U.S. patent number RE34,145 [Application Number 07/581,545] was granted by the patent office on 1992-12-15 for encapsulating finely divided solid particles in stable suspensions.
This patent grant is currently assigned to Union Carbide Chemicals & Plastics Technology Corporation. Invention is credited to Robert W. Martin.
United States Patent |
RE34,145 |
Martin |
December 15, 1992 |
Encapsulating finely divided solid particles in stable
suspensions
Abstract
Finely divided water insoluble solid particles free of ionic
charges and ranging in size from about 0.01 to several hundred
microns or higher, including but not limited to paint pigment
particles, are given a generally uniform polymeric encapsulation by
admixing such particles in an aqueous reaction medium with a water
insoluble monomer polymerizable to form a generally water insoluble
polymer free of ionic charges in the presence of a nonionic surface
active stabilizing agent, preferably a polyethoxylated alkyl phenol
containing at least about eight carbon atoms in the alkyl group
thereof and preferably at least about 40-50 ethylene oxide groups
per molecule, and polymerization of the monomer is then initiated,
usually with heating, with a redox polymerization initiating system
which is free of ionic groups and does not decompose to release
ionic groups in the reaction medium. Naturally agglomerated
particulate materials are effectively dispersed in situ during
polymerization, eliminating the necessity for preliminary grinding
and/or dispersion treatments. Monomers generally useful for
emulsion polymerization and free of ionic groups are effective and
reaction conditions are generally the same as employed in emulsion
polymerization. The polymerization product is a suspension of
generally discrete particles enveloped within a polymeric coating
which exhibits remarkable stability against flocculation or
settling. White paint pigments, e.g., titanium dioxide,
encapsulated in this manner exhibit greatly increased hiding power,
while colored pigments exhibit greater brilliance and depth of
color in dried films, and the film in either case has much improved
abrasion or scrub resistance, and much improved stain resistance
due to reduced porosity, all compared to equivalent conventional
latex paints.
Inventors: |
Martin; Robert W. (S.
Charleston, WV) |
Assignee: |
Union Carbide Chemicals &
Plastics Technology Corporation (Danbury, CT)
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Family
ID: |
27541520 |
Appl.
No.: |
07/581,545 |
Filed: |
September 12, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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758850 |
Jul 26, 1985 |
4608401 |
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583818 |
Feb 27, 1984 |
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516859 |
Jul 25, 1983 |
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414392 |
Sep 2, 1982 |
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Reissue of: |
899207 |
Aug 22, 1986 |
04771086 |
Sep 13, 1988 |
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Current U.S.
Class: |
523/205; 523/137;
524/762; 524/832; 523/200; 523/209; 524/413; 524/497; 524/733;
524/760; 524/766; 524/767; 524/780; 524/785; 524/808 |
Current CPC
Class: |
C09D
7/62 (20180101); B82Y 30/00 (20130101); C08F
292/00 (20130101); C08K 9/10 (20130101); C01P
2004/32 (20130101); C08K 3/22 (20130101); C01P
2006/60 (20130101); C01P 2006/12 (20130101); C01P
2002/85 (20130101); C01P 2004/62 (20130101); C01P
2004/61 (20130101); C01P 2004/51 (20130101); C01P
2004/50 (20130101); C08K 3/34 (20130101); C01P
2004/64 (20130101); C08K 9/08 (20130101); C01P
2004/03 (20130101); C01P 2006/22 (20130101) |
Current International
Class: |
C08F
292/00 (20060101); C08K 9/00 (20060101); C08K
9/10 (20060101); C09D 7/12 (20060101); C08K
009/04 (); C08K 003/10 (); C08K 009/10 () |
Field of
Search: |
;523/205 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0603430 |
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Aug 1960 |
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CA |
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714113 |
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Jul 1965 |
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740823 |
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Aug 1966 |
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CA |
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829842 |
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Dec 1969 |
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CA |
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829843 |
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Dec 1969 |
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CA |
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0899723 |
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May 1972 |
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CA |
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7514188 |
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Dec 1981 |
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DE |
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1330500 |
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May 1963 |
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FR |
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0773325 |
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Apr 1985 |
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ZA |
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1005434 |
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Sep 1965 |
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GB |
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1117224 |
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Jun 1968 |
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GB |
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1506236 |
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Apr 1978 |
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GB |
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1536443 |
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Dec 1978 |
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GB |
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1537986 |
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Jan 1979 |
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GB |
|
2057457 |
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Apr 1981 |
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GB |
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Primary Examiner: Michl; Paul R.
Assistant Examiner: Merriam; Andrew E. C.
Attorney, Agent or Firm: Hegedus; Sharon H.
Parent Case Text
This application is a divisional application of application Ser.
No. 758,850, filed Jul. 26, 1985, now U.S. Pat. No. 4,608,401,
which is a continuation-in-part application of application Ser. No.
583,818, filed Feb. 27, 1984, now abandoned, which is a
continuation-in-part application of Ser. No. 516,859, filed July
25, 1983, now abandoned, which is a continuation-in-part
application of Ser. No. 414,392, filed Sept. 2, 1982, now
abandoned.
Claims
What is claimed is:
1. A stable aqueous suspension of discrete finely divided solid
particles each encapsulated, under agitation less than high shear
mixing, within an envelope of a water-insoluble addition polymer
which is substantially free of ionically charged groups, the shape
of said encapsulated particles generally conforming to the shape of
the starting particles and the size of said encapsulated particles
.[.being determined by the starting solid partciles.]. being
determined by the starting solid particles plus the polymer layer
formed thereon, said polymer layer being substantially uniform
.Iadd.in thickness .Iaddend.over the entirety of the surface of
said solid particles, .Iadd.said .Iaddend.aqueous suspension being
stabilized by a nonionic, surface active, stabilizing agent, said
suspension being free of anionic or cationic surface active or
dispersing agents, having high hiding power and being resistant to
settling.
2. Stable aqueous suspension as claimed in claim 1, wherein said
finely divided particles are titanium dioxide particles.
3. Stable aqueous suspension as claimed in claim .[.1.].
.Iadd.12.Iaddend., wherein said finely divided particles are clay
particles.
4. Stable aqueous suspension as claimed in claim .[.1.].
.Iadd.12.Iaddend., wherein said finely divided particles are talc
particles.
5. Stable aqueous suspension as claimed in claim 1, wherein said
polymer is a copolymer of vinyl acetate and n-butyl acrylate.
6. Stable aqueous suspension as claimed in claim 1, wherein said
polymer is a vinyl acetate polymer.
7. Stable aqueous suspension as claimed in claim 1, wherein said
.[.polymer is.]. .Iadd.particles are within an envelope comprising
.Iaddend.an N-vinyl-2-pyrrolidone polymer .Iadd.layer and a
water-insoluble polymer layer.Iaddend..
8. Stable aqueous suspension as claimed in claim 1, wherein said
polymer is a copolymer of vinyl acetate and dibutyl maleate.
9. Stable aqueous suspension as claimed in claim 1, wherein said
polymer envelope comprises an N-vinyl-2-pyrrolidone polymer layer
and a layer of a copolymer of vinyl acetate and n-butyl
acrylate.
10. Stable aqueous suspension as claimed in claim 1, wherein said
polymer envelope comprises a polymer of N-vinyl-2-pyrrolidone
polymer layer and a layer of a copolymer of vinyl acetate and
dibutyl maleate.
11. Stable aqueous suspension as claimed in claim 1, wherein said
polymer envelope comprises a vinyl acetate polymer layer and a
layer of a copolymer of vinyl acetate and n-butyl acrylate.
.Iadd.12. A stable aqueous suspension of discrete finely divided
solid particles each encapsulated, under agitation less than high
shear mixing, within an envelope of a water-insoluble addition
polymer which is substantially free of ionically charged groups,
the shape of said encapsulated particles generally conforming to
the shape of the starting particles and the size of said
encapsulated particles being determined by the starting solid
particles plus the polymer layer formed thereon, said polymer layer
being substantially uniform in thickness over the entirety of the
surface of said solid particles, said aqueous suspension being
stabilized by a nonionic, surface active, stabilizing agent, said
suspension being free of anionic or cationic surface active or
dispersing agents, said suspension being substantially free of
polymer particles containing none of said finely divided solids
particles, and being resistant to settling.
Description
1. FIELD OF THE INVENTION
This invention relates to the encapsulation with polymer of finely
divided solid particles and is concerned more particularly with a
direct and simply executed method for applying a polymeric envelope
or coating to finely divided solid particles, especially pigment
particles utilized in paints and similar opacified coating
compositions.
2. BACKGROUND OF THE INVENTION
There exists a great need in the art for a simple, uncomplicated
and readily executed procedure for applying a polymer coating or
envelope around finely divided solid particles. While such a
technique would be valuable in a variety of fields, its value is
especially strong in the field of paint and similar coating
formulations. Paint formulations separate basically into two types:
The oil-based paint where the polymeric or resinous binder is
dissolved in an organic solvent as a continuous phase with the
pigment particles dispersed therein as discrete particles and a
latex-based paint wherein the polymer or resinous binder exists as
a dispersed phase or latex separately prepared by emulsion
polymerization within an aqueous medium and paint pigment particles
are dispersed within the aqueous medium as a dispersed phase
separate and independent from the binder latex phase. Both
formulations would, of course, typically contain a variety of other
additives for various purposes, mostly unrelated to this invention.
The latex-based paint has the advantage of lower costs and better
odor since solvents are absent, and easy clean up by simple water
washing instead of organic solvents, and it is in the context of
latex-based paints that the present improvement finds especially
advantageous application.
Although modern paints have been much improved in stability against
settling or flocculation of dispersed material therein by means of
various stabilizing additives and advanced dispersing techniques,
latex paints are inherently subject to setling and flocculation
with consequential undesirable effects upon their properties. For
example, virtually any latex paint will undergo separation of the
dispersed phase from the continuous phase if subjected to
centrifugation even for a relatively short time. Such
centrifugation represents an artifically exaggerated condition
accelerating the unsuitable effects of gravity over longer periods
of time. Flocculation and settling of the pigment phase are
particularly undesirable since they lead to the clinging together
of the packed pigment particles into agglomerates or clusters that
tend to resist subsequent redispersion by agitation and degrade the
hiding power of the resultant paint.
The opacifying capability or hiding power of a latex paint, or for
that matter virtually any paint depends mainly upon three factors.
