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Description  |
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DESCRIPTION OF THE INVENTION
The present invention relates to particulate compositions. More particularly, the present invention relates to free flowing particulate compositions comprising particulate amorphous precipitated silica and, in combination, hydrocarbon process
oil and organic carboxylic acid. The particulate compositions of the present invention have measurably improved dispersibility when used for elastomer reinforcement.
The use of particulate amorphous precipitated silica as a reinforcing filler in elastomers, e.g., rubbers used in vehicle tire manufacture and the soles of athletic foot wear, is widely practiced. Particulate amorphous precipitated silica is
typically used to impart improved physical properties, e.g., tensile strength, tear resistance and abrasion resistance, to the elastomers into which it is dispersed. As is known to those of ordinary skill in the art, an optimum improvement in elastomer
physical properties, i.e., reinforcing properties, can be achieved when the particulate amorphous precipitated silica is thoroughly and homogeneously dispersed in the elastomer. A quantitative measure of the degree of silica dispersion within an
elastomer matrix is percent white area. Lower values of percent white area are indicative of an improved degree of silica dispersion within the elastomer.
Particulate amorphous precipitated silica is typically dispersed in an elastomer using energy intensive mixing means, such as, internal mixers, a particularly common example of which is a Banbury mixer. In vehicle tire manufacture, the rubber,
e.g., styrene-butadiene rubber, is first introduced into an internal mixer and then particulate amorphous precipitated silica is added while the mixer is running. While prolonged mixing can result in improved silica dispersion, there is a finite limit
to the level of dispersion that is possible, this finite level being intrinsic to the particular silica used. Regardless of the specific type of reinforcing silica employed, it is desirable, with regard to manufacturing costs in particular, that this
finite or maximum level of silica particle dispersion be achieved in a minimum amount of time.
It can be appreciated by those of ordinary skill in the art that the combination of optimum silica dispersion and minimum processing time are not readily compatible. It would be desirable to identify a free flowing composition comprising
particulate amorphous precipitated silica that, when used as a reinforcing filler in an elastomer, provides an optimum level of dispersion in a minimum amount of processing time. Co-pending and commonly assigned U.S. patent application Ser. No.
08/994,255 discloses a method and apparatus for cracking amorphous precipitated silica particles. Also disclosed is a composition produced by mixing together rubber and the amorphous precipitated silica particles produced by the cracking method.
International patent publication number WO 96/32949 discloses substantially dry free-flowing particles of amorphous precipitated silica containing adsorbed organic liquid, e.g., plastisizers and/or softeners such as paraffinic oil and aromatic
oil. The silica particles are described as having a size distribution such that at least 80 percent of the weight of said particles is retained on a 150 mesh screen and at least 90 percent by weight is retained on a 200 mesh screen.
International patent publication number WO 97/24396 discloses a particulate silica, which has been treated with from 2.5 percent to 40 percent by weight, based on silica weight, of polymer oil jelly. The polymer oil jelly is described as a
substantially homogenous blend of oil extender, e.g., naphthenic process oil, and a thermoelastic polymer, e.g., a copolymer of ethylene, propylene and, optionally, a diene. When used as a reinforcing filler for elastomeric materials, the treated
particles are described as being incorporated more quickly into the elastomer, resulting in shorter mixing times and, in consequence, less power.
U.S. Pat. Nos. 4,436,847 and 4,474,908 disclose a substantially dry, free-flowing rubber compounding additive comprising finely-divided siliceous pigment, organic silane coupling composition and rubber process oil. The siliceous pigment is
described as having an ultimate particle size of from 5 to 100 nanometers.
It has now been surprisingly discovered that free flowing culate compositions according to the present invention have measurably improved dispersibility in an elastomer relative to either particulate amorphous precipitated silica alone or
particulate amorphous precipitated silica containing hydrocarbon process oil. In accordance with the present invention, there is provided a free flowing particulate composition comprising in combination:
(a) particulate amorphous precipitated silica;
(b) hydrocarbon process oil; and
(c) organic carboxylic acid having from 2 to 30 carbon atoms, preferably from 3 to 25 carbon atoms, and more preferably from 10 to 20 carbon atoms, said hydrocarbon process oil and said organic carboxylic acid being sorbed on said silica and
being present in amounts sufficient to measurably improve the dispersibility of said particulate amorphous precipitated silica into an elastomer, as measured by percent white area of the cured elastomer. Percent white area is measured in accordance with
a method as further described herein.
