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FIELD OF THE INVENTION
The present invention relates to a method for converting normally
hydrophobic polymeric foams into hydrophilic foams. The foams thus
"hydrophilized" are suitable for use in absorbent devices such as diapers,
adult incontinence garments, sanitary napkins, bandages, and the like,
which are especially adapted for absorbing various aqueous bodily fluids.
BACKGROUND OF THE INVENTION
A wide variety of foam materials, or common "sponges", which effectively
absorb moisture are well-known in commercial practice. Typically, such
foams are open-cell structures and comprise various cellulosic or
polymeric materials. For example, various polyurethanes and like materials
have long been used to prepare synthetic foams. As is known in the art,
foam materials function most efficiently as absorbents for aqueous liquids
when their surfaces are substantially hydrophilic. However, many synthetic
foams are prepared by the polymerization of organic monomers which yield
polymeric foams which are substantially hydrophobic in nature.
Accordingly, considerable attention has been given to finding means
whereby otherwise hydrophobic synthetic foams can be rendered hydrophilic.
For example, it is known that some types of foams have been prepared using
certain selected monomers which, themselves, impart at least some degree
of hydrophilic character to the resulting polymerized foam. Such monomers
are then incorporated into the basic structure of the foam network during
the polymerization process. Unfortunately, the hydrophilic substituents
present in the monomers can undesirably modify the basic characteristics
of the resulting foam. Thus, while the resulting foam may have the desired
hydrophilic character, it may lose some of its other desirable structural
features or performance qualities. Moreover, such specialized, hydrophilic
monomers can be expensive relative to standard monomers used to prepare
foams, and thus their use can increase the overall cost of the foam.
In other processes, some foams have been treated to provide anionic
substituent groups such as carboxylate or sulfonate moleties on their
polymeric structures. Such anionic substituents can be effective in
hydrophilizing the surface of the foams, but, unfortunately, their
utilization can result in foams that are rather stiff and lack resilience.
Such foams are not optimally comfortable when used in close contact with
human skin, as, for example, in diapers and sanitary articles.
In some instances, synthetic hydrophobic foams can be rendered hydrophilic
by incorporating small quantities of surfactants into the foam matrix.
While this can render a foam hydrophilic and quite useful for some
purposes, surfactant-containing foams are not always suitable for use in
prolonged contact with skin, since the surfactant can cause skin
irritation. In addition, some surfactants, e.g., water-soluble ones, can
desorb from the foam and dissolve into the fluid being absorbed by the
foam. This can significantly change the surface tension of the fluid and
dramatically affect the strength with which it is held by the foam.
The manufacture of hydrophilic foams for use as fluid absorbents in
sanitary articles, especially disposable diapers and sanitary napkins,
requires that the foams not only have superior fluid-handling properties,
but also be comfortable to the wearer and safe when used in close
proximity to human skin over prolonged periods of wear. Moreover, it is
important to the performance of foams designed for use in diapers and
catamenials that the fluidity properties of body fluids such as urine and
menses not be substantially affected by the hydrophilizing agent, such as
could happen when some surfactants, e.g., water-soluble ones, are used to
hydrophilize absorbent foams. Accordingly, safe, effective, economical
means for hydrophilizing absorbent foams is of substantial interest to the
manufacturer of such items. The present invention provides a safe and
effective foam hydrophilization method which meets the foregoing
requirements.
BACKGROUND ART
Lindquist; U.S. Pat. No. 3,563,243; Issued Feb. 16, 1971 relates to the use
of oxyalkylene-substituted polyurethane foams in diapers. See also Kao;
Japanese Patent Application 02-239863; Laid Open Sep. 21, 1990.
Jones et. al.; U.S. Pat. No. 4,612,334; Issued Sep. 16, 1986 and Haq et.
al.; U.S. Pat. No. 4,606,958; Issued Aug. 19, 1986 both relate to certain
foams having carboxy and other artionic substituent groups.
Unilever; EPO Patent Application 299762; Published Jan. 18, 1989 relates to
the use of calcium chloride in the manufacture of high internal phase
emulsion foams.
