 |
Description  |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for making a biodegradable foamed
product from starch using an extruder with traveling molds which close
around the nozzle of the extruder to provide a continuous molding process
using a plurality of individual molds.
2. Related Art
Foamed products, which are often referred to as structural foam, cellular
polymer, cellular plastic, plastic foam, foamed plastic and expanded
plastic, are products generally made from a plastic composition and have a
cellular structure.
A major commercial use of foamed products is in the field of packaging.
Styrofoam, a foamed polystyrene product, is a well-known foamed product
used for packaging. Such packaging material is used to cushion and prevent
damage to the contents of a package.
Conventionally, foamed products are made by fluidizing a plastic
composition, forming a cellular structure by means of a blowing agent in
the fluidized plastic composition, and then hardening the composition to
retain the cellular structure created by the blowing agent.
Blowing agents, sometimes referred to as foaming agents, are substances
that produce the cellular structure in the fluidized plastic composition.
Conventional blowing agents include fluorocarbons, nitrogen gas, hydrazine
derivatives, trihydrazide triazine, 5-phenyl tetrazole, p-toluene sulfonyl
semicarbazide, modified azodicarbonamide, and azodicarbonamide. Chemical
blowing agents are a class of blowing agents which are solids or liquids
at room temperature and, upon heating, release a gas. Typically, they are
employed by mixing the chemical blowing agent with the solid plastic
material and heating the mixture while maintaining the mix under pressure.
Heating fluidizes the plastic material and causes the blowing agents to
release a gas. The gas forms bubbles or cells in the fluid plastic
material. The mixture is maintained under pressure until it is placed into
a mold where the gas expands and in turn causes the fluid plastic material
to expand. Upon solidifying, the cell structure remains. Different blowing
agents release gas at different temperatures and the choice of blowing
agent depends on the processing conditions, plastic material and the size
of the desired gas cells. When starch is used to make the foamed product,
water is typically used as the blowing agent.
Chemical blowing agents have the advantage over gaseous blowing agents in
that they can be added to the solid plastic material prior to heating.
Gaseous blowing agents such as nitrogen and some fluorocarbons must be
injected into the already fluidized plastic material.
Packaging materials made from synthetic polymers, such as polystyrene, have
come under attack due to their non-biodegradable nature. With mounting
pressure on manufacturers to produce a biodegradable product, more and
more manufacturers are seeking alternative sources for conventional
synthetic polymers. Natural polymers such as cellulose and starch have
been the main contenders as replacements for the synthetic polymers.
It has been suggested to use starch and modified starch to make foamed
products. For example, U.S. Pat. No. 3,137,592 teaches extruding starch
with a plasticizer, preferably water, such that the plasticizer flashes as
the fluidized starch exits the die of the extruder. The flashing of the
plasticizer causes puffing or expanding of the fluidized starch and the
formation of a porous mass. The amount of plasticizer ranges from 22% to
67% based on the dry weight of starch. The temperature during extrusion
ranges from 125.degree. C. to 250.degree. C. with a working temperature
range of 150.degree. C. to 160.degree. C.
International Application Publication No. WO83/02955 teaches making a
foamed starch product by extruding a composition of starch with a moisture
content of 12-20% by weight starch with a gas forming or gas generating
agent (blowing agent) at a temperature of 60-220.degree. C. Such an
expanded starch product is taught as being useful as a packaging material
to replace polystyrene foam or as a snack food if mixed with flavorings
and the like. Potato starch is used in the example to produce the foamed
product.
U.S. Pat. Nos. 4,863,655; 5,035,930; and 5,043,196 teach making a
biodegradable foamed packaging material from a high amylose starch. These
patents teach that the foamed packaging material is prepared by extruding
high amylose starch having a moisture content of 10% to 21% by weight at a
temperature of 150.degree. C. to 250.degree. C. According to these
patents, both unmodified and modified starch can be used to make the
packaging material. The types of modified starches taught by these patents
are esterified, etherified, oxidated, crosslinked, enzyme converted and
acid hydrolysized. The etherified high amylose starches, and specifically
hydroxypropylated high amylose starch, are the main focus of these
patents.
Starch graft copolymers have been suggested for use in making a foamed
starch product, see for example, U.S. Pat. Nos. 5,523,372 and 5,853,848.
