Method of achieving superior dispersions of polymeric sulfur and products thereof

A method of achieving a superior dispersion of polymeric sulfur comprises mixing the polymeric sulfur while it is in the rubbery state with a diluent, such as rubber, to obtain a uniform dispersion of the sulfur in the diluent.

FIELD OF THE INVENTION 
The invention relates generally to sulfur. More particularly, it relates to 
a method of achieving novel superior dispersions of polymeric sulfur in 
unvulcanized rubber by adding the polymeric sulfur to the rubber while the 
polymeric sulfur is in the rubbery state and to the resulting product. 
BACKGROUND OF THE INVENTION 
Rubber and similar polymers must be mixed with a selection of other 
ingredients to develop the properties necessary for specific applications. 
One of these ingredients is a vulcanizing agent. Elemental sulfur is by 
far the most widely used vulcanizing agent, especially in tires and other 
dynamic applications. While certain chemical compounds of sulfur can be 
used as sulfur donors to accomplish vulcanization, only elemental sulfur 
is believed to impart the optimum combination of properties for most of 
the tire. One of the most important properties required in tire rubber is 
fatigue resistance. The superior fatigue resistance achieved when using 
elemental sulfur instead of sulfur donors is reported in the "Natural 
Rubber Formulary" pp128-129, and again on pp180-181. Elemental sulfur is 
used in the rubber industry in two basic forms: 
1. Ortho-rhombic (commonly called rhombic) crystals, consisting of 
molecules containing eight sulfur atoms per molecule in a ring-like 
structure. This form is referred to as normal, or soluble sulfur. Its main 
disadvantage is that it "blooms" in the unvulcanized rubber compound. 
2. Polymeric sulfur, consisting of molecules that contain long chains of 
sulfur atoms, usually thousands of sulfur atoms per molecule. At room and 
processing temperatures, these chains tend to revert to normal sulfur. 
This reversion can be deterred by adding certain stabilizing agents in 
small quantities. The stabilized forms have dominated the market. 
Polymerized sulfur is referred to as insoluble sulfur. There are no known 
solvents for insoluble sulfur; hence its name. Its main disadvantage is 
that it is hard to disperse well in the unvulcanized rubber compound. It 
is also quite expensive compared to normal sulfur. 
The degree of dispersion, in the unvulcanized rubber compound, of all of 
these compounding ingredients affects the properties of the vulcanized 
product. This is especially true of the vulcanizing agent. For the vast 
majority of products, such as tires, the best dispersion gives the best 
product because of the homogeneity achieved. Sulfur exists at room 
temperatures primarily as rhombic crystals. Other forms of sulfur, such as 
monoclinic crystalline sulfur, or polymeric sulfur, are the normal primary 
forms which elemental sulfur assumes at certain higher temperature ranges. 
At room temperatures, these forms convert, or revert, to rhombic sulfur. 
Polymeric sulfur is called insoluble sulfur, especially in the rubber 
industry, and normal, non-polymeric or rhombic sulfur is called soluble 
sulfur, because it is soluble to a limited extent in most rubbers. The 
term "rubber" as used herein means any sulfur vulcanizable polymer. Sulfur 
vulcanizable polymers are primarily those polymers having carbon to carbon 
molecular chain structures, with some double bonds existing in their 
structure. These polymers are called unsaturated. The double bonds are the 
sites for sulfur vulcanization. The term "rubbery" as used herein means 
masses of matter that are not hard, brittle, or friable, but are plastic 
and/or elastic. The term "saturated rubbery polymers" means those rubbery 
polymers that do not contain sulfur vulcanizable bonds, such as 
ethylene-propylene rubber. 
The amount of normal or rhombic sulfur that is soluble in rubber increases 
as the temperature increases. Typical rubber compounds contain from one to 
three parts of sulfur per one hundred parts of rubber hydrocarbon (rhc). 
