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Description  |
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FIELD OF THE INVENTION
This invention relates to a process to prepare foam compositions from high
internal phase emulsions wherein the non-internal phase is a polymerizable
composition.
BACKGROUND OF THE INVENTION
Polymeric foams can be generally classified as either closed-cell foams or
as open-cell foams. Open-cell foams can be used as a matrix to contain
various liquids and gases. They are suitable for various applications such
as, for example, use in wipes and diapers, as carriers and ion exchange
resins. For some of these applications, it is desirable to have porous
crosslinked polymer blocks which have a very low density and a high
capacity of absorbing and retaining liquids. Such high absorption
capacity, low density, porous polymer blocks can be prepared by
polymerizing a specific type of water-in-oil emulsion known as high
internal phase emulsion having relatively small amounts of a continuous
oil phase and relatively greater amounts of an internal water phase.
Such high absorption capacity, low density, foams are prepared by a process
disclosed in U.S. Pat. No. 4,522,953 by polymerizing and crosslinking
monomers in the continuous oil phase of a high internal phase water-in-oil
emulsion with a polymerization initiator such as potassium persulfate.
Generally, these high internal phase water-in-oil emulsions contain at
least 90 weight percent of a water phase as the internal phase. The high
ratio water-in-oil emulsions are formed by combining the oil phase with
water under moderate shear. In order to obtain this high internal phase
water-in-oil emulsion, a surfactant is used to stabilize the emulsion. It
is also advantageous to incorporate an electrolyte into the aqueous phase.
The amount and type of electrolyte, along with the amount and type of
surfactant, effects the pore size, and wicking ability of the cured foam.
Foams prepared from high internal phase emulsions are generally cured at
temperatures of forty to seventy degrees centigrade for periods of several
hours. In a laboratory scale, this is easily accomplished. Scale up to a
reasonable commercial scale is a significant challenge. Curing and
handling individual tubs of emulsion and foam would be slow and labor
intensive.
U.S. Pat. No. 5,250,576, issued to Des Marais et al. on Oct. 5, 1993,
5,198,472, issued to Des Marais et al. on Mar. 30, 1993, and 5,149,720,
issued to Des Marais et al. on Sep. 22, 1992, disclose a continuous
process to prepare a high internal phase emulsion for curing into foam
compositions, but the emulsions are continuously poured into
"polymerization containers" and cured in the individual containers. Thus,
preparation of the emulsion in a continuous fashion is known, but curing
remains a slow and labor intensive operation.
It is therefore an object of the present invention to provide a method to
prepare foams from of high internal phase emulsions wherein the method can
be readily automated and operated on a large scale.
In another aspect, it is an object of the present invention to provide an
apparatus wherein emulsions can be cured in large volumes.
SUMMARY OF THE INVENTION
According to the present invention, a process for curing a high internal
phase emulsion to form a porous crosslinked polymeric material is
provided, the process comprising the steps of:
forming a high internal phase emulsion having at least one curable phase;
providing a continuous strip of a polymeric film wherein the polymeric film
is incompatible with each of the phases of the emulsion;
placing at least a portion of the emulsion continuously on the polymeric
film;
closing the polymeric film around the film; and
curing the emulsion within the polymeric film.
In another aspect of the present invention, apparatuses capable of
performing this method are also claimed.
This method provides for continuous curing of an emulsion, and therefore is
more readily scaled up to commercial operations.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of on embodiment of the present invention.
FIG. 1A is a crossection of emulsion in a polymeric film of FIG. 1.
FIG. 2 is a top view of the embodiment of the present invention of FIG. 1.
FIGS. 3 and 4 are, respectively, side and top schematic views of an
alternative embodiment of the present invention.
FIG. 3A is a crosssectional view of emulsion in a polymeric film of FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
An emulsion according to the present invention is produced by forming a
first curable water-in-oil high internal phase emulsion by gradually
adding and mixing an aqueous solution optionally containing an electrolyte
into a monomer solution (oil phase) containing a mixture of polymerizable
monomers and a surfactant. A polymerization initiator is added either in
the monomer solution or the aqueous solution before mixing or after
formation of the emulsion depending on the desired process conditions. The
curable water-in-oil high internal phase emulsion is then cured
(polymerized and crosslinked) by heating the emulsion at a temperature of
at least about 25.degree. C. for a time effective to cure the monomers.