Kelly et. al.; U.S. Pat. No. 4,985,467; Issued Jan. 15, 1991 discloses a
hydrophilic polyurethane foam comprising superabsorbent material. This
patent also cites the following references relating to absorbent foams
and/or other absorbent materials: U.S. Pat. Nos. 4,104,435; 4,717,738;
4,725,629; 4,076,663; 4,454,268; 4,337,181; 4,133,784; 3,669,103;
4,464,428; 4,394,930; 3,900,030; 4,239,043; 4,731,391 and Japanese
55-168104 (1982); 57-92032 (1982); also U.S. Pat. Nos. 3,021,290;
3,171,820; 3,175,025; 4,359,558; and 4,521,544.
Barby et. al.; U.S. Pat. No. 4,797,310; Issued Jan. 10, 1989; Edwards et.
al.; U.S. Pat. No. 4,788,225; Issued Nov. 29, 1988 and Barby et. al.; U.S.
Pat. No. 4,522,953; Issued Jun. 11, 1985 all relate to porous polymeric
materials (foams), some of which contain surfactants and which presumably
are hydrophilic.
SUMMARY OF THE INVENTION
The present invention provides a method for rendering substantially
hydrophobic polymeric foams suitable for absorbing hydrophilic liquids. In
the first step of such a method, both a certain type of surfactant and a
solution formed from a solvent such as water and a certain type of
hydrophilizing agent salt are incorporated into a polymeric foam material
which is substantially hydrophobic in the absence of added or residual
hydrophilizing agents. In a second step of the method herein, this treated
polymeric foam material is dried to remove solvent therefrom and to
thereby leave incorporated within the foam material a substantially
uniformly distributed, hydrophilizing amount of both the surfactant and
the hydrophilizing agent salt. The surfactant which is employed is one
which is substantially water-insoluble and is mild and relatively
non-irritating to the skin. The specific hydrophilizing agent salts which
are essentially employed in this method are the toxicologically
acceptable, hydrated or hydratable calcium and magnesium salts such as
calcium chloride and magnesium chloride.
The present invention is also directed to hydrophilized polymeric foam
materials themselves which are suitable for absorbing hydrophilic liquids.
Such foams have the above mentioned surfactants and hydrophilizing agent
salts incorporated therein in substantially uniformly distributed,
hydrophilizing amounts comprising at least about 0.05% by weight of the
foam. Such foams furthermore contain no more than about 50% by weight of
the foam of free water.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 of the drawings is a photomicrograph of the interstices of a typical
hydrophilizable absorbent foam of the present invention.
FIG. 2 of the drawings represents a blown-apart view of the components of a
diaper structure which has a dual layer absorbent core configuration and
which employs a hydrophilized absorbent foam material as one of its
elements.
DETAILED DESCRIPTION OF THE INVENTION
The hydrophilization method of the present invention deals with the
treatment of polymeric foam materials which are suitable for absorbing
liquids into their foam structures. Polymeric foams can in general be
characterized as the structures which result when a relatively
monomer-free gas or relatively monomer-free liquid is dispersed as bubbles
in a polymerizable monomer-containing liquid, followed by polymerization
of the polymerizable monomers in the monomer-containing liquid which
surrounds the bubbles. The resulting polymerized dispersion can be in the
form of a porous solidified structure which is an aggregate of cells, the
boundaries or walls of which cells comprise solid polymerized material.
The cells themselves contain the relatively toohomer-free gas or
relatively toohomer-free liquid which, prior to polymerization, had formed
the "bubbles" in the liquid dispersion.
As described more fully hereafter, preferred polymeric foam materials
useful in the present invention are those prepared by polymerizing a
particular type of water-in-oil emulsion. Such an emulsion is formed from
a relatively small amount of a polymerizable monomer-containing oil phase
and a relatively larger amount of a relatively toohomer-free water phase.
The relatively monomer-free, discontinuous "internal" water phase thus
forms the dispersed "bubbles" surrounded by the continuous polymerizable
monomer-containing oil phase. Subsequent polymerization of the monomers in
the continuous oil phase forms the cellular foam structure. The aqueous
liquid remaining in the foam structure formed upon polymerization can be
removed by pressing and/or drying the foam.