The processing of unvulcanized rubber requires mechanical working of the 
rubber, which generates heat. The temperatures developed as a result of 
this processing are usually sufficient to dissolve the typical normal 
sulfur content. When the rubber cools to room temperature the solubility 
of the sulfur in rubber is exceeded, and a supersaturated solution ensues. 
This supersaturated portion of the sulfur tends to migrate to the surface 
of the rubber and crystallize. This condition is called "bloom" and is 
highly undesirable. 
At room temperatures, surface blooming occurs primarily when the 
concentration of soluble sulfur in the rubber is between the limits of 
about 0.8 parts and 8.0 parts per 100 parts of rubber hydrocarbon. These 
limits vary among different compounds. Below the lower limit the sulfur is 
soluble. Above the upper limit the sulfur drops out of solution in the 
interior of most rubber compounds, forming micro-crystals throughout the 
mix. In some rubber compounds these micro-crystals grow to objectionable 
size, causing nonhomogeneity of properties throughout the vulcanized 
product. 
Polymeric or insoluble sulfur does not dissolve in rubber, and therefore 
does not bloom. However, the insoluble sulfur can revert to normal sulfur, 
and the rate of reversion is a time-temperature phenomenon which increases 
with temperature. Elemental insoluble sulfur can be stabilized by the 
addition of various substances, notably the halogens. This stabilized 
insoluble sulfur tends to remain polymeric at room and processing 
temperatures but it reverts to normal sulfur at the higher vulcanizing 
temperatures, thus becoming available for the vulcanization reaction. 
Insoluble sulfur is normally supplied by the sulfur manufacturers in 
discrete particles, or powder. This powder is extremely fine, classically 
having a reported average particle size of 3 microns. These particles are 
considerably smaller than the particles usually supplied of normal sulfur. 
These smaller particles are desired because the dispersion of this form of 
insoluble sulfur is limited by the particle size supplied, unlike the 
dispersion of soluble sulfur. This very fine powder presents various 
processing difficulties. It tends to form dust clouds in the mixing room, 
which are both a health hazard and a safety hazard. Sulfur dust explosions 
are a known hazard in the rubber industry. A number of ways to reduce this 
dusting are mentioned in the literature. Also, the sulfur powder is 
difficult to disperse in rubber. The individual particles tend to 
agglomerate. Because of this, the powders are frequently mixed with a 
portion of a polymer or other matrix materials to form a masterbatch 
before being added to the final compound. These masterbatches usually 
contain fifty percent or more sulfur. This processing step adds to the 
cost. Since these discrete particles retain their identity during mixing, 
the best possible dispersion is limited by the size of the particles, 
unless their melting point is exceeded. However, when melted, the rate of 
reversion is very rapid and the reverted sulfur, of course, blooms, and 
the advantages of using insoluble sulfur are negated. 
The prior art falls in three categories: 
1. Insoluble sulfur powders 
U.S. Patent No. 2,419,310 to Belchetz 
U.S. Patent No. 2,419,309 to Belchetz 
U.S. Patent No. 2,579,375 to Grove 
These patents deal with insoluble sulfur in a form that has distinct 
disadvantages. The present invention overcomes these disadvantages. 
2. Sulfur donors 
U.S. Patent No. 4,621,118 to Schloman 
U.S. Patent No. 2,989,513 to Hendry 
U.S. Patent No. 2,481,140 to Morris 
All of these patents teach a chemical reaction of sulfur with an organic 
compound to form sulfur donors. The crosslinking achieved using sulfur 
donors is distinctly different from that achieved using elemental sulfur. 
No long chain polymers of sulfur are contemplated or achieved. Therefore 
they are not pertinent. 
3. Solutions of normal sulfur 
U.S. Patent No. 1,782,693 to Miller 
This patent teaches solutions of normal sulfur in an organic resin. Long 
chain polymers of sulfur do not go into solution in any known substance. 
