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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to biodegradable packaging material derived from
high amylose starch and to the method of preparation thereof.
Problems associated with the handling of environmental waste, particularly
the large amount of discardable plastic products and the limited volume of
land fill facilities, has placed added emphasis on developing products
which are either biodegradable or recyclable. This is particularly true in
the packaging areas where large volumes of discardable plastic packaging
materials are used in various forms, including containers, sheets, films,
tubing and fillers. Because of this large increase in the use of plastic
materials, it has been proposed to make throwaway materials from
biodegradable plastics to alleviate the waste disposal problems. Several
reasons have prevented the development and likelihood of developing this
technology except in special situations. First of all, the high volume
packaging plastics such as polyethylene, polystyrene, polypropylene and
polyethylene terephthalate are low cost and are not biodegradable.
Attempts to make such materials biodegradable by blending them with
biodegradable fillers or additives have not been overly successful. Those
existing plastics which are biodegradable, are deficient in properties
required in most packaging applications and are more expensive than
commonly used packaging plastics. Degradable plastics are more difficult
to recycle than nondegradable plastics. Furthermore, another reason the
nondegradable plastics are preferred in landfill sites is because they do
not generate noxious or toxic gases.
Starch, a readily available, known biodegradable material, has been used to
prepare foamed and film products as well as other shaped products for
different purposes including selected packaging applications. In Patent
Cooperation Treaty (PCT) Publication No. WO 83/02955, a foamed starch
product is formed by extruding starch in the presence of a gas expanding
agent, the product being useful in various applications such as foam
sheets or fillers for packing.
The use of starch materials to form film products is well known, as shown
e.g., in British Patent No. 965,349 which discloses the extrusion of
amylose material without using solvents, to form films having excellent
tensile strength. Another film forming operation using starch is shown in
U.S. Pat. No. 3,116,351 where an unsupported amylose film is made by
extruding an aqueous alkali-amylose solution into a coagulation mixture of
ammonium sulfate and sodium sulfate.
U.S. Pat. No. 4,156,759 discloses a process for preparing low cost
polyurethane foam by incorporating a starch containing amylaceous material
into the foamed material yielding rigid or flexible and high resilient
products.
U.S. Pat. No. 3,137,592 shows the extrusion of starch to produce an
expanded gelatinized product in different shapes and forms, such as
ribbon, ropes and tubes, which are useful in a variety of applications.
U.S. Pat. No. 3,336,429 involves a method for producing clear, thin.
elongated shaped structures of amylose in forms such as film, tubes, bands
and filament, by extruding an aqueous caustic solution of high amylose
material through an aqueous acid bath.
U.S. Pat. No. 3,891,624 discloses the preparation of a dispersible,
hydrophobic porous starch product by extrusion of a selected hydrophobic
starch material at a temperature of 100.degree. to 250.degree. C. and a
moisture content of 4 to 15 percent.
The use of starch in foods and confectionery products is well known. One
area where starch use has been of particular interest involves expanded
products such as snack foods and dry pet foods. The quality of such
products, as evidenced by their crispiness, is affected by expansion
volume which was studied and reviewed in two recent articles by R.
Chinnaswamy and M. A. Hanna: "Relationship Between Amylose Content and
Extrusion-Expansion Properties of Corn Starch", Cereal Chemistry, Vol. 65,
No. 2, 1988, pp. 138 to 143 and "Optimum Extrusion-Cooking Conditions for
Maximum Expansion of Corn Starch", Journal of Food Science, Vol. 53, No.
3, 1988, pp. 834 to 840.
The use of starch in the manufacture of confectionery products is disclosed
in U.S. Pat. No. 3,265,509 where a mixture of high amylose starch and
sugar is passed through an extruder in the presence of less than 25%
moisture, to form a solid, plastic, shape-retaining confectionery mass.