There is further provided a method of improving the dispersibility of precipitated amorphous silica particles in an elastomer comprising dispersing into said elastomer the above described free flowing particulate composition of the present
invention.
The features that characterize the present invention are pointed out with particularity in the claims which are annexed to and form a part of this disclosure. These and other features of the invention, its operating advantages and the specific
objects obtained by its use will be more fully understood from the following detailed description and the accompanying drawings in which preferred embodiments of the invention are illustrated and described. In the accompanying drawings, like reference
numerals represent the same structural parts, and the same process streams.
Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used in the specification and claims are to be understood as modified in all instances by the term
"about". As used herein, values of mesh sieve screen, e.g., 200 mesh sieve screen, are made with reference to screens of the U.S. Standard Sieve Series.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing plots of torque versus time corresponding to the separate dispersing of comparative particulate compositions and a particulate composition according to the present invention into an elastomer; and
FIG. 2 is a schematic representation of an apparatus used to prepare cracked particulate amorphous precipitated silica granules useful in the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The hydrocarbon process oil and organic carboxylic acid of the free flowing particulate composition of the present invention may be added to, e.g., sorbed onto, the particulate amorphous precipitated silica either together, e.g., from a
homogenous mixture, or separately. If added to the silica separately, they may be added in any order. The hydrocarbon process oil and organic carboxylic acid are generally added together to the silica. The hydrocarbon process oil and organic
carboxylic acid may be added to the silica by any convenient means, e.g., spray application, curtain application, immersion and pouring.
As used herein, the term "free flowing" is intended to mean a particulate composition having the handling characteristics of a substantially dry particulate composition. Particulate compositions according to the present invention will preferably
have a minimum of clumping or aggregation between individual particles and will accordingly be free flowing.
The hydrocarbon process oil and organic carboxylic acid are present in amounts sufficient to measurably improve the dispersibility of the particulate amorphous precipitated silica into an elastomer as measured by percent white area of the cured
elastomer. When used to prepare rubbers having the same final compositions, e.g., the same silica, hydrocarbon process oil and organic carboxylic acid levels measured in parts per hundred parts rubber (phr), the particulate composition of the present
invention has been found to have measurably improved dispersibility when compared to: (a) the same particulate amorphous precipitated silica having neither hydrocarbon process oil nor organic carboxylic acid sorbed thereon; and (b) the same particulate
amorphous precipitated silica having only hydrocarbon process oil sorbed thereon.
Percent white areas of cured elastomers, e.g., cured vehicle tire rubber compositions, as referenced herein were determined according to the method protocols enumerated in the Examples below. Cured elastomers reinforced with free flowing
particulate compositions according to the present invention will typically have percent white areas of, for example, less than 2 percent. Often the percent white areas are less than 1 percent. Lower values of percent white area are indicative of an
improved level of silica dispersion within the elastomer. A higher level of silica dispersion can result in a subsequent improvement in elastomer physical properties. In the case of rubbers used for vehicle tire manufacture, improved tread wear, e.g.,
as measured by running the tire for hundreds of miles on a track, is known to be related to an improved level of silica dispersion within the tread rubber.
As used herein, the term "hydrocarbon process oil" is intended to mean and include oils used in the processing of elastomer compositions, e.g., rubber compositions used for vehicle tires and the soles of foot wear. Classes of hydrocarbon process
oils useful in the present invention include, but are not limited to: the aromatic, naphthenic and paraffinic hydrocarbon fractions defined in America Society of Testing Materials (ASTM) designation D 2226; and oils obtained from natural, e.g., plant,
sources, such as, rapseed oil. In a preferred embodiment of the present invention, the hydrocarbon process oil is an aromatic process oil.
