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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The United States produces more than 5 billion lbs. of plastic film each
year, virtually all of which is made from petroleum-based raw materials.
These films are most economically made by an economical extrusion blowing
process in which a tubular extruded bubble is expanded and shaped by air
streams at the die exit. In the field of agriculture, approximately 130
million lbs. of polyethylene (PE) film is used annually as mulch to
improve crop yields by controlling weeds, retaining soil moisture, and
reducing nutrient leaching. Since PE mulch cannot be reused and does not
degrade between growing seasons, it must be removed from the field and
disposed at a current estimated cost of $100 per acre. Other agricultural
uses for plastic film include seedling containers and the protection of
roots during transplanting. These films have also become an important
factor in the packaging of consumer products and as containers for the
disposal of waste.
Rapidly increasing prices, dwindling supplies of petroleum, and the need
for economically feasible biodegradable films that do not adversely affect
the environment upon disposal have intensified the need for alternate
sources of raw materials for making plastics. This invention relates to
the preparation of such films by the blowing of formulations based upon a
renewable resource.
2. Description of the Prior Art
Numerous attempts have been made to produce degradable films from petroleum
and cellulose-derived materials [Chemical Week 109: 45-46 (1971)]
including PE-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 were too costly, or they decompose too slowly for
many applications. Starch is probably the most abundant, low-cost,
biodegradable polymer available and its use in plastic film production
would greatly reduce the demand for petrochemicals and the negative impact
on the environment now caused by discarding nonbiodegradable films. Since
starch alone forms a brittle film that is sensitive to water, it is
generally understood that starch must be combined with other materials in
order to produce a satisfactory product. PE is the most widely used
material for producing films that have desirable physical properties for
packaging and mulch applications, and it is available at a relatively low
cost. It is therefore a particularly desirable material to combine with
starch to achieve the desired flexibility, water resistance, and strength.
However, previous attempts to produce blown films from compositions
containing high levels of starch combined with PE have been unsuccessful.
Griffin (U.S. Pat. No. 4,016,117) teaches that about 8% predried starch
(0.5% moisture), 90% PE, 1.6% ethyl oleate, and less than 1% oleic acid
compositions can be converted to blown films (Example I). However,
essentially the same composition could not be blown into a satisfactory
film if the starch contained as much as 2% moisture (paragraph bridging
columns 3 and 4; Example VII). The product became disfigured and weakened
by the presence of numerous small bubbles created by the conversion of the
free moisture to steam. This limitation on the moisture content requires
special drying, handling, and storage techniques preparatory to film
formation. Griffin further observed that both gelatinizing the starch
(column 3, lines 36-39) and increasing the starch content of film
formulations from 5 to 15% (Example XI) resulted in feel and crease
retention properties much more paperlike than unmodified PE film.
The discovery by Otey et al. (U.S. Pat. No. 4,133,784) that compositions of
ethylene arylic acid copolymer (EAA) and a starchy material can be formed
into films that are flexible, water resistant, heat sealable, and
biodegradable has intensified interest in the possibility of making
starch-based films. These films were formed by either casting, simple
extruding, or milling the starch-EAA composition. All are relatively slow
processes that are considerably more expensive than the more conventional
extrusion blowing technique. The relatively high processing cost coupled
with the high price of EAA compared to PE tend to diminish this
composition's potential for achieving large-scale commercial success.
Also, at certain starch levels needed for achieving desired mechanical
properties, the optimum degrees of biodegradability and UV stability are
compromised.
Our attempts to incorporate pelletized PE into the pelletized EAA and
starch composition described by Otey et al. (U.S. Pat. No. 4,133,734) and
to convert the composite into blown films were not successful. Continuous
blowing was difficult because the films ruptured. Visible striations and
other evidence of poor compatibility between the starch and resin
components were also indicative of an inferior product.
