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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to biodegradable shaped products including packaging
products and 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.
U.S. Pat. No. 3,962,155 discloses a molded foam product of different forms
obtained by blending pullulan, a polysaccharide produced by Pullularis
pullulans, or modified pullulan with a gas yielding foaming agent.
While many of 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 of environmental waste.
SUMMARY OF THE INVENTION
The present invention provides a biodegradable shaped product comprising an
expanded high amylose starch product having a low density, closed cell
structure with good resilience and compressibility properties. More
particularly, the expanded starch product of this invention, which
includes packaging products and packaging material, has at least 45% by
weight amylose content, a bulk density of 0.1 to 5 lb/ft.sup.3, a
resiliency of at least about 20% and a compressibility of from about 100
to 800 g/cm.sup.2.
In one embodiment of this invention, the biodegradable product is a
packaging material, such as an insert or loose fill, having 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 shaped products such as packaging products and
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.
Another embodiment of this invention relates to a shaped product prepared
by extruding a modified high amylose starch which contains about 2% or
more by weight of salt content.
DETAILED DESCRIPTION OF THE INVENTION
The ability to provide a shaped product for packaging applications 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 loose fill. 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 shaped product 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.
Depending on the particular end use application, the expanded product can
be used in the form it is in after extrusion, e.g., a sheet, cylindrical
or rope-like product or it can be further formed and configured into
different shapes, such as a carton, container or tray. In one embodiment,
the expanded, high amylose starch material exits the extruder in the form
of a sheet or cylindrical rope, 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. Therefore, the expanded biodegradable starch product
can be provided in various forms and shapes making it useful as a
packaging product such as containers, cartons, trays, cups, dishes,
sheets, etc., or as packaging material for uses such as loose fill or a
filler, insulator, sheet or as protective packaging. e.g., cushioning for
sensitive equipment, apparatus and components.
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, particularly the alkenylsuccinic acids;
ethers, such as the hydroxyethyl- and hydroxypropyl starches and starches
reacted with hydrophobic cationic epoxides; starches oxidized with
hypochlorite; starches reacted with cross-linking agents such as
phosphorus oxychloride, epichlorohydrin, 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, flame retardancy, density, color, etc. as well as
to provide repellency to insects and rodents, if needed or desired.
Compounds such as polyvinyl alcohol, monoglycerides, and poly(ethylene
vinyl acetate) are typical additives which may be used. Regarding flame
retardants, there are many known compounds and classes of compounds which
may be used including phosphorus containing compounds such as sodium
phosphate, sodium triphosphate and ammonium phosphate, as well as sulfur
containing compounds such as ammonium sulfate and ammonium sulfamate.
Water resistance can also be improved using additives with styrene
acrylate resins being one type that was found particularly effective.
Density as well as resiliency and flexibility can be improved by
incorporation of synthetics such as polyvinyl alcohol, polyvinyl acetate,
polyurethane, polystyrene, poly(ethylene vinyl acetate) and
polyvinylpyrrolidone. These additives may be used in any amount that will
effectively satisfy the desired property, provided the extrusion of the
starch and the overall 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.
It has also been found that when using modified starch compounds, shaped
products having especially improved uniform, closed cell structure can be
obtained when the modified starch starting material contains about 2% or
more by weight of salt as well as the required high amylose content. The
salt in the modified starch can either be added to the already prepared
starch starting material or it can be residual salt, as determined by ash
content, that remains after the preparation of the modified starch, e.g.,
in the well known method of producing hydroxyalkyl starch ethers using
alkaline conditions where salts such as sodium sulfate and sodium chloride
are used. The amount of residual salt in the starch can be controlled by
the amount of washing after the modified starch product is produced.
The salt used in the above described improvement can be any inorganic,
water soluble salt or mixtures thereof and more particularly, an alkali
metal or alkaline earth metal salt with the sodium salts such as sodium
sulfate and sodium chloride being preferred. The amount of salt used will
be 2% or more and preferably 3% or more by weight based on the weight of
the starch. The particularly useful modified starch materials for this
improved embodiment are the etherified materials such as the hydroxyalkyl
starches produced by the etherification with alkylene oxides and the
esterified materials such as those acetylated with acetic anhydride, with
the etherified materials being preferred.
In preparing the shaped products of this invention, an extrusion process,
either alone or in combination with other forming operations, may be used
depending on the type of final product desired. The expanded starch
product leaving the extruder is typically in a rope or cylindrical form.
By varying the size and configuration of the die opening of the extruder,
different forms such as sheets of varying thickness and widths, irregular
profiles and other shapes may be obtained. Products of this type may have
particular application as packaging materials such as inserts and loose
fill or fillers and as protective packaging for electrical and other
sensitive equipment. When expanded products of different shapes and design
are desired, other forming operations subsequent to the extrusion
operation may be utilized. One such readily adaptable technique involves
thermoforming. In this operation, a material is heated to a temperature at
which it is pliable or shapable and then forced against a mold by applying
vacuum, air or mechanical pressure. After the expanded starch product of
this invention leaves the extruder, it is still quite hot and malleable
and therefore well suited for the thermoforming step. Shaped products such
as containers, cartons, trays, dishes, cups, etc., can be formed by
thermoforming an extruded starch sheet. Additionally, products | | |