Hence it fails to teach or suggest anything concerning polymeric sulfur. 
Recently an "improved product" has been introduced, that has a reported 
average particle size of 2 microns. The improvement is in the degree of 
dispersion afforded by the smaller particle size. However, as could be 
expected, the dusting problem has gotten worse with the "improved 
product", and the dispersion is, of course, still limited by the particle 
size. There is still a need for much better dispersion than is achieved by 
the "improved product", or that can be achieved by any product that must 
rely on the size of a particulate to maximize its dispersion. 
It is generally known that rapidly cooling molten sulfur from a 
sufficiently high temperature produces a mass of polymeric sulfur that is 
in a rubbery state. This rubbery state is a metastable condition, and upon 
standing, the mass becomes hard and brittle. It can then be ground to form 
a powder. In some processes the molten sulfur is sprayed and thereby 
cooled, the individual droplets formed in the spraying are also initially 
rubbery. Special handling is necessary to keep them as individual 
particles since the rubbery state tends to make them agglomerate. In other 
processes, sulfur vapor is forced under pressure into a liquid cooling 
medium. Again rubbery particles are first formed. This rubbery state has 
been considered undesirable because it is a free flowing powder which has 
been desired. 
Other elements in Group VI.sub.B of the Periodic Table of the Elements, 
notably Selenium and Tellurium, have many properties that are similar to 
Sulfur. They have found limited use as vulcanizing agents for rubber, and 
they polymerize. It is well known that they polymerize themselves and that 
they form copolymers and terpolymers with sulfur. These polymers exhibit 
rubbery properties similar to those of the monopolymer of sulfur, and 
hence can be dispersed in similar fashions as the rubbery polymer of 
sulfur. 
Other elements, such as Arsenic, can be included in the sulfur polymer 
chain, and a rubbery state can still be obtained. One skilled in the art 
could develop many such modifications of the sulfur polymer, and still 
obtain a rubbery polymer. These modifications are included in the 
substance of this invention. The term "polymeric sulfur," as used herein, 
includes all of the modifications of the polymer that can be obtained in 
the rubbery state. 
There is a need for ways to achieve superior dispersion of polymeric sulfur 
in rubber in a dust free manner. This need exists whether the sulfur is 
added to a rubber batch in the lower amounts needed for proper 
vulcanization, or in the higher amounts used in preparing masterbatches or 
intermediate batches, which are later added in the proportion needed to 
achieve the proper quantity of sulfur needed for vulcanization. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to disclose how to achieve 
superior dispersions of polymeric sulfur in unvulcanized rubber, in a 
completely dust free manner. 
It is a further object of the present invention to disclose novel blends 
made of polymeric sulfur with a diluent which is compatible with sulfur 
vulcanizable polymers. 
It is a further object to disclose a novel dust-free method of preparing 
such blends. 
I have discovered that superior dispersion of polymeric sulfur is attained 
if the polymeric sulfur is added to the rubber while the polymeric sulfur 
is in the metastable rubbery state. Previous to the present invention, 
this state has always been regarded as undesirable. The literature 
contains many descriptions of ways to harden this rubbery mass, so as to 
make it usable by prior known art. 
I have further discovered that superior preparations of polymeric sulfur 
can be prepared by a method which comprises mixing polymeric sulfur while 
in the rubbery state with a compatible diluent, which is compatible with 
the polymer to be vulcanized. The amount of polymeric sulfur in the blend 
can vary from about 2% to about 95% by weight of the total blend. However, 
the preferred blends will contain about 20% to about 95% of the rubbery 
polymeric sulfur by weight. 
