WO1991015542A1

WO1991015542A1 - Starch-based degradable plastic containing polypropylene glycol

Info

Publication number: WO1991015542A1
Application number: PCT/US1991/002361
Authority: WO
Inventors: John S. Peanasky; John M. Long; Richard P. Wool
Original assignee: Agri-Tech Industries, Inc.
Priority date: 1990-04-06
Filing date: 1991-04-05
Publication date: 1991-10-17
Also published as: AU7742091A

Abstract

A degradable plastic which contains starch and polypropylene glycol. High starch percentages can be combined with conventional plastics, such as polyethylene, by using polypropylene glycol to produce biodegradable materials with enhanced mechanical properties for the plastic product.

Description

STARCH-BASED DEGRADABLE PLASTIC

CONTAINING POLYPROPYLENE GLYCOL SPECIFICATION BACKGROUND OF THE INVENTION

This invention relates generally to plastics and, more particularly, to degradable plastics containing starch and polyethylene with selected additives for producing enhanced mechanical properties.

Plastic products make up a substantial proportion of the solid wastes that are disposed of each year.

Because many plastics degrade extremely slowly, plastics disposal is a major problem, particularly in advanced, consumer oriented societies.

One possible solution to the plastic disposal problem lies in the development of naturally degradable plastics. Various proposed formulations for degradable plastics are based on the mixture of polyethylene with cornstarch. Although such mixtures can show greatly enhanced degradability over pure polyethylene, this often is obtained at the expense of other important mechanical properties, such as strength and flexibility. As a general rule, compositions containing a higher percentage of starch degrade faster than compositions containing less starch. However, higher starch concentrations tend to exhibit substantially reduced strength and flexibility, thereby limiting their usefulness. At present, there exists no starch and polyethylene based plastic that exhibits a good balance of meaningful degradability in combination with useful and practical mechanical

properties.

In view of the foregoing, it is a general object of the present invention to provide a new and improved starch-based degradable plastic.

It is a further object of the present invention to provide a starch-based degradable plastic and method of manufacture that exhibits meaningful degradability in combination with useful and practical mechanical

properties.

It is a still further object of the present invention to provide a starch-based degradable plastic having the degradability characteristics of a high starch concentration compound in combination with the strength and flexibility characteristics of a low starch

concentration compound.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with the further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings wherein like reference numerals identify like elements, and wherein:

FIGURE 1 is a graph showing the percentage of accessed starch, as determined through acid hydrolysis, versus the volume percentage of starch in various

starch/polyethylene mixtures.

FIGURE 2 is a graph of average tensile modulus versus volume percent of polypropylene glycol (PPG) for several experimental samples containing starch,

polyethylene and PPGs of various molecular weights.

FIGURE 3 is a graph of average yield stress versus volume percent PPG for various samples containing polyethylene, starch and PPGs of various molecular weights

FIGURE 4 is a graph of average ultimate stress versus volume percent PPG for various samples containing polyethylene, starch and PPGs of various molecular weights

FIGURE 5 is a graph of average elongation to break versus volume percent PPG for various samples containing polyethylene, starch and PPGs of various molecular weights.

FIGURE 6 is a graph of average fracture energy versus volume percent PPG for various samples containing polyethylene, starch and PPGs of various molecular weiglits

FIGURE 7 is a graph representation of tensile modulus for various samples containing starch and PPG in combination with high density polyethylene (HDPE), linear low density polyethylene (LLDPE) or polypropylene (PP).

FIGURE 8 is a graph representation of yield stress for various samples containing starch and PPG in combination with HDPE, LLDPE or PP.

FIGURE 9 is a graph representation of ultimate stress for various samples containing starch and PPG in combination with HDPE, LLDPE or PP.

FIGURE 10 is a graph representation of elongation to break for varrious samples containing starch, PPG and HDPE, LLDPE or PP.

FIGURE 11 is a graph representation of fracture energy for various samples containing starch, PPG and HDPE, LLDPE or PP.

