance peaks obtained

are 3200 cm ' (NH-sh.) I ti00 cm~' INH-det), 1200 cm-' (CS-sh.), 800-900 cm-'

(CH-def). 2950 cm ' (C'H-str). This shows the presence of NHZ, CH, CS

and phenyl rtng shuctures present in DTB.

Figure 2.1. lnfra red spectrum of DTB

2.3.2. H-NMR Spectm

The proton NMK spectrum of the newly synthesized compound is

given in Figure 2 . 2 . The spectrum was taken on Bruckes WM 300 MHz and

the chemical shifts values are reported in parts per million (ppm) relative to

tetra methyl silane (0.00 ppm) The spectrum was taken by dissolving the

purified compound in DMSO-D6 solvent. Characteristic peaks

corresponding to protons of different environment was obtained. Peaks

corresponding to NH protons (9-12 ppm), phenyl ring protons (7-8 ppm)

imd NH, protons (7 ppm) are clearly dishguishable from the figure. The peak

at 2.5 corresponds to the solvent DMSO and that corresponding to absorbed

moisture is observed at 3 .5 ppm. This spectsum confirms the structure of DTB.

Figure 2.2. H-NMR of D m

2.3.3. Mass Spectra

Electron ~mpact mass spectra were recorded on a Finnigan MAT

MS 8230 mass spectroli-~eter. The mass spectrum of DTI3 is given in Figure 2.3.

The DTB molecular ion shows a stable peak at 21 l(M+l). The spectrum re-

confirms the structure of DTB. The other peaks are at 135 (base peak), 93,

77,5 1 correspond to fragments formed from it.

Figure 2.3. Mass Spectra of DTH

2.4. Compounding of Rubber

Compounding of rubber was carried out on a two roll open mill

(300 x 150mm) at a friction ratio 1 : 1.14 according to ASTM-D-3 182-94.

First the rubber was masticated in the mill in order to lower its viscosity. In

the case of natural rubber mastication was required for a longer time due to

the higher molecular weight. After lowering the viscosity ingredients like

zinc oxide, stearic acid, accelerators, sulphur and fillers were added. After

incorporating the ingredients, the batch was homogenized by passing it in

single d~rectlon in order to ensure the orientation of chains and to preserve

the mill direction before moulding. The total time of mixing and roll

temperature were k.ept constant through out the study.

2.5. Cure Characteristics

Monsanto Rheometer R-100 was used to measure the curing

behaviour of the rubber compounds. In this instrument, the rubber

compound IS placed in a cylindrical cavity (50mm x 10mm) and a biconical

rotor of diameter 17mm rs embedded in it, which is oscillated sinusoidally

tluough a srlrall arc ampli,tude (I to 3 degree). The cavity and specimen are

maintained to within + 0.5 OC and the force required to oscillate the disc is

measured. A typical torque-time curve (vulcanization curve) also known as

rheograph is shown in Figure.2.4.

L-- . Time

Figure 2.4. A typi~al rheograph obtained from Monsanto Rheometer (R-100).

The reler mt ddta !hat could be taken from the torqur-time curve are:

a) Minimum torque (M.): r h ~ s IS the torque attamed by the mlx after

homogen~/inp at the te\t temperature before the onset of cure.

b) Maximum Torque (M,): This is the torque recorded after the

curing of the mix is completed

c) Scorch time (t,,,): This 1s the time taken for two units (0.2Nm) rise

above the rnin,im~um torque (about 10 % vulcanization).

d) Optimum cure time (tP,): This is the time taken for attaining 90%

of the maxlmumt torque (90% vulcanization).

e) Cure rate Nntiex: Cure rate Index was determined from the

rheographs 03f the respectlve mixes.

Cure rate Index = 100/ tgo-tlo .................... (2.1)

where tq, and t,, are times corresponding to optimum cure and scorch time

respectively.

2.6. Vulcanization

Vulcanization of the samples was camed out in an electrically

heated Hydraulic Press at 1 5 0 ' ~ at a pressure of 120 kg/cm2 in a mould for

optimum cure time. Mouldings were cooled quickly in water at the end of

the curing cycle and stored in a cold and dark place for 24 hours, and were

used for subsequent physical tests and chemical analysis. For samples

having thickness more than 6 mm (for compression set) additional timings

based on the sample thickness were used to obtain satisfactory mouldings.

2.7. Preparation of Double Networks

Curing of the compounds was done in two steps. In the first step the

rubber compound was cured for TS0 (50% of the optimum cure time) at

1 2 0 ' ~ in a hydraulic press at 120 kg/cmz pressure. In the second step,

partially crosslinked rubber was extended uniaxially to various desired

lengths using a metal holder. The experiment was also conducted by

preparing initially cur'ed sheets at TTO (70% of the optimum cure time).

