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BACKGROUND OF THE INVENTION
The present invention relates to biodegradable starch acetate polymers,
blends, and compositions, and methods for making them.
The term "biodegradable" does not yet have a generally accepted meaning in
the plastics industry. Agencies such as the Food & Drug Administration
("FDA") and the Environmental Protection Agency ("EPA") have not, to date,
promulgated a test for establishing which products are "biodegradable." In
general, the term biodegradable has been applied to any material which is
meant to decompose significantly when placed in land fills. Unfortunately,
many plastic formulations are said to be biodegradable even when composed
of mostly nonbiodegradable oil-based polymers. For example, a formulation
of 98% by weight polyethylene and 2% by weight corn starch is called
"biodegradable" because over time the corn starch binder will decompose
and cause the material to break into smaller pieces or chunks of
polyethylene. Unfortunately, the resultant pieces of polyethylene will not
biodegrade further. Thus, truly biodegradable plastics that safely
decompose into primarily carbon dioxide and water are needed.
The use of oil and hydrocarbon gases as the raw material for plastics has
dominated the industry. Substantial amounts of carbon dioxide and other
toxic gaseous pollutants are released into the atmosphere during the
processing to make these raw materials as well as the ultimate plastic
products Further, decomposition by-products of oil and hydrocarbon gas
based plastics sometimes contaminate ground water. Thus, environmentally
safe plastics and processes for making them from benign starting materials
are needed.
Plant-derived products are appropriate starting materials for the desired
biodegradable plastics. Agricultural plants and their by-products absorb
large amounts of carbon dioxide and release large amounts of oxygen during
growth. When decomposed, most of the carbon dioxide and water will be
recycled to the earth and atmosphere. Starch based polymers such as starch
acetate will biodegrade completely and can be made from natural
plant-derived materials. However, their potential as environmentally sound
commercial materials has not, until now, been realized.
Earlier attempts to make starch-based plastics employed purified starches
(usually corn starch) rather than unprocessed flour Purified starches were
preferred because they generally produced whiter plastics having broad
market appeal. In some cases, whiteness was enhanced by bleaching flour
starting materials with sodium hypochlorite or other agents. Purified
starches were also preferred because they do not contain simple or complex
sugars such as glucose, sucrose, fructose and, in the case of sweet
potatoes, maltose. Under conditions previously employed to make starch
acetate, these sugars became dark, turning to a sticky char which spoiled
the starch acetate product. Thus, pure starch was not only desirable but
required to make useful starch acetate.
Unfortunately, additional processing is required to obtain purified starch
from flour. Further, the native starch granule size and hence the number
of monomer units in each starch molecule decreases when flour is converted
to starch. Thus, polymers made from purified starch generally have low
molecular weights and tensile strengths. They have not been suitable for
consumer products such as containers or wrappings.
In many prior starch acetate synthesis processes, starch is initially
dissolved in a low pH acid solution. The heat given off by this exothermic
reaction is so great that a low boiling point solvent is often used to
prevent a run away reaction. These solvents may be carcinogenic or
otherwise hazardous to use in a manufacturing facility. From a practical
standpoint, solvents increase the cost of the overall process and thus the
price of the final plastic products. Further, if the reactor temperature
exceeds an optimum value for too long, low molecular weight plastics will
be formed. Charring and/or decomposition of the raw material are also
possible if temperatures remain high for too long.
SUMMARY OF THE INVENTION
The present invention provides a method for producing truly biodegradable
plastic materials. The improved novel biodegradable plastic materials and
compositions employing these plastic materials are also part of the
present invention. The methods of the present invention produce starch
acetate polymers of higher quality, and particularly higher molecular
weight, than was previously realizable. In this invention, no solvents are
used in the synthesis or blending stages of plastic formation. Further,
the starting materials are all completely natural or environmentally safe
and can be obtained with minimal processing steps.
One aspect of the present invention is a process for converting whole
agricultural flour into a high-quality biodegradable starch acetate
polymer by combining whole flour with an acetylation agent and
subsequently adding a catalyst while the reaction temperature is held
between 50.degree. C. and 90.degree. C. Preferably, the catalyst, which
may be a common mineral acid, or mixture of mineral acids or
methanesulfonic acid, is added gradually over the course of the reaction,
i.e., the catalyst addition is "staged." The reactant mixture may be first
heated to about 50.degree. C. before adding the catalyst. After the
reaction is complete, the starch acetate product may be precipitated by,
for example, adding cold water. Typically, the pH of the reactant mixture
will be maintained below 1.0 during the reaction.
