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134
Chapter 4 RESULTS AND DISCUSSION
4.1. Introduction4.2. Study of thermal properties4.3. Study of electrical properties4.4. Study of mechanical properties4.5. Study of FRLS properties4.6. Calculation steps4.7. Conclusion
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4.1. Introduction
The ideal, that is, an ever-continuing search for a polymer
composition that produces no smoke at all and shows zero affinity for fire
shall always remain. This chapter deals with the experimental results
obtained during the course of the work and a systematic study of the
observations.
The experimental work comprises preparation of polymer
‘compounds’ of varying compositions and study of various properties of
these compounds. The study of various properties can broadly be organized
under four headings: study of thermal properties, study of electrical
properties, study of mechanical properties and study of FRLS properties.
Polymeric materials can be classified in a variety of ways: (i) based
on their origin, into natural and synthetic and, (ii) based on their physical
properties, into elastomers, plastics and fibers. Elastomers (or rubbers) are
characterized by a long-range extensibility that is almost completely
reversible at room temperature. Plastics have only partially reversible
deformability, while fibers have very high tensile strength but low
extensibility. Plastics can be further subdivided into thermoplastics (whose
deformation at elevated temperatures is reversible) and thermosets (which
undergo irreversible changes when heated). Elastomers have elastic modulii
between 105 and 106 N/m2, while plastics have modulii between 107 and 108
N/m2, and fibers have modulii between 109 and 1010 N/m2. In terms of
elongation, elastomers can be stretched roughly up to 500 to 1000 percent,
plastics between 100 to 200 percent, and fibers only 10 to 30 percent before
fracture of their material is complete.
Polymers can also be classified in terms of their chemical
composition; this gives a very important indication as to their reactivity,
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including their mechanism of thermal decomposition and their fire
performance.
The main carbonaceous polymers with no heteroatom are polyolefins,
polydienes and aromatic hydrocarbon polymers (typically styrenics). The
main polyolefins are thermoplastics: polyethylene and polypropylene, which
are two of the three most widely used synthetic polymers. Polydienes are
generally elastomeric and contain one double bond per repeating unit; the
common examples are ABS (Acrylic Butadiene Styrene terpolymers) and
EPDM (Ethylene Propylene Diene rubbers). The most important aromatic
hydrocarbon polymers are based on polystyrene.
The most important oxygen-containing polymers are cellulosics,
polyacrylics, and polyesters.
Nitrogen-containing materials include nylons, polyurethanes,
polyamides and polyacrylonitrile.
Chlorine-containing polymers are exemplified by PVC. It is the most
widely used synthetic polymer, together with polyethylene and
polypropylene. It is unique in that it is used both as a rigid material
(unplasticized) and as a flexible material (plasticized). Flexibility is achieved
by adding plasticizers or flexibilizers. Through additional chlorination of
PVC, another member of the family of vinyl materials is made: CPVC, with
different physical and fire properties from PVC.
Fluorine-containing polymers are characterized by high thermal and
chemical inertness and low coefficient of friction. The most important in the
family is PTFE [108(i)].
4.2. Study of thermal properties
Solid polymeric materials undergo both physical and chemical
changes when heat is applied; this usually results in undesirable changes to
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the properties of the polymer. A clear distinction needs to be made between
thermal decomposition and thermal degradation. Thermal decomposition is
‘a process of extensive chemical species change caused by heat’. Thermal
degradation is ‘a process whereby the action of heat or elevated temperature
on a material, product or assembly causes a loss of physical, mechanical or
electrical properties’.
In the context of fire, the important change is thermal decomposition,
whereby the chemical decomposition of a solid material generates volatile
gaseous fuel vapors, which can burn above the solid material. In order for the
process to be self-sustaining, it is necessary for the burning gases to feed
back sufficient heat to the material to continue the production of gaseous fuel
vapors or volatiles. As such, the process can be a continuous feedback loop if
the material continues burning. In that case, heat transferred to the polymer,
ΔH2, causes the generation of flammable volatiles; these volatiles react with
the oxygen in the air above the polymer to generate heat, ΔH1, and a part of
this heat is transferred back to the polymer to continue the process [108(ii)],
as shown in the figure 4.1.
Figure 4.1. Polymer combustion feed-back loop
However, both, the chemical and the physical aspects of thermal
decomposition of polymers are important. The chemical processes are
responsible for the generation of flammable volatiles while physical changes,
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such as melting and charring, can markedly alter the decomposition and
burning characteristics of a material.
The dependence of thermal decomposition on heating rate is due to
the fact that the rate of thermal decomposition is not only a function of the
temperature, but also of the amount and nature of the decomposition process
that has preceded it.
Materials that are stable at high temperatures are likely to be better
performers as far as fire properties are concerned. However, there are several
reasons why the relevance of TGA studies to fire performance is being
questioned: heating rate, amount of material and lack of heat feedback are
the major ones. For example, it is well-known that heating rates of 10-100
K/s are common under fire conditions, but are rare in thermal analysis. More
seriously, TGA studies are incapable of simulating the thermal effects due to
large amount of material burning and resupplying energy to the decomposing
materials at different rates. However, analytical TGA studies do give
important information about the decomposition process, even though
extreme caution needs to be exercised in their direct application to fire
behavior.
Differential thermogravimetry (DTG) is exactly the same as TGA,
except that the mass loss versus time output is differentiated automatically to
give the mass loss rate versus time. DTG is the best indicator of the
temperatures at which the various stages of thermal decomposition take place
and the order in which they occur [108(iii)].
The major reason why thermal decomposition of polymers is studied
is because of its importance to fire performance. Early on, comparisons were
attempted between the minimum decomposition temperature, Td (or, even
better, the temperature for 1% thermal decomposition, T1%) and the Limiting
Oxygen Index. The conclusion was that, although low flammability resulted
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from high minimum thermal decomposition temperatures, no easy
comparison could be found between the two. There were some notable cases
of polymers with both low thermal stability and low flammability. From the
considerable amount of work done in this area, it can be safely deduced that
thermal decomposition cannot be a stand-alone means of predicting fire
performance. A high LOI does not always mean a high T1% or vice-versa.
But, whatever the degree of predictability of fire performance data
from thermal decomposition data, its importance should not be
underestimated: Polymers cannot burn if they do not break down.
Table 4.1. Thermal Stability and Flammability of Polymers
Polymer aTdbT1%
cLOIPolypropylene 531 588 17.4
LDPE 490 591 17.4
HDPE 506 548 17.4
Poly(vinylidenefluoride) 628 683 43.7PVC 356 457 47.0
aTd: Minimum thermal decomposition temperature from TGA (10 mg sample, 10 K.min-1 heating rate, N2 atm.), bT1%: Temperature for 1% thermal decomposition, cLOI: Limiting Oxygen Index
More complete and detailed surveys of polymers and their thermal
decomposition are available in the literature [108(iv),(v)].
Thermal decomposition of PVC has been one of the most widely
studied, given its commercial importance and range of applications. Between
225-275oC, HCl gas is evolved almost quantitatively, by a chain-stripping
mechanism. It is very important to point out, however, that the temperature at
which HCl is evolved in a measurable way is highly dependent on the
stabilization package used. Thus, commercial PVC compounds have been
shown, in recent work, not to evolve HCl until temperatures exceed 250 oC
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and to have a dehydrochlorination stage starting at 325oC. Between 425-475 oC, hydrogen is evolved during carbonization, following cyclization of the
species involved. At temperatures above 475oC, the char (resulting from
dehydrochlorination and further dehydrogenation) is oxidized, leaving no
residue. Dehydrochlorination stabilizers include zinc, cadmium, lead,
calcium and barium salts and organotin derivatives [109-112].
In the present work, thermograms were recorded for different
polymer compounds with a view to understand their thermal behavior and
the thermal rating values and kinetic properties like activation energy and
rate constant were calculated. In a few cases, glass transition temperatures
were also measured using Thermo Mechanical Analysis (TMA) apparatus.
Thermal stability-time was measured in the relevant cases.
4.2.1. The thermal stability-time recording procedure
The thermostat is set at 199.9oC. The previously weighed (~ 50 mg)
samples (in small pieces), taken in sample tubes, are places in the designated
‘cavities’ on the thermostatically controlled heating chamber. Three small
pieces of ph paper are rolled into cylindrical shape and inserted into the top
portion of the sample tubes. Every fifteen minutes for the first one hour, and
every five –minute intervals thereafter, the ph paper pieces are examined and
rated visually for any color change. The color change indicates
decomposition of the sample evolving HCl vapors. The observations are
tabulated.
