Note: Descriptions are shown in the official language in which they were submitted.
13392~7
TITLE:
OPAC~ KS FOR PAINTS AND COATINGS
~CRG~QUND OF THE lNv~..lON
~ CnNlCAL FIELD:
The technical field of the present invention is
coatings and paints and to new and improved opacifiers
affording enhanced hiding power and of opacifier materials
having a specific gravity controllable to values
significantly lower than typical in the prior art and often
substantially the same as the coating vehicle and thus
having little tendency to settle or float or otherwise
separate from the coating formulation.
R~RG~UND ART
A wide diversity of opacifiers are known to paint and
coating technology, and the present invention can be
employed, with suitable adjustments for the specific
characteristics of the materials selected, with any of
them.
As those of ordinary skill in the art will well
understand, the present invention will be described in
relation to the most commonly employed of such materials.
While this is not intended to exclude the employment of
still other opacifier materials, it is believed that the
full nature of the present invention and the parameters
which should guide the art in its use will be most
conveniently and fully understood in relation to such
materials. These include, as mentioned above, such
materials as titanium dioxide, in both anatase and rutile
forms, zinc oxide, calcium carbonate, talc, and where
appropriate to the discussion, other related materials.
The opacifiers of the prior art have specific
gravities which are generally quite high, ranging from
about 1.75 up to as much as about 4.5. As those of
ordinary skill in the art are well aware, these materials
have a decided tendency to separate from the medium and
settle, often as a hard settlement.
1~3~2~7
It is also common, in order to achieve the opacities
desired, to employ the opacifiers in quite substantial
proportions, most often in excess of the "critical pigment
volume" required in order to achieve a degree of porosity
which produces a pigment or opacifier air interface which
enhances the opacity of the coating when dry. Such
porosity, however, can lead to infiltration of
environmental liquids, i.e. water from rain or the like,
which operates to displace air from the pores, wet the
pigment, thus reducing the effectiveness of the opacifier
and greatly reducing the durability of the coating.
The role of opacifiers in coating and paint technology
are generally well understood by practitioners in the art.
The opacifiers of the present invention are well behaved
specimens within the spectrum of familiar, and well known
materiàls, and will give those of ordinary skill in the art
little difficulty once the central properties and
parameters which distinguish the materials from the
conventions of the known materials are clearly defined and
understood.
As the art well understands, it is ordinarily the
difference in refractive indices between the coating binder
and the opacifier material which dictates the hiding power
of a particular coating. That is, the greater the
difference in refractive indices at each occurance of
interface between the coating binder and the opacifier
material, the greater the hiding power of the coating.
Coating binders have refractive indices which typically are
in the vicinity of about 1.5 or 1.6. Opacifiers are most
often materials having refractive indices greater than
about 1.8, and are more effective, generally, as the
refractive index increases. Titanium dioxides, having
refractive indices greater than 2, are among the most
effective opacifiers in general use, and are often
preferred for that reason.
It has long been known that air, having a refractive
index of 1.0, makes a superior interface with opacifiers,
1333~7
and that making paints and coatings porous by exceeding the
"critical pigment volume" or "CPV", loading opacifiers at
the surface, and the entrainment of air in the coating
formulation can all enhance the hiding power. This
technique, by creating an interface of air and opacifier,
is known to be quite effective, but in some contexts
results in the compromise of other properties of the
coating formulation or the resultant coating.
The inclusion of air also enhances opacity through the
air-binder interface, since the binder has refractive index
which is materially different from that of air. This
attribute of such systems is lesser than the air-opacifier
interface effect.
The behavior of binders, opacifiers, and the inclusion
of air through one or more of the techniques known to the
art are all well known, and are in fact well quantifiable
through the application of the Lorentz-Lorenz equations and
the Fresnel equations. Both diffraction and dispersive
effects are accounted for through these techniques.
Through the application of the techniques known to the art,
hiding power of a particular coating formulation can be
predicted quantitatively with considerable reliability.
The materials referred to herein as expanded or
expandable thermoplastic microspheres are most often the
materials described in Morehouse, U.S. Patent 3,615,972,
and like materials. These materials are per se known in
the art, and do not as such form a part of the present
invention. While such materials are disclosed in a
substantial number of prior art teachings, the Morehouse
patent cited above is the most complete description of the
materials and their formation, and is hence the most
relevant and material such teaching in relation to the
present invention.
The microspheres described in the Morehouse Patent
have been employed in coatings of a variety of types.
Representative of such teachings are Wolinski, U.S. Patent
4,006,273, and Wolinski, U.S. Patent 4,044,176. These,
1339247
like all known teachings relating to the inclusion of
microspheres in coatings, do not relate directly to
opacifiers or hiding power of the coatings, and are thus
materially different from the present invention.
The use of opacifiers, as discussed herein, is a vast,
well documented practice in the art. The use of such
materials to render coatings opaque is not per se a part of
the present invention.
8UNMARY OF THE lN V~. ~ION
According to this invention, traditional opacifiers
are combined with thermoplastic, expandable microspheres
under conditions which result in the solid, opacifiers
being adhered to or embedded in the surface of the
microspheres, which are expanded to afford a specific
gravity of the coating formulation and the composite which
taken together reduces the tendency for "floating" or
"settling". The composite has exceptional hiding power
when formulated into paints. Coatings based on the
composites have exceptional performance at materially
reduced opacifier loadings and cost.
Other aspects of this invention are as follows:
The method of making a composite opacifier for
coatings, comprising:
A. mixing an particulate coating opacifier component
with expandable thermoplastic microspheres, said opacifier
component comprising from 20 to 97 weight percent of the
mixture, with a material proportion of said opacifier
component having a particle size of from about 200 to about
2000 millimicrons;
B. heating the mixture of said opacifier component
and said microspheres under conditions of time, temperature
and pressure to cause said opacifier component to adhere to
the surface of said microspheres;
C. thermally expanding said composite to attain a
composite specific gravity of from about 0.1 to about 2.8
gm/cc;
D. recovering said opacifier for incorporation into
coating formulations.
