Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: ISOBUTYLENE-BASED ELASTOMER BLENDS
FIELD OF INVENTION
This invention relates to isobutylene-based polymers, particularly halogenated
isobutylene-based polymers, and more particularly to blends of semi-
crystalline
polymers with brominated butyl rubber having improved green (pre-cure)
strength
and improved impermeability, and methods for its preparation.
BACKGROUND OF THE INVENTION
to Isobutylene-based polymers have been blended with numerous compounds
such as natural rubber in order to improve its various properties, such as
elasticity,
strength, air impermeability, etc. Natural rubber (NR) is known to crystallize
upon
extension and is known to have very high molecular weight fractions, both of
which
help improve its green properties. The terms "green properties" and "green
strength"
are terms applied to denote the strength, cohesiveness and dimensional
stability of
rubber compounds before they are vulcanized or cured. Such properties axe
important
in fabricating rubber articles from green compounds, particularly composites
such as
tires, but also can be important in extruded items such as innertubes and
molded
articles such as pharmaceutical stoppers. Isobutylene-based polymers are
therefore
2o blended with natural rubber when green properties need to be improved.
However,
green strength properties of isobutylene-based polymers are often adverse to
those of
natural rubber, particularly at elevated temperatures of up to 40 to
70°C. Addition of
natural rubber reduces the barrier properties of isobutylene-based
polymer/natural
rubber blends significantly, which is undesirable for applications requiring
low
permeability to gases, such as in tires and in bladder applications. Heat
stability of
cured compounds is also diminished in natural rubber blends.
Isobutylene-based polymers, particularly halogenated isobutylene-based
polymers, and more particularly brominated butyl rubber are the primary blends
of
3o most tire liners, heat resistant tubes, bladders and other commercially
known products
such as pharmaceutical ware. The term "butyl rubber" as employed herein is
intended
to refer to a vulcaxuzable rubbery copolymer containing, from 85 wt% to 99.5
wt%
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combined isoolefin derived units having from 4 to 8 carbon atoms. Such
copolymers
and their preparation are well known. See, e.g., RUBBER TECHNOLOGY 284-321
(Chaprrian & Hall 1995). Halogenated butyl rubber, particularly brominated
butyl
rubber, is also well known. It may be prepared by treating a solution of butyl
rubber,
in an organic solvent, with bromine and recovering the brominated butyl rubber
by
contacting it with steam amd drying the resulting aqueous slurry.
Brominated butyl rubber typically contains less than one bromine atom per
carbon-carbon double bond originally present in the polymer or from less than
3 wt%
l0 (weight percent) bromine. The Mooney viscosity of the halobutyl rubbers
useful in
the instant invention, measured at 125°C (ML 1+8), range from 20 to 80,
more
preferably from 25 to 55, and most preferably from 30 to 50. It is a
relatively
chemically resistant, rubbery polymer which can be compounded and cured to
produce synthetic rubber with an outstanding air impermeability, useful in
making tire
innerliners and innertubes.
Brominated butyl rubber has a greater degree of reactivity than butyl rubber,
so that it can be blended with other unsaturated polymers and co-vulcanized
therewith, which the unreactivity of butyl precludes. Brominated butyl rubber
vulcanizates, however, show good air impermeability, heat aging
characteristics and
general chemical resistance. It finds one of its principal uses in the
tubeless tire
innerliners. Such liners are in effect thin sheets of rubber, adhered to the
tire carcass
by co-vulcanization with the rubbers comprising the tire carcass. The heat
aging
characteristics air impermeability and co-vulcanizability of brominated butyl
rubber
render it suitable for use in such tire innerliners. Other known uses for
halogenated
butyl rubber include white sidewall compounds for tires, heat resistant tubes
and
bladders.
A deficiency of butyl and halobutyl rubber is its lack of green strength when
3o alone. In addition, the elongation characteristics of the uncured compounds
can be
used as a valuation of green strength. Lack of green strength renders
difficult the
processing and molding of rubber compounds based on butyl rubber. Green
strength,
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viscosity and elastic memory are important properties influencing the
processability
of polymers and compounds in various end-use applications, e.g., tire
fabrication. For
example, in the manufacture of tire liners, very thin sheets of butyl rubber
compound
have to be prepared, applied to the green tire carcass and then cured. If the
butyl or
halobutyl rubber compound is deficient in green strength, there is risk of
rupturing the
thin sheets during processing unless very careful handling thereof is
undertaken.
