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Description  |
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BACKGROUND OF THE INVENTION
This invention pertains to fire prevention and, more particularly, to a
process for preventing grease fires in steel mills.
In steel mills, hot molten steel is formed into slabs in a hot steel slab
caster. In slab casters, molten steel enters a formation chamber. One or
more steel slabs emerge from the formation chamber with a thin skin of
solidified steel holding them together. The steel emerging from the
formation chamber can be in the form of a series of discrete slabs or,
alternatively, as one unbroken slab which is cut into discrete slabs at
the far end of the slab caster. This latter process is characteristic of
the more modern facilities and is usually referred to as a continuous
caster. Steel slabs can vary in width and thickness depending on the
particles steel mill, but a standard width for a single strand of steel on
a continuous caster is about six feet with a thickness of 9-12 inches.
Steel slabs, once cut, are typically about 25 feet long.
In order to convey the steel slab from the formation chamber, the slab is
supported by a series of rotatable caster rollers. Each of these caster
rollers has a bushing or bearing, usually a tapered roller bearing, at
each end which allows the caster roller to turn. The line or lines of
caster rollers in steel mills can be as long as three miles with a caster
roller every two feet. Such a line or lines can use three million pounds
of grease per year. Because the caster rollers are not much wider than the
steel slab it supports, the steel slab typically comes within only a very
few inches of the bearings. The bearings and grease used to lubricate
those bearings experience very high thermal stress, with the steel slab
surface often irradiating at temperatures of 1,500.degree. F. to
2,000.degree. F. Also, steel slabs exert a large force on each caster
roller due to the heavy weight of the slabs causing high loading pressures
on the bearing and bearing grease.
High performance greases are important to minimize failure of the caster
bearings. Such bearing failures will cause the caster to stop rotating
under the progressing steel slab. If this occurs, the dragging force
between the slab surface and the nonrotating caster roller can rupture the
slab skin causing a breakout which can endanger operating personnel,
damage property and interrupt steel mill operations and production.
For example, when the hot steel slab moves along the series of caster
rollers, the slab is quickly quenched and cooled to strengthen and thicken
the solid skin of the slab. If quenching is not done properly, the tenuous
skin can rupture causing molten steel to flow out onto the caster rollers,
bearing housings, and eventually the plant floor. Such an occurrence
(breakout) is very costly in terms of plant downtime and maintenance cost.
To minimize breakouts, rapid quenching, cooling and strengthening of the
skin is accomplished by high velocity water spray from all directions. The
spray velocity can be as high as 1,000 gallons per minute. With such water
spray force, even well sealed bearings will not totally exclude water.
Therefore, the bearing grease will experience water contamination with a
physical force that tends to wash (flush) the grease out of the bearings.
A significant problem associated with conventional steel mill greases which
is becoming of great concern is the increasing number and intensity of
grease fires. Grease fires can occur from hot molten metal, from acetylene
torches during periodic maintenance, and from other sources of ignition.
Grease fires can be costly in terms of loss of equipment, operational
downtime, and loss of life. It is highly desirable to have a high
performance steel mill grease which also reduces the occurrence of grease
fires.
Once formed and sufficiently cooled, steel slabs can be fabricated into
other more commercially useful forms in process mills, such as hot strip
mills, cold strip mills, billet mills, plate mills, and rod mills.
Although the lubricant environment for process mills are not as severe as
slab casters, grease specifications are quite stringent because of the
high operating temperature and extreme pressure, antiwear requirements.
Grease mills which purify, form, and process other metals such as aluminum
encounter many similar problems as steel mill greases.
Preferably, the grease used to lubricate the bearings of hot slab casters
should: (a) reduce wear and friction; (b) prevent rusting even in presence
of water sprays; (c) be passive, non-corrosive, and unreactive with the
bearing material; (d) resist being displaced by high velocity water
sprays; and (e) maintain the integrity of its chemical composition and
resulting performance properties under operating conditions near thermal
sources which irradiate at temperatures of 1,500.degree. F. to
2,000.degree. F.
In order to enhance the safety, health, and welfare of operating personnel,
greases used in steel mills should be non-toxic, reduce the incidence of
grease fires, and be of a safe composition. Materials known to be serious
skin irritants, carcenogenic, and mutogenic should be avoided in steel
mill greases.
