Note: Descriptions are shown in the official language in which they were submitted.
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60455-549D
RUBBER~VULCANIZATION AGENTS AND METHODS
FOR THEIR PREPARATION
The present divisional application is divided out of
patent application serial No. 544,549 filed on August 14, 1987.
According to one aspect of the present divisional
application there is provided a process for preparing a bloom
resistant vulcanizate which comprises 1) mixing a sulfur curable
rubber with from 0.5 to 12% by weight of said rubber of a curing
agent prepared by heating a mixture of water, sulfur and olefin
to 120-200C with agitation optionally in the presence of a
basic catalyst and a dispersing agent; and then 2) vulcanizing
the rubber curing agent mixture.
According to another aspect of the present divisional
application there is provided a process for preparing a rubbery
vulcanizate comprising A) mixing a major proportion of a
rubbery material having available unsaturation selected from
the group consisting of natural and synthetic polyisoprenes,
polybutadienes, polychloroprenes, copolymers of isobutylene and
isoprene, copolymers of butadiene-1,3 and styrene and copolymers
of butadiene-1,3 and acrylonitrile with from 0.5 to about 12
by weight based on the weight of said rubbery material of an
adduct of sulfur and at least one olefinic hydrocarbon selected
from the group consisting of styrene, alpha-methylstyrene,
butene-l, butene-2, isobutylene, diisobutylene, dicyclo-
pentadiene, cyclooctadiene, cyclopentadiene and propylene; said
adduct being prepared by heating a mixture of water, sulfur and
olefinic hydrocarbon from 120C to 200C for 1 to 3 hours with
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60455-549D
stirring; B) vulcanizing the rubbery material-sulfur adduct
mixture.
According to one aspect of the parent application
there is provided a process for preparing a vulcanizing agent
for natural and synthetic rubbers which comprises heating
together with agitation at a temperature of from 120-200C
between 1 and 50 parts by weight of sulfur and one part by
weight of an olefin or olefins in an a~ueous media in the
presence of a basic catalyst and a dispersing agent.
According to another aspect of the parent application
there is provided a process for making a vulcanizing agent which
comprises 1) preparing a mixture of water, sulfur and an olefin
wherein from 1 to 50 parts by weight of sulfur per part by
weight of olefin and wherein the weight ratio of olefin plus
sulfur to water ranges from 1:2.5 to 1:60; 2) heating the
mixture to 120-200C with agitation to form the vulcanizing
agent; 3) cooling and isolating the granular vulcanizing agent.
Technical Field
The invention of this parent application relates to
vulcanization agents for rubber and a novel method for their
preparation. The invention of the divisional application relates
to vulcanizers.
Background Art
Obtaining a uniform dispersion of elemental sulfur
in rubber stocks has been a serious problem for the rubber
industry and one which has received a great deal of attention.
Problems attributable to poor dispersion of sulfur in the rubber
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60455-549D
include migration Gf the sulfur to the surface of the rubber
stock commonly referred to as "bloom"; decrease of tack at the
surface of the rubber stock; variatiGn of the physical properties
of vulcanizates from batch to batch; and others.
Compounds containing high sulfur levels can exhibit
problems with sulfur bloom on the surface of the unvulcanized
rubber. This surface layer of sulfur crystallizes causing a
loss of building tack which can cause problems in tire building.
Numerous modifications of standard rubber processing
techniques have been utilized to minimize the sulfur bloom
tendencies. These prior methodologies include the use Gf
insoluble sulfur in the compound; limiting the compound mixing
temperatures during the sulfur addition stage; and minimizing
the heat history that the compound is exposed to during
processing.
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_
--2--
Insoluble sulfur is formed by rapidly quenching
molten sulfur that is above 159C (preferably
200-250C). This product consists primarily of long
chain sulfur molecules and a lesser amount of soluble
S8 rings. There is a tendency for the long chain
molecules to revert to the more stable solub~e form if
exposed to higher temperatures, long storage times
and/or hostile storage environments.
Commercial insoluble sulfur products contain a
stabilizer to reduce this tendency. When insoluble
sulfur is mixed in a rubber compound, it exists as more
or less discreet particles of varying size in the
rubber phase. Above about 118C substantial reversion
to the soluble sulfur form occurs with resulting sulfur
bloom.
An approach taken over the years has been to
combine sulfur with an unconjugated diene which is
believed to enhance the compatibility with the rubber.