U.S. 4,256,857 discloses the improvement of green strength by treating the
brominated butyl rubber with relatively small amounts of certain organic amine
1o compounds. Examples of suitable amine compounds include N,N-dimethyl
hexylamine, N,N-dimethyldodecylamine, N,N-dimethyloctadecylamine, N,N-
diethyldecylamine and N,N-dimethylbenzylamine. These amine compounds have
been found to provide green strength and allow the retention of good
processing
properties. While other amine compounds may be reacted with brominated butyl
rubber to improve the strength of the rubber compound they generally also
cause the
rubber compound to be of inferior processing properties. In either case,
heating and
time requirements that are not efficient or practical for quick application
for
compounding in industrial applications.
2o U.S. 5,162,409 to Mo~ocskowski describes a rubber blend suitable for use in
automobile tire treads wherein the blend comprises a halogenated isobutylene
rubber
which can be the sole rubber of the blend or one of a combination of rubbers.
A
preferred embodiment comprises a rubber component comprising 20 to 60 wt%
styrene/butadiene rubber, 20 to 60 wt% butadiene rubber, and 10 to 30 wt% of a
halogenated rubber, a silica filler, and an organosilane cross-linking agent.
It is disclosed
that in a preferred embodiment, the rubber blends comprise 10 to 30 parts per
100 parts
rubber (phr) of untreated, precipitated silica employed with an effective
amount of
organosilane coupling agent, for example, 1 to 8 phr. However, the green
strength
properties of the isobutylene rubber or blends thereof are not significantly
improved.
The prior art has not addressed the full complement of green strength
properties. In particular, what is needed is a blend that has improved
relaxation and
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other properties to allow the blend to be processed at elevated temperatures,
for
example, at around 50°C, or from about 40 to 70°C. The present
invention provides
for a novel blend which addresses the present need for improved green strength
while
maintaining adequate impermeability.
SUMMARY OF THE INVENTION
In accordance with the present invention, an embodiment is directed to a blend
comprising an isobutylene-based polymer and a semi-crystalline copolymer (SCC)
which improves green strength properties while maintaining the barrier and
oxidative
to heat aging properties. The SCCs are generally semi-compatible with
isobutylene-
based polymers and have crystalline melting points below the temperatures used
in
the mixing and shaping operations. An embodiment is a barrier membrane having
an
isobutylene-based polymer and a SCC, the SCC being a semi-crystalline ethylene
copolymer having a melting point of from 25°C to 105°C and a
heat of fusion from 2
J/g to 120 J/g as determined by differential scanning calorimetry (DSC) in one
embodiment, from 10 J/g to 90 J/g in another embodiment, and from 20 J/g to 80
J/g
in yet another embodiment.
In accordance with another embodiment of the present invention, the blend is
2o an isobutylene-based polymer and a SCC, the SCC being a semi-crystalline
ethylene
copolymer wherein the ethylene content is at least from 45 wt% by weight of
the
SCC. Generally, the SCC is a copolymer of ethylene derived units and alpha-
olefin
derived units, the alpha-olefin having from 4 to 16 carbon atoms in one
embodiment.
The crystallinity arises from the ethylene derived units.
In accordance with a further embodiment, a tire innerliner or innertube is a
brominated butyl rubber polymer and a SCC, the SCC being a semi-crystalline
ethylene copolymer blend wherein the semi-crystalline ethylene copolymer has a
melting point by DSC of from 25°C to 105°C in one embodiment,
from 25°C to 90°C
3o in another embodiment, and from 35°C to 80°C in yet another
embodiment, and an
average ethylene content of at least 45 wt% in one embodiment, and at least 60
wt%
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in another embodiment, and at least 70 wt% in yet another embodiment, the wt%
relative to the total weight of the SCC.
DETAILED DESCRIPTION OF THE INVENTION
5 The present invention is a blend of an isobutylene-based elastomer (such
as,,
for example, butyl rubber) and a semi-crystalline copolymer (SCC) that
exhibits
improved green strength, green elongation, and green relaxation properties. A
further
embodiment of the present invention is a blend of any isobutylene-based
elastomer
and SCC which exhibits improved aging properties and improved barrier
properties.
to The improvement in green strength according to the invention is achieved
without
substantial sacrifice of any of the other desirable properties or
processability of
isobutylene-based elastomers and does not interfere with the subsequent curing
operations conventionally conducted with isobutylene-based elastomers or the
usefulness of the vulcanizates so obtained.