The mixture of polymerizable monomers generally contains one or more vinyl
monomers and one or more crosslinking agents. Various monomers may be used
in the preparation of the foams, provided the monomers can be dispersed in
or form an oil phase of a water-in-oil high internal phase emulsion
(oil-soluble) and have a polymerizable vinyl group. Suitable vinyl
monomers include, for example, monoalkenyl arene monomers such as styrene,
.alpha.-methylstyrene, chloromethylstyrene, vinylethylbenzene and vinyl
toluene; acrylate or methacrylate esters such as 2-ethylhexyl acrylate,
n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, hexyl acrylate,
n-butyl methacrylate, lauryl methacrylate, and isodecyl methacrylate;
conjugated diolefins such as butadiene, isoprene, and piperylene; allenes
such as allene, methyl allene and chloroallene; olefin halides such as
vinyl chloride, vinyl fluoride and polyfluoro-olefins; and mixtures
thereof.
Suitable crosslinking agents can be any multifunctional unsaturated
monomers capable of reacting with the vinyl monomers. The crosslinking
agents contain at least two functional groups. The functionality can be,
for example, vinyl groups, acrylate groups and methacrylate groups.
Multifunctional unsaturated crosslinking monomers include, for example,
difunctional unsaturated crosslinking monomers such as divinylbenzene,
diethylene glycol dimethacrylate, 1,3-butanediol dimethacrylate, and allyl
methacrylate and tri-, tetra- and penta-functional unsaturated
crosslinking monomers such as trimethylolpropane trimethacrylate,
pentaerythritol tetramethacrylate, trimethylolpropane triacrylate, and
pentaerythritol tetraacrylate, glucose pentaacrylate, glucose
diethylmercaptal pentaacrylate, and sorbitan triacrylate; and
poly-functional unsaturated crosslinking monomers such as polyacrylates
(e.g. sucrose per(meth)acrylate and cellulose (meth)acrylate).
Crosslinking monomers are typically present in each emulsion in an amount
of from about 2 weight percent to about 70 weight percent, preferably from
about 5 weight percent to about 40 weight percent based on the total
monomer mixture. Some of these crosslinking monomers can be incorporated
as a non-crosslinked monomer as long as at least about 2 weight percent of
the crosslinking monomers are crosslinked.
Divinylbenzene is a preferred crosslinking monomer, and is typically
available as a mixture with ethylstyrene in proportions of about 55:45 by
weight. These proportions can be modified so as to enrich the oil phase
with one or the other component. Generally, it is advantageous to enrich
the mixture with ethylstyrene which simultaneously reducing the amount of
styrene in the monomer blend. The preferred ratio of divinylbenzene to
ethyl styrene is from about 30:70 to 55:45, and most preferably from about
35:65 to about 45:55, by weight. The inclusion of higher levels of
ethylstyrene imparts greater toughness without increasing the T.sub.g of
the resulting copolymer to the degree that styrene does.
Suitable polymerization initiators can be water-soluble or oil-soluble.
Water-soluble initiators include, for example, persulfates such as
potassium or sodium persulfate and various redox systems such as ammonium
persulfate together with sodium metabisulfite. Oil soluble (monomer
soluble) initiators include, for example, azo compounds such as
azobisisobutyro-nitrile; and peroxides such as benzoyl peroxide, methyl
ethyl ketone peroxide, alkylperoxycarbonates such as di-2-ethylhexyl
peroxydicarbonate and di(sec-butyl)peroxydicarbonate and
alkyl-peroxycarboxylates such as t-butyl peroxyisobutyrate,
2,5-dimethyl-2,5-bis(2,3-ethylhexanoylperoxy)hexane, and t-butyl
peroctoate. The preferred water-soluble polymerization initiator is
potassium persulfate and the preferred oil-soluble polymerization
initiators are alkylperoxycarbonates and alkylperoxycarboxylates for fast
curing time.
