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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
While cellulose-based films have achieved prominence as semipermeable
membranes, starch-based films have not been available that would withstand
prolonged exposure to water. Starch and cellulose are high polymers
composed of D-glucose units. Their molecules differ only in weight and in
the manner in which the glucose units are joined together. Cellulose is a
linear polysaccharide consisting of 6,000 to 8,000 1,4-linked
.beta.-D-glucose units. Because of this 1,4-.beta.-linkage, these chain
molecules can align themselves alongside each other to form linear
crystals or microfibrils. These structural properties contribute to strong
hydrogen bonding, film-forming capabilities, and high resistance to
gelatinization in water. In contrast, most common starches contain 17-27%
linear polysaccharide (amylose) consisting of 400 to 1,000 1,4-linked
.alpha.-D-glucose units and the remaining composition is a branched
molecule (amylopectin) having 10,000 to 40,000 1,4- and 1,6-linked
.alpha.-D-glucose linkage. Because of the 1,4-.alpha.-linkage, the amylose
molecules assume a spiral or helical shape having six glucose units per
spiral. Starch readily disperses in hot water to form starch-pastes
possessing unique viscosity characteristics and film-forming behavior.
However, such films are very brittle upon drying and are very sensitive to
water. This invention relates to certain starch-based formulations which
yield durable semipermeable films having potential applications as
dialyzing membranes.
2. Description of the Prior Art
In U.S. Pat. No. 4,337,181, Otey et al. teach the preparation of flexible,
starch-based films which are water resistant, yet biodegradable. The films
are formed by shaping a composite of a water-dispersible ethylene acrylic
acid (EAA) copolymer and a starchy material by the methods of extrusion
blowing, simple extrusion, or molding. The resultant products have
disclosed utilities as agricultural mulch, as well as disposable packaging
and bagging materials. In this procedure, gelatinized starch and EAA are
combined in a plasticized matrix, and then the acidic portion of the EAA
is at least partially neutralized with ammonia or an amine. It is by
virtue of the neutralization and adjustment of the moisture content that
the matrix can be extrusion blown. Films prepared by this process have no
observed permeability properties.
SUMMARY OF THE INVENTION
It has now been surprisingly discovered that when starch-EAA formulations
such as those of Otey et al., supra, are neutralized with a strong alkali
as opposed to ammonia or an amine, it is possible to prepare films having
semipermeable characteristics. These results are accomplished by
converting to a plasticized matrix a mixture of highly gelatinized starch,
EAA, and sufficient strong alkali to neutralize substantially all of the
EAA. The matrix is thereafter shaped into a film by any method known in
the art.
In accordance with this discovery, it is an object of the invention to
prepare starch-containing semipermeable films useful as dialysis
membranes.
It is also an object of the invention to prepare highly transparent,
starch-based films which are resistant to degradation under conditions of
dialysis.
It is a further object of the invention to tailor the permeability of the
films of the invention to predetermined molecular sizes by controlling the
formulation constituents.
Other objects and advantages of the invention will become readily apparent
from the ensuing disclosure.
DETAILED DESCRIPTION OF THE INVENTION
"Films," such as those made in accordance with the invention, are defined
by the polymer industry (Encyclopedia of Polymer Science and Technology,
John Wiley & Sons, Inc., 1967, Vol. 6, page 764) as "shaped plastics that
are comparatively thin in relation to their breadth and width and have a
maximum thickness of 0.010 in." Self-supporting films are those capable of
supporting their own weight. "Uniform films," as used in this application,
refer to those which are virtually free of breaks, tears, holes, bubbles,
and striations.
"Semipermeable membranes" are those films through which certain substances
pass while others are retained. Typically, it is at the molecular level
that such membranes are semipermeable.
"Composite" is defined herein in accordance with The American Heritage
Dictionary of the English Language, New College Edition, published by
Houghton Mifflin Company, page 27, to mean "a complex material . . . in
which two of more distinct, structurally complementary substances,
especially . . . polymers, combine to produce some structural or
functional properties not present in any individual component."