The Semi-Crystalline Copolymer
It was found that a class of preferably saturated (no backbone unsaturation),
SCCs can be added to isobutylene-based polymers to improve green strength
properties while maintaining the barrier and oxidative heat aging properties.
These
2o polymers are generally at least semi-compatible with isobutylene-based
polymers and
have crystalline melting points below the temperatures used in mixing and
shaping
operation. At handling and some further processing operations, such as tire
building,
done at temperatures below the crystalline melting point of the SCCs, the
green
properties are enhanced in blends of the present invention.
Generally, the SCC is a copolymer of ethylene derived units and alpha-olefin
derived units, the alpha-olefin having from 4 to 16 carbon atoms in one
embodiment,
and in another embodiment the SCC is a copolymer of ethylene derived units and
alpha-olefin derived units, the alpha-olefin having from 4 to 10 carbon atoms,
wherein
the SCC has some degree of crystallinity. In a further embodiment, the SCC is
a
copolymer of 1-butene derived units and another alpha-olefin derived unit, the
other
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alpha-olefin having from 5 to 16 carbon atoms, wherein the SCC also has some
degree of crystallinity. The SCC can also be a copolymer of ethylene and
styrene.
More specifically, one embodiment of the SCC is, a thermoplastic copolymer,
preferably random, of ethylene derived units an alpha-olefin derived units
having
from 4 to 16 carbon atoms, the SCC having a melting point by DSC analysis of
from
25°C to 105°C in one embodiment, from 25°C to 90°C
in another embodiment, and
from 35°C to 80°C in yet another embodiment, and an average
ethylene content by
weight of from at least 45% in one embodiment, and from at least 60% in
another
to embodiment, and from at least 70% in yet another embodiment, the wt%
relative to
the total weight of the SCC. The SGG preferably has a heat of fusion from 2
J/g to
120 J/g as determined by DSC (differential scanning calorimetry) in one
embodiment,
from 10 J/g to 90 J/g as determined by DSC in another embodiment, and from 20
J/g
to 80 J/g in yet another embodiment.
The SCC of the polymer blends of the present invention is a crystallizable
copolymer of ethylene (or 1-butene) derived units, and another alpha-olefin
derived
unit having from 4 to 16 carbon atoms in one embodiment, and from 4 to 10
carbon
atoms in another embodiment. In one embodiment when ethylene is one copolymer,
the alpha-olefin unit is derived from 1-butene, and 1-octene in yet another
embodiment. The crystallinity of the SCC axises from crystallizable ethylene
sequences. Blends, and the process for producing blends wherein ethylene and
C4 to
C16 alpha-olefins are used are more fully described in U.S. 5,272,236,
5,665,800,
5,783,638, 5,191,052, 5,382,630, 5,382,631, and 5,084,534. In yet another
embodiment, the SCC is copolymer of 1-butene derived units and a CS to Clo
alpha
olefin derived units, the crystallizable units being derived from the 1-
butene.
The SCC of the present invention preferably includes a random crystallizable
copolymer having a narrow compositional distribution. The SCC is statistically
3o random in the distribution of the ethylene and alpha-olefin comonomer
sequences
along the chain. There is substantially no statistically significant
difference in the
SCC, either among two polymer chains or along any one chain. Crystallization
is
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measured by DSC, as described herein. In all SCC, the length and distribution
of
polyethylene sequences is consistent with the substantially random statistical
crystallizable copolymerization.
The SCC desirably has a single broad melting transition as determined by
DSC. Typically, a sample of the SCC will show secondary melting peaks adjacent
to
principal peak. These are considered together as single melting point and the
highest
of these peaks is considered the melting point. These SCC polymers have a
melting
point of less than 105°C in one embodiment, less than 100°C in
another embodiment,
to and between 25°C and 105°C in another embodiment, and between
25°C and 90°C in
yet another embodiment, and between 35 °C and 80 °C in yet
another embodiment,
and a heat of fusion of less than 120 J/g in one embodiment, less than 90 J/g
in
another embodiment, and less than 80 J/g in yet another embodiment, as
determined
by DSC.
