Note: Descriptions are shown in the official language in which they were submitted.
336
This invention relates to thickening agents and
organic liquids thickened with such agents. More
particularly, the thickening agents of this invention
comprise a mixture of a finely divided silica and a
finely divided fibrous polyolefin.
A wide variety of finely divided inorganic and
organic materials have been used to increase the
viscosity of organic liquids for use in various
applications such as paints, coatings, lubricating
oils, and molding compositions. Synthetic amorphous
silicas such as silica aerogels and pyrogenic silicas
have commonly been used, for example, to thicken
liquids such as paraffin oils and polyester, alkyd,
and epoxy resins in the production of greases from
oils, resinous gel coats, and other similar applications.
Relatively large amounts of many silica and other
conventional thickening agents have been required to
provide the increase in viscosity of organic liquids
required for the formation of thixotropic gels and
certain other applications. The use of such amounts
may adversely affect th-: properties of the thickened
organic liquid that are desirable for the application
and make the cost of the thickening agent economically
prohibitive. Thus, there is a great need for
materials with improved thickening efficiency to
provide a greater viscosity increase when used in
the same or smaller proportions than known thickeners.
Silica thickeners of improved thickening
efficiency are disclosed in U. S. Patents 3,293,205
of Doyle andYoung and 3,354,114 of Doyle. The Doyle
336
and Young thickener may comprise a mixture of finely
divided polyoxymethylene fibers and fine sized
materials such as finely divided polyethylene,
oxidized polyethylene, silica aerogels, or other
synthetic and natural silicas~ The ~hickener of the
Doyle patent isan intimate mixture of finely divided
fibers of polystyrene and fine sized silica which is
of the aerogel type.
In accordance with the present invention, it has
been discovered that a composition which comprises a
mixture of flne'.y divided silica and finel~ divided
pblyolefin fibers has unexpectedly superior efficiency
in thixotropic thickening of a wide variety of organic
liquids compared to the thickening efficiencv of the
silica component when used alone and avoids the
agglomeration that can occur when the polyolefin
component is used alone The fibers have a fibrillar
structure and an exceptionally high surface area.
The finely divided silica used in the present
invention is generally a substantially dehydrated
synthetic amorphous silica~ The water content is
generally from about 1 to about 15 weight percent as
measured by loss in weight after heating for 1 hour
at 1750F. (955C.). These synthetic ar,lorphous
silicas generally have surface areas of greater than
about 50 square meters per gram and commonly of
greater than about 150 square meters per gram. The
surface areas are determined by the nitrogen adsorp-
tion method described in Brunauer, Emmett, and Teller,
30 60 J. Am. Chem._Soc. 309 (1933~. The method is run
to a P/PO of 0.967 so that pores of from 14 to
--3--
336
600 angstroms in diameter are measured.
The synthetic amorphous silicas generally have an
aggregate weight median particle diameter of less than
about 50 microns and preferably of less than
about 10 r,licrons. This aggregate sili_a
particle diameter is the si?e to which the ultimate
silica particles having an average size of from about
10 to about 50 millimicrons coalesce by a combination
of chemical reaction, physical attraction and mechanical
interaction.
Silica aerogels, pyrogenic silicas, and mixtures
thereof are highly preferred synthetic amorphous silicas
for use in th~ thickening a~ents of this invention because
their mixtures with a finely divided fibrous polyolefin
have significantly superior thickening efficiency.
These amorphous silicas comprise chemically
similar polymerized silica molecules and have some
differing and some similar physical properties.
Because of this basic chemical similarity, the
silica art has adopted the method of synthesis as the
principal means of differentiating between the
various types of synthetic amorphous silicas.
Silica aerogels are the most preferred synthetic
amorphous silica for use in the thickening agent of this
invention. A silica aerogel is typically prepared
by mixing sodium silicate and sulfuric acid to form
an acidic silica hydrosol, allowing the hydrosol to
set to a hydrogel, treating the hydrogel with ammonium
hydroxide, washing the hydrogel substantially free
of sodium and ammonium compounds, and drying the
washed hydrogel in a manner so that there is no
substantial shrinkage of the silica ~truc-ture.
--4--
33~6
A useful drying technique employs a fluid energy mill
which concurrently dries and sizes the silica aerogel
to the desired particle size range. Silica aerogels
may also be prepared without ammonium hydroxide
treatment by use of a drying step in which the hydrosol
or the washed hydrogel is heated in the presence of
an organic solvent, such as ethyl acetate, to at least
the critical temperature of the solvent and thereafter
the solvent is slowly released from the system. The
silica aerogel products have relatively low surface
areas and large pore volumes and average pore diameters.