The starch acetate products produced by this process can be used alone or
in combination with other starch or cellulose acetates to form a polymer
blend. These materials can be made soft and pliable by the addition of one
or more plasticizers, preferably natural, environmentally safe
plasticizers In addition, plastic compositions may include natural fillers
such as nutshells or mollusk shells. Natural colorants or dyes may also be
added to impart pleasing colors to the final material.
A further understanding of the nature and advantages of the invention may
be realized by reference to the remaining portions of the specification.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flow chart depicting the process steps employed in a preferred
embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Contents
I. Definitions
II. Flour Starting Materials
III. Reaction Process
A. Catalysts
B. Reaction Temperature
C. Waste Water Treatment
IV. Biodegradable Additives
A. Plasticizers
B. Agricultural Fillers
C. Colorants
V. Examples
I. Definitions
The following definitions are presented to aid in understanding the present
invention but are not intended to limit the meaning in the claims. The
specific embodiments are only examples within broader classes.
Biodegradable: Although no generally accepted definition of "biodegradable"
exists in the art, for purposes of this application, it refers to a
material that decomposes over a period of less than about 10-15 years to
primarily carbon dioxide and water or minerals commonly found in the
environment. Preferably the material will completely decompose in less
than about 5 years when buried in soil containing natural microorganisms.
Staged Addition: This describes the gradual addition of a material, such as
a catalyst, to another material, such as a reactant mixture, over some
period of time. For purposes of this invention, the addition will
preferably take place over a period of less than about 10 hours.
Agricultural Filler: A material added to a polymeric material for purposes
of altering the properties of the final material. Agricultural fillers are
made from materials commonly obtained by traditional farming or harvesting
activities. For example, commonly cultivated plant materials such as nuts,
berries, corn, and other grains contain waste materials such as shells or
husks which can be used as agricultural fillers. In addition to other
natural materials, the shells of mollusks or other seafood can be used as
agricultural fillers.
Colorant: An inorganic or organic dye, pigment, lake, or other material
which when blended with a polymeric material imparts color to the final
product.
Acetylation Agent: A compound or mixture of compounds which contain acetyl
groups (CH.sub.3 C(O)O--) available for addition to another compound such
as a hydroxyl containing compound. The reaction of an acetylation agent
with a hydroxyl containing compound will under the right conditions
sometimes yield an acetate.
Solvent-free: A material that contains almost no traditional solvent except
for water or low pH acid solution. A solvent-free material will preferably
include at most traces of ether solvents, aromatic solvents, nitrogenous
base solvents, or other toxic or flammable organic solvent.
II. Flour Starting Materials
The present invention provides a method by which complex, cross-linked,
high molecular weight, starch acetates and their associated by-products
are produced from one or more of the following agricultural materials:
rice flour, potato flour, corn flour, oat flour and wheat flour. When dry,
each of these contains a high weight percentage of starch. Almost any
common flour commodity can be used to produce biodegradable polymers in
accordance with this invention. Preferred flour starting materials,
however, will have higher percentages of starch than might be desired for
other applications. For example, potatoes can be specifically bred to have
high yields of starch and a low amount of sugar and a high amount of total
solids. In rice, the amount of starch, sugar and protein can be controlled
by using specific fertilizers, and by timing watering and other parameters
which can be controlled by the farmer, as well as by breeding.
An important parameter in agriculturally based starches is the ratio of
amylose to amylopectin. Amylose is preferred over amylopectin in plastics
synthesis since it results in polymers that typically have a higher
molecular weight and a corresponding higher tensile strength. In addition,
amylose derived plastics are more often colorless. Unfortunately, in most
food grade potatoes, the relative amount of amylose is relatively low, and
waxy corn can contain almost 100% amylopectin. However, specially bred
corn can contain 70% to 80% amylose by weight in the isolated starch.
The starch granule size is another important parameter. Larger granule
sizes are generally desired because they provide a higher molecular weight
starting material. The molecules of amylose and amylopectin are
synthesized from amyloplast enzymes and deposited as starch granules in
the root, tuber or grain. The granule size is typically 3 to 8 microns for
rice, 8 to 25 microns for corn and 25 to 100 microns for potatoes.