The weight average molecular weight of the SCC can be between 10,000 to
5,000,000 in one embodiment, and from 80,000 to 500,000 in another embodiment,
with a polydispersity index (PDI) between 1.5 to 40.0 in one embodiment,
between
1.8 to 5 in another embodiment, and between 1.8 to 3 in yet another
embodiment. It
2o is desirable that the SCC have a Melt Index (MI) that is at a level of
greater than l, so
long as the crystallinity of the SCC is within the ranges stated above. The MI
can be
between 0.1 and 5000 in one embodiment, greater than 35 in another embodiment,
greater than 100 in yet another embodiment, between 0.1 and 1000 in yet
another
embodiment, and between 0.5 and 100 in yet another embodiment as measured by
ASTM D1238.
The semi-crystalline nature, or diminished level of crystallinity, in the SCC
of
the invention relative to a crystalline homopolymer of ethylene derived units,
is
obtained by incorporating from 5 to 55 wt% by weight alpha-olefin in one
3o embodiment, from 6 to 40 wt% by weight alpha-olefin in another embodiment,
and
from 8 to 30 wt% by weight alpha-olefin derived units in yet another
embodiment into
the copolymer, the wt% relative to the total weight of the SCC. The alpha-
olefins
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comprise one or more members of the group C2, C4-C 16 alpha-olefins, and
styrenes
in one embodiment. As discussed above, one desirable alpha-olefin is a C4 (the
EXACTTM 4033 plastomer, a C2, C4 copolymer, commercially available from
ExxonMobil Chemical Company) and another is a Cg (the EXACTTM 8201
plastomer, a C2, Cg copolymer, commercially available from ExxonMobil Chemical
Company).
More than one SCC as defined in the present application may be used as the
SCC component of the invention. The different SCCs may differ in their
crystalliiuty
1o so long as the crystallinity falls within the described ranges.
The semicrystalline polymer component may contain small quantities of at
least one dime, desirably at least one of the dimes is a non-conjugated dime
to aid in
the vulcanization and other chemical modification when present. The amount of
dime is limited to be no greater than 10 wt% and desirably no greater than 5
wt%.
The dime may be selected from the group consisting of those that are used for
the
vulcanization of ethylene propylene rubbers and desirably ethylidene
norbornene,
vinyl norbornene, dicyclopentadiene, and 1,4-hexadiene (available from DuPont
Chemicals).
Rubber Component
A second component of the blends of the invention is an isobutylene-based
elastomeric copolymer or other rubber component. In one embodiment,
isobutylene-
based polymers are employed in the blend of the invention, while halogenated
isobutylene-based polymers are employed in another embodiment of the blend,
and
brominated butyl rubber, including star branched butyl rubber, are employed in
yet
another embodiment of the blend of the invention. The aforementioned list of
isobutylene-based polymers are available from ExxonMobil Chemical Co.
(Houston,
TX) and described in LT.S. 2,631,984, 2,964,489, 3,099,644, and 5,021,509. The
isobutylene-based polymer may be selected from the group consisting of butyl
rubber,
polyisobutylene, random copolymers of a C4 to C7 isomonoolefin and a para-
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alkylstyrene, such as EXXPROTM, available from ExxonMobil Chemical Co. and
described in U.S. 5,162,445, 5,430,118, 5,426,167, 5,548,023, 5,548,029, and
5,654,379. However, the scope of the present invention is not limited to the
aforementioned blends and may include any isobutylene-based elastomeric
polymer.
Such other rubbers with which the brominated butyl of this invention may be
blended include natural rubber, polyisobutylene rubber, ethylene co-polymers,
such as
ethylene cycloolefin and ethylene isobutylene copolymers, styrene-butadiene
rubber,
polybutadiene, polyisoprene and styrene-butadiene polymers and the lesser
to unsaturated rubbers such as ethylene-propylene-dime polymers (EPDM). EPDM
is
the ASTM designation for a terpolymer of ethylene, propylene and a non-
conjugated
diolefin. One embodiment of the EPDM terpolymer is VISTALON 2200TM grade,
available from ExxonMobil Chemical Company. Additional acceptable polymers are
described in U.S. 5,763,556 and 5,866,665.
An embodiment of the primary rubber component present is natural rubber.
Natural rubbers are described in detail by Subramaniam in RUBBER TECHNOLOGY
179-208 (Van Nostrand Reinhold Co. Inc., Maurice Morton, ed. 1987). Desirable
embodiments of the natural rubbers of the present invention are selected from
the
2o group consisting of Malaysian rubber such as SMR CV, SMR 5, SMR 10, SMR 20,
and SMR 50 and mixtures thereof, wherein the natural rubbers have a Mooney
viscosity at 100°C (ML 1+4) of from 30 to 120, more preferably from 40
to 65. The
Mooney viscosity test referred to herein is in accordance with ASTM D-1646.