Especially preferred silica aerogels for use in
the thickening agent of this invention have a weight
median particle diameter of from about 2 to about lO
microns, a surface area of from about 300 to about
400 square meters per gram, a pore volume of at least
about 1.2 cubic centimeters per gram, and an average
pore diameter of from about 150 to about 250 angstroms.
The pore volume is determined by the same B.E.T. nitrogen
adsorption method used to determine surface area.
The average pore diameter in angstroms is calculated
from the pore volume in cubic centimeters per gram
and surface area in square meters per gram in accor-
dance with the equation 4
. 4 x ~ore volume x lO
average pore dlameter=
surface area
The preferred pyrogenic silicas are sometimes
also referred to as fumed silicas. Pyrogenic silicas
are prepared by volatilizing and recondensing silica
--5--
33~
in high temperature arc or plasma jet processes or by
charging vapors of a silicon compound, such as silicon
tetrachloride, silicon tetrafluoride, or silicon sulfide
into a high temperature hydrolyzing flame.
The fibrous polyolefin used in the thickening
agent of this invention may be a polymer of a variety
of olefins and is generally a crystalline or partially
crystalline high density polyalkene. Fibrous polymers
of lower aliphatic alkenes containing from about 2 to
about 6 carbon atoms are generally employed. Preferably,the polyolefin is selected from the group consisting
of polyethylene, polypropylene, and mixtures
thereof. Fibrous polyethylene is especially preferred.
Other olefins which may be employed include diolefins
such as butadiene and isoprene and alpha-olefins such
as l-butene, l-pentene, l-dodecene, and 4-methyl-1-
pentene. In addition to fibrous homopolymers of these
olefins, fibrous copolymers and bloc~ copolymers may
be formed by polymerization of olefin mixtures. Prefer-
ably, the fibrous polyolefin has a viscosity averagemolecular weight of greater than about 400,000 and more
preferably of greater than about one-half million.
These molecular weights correspond to a preferred
intrinsic viscosity of greater than about 4.0 dl/gram
and a more preferred intrinsic viscosity of greater
than about 5.0 dl./gram and a melt index of zero as
measured by ASTMD-1238-62T. The preferred fibrous
polyethylene softens at a temperature of from about
120 to about 130C. (248-266F.) and melts at a
30 temperature of from about 130 to about 135C.
(266-275F.)
The fibers are made up at least in part of fibrils
and thus have a fibrillar structure. Some of the fibers
--6--
336
are made up of bundles of macrofibrils which are generally
larger than about 1 micron in diameter and some of the
macrofibrils have portions that are made up of micro-
fibrils having a diameter of less than about 1 micron.
5 Preferably, the polyolefin fibers are highly
fibrillated (i.e. branched) and have an exceptionally
high surface area of greater than about 1 square meter
per gram and preferably of ~reater than about 5 square
meters per gram. The surface area of the fibers
typically ranges from about 5 to about 15 square meters
per gram. The surface area is measured by-gas adsorption
techniques, such as the nitrogen B.E.T. method, of samples
rinsed in isopropanol, dried in a 45C. (113F) oven,
and vacuum dried.
Suitable high surface area fibrous polyolefins
may be prepared, for example, by direct conversion of
an olefin monomer gas. In these processes, a monomeric
olefin is polymerized at a relatively rapid reaction
rate in a reaction medium in which the polyolefin to
be formed is swellable or soluble to a significantly
measurable extent in the presence of a coordination
catalyst under conditions of high shear stress.
Representative polymerization processes of this type
are disclosed in U. S. Patents 3,891,610 and 3,849,387, -
herein incorporated by reference. The fibrous polyolefinmay also be prepared by the process of U. S. Patent
3,743,272, in which a polyolefin is dispersed in a
precipitant under conditions of shear stress to form poly-
olefin fibers having a micro-fibrillar structure, a high
surface area, and a size and morphology similar to natural
cellulosic fibers.
The fibers produced by the polymerization tend
to be interconnected or bundled together. The fibers
can be refined or beaten to separate discrete fibers
from the bundles by conventional defibering or
--7--
~1336
shredding techniques in an apparatus such as a disc
* *
refiner, Claflin refiner, Hollander beater,
Dynapulper and the like.
The fibrous polyolefin can be fluffed by passing
the fibers several times through a high-speed material
fan. The fluffing operation by itself does not dry
the fibers to any great extent but the fluffed fibers
can be dried by various hot air systems to a moisture
content of less than about 2 weight percent. The
fluffed fibrous polyolefin having a moisture content
of from about 45 to about 55 weight percent is preferred
for preparing the thickening agent of this invention
because of its convenience in handling.
