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FIELD OF THE INVENTION
This application relates to rapid curing of high internal phase emulsions
to produce microporous, open-celled polymeric foam materials with physical
characteristics that make them suitable for a variety of uses.
BACKGROUND OF THE INVENTION
The development of microporous foams is the subject of substantial
commercial interest. Such foams have found utility in various
applications, such as thermal, acoustic, electrical, and mechanical (e.g.,
for cushioning or packaging) insulators, absorbent materials, filters,
membranes, floor mats, toys, carriers for inks, dyes, lubricants, and
lotions, and the like. References describing such uses and properties of
foams include Oertel, G., Polyurethane Handbook, Hanser Publishers,
Munich, 1985, and Gibson, L. J.; Ashby, M. F., Cellular Solids. Structure
and Properties, Pergamon Press, Oxford, 1988. The term "insulator" refers
to any material which reduces the transfer of energy from one location to
another. The term "absorbent" refers to materials which imbibe and hold or
distribute fluids, usually liquids, an example being a sponge. The term
"filter" refers to materials which pass a fluid, either gas or liquid,
while retaining impurities within the material by size exclusion. Other
uses for foams are generally obvious to one skilled in the art.
Open-celled foams prepared from High Internal Phase Emulsions (hereinafter
referred to as "HIPEs") are particularly useful in a variety of
applications including absorbent disposable articles (U.S. Pat. Nos.
5,331,015 (DesMarais et al.) issued Jul. 19, 1994, 5,260,345 (DesMarais et
al.) issued Nov. 9, 1993, 5,268,224 (DesMarais et al.) issued Dec. 7,
1993, 5,632,737 (Stone et al.) issued May 27, 1997, 5,387,207 (Dyer et
al.) issued Feb. 7, 1995, 5,786,395 (Stone et al.) Jul. 28, 1998,
5,795,921 (Dyer et al.) issued Aug. 18, 1998), insulation (thermal,
acoustic, mechanical) (U.S. Pat. Nos. 5,770,634 (Dyer et al.) issued Jun.
23, 1998, 5,753,359 (Dyer et al.) issued May 19, 1998, and 5,633,291 (Dyer
et al.) issued May 27, 1997), filtration (Bhumgara, Z. Filtration &
Separation 1995, March, 245-251; Walsh et al. J. Aerosol Sci. 1996, 27,
5629-5630; published PCT application W/O 97/37745, published on Oct. 16,
1997, in the name of Shell Oil Co.), and various other uses. The cited
patents and references above are incorporated herein by reference. The
HIPE process provides facile control over the density, cell and pore size
and distribution, proportion of cell struts to windows, and porosity in
these foams.
An important issue in making HIPE foams commercially attractive is
economics. The economics of HIPE foams depend on the amount and cost of
the monomers used per unit volume of the foam, as well as the cost of
converting the monomers to a usable polymeric foam (process costs).
Making, HIPE foams economically attractive can require using: (1) less
total monomer per unit volume of foam, (2) less expensive monomers, (3) a
less expensive process for converting these monomers to a usable HIPE
foam, or (4) combinations of these factors. The monomer formulation and
process conditions must be such that the properties of the HIPE foam meet
the requirements for the particular application.
The physical properties of the foam are governed by: (1) the properties of
the polymer from which the foam is comprised, (2) the density of the foam,
(3) the structure of the foam (i.e. the thickness, shape and aspect ratio
of the polymer struts, cell size, pore size, pore size distribution,
etc.), and (4) the surface properties of the foam (e.g., whether the
surface of the foam is hydrophilic or hydrophobic). Once these parameters
have been defined and achieved for a particular application, an
economically attractive process for preparing the material is desired. A
key aspect of this process is the rate of polymerization and crosslinking,
together referred to as curing, of the oil phase of a HIPE to form a
crosslinked polymer network. Previously, this curing step required that
the emulsion be held at an elevated temperature (40.degree. C.-82.degree.
C.) for a relatively long period of time (typically from 2 hours to 18
hours or longer). Such long cure times necessitate relatively low
throughput rates, as well as high capital and production costs.