Fillers
The blends of the present invention may have one or more filler components
such as calcium carbonate, clay, silica, talc, titanium dioxide, and carbon
black. In
one embodiment, the filler is carbon black or modified carbon black. The
filler is
reinforcing grade carbon black present at a level of from 10 to 100 phr of the
blend,
3o more preferably from 30 to 80 phr. Useful grades of carbon black as
described in
RUBBER TECHNOLOGY 59-85 (Van Nostrand Reinhold Co. Inc., Maurice Morton, ed.
1987) range from N110 to N990. More desirably, embodiments of the carbon black
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useful in, for example, tire treads are N229, N351, N339, N220, N234 and N110
provided in ASTM (D3037, D1510, and D3765). Embodiments of the carbon black
useful in, for example, sidewalls in tires, are N330, N351, N550, N650, N660,
and
N762.
5
Preparation of the Blend
The blends of the present invention typically have from 3 to 95 wt% of the
SCC by weight of the blend in one embodiment, and from 5 to 30 wt% by weight
of
the blend in another embodiment. The components are blended by techniques
known
l0 to those skilled in the art, and is not limited therein by the method of
blending or
mixing.
The following data demonstrates improvements in green properties with little
consequence to barrier or cured properties for the inventive blends. Moreover,
the
data suggest that blending of low molecular weight SCC with isobutylene-based
polymers may enable reduction in plasticizer levels, such as oil and
STRUKTOLTM
MS-40, (Struktol Chemicals, Akron, Ohio) to further reduce barrier
disadvantages
while maintaining good compound processability. Also, low molecular weight
polyisobutylene polymer, i.e. polyisobutylene oil, can be used as a
plasticizer in place
of processing oils such as FLEXONTM 876 used in the present examples.
Plasticizers
are added for obtaining acceptable processing characteristics such as mixing,
milling,
calendering, extrusion and molding. When low molecular weight SCC are added
they
can also act as plasticizers while the crystallinity of the SCC maintains
improved
green properties even at lower molecular weight.
Suitable barrier membranes, such as tire innerliner and innertube blends, may
be prepared by using conventional mixing techniques including, e.g., kneading,
roller
milling, extruder mixing, internal mixing (such as with a BANBURYTM mixer),
etc.
The sequence of mixing and temperatures employed are well known to the skilled
rubber compounder, the objective being the dispersion of polymers, fillers,
activators
and curatives in the polymer matrix without excessive heat buildup. A useful
mixing
procedure utilizes a BANBURYTM mixer in which the polymeric components,
fillers,
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and other resins, plasticizers, and oils are added and the blend mixed for the
desired
time to a particular temperature to achieve adequate dispersion of the
ingredients.
Alternatively, the polymers and a portion of the fillers (e.g., one-third to
two-thirds)
are mixed for a short time (e.g., 1 to 3 minutes) followed by the remainder of
the
fillers and oil. Mixing is continued for 5 to 10 minutes at high rotor speed
during
which time the mixed components reach a temperature of 150°C. Following
cooling,
the components are mixed in a second step on a rubber mill or in a BANBURYTM
mixer during which the curing agent and optional accelerator are thoroughly
and
uniformly dispersed at a relatively low temperature, e.g., 80° to
105° C. Variations
l0 in mixing will be readily apparent to those skilled in the art and the
present invention
is not limited to any specific mixing procedure. The mixing is performed to
disperse
all components of the blend thoroughly and uniformly.
The improved green strength blend of the present invention can be
compounded alone or blended with other rubbers and processed with the same
ingredients and the same procedures as used with conventional brominated butyl
rubber, i.e. with fillers such as carbon black, silica or clay, with
plasticizers, extender
oils, such as isobutylene oil, low molecular weight polybutenes, and
tackifiers and
with vulcanizing agents such as zinc oxide and/or sulfur with or without
additional
2o vulcanization accelerations. Such other rubbers with which the brominated
butyl of
this invention may be blended include natural rubber, polyisobutylene rubber,
ethylene co-polymers, such as ethylene cycloolefin and ethylene isobutylene
copolymers, styrene-butadiene rubber, polybutadiene, polyisoprene and styrene-
butadiene polymers and the lesser unsaturated rubbers such as ethylene-
propylene-
diene polymers (EPDM). EPDM is the ASTM designation for a terpolymer of
ethylene, propylene and a non-conjugated diolefin. An embodiment of an EPDM
terpolymer is VISTALON 2200TM grade, (ExxonMobil Chemical Company, Houston,
TX). Additional acceptable polymers are described in U.S. 5,763,556 and
5,866,665.