Preferred starting materials include white long grain rice flour and
Japonica rice flour both of which have a low sugar and high starch
content. In the industry, a premium is sometimes paid for rice with a high
protein content. For purposes of this invention, however, the preferred
rice will have a low sugar and protein content and the highest possible
starch content and starch granule size. Any whole or peeled potato can
also be used as a source of flour for this invention. Preferred potatoes
will have over 85% starch in the solids content and less than 75% total
water content. Potatoes meeting these requirements can be obtained by
special breeding. It should be noted that polymers produced from potatoes
with peels are sometimes colored brown from the natural pigments in the
skin. Suitable corn flour can come from any type of corn including white,
yellow or blue varieties. Preferably, the corn is selectively bred for a
high amylose content. The wheat flour, likewise, can come from any variety
of wheat. The favored wheat flour will come from a "low grade" (or low
protein content) wheat that can be obtained from manufacturing plants
which grade the wheat by protein and sugar levels. The lowest grade wheat,
which has the lowest protein content is preferred in this invention.
Although light colored wheat flours are sometimes preferred to give the
lightest colored polymer products, any color or shade of wheat can be
used, recognizing that the final product color will be affected.
Properties of the final product can be tailored by blending whole flours
and isolated starches from potatoes and corn. In a preferred embodiment of
this invention, the flours are selected from among a diverse origin of
agriculturally based products.
The flour starting material is typically milled to a particle size less
than 40 mesh for ease of processing and to accelerate starch dissolution.
Smaller particle sizes (down to about a 40 mesh size) allow the material
to go into solution faster. Beyond 40 mesh size no improvement in process
speed or product quality is realized. In fact, some reduction in the
starch granule size may occur if excessive milling is applied to the flour
materials. Further, milling to very small particle sizes tends to overheat
and damage the starch.
In some previous processes the starch starting material was "activated" by
various solvents such as pyridine or treated in other ways. In this
invention, however, the only pretreatment is milling the raw material to a
flour and then drying the flour to remove residual moisture. Thus, the
expense and danger of additional processing is avoided.
As noted, previous processes required purified starch because simple or
complex sugars such as glucose, sucrose, maltose and fructose (found in
whole flour) quickly become dark and turned to a sticky char. In this
invention, sugars can be tolerated and do not have to be isolated prior to
reaction. In the polymer washing stage of this invention, residual sugars
are dissolved into the cold water and removed from the end product.
In earlier processes, the wash materials often contained chlorites,
solvents or other materials having negative environmental effects. In this
invention, all waste and wash materials are treated in water lagoons or
water aeration tanks to produce a waste stream that can be used for
agricultural purposes. In fact, the residual materials are generally safe
for spraying onto agricultural lands to grow more crops that might be used
to produce more flour starting material.
III. Reaction Process
The major process steps of a preferred embodiment are shown in FIG. 1. To
minimize the water content in the reaction solution, the flour is first
dried until no further weight loss is observed Preferably, the flours are
dried immediately prior to being charged into the reaction vessel. The
drying temperature may range between 50 to 90 degrees Centigrade.
Next, one or more of the flours described above are added to acetic
anhydride or a mixture of acetic acid and acetic anhydride. Mechanical
mixing is required during this step to completely blend the reactant
mixtures which often become highly viscous. Typically, the viscosity of
the reaction mixture changes as the flour goes into solution and as the
starches are converted into acetates.
By using hot water or other heat sources, the temperature of the reaction
mixture is brought up to a minimum of 50 degrees Centigrade. This energy
will preferably come from nearby waste heat sources. During the entire
process, mechanical mixing is continued in order to maintain a uniform
reaction composition.
When the reactant mixture reaches 50 degrees, methanesulfonic acid,
concentrated sulfuric acid or a mixture of sulfuric and hydrochloric acid
or other catalyst blends is slowly added to the reaction mixture over a
one to eight hour period During this time the temperature of the reaction
mixture is either maintained at 50 degrees or slowly increased to between
about 60 and 90 degrees Centigrade.
In a preferred embodiment, the catalyst(s) is mixed with acetic acid prior
to being added to the reactor. The acid mixture is then added at a rate to
match the acetylation and solution processes to optimize polymer
properties and minimize oxidation and other undesired side reactions.
Excessive amounts of catalyst or rapid acid catalyst addition causes
charring. By mixing the acid catalyst with acetic acid or acetic
anhydride, the concentrated acid becomes diluted and thus eliminates the
negative effects of decomposition or charring.