Highly preferred polymeric foam materials for use in the present invention
are those prepared by polymerizing water-in-oil emulsions containing
certain polymerizable monomers, such as styrene, alkyl(meth)acrylates
and/or divinylbenzene, in the oil phase of such emulsions. The most
preferred polymeric foam materials of this type are those described in the
concurrently filed patent application of DesMarais, Stone, Thompson,
Young, LaVon, and Dyer having U.S. application Ser. No. 07/743,839, U.S.
Pat. No. 5,260,345 entitled "Absorbent Foam Materials for Aqueous Body
Fluids and Absorbent Articles Containing Such Materials," which
application is incorporated herein by reference. Such highly preferred
foam materials will generally have a pore volume of from about 12 to 100
ml/g and a capillary suction specific surface area of from about 0.5 to
5.0 m.sup.2 /g. These foams can be prepared from water-in-oil emulsions
wherein the water to oil weight ratio ranges from about 12:1 to 100:1,
more preferably from about 20:1 to 70:1.
Another common type of polymeric foam material useful in the present
invention comprises the polyurethanes. Polyurethane foams are those
prepared by reacting a polyisocyanate such as a diisocyanate with a
hydroxyl-containing material such as a polyether polyol in the presence of
water and a catalyst. As the polymer forms, the water reacts with the
isocyanate groups to cause crosslinking. Carbon dioxide is also produced,
and this causes foaming. Trifluoromethane or other volatile materials may
also be employed as a blowing agent.
Polymeric foams, including the preferred foams herein prepared from
polymerizable water-in-oil emulsions, may be relatively closed-celled or
relatively open-celled in character, depending upon whether and/or the
extent to which, the cell walls or boundaries, i.e., the cell windows, are
filled or taken up with polymeric material. The polymeric foam materials
useful in the method of the present invention are those which are
relatively open-celled in that the individual cells of the foam are for
the most part not completely isolated from each other by polymeric
material of the cell walls. Thus the cells in such substantially
open-celled foam structures have intercellular openings or "windows" which
are large enough to permit ready fluid transfer from one cell to the other
within the foam structure.
In substantially open-celled structures of the type useful herein, the foam
will generally have a reticulated character with the individual cells
being defined by a plurality of mutually connected, three dimensionally
branched webs. The strands of polymeric material which make up the
branched webs of the open-cell foam structure can be referred to as
"struts." Open-celled foams having a typical strut-type structure are
shown by way of example in the photomicrograph set forth as FIG. 1. For
purposes of the present invention, a foam material is "open-celled" if at
least 80% of the cells in the foam structure are in fluid communication
with at least one adjacent cell.
The polymeric materials that form the foams which are used as the starting
materials in the method of this invention will generally be non-swellable
in aqueous liquids and will also generally be substantially free of polar
functional groups on their polymer structures. Thus after the structures
of such foams have been formed, the foam structure surfaces comprise
polymeric materials which, in the absence of any residual or added
surfactants or other hydrophilizing agents, would be substantially
hydrophobic in character.
The extent to which polymeric foam materials are either "hydrophobic" or
"hydrophilic" can be quantified by referencing the "adhesion tension"
exhibited by such foams in contact with an absorbable test liquid.
Adhesion tension is defined by the formula
AT=.gamma.COS.theta.
wherein
AT is adhesion tension in dynes/cm;
.gamma.is the surface tension of a test liquid absorbed by the foam
material in dynes/cm;
.theta. is the contact angle in degrees between the surface of foam polymer
material and the vector which is tangent to the test liquid at the point
that the test liquid contacts the foam polymer surface.
For any given foam material, the adhesion tension exhibited by the foam can
be determined experimentally using a procedure whereby weight uptake of a
hydrophilic test liquid, e.g., synthetic urine, is measured for a foam
sample of known dimensions and capillary suction specific surface area.
Such a procedure is described in greater detail in the TEST METHODS
section hereinafter.