Figure 4.2. The sample tube used for thermal stability-time test
4.2.2. Studies on PVC K-70 compositions
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Seven samples were prepared using fixed concentrations of K-70
grade PVC, CaCO3, martinal, Mg(OH)2, Sb2O3, tribasic lead sulfate (TBLS),
calcium stearate and paraffin wax and variable concentrations of the two
plasticizers, DOP and TOTM and the two additives, glass and cenospheres.
The objective of the study was to examine the comparative effect of
the two plasticizers, DOP and TOTM, and the additives glass and
cenospheres on the thermal stability of the compositions.
Table 4.2 gives details of the seven compositions prepared using the
various ingredients.
Table 4.2. Compositions of the PVC systems with DOP and TOTM as plasticizers with 100 phr PVC
SlNo
Components (in phr units)(a) (b) (c) (d) (e) (f
)(g) (h) (i) (j) (k) Sample
Code1 30 06 16 28 4 2 8 0.6 0.4 09 09 PM012 12 24 16 28 4 2 8 0.6 0.4 18 - PM023 24 12 16 28 4 2 8 0.6 0.4 18 - PM034 06 30 16 28 4 2 8 0.6 0.4 09 09 PM045 - 36 16 28 4 2 8 0.6 0.4 - 18 PM056 36 - 16 28 4 2 8 0.6 0.4 18 - PM067 - 36 16 28 4 2 8 0.6 0.4 18 - PM07
(a).DOP, (b). TOTM, (c). CaCO3, (d). martinal (e). Mg(OH)2,(f). Sb2O3, (g). TBLS, (h). calcium stearate, (i). paraffin wax, (j). glass, (k).cenospheres
TGA studies were carried out for all the samples using the
thermogravimetric analyzer, using different heating rates. From the
thermograms, the T5% (temperature at 5% weight loss) and T10% (temperature
at 5% weight loss), were noted. However, for the present purposes, only T5%
values would be sufficient and these are presented in table 4.3.
Table 4.3. TGA and Tg data and kinetic properties for PM01 through PM07
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Heating rateβ (o C . min –1 )
T5%
PM01 PM02 PM03 PM04 PM05 PM06 PM07T
GA
2 205 206 204 264 267 252 2584 219 214 213 274 288 256 2818 225 234 235 286 293 273 28912 - 236 243 291 303 285 295
Thermal rating, θ (o C)
107.5 91.7 91.75 162.3 120 109 155
E (Kcal. K–1. mol–1)
29.13 24.00 23.55 36.22 21.74 21.74 23.48
Tg (o C) - - - - 110.1 83.14 79.38
From the above table, it is clear that PM05 shows the highest value
for T5% for all the rates of heating carried out. The composition PM03 shows
the lowest value for T5% for lower heating rates. However, no regular trend in
the variation of T5% is discernable with different heating rates.
Using Ozawa’s method and Toop’s equation, respectively, the
activation energy for thermal decomposition, E, and the thermal rating, θ,
were calculated (the details of the equations are dealt with later in this section
and also in Chapter 5).
In table 4.2., first, let us consider the two compositions, PM01 and
PM04. In these, the quantities of plasticizers, DOP and TOTM, are reversed,
concentrations of all other ingredients remaining same in both. From table
4.3., it can be seen that E and θ values for PM01 and PM04 are 29.13
Kcal.K–1.mol–1 and 36.22 Kcal.K–1.mol–1 and 107.5oC and 162.3oC,
respectively. The data suggest that TOTM, compared to DOP, at the same
level of phr, imparts better thermal stability on the polymer system
considered. Further, incidentally, it is for PM04 that the θ value, at 162.3 o C,
is maximum, compared to that of the other compositions listed in the table
4.2. Thus, from standpoint of thermal stability alone, TOTM seems to be a
better plasticizer than DOP in this context.
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Considering the other two comparable compositions, PM02 and
PM04, the distinguishing feature, again, is plasticizer concentration: PM02
having TOTM concentration double that of DOP, and PM03, having the
concentrations of the two plasticizers reversed. In these two cases, E and θ
values appear to be nearly same and somewhat low, probably because glass
additive, at 18 phr, has made a difference, masking the otherwise possible
difference in θ value, and also by lowering the thermal rating value to some
extent.
A closer look at the table 4.2. would reveal that whereas the four
compositions, PM01 to PM04, contain two plasticizers, the remaining three
compositions, PM05, PM06 and PM07, have only one of the two
plasticizers, DOP or TOTM, and at higher loadings (36 phr). Among PM05
and PM07, the two differing only in respect of glass/cenosphere
concentration, the additive glass has led to higher thermal stability compared
to cenospheres. Between PM06 and PM07, the composition PM07, having
TOTM as plasticizer, has a larger θ value than the composition PM06,
having DOP at the same concentration. This observation reinforces the
inference made above: thermal stability is better with TOTM compared to
that with DOP.
The most desirable combination from thermal stability standpoint is
that of PM04, which contains two plasticizers and two filler additives,
probably suggesting that there is some synergism at work here.
For the compositions PM05, PM06 and PM07, the glass transition
temperatures (Tg oC) were recorded. The composition PM07 has the lowest
Tg of the three, possibly suggesting that the absence of DOP and the additive
cenospheres could be the reason for the lower value.
A set of five compositions, PM08 to PM12, were prepared using two
specially procured PVC resins: CP 172 SG and K6701. The former is a
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specialty emulsion polymerized resin made for the manufacture of sintered
battery separators. Its carefully controlled particle size and shape enable the
production of separators with minimum thickness and optimum mechanical
properties. A recent study [113] suggests that while both the samples CP 172
SG and K6701 are usable for LT PVC cable sheathing, only sample K 6701
can be used for insulation.
The two resins were mixed in different ratios, as given in table 4.4.
The anti-oxidant bisphenol-A has been added here; only DOP is the
plasticizer used. The objective was to investigate the thermal behavior of
these resin samples, when the polymer matrix contained no flame-retardants
or filler additives and contained only the plasticizer DOP, calcium carbonate
filler, stabilizer tribasic lead sulfate, stabilizer lubricant calcium stearate and
the anti-oxidant bisphenol-A.
Table 4.4. Compositions of the PVC systems (Specially procured PVC resins)
Sample Code
Components (in phr units)
a1 a2 b c d e f g h i j k l m
PM08 100 -35
- 20 - - - 5 0.1 - - - 0.1
PM09 - 10035
- 20 - - - 5 0.1 - - - 0.1
PM10 50 5035
- 20 - - - 5 0.1 - - - 0.1
PM11 30 7035
- 20 - - - 5 0.1 - - - 0.1
PM12 20 8035
- 20 - - - 5 0.1 - - - 0.1
(a1). PVC6701, (a2). PVC CP 172 SG, (b).DOP, (c). TOTM, (d). CaCO3, (e). martinal (f). Mg(OH)2, (g). Sb2O3, (h). TBLS, (i). calcium stearate, (j). paraffin wax, (k). glass, (l).cenospheres, (m). bisphenol A
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Table 4.5. TGA and Tg data for PM08 through PM12
β(o C . min –1 )
T5%
PM08 PM09 PM10 PM11 PM12
TG
A
2 202 207 204 207 2114 220 220 222 222 2278 224 248 242 240 23912 262 263 260 244 241
Tg (o C) 141.11 139.88 139.98 138.93 139.62
Though the thermal stability in the first three cases, that is, PM08 to
PM 10, is almost same, it shows a decrease in the latter two cases, if one
were to consider the higher heating rate of 12o C.min –1. However, it is not
possible to arrive at any significant correlation between the compositions and
their thermal behavior, as the T5% readings at different heating rates given in
the table 4.4. clearly show an erratic trend. As no meaningful deductions
were possible from TGA studies, the transition temperature studies were
carried out. The data show that all the five compositions have virtually the
same Tg value and the value is considerably higher than that of the pure PVC
resin (75 – 85 o C). In the light of these observations, the two resins were
considered ‘not suitable’ for further investigations and no other properties
were evaluated for them.
Nine more compositions were prepared using DOP and TCP (in place
of TOTM) as plasticizers; Sb2O3 was not included; zinc borate, a smoke-
suppressant, was included. Different ratios of various additives were tried
out, taking the concentrations of only DOP, martinal and glass as variables
and keeping concentrations of all other components constant. The
compositions are given in table 4.6.