The extended rubber, placed in between the metallic holder was kept in an

air oven at 100 "C in order to complete the cure. Lower temperature was

used for completing the cure, in order to minimize the degradation during

curing under tension, necessary to produce double networks.

The exper~mental set up for the preparation of double networks is

shown in Figure 2.5, The set up (a) represents the rubber specimen kept

within holders. The extended rubber, (b) placed in between the metallic

holder was kept in an air oven at I OO'C in order to complete the cure. After

completing the cure, (c) the force with which the sample extended is

released.

Figure 2.5. Experimental set up for preparation of double networks. (a) rubber specimen kept within holders. (b) extended rubber, placed in between the metallic holder. (c) extended sample released after double net work formation.[L-initial unstretched length. L- stretched length, If.. final length]

2.8. Mechanical Propert ies

2.8.1. Tensile Strength, Modulus and Elongation a t Break

Tensile tests were carried out according to ASTM designation

D 412-98a uslng dumb-bell specimens at 28 + 2 ' ~ . Samples were punched

from vulcanized sheets parallel to the grain direction using a dumbbell die

(C-type). The thickness of the narrow portion was measured by thickness

gauge. The sample was held tight by the two grips in a tensile testing

machme, (TNE series 5T) the lower of which being fued. The testing

speed was 500 m~drninute. Young's modulus and modulus at 300 %

elongation bere meaiiured from the load displacement curves. The tensile

strength dnd modulus are reported in MPa and elongation at break in

percentage

Force at break (N) 'Tensile Strength ( ~ i m m ~ ) =

2 (2.2) Area of cross section (mm )

2.8.2. Tear Resistance

The test was camled out as per ASTM method D 624-98; unnicked,

90' angle test pieces viere used. The samples were cut from the vulcanized

sheets parallel to the grain direction. The test was carried out on Universal

Testing Machine (TNE series 5T). The speed of extension was 500 mm per

minute and the temperature of testing was 28 k 2 ' ~ . Tear resistance has

been reported in Nlmm.

Tear Strength (Nlmm) = Ult~mate Load RJ) .................... Thickness (mm) (2.3)

2.8.3. Compression Set

The samples (1.25 cm thick and 2.8 cm diameter) in duplicate

compressed to constant deflection (25%) (method B) were kept for 22

hours in an air oven at 70°C (ASTM D 395-98). After the heating period,

the samples were take1.1 out, cooled to room temperature for half an hour

and the final th~ckncss wals measured. The compression set was calculated

as follows.

where t,, and t , are the initial and final thickness of the specimen and t, is

the thickness of the space bar used.

2.8.4. Rebound Resilience

Dunlop Tripsometer (ASTM D 1054-91) was used to measure

rebound resilience. The sample was held in positic~n 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'~:.

Rebound resilience was calculated as follows,

I - coso Rebound Resilience (96) = x IOCI-------------------- (2.5)

1 - coso

where 8, and O2 are the initial and rebound angles respectively. 0, was 45'

in all tests.

2.8.5. Hardness

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

the vulcan~zates. PL calibrated spring is used in the instrument to provide

the ~ndent~ng force Readings were taken after 15 seconds of the

indentation when fum contact has been established with the specimens.

The method employed is the same as that in ASTM D 2240-97.

2.8.6. Ageing

Dumb-bell samples for tensile testing were prepared and kept in an air

oven at predetemuned temperature (70 '~) for specified periods as per ASTM

D 572-99. Mechanical properties were measured before and after ageing.

2.9. Network Char-acterization

2.9.1. Determination of Total Crosslink Density

A circular test piece with 2cm diameter, weighing about 0.2g was

cut from the compression-molded mbber sample using a steel edged die.

The sample was immersed in pure toluene at room temperature to allow the

swelling to reach diffua,ion equilibrium [2]. At the end of this period the

test piece was taken out and the adhered liquid was rapidly removed by

blotting with filter paper and the swollen weight was immediately

measured. The samples were dried in vacuum to constarit weight and the

desorbed weight was taken.

The crosslink density was determined using the equilibrium

swelling data 131. The volume fraction of rubber (Vr) in the swollen

network was then calculated by the method reported by Ellis and Welding

[4] using the follow~ng equation.

where D is the deswollen weight of the test specimen, A,, is the weight of

the solvent absorbed, p, ar~d p, are the density of the polymer and solvent

--- -- Expenmental 77 --

respect~~el! The molecular \\eight between crossliriks was determined by

the Flory-Rehner equation 151

where V, I S the molar volume of the solvent and X, the interaction

parameter. I S obtairied using the Hildebrand equation.

where IS the latlicc constant, V, is the molar volume of the solvent, R is

the gas constant. 1' is the temperature, 6, and 6, are the solubility parameters

of the solvent ancl polymers respectively. Crosslink density (v) is obtained

using the relation.