The polymeric structure is also believed to improve ~he
stability of the sulfur chains against breakdown to
soluble S8 units at normal storage and processing
temperatures yet readily allow the release of sulfur
for crosslinking at vulcanizing temperatures.
U.K. Patent 1,232,056 discloses a method of
preparing a vulcanizing agent for natural and synthetic
rubbers which comprises heating together at a
temperature of from 100-250C between 3 and 50 parts by
weight of sulfur and one part by weight of a conjugated
diolefin in the presence of a catalytic amount of an
amine, such as a dimethyl-substituted tertiary amine.
U.S. Patent 2,989,513 discloses a rubber
composition comprising natural or synthetic elastomers
and from about 1 to 12% by weight based on the weight
of said rubber of a curing agent comprising at least
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one interpolymer of sulfur and an olefinic hydrocarbon
selected from styrene, alpha-methylstyrene, butene,
isobutylene, diisobutylene, triisobutylene, ethylene
and propylene.
U.S. Patent 3,544,492 discloses an improved curing
agent which is a resinous composition formed by the
reaction of one or more olefinic materials and a
s-triazine, substituted with three groups containing
activated terminal unsaturation and sulfur.
U.S. Patent 3,264,239 discloses a process and a
vulcanizing agent which comprises preparing a mixture
of sulfur, linseed oil and dicyclopentadiene, heating
the resulting mixture at 125-135C for at least 5 hours
to form an interpolymer, and cooling and isolating the
interpolymeric product.
U.S. Patent 3,523,926 discloses a vulcanizing agent
for rubbers which is prepared by heating conjugated
diolefins with sulfur in the presence of catalytic
amounts of amines.
U.S. Patent 4,564,670 describes a disperse sulfur
product formed by dispersing particulate sulfur in a
liquid poly(cis-isoprene) dispersion agent. The
product can be formed by simply mixing the liquid
poly(cis-isoprene) dispersion agent with a major amount
of sulfur until the desired product results.
Canadian Patent 965,231 claims a method for
improving the dispersibility of insoluble sulfur in
rubber which comprises admixing insoluble sulfur
containing up to about 70% by weight soluble sulfur
with from about 0.3 to 5~ by weight based on the total
weight of the sulfur of a dispersing aid selected from
a specific group of alkyl-phenoxypoly(ethyleneoxy)-
ethanol compounds.
13332~5
Japanese Publication No. 57-133135 discloses a
rubber composition with improved sulfur bloom
characterized by the addition of triisopropanolamine,
diisopropanolamine, monoisopropanaolamine or blends
thereof to a rubber composition composed of 2 to 10
weight parts of sulfur as a vulcanizing agent blended
in 100 parts of rubber selected from natural rubber,
synthetic rubber or rubber blended from the two.
From a review of the prior art methodologies for
preparing sulfur-olefin adducts, it is quite apparent
that the reaction product of sulfur and an olefin
results or can result in materials that are viscous
liquids or solids. For example, U.S. Patent 3,259,598
teaches that a sulfur, linseed oil, styrene reaction
product can be used to vulcanize rubber. The product
from this reaction mixture must be pulverized before it
can be incorporated into the elastomer due to the
physical properties of the sulfur-olefin adduct.
Uniform dispersion of the sulfur in the rubber is a
prerequisite for uniform vulcanization and vulcanizates
with optimum mechanical properties and many
sulfur-olefin vulcanizing compositions of the prior art
require that grinding or milling steps be performed on
the sulfur-olefin adduct prior to its use in rubber.
This problem has been overcome through the instant
invention wherein the sulfur~olefin adducts are
prepared by reacting sulfur and an olefin at 140-160C
with agitation in water which optionally contains a
base as a catalyst and a dispersing agent. The prior
art does not suggest or disclose a process for the
preparation of sulfur-olefin adducts which is
characterized as heating sulfur, an olefin, water, base
and optionally a dispersing agent to 120-200C with
agitation, cooling the reaction mixture and filtering
the sulfur/olefin adduct beads.
1333205
The water serves as a medium in which the sulfur
can melt and react with the olefin in a particle form.
When the reaction mixture is cooled, the sulfur/olefin
adduct is frozen into a bead form. The water also acts
as a heat sink for the exothermic reaction.