In some cases, it may take as long as three to four hours for the starch
materials to dissolve. In the case of potato flour which contains peels,
complete dissolution will never be achieved during the reaction because
the potato peels contain insoluble inerts. If all material does not go
into a homogenous solution after four hours, the reaction mixture can be
filtered hot to remove undissolved matter. The materials removed can be
water washed and spread on to soil to act as a natural soil builder.
It is important to note that each crop and each batch behaves slightly
differently. Thus, for example, the time required to completely dissolve
the starch and the amount of energy generated by the heats of solution and
reaction will sometimes vary from one batch to another. These variations
can be accommodated by constantly monitoring the reaction mixture. The
most important reaction parameter to be monitored is pH. This determines
when all or most of flour has gone into solution. At the critical pH no
further homogenous acid or blend of acids is added to the mixture. Thus,
the amount of acid required for each run is minimized. Because flours
often act as buffers, it is usually easy to control the pH during
acetylation. Different blends of flours will, of course, have different
buffering properties.
To obtain high molecular weight and high melting temperature agricultural
flour acetate polymers the addition of the acid catalyst must be made
slowly to prevent excessive oxidation or charring of the flour materials.
In practice this is achieved by metering the acid into the reactor over a
period of two to four hours. It is vital to add the catalyst and acid
mixture at such a rate that no visible signs of darkening are noted as the
acid mixes with the flour slurry mixture. The rate is dependent on both
time and temperature. Longer addition periods in general lead to higher
polymer yields.
For the agricultural flours described herein the weight ratio of acid
catalyst to flour is typically in the range of 1:100 to 3:100. The rate of
catalyst addition is typically in the range of two to four hours.
Once complete dissolution of the flour is achieved, the reaction mixture
may be either precipitated or held at temperatures of 50 to 90 degrees
Centigrade for up to eight more hours. The degree of acetyl substitution
will depend on type of flour and the reaction conditions. Varying degrees
of esterification are achieved by the staged addition of catalyst to a
reactant mixture of flour and acetic acid or acetic anhydride. The percent
substitution of hydroxyl groups by the acetyl groups in the product can be
determined experimentally by the Ost distillation method. With the
processes and starting materials of this invention, the acetyl group
substitution typically ranges from 30% to 55%. The degree of discoloration
in the reaction mixture is a function of the sugar and protein content of
the reactant flours as well as the reaction temperature history and the
rate of acid catalyst addition.
At the end of the reaction period, the heat source is removed from the
reaction mixture. The mixture can then be allowed to cool slowly or
rapidly (by, for example, immediately adding cold water). The ratio of
reaction mixture to water can range from 1:1 to 1:4 or higher depending on
the temperatures of the final reaction mixture. The cooled reaction
mixture initially acts as a gel and must be agitated continuously to
generate a uniform mixture. Mixing also accelerates precipitation.
In some cases a white to off-white or yellow starch acetate precipitate
will form immediately. In other cases (such as with rice flours)
precipitation may take place over a span of several minutes to an hour.
Separation of the starch acetates from the reaction mixture can be done by
direct filtration, the use of settling tanks, or by centrifugation.
Once the precipitate is concentrated from the water solution, it is
necessary to wash the plastic to remove any acid residues. This can
sometimes best be achieved by the addition of a mild solution of an
alkaline compound such as dilute ammonium hydroxide.
A final clean water wash is then employed to remove all water soluble
materials including residual alkaline wash, sugars and other water soluble
compounds. All wash and waste streams are combined and fed into a staged
lagoon system or into aeration tanks. Depending on ambient temperatures,
treating the waste water may require three to ten days. For the process to
function properly, water temperatures should be kept within the range of
15 to 35 degrees Centigrade. Lime or other basic compounds may be added to
the lagoon or aeration tanks to bring the final water pH to a level
suitable for agricultural purposes. The pH should preferably be maintained
between 4.5 and 6.5.
For the microbial population to be maintained at a high enough level, there
must be a sufficient and constant flow of air through the aeration tank
reactor or in the aeration lagoon, as well as a constant flow of waste
from the manufacturing plant. Residual sugars and other agricultural
by-product wastes from the process can maintain a natural microbial
population if the temperature and pH are maintained within the ranges
specified above.
