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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to biodegradable films, particularly, those useful
for agricultural mulch and heat-sealable packaging films that are stable
to weathering conditions for a predetermined period and then decompose.
Applying plastic film for agricultural mulching has become an important
technique for increasing the yield and quality of vegetable and fruit
crops and for decreasing the production costs of these crops. Principal
benefits of mulching are to provide weed control, warm the soil for early
crop production, control soil moisture, and reduce nutrient leaching.
The packaging of various items in plastic films has become an important
factor in the safe and economical distribution and use of many consumer
products.
2. Description of the Prior Art
Polyethylene is the most common resin used for making mulch and packaging
films. However, when used as a mulch, it must be removed from the field
and burned or buried at the end of each fruiting season since it does not
decompose in time to start the subsequent crop. In fact, the film persists
for several years if not removed. Because the removal and burying or
burning of mulch film is both costly and has an adverse effect on the
ecology, the need for a plastic mulch that will decompose by the end of a
growing season has recently become apparent. Likewise, the buildup and
pollution caused by plastic packaging films in municipal landfill and
disposal areas could be reduced if such films were degradable.
Numerous attempts have been made to produce degradable films from petroleum
and cellulose derived materials (Chemical Week 109: 45-46 (1971) including
polyethylene-coated paper (Chemical Week 110: 44 (1972) and polybutene-1
films (U.S. Pat. No. 3,590,528). None has been completely successful,
apparently because they are too costly or they decompose too slowly for
many applications.
Although whole starch has been investigated for many years as a potential
raw material for nonsupported films, it has never achieved large-scale
commercial success because its films are brittle and are greatly affected
by moisture (Cereal Chemistry 40: 154-161 (1963). Large amounts of
compatible water-soluble plasticizers, such as glycerol, are effective
softening agents for whole starch, but the resulting dried films are too
soft and tacky at high humidity and have virtually no wet strength. Also,
upon contact with water, the plasticizers are readily leached out leaving
a brittle product upon drying. A solution to the tackiness and wet
strength problem is disclosed by Otey et al. in U.S. Pat. No. 3,949,145
wherein a starch-polyvinyl alcohol glycerol film is coated with a
water-resistant coating. However, the application of coatings is expensive
and does not completely prevent the leaching of plasticizers. In the area
of packaging, starch-based films heretofore known in the prior art have
not been heat-sealable.
SUMMARY OF THE INVENTION
It has now been unexpectedly discovered that by producing a composite of a
water-dispersible ethylene acrylic acid (EAA) copolymer with a starchy
material, flexible and water-resistant films can be prepared which are
also self-supporting, biodegradable, and heat-sealable. These films
obviate the need for plasticizers and film coatings and eliminate the
problems associated therewith as experienced in the prior art. Methods of
preparation include casting and extruding heated aqueous dispersions of
the starchy material and EAA, and also milling and rolling dry mixtures
into sheets or films.
In accordance with this discovery, it is an object of this invention to
provide a synthetic agricultural mulch which is biodegradable and does not
need to be taken up after the growing season.
It is also an object of the invention to prepare an inexpensive
agricultural mulch without the need of plasticizers or coatings.
Another object of the invention is to provide an agricultural mulch which
can be readily tailored to withstand outdoor weathering conditions for a
predetermined period of exposure.
A further object of the invention is to provide a biodegradable and
heat-sealable packaging film which can be readily disposed of without
detriment to the environment.
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 and 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."
"Composite" is defined herein in accordance with The American Heritage
Dictionary of the English Language, New College Edition, published by
Houghton Mifflin Company, page 273, to mean "a complex material . . . in
which two or more distinct, structurally complementary substances,
especially . . . polymers, combine to produce some structural or
functional properties not present in any individual component."
The starch-EAA films of the invention are prepared from any unmodifed
starch from cereal grains or root crops such as corn, wheat, rice, potato,
and tapioca. The amylose and amylopectin components of starch as well as
modified starch products such as partially depolymerized starches and
derivatized starches may also be used. The term "starchy materials" as
used in the specification and in the claims is defined herein to include
all starches, starch components, and modified starch products as described
above.
In the preparation of the instant starch-based films, it is preferred that
the starchy materials be gelatinized. Gelatinization is effected by any
known procedure such as heating in the presence of water or an aqueous
solution at temperatures of above about 60.degree. C. until the starch
granules are sufficiently swollen and disrupted that they form a smooth
viscous dispersion in the water. The gelatinization may be carried out
either before or after admixing the starchy material with the EAA as
discussed further below.
The EAA is a critical component in the preparation of the starch-based
films having the properties disclosed herein. It is believed that the
pendant carboxyl groups supplied by the acrylic acid component contribute
to the water dispersibility of the copolymer. It is also believed that the
carboxyl groups associate with the hydroxyl groups of the starch, thereby
contributing to the compatibility and composite formation of the starch
and the EAA. We have found as a rule of thumb that if the EAA is water
dispersible, it will also be compatible with starch for purposes of
preparing the disclosed films.