Previous efforts to devise commercially successful schemes for producing
HIPE foams have involved, for example, pouring the HIPE into a large
holding vessel which is then placed in a heated area for curing (see for
example U.S. Pat. No. 5,250,576 (Desmarais et al.) issued Oct. 5, 1993).
U.S. Pat. Nos. 5,189,070 (Brownscombe et al), issued Feb. 23, 1993;
5,290,820 (Brownscombe et al.) issued Mar. 1, 1994; and 5,252,619
(Brownscombe, et al.) issued Oct. 12, 1993 disclose curing the HIPE in
multiple stages. The first stage is conducted at a temperature of less
than about 65.degree. C. until the foam reaches a partial state of cure.
Then the temperature is increased to between 70.degree. C. and 175.degree.
C. to effect final curing rapidly. The whole process takes about 3 hours.
Another scheme to produce HIPE foams envisaged placing the emulsion on a
layer of impermeable film which would then be coiled and placed in a
curing chamber (U.S. Pat. No. 5,670,101 (Nathoo, et al.) issued Sep. 23,
1997). The coiled film/emulsion sandwich could then be cured using the
sequential temperature sequence disclosed in the Brownscombe, et al
patents discussed above. U.S. Pat. No. 5,849,805 issued in the name of
Dyer on Dec. 15, 1998 discloses forming the HIPE at a temperature of
82.degree. C. (pour temperature in Example 2) and curing the HIPE at
82.degree. C. for 2 hours. However, none of these approaches offer the
combination of very fast conversion (e.g., in minutes or seconds) from
HIPE to polymeric foam that would provide for a relatively simple, low
capital process for producing HIPE foams both economically and with the
desired set of properties.
The art also discloses using pressure to control the volatility of monomers
that, otherwise, would boil off at a suitable polymerization/curing
temperature. For example, commonly assigned U.S. Pat. No. 5,767,168,
issued to Dyer, et al. on Jun. 16, 1998, discloses the suitability of
pressurization to control the volatility of relatively volatile conjugated
diene monomers. However, the cure time for the foams disclosed therein is
still greater than two hours so there is still substantial opportunity for
substantial improvement in curing rate that would improve the economic
attractiveness of HIPE foams.
Accordingly, it would be desirable to develop a rapid and efficient process
for preparing open-celled polymeric HIPE foam materials with the desired
properties.
SUMMARY OF THE INVENTION
The present invention relates to a process for obtaining open-celled foams
by polymerizing a High Internal Phase Emulsion, or HIPE, which has a
relatively small amount of a continuous oil phase and a relatively greater
amount of a discontinuous aqueous phase. The present invention
particularly relates to relatively high temperature processes for curing
the oil phase. This enables the foam to be prepared in a much shorter
interval than has heretofore been possible. This enables practical
continuous production processes of HIPE foams which have to this point
been made via batch processes.
The process of the present invention generally comprises the steps of: 1)
forming a water in oil emulsion (HIPE) wherein the oil phase comprises
polymerizable monomers; and 2) polymerizing and crosslinking the monomers
at temperatures greater than 90.degree. C. to form a HIPE foam.
Specifically, the oil phase comprises: 1) from about 85 to 99% by weight
of a monomer component capable of forming a copolymer having a Tg of about
90.degree. C. or lower, wherein the monomer component comprises a blend of
monofunctional monomers, crosslinkng agents, and comonomers capable of
modifying foam properties, and 2) from about I to about 20% of an
emulsifier component capable of forming a stable HIPE. The aqueous phase
comprises from about 0.2 to about 40% by weight of a water soluble
electrolyte and an effective amount of a polymerization initiator. The
volume to weight ratio of aqueous phase to oil phase is between about 8:1
and about 140:1. After polymerization, the aqueous fraction of the HIPE
foam may be removed by a variety of techniques to yield the open-celled,
microporous, low density product.