First, light absorption due to the inherent coloration of the
pigment particles, which is of minor importance, and then primarily
only for tinted or nonwhite paints; second, light refractance which
is fixed for any given combination of binder and pigment; and
third, light reflection and dispersion or scattering by the
surfaces of the pigment particles in the eventual solidified paint
film. Every latex paint has a minimum filming temperature,
characteristic of its particular binder and possibly other
constituents, which is the minimum ambient temperature at which the
discrete latex particles or globules in a film thereof will
colaesce together during drying so as to result in a solid binder
film. The binder film is in itself essentially colorless, or water
white in color, and serves then as the vehicle for holding the
pigment particles dispersed therethrough. The net hiding power is
determined particularly in a white paint essentially by the
scattering effectiveness of the thus dispersed pigment layer which
is in turn highly influenced by the regularity of the arrangement
of the pigment particles in the layer as well as the regularity of
the particle sizes themselves. The latter can to some extent be
contained within acceptable limits by proper control of grinding
and dispersing techniques but the former is dependent virtually
entirely upon the relative disposition of the myriad pigment
particles throughout the solidified binder layer. If the pigment
particles are uniformly spaced apart an ideal distance, their light
scattering power will be optimum as will be the hiding coverage of
the paint. On the other hand, if the particle spacing is irregular
and if pigment agglomerates are present that deviate considerably
from the desired uniform pigment size, the light scattering will be
degraded as will the hiding power. Various ways have been attempted
in the paint field to achieve an optimum physical disposition of
the pigment particles within a paint film including the use of
so-called extender pigments which essentially function as
mechanical spacing elements for the opacifying pigment particles so
as to thereby produce a uniformly and properly spaced pigment layer
with optimum light scattering and hiding power. However,
flocculation forces are particularly acute during colaescence of
the binder latex upon drying, promoting the creation of irregular
clumps and clusters despite the presence of extender pigments.
It will be apparent that if a solid polymeric coating or envelope
could be applied with controllable generally uniform thickness
around discrete separate particles of a finely divided solid, such
as a paint pigment, such an envelope could act to precisely
determine the spacing between contiguous particles in the ultimate
paint film, provided that a dispersion of such uniformly enveloped
particles was stable against settling, reagglomeration or
coalescence of contiguous coated particles. Such coated particles
would for purposes of paint formulation offer further advantages of
great importance. For example, deterioration in color, particularly
of tinted paints is largely caused by the attack of light and air
upon the pigment particles within the dried latex film which
normally exhibits some degree of porosity as to allow access of
atmosphere to the pigment particles it contains. If, on the other
hand, the pigment particles were completely enveloped within a
continuous polymeric coating, they would be shielded against
contact with atmospheric air and at least to some degree protected
against the photolytic effect of sunlight due to the reflecting
quality of the polymer coating. Thus, not only would a latex paint,
wherein at least a significant portion of the pigment component
thereof was united with at least a significant portion of the
binder phase into a composite dispersed phase in which each
particle formed a core within a uniform polymeric envelope, offer
the advantage of substantially enhanced hiding or covering power,
that power would be retained for a substantially longer period of
time than with conventional latex paints.
Again, conventional latex paint films rarely exhibit the "scrub"
resistance, i.e., resistance to abrasion, of a good oil-base paint
since the necessity for escape of water from the film during drying
creates inevitable channels or pores therethrough which constitute
points of structural weakness and the coalescence and coagulation
of the latex phase particles cannot lead to proper envelopment of
the pigment particles without voids and spaces therebetween. If, on
the other hand, each individual pigment particle were completely
enclosed within an envelope of the binder polymer, total
integration of the pigment into the dried paint film results upon
drying with greatly increased scrub and abrasion resistance.
Furthermore, it is commonly recognized that conventional latex
paint films are subject to staining, both in the sense of absorbing
extraneous colored matter from the environment as well as allowing
chalking or the migration of pigment particles onto adjacent
unpainted surfaces, such as housing foundations. Staining is due
primarily to the porosity of the film which allows the pigment
particles to absorb extraneous colored matter while chalking is
caused in part by porosity. If, however, the pigment particles were
totally enveloped in a polymeric coating, solid paint film
containing the same would necessarily exhibit very substantially
reduced staining and chalking tendencies.
In addition, one of the most difficult operations in paint
formulation is the effective dispersion of the pigment particles
into the paint system, requiring expensive complex grinding and
milling equipment together with a formulary of dispersant additives
and stabilizers which add significantly to the overall cost of
making paint. If it were possible to prepare pigmented dispersions;
particularly already carrying polymeric envelopes, without the
necessity for such elaborate and prolonged treatment, there would
result a substantial decrease in the cost of paint manufacture and
thus the ultimate cost to the paint consumer.
With these compelling advantages so clearly foreseeable, it is not
surprising that many attempts have been made in the art to develop
techniques for the polymeric encapsulation of finely divided solids
such as paint pigments. The following prior art may be mentioned to
illustrate such attempts:
U.S. Pat. No. 3,068,185--preliminary treatment of clay particles to
sorb on at least the surfaces thereof free radical generating
addition polymerization initiating agent, e.g., by exposing the
clay under vacuum to a gaseous atmosphere containing the initiating
agent, followed by admixture to a water suspension of the thus
treated clay particles at least one addition polymerizable
unsaturated monomer in amount up to about 30% by weight of the clay
and heating the mixture to effect polymerization of the
monomer;
Canadian Pat. No. 714,113--mixing a pigment, water and cationic
surface active agent to render the pigment hydrophobic, plus an
organic phase containing a polymerizable monomer, to cause transfer
of the pigment from the water to the monomer phase, and then
effecting the polymerization of the pigment-containing monomer
while dispersed in an aqueous medium;
U.S. Pat. No. 3,544,500--water soluble polymer is preliminarily
absorbed on the surface of solid particles which polymer either
includes hydrophilic polymeric chains, e.g., in grafted form, or
has associated therewith a surface active agent having one end
adapted to be anchored to the adsorbed prepolymer layer with the
other end providing a steric stabilizing effect around the
particles, then a monomer which is a swelling agent or solvent for
the preabsorbed polymer is added and caused to undergo
polymerization;
U.S. Pat. No. 3,714,102--a cationic charge is established on an
aqueous dispersion of finely divided solid particles by
acidification of the medium and adsorption of multivalent aluminum
cations from a compound releasing such cations present in amount to
decrease the viscosity of the dispersion, then a polymerizable
vinyl monomer is added and caused to undergo polymerization with
the aid of a free radical polymerization initiator, the weight
ratio of total monomer to solid not exceeding about 2.5:1;
U.S. Pat. No. 4,421,660--inorganic particles, e.g., pigment
particles, are passed through a high shear mixing device, e.g., a
Waring blender, homogenizer or ultrasonic mixer (col. 4, lines
38-43, Examples 1-6) and monomer is polymerized in the resulting
dispersion forming a very low percent solids latex (less than 30%,
as low as 7.4%) which is not suitable for paints or adhesives
unless concentrated, e.g., by vacuum distillation. The resulting
latexes also contain substantial amounts of particles having an
average particle diameter which is only a fraction of the average
particle diameter of the pigment thus indicating the presence of
substantial amounts of pigment-less polymer particles. There is no
disclosure or suggestion in the Solc patent of the use of
substantially nonionic polymerization conditions, including the use
of nonionic surfactants, particles (e.g., pigment) substantially
free of ionic charges, monomer capable of forming a polymer free of
ionically charged groups, and an initiating agent which is free of
strong anionic groups and does not decompose to form such strong
anionic groups. By contrast, the present invention does not require
high shear mixing, produces high percent solids latexes, produces
latexes containing predominantly polymer coated pigment particles
with little or no pigment-less polymer particles, and utilizes
substantially nonionic conditions at least in the early and
intermediate stages of polymerization.
While each of these techniques might conceivably have accomplished
their intended purpose, it is obvious that none of them is well
suited for execution on a commercial scale. In particular, those
techniques which involve the generation of cationic charges on the
finely divided particles create serious practical difficulties. The
resultant latexes cannot, for example, be mixed with conventional
latex paints since if so combined, either deliberately or
accidentally, flocculation can result as a consequence of the
anionic nature of the conventional latex, dependent upon the
relative degree of polarity of the two latexes. This problem can
frequently be avoided by very careful mixing of the two latexes but
in this case, the water sensitivity of the ultimate dried film will
suffer. Also, cationic paint systems even when dried cause rusting
of ferrous materials in contact therewith which precludes the
application of such coatings over exterior surfaces of iron or even
having exposed nails or other iron fasteners since otherwise rust
would quickly develop. It, therefore, is perhaps not surprising
that insofar as I am aware up to the present time, the goal of a
simple, direct and effective polymer encapsulation technique for
solid particles remains an elusive one for the paint and other
industries.
GENERAL SUMMARY OF THE INVENTION
In accordance with the invention, a water insoluble monomer
polymerizable by appropriate initiation to a water insoluble
polymer free of ionic groups, normally selected from among the
vinyl type monomers, is added with mixing to an aqueous suspension
medium in the presence of a nonionic surface active stabilizing
agent, the medium containing or having then added thereto with
mixing finely divided solid particles, e.g., a paint pigment free
of surface charges thereon. A water soluble initiating agent is
present in or added to the aqueous medium to initiate the
polymerization of the monomer, preferably in gradual steps to
maintain the polymerization rate at a manageable level until the
monomer has been reacted. The novel latex-forming system of this
invention, must be substantially nonionic, that is, it must be
substantially free of strong anions or cations in the early and
intermediate stages of polymerization, although such strong ionic
groups can be added or formed during the final stages of
polymerization or subsequent to completion of polymerization.
Although the behavior of the present system has not been fully
rationalized, it appears that the nonionic surface active agent,
consisting of strongly hydrophilic and hydrophobic groups, has its
hydrophobic groups sufficiently strongly repelled by the aqueous
medium as to become deposited upon or adsorbed superficially by the
solid particles even though the latter are not necessarily or even
particularly considered hydrophobic in the usual sense, while the
hydrophilic groups extend into the surrounding margins of the
aqueous medium. Given an adequate amount of surface active
stabilizing agent present, the solid particles become surrounded by
a sheath of oriented molecules of such agent, and it appears to be
important to the achievement of good results that this sheath be
essentially continuous over the particles surface and, furthermore,
have substantially uniform thickness either as a monomolecular
layer or as a multimolecular layer. For reasons that are not
understood, but which again appear to involve preferential
repulsion of the water insoluble monomer by the aqueous medium,
monomer becomes attracted to and deposited as a layer upon the
surfactant-sheathed solid particles, notwithstanding the
hydrophilic nature of the sheath possibly by de-sorbing the
surfactant molecules for migration to the interface of the monomer
layer and the medium.