Disclosure of the Invention
There is disclosed a novel w lcanizing agent for
natural and synthetic rubbers, said vulcanizing agent
is prepared by heating together with agitation at a
temperature of from 120-200C between 1 and 50 parts by
weight of sulfur and 1 part by weight of an olefin or
olefins in an aqueous reaction media in the presence of
a basic catalyst and a dispersing agent.
There is further disclosed a process for making a
w lcanizing agent which comprises 1) preparing a
mixture of water, sulfur and an olefin wherein from 1.0
to 50 parts by weight of sulfur per part by weight of
olefin and wherein the weight ratio of olefin plus
sulfur to water ranges from 1:2.5 to 1:60, 2) heating
the mixture to 120-200C with agitation to form the
vulcanizing agent 3) cooling the reaction mixture and
isolating the granular vulcanizing agent.
There is also disclosed a method for preparing a
bloom resistant vulcanizate which comprises 1) mixing a
sulfur curable rubber with from 0.5 to 12~ by weight of
said rubber of a curing agent prepared by heating a
mixture of water, sulfur and an olefin to 120-200C
with agitation optionally in the presence of a basic
catalyst and a dispersing agent; and then 2)
vulcanizing the rubber/curing agen~ mixture.
There is also disclosed a method for preparing a
rubbery vulcanizate comprising A) mixing a major
proportion of a rubbery material having available
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~ 60455-549D
unsaturation selected from the group comprising natural and
synthetic polyisoprenes, polybutadienes, polychloroprenes,
copolymers of isobutylene and isoprene, copolymers of
butadiene-1,3 and styrene, copolymers of butadiene-1,3 and
acrylonitrile; with from 0.5 to 12% by weight based on the weight
of said rubber material of an adduct of sulfur and at least one
olefinic hydrocarbon selected from the group consisting of
styrene, alpha-methylstyrene, butene-l, butene-2, isobutylene,
diisobutylene, cyclooctadiene, dicyclopentadiene, cyclopentadiene
and propylene; said adduct being prepared by heating a mixture
of water, sulfur and olefinic hydrocarbon to 120C to 200C for
1 to 3 hours with stirring; B) vulcanizing the rubbery material-
sulfur adduct mixture.
The rubbers useful as ingredients in the vulcanizates
include rubbery materials having available unsaturation such as
natural and synthetic vulcanizable rubbers and rubbery polymers
of dienes preferably of open chain conjugated dienes having from
4 to 8 carbon atoms. Specific examples of rubbery materials
which may benefit from the sulfur-olefin adduct of this invention
are natural rubber, polybutadiene-1,3, polyisoprene, poly-2,3-
dimethylbutadiene-1,3, poly-2-chlorobutadiene-1,3 and the like.
Other rubbers useful are the synthetic rubbers such as those
obtained from 1,3-dienes by means of polymerization or the
rubbery copolymers, terpolymers and the like of these and similar
conjugated dienes with each other or with at least one copolymer-
izable monomer such as isobutylene, styrene, acrylonitrile,
methylacrylate, ethylacrylate, methyl methacrylate, 4-vinyl
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60455-549~
pyridine and the like. The polymeric diene rubbers generally
contain at least 50% by weight of the diene and preferably
contain from about 55-85% by weight of the diene. However,
copolymers, terpolymers and other multicomponent polymers
containing as little as 35% or less by weight of diene can also
be employed. Still other rubbery materials can be used such as
unsaturated polymers containing acid groups obtained by the
copolymerization of a major amount of a conjugated diene with
an olefinically unsaturated carboxylic acid. Still other
rubbers can be employed such as those formed by the copolymer-
ization of dienes with alkyl acrylates and by the polymerization
of an alkyl acrylate with at least one other unsaturated monomer
followed by hydrolysis. Rubbery polyesterurethanes, polyether-
urethanes and polyesteramideurethanes having curable double
bonds or available unsaturation and rubber reclaimed from the
foregoing can also be used. Mixtures of two or more of the
foregoing rubbers may be employed as ingredients in the
vulcanizates of this invention. The preferred rubbers are the
natural and synthetic polyisoprenes, the polybutadienes, the
polychloroprenes, the copolymers of isobutylene with isoprene,
butadiene-1,3 with styrene, and butadiene-1,3 with acrylonitrile.