Generally, the finely divided polyolefin used in
this invention has an average fiber length of less than
about 900 microns and a diameter of less than about 10
microns. Average fiber length is the average by weight
measured in a Bauer-McNett classifier in accordance
with TAPPI Standard Test No. T-233 S~-64. The length to
diameter ratio of the fibers is greater than about 1
to l~and generally is greater than about 5 to 1.
Preferably, the fibrous polyolefin is reduced in
size for use in the thickening agent of this invention
so that it has a major dimension of not greater than
about 50 microns and preferably of less than about 10
microns. The minor dimension of the preferred fibers
xanges from less than about 5 to less than about 1 micron.
The length to diameter ratio of the fiber aggregate
particles is preferably greater than about 10 to
1 and more preferably greater than about 50 to 1.
--8--
*Trademark
~,
l336~
The fibrous polyolefin generally contains a major
amount, e.g., greater than 90 percent by weight, of
fibers having lengths of from about 5 to about 10
microns and diameters of less than about 1 micron.
Minor amounts, e.g., less than about 10 percent, of
larger fibers or agglomerates of the smaller fibers
having major dimensions of up to about 50 microns
and minor dimensions of from about 5 to about 10
microns can bedetected in a microscopic examination.
The finely divided silica used in the thickening agent
of this invention generally has a weight median
particle diameter of less than about 50 microns and
preferably less than about 10 microns.
Any apparatus suitable for the reduction of the
silica and fibrous polyolefin to the desired size may
be used. The feed to the apparatus may be a silica
hydrogel or a silica aerogel produced by drying the
hydrogel. The gel and polyolefin components are
preferably broken up, as by cutting or shredding,
into pieces of about 1/8 inch or less in size to aid
feeding into the apparatus. The polyolefin fibers
and silica may be mixed in a conventional manner to
form the thickening agents of this invention. Prefer-
ably, the components are mixed prior to incorporation
in the organic liquid and mixing methods, such as blending
or tumble mixing, are sufficient. Although lower thick-
ening results, the silica and polyolefin components may
be mixed in the liquid to be thickened if desired.
In a preferred embodiment, the mixture of the
thickening agent of this invention is prepared by
3~
simultaneous size reduction of the silica and polyolefin
in the presence of the other by a crushing, milling,
or grinding operation in such a manner as to cause
fracture of the particles and formation of freshly
exposed surfaces and provide an intimate mixture of
the components. Suitable apparatus for simultaneous
size reduction in which freshly formed surfaces are
exposed includes, for example, ball mills, vibration
mills, pot mills, hammer mills, gyratory crushers,
pulverizers, speedline mills, sand grinders, colloid
- mills, mic~on mills, and the like.
The preferred method for preparing the mixture
comprises simultaneous fluid energy milling of the
components. In this method, the polyolefin and
silica are suspended in a moving gaseous medium and
additional gas is continuously introduced in a
plurality of high velocity streams directed inwardly
into the mill in such a way as to cause extreme
turbulence and attrition and fracturing of the suspended
silica and polyolefin. The comminuted fibrous
polyolefin-silica mixture is continuously removed
from the mill along with the gaseous medium and
separated from the suspending gas. Air and steam
are the preferred suspending gases and are also
preferably used as the supplemental turbulence-
creating gas because of inexpensiveness and ready
availability.
In the operation of the fluid energy mill,
using air as the gaseous grinding medium, suitable
pressures of the suspension air range from about 100
--10--
1~1336
to about 500 pounds per square inch gauge
and preferably from about 110 to about 300
pounds per square inch gauge. The auxiliary
turbulence-creating air can be injected into
the whirling body of polyolefin and silica at
pressures which may range from about 1~0 to
about 500 pounds per s~uare inch gauge and
preferably are between about 110 to about 250
pounds per square inch gauge. The suspension
and auxiliary air is at a temperature low enough to
avoid softening or melting of the polyolefin. Prefer-
ably, the air temperature is from about 50 to about
120F. (10-49C.). The average particle size of the
product can be varied by controlling the air velocity,
temperature, and feed rate. The product can be
separated from the suspending air in any suitable
manner, preferably by the use of bag collectors, though
cyclone and other kinds of separator can also be used.
The preferred fluid energy mill is the micronizer,
in which relatively large particles are suspending in
a gaseous medium and whirled around an enclosed base
with additional gases introduced into the whirling
body in a manner causing turbulence within the body
and comminution and fracturing of the particles.
