Experimental Techniques

2.1. Materials

2.1.1. Natural rubber (NR)

N atural rubber, ISNR5, (Indian Standard Natural Rubber-5) was supplied by

the Rubber Research Institute of India, Kottayam. NR, and NR8 are

obtained by mastication of NR for 3 and 8 min, respectively. The physical

characteristics of natural rubber are given in Table 2.1. The rubber from the

same lot has been used in a particular experiment, since the basic properties

such as molecular weight, distribution and the contents of the non-rubber

constituents of NR are affected by clonal variation, season, use of yield stimulants

and methods of preparation [1,2].

2.1.2. Poly(methy1 methacrylate) (PMMA)

Poly(methy1 methactylate) (PMMA1) was synthesised in our laboratoty by

polymerising methyl methactylate using benzoyl peroxide. PMMA2 was supplied

by Gujarat State Fertiiiser Corporation Limited. The characteristics of PMMA

used are given in Table 2.1.

2.1.3. Graft copolymer (NR-g-PMMA)

Graft copolymer of NR and PMMA was prepared in our laboratory by

polymerising methyl methacrylate in the presence of natural rubber latex using a

redox initiator. NR latex particles were swollen with thk.:rqonomer, methyl , %\,,

,<\., methacrylate, which is then polymerised [3]. Cumene &dr+eroxide and 5'' 4 ,

tetraethylene pentammine were the initiator systems which . . pep i t . . the^?^;.' .,. - .- ~ ~

, - . ..

polymerisation to go to completion at room temperature. The NR latex stability

was maintained by the addition of oleic acid with monomer to the ammoniated

NR latex. The graft was purified by fractional precipitation method. Free

PMMA and NR were removed using acetone and petroleum ether respectively.

The purified graft copolymer was dried in vacuum oven for a period of 48 h.

NR-g-PMMA5 and NR-g-PMMAlo were obtained by the mastication of graft

copolymer for 5 and 10 min, respectively. The characteristics of the various graft

copolymers are given in Table 2.1

Table 2.1. Characteristics of the polymers.

Materials -

Density Solubility Molecular ~ w f i n (dcm3) parameter weight

(caycm3)" (Rw)

2.1.4. Solvents

Toluene and chlorobenzene (supplied by the BDH chemicals) distilled and

dried over CaC12 were used. The other solvents acetone, chloroform and

petroleum ether were of analytical grade.

2.1.5. Chemicals

Dicumyl peroxide and tetraethylene pentammine were supplied by RT

Vanderbilt Company, No~walk, USA. Benzoyl peroxide was obtained from BDH

Chemicals Limited.

2.2 Preparation of the blends

2.2.1. Solution cast blends

Binary blends of NR and PMMA with and without graft copolymer were

prepared by solution mixing technique. The polymer components were

dissolved in toluene at a total concentration of 5 % and stirred for about 16 h

using a magnetic stirrer at ambient temperature. The polymer solution was cast

into films and dried at about 1lO"C for several weeks in a vacuum oven to

eliminate the solvent. The solution cast NFUPMMA blends with 0 , 30, 50, 60, 70

and 100% NR are denoted by No, N,, Nw, N,, N,, and Nlm, respectively.

50150 NRPMMA blends with 0, 5, 10 and 1 5 % graft copolymer were denoted by

ON,, 5N50. ION, and 15N,, respectively.

2.2.2. Melt mixed blends

Samples were also prepared by melt mixing the components. Melt mixing

was carried out using a Brabender Plasticorder-PLE 651 at a temperature of

160°C and at a rotor speed of 80 rpm. PMMA was melted for two minutes and

blended with NR for another two minutes. Finally compatibiliser was added and

blended for another four minutes. The molten mixture was removed from the

Brabender mixing bowl, sheeted on a mill and compression moulded into thin

sheets of 2 mm thickness. NFUPMMA melt blends with 0, 50, 70 and 100% of

NR are denoted by No, N',, N'70 and N',oo, respectively. The dynamically

vulcanised blends using DCP and sulphur are denoted by D . The 50150

NRiPMMA melt blends with 0, 5 and 10% graft copolymer are denoted by ON'%,

5N', and ION',, respectively.