3o The improved green strength blend of the present invention, alone or
blended
with other rubbers, may be cured by reaction with curatives well known in the
art; the
amounts of such curatives being those conventionally used. Generally, polymer
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blends, e.g., those used to produce tires, are often crosslinked. It is known
that the
physical properties, performance characteristics, and durability of vulcanized
rubber
compounds are directly related to the number (crosslink density) and type of
crosslinlcs
formed during the vulcanization reaction. (See, e.g., W.F. Helt, B.H. To & W.W
Paris,
The Post Irulcahization Stabilization for NR, RUBBER WORLD 18-23 (1991))
Generally,
polymer blends may be crosslinked by adding curative molecules, for example
sulfur,
zinc, metals, radical initiators, etc. followed by heating. This method may be
accelerated
and is often used for the vulcanization of elastomer blends. The mechanism for
accelerated vulcanization of natural rubber involves complex interactions
between the
e o curative, accelerator, activators and polymers. Ideally, the entire
available curative is
consumed in the formation of effective crossliiiks that join together two
polymer chains
and enhance the overall strength of the polymer matrix. Numerous curatives are
known
in the art and include, but are not limited to, the following: zinc oxide,
stearic acid,
tetramethylthiuram disulfide (TMTD), 4,4'-dithiodimorpholine (DTDM),
tetrabutylthiuram disulfide (TBTD), 2,2'-benzothiazyl disulfide (MBTS),
hexamethylene-1,6-bisthiosulfate disodium salt dehydrate (ERP 390), 2-
(morpholinothio)
benzothiazole (MBS or MOR), blends of 90% MOR and 10% MBTS (MOR 90), N-
oxydiethylene thiocarbamyl-N-oxydiethylene sulfonamide (OTOS) zinc 2-ethyl
hexanoate (ZEH); and MC sulfur. In addition, various vulcanization systems are
known
2o in the art. (For example, see Formulation Design and Curing Characteristics
of NBR
Mixes for Seals, RUBBER WORLD 25-30 (1993)). The amount of other compounding
ingredients is within the ranges known in the axt.
The following examples include data that illustrates the improvements found
to green elongation, green strength and relaxation integrity in barrier
membranes and
blends in general, such as tore innerliner model compounds. The barrier
membranes
and blends produced with the aforementioned blends may be used in the
manufacture
of articles, such as curable articles and/or vulcanizates, such tire
innerliners, tire
innertubes, pharmaceutical stoppers, roof sheeting, belts, tubes, hoses, and
so on. The
3o barrier membrane may be used to prevent gas or fluid intrusion or leakage.
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Test Methods
I. Green strengthlstress relaxation
The green strength tests follow the guidelines set by ASTM D4I2-87, and
modified as described.
A. Sample preparation. Test pad samples are prepared from a 102 x 102 x 6.0 mm
milled sample weighing approximately 85 + 5 grams. The uncured sample is
placed
between MYLARTM sheets in a room temperature mold, noting the direction of the
mill grain. The mold is loaded in a curing press set at approximately
100°C and
l0 pressed for a total of approximately five minutes; two minutes at low
pressure
(approximately 7800 lbs.) and three minutes at high pressure (30,000 lbs.).
The
molded pad is then removed and allowed to sit at room temperature for at least
approximately 24 hours before testing. The testing samples are cut in 12 mm
wide x
75-mm long samples without removing the MYLARTM backing, with mill grain along
the sample length.
B. Testihg. The standard test temperature is preferably 23 + 2°C (open
laboratory
atmosphere) or 50°C. Samples are tested using an Instron tester having
the following
settings:
2o Load Cell: 1000 Newtons
Pneumatic jaws: set at 30-psi air pressure
Crosshead speed: 127 mm/min
Chart Speed: 50 mm/min
Full Scale: 25 Newtons
Jaw separation: 25 mm
The MYLARTM backing is removed from each side of the sample, for example
by using acetone. The sample thickness is measured and marked with a 25-mm
benchmark. The ends of the sample are covered with MYLARTM on each side to
prevent adhesion to the jaws. The sample is placed in the jaws of the tester,
aligning
the benchmark with the top and bottom edges of the jaws. The sample is
stretched
100% (from 25 to 50 mm jaw separation). The tensile force is monitored after
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deformation is stopped until the tensile force exceeds the point at which the
force
decays by 75% (to 25% of the value after the crosshead is stopped).