Upon initiation of the polymerization reaction, the deposited
monomer begins to polymerize. As a consequence of this initial
polymerization, and this is one of the remarkable effects observed
in the present invention, flocculates or agglomerates in the
original solid particles, which are impossible to avoid in
practice, particularly since the solids used herein need not be
thoroughly dispersed in the usual manner, become immediately and
remarkably broken up and uniformly dispersed. This is believed due
to generation of heat by the polymerization, which is an exothermic
reaction, localized at the surface of the particles which in effect
"explodes" agglomerates apart into isolated individual particles.
The system contains additional monomer dispersed therein and/or
more monomer is added, and as the polymerization proceeds by
further initiation, monomer migrates from the monomer particles
onto the polymer coated solid particles with consequential growth
of the polymer layer. Given solid particles relatively free of
surface contaminants, the thus created polymer layer is
surprisingly uniform over the entirety of the particle surface,
following closely the contours of that surface and such uniformity
can be more or less preserved during subsequent growth of the
polymeric envelope, the ultimate thickness of which will be
selected primarily according to the particular application of the
polymer encapsulated particles.
DETAILED DESCRIPTION OF SEPARATE ASPECTS OF THE INVENTION
Reaction Medium
It is essential that the reaction medium be substantially aqueous
in character in the sense of retaining substantial polar character
relative to the nonpolarity of the polymerizable monomeric
component present. Generally in practice, this will mean the
selection of an entirely aqueous reaction medium which preferably,
as is well known in the art of emulsion polymerization generally,
is deionized so as to be free of ions of metals and other
contaminants which could lead to undersirable consequences. It
would presumably be acceptable, depending upon the hydrophobic
strength of the monomeric component, to include minor amounts of
watermiscible organic liquids, such as the alcohols, particularly
the lower alcohols, provided the medium remains strongly
hydrophilic relatively to the hydrophobic monomeric component, but
such alcohols act as chain transfer agents terminating the
polymerization and yielding lower molecular weight polymers. With
aqueous media free of such diluents, high molecular weight polymer
formation is promoted which is normally advantageous.
Monomeric Component
Broadly speaking, virtually any monomer capable of undergoing
addition polymerization in emulsion form to produce a polymer free
of ionically charged groups is at least in principle useful in the
present invention, but for virtually all practical purposes, the
present monomers are selected from among the so-called vinyl
monomers, including vinylidene and acrylate monomers, which are
substantially water insoluble and which polymerize to substantially
water insoluble polymers free of ionic groups. The requirements of
water insolubility does not dictate absolute insolubility in water
since useful monomers, and indeed some preferred monomers are
characterized by a low degree of water solubility say up to about
3% or so. Typical useful monomers thus include the following, to
mention only a few; vinyl monomers, such as vinyl acetate, vinyl
chloride, vinylidene chloride, acrylonitrile, methacrylonitrile,
vinylidene cyanide, styrene, alpha-methyl-styrene, vinyl benzene,
isobutylene, vinyl toluene, and divinyl benzene plus various vinyl
ethers and ketones, the acrylic and methacrylic esters such as
methyl acrylate, n-butyl acrylate, isobutyl acrylate, methyl
methacrylate, ethyl methacrylate, n-butyl and isobutyl methacrylate
and olefins, such as ethylene, propylene, butene, 1-hexane and
butadiene, etc. Monomers such as acrylic or methacrylic acid which
give polymers giving ionic groups, i.e., carboxyl groups, are not
acceptable for this invention. Combinations of plural monomers are
entirely permissible or even desirable so as to form co- or
terpolymers which by virtue of the combination of proportions of
monomers conferring significantly different properties, make
possible a tailoring of the overall properties for the polymeric
coating between the extremes represented by the individual
components themselves. Different monomers can be polymerized in
superposed layers, following the known core and shell principle, so
as to again achieve special effects such as, for example, the
provision of hard, thin exterior shell around a soft, thick
polymeric layer which might have peculiar value for adhesive
purposes.
The selection of particular monomers will depend upon the end use
of the ultimate encapsulated material and, obviously, not all of
the monomers listed above will be equally suitable for all end
uses. For example, styrene is unsuitable as the sole binder for a
paint composition, as is methylmethacrylate under most conditions,
due to excessive hardness and brittleness causing a film thereof to
crack and break apart. However, styrene is useful for other
purposes where hardness is a virtue, and styrene in any case is
very desirable for copolymerization with one or more monomers
forming softer polymers. Similarly, some of the monomers mentioned
above form polymers which are relatively soft such as vinylidene
chloride and n-butylacrylate and would not, therefore, be well
adapted as a paint binder unless copolymerized with other harder
monomers such as styrene or methylmethacrylate. On the other hand,
for purposes of an adhesive, the softer monomers might be more
desirable.
The monomers can, of course, be substituted with a variety of
substituents including any substituents known with monomers
employed in conventional emulsion polymerization which do not
introduce ionic groups or interfere with the polymerization
mechanism.
Monomers that form water soluble polymers are not useful here as
the exclusive monomeric component or, alternatively, as the
exterior component of a multi-component system since such polymers,
although they appear to deposit upon certain types of particles but
not upon all types, lead to flocculation and agglomeration of the
thus coated particles which tend to stick together due to the
sticky water-swollen coating thereon. However, water soluble
polymers can be applied if followed by the application of a
compatible polymer coating which is sufficiently water insoluble.
However, up to now, no significant benefit has been found in the
preliminary application of a water soluble polymer since it appears
neither to facilitate the envelopment of difficulty treatable
particulate solids nor to promote the subsequent deposition of the
exterior water insoluble polymeric component. The only reason now
perceived for the possible inclusion of a water soluble monomer
would be with other monomers as part of a co- or terpolymeric
system having water insoluble properties, functioning in such a
system, for example, as a coupling unit between otherwise
incompatible monomeric components.
The Finely Divided Solid Particles
As presently understood, virtually any solid particulate matter,
including the usual variety of natural and synthetic pigment
materials, qualify for treatment by the present method, provided
that such particles are free of significant levels of ionic charge,
either anionic or cationic, existing either from their structure or
generated during their preparation and handling through electrolyte
additions. Particles which are charged have been found not to
participate in the present encapsulation mechanism but, in fact, to
severely inhibit the same, resulting in virtually immediate
flocculation of the entire solids into mass resembling cottage
cheese or worse. This strong inhibiting action has been found to
occur, for example, in the attempted treatment of pigment particles
subjected to preliminarly dispersion in the usual way with
conventional strongly anionic dispersing aids, e.g.,
polyphosphates, or when an anionically charged agent, such as a
strongly anionic surface active stabilizing agent of the type such
as alkali metal alkyl sulfates commonly utilized in conventional
emulsion polymerization, is added to or substituted for the
nonionic stabilizing agent of the present invention.
Titanium dioxide is of special importance in the paint field for
the production of white paints, either for use as such or as the
base for subsequent tinting, and which constitutes a major volume
of commercial paint production. As manufactured, titanium dioxide
exhibits certain undersirable properties from the standpoint of its
utilization in conventional paint manufacture, being difficult to
maintain in suspension and susceptible to excessive chalking when
used in exterior finishes and, consequently, through the years
titanium dioxide manufacturers have developed surface treatments
for their product to improve these and other properties. Such
treatments include the addition to the regular titanium dioxide of
metal and metaloid compounds such as aluminum oxide silicon
dioxide, and zinc oxide to mention the most common, and such
treatment compounds can be incorporated in varying amounts up to
about 20% and in various ways so as to suit the needs of particular
paints destined for particular purposes. Many of these treatments
are proprietary in nature and precise descriptive information
concerning the same is difficult to obtain, the commercial grades
of the treated product usually being merely designated by its
content of titanium dioxide with a general identification of the
additive.
Paradoxically, these treatments for modifying titanium dioxide to
improve their behavior in conventional paint systems prove to have
quite the opposite consequences in the inventive system, resulting
in inferior encapsulated products. This does not mean that such
treated titanium pigments, even heavily treated ones, cannot in
principle be encapsulated following the steps of the invention but
rather that the resultant encapsulated product exhibits relatively
poor properties compared with other untreated pigments and the
behavior of the treated pigments tends to vary widely from batch to
batch. It appears that these chemical treatments of titanium
dioxide are far from uniform considered either as a whole from
batch to batch, or within a given batch in their effect upon the
surface of the titanium dioxide particles. Thus, the deposition of
the treatment compound upon the titanium dioxide particles is not
uniform, perhaps due to the incomplete initial coverage or
breakdown during grinding of initially covered larger particles
into smaller particles with uncovered areas or other reasons, and
the extent of such nonuniformity with its consequential influence
on the progress of the polymerization reaction during evaporation
is impossible to predict among various batches of presumably
identically treated titanium pigment. If the surface treatment
could be applied uniformly over the titanium pigments or if a
monomeric component, including plural monomers if need be, could be
formed that was equally compatible with the titanium dioxide base
material and the surface treatment material, then encapsulation by
the present method should proceed satisfactorily.
For example, equivalent dried paint films containing equal amounts
of comparably encapsulated treated and untreated titanium dioxide
can show extraordinary differences in hiding power, the values
obtained from treated pigment being 50% less than that obtained
from minimally treated pigment. This behavior can be explained by
unevenness of the polymeric envelope around the treated titanium
pigment particles which results in nonuniform spacing of these
particles in the dried paint film and consequential reduction in
light scattering and hiding power. On the other hand, the benefits
of the invention in obtaining highly uniform dispersion of the
treated pigment particles and of imparting high stability to the
resultant latexes are as evident with the treated materials as with
the untreated ones. Obviously, where a paint is to be formulated,
it becomes advantageous to select a titanium pigment with minimum
chemical treatment, preferably as close to 100% titanium dioxide as
possible. With these preferred pigments, encapsulation according to
this invention makes possible as much as a 50% reduction in the
content of prime pigment to obtain the same degree of hiding power
in the paint produced therefrom.
While for use in white paints, titanium pigments are much
preferred, other pigments such as antimony oxide and zinc oxide
have value for this purpose and can be treated equally well by this
invention. Other pigments which have a relatively low refractive
index compared to that of the usual polymeric binders with little
or no light refraction and consequential weak light scattering
power, such as barytes, whiting, talc, China clay, mica, calcium
carbonate, and the like are often incorporated in paints for other
reasons, such as fillers, extenders, flatting agents,
reinforcement, etc., and can be advantageously encapsulated for
these same effects. Certain pigments have acicular (i.e.
needlelike) or lamellar (i.e. platelike) shapes and thus have poor
hiding power in a paint film but can serve well for other purposes
such as reinforcement as mentioned. Colored pigments, mostly
organic in nature, are entirely suitable for encapsulation and
inasmuch as complete information as to the identities and sources
of the myriad variety of such pigments available in the field is
provided in The Raw Materials Index published periodically by the
National Paint and Coating Association, Inc., Washington, D.C., no
attempt will be made to list examples of this kind of material.