146
The compositions PM13, PM14 and PM15 differ only in the
concentration of one component, glass. The next three, PM16, PM17 and
PM18, differ only in the concentration of martinal. Similar observations can
be made for the other compositions as well.
Since some processing issues came up hindering smooth blending
operation, and proper blending required 4 to 6 phr of additional DOP than
originally intended, further investigations were not taken up for these
compositions. Only electrical properties were recorded.
Table 4.6. Compositions of the PVC systems with different additives
Sl. No.
Components (in phr units) Sample Code
(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l)
1 100
36 8 14 24 3 1 10 0.2 0.2 10 0.5 PM13
2 100
36 8 14 24 3 1 10 0.2 0.2 15 0.5 PM14
3 100
36 8 14 24 3 1 10 0.2 0.2 20 0.5 PM15
4 100
36 8 14 28 3 1 10 0.2 0.2 10 0.5 PM16
5 100
36 8 14 32 3 1 10 0.2 0.2 10 0.5 PM17
6 100
36 8 14 26 3 1 10 0.2 0.2 10 0.5 PM18
7 100
30 8 16 26 3 1 10 0.2 0.2 10 0.5 PM19
8 100
36 8 16 26 3 1 10 0.2 0.2 10 0.5 PM20
9 100
36 8 14 26 3 1 10 0.2 0.2 15 0.5 PM21
(a). PVC, (b).DOP, (c). TCP, (d). CaCO3, (e). martinal (f). Mg (OH) 2, (g). zinc borate, (h). Ca/Zn stabilizer, (i). calcium stearate, (j). stearic acid, (k). glass, (l). bisphenol A
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After a thorough consideration of the functions of each of the
constituents, suitable alterations were made in their relative proportions, and
three compositions, PM22, PM23 and PM24, formulated. These are given in
table 4.7. These compositions showed better performance on the whole; TGA
data were not recorded as it was not considered necessary. Electrical,
mechanical and FRLS properties were recorded.
Table 4.7. Compositions of the PVC systems with different ratios of additives
Sl. No.
Components (in phr units) Sample Code
(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l)
1 100
30 8 16 34.3
4.3 1 10 0.2 0.2 10
0.5 PM22
2 100
30 8 16 34.3
4.3 1 10 0.2 0.2 20
0.5 PM23
3 100
30 8 16 34.3
4.3 1 10 0.2 0.2 30
0.5 PM24
(a). PVC, (b).DOP, (c). TCP, (d). CaCO3, (e). martinal (f). Mg (OH)2, (g). zinc borate, (h). Ca/Zn stabilizer, (i). calcium stearate, (j). stearic acid, (k). glass, (l). bisphenol A
On further considerations, a batch of five compositions, PM25 to
PM29, with cenospheres, the inert filler, as the only variable, were prepared.
DOP and TOTM were the plasticizers used. These compositions showed
impressive results in all the aspects studied.
Table 4.8. Compositions of the PVC systems with cenospheres as filler additives with 100 phr of PVC
Sample Code
Components (in phr units)
a b c d e f g h I j k
PM25 - 36 16 28 4 3 8 0.6 0.4 - -
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PM26 - 36 16 28 4 3 8 0.6 0.4 - 6PM27 - 36 16 28 4 3 8 0.6 0.4 - 12PM28 - 36 16 28 4 3 8 0.6 0.4 - 18PM29 - 36 16 28 4 3 8 0.6 0.4 - 24
(a). DOP, (b). TOTM, (c). CaCO3, (d). martinal (e). Mg(OH)2, (f). Sb2O3, (g). TBLS, (h). calcium stearate, (i). paraffin wax, (j). glass, (k). cenospheres
The TGA thermograms were recorded for each of these five
compositions and using the thermal data, the kinetic parameters for the
thermal decomposition and the thermal rating values were calculated.
Table 4.9. TGA and Tg data and kinetic properties for pM25 through PM29
β(o C.min –1)
Sample Code
PM25 PM26 PM27 PM28 PM29T5% (o C)
TG
A
2 270 260 263 265 266
4 279 272 273 279 280
8 294 289 289 289 293
12 300 293 298 299 299Thermal rating,
θ (oC )183 148 153 162 184
E (Kcal. K–1. mol–1)
44.31 31.04 31.96 35.18 38.89
Tg (o C) 82.13 66.55 - 77.63 -
4.2.2.1. Energetics of the reaction
Using different heating rates, a series of curves (shown in the figure
4.2.) are recorded, the 5% loss temperatures shifting to lower values with
reduced scan rate.
The temperatures at 5% weight loss, as noted in table 4.9, are used to
calculate the activation energy, E, using the following equation (Ozawa’s
method):
- log β – 0.457 E/R ( 1/Tp ) = constant or
149
- log β = constant + (– 0.457 E/R) (1 / 103). (1/Tp *103).
The slope (m) = (– 0.457 E/R) (1/103)
Knowing the slope, the activation energy, E, is calculated.
4.2.2.2. Evaluation of thermal condition
Reports on the application of thermal analysis for estimation of
thermal life of wire enamels from the decomposition reactions are available
in literature. The mathematical expression based on life theory and
thermogravimetric theory is given by Toop:
log tf = E/2.303 Rθ + log E p(xf)/ βR
where log p(xf) = -2.315 – 0.457 E/R Tf
E = energy of activation, R = gas constant, Tf = temperature at which specific change is observed, β = heating rate, θ = thermal condition and tf
= time to condition at temperature θ Because of the simplicity and reliability, this approach is generally
preferred for determining thermal rating where thermal degradation is the
result of simple chemical reaction.
Figure 4.3. Representative TGA Curves
150
In the filler concentration range studied, both, the 5% and 10% loss
temperatures, somewhat increase with the filler concentration. The activation
energy for decomposition of the compound decreases in presence of
cenospheres, the magnitude of this decrease being maximum at 6 phr of filler
and becoming progressively less pronounced at higher filler concentrations.
A similar trend is noted for thermal rating values as well.
4.2.3. Glass transition temperature (Tg) studies
At a relatively simple-minded practical and operational (and thus
theoretically non-rigorous) level of treatment, glass transition temperature
may be defined as the temperature at which the forces holding the distinct
components of an amorphous solid together are overcome by thermally
induced motions within the time-scale of the experiment, so that these
components become able to undergo large-scale molecular motions on this
time-scale, limited mainly by the inherent resistance of each component to
such flow. The practical effects of the glass transition on the processing and
performance characteristics of polymers are implicit in this definition. The
standard Tg value for PVC from literature is 75 - 82 oC.
For the three compositions PM25, PM26 and PM28, the Tg values
were measured using the thermo mechanical analyzer (TA Instruments: TMA
Q400 Model). The curves are shown in the figures 4.3., 4.4. and 4.5. and
readings are given in table 4.9. The Tg value decreases in presence of
cenospheres, again the decrease being maximum at the lowest concentration
studied. Lower Tg would mean increased polymer flexibility.
Glass transition temperatures
Sample Tg ( o C) PM 25 82.13PM 26 66.55
151
PM 28 77.63
A careful consideration of the consolidated data obtained so far led to
the following three formulations, PM30, PM31 and PM32. Their mechanical
properties were measured and are given in table 4.20 of this chapter.
Figure 4.4. TMA curve for the composition PM25
Figure 4.5. TMA curve for the composition PM26
152
Figure 4.6. TMA curve for the composition PM28
Table 4.10. Compositions of the PVC systems PM30, PM31 and PM32
Sample Code
Components (in phr units)a b c d e f g h i j k l
PM30 100 30 8 14 30 3 1 10 0.2 0.2 14 0.6PM31 100 30 8 14 30 3 1 10 0.2 0.2 15 0.6PM32 100 30 8 12 30 3 1 10 0.2 0.2 15 0.6
(a). PVC, (b).DOP, (c). TCP, (d). CaCO3, (e). martinal (f). Mg(OH)2, (g). zinc borate, (h). Ca/Zn stabilizer, (i). calcium stearate, (j). stearic acid, (k). glass, (l). bisphenol A
To understand the effect of drying on properties, the components
PVC, CaCO3, martinal and glass were dried at 105 oC for one hour and five
compositions were prepared, choosing the composition PM31 as basis.