For purposes of the present invention, a particular foam material is
considered to be substantially hydrophobic if, in the substantial absence
of any added or residual surfactants or other hydrophilizing agents, it
exhibits an adhesion tension of less than about 15 dynes/cm as determined
by capillary suction uptake of synthetic urine having a surface tension of
65.+-.5 dynes/cm. Conversely, a polymeric foam material is considered to
be relatively hydrophilic when it exhibits an adhesion tension of 15
dynes/cm or greater, preferably 20 dynes/cm or greater, as determined by
capillary suction uptake of this same synthetic urine.
In a first step of the method herein, a substantially hydrophobic polymeric
foam is treated so as to incorporate into the foam material both a certain
type of surfactant and a solution comprising a particular type of
hydrophilizing agent which is dissolved in a suitable solvent. The
surfactant which is incorporated into the foam material can comprise any
substantially water-insoluble, mild, relatively non-irritating surfactant
compound which tends to enhance the wettability of the polymeric surfaces
with which it is contacted and onto which it may be deposited. Such
surfactants can include, for example, sorbitan fatty acid esters,
polyglycerol fatty acid esters and polyoxyethylene (POE) fatty acids and
esters. Examples of surfactants of these types include TRIODAN.RTM.20
which is a commercially available polyglycerol ester marketed by Grindsted
and EMSORB.RTM.2502 which is a sorbitan sesquioleate marketed by Henkel.
Especially preferred are the 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
(SPAN.RTM.80). One such particularly preferred surfactant combination
comprises the combination of sorbitan monooleate and sorbitan trioleate in
a weight ratio greater than or equal to about 3:1, more preferably greater
than about 4:1.
Another particularly preferred sorbitan fatty acid ester is, as indicated,
sorbitan monolaurate (SPAN.RTM.20). Sorbitan monolaurate is, in fact, so
beneficial in imparting hydrophilicity characteristics to absorbant foams
that its use as a hydrophilizing agent is the subject of the separate,
concurrently filed U.S. Pat. application of DesMarais and Stone, having
Ser. No. 07/743,838, which application is incorporated herein by
reference. This sorbitan monolaurate material is accordingly highly
preferred for use as the surfactant material to be incorporated into the
foams herein in the context of the present invention.
The surfactant materials of the foregoing type can be incorporated into the
foam materials herein by any suitable means which will result in the
surfactant(s) contacting the polymeric surfaces of the foam material. Most
preferably, this can be brought about by employing the surfactant
material(s) as a component in the process which is used to prepare the
foam materials herein. For the preferred foams herein which are prepared
by polymerizing water-in-oil emulsions, the substantially water-insoluble
surfactant materials can be added as emulsifiers to the monomer-containing
oil phase of such emulsions. In this manner, the surfactant materials
perform the dual role of stabilizing the emulsions to be polymerized and
acting as residual hydrophilizing agents which contact and preferably coat
the polymeric surfaces of the foam structure after this structure is
formed. Surfactant materials can be added to the polymerizable
monomer-containing oil phase to the extent of from about 0.5% to 20% by
weight of the polymerizable monomer materials in the oil phase.
Alternatively, the surfactant materials used in the present invention can
be introduced or reintroduced into the foam material which contains no
surfactant as made or from which residual surfactants have been removed.
Such introduction or reintroduction of surfactant can be carried out by
treating such foams with an appropriate surfactant solution or suspension.
Thus, the water-insoluble surfactants useful herein can be dissolved or
dispersed in a suitable solvent or carrier such as isopropanol, and the
resulting solution or suspension can be contacted with the foam material
to be treated therewith. In this manner, the surfactant materials can be
incorporated into the interstices of the foam structure.
The surfactant materials used in the present invention are generally
incorporated into foam materials in amounts which, in conjunction with the
hydrophilizing agent salt component, impart suitable hydrophilicity
characteristics to the foams so treated. Frequently such amounts of
incorporated surfactant will range from about 0.5% to 20% by weight of the
polymerized foam material, more preferably from about 1% to 16% by weight
of the polymeric foam material.