133~ 17
A composite opacifier for coatings comprising:
A. thermoplastic polymer microspheres; and
B. particulate opacifier component, a material
portion thereof having a particle size of from about 200 to
about 2000 millimicrons in diameter;
C. said particulate opacifier component comprising
from 20 to 97 weight percent of the mixture and being
adhered to and embedded in the surface of said
microspheres; and
D. said composite opacifier having a composite
specific gravity of from about 0.1 to about 2.8 gm/cc.
The method of making an opacifier intermediate for
incorporation into coatings, comprising:
A. mixing a particulate coating opacifier component
with expandable thermoplastic microspheres, said opacifier
component comprising from 20 to 97 weight percent of the
mixture, with a material proportion of said opacifier
component having a particle size of from about 200 to about
2000 millimicrons;
B. heating the mixture of said opacifier component
and said microspheres under conditions of time, temperature
and pressure to cause said opacifier component to adhere to
the surface of said microspheres;
C. thermally expanding said composite to attain a
composite specific gravity of from about 0.1 to about 2.8
gm/cc;
D. recovering said opacifier;
E. combining said composite opacifier into a carrier
vehicle therefor for incorporation into coating
formulations.
A composition for use in the making of paints and
coatings comprising a film forming coating binder and an
opacifier, said opacifier comprising an expanded
thermoplastic microspheres having an opacifying constituent
adhered to or embedded in the surface thereof.
The method of making an opaque coating comprising
combining a film forming coating binder and a composite
opacifier, said composite opacifier comprising dry expanded
thermoplastic microspheres having an opacifyingl3con9s~i~uent
adhered to or embedded in the surface thereof, said
opacifying constituent having a refractive index of greater
than about 1.6, and said opacifier having a hiding power
substantially equivalent to that of said opacifying
constituent having a refractive interface with air.
A method of applying an opaque coating to a substrate
comprising forming a coating medium comprising a film
forming coating binder and a composite opacifier, said
composite opacifier comprising expanded thermoplastic
microspheres having an opacifying constituent adhered to or
embedded in the surface thereof, and applying said coating
medium to said substrate.
A coating medium for application to a substrate
comprising a film forming coating binder and a composite
opacifier, said composite opacifier comprising expanded
thermoplastic microspheres having an opacifying constituent
adhered to or embedded in the surface thereof.
SUMMARY DESCRIPTION OF THE DRAWINGS
The attached single drawing is an illustration of a
typical microsphere-opacifier composite according to this
invention.
DISCLOSURE OF l~v~L.lION
The present invention is based on the employment of a
new form of opacifier. The opacifier of the present
invention is a composite of expanded thermoplastic
microspheres, having adhered to or embedded in the surface
thereof an particulate opacifying component.
The composite is formed by the procedure disclosed in
Wolinski et al, U.S. Patent No. 4,722,943. The present
composite opacifier is a species of the product disclosed
and claimed in that application, with the following points
of distinction:
In the present invention, the solid particulate
materials are limited to those which can serve the
opacifying function in paints and related coatings. In the
parent, there are many organic particulate materials which
would not be effective in the present invention, having
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133~247
refractive indices too low to be effective in the this
context.
In the present invention, the proportion of
microspheres and opacifying component, and the extent of
expansion of the beads, are carefully balanced to afford a
controlled specific gravity of the composite after
expansion. This feature reduces the tendency for
separation of the composite from the paint formulation on
standing.
In furtherance of the same objective, the degree of
expansion of the microspheres in the procedure is
controlled to afford the target specific gravity. This
will, in some cases, require some expansion of the
microspheres, but rather less than the full expansion of
which those materials are capable.
As used in the present application, microspheres are
any of the thermoplastic hollow spheres containing a
blowing agent and which are thermally expandable to form
light weight hollow structures. Most often, the
microspheres of interest are those formed in accordance
with the Morehouse Patent cited herein above, which
materials are generally available as commercial materials.
The commercial versions are made of polyvinylidene
chloride, and contain alkane blowing agents. Unless
otherwise indicated herein, these are the preferred
materials, and are those referred to, unless otherwise
specifically identified as some other material. As an
example of such other material, good results have also been
achieved with "ROPAQUE OP-62TM", manufactured by Rohm and
Haas Company, Independence Mall West, Philadelphia,
Pennsylvania, under United States Patent 4,427,836
Kowalski, January 24, 1984. As commercially sold, this
--5b--
13~924~
product has a thin shell of an expandable polymer with a
particle size of 0.40 microns and contains water as the
blowing agent.
In the context of the present invention, it is
generally preferred to utilize microspheres of the smallest
available sizes, on the order of 0.2 to 0.5 microns in
diameter.
The opacifier component may be any of the solid
particulate materials commonly employed in the technology
of paint opacifiers. Such materials include rutile and
anatase TiO2, ZnO, CaCo3 talc, clay materials, and the
like. The particle size requirements of such materials is
observed to be of less significance in the present
invention than in usual circumstances, although it will
generally be most effective to employ particle sizes of
diameter near that of the wave lengths of visible light, as
is common to the art. Because of the enhanced opacities
achieved with the opacifiers of the present invention, a
desired degree of opacity will often be attained with a
lesser grade of material, or even a less effective material
than is ordinarily required in customary formulations.
The characteristics of the microspheres has precluded
many approaches to their drying and pre-expansion. Severe
agglomeration and adherence of the materials to warm
surfaces of equipment have eliminated from serious
consideration most approaches to such procedures. Wet
expansion in steam is of limited use when dry microspheres
are needed, and the spray drying procedure is so expensive,
and the product so prone to excessive, and extremely
difficult, dusting problems, that the effective development
of the potential markets has been limited by such factors.
It has been observed that particulate opacifier
components can be employed, in substantial proportions by
weight, which prevent agglomeration of the microspheres
upon drying and expansion, and that such materials actively
and effectively suppress dusting of the expanded products
as well. This combination of features and observations has
1339~
led to the development of effective drying, and optional
expansion, of microspheres by mixing such particulate
opacifier components into the wet cake, followed by drying,
optionally vacuum drying, and recovery of the dry, free-
flowing product. As an alternative, it is believed that amore reproducible composite can be produced by pre-drying
the microspheres at about 60 degrees C, and them mixing the
dried microspheres with particulate opacifier. With either
approach, the microspheres remain in the desired uni-
cellular condition, and substantially free of undesirableagglomeration. The expansion can be up to the very limits
of the microspheres, as established by prior efforts in the
art, although the desired specific gravity may often be
achieved with a lesser degree of expansion.