These were designated as PM31.1 (30 minutes blending), PM31.2 (25
minutes blending), PM31.3 (20 minutes blending), PM31.4 (15 minutes
blending) and PM31.5 (10 minutes blending), respectively. As an additional
confirmatory measure, four more compositions were prepared (the
components are as taken for PM31), two, using components without drying
153
and two, using components with drying. Blending times were also different.
They were designated as PM31 (R1), PM31 (R2), PM31 (R1-D) and PM31
(R2-D), ‘D’ indicating that dried components were used.
Their electrical and mechanical properties for all the nine cases were
evaluated and the values are given in tables 4.23. and 4.24.
Further considerations led to the following 3 formulations.
Table 4.11. Compositions of the PVC systems PM33, PM34 and PM35
Sample Code
Components (in phr units)(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l)
PM33 100 30 10 12 30 3 1 10 0.2 0.2 15 0.6PM34 100 30 9 12 30 3 1 10 0.2 0.2 14 0.6PM35 100 30 10 14 30 3 1 10 0.2 0.2 15 0.6
(a). PVC, (b).DOP, (c). TCP, (d). CaCO3, (e). martinal (f). Mg(OH)2, (g). zinc borate, (h). Ca/Zn stabilizer, (i). calcium stearate, (j). stearic acid, (k). glass, (l). bisphenol A
On considering the entire evaluation data, the composition PM34 was
selected as the preferred choice and repeat trials (nine times) conducted.
Three of them (the relevant data for them are shown in later pages) are
designated as PM34.1, PM34.2 and PM34.3.
4.2.4. DTG studies
DTG curves were recorded for some compositions in order to
supplement the thermal decomposition data obtained using TGA. The
representative curves are shown in the figures 4.7. to 4.10.
154
Figure 4.7DTG curve for a PVC composition at 2oC heating rate
Figure 4.8DTG curve for a PVC composition at 4oC heating rate
Figure 4.9DTG curve for a PVC composition at 8oC heating rate
155
Figure 4.10DTG curve for a PVC composition at 12oC heating rate
The curves give accurate values for the decomposition temperature at
each heating rate and also show that the decomposition temperature shifts to
lower values with reduced heating rate.
4.3. Study of electrical propertiesSurface resistivity, volume resistivity, capacitance and tan δ: these
properties together characterize dielectric properties of electrical insulation
materials (conductive materials are those having a surface resistivity < 1x105
Ω/sq (or Ω) or volume resistivity < 1x104 Ω-cm).
The higher the values for surface and volume resistivities, the better it
is for an insulation material.
Capacitance is the measure of a capacitor's ability to store an electric
charge on its plates. Thus, for an insulation material, the lower the
capacitance value, the better.
The greater the ‘phase defect angle’, δ, the greater is the tan δ value
and the worse is the cable.
The insulating material is prone to stresses like thermal stress,
electrical stress, mechanicals stress, and environment stress etc. The normal
practice is that capacitance and tan δ values are obtained on new insulation
are treated as benchmark readings. By measuring and comparing the
periodical readings of the capacitance and tan δ of the insulating material
156
with the benchmark readings, one can know the rate of deterioration of the
health of the insulation.
The electrical properties recorded for different compositions are
tabulated in tables 4.12. to 4.17.
Table 4.12. Electrical properties for compositions PM01 to PM05
Sample Code Electrical propertiesCapacitance (30 o C) (pF) tan δ (30 o C)
PM01 - -PM02 22.259 0.0644PM03 24.095 0.0600PM04 34.16 0.06989PM05 26.15 0.0596
From table 4.12., it is observed that at 30 o C, maximum value for
both capacitance and tan δ is shown by the composition PM04, whereas
minimum value for capacitance and tan δ is shown by the compositions
PM02 and PM05, respectively.
Table 4.13. Electrical properties for compositions PM13 to PM21
Sample Code Electrical propertiesCapacitance (30 o C) (pF) tan δ (30 o C)
PM13 47.323337 0.1375767PM14 47.63434 0.1315667PM15 49.788033 0.1384933PM16 48.79841 0.1418267PM17 47.259007 0.1395333PM18 47.473043 0.1312933PM19 43.772187 0.1166433PM20 49.975113 0.1471333PM21 46.58698 0.130900
The electrical properties recorded at 30oC for the next batch of nine
compositions are tabulated in table 4.13. It is observed that the minimum
value for both capacitance and tan δ is shown by the composition PM19.
157
Table 4.14. Electrical properties for compositions PM22 to PM24
Sample Code
Electrical propertiesSurface
resistivity
Volume
resistivity
Capacitance (pF)
(30 o C)tan δ
(30 o C)
PM22 21.44E13 6.0241E12 39.7236 0.1031133PM23 19.05E13 3.6533E12 40.0366 0.1083233PM24 14.104E13 2.3130E12 35.9868 0.11658
The three compositions listed in table 4.14. above showed much
better (lower) values for capacitance and tan δ. The lowest value for
capacitance tan δ is shown by PM24. The surface resistivity and volume
resistivity values are also good; the highest for both these values is shown by
the composition PM22. The anti-oxidant BPA, present in the three
compositions, makes a strong positive contribution to volume resistivity.
Table 4.15. Electrical properties for compositions PM125 to PM29
Sample CodeElectrical properties
Surface resistivity
Volume resistivity
Capacitance (pF)(30 o C)
tan δ(30 o C)
PM25 94.91E13 5.95E13 34.82 0.07619PM26 73.66E13 5.747E13 30.902 0.07769PM27 70.69E13 4.546E13 30.490 0.07654PM28 72.30E13 6.86E13 30.401 0.07592PM29 83.37E13 1.569E13 30.051 0.07500
The next batch of five compositions prepared using cenospheres as
fillers showed further improvement in electrical characteristics. From table
4.15., it can be noticed that the capacitance and tan δ values are still lower
compared to the values for the compositions listed in the tables so far. The
lowest value for capacitance and tan δ is shown by the composition PM29,
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containing maximum filler concentration of 24 phr, possibly implying the
ability of this filler to improve electrical characteristics. It is also clear that
the volume resistivity and surface resistivity values are quite good, though,
the highest values for these properties are shown by the composition not
containing cenospheres.
Effect of temperature on tan δ and capacitance was studied for a few cases.
Table 4.16. Effect of temperature on capacitance and tan δ for compositions PM22, PM23 and PM24
Temperature Sample CodePM22 PM23 PM24
30 o C Capacitance (pF) 39.7236 40.0366 35.9868
tan δ 0.1031133 0.1083233 0.11658
60 o C Capacitance (pF) 58.3899 59.5654 58.4614
tan δ 0.3761167 0.4137867 0.5678367
90 o C Capacitance (pF) 109.9661 101.8278 -tan δ 2.066 2.2930 -
As can be expected, the capacitance and tan δ values show a regular
increase with increase in temperature from 30 o C to 90 o C, in case of the
three compositions studied.
Table 4.17. Effect of temperature on capacitance and tan δ for compositions PM25 through PM29
Temperature Sample CodePM25 PM26 PM27 PM28 PM29
30 o C Capacitance (pF)
34.817 30.902 30.490 30.401 30.051
tan δ 0.07619 0.0776 0.07592 0.0759 30.051
159
9 2
60 o C Capacitance (pF)
50.417 44.697 47.231 44.744 42.734
tan δ 0.05901 0.06060
0.06159 0.06143
0.06260
90 o C Capacitance (pF)
55.956 45.111 50.271 45.609 45.132
tan δ 0.06869 0.06850
0.08395 0.07967
0.08491
The temperature effects on the five compositions listed in the table
4.17. show that the capacitance values register a regular increase with
increase in temperature from 30 oC to 90 oC for each of the five
compositions, though the increase in the tan δ values is not regular in the
temperature range studied.
4.4. Study of mechanical properties
Tensile strength (T.S.) and % elongation at break: these are the main
mechanical properties of interest in the present context. These properties are
measured in relevant cases and the data tabulated below. Target values are:
Min: 12.5N/mm2 and Min: 150 %.
Table 4.18. Tensile strength and elongation at break for the three compositions PM22, PM23 and PM24
Composition Tensile Strength, (N/mm2) % Elongation at break
trials average Trials averagePM22
Specimen 1 14.0914.49
125142Specimen 2 15.39 160
Specimen 3 13.99 140PM23
Specimen 1 13.9114.66
130143Specimen 2 14.95 135
Specimen 3 15.12 165
160
PM24Specimen 1 13.16
12.65170
153Specimen 2 12.81 165Specimen 3 11.97 125
Mechanical properties were studied for the compositions PM22,
PM23 and PM24, that showed good electrical properties, and the data are
given in table 4.18. As can be seen, for these compositions, mechanical
properties are not in the fully acceptable range.