FIGURE 12 is a graph of change in mass/initial mass versus time for various samples used in a soil degradation study of samples containing polyethylene, starch and PPG.

FIGURE 13 is a graph of concentration of components (s) to C-O stretch band versus time for various samples used in the soil degradation study.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In one aspect of the present invention there is provided a degradable plastic compound blended from

polyethylene (PE), starch and polypropylene glycol (PPG) in liquid form as an interfacial additive. It is believed that the PPG acts as an interfacial lubricant which

lubricates contact points between starch and starch, starch and PE, as well as between PE and PE. This results in degradable plastics that have relatively high starch concentrations but that nevertheless exhibit improved strength and flexibility characteristics compared with similar compounds lacking the critical interfacial additive. Additional details and examples of the

advantage of the PPG type additive will be described hereinafter. As used herein, "relatively high" starch concentrations are those that result in commercially significant improvement in degradation characteristics as compared against materials containing substantially pure polyethylene.

In accordance with another aspect of the invention, it has been determined that blended starch and polyethylene compounds exhibit measurably improved

degradation characteristics when the starch concentration comprises roughly 30% or more, by volume, of the starch and polyethylene blend (hereinafter all percentages will be by volume unless specifically indicated). It is believed that the 30% concentration represents a critical and surprising "percolation threshold" at which the starch forms a fully connected substructure within a given starch and polyethylene component. This results in a majority of the starch in the manufactured component being accessible to bioorganisms and environmental chemical action. This level of starch leads further to more rapid degradation than would occur at concentrations below the percolation threshold where a fully connected starch lattice does not normally exist.

The percolation threshold of approximately 30% starch has been experimentally verified through acid hydrolysis techniques. The results of this verification are illustrated graphically in FIG. 1. As shown therein, the percentage of accessible starch, as determined through acid hydrolysis techniques, increases sharply between substantially 20% and 36% starch, with substantially 50% of the starch being accessible at a starch concentration of approximately 30%.

In accordance with the invention related to PPG additive, it has been discovered that the addition of the PPG to the blended starch and polyethylene results in a noticeable improvement in a balance of the properties of strength and flexibility of the resulting compound.

Without being limited in the scope of the claims, it is believed that, without the presence of PPG, the lack of adhesion between the polyethylene and the starch results in the ready formation of microvoids at the poles of the starch granules when the combined starch and PE undergoes uniaxial extension. As the extension increases, the microvoids coalesce until the material fails. As the starch concentration is increased and approaches the percolation threshold, the microvoids coalesce at lower extensions in proportion to the size of an invading cluster representing the accessible starch. Accordingly, when complete connectivity through the material is achieved by increasing starch concentration to the

percolation threshold, the strength and flexibility of the material deteriorates. A nonlimiting explanation is that the addition of PPG retards the coalescence of these voids by lubrication. In the final stages of deformation prior to material failure, crack initiation and crack growth may be delayed via an energy dissipation mechanism aided by the presence of PPG.

The substantial, beneficial effects of the addition of PPG have been observed at starch concentration as high as 63% by volume. In addition, the beneficial effects of the addition of PPG have been observed at PPG levels as low as at least 0.5% by volume. The beneficial effects of the addition of PPG have also been observed at PPG concentrations as great as 10% by volume although it is believed that PPG concentrations in excess of 3% by volume merely result in an excess of PPG with little corresponding improvement in overall mechanical

properties. The preferred concentration of PPG is between about 1 and 3% by volume. This range of concentration achieves the desired improvement in physical properties without exhibiting the characteristics of excess PPG loading, such as an odor and oily feel. It will be appreciated that known ultra violet degradation enhancers can also be added to enhance degradation of the compounds in the presence of ultra violet light.

Various polypropylene glycols having molecular weights between 403 and 5000 have been investigated. The addition of PPG substantially enhanced the physical properties of the starch and polyethylene blend. For example, compounds which contained no PPG and could not even survive being cut into dogbone samples, could, with the addition of as little as 0.5% PPG be cut into such samples and tested.