The preferred EAA is a water-dispersible 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 relative amounts of starch and EAA used to make the films are dependent
upon the particular film applications. As the percentage of starch is
increased, the films degrade more rapidly when in contact with soil
microorganisms, become more sensitive to water, become stronger and less
flexible.
For biodegradable mulch applications, where soil coverage must last for at
least 30 days, the films should contain starch in the range of about
20-40%. When a preservative is added to the film, such as paraformaldehyde
at a level of 2%, the amount of starch can be increased to about 50-60%.
Films with up to 50% starch and 2% paraformaldehyde have been found to
last more than 70 days in outdoor soil exposure.
The preferred life of a mulch film is dependent upon the crop application.
For quick maturing crops such as lettuce and radishes and for crops that
require mulch film for frost protection, the film needs to last only a few
days or weeks. Other crops such as tomatoes need a film that will provide
soil coverage for a few months.
For packaging applications in which the films are not subjected to
biodegradation conditions until after disposal, the proportions of the
components can be varied over an even wider range depending upon the
physical requirements of the films. For the packaging of dry items where a
high degree of flexibility is not required, the starch level may range
from 10-90% of the total film composition.
In addition to improving the mechanical properties of starch films, the EAA
improves the water resistance and provides heat-sealing properties. EAA
levels may range from 10-90% and constitute the full balance of the film
compositions not comprised of starch. However, in order for the films to
retain flexibility after repeated contacts with water and in order for
them to have good heat-sealing qualities, they must contain more than
about 20% EAA.
The films are formed by either casting, extruding, or milling the film
formulations. In casting, the EAA copolymer and the starch are dispersed
in water in an amount of about 5-15 times the weight of the starch, and
the resultant suspension is then heated with stirring, cast as a film, and
dried in a suitable manner. The EAA and the starch may both be dispersed
separately or together by mechanical means such as with a high shear
blender. Alternatively, the EAA may first be chemically dispersed in an
aqueous ammonium solution wherein the pendant carboxyl groups are
converted to their ammonium salts. After casting, the salts are reverted
to the acid form by drying at elevated temperatures, thereby driving off
ammonia. Preferred drying temperatures are in the range of
110.degree.-130.degree. C. The heating step prior to casting is intended
to gelatinize the starch, if necessary, and to remove any bubbles from the
dispersion which would otherwise reduce the quality of the film.
Temperatures in the range of 60.degree.-100.degree. C., and preferably
80.degree.-100.degree. C., for a period of 0.5-2 hours are sufficient to
effect gelatinization. If the starch were gelatinized prior to dispersing
with the EAA, milder conditions such as on the order of
50.degree.-100.degree. C. for 0.5-1 hour would be effective for removing
the bubbles. Any of the conventional methods of casting and drying films
known to those skilled in the art are suitable for the purpose of the
invention. Forced air drying and oven drying have been found to be
particularly useful.
In an extrusion operation, the starch is mixed with water and heated until
it attains a viscosity suitable for producing an extrudable matrix when
admixed with the EAA. Heating is continued during admixture with the EAA,
and the composite is then extruded through a die as a film. Either the
starch may be pregelatinized, or else granular starch is used and the
water content and temperatures during the mixing and/or extrusion steps
are selected, as within the skill of a person in the art, to effect its
gelatinization.
When milled into films, the starch-EAA composites may be prepared from
either granular or gelatinized starch. A dry mix of the components is
passed through a conventional mill such as a rubber mill at plasticizing
temperatures, preferably in the range of about 128.degree.-135.degree. C.,
and the resultant plasticized matrix is rolled into a thin sheet or film.
Additives may be incorporated into the starch-EAA films to alter the
properties during preparation or in use. An organic acid, preferably
stearic acid, is added to the formulation to reduce the viscosity of the
paste before casting, and to provide good mold release. Amounts in the
range of about 0.1-10% are suitable, though amounts of 1-5% based on the
dry weight of the film are normally preferred. Other additives include
those conventionally incorporated into agricultural mulches and packaging
films including fungicides, herbicides, fertilizers, opacifying agents,
stabilizers, etc. These additives may be employed in conventional amounts
as determined by the skilled artisan.
The reason that these starch-based films have good water resistance and
require no plasticizer is not fully understood. Normally, starch molecules
are expanded and quite flexible when first cast or extruded from an
aqueous dispersion. Upon drying they contract, and various bonding forces
cause them to become brittle. Without desiring to be bound to a particular
theory, it is possible that the added EAA in accordance with this
invention may be associating with the starch molecules enough to hold them
in their expanded, flexible state.
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 1-11
Preparation by casting
In a round bottom flask, equipped with a heated water bath, were mixed
air-dried granular corn starch, aqueous ammonium dispersion containing 25%
EAA (PCX-100 manufactured by Union Carbide Corporation), enough water to
equal 10 times the dry weight of starch, and stearic acid. In Examples 6
and 9, enough paraformaldehyde was added to equal 2% of the dry weight of
product. The mixtures were then heated by rotating the flask in an
80.degree.-90.degree. C. water bath for 1 hour. Then the resulting thick
dispersions were cast with a doctor blade at 30-mil. wet-thickness onto
silicone-coated plate glass preheated to 80.degree.-90.degree. C. The
products were then dried to clear films in a forced air oven at
120.degree. C. for 5 minutes. The films were immediately removed from the
glass plates and equilibrated at 50% relative humidity.