The no~-el sulfur-olefin adducts comprise adducts of
sulfur and one or more olefinic compounds. Specific olefinic
compounds include but are not limited to Glefinic hydrocarbons
such as ethylene, propylene, butene, isobutylene, isopentene,
diisobutylene, triisobutylene, hexyl-2-decene-1, and
heptadecene-7; cycloalkenes such as cyclopentene and cyclohexene;
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60455-549D
unsaturated terpenes such as pinene, camphene, and myrcene;
aralkenes such as styrene, dihydronaphthalene, indene, alpha-
methylstyrene and polyolefins such as butadiene, isoprene,
chloroprene, cyclopentadiene, dicyclopentadiene, cyclohexadiene,
vinyl cyclohexene, 1,7-octadiene, cyclooctadiene and the like.
The preferred olefinic compounds are styrene, alpha-methyl-
styrene, cyclopentadiene and dicyclopentadiene.
Elemental sulfur is used in preparing the sulfur-olefin
adduct curing agents.
A catalyst may be employed to facilitate the formation
of the sulfur-olefin adduct although they are not necessary.
Representative of the basic materials which may be used to
catalyze the formation of the sulfur-olefin adduct include
calcium carbonate, sodium carbonate, sodium hydroxide, tetra-
methylethylene diamine and the like.
Representative of the dispersing agents that may be
used in the preparation of the sulfur-olefin adduct include
polyethylene oxides, carboxymethyl cellulose, polyvinylalcohol
and the like.
The reaction to form the sulfur-olefin adduct is
advantageously and necessarily carried out in water.
The weight ratio of water to sulfur plus olefin can
vary from 2.5:1 to 60:1. Preferably the ratio is 7:1 to 4:1
with 5:1 being most preferred. The weight ratio of sulfur to
olefin can vary from 1:1 to 50:1, with 6:1 being preferred.
The sulfur-olefin adduct curing agents are prepared at
temperatures in the range of from 120C to 200C and preferably
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60455-549D
from 140-160C. Most preferably, the reaction is conducted at
least 150C. At temperatures below 145C, the beads after
isolation will agglomerate unless a cross-linkable monomer such
as divinylbenzene is used in conjunction with a peroxide in the
reaction mixture. The reaction, since it is conducted above the
boiling point of water, is necessarily conducted in a closed
vessel such as an autoclave.
The vulcanizates comprise as essential ingredients a
major proportion of a single rubbery material or a mixture of
two or more rubbery materials and a minor proportion of a single
or a mixture of sulfur-olefin adduct curing agents. Other
appropriate compounding ingredients such as reinforcing agents,
fillers, accelerators, plasticizers, antioxidants, age
resistors, resins, dyes, color pigments and the like may be
employed with the rubbery compositions disclosed herein in the
amounts customarily employed in the rubber industry.
The sulfur-olefin curing agent is useful in the range
of from about 0.5 to about 12 parts by weight per 100 parts by
weight of the rubbery material to be vulcanized. The preferred
range is from about 1 to about 5 parts by weight per 100 parts
of rubbery material. The rubber stocks containing the curing
agent and other appropriate compounding ingredients can be mixed
on a mill or other suitable mixing device such as a Banbury and
the resulting blend can be cured at temperatures of from about
120C to about 180C and preferably at from about 130-160C.
The vulcanizates are useful in tire treads, tire
sidewalls, carcass stocks, V-belts, gloves, shoe heels, shoe
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60455-549D
soles, printing rollers, colored rubber articles and wherever
it is desired to provide a stable elastomer that does not
exhibit sulfur bloom.
In the following examples, which illustrate the
inventions of the parent and the divisional applications, the
amounts of ingredients are given in parts by weight unless
otherwise specified.
Prior Art Process
Comparative Example 1 -
Reaction of DCPD with Sulfur
Into a 500 ml flask equipped with a temperature
controller, nitrogen inlet tube and mechanical stirrer were
placed 225 g of sulfur, 75 g of dicyclopentadiene (DCPD) and
3 g of N,N,N ,N -tetramethylethylene diamine (TMEDA). The
flask was flushed with nitrogen and the mixture was heated to
140C. Stirring was started when the sulfur had melted. The
reaction exothermed to 165C and became too viscous to stir in
25 minutes. The product solidified at 165C and upon cooling
resembled concrete.