While the disclosures noted above show the use of amylose containing starch
materials in forming films and various other shaped products, the use of
such materials in packaging has generally been limited to selected
applications such as film wrappings for food. The area involving
resilient, compressible, low density packaging materials for uses such as
protective packaging, has been generally left to lightweight plastics,
including expanded polystrene, more particularly Styrofoam (registered
trademark of Dow Chemical Co.). However, as noted earlier, these materials
are not biodegradable and, therefore, the need still exists for a material
which will meet the demanding requirements of the packaging industry while
satisfying the ever increasing governmental regulations and controls for
environmental waste.
SUMMARY OF THE INVENTION
The present invention provides a biodegradable packaging material
comprising an expanded amylose starch product having at least 45% by
weight amylose content, said expanded product having a low density, closed
cell structure with good resilience and compressibility properties. More
particularly, the expanded packaging material of this invention has a
uniform closed cell structure with a bulk density of less than about 2.0
lb/ft.sup.3, a resiliency of at least about 50% and a compressibility of
from about 100 to 800 g/cm.sup.2.
Another embodiment of this invention relates to a method of preparing low
density, biodegradable packaging material comprising extruding an amylose
starch having at least 45% amylose content, in the presence of a total
moisture content of 21% or less by weight, at a temperature of about
150.degree. to 250.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
The ability to provide a packaging material which is biodegradable, is an
important feature of this invention. The term "biodegradable"as used
herein refers to the susceptibility of a substance to decomposition by
living things (organisms/microorganisms) and/or natural environmental
factors, e.g., the ability of compounds to be chemically broken down by
bacteria, fungi, molds and yeast. Plastics used in packaging, especially
polystyrene are not biodegradable. This creates a problem in the area of
low density packaging, where expanded polystrene such as Styrofoam is used
in large volumes in many applications, particularly protective packaging
or fillers. While starch is a material with known biodegradable
properties, its use in packaging has not been widespread primarily because
it lacked many of the physical attributes required of packaging materials.
Now, in accordance with this invention, a biodegradable, low density, low
cost packaging material is obtained by expanding a high amylose starch
material, having at least 45% by weight of amylose content, through an
extruder in the presence of a total moisture content of 21% or less by
weight, at a temperature of from about 150.degree. to 250.degree. C. The
expanded, high amylose starch material has excellent resilience and
compressibility properties, which coupled with its low density, make it
attractive for use as a packaging material, particularly in the area of
protective packaging.
The starting starch material useful in this invention must be a high
amylose starch, i.e., one containing at least 45% by weight of amylose. It
is well known that starch is composed of two fractions, the molecular
arrangement of one being linear and the other being branched. The linear
fraction of starch is known as amylose and the branched fraction
amylopectin. Starches from different sources, e.g., potato, corn, tapioca,
and rice, etc., are characterized by different relative proportions of the
amylose and amylopectin components. Some plant species have been
genetically developed which are characterized by a large preponderance of
one fraction over the other. For instance, certain varieties of corn which
normally contain about 22-28% amylose have been developed which yield
starch composed of over 45% amylose. These hybrid varieties have been
referred to as high amylose or amylomaize.
High amylose corn hybrids were developed in order to naturally provide
starches of high amylose content and have been available commercially
since about 1963. Suitable high amylose starches useful herein are any
starches with an amylose content of at least 45% and preferably at least
65% by weight. While high amylose corn starch has been especially
suitable, other starches which are useful include those derived from any
plant species which produces or can be made to produce a high amylose
content starch, e.g., corn, peas, barley and rice. Additionally, high
amylose starch can be obtained by separation or isolation such as the
fractionation of a native starch material or by blending isolated amylose
with a native starch.
The high amylose starch used in this invention may be unmodified or
modified and the term starch as used herein includes both types. By
modified it is meant that the starch can be derivatized or modified by
typical processes known in the art, e.g., esterification, etherification.
oxidation, acid hydrolysis, cross-linking and enzyme conversion.
