Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to the prevention of
scorching, prior to w lcanization, of peroxide curable
ethylene polymer based compositions.
Description of the Prior Art
Insulation compositions which are employed on
electrical wire and cable are, in many cases, prepared
from compositions which are based on vulcanizable, or
cross-linkable, ethylene polymers. These ethylene poly-
mer based compositions may be vulcanized, or cured, or
crosslinked, with various organic peroxide compounds, as
disclosed for example in United States Patents, 2,826,570;
2,888,424; Z,916,481; 3,079,370 and 3,296,189.
In the organic peroxide compounds which have
been used to date for commercial purposes in these vul-
canizable ethylene polymer ~ased compositions, each
oxygen atom in the peroxide group, i.e., -O-O-, of such
compounds is directly attached to a carbon atom of an
organic radical. The commerçially useful compositions
do not employ hydroperoxide compounds thereln as curing
agents because they have relatively high decomposition
temperatures, and the free radicals provided by the de-
composed hydroperoxides are not effective for cross-
linking ethylene polymers.
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In order to process the organic peroxide con-
taining compositions so as to adapt them to be placed,
as insulation, on the electrical conductor components
of the wire and cable it is usually necessary to admix
the components of the compositions at high temperatures,
and to extrude them, again at high temperatures, onto
the electrical conductor. These processing activities
occur prior to the intended vulcanization of the per-
oxide containing compositions, which is usually accomp-
lished after such compositions are extruded onto theelectrical conductor.
It has been found, however, that when certain
of the organic peroxide compounds, such as dicumyl
peroxide, as used in combination wither certain types of
ethylene polymers or in certain types of ethylene poly-
mer based compositions, that the entire curable compo-
sition is susceptible to scorching during the high temp-
erature processing thereof prior to the vulcanization
of the composition on the electrical conductor.
Scorching is, in effect, the premature vulcanization of
the insulation composition. This premature vulcani-
zation usually occurs, when it occurs, in the barrel
or die head of the extruder in which the insulation
composition is being processed, at elevated tempera-
tures, prior to its being extruded onto an electrical
conductor, and prior to its intended vulcanization.
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~en an insulation composition is scorched in the extrud-
er, the e~truded composition ~ill have imperfections in
the form of discontinuity and roughness in the surface
of the extrudate; and lumps or surface ripples caused
by gel particles in the body of the extrudate. In
addition, excessive scorching ma~ cause enough of a
pressure build-up in the extrusion device to require a
cessation of the extrusion operation entirely.
The tendency of a composition to experience
scorch is a relative matter, since any vulcanizable
ethylene polymer based composition can be made to scorch
if processed under conditions designed to produce such
result. Under a given set of conditions some compo-
sitions are more prone to scorching than are~others.
Compositions which have been found to be more
susceptible to scorching under a given set of conditions
are those in which the ethylene polymer has a relatively
low melt index and/or a relatively narrow molecular
weight distribution.
The tendency of a composition to scorch under
commercial operating conditions may be measured by
means of the Monsanto Rheometer Test Procedure. The
Monsanto Rheometer Test Procedure is described in
AST~-D-2084-71T.
Prior to the work of the present inventor
as disclosed in this patent application, scorch prevention
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has been accomplished by the use o~ additives such as
nitrites as disclosed in U.S. 3,202,648; the specific
antioxidants and vulcanization accelerators disclosed in
U.S. 3,335,124; and the chain transfer agents disclosed
in U.S. 3,578,647. A mixture of two specific peroxides
has also been used to provide a rate of cure that is
intermàdiate the rate of cure of either of such peroxides,
as disclosed in U.S. 3,661,877.
Summar~ of the Invention -
It has now been found that vulcanizable ethy-
lene polymer based compositions which employ bis-
(tertiary-butyl peroxy-isopropyl) benzene and 2,5-
dimethyl-2,5-di-(tertiary butyl peroxy) hexane as
vulcaniz ing agents, and which compositions are susceptible
to scorching under a given set of conditions, can be pro-
tected against scorching under such conditions by in-
corporating in such compositions cumene hydroperoxide
and/or tertiary butyl hydroperoxide.