The hydrocarbon process oil is typically present in the composition of the present invention in an amount of at least 10 percent by weight, preferably at least 20 percent by weight, and more preferably at least 30 percent by weight, based on the
weight of the particulate amorphous precipitated silica. The hydrocarbon process oil is also typically present in the composition in an amount of less than 70 percent by weight, preferably less than 60 percent by weight, and more preferably less than 55
percent by weight, based on the weight of the particulate amorphous precipitated silica. The amount of hydrocarbon process oil present in the free flowing particulate composition of the present invention may range between any combination of these values
inclusive of the recited values. In a particularly preferred embodiment of the present invention, the hydrocarbon process oil is present in the particulate composition in an amount of from 40 percent to 50 percent by weight, e.g., 46 percent by weight,
based on the weight of the particulate amorphous precipitated silica.
Classes of organic carboxylic acids that may comprise the free flowing particulate composition of the present invention include, but are not limited to: straight or branch chain carboxylic acids having no ethylenic unsaturation, e.g., acetic
acid, propionic acid, 2-methyl propionic acid, butanoic acid, pentanoic acid, pentanedioc acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, lauric acid, myristic acid, palmitic acid,
stearic acid, icosanoic acid, pentacosanoic acid and triacontanoic acid; straight or branched chain carboxylic acids having ethylenic unsaturation, e.g., 9,10-octadecanoic acid, 9,12-octadecadienoic acid and 9,12,15-octadecatrienoic acid; cyclic
aliphatic carboxylic acids, e.g., cyclopentane carboxylic acid, cyclohexane carboxylic acid, 1,4-cyclohexane dicarboxylic acid and 2-bicyclo [2,2,1]heptanoic acid; aromatic carboxylic acids, e.g., benzoic acid, terephthalic acid and naphthalene
dicarboxylic acid; and substituted organic carboxylic acids, e.g., lactic acid, leucic acid, 12-hydroxystearic acid and 3-bromo-henzoic acid.
Preferred organic carboxylic acids include, propionic acid, lauric acid, oleic acid, stearic acid and mixtures thereof. In a particularly preferred embodiment of the present invention, the organic carboxylic acid is stearic acid.
The organic carboxylic acid is typically present in the composition of the present invention in an amount of at least 0.5 percent by weight, preferably at least 0.75 percent by weight, and more preferably at least 1 percent by weight, based on
the weight of the particulate amorphous precipitated silica. The organic carboxylic acid is also typically present in the composition in an amount of less than 10 percent by weight, preferably less than 5 percent by weight, and more preferably less than
3 percent by weight, based on the weight of the particulate amorphous precipitated silica. The amount of organic carboxylic acid present in the free flowing particulate composition of the present invention may range between any combination of these
values inclusive of the recited values.
The free flowing particulate composition of the present invention, comprises particulate amorphous precipitated silica. Particulate amorphous precipitated silica is usually produced commercially from an aqueous
solution of alkali metal silicate, e.g., sodium silicate. Typically an acid, e.g., sulfuric acid, hydrochloric acid or carbon dioxide, is added to the alkali metal silicate to precipitate the silica particles. Alternatively, a polyvalent metal
cation, e.g., calcium ions, can be used to precipitate the silica from solution.
Following precipitation, the amorphous precipitated silica is usually washed and then dried. Examples of methods by which amorphous precipitated silica may be dried include, for example, tray drying in an oven, drying in a fluidized bed, and
drying in a rotary dryer. Particulate amorphous precipitated silica useful in the present invention is preferably dried using a spray dryer in which a feed of an aqueous dispersion of the silica is sprayed into a column of moving hot air. Spray dryers
and their operation are well known to those of ordinary skill in the art. Typically, the spray dryer is operated at an outlet temperature of at least 100.degree. C., although lower outlet temperatures may be employed when, for example, lower
evaporation rates can be tolerated. More often, the spray dryer is operated at an outlet temperature ranging from 100.degree. C. to 170.degree. C., and preferably from 100.degree. C. to 130.degree. C., the ranges being inclusive of the recited
values.