The starch-based films of the invention are prepared from any unmodified
starch from cereal grains or root crops such as corn, wheat, rice, potato,
and tapioca, from the amylose and amylopectin components of starch, from
modified starch products such as partially depolymerized starches and
derivatized starches, and also from starch graft copolymers. The term
"starchy materials" as used in the specification and in the claims is
defined herein to include all starches, starch flours, starch components,
and other starch products as described above. The term "starch-based" in
reference to the products of the invention is used in the broad sense of
containing a starchy material.
In the preparation of the instant starch-based films, the starchy materials
must be highly gelatinized by the time of the shaping steps. By "highly"
gelatinized, it is meant that all or substantially all of the starch
granules are sufficiently swollen and disrupted that they form a smooth
viscous dispersion in the water. Gelatinization is effected by any known
procedure such as heating in the presence of water or an aqueous solution
at temperatures of about 60.degree. C. The presence of strong alkali is
known to facilitate this process. The gelatinization may be carried out
either before or after admixing the starchy material with the EAA as
discussed further below.
The EAA copolymer must have sufficient carboxyl functionality so as to be
compatible with the starch for purposes of preparing the disclosed films.
It is believed that the pendant carboxyl groups supplied by the acylic
acid component associate with the hydroxyl groups of the starch, thereby
contributing to the compatibility and composite formation of the starch
and the EAA. These carboxyl groups coincidentally contribute to the water
dispersibility of the copolymer. We have found as a rule of thumb that if
the EAA is water dispersible, it will also be sufficiently compatible with
the starch.
The preferred EAA is a product prepared by copolymerizing a mixture
comprising about 20% acrylic acid and 80% ethylene, by weight. However, it
is to be understood that EAA copolymers having somewhat different
proportions of polymerized acrylic acid and ethylene would also yield
acceptable starch-based films provided that they contain a sufficient
number of carboxyl groups to be water dispersible.
The strong alkali of choice for use in the invention is sodium hydroxide,
though potassium hydroxide would also be effective. The amount added to
the film compositions may be varied over a wide range so long as enough is
initially present to equal at least one equivalent per equivalent of acid
in the EAA. Normally, the level of alkali addition will be about 2 to 8
weight percent based on the dry weight of the starch-EAA formulation. The
alkali cation is believed to form a salt with the acid as evidenced by the
weight change observed upon soaking and redrying the film. Addition of an
amount of alkali in excess of that required for neutralization insures
that the starch attains an adequate degree of gelatinization.
On a dry weight basis, the starch:EAA ratio must be at least 20:80, and
should not exceed about 60:40. The range is preferably from about 30:70 to
about 50:50. Below ratios of 20:80, the permeability of the film to even
the smallest of molecules becomes insignificant. At this level, we have
found that only trace amounts of urea (MW 60) will pass through. As the
starch:EAA ratio approaches 60:40, the degradation and tear resistances
drop considerably, the film becomes translucent, and the other physical
properties become fair to poor. Within the designated ratios, the
proportions may be varied to tailor the films' permeability to the desired
end-use. Permeability may also be controlled by selection of the starchy
material type.
If the starch has been pregelatinized, its moisture content at the time of
addition to the starting mixture is not particularly critical provided
that enough moisture is available in the system to permit dispersing the
EAA. If the added starch is granular, sufficient moisture must be provided
to allow partial or complete gelatinization. Either way, during the
initial mixing of the formulation components, at least 10-20% water based
on total solids should be present. Excess moisture is then removed from
the composition by evaporation during the subsequent processing
operations.
In the preferred embodiment of the invention, the starch is gelatinized in
the presence of the strong alkali during the mixing operation. The mixture
should be heated sufficiently to simultaneously gelatinize the starch and
melt the EAA, resulting in the formation of a plasticized matrix. With the
aforementioned EAA copolymer comprising 20% acrylic and 80% ethylene, this
can be accomplished at temperatures as low as 60.degree. C., though the
operation is more readily conducted at 95.degree.-130.degree. C.