General classes of organic pigments from which specific pigments
can be selected for use here include the following: Insoluble azo,
anthraquinone, Indigoid, phthalocyanine, basic, as well as more
modern types such as those obtained from the du Pont company under
the trade name Monstral. Helpful general information regarding the
selection of specific organic pigments can also be found in Organic
Coating Technology by Payne, Volume 2, John Wiley & Sons, Inc.,
copyright 1961, chapter 20, pages 853 et seq. Obviously, the
selected pigment must be substantially water insoluble and must not
interfere significantly in the polymerization reaction. Some
pigments, particularly those containing alumiunm or iron, such as
iron oxide, may require adjustment of the suspension medium pH to
avoid possible adverse reaction but with such adjustment are
entirely satisfactory. So-called "reactive" pigments such as zinc
oxide, already mentioned, cause no problems when used in this
method.
Nor is the invention limited to materials generally considered to
be pigments; rather it extends to other types of finely divided
particulate matter that may have very different utility. Thus, it
is possible to encapsulate particles of sand, clay, glass beads,
short glass fibers, beads of various metals, such as iron, steel,
brass, titanium, cobalt, nickel, gold, platinum, chromium, zinc,
palladium, silver, ruthenium, platinum, rhodium, or copper or the
like and oxides thereof, to specify just a few. Some particulate
solids, such as calcined clay, zeolites, diatomaceous earths and
the like are characterized by a more or less highly porous
structure providing a high volume of interior spaces. While such
materials can be processed by this invention, during the reaction,
the monomer deposits and polymerizes equally on the interior and
exterior surface areas thereof and the amount of polymer within the
interior areas is wasted as far as making any contribution to the
binding action of the polymer is concerned. In addition, the
primary function of these materials as paint pigment is to increase
dry hiding power due to air held in their pores and as this air is
displaced by the polymer, this function is defeated. Of course, for
other purposes where total coverage of the surface area, but
interior and exterior, of porous materials is advantageous for a
particular end use, for example, in withstanding attack from an
otherwise corrosive environment, porous materials may be quite
useful.
The size of the particulate matter to be encapsulated may vary
widely. Colored organic pigments as a group tend to have extremely
small particle size in the order of 0.01 microns average and can be
processed effectively with the proviso that because of the
enormously increased surface area of extremely small pigment
particles, larger amounts of surfactant and/or monomer will
normally be required in order for the latter to deposit over the
entire surface area of such particles. Indeed, the invention may be
specially suitable for the treatment of such extremely finely
divided matter which because of the enormous surface forces arising
from the increased surface area exhibit a strong natural tendency
toward the formation of agglomerates which tendency is overcome in
the course of the polymerization, as will be explained further.
Larger particles with lesser surface area are easier to put into
good suspension and can hence be processed more easily. As regards
the upper size limit, there appears to be no maximum other than
that imposed by practical considerations; namely, the creation of a
generally uniform suspension within the medium. Thus, particles in
the order of several hundred microns or even larger could be
treated without difficulty. By way of illustration of the typical
size of preferred pigments, titanium pigments usually average about
0.2 microns, calcium carbonate particles are somewhat larger in the
range of 0.5-60 microns, while carbon black runs about 0.01-0.05
microns, and sizes of this magnitude respond equally well to the
present treatment.
To avoid confusion or misunderstanding, it should perhaps be
mentioned that virtually all natural pigment material can be
expected to carry some small amount of charge thereon, especially
when dispersed in water, which would usually be anionic in
character but as is apparent from the above description and the
working examples to follow, such small natural background charge
levels impose no difficulty in the execution of the present
process, and in the above general characteristics of operative
particulate material, the term "significant" is deliberately
employed so as to encompass such small insignificant levels of
background charge while excluding high levels of charge which are
not contemplated within the scope of this invention.
THE NONIONIC STABILIZING AGENT
A critical feature of the present invention is the generation
around the finely divided solid particles suspended in the reaction
medium of a steric barrier or sheath that persists during the
course of polymerization reaction by means of a nonionic
stabilizing agent which adsorbs on the surface of the particles and
does not contain any ionic groups, either cationic or anionic. It
is known in conventional emulsification polymerization as well as
in the suspension of paint pigment particles in the preparation of
paint compositions to create around the particles, either latex or
pigment, an electrostatic barrier constituted by a cloud of
electrostatically charged ions of a given polarity which, due to
the repulsive effect of like charged particles, exerts a dispersing
action on the suspended particles which tends to stabilize the
resultant suspensions. Typical compounds used for this purpose of
pigment are strongly anionic polyelectrolytes, such as sodium
tripolyphosphates or other so-called molecularly dehydrated
phosphates, and for latexes, strongly anionic emulsifying agents,
such as sodium lauryl sulfate. Such stabilization by electrostatic
charges carried by ionic groups is not acceptable in the present
invention, resulting in highly undesirable and strong flocculation
of the polymer and particulate matter together into a more or less
solid mass. For example, if conventional emulsion polymerization
using an anionic emulsifying and stabilizing agent is attempted to
be carried out in the presence of a dispersed phase of solid
particles, the polymerization is essentially like bulk
polymerization, resulting in the solids of the system flocculating
or setting up within a few seconds or less into a mass varying in
consistency from cottage cheese to a lumpy dough to a sticky
plastic, any of which are entirely worthless for any practical
use.
It has been found, quite surprisingly in the light of this
experience, that if the anionic emulsifying and stabilizing agent
is replaced in entirety with a nonionic stabilizing agent of
sufficient hydrophilic-lipophilic power as to possess good
emulsifying action, the added monomer present deposits or is
adsorbed preferentially on the particle surfaces and polymerizes
exclusively upon the surface of the solid particles present, at
least in the absence of a large access of nonionic stabilizing
agent, forming a polymer envelope around the particle surface that
is remarkably uniform in thickness. The result is a suspension of
polymeric encapsulating solid particles having extraordinary
stability against flocculation or settling compared with
conventional latex systems.
The nonionic stabilizing agent of the invention is by definition
free of ionically charged pumps and does not dissociate into such
groups upon solution in the aqueous medium. In order to exert a
sufficiently high degree of dispersing action, it is considered
that this agent needs to have a so-called HLB number of at least
about 13. As is well known in the surfactant field, it is possible
to determine empirically, as well as estimate or approximate by
calculation, the surface active strength of a given agent, which
strength is referred to as the HLB number. To be effective here,
the nonionic stabilizing agent should have an HLB number of at
least about 13 up to 20 or higher. Conceivably nonionic surfactants
with somewhat lower HLB numbers than 13 could be employed with less
efficient results, particularly if their suspending power was
augmented by the addition of a protective colloid or thickening
agent, e.c. polyvinyl alcohol, hydroxyethylcellulose or the like,
which, as is known in the field of emulsions are able to enhance or
strengthen the protective barrier created by surface active agents
around dispersed phase globules or particles, but for most
practical purposes, a minimum HLB number of 13 is indicated so as
to avoid any necessity for a protective colloid.
There exist a variety of nonionic surface active stabilizing agents
which have HLB numbers of about 13 or higher and these can be
identified by resort to any text relating to emulsions: Emulsions:
Theory and Practice by Becher, 2nd edit., Reinhold Publishing
Corp., especially at pages 235-238, or surfactant handbook such as
McCutcheon's Detergents and Emulsifiers. Any nonionic surface
active agent meeting the above criteria would presumably be
suitable for use in the present invention but, as a general rule,
these agents will be polyethoxylated derivatives of various
hydrophobic groups, including poly-ethoxylated esters of fatty
acids, and polyethoxylated ethers of fatty alcohols and
alkyl-substituted phenols or the like having a sufficient carbon
chain length as to impart adequate hydrophobic power, for
adsorption upon the particle surface generally at least about 8 and
preferably 12 or more, the phenyl group being equivalent to a
carbon chain of 4. The number of carbon chains in the hydrophobic
chain can, of course, go much higher to include virtually any of
the available fatty acids and fatty alcohols as the hydrophobic
group.
The number of ethylene oxide (EtO) groups in the polyethylene oxide
chain can vary from about 10 up to 200 or more and it is preferred
to have at least 40 to 50 EtO groups up to about 150. A
particularly preferred stabilizing agent is an octyl- or
nonyl-phenol polyethyloxylate containing 50 to 150 EtO groups.
To illustrate the relationship between the number of EtO groups and
the HLB number, a polyethoxylated nonyl-phenol with 10 EtO groups
has an HLB number of 13, with 50 EtO groups, and HLB number of 18,
and with 100 EtO groups, and HLB number of 19. The analogs of such
alkyl phenol derivatives derived from fatty alcohols or fatty acids
of equivalent carbon number could be substituted with equivalent
effectiveness in the present method. It does not appear to be
required that the hydrophilic chain be constituted exclusively of
EtO groups provided the requisite minimum HLB number is achieved by
the selected surface active agent; however as practical matter
virtually all such agents available with an HLB number of this
magnitude do depend upon ethylene oxide chains for their
hydrophilicity.
It is extremely difficult to specify precisely the amount of the
nonionic surface active stabilizing agent that is needed in the
practice of this method inasmuch as the amount of the agent
required to deposit around a suspended phase of solid particles is
basically determined by both the particle size and the number of
particles present, the multiple of which gives the aggregate
surface area present which has to be covered by adsorbed
stabilizing agent so as to form a steric barrier around each such
particle. For example, for particles varying in mean diameter
merely from 0.007-0.07 micron, which is only a fraction of the size
range possible in the invention, there is a 50-fold variation in
surface area from about 1,000 to about 23 square meters per gram.
It is certain that with the very finely divided pigment particles,
such as carbon blacks or precipitated calcium carbonate with a size
in the order of 0.01 micron, the amount of surface active agent
must be increased considerably if effective stabilizing action is
to be achieved. Broadly speaking, the amount of stabilizing agent
will fall within the range of about 0.5 to about 20%, and more
usually 1-10%, of the total weight of the monomer and solid
particles present, the lower end of the broader range being
applicable to large sized particles, such as fibrous talc, while
the upper end applies to the very fine particles of the type just
mentioned.