Comparative Example 2
Into a 500 ml flask equipped with a temperature
controller, nitrogen inlet tube and mechanical stirrer were
placed 270 g of sulfur, 30 g of DCPD and 0.6 g of TMEDA. The
flask was flushed with nitrogen and the mixture was heated to
140C for 45 minutes. The hot product was poured into an
aluminum pan where it eventually solidified. The reaction flask
was discarded because of the difficulty involved in cleaning off
the product.
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60455-549D
Process of the Invention
Example 1
Into a one liter autoclave were placed 75 g of sulfur,
25 g of DCPD, 500 ml of water, 10 g of calcium carbonate as a
catalyst, and 6.5 g of carboxymethyl
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11
cellulose as a dispersing agent. The autoclave was
flushed with nitrogen and heated to 140C for three
hours with stirring. The autoclave was cooled to 13C
and small brown beads of product were filtered from the
water. Differential scanning calorimetry showed that a
reaction had occurred between the sulfur and the
olefin.
Example 2
Into a 4 liter autoclave were placed 425 g of
sulfur, 75 g of DCPD, 2500 ml of water, 20 g of calcium
carbonate as a catalyst and 32 g of carboxymethyl
cellulose as a dispersing agent. The autoclave was
flushed with nitrogen and heated to 150C for three
hours with stirring. After cooling to room
temperature, small beads of brown product were filtered
from the water. Differential scanning calorimetry
indicated that a reaction had occurred.
Example 3
Into a 4 liter autoclave were placed 425 g of
sulfur, 75 g of alpha-methylstyrene, 2500 ml of water,
20 g of calcium carbonate as a catalyst and 32 g of
carboxymethyl cellulose as a dispersing agent. The
autoclave was flushed with nitrogen and heated to 150C
for three hours with stirring. After cooling to room
temperature, the product was isolated as an orange
powder.
Example 4
1.0 g of polyvinylalcohol (Vinol~ 203 from Air
Products) was dissolved in 2500 ml of water. This
solution was added to a 4 liter reactor with 425 g of
sulfur, 75 g of DCPD, and 20 g of calcium carbonate.
13332û.5
The reactor was sealed and heated with stirring to
150C for three hours. After cooling to room
temperature, the small beads of sulfur/DCPD adduct were
filtered.
Example 5
Into a 4 liter autoclave were placed 425 g of
sulfur, 75 g of DCPD, 2500 ml of water, 20 g of calcium
carbonate and 5.0 g of Igepal~-630 (GAF). The reactor
was sealed and heated to 150C for three hours with
stirring. After cooling to room temperature, the
sulfur/DCPD adduct was isolated as small ribbons.
Example 6
No Dispersing Agent
Into a 4 liter autoclave were placed 425 g of
sulfur, 75 g of DCPD, 2500 ml of water and 20 g of
calcium carbonate. The reactor was then sealed and
heated to 150C for three hours with stirring. After
cooling to ambient temperature, the sulfur/DCPD adduct
was isolated as small beads.
Example 7
Peroxide and Divinylbenzene
Into a 4 liter autoclave were placed 425 g sulfur,
50 g DCPD, 20 g styrene, 5 g divinylbenzene, 2500 ml of
water, 20 g CaCo3, 32 g carboxymethyl cellulose and 7.1
g of a 70% solution of t-butylhydroperoxide in water.
The reactor was sealed and heated to 140C for three
hours. After cooling to ambient temperature, the
sulfur/olefin adduct was isolated as beads.
- 1~3320~
-13-
Example 8
No Catalyst
Into a 4 liter autoclave were placed 425 g sulfur,
75 g DCPD, 2500 g water and 32 g carboxymethyl
cellulose. The autoclave was sealed and heated to
150C for three hours with stirring. After cooling to
ambient temperature, the sulfur/olefin adduct was
isolated as beads.
Example 9
Into a 4 liter autoclave were charged 1800 g of
water, 478 g of sulfur, 84 g DCPD, 36 g of
carboxymethyl cellulose and 15 g of calcium carbonate.
The reactor was sealed and heated to 150C for 3 hours
with stirring. After cooling to room temperature,
beads of sulfur/olefin adduct were isolated.
Example 10
Into a 4 liter autoclavè were charged 425 g of
sulfur, 75 g DCPD, 2500 ml of water, 20 g of calcium
carbonate and 32 g of carboxymethyl cellulose. The
reactor was sealed and heated to 150C for three hours
with stirring. After cooling to ambient temperature,
the beads were filtered.