Grease used to lubricate tapered roller bearings of slab casters and
process mills in steel mills should desirably have good adherence
properties as well as resist displacement by water spray. The grease
should retain these properties during use without exhibiting any adverse
effects such as lacquer deposition on the tapered roller bearing parts due
to high temperature oxidation, thermal breakdown, and polymerization of
the lubricating grease. Such lacquering problems have been a common
occurrence in hot slab casters especially where aluminum complex and
lithium complex thickened greases have been used. When such lacquering
becomes severe enough, the results are similar to rusting: the caster
bearings fails and a breakout can occur.
Since hot slab caster bearing grease may be used in other applications in
the steel mill, additional properties such as good elastomer compatibility
and protection against other types of wear such as fretting wear is
desirable. Also, many steel manufacturers prefer a grease which would work
well in slab casters and in process mills, thereby allowing a multi-use
consolidation of lubricants and a reduction in lubricant inventory.
Over the years, a variety of greases and processes have been suggested for
use in steel mills and other applications. Typifying such greases and
processes are those found in U.S. Pat. Nos. 2,964,475, 2,967,151,
3,344,065, 3,843,528, 3,846,314, 3,920,571, 4,107,058, 4,305,831,
4,431,552, 4,440,658, 4,514,312, and Re. 31,611. These prior art greases
and processes have met with varying degrees of success. None of these
prior art greases and processes, however, have been successful in
simultaneously providing all the above stated properties at the high
performance levels required in steel mills.
It is, therefore, desirable to provide an improved process for minimizing
grease fires in steel mills which overcomes many, if not all, of the
preceding problems.
SUMMARY OF THE INVENTION
An improved process is provided for preventing grease fires, which is
especially useful in steel mills and other metal processing mills, such as
strip mills, billet mills, plate mills, and rod mills. In the novel
process, when a flame is ignited, such as from molten steel or other hot
metal or from acetylene torches, or other welding equipment, and
approaches near and contacts the described special grease, which can be
injected into the caster bearings or rollers in a metal processing mill,
the special grease emits a sufficient amount of carbon dioxide to blanket
and extinguish the flame or otherwise substantially prevent the grease
from igniting, burning, and combusting. In the preferred process, carbon
dioxide is emitted from thermal decomposition of calcium carbonate in the
grease.
This patent application also discloses an improved high performance
lubricating grease which is particularly useful to lubricate caster
bearings in hot slab casters and process mills, especially of the type
used in steel mills. This novel grease composition exhibited surprisingly
good results over prior art grease compositions.
Desirably, the new grease provides superior wear protection under low loads
as well as under high loads. The new grease also reduces friction and
prevents rusting under prolonged wet conditions. Desirably, the novel
grease is substantially nonreactive, non-corrosive, and passive to ferrous
and nonferrous metals at ambient and metal processing temperatures,
resists displacement by water spray, and minimizes water contamination.
The grease also retains its chemical composition for extended periods of
time under operating conditions.
Advantageously, the novel grease and process produced unexpectedly good
results and achieved unprecedented levels of high performance during
extensive testing on hot steel slab casters by a major U.S. steel
producer. Significantly, during the tests water contamination levels in
the caster bearings and rotatable caster rollers were reduced by about 90%
with the novel grease, thereby virtually eliminating wear, rust, and
corrosion in the bearings of the slab casters. Also, breakouts on the
casting line were prevented and downtime was significantly decreased with
the subject grease.
Another significant benefit of the subject steel mill grease and process
are that they decrease the amount of grease used (grease consumption) by
over 80% in comparison to the amount of conventional steel mill greases
previously used.
Desirably, the novel grease and process perform well at high temperatures
and over long periods of time. The grease also exhibits excellent
stability, superior wear prevention qualities, and good oil separation
properties even at high temperatures. Furthermore, the grease is
economical to manufacture and can be produced in large quantities.
In use, the improved lubricating grease is periodically and frequently
injected into rotatable caster rollers and particularly the tapered caster
roller bearings of slab casters in steel mills which are subject to
extreme thermal stresses by supporting the heavy loads of hot steel slabs
while being substantially continuously quenched (sprayed) with water or
some other liquid at high pressure and velocities. The improved
lubricating grease can also be injected into the bearings and caster
rollers of process mills, such as hot strip mills, cold strip mills, strip
mills, billet mills, plate mills, and rod mills, or other metal forming
mills, such as aluminum mills.