Typically, modified starches include esters, such as the acetate and the
half-esters of dicarboxylic acids/anhydrides, particularly the
alkenylsuccinic acids/anhydrides; ethers, such as the hydroxyethyl- and
hydroxypropyl starches; starches oxidized with hypochlorite; starches
reacted with cross-linking agents such as phosphorus oxychloride,
epichlorohydrin, hydrophobic cationic epoxides, and phosphate derivatives
prepared by reaction with sodium or potassium orthophosphate or
tripolyphosphate and combinations thereof. These and other conventional
modifications of starch are described in publications such as "Starch:
Chemistry and Technology", Second Edition, edited by Roy L. Whistler et
al., Chapter X; Starch Derivatives: Production and Uses by M. W. Rutenberg
et al., Academic Press, Inc., 1984.
One modification of the high amylose starches used in this invention that
is especially advantageous, is the etherification with alkylene oxides,
particularly those containing 2 to 6, preferably 2 to 4, carbon atoms.
Ethylene oxide, propylene oxide and butylene oxide are exemplary compounds
useful in etherifying the starting starch materials with propylene oxide
being especially preferred. Varying amounts of such compounds may be used
depending on the desired properties and economics. Generally, up to 15% or
more and preferably, up to about 10%, by weight, based on the weight of
starch will be used. Extruded starches modified in this manner, showed
improved expansion, uniformity and resiliency.
Additive compounds may also be combined or blended with the starch starting
material to improve properties such as strength, flexibility, water
resistance, resiliency, color, etc. as well as to provide repellency to
insects and rodents, if needed or desired. Compounds such as polyvinyl
alcohol, monoglyceride, and polyethylene vinyl acetate are typical
additives which may be used. They are used in any amount provided the
extrusion of the starch and the properties of the expanded product are
suitable. Typically, up to about 50% by weight of such additives, and
preferably up to about 10% by weight, may be used.
In addition to the above noted modified starches and additive compounds, a
pregelatinized form of the starch starting material may be used, if
desired.
The method used in preparing the packaging materials of this invention is
an extrusion process wherein the starting high amylose starch is fed into
an extruder and conveyed through the apparatus under select conditions.
The product emerging from the extruder is an expanded, closed cell, low
density material with good resilience and compression properties making it
particularly suitable for packaging applications. Extrusion is a
conventional well known technique used in many applications for processing
plastics and has been used to a lesser or limited extent in processing
food starches as noted in some of the disclosures cited earlier which show
extrusion of starch materials to produce products such as films, foods and
confectioneries and gelatinized starches.
The essential feature of this invention is the ability to produce an
expanded, biodegradable starch product having a uniform, closed cell
structure with low density and good resilience and compressibility
properties. This is accomplished by the extrusion of a high amylose
starch, i.e., starch having at least 45% and preferably at least 65% by
weight amylose content, at a total moisture or water content of 21% or
less by weight and at a temperature of from about 150.degree. to
250.degree. C.
The important property characteristics of the extruded product of this
invention are its relatively light weight, as evidenced by bulk density,
as well as its resilience and compressibility. The uniform, closed cell
structure of the product with its characteristic tiny bubble formation,
not only results in a Styrofoam-like appearance and density, but gives it
the necessary resilience and compressibility needed for different
packaging applications. A closed cell structure is defined as one having
largely nonconnecting cells, as opposed to open cells which are largely
interconnecting or defined as two or more cells interconnected by broken,
punctured or missing cell walls. The tiny bubble formation generally
results in a small cell size of typically about 100 to 600 microns.
The bulk density, resilience and compressibility properties of the product
are measured in accordance with procedures described hereinafter The bulk
density of the product is less than about 2.0 lb/ft.sup.3, preferably less
than about 1.0 and more preferably less than about 0.6 lb/ft.sup.3. The
resilience is at least about 50% and preferably at least about 60% and the
compressibility will range from about 100 to 800, preferably about 150 to
700 and more preferably from about 400 to 600 g/cm.sup.2.