Most preferable alkylperoxycarbonates are branched at the 1-carbon position
and most preferable alkylperoxycarboxyl-ates are branched at the
.alpha.-carbon position and/or 1-carbon position. These branched
alkylperoxycarbonate peroxide can be represented by the formula:
##STR1##
where R.sup.1 is independently C.sub.1 to C.sub.16 hydrocarbons or
hydrogen in which at least two of the R.sup.1 are hydrocarbon groups.
The preferred branched alkyl carboxylate peroxide can be represented by the
formula:
##STR2##
where R.sup.1 and R.sup.2 are independently C.sub.1 to C.sub.16
hydrocarbon groups or hydrogen in which at least two of the R.sup.1 or
R.sup.2 are hydrocarbon groups. Preferably at least two of both R.sup.1
and R.sup.2 are hydrocarbon groups. Hydrocarbon groups can be alkyl,
alkenyl or aryl groups.
The water-soluble initiators and/or oil-soluble initiators should be
present in an effective amount to cure (polymerize and to crosslink) the
monomers so that the monomers are substantially polymerized and
crosslinked prior to significant diffusion of monomers between the two
emulsions. Typically the initiator can be present from about 0.005 to
about 15 weight percent based on the monomers. The initiators can be
introduced with the oil phase or the aqueous phase before or after
formation of the high internal phase emulsion.
A water-soluble initiator such as potassium persulfate can be added to the
aqueous solution before forming the emulsions or to the emulsions. An
oil-soluble initiator can be added to the monomer solution or an advanced
monomer solution or to the emulsion. Addition of a polymerization
initiator to an high internal phase water-in-oil emulsion is described in
U.S. Pat. No. 5,210,104, the disclosure of which is herein incorporated by
reference. The initiator added to the emulsion can optionally be blended
into the emulsion by any blending technique such as, for example, static
mixer or a pin mixer at a low shear rate, to form a curable water-in-oil
high internal phase emulsion. The rate of shear must be high enough to
blend the initiator but low enough not to allow the emulsion to coalesce
or liquify.
Conveniently, the initiators can be added to the oil phase (monomer phase)
or aqueous phase prior to formation of the emulsion. Alternatively, at
least a portion of the monomer solution can be advanced (partially
polymerized) in the presence of an effective amount of an advancement
initiator or by a free-radical-producing radiation source to produce an
advanced monomer component prior to formation of the emulsion to reduce
curing time.
To form a stable high internal phase emulsion requires that a surfactant be
included in the emulsion. Such surfactant must be soluble in the oil phase
used to form the emulsion. The surfactant may be nonionic, cationic,
anionic or amphoteric provided the surfactant or combination of
surfactants are effective to form a stable high internal phase emulsion.
Preferred types of surfactants that can be used include sorbitan fatty
acid esters, polyglycerol fatty acid esters, polyglycerol fatty acid
esters, polyoxyethylene fatty acids and esters. In particular, sorbitan
fatty acid esters such as sorbitan monolaurate ("SPAN.RTM. 20"), sorbitan
monooleate ("SPAN.RTM. 80"), and combinations of sorbitan trioleate
("SPAN.RTM. 85"), and sorbitan monooleate. One such surfactant combination
is the combination of sorbitan monooleate and sorbitan trioleate in a
weight ration greater than or equal to about 3:1, more preferably about
4:1. Another acceptable surfactant in "TRIODAN.RTM. 20" which is a
polyglycerol ester available from Grindsted and "EMSORB 252" which is a
sorbitan sesquioleate available from Henkel.
Between about one and about thirty percent by weight of surfactant, based
on the monomers in the oil phase, is generally sufficient, with higher oil
to water ratios and higher mixing and curing temperatures generally
requiring more surfactant than lower oil to water ratios and temperatures.
The type of surfactant used in making the high internal phase emulsions
that are to be polymerized is important in forming water-in-oil high
internal phase emulsion and the final properties of the polymerized foams
obtained. The surfactants are typically added to the monomer phase (oil
phase).