An object of the present invention is to pro-
vide scorch resistant, vulcanizable, ethylene polymer
based compositions.
Another object of the present invention is to
provide a process for protecting against scorching
vulcanizable ethylene polymer based compositions which
employ bis-(tertiary-butyl peroxy-isopropyl) benzene and
2,5-dimethyl-2,5-di-(tertiary butyl peroxy) hexane peroxides
10472Q0
as vulcanizing agents and which are susceptible to scorch-
ing.
A further object of the present invention is
to provide scorch resistant insulation for electrical
wire and cable.
A further object of the present invention is
to provide a process whereby vulcanizable ethylene
polymer based compositions which employ therein
bis-(tertiary-butyl peroxy-isopropyl) benzene and
2,5-dimethyl-2,5-di-tertiary butyl peroxy hexane as
vulcanizing agents and which compositions are susceptible
to scorching, may be processed in mixing and extruding
devices, prior to ~he w lcanization thereof, at fast
throughput rates and at relatively high processing
temperatures without experiencing scorching.
These and other objects of the present
invention are achieved by employing cumene hydroperoxide
and/or tertiary butyl hydroperoxide as scorch preventing
agents in the compositions of the present inventions.
THE DRAWINGS
Figures 1 and 2 of the drawings show, graphically,
Monsanto Rheometer Test curves which were used to illustrate
the derivation of an efficiency factor as described below.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
The Scorch Resistant Composition
The scorch resistant compositions of the present
invention comprise, in weight ratio,
100 parts by weight of ethylene polymer,
about 0.1 to 5.0, and preferably 0.2 to 2.0,
parts by weight of a first peroxide which is bis(tertiary-
butyl peroxy isopropyl) benzene and/or 2,5-dimethyl-2,5-
di-(tertiary butyl peroxy) hexane, and
about 0.1 to 2.0, and preferably about 0.05 to
1.0, parts by weight of at least one hydroperoxide compound
wh~ch is cumene hydroperoxide and/or tertiary butyl
hydroperoxide.
About one part by weight of either or both of
the hydroperoxides is used per 5 to 40 parts by weight
of the first peroxide.
A minor amount, i.e., less than about 50 weight
percent, of the first peroxide can be replaced by dicumyl
peroxide.
Ethylene Polymer
The ethylene polymers which are used in the
compositions of the present invention are solid (at
25C.) materials which may be homopolymers, or copoly-
mers, of ethylene. The ethylene copolymers contain at
least 30 weight percent of ethylene and -up to about 70
weight percent of propylene, and/or up to about 50
weight perce~t of one or more other organic compounds
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which are interpolymerizable witn ethylene. These other
compounds which are interpolymerizable with ethylene
are preferably those which contain polymerizable
unsaturation, such as is present in compounds containing
an ethylene linkage, > C = C < . These other inter-
polymerizable compounds may be hydrocarbon compounds
such as, butene-l, pentene-l, isoprene, butadiene,
bicycloheptene, bicycloheptadiene and styrene, as
well as vinyl compounds such as vinyl acetate and
ethyl acrylate.
These copolymers could thus include those
containing > 0 to 70 weight percent of propylene and
30 to < 100 weight percent of ethylene; and ~0 to < 50
weight percent of butene-l or ethylene vinyl acetate and
50 to < 100 weight percent of ethylene; and > 0 to < 30
weight percent of propylene, > 0 to 20 weight percent of
butene-l and 50 to < 100 weight percent of ethylene.
The ethylene polymers may be used individ-
ually, or in combination thereof. The ethylene poly-
mers have a density (ASTM 1505 test procedure withconditioning as in ASTM D-1248-72) of about 0.86 to
0.96 and a melt index (ASTM D-1238 at 44 psi test
pressure) of about 0.1 to 20 decigrams per minute.
Adiuvants
In addition to the ethylene polymer, and the
peroxide compounds, the compositions of the present
.
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invention also advantageously ir.clude about 0.01 to 3.0
and, preferably 0.05 to 1.0, parts by weight of one
or more suitable high temperature antioxidants for the
ethylene polymers, per 100 parts by weight of ethylene
polymer in such compositions.