In addition to PPG, various related copolymers of PPG and polyethylene glycol (PEG), as well as PPGs having ends capped with PEG, have been investigated as

additives.

In addition to low density polyethylene, the efficacy of PPG as an additive in starch-based degradable compositions has also been demonstrated in compositions containing high density polyethylene (HDPE), linear low density polyethylene (LDPE) and polypropylene (PP) in place of LDPE.

The use of PPG lubricants in starch-containing polyethylene compounds to achieve unique mechanical properties has been demonstrated through experiment (see Example) . The benefits derived from the use of PPG and its lubricating effects are observed in a wide range of PE/starch formulations. The use of PPG appears to be most beneficial in PE/starch compounds having starch

concentrations above the percolation threshold level of approximately 30 volume percent. This percolation

threshold has been shown to be the critical starch level for significant biodegradability. The use of PPG provides useful, flexible materials containing high starch levels and that are suitable for the manufacture of a wide range of plastic products with greatly enhanced biodgradability.

Further features, advantages and characteristics of the invention are demonstrated in the examples that follow. These examples are meant to be illustrative of the inventions and are not intended to limit the scope thereof .

EXAMPLES

Verification Of Percolation Threshold

The following Examples 1 through 8 verify the existence of the percolation threshold through acid hydrolysis techniques. In these examples, low density polyethylene (LDPE) supplied by Quantum Chemical Company was mixed with cornstarch, supplied by Cargill Inc., in starch concentrations ranging from 5 to 60 volume

percent. The fractional volume of starch p, in these blends was determined using the formula,

p = 1/[1 + yq_s/xq_p] where x and y are the weight fractions of starch and PE, and q_s and q_p are the densities of starch and PE, respectively. The density of starch was determined using pycnometry. Melt blending was performed in a Brabender Data Processing Plasti-Corder Model PL2000 with a 350cc capacity mixing head attachment. Samples were mixed at 140°C and 60rpm for 30 minutes. The blends were then compression molded into plates of thickness 0.05-0.2 inches using 5 inch by 5 inch window frame molds. Samples were cut from this plate using an ASTM D-412c (D-638IV) half size dogbone cutter.

Following preparation, each of these samples was refluxed in hydrochloric acid. When the color of the solution was noticeably amber, indicating the presence of hydroxymethylfuranose formed through acid hydrolysis of the starch component of each sample, the acid solution was replaced with fresh solution. This process was repeated over a period of 24 to 72 hours until no visually

detectable color change in the solution was noted. The samples were then removed to deionized water for about 72 hours. The water solution was changed every 6 to 5 hours. The samples were then placed under vacuum in an oven at 60°C for another 72 hours, and then removed to a dessicator under vacuum for two months. At the end of two months, a final weight was obtained. The loss of weight experienced by each sample provides an indication of the accessable starch in each sample.

EXAMPLE ONE

This sample consisted of 95% polyethylene and 5% starch. The percentage of accessible starch as determined through acid hydrolysis was approximately 5%.

EXAMPLE TWO

This sample consisted of 85% polyethylene and 15% starch. The percentage of accessible starch as determined through acid hydrolysis was approximately 4%.

EXAMPLE THREE

This sample consisted of 75% polyethylene and 25% starch. The percentage of accessible starch as determined through acid hydrolysis was approximately 14%.

EXAMPLE FOUR

This sample consisted of 70% polyethylene and 30% starch. The percentage of accessible starch as determined through acid hydrolysis was approximately 35%.

EXAMPLE FIVE

This sample consisted of 67.5% polyethylene and

32.5% starch. The percentage of accessible starch as determined through acid hydrolysis was approximately 58%.

EXAMPLE SIX

This sample consisted of 65% polyethylene and 35% starch. The percentage of accessible starch as determined through acid hydrolysis was approximately 76%.