After drying, e.g., spray drying, the amorphous precipitated silica has the characteristics of a dry solid when handled. While the dried amorphous precipitated silica may be described as substantially dry, it is ordinarily not absolutely
anhydrous, in that it typically contains: bound water, present in an amount of from, for example, 2 percent to 5 percent by weight, based on silica weight; and adsorbed (or free) water, present in an amount of from, for example, 1 percent to 7 percent by
weight, based on silica weight. Adsorbed water is that water which is removed from the silica by heating at a temperature of 105.degree. for a period of 24 hours at atmospheric pressure in a laboratory oven. The amount of adsorbed water present on the
silica is often partly dependent upon the relative humidity of the atmosphere to which the silica is exposed. Bound water is that water which is removed by additionally heating the silica at temperatures ranging from, for example, 600.degree. C. to
1200.degree. C., i.e., calcination temperatures. The water content (inclusive of both adsorbed and bound water) of particulate amorphous precipitated silica useful in the present invention is usually less than 12 percent by weight, preferably less than
9 percent by weight, and more preferably less than 7 percent by weight, all percent weights being based on silica weight.
Particulate amorphous precipitated silica having particle sizes smaller than those obtained from, for example, spray drying, are also useful in the present invention. Size reduction of particulate amorphous precipitated silica can be
accomplished by several methods, including for example, fluid energy mills, roller mills and hammer mills.
The present invention is applicable to particulate amorphous precipitated silica having a variety of physical forms, e.g., powders, granules, beads and spherules. In a preferred embodiment of the present invention, the particulate amorphous
precipitated silica is in the form of granules. In a particularly preferred embodiment of the present invention, the particulate amorphous precipitated silica is in the form of cracked amorphous precipitated silica granules.
Cracked, i.e., size reduced, amorphous precipitated silica granules useful in the present invention may be produced according to the following procedure using an apparatus similar to that represented schematically in FIG. 2. A feedstock of a
granulated amorphous precipitated silica, e.g., Hi-Sil.RTM. granulated silica available commercially from PPG Industries Inc., is added to hopper 12 of apparatus 10. The granulated silica feedstock is transferred uniformly from hopper 12 to conveyor
belt 14, as indicated by arrow 80, at a steady rate. Conveyor belt 14 is driven by driving rollers 92, which are rotated in the direction indicated by arcuate arrows 95.
The silica feedstock is transported along conveyor belt 14 to a first rotatable roll 20 positioned above conveyor belt 14. First rotatable roll 20, having closely spaced sharply pointed spikes 22 extending from its surface, is rotated in the
direction indicated by arcuate arrow 83. Spikes 22 engage intimately with the granulated silica feedstock to produce intermediate cracked amorphous precipitated silica granules on the immediate downstream side of roll 20.
The intermediate cracked amorphous precipitated silica granules are then further transported downstream by conveyor belt 14, as indicated by arrow 90, to second rotatable roll 30 located above conveyor belt 14. Second rotatable roll 30, having
closely spaced sharply pointed spikes 32 extending from its surface, is rotated in the direction indicated by arcuate arrow 86. Spikes 32 engage intimately with the intermediate cracked granules carried on conveyor belt 14 and produce product cracked
amorphous precipitated silica granules on the immediate downstream side of roll 30. The product cracked amorphous precipitated silica granules have generally smaller sizes than those of the intermediate cracked granules.
The product cracked amorphous precipitated silica granules are further transported downstream by conveyor belt 14 and deposited into receiving hopper 40, as depicted by arrow 89. Receiving hopper 40 includes a vacuum system 42 for removing dust
from the product cracked amorphous precipitated silica granules through conduit 98.
The of outwardly extending sharply pointed spikes 32 of roll 30 are arranged in a plurality of circumferential rows (not shown). Within a circumferential row, the distance between adjacent spikes 32 is approximately 3.18 mm, based on the
intersection of spike centerlines with the outer surface of roll 30. Additionally, the distance between adjacent rows of spikes is approximately 3.18 mm, based on the intersection of spike centerlines with the outer surface of roll 30. Sharply pointed
spikes 32 themselves extend the same distance from the outer surface of roll 30.