Table 4.19. Mechanical properties for compositions PM25 through PM29
Sample Code
Mechanical propertiesT.S. (MPa) Elongation at break, %
PM25 19.0 127.7PM26 16.8 139.6PM27 16.9 71PM28 15.0 58.5PM29 12.8 53.3
The compositions PM25 to PM29, that showed promise in terms of
thermal and electrical properties, were studied. Their mechanical properties,
given in table 4.19., show that the more desirable values are showed by the
composition PM26, containing cenospheres concentration at 6 phr level (but,
lowest capacitance and tan δ values were shown at cenospheres
concentration of 24 phr).
Table 4.20. Tensile strength , elongation & thermal stability for the three compositions PM30, PM31and PM32
Composition Tensile Strength, (N/mm2)
% Elongation at break
Thermal Stability,(minutes)
trials average trials averagePM30 Specimen 1 15.78 100
161
15.69 120 95 – 100Specimen 2 14.62 110Specimen 3 16.21 115Specimen 4 16.14 155
PM31
Specimen 1 15.2514.92
165164 75 – 80Specimen 2 14.00 175
Specimen 3 14.86 145Specimen 4 15.56 170
PM32
Specimen 1 15.6015.60
95135 95 – 100Specimen 2 15.34 155
Specimen 3 15.65 155Specimen 4 15.80 135
The set of three compositions PM30, PM31 and PM32 were studied.
Their mechanical properties and also thermal stability time are given in table
4.20. The more desirable values are shown by the composition PM31.
However, the thermal stability value for this composition needs
improvement.
Table 4.21. Mechanical properties and thermal stability-time for the three compositions PM33, PM34 and PM35
Composition Tensile Strength, (N/mm2)
% Elongation at break
Thermal Stability,(minutes)trials average trials average
PM33
Specimen 1 14.9714.93
9086 85 – 90Specimen 2 14.81 65
Specimen 3 15.23 90Specimen 4 14.70 100
PM34
Specimen 1 18.1217.52
95100 95 – 100Specimen 2 16.91 105
Specimen 3 17.62 105Specimen 4 17.42 95
PM35
Specimen 1 14.7415.54
85106 90 – 95Specimen 2 16.51 120
Specimen 3 15.52 110Specimen 4 15.37 110
162
From table 4.21., it is observed that maximum tensile strength value is
shown by the composition PM34. The thermal stability time also meets the
requirement better than the other two compositions.
The data in tables 4.20. and 4.21. illustrate that small variations in
concentrations of plasticizer, filler and flame-retardant additives can together
bring about significant differences in tensile properties. The synergistic effect
is probably better at 9 phr TCP, 12 phr CaCO3 and 14 phr glass.
Table 4.22. Mechanical properties and thermal stability-time for the composition PM34 to verify repeatability
CompositionTensile Strength,
(N/mm2)% Elongation at
breakThermal Stability,(minutes)trials average trials average
PM34.1
Specimen 1 14.2314.49
95149 85 – 90Specimen 2 14.32 165
Specimen 3 14.48 165Specimen 4 14.92 170
PM34.2
Specimen 1 14.0515.06
100139 85 – 90Specimen 2 16.82 170
Specimen 3 13.13 110Specimen 4 16.23 175
PM34.3
Specimen 1 14.9614.91
135153 80 – 85Specimen 2 14.27 135
Specimen 3 14.92 170Specimen 4 15.49 170
In order to verify the repeatability of the performance characteristics
by the composition PM34, three more samples were prepared and their
mechanical properties and thermal stability studied. The data are given in
table 4.22. It is clear that the variation in properties is within acceptable
range. The composition is being considered for scale-up studies and
extrusion trials.
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Table 4.23. Mechanical properties and thermal stability-time for the composition PM31with varying blending times and drying of components
Composition Tensile Strength, (N/mm2)
% Elongation at break
Thermal Stability,(minutes)trials average trials average
PM31.1 (30 min. blending)Specimen 1 15.64
16.19175
158 95 - 100Specimen 2 16.75 130Specimen 3 16.72 195Specimen 4 15.64 130
PM31.2 (25 min. blending)Specimen 1 15.40
15.67185
170 95 – 100Specimen 2 16.29 160Specimen 3 15.85 165Specimen 4 15.12 170
PM31.3 (20 minutes blending)Specimen 1 13.18
14.48155
165 90 - 95Specimen 2 14.49 180Specimen 3 14.72 165Specimen 4 15.53 160
PM31.4 (15 minutes blending)Specimen 1 15.40
15.30195
184 90 – 95Specimen 2 15.89 190Specimen 3 15.07 175Specimen 4 14.82 175
PM31.5 (10 minutes blending)Specimen 1 15.80
14.23170
155 90 - 95Specimen 2 12.88 150Specimen 3 14.71 155Specimen 4 13.54 145
The above compositions are five times repetitions of PM31 [Drying PVC, CaCO3, martinal and glass at 1050C for 1.0 hour].
164
Table 4.24. Mechanical properties and thermal stability-time for the composition PM31with different blending times and drying of components (cross-verification)
Composition Tensile Strength, (N/mm2)
% Elongation at break
Thermal Stability (minutes)
trials average trials average trail 1 trail 2PM31(R1) Specimen 1 14.21
13.74
135
159 75 – 80 75 - 80
Specimen 2 14.46 185Specimen 3 13.64 185Specimen 4 12.38 140Specimen 5 14.09 150
PM31(R2)Specimen 1 15.27
14.11
180
182 85 – 90 85 – 90
Specimen 2 12.51 160Specimen 3 12.84 185Specimen 4 14.84 195Specimen 5 15.08 190
PM31(R1-D)Specimen 1 13.70
13.99
165
173 85 – 90 90 - 95
Specimen 2 13.55 150Specimen 3 14.17 175Specimen 4 14.16 165Specimen 5 14.37 210
PM31(R2-D)Specimen 1 14.67
14.02
220
188 85 – 90 85 – 90
Specimen 2 13.90 210Specimen 3 13.03 140Specimen 4 14.25 215Specimen 5 14.25 155
R1 = 10 minutes blending; R2 = 15 minutes blending; (R1-D) =10 minutes blending [Drying PVC, CaCO3, martinal and glass at 1050C for 1 hour] and (R2-D) =15minutes blending [Drying PVC, CaCO3, martinal & glass at 1050C for 1 hour]
As PM31 showed good mechanical properties though somewhat less
than required thermal stability time, it was considered appropriate to study
165
the effect of drying the major ingredients that go into the resin matrix on
these properties. Drying components before blending seems to have an
effect in that the mechanical properties apparently improve while there is an
almost definite increase in the thermal stability time. The improvement in
mechanical properties appears better at higher blending times upto a limit.
The data seem to suggest that here the optimum blending time is fifteen
minutes.
To reaffirm the observations just made, another batch of four samples
with the same composition as that of PM31, but with dried ingredients were
prepared. As can be seen from table 4.24., it turns out that somewhat better
properties are actually obtainable at fifteen minutes blending time.
4.4.1. Study of specific gravity
The specific gravity (S.G.) measurements (ASTM D792-00) for
PM22 to PM29 are given in table 4.25.
Table 4.25. Specific gravity data
Sample Code PM22 PM23 PM24S.G. 1.5079 1.5152 1.5458
Sample Code PM25 PM26 PM27 PM28 PM29S.G. 1.5032 1.4485 1.4282 1.3701 1.3792
(Temperature of testing: 25.8 o C; ρ (H2O) at 25.8 o C is 0.996836)
Among the three compositions, PM22, PM23 and PM24, there is a
regular increase in the specific gravity value, as the concentration of the
additive increases from 10 phr to 30 phr in going from PM22 to PM24. This
is expected as glass is a somewhat dense additive. In the case of the
compositions PM25 to PM29, there is a gradual decrease in the specific
gravity value as the concentration of cenospheres increases; again, this is
expected as cenospheres are lightweight filler additives.
166
4.4.2. Study of surface hardness
Shore A: Shore A is used for hardness testing of softer plastics such as
rubbers and fluoropolymers found in products such as tires, wiper bladers,
gasket seals, and much more. Shore A is used when Shore D results are less
than 20.