EXAMPLE SEVEN

This sample consisted of 60% polyethylene and 40% starch. The percentage of accessible starch as determined through acid hydrolysis was approximately 94%. EXAMPLE EIGHT

This sample consisted of 50% polyethylene and 50% starch. The percentage of accessible starch as determined through acid hydrolysis was approximately 96%.

Examples Including Addition of PPG

The following examples demonstrate the efficacy of PPG in significantly improving the strength and flexibility characteristics of blended polyethylene and starch.

A number of samples containing various

combinations of starch, polyethylene and PPG were prepared and tested. For each combination, a total of six samples were prepared and tested for tensile (Young's) modulus, yield stress, ultimate stress, elongation to break and fracture energy. The various combinations, and the average test result data for the combinations are

summarized in Tables 1, 2, and 3 below.

The following material processing equipment, raw materials and data processing equipment and software were used in preparing and testing the samples:

Material Processing Equipment Used:

1. Mettler Balance Model PM4600

2. Pasteur Transfer Pipettes, 5.75 inch

3. Brabender Data Processing Plasti-Corder Model PL2000 350cm Cubed Mixing Head with Roller Blades

4. Brabender Data Processing Plasti-Corder

Model PL331 350cm Cubed Mixing Head with roller Blades

5. Carver Laboratory Hotpress Model M25

6. Custom Scientific Instrument Half Size Dogbone Cutter, ASTM D-412c (D-638IV)

7. MTS 812 Series with Hydraulically

Driven Actuator

Raw Materials:

1. LDPE Chemplex 3404B

2. Unmodified cornstarch from Cargill

Corporation

3. Food grade potato starch from Western Polymer Corp.

4. Aldrich 20, 230-4 PPG-Molecular weight 425

5. Aldrich 20, 232-0 PPG-Molecular weight 1000

6. Aldrich 20, 233-9 PPG-Molecular weight 2000

Data Processing Equipment and Software:

1. IBM PC/AT

2. IBM PS2 Model 80

3. HP7475A Six Pen Plotter 4. Uniform-PC from Micro Solutions

Computer Products

5. Graphing equipment computer software

from Golden Software, Inc.

Each of the samples was prepared as follows:

Tare weights of the starch and PE were measured using the Mettler Balance. PPG was added to the tared starch using the transfer pipette.

The PE was introduced to the mixing bowl of the Brabender Plasti-Corder at 140°C and 60rpm and allowed to melt. Once melting was achieved, as determined by having reached a peak torque value, the starch/PPG mixture was added.

Mixing continued for 15 minutes and a plateau torque and temperature was achieved a few minutes after loading the raw materials. The blends were then removed from the mixing head attachment and cut into pieces

approximately 1/4 inch in diameter using scissors. The resulting "pellets" were wrapped in aluminum foil and set aside for a minimum of two days before molding into plates.

Torque and temperature data were recorded for each run using the Brabender software and were stored on disk in CPM format. The data on the CPM disks were read usiny the uniform-PC software in an IBM PC/AT and stored in DOS ASCII format.

The plates were compression molded in window frame molds (4 inches by 4 inches) using the Carver Press operating at a temperature of 140°C. Molding runs

consisted of a 450 second heating period, and a 450 second molding period at 20,000 pounds force (1300 PSI0). After the molding period, the press was cooled to room

temperature prior to releasing slowly the pressure. The plastic was removed from the molds using a utility knife.

Dogbone samples were cut for tensile tests using the ASTM D-412 half size cutting die. The thickness of the samples was measured with a micrometer using the method described by ASTM D-638. Thickness among the samples varied from 1.8 milJimeters to 2.2 millimeters. All samples were placed in the grips of the MTS 812 at room temperature and elongated at a rate of 5 millimeters per minute. Data were collected using an IBM analog to digital data acquisition board and stored in DOS ASCII format. For each sample, the tensile modulus (PSI), yield stress (PSI), ultimate stress (PSI), elongation to break and fracture energy were determined.