In order to obtain the expanded, closed cell structure characteristic of
the desired product , it is important that the total moisture content of
the high amylose starch material feed be at a level of 21% or less by
weight, based on the dry weight of starch material. By total moisture or
water content is meant both the residual moisture of the starch, that is
the amount picked up while stored at ambient conditions, and the amount of
water fed to the extruder. Typically, starch, and particularly high
amylose starch, will contain about 9 to 12% residual moisture. Enough
water must be present to allow the material to be processed, mixed and
heated to the desired temperatures. While some water may be added to the
extruder, only an amount which will bring the total moisture level to 21%
or less can be added. This is necessary to allow for the desired expansion
and cell structure formation in the prepared product. Accordingly, while
the total moisture content that is used for carrying out the process may
vary somewhat, depending on the actual material used and other process
variations, a range of from about 10 to 21%, preferably from about 13 to
19% and more preferably from about 14 to 17% by weight, will generally be
suitable. The temperature of the material in the extruder will be
increased to reach about 150.degree. to 250.degree. C. This temperature
must be maintained in at least the section of the extruder closest to the
die and just before the material leaves the extruder. The die is
positioned at the point or location at the end of the extruder from which
the extruded material emerges or exits the apparatus into the ambient air.
Depending on the particular material being processed, as well as other
process variations, this temperature can vary somewhat within the noted
range and preferably will be from about 160.degree. to 210.degree. C. When
modified starch such as the etherified material is used, the temperature
used will preferably be from 160.degree. to 180.degree. C. while the use
of unmodified starch will have a preferred temperature of from about
170.degree. to 210.degree. C. in at least the section of the extruder
closest to the die. By maintaining these conditions in the extruder, the
material upon leaving the die and extruder outlet into the open air,
expands and cools to form an expanded, low density, resilient and
compressible starch product.
The apparatus used in carrying out this process may be any screw. type
extruder. While the use of a single, or twin-screw extruder may be used,
it is preferred to use a twin-screw extruder. Such extruders will
typically have rotating screws in a horizontal cylindrical barrel with an
entry port mounted over one end and a shaping die mounted at the discharge
end. When twin screws are used, they may be corotating and intermeshing or
nonintermeshing. Each screw will comprise a helical flight or threaded
section and typically will have a relatively deep feed section followed by
a tapered transition section and a comparatively shallow constant-depth
meter section. The screws, which are motor driven, generally fit snuggly
into the cylinder or barrel to allow mixing, heating and shearing of the
material as it passes through the extruder.
Control of the temperature along the length of the extruder barrel is
important and is controlled in zones along the length of the screw. Heat
exchange means, typically a passage, such as a channel, chamber or bore
located in the barrel wall, for circulating a heated media such as oil, or
an electrical heater such as calrod or coil type heaters, is often used.
Additionally, heat exchange means may also be placed in or along the shaft
of the screw device.
Variations in any of the elements used in the extruder may be made as
desired in accordance with conventional design practices in the field. A
further description of extrusion and typical design variations can be
found in "Encyclopedia of Polymer Science and Engineering", Vol. 6, 1986,
Pp. 571 to 631.
The expanded product resulting from the extrusion of the high amylose
starch has excellent properties for packaging, particularly in the areas
of protective packaging. The finished product has properties making it
comparable in most aspects to Styrofoam, or expanded polystyrene with the
added feature that it is biodegradable.
An additional and important feature of the product of this invention is
that is does not retain an electrostatic charge buildup as commonly found
in plastics. This static-free characteristic, makes the material
especially attractive for the protective packaging of sensitive electrical
apparatus or devices, unlike the traditional commercially available
Styrofoam material which requires a special or different grade product for
this purpose.
It is also noted that the expanded starch product may be formed in
different shapes by varying the size and configuration of the die opening.
The product thus may be obtained in forms, such as sheets which differ
from the typical rope or cylindrical product thereby extending the type of
packaging and configuration in which it might be used. The product,
accordingly, may be used as a filler or as a protective cushioning
packaging material, particularly for sensitive electrical equipment and
devices.
In the following examples which are merely illustrative of the various
embodiments of this invention, all parts and percentages are given by
weight and all temperatures are in degrees Celsius unless otherwise noted.
The following procedures were used to determine the characteristic
properties of material being evaluated and as specified throughout the
specification and claims:
Bulk Density
The method used to determine the bulk density of the material was the
volume replacement method described by M. Hwang and K. Hayakawa in "Bulk
Densities of Cookies Undergoing Commercial Baking Processes", Journal of
Food Science, Vol. 45, 1980, pp. 1400-1407. Essentially, this involved
taking a beaker of known volume, i.e., 500 ml. and determining the weight
of small glass beads (diameter 0.15-0.16 mm) needed to fill the beaker.