As indicated, the first step of the method of the present invention also
involves the incorporation into the foam materials herein of a certain
type of hydrophilizing agent salt solution. The essential component of
such a hydrophilizing agent salt solution is a hydrophilizing agent salt
which is selected from the toxicologically acceptable, hydrated or
hydratable calcium and magnesium salts.
Nonlimiting examples of the hydrophilizing agent salts useful herein
include hydrated and hydratable material s such as the following: calcium
tartrate tetrahydrate; calcium thiosulfate hexahydrate; calcium chloride
hexahydrate; calcium chloride tetrahydrate; calcium citrate tetrahydrate;
calcium bromide trihydrate; calcium bromide hexahydrate; calcium sulfate
dihydrate; magnesium orthophosphate octahydrate; magnesium tartrate
pentahydrate; magnesium chloride hexahydrate; magnesium citrate
pentahydrate; magnesium iodide octahydrate; magnesium sulfate
heptahydrate; and magnesium salicylate tetrahydrate. Preferred
hydrophilizing agents herein include hygroscopic or deliquescent salts
such as the following: calcium chloride, calcium bromide, magnesium
chloride and magnesium iodide. Mixtures of these salts may also be
employed.
The calcium and magnesium salts used in the present invention should, of
course, be toxicologically acceptable. Toxicologically acceptable salts
are those which present little or no risk to humans or animals if they are
accidentally ingested or inhaled in amounts which might be encountered
during use or manufacture or after disposal of the hydrophilized foams
herein. Thus, for example, hydrated magnesium arsenate might very well
provide foams of suitable hydrophilicity. This salt, however, is toxic if
ingested or inhaled and would therefore not be encompassed within the
scope of the present invention.
The hydrophilizing agent salts as hereinbefore described will generally be
dissolved in a suitable solvent to form a solution which can be
incorporated into the polymeric foam material to be treated in accordance
with the method herein. Water is the preferred solvent for use in
preparing this treating solution, but various alcohol or water/alcohol
solvents can also be employed. The hydrophilizing agent can be
incorporated into the solution at any convenient concentration. Typically,
solutions containing from about 1% to 10% by weight of solution of the
hydrophilizing agent are used, but higher concentrations, and even
saturated solutions, can be employed. When calcium chloride is used as the
hydrophilizing agent, it is generally employed in aqueous solution at a
concentration of from about 1% to 5% by weight.
As with the surfactant, the solution containing the hydrophilizing agent
salt can be incorporated into the structure of the substantially
hydrophobic polymeric foam material by any convenient procedure which will
result in the solution filling a significant number of the cells within
the foam. Most preferably, this can be brought about by actually employing
the hydrophilizing agent solution in the process which is used to prepare
the foam structure. Thus, for example, an appropriate calcium and/or
magnesium salt may be added to the water phase of a water-in-oil emulsion
which is to be used to prepare the polymeric foam. When such an emulsion
is subsequently polymerized, the solid cellular structure of the foam will
be formed around residual water phase material having the desired
hydrophilizing agent salt dissolved therein.
Alternatively, foams made without any hydrophilizing agent solution used in
their preparation process may, after formation, be treated by repeated
contact and washing with an appropriate solution of hydrophilizing agent
salt to thereby incorporate the solution into the foam. Such treatment of
foam materials which are substantially hydrophobic as formed may be
difficult, however, because hydrophilizing agent solutions, which are
frequently aqueous, may not be readily absorbed into hydrophobic foams. In
such instances, it may be necessary to force hydrophilizing agent solution
into the foam structure by application of pressure or by means of repeated
washing and/or foam squeezing steps. It may also be necessary, for example
in the case of relatively large cell (>90.mu.) hydrophobic foams such as
polyurethanes, to employ an alcohol or water/alcohol solvent for the
hydrophilizing agent in order to realize acceptably uniform distribution
of the hydrophilizing agent salt within the foam structure.
It is, of course, possible to employ a combination of the foregoing
techniques for incorporating hydrophilizing agent salt solution into the
foam structure being treated. Thus, for example, a portion of the
eventually desired hydrophilizing agent salt may be incorporated into the
process liquids used in the preparation of the foam. After formation of
such a foam, additional or replacement hydrophilizing agent salt may be
incorporated during subsequent post-formation treatment, e.g., washing, of
the foam with hydrophilizing agent solution.