Furthermore, the amount of the surface active stabilizing agent is
affected by the existence of an equilibrium concentration of that
agent between the aqueous medium and the dispersed phase. Thus, as
the solids content of the system is varied, appropriate adjustment
needs to be made in the amount of stabilizing agent employed in the
initiation stage so as to achieve an optimum equilibrium
concentration. For example, if the solids content of a given system
is reduced to give a more dilute condition, then more of the
stabilizing agent needs to be present to maintain the needed
equilibrium concentration in the greater volume of suspending
medium.
The ultimate indication or test of a proper armount of nonionic
surface active stabilizing agent is the actual production of a
suspension of polymer-encapsulated solid particles having strong
stability against flocculation or setting. Various instruments and
procedures are known for determining the stability of suspensions
and any of these is in principle suitable here. However, the most
widely accepted test in actual practice is the so-called "hand rub"
test in which a small amount of the suspension is placed in the
palm of one hand and is rubbed or spread out with the fingers of
the other hand in a circular motion. If the emulsion spreads
smoothly and uniformly with a cream-like feeling, it has good
stability; whereas, if it coagulates or flocculates into lumps,
then the suspension is clearly unstable. Once a suspension has been
found to be in a de-stabilized condition, this condition cannot
thereafter be corrected by adding more or less stabilizing agent or
any reaction ingredient. If as preferred the polymerization is
carried out in stages, i.e., an initial stage in which a fraction,
say about 5-10%, of the monomer is caused to undergo monomer are
thereafter fed gradually or step by step together with
corresponding amount of polymerization initiating agent so as to
continue the polymerization, then as a general rule the initiation
stage is the more critical as regards stabilization and a sample of
the reaction product is desirably tested at the end of the
initiation stage. If de-stabilization has already occurred, the
reaction mass must be discarded and a new reaction procedure set up
with appropriate adjustments in the amounts of stabilizing agent,
and possibly other ingredients, as dictated by the results of the
original run and one's experience in the process.
THE POLYMERIZATION INITIATING AGENT
The polymerization reaction involved in the present invention is
initiated and maintained with an initiating agent or catalyst, as
it is sometimes referred to, which is generally similar to those
employed in conventional latex polymerization except that the most
commonly used conventional initiating agent; namely, the
persulfates, are not acceptable for use here. It appears that
persulfate type initiators are either highly anionic or decompose
during the polymerization reaction into decomposition products
which are highly anionic and cause severe inhibition of the
deposition of the polymer upon the pigment particles, resulting in
an uncontrolled polymerization leading to flocculation and gelation
of the entire body of solids. The initiating agent comprises an
oxidant and any of the usual peroxides and hydroperoxides serve
very well including hydrogen peroxide, n-butyl hydrogen peroxide,
cumyl hydrogen peroxide, benzoyl peroxide and the like. As with any
initiating agent, the peroxides decompose or disassociate on
addition to water, generating free radicals which then activate the
polymerization of the monomer in accordance with well-known
principles of addition polymerization. The disassociation of these
compounds is a function of temperature and higher temperatures are
desirable in increasing the rate of initiation. However, in
practice, the disassociation rate of these compounds even with
heating is undesirably slow, and it is thus desirable to include a
reductant to accelerate the initiating effect of the above
oxidants. Typical reductants include sodium formaldehyde
sulfoxylate and compounds releasing ferrous or ferric ions, with
the proviso that the amount of free ferrous or ferric ions released
is not sufficient to react with and consume all of the peroxide
oxidant through formation of ion hydrates. Other recognized
oxidants are azo and diazo compounds, for example, alpha,
alpha'-azo diisobutyronitrile and other common reductants include
the water soluble sulfides, bisulfides, and hydrosulfides, such as
sodium bisulfite.
It will be obvious that the oxidant and reductant together
constitute a redox system and, the term "polymerization initiating
agent" as employed here is intended to cover such redox systems as
well as in the use of the oxidant alone when appropriate.
The amount of the initiating agent employed follows generally the
practice in conventional emulsion polymerization which is
ordinarily initiated in the same manner. In general, the amounts of
each of the oxidant and reductant can vary within the range of
about 0.25% up to 3 or 4% or possibly higher by weight of the total
monomer, it being generally recognized that higher levels of
initiating agent tend to result in lowered molecular weight for the
ultimate polymer. Preferably, the oxidant is introduced into the
reaction vessel before the reductant since if the reductant were to
be added first without purging the reaction vessel free of oxygen,
the reductant reacts with extraneous oxygen present and is
unavailable when the actual oxidant is introduced. Similarly, the
oxidant and reductant should not be mixed together before
introduction to avoid premature reaction and loss of effective
oxidant. The amount of initiating agent is also influenced to some
extend by any condition which tends to inhibit the polymerization
reaction, more of the initiating agent being required in order to
overcome such inhibition.
If the polymerization is carried out in multiple stages, the amount
of initiating agent in the beginning or initiating stage is
adjusted to match the proportion of the monomer then present, and
further initiating agent is fed during the delayed feed stage to
correspond to the delayed feed of the monomer. Basically, in any
case, the initiating agent is supplied as needed to maintain the
reaction in a smooth and easily controlled condition.
REACTION CONDITIONS
Generally speaking, the reaction conditions employed in the
execution of the present method parallels those utilized in
conventional emulsion polymerization as regards such variables as
temperature, time, agitation, equipment, etc. The starting
temperature, at which the oxidant is added, is usually around
50.degree. to 55.degree. C. and as the reaction proceeds
exothermically, the temperature rises. It is preferred to control
reaction temperature during the exothermic phase to around
65.degree.-70.degree., plus or minus a few degrees. Higher
temperatures are possible, e.g., up to 80.degree.-90.degree. C., as
is known in conventional emulsion polymerization, but tend to
result in chain branching of the polymer and crosslinking and are
usually less desirable. At these higher temperatures, some
monomers, such as vinyl acetate, are above their boiling points and
undergo refluxing which tends to be undesirable in removing
monomers from the actual reaction site and favoring
homopolymerization.
The time of the reaction is difficult to predict since it will
depend upon other variables, such as the amount of initiating agent
introduced, the reaction temperature, etc. If the amount of monomer
is small, the reaction may be finished within about an hour but
with larger amounts, say a 1:1 monomer/pigment ratio or higher, the
reaction will usually continue for 3 to 4 hours, about half the
duration of the usual conventional emulsion polymerization
including 1/2 to 1 hour of post-heating stage after all monomer has
been added so as to insure that the polymerization has gone to
completion and no free monomer is present.
The sequence of addition of the various ingredients is not critical
and can be varied. Usually, aqueous medium is first added to the
reactor, then the nonionic stabilizing agent, particulate matter
and monomer in that order, all being added while the medium is
thoroughly agitated, followed by the oxidant and finally the
reductant, but other sequences are possible. As the particulate
matter is introduced into the medium before the surfactant, it may
not be well dispersed within the medium but the quality of the
dispersion improves when the stabilizing agent and monomer is
added. Some agglomerates normally will still remain but will be
separated before, during and after to initiation of
polymerization.
The agitation is similar to that applied in conventional emulsion
polymerization using preferably a turbine type impeller rotating
around 200-300 rpm under lab conditions and at considerably slower
speeds say about 50 rpm or so, under plant conditions. High shear
mixing is neither needed nor desirable.
An important feature of the present invention is the wide variation
in the relative amounts of monomer to particulate solids that is
possible, including ratios of monomer/particles which are much
greater than the capability of other methods of encapsulating solid
particles. Good results are obtained at monomer/particle ratios of
about 10:1-1:10, influenced to some extent by the type and particle
size of the solid matter being treated. Thus, for very fine
particle sizes, larger amounts of monomer are desirable; while,
conversely, small amounts of polymer are suitable for encapsulating
larger particles. The minimum amount of monomer cannot be easily
specified since it will be that amount which is necessary to form a
monomolecular polymer envelope around the given particles, which is
solely dependent upon surface area but such amount is calculable
from known equations. In addition, the nature of the application of
the final material will affect the proper amount of monomer. Except
for very fine particle sizes, 5% monomer by weight of particle
solids will be effective, especially for very large-sized
materials, and perhaps even lower levels of monomer would be useful
in such cases although not for finer particles. The upper limit
possible for the monomer will only be limited by practical
considerations, again determined by particle size, being reached
when the encapsulated particles have grown so large as to
completely fill the available space.
Important Features of the Invention
The product of the present encapsulation method is a creamy
suspension of encapsulated particles which is either white or
colored (if the particles are colored) and is characterized by an
extremely strong stability or resistance to flocculation. For
example, products formulated within reasonable limits of the ranges
described are almost impossible to separate by high speed
centrifugation at several thousand rpm or so for several hours and
usually exhibit no perceptible selting or syneresis; they are
stable against settling on shelf storage for many months. In
contrast, a conventional polymer latex or latex paint will separate
under the same conditions of centrifugation within a few minutes.
At very low levels of monomer with certain pigments, such as 5%
monomer for calcium carbonate, a slight syneresis may be observed
after a few days shelf storage but the product is nevertheless
readily redispersable with agitation into a fully uniform
suspension.
One of the surprising aspects of this invention is the inherent
strong dispersing effect that is exerted upon the almost inevitably
present agglomerates and clusters of pigment particles during the
initiation of the polymerization reaction. Following the course of
the reaction by light microscopic inspection of periodically
withdrawn samples, this phenomenon can be readily observed. Before
polymerization, the solid particles will be seen as small
agglomerates or clusters of varying sizes but upon the initiation
of polymerization, these clusters seem in effect to "explode" apart
so that individual solid particles now carrying a very thin
envelope of polymer will appear as fine points of light scattered
uniformly throughout the medium until all of the particle
agglomerates are broken down after a few , say 5 minutes or so.
This in-situ dispersion action is an important advantage and makes
unnecessary the usual lengthy and cumbersome dispersing
manipulations ordinarily required in preparing pigment dispersions
for incorporation in a paint and like coatings.
Although, as already mentioned, changes in the initial proportion
of the aqueous medium will influence the amount of nonionic
stabilizing agent required for effective processing, once the
polymerization reaction has been completed and a suspension of
acceptable stability achieved, the system has high tolerance for
further dilution with water as may be needed to adjust the solids
content to a desired level.
As noted above, compounds which are strong anionic or cationic
surface active or dispersing agent cannot be present in the present
method at the beginning of or during the polymerization reaction.
It is preferable, also, that the entire polymerization reaction
system be substantially free of strong anionic or cationic
materials and materials that form strong anions or cations at the
beginning of or during the major portion of the polymerization
reaction. However, once the polymerization reaction has been fully
completed and a stable suspension produced, such compounds can be
added without serious impairment of desirable product properties
or, alternatively, the present products can be combined with
conventional latex paints, especially those which are anionic in
nature, without encountering problems.