It is important to the present invention that in the
context of most uses of the dry, expanded microspheres, it
is the specific gravity or composite density considerations
which are most often of substantial importance. Even quite
substantial proportions of the particulate opacifier
components on a weight basis form a negligible or very
minor component on a volumetric basis. For example,
employing talc as the particulate opacifier component, the
volume and weight relationships of the dry, expanded
microspheres with varying amounts of talc show the
relationships detailed in TABLE I.
TABLE I
EXPANDED MICROSPHERES BLENDED WITH TALC
30MICROSPHERE CONTENT OF PRODUCT
WEIGHT % VOLUME %
99.6
98.6
94 4
88.2
78.0
3 67.6
NOTES:Data are based on Microspheres at 0.04 gm/cc
and the talc at 2.70 gm/cc.
133~2'~7
As the relationships in Table I show, even quite large
proportions of talc by weight represent a minor fraction of
the volume of the dry expanded product. It may be
advantageous to employ more than one type of particulate
opacifier component in mixtures and combinations with one
another.
It has been observed that with appropriate levels of
such particulate opacifier components, the tendency of the
microspheres to agglomerate, or to stick to heated surfaces
of drying equipment is effectively eliminated, and the
dusting of the final expanded product is materially
reduced, if not effectively eliminated.
As those of ordinary skill in the art will readily
recognize, there are a substantial number of parameters
which govern the method and the products produced in the
present invention. Each of the known parameters is
hereafter discussed in turn in relation to the present
invention.
Microspheres are generally available in the form of a
wet cake, which is typically about 35 percent water, about
65 percent unexpanded microsphere beads, and minor
additional amounts of the materials employed in the
manufacture of the microsphere beads by the process of the
Morehouse patent, i.e., "wetting agents."
The most readily available-domestic microspheres are
those available from Pierce ~ Stevens Corporation, 4475
Genesee Street, Buffalo, New York, under the trademark
"EXPANCEL" which are polyvinylidene chloride microspheres
with an inclusion of iso-butane as the blowing agent.
(EXPANCEL is a registered trademark of Casco Nobel AB, a
corporation of Stockholm, Sweden.) The available materials
are preferred in the present invention, primarily for their
availability and reasonable cost.
As the Morehouse patent indicates, microspheres can be
made from a rather wide diversity of thermoplastic
polymers, In practice, the commercially available
microspheres are generally limited to polyvinylidene
1339247
chloride. Microspheres of other materials, such as
polyacrylonitrile, poly-alkyl methacrylates, polystyrene,
or vinyl chloride, are known, but these materials are not
widely and generally available. The present invention is
applicable to any thermoplastic of which microspheres is
made, but since the polyvinylidene chloride materials are
those most available to the art, the discussion herein will
be directed predominantly to that material, and to "ROPAQUE
OP-62" as mentioned above. As those of ordinary skill in
the art will readily recognize, the processing parameters
will require adjustment to accommodate differing polymer
materials.
A wide variety of blowing agents can be employed in
microspheres. Again, the commercially available materials
are more limited in range, most often being selected from
the lower alkanes, particularly propane, butane, pentane,
and mixtures thereof, suited to the polyvinylidene chloride
polymer. As the Morehouse patent clearly sets forth, the
selection of the blowing agent is a function of the
particular thermoplastic polymer employed, and in the
context of the present discussion, those ordinarily used
with the commercially available microspheres are given the
greatest attention. Isobutane is most often used with
polyvinylidene chloride microspheres, while water is the
blowing agent in "ROPAQUE OP-62".
In unexpanded form, the microspheres can be made in a
variety of sizes, those readily available in commerce being
most often on the order of 2 to 20 microns, particularly 3
to 10 microns. It has been demonstrated, for example, that
microspheres can be made from as small as about 0.1 micron,
up to as large as about 1 millimeter, in diameter, before
expansion. In the present invention, lower particle sizes,
i.e. in the range of from about 0.1 to about 10
micrometers, preferably about 0.2 to about 3 microns, are
generally preferred, as an aid in leveling of the coating
formulations into which the opacifier is incorporated.
1~392~17
While variations in shape are possible, the available
microspheres are characteristically spherical, with the
central cavity containing the blowing agent being generally
centrally located.
Dry, unexpanded microspheres typically have a
displacement density of just greater than 1 gm/cc,
typically about 1.1 gm/cc.
When such microspheres are fully expanded, they are
typically enlarged in diameter by a factor of 5 to 10 times
the diameter of the unexpanded beads, giving rise to a
displacement density, when dry, of 0.1 gm/cc or less, often
about 0.015 to 0.06 gm/cc.
While the microspheres are produced in an aqueous
suspension, it is common to break and de-water the
suspension, and to supply the microspheres in the form of a
"wet cake." This avoids shipping larger than necessary
quantities of the aqueous system.
The solids content of the wet cake is substantially
all unexpanded microspheres, but also includes the
suspension components, including the wetting agents, so
that the remaining water in the wet cake is extremely
difficult to remove.
The present invention is based on the use of
conventional contact type, indirect heat exchange mixing
driers. A wide diversity of types of equipment are
applicable. In general terms, the requirements are for
good temperature control, good mixing of powder and
granular materials, optionally with operation at reduced
pressure provided, and the removal and recovery, preferably
with condensation of the evaporated water and entrained
blowing agent. Cooling of the microspheres, either in the
mixing drier itself, or in ancillary equipment is also
preferred.
There is a great diversity of driers available, at
almost any desired scale of operations which meet the
foregoing criteria with a capability of either batch or
continuous operation in the context of the present
----10----
1~392~7
invention. As a general rule continuous operation is
preferred.