These antioxidants are preferably sterically
hindered phenols. Such compounds would include 1,3,5-
trimethyl-2,4,6-tris(3,5-ditertiary butyl-4-hydroxy
benzyl) benzene; 1,3,5-tris(3,5-ditertiary butyl -4-
hydroxy benzyl)-5-triazine-2,4,6-(lH,3H,5H)trione;
tetrakis-[methylene-3-(3',5-di-t-butyl-4'-hydroxy
phenyl)-propionate]methane; and di(2-~nethyl-4-hydroxy-
5-t-butyl phenyl)sulfide. Polymerized 2,2,4-trimethyl
dihydroquinoline may also be used.
Other adjuvants which may be employed in the
compositions of the present invention would include
adjuvants commonly employed in vulcanizable ethylene
polymer based compositions including fillers, such as
carbon black, clay, talc and calcium carbonate; blowing
agents; nucleating agents for blown systems;
lubricants; UV stabilizers; dyes and colorants; voltage
stabilizers; metal deactivators and coupling agents.
These adjuvants would be used in amounts de-
signed to provide the intended effect in the resulting
composition.
The compositions of the present invention
may also be extended, or filled, with polymers other
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than the ethylene polymer which are compatible, i.e.,
can be ?hysically blended or alloyed, with the ethylene
polymer. The resulting compos~tions should contain
at least about 30 weight percent o~ interpolymerized
ethylene in all the poly~ers that may be present in the
composition, based on the to~al ~eight of the resulting
composition. The other polymers which may be used would
~nclude polyvinyl chloride and polypropylene.
The total amount of adjuvants used will range
from 0 to about 60 weight percent based on the total
weight of the composition.
Processin~ of the Compositions
All of the components of the compositions of
the present invention are usually blended or compounded
together prior to their introduction into the extrusion
device from which they are to be extruded onto an electri-
cal conductor. The ethylene polymer and the other
desired constituents may be blended together by any of
the technîques used in the art to blend and compound
thermoplastics to homogeneous masses. For instance,
the components may be fluxed on a variety of apparatus
including multi-roll mills, screw mills, continuous
mixers, compounding extruders and Banbury* mixers, or
dissolved in mutual or compatible solvents.
When all the solid components of the composi-
tion are a~ailable in the form of a powder, or as small
particles, the compositions are most conveniently pre-
*Trade Mark or Trade Name
'-" 10.
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pared by first making a blend of the components, say
in a Banbury mixer or a continuous extruder, and then
masticating this blend on a heated mill, for instance
a two-roll mill, and the milling continued until an
intimate mixture of the components is obtained.
Alternatively, a master batch containing the ethylene
polymer(s) and the antioxidants(s) and, if desired, some
or all of the other components, may be added to the
mass of polymer. Where the ethylene polymer is not
available in powder form, the compositions may be made
by introducing the polymer to the mill, masticating it
until it forms a band around one roll, after which a
blend of the remaining components is added and the
milling continued until an intimate mixture is obtained.
The rolls are preferably maintained at a temperature
which is within the range 80C to 150C and which is
below the decomposition temperatures of the first per-
oxide compound(s). The composition, in the form of a
sheet, is removed from the mill and then brought into
a form, typically dice-like pieces, suitable for sub-
sequent processing.
After the various components of the compositions
of the present invention are uniformly admixed and blended
together, they are further processed, in accordance with
the process of the present invention, in conventional
extrusion apparatus at about 120 tol60C.
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After being extruded onto a wire or cable, or
ol~her substrate, the compositions of the present invention
are w lcanized at elevated temperatures of about > 180C.
and preferably at > 215-230C. using conventional
vulcanizing procedures.
Derivation of Curing System Efficiency Factor
In the Monsanto Rheometer Test Procedure a ``
sample of the vulcanizable composition is measured in
a rheometer before the composition is subject to high
temperature mixing or extrusion conditions. The test
results are plotted as functions of inch-pounds of
torque versus time. The compositions which are
less susceptible to scorching are those that experience,
after the minimum torque value is achieved, a delay in
the rise of the torque values followed by a fast rise
in the torque values to the level required for the in-
tended end use of the composition being evaluated.