The improved lubricating grease has: (a) a substantial proportion of a base
oil, (b) a thickener, such as polyurea, triurea, biurea or combinations
thereof, (c) a sufficient amount of an additive package to impart extreme
pressure antiwear properties to the grease, (d) a boron-containing
material to inhibit oil separation especially at high temperatures, and
(e) a sufficient amount of a high temperature, non-corrosive, oxidatively
stable, thermally stable, water-resistant, hydrophobic, adhesive-imparting
polymeric additive in the absence of sulfur. The polymeric additive
cooperates and is compatible (non-interfering) with the extreme pressure
antiwear additive package to minimize water contamination in the grease as
well as resist displacement by water spray while not adversely affecting
low temperature mobility properties of the grease.
The polymeric additive can comprise: polyesters, polyamides, polyurethanes,
polyoxides, polyamines, polyacrylamides, polyvinyl alcohol, ethylene vinyl
acetate, or polyvinyl pyrrolidone, or copolymers, combinations, or
boronated analogs (compounds) of the preceding. Preferably, the polymeric
additive comprises: olefins (polyalkylenes), such as polyethylene,
polypropylene, polyisobutylene, ethylene propylene, and ethylene butylene;
or olefin (polyalkylene) arylenes, such as ethylene styrene and styrene
isoprene; polyarylene such as polystyrene; or polymethacrylate.
In one form, the extreme pressure antiwear (wear-resistant) additive
package comprises tricalcium phosphate in the absence of sulfur compounds,
especially oil soluble sulfur compounds. Tricalcium phosphate provides
many unexpected advantages over monocalcium phosphate and dicalcium
phosphate. For example, tricalcium phosphate is water insoluble and will
not be extracted from the grease if contacted with water. Tricalcium
phosphate is also very nonreactive and non-corrosive to ferrous and
nonferrous metals even at very high temperatures. It is also nonreactive
and compatible with most if not all of the elastomers in which lubricants
may contact.
On the other hand, monocalcium phosphate and dicalcium phosphate are water
soluble. When water comes into significant contact with monocalcium or
dicalcium phosphate, they have a tendency to leach, run, extract, and
washout of the grease. This destroys any significant antiwear and extreme
pressure qualities of the grease. Monocalcium phosphate and dicalcium
phosphate are also protonated and have acidic hydrogen present which can
at high temperature adversely react and corrode ferrous and nonferrous
metals as well as degrade many elastomers.
In another form, the extreme pressure antiwear additive package comprises
carbonates and phosphates together in the absence of sulfur compounds
including oil soluble sulfur compounds and insoluble arylene sulfide
polymers. The carbonates and phosphates are of a Group 2a alkaline earth
metal, such as beryllium, magnesium, calcium, strontium, and barium, or of
a Group 1a alkali metal, such as lithium, sodium, potassium, rubidium,
cesium, and francium. Calcium carbonate and tricalcium phosphate are
preferred for best results because they are economical, stable, nontoxic,
water insoluble, and safe.
The use of both carbonates and phosphates in the additive package produced
unexpected surprisingly good results over the use of greater amounts of
either carbonates alone or phosphates alone. For example, the use of both
carbonates and phosphates produced superior wear protection in comparison
to a similar grease with a greater amount of carbonates in the absence of
phosphates, or a similar grease with a greater amount of phosphates in the
absence of carbonates. Furthermore, the synergistic combination of calcium
carbonate and tricalcium phosphate can reduce the total additive level
over a single additive and still maintain superior performance over a
single additive.
Furthermore, the combination of the above carbonates and phosphates in the
absence of insoluble arylene sulfide polymers achieved unexpected
surprisingly good results over that combination with insoluble arylene
sulfide polymers. It was found that applicant's combination attained
superior extreme pressure properties and antiwear qualities as well as
superior elastomer compatibility and non-corrosivity to metals, while the
addition of insoluble arylene sulfide polymers caused abrasion, corroded
copper, degraded elastomers and seals, and significantly weakened their
tensile strength and elastomeric qualities. Insoluble arylene sulfide
polymers are also very expensive, making their use in lubricants
prohibitively costly.
The use of sulfur compounds, such as oil soluble sulfur-containing
compounds, should generally be avoided in the additive package of steel
mill greases because they are chemically very corrosive and detrimental to
the metal bearing surfaces at the high temperatures encountered in hot
slab casters. Oil soluble sulfur compounds often destroy, degrade, or
otherwise damage caster bearings by high temperature reaction of the
sulfur with the internal bearing parts, thereby promoting wear, corrosion,
and ultimately failure of the bearings. Such bearing failures can actually
cause a breakout which can result in complete shut-down of the hot slab
caster. Oil soluble sulfur compounds are also very incompatible with
elastomers and will typically destroy them at elevated temperatures.