C. Calculations. Using sample dimensions (width and thickness) and force data,
the
following can be calculated:
(i) Green strength: stress at 100% (at the point the crosshead stopped).
N/mm2 = Force (N)/(sample width (mm))(tluckness (mm)).
(ii) Relaxation Time (t75): from the recorder chart compute the time it took
for
the stress (force) to decay by 75% (from its value when relaxation started to
the point
l0 the stress decayed to 25% of this value). The time should be counted after
the
crosshead stopped (it should exclude the deformation time of 12 sec).
(iii) Test three (3) specimens for each compound. Report the median as green
strength and time to 75% decay.
D. Normalization. Green strength and stress relaxation values obtained for
various
materials can be normalized against a given material. This is accomplished by
dividing each resulting reference stress by the reference stress for the
standard
material. The normalization should be done for measurements using identical
parameters. However, after normalization, materials measured with one or more
2o changes in the test parameters can still be compared if the standard
material is the
same material and was measured with both sets of parameters. For example, if
the
extent of decay is 75 % in one set of tests and 50 % in another set, both sets
can be
normalized against the same standard which is itself measured under both
conditions.
Since decay follows an exponential form, a normalized relaxation time is not
strongly
dependent on the extent of decay.
Permeability was tested by the following method. Thin, vulcanized test
specimens from the sample blends were mounted in diffusion cells and
conditioned in
an oil bath at 65°C. The time required for air to permeate through a
given specimen is
3o recorded to determine its air permeability. Test specimens were circular
plates with
12.7-cm diameter and 0.38-mm thickness.
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The procedure used in the present application for Differential Scanning
Calorimetry (DSC) is described as follows. Preferably, 6 mg to 10 mg of
pellets is
placed in a Differential Scanning Calorimeter and cooled to -50°C to -
70°C. The
sample is then heated at 20°C/min to attain a final temperature of
200°C to 220°C.
5 The thermal output is recorded as the area under the melting peak of the
sample which
IO
is typically at a maximum peak at 30°C to 175°C and occurs
between the
temperatures of 0°C and 200°C. The thermal output is measured in
Joules as a
measure of the heat of fusion. The melting point is recorded as the
temperature of the
greatest heat absorption within the range of melting temperature of the
sample.
Further descriptions of the testing procedures are described in U.S.
5,071,913.
Green strength, viscosity and elastic memory are important properties
affecting the
processability of polymers and compounds in various end-use applications,
e.g., tire
fabrication. Tire innerliner compounds, for example, require low elastic
memory. It
15 would be expected that this property would be enhanced by lower viscosity,
but it
must be balanced against the need to maintain acceptable green strength that
directionally increases as viscosity increases. Lower viscosity polymers are
also
preferred for easier mixing and calendering.
Examples
A series of compounds were mixed in an internal mixer using a model
formulation. The compounds were based on one bromobutyl rubber (Sample 1), or
blends of the bromobutyl rubber with the SCC according to the present
invention
(Samples 3-6), or with natural rubber (Sample 2). The SCC was introduced at
levels
of either 10 or 20 phr (parts per hundred rubber), the NR was introduced at 20
phr
(Sample 2) as shown in the Table 1. Oil (STRUCTOLTM 40 MS) was introduced at 5
phr and carbon black was introduced at 60 phr in Samples 1-6.
The cure system shown in the Table 1 was incorporated on a mill. Table 2
3o describes two embodiments of the SCC used in the Sample blends 3-6.
CA 02406575 2002-10-17
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16
Green properties of the compounds without the cure system components were
measured at room temperature and at SO°C and shown in Table 3. It was
found that
for consistent results the samples for green properties testing had to be
carefully
calendered before molding. Samples were cut out of molded pads into strips of
2.S
inches (6.35 cm) long, O.S inch wide (1.27 cm) and 0.1 inch thick (0.254 cm),
such
that the length of the samples corresponded to the direction of the calendered
sheet
leaving the calender. In all tests the strips were clamped in a tensile tester
such that
the distance between the grips was 1 inch (2.54 cm). A green tensile test was
also
conducted at SO°C as shown in Table 3. For the tensile tests the same
sample and test
to configuration was used with the sample pulled to break at a rate of 10
inches per
minute (25.4 cm/min). All tests were carried on in triplicates with the median
value
being the recorded one.