The curing of HIPEs in a relatively short time period at elevated
temperatures allows increased production and improved economics relative
to previously described methods. Either batch or continuous processes can
be used. In either case, because the vapor pressure of both phases in the
emulsion increases as the temperature is increased, some containment
and/or pressurized system is generally required to prevent volatilization
of the HIPE components during the high temperature curing and or
emulsification steps. Volatilization to form gas or vapor bubbles may
damage the fine structure, particularly the cell size distribution, of the
HIPE and resulting HIPE foam, and is generally to be avoided. This may be
accomplished by applying pressure from an external source such as a pump
or pressurized gas cylinder, by heating the emulsion in a closed container
with relatively small headspace volume, by heating a portion of the
composition under the surface of the emulsion in an open container such
that the "hydrostatic" pressure prevents volatilization of the liquid
comprising the emulsion, or by any other method or device generally known
to those skilled in the art. Elements of these approaches may be combined
to develop a suitable process for rapid curing of HIPE foams.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is an electron photomicrograph at 500 X magnification of a control
HIPE foam in its expanded state wherein the emulsion was formed at
47.degree. C. and cured at 65.degree. C. under ambient pressure according
to the prior art.
FIG. 2 is an electron photomicrographs at 500 X magnification of a
representative polymeric foam in its expanded state according to the
present invention prepared as described in Example 1.
FIG. 3 is an electron photomicrograph at 1000 X magnification of a control
HIPE foam in its expanded state wherein the emulsion was formed at
47.degree. C. and cured at 65.degree. C. under ambient pressure according
to the prior art.
FIG. 4 is an electron photomicrographs at 1000 X magnification of a
representative polymeric foam in its expanded state according to the
present invention prepared as described in Example 1.
FIG. 5 is an electron photomicrograph at 2500 X magnification of a control
HIPE foam in its expanded state wherein the emulsion was formed at
47.degree. C. and cured at 65.degree. C. under ambient pressure according
to the prior art.
FIG. 6 is an electron photomicrographs at 2500 X magnification of a
representative polymeric foam in its expanded state according to the
present invention prepared as described in Example 1.
FIG. 7 is a schematic diagram of the curing chamber used to prepare foams
depicted in FIGS. 1 and 2.
FIG. 8 is a schematic diagram of a continuous process for preparing HIPE
foams
DETAILED DESCRIPTION OF THE INVENTION
I. Definitions
The following definitions are offered relative to the current invention.
"Curing" is the process of converting a HIPE to a HIPE foam. Curing
involves the polymerization of monomers into polymers. A further step
included in the curing process is crosslinking. A cured HIPE foam is one
which has the physical properties, e.g., mechanical integrity, to be
handled in subsequent processing steps (which may include a post-curing
treatment to confer the final properties desired). Generally, curing is
effected via the application of heat. An indication of the extent of cure
is the mechanical strength of the foam, as measured by the yield stress
described in the Test Methods section below.
"Polymerization" is the part of the curing process whereby the monomers of
the oil phase are converted to a relatively high molecular weight polymer.
"Crosslinking" is the part of the curing process whereby the monomers
having more than one functional group with respect to free radical
polymerization are copolymerized into more than one chain of the growing
polymer.
"Hydrostatic" relates to pressure conferred by a column of liquid in a
gravitational field, sometimes referred to as "hydrostatic head". The
liquid is not necessarily water, but may be an aqueous solution, emulsion,
suspension or other liquid.