While the novel lubricating grease is particularly useful for steel mill
and process mill lubrication, especially lubrication of caster bearings,
it may also be advantageously used in the constant velocity joints of
front-wheel or four-wheel drive cars. The grease may also be used in
universal joints and bearings which are subjected to heavy shock loads,
fretting, and oscillating motions. It may also be used as the lubricant in
sealed-for-life automotive wheel bearings. Furthermore, the subject grease
can also be used as a railroad track lubricant on the sides of a railroad
track.
As described herein, steel or other metal can be formed, treated,
fabricated, worked, or otherwise processed in a steel mill or a process
mill, such as a hot strip mill, cold strip mill, billet mill, plate mill,
or rod mill, and conveyed on caster rollers with bearings. In the
preferred process, the described special high performance grease is
injected into and prevented from leaking out of the bearings so as to
lubricate and enhance the longevity and useful life of the bearings.
Desirably, the bearings are protected against rust and corrosion at high
temperatures during casting, working, fabricating, and other processing,
as well as at lower and ambient temperatures. In the preferred process,
this is accomplished by the described special non-corrosive, oxidatively
stable, thermally stable, adhesive-imparting grease which also
hermetically seals the bearings, substantially eliminates grease leakage
and toxic emissions, and does not normally irritate the skin or eyes of
workers in the mill. Advantageously, substantially less grease is
required, consumed, and used with the described special grease.
In steel mills, molten steel is fed to a formation chamber where it is
formed into a hot steel slab and discharged on a slab caster. The hot
steel slab is conveyed on caster rollers with tapered roller bearings. The
hot steel slab is quenched and cooled with a high velocity water spray
from above and below the caster rollers and bearings. Advantageously, the
special high performance grease prevents the grease from being flushed and
washed out of the bearings.
As used in this application, the term "polymer" means a molecule comprising
one or more types of monomeric units chemically bonded together to provide
a molecule with at least six total monomeric units. The monomeric units
incorporated within the polymer may or may not be the same. If more than
one type of monomer unit is present in the polymer the resulting molecule
may be also referred to as a copolymer.
The term "bearing" as used in this application includes bushings.
A more detailed explanation of the invention is provided in the following
description and appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A high performance lubricating grease and process are provided to
effectively lubricate the caster bearings of hot steel slab casters, hot
strip mills, cold strip mills, billet mills, plate mills, rod mills, and
other process units used in commercial steel mills. The novel steel mill
grease exhibits excellent extreme pressure (EP) properties and antiwear
qualities, resists displacement by water, prevents rusting even in a
constant or prolonged wet environment, and is economical, nontoxic, and
safe. Desirably, the steel mill grease is chemically inert to steel even
at the high temperatures which can be encountered in hot steel slab
casters.
Advantageously, the steel mill grease is chemically compatible and
substantially inert to the elastomers and seals commonly used in other
parts and operations common to steel mills, thereby increasing its
utility. Also, the grease will not significantly corrode, deform, or
degrade silicon-based elastomers nor will it significantly corrode,
deform, or degrade silicone-based seals with minimal overbasing from
calcium oxide or calcium hydroxide. Furthermore, the grease will not
corrode, deform, or degrade polyester and neoprene elastomers.
The preferred lubricating grease comprises by weight: 45% to 85% base oil,
6% to 16% polyurea thickener, 2% to 30% extreme pressure wear-resistant
additives, 0.1% to 5% boron-containing material for inhibiting oil
separation, and 1% to 10% of a high temperature non-corrosive, thermally
stable, oxidatively stable water-resistant, hydrophobic,
adhesive-imparting, high performance polymeric additive. The polymeric
additive also promotes good low temperature grease mobility for outside
tank storage and transportation. For best results, the steel mill
lubricating grease comprises by weight: at least 70% base oil, 8% to 14%
polyurea thickener, 4% to 16% extreme pressure wear-resistant additives,
0.25% to 2.5% boron-containing material for inhibiting oil separation, and
2% to 6% polymeric additives. The polymeric additives are compatible
(non-interfering) with the extreme pressure wear-resistant additives so as
to not adversely affect the positive performance characteristics of the
extreme pressure wear-resistant additives.