If desired, with sufficiently hydrophilic pigments such as China
clay, talc, or titanium dioxide treated with aluminum silicate or
alumina, it is possible to apply by this method an initial coating
of a water soluble polymer, such as polyvinyl pyrrolidone, followed
by the application of an exterior envelope of a water-insoluble
polymer by the procedure described above. This variation is not,
however, applicable to more hydrophobic pigments, such as calcium
carbonate, silica, or silica-treated titanium dioxide pigments, and
if an attempt is made to apply a preliminary envelope of a water
soluble polymer to the latter pigments, the entire system can
undergo coagulation. While this option of initially coating
hydrophilic particles with a water-soluble polymer is available, up
to now no benefit or advantage has been found for pursuing it since
it does not simplify or promote the formation of the ultimate
water-insoluble polymeric envelope and does not appear to aid any
other aspect of the procedure.
When a preliminary coating of water-soluble polymer is used, it
does not alter the viscosity of the medium since the water-soluble
polymer deposits preferentially upon the particles and is
subsequently completely overcoated by the exterior water insoluble
polymeric envelope, notwithstanding the fact that in the absence of
a suspended phase of solid particles, polymerization of vinyl
pyrrolidone under the same conditions would result in the water
soluble pyrrolidone polymer dissolving in the aqueous medium and
increasing its viscosity.
The usual additives or adjuvants for polymerization and/or
stabilization in conventional emulsion polylmerizations can
ordinarily be employed here except as otherwise specified. Thus,
any of the usual protective colloids, such as polyvinyl alcohol,
hydroxyethyl cellulose, casein and the like, can be incorporated in
the amount of about 1-5% by weight of solids to augment the
stabilizing power of the non-ionic stabilizing agent and often
permit a reduction in the amount of such agent needed to accomplish
good stabilization. Similarly, small amounts of organic and
inorganic acids, such as acetic, hydrochloric or sulfuric acid, can
be added to adjust the pH to the optimum point for the
polymerization reaction. In like manner, the additives, aids and
adjuvants usually incorporated in the ultimately formed paints or
coatings, can be incorporated here such as rheology-modifying
compounds, including those carrying ionically charged carboxyl
groups, typical thickening agents including carboxymethyl
cellulose, glycols for increased freeze-thaw resistance and
wet-edge retention, various wetting agents to improve substrate
rewettability, defoamers, filming aids to reduce the polymer
softening temperature, and so on.
In summary, the microencapsulation process according to the present
invention offers, among others, the following advantages:
(1) It is carried out in an aqueous medium,
(2) It requires no pre-dispersion or other pre-treatment steps for
the solid particles,
(3) It involves conventional emulsion polymerization
conditions,
(b 4) Monomers commonly employed in the coatings industry can be
utilized,
(5) A wide variety of pigments and similar materials can readily be
encapsulated,
(6) Polymer to pigment ratios can range at least from 1:10 to
10:1,
(7) Under any reasonable conditions 100% sheathing efficiency is
achieved, and
(8) The number and complexity of the operational steps are no
greater than those normally required in conventional latex
manufacture.
EXAMPLES
EXAMPLES 1-3
Examples 1-3 illustrate the extremely simple nature of the basic
method of the present invention, permitting it to be carried out
with quite primitive and meager equipment.
Example 1
______________________________________ Initial Charge Ingredient Wt
(gms) ______________________________________ Medium Tap water 100
Stabilizing Agent Polyethoxylated nonyl phenol, 6 100 EtO, 70% aq.
sol., Igepal CO-997, GAF Particles China clay, 0.5 micron, 50
ASP-170, Engelhard Monomer N-vinyl-2-pyrrolidone, GAF 10 Protective
Colloid None -- Delayed Feed: Monomer Mixture: Vinyl acetate 32
n-butylacrylate 8 Polym. Initiator: Oxidant t-butyl hydroperoxide
(t-BHP), as needed 70% aq. sol. diluted 1.3 parts in 5 parts water
Reductant sodium formaldehyde as needed sulfoxylate (SFS) solution
1.3 parts in 8 parts water
______________________________________
The initial charge ingredients are added to 1 liter, tall-form
beakers or equivalent vessels while agitation is maintained using a
"Lightnin'" type stirrer with a three-blade impeller or equivalent.
The reaction vessel is heated with a hot water bath while the
temperature of the charge is monitored with a long stem mercury
thermometer having a scale from 0.degree.-100.degree. C. A small
amount, say 1/2 cc, of the oxidant is added to the initial charge
and heating is continued until the charge mixture has reached
approximately 55.degree. C. at which point an equivalent amount of
the reductant is added with immediate initiation of polymerization
of the initial monomer. The polymerization reaction is exothermic
so that the water bath heating can be discontinued and the reaction
temperature rises quickly to about 70.degree.. When the reaction
decays as indicated by a drop in charge temperature to about
65.degree., a metered amount of the delayed feed monomer, say about
10 ml and about 0.5 ml of each of the oxidant and reductant are
introduced in that order by means of a hypodermic syringe or
equivalent, such introduction being accompanied by a drop in charge
temperature to about 60.degree. due to the cooling of the monomer
at which point the polymerization reaction is reinitiated with a
consequential temperature rise to about 70.degree. C. When the
injected portion of delayed feed monomer is polymerized, the
reaction temperature again decays to about 65.degree. C. at which
point similar fresh amounts of monomer and oxidant and reductant
are added to repeat the cycle until the total amount of delayed
feed monomer has been introduced and polymerized. With such an open
vessel experiment, care must be exercised to avoid inhalation of
monomer vapors which in some instances are known to be carcinogenic
and ideally the experiment can be carried out under an exhaust hood
as a protective measure. When, as in this example, the reductant is
sodium formaldehyde sulfoxylate, the solutions thereof should be
fresh and pure with absolute clarity since even slight turbidity,
which can develop with aging, may result in undesirable foam
formation in coating applications.
EXAMPLE 2
______________________________________ Initial Charge Ingredient
Wt. (gms) ______________________________________ Medium Tap water
100 Stabilizing Agent Polyethoxylated nonyl phenol, 5 50 EtO, 70%
aq. sol., T-Det-N-507, Thompson Hayward Particles Titanium dioxide,
0.2 micron, 40 Ti-Pure R-960, du Pont Monomer
N-vinyl-2-pyrrolidone, 6 V-Pyrol, GAF Protective Colloid
Hydroxyethyl cellulose, 0.5 CELLOSIZE QP-300, Union Carbide Delayed
Feed: Monomer Mixture: vinyl acetate 41 isobutyl acrylate 13 Polym.
Initiator: Oxidant t-BHP solution 1.3 parts in 6.3 5 parts water
Reductant SFS solution 1.3 parts in 9.3 8 parts water
______________________________________
EXAMPLE 3
______________________________________ Initial Charge Ingredient
Wt. (gms) ______________________________________ Medium Tap water
100 Stabilizing Agent Polyethoxylated nonyl phenol, 5 100 EtO, 70%
aq. sol, T-Det N-1007 Thompson Hayward Particles Talc, 6 micron. 20
R. T. Vanderbilt Co. Monomer Vinyl acetate 10 Protective Colloid
None -- Delayed Feed: Monomer Vinyl acetate 70 Polym. Initiator:
Oxidant t-BHP solution 1.3 parts in 6.3 5 parts water Reductant SFS
solution 1.3 parts in 9.3 8 parts water
______________________________________
Example 1 is repeated using talc as the particle material to be
encapuslated.
The result of each of Examples 1-3 is a stable suspension of
encapsulated particels in water similar in appearance to a
conventional latex. These suspensions were evaluated by crude
staining tests in which dried coatings of the same were prepared
and contacted with materials known to have high staining
propensity; namely, mustard, ketchup, grape juice, and chocolate
syrup, and all were found to be highly resistant to staining from
any of these materials. Samples of these suspensions were examined
by electron microscopy and no evidence could be found of the
formation of any separate polymer particles which if present would
be readily discernible by a distinctive shape; whereas, all
particles observable in these tests conformed to the shape of the
original pigment particles.
EXAMPLES 4 and 5
Examples 4 and 5 illustrate the practice of the present method with
more sophisticated laboratory equipment; namely, three-liter round
bottom flasks heated in a water bath equipped with immersion
heaters for temperature control, an electric mixer having a turbine
type impeller and dropping funnels for introducing the delayed feed
ingredients.
EXAMPLE 4
______________________________________ Initial Charge Ingredient
Wt. (gms) ______________________________________ Medium Deionized
water 985 Stabilizing Agent Polyethoxylated nonyl phenol, 50 30
EtO, 70% aq. sol, Renex 300, ICI do Brasil Particles Rutile
titanium dioxide 80 pigment, 0.2 mi., 1385-RN-59, Hoechst do Brasil
Monomer N-vinyl-2-pyrrolidone, 80 V-Pyrol, GAF Protective Colloid
Hydroxy ethyl cellulose 10 CELLOSIZE QP-09, Union Carbide Initial
Polym. Initiator: Oxidant t-BHP solution 1 part in 5 parts 6
deionized water Reductant SFS solution 1 part in 8 parts 9
deionized water Delayed Feed: Monomer Mixture: vinyl acetate 574
dibutyl maleate 143 Polym. Initiator: Oxidant t-BHP solution 3.8
parts in 37.8 34 parts deionized water Reductant SFS solution 2.4
parts in 36.4 34 parts deionized water
______________________________________
The initial charge ingredients were mixed for approximately 20
minutes prior to initation of heating with the water bath
maintained at about 80.degree. C. When the charge temperature
reached about 50.degree. C., initial oxidant was added and at about
56.degree. C. the initial reductant was added and the water bath
temperature dropped substantially by adding cold water, the heaters
being subsequently adjusted to maintain a bath temperature of about
50.degree. C. The reaction temperature increased exothermically to
around 70.degree. C. and then decayed, and when decay was observed,
the delayed feed monomer and catalyst were introduced at the rates
of about 6 ml/min for monomer and about 0.25 ml.min for each of the
oxidant and reductant simultaneously. The reaction temperature
varied roughly between 65.degree. C. and 70.degree. C. After all of
the monomer had been introduced and reacted and the final
temperature decay observed, water bath temperature was increased to
give a charge temperature of about 70.degree. C. which was
maintained for about an hour, which feed rates for the oxidant and
reductant being continued for about 20 minutes of this post-heat
period to insure complete reaction.
