 |
Description  |
|
|
FIELD OF THE INVENTION
The invention relates to the preparation of starch-based biodegradable
products and more particularly to processes of manufacturing substantially
completely biodegradable blown films from unprocessed raw starch.
BACKGROUND OF THE INVENTION
Waste disposal has become increasingly problematic in recent years, and
plastic packaging is perceived to be a major contributor to the problem.
The availability of landfill space is decreasing, and international
restrictions on the disposal of plastics at sea will be imposed in 1994.
According to some estimates, conventional plastics require hundreds of
years to decompose. In response to these concerns, there has been much
activity in recent years in the development of biodegradable packaging
materials. Polymers such as chitosan, polyvinyl alcohol (PVOH) and
polyhydroxybutyrate-co-hydroxyvalerate have been investigated for these
applications.
Many attempts have been made to produce biodegradable films from petroleum
and cellulose derived materials. None has been completely successful
because either they are too costly or decompose too slowly for most
applications. Starch is probably the most abundant, low-cost,
biodegradable polymer available, and its use in plastic film production
could greatly reduce the demand for petrochemicals and the harmful impact
on the environment caused by discarding nonbiodegradable plastic films.
As described in U.S. Pat. No. 4,673,438 to Wittwer et al., and U.S. Pat.
Nos. 4,133,784, 4,337,181, and 4,454,268 to Otey et al., many starch-based
biodegradable formulations use starch that has first been "destructurized"
or "gelatinized". Destructurization or gelatinization is accomplished by
heating the raw starch granules in the presence of water under elevated
pressure. This treatment produces a disordering of the starch granules and
allows the starch to be more effectively blended in conventional
processing and production steps. However, destructurizing or gelatinizing
the starch requires an additional pass through an extruder which increases
the time and cost required to make starch-based biodegradable articles.
Starch must be combined with other materials (copolymers) in order to
produce a satisfactory extruded film because extrusion of starch alone
produces a brittle, water-sensitive foam. Addition of polyethylene or
polypropylene are known to add water stability, elasticity, and toughness
to processed starch-filled films. Unfortunately, polyethylene and
polypropylene are compounds which have been shown not to be biodegradable.
As a result, only the starch portion of the composite film biodegrades
while the remaining copolymers remain intact. In U.S. Pat. No. 5,087,650
to Willett et al., for example, olefins such as ethylene and propylene are
copolymerized with comonomers such as methyl acrylate, ethyl acrylate, and
hexyl acrylate to produce a graft copolymer which is then combined with
starch. The graft copolymer is not biodegradable, and thus the final
product is actually only partially biodegradable due to decomposition of
the starch component.
Conventional preparation of starch-based films, such as that described in
U.S. Pat. No. 4,337,181 to Otey et al., requires a multiple step process
in which starch is gelatinized, combined with pellets of nonbiodegradable
copolymer, and processed into a film or other shaped article using a
single screw extrusion process. In the process described in U.S. Pat. No.
5,087,650 to Willett, a starch-based "masterbatch" is formed by combining
starch with copolymers, chopping the masterbatch extrudate into pellets,
then blending the masterbatch pellets with polyolefins such as
polyethylene or polypropylene which then can be injection molded or blown
into film. Due to the fixed proportion of starch and copolymer in the
pellets utilized in the prior art, ratios of starch and copolymer are not
easily tailored to specific product requirements. Therefore, production of
biodegradable starch-based products with different ratios of starch and
copolymer are not easily manufactured. Furthermore, these conventional
techniques of producing a starch-based film require additional steps that
take extra time and add to the cost of the final articles.
SUMMARY OF THE INVENTION
The present invention provides for an improved process of manufacturing
biodegradable products from unprocessed raw starch materials and
biodegradable copolymers. This process improves existing manufacturing
techniques by utilizing unprocessed raw starch materials that have not
been pretreated or prepelletized, thus allowing for production of final
products from the raw materials directly. This simplified process reduces
the number of manufacturing steps, lowers the cost and time required to
produce the product, allows for on-site changes to formulations optimized
for given product applications where biodegradable products are desired,
and allows for substantially complete biodegradablility of the final
product. In one embodiment, unprocessed raw starch is mixed directly with
a biodegradable copolymer powder, lubricant, plasticizer, and water, and
the resulting mixture is blown into a film. In a second embodiment, a twin
screw mixer is used to continuously combine unprocessed raw starch,
biodegradable copolymer powder, lubricant, plasticizer, and water. The
resulting mixture then is continuously processed into a blown film. In
either embodiment, the resultant films exhibit excellent mechanical and
biodegradability properties and are suitable for, among other things,
biodegradable packaging, agricultural mulch films, disposable diaper
liners, and trash bags.