The washed polymers are dried using any conventional method including spray
drying, oven drying or rotary drum drying. Since the polymers are in
direct contact with air, drying times should be kept to a minimum.
Once dried, the polymers can then be further blended with other
biodegradable plastics or blended with various plasticizers, fillers and
coloring agents. Preferred plastics for blending include cellulose acetate
and or flour based plastics The blending operation can be done simply by
using cold or hot extrusion, rotary powder mixing or other methods common
in the art. When pellets are the desired end product, extrusion is
convenient because both liquids and solids can be processed and blended
using the same equipment.
Preferred fillers include waste nut shell flour, calcium carbonate from
mollusk shells or other sources, and dried lobster, crab and shrimp
shells. The specific nut shells include, for example, walnut, pecan and
pistachio.
Various coloring agents can be added during the final blending process as
desired. For plastics materials which may be in direct contact with foods,
the seven FD&C colors can be used including red #3, red #40, yellow #5,
yellow #6, green #3, blue #1 and blue #2. Natural extracts from
agricultural products can also be used including extracts from beets and
hibiscus flowers (reds), carrots (yellows to oranges), grape skins
(purples and reds), berries (pinks to reds) and whole flours such as
potato peels and walnut shells (browns).
Preferred plasticizers include soy bean oil, epoxidized soy bean oil,
peanut oil, olive oil, corn oil, walnut oil, safflower oil, sunflower oil,
cotton seed oil, glycerin, monoacetin, diacetin, triacetin, sucrose
acetate and glucose acetate. Epoxidized soy bean oil is a naturally
occurring oil having oxygen atoms incorporated into the long hydrocarbon
chains. Glycerin and related compounds may be obtained from natural
sources and may have varying degrees of acetyl substitution. The sugar
acetate compounds can also contain varying degrees of acetyl substitution.
All of the above oils can be used in the hydrogenated form such as a blend
of cottonseed oil and soy bean oil commonly known as baking "shortening".
The plasticizers which are most compatible with the flour acetate polymers
and which give formulated blends the highest physical strength are the
glycerine acetates, specifically triacetin, diacetin and monoacetin or
blends of these three compounds.
A. Catalysts
In the prior art, a number of catalysts have been used in the
esterification of starches. Typically, catalysts were chosen to promote
immediate dissolution of the purified starch materials and quick reactions
(sometimes less than five minutes). However, the rapid reaction times were
accomplished at high acid concentrations and reaction temperatures in
excess of 90 degrees Centigrade. The resulting polymers had numerous
undesirable properties such as reduced molecular weights, and increased
branching resulting from cleavage during synthesis.
Also, by using high concentrations of acids with highly purified starches
at high reaction temperatures, a large, difficult to handle exotherm
resulted. Further, high concentrations of acids resulted in waste disposal
and treatment problems. Those processes using solvents to handle the
exotherm required an additional step of removing the solvent from the
effluent stream. Most such processes would have difficulty meeting new EPA
standards for waste streams from chemical plants.
Some previous methods employed cation exchange resins with sulfonic groups
as a substitute for acids. However, when using agriculturally based flours
that contain sugars and proteins in addition to starches, both the micro
and macro pore structures in the commercial resins quickly plug, rendering
them ineffective as catalysts for synthesis of complex starch acetates.
In this invention, low concentrations of homogenous or mixed catalysts are
used. The catalysts include methanesulfonic acid, concentrated sulfuric,
hydrochloric and phosphoric acids used individually and as blends.
Methanesulfonic acid is especially desirable because it biodegrades easily
in the lagoon or aeration tank. EPA process licenses are also easier to
obtain when methanesulfonic acid is used as the catalyst.
In this invention, the catalyst is introduced into the reaction mixture in
a staged or timed manner. In other words, it is added over a period of
time instead of all at once, thus eliminating the need for solvents and
increasing the product quality by reducing cleavage in the early stages of
the acetate or triacetate synthesis.
By maintaining a moderate acid catalyst concentration during acetylation,
the charring associated with the use of agricultural flours is minimized.
Further improvement can be obtained by using flour blends specially
formulated to act as buffers, permitting acetylation to proceed while at
the same time preventing excessive side reactions associated with sugars
and proteins and excessive oxidation from strong acids.
B. Reaction Temperature
In previous methods, preferred reaction temperatures for starch acetate
synthesis were typically in the range of 95 to 105 degrees Centigrade.