Shaping of the matrix into the semipermeable films is most expeditiously
achieved by means of extrusion blowing. The term "extrusion blowing" is
well known in the art and distinguishes from simple extrusion in that it
relates to shaping a tubular extrudate, or "bubble" into its form by
internal and external cooling streams of air, the internal stream causing
expansion of the bubble to several times the size of the die opening.
Films prepared by this technique are commonly referred to as "blown
films." By continuous feeding of the plasticized formulations of this
invention into the blowing apparatus, uniform, continuous blown films can
be readily obtained.
The moisture content of the film formulation just prior to and after
blowing must be maintained within the range of about 2 to 10% (w/w) and
preferably between 5 and 8%. Compositions with moisture contents outside
of this range do not produce a uniform, continuous film.
A second stage of mixing at temperatures of 125.degree.-145.degree. C. is
suitable for adjusting the moisture content to the appropriate level.
Since the formulations are readily blown at these temperatures, further
temperature adjustment is unnecessary. Of course, the gelatinization,
mixing, moisture reduction, and film blowing could all be conducted in one
continuous operation using commercial equpiment with heating, mixing,
venting, and extrusion blowing capability.
It is envisioned that the films could also be formed by other known methods
to include simple extrusion, milling, and casting provided that the
plasticized matrix is prepared accordingly. Typically, the formulations
and preparatory steps for simple extrusion would be substantially the same
as those set forth above. In casting, the EAA, starch, and alkali are
dispersed in water in an amount of about 5-15 times the weight of the
starch. Treatment in a high-shear blender accelerates dispersion. When
heated, the resultant suspension is converted to a thin plasticized matrix
which is readily cast and dried in a suitable manner. For purposes of
milling, it is advisable to hold the moisture content to the minimal level
required for gelatinization. The formulation of ingredients is passed
through a conventional mill such as a rubber mill, and the resultant
plasticized matrix is rolled into a thin sheet or film.
The product as produced by any of the aforementioned procedures is a
flexible composite of the gelatinized starch and the EAA salt. Without
desiring to be bound to any particular theory, it is believed that the EAA
salt associates with the gelatinized starch molecules and holds them in
the same expanded flexible state in which they exist in the heated matrix.
The instant films are transparent.
As indicated by the relative diffusion rates of diverse molecules, these
films have semipermeable properties useful in a variety of applications as
known in the art. The absence of these properties in films produced by the
process of Otey et al., supra, establishes the criticality of
incorporating strong alkali into the formulation.
The films are prepared for use as dialyzing membranes by conditioning them
in water or other suitable aqueous solution. During soaking, they imbibe a
considerable amount of water, causing expansion and a leaching out of any
excess alkali. When mounted in test dialysis cells in the presence of
water, the expanded membranes tend to remain stable for several weeks with
no apparent distortion or change in appearance.
Additives may be incorporated into the composites to alter their properties
during preparation or in use. It was mentioned above that these films
could be tailored for specific permeability characteristics by controlling
the relative proportions of starch and EAA, as well as by the particular
choice of starchy material. It is also possible to enhance permeability by
incorporation of water-soluble additives which are leached out during the
conditioning step. Many conventional plasticizers and other extractable
materials as determined by the skilled artisan are suitable for this
purpose. However, the principle by which their extraction from the
membrane alters the permeability characteristics is not currently
understood.
The following examples further illustrate the invention but should not be
construed as limiting the invention which is defined by the claims.
All percents herein disclosed are "by weight" unless otherwise specified.