The Monsanto Rheometer Test Procedure is, in
effect, a means for comparitively evaluating, graphically,
the susceptibility of different vulcanizable compositions
to scorch. In this way the use of different curing agents,
or curing agent compositions, in such vulcanizable com-
positions, can also be graphically compared.
For the purposes of the present invention, a
procedure has now been devised whereby, using the
graphical results of Monsanto Rheometer Test procedures,
the efficiency of different curable compositions, relative
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to the susceptibility of such compositions to scorching,
can also be numerically compared. By using this new
evaluation procedure, a separate and distinct numerical-
efficiency factor (E) can be assigned to each curable
composition. To make these efficiency factors more
meaningful, for comparison purposes, they should be based
on rheometer curves which are all obtained when the
curable compositions being comparedare evaluated under
the same test conditions. In all the experiments reported
herein the test samples were evaluated in a Monsanto
Rheometer at a cure temperature of 360F., using a rheometer
oscillation of 110 CPM and an arc of + 5.
There is also provided here below, the derivation
of a numerical efficiency factor (E) for vulcanizable
compositions. The derivation employs typical rheometer
curves that were arbitrarily drawn, and which are not
based on actual experiments. Such curves are shown in
Figures 1 and 2 of the drawings.
A typical Monsanto Rheometer curve, as shown
graphically in Figure 1, contains several parameters which
are used in the derivation of the efficiency factor (E).
The optimum cure level (highest cross-link density) is
designated as H. H is measured in terms of inc~-pounds
of torque on the rheometer test equipment. A higher value
for H corresponds to a higher cross-link density.
The time, in minutes, required to reach 90% of the
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maximum cure (H) is designated as CT. Thus, in Figure 1,
H is S0 inch-pounds and CT is 5.5 minutes, which is the
time required to reach a level of 45 (or 90% of 50) inch-
poun~s of torque during the test procedure. ~ -
The scorch time, ST, is defined as the point in
time, in minutes, at which the curve reaches a rheometer
level of 10 inch-pounds of torque on the upswing of the
curve. In Figure 1, ST is about 2.1 minutes.
In general, one is interested in getting to the
maximum cure (H) as soon as possible. In other words,
a short CT is desirable. At the same time, one would
like ST to be as long as possible since a longer ST means -~
the vulcanizable composition being evaluated can be
processed at a higher speed or at a higher temperature.
That is, it would be less scorchy. Thus it is important
to discuss the time intervals between CT and ST, or
CT ~ ST since CT is, arbitrarily, always longer than ST.
Then, too, it is of interest to compare ST with
CT ~ ST since the best vulcanizable system would be one
whose ST is relatively long, and whose difference between
CT and ST, (CT - ST), would be relatively short. Thus,
the ratio ST/CT - ST is of importance. The larger is this
ratio, the less susceptible is the vulcanizable composition
to scorching.
Finally, the times (CT and ST) are related to
the maximum cure point, H. Thus, if one can maintain
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the same ST, and yet reach a higher H, one can thereby
provide a vulcanizable composition that is less
susceptible to scorch. When vulcanizable compositions
are cured by peroxide curing agent systems, particularly
those using individual peroxides such as dicumyl
peroxide, as you increase the value of H, by simply adding
more of the peroxide curing agent, you decrease ST. :
The efficiency of a particular curing agent
system, therefore, when used with a given vulcanizable
composition, and cured at a given temperature, can be
determined by multiplying H by ST / CT - ST or, as
shown in Equation I;
E = H x ST (I)
CT ~ ST -
The numerical efficiency (E) of the arbitrary
curing agent system shown graphically in Figure 1
therefore, would, be
E = H x ST = (50) (2.1) = 30.9
CT ST 5.5 - 2.1
To further illustrate the utility of this method,
for the purposes of comparitively evaluating different
vulcanizable compositions, reference is made to Figure 2
of the drawings in which there is graphically presented
typical Monsanto ~heometer curves 1 and 2, that were also
arbitrarily drawn, and which are not based on actual
experiments.