Co-milling of the fibers and the silica
concurrently decreases the particle size of the
silica, defibrillates (opens or unwinds) the fibers,
and produces an intimate mixture. In the intimate
mixture, the fibers and the silica particles are held
together and cannot be separated by normal mechanical
--11--
~5~336
means so that something more tha~ mere electrostatic
attraction or mechanical ~mpingement is present. It
is believed that co-milling exposes active surfaces
which bond the polyolefin and si]ica. Various methods
may be used to produce such an intimate mixture but
fluid energy co-milling is preferred.
The polyolefin should be present in the mixture
in an amount sufficient to provide a substantial
increase in the thickening efficiency of the silica.
The mixture generally contains from about 95 to about
5 weight percent polyolefin and from about 5 to about
95 percent silica. Preferahly, the mixture comprises
from about 25 to about 35 weight percent polyolefin
and from about 75 to about 65 weight percent silica.
lj A preferred use for the mixtures of this inven-
tion is as an agent to thicken, i.e., form thixotropic
gels, and/or increase the viscositv of organic liquids.
The organic liquids which may be employed in the
compositions of this invention are, for example,
organic solvents, liquid organic film-formers, liquid
organic resins, oleaginous liquids, and mixtures
thereof. Such organic solvents may be solvents used
in paint, varnish, or lacquer removers and include
aliphatic and aromatic alcohols, ketones, and esters,
such as ethanol, acetone, methyl ethyl ketone,
ethyl acetate, or amyl acetate. The liquid organic
film-formers generally comprise solutions of high
molecular weight film-formers dissolved in organic solvents
and are generally employed as adhesives, films,
-12-
336
foils, paints, lacquers, and dopes. Such
high molecular weight organic film-formers are
exemplified by nitrocellulose, cellulose acetate,
chlorinated rubber polyvinyl acetate, polyvinyl
chloride, polyacrylic esters, cellulose butyrate,
and cellulose propionate. When these liquid compo-
sitions are sprayed or spread on objects, the
thickening agent of this invention will cause the
formation of a thixotropic gel almost immediately
on contact with the object and the gel will not run
o~r drain.
The liquid resin compositions which may be
employed with the silica and fibrous polyolefin
thickening agent include plastisol compositions
comprising halogenated vinyl or vinylidene resins.
The thickening agent of this invention is especially
useful for thickening thixotropio, polymerizable
organic liquid resin compositions which are
used in coating, filling, adhesive, and laminating
operations. Such compositions include liquid alkyd
or epoxy resins or solutions of solid alkyd, epoxy
or polyester resins dispersed in a solvent (for
example, styrene) which is usually copolymerizable
with the polyester resin. The mixtures of this invention
are readily wetted and dispersed and give very great in-
creases in viscosity at generally lower concentrations in
curable liquid resins such as polyesters and
polyepoxides and resin latices such as paints.
The oleaginous liquids in which the present thick-0 ening agent may ~e used`include oils of animal
-13-
L336
and vegetable origin such as, for example, cod liver oils,olive oil, corn oil, and lubricating oils such as hydro- -
carbon motor oils and mixtures thereof. The lubricating
oils may be thickenecl with the thic7~ening agent o~ this
invention to provide ~el-like bodies having a grease
consistency.
The thickening agent of this invention may be
incorporated into the organic liquid by any conventional
dispersion method. Relatively low shear mixing
methods such as hand stirring are often satisfactory
but high shear dispersion mixers, such as roll
mills, high speed blenders, or ultrasonic
mixers may be preferred for certain organic
liquids.
The amount of the thickening composition utilized
in the liquid to be treated can vary greatly depending
on the nature of the organic liquid, the dispersion
method, and the degree of thickening desired and is a
minor amount sufficient to increase the viscosity of
the organic liquid. The amount of the thickening
agent can generally vary from about 0.05 to about
10 percent but usually is from 1 to about 5 percent by
weight of the liquid to be thickened.
EXAMPLE 1
250 grams of a silica aerogel and 200 grams of
shredded polyethylene fibers having the properties
shown in Table I were placed in a blender and mixed
at high speed for 1 minute. This blending operation
- was repeated many times to accumulate 15 pounds of
the mixture.
