Materials &a/ Meuiodsshodhganga.inflibnet.ac.in/bitstream/10603/589/8/08_chapter 2.pdf · Glass...
Transcript of Materials &a/ Meuiodsshodhganga.inflibnet.ac.in/bitstream/10603/589/8/08_chapter 2.pdf · Glass...
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).