The results for room temperature green stress relaxation are expressed as the
1S green strength in Table 3. The green strength may be defined as being the
stress at the
end of extension (100% extension) after extension at S inch/min (12.7 cm/min),
and
the time to relax the stress by 7S% from the stress at the end of extension.
The time
was measured from the instant the extension was stopped. The green strength
test at
SO°C uses similar parameters to the room temperature test. The
parameters recorded
2o for the SO°C green tensile test were 100% modulus, and % elongation
at break. The
parameters were also reported as the median sample out of three runs. The
results for
green properties are given in Table 3 and for selected samples according to
the present
invention in the stress-time and stress-strain traces at SO°C in Table
3.
25 For room temperature green strength it can be seen that all samples with
SCC
have higher green strength. Sample 1 representing the lowest commercially
available
molecular weight bromobutyl (commercially available as BR 2222, ExxonMobil
Chemical Company, Houston TX) without a second polymer showed the fastest
relaxation time. For the SO°C green strength, the relative ranking is
similar; the
30 relaxation times are different between the Sample 1 and Samples 3-6 in that
the values
are less at SO°C than at room temperature. This is an advantageous
property for high
temperature processing of the uncured blends to form such articles as tires,
where the
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17
uncured blend must be stretched and deformed into the desired shape, then
maintain
that shape after a desirably short period of time. An increase in elongation
to break at
50°C is shown by all compounds containing 20 phr SCC. The higher
elongation is
important for maintaining material integrity when processing calendered sheets
at
higher temperature, such as between 40 and 70°C. The higher green
strength also
helps in handling by helping reducing deformation during processing.
Processability during mixing, milling and calendering (or other high
temperature green compound shaping such as extrusion) was not adversely
affected
to by the addition of the SCC blends as experienced during sample preparation
for this
example. This behavior can be demonstrated in capillary flow as shown in the
Table
4. Capillary extrusion was conducted in a Monsanto Processability Tester (MPT)
instrument commercially available from Alpha Technologies of Akron, Ohio, at
100°C. The lower die swell at high shear rates of the SCC blends vs.
the 100%
bromobutyl formulation indicate reduced elasticity in processing which is
helpful in
shaping operations, even when high shear viscosities are similar. Viscosities
may be
further reduced if the molecular weight of the SCC is lowered, or the MI is
increased.
Cured physical properties shown in the Tables 5 show that the modulus and
2o elongation at break are is not adversely affected upon addition of the SCC.
An important property for isobutylene-based polymers in gas containing
applications is air impermeability. For example, barrier membranes, or
innerliners in
tires, and innertubes for tires and bicycles, must contain pressurized gas for
extended
periods of time, and thus must have a high degree of impermeability. The data
in
Table 5 establishes improved air° permeability at 65°C when
blending isobutylene-
based polymers with the SCC disclosed in this application as compared to
natural
rubber or amorphous polymers and blends thereof. Reducing oil levels can
reduce
permeability significantly, while an increase in carbon black level is only
slightly
advantageous. Since some low molecular weight (or high MI) semi crystalline
polymers and copolymers can act as plasticizers, oil levels can be reduced to
improve
barrier properties without impacting processing operations. Thus, the blends
of the
CA 02406575 2002-10-17
WO 01/85837 PCT/USO1/13588
18
present invention are useful for articles that require a. low gas permeability
such as
inner tubes and inner liners for automobile tires, truck tires, and tubes for
bicycles,
motorcycles, and other applications.
Although the invention has been described with reference to particular
embodiments, it is to be understood that these embodiments are merely
illustrative of
the principles and applications of the present invention. It is therefore to
be
understood that numerous modifications may be made to the illustrative
embodiments
and that other arrangements may be devised without departing from the spirit
and
to scope of the present invention as defined by the appended claims.
All priority documents are herein fully incorporated by reference for all
jurisdictions in which such incorporation is permitted. Further, all documents
cited
herein, including testing procedures, are herein fully incorporated by
reference for all
jurisdictions in which such incorporation is permitted.
CA 02406575 2002-10-17
WO 01/85837 PCT/USO1/13588
19
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