Composition and properties of these films are listed in Table I.
Water resistance
Film samples from each of the Examples listed in Table I were repeatedly
soaked in water for 24 hours, then dried for 24 hours for a total of 13
times. Those films (Examples 1-4) that contained 40% or less starch were
flexible and strong after the 13 cycles of soaking and drying; those films
(Examples 5, 7, and 8) with 50%, 60%, and 70% starch retained a film
characteristic but became less flexible as the level of starch was
increased; and those films (Examples 10 and 11) with 80% and 90% starch
lost their film characteristic during the soaking and drying treatments
and became brittle.
Outdoor exposure
Film samples from Examples 3-6, 8, 9, and 11, which contained 30%, 40%,
50%, 70%, and 90% starch were exposed to outdoor soil contact, with their
ends buried in soil, to observe their resistance to sunshine, rain, and
soil microorganisms. If no rain fell during any week, the samples were
sprinkled with enough water to equal 1/2 inch of rain. Example 11, with
90% starch was badly torn within 1-3 days after water soaking primarily
because of embrittlement and shrinkage upon drying. Heavy mold growth
occurred on the buried portion of this sample within 3 to 5 days. Similar
deterioration was observed for samples containing 50 and 70% starch
(Examples 5 and 8) but not until about 5 to 7 days of soil exposure.
Examples 3 and 4 that contained 30 and 40% starch remained flexible and
intact for more than 30 days. Deterioration of the latter films was
attributed primarily to microbial attack, as evidenced by heavy mold
growth, rather than embrittlement and shrinkage. Examples 6 and 9, that
were formulated with 50% and 70% starch and 2% paraformaldehyde,
demonstrated much better resistance to microbial attack. Those with 70%
starch and 2% paraformaldehyde (Example 9) became somewhat brittle and had
small tears due to shrinkage within 5 to 7 days but they continued to
provide soil coverage for about 15 days, while those with 50% starch and
2% paraformaldehyde (Example 6) remained flexible and provided good soil
coverage throughout the test period of 70 days.
As long as the films remained in good condition, the soil under them was
moist even when the surrounding area became dry. Within a few weeks after
the exposed surface of the films became severely damaged, the buried
portion had degraded into small particles.
Heat sealing
Film samples from Examples 1-5, 7, 8, 10, and 11 of Table I were folded and
placed in a Clamco Handy Bag Sealer, Model 70, made by the
Cleveland-Detroit Corporation. Those samples that contained more than 19%
EAA (Examples 1-5, 7, and 8) readily formed a tight seal due to melting of
the film, while those with 9 and 19% EAA did not melt sufficiently enough
to form a strong seal.
EXAMPLE 12
Preparation by extruding
In a Brabender Plastograph was mixed 67.1 g. corn starch and 70 g. water at
115.degree. C. until the viscosity of the mixture was 100 meter-grams of
torque. Water evaporated during this mixing such that the mixture was 83%
starch and 17% water. Then 42 g. (containing 35 g. of starch) of the
mixture was mixed with 35 g. of dry EAA (PCX-300, Union Carbide
Corporation) and the composite mixed in the Brabender Plastograph at
115.degree. C. and then extruded through a die as a thin film. The film
was flexible and strong.
EXAMPLE 13
Preparation by milling
Various proportions of dry starch and dry EAA (PCX-300, Union Carbide
Corporation), ranging from 10-50% starch and 90-50% EAA, were fluxed on a
conventional rubber mill at 121.degree.-132.degree. C. for about 10
minutes and then pulled from the rolls as thin sheets or films. All films
were flexible and strong.
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 I
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Properties
Composition, wt. % Tensile.sup.1
Example Stearic
Formal-
Thickness
strength
Elongation
Fold.sup.2
Burst.sup.3
Fungi.sup.4
No. Starch
EAA acid dehyde
(mils.)
(p.s.i.)
(%) endurance
factor
resistance
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1 10 84 4 0 3.1 2395 181 5025 18.4
0
2 20 76 4 0 2.3 2483 116 2972 20.7
2
3 30 66 4 0 2.1 2525 56 1262 22.2
4
4 40 56 4 0 1.9 2528 44 531 21.7
4
5 50 46 4 0 2.2 3003 35 420 18.8
4
6 50 44 4 2 -- -- -- -- -- --
7 60 37 3 0 2.0 3186 21 349 20.6
4
8 70 29 1 0 1.5 3933 10 194 19.9
4
9 70 27 1 2 -- -- -- -- -- --
10 80 19 1 0 1.5 5345 10 61 15.0
4
11 90 9 1 0 1.5 7190 15 94 17.9
4
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.sup.1 Scott Tester, Scott Testers, Inc., Providence, Rhode Island.
.sup.2 Folding Endurance Tester, Tinius Olsen Testing Machine Co., Willow
Grove Pennsylvania.
.sup.3 The Mullen Tester, B. F. Perkins and Son, Inc., Holyoke,
Massachusetts.
.sup.4 ASTM D 1924-70.
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