2.9.2. Determination of Mono, Di and Polysulphidic Linkages

a) Concentration of Polysulphidic Linkages

The concentration of polysulphidic crosslinks was estimated from

the change in the crosslink density of the vulcanizates before and after

treatment with propane thiol and piperidine, which cleave the

polysulphidic cr~ssslinks in the network [6].

Vulcanizate sample weighing about 0.2-0.3 g was allowed to stand

in excess of solvent (toluene) containing 0.1 % PBN for 24 hours at room

temperature Thsen the solvent was replaced by a solution (100ml) of 0.4 M

propane thlol and pipendine in toluene containing 0.5% PBN for two

hours. On complct~on of reaction, the sample was removed from the

reagent solut~on washecl with petroleum ether (four times), surface dried

on filter paper as quickly as possible and dried in vacuum to constant

weight at room temperature. The specimen was kept in excess of the

solvent with O.lO/o PBN for 24 hours, and finally extracted for two hours in

pure solvent. The swollen sample was weighed, solvent removed in

vacuum and the sample weighed again. The volume fraction of rubber (V,)

was then determined.

b) Concentration of Disulphidic and Monosulphidic Linkages

Both polysulphidic and disulphidic crosslinks in the vulcanizates

could be cleaved by treatment with 1-hexane-thiol in piperidine. The

determination of crosslink density before and after this treatment gives the

concentration of rnonosulphidic linkages, assuming carbon-carbon linkages

to be negligible. Since the concentration of polysulphidic linkages was

determined before, the concentration of disulphidic linkages was also

estimated [7]

Vulcanizate sample weighing about 0.2-0.3g was allowed to stand

m 100 ml of I-hexar~e thiol in piperidine (1M solution) containing 0.5%

PBN for 48 hours at room temperature. The mixture was agitated

occasionally. On completion of reaction the sample was removed from the

reagent solut~on. washed with petroleum ether (four times) surface dried on

filter paper as qu~ckly as possible and dried in vacuum to constant weight at

room temperature 'Then the speclmen was kept in excess solvent (toluene)

containing I). I'!4 P H N for 24 hours. Finally the specimen was kept in pure

solvent for 2 hours and weighed. Then the solvent was removed in vacuum

and the de-swollen sample was weighed. The volume fraction of rubber in

the swollen network was then determined as before and the crosslink

density was calculate'd.

2.10. Dynamic Mechanical Thermal Analysis (DMTA)

DMTA tests were conducted on an EplexorTM 150 N (Gabo

Qualiineter, Ahlden, Germany). Viscoelastic material parameters such as

mechanical loss factor, storage modulus and loss modulus (tan6 and E' and E

respectively) were nieasured over a broad temperature range (-1 10 to m O c ) at

a heating rate of 0.8~~:lmin. Rectangular specimens 60mm x lOmm x 6mm

(length x width x tl~ickness) were subjected to tensile loading consisting of

a static preload of' 3 f 1 N at frequencies of 1, 10, 50 and 100 Hz.

Measurements were also done on DMTA-MK I1 at a strain of 4% under the

tension mode within the temperature range -100 to + 6 0 ' ~ .

2.11. Scanning Ellectron Microscopy

Scanning electron microscopy was carried out on a Philips XL 20 at

an accelerating voltage of 15 kV. The fracture surfaces were carefully cut

from the failed test specimens without touching the surface and were

sputter coated with gold. The fractured specimens and the gold-coated

samples were kept in a ciessicator till the SEM observations were made.

2.12. References

1 . C. P Joshua, E Prasannan and S. K. Thomas, Indian J. Chem., 21 8,

649 (1982)

2. G. N. Byran, G. W. Weld~ng, "Techniques of Polymer Science" by

Soc. Chem. Lnd, Vo1.17. p.75 (1963).

3. A. D. T. Ciorton, T. D. Pendle, NR Technology, 7(4), 77 (1976)

4. B. Ellis, G. W. 'Welding, Rubber Chem. Technol., 37, 571 (1964).

5. P. J . Flory, J . Rsehner, J.Chem.Phys.,ll, 5120 (1943).

6. D. S. Campbell, J. Appl. Polym. Sci., B13, 120 (1969).

'7. V. Brajko, V. I)uc:hacek, J. Taue, E. Tumora, Int. Polym. Sci. Technol.,

7, B4 ( 1980).