Sulfide polymers, such as insoluble arylene sulfide polymers, should be
avoided in the grease because they: (1) corrode copper, steel, and other
metals, especially at high temperatures, (2) degrade, deform, and corrode
silicon seals, (3) significantly diminish the tensile strength and
elastomeric properties of many elastomers, (4) exhibit inferior fretting
wear, and (5) are abrasive.
Sulfur compounds, such as oil soluble sulfur compounds, can be even more
aggravating, troublesome, and worse than oil insoluble sulfur compounds.
Sulfur compounds and especially oil soluble sulfur compounds should be
generally avoided in the grease because they are often chemically
incompatible and detrimental to silicone, polyester, and other types of
elastomers and seals. Oil soluble sulfur compounds can destroy, degrade,
deform, chemically corrode, or otherwise damage elastomers and seals by
significantly diminishing their tensile strength and elasticity.
Furthermore, oil soluble sulfur compounds are extremely corrosive to
copper, steel and other metals at the very high temperatures experienced
in steel mills. Such chemical corrosivity is unacceptable in steel mills.
Generally, any sulfur-containing compounds should be avoided in the
additive composition of the steel mill grease, especially the sulfurized
hydrocarbons and organometallic sulfur salts. Sulfur compounds of the type
to be avoided in the grease include saturated and unsaturated aliphatic as
well as aromatic derivatives that have from 1 to 32 or 1 to 22 carbon
atoms. Included in this group of oil soluble sulfur compounds to be
avoided in the grease are alkyl sulfides and alkyl polysulfides, aromatic
sulfides and aromatic polysulfides, e.g. benzyl sulfide and dibenzyl
disulfide, organometallic salts of sulfur containing acids such as the
metal neutralized salts of dialkyl dithiophosphoric acid, e.g. zinc
dialkyl dithiophosphate, as well as phosphosulfurized hydrocarbons and
sulfurized oils and fats. Sulfurized and phosphosulfurized products of
polyolefins are very detrimental and should be avoided in the grease. A
particularly detrimental group of sulfurized olefins or polyolefins are
those prepared from aliphatic or terpenic olefins having a total of 10 to
32 carbon atoms in the molecule and such materials are generally
sulfurized such that they contain from about 10 to about 60 weight percent
sulfur.
The aliphatic olefins to be avoided in the grease include mixed olefins
such as cracked wax, cracked petrolatum or single olefins such as
tridecene-2, octadecene-1, eikosene-1 as well as polymers of aliphatic
olefins having from 2 to 5 carbon atoms per monomer such as ethylene,
propylene, butylene, isobutylene and pentene.
The terpenic olefins to be avoided in the grease include terpenes (C.sub.10
H.sub.32), sesquiterpenes (C.sub.15 H.sub.24) and diterpenes (C.sub.20
H.sub.32). Of the terpenes, the monocyclic terpenes having the general
formula C.sub.10 H.sub.16 and their monocyclic isomers are particularly
detrimental.
Inhibitors
The additive package may be complemented by the addition of small amounts
of an antioxidant and a corrosion inhibiting agent, as well as dyes and
pigments to impart a desired color to the composition.
Antioxidants or oxidation inhibitors prevent varnish and sludge formation
and oxidation of metal parts. Typical antioxidants are organic compounds
containing nitrogen, such as organic amines, sulfides, hydroxy sulfides,
phenols, etc., alone or in combination with metals like zinc, tin, or
barium, as well as phenyl-alpha-naphthyl amine, bis(alkylphenyl)amine,
N,N-diphenyl-p-phenylenediamine, 2,2,4-trimethyldihydroquinoline oligomer,
bis(4-isopropylaminophenyl)-ether, N-acyl-p-aminophenol,
N-acylphenothiazines, N of ethylenediamine tetraacetic acid, and
alkylphenol-formaldehyde-amine polycondensates.
Corrosion inhibiting agents or anticorrodants prevent rusting of iron by
water, suppress attack by acidic bodies, and form protective film over
metal surfaces to diminish corrosion of exposed metallic parts. A typical
corrosion inhibiting agent is an alkali metal nitrite, such as sodium
nitrite. Other ferrous corrosion inhibitors include metal sulfonate salts,
alkyl and aryl succinic acids, and alkyl and aryl succinate esters,
amides, and other related derivatives. Borated esters, amines, ethers, and
alcohols can also be used with varying success to limit ferrous corrosion.