This allowed the density of the glass beads to be established (formula
below). The weight of a sample was measured and by measuring the weight of
glass beads that were needed to replace the volume of that sample, the
density of the sample was calculated using the following equations:
##EQU1##
where d.sub.s =density or sample
W.sub.s =weight of sample
W.sub.gr =weight of glass beads needed to replace volume of sample
d.sub.g =density of glass beads
W.sub.gb =weight of glass beads needed to fill beaker
V.sub.b =volume of beaker
Resiliency
The resiliency (also called rebound resilience or relaxation) refers to the
ability of a material to recover to its original shape after it has been
deformed by a force and was determined using a Stevens LFRA Texture
Analyzer employing a cylindrical probe (TA-6, 0.25"diameter) run at a
probe speed of 0.5 mm/sec. and a probe distance of 0.1 mm. Sample
extrudates were cut into 1-inch long pieces, placed on the texture
analyzer's sample table, and secured with pins. The probe was lowered
automatically using the above conditions. After the probe was fully
lowered, it was held at that distance for one minute before it was
released. The force required to initially compress the sample and the
force required to compress the sample after one minute were determined.
The percent recovery of the sample is determined by dividing the
compression force after one minute by the initial compression force and
multiplying by 100. A higher percent recovery corresponds to a material
having a better resiliency.
Compressibility
The compressibility, i.e., the force necessary to deform a material, of a
sample was determined using a Stevens LFRA Texture Anmalyzer employing the
conditions as noted above in measuring resiliency.
Sample extrudates cut into 1-inch long pieces were placed on the analyzer's
sample table and secured with pins. The probe was lowered and raised
automatically with the force required to compress the sample being
measured in g/cm.sup.2. This analysis was repeated two additional times
using a fresh piece of sample extrudate each time. The average of the
three measurements was taken as the compressibility value. A high value is
attributed to a sample that is relatively hard, i.e., less compressible,
while a lower value is attributed to a sample that is easily compressible.
EXAMPLE I
Several samples of unmodified starch materials containing varying amounts
of amylose content, i.e., corn (.about.25-28% amylose), waxy maize corn
(.about.0-1% amylose), potato (.about.23% amylose), Hylon V (.about.50%
amylose) and Hylon VII (.about.70% amylose) were fed to a Werner and
Pfleiderer twin screw corotating extruder, model ZSK30. Hylon is a
registered trademark of National Starch and Chemical Corporation for
starches. The extruder had a screw having a high shear screw design, a
barrel diameter of 30 mm, two die openings of 4 mm diameter each, a L/D of
21:1, and oil heated barrels The samples were fed to the extruder which
had a screw speed of 250 rpm, at a rate of 10 kg/hr with input moisture of
about 6.7% based on weight of starch added (residual moisture of starting
starch materials was .about.9 to 12%). The temperature in the extruder was
increased to a level of about 200.degree. C. in the barrel or section
nearest or just before the die and the extruder pressure was between about
200 to 500 psi.
The expanded products leaving the extruder were collected and evaluated for
different characteristics as shown in Table 1. The high amylose starches,
i.e., Hylon V and VII had an essentially uniform, closed cell structure
with tiny bubble formation quite evident. The base starches which
contained significantly lower than 45% amylose content, i.e., corn starch,
waxy maize starch and potato starch, all gave an expanded product but each
had a poor, relatively open cell structure, and were brittle and easily
crushed as typified by the results for corn starch shown in Table 1.
TABLE 1
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Sample Bulk Compress-
Material Density (lb/ft.sup.3)
Resilience (%)
ibility (g/cm.sup.2)
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Corn Starch
0.328 0 (no recovery
1000
crushed)
Hylon V 0.461 -- 192
Hylon VII
0.105 68.3 128
Styrofoam
0.10 73.8 588
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EXAMPLE II
Additional samples of corn starch and the high amylose starches, Hylon V
and Hylon VII each modified by hydroxypropylating with propylene oxide
(P.O.) were used to prepare expanded products using the same procedure as
Example I with a temperature of about 175.degree. C. in the barrel or
section just before the die.