Some of the more notable combinations shown in

Tables 1, 2, and 3 are discussed in greater detail below:

EXAMPLE NINE

This sample consisted of 100% Chemplex 3404B LDPE and served as a control. This sample exhibited an average modulus of 14844, and average yield stress of 1108, an average ultimate stress of 1608, an average elongation to break of 6.10 and an average fracture energy of 8038.

EXAMPLE TEN

This sample was prepared using 75% Chemplex

3404B LDPE, 23% cornstarch and 2% Aldrich 20,230-4 PPG having a molecular weight of 425. This sample exhibited an average modulus of 9809, an average yield stress of 1029, an average ultimate stress of 1036, an average elongation to break of 1.39 and an average fracture energy of 1252.

EXAMPLE ELEVEN

This sample was prepared using 60% Chemplex

3404BH LDPE, 35% cornstarch and 5% Aldrich 20,232-0 PPG having a molecular weight of 1000. This sample exhibited an average modulus of 7613.28, and average yield stress of 464.10, an average ultimate stress of 592.35, and average elongation to break of 1.744 and an average fracture energy of 912.61.

EXAMPLE TWELVE

This sample was prepared using 60% Chemplex

3404B LDPE, 30% cornstarch and 10% Aldrich 20,232-0 PPG having a molecular weight of 1000. This sample exhibited an average modulus of 6963.99, and average yield stress of 478.30, an average ultimate stress of 586.52, and average elongation to break of 1.24 and an average fracture energy of 646.61.

EXAMPLE THIRTEEN

This sample was prepared using 50% Chemplex

3404B LDPE, 48% cornstarch and 2% Aldrich 20,230-4 PPG having a molecular weight of 425. This sample exhibited an average modulus of 5147.54, and average yield stress of 316.90, an average ultimate stress of 473.37, and average elongation to break of 2.10 and an average fracture energy of 867.76.

EXAMPLE FOURTEEN

This sample was prepared using 60% Chemplex

3404B LDPE, 39.5% cornstarch and 0.5% Aldrich 20,230-9 PPG having a molecular weight of 2000. This sample exhibited an average modulus of 14371.41, and average yield stress of 537.23, an average ultimate stress of 610.5591, and average elongation to break of 1.32 and an average fracture energy of 750.88.

The following examples demonstrate the efficacy of PPG as an additive in starch and polyethylene blends containing a starch other then cornstarch. In the

following examples, potato starch was used.

EXAMPLE FIFTEEN

This sample consisted of 35% Chemplex 3404B LDPE, 63% potato starch and 2% Aldrich 20,233-9 PPG having a molecular weight of 2000. The resulting sample exhibited an average modulus of 2716.94, and average yield stress of 173.71, an average ultimate stress of 271.25, an average elongation to break of 1.31 and a fracture energy of

300.66.

EXAMPLE SIXTEEN

This sample consisted of 35% Chemplex 3404B LDPE,

61.5% potato starch and 3.5% Aldrich 20,233-9 PPG having a molecular weight of 2000. The resulting sample exhibited an average modulus of 2393.66, and average yield stress of 156.94, an average ultimate stress of 243.97, an average elongation to break of 1.05 and a fracture energy of

215.30.

EXAMPLE SEVENTEEN

This sample consisted of 35% Chemplex 3404B LDPE, 63% potato starch and 2% Aldrich 20,230-4 PPG having a molecular weight of 425. The resulting sample exhibited an average modulus of 2813.54, and average yield stress of 180.72, an average ultimate stress of 284.22, an average elongation to break of 1.61 and a fracture energy of

388.01,

EXAMPLE EIGHTEEN

This sample consisted of 60% Chemplex 3404B LDPE,

38% potato starch and 2% Aldrich 20,230-4 PPG having a molecular weight of 425. The resulting sample exhibited an average modulus of 8690.58, and average yield stress of 600.95, an average ultimate stress of 639.02, an average elongation to break of 1.28 and a fracture energy of

767.44.