Likewise, substituted amides, imides, amidines, and imidazolines can be
used to limit ferrous corrosion. Other ferrous corrosion inhibitors
include certain salts of aromatic acids and polyaromatic acids, such as
zinc naphthenate.
Metal deactivators can also be added to further prevent or diminish copper
corrosion and counteract the effects of metal on oxidation by forming
catalytically inactive compounds with soluble or insoluble metal ions.
Typical metal deactivators include mercaptobenzothiazole, complex organic
nitrogen, and amines. Although such metal deactivators can be added to the
grease, their presence is not normally required due to the extreme
nonreactive, non-corrosive nature of the steel mill grease composition.
Stabilizers, tackiness agents, dropping-point improvers, lubricating
agents, color correctors, and/or odor control agents can also be added to
the additive package.
Base Oil
The base oil can be naphthenic oil, paraffinic oil, aromatic oil, or a
synthetic oil such as a polyalphaolefin polyolester, diester, polyalkyl
ethers, polyaryl ethers, silicone polymer fluids, or combinations thereof.
The viscosity of the base oil can range from 50 to 10,000 SUS at
100.degree. F.
Other hydrocarbon oils can also be used, such as: (a) oil derived from coal
products, (b) alkylene polymers, such as polymers of propylene, butylene,
etc., (c) olefin (alkylene) oxide-type polymers, such as olefin (alkylene)
oxide polymers prepared by polymerizing alkylene oxide (e.g., propylene
oxide polymers, etc., in the presence of water or alcohols, e.g., ethyl
alcohol), (d) carboxylic acid esters, such as those which were prepared by
esterifying such carboxylic acids as adipic acid, azelaic acid, suberic
acid, sebacic acid, alkenyl succinic acid, fumaric acid, maleic acid,
etc., with alcohols such as butyl alcohol, hexyl alcohol, 2-ethylhexyl
alcohol, etc., (e) liquid esters of acid of phosphorus, (f) alkyl
benzenes, (g) polyphenols such as biphenols and terphenols, (h) alkyl
biphenol ethers, and (i) polymers of silicon, such as tetraethyl silicate,
tetraisopropyl silicate, tetra(4-methyl-2-tetraethyl) silicate,
hexyl(4-methol-2-pentoxy) disilicone, poly(methyl)siloxane, and
poly(methyl)phenylsiloxane.
The preferred base oil comprises about 60% by weight of a refined
solvent-extracted hydrogenated dewaxed base oil, preferably 850 SUS oil,
and about 40% by weight of another refined solvent-extracted hydrogenated
dewaxed base oil, preferably 350 SUS oil, for better results.
Thickener
Polyurea thickeners are preferred over other types of thickeners because
they have high dropping points, typically 460.degree. F. to 500.degree.
F., or higher. Polyurea thickeners are also advantageous because they have
inherent antioxidant characteristics, work well with other antioxidants,
and are compatible with all elastomers and seals.
The polyurea comprising the thickener can be prepared in a pot, kettle,
bin, or other vessel by reacting an amine, such as a fatty amine, with
diisocyanate, or a polymerized diisocyanate, and water. Other amines can
also be used.
Biurea (diurea) may be used as a thickener, but it is not as stable as
polyurea and may shear and loose consistency when pumped. If desired,
triurea can also be included with or used in lieu of polyurea or biurea.
Additives
In order to attain extreme pressure properties, antiwear qualities, and
elastomeric compatibility, the additives in the additive package comprise
tricalcium phosphate and calcium carbonate in the absence of sulfur
compounds. Advantageously, the use of both calcium carbonate and
tricalcium phosphate in the additive package adsorbs oil in a manner
similar to polyurea and, therefore, less polyurea thickener is required to
achieve the desired grease consistency. Typically, the cost of tricalcium
phosphate and calcium carbonate are much less than polyurea and,
therefore, the grease can be formulated at lower costs.
Preferably, the tricalcium phosphate and the calcium carbonate are each
present in the additive package in an amount ranging from 1% to 15% by
weight of the grease. For ease of handling and manufacture, the tricalcium
phosphate and calcium carbonate are each most preferably present in the
additive package in an amount ranging from 2% to 8% by weight of the
grease.