Shore D: Shore D is used to characterize harder plastics such as
polyester, PVC, and acrylics. Shore D is used when Shore A results are over
90. Surface hardness was measured for various compositions, using Shore D
Durometer. The values lie in the range 55 to 60 for all the compositions.
4.5. Study of FRLS properties
Limiting Oxygen Index and Smoke Density Rating: these two
constitute FRLS properties. These properties are measured in required cases
and the data tabulated below. The higher the LOI, the better. The lower the
SDR, the better. Target values are: LOI>30 and SDR < 60.
Table 4.26. FRLS properties for compositions PM22, PM23 and PM24
Samplecode
Limiting Oxygen Index (%) Smoke Density Rating (%)Trials Average Trials Average
PM22 31.4 31.2 54.45 55.9731.2 60.5531.0 52.92
PM23 31.2 31.0 51.62 52.3031.0 57.6330.8 47.65
PM24 31.0 31.5 53.69 51.3031.8 51.2231.2 48.99
All the three compositions, PM22, PM23 and PM24, shown in table
4.26., show acceptable FRLS properties. TCP, one of the plasticizers present
in them is an effective flame-retardant, but generates high smoke under fire
conditions. Martinal is an additive that performs the dual role of a flame-
167
retardant and a filler. Zinc borate present in these compositions is a smoke-
suppressant. The LOI and SDR values reflect the net outcome of the rivaling
effects of the various additives present.
Table 4.27. FRLS properties of compositions PM25 through PM29
Sample Code FRLS propertiesLOI (%) SDR (%)
PM25 32.6 55.6PM26 34.4 68.8PM27 33.9 70.6PM28 33.5 56.3PM29 32.9 56.8
It was discussed earlier that more acceptable thermal, electrical and
mechanical properties are shown by these compositions containing
cenospheres as filler additives, but not all at the same concentration of
cenospheres. It is seen from the FRLS data in table 4.27. that such behavior
continues in the case of FRLS properties as well. Better FRLS performance
is shown by PM28 and PM29, containing 18 phr and 24 phr of cenospheres.
For the compositions listed in table 4.28., it appears that though
flammability is acceptable, consistently the SDR values are rather high. Also,
the blending time and SDR values show no perceivable trend.
Table 4.28. FRLS properties of composition PM31 repeated with different blending times and drying of components
Sample Code FRLS propertiesLOI (%) SDR (%)
PM31.1 31.2 62.03PM31.2 31.6 57.02
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PM31.3 31.7 67.94PM31.4 32.0 62.35PM31.5 - 69.96
Table 4.29. FRLS properties of PM31(R1), PM(R2), PM(R1-D) and PM(R2-D)
Sample Code FRLS properties
LOI (%) SDR (%)
PM31(R1) 31.3 69.55PM31(R2) 31.5 70.36PM31(R1-D) 30.7 70.99PM31(R2-D) 30.0 71.95
The FRLS properties for the ‘repeat’ compositions are given in table
4.29. The values for PM31(R1-D) and PM31.5 are similar. For PM31(R2-D)
and PM31.4., LOI values correlate better than the SDR values.
4.6. Calculation steps
An illustrative example for detailed calculation of kinetic parameters
using Ozawa’s method from TGA data at various heating rates is given for
the composition PM02 below.
Sample: PM02, temp. range: 30-700 oC, atmosphere: N2, weight: 8-10 mg
Sl No. β (o C . min –1 ) T5% T10%
1 2 206 2312 4 214 2433 8 234 2554 12 236 261
4.6.1. Calculation of E valueA. Calculations for 5% weight loss
Sl No.
β(o C . min –1 )
log β T5%
(oC)Tp
(t o C + 273) K1/Tp*10
001 2 0.3010 206 479 2.08772 4 0.6021 214 487 2.0534
169
3 8 0.9031 234 507 1.97244 12 1.0792 236 509 1.9646
Slope: - 5.52The related equation is:- log β – 0.457 E/R ( 1/Tp ) = constant or
- log β = constant + (– 0.457 E/R) (1 / 10 3 ). ( 1/Tp *10 3 ). Thus, slope (m) = (– 0.457 E/R) (1 / 10 3 ) = - 5.52Therefore, E = 5.52 x R x 10 3 / 0.457 = 24 Kcal. K–1. mol–1
B. Calculations for 10% weight loss
Sl No.
β(o C . min –1 )
log β T10%
(o C )Tp (t o C + 273)
K1/Tp*1000
1 2 0.3010 231 504 1.98412 4 0.6021 243 516 1.93803 8 0.9031 255 528 1.89394 12 1.0792 261 534 1.8727
Slope: - 6.92Thus: E = 6.92 x R x 10 3 / 0.457 = 30.09 Kcal. K–1. mol–1
The activation energy values for the 5% decomposition step and 10%
decomposition step are 24 Kcal. K–1. mol–1 and 30.09 Kcal. K–1. mol–1,
repectively. This is as expected.
4.6.2. Calculation of Arrhenius factor (5% weight loss data considered)
A= (βE/R Tp 2) . exp E/R Tp
Sl. No. β (o C . min –1 )
Tp (K) A (min –1 )(x 10 10 )
1 2 479 0.942 4 487 1.203 8 507 0.84
170
4 12 509 1.13
A average = 1.03 x 10 10 min –1
The Arrehenius factor, A, also called frequency actor is a measure of
inter-molecular collisions occurring per unit time in the reaction phase.
4.6.3. Calculation of rate constant, k (5% weight loss data considered)
Sl. No. Tp (K) A (min –1 )(x 10 9 )
k (min –1 )(x 10 –1 )
1 479 0.94 0.11532 487 1.20 0.17443 507 0.84 0.46394 509 1.13 0.5094
k = A. exp -E/R Tp
k average = 0.3158 min –1
The rate constant shows that thermal decomposition of the polymer
compounds under investigation can be treated as a first order reaction.
4.7. Conclusion
A number of polymer compounds of varying compositions were
prepared and their various properties studied. The properties studied include
thermal properties, electrical properties, mechanical properties and FRLS
properties. Compositions holding out promise for scale-up studies were
arrived at.
In relevant cases, thermogravimetric data were obtained using TGA,
and kinetic parameters and thermal rating values calculated. DTG curves
were recorded for a representative sample.
171
172
Chapter 5 STUDIES ON
EPDM COMPOSITIONS
5.1 Introduction5.2 Relevance of the present investigation5.3 Materials used5.4 Compounding of the resin systems5.5 Results and Discussion5.6 Conclusion
173
5.1. Introduction
EPDM rubber (ethylene propylene diene monomer (M-class) rubber
[114,115], a type of synthetic rubber, is an elastomer which is used for a
wide range of applications. The E refers to ethylene, P to propylene, D to
diene and M refers to its classification in ASTM standard D-1418. The M
class includes rubbers having a saturated chain of the polymethylene type.
Dienes currently used in the manufacture of EPDM rubbers are
dicyclopentadiene (DCPD), ethylidene norbornene (ENB), and vinyl
norbornene (VNB). EPDM rubber is closely related to ethylene propylene
rubber (ethylene propylene rubber is a copolymer of ethylene and propylene
whereas EPDM rubber is a terpolymer of ethylene, propylene and a diene
component).
In EPDM rubbers, the ethylene content is around 45% to 80%. The
higher the ethylene content, the higher the loading possibilities of the
polymer, better the mixing and the extrusion. Peroxide curing of these
polymers gives a higher crosslink density compared to their amorphous
counterpart. The amorphous polymers are also excellent in processing, which
is strongly influenced by their molecular structure. The dienes, typically
comprising from 2.5% to 12% by weight of the composition, serve as
crosslinks when curing with sulphur and resin, with peroxide cures the diene
(or third monomer) functions as a co-agent, which provides resistance to
unwanted tackiness, creep or flow during end use.
5.1.1. General Chemistry of EPDM
Ethylene-propylene rubbers use the same chemical building blocks or
monomers as polyethylene (PE) and polypropylene (PP) thermoplastic
polymers. These ethylene (C2) and propylene (C3) monomers are combined
in a random manner to produce rubbery and stable polymers. A wide family
174
of ethylene-propylene elastomers can be produced ranging from amorphous,
non-crystalline to semi-crystalline structures depending on polymer
composition and how the monomers are combined [116].