Among the commercially available driers with which the
present invention has been employed are the following:
(1) Luwa Corp: Horizontal Thin Film Contact Driers
(2) Charles Ross ~ Son: Ross-Bolz Cone Screw Drier
These quite different units have performed quite
satisfactorily in the practice of the present invention, as
shown in the examples, infra.
The particulate opacifier component in the present
invention is any one of a wide diversity of materials which
meet the requirements of the intended function. It is
required that the particulate opacifier component be a free
flowing solid at the temperature and pressure of the drying
operation, that it not react chemically with the
microspheres, or with the other constituents of the system,
e.g. the wetting agents and related components of the wet
cake, and that at the temperature of the expansion, that it
function to separate the microspheres undergoing expansion
so that they do not come into contact and bond to one
another. It is also required that the particulate
contribute opacity to the coatings to which the composite
is incorporated. Substantially all known opacifiers
commonly employed in the coating industry will meet these
criteria, and thus, can be employed in the present
invention with the adaptations required herein.
The particulate opacifier component may be selected
from one or more components meeting the following general
characteristics:
The opacifier component should be a finely divided
particulate solid material, and should be a free-flowing
solid under the processing conditions of the present
invention. It should have a melting point, for example,
above the temperature of the drying process, generally
above about 250 degrees C. Most opacifier materials will
have no difficulty meeting this requirement, of course.
----11----
133g~'~7
The opacifier component must be finely divided enough
to be able to effectively blend with and adhere to the
surfaces of the microspheres. The maximum major dimension
of the particle size should be no larger than about the
diameter of the expanded microspheres, and preferably less.
The minor dimensions will generally be as small as
possible, effectively from about 200 millimicrons or less,
up to as much as about 2.0 microns. Particle sizes having
dimensions near the wave lengths of visible light, i.e.,
about 400 to 800 millimicrons, are particularly preferred.
It is desirable in many situations to employ a blend of
particle sizes, and it may be desirable in such
circumstances to employ an increment of the opacifier
component having even larger particle sizes as an aid to
the drying process as taught in the prior parent
applications.
The particulate opacifier components are desirably
materials which are known opacifiers in coating
formulations and thus are commonly used in the formulations
where the microsphere composite materials are to be used.
For example, titanium dioxide, talc, calcium carbonate,
barium sulfate, alumina, silica, zinc oxide, mineral clays
and the like may be employed. Other materials of interest
may include spherical beads, or hollow beads, of ceramics,
quartz, or glass. All these are typical and illustrative
of the commonly employed materials in coating compositions,
and those of ordinary skill in the art will be familiar
with others that can also be suitably employed. Blends of
such materials can be employed in many cases.
The selection of suitable particulate opacifier
components among the wide diversity of materials that meet
the general characteristics required of such materials is
generally a matter of balancing a number of functional
requirements in the procedure of the invention and in the
context of the intended uses of the product. Among the
criteria that will guide those of ordinary skill in the art
are the following:
12--
13392~I7
The primary function of the particulate opacifier
component during the manufacture of the composite is to
prevent the microspheres from coming into direct contact
with one another and with the surfaces of the processing
equipment while in a tacky, thermoplastic state, and thus
to prevent them adhering. The opacifier provides this
result by virtue of adhering to the tacky surfaces of the
microspheres as soon as they reach a tacky state, and
continuing to adhere throughout the process. The opacifier
component thus becomes adhered to or partially embedded in
the surface of the microspheres, and forms a buffer between
the thermoplastic material and any other materials with
which it might otherwise come into contact.
When combinations of different materials are employed
as the particulate opacifier component, it is possible to
stay within the compounding requirements of virtually any
designed formulation.
By virtue of the higher density of the particulate
opacifier component than that of the expanded microspheres,
the composite product has a greatly reduced tendency to
become entrained in gas streams or in the environmental
atmosphere. As those of ordinary skill in the art will
readily appreciate, the tendency to dusting is a material
safety hazard, both in terms of exposure of workers and in
terms of fire and explosive hazards. Since the
microspheres may contain an alkane blowing agent in
substantial proportions, large quantities of these
materials in the atmosphere presents a substantial problem
in some circumstances. These difficulties, and the effort
and expense of their resolution are minimized or eliminated
altogether in the present invention.
Generally, the greater the density of the particulate
opacifier component, the greater the reduction in the
dusting problem. Since the major proportion of the product
on a weight basis is the particulate opacifier component,
addition of a high density opacifier component to the
system can effectively eliminate any dusting problems.
133~247
By virtue of the increased density of the composite,
the demands on the processing equipment and system in
recovering the expanded and dried microspheres from fluid
streams is greatly facilitated, and product losses are
substantially reduced.
The particulate opacifier component is used in the
present invention in an amount sufficient to permit the
drying and expansion of the microspheres without sticking
to the equipment employed or forming agglomerations of
microspheres. While this amount will vary depending on the
particular equipment employed, and with the particular
processing conditions, it will most often be on the range
of about 20 to 97 weight percent of the mixture of
opacifier component and microspheres, on a dry weight
basis. As a general rule, in most circumstances the amount
employed should be the amount that will reliably and
consistently achieve the target specific gravity of the
composite after any expansion that may be planned. It is
generally preferred that the opacifier component be
employed in amounts less than 90, and preferably less than
80 weight percent of the blend. This normally results in a
dry expanded product which is more than 90 volume percent
microspheres.
Since the predominant concerns in most uses of
microspheres is with the volumetric proportions, even quite
considerable proportions by weight of the inorganic
particulate opacifier component can be included without
detriment in the end uses. When substantial amounts of the
particulate opacifier component are introduced as a
component of the composite microsphere formulation,
appropriate allowances for this component should be made in
the compounding of coating materials. Thus, the proportion
of the volatile water or solvent system in the coating
composition can often be reduced as well as the proportion
of the opacifier agent required for the desired opacity.
In the present invention, contact drying of the
microspheres is accomplished with active mixing, optionally
--14--
13~9247
at low pressure, in the presence of the particulate
opacifier component. The term contact heating is employed
in the present application to connote heating or drying
involving procedures other than direct heat exchange in a
heated fluid, particularly in a heated gas stream. Contact
drying processes employing indirect heat exchange are
generally well known in other contexts, but in the context
of the present invention, must be adapted to accommodate
the particular and unusual conditions of operation, as
described infra.