The amount of surfactant system must be such that a water-in-oil high
internal phase emulsion will form. Generally, the surfactant system is
present in an amount effective to form a water-in-oil high internal phase
emulsion. Preferably, the surfactant system can be present in
concentrations of from about 0.1 to about 40 weight percent, more
preferably about one to about thirty weight percent based on the monomers
of the particular emulsion. When saccharide fatty acid esters are used as
a component of the surfactant the saccharide fatty acid surfactants are
preferably present from about 0.1 weight percent to about 36 weight
percent, more preferably from about 0.1 to about 25 weight percent based
on the monomers. When sorbitan fatty acid esters are used as a component
of the surfactant the sorbitan fatty acid ester surfactants are preferably
present from about 2 weight percent to about 36 weight percent, more
preferably from about 5 weight percent to about 25 weight percent based on
the monomers.
The relative amounts of the aqueous phase containing water and an
electrolyte and monomer phase containing monomers and surfactants used to
form the high internal phase emulsions are a factor in determining the
structural, mechanical and performance properties of the resulting
polymeric foam phases. The ratio of water and oil in the emulsions can
influence the density, cell size, and specific surface area of the foam
phase. To form a polymeric foam phase with suitable density and high
absorption capacity, the water-in-oil high internal phase emulsions
typically contain as the internal phase, at least about 90 weight percent
of water, based on the emulsion, corresponding to a water to oil weight
ratio of at least about 9:1, more preferably at least about 95 weight
percent of water, most preferably at least about 97 weight percent of
water, corresponding to a water to oil weight ratio of at least about
33:1.
The internal aqueous phase preferably contains a water-soluble electrolyte
to stabilize the emulsion and to make the foam more water wettable.
Suitable electrolytes include inorganic salts (monovalent, divalent,
trivalent or mixtures thereof), for example, alkali metal salts, alkaline
earth metal salts and heavy metal salts such as halides, sulfates,
carbonates, phosphates and mixtures thereof. Such electrolyte includes,
for example, sodium chloride, sodium sulfate, potassium chloride,
potassium sulfate, lithium chloride, magnesium chloride, calcium chloride,
magnesium sulfate, aluminum chloride and mixtures thereof. Mono- or
divalent salts with monovalent anions such as halides are preferred.
The formation of a water-in-oil high internal phase emulsion is dependent
on a number of factors such as the monomers used, water to oil ratio, type
and amount of surfactant used, mixing conditions, presence and the amount
of water-soluble electrolyte. It has been found that by adding a
quaternary salt to a primary surfactant such as sorbitan fatty acid ester
or saccharide fatty acid ester, a stable emulsion can be formed and high
water to oil ratio can be achieved resulting in high fluid absorption
capacity foams.
The formation of a water-in-oil emulsion is described in U.S. Pat. Nos.
4,522,953, and 5,149,720, the disclosures of which are incorporated herein
by reference. In general, to form the water-in-oil emulsion, the water can
be mixed in any way up to a water to oil ratio of about 4:1. An
oil-in-water emulsion may result if the water was added all at once beyond
a water to oil ratio of about 4:1. Typically, the water is therefore added
gradually with a moderate rate of shear. A small capacity mixer such as a
paint mixer with a shear rate of at least about 5 s.sup.-1, preferably at
least about 10 s.sup.-1 can be used to mix the water-in-oil emulsion. A
larger mixer equipped with an impeller with a shear rate of at least about
10 s.sup.-1 or a pin gap mixer with a shear rate of at least about 50
s.sup.-1, preferably at least about 100 s.sup.-1 can also be used. If the
shear rate is too low, the water-in-oil emulsion may revert to a
oil-in-water emulsion. It is desirable to at least have a water to oil
ratio of about 9:1, preferably at least about 19:1, more preferably at
least about 30:1 for a high absorbency capacity foam.
A high internal phase emulsion can be prepared in batches or continuously.