-14-
336
o ~
o ~ Ct~ o o o
. o ~ o ,~-, ~ ,~
o cr~
U~
"~.
a~ ~ - ~
.a ~ c
.,~ ,a ~ ~^ c o c
3 ~ C h C '; ;~ ~ C
a~,~ a~ Q~ O ~ ^ C
_I ~ c-~ ~ ~ C) tr, ~ ~ c _ ~ _
O ~ ~ rt ~1 a.~ ~ ~r1
_~ Q) a~ r~ ~ o ,-~ o
~ P~ r ~ U~ ~ O ~ O _ ~ _
5-/
O O ~ O
O
~ r1 ~ h
C ~ iq
~ 3 ^ ~ R ~ L: 3
O ~ o ~ 3 u,~ ~ ~ cq a,~
h ~ o ~ , S~ F
s~ ~q ~ ~ J o :5
t`5C ~4 C O O Cr~l r~ S~ ,~
C r-l r~ O a) u~ o o ~ ~ ~ ~ o ~ o
~ O v),,~ ~ S~ ~ R ~1 a ~ c ~ ,~
U ~ ~ ~s ~ ~ ~ ~I~
,~ 07 Y'~ o~ Q ,~ .~ ~ Q ~ a) (3
_~ Cq dP U~ h`~ r~ U
rl O ~_ ~ ~r~ O--~ ~ O--
~q ~ ~ o m c~
.
~13~
6 pounds of the mixture was fed through a vibrating
screw feeder and injected with compressed air at 127
pounds per s~uare inch gauge and 85-90F. (30-32C.)
into an 8 inch micronizer at a rate of 7.5 pounds per
hour. Air at 95 to 100F. (35-38C.) and under a
pressure of 115 to 116 pounds per square inch gauge
was injected into the whirling body of polyethylene
fibers and silica aerogel to create a turbulent
mass in the mill. The outlet air temperature was
lQ 95 to 105F. ~35-41C.) and 4 pounds of the milled
product were separated from the air stream in a bag
collector.
The product was a white, odorless, flocculated
powder and consisted of finely divided polyethylene
fibers intimately mixed with finely divided silica.
Microscopic examination of the powder showed
that the fibers had a major dimension of
less than 1 micron and a smaller dimension in a
low millimicron range.
The blended mixture, the blended and milled
mixture,and a sample of the same silica aerogel per
se were used to thicken a liquid styrene-unsaturated
polyester resin. ~he resin contained about 45
parts by weight of styrene and about S5 parts by
weiqht of the unsaturated polyester resin.
The liquid resin composition had a viscosity
of 110 centipoises at 77F. (25C.~ as measured
in a Brookfield viscometer using a No. 4 spindle
at 20 revolutions per minute.
--16-
~13~tj
Various concentrations by weight of each o~ the
thickening agents were mi~ed with separate portions
- of the liquid resin for ~ minutes at 4C00 revolutions
per minute in an Eppenbach homogenizer. The viscosit~
was then immediately measured in the same viscometer
under the same conditions as the unthic~ened resin~
The viscosities of the various thlc.~ened samples
were also determined at 20 revolutions per minute
in the same apparatus at 77~. (25C.) using a No. 4
spindle and ~he thixotropic index was calculated as
the viscosit~ at 2 revolutions per minute divided by
the viscosity at 20 revolutions per minute. The
results are shown in Table II.
-17-
3~6
t.
H
~0; ~ ~ ~ ~r ,~ co ~ ~ co
E~ ~ .... ... ...
O :~;
H
, E~
H ~ O O O O O O O O O O
~ ~ ~ ~J N ~ O Ll') u~ ~ ~ c~ ~
E~ o ~J ~1 ~ .-1
p:; H
U~
E_l ~
P~ U~ ~ O O U) O 11') Irl Ul t~l O O
~ H 1~ ~ ~D O
O ~ ~ ~ ~ ~ ~r
~ ~1 .
z
E~ ~ a)-,l
u~ ~y h al
H Z Z C~
H H O -1 ~
~'1 Z ~ ~ O
~ O
m H
~¢ ~ z ~J O ~
1~
~ Z;~
t~
Z ~ L~
o ,~
O 3~3
U~
H
g ~
o ' a
O ~ ~ ~
X O ~ X
5H a~ -1 e
E~ ~ e
~ a~
v a
.,1 ~ ~
-18-
~3~5:~336
The results clearly demonstrate that the use
of the thickening agent of the present in~ention
provides an unexpectedly higher increase in
viscosity and in thixotropy when incorporated in
organic liquids compared to the use o~ a silica
aerogel per se in such organic liqui~s. The inclu-
sion af the polyethylene fibers significantly reduced
the amount of silica material required to thicken
the liquid. A substantially higher viscosity and
thixotropic index were achieved with the co-milled
polyolefin and silica thickening agent compared
to the blended mixture. It was also found that
the polyethylene fibers agglomerated and floated
to the surface of th,e resin when used alonff.
, !