I. Polymeric Foam Derived From a High Internal Phase Emulsion
A. General Foam Characteristics
1. Oil Phase Components
The continuous oil phase of the HIPE comprises monomers that are
polymerized to form the solid foam structure and the emulsifier necessary
to stabilize the emulsion. In general, the monomers will include from
about 20 to about 95% by weight of at least one substantially
water-insoluble monofunctional monomer capable of forming an atactic
amorphous polymer having a glass transition temperature (Tg) of about
35.degree. C. or lower. This comonomer is added to lower the overall Tg of
the resulting HIPE foam. Exemplary monomers of this type include C.sub.4
-C.sub.14 alkyl acrylates and C.sub.6 -C.sub.16 methacrylates such as
2-ethylhexyl acrylate, n-butyl acrylate, hexyl acrylate, n-octyl acrylate,
nonyl acrylate, decyl acrylate, isodecyl acrylate, tetradecyl acrylate,
benzyl acrylate, nonyl phenyl acrylate, hexyl methacrylate, octyl
methacrylate, nonyl methacrylate, decyl methacrylate, isodecyl
methacrylate, dodecyl methacrylate, and tetradecyl methacrylate;
substituted acrylamides, such as N-octadecyl acrylamide; dienes such as
isoprene, butadiene, chloroprene, piperylene, 1,3,7-octatriene,
.beta.-myrcene and amyl butadiene; substituted C.sub.4 -C.sub.12 styrenics
such as p-n-octyl styrene; and combinations of such monomers. The Tg
lowering monofunctional monomers will generally comprise 20% to about 95%,
more preferably 45% to about 65%, by weight of the monomer component.
The oil phase will also comprise from about 5 to about 80% by weight of a
first substantially water-insoluble, polyfunctional crosslinking agent.
This comonomer is added to confer strength to the resulting HIPE foam.
Exemplary crosslinking monomers of this type encompass a wide variety of
monomers containing two or more activated vinyl groups, such as the
divinyl benzenes and analogs thereof. These analogs include m,p-divinyl
benzene mixtures with ethyl styrene, divinyl naphthalene, trivinyl
benzene, divinyl alkyl benzenes, divinyl biphenyls, divinyl phenyl ethers,
divinyl ferrocenes, divinyl furans, and the like. Other useful
crosslinking agents may be selected from a group derived from the reaction
of acrylic acid or methacrylic acid with polyfunctional alcohols and
amines. Nonlimiting examples of this group include
1,6-hexanedioldiacrylate, 1,4-butanedioldimethacrylate, trimethylolpropane
triacrylate, hexamethylene bisacrylamide, and the like. Other examples of
crosslinking monomers include divinyl sulfide, divinyl sulfone, and
trivinyl phosphine. Other crosslinkers useful in this regard are well
known to those skilled in the art. It should be noted that the weight
fraction of the crosslinking component is calculated on the basis of the
pure crosslinker in cases wherein the crosslinking monomer is commonly
used as a mixture (e.g., divinyl benzene often is a 55% pure mixture with
the balance being ethyl styrene).
Any third substantially water-insoluble comonomer may be added to the oil
phase in weight percentages of from about 0% to about 70%, preferably from
about 15% to about 40%, to modify properties in other ways. In certain
cases, "toughening" monomers may be desired which impart toughness to the
resulting HIPE foam equivalent to that provided by styrene. These include
styrenics such as styrene and ethyl styrene and methyl methacrylate. Also
include are styrenics and other compounds which may also help reduce the
Tg or enhance the strength of the resulting HIPE foam such as p-n-octyl
styrene. Monomers may be added to confer flame retardancy as disclosed in
commonly assigned copending application 09/118,613 (Dyer) filed Jul. 17,
1998. Monomers may be added to confer color, fluorescent properties,
radiation resistance, opacity to radiation (e.g., lead tetraacrylate), to
disperse charge, to reflect incident infrared light, to absorb radio
waves, to form a wettable surface on the HIPE foam struts, or for any
other purpose.
2. Aqueous Phase Components
The discontinuous aqueous internal phase of the HIPE is generally an
aqueous solution containing one or more dissolved components. One
essential dissolved component of the water phase is a water-soluble
electrolyte. The dissolved electrolyte minimizes the tendency of monomers,
comonomers, and crosslinkers that are primarily oil soluble to also
dissolve in the water phase.
Another component of the aqueous phase is a water-soluble free-radical
initiator as may be known to the art. The initiator can be present at up
to about 20 mole percent based on the total moles of polymerizable
monomers present in the oil phase. More preferably, the initiator is
present in an amount of from about 0.001 to about 10 mole percent based on
the total moles of polymerizable monomers in the oil phase. Suitable
initiators include ammonium persulfate and potassium persulfate.