EXAMPLE 5
______________________________________ Initial Charge Ingredient
Wt. (gms) ______________________________________ Medium Deionized
water 985 Stabilizing Agent Polyethoxylated nonyl phenol, 57 100
EtO, 70% aq. sol., Renex 1000, ICI do Brasil Particles Titanium
dioxide pigment, 265 0.2 micron, 1385-RN-59, Hoechst do Brasil
Monomer Vinyl acetate 80 Protective Colloid None -- Initial Polym.
Initiator: Oxidant Same as Example 4 6 Reductant Same as Example 4
9 Delayed Feed: Monomer Mixture: vinyl acetate 599 n-butylacrylate
Polym. Initiator: Oxidant Same as Example 4 37.8 Reductant Same as
Example 4 36.4 ______________________________________
This products of Examples 4 and 5 were tested by bromine titration
for residual free vinyl acetate monomer and were found to contain
less than 0.5% by weight thereof. The product suspensions showed
excellent mechanical stability and degree of dispersion as
determined by both hand rub and Waring blender testing. For
comparison, a conventional latex paint was prepared from the same
pigment and monomers at the same solids content using a Cowles type
disperser and dry coatings of the same were evaluated. Coatings
obtained with the products of Examples 4 and 5 above displayed
significantly better hiding power and surface gloss determined by
visual observation.
EXAMPLES 6-9
These examples illustrate the application of the inventive method
to so-called U.S. gloss grade titanium dioxide pigments
manufactured for paint purposes and treated with various surface
treatments by the respective manufacturers.
EXAMPLES 6-9
______________________________________ Initial Charge Ingredient Wt
(gms) ______________________________________ Medium Deionized water
960 Stabilizing agent Polyethoxylated nonyl phenol, 60 100 EtO, 70%
aq. sol, Igepal, CO-997, GAF Particles Titanium dioxide pigment,
500 0.2 micron. Ex. 6 - Tronox CR-800, Kerr-McGee, 95% TiO.sub.2,
Alumina-treated.sup.1 Ex. 7 - Titanox 2020, N.L. Industries, 94%
TiO.sub.2, Alumina-treated.sup.1 Ex. 8 - TiPure R-900, du Pont, 94%
TiO.sub.2, Alumina-treated.sup.1 Ex. 9 - Zopaque RCL-9, Gidden 95%
TiO.sub.2, Alumina treated.sup.1 Monomer Vinyl acetate 90
Protective Colloid None -- Initial Polym. Initiator: Oxidant t-BHP
solution 0.8 parts in 5.8 5 parts deionized water Reductant SFS 0.8
parts in 7 parts 7.8 deionized water Delayed Feed: Monomer Vinyl
acetate 410 Polym. Initiator: Oxidant t-BHP solution 3.8 parts in
37.8 34 parts deionized water Reductant SFS 2.4 parts dissolved in
36.4 34 parts deionized water
______________________________________ .sup.1 Type of surface
treatment as identified by manufacturer
In Examples 6-9, the delay feed monomer was introduced at the rate
of 5.6 ml/min while the oxidant and reductant feed rates were 0.19
ml/min in each case, the reaction temperature being held at about
68.degree. C. In each experiment, the beginning agitation was at
about 350 rpm and was increased to the end of the example within
the range of 490-605 rmp. The initial oxidant was added when the
charge temperature had reached 45.degree. C. and the initial
reductant when the temperature reached 55.degree. C. After the
introduction of all delayed feed monomer, the charge temperature
was raised to 75.degree. C. in a post-heat stage of 30 minutes and
during the first 15 minutes of that period, the same oxidant and
reductant feed rates were maintained. The reaction vessel was
blanketed with nitrogen gas throughout all four experiments. The
polymerization reaction during each experiment was followed by
observing periodic samples through a light microscope and good
deflocculation was apparent in all experiments after the lapse of 7
minutes after the addition of the initial reductant, at which point
the delayed feed ingredient introduction was begun.
Although all four titanium dioxide pigments treated in Examples 6-9
were sold as fully equivalent to one another as a gloss grade paint
pigment, they exhibited diverse behavior in the process of the
present invention as indicated by the respective measured viscosity
summarized in the following Table 1.
TABLE I ______________________________________ Stormer, Ex. pH
K.U..sup.1 Low Shear, cps.sup.2 High Shear, poise.sup.3
______________________________________ 6 5.8 72 12,500 0.58 7 5.7
70 7,500 0.60 8 6.3 61 1,500 0.41 9 6.3 65 14,000 0.45
______________________________________ .sup.1 Stormer Viscometer
.sup.2 Brookfield Viscometer, Model LVT, 0.3 rpm, 25.degree. C.
.sup.3 ICI Viscometer, 25.degree. C.
The suspensions obtained in Examples 6-9 were used for applying
films to Leneta 3B chart, which are slick surfaced sheets having a
solid black band between two solid white bands and are useful in
determining the hiding power of paint films. The respective films
were applied with a drawdown blade having a 6 mil cap and oven
dried at 40.degree. C. The dried films were measured for gloss
using a Gardner gloss meter and the results of these measurements
are summarized in the following Table II:
TABLE II ______________________________________ Ex.
60.degree..sup.4 Gloss 20.degree..sup.5 Gloss Contrast Ratio
______________________________________ 6 53 15 0.990 7 55 18 0.985
8 26 3 0.985 9 52 12 0.991 ______________________________________
.sup.4 Angle of measuring glass from the vertical on dried film
.sup.5 Contained considerable microscopic air apparent as foam
Similar films were prepared and dried under ambient temperature,
i.e., about 25.degree. C., and it as observed that the film of the
product of Example 6 exhibited slight "mud cracking", that for
Example 7 exhibited severe "mud cracking" and flaking, while the
films obtained with the products of Examples 8 and 9 were smooth
and continuous without signs of cracking. It can be commented in
this connection that the "mud cracking" phenomenon (which resembles
the surface appearance of air dried mud layers) is indicative in
the case of polyvinyl acetate of a high molecular weight polymer
inasmuch as the minimum filming temperature for polyvinyl acetate
is about 28.degree. C. so that coalescence into a continuous film
would not be expected in the ambient drying temperature used
here.
Samples of the suspensions of Examples 6-9 were observed through an
electron microscope and showed no formation of any separate polymer
particles.
EXAMPLES 10 and 11
These examples illustrate the complexity in treating particulate
matter of very fine particle size; namely, a German anatase
titanium dioxide pigment with an average particle size of 0.03
micron sold under the trade designation P-25 by DeGussa.
Examples 10 and 11
______________________________________ Ex. 10 Ex. 11 Initial Charge
Ingredient Wt (gms) Wt (gms) ______________________________________
Medium Deionized water 1450 " Stabilizing Igepal CO-997 79 107
agent Particles P-25 pigment 500 200 Monomer Vinyl acetate 90 "
Protective None -- -- Colloid Initial Polym. Initiator Oxidant
t-BHP solution 5.8 " 0.8 parts in 5 parts deion. water Reductant
SFS solution 0.8 7.8 " parts in 7 parts deion. water Delayed Feed:
Monomer Mixture: vinyl 344 672 acetate n-butylacrylate 66 128
Polym. Initiator: Oxidant Same as Ex. 6-9 37.8 " Reductant Same as
Ex. 6-9 36.4 " ______________________________________
The above examples were carried out in equipment simiar to that use
for Examples 6-9, the initiation oxidant being added when the
charge temperature reached 50.degree. C. and the initiation
reductant when that temperature reached 55.degree. C. The delayed
monomer was fed at the rate of 5.6 ml/min and each of the delayed
oxidant and reductant were fed in Example 10 at the rate of 0.19
ml/min, and in Example 11 at the rate of 0.30 ml/min. After only a
few minutes into Example 10, the suspension underwent complete
destabilization into a sticky solid mass. It was possible to carry
Example 11 to reaction completion but the reaction product
exhibited inadequate stabilization when tested by the hand rubbing
text by approximately the midpoint of the run.
The course of Example 11 was monitored by periodic removal of
samples for electron microscopic observation, magnification
.times.5400. The sample obtained after the end of the initial
reaction stage, as indicated by drop in the reaction temperature,
showed evidence of flocculation in the form of flocs of a regular
contour along with individual particles having the approximate
diameter of the original pigment, i.e., about 0.03 micron. Fifteen
minutes after beginning the delayed feed, one could observe the
presence of flocs appearing as irregular sponge like masses
together with individual particles of apparently increased
diameter. After the introduction of 40% of total monomer, the
sponge-like appearance of the flocs diminished and the flocs became
less irregular in contour while individual particles appeared to
have grown to about 0.1-0.3 micron diameter. When 75% of the total
monomer had been introduced, the sponge-like flocs had disappeared
and one could now observe spheroidal agglomerates of fairly uniform
size distribution of approximately 3.0-5.0 micron diameter with
occluded particles of about 0.5-0.8 micron diameter. At the end of
the treatment, the agglomerates appeared distinctly spheroidal with
a size in the range of 3-10 micron or larger, while the individual
particles appeared to have diameters greater than 1 micron. At no
point could separate polymer particles to be detected in the
samples periodically taken during Example 11.
It is evident from Example 10 and 11 that the finely divided
pigment processed there required increased amounts of surfactant
and that the optimum amount had still not been reached in Example
11, at which point the experiments had to be terminated due to
exhaustion of the pigment supply. Obviously, however, the increase
in the amount of surfactant and the decrease in the amount of
pigment being processed in Example 11 compared to Example 10 showed
that substantial improvement in stabilization- had been achieved
even though not to an ideal level.
EXAMPLES 12-14
These three examples illustrate the effects of changing the
polymer/pigment ratio from 3:1 to 1:1 to 1:3, respectively.
EXAMPLES 12-14
______________________________________ Ex. 12 Ex. 13 Ex. 14 Initial
Charge Ingredient Wt (gms) Wt (gms) Wt (gms)
______________________________________ Medium Deion. water 940 " "
Stabilizing T-Det N-107 56 58 60 Agent Particles Titanox 2010.sup.1
250 500 750 Monomer Vinyl acetate 75 " " Protective None -- -- --
Colloid Initial Polym. Initiator: Oxidant Same as 5.3 " " Ex. 1-3
Reductant Same as 8.3 " " Ex. 1-3 Delayed Feed: Monomer Mixture:
550 346 143 Vinyl acetate n-butyl 125 79 32 acrylate Polym.
Initiator: Oxidant Same as Ex. 37.8 " " 10 & 11 Reductant Same
as Ex. 36.4 " " 10 & 11 ______________________________________
.sup.1 A titanium dioxide pigment of 0.2 micron size containing 97%
TiO.sub.2 titanium dioxide and designated by the manufacturer N.L.