DETAILED DESCRIPTION OF THE INVENTION
The starch-based biodegradable films of this invention are prepared from
any unprocessed raw starch, including but not limited to cereal grains or
root crops such as wheat, corn, rice, oat, arrowroot, pea and potato.
Unlike many other starch-based "biodegradable" films, the starch used in
this invention is not required to be pretreated by destructurizing or
gelatinizing. Instead, dry, unprocessed raw starch is fed into the
manufacturing apparatus directly. Furthermore, starches from different
sources may be blended to obtain desired physical properties of the blown
film product. The term "unprocessed raw starch" as used in the
specification and claims is defined herein to include, but is not limited
to, starches that have been neither destructurized nor gelatinized.
A copolymer of polyvinyl alcohol (PVOH) or ethylene vinyl alcohol (EVOH)
provides the starch blend film with desirable structural properties
including flexibility, durability, and dimensional stability. In addition,
these copolymers have been shown to be substantially completely
biodegradable, unlike the polyethylene and polypropylene copolymers used
in blown films described in prior art.
The copolymers of EVOH or PVOH can be used in a powder or pellet form. When
the pellet form is used, the raw materials will include a starch powder,
one or more plasticizers in a liquid form, and copolymer pellets. In this
invention, the liquids and powders can be mixed prior to their addition to
the extruder or fed directly into the feed hopper. Liquid plasticizers may
be metered by controlling flow rate. Commercial equipment is available to
blend and feed powders and pellets over a wide variety of composition
ratios of powder to pellet weights. EVOH and PVOH are commercially
available with a broad range of physical properties varying predominantly
in melting point, melt flow index and gas permeability. In one embodiment
the invention uses PVOH with molecular weights ranging from 30 to 70
kilodaltons and from 124 to 186 kilodaltons. However, the product film
exhibits the best combination of tensile strength and extensibility when
PVOH in the molecular weight range of 70 to 100 kilodaltons is used.
Chemically, PVOH is characterized by molecular weight and degree of
hydrolysis, while EVOH is chemically differentiated by molar percent of
ethylene in the copolymers.
Glycerol is included as a plasticizing agent for starch and the copolymer.
Talc can also be added as a nucleating agent and lubricant during the
extrusion. Alternatively, stearates, such as calcium or zinc stearate may
be used as a nucleating agent. Water is added to the blend to facilitate
melting of the starch and blending with the copolymer during production.
The final moisture content of the film is between five (5) and eight (8)
percent by weight immediately after manufacture. The water content may
change on exposure to varying temperature and humidity conditions.
The proportions of starch and PVOH or EVOH may be varied in order to
produce a film with specific characteristics. For blown films, starch
content is in the range of 20 to 80% by weight, and preferably on the
order of 50 to 70%. Useful levels of PVOH or EVOH range from 20 to 80% by
weight, with the most preferable range being 30 to 50%. Optimal range of
glycerol is from 10 to 20%.
The processing of the raw materials into a finished film is accomplished
with a twin-screw mixer (TSM) fitted with a blown film die. A twin-screw
mixer is preferred due to the high shear forces needed to break up the
starch granule and completely mix it with the other components. Unlike
prior methods using single screw mixers and pelletized copolymers, starch,
plasticizers, and copolymers of the present invention are efficiently
blended by the twin-screw mixing process to yield a highly homogeneous
product. In addition, the process temperatures used are thirty to forty
degrees Centigrade lower than those associated with known commercially
available materials of similar formulation. The preferable zone and die
temperatures are 70.degree.-180.degree. C., more preferably 80.degree.-
160.degree. C., and most preferably 90.degree.-145.degree. C. The
preferable extruder speed depends on the size and type of commercial twin
screw mixer used in the manufacture of blown film. Typical rotational
speeds vary from 20 to 120 RPM. Twin screw mixer units which are used for
this process, in general, can be empirically adjusted by the operator in
order to produce a film with particular desired characteristics. The
operator may control RPM, raw material feed rates, zone temperatures,
parison internal air pressure, and other parameters.