3. Emulsifier
The emulsifier is necessary for forming and stabilizing the HIPE. The
emulsifier is generally included in the oil phase and tends to be
relatively hydrophobic in character. (See for example Williams, J. M.,
Langmuir 1991, 7, 1370-1377, incorporated herein by reference.) An example
emulsifier which functions very well is diglycerol monooleate. Other
emulsifiers of this general sort also include diglycerol monomyristate,
diglycerol monoisostearate, diglycerol monoesters of coconut fatty acids,
sorbitan monooleate, sorbitan monomyristate, sorbitan monoesters of
coconut fatty acids, sorbitan isostearate, and like compounds and mixtures
thereof. U.S. Pat. No. 5,786,395 (Stone et al.) issued Jul. 28, 1998 offer
further examples of these emulsifiers and is incorporated herein by
reference. Such emulsifiers are advantageously added to the oil phase so
that it comprises between about 1% and about 15% thereof. Obviously,
emulsifiers that are particularly able to stabilize HIPEs at high
temperatures are preferred. Diglycerol monooleate is exemplary in this
respect.
Coemulsifiers may also be used to provide additional control of cell size,
cell size distribution, and emulsion stability. Exemplary coemulsifiers
include phosphatidyl cholines and phosphatidyl choline-containing
compositions, aliphatic betaines, long chain C.sub.12 -C.sub.22
dialiphatic, short chain C.sub.1 -C.sub.4 dialiphatic quaternary ammonium
salts, long chain C.sub.12 -C.sub.22 dialkoyl(alkenoyl)-2-hydroxyethyl,
short chain C.sub.1 -C.sub.4 dialiphatic quaternary ammonium salts, long
chain C.sub.12 -C.sub.22 dialiphatic imidazolinium quaternary ammonium
salts, short chain C.sub.1 -C.sub.4 dialiphatic, long chain C.sub.12
-C.sub.22 monoaliphatic benzyl quaternary ammonium salts, the long chain
C.sub.12 -C.sub.22 dialkoyl(alkenoyl)-2-aminoethyl, short chain C.sub.1
-C.sub.4 monoaliphatic, short chain C.sub.1 -C.sub.4 monohydroxyaliphatic
quaternary ammonium salts Particularly preferred is ditallow, dimethyl
ammonium methyl sulfate. Such coemulsifiers and additional examples are
described in greater detail in U.S. Pat. No. 5,650,222, issued in the name
of DesMarais, et al. on Jul. 22, 1997, the disclosure of which is
incorporated herein by reference.
4 Optional Ingredients
Various optional ingredients may also be included in either the water or
oil phase for various reasons. Examples include antioxidants (e.g.,
hindered phenolics, hindered amine light stabilizers, UV absorbers),
plasticizers (e.g., dioctyl phthalate, dinonyl sebacate), flame retardants
(e.g., halogenated hydrocarbons, phosphates, borates, inorganic salts such
as antimony trioxide or ammonium phosphate or magnesium hydroxide), dyes
and pigments, fluorescers, filler particles (e.g., starch, titanium
dioxide, carbon black, or calcium carbonate) fibers, chain transfer
agents, odor absorbers such as activated carbon particulates, dissolved
polymers and oliogomers, and such other agents as are commonly added to
polymers for a variety of reasons. Such additives may be added to confer
color, fluorescent properties, radiation resistance, opacity to radiation
(e.g., lead compounds), to disperse charge, to reflect incident infrared
light, to absorb radio waves, to form a wettable surface on the HIPE foam
struts, or for any other purpose.
B. Processing Conditions for Obtaining HIPE Foams
Foam preparation typically involves the steps of: 1) forming a stable high
internal phase emulsion (HIPE); 2) curing this stable emulsion under
conditions suitable for forming a cellular polymeric structure; 3)
optionally squeezing and washing the cellular polymeric structure to
remove the original residual water phase from the polymeric foam structure
and, if necessary, treating the polymeric foam structure with a
hydrophilizing surfactant and/or hydratable salt to deposit any needed
hydrophilizing surfactant/hydratable salt, and 4) thereafter dewatering
this polymeric foam structure.