Contact drying, including vacuum drying, is widely
practiced for very diverse and demanding operations which
are temperature sensitive. Reducing solutions,
suspensions, dispersions, slurries and semisolid wet cake
to dry, free-flowing granular solids is commonly achieved
in many industries with a great diversity of products.
There are a substantial number of types of equipment in
common use, substantially any of which can be adapted to
use in the present invention. Most such equipment employs
indirect heat exchange, using steam, heated oil, or the
like as a heat transfer medium. Such drying operations
commonly employ mixing means to distribute the material
within the drier, and to prevent agglomeration of the
material. Reduced pressures range from atmospheric
downward below atmospheric to as low as 1 mm Hg absolute in
such operations.
Such drying operations are employed in some contexts
with thermoplastic materials, although not at temperatures
at which the thermoplastic melts or softens, since at a
point near the melting point or the glass transition
temperature of thermoplastic polymers, a highly tacky state
arises, which would result in severe agglomeration into a
relatively monolithic mass and sticking to the equipment.
It has now been discovered that such equipment can be
employed for drying and expanding thermoplastic
microspheres, at temperatures at which the thermoplastic
material becomes tacky and adherent, by virtue of the
--15
1339247
action of the particulate opacifier component and the
continuous mixing, which combine to prevent sticking to the
equipment and agglomeration of the microspheres.
It is also common in such equipment to remove and
condense the "distillate" removed from the solid. Since
this is done on a continuous basis, the hazards in the
present system as a consequence of accumulations of the
highly flammable or explosive blowing agent are avoided.
The blowing agent, typically iso-butane, is continuously
removed and condensed in such equipment. This eliminates
the need, as has been common in the drying of microspheres
by spray drying procedures, of employing a non-oxidizing
atmosphere in the drying chamber. Use of air, or other
oxygen containing gases has proved an unacceptable fire and
explosion hazard in such systems, and most are operated by
- employing nitrogen or some other inert gas as the heat
exchange medium. Inert gas direct heat exchange is quite
expensive, and still requires care in the handling of the
substantial gas stream with the blowing agent carried with
it, and thus solves only a part of the hazard.
The equipment selected for use must, rather evidently,
provide for adequate heat transfer to remove substantially
all the water from the feed stock. The significant control
parameters for any given equipment will be residence time,
pressure, and heat input, normally based on operating
temperature for convenience. At the residence time and
pressure employed, heat exchange must be accomplished
within the constraints of the temperature limitations of
the microspheres, which cannot be permitted to reach a
temperature at which the blowing agent bursts the sphere.
The equipment must also provide the energy for the
expansion itself. This is not large, and in most
circumstances achieving a bead temperature (depending on
the specific polymer) at which expansion occurs, as
previously defined, there will be little difficulty in
attaining the desired degree of expansion. In most
circumstances, full expansion is desired, i.e., to a
--16--
1339247
microsphere density of less than 0.06 gm/cc, preferably
about 0.02 gm/cc (without the contribution of the
particulate opacifier component). This refers to the true
densities of the microspheres and not the bulk or apparent
densities.
The important temperature limitations are defined by
the thermoplastic polymer. It is important not to melt the
polymer mass, so that the hollow spherical structure is
lost through over expansion. On the other hand, if the
temperature is not high enough to soften the polymer and to
develop an adequate pressure of the blowing agent,
expansion may not occur, or may be insufficient. Residence
time at the appropriate temperature is also an important
control parameter, since there is a definite duration for
the expansion process. Even when adequate temperatures are
achieved, if the residence time at temperature is too
short, the expansion may be insufficient. If the time is
too long, the microspheres themselves may be disrupted,
leaving broken spheres and polymer fragments and grit in
the product, with attendant losses of production.
As a general parameter, the time and temperature to be
achieved is determined by the nature of the polymer of
which the microspheres are made, and the degree of
expansion required to attain the target specific gravity.
The temperatures are generally near, but not materially
above, the glass transition temperature of amorphous
materials and the melting temperature of crystalline
polymers. These matters are discussed in more detail in
the Morehouse patent.
It is the function of the particulate opacifier
component to prevent the formation of aggregates of the
microspheres to the maximum attainable degree. In most
drying equipment this particular requirement is facilitated
by the use of continuous, often relatively high speed, low
shear mixing of the material in the drier. It is worth
note that excessive shear in the mixing operation may
result in disrupting the microspheres, and must be avoided.
--17--
1339~47
It is generally believed, although applicants have no
wish to be bound thereby, that the particulate opacifier
components in the present invention function to adhere to
the surface of the microspheres as they reach a temperature
at which the polymer material becomes tacky. By such
adherence over the surface of the particles, the
superficial layer of the opacifier component precludes
surface bonding between microspheres as they come into
contact.
It is one of the unique features of the present
invention that the microsphere beads can be dried without
expansion. This has not been possible in any effective
process in the prior art. Such a result is achieved by
drying at temperatures below that at which the microspheres
soften, and where the internal pressure of the blowing
agent is less than that needed to cause expansion. Since
the microspheres typically expand at temperatures on the
order of about 120 degrees C, drying can proceed
effectively at lower temperatures. By use of reduced
pressures, the drying can proceed at considerable rates.
The degree of expansion can range from substantially
none, to the known limits of expansion. This parameter is
determined by the temperature, the residence time at
temperature, and to a lesser degree, by the pressure in the
system. By balancing these. parameters against the
requirements for evaporating the water, substantially any
degree of expansion and the targeted specific gravity for
the intended use can be attained.
If the particulate opacifier is to be added to wet
cake, it is important to have the opacifier component well
dispersed in the continuous phase during the drying
operation. This requirement ordinarily mandates a pre-
mixing operation to disperse the particulate opacifier
component into the wet cake before it is fed to the drier.