Starch graft copolymers are biodegradable. They are made from starch and a
synthetic monomer such as vinyl or acrylic.
Additionally, because starch and starch based materials can be combined
with other synthetic plastic materials, there has been a great interest in
the use of these materials to make a biodegradable material.
The use of natural polymers such as starch and starch graft copolymers to
make biodegradable foamed products, however, have not been widely utilized
because of certain problems experienced during their extrusion.
One of the problems that inhibits starch foam technology is the manner by
which starch materials react upon emergence from the die in the extrusion
process. For the foaming action to take place, the temperature, shear and
pressures must be achieved in the barrel of the extruder so as to cause
the starch to liquefy. In this liquefied state, starch is capable of
forming a film. Various polymers and additives are often combined with the
starch. They are used to encourage thin yet stronger cell walls, thus
enabling lower foam bulk densities with acceptable properties. The foaming
action occurs as the material emerges from the die resulting in immediate
expansion after emerging from the extruder die.
Two distinct problems arise when molding these foams. First, the conversion
of these materials into foams must occur at a continuous rate because of
the nature of the extrusion expansion process, however, molds are
generally not available on a continuous basis. Typically, an individual
mold is supplied to the die, fluidized material is injected into the mold
via the die, the mold is moved away from the die, and another mold is then
provided to the die. Thus, the die does not continuously eject the starch.
The second problem is the fact that upon emergence from the extruder these
foams immediately cool on the surface and tend to retain their shape. This
inhibits the manufacture of parts that are larger than the initial
expansion size reached as the material emerges from the die opening. It
also inhibits the production of parts that are complicated designs.
Additionally, starch is not a thermoplastic material, thus, although it is
compatible with synthetic polymers, it does not melt in the same manner or
behave in the same way as the synthetic polymers do in the extruder.
In part, this problem has been overcome through the use of processing aids
such as water, glycerin products, urea, and many other compounds known to
soften the starch granule in the art of starch processing.
Water is especially useful in the production of foamed articles as it
causes rapid gelatinization of starch. This also provides for extensive
foaming action at the low moisture levels necessary for desired foam
characteristics, even when using high percentages of starch. During the
foaming process, the elevated temperatures, shear and pressures associated
with extrusion creates an excellent environment for rapid cell
development. Specific moisture percentages and nucleating agents are used
to control cell size. Foaming agents may also be used, however, during
most extrusion processes, many foaming agents that are compatible to
biodegradable materials and starch do not activate quickly enough to have
measurable effect on items such as loose fill.
Thus far, these difficulties in the extrusion process have limited the use
of materials containing high load levels of starch to elementary foam
products for basic foam applications such as loose fill. Some rigid sheet
products are being produced in sizes and properties of limited scope.
Until now, molded articles produced in economical continuous or large
scale production techniques have been very difficult to achieve.
Extruders with traveling molds are know, see for example, U.S. Pat. No.
5,645,871. The '871 patent teaches making plastic pipes from synthetic
polymers. The teachings of the '871 patent are incorporated herein by
reference. Traveling molds are generally used for production of continuous
parts such as pipes, where the finished product has a continuous shape.
There is a need for commercially feasible operation for making a
biodegradable foamed product from starch and starch based compositions.
SUMMARY OF THE INVENTION
Applicant has discovered that, by using an extruder with a plurality of
individual traveling molds which close around the extruder's nozzle and
are interconnected to allow for continuous extrusion of the starch
material, a commercially feasible process for making foamed starch
products is obtained. These individual traveling mold blocks prevent the
formation of a skin and slow the loss of moisture, thereby aiding the
adhesion of the foam into a continuous phase and providing for a uniform
molded foam starch product with complicated shapes.
Water can be injected into the die, preferably as a mist or steam, to keep
the foam wet enough to allow it to congeal into one uninterrupted phase in
the mold. Foaming agents can also be used to delay the expansion of the
foam in the mold and to assist the foam in completely filling the mold
cavity. These foaming agents are known to modify the properties of the
foam product such as resiliency, compression rate, and surface tension.