Desirably, the maximum particle sizes of the tricalcium phosphate and the
calcium carbonate are 100 microns and the tricalcium phosphate and the
calcium carbonate are of food-grade quality to minimize abrasive
contaminants and promote homogenization. Calcium carbonate can be provided
in dry solid form as CaCO.sub.3. Tricalcium phosphate can be provided in
dry solid form as Ca.sub.3 (PO.sub.4).sub.2 or 3Ca.sub.3 (PO.sub.4).sub.2
.multidot.Ca(OH).sub.2.
If desired, the calcium carbonate and/or tricalcium phosphate can be added,
formed, or created in situ in the grease as by-products of chemical
reactions. For example, calcium carbonate can be produced by bubbling
carbon dioxide through calcium hydroxide in the grease. Tricalcium
phosphate can be produced by reacting phosphoric acid with calcium oxide
or calcium hydroxide in the grease. Other methods for forming calcium
carbonate and/or tricalcium phosphate can also be used.
The preferred phosphate additive is tricalcium phosphate for best results.
While tricalcium phosphate is preferred, other phosphate additives can be
used, if desired, in conjunction with or in lieu of tricalcium phosphate,
such as the phosphates of a Group 2a alkaline earth metal, such as
beryllium, magnesium, calcium, strontium, and barium, or the phosphates of
a Group 1a alkali metal, such as lithium, sodium, and potassium.
Desirably, tricalcium phosphate is less expensive, less toxic, more readily
available, safer, and more stable than other phosphates. Tricalcium
phosphate is also superior to monocalcium phosphate and dicalcium
phosphate. Tricalcium phosphate has unexpectedly been found to be
noncorrosive to metals and compatible with elastomers and seals.
Tricalcium phosphate is also water insoluble and will not washout of the
grease when contamination by water occurs. Monocalcium phosphate and
dicalcium phosphate, however, have acidic protons which at high
temperatures can corrosively attack metal surfaces such as found in the
caster bearings of hot steel slab casters. Monocalcium phosphate and
dicalcium phosphate were also found to corrode, crack, and/or degrade some
elastomers and seals. Monocalcium phosphate and dicalcium phosphate were
also undesirably found to be water soluble and can washout of the grease
when the caster bearing is exposed to the constant high velocity water
spray of slab casters, which would significantly decrease the antiwear and
extreme pressure qualities of the grease.
The preferred carbonate additive is calcium carbonate for best results.
While calcium carbonate is preferred, other carbonate additives can be
used, if desired, in conjunction with or in lieu of calcium carbonate,
such as the carbonates or Group 2a alkaline earth metal, such as
beryllium, magnesium, calcium, strontium, and barium.
Desirably, calcium carbonate is less expensive, less toxic, more readily
available, safer, and more stable than other carbonates. Calcium carbonate
is also superior to calcium bicarbonate. Calcium carbonate has been
unexpectedly found to be non-corrosive to metals and compatible to
elastomers and seals. Calcium carbonate is also water insoluble. Calcium
bicarbonate, however, has an acidic proton which at high temperatures can
corrosively attack metal surfaces such as found in the caster bearings of
hot steel slab casters. Also, calcium bicarbonate has been found to
corrode, crack, and/or degrade many elastomers and seals. Calcium
bicarbonate has also been undesirably found to be water soluble and
experiences many of the same problems as monocalcium phosphate and
dicalcium phosphate discussed above. Also, calcium bicarbonate is
disadvantageous for another reason. During normal use, either the base oil
or antioxidant additives will undergo a certain amount of oxidation. The
end products of this oxidation are invariably acidic. These acid oxidation
products can react with calcium bicarbonate to undesirably produce gaseous
carbon dioxide. If the grease is used in a moderately sealed application
such as slab caster bearings, the calcium carbonate generated would build
up pressure and eventually weaken the seal in order to escape. Once
weakened, the seal would be much less effective in minimizing water
contamination of the bearing.
The use of both tricalcium phosphate and calcium carbonate together in the
extreme pressure antiwear (wear-resistant) additive package of the steel
mill grease was found to produce unexpected superior results.
Borates
It was found that borates or boron-containing materials such as borated
amine, when used in polyurea greases in the presence of calcium phosphates
and calcium carbonates, act as an oil separation inhibitor, which is
especially useful at high temperatures, such as occurs in slab casting and
other operations in steel mills. This discovery is also highly
advantageous since oil separation, or bleed, as to which it is sometimes
referred, is a property which needs to be minimized in steel mill greases.