SUMMARY OF THE INVENTION
It has now been unexpectedly discovered that formulations containing up to
about 60% gelatinized starch and various levels of EAA, and optionally PE,
can be readily blown into high-quality biodegradable films having the feel
and general appearance of conventional plastic films. These results are
accomplished by the addition of a sufficient amount of neutralizing agent
to neutralize part or all of the acidic portion of the EAA and by blowing
the formulation at a moisture content in the range of about 2-10%. This
discovery is entirely unexpected especially in view of Griffin's
observations reported above. It was also a surprising discovery that when
PE was incorporated into the composition, it increased both UV stability
and biodegradability of the films.
In accordance with this discovery, it is an object of this invention to
provide a method for blowing film-forming compositions at starch and
moisture levels higher than heretofore possible.
It is also an object of the invention to prepare films that are stable to
weathering conditions for a predetermined period and then decompose.
It is a further object of the invention to incorporate PE into starch-based
films while maintaining or enhancing their biodegradability.
Other objectives 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." "Uniform films" as used in this
application refer to those which are virtually free of breaks, tears,
holes, bubbles, and striations.
"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 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 final 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."
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. 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 flours, starch components, and modified starch
products as described above.
In the preparation of the instant starch-based films, the starchy materials
must be partially or completely gelatinized. Gelatinization is effected by
any known procedure such as heating in the presence of water or an aqueous
solution at temperatures of about about 60.degree. C. until the starch
granules are suffiently 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 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 acrylic
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 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 preferred neutralizing agent for use in the invention is ammonia in
either its anhydrous or aqueous form. 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 about one-half equivalent per
equivalent of acid in the EAA. Normally the level of ammonia addition will
be about 0.8-5 weight percent based on the dry weight of the starch-EAA-PE
formulation. The ammonia is believed to form an ammonium salt with the
acid as evidenced by an infrared spectrophotometer peak in the range of a
carbonyl salt observed in the final film product. Any excess ammonia added
to the formulation tends to be driven off during the processing steps
described below. Likewise, it is expected that a portion of the ammonia
associated with the EAA volatilizes during blowing. Other suitable
neutralizing agents would include simple amines which are substantially
similar to ammonia in their tendency to form salts with organic acids.
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. If the starch has
been pregelatinized, its moisture content at the time of addition 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% and preferably 20 to 50% by weight moisture,
based on total solids, should be present, Excess moisture is then removed
from the composition by evaporation during the processing operations.
While the inclusion of PE in the film formulation is desirable from an
economic standpoint, it suprisingly increases the UV stability and the
rate of biodegradation of the resulting products. Any grade of PE that can
be blown into a film is suitable for the instant process. Low density PE
is normally used for this purpose.
The proportions of starchy material, EAA, and PE may be varied over wide
ranges in order to tailor the resultant film properties to the desired end
use. Based upon the combined weight of these three components, the starchy
material content may be in the range of 10-60%, and preferably on the
order of 30-40%. As the starch level approaches 60%, the weather and tear
resistance drop considerably, the film becomes translucent, and the other
physical properties become fair to poor. Acceptable levels of EAA
copolymer are in the range of 10-90%, with the preferred amount being in
the range of about 30-70%, depending on the proportion of PE. PE levels
may range from 0-80%, but at a starch content of 30-40%, PE amounts in the
range of 10-40% are preferred for acceptable physical properties and
blowing characteristics.
If the starch is to be gelatinized during the mixing operation, the
formulation should be heated to at least 60.degree. C. Simultaneous
gelatinization and EAA melting are preferably conducted at temperatures of
95.degree.-100.degree. C. The gelatinized starch and melted EAA form a
homogeneous plasticized matrix. In the second stage of heating and mixing,
temperatures of 125.degree.-145.degree. C. are suitable for adjusting the
moisture content to the appropriate range for blowing, and for fluxing any
added PE into the matrix. 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
equipment with heating, mixing, venting, and extrusion blowing capability.