To appreciate long-term performance of EPDM rubbers in various
applications, one needs to examine the chemical structure of the parent
EPDM polymer, which is synthesized from three building block monomers-
ethylene, propylene, and a diene [117]. The ethylene and propylene
monomers combine to form a chemically saturated, stable polymer
backbone. The third, a non-conjugated diene monomer can be
terpolymerized in a controlled manner to maintain the saturated backbone
and place the reactive unsaturation in a side chain, making it available for
vulcanization or polymer modification chemistry. Thus, since the
unsaturation is protected, EPDM is inherently ozone-resistant as compared to
other rubber materials. It is also resistant to acid and base attack, and
possesses excellent weathering properties. [118].
The two most widely used diene termonomers are primarily
ethylidene norbornene (ENB) followed by dicyclopentadiene (DCPD). An
EPDM polymer structure is illustrated in figure 5.1. The ethylene-propylene
copolymers are called EPM.
Figure 5.1. Structure of EPDM containing ENB.
As mentioned above, the diene is the reactive part of the EPDM
molecule - cross linking agents react with the diene to 'tie' the polymer
175
molecules together, increasing physical and mechanical properties, such as
heat and solvent resistance, tensile and tearing strength. Most importantly,
cross-linking increases the material’s service-life [119].
5.2 Relevance of the present investigation
In the wire and cable industry, for some years now, interest in
elastomers has been increasing. Spurred by legislative changes and customer
demands [120], particularly in the developed world where the PVC claims
more than 50% market share, the work for cleaner technologies using
halogen-free resins and lead-free stabilizers has acquired high priority. In the
wake of rising environmental, health and safety concerns, the recent research
has been directed towards low-smoke emission, low corrosivity, low toxicity,
low heat release, flame-retardant, and zero-halogen polymeric compositions
to substitute halogen-based systems. This is reflected in the growing
requirement for low-smoke zero-halogen (LSZH) materials manufactured in
a wide variety of compositions. Some of the materials that have held out
promise include silicone, nylon, polypropylene, acrylic, and thermoplastic
elastomers such as EPDM.
The main advantageous properties of EPDM are its great heat, ozone
and weather resistance; its resistance to polar substances and steam is also
good. It has very good electrical insulating properties. It does not contain any
halogen. Hence, it is almost toxicity and corrosion free. The pure resin has an
LOI of about 21%, somewhat close to the threshold value of 30%, the
minimum required for a flam-retardant composition. In addition, it is already
being used as a material for the outer casing on wires in electrical appliances
for outdoor installation, or in those exposed to UV light. In particular, in
cost, it is more or less comparable to PVC. For all these reasons, EPDM was
considered a suitable resin for present investigations.
176
These polymers respond well to high filler and plasticizer loading,
providing economical compounds. They can develop high tensile and tear
properties, excellent abrasion resistance, as well as improved oil swell
resistance and flame-retardance. A general summary of properties is shown
in table 5.1.
Table 5.1. Properties of Ethylene-Propylene Elastomers
Polymer Properties
Mooney Viscosity, ML 1+4 @ 125°C
5 – 200+
Ethylene Content, wt. % 45 to 80 wt%Diene Content, wt. % 0 to 15 wt%
Specific Gravity, gm/ml 0.855 – 0.88#
Vulcanizate Properties*
Hardness, Shore A Durometer 30A to 95ATensile Strength, MPa 7 to 21
Elongation, % 100 to 600Useful temperature range, °C -50° to +160°
Tear Resistance Fair to GoodAbrasion Resistance Good to
ExcellentElectrical Properties Excellent
# Depending on polymer composition, * Range can be extended by proper
compounding. Not all of these properties can be obtained in one compound.
5.3 Materials used
HERLENE 512 grade EPDM, with Ethylene/Propylene weight ratio
68 / 32 and Mooney viscosity of 55 value (ML 1+4) at 125 0C, has been used
as such. The locally available additives have been used. The DOP has been
used as primary plasticizer, paraffin wax, as an external lubricant, sulphur, as
vulcanizing agent, and Dibenzothiazole disulphide (MBTS, molecular
formula: C14H8N2S4) and Tetramethyl thiuram disulphide (TMTD, molecular
formula: C6H12N2S4), as accelerators. The accelerator activators used in this
study are zinc oxide and stearic acid. Stearic acid also acts as an internal
177
lubricant, thereby improving processability. Clay has been used as filler,
litharge (PbO), as an acid scavenger and bisphenol-A, as an anti-oxidant.
Flame-retardant additives, antimony trioxide and trihydrated alumina, were
procured locally and used as such. The modified transition metal sulphate
glass, prepared in the laboratory [121], was ground to fine powder and used
in the EPDM system.
5.4. Compounding of the resin systems
The compositions were prepared using the Brabender Plasticordor
(PLE 331) and cured using a compression molding machine. Test specimens
as per various test requirements were cut from the cured sheets.
5.4.1. Toxicity index measurement studies
In toxicity index measurement studies, the specimen holder is a non-
combustible device and holds the specimen at its top over the burner without
masking the specimen from the flame by more than 5% of its surface area.
Fourteen gases can be detected using the apparatus. A maximum of one gram
of the sample (EPDM-E) is burnt for a maximum of three minutes. ‘Gas
reaction tubes’ are used to determine the quantity of a gas evolved. The
sample is placed on the glass wool to prevent char particles, formed if any,
from falling into the Bunsen burner below, inside the Test Chamber.
5.5. Results and Discussion
An outline of the compositions prepared: EPDM: 100, ZnO: 5-10,
stearic acid: 1-5, clay: 40-80, DOP: 2-10, bisphenol-A: 1-5, red lead: 2-5,
paraffin wax: 1-5, Sb2O3: 5-15, trihydrated alumina; 50-100, sulfur: 1-2,
TMTD: 0.1-0.5, MBTS; 0.5-2.0 and sulfate glass: 5-30 (all in phr). The
compositions of the samples taken for analysis are given in table 5.2, the
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variant being the mixture (M1) of the flame-retardant additives Sb2O3 and
trihydrated alumina combined in a specified ratio.
Table 5.2. The EPDM-based compositions
Sample code M1 (in phr)EPDM-A 20EPDM-B 40EPDM-C 60EPDM-D 80EPDM-E 100
EPDM-based compositions using the ingredients in the ranges
outlined above were prepared and their electrical, mechanical and FRLS
properties evaluated [122]. The addition of trihydrated alumina along with
antimony trioxide to EPDM composition enhances the flame-retardancy by
improving the oxygen index values from a minimum of 21 % to a level of
32%.
The addition of antimony oxide increases the oxygen index, but the
flame-retardant efficiency starts to decrease when the antimony oxide
concentration exceeds about 5%. Addition of 20-30% of aluminum
trihydroxide to these formulations raises the oxygen index above the values
obtained using antimony oxide alone. The magnitude of increase depends on
the amount of plasticizer and antimony oxide used in the formulation. The
increase is more pronounced at low to moderate plasticizer concentrations.
Formulations containing higher levels of antimony oxide also benefit by the
addition of ATH or other inorganic hydroxides. However, proper formulation
can achieve flame-retardant systems without incorporation of antimony
oxide. These systems produce less smoke than those containing antimony
oxides. The particle size of the ATH used also has a definite effect on the
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flame-retardant performance of flexible PVC. The use of 1 -µm precipitated
ATH gives a two or three oxygen index unit increase over coarser products.
The thermal data for the five compositions are given in tables 5.3a
and 5.3b. TG curves at different heating rates and DTG curve for EPDM-E
are given in figures 5.2. and 5.3.
Table 5.3a. Temperature of 5% weight loss of EPDM-based compositions at different heating rates (obtained from thermogravimetric data).
β (o C. min–1 )
EPDM-A
EPDM-B EPDM -C
EPDM-D EPDM-E
2 234 232 225 251 2104 247 240 244 264 2266 258 244 253 272 --8 -- 255 -- -- 23110 274 -- 254 254 --12 -- 271 -- -- 248
Table 5.3b. Temperature of 10% weight loss of EPDM-based compositions at different heating rates (obtained from thermogravimetric data).