Roll 20 is substantially the same as roll 30 except for the spacing of outwardly extending sharply pointed spikes 22. Within a circumferential row (not shown), the distance between adjacent spikes 22 is approximately 6.35 mm, based on the
intersection of spike centerlines with the outer surface of roll 20. Additionally, the distance between adjacent rows of spikes is approximately 6.35 mm, based on the intersection of spike centerlines with the outer surface of roll 20. Sharply pointed
spikes 22 extend the same distance from the outer surface of roll 20 and are evenly spaced across the surface of roll 20. More specific details concerning rolls 20 and 30, are found in co-pending and commonly assigned U.S. patent application Ser. No.
08/994,255, the disclosure of which is incorporated herein in its entirety.
In apparatus 10 illustrated in FIG. 2, a common motor can be utilized for driving conveyor belt 14, roll 20, and roll 30. Appropriate gearing can be used to select the desired rotational speeds for the individual rolls. Sharply pointed spikes
22 extend the same radial distance from the surface of roll 20 and sharply pointed spikes 32 extend the same radial distance from the surface of roll 30. Roll 20 and roll 30 are positioned so that sharply pointed spikes 22 and sharply pointed spikes 32
almost engage the upper surface of conveyor belt 14 to assure that sharply pointed spikes 22 and sharply pointed spikes 32 will engage the particles to be reduced in size. The speeds of roll 20 and roll 30 should provide linear speeds of the ends of
sharply pointed spikes 22 and sharply pointed spikes 32 that are at least equal to and even greater than the speed of conveyor belt 14 to ensure that sharply pointed spikes 22 and sharply pointed spikes 32, respectively, will properly engage and crack
the particulate precipitated amorphous silica.
Cracked precipitated amorphous silica granules prepared as described with reference to FIG. 2, will have a reduced level of dust. As used herein, by "dust" is meant particles of precipitated amorphous silica that will pass through a 200 mesh
sieve screen, i.e., a sieve screen having openings of 0.075 mm. Cracked granules prepared as described herein typically contain less than 1 percent by weight of dust, and more often less than 0.6 percent by weight of dust.
While not intending to be bound by any theory, it is believed that cracking is enhanced and dust is reduced by minimizing the surface area of individual spikes that come into contact with the particulate amorphous precipitated silica granules
being cracked. Generally a larger contact surface area will result in a higher level of dust produced during the cracking process. Consequently, the use of sharply pointed spikes is believed to greatly assist in the reduction of dust in the cracking
process described with reference to FIG. 2.
The particulate amorphous precipitated silica of the composition of the present invention may be of any suitable particle size or particle size distribution. If the particulate silica is in the form of a powder, the average particle size may
range from, for example, 60 to 600 microns. In a preferred embodiment of the invention, at least 50 percent by weight of the particulate amorphous precipitated silica is in a size range of from 0.15 mm to 2.8 mm. Preferably at least 50 percent by
weight of the particulate silica is in a size range of from 0.3 mm to 2.8 mm. It is especially preferred that at least 65 percent by weight of the particulate silica is in a size range of from 0.85 mm to 2.8 mm. Typically, less than 30 percent by
weight of the particulate silica is greater than 2.8 mm. The particulate amorphous precipitated silica is preferably in the form of granules, e.g., cracked granules prepared as described previously herein.
Particulate amorphous precipitated silica useful in the present invention, and particularly when in the form of granules, will typically have a friability of less than 2 percent and preferably less than 1.7 percent. As used herein by
"friability" is meant the tendency of a material to break up during its preparation and use, e.g., packaging, transportation, conveying and weighing. For a description of the method used to calculate friability, see Example A herein.
The free flowing particulate composition of the present invention may optionally include one or more organosilane coupling agents. The organosilane coupling agent(s) is preferably present in at least a coupling amount. By "coupling amount" is
meant an amount sufficient to provide adequate coupling between the particulate am | | |