The expanded products leaving the extruder were collected and evaluated for
different characteristics as shown in Table 2. The modified corn starch
material expanded into a product which appeared better than the product
made from corn starch alone, shown in Example I, but had an open cell
structure, was brittle and crushed easily and disintegrated when
compressed. The modified high amylose starches, i.e., Hylon V and VII had
the desired uniform, closed cell structure which compared favorably with
the products previously made from the unmodified starch and showed
satisfactory bulk density, resilience and compressibility properties as
well as increased strength and expansion diameter. Other expanded products
were prepared from the same high amylose starches modified with amounts of
from 2 to 10% of propylene oxide by weight and these products also
exhibited satisfactory property characteristics and a uniform, closed cell
structure.
TABLE 2
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Sample Bulk Compress-
Material Density (lb/ft.sup.3)
Resilience (%)
ibility (g/cm.sup.2)
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Hylon V, 5%
0.488 66.4 703
P.O.
Hylon VII,
0.321 73.2 508
5% P.O.
Styrofoam
0.1 73.8 588
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EXAMPLE III
Expanded products were prepared as in Example II using the high amylose
Hylon VII (70% amylose) starch modified with propylene oxide (5%) with the
addition of polyvinyl alcohol (2.40% by weight). Good expanded products
were made as illustrated by the product containing 8% polyvinyl alcohol
which had a bulk density of 0.351 lb/ft.sup.3, a resilience of 70.0% and
compressibility of 421 g/cm.sup.2. All products gave improved strength and
flexibility.
EXAMPLE IV
Additional products were prepared using the Hylon VII (70% amylose) starch
with different modifiers, i.e., acetic anhydride, octenyl succinic
anhydride, phosphorus oxychloride and diethyl aminoethyl chloride. and
additives, i.e., monoglyceride and urea. Expanded products were made
having some improved properties over the product derived from the
unmodified starch while exhibiting a similar relatively uniform, closed
cell structure.
EXAMPLE V
The effect of total moisture or water content on the extruded product was
demonstrated by preparing a product using conditions as in Example II. The
starting material was a high amylose, Hylon VII (70% amylose) starch
containing a known amount of residual moisture. The level of total
moisture was varied by adding different amounts of water to the extruder.
The resulting products were collected and evaluated for different
characteristics as shown in Table 3. All the products gave a uniform close
cell structure except the one having total moisture at 12.8% was
nonuniform and not evaluated for resilience and compressibility and those
having 24.0 and 26.4% total moisture were non-expanded rope-like products
having unsuitable properties.
TABLE 3
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Starting Initial Moisture
Input Total Bulk Density Compressi-
Material Content (%)
Moisture (%)
Moisture (%)
(1b/ft.sup.3)
Resilience (%)
bility (g/cm.sup.2)
__________________________________________________________________________
Hylon VII, 5% P.O.
8.0 4.8 12.8 0.22 Not Uniform
" 8.0 5.5 13.5 0.26 62.15 791
" 8.0 6.1 14.1 0.24 62.77 264
" 8.0 6.7 14.7 0.31 62.34 473
" 8.0 7.4 15.4 0.27 70.74 542
" 8.0 8.0 16.0 0.29 70.68 493
" 8.0 8.6 16.6 0.39 72.34 519
" 8.0 9.2 17.2 0.36 62.74 669
" 8.0 9.8 17.8 0.36 62.39 668
" 8.0 10.4 18.4 0.45 62.15 723
" 8.0 10.9 18.9 0.53 64.35 521
" 8.0 11.5 19.5 0.47 63.66 632
" 8.0 12.1 20.1 0.42 61.95 543
" 8.0 12.6 20.6 0.38 63.42 609
" 20.4 -- 20.4 -- -- --
" 20.4 3.6 24.0 Non-Expanded Product
" 20.4 6.0 26.4 Non-Expanded Product
__________________________________________________________________________
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