EXAMPLES 1-3
A. Film Preparation. A mixture of air-dried corn starch (11% moisture) and
sodium hydroxide dissolved in an amount of water to provide 50% solids in
the complete formulation were blended for 2-5 min. at
95.degree.-100.degree. C. in a steam-heated Readco mixer (type: 1-qt. lab
made by Read Standard Div., Capitol Products Corp.) to gelatinize the
starch. EAA pellets (type: 2375.33 manufactured by Dow Chemical Co.) were
added while heating and mixing were continued. After the mixture was
stirred and heated for a total of about 45 min., the resulting plasticized
matrix was extrusion processed with an extrusion head attached to a
Brabender Plasti-Corder (type: PL-V300, manufactured by C. W. Brabender
Instruments, Inc., South Hackensack, N.J.). The screw of the extruder was
3/4-in. in diameter, 9 in. long, and had a compression ratio of 2:1. The
die consisted of 24 circular holes of 1/32-in. diameter. This extrusion
process was repeated usually one or two more times until the moisture
content of the exudate was between about 5 to 10%. This exudate was then
blown into a film by passing it through the same extruder except that the
die was replaced with a heated 1/2-in. blown film die. The barrel and die
temperature ranged from 105.degree.-110.degree. C. As the level of starch
was increased, extrusion blowing became increasingly more difficult until
at about 60% the limits of the equipment were attained.
B. Semipermeability Analysis. Rotating dialysis cells were constructed from
"Plexiglas" as follows: Each half of the cell was prepared by cutting a
round 9.4-cm. diameter hole from a 0.6-cm. thick "Plexiglas" acrylic disc
(13 cm. diameter), and this disc was then laminated to the same diameter
"Plexiglas" disc that was 1.3-cm. thick. A "Plexiglas" rod (3.5 cm.
diameter and 3 cm. long) was glued to the outside corner of each half for
attaching the motor chuck. Two filling holes were drilled 180.degree. C.
apart on the edge of each compartment and provided with threaded plugs
fitted with O-rings. Four-inch O-rings were fitted into grooves machined
in the face of each compartment to seal the membranes clamped between the
two halves.
The maximum available volume in each compartment was 46 cm..sup.3 and the
area of exposed film, measured across contact with the large O-ring (100
mm. diameter) was 78.5 cm..sup.2 The entire measured area of the membrane
was used to calculate observed permeabilities (P.sub.o). Even though the
cell is only partly filled, the diffusion process apparently continues due
to a thin film of solution being carried over the arc above the solution
during rotation of the cell.
Thickness of air-dried film specimens was measured at nine locations and
reported as average thickness. The films were then soaked in water for
several hours, and clamped, while wet, between the two cell halves. The
cell was then attached to a motor chuck and the complete assembly
positioned so that the axis of cell rotation was horizontal. A weighed
syringe was used to introduce about 34 cm..sup.3 water into one side (B)
and the same volume of solution into the other side (A). The filling holes
were immediately plugged and the motor started. Since volumes A and B were
essentially equal, P.sub.o was calculated from the following equation:
P.sub.o =V ln (.DELTA.C.sub.o /.DELTA.C)/2At
where .DELTA.C.sub.o /.DELTA.C is the ratio of the concentration
differences initially and at time t, A is the membrane area (78.5
cm..sup.2) through which transport took place, and V is the volume of
liquids (about 34 cm..sup.3) in each side of the cell. Initial
concentration, C.sub.o, of each solute was 1.5% (w/w).
Diffusion rates of solutes through the membranes were followed by
colorimetric analyses for the individual runs of urea and sugars. Sodium
chloride concentrations were measured by atomic absorption analysis of
sodium on a Varian Techtron AA120 spectrophotometer. The diffusivity of
each of the tested solutes was individually determined. The results are
reported in Table I, below.
EXAMPLE 4
The procedure of Examples 1-3 was repeated except that the corn starch was
gelatinized in aqueous NaOH at 90.degree. C., freeze dried, and then
blended with the EAA and additional water to provide a formulation having
50% solids. The diffusivity of the sugars and alanine were simultaneously
determined by introducing a mixed solution thereof [each at 1.5% (w/w)] to
side A of the test cell. Solutions withdrawn from the cells at the
conclusion of each run were analyzed by HPLC. The results are reported in
Table I.
EXAMPLES 5-6
The procedure of Examples 1-3 was repeated except that water-soluble
additives (20% ethylene glycol, Example 5; 2% glycerol and 6% glycol
glucoside, Example 6) were blended into the mixture at the time of
formulation, and in Example 5 the amount of water added with the NaOH
provided a solids content of 77%. The diffusivity of the sugars and
alanine were simultaneously determined as in Example 4. The results are
reported in Table I.