While the ammonia may be added at almost any time prior to blowing, it is
most advantageously incorporated toward the end of the heating operation
in order to minimize losses by evaporation. Immediately upon addition of
the ammonia, the viscosity of the matrix increases rapidly, suggesting a
significant change in the composition due to its presence.
The blown film product is a flexible composite of the gelatinized starch,
the EAA ammonium salt, and the PE (if present). 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.
Other materials, either polymeric or monomeric, may be added to the
composition in order to achieve specific properties in the film. For
example, polyvinyl alcohol may be added in varying amounts to improve the
rate of biodegradation, and UV stabilizers such as carbon black can be
added to greatly improve resistance of the film to sunlight. Other
additives include those conventionally incorporated into agricultural
mulches and packaging films including fungicides, herbicides,
antioxidants, fertilizers, opacifying agents, stabilizers, etc. These
materials and additives may be employed in conventional amounts as
determined by the skilled artisan, and may collectively comprise up to 80%
of the film composition.
By continuous feeding of the plasticized formulations of this invention
into the blowing apparatus, continuous blown films can be readily
obtained. It is also obvious to those skilled in the art that these
formulations could be extruded into thin film, rods, or hollow tubing or
that they could be injection-molded into finished products that would be
biodegradable.
The following examples further illustrate the invention but should not be
construed as limited the invention which is defined by the claims.
All percents herein disclosed are "by weight" unless otherwise specified.
EXAMPLES 1-5
Blown Film Preparations: Starch-EAA.
A mixture of air-dried corn starch (11% moisture) and enough water to equal
the total solids in the final composition were blended for 2-5 min.. at
95.degree. C. in a steam-heated Readco mixer (type: 1 qt. Lab. made by
Read Standard Div., Capitol Products Corp., York, PA) to initiate
gelatinization of the starch. EAA pellets (type: 2375.33 manufactured by
Dow Chemical Co.) were added, and heating at 95.degree. C. to 100.degree.
C. and mixing were continued for about 45 min. during which time the EAA
melted and the formulation was converted into a uniform matrix. Aqueous
ammonia was then added and the viscosity of the matrix rapidly increased.
Mixing was continued for about 5 min. Due to water loss by evaporation,
the resultant matrix contained about 25 to 35% moisture. To further reduce
the moisture content, the 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%. The
exudate was a transparent, flexible, strong plastic. This material was
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 screw r.p.m.
was about 70-80, torque reading was 400-500 meter-grams, barrel
temperature was 120.degree.-130.degree. C., and the die temperature was
set in the range of 125.degree.-145.degree. C.
Compositions and properties of films prepared by this procedure are
reported in the Table, below. The physical properties were determined by
standard procedures. Tensile strength was measured on a "Scott Tester" and
is reported as the maximum load per unit area of original cross-section
required to break a test specimen. The percent elongation is the extension
recorded when the specimen ruptured, expressed as a percentage of the
original length of the section under test. The "Weather-Ometer" data
indicates the number of hours until the sample showed cracks in a twin arc
model DMC-HR Weather-Ometer (Atlas Electric Devices Co.) operated on a
cycle of 120 min. of light only followed by 18 min. of light and water
spray using a black panel temperature of 63.degree. C.
EXAMPLES 6-10
Blown Film Preparations: Starch-EAA-PE.
The procedure of Examples 1-5 was repeated except that PE pellets were
added to the formulation about 15 min. after EAA addition. Since the
temperature conditions of the Readco mixer were insufficient to melt the
pellets, the extrusion through the 24-hole die was preceded by two or
three extrusions through a 1/4-in. orifice at a barrel temperature of
about 135.degree. C. to flux the PE. During the extrusion blowing
operation, higher levels of PE required temperatures near the upper end of
the 125.degree.-145.degree. C. range.
Compositions and properties of films prepared by this procedure are
reported in the Table. The MIT fold test conducted with a "Folding
Endurance Tester" (Tinius Olson Testing Machine Co.) shows the number of
times a specimen can be folded before breaking when subjected to
continuous folding through an angle of 135.degree. under a tension of 500
g. The burst factor data was collected with a "Mullen Tester" (B. F.