1. Formation of HIPE
The HIPE is formed by combining the water and oil phase components in a
ratio between about 8:1 and 140:1. Preferably, the ratio is between about
10:1 and about 75:1, more preferably between about 13:1 and about 65:1. As
discussed above, the oil phase will typically contain the requisite
monomers, comonomers, crosslinkers, and emulsifiers, as well as optional
components. The water phase will typically contain electrolyte or
electrolytes and polymerization initiator or initiators.
The HIPE can be formed from the combined oil and water phases by subjecting
these combined phases to shear agitation. Shear agitation is generally
applied to the extent and for a time period necessary to form a stable
emulsion. Such a process can be conducted in either batchwise or
continuous fashion and is generally carried out under conditions suitable
for forming an emulsion where the water phase droplets are dispersed to
such an extent that the resulting polymeric foam will have the requisite
structural characteristics. Emulsification of the oil and water phase
combination will frequently involve the use of a mixing or agitation
device such as an impeller.
One preferred method of forming HIPE involves a continuous process that
combines and emulsifies the requisite oil and water phases. In such a
process, a liquid stream comprising the oil phase is formed. Concurrently,
a separate liquid stream comprising the water phase is also formed. The
two separate streams are provided to a suitable mixing chamber or zone at
a suitable emulsification pressure and combined therein such that the
requisite water to oil phase weight ratios previously specified are
achieved.
In the mixing chamber or zone, the combined streams are generally subjected
to shear agitation provided, for example, by an impeller of suitable
configuration and dimensions, or by any other means of imparting shear or
turbulent mixing generally known to those skilled in the art. Shear will
typically be applied to the combined oil/water phase stream at an
appropriate rate and extent. Once formed, the stable liquid HIPE can then
be withdrawn or pumped from the mixing chamber or zone. This preferred
method for forming HIPEs via a continuous process is described in greater
detail in U.S. Pat. No. 5,149,720 (DesMarais et al), issued Sep. 22, 1992,
which is incorporated by reference. See also commonly assigned U.S. Pat.
No. 5,827,909 (DesMarais) issued on Oct. 27, 1998 (incorporated herein by
reference), which describes an improved continuous process having a
recirculation loop for the HIPE. The process also allows for the formation
of two or more different kinds of HIPEs in the same vessel as disclosed in
U.S. Pat. No. 5,817,704 (Shiveley et al.) issued Oct. 6, 1998,
incorporated herein by reference. In this example, two or more pairs of
oil and water streams may be independently mixed and then blended as
required.
2. Polymerization/Curing of the oil phase of the HIPE
The present invention relates to polymerization/curing of the oil phase of
the emulsion at high curing temperatures for short periods. The HIPE
formed as described above may be polymerized/cured in a batch process, or
in a continuous process.
A measure of the extent of cure of the polymer is the strength of the foam,
as measured by the yield stress described in the Test Methods section
below. Another measure of the extent of cure of the polymer is the extent
to which it swells in a good solvent such as toluene (being crosslinked,
the HIPE foam does not dissolve without being chemically altered).
Without being bound by theory, it is believed that curing comprises two
overlapping but distinct processes. The first involves polymerization of
the monomers. The second the formation of crosslinks between active sites
on adjacent polymer backbones. Crosslinking is essential to the formation
of HIPE foams with strength and integrity essential to their further
handling and use. The rate controlling step of this crosslinking reaction
is believed to be related to the rate of diffusion of the active sites
attached to the polymer chains. It has been discovered surprisingly that
an increase in the rate of production of free-radicals in the HIPE does
not accelerate curing usefully. However, increasing the diffusion rate of
the active sites by increasing the temperature of the system in a
conventional curing process is limited by the volatility of the components
of the emulsion. The current invention provides for curing the emulsion
under an elevated pressure in order to allow high temperatures and rapid
cure times to be attained without excessive volatilizatio | | |