Example 11
Into a 4 liter au.oclave were charged 425 g sulfur,
60 g of DCPD, 15 g styrene, 2500 ml of water, 20 g of
calcium carbonate and 32 g of carboxymethyl cellulose.
The reactor was sealed and heated to 150C for three
hours with stirring. After cooling to ambient
temperature, the beads were filtered.
The ability of the sulfur-olefin adduct prepared
according to Examples 10 and 11 to cure rubbers was
compared with that of insoluble sulfur using a prepared
_ -14- 133320~
unw lcanized blend of natural and synthetic
polyisoprene rubber.
Two rubber stocks were prepared by employing the
following recipe set out in Table I. All parts are
parts by weight.
Table I
Component Control Experimental
Polyisoprene/Natural
Rubber (Non-productive)176.05 176.05
methoxy methyl-
melamine 2.80 2.80
Retarder 0.10 0.10
Antioxidant 0.75 0.75
Zinc Oxide 3.00 3.00
Accelerator 0.60 0.60
Secondary Accelerator0.60 0.60
Insoluble Sulfur 3.00 -0-
Sulfur-Olefin Adduct
from Example 10/11 -0- 2.83
The components were mixed in a Banbury~. The
quantities and materials used were those typically
used in the art for producing w lcanized rubber. The
amount of sulfur-olefin adduct used was such that its
available sulfur was substantially equivalent in
weight to the weight of sulfur used in the control.
The Banbury~ mix time required to incorporate the
sulfur-olefin adduct was similar to that required to
incorporate the insoluble sulfur therein.
As an additional comparative, a commercially
available sulfur-olefin adduct obtained from Wyrough
and Loser, Inc. known as Thio-Cure-BR was used in
place of the insoluble sulfur and the adducts from
Examples 10 and 11; however, at a level of 3.0 instead
of 2.83.
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60455-549D
The vulcanizable compositions were pressed out and
cured in a rubber press for about 40 minutes at 150C to provide
cured slabs for testing. The physical properties for the controls
and the experimental samples are set out in Table II.
A critical factor in the utility of rubber chemicals
is how well they can be dispersed into the rubber. Clearly, if
the chemicals are not well dispersed, they cannot fully take part
in the cure. It has been found, through a dispersion test,that
the sulfur/DCPD/styrene adduct prepared as disclosed herein is
dispersed into rubber better than a sulfur/DCPD adduct. The test
consists of mixing the material into the rubber using a Banbury,
sheeting the rubber out on a mill and visually observing
unbroken beads in a standard size area. The dispersion values
ranged from 0-5 with 0 being complete dispersion and 5 being the
poorest dispersion. The dispersion values for the controls and
Examples 10 and 11 in Table II were control - 0, sulfur/DCPD - 4,
sulfur/DCPD/styrene - 0, Thio-Cure-BR - 5. The test
demonstrates that the sulfur curatives prepared as diclosed
herein, when DCPD and styrene are used as the olefins, had
comparable dispersion ability to insoluble sulfur and much better
dispersability than the commercially available Thio-Cure.
The data indicates that the sulfur-olefin adduct
prepared as disclosed herein had better scorch delay than the
control but cured at a slower rate. In addition, the curative
from Examples 10 and 11 reached a much higher state of cure than
the commercially available Thio-Cure-BR
It was further found that the vulcanizable compositions
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60455-549D
containing the sulfur-olefin adducts prepared as disclosed herein
had bloom resistance similar to compositions made with insoluble
sulfur.
From the studies conducted on the sulfur-olefin adducts
prepared as disclosed herein, it was demonstrated that elastomers
cured with the product disclosed herein exhibited much less
bloom than orthorhombic sulfur and equivalent bloom to insoluble
sulfur.
Industrial Applicability
The above Examples clearly demonstrate the utility of
the sulfur-olefin curing agents. These curing agents can be
employed in a wide variety of rubber stocks and the resulting
vulcanizates have physical properties comparable to those
achieved with a conventional sulfur cure but with none of the
attended disadvantages related to bloom. The sulfur-olefin
adduct curing agents thus offer improved results over presently
known commercial sulfur containing curing agents.
It is, of course, to be understood that the foregoing
examples are intended to be illustrative only and that numerous
changes can be made in the ingredients, proportions and conditions
specifically disclosed therein without departing from the spirit
of the invention as defined in the appended claims.
1333205
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