To form the high internal phase emulsion in batches, the emulsion is
formed in a vessel or a container by gradually adding an aqueous phase to
a monomer mixture and/or advanced monomer mixture under a moderate rate of
shear until the desired water to oil ratio is reached.
An individual high internal phase emulsion can be prepared continuously by
initially preparing a preformed emulsion of approximately the same
character as the desired emulsion by the method described above, then
introducing into the preformed emulsion, both the aqueous phase and/or the
oil phase in such proportions so as to produce the desired emulsion at a
desired rate of production of emulsion. The emulsified mass is maintained
in a state of continuous shear sufficient to reduce the effective
viscosity of the mass near to that of the introduced phase but not above
the inherent shear stability point of the desired emulsion. The prepared
emulsion is then withdrawn at the desired rate.
The aqueous phase and the monomer phase for the batch process and the
continuous process can be introduced in a mixing vessel by an aqueous
stream or a monomer stream, respectively, through one or more inlets. The
streams can be combined prior to or after entering the mixing vessel then
mixed in such a way to produce the desired emulsion. The mixing vessel is
any container in which the high internal phase emulsion is made regardless
of the type of mixer or mixer head used.
The emulsion is preferably polymerized and cured at a temperature within
the range of about 25.degree. C. to about 90.degree. C., as long as the
emulsion is stable at the curing temperature. Alternatively, a
multiple-step process as described in U.S. Pat. No. 5,189,070 issued Feb.
23, 1993, the disclosure of which is herein incorporated by reference, can
also be used. In the multi-step process the emulsion is pre-cured at a
temperature of less than about 65.degree. C. until the emulsion has a
Rheometrics dynamic shear modulus of greater than about 500 pascal,
(lightly gelled, having a consistency like a jelly or a gelatin referred
to as "gel"), then cured at a temperature of above about 70.degree. C. for
a time effective to cure the gel. The cure can be as high as about
175.degree. C. under a pressure sufficient to prevent the aqueous phase
from boiling.
The emulsions can be heated, for example, by hot water, hot air, steam,
electron beam radiation ("EBR"), radio frequency ("RF"), microwave or
ohmic heating. The emulsions should be cured until the desired properties
are obtained. Typically, to obtain a cured foam, the emulsions should be
cured for at least about 8 hours, at 60.degree. C. or at least about 1
hour at 60.degree. C. then 3 hours at a temperature of above about
70.degree. C. Generally, the extent of reaction after curing is at least
about 85% of the monomers, preferably at least about 90%, more preferably
at least about 95% (i.e., less than about of free monomers), most
preferably at least about 99% (i.e., less than about 1% of free monomers)
in order to obtain good properties.
In the practice of the present invention, the emulsion may be formed in
batches or continuously, but the emulsion is placed on a continuous
polymer film continuously in order to have a relatively continuous curing
of the emulsion. The polymeric film is preferably polyproplyene, but could
be another material that does not adhere to the cured emulsion, and does
not cause the emulsion to break at the surface of the film. Polypropylene
is convenient because it has these properties, is readily formed into a
film, is inexpensive and can be recycled.
After being placed on a continuous film, the film is closed around the
emulsion. The film can be simply laid over the emulsion in an overlapping
fashion, or edges of the film can be interlocked with a mechanical closing
zip-lock. Alternatively, the film edges can be sealed together using, for
example, a heat seal or adhesive. A continuous tube of the emulsion in the
film is therefore created. This continuous film is then more easily cured
and handled after curing. The film is then preferably removed from outside
of the cured emulsion, and the cured emulsion is then further processed,
by for example, slicing the emulsion into thinner pieces, removing water
and drying the emulsion.
The emulsion could be placed on the continuous sheet in a thickness such
that slicing of the resultant cured foam to thinner sheets is not
necessary. If a relatively thin sheet of emulsion is cured on the
continuous sheet so that slicing is not necessary, the continuous sheet
could be a material that adheres to the cured foam, and the continuous
sheet could then function as a backing sheet to the foam. Slicing of the
cured foam could also be avoided by layering films and emulsion repeadily,
so that a significant thickness, for example four inches, of emulsions
could be cured at one time, and then separated by pulling apart film
sheets between the layers. In some end-uses, it could be beneficial to
have such backing sheet be impervious to liquids. For example, having the
impervious backing sheet impervious to liquids could be useful in a
diaper.