In a second process step of the method herein, the polymeric foam material,
with its incorporated surfactant and incorporated solution of
hydrophilizing agent salt, is subjected to drying procedures to remove
therefrom solvent from the hydrophilizing agent salt solution. Drying can
be effected by air, heat, or microwave treatment or by other conventional
methods which serve to remove the solvent, but not excessive amounts of
the hydrophilizing agent salt itself, from the foam structure.
It would be possible in accordance with this invention to completely remove
from the foam solvent which has been used to deliver the hydrophilizing
agent salt into the foam. Such complete removal of the solvent by the
drying step would leave fine particles of the hydrophilizing agent salt
deposited onto the foam surfaces. It will be appreciated from the
discussion hereinafter that foam treated in this manner might be described
as "over-dried" because, upon complete drying, the hydrophilizing agent
salt loses its waters of hydration. Accordingly, the resulting foam
appears to be hydrophobic. However, in the presence of relative humidities
above about 40%, the hydrophilic surface of such completely dried foam
will be restored. More preferably, the foam should not be dried completely
to such a state. Rather, the drying conditions will preferably be adjusted
such that the hydrophilizing agent salt retains its waters of hydration,
and, as such, the resulting foam is hydrophilic as-made. Frequently,
solvent will be removed from the foam structures treated by the method
herein such that residual solvent, e.g., free water, in the foam comprises
no more than about 50% by weight of the (dry) foam, more preferably no
more than about 10% by weight of the (dry) foam.
It will be appreciated that the amount of hydrophilizing agent salt
introduced throughout the foam network and onto the surfaces of the
network structure of the foam via the instant method can vary, according
to the degree of hydrophilization desired and according to the
effectiveness of the hydrophilizing agent salt chosen. For example, with
truly deliquescent hydrophilizing agent salts, a smaller proportion may be
needed for satisfactory results. With salt materials that form lower
hydrates, somewhat more of the hydrophilizing agent salt may have to be
used. In general, the object of the present invention is to incorporate a
hydrophilizing amount of the surfactant and the calcium and/or magnesium
salt(s) into and onto the foam structure. For purposes of this invention,
such an amount can be defined as that quantity of surfactant and of
calcium and/or magnesium salt(s) which, when the salts are fully hydrated,
provides a foam that exhibits an adhesion tension of at least about 15
dynes/cm, preferably at least about 20 dynes/cm, as determined by the
capillary suction uptake at 37.degree. C. of synthetic urine having a
surface tension of 65.+-.dynes/cm.
Typically, the dried foam will comprise at least about 0.05% by weight of
the surfactant plus the hydrophilizing agent salt. When using materials
such as calcium chloride, the foams will generally contain from about 0.1%
to 7% by weight of the hydrophilizing agent salt based on the weight of
the foam material. Higher levels can be used, but it should be appreciated
that excessive amounts of the hydrophilizing agent salt will not, in
general, serve any useful purpose and may cause the resulting foam to shed
dusty particles due to an overloading of the dried hydrophilizing agent
salt.
It will be appreciated that for the surfactant and the hydrophilizing agent
salt to be effective in imparting hydrophilic characteristics to the foam
being treated therewith, both the surfactant and the hydrophilizing agent
salt must be substantially uniformly distributed within the internal
structure of the foam. If the surfactant and/or the hydrophilizing agent
salt is deposited within the foam only in discrete discontinuous zones,
such as may happen for example if the surfactant or hydrophilizing agent
salt solution forms beads or droplets within the foam structure formed by
the polymeric struts, then the full foam hydrophilization effect provided
by the method herein may not be realized.
Treatment of polymeric foam materials in accordance with the method of the
present invention renders such foam materials suitable for absorbing
hydrophilic liquids. While not limited by theory, it appears that the
surfactant and the specific hydrophilizing agent salts used herein, in
fact, do not chemically interact with the foam polymer material. Rather,
it appears that the hydrophilizing agent salts herein function by simply
providing molecules of water of hydration uniformly distributed at the
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