Electron Micrographs of a cross section of this superior dispersion, taken 
by a Leica Cambridge Ltd. S360 Scanning Electron Microscope, Version 
V0302A, equipped with Dynamic Stereo and Remote Control options, failed to 
discern any areas of unblended polymeric sulfur. The same device clearly 
discerned the &lt;1 micron to 3 micron particulates of Crystex Sulfur, the 
brand of insoluble sulfur that presently dominates the market because it 
is the best available, in a 50/50 masterbatch dispersion of Crystex/EPR 
707 (a high Mooney Viscosity Ethylene Propylene Rubber). These micrographs 
also showed the non-uniform dispersion of the sulfur particulates, and 
showed three-dimensional agglomerates of these particulates up to 29 
microns across. 
The blends of the present invention, since they are formed with the 
polymeric sulfur in the rubbery state, eliminate the need for comminution 
into fine powders, thereby effecting significant economies and eliminating 
the hazardous formation of dust. It also eliminates the need for the 
rubbery sulfur polymer to harden, so that it can be processed into a 
powder, thereby effecting more economies. The blends of the present 
invention when properly prepared do not harden. They remain in the viscous 
or rubbery state so as to be readily dispersable by the commercial methods 
used to mix rubber. 
In the preferred practice of the method of the present invention, the 
conversion process by which soluble sulfur is converted to polymeric 
sulfur is interrupted when the polymeric sulfur is in the rubbery state, 
and the rubbery polymeric sulfur is mixed with the unvulcanized rubber, in 
the proportion needed for proper vulcanization, or with the compatible 
diluent, at that time. 
In another embodiment, a portion of the polymer that is to be vulcanized is 
mixed or masticated and used as the compatible diluent for the rubbery 
polymeric sulfur which is blended into it, thereby reducing the number of 
processing steps even further. A two roll rubber mill or other masticating 
equipment can be used. 
In still another embodiment, normal or rhombic sulfur can be mixed in a 
relatively high percentage (e.g. 80%) with a compatible diluent, such as 
EPR (ethylene-propylene rubber, a saturated rubbery polymer) and the 
sulfur polymerized in situ, by raising the temperature of the mixture to 
melt the sulfur and then rapidly cooling the mixture. This diluent must be 
of such nature that it will not chemically react with the sulfur. This 
process is best done in a chambered mixer, such as an extended barrel 
extruder rather than on an open mill. When using the preferred high 
percentage of sulfur, the molten sulfur tends to separate out of the 
mixture and runs off the mill. In the preferred extruder, the first zone 
is heated and subsequent zones cooled. 
In another preferred embodiment, the sulfur conversion unit is set up 
convenient to the intensive internal mixer, of the Banbury type. The 
rubbery sulfur is added to the final or intermediate batch in the 
quantities needed for proper vulcanization of the intended final batch. A 
3 lb. capacity laboratory mixer was used. 
If desired, the polymerized molten sulfur can be added directly to the 
rubber batch being mixed in the Banbury, using the rubber matrix as the 
rapid cooling medium for the sulfur. 
It will be apparent to those skilled in the art that the above objects and 
other advantages will accompany the practice of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In the preferred practice of the method of the present invention, polymeric 
sulfur in the rubbery state is blended with the compatible diluent, which 
is rubber, by mixing the rubbery polymeric sulfur and the diluent on a two 
roll rubber mill and continuing the blending or mastication until a 
uniform blend is obtained. The amount of the rubbery polymeric sulfur 
which is used is preferably enough to produce a uniform blend which 
contains about 40% to about 80% of the polymeric sulfur as based on the 
total weight of the blend. 
The rubbery polymeric sulfur can be prepared by any known manner. It is 
known that at about 113 degrees C. and above, sulfur melts. The molten 
sulfur begins to polymerize as the temperature is raised to about 159 
degrees C. This is evidenced by a pronounced increase in viscosity. As the 
temperature is further increased, the viscosity increases sharply to a 
maximum in a rise of only a few degrees. It then drops off at a fairly 
steady rate as the temperature is still further increased. It is believed 
that the mean chain length is at its maximum at the point of maximum 
viscosity, and that the mean chain length decreases as the temperature 
rises further. The higher viscosities apparently have not been shown to 
inversely correlate with the percent of unpolymerized sulfur present. The 
purer the sulfur, the higher is the maximum viscosity. The curve of the 
viscosity versus temperature of pure sulfur is reversible if the 
temperature changes are gradual. This reversibility does not necessarily 
hold true when other substances, such as certain organic compounds or 
halogens, are present in small amounts. In pure sulfur, the end of the 
polymer chain is a free radical. When these other substances are present, 
either purposefully or accidentally, the free radical is probably "capped" 
by the other substances. 