In some cases, there may be adequate mixing in the drier to
achieve adequate dispersion before the point at which the
drying proceeds to the extent that requires uniform
--18-
133924~
dispersion, but in most circumstances, those of ordinary
skill in the art will recognize, a pre-mixing step will
insure better results. It will generally not be necessary
to add wetting agents or surfactants into the mixture in
order to attain adequate dispersion because of the wetting
agents already present in the wet cake.
The microsphere beads expand at a temperature which is
a function of the specific polymer and blowing agent
employed. Typically, expansion occurs at about 120 degrees
Centigrade. At reduced pressure, expansion may occur at
slightly lower temperatures.
Expansion requires that the blowing agent develop a
substantial internal pressure (as compared with the
external pressure), and that the polymer become softened
enough to flow under the effect of the internal pressure.
This generally means that the polymer must be heated to a
point near its melting or glass transition temperature, or
very slightly above. If the polymer temperature is too
high, the microspheres will over-expand, burst, and
collapse. The range of actual temperatures necessary will
depend upon the specific microspheres utilized, i.e. the
polymer therein. At temperatures near the upper limit, the
residence time at temperature should be brief.
It will often be desirable to conduct the drying
operation at reduced pressure to accelerate the rate of the
water removal. Thus, in the present invention, pressures
from ambient to as low as 1 mm Hg absolute have been
employed with success. As those of ordinary skill in the
art will readily recognize, the balancing of time,
temperature, and pressure can be readily adapted to the
substantially complete removal of the water and the
appropriate expansion of the microsphere beads.
Particularly when little or no expansion is wanted, low
pressure drying greatly facilitates low temperature
operations at which the expansion of the microspheres does
not occur.
----19--
1339247
As the temperature is raised to the point at which the
microspheres begin to soften and expand, and their surface
area becomes tacky, the particulate opacifier component
will adhere to the surface. Good mixing operates to
maximize the extent of contact between the particulate
opacifier component and the microspheres at this stage in
the process. The extent of the mixing is not narrowly
critical, so long as a relatively homogeneous dispersion of
the opacifier component and the microspheres is maintained,
and so long as the mixing does not disrupt the structure of
the microspheres.
It is generally preferred to actively cool the dried
and expanded microspheres before they are collected and
packaged or otherwise handled. When reduced pressure is
employed in the drier, it is preferred that the
microspheres be stabilized by cooling before the pressure
is increased. This minimizes the degree to which the
pressure change can operate on the polymer and possibly
disrupt the system while the polymer is in the plastic
state.
The resulting dry microspheres can be conveniently
recovered from the drier, collected and handled by entirely
convention procedures and equipment usually employed in
such drying operations for dealing with powdered or
granular materials.
The result of the process is the production of a
unique form of the composite opacifier-microspheres. The
composite will comprise the microspheres which have an
adherent surface deposit of the particulate opacifier
component, ordinarily adhered to or partially embedded in
the surface of the polymer material. When an excess of the
particulate opacifier component is used, there may be an
additional amount of free material entrained in, but not
bound to the surface of, the microspheres. The particulate
material may form a discontinuous layer on the surface, or
in other circumstances may completely coat the surface in a
continuous layer. By varying the proportions of the
--20-
133~2 17
opacifier component and the microspheres, either condition
may be attained. Depending on the intended environment of
use, either condition may be preferred. For example, when
the microspheres are to be incorporated into a coating
polymer matrix which does not readily wet and bond to the
polyvinylidene chloride, the adhered or embedded particles
of the particulate opacifier component can function
effectively as a "primer" or "key" coating on the beads,
resulting in improved bond strength in such circumstances.
In other cases, where the polymer binder forms strong bonds
directly to the polyvinylidene chloride, a discontinuous
coating of the opacifier component may result in better
bonding.
The composite opacifier-microspheres of the present
invention are essentially a dry, free-flowing powder, but
can contain up to about 5 % moisture and retain its free
flowing characteristic. Because there will still be a
residuum of the "wetting agents" remaining from the limited
coalescence process by which the microspheres were made,
the product will be slightly hygroscopic, and unless
protected from ambient moisture, will gradually take up
additional water. The materials involved are not so
strongly hygroscopic, however, that this is a major
problem. In most circumstances, unprotected microspheres
will tend to stabilize at a water content of about 1.5
weight percent. The microspheres will remain a free
flowing powder even under such conditions. When
formulating aqueous based coatings, the moisture content of
the opacifier can generally be ignored.
The microsphere product of the present invention can
be un-expanded, or can be expanded to very near the limit
of expandability, i.e., to a density of between 0.010 and
0.015 gm/cc. Intermediate values are also possible. When
the particulate opacifier is taken into account, the
composite density will, of course, be higher, and should
have a density within the broad range of 0.1 to 2.8 gm/cc,
and preferably within the range 0.15 to 1.5 gm/cc to make a
--21--
92~7
non-floating, non-settling product. Utilizing "ROPAQUE OP-
62", excellent composite has been produced having a density
of 2.8 gm/cc. Thus the composite density of the product
will be determined by the density of the particular
opacifier component employed, the amount of the opacifier
component included, and the degree of expansion. Those of
ordinary skill in the art will be able to readily determine
the composite density of the product from the information
provided in Table I, hereinabove.
The opacifier component of the composite is adhered to
or embedded in the surface of the microsphere, and is
believed not to be in direct contact with the gases on the
interior of the microsphere structure. Thus, so far as is
presently known, there is no opacifier-air interface
present. In that context, it has been surprising to
observe that the performance of the composite is fully
equivalent to that which would be anticipated if there were
- such an interface. Observed hiding power of the
formulations with the composite of the present invention
are typically about 225 to 250 percent of the values
predicted by the Lorentz-Lorenz equations for titanium
dioxide-polyvinylidene chloride interfaces, and are fully
equivalent to those anticipated for titanium dioxide-air
interfaces. The degree of enhanced hiding power is even
greater for other, less efficient opacifiers. In the case
of talc-microsphere composite opacifier, for example, the
hiding power is often in excess of 100 times that for the
talc alone.