These temperatures may be appropriate for purified starch starting
materials but are too high for whole agricultural flours. Immediate
charring of the flours is caused by side reactions with sugars and
proteins.
The high temperatures also adversely impacted the molecular weight, flow
temperature and other properties of the end product. Thus, the resulting
polymers were difficult to mold and extrude. To alleviate these problems,
the starch acetate was typically blended with non-biodegradable oil-based
polymers. Another disadvantage associated with the high reaction
temperature is the need for a heat source near or above the boiling point
of water. Steam generation places a high variable cost in the
manufacturing plant. This is especially problematic given the trend today
toward increased energy costs in plastics manufacturing.
The energy requirement for the present invention can be met by waste heat
from refineries or other chemical operations. Thus, the energy cost
associated with the primary synthesis is minimal. The reaction
temperatures employed in the present invention range from 50 to 90 degrees
Centigrade, well within the operational limits of low cost jacketed mixing
kettles common in the industry. Further, the blending of polymers and
addition of plasticizers can be done at ambient or slightly elevated
temperatures without the use of solvents in heated, jacketed extrusion
equipment.
C. Waste Water Treatment
To complete this environmentally-sound process, all waste streams generated
in the synthesis should be treated appropriately. The use of agricultural
flours instead of purified starches presents unique waste stream
compositions. The major waste stream components of the present invention
are acetic acid, sugars, denatured proteins, methanesulfonic acid,
sulfuric acid, and hydrochloric acid. It is possible to recover the acetic
acid by various methods including distillation and reverse osmosis.
However, some recovery operations are expensive, and alternative processes
are sometimes necessary to handle the waste streams. In one approach, the
waste water generated from polymer precipitation is treated in staged
lagoons or aeration tanks for several days until bacteria can directly act
on the acetic acid, converting it to carbon dioxide and water. In most
cases, it is necessary to first increase the pH (to approximately 5.0 to
6.5) of the waste water by adding an alkaline material such as lime, quick
lime, ammonia, ammonium hydroxide or calcium hydroxide. Then the waste
water can be mixed with clean water and sprayed onto agricultural products
via irrigation systems.
IV. Biodegradable Additives
A. Plasticizers
A number of plasticizers have been reported for use with starch and
cellulose acetates. These have included such materials as dimethyl
phthalate, various co-polymers derived from oil processing and other
materials which are not biodegradable and which in general are not safe
for direct contact with foods under FDA regulations. As noted above,
preferred plasticizers for the plastics of this invention include various
natural oils and acetate derivatives of natural substances. Each of these
is safe and biodegradable.
The purpose of plasticizers are, in general, to make stiff starch acetates
more flexible. They accomplish this by lowering the plastic's transition
temperature melt viscosity. Thus, the final plastics are more amenable to
processing with extruding and molding equipment.
The type of plasticizer used and the weight percent loading in the polymer
depend on the processing conditions employed to produce the final plastic
article. Processing may include injection molding, extrusion, blow
molding, rotational molding, thermoforming and other applications. Each
has an optimal pressure ranging from ambient to 40,000 psi, an optimal
temperature ranging from ambient to 250 degrees Centigrade, and optimal
cycling times ranging from seconds to several minutes.
Plasticizers chosen for use in this invention should be able to withstand
these conditions. In addition, they should have a long shelf-life,
remaining stable for extended periods of time prior to product use, and be
free of any offensive odors. The plasticizers should also be chemically
compatible with the selected starch acetate polymer over a variety of
conditions including high and low temperature, pressure and pH associated
with use and degradation of the end product.
Most natural plasticizers will preferably be present at concentrations of
between 5% to 35% by weight of the final starch acetate polymer. To
achieve specific plastic properties, various plasticizers may be blended
with two or more plastics and natural fillers. For some plasticizers,
small incremental changes in the weight percent loading in the plastic
blend can cause dramatic differences in tensile strength and flow
properties of the polymer.
The concentration of the plasticizer and blend of plastics determines the
flow temperature which in turn determines the flow designation as defined
in ASTM Test Method D-569. The flow temperature may be further altered by
the use of various fillers as described below in the examples.
B. Agricultural Fillers
Fillers are, in general, materials that add bulk, color, or texture to the
plastic product and some cases also increase the physical strength
Sometimes fillers, especially inexpensive materials, are used to lower the
unit cost of the plastic. For example, calcium carbonate is often used
with LDPE (Low Density Polyethylene).