Such useful borated additives and inhibitors include: (1) borated amine,
such as is sold under the brand name of Lubrizol 5391 by the Lubrizol
Corp., and (2) potassium triborate, such as a microdispersion of potassium
triborate in mineral oil sold under the brand name of OLOA 9750 by the
Oronite Additive Division of Chevron Company.
Other useful borates include borates of Group 1a alkali metals, borates of
Group 2a alkaline earth metals, stable borates of transition metals
(elements), such as zinc, copper, and tin, boric oxide, and combinations
of the above.
The steel mill grease contains 0.01% to 10%, preferably 0.1% to 5%, and
most preferably 0.25% to 2.5%, by weight percent material.
It was also found that borated inhibitors minimized oil separation even
when temperatures were increased from 210.degree. F. to 300.degree. F.
Advantageously, borated inhibitors restrict oil separation over a wide
temperature range. This is in direct contrast to the traditional oil
separation inhibitors, such as high molecular weight polymer inhibitors
such as that sold under the brand name of Paratac by Exxon Chemical
Company U.S.A. Traditional polymeric additives often impart an undesirable
stringy or tacky texture to the lubricating grease because of the
extremely high viscosity and long length of their molecules. As the
temperature of the grease is raised, the viscosity of the polymeric
additive within the grease is substantially reduced as is its tackiness.
Tackiness restricts oil bleed. As the tackiness is reduced, the beneficial
effect on oil separation is also reduced. Borated amine additives do not
suffer from this flaw since their effectiveness does not depend on
imparted tackiness. Borated amines do not cause the lubricating grease to
become tacky and stringy. This is desirable since, in many applications of
lubricating greases, oil bleed should be minimized while avoiding any
tacky or stringy texture.
It is believed that borated amines chemically interact with the tricalcium
phosphate and/or calcium carbonate in the grease. The resulting species
then interacts with the polyurea thickener system in the grease to form an
intricate, complex system which effectively binds the lubricating oil.
Another benefit of borated oil separation inhibitors and additives over
conventional "tackifier" oil separation additives is their substantially
complete shear stability. Conventional tackifier additives comprise high
molecular weight polymers with very long molecules. Under conditions of
shear used to physically process and mill lubricating greases, these long
molecules are highly prone to being broken into much smaller fragments.
The resulting fragmentary molecules are greatly reduced in their ability
to restrict oil separation. To avoid this problem, when conventional
tackifiers are used to restrict oil separation in lubricating greases,
they are usually mixed into the grease after the grease has been milled.
This requires an additional processing step in the lubricating grease
manufacturing procedure. Advantageously, borated amines and other borated
additives can be added to the base grease with the other additives, before
milling, and their properties are not adversely affected by different
types of milling operations.
In contrast to conventional tackifiers, borated amines can be pumped at
ordinary ambient temperature into manufacturing kettles from barrels or
bulk storage tanks without preheating.
Inorganic borate salts, such as potassium triborate, provide an oil
separation inhibiting effect similar to borated amines when used in
polyurea greases in which calcium phosphate and calcium carbonate are also
present. It is believed that the physio-chemical reason for this oil
separation inhibiting effect is similar to that for borated amines. The
advantages of borated amines over conventional tackifier additives are
also applicable in the case of inorganic borate salts.
Polymers
It has been unexpectedly and surprisingly found that the polymeric
additives comprising the polymers described below, in the absence of
sulfur and particularly in the absence of organically bonded sulfur, when
used in the presence of and in combination and conjunction with the above
described tricalcium phosphate and calcium carbonate extreme pressure
wear-resistant additives and preferably with the above described
boron-containing material, imparts requisite adhesive strength and water
resistance properties to the finished grease to substantially prevent the
grease from running, bleeding, and being washed (flushed) out of caster
bearings and caster rollers of hot slab casters in steel mills when the
hot steel slab is substantially continuously quenched with high velocity,
high pressure water sprays. The polymers are thermally stable and
substantially minimize high temperature oxidation, corrosion, thermal
breakdown, detrimental polymerization of the grease, and lacquering
(lacquer deposition) of tapered roller bearing (caster bearings) in steel
mills and process mills from the heat, load, and stress of the hot steel
slabs. Advantageously, such polymers are hydrophobic and also extend the
useful life of the grease and decrease overall grease consumption in steel
and process mills. Polymers containing organically bonded sulfur should be
avoided due to their high temperature corrosive nature.
It has also been unexpectedly found that the preferred and most preferred
polymers described below, when used in the presence of and in combination
and conjunction with the described tricalcium pho | | |