These observations are not fully understood, and have
been incompletely investigated at the present, but it is
believed possible that the expansion of the microspheres
preferentially reduces the thickness of the polyvinylidene
chloride film at the points at which the embedments of the
solid particles are located, so that the film is reduced to
dimensions of thickness substantially less than the wave
length of visible light, where refractive behavior is
substantially altered, and in the present context ceases to
--22--
13392~7
be a material factor in the refraction characteristics of
the composite. The thickness of the microsphere wall has
been observed to be on the order of about 30 to 50,
typically about 40 millimicrons, while the wave lengths of
visible light are, of course, from 400 to 800 millimicrons.
As a consequence, the behavior of the opacifiers in
the present invention is fully consistent with the
attainment of high levels of porosity and air entrainment
in coatings, fully equivalent to the behavior of such
systems with opacifier loadings substantially in excess of
the "CPV" even when materially less opacifier than that
required to attain the CPV is employed. On the other hand,
the deleterious effects on the performance of coatings as a
consequence of the entrainment of air and the introduction
of porosity is substantially completely avoided.
The composites are compatible with a very wide and
broad diversity of paint binders, vehicles, and ancillary
components. The sole limitations are that the formulation
must not dissolve the material of the microspheres (which
may in some cases be assured by treating the microspheres
to assure such a result), and the conditions of the making
up of the formulation must not disrupt the physical
structure of the microspheres. Hence, if any components
require grinding or milling, those steps should be
conducted before the inclusion ~of the microsphere based
composite. The opacifying constituents are quite
insensitive to such matters and do not pose any limitations
on the make up of the formulations into which the composite
is added.
The composite opacifiers of the present invention are
readily incorporated into paints, inks, and other coating
formulations with little effort or difficulty. When the
vehicle of the formulation wets the polyvinylidene chloride
or the opacifying constituent, the composite will be quite
readily dispersed in the formulation with nothing more than
simple mixing. In an aqueous media, of course, the wetting
agents employed in the manufacture of the microspheres will
--23-
13392~
generally remain present, so that dispersion in aqueous
systems is generally quite simple. In other vehicles, it
will ordinarily suffice to wet the composite with a
surfactant or wetting agent appropriate to and compatible
with the vehicle.
In some cases, it may be preferred to formulate the
composite into a base with the coating binder, vehicle, and
other suitable ingredients, designed to be mixed with other
components which are formulated separately, such as color
concentrates, and the like.
The specific gravity of the composite opacifier is
regulated, by determining the relative proportions of
microspheres and components and the degree of expansion of
the microspheres to provide a specific gravity which
approximates that of the coating vehicle. By this feature
of the invention, the tendency of the opacifier components
to separate from the coating formulation is very slight or
non-existent. This is a particular advantage, as those of
ordinary skill in the art will readily recognize.
As a consequence of the high volume of the composite
in relation to the amount of the opacifying power, and the
hiding power of the coating formulation into which it is
incorporated, it has been found that the weight per unit
volume for a given hiding value will be greatly reduced,
and in substantially all cases will be less than the CPV or
"critical pigment volume". While there is no fundamental
reason the CPV of the coating system may not be exceeded,
there will rarely be any benefit in doing so when utilizing
the composite of this invention. Furthermore, some of the
advantage of the present invention, predicated on the
avoidance of the introduction of porosity and the physical
effects on the coating film will be lost by exceeding the
CPV, so that such loading levels will not often be
desirable for economic reasons and for the degradation of
the coating that comes with such procedures. Since weight
is a primary characteristic of paints and coating
formulations, there can be material savings as a
--24--
1339247
consequence of these effects, in reducing shipping and
handling costs, and in affording greater ease of handling
of such materials.
In light of the enhanced performance of the opacifiers
of the present invention when contrasted to the opacifying
component as used alone in the prior art, it is often
possible to reduce the amount of such component by a
material proportion, often as little as half or less of the
amount formerly required being effective to attain the same
opacity and hiding power. It is also frequently possible
to employ a less expensive grade of opacifier material, or
even a different material of lower opacifying capacity, as
a consequence of the enhanced effectiveness of the
composite form. Since the opacifiers are, as a general
rule, the single greatest cost component of paints and
other coating formulations, the savings made possible by
the present invention can be quite substantial. The
inclusion of the microsphere component is a quite minor and
modest cost factor in relation to the savings made possible
by the reduction of the titanium dioxide, say, by one half
or thereabouts, or the substitution, in whole or in
material part, of inexpensive talc for titanium dioxide.
It is of course necessary to take appropriate steps to
adapt the composite to the specific formulation into which
it is to be formulated. As those of ordinary skill in the
art will readily recognize, the polymer of the microsphere
component must not be soluble in the formulation or any
component therein. For most common paint vehicles, this is
a readily attained objective.
MODES FOR CARRYING OUT THE INVENTION
Te~t 1.
As already noted, the use of the composite opacifier-
microspheres in paints and other coatings has given greater
than predicted opacity. This synergistic effect may be due
to the light scattering power of the microspheres
themselves, the dilution effect of titanium dioxide (Above
ten volume percent titanium dioxide the efficiency of
--25--
13392~7
hiding is drastically reduced), or the unexpected synergism
of titanium dioxide attached to the microspheres, or a
combination of all or any of these effects.
In order to obtain a quantitative measure of the
synergistic effect, the film scattering coefficient of
several samples were first determined, and then these
values contrasted to those predicted so that the percent
enhancement in opacity could be calculated. The film
scattering coefficient of seven test samples were
determined using the procedure described in the Official
Digest, September 1963, pp. 871-911, P. B. Mitton and A. E.
Jacobson, giving a Kubelka-Munk scattering coefficient in
mil~l. Table II below lists the values for the scattering
coefficients for the seven samples tested.
TABLE II
___________________________________________________________
FILM SCATTERING CO~clCIENT (Mil-l)
OF SEVEN TEST SAMPLES
___________________________________________________________
Scattering Titanium Dioxide Microspheres
Coefficielnt Volume Fraction Volume Fraction
(Mil _)
1 4.52 0.120 0
2 8.34 0.215 0
3 8.78 0.291 0
4 1.76 ~ 0.350
1.60 0 0.410
6 7.34 0.075 0.405
7 9.01 0.070 0.520
From Table II can be seen the decreasing efficiency of
titanium dioxide as the volume fraction increases. A
similar effect can be seen for volume decreases in
microspheres.