If the molten sulfur is cooled gradually from the temperature of maximum 
viscosity, the drop in viscosity is attributed to the reversion of the 
polymeric sulfur to predominantly S.sub.8. Other sulfur molecules, notably 
S.sub.6, but others from S.sub.2 to S.sub.23 have been identified. 
When the molten sulfur is cooled rapidly from temperatures whereat a 
significant portion of the sulfur is polymeric, the polymeric form 
survives to a metastable form, which is rubbery, at room temperature. In 
pure sulfur, the polymer reverts to predominantly S.sub.8 in a relatively 
short time. This reversion is hindered by the presence of halogens or 
certain other substances, and seems to be accelerated by the presence of 
moisture, or alkalinity. 
Upon rapid cooling, the molten polymerized sulfur forms a rubbery mass or 
rubbery polymeric sulfur, which hardens by itself upon standing. This 
hardening does not necessarily indicate de-polymerization, or reversion. 
Certain treatments can accelerate this hardening. 
The compatible diluent which is blended with the rubbery polymeric sulfur 
to form the blends of the present invention can be any substance which is 
compatible with both the rubbery polymeric sulfur and the polymer which is 
to be vulcanized with the blend. Representative diluents include natural 
and synthetic rubbers, soaps, petroleum fractions, wood tar products and 
plasticizers, such as ethers and esters and their polymers. 
The method of preparing the blends can be any of those described herein, as 
well as other methods which can be used to form sufficiently uniform 
blends. 
In the following examples 1 to 6, several techniques are described which 
demonstrate the wide variety of sulfur conversion processes that can be 
used in the practice of this invention. 
EXAMPLE 1 
One part of bromine is added to one hundred parts of sulfur in a glass 
retort. Heat is applied to melt and then vaporize the sulfur. When the 
distillate runs in a steady stream, which must be a very thin stream, it 
is run into a pot of ice water which is rotating at forty-five rpm. It is 
collected until the stream becomes unsteady, which indicates that it will 
soon break. The sulfur forms a rubbery mass of strings in the ice water. 
It is removed from the ice water, and shook to remove most of the water. 
The remaining water is blown off with compressed air. The rubbery 
polymeric sulfur is then blended with an equal amount of natural rubber, 
the compatible diluent, on a two roll mill. An excellent uniform blend of 
the polymeric sulfur and the rubber is obtained. 
EXAMPLE 2 
The procedure of Example 1 is repeated except in place of 1 part bromine, 
0.8 parts of bromine and 0.2 parts of iodine are used. An excellent 
uniform blend of the polymeric sulfur and the rubber is obtained. 
EXAMPLE 3 
The procedure of Example 2 is repeated, but the distillate of polymeric 
sulfur is run directly into the nip of a cooled two roll rubber mill upon 
which the rubber, the compatible polymer, has already been banded and has 
formed a rolling nip. An excellent uniform blend of the polymeric sulfur 
and the rubber is obtained. 
EXAMPLE 4 
One hundred parts of sulfur and 0.25 parts of bromine are melted in a 
beaker and heated to a point (e.g. 200 degrees C.) above the maximum 
viscosity of the sulfur to form a liquid. The liquid is slowly poured into 
a rotating ice water bath to form rubbery polymeric sulfur which after 
isolation is blended with rubber, the compatible diluent, as in Example 1. 