Through constant feeding of the components into the blowing apparatus, a
blown film can be continuously extruded. These formulations can also be
extruded into thin film, rods, hollow tubing, or other shapes with axial
symmetry. Articles that may be manufactured using the formulation and
process of this invention include, but are not limited to, blown films,
agricultural mulch films, trash bags, and biodegradable packaging.
The following examples further illustrate the invention but should not be
construed as limiting the invention which is defined by the claims. Other
formulations containing starch with alternate copolymers and additives
(plasticizers, nucleating agents) are processible by this method.
EXAMPLE 1
This example describes preparation of a substantially completely
biodegradable blown film from a small batch of manually prepared raw
ingredients.
A mixture of dry ingredients is first prepared by combining unprocessed raw
starch (200 g), powdered ethylene vinyl alcohol (200 g with a 32% ethylene
copolymer ratio), and zinc stearate (20 g). A separate mixture of glycerol
(137 g) and water (63 g) is prepared, added to the combined dry
ingredients, and stirred to yield a semi-dry powder. The mixture is then
fed into the extruder, and processed using a Brabender 42 mm
Counter-rotating Twin Screw Mixer fitted with a blown film die (2.54 cm
i.d. with a 0.05 cm gap). No water is added to the extrusion process
through injection ports nor is water removed via venting.
The film was formed under the following conditions:
______________________________________
Zone 1 Temp. 115.degree. C.
Zone 2 Temp. 125.degree. C.
Die Temp. 126.degree. C.
Melt Temp. 138.degree. C.
Extruder Speed 40 RPM
Amps 15
Die pressure 200 psi
______________________________________
The samples were evaluated for moisture content, tensile strength, and
ultimate elongation:
______________________________________
Moisture before extrusion
13%
Moisture after extrusion
5%
Tensile Strength (MPa) 5.6
Ultimate Elongation 195%
______________________________________
EXAMPLE 2
The powdered ethylene vinyl alcohol with a 32% copolymer ratio used in
Example 1 is substituted with powdered ethylene vinyl alcohol with a 38%
copolymer ratio and processed under the same conditions as in Example 1.
The samples were evaluated for moisture content, tensile strength, and
ultimate elongation:
______________________________________
Moisture before extrusion
13%
Moisture after extrusion
5%
Tensile Strength (MPa) 15.1
Ultimate Elongation 338%
______________________________________
EXAMPLE 3
This example describes preparation of a substantially completely
biodegradable blown film from a small batch of manually prepared raw
ingredients.
A mixture of dry ingredients is first prepared by combining unprocessed raw
starch (200 g), powderized polyvinyl alcohol (200 g of 70-100K molecular
weight), and talc (20 g). A separate mixture of glycerol (137 g) and water
(63 g) is prepared, added to the combined dry ingredients, and stirred to
yield a semi-dry powder. The mixture is then fed into the extruder, and
processed using a Brabender 42 mm Counter-rotating Twin Screw Mixer fitted
with a blown film die (2.54 cm i.d. with a 0.05 cm gap). No water was
added to the extrusion process through injection ports nor is water
removed via venting.
The film was formed under the following conditions:
______________________________________
Zone 1 Temp. 105.degree. C.
Zone 2 Temp. 105.degree. C.
Die Temp. 110.degree. C.
Melt Temp. 113.degree. C.
Extruder Speed 50 RPM
Amps 7
Die Pressure 200 psi
______________________________________
The sample was evaluated for moisture content and crystal melting profile
by differential scanning calorimetry (DSC) before and after extrusion. In
addition, the blown film sample was evaluated for physical properties,
moisture content, oxygen permeability, tensile strength, and ultimate
elongation:
______________________________________
Moisture before extrusion
13%
Moisture after extrusion
5%
Oxygen Barrier 1.5 g/m.sup.2 /24 hrs.
Tensile Strength 28.8 MPa
Ultimate Elongation 168%
______________________________________
It is possible to scale up the amounts of ingredients utilized in Examples
1-3 to prepare biodegradable blown films continuously from raw materials.
Stocks of dry and wet ingredient mixtures are held in respective storage
hoppers. Mixing can be accomplished using a blending hopper system with
liquid injection capabilities which feeds directly into the extrusion
system. This method allows for continuous production of biodegradable
blown film.
Although the invention has been shown and described with respect to
exemplary embodiments thereof, various other changes, omissions and
additions in form and detail thereof may be made therein without departing
from the spirit and scope of the invention.
* * * * *
|
|
 |
|
 |
Description |
|