Industries as a general purpose minimum surface treated
pigment.
Examples 12-14 were carried out using the same equipment and
techniques as employed in Examples 6-11, the initial oxidant being
added at a charge temperature of 52.degree. C. and the initial
reductant at a temperature of 56.degree. C. The delayed monomer
feed ratea was 5.8 ml/min and the delayed oxidant and reductant
feed rate were each 0.19 ml/min. A post-heat stage was applied for
45 minutes at 70.degree. C. following introduction of all monomer,
and the catalyst feeds were evident 20 minutes into this post-heat
stage. Agitation was applied throughout all runs in the range of
180-540 rpm. The vinyl products were checked for free vinyl acetate
monomer by bromine titration and were found to contain less than
0.5% free monomer.
To each of the resultant products, 80-120 ml water as added as
necessary to reduce the total solids weight to 48% for comparative
evaluation with a commercial acrylic semi-gloss latex enamel paint
containing polymer binder and titanium pigment in the weight ratio
of 46.5-43.5 for 2.4 pounds per gal titanium dioxide pigment, with
no extender pigment, and containing 48% solids by weight. Films
were cast from the dilute products of Examples 12-14 and this
commercial paint on Leneta 3B charts with a drawdown blade having a
6 mil gap and after air drying for 24 hours were measured for gloss
with a Gardner portable gloss meter with the results summarized in
Table III.
TABLE III ______________________________________ Commercial Paint
Ex. 12 Ex. 13 Ex. 14 ______________________________________
60.degree. 34 74 72 36 20.degree. 4 36 34 5
______________________________________
While the values set forth in Table III above state the actual
readings obtained with the Glossmeter, to the eye of the observer,
the apparent gloss of the films produced from the products of
Example 12 and 13, where the polymer/pigment ratio is 3:1 and 1:1,
respectively, exhibited a distinctly higher and more brilliant
gloss than these values would suggest and were vastly superior to
those obtained with the commercial latex paint film, particularly
in sharpness of image reflection from the surfaces thereof.
Although the commercial latex paint had been passed twice through
"silkalene" (a fine mesh fabric) to remove the larger particles
present, the films obtained with the conventional paint were to the
touch unmistakably higher in surface roughness than films produced
from the unfiltered products of Examples 12-14.
When evaluated on Leneta 3-B charts, the films of Examples 12-14
possessed excellent hiding power when observed by the eye, the
product of Example 14, at the 3:1 polymer/pigment ratio, equivalent
to about 0.8 lbs pigment/gallon, matching the hiding power of the
commercial paint film despite the latter's content of about 2:4 lbs
pigment/gallon. Of the higher ratio films of the invention, i.e.
Examples 12 and 13, hiding power appeared to be maximum for the
film at the 1:1 ratio, equal to about 2.1 lbs pigment/gallon; the
film at the 3:1 ratio, equal to more than 5 lbs pigment/gallon,
were almost as good but not superior, as might be suspected from
the greater pigment content, due perhaps to higher particle
packing. Only with the addition of heavy blue-black toning could
the commercial latex film even approach the hiding power of the
film of the invention at the 1:1 ratio.
An interesting characteristic of the film of the invention detected
on the Leneta charts was the definite tendency of their surfaces to
closely follow or transmit the very slight irregularities in the
surfaces of these charts, even when applied with 6 mil. thickness,
whereas the commercial latex paint film had a smooth surface
flatness independent of irregularities in the chart surface. This
indicates a desirable tightness of bonding of the instant
films.
Films of Examples 12-14 and of the commercial latex paint were
tested for scrub resistance following the ASTM scrub test procedure
using a Gardner-Straight-Line Washability Tester and the scrub
resistance values obtained from such tests are summarized in the
following Table IV, the values there representing the average of
side-by-side scrubs for the several films:
TABLE IV ______________________________________ Scrub Resistance
Commercial Paint Ex. 12 Ex. 13 Ex. 14
______________________________________ 24 hr. air dry 290 1165 1160
340 Cycles to initial failure 2 week air dry 260 2260 2140 --
cycles to init. failure 4 week air dry 295 3480 2400 -- cycles to
init. failure ______________________________________
The striking improvement in the scrub resistance values for the
films of the invention is evident from Table IV, recalling that the
product of Example 13 is substantially equivalent in polymer
content with the commercial latex paint. It is notable that there
is very little difference between the values for Examples 12 and 13
despite the three-fold difference in polymer which indicates that
once the particles have been well encapsulated, the addition of
further polymer does not impart further scrub resistance. Even for
Example 14 where the polymer content had been reduced to about
one-third of the commercial paint, the scrub resistance values were
fully comparable to those of the commercial paint.
Product samples from Examples 12-14 were observed with a
conventional electron microscope and no evidence of the presence of
separate pure polymer particles could be seen, even for Example 12
where a great excess of polymer is present. A sample of Example 13
was also examined by transmission electron microscopy (with the
light shining through the sample from beneath) and the existence of
a polymer sheath or envelope around individual pigment particles
could be readily perceived. Film samples of the Example 13 product
and the commercial latex paint were further observed by electron
microscope (mag. .times.10,000) in cross-sections prepared both by
microtoming and "cold-fracturing", and distinct differences in
structural regularity and the spacing of the pigment particles were
readily visible. The inventive film displayed a dense, coherent,
virtually void-free structure with the pigment particles totally
integrated therein; in contrast, the commercial paint film
contained a substantial proportion of void spaces, giving the
polymer binder a kind of matrix structure and distinct pigment
particles together with occasional isolated polymer particles were
easily recognized, the pigment particles tending to be grouped into
clusters rather than uniformly and homogeneously distributed
through a continuous polymer layer in the inventive material.
EXAMPLES 15 and 16
These examples illustrate the practice of the present invention in
preparing 76.2% PVC paints using standard emulsion polymerization
techniques.
______________________________________ Specific Example 15 Example
16 Material Ingredient Wt. (gms) Wt. (gms.)
______________________________________ Initial Charge Medium
deionized water 430 430 Stabilizing Igepal CO-997 57 15.2 Agent
Monomer vinyl acetate 89 89 n-butyl acrylate 25 25
n-vinyl-2-pyrrolidone C 25 (V-Pyrol) Mix for 15 min.. at room temp.
at 200 rpm, then add: Particles TiPure R-902, du Pont, 210 210 90%
TiO.sub.2, alumina-treated Mix for 15 min. at room temp. at 300
rpm, then add: Particles Celite 281, silicate, 96 96 Johns-Manville
Mix for 15 min. at room temp. at 400 rpm, then add: Particles Gold
Bond R silica, 288 288 (288 gms. silica mixed with 200 gms. water)
Medium water 200 200 Mix for 15 min. at room temp. at 400 rpm, then
add: Particles ASP-170, china clay 292 292 slurry, Engelhard (292
gms. in 200 gms. water in Example 15, 292 gms. in 300 gms. water
for Example 16) Medium water 200 300 Mix for 15 min. at room temp.
at 500 rpm, then add: Oxidant t-BHP solution 1.3 wt. 1.3 1.3 parts
(in 5 wt. parts deionized water) Medium deionized water 5.0 5.0
Raise temp. to 50.degree. C., then add: Reductant SFS solution (1.3
wt. 1.3 1.3 parts in 8 wt. parts deionized water) Medium deionized
water 8.0 8.0 React for 1 hr. at 68.degree. C., then add: Colloid
Colloids 581-B 2 0 ______________________________________
The resulting reaction mixture was cooled to room temperature. The
resulting polymer encapsulated pigment latexes of these examples
has the following properties:
______________________________________ Property Example 15 Example
16 ______________________________________ wt. % solids 53.1 52.1
Stormer viscosity, KU 93 100 Brookfield viscosity, 64000 56000 0.3
rmp, cps ICI viscosity, poise 0.5 0.5 pH 7.5 7.9 Film appearance,
smooth smooth 6 mil gap, 3B chart non-grainy non-grainy Contrast
ratio, dry 0.99 0.99 Contrast ratio, wet 0.985 0.99
______________________________________
EXAMPLE 17
This example illustrates an emulsion polymerization technique for
producing a latex of polymer encapsulated pigments. The latex was
prepared by admixing in a reactor purged with nitrogen 485 gms. of
water, 26 gms. of titanium dioxide pigment (TiPure R-900) and 238
gms. of kaolin clay as an extender (Satintone #5) and the resulting
mixture was agitated for a short period at about 200 rpm.
Thereafter, 87 gms of V-Pyrol were added following which the
resulting mixture was agitated for about 5 minutes at 250-270 rpm.
Then 51 gms of polyethoxylated nonyl phenol containing about 50
ethylene oxide units per molecule as a 70 wt. % aqueous solution
(T-DET-N-507) were added and agitation at 270 rpm was continued for
5 minutes. Thereafter, 1 gm. of Nalco 71-D5 antifoam was added.
Then, the initial catalyst comprising 1.3 gms. of t-BHP dissolved
in 5 gms. of deionized water was added to the mixture and further
mixed for 5 minutes at 270 rpm. The initial reductant comprising
1.3 gms. of SFS dissolved in 8 gms. of deionized water were added
to the reaction vessel and the temperature was increased at about
50.degree. C. (immersing the vessel in a bath maintained at about
60.degree. C.) When the reactor exotherm, produced by the
polymerizing V-Pyrol, caused the temperature to reach 60.degree. C.
to 65.degree. C. in the reactor, the monomer and catalyst/reductant
feeds were begun. The monomer feed comprised a mixture of 550 gms.
of vinyl acetate, 155 gms. of n-butyl acrylate and 7 gms. of Nalco
71-D5 antifoam agent and was fed in over a four hour period. The
catalyst (oxidant) feed contained 3.63 gms. of t-BHP in 34.4 gms.
of deionized water (38 ml. of 9.57 wt. % t-BHP in water) and the
reductant feed contained 2.3 gms. SFS dissolved in about 32.7 gms.
of water (35 ml. of a 6.59 wt. % SFS aqueous solution). Each of
these feeds were linearly fed in concurrently over a period of 33/4
to 4 hours while the bath temperature was maintained at about
70.degree. C. The temperature during the 4 hour period was
maintained at about 65.degree. C. to 70.degree. C. and agitation
was increased to maintain a vortex for good mixing. The reaction
mixture then was post heated at 70.degree. C. to 75.degree. C. for
an additional 30 minutes under minimal agitation. Thereafter 0.75
gm. of Dowicil 75 biostat in 3 gms. of deionized water was added to
the reaction mixture which was then cooled. There resulted a stable
latex resembling a latex paint.
* * * * *