In this invention, preferred fillers are whole, dried, agriculturally based
flours milled to less than a 80 mesh particle size. Examples of
appropriate fillers include various flours made from waste nut shells as
well as dried potato skins. Calcium carbonate made by grinding shells from
oysters, clams and other mollusks may also be used as a natural filler.
Further, dried lobster, crab, and shrimp shells, which include a blend of
chitin, calcium carbonate, red coloring and other materials, are another
source of natural fillers. Flours from these shells in general have a low
density.
The use of waste nut shells is of particular interest because some of these
are inexpensive waste products from shelling operations that add strength
to the final polymer blend. For example, strength of pressed polymer disks
may increase by as much as 5% to over 30% (as measured by durometer)
depending on the particular flour starch blend, the specific plasticizer
and the weight loading of the filler flour material. Preferred shell
flours include material derived from walnut, pistachio, pecans and other
food nut products common in the industry.
Due to the hardness of some of these fillers, milling operations must take
place in several steps. In this invention, a preferred method was found to
be first crushing the shells to a -10 mesh size followed by successive
hammer milling passes of -50 and then -80 mesh. The -80 mesh flours thus
obtained have a specific gravity in air as a dried powder ranging from
0.79 to 0.92 grams/cubic centimeter. By comparison calcium carbonate
filler has a smaller particle size with a specific gravity ranging from
2.71 to 2.93 grams/cubic centimeter.
Dried agricultural fillers are generally very stable and can be kept for
extended periods without spoilage Weight loadings in the final plastics
blend depend on the desired physical properties. Weight loading ranges,
however, are expected to be in the range of 1% to 15% for most plastics
Weight loadings higher than this may adversely affect physical strength
and the flow properties of the plastic. Low weight loadings, in general,
should increase tensile strength and reduce unit costs of the plastics.
When a coloring agent is added to a blend of a starch acetate and an
agricultural fillers, the filler tends to selectively adsorb more of the
coloring compound than the bulk plastic or plasticizer. In such cases the
filler and the coloring agent may have to be preblended with the
plasticizer before mixing with the bulk starch acetate plastic to obtain
uniform colors throughout the final product. Alternatively, the filler may
also be a coloring agent. Potato peels are one material that serves both
as a coloring agent and a filler. Potato peels are a readily available
waste associated with most potato processing plants including plants for
making french fries, potato flakes, potato flour and other end products.
The traditional method of handling these wastes is to spread them over
given tracts of land.
C. Plastics Colorization
In the formulation of plastics, many options are available to add color and
texture to the final product. For example, calcium carbonate and titanium
dioxide are sometimes used to add a white color or opaque quality to
certain plastics. Other inorganic and organic pigments and dyes have also
been used. When organics are used, they must be able to survive the
operating temperatures of the molding or extrusion equipment.
Many inorganic pigments containing metals harmful to the environment have
been used. For example, some pigments employ cadmium and mercury to
achieve yellows, oranges and reds. Others pigments often employ these and
other heavy metals which remain as residues after the material has
decomposed or is exposed to acid ground water.
For this invention, most any inorganic pigment can be used, but the
preferred compounds are safe to the environment and add to the
agricultural viability of the soil. Most any organic pigment or dye can be
used, but the preferred compounds are safe for direct contact with foods
and can survive processing temperatures as high as 250 degrees Centigrade
for at least one minute (conditions, typical in commercial molding and
extruding equipment).
The list of safe inorganic and organic dyes and pigments is constantly
shrinking as the EPA and FDA find further reason to eliminate various
compounds from the approved list. Labeling laws require full listing of
FD&C (Food & Drug Cosmetic Act of 1938) colors. Previously, the ingredient
could be identified simply as "artificial color". Color compounds which
are isolates of extracts found in plants and flowers approved for human
consumption are the most preferred safe coloring agents. Included among
these are extracts from carrots, beets, grape skins and others. Some
starch raw materials contain natural color which appears in the final
product as various shades of yellow.
To be used as a coloring agent, the dye or natural agricultural extract is
usually mixed with the plasticizer prior to blending. When this is not
possible, the coloring agent is mixed and blended in the final preparation
of plastic pellets used for manufacturing.
V. Examples
Example 1--Rice Flour
Japonica rice flour milled to -50 mesh was oven dried at 70 degrees
Centigrade until no further weight loss from water was | | |