EXAMPLES 7-10
The procedure of Example 1 was repeated except hydroxyethyl starch (a
modified corn starch sold under the trade name "Amaizo 742D," American
Maize Products Co.) was substituted for the unmodified corn starch. In
Examples 8-10, various levels of sucrose were blended into the mixtures at
the time of formulation. Diffusivity of the sugars and alanine were
simultaneously determined as in Example 4. The results are reported in
Table I.
EXAMPLES 11-16
The procedure of Example 1 was repeated except that starch graft copolymers
were substituted for the unmodified corn starch. The copolymers were
prepared by grafting either methyl methacrylate, methyl acrylate, or
acrylonitrile onto corn starch by the procedure reported in Polym. Lett.
6: 599-602 (1968). Diffusivity of the sugars and alanine were
simultaneously determined as in Example 4. The results are reported in
Table I.
EXAMPLES 17-19
For purposes of comparison, the procedure of Example 1 was repeated except
ammonia as ammonium hydroxide was substituted for the NaOH in the
specified amounts.
TABLE I
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Film
Ex-
Starchy thick-
P.sub.o .times. 10.sup.6
(cm./sec.)
am-
material, EAA, NaOH,
ness, Glu-
Fruc-
Su-
Raf-
Ala-
ple
%.sup.a %.sup.a
Additives, %.sup.a
p.p.h..sup.b
mils
NaCl
Urea
cose
tose
crose
finose
nine
__________________________________________________________________________
1 starch, 20 80 . . . 5 1.6 . . .
0.01
. . .
. . .
. .
. .
. . .
2 starch, 60 40 . . . 7.5
2.5 31.6
52.8
6.3
10 2.9
2.5 . . .
3 starch, 40 60 . . . 5 1.9 7.2 12 1.5
1.1 0.4
0.08
. . .
4 starch, (freeze
40 60 . . . 5 1.5 5.1 17.5
0.9
1.2 0.6
0.1 2.4
dried)
5 starch, 30 50 ethylene glycol,
20
5 1.7 11.8
33.6
2.0
2.3 0.9
0.2 4.0
6 starch, 40 52 glycerol,
2
5 2.3 . . .
21.6
4.1
. . .
2.0
1.1 . . .
glycol glucoside,
6
7 hydroxyethyl,
40 60 . . . 5 1.2 4.6 20 1.0
1.1 0.4
0.1 2.2
8 hydroxyethyl,
40 50 sucrose, 10
5 1.9 8.7 37 0.8
1.0 0.4
0.1 1.9
9 hydroxyethyl,
40 40 sucrose, 20
5 2.2 20.4
66 4.8
5.5 2.6
1.2 4.3
10 hydroxyethyl,
40 30 sucrose, 30
5 1.9 34.7
112 5.1
5.7 2.6
1.3 6.5
11 starch/methyl
40/10
50 . . . 5 1.8 9.7 25.4
1.3
1.7 0.8
0.4 2.7
methacrylate,
12 starch/methyl
40/20
40 . . . 5 3.1 5.1 15 0.3
0.4 0.3
0 1.1
methacrylate,
13 starch/methyl
40/10
50 . . . 5 2.5 20.8
88 3.7
4.2 1.7
0.6 6.1
acrylate,
14 starch/methyl
40/20
40 . . . 5 1.9 11.1
34.7
4.4
5.2 2.7
1.4 6.8
acrylate,
15 starch/acryloni-
40/10
50 . . . 5 1.3 11.1
30 1.5
1.8 0.7
0.3 3.2
trile,
16 starch/acryloni-
40/20
40 . . . 5 2.2 11.2
26.0
0.8
1.0 0.7
0.3 2.2
trile,
17 starch, 20 80 . . . .sup. 0.sup.c
0.8 . . .
0 0 0 0 0 0
18 starch, 40 60 . . . .sup. 0.sup.d
1.0 . . .