It should be noted from a review of Figure 2
that the cure times CT_l for c~mposition 1 and CT_2 for
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composition 2, are the same for both compositions and
each curve reaches a relatively high torque level with
t:he value of Hl (for composition 1) which is 70, being
relatively close to the value of H2 (for composition 2)
which is 62. ST 2 (for composition 2) however, is more
than a minute longer than ST_l (for composition 1),
3.2 vs 2.0 minutes. Thus, it is quite obvious from a
review of these two curves that curve 2 represents the
better cure system. If one maintains the same CT, and
reaches almost the same maximum cross-link density (H),
then increasing ST must lead to a better curing system,
in accordance with the above definition of E.
A calculation of the relative numerical effi-
ciencies of the curable compositions shown graphically
in Figure 2 is shown below:
Efficiency (El) of composition 1, based on curve 1:
El = Hl x STl = (70) (2) = 140 = 35.0
C - S 1 (6 - 2) 4
Efficiency (E2) of composition 2, based on curve 2:
E2 = H2 x ST2 = (62) (3.2) = 198.4 = 70.8
CT2 - ST2 (6 - 3.2) 2.8
Thus, this efficiency factor, E, is a useful
parameter and it can be shown that in fact a higher value
for E represents a better system, as defined above, and
represents improved utility for such better system. The
use of this efficiency factor, E, can also apply to
comparisons of Rheometer test curves where the maximum
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cure (H) shown in each curve is vastly different, since the
calculation of E is, in effect, a normalization procedure.
The compositions of the present invention have an efficiency
factor (E), as determined above, which is at least about 3,
and is preferably more than 10 to 15, units of such
efficiency factor above the efficiency factor of such
compositions in the absence of the cumene hydroperoxide
and/or tertiary butyl hydroperoxide.
The following examples are merely illustrative -
of the present invention and are not int~nded as a limit-
ation upon the scope thereof.
General Admixing Procedure
The vulcanizable compositions used in Examples
1-11 were all prepared by the following procedure:
100 parts by weight of the ethylene polymer were
fluxed in a Banbury mixer at approximately 120C. The
additives, i.e., anti-oxidant, and the peroxides and,
where used, other adjuvants, were then added to the
fluxed mixture. The resulting composition was then
blended for 2-3 minutes and then transferred to a
2-roll mill for sheeting. The hot rolled sheet was then
chopped on a hot granulator to yield a chipped product.
The chips were then compression molded into
plaques for use in Monsanto Rheometer test procedures.
All of the rheometer data which was then obtained on the
samples, unless otherwise stipulated, was obtained at
360F. (182.2C.).
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Examples 1-4
The following four vulcanizable compositions
were prepared as in The General Admixing Procedure
utilizing bis(tertiary butyl peroxy isopropyl) benzene
[Peroxide I] as the vulcanizing agent with a low density
ethylene homopolymer I [having a density of 0.919, a melt
index of 1.6 to 2.2 (lP 190C.)] and the hydroperoxides
shown in Table I. All of the compositions contained 0.2
parts by weight of di(2-methyl-4-hydroxy-5-t-butyl phenyl)
sulfide as an antioxidant.
TABLE I
Compositions of Examples 1-4 in
parts by weight
Component 1 2 3 4
Ethylene homopolymer I 100.0 100.0 100.0 100.0
Peroxide I 1.2 1.2 1.2 2.3
cumene hydroperoxide - 0.5
t-butyl hydroperoxide` - - 0.3
2,5-dimethyl-2,5-`
dihydroperoxy hexane - - - 0.3
Efficiency factor 16.3 26.5 20.6 18.3
me Efficiency Factors for each of the
compositions of Examples 1-4 were obtained on the
basis of Monsanto Rheometer Test Curves (not shown)
which disclosed the following test data for each of
the test compositions:
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TABLE II
Test Data - for the compositions
of Examples 1 2 3 4
ST. minutes 2.9 5.64.7 3.0
CT. minutes 10.2 9.49.5 11.3
H inch-pounds 41.0 18.021.0 47.0
The Efficiency Factors for these compositions
indicates that the addition of any of either of the mono-
hydroperoxides of the present invention leads to a less
scorchy system in an ethylene homopolymer based
composition containing Peroxide I alone , or in combination
with the di-hydroperoxide.