2.3. Morphology of the blends

Scanning electron microscopy has been found to be a valuable tool in

studying the phase morphology of high impact strength of blends PP and EPDM

[41.

The morphology of the sample was examined under optical (Inco

Ambala) and scanning electron microscopies (Joel 35 CF). For SEM studies,

samples were fractured under liquid nitrogen and examined under the

microscope. Thin films of 1 0 pm thickness were used for optical microscopy

studies.

2.4. Viscosity measurements

Interactions in binary polymer systems by viscomehy have been reported

by Kulshreshtha et al. [5 ] . The relative viscosities of the polymer solution of

different concentrations and their mixtures were determined by Ubbelodhe type

viscometer (Schott Gerate AVS 400 viscometer). The measurements were

carried out at constant temperature of 28.9 +. 0.010C and was achieved in a

water bath with a thermostat (Schott Gerate CT 145012 thermostat). Blends of

NR and PMMA having 100, 70, 50, 30 and 0% PMMA at maximum

concentration of 0.1 g/dm3 in toluene were prepared for viscometric

experiments.

2.5. Phase separation experiments

Phase separation experiments were carried out by preparing the solution

of 50150, 60140 and 70130 W M M A in toluene with and without the addition

of graft copolymer. The blend solution was stirred for 12 h and kept standing.

The sample was examined for phase separation as a function of time and graft

copolymer concentration. The volume fraction of the phase separated PMMA

layer was measured at different time intervals and graft copolymer concentration.

The experiment was repeated with chlorobenzene solvent, with graft and

homopolymers of various molecular weights and also by changing the mode of

addition of the graft copolymer. Similar phase separation experiments have been

reported by Molau et al. [6,7]

2.6. Mechanical properties

2.6.1. Tensile strength

Tensile strength and elongation at break of the samples were measured at

25°C according to ASTM D6.38 specification using dumb-bell shaped test pieces

at a cross head speed of 500 m d m i n using a Zwick U n i v e ~ l testing machine.

The Young's modulus were determined from the linear portion of the stress-

strain curve.

2.6.2. Tear strength

The tear strength of the sample was determined according to ASTM

D624-81 using 90" angle test pieces. The temperature and cross head speed

used are the same as that for tensile strength measurements.

2.6.3. lzod impact strength

The izod impact strength of samples was measured according to the

ASTM D256 test method. The dimensions of the specimen used were 6.13 x

1.20 x 1.23 cm. The impad energy was obtained by the difference in the

potential energy of the falling hammer before and after impact.

2.6.4. Morphology of the failure surface

Scanning electron microscopy (SEM) has been successi~ib used for

studying the failure surface of rubber composites 18). The SEM observations of

the tensile and tear failure surface were made using scanning electron

microscope. The failure surfaces of the test sampler were carefully cut and

sputter coated with gold and then examined under the microscope.

2.7. Rheological properties

2.7.1. Measurements

The rheological studies were carried out using a capillay rheometer

attached to a Zwick Universal Testing machine model 1474. The capillay used

was made of tungsten carbide and has an Vd ratio 40 and an angle of entry 180".

The sample for testing was placed inside the barrel of the extmsion assembly and

forced down to the capillay with a plunger attached to the moving cross head.

The studies were carried out in the shear range of 1.6 to 833 s-'. The

temperature controller provided the facility to increase the temperature

gradually across the length of the barrel. W~th a single charge of the material the

machine was operated to give nine dierent plunger speed. A warm up period

of 3 min was given to the sample before starting the experiment. The melt was

extruded through the capillary at preselected speed of the cross head. The

forces corresponding to specific plunger speed were recorded using a strip chart

assembly. The force and the cross head speed were converted into shear stres

(T,) and shear rate (f,) at wall, respectively using the following equations

involving the geometry of the capillary and the plunger

where F = force applied a t a particular shear rate

A, = cross sectional area of the plunger (mm2)

6 = length of the capillary (mm)

dc = diameter of the capillary (mm)

Q = the volume flow rate (mrn3/s)

Q is calculated from the velocity of the cross-head and the diameter of the

plunger. The flow behaviour index n' is defined by

and was determined by regression analysis of the values of rw and Y,, obtained

from the experimental data. The apparent wall shear rate (yw,) was calculated as

32Q/nd;. The shear viscosity q was calculated using the equation

Rheological measurements were carried out at temperatures of 130, 140 and

150°C.