Perkins and Son, Inc.) and indicates the amount of pressure required to
rupture a specimen.
TABLE
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Tensile
Elonga- "Weather-
Fungi susceptibility,
Example
Formulation, %.sup.a,b
strength,
tion, MIT fold,
Burst
Ometer,"
weeks.sup.d
No. Starch
EAA PE p.s.i.
% Ammonia.sup.c
No. folds
factor
hours 1 2 3 4
__________________________________________________________________________
1 10 90 0 3470 260 4.9 -- -- 402 0 0 0 0
2 20 80 0 4140 120 4.3 -- -- 212 0 0 0 0
3 30 70 0 3225 150 3.8 -- -- 168 0 0 0 0
4 40 60 0 3870 92 3.3 -- -- 90 1- 1 1 1
5 50 50 0 3940 61 2.7 -- -- 90 1+ 2+ 3 3+
6 40 50 10 3570 80 3.6 3800 24 111 2 3 4 4
7 40 40 20 3477 66 2.2 7000 24 134 1 2 3 4
8 40 30 30 3150 36 1.7 2700 21 134 1 2 3 4
9 40 20 40 2920 34 2.8 4800 19 199 -- -- -- 4
10 40 10 50 1840 10 2.8 470 9 559 -- -- -- 4
__________________________________________________________________________
.sup.a Based on combined dry weight of starch, EAA, and PE, exclusive of
water and NH.sub.3.
.sup.b Formulations of Examples 4, 6, 9, and 10 additionally contained
about 1% antioxidant ("Irganox 1035," Ciba Geigy Corp.).
.sup.c Parts of ammonia per 100 parts of formulation dry weight.
.sup.d ASTM D 192470. Larger numbers indicate more fungal attack.
Blown Film Properties
Properties of film samples from Examples 1-5 reveal effects of increasing
the starch level from 10% to 50% without any PE in the formulation. All of
the films containing up to 50% starch were transparent, flexible,
self-supporting, and generally were considered to have good physical
properties. However, the degree of transparency and flexibility decreased
slightly as the level of starch was increased. All of the samples were
uniform and indicated that good compatibility existed between the starch
and EAA. It was apparent from the general appearance of these films, their
blowing rate, and the flow characteristics of their plasticized
formulations that the maximum level of starch which could be incorporated
to achieve acceptable films was about 60%, with the preferred level at
about 40%. As the starch level increased, there was a significant decrease
in film resistance to artificial weathering in a "Weather-Ometer."
Deterioration was attributed to UV instability which caused small cracks
or tears in the film. More significantly was the lack of fungal attack
under controlled conditions with up to 30% starch and a very slow attack
with 40 % starch present. While it is expected that all the films are
biodegradable, the ASTM method used for measuring fungal attack did not
extend beyond 4 wks.
Examples 6-10 reveal that films with 40% starch and up to about 40% PE were
clear, flexible, self-supporting, and uniform, indicating good
compatibility. Above about 40% levels of PE, the films were less
transparent and in some instances translucent, and were observed to have
less tear resistance. In contrast, to the film samples without PE, the
films prepared in Examples 6-10 reflect a substantial increase in
resistance to "Weather-Ometer" exposure as increasing amounts of PE were
incorporated into formulations. Furthermore, the addition of PE greatly
increased the fungal attack on the samples showing that the film would
biodegrade more readily when exposed to outdoor soil contact.
Films corresponding to those prepared in Examples 4 and 8 were subjected to
a 35-da. outdoor exposure test. The film with 40% starch and 60% EAA
developed cracks within 11-13 da. while that containing 40% starch, 30%
EAA, and 30% PE did not develop any cracks.