Examples 5-7
The following three mixtures were prepared
utilizing the same ethylene polymer and anti-oxidant, and
amounts thereof, as was used in Example 1-4, but peroxide
was 2,5 dimethyl -2,5-di(t-butyl peroxy)hexane (Peroxide II).
The data are shown in Table III.
TABLE III
Compositions of Examples 5-7
in parts by weight
Component _ 6 7_
Ethylene homopolymer I 100.0100.0 100.0
Peroxide II 2.0 2.0 2.0
Cumene Hydroperoxide - 0.5
t-butyl Hydroperoxide - - 0.3
Efficiency Factor 15.4 18.8 19.6
The Efficiency Factor for each of the compositions
of Examples 5-7 were obtained on the basis of Monsanto
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Rheometer Test Curves (not shown) which disclosed the
foll.owing test data for each of the test compositions:
TABLE IV -
Test Data - For the compositions
of Examples 5 6 7
ST, minutes 2.4 3.7 2.9 .
CT, minutes 10.2 lO.0 9.8
H, inch-pounds 50.0 '32.0 46.0
The Efficiency Factors for these compositions
indicates that the addition of either of the hydroperoxides
improves the scorch resistance of the ethylene polymer -
based composition containing Peroxide II.
Examples 8-9
The following two vulcanizable compositions
were prepared as in Examples 1-4 with Peroxide I and
with an ethylene-ethyl acrylate copolymer (Copolymer I)
which contained 15% by weight of ethyl acrylate and which
had a density of about 0.92 and a melt index of 1.6-2.2
(lP. 190C) and tertiary butyl hydroperoxide as shown .
in Table V. Di(2-methyl-4-hydroxy-5-t-butyl phenyl)
sulfide was used as the antioxidant.
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TABLE V
Compositions of Examples 8-9 in ~-
parts by weight -
Component 8 9 -
Copolymer IlO0.0 \ lO0.0
Antioxidant 0.2 0.2
Peroxide I l.2 l.2
t-butyl hydroperoxide0 0.2
Efficiency Factor19.2 23.2
The Efficiency Factor for each of the compositions
of Examples 8-9 were obtained on the basis of Monsanto
Rheometer Test Curves (not shown). The test curves for
thesé four compositions disclosed the following test data
for each of the test compositions:
TABLE VI
Test Data - for the compositions
of Examples
Example 8 - 9
S minutes 2.4 2.9
T,
CT minutes lO.8 lO.2
H, inch-pounds 67.0 58.5
The Efficiency Factors for these compositions
indicates that the addition of the hydroperoxide to
the copolymer I based system can provide a good
improvement in scorch resistance.
Examples lQ-ll
The following two vulcanizable compositions
were prepared utilizing an ethylene-vinyl acetate copolymer
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(('opolymer II) which contained 18% by weight of vinyl -
acetate and which had a melt index of 2.5 (lP, 190C,)
al~d a density of about 0. 92. In addition, the compositions
contained hydrated alumina as a filler, a silane coupling
agent tri(methoxy ethoxy) vinyl silane and di(2-methyl-4-hydroxy
-5-t-butyl phenyl) sulfide as antioxidant, Peroxide I and
t-butyl hydroperoxide, as shown in Table VII.
TABLE VII
Compositions of Examples 10-11 in
parts by weight
Example 10 11
Component
Copolymer II 100.0 100.0
A1203 3H20 128.0 128.0
Silane coupling agent 1.3 1.3
Antioxidant 0.8 0.8
Peroxide I 0.65 1.1
t-butyl hydroperoxide - 0.25
Efficiency Factor 14.5 28.2
The Efficiency Factors for each of the
compositions of Examples 10-11 were obtained on the basis
of Monsanto Rheometer Test Curves (not shown) which disclosed
the following data for each of the test compositions:
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TABLE VIII
Test Data - For the compositions
of Examples 10 11
ST; minutes 0.63 0.9
CT minutes 5.30 6.0
H~ inch-pounds 110 160
The Efficiency Factors for these compositions -
indicates that the addition of t-butyl hydroperoxide to the
highly filled formulation of Example 10 greatly improves
the scorch resistance of the latter composition.
In all cases the TBH was used in the form of a
mixture of 90% tertiary butyl hydroperoxide and 10%
tertiary butyl alcohol.