When two scattering particles are present, the
contribution of each particle should in theory depend on
the volume fraction of each multiplied by the scattering
coefficient. The equation for this is S = SlVl + S2V2,
where S is the scattering coefficient and V is the volume
fraction present. Using this equation one can calculate
the individual scattering contributions of each constituent
--26--
13~9~7
and the theoretical total. By contrasting this total to
the above values, any synergistic effect can be determined.
The film scattering coefficients for titanium dioxide and
for microspheres was determined over a volume percent of
0.05 to 0.60 to be 38 and 6 mil~1 respectively. For the
sample 6 composite in Table II, S = (0.075)(38) +
(0.405)(6) = 2.85 + 2.43 = 5.28. On the other hand, the
value measured as shown in Table II was 7.35. Therefore,
the synergistic effect was 7.35 - 5.28 = 2.07.
Accordingly, the percent enhancement in opacity was
(2.07/2.85)100% = 73%, while the total enhancement ratio
over that of titanium dioxide was (7.35/2.85)100% = 258%.
Similarly for sample 7 composite in Table II, the
calculations show: S =(0.07)(38) + (0.52)(6) = 2.66 + 3.12
= 5.78 mil~1 However, the measured value was 9.01.
Therefore, the enhancement in coefficient was 9.01 - 5.78 =
3.23. Accordingly, the enhancement in opacity was
(3.23/2.66)100% = 121%, while the total enhancement ratio
over that of titanium dioxide was (9.01/2.66)100% = 399%.
TeQt 2.
The lower value for the scattering coefficients for
microspheres and the inventive composites with microspheres
in contrast to titanium dioxide, requires that a thicker
film be used to obtain 98% hiding power typically required
for paint films and other coatings. A contrast ratio of
.98 is required where the contrast ratio is defined as the
ratio of the reflectance over a black/white Leneta chart.
Judd and Wyszecki in their book Color in Business, Science
and Industry, Wiley, N.Y, N.Y., 1963, defined equations
which allow film thicknesses, X, to be calculated if the
film scattering coefficients, S, are known, where film
reflectivity R = 94% and substrate reflectivity Rs = 80%.
Rearranging the book's equation for ease of
calculating gives the following equation:
X =(R -Rs)/S(l-Rs)(l-Ro)
The Kubelka-Munk equation was derived with terms that
quantize the absorption of incident energy. The average of
1339247
visible light, 0.55 microns, was used for this equation.
Titanium dioxide and microspheres do not absorb energy at
this wave length. Terms relating to the absorption of
energy in the original Kubelka-Munk equation were
eliminated to permit use of the simplified equation shown
above. Table III below provides film thicknesses required
for 94% hiding when the scattering coefficient is known.
These data are for film reflectivity of 94%, substrate
reflectivity of 80%.
TABLE III
___________________________________________________________
FILM THI~KN~S-S FOR
98% HIDING POWER
___________________________________________________________
Scattering Film Thickness Material Volume
Coeffic~ent (mils) Fraction
(Mil _)
38 0.61 TiO2 0.50
6 3.89 MS 0.50
7.34 1.59 Sample 6 Table II
9.01 1.29 Sample 7 Table II
________________________
Different film thickness would be obtained with different
reflectivities.
Te~t 3.
In another study, a 50PVC paint was made using the
composite opacifier-microspheres. (50PVC O/MS) and compared
to a 50PVC prior art paint using only titanium dioxide
(50PVC TiO2). The composite opacifier-microspheres,
identified as ~M6018~', was a composite of titanium
dioxide/microspheres at a ratio of 95.46/4.54. Table IV
below shows how they compared.
--28-
133~2~
TABLE IV
___________________________________________________________
Grind 50PVC O/MS 50PVC TiO~
Propylene Glycol 8.1 5.8
Tamol 850 0.9 1.9
Colloid 643 0.28 0.2
TiO~ 43.6
M60~8 (O/MS) 23.7
H2O 19.2 6.93
SB8208 0.017
M73 0.003
Texanol 1.8
Letdown
Rhoplex E1953 38.7 29.3
H2O 4.1
Texanol 0.8
Nuosept 95 0.1 0.08
Triton GR-7M 0.1 0.08
Thirl-.-n~r
H2O 6.9 7.1
Natrosol 250 MHBR 0.2 0.2
100.00 100.00
Hiding Power 1 mil dry 85 97
Volume Solids % 35 35
Density Opacifier gm/cc 1 4.2
Thickness for 98%
Hiding Power 1.90 1.0
___________________________________________________________
Resistance to loss of hiding and opacity was measured
with water. The dry film, about 2 mils dry, was applied to
a Leneta chart. After three days the contrast ratio was
98~. The samples were then covered with water and a watch
glass was placed over the water to prevent evaporation.
After 24 hours, the watch glass was removed and the hiding
was measured. A reading ratio of 98% indicated no loss of
hiding or opacity as is experienced with coatings that have
"dry" hiding, where the critical pigment volume has been
exceeded and where air is present. Replacement of air by
water causes loss of hiding in other paint films.
--29-
133 9 2 ~r~
Example 1 - Alkyd Paint.
An alkyd paint was made using the M6018 composite of
this invention as follows:
M6018 24.0%
Soya Alkyd Resin 28.8%
Drier 00.4%
Mineral Spirits 46.8%
100 . 0%
This was a solvent based paint, 40 PVC. 98% hiding
required a coating of about 2.6 mils ~ry.
Example 2 - Epoxy Coating With Activator.
A coating mixture was prepared as follows:
M6018 20.0%
Polyamid Resin 20.0%
Urea Resin 00.6%
Mineral Spirits 46.2%
Glycol Ether 13.2%
100 . 0%
The activator was prepared as follows:
Epoxy Resin 61.2%
Propylene Glycol Ether 38.8%
100 . 0%
Equal parts of the coating and the activator were mixed
thoroughly and allowed to pre-react for one hour. A
coating was applied by brush to a dry thickness of about 4
mils. Hiding was 98% after a cure for three days.
--30--