An excellent uniform blend of the polymeric sulfur and the rubber is 
obtained. 
EXAMPLE 5 
The procedure of Example 4 is repeated except that the molten liquid sulfur 
is fed into the compatible diluent, rubber, in the nip of the rollers, as 
in Example 3. An excellent uniform blend of the polymeric sulfur and the 
rubber is obtained. 
EXAMPLE 6 
One hundred parts of sulfur and one part of iodine are melted in an 
aluminum dish on a hotplate and heated to spontaneous combustion. The 
molten sulfur is sprayed with a hot spray gun onto a cool aluminum sheet 
to form a thin coating of rubbery polymeric sulfur. The coating is peeled 
from the aluminum sheet, and laminated with a thin sheet of broken down 
rubber, the compatible diluent. The laminate is rolled up and passed 
through the nip of a two roll rubber mill. The mixture is cross rolled 
several times and banded on the mill. An excellent uniform blend of the 
polymeric sulfur and the rubber is obtained. 
EXAMPLE 7 
The uniform blends prepared as described in Examples 1-6 can be used as 
vulcanizing agents for sulfur vulcanizable polymers. The vulcanizing 
agents can be readily and uniformly dispersed in the polymer to be 
vulcanized. The resulting products are free of defects caused when sulfur 
is not uniformly dispersed in the compound. 
EXAMPLE 8 
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parts per 100 RHC 
gms. per 3 lb. batch 
______________________________________ 
1. NR 50.0 420.0 
2. SBR 50.0 420.0 
3. HAF Carbon 50.0 420.0 
Black 
4. Process Oil 3.0 25.2 
5. Zinc Oxide 3.0 25.2 
6. Stearic Acid 2.0 16.8 
7. Antioxidant 1.0 8.4 
8. Accelerator 1.0 8.4 
9. Rubbery Sulfur 
2.5 21.0 
TOTAL 162.5 pts. 1365.0 gms. 
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If it is desired to initially keep selected ingredients apart, such as the 
sulfur and accelerator, two or more intermediate batches could be mixed 
separately, and later blended together, to give the desired finished batch 
formulation. Typical recipes for accomplishing this would be: 
______________________________________ 
Batch A 
Batch B 
______________________________________ 
1. NR 50.0 50.0 
2. SBR 50.0 50.0 
3. HAF 50.0 50.0 
4. Process Oil 3.0 3.0 
5. Zinc Oxide 3.0 3.0 
6. Stearic Acid 2.0 2.0 
7. Antioxidant 1.0 1.0 
8. Accelerator 2.0 0.0 
9. Rubbery Sulfur 
0.0 5.0 
TOTAL 161.0 164.0 
______________________________________ 
Batch A and Batch B are each mixed. After cooling, they are blended in 
proportion to their batch weight to give the same finished batch as in the 
formula immediately above. 
EXAMPLE 9 
95 parts of sulfur and 5 parts of Selenium are melted together and heated 
to 260.degree. C. Upon rapid cooling, a rubbery mass is formed. This is 
then blended with rubber. An excellent dispersion is obtained. 
As previously described, the rubbery polymeric sulfur used to make the 
blends may be prepared by any known method. However, it also has been 
found that if rubbery polymeric sulfur is treated, either after it is 
formed, or as it is formed, with a solvent for soluble sulfur, such as 
carbon disulfide, a chlorinated hydrocarbon, an aromatic hydrocarbon, or 
other suitable solvent, it is possible to dissolve out some or 
substantially all of the soluble sulfur present. As a result, the 
polymeric sulfur content of the rubbery mass can thereby be increased on a 
percentage basis. As a result, blends prepared from polymeric sulfur thus 
treated contain less soluble or normal sulfur. 
It will be apparent to those skilled in the art that a number of 
modifications and changes can be made without departing from the spirit 
and scope of the invention. Therefore, the invention is not to be limited 
except by the claims.