2.7.2. Extrudate swell

The extrudates were collected from the capillary die and care was taken

to avoid any deformation. The diameter of the extrudate was measured after

24 h. of extrusion using a travelling microscope. The ratios of the diameter of

the extrudate to that of the capillary was calculated as the die swell (ddd,). The

distortion of the extrudate was studied by taking the photograph of the extrudate

at different shear rates.

2.7.3. Melt flow indices

The melt flow indices (MR) were determined using a Ceast melt flow

indexer model 6542 using 49.05 N load as per ASTM D 123873. The

measurements were made at 200, 210, 220 and 230°C for 50150, 60140 and

70130 W M M A blends with and without the addition of graft copolymer.

2.7.4. Morphology of the extrudate

Morphological characterisation of the extrudate was performed using

scanning electron microscope. The blends with 0, 10 and 15% of graft

copolymer was fractured under liquid nitrogen and examined under scanning

electron microscope. The influence of annealing on the morphology of the

blends has been studied. The blends with 0 and 10% graft copolymer were

annealed by keeping it inside the barrel of the MFl apparatus for 1 h at 220°C

and then extruded. The extrudates before and after annealing were fractured

under liquid nitrogen and the morphology was examined by scanning electron

microscope.

2.8. Thermal analysis

2.8.1. Dynamic mechanical analysis

The dynamic mechanical properties of the NR, PMMA and NRIPMMA

blends were measured using a Rheovibron DDV-IIIC. Moulded samples of

dimensions 7 x 1 x 0.5 cm were used for testing. The samples were tested at a

strain amplitude of 0.0025 cm and at a frequency of 35 Hz. The heating rate of

the sample was 1°C rise in temperature per minute. The complex modulus E"

was calculated with a microcomputer using the following equation.

(L + AL) 1$ E* - - dy nes/cm2

8 x S x A ( D - K )

where F = dynamic complex modulus

L = length of the sample between the clamps

AL = oscillating displacement

S = cross sectional area of the sample

A = amplitude factor

D = value of dynamic force dial

K - - error constant.

The storage modulus E' and loss modulus E are obtained from EH and F

using the following equations.

E - - F S i n 6

E' = F C o s 6

The loss tangent

tan 6 = F I E '

2.8.2. Differential scanning calorimetry

Perkin-Elmer differential scanning calorimeter was used to study the

thermal behaviour of NR. PMMA and W M M A blends with 0, 5 and i C % graft

copolymer. The samples were inserted into the apparatus at room temperature

and immediately heated to 200°C at a rate of WCImin and kept for 1 min at thii

temperature. The samples were quenched to 80°C at a rate of 320"C/min. The

DSC scan was made from -80 to 1lO"C at a heating rate of 1O0C/rnin in the

presence of helium atmosphere.

2.8.3. Thermogravimetry (TG)

Therrnogravimety and derivative thermogravimeby (DTG) were carried

out in a Schirnadzu D T 4 , thermal analyser in nitrogen at a heating rate of

1O0Urnin.

References

1. Subramanyam, A. Rubber Chem. Technol. 1972,15,346.

2. Subramanyam, A. Proc. of the RRIM Planters Conference, Kualumpur, 1971, p. 225.

3. Bloomfield, G. F. and McL Swift, P. J. Appl. Chem. 1955,5, 609.

4 . N. M. Mathew and S. K. De Polymer 1982,23,632.

5 . Kulshreshtha, A. K.; Singh, B. P. and Sharma, Y. N. Polym. J. 1988,24,29.

6 . Molau, G. E. J. Polym. Sci. Part A 1965, 3, 1267.

7 . Molau, G. E. J. Polym. Sci. Part A 1%5, 3,4235.

8. Bas.com, W . D. Rubber Chem. Technol. 1977,50,327.