β (o C. min–1 )
EPDM-A
EPDM-B EPDM -C
EPDM-D EPDM-E
2 266 264 267 282 2484 283 275 287 297 2646 293 277 294 309 --8 -- 289 -- -- 27310 313 -- 295 295 --12 -- 308 -- -- 290
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Figure 5.2. TGA curves of the EPDM-E composition at different heating rates
Figure 5.3. DTG curve of the EPDM-E composition
From the tables and the figures, it is clear that the compositions are
thermally stable upto 200 0C. This is the temperature at which the EPDM, in
its pure form is flammable. This implies that the additives have not had any
undesirable influence on the thermal stability of the compositions. It is also
observed that for each composition, the temperatures at which 5% and 10%
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weight losses are shifted to lower values with reduced scan rate. It may
further be seen that for lower heating rates the maximum values for
temperatures at 5% and 10% weight losses are observed in the case of
EPDM-D, which contains 80 phr of the flame-retardant mixture. For higher
heating rates, EPDM-A shows higher thermal stability. In all cases, the
thermal degradation occurs at nearly 500 o C.
5.5.1. Calculation of activation energy for thermal decomposition, E
Ozawa’s method [123], based on variable heating rate, has been
normally used to derive kinetic parameters for many industrial materials. A
relationship connecting peak temperature, Tp and heating rate, β, has been
derived:
- log β – 0.457 E/R (1/Tp ) = constant (5.1)
The kinetic parameters, ‘E’, ‘A’ and ‘k’ are calculated for all samples
using equation (5.1) [123,124-126].
- log β – 0.457 E/R (1/Tp ) = constant or
- log β = constant + (– 0.457 E/R) (1 / 10 3). (1/Tp *10 3).
Slope = (– 0.457 E/R) (1 / 10 3) = -5.7
E = 5.7 x R x 10 3 / 0.457 = 24.78 Kcal. K–1. mol–1
As a representative case, the details of calculations for EPDM-A
along with relevant data are given in tables 5.4. and 5.5.
Table 5.4. Calculations for 5% weight loss
Sl No.
β(o C. min –1 )
log β Temp of 5% wt loss (oC )
T (t o C + 273) K
1/Tp*1000
1 2 0.3010 234 507 1.972 4 0.6021 247 520 1.923 6 0.7782 258 531 1.884 10 1.0000 274 547 1.83
(Sample: EPDM-A, temperature range: 40 o C – 600 o C, atmosphere: N2)
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Table 5.5. Calculations for 10% weight loss
Sl No.
β(oC . min –1 )
log βTemp of 10% wt loss (oC )
T (t o C + 273) K
1/Tp*1000
1 2 0.3010 266 539 1.862 4 0.6021 283 556 1.803 6 0.7782 293 566 1.784 10 1.0000 313 586 1.71
E = 3.23 x R x 10 3 / 0.457 = 14.04 Kcal. K–1. mol–1
The activation energy E value is 24.78 Kcal. K–1.mol–1 and 14.04
Kcal.K–1.mol–1 for 5% weight loss step and the 10% weight loss step,
respectively.
5.5.2 . Calculation of Arrhenius factor, A
The Arrhenius factor, a measure of the frequency of molecular
collisions during a reaction, is calculated as per the following formula [124]:
A= (βE/R Tp 2). exp E/R Tp (5.2)
Taking β = 2 o C. min –1, A= 4.6 x 10 9 min –1
Taking β = 6 o C. min –1, A= 4.2 x 10 9 min –1
Taking β = 10 o C. min –1, A= 3.3 x 10 9 min –1
A average = 4.03 x 10 9 min –1
5.5.3. Calculation of rate constant, k
Knowing the ‘A’ value, the rate constant is calculated using the
familiar Arrhenius equation.
k = A. exp -E/R Tp (5.3)
= 4.03 x 10 9 min –1. exp -24.78 x 1000/1.987 x 520
= 4.03 x 10 9 x (3.8 x 10 -11)
= 1.53 x 10 -1 min –1
It can be seen that the decomposition follows first-order kinetics.
5.5.4. Evaluation of thermal condition, θ
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The mathematical expression to evaluate θ is given by Toop
(discussed earlier in the thesis) [125-127]:
log tf = E/2.303 Rθ + log E p (xf)/ βR (5.4)
where log p (xf) = -2.315 – 0.457 E/R Tf , E = energy of activation,
R = gas constant, Tf = temperature at which specific change is
observed, β = heating rate, θ = thermal condition and tf = time to
condition at temperature θ.
Because of the simplicity and reliability, this approach is generally
preferred for determining thermal rating where thermal degradation is the
result of simple chemical reaction [128, 129].
5.5.4.1. Thermal condition for EPDM-based compositions
As representative cases, for the samples EPDM-C and EPDM-D, the
details of calculation of thermal rating values are given in tables 5.6 and 5.7.
The ‘E’ and the corresponding ‘θ’ values for all the five compositions are
summarized in table 5.8.
Table 5.6. Thermal index values for EPDM-C
Sl. No. β (oC. min –1) Tf (K) E/R Tf p(xf) (x10 –13) θ (o C )1 2 498 23.90 0.67593 104.342 4 517 23.02 1.7518 107.463 6 526 22.63 2.6733 107.674 10 527 22.59 2.7923 102.07
(5% wt loss data used; E= 23.65 Kcal. K–1 mol–1; tf = 20000 hrs; R= 1.987 Cal. K–1. mol–1)
Table 5.7. Thermal index values for EPDM-D
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Sl. No. β (oC. min –1 ) Tf (K) E/R Tf p(xf) (x10 –17) θ (o C )1 2 524 31.73 1.5526 147.022 4 537 30.97 3.4798 149.173 6 545 30.51 5.6752 149.134 10 549 30.29 7.1717 146.18
(5% wt loss data used; E= 33.04 Kcal. K–1.mol–1; tf = 20000 hrs; R= 1.987 Cal. K–1. mol–1)
Table 5.8. Thermal Rating Summary
Sl. No
Sample name
Kinetic parameter θ(o C )E (Kcal. K–1.
mol–1)*A
(min–1)k
(min–1)
1 EPDM-A 24.78 4.03 x 10 9 1.53 x 10 -1 1152 EPDM-B 23.04 1.07 x 10 9 1.15 x 10 -1 103.63 EPDM-C 23.65 2.14 x 10 9 1.77 x 10 -1 105.394 EPDM-D 33.04 7.768 x 1012 3.58 x 10 -1 147.95 EPDM-E 27.57 - - 108.1
*calculated at 5% wt loss
From thermal rating summary, it is observed that for all the
compositions, the activation energy for decomposition lies in the range 23 to
33 Kcal. K–1. mol–1. Also, the thermal rating, that is, the maximum and/or
minimum temperature at which a material will perform its function without
undue degradation, lies within the acceptable range.
5.5.5. Toxicity Index (T. I.) test
The TI is a measure of the amount of toxic gases a composion would
evolve on burning. The products of combustion in the test chamber, the
following gases, are quantitatively measured and the T.I. value is calculated
using the formula given below:
T. I. C = (5.5)
185
where, Cf = concentration of the gas considered fatal to man for a 30 minute exposure time (ppm), C = concentration of gas detected (ppm), m= fire test mass (g) and v = volume of test chamber (m³).Here, v = 0.7 m³
m = 0.9622 g
Now, C w.r.t. CO2 = (0.4 x 104 x 0.7x 100)/ 0.9622/105 = 2.91
C w.r.t. H2CO = (0.5 x 0.7x 100)/ 0.9622/500 = 0.073
Total T. I. for the sample = 2.91 + 0.073 = 2.983 (i.e. < 3)
Table 5.9. The T. I. test data
Sl. No.
Gas Initial reading
Final Reading
Quantity of gas evolved
Cf values for gases (ppm)
1 CO2 0.5% 0.9% 4000 ppm 1000002 CO 5 5 Nil 40003 NOx 2 2 Nil 2504 H2CO 0.5 1.0 0.5 5005 CH2CHCN 1 1 Nil 4006 HCl 0 0 Nil 4007 H2S 0 0 Nil 7508 NH3
Not measured as EPDM has none of N,S,Ph,or Br/F
5509 SO2 40010 PhOH 25011 HCN 15012 HBr 15013 HF 10014 COCl2 25
Sample description on burning: not burnt fully; only slightly charred.
It is evident from the table that no HCl is released during the combustion of
the EPDM sample.
5.6. Conclusion
In the range investigated, for the EPDM-based compositions, the
thermal rating values are acceptable and have toxicity index less than three.
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They do not release any toxic or corrosive gases on burning. The FRLS,
mechanical and electrical properties are also within range. In line with the
current trend of zero-halogen polymeric compositions substituting halogen-
based systems, they hold out promise as commercially viable compositions
for electrical cable sheathing applications.
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