Referring now to FIGS. 1, 1A,of the an embodiment of the present invention
is shown schematically. Emulsion 1 is placed in a film 2. The film is
initially on a roll 3, and is pulled off the roll in as a doubled sheet.
The doubled sheet is spread by spreading rollers 4, to provide a vertical
pocket 5 for the emulsion. The film could alternatively be on a single
layer roll, and doubled to form a pocket by guiding rollers or wires. The
emulsion is placed in the pocket by, for example, a distribution manifold
6. A tank 7 is provided for either holding an emulsion made by a batch
process or for holding a volume of emulsion that is being prepared
continuously. The emulsion can be pumped by pump 8 through recycle piping
11 to a static mixer 9 with a slip stream of emulsion being routed through
a control valve 10 to the distribution manifold 6.
The film 2 is provided with an interlocking seal along the edges of the
film, with a bead 12 on one edge and a locking edge 13 along the other
edge. After the emulsion is placed in the vertical pocket 5, the edges are
locked by a zipper fitting 14. The emulsion in the locked film is then
wrapped onto a spool 15 for curing. Multiple spools 15 can be provided so
that when one spool is full, the emulsion loaded onto that spool can be
cured and another spool can be used to store the emulsion that is being
place, at that time, onto the continuous polymeric sheet 2. Changing
spools can be accomplished, for example, by providing two clamps to seal a
section of the sealed film that contains the polymer, and cutting the film
between the clamps. The spools can be rotatable to enable the spools to
pull the emulsion filled film along.
The pocket formed by the spreader rollers can be, for example, four inches
wide and four feet high. A pocket of this dimension can hold the emulsion
with a reasonable thickness of polypropylene film. When a spool is filled
with emulsion filled film, a stiff band or a series of stiff bands can be
placed around the outer most film pocket, and this stiff band will hold
the outer most film pocket nearly vertical on the spool. It is preferably
that the spool have a diameter of at least 85 feet in order to accommodate
a significant volume of emulsion.
After the emulsion is cured, the cured emulsion can be unwrapped from the
spool and further processed by, for example, slicing the cured emulsion
into thinner slices, removing water, rinsing, and drying the cured
emulsion.
Referring now to FIGS, 3, 3A, and 4, an alternative embodiment of the
present invention is schematically shown. Emulsion 21 is placed on film 22
through a manifold 24. The film can be provided on a single layer roll 23.
In the embodiment of these figures, the film is doubled over on top of the
emulsion rarer than sealed, although the edges of the film could
alternatively be sealed as in the embodiment of FIGS. 1, 1A. and 2. In the
embodiment of FIGS. 3, 3A, and 4, the film is supported on horizontal
conveyor belt 27 and a pocket is formed by vertical side conveyor belts 28
and 29. These conveyor belts can be, for example, interlocking wire belts,
or reinforced elastomer belts that can withstand curing temperatures. The
conveyor belts pull the emulsion filled film through a curing oven, 30.
The emulsion in the film on the conveyor belt can be, for example, 4 to 6
inches deep, and about 4 to 6 feet wide. The curing oven is a very long
oven, for example, 360 to 480 feet long, to enable a sufficient curing
time with a reasonable conveyor belt speed, such as about one quarter to
about two feet per second. Processing at the exit end of the curing oven
31 is continuous and can include slicing the cured emulsion using slicer
32, water removal in a squeezer 33, and collection of thin layers of
dewatered foam on rolls 34.
These foams can be optionally post-cured to improve the foam properties.
Post-curing of the foam can be carried out by heating the foams to a
temperature of above about 75.degree. C., preferably greater than
90.degree. C. by steam, hot air or other heating source. Such heating may
be performed in a heat exchanger, oven, over heated rollers, hot water,
hot air, steam, electron beam radiation ("EBR"), radio frequency ("RF"),
microwave or ohmic heating or by other means.
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