EXAMPLE 11
Starch-EAA: The Effects of Ammonia Omission and Vapor Treatments
A composition was prepared essentially as described in Examples 1-5 except
that ammonia was omitted from the formula. Exclusive of moisture, the
formulation contained 40% starch, 59.5% EAA, and 0.5% pentachlorophenol
(fungicide). After the mixing and extruding through the 24-hole die, the
matrix was blended cold on a rubber mill for 2-3 min. One-half of this
product was passed through the blown film die. The film contained streaks
indicating poor compatibility. The blown film was then exposed to ammonia
vapors in a closed container for a few minutes and again passed through
the blown film die to produce a clear, uniform film with good physical
properties.
The remaining half of the rubber-milled products was sealed in a plastic
bag containing aqueous ammonia for a few minutes and then passed through
the blown film die to produce a good quality, uniform, clear film.
EXAMPLE 12
Starch-EAA-PE: The Effect of Ammonia Omission
A composition was prepared as described in Examples 6-10 except that
ammonia was omitted from the formula. Exclusive of moisture, the
formulation contained 40% starch, 30% EAA, and 30% PE. The matrix was
repeatedly passed through the blown film die but a clear, uniform film
could not be obtained. The film contained white spots and frequently
ruptured during the blowing attempts.
EXAMPLE 13
Composition Containing Carbon as a UV Stabilizer
A composition was prepared as described in Examples 6-10 except that carbon
black (Industrial Reference black No. 3) was blended into melted EAA prior
to blending the other components. Composition of the blown film, exclusive
of moisture and ammonia, was 5% carbon black, 32.5% EAA, 32.5% PE, and 30%
starch. The blown film had a tensile strength of 2000 p.s.i., elongation
of 62%, and withstood "Weather-Ometer" exposure for 710 hr. before any
cracks or evidence of deterioriation occurred.
EXAMPLE 14
Composition Containing Polyvinyl Alcohol, Sorbitol, and Glycerol
A mixture of air-dried corn starch (11% moisture) polyvinyl alcohol (Vinol
425 made by Air Products and Chemicals, Calvert City, KY), sorbitol,
glycerol, and enough water to equal the weight of total solids in the
formula was blended at 95.degree. C. in the Readco mixer for 1 1/6 hr.
Then enough aqueous ammonia was added to equal about 2.6% ammonia based on
the dry solids weight of the composition. After another 1/3 hr. of mixing,
the composition was passed twice through the 24-hole extrusion head and
then the blown film die as described in Examples 1-10. A transparent
flexible film was obtained that had a tensile strength of 3500 p.s.i. and
an elongation of 300%. The dry film composition was 25% starch, 25% PVA,
18% sorbitol, 2% glycerol, and 30% EAA. When exposed to soil
microorganisms according to ASTM D 1924-70, 100% of the sample was covered
with mold growth within 1 wk.
EXAMPLE 15
A film was prepared essentially as described in Examples 6-10, except the
composition of the final dry film was 30% starch, 10% polyvinyl alcohol,
30% EAA, and 30% PE. Tensile strength of the film was 2096 p.s.i.,
elongation was 25.5%, the fungi susceptibility was 4 after 4 wk. as
described by ASTM D 1924-70, and the blown film was resistant to
"Weather-Ometer" conditions for 146 hr.
EXAMPLE 16
In the Brabender mixer was melted a quantity of EAA pellets. The air-dried
starch (12.2% moisture) was slowly added with mixing to the molten EAA at
about 90.degree. C. and the formulation stirred for 15 min. to form a
matrix. Aqueous ammonia was added to the matrix and stirring was continued
for another 1/4 hr. The product was then blown into a film using the same
1/2-in. blown film die and procedure as described in Examples 1-5.
Composition of the film exclusive of moisture and ammonia was 40% starch
and 60% EAA. The amount of ammonia added was equal to 3.5 parts per
hundred parts of starch plus EAA. The blown film had a tensile strength of
2404 p.s.i. and an elongation of 82%.
* * * * *
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