0 0 0 0 0 0
19 hydroxyethyl,
40 60 . . . .sup. 0.sup.d
1.9 0 0 . . .
. . .
. .
. .
. .
__________________________________________________________________________
.
.sup.a All percents are expressed on a dry weight basis.
.sup.b P.p.h. is parts per hundred of starchy material plus EAA plus othe
additives, on a dry weight basis.
.sup.c Formulation was neutralized with 4.3 p.p.h. NH.sub.3.
.sup.d Formulation was neutralized with 3.2 p.p.h. NH.sub.3.
With respect to the NaOH incorporated at 5 p.p.h. in the preceding
examples, the molar amount of NH.sub.3 was 100% more for Example 17, and
50% more for Examples 18 and 19. In Example 19, the starchy material was
the hydroxyethyl starch used in Examples 7-10. Diffusivity determination
was conducted as in Example 4, except for Examples 17 and 18 in which the
concentration of each solute tested was 2% (w/w).
EXAMPLE 20
The procedure of Examples 1-3 was repeated wherein the formulation on a dry
basis was 40% starch, 20% water-soluble sucrose, 40% EAA, and 5 p.p.h.
NaOH, and the diffusivity of 7.65% ethanol was determined and analyzed by
gas chromatography. Diffusivity results were: sodium chloride, P.sub.o
=14.4.times.10.sup.-6 cm./sec.; urea, P.sub.o =26.7.times.10.sup.-6
cm./sec.; and ethanol, P.sub.o =17.9.times.10.sup.-6 cm./sec.
EXAMPLE 21
The procedure of Examples 1-3 was repeated wherein the formulation on a dry
basis was 40% starch, 15% urea, 45% EAA, and 5 p.p.h. NaOH. The resulting
film was especially pliable, uniform, and transparent. Diffusivity results
were: sodium chloride, P.sub.o =13.5.times.10.sup.-6 and urea, P.sub.o
=24.times.10.sup.-6 cm./sec.
EXAMPLE 22
The films prepared in accordance with Examples 2, 3, and 6 were subjected
to wet and dry tensile measurements using an Instron. Thirty 0.635
cm..times.10 cm. strips were cut from each film. Ten were used for dry
tensile and elongation measurements and the other 20 were soaked 4 hr. in
water, after which excess water was removed by blotting lightly with paper
towels. Ten of the soaked strips were used for wet tensile and elongation
measurements. The effects of soaking were determined from the remaining 10
strips. All of the films expanded and imbibed a considerable amount of
water as indicated by the weight increase. After the wet samples were
taken to dryness, they were reweighed. Weight losses indicate that the
sodium hydroxide, in excess of that needed to neutralize the 20% acrylic
acid in the EAA, and the glycerol and glycol glucoside (Example 6) are
extracted within 4 hr. The results are reported in Table II, below.
It is understood that the foregoing detailed description is given merely by
way of illustration and that modification and variations may be made
therein without departing from the spirit and scope of the invention.
TABLE II
__________________________________________________________________________
Tensile
Formulation.sup.a strength,
Elongation,
Effect of water soaking.sup.c
Starch, EAA,
NaOH,
kg./cm..sup.2.spsp.b
%.sup.c
Length
Weight
Weight
Film % % p.p.h.
Dry
Wet
Dry
Wet
increase, %
increase, %
loss, %
__________________________________________________________________________
Example 2
60 40 7.5 255
24.4
7.6
27.5
12.4 58.9 7.5
Example 3
40 60 5 238
44.9
50.4
48.3
9.5 40.4 1.8
Example 6.sup.d
40 52 5 198
44.5
35.1
45.0
8.4 33.8 8.7
__________________________________________________________________________
.sup.a Based on dry weight exclusive of water. NaOH given in parts per 10
parts formulation.
.sup.b Values are average of 10 specimens. Wet strengths based on dry
crosssectional areas; all samples broke at jaws.
.sup.c Values are average of ten specimens.
.sup.d Contained 2% glycerol and 6% glycol glucoside.
* * * * *
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