Traveling molds are conventional pieces of equipment which are operated in
a conventional manner. The traveling molds comprise a plurality of
individual, separate molds on a continuous track. Each mold is made up of
a pair of mold halves which, when joined, form a cavity therein. The
cavity has the shape of the foamed article. The pair of mold halves are
joined about the nozzle of the extruder. The extruded starch material is
injected into the mold such that the cavity is filled. A vacuum can be
applied to the mold cavity to assist in the filling of the cavity. The
closed mold continues to travel along the track while the starch material
in the mold cools and cures to form the finished foamed starch product.
Once cooling is completed, the mold pair opens and the formed individual
foamed starch product is removed. Ejection devices can be employed to
assist in the removal of the product from the mold cavity. The two mold
halves then travel back to the extruder nozzle to be closed and start the
process again.
A plurality of molds are employed such that individual molds are
continuously supplied to the nozzle. Each mold, at a center point, has an
opening which, when it is aligned with the mold in front of it and behind
it, is aligned with an identical opening in the other mold. In this way
each mold forms a separate mold cavity, yet each cavity is connected to
the cavity in the other molds when they are in the closed state. The
starch material is continuously extruded from the nozzle without any
interruption and each formed product in each cavity is connected to the
other foamed product in the immediate adjacent cavity via the starch which
accumulates in the openings between adjacent molds. The tailing which is
formed in the opening will generally break off of the cured foamed starch
product when the mold is opened to release the foamed product.
Alternatively, it is trimmed off of the product using a knife or other
conventional cutting utensil. The speed of travel of the molds will be
determined by and synchronized with the throughput of the extruder and the
volume of the foam required by the size of the part. The speed of the
ejection of the foamed product by the die can also vary in speed to
coincide with the speed of travel of the molds.
Broadly, the process of the present invention entails forming a shaped
foamed article from starch comprising:
(a) forming a composition comprising starch, water and a blowing agent;
(b) heating said composition under pressure in an extruder to gelatinize
said starch wherein said extruder has a nozzle;
(c) providing a plurality of separate molds on a continuous path to said
extruder, each said mold comprising a pair of mold halves with a void
therein such that when said mold halves are joined to form said mold, said
void has the shape of said shaped foamed article;
(d) joining each of said mold halves to form said mold about said nozzle of
said extruder;
(e) extruding said composition through said nozzle into said void of said
mold such that said composition foams and fills said void;
(f) cooling said foamed composition in said mold to form a shaped foamed
article; and
(g) opening said mold into said pair of mold halves to recover said foamed
shaped article and returning said pair of mold halves to said nozzle.
The starch used in the process can be an unmodified starch, a modified
starch, or a mixture thereof. More preferably, the modified starch is an
esterified starch, etherified starch, oxidized starch, crosslinked starch,
or a combination thereof. The more preferred modified starch is a starch
graft copolymer.
The composition can also include other components such as synthetic
polymers, processing aids, fire retardants, emulsifiers or mixtures
thereof.
Suitably, the starch can be a high amylose (greater than 30%) starch, an
esterified high amylose starch, an etherified high amylose starch, an
oxidized high amylose starch, a cross-linked high amylose starch, or a
combination thereof.
Suitable synthetic polymers which can be used in the composition include
polystyrene, polyethylene, polypropylene, poly(ethylene terephthalate)
(PET). The synthetic polymer is combined with the starch to form the
foamed product.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects of the present invention may be more fully
understood by reference to one or more of the following drawings wherein:
FIG. 1 is a side view of the process and equipment employed in the present
invention; and
FIG. 2 is a front view of the equipment in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates extruder 10 having nozzle 12 through which starch
material is ejected into mold 20. Mold 20 travels a continuous loop 22
which is supported by frame 24.
FIG. 2 illustrates a front view where mold 20 is divided between mold pair
26 having cavity 28 therein. Each mold 20 has opening 30. Opening 30 in
one mold 20 is aligned with opening 30 in adjacent mold 20 such that they
align to create one continuous hole throughout all mold 20 as shown in
FIG. 1.
Traveling molds are conventional pieces of equipment which are employed in
accordance with the present invention in a conventional manner.
It will be understood that the claims are intended to cover all changes and
modifications of the preferred embodiments of the invention herein chosen
for the purpose of illustration which do not constitute a departure from
the spirit and scope of the invention.
* * * * *
|
|
 |
|
 |
Description |
|