EXAMPLE NINETEEN

This sample consisted of 40% Chemplex 3404B LDPE, 58% potato starch and 2% Aldrich 20,230-4 PPG having a molecular weight of 425. The resulting sample exhibited an average modulus of 3599.19, and average yield stress of 250.36, an average ultimate stress of 357.82, an average elongation to break of 1.56 and a fracture energy of

481.60.

EXAMPLE TWENTY

This sample consisted of 60% Chemplex 3404B LDPE, 38% potato starch and 2% Aldrich 20,230-4 PPG having a molecular weight of 425. The resulting sample exhibited an average modulus of 6542.78, and average yield stress of 530.08, an average ultimate stress of 611.88, an average elongation to break of 1.15 and a fracture energy of

636.80. EXAMPLE TWENTY-ONE

This sample consisted of 50% Chemplex 3404B LDPE, 48% potato starch and 2% Aldrich 20,230-4 PPG having a molecular weight of 425. The resulting sample exhibited an average modulus of 4746.73, an average yield stress of 341.76, an average ultimate stress of 481.21, an average elongation to break of 1.70 and a fracture energy of

715.66.

The following examples consist of polyethylene in combination with starch without the addition of any PPG additive. These examples serve as controls to demonstrate the effectiveness of the PPG additive.

EXAMPLE TWENTY-TWO

This sample consisted of 25% cornstarch in combination with 75% Chemplex 3404B LDPE. The resulting sample exhibited an average modulus of 14170, and average yield stress of 739, and average ultimate stress of 942, and average elongation to break of 2.57 and an average fracture energy of 2279.

EXAMPLE TWENTY-THREE

This sample consisted of 40% cornstarch in combination with 60% Chemplex 3404B LDPE. The resulting sample exhibited an average modulus of 22539, and average yield stress of 653, and average ultimate stress of 665, and average elongation to break of .738 and an average fracture energy of 448.

EXAMPLE TWENTY-FOUR

This sample consisted of 65% cornstarch in combination with 65% Chemplex 3404B LDPE. The resulting sample was too brittle to be cut into dog bones and hence could not be tested for mechanical properties.

EXAMΓLE TWENTY-FIVE

This sample consisted of 25% potato starch in combination with 75% Chemplex 3404B LDPE. The resulting sample exhibited an average modulus of 10540, and average yield stress of 773, and average ultimate stress of 855, and average elongation to break of 1.19 and an average fracture energy of 936.

EXAMPLE TWENTY-SIX

This sample consisted of 40% potato starch in combination with 60% Chemplex 3404B LDPE. The resulting sample exhibited an average modulus of 9626, and average yield stress of 510, and average ultimate stress of 650, and average elongation to break of 1.34 and an average fracture energy of 798.

EXAMPLE TWENTY-SEVEN

This sample consisted of 65% potato starch in combination with 35% Chemplex 3404B LDPE. The resulting sample exhibited an average modulus of 6840, and average yield stress of 187, and average ultimate stress of 311, and average elongation to break of 1.15 and an average fracture eneryy of 324.

Addition of PPG And PEG Copolymers and

PEG Capped PPGS

The following examples demonstrate the efficacy of, copolymers of PPG and PEG, and the efficacy of PPGs having ends capped with PEG, in improving the strength and flexiblity characteristics of blended polyethylene and starch.

In the examples that follow, the following additives were utilized:

1. Dow Voranol 235-044, a trifunctional copolymer of PPG and PEG having a molecular weight of 3800.

2. Texaco Jeffamine M-2070, a monofunctional copolymer of predominentaly PEG with some PPG and having a molecular weight of 2070.

3. Dow Voranol 222-056, a PEG end capped PPG copolymer having a homogeneous backbone and a

molecular weight of 2000. Between 10 and 20% of the chains are capped with ethylene oxide rather polypylene oxide. 4. Dow Voranol 232-023, a trifunctional additive having between 10 and 20% of the chains capped with ethylene oxide and having a molecular weight of 7200.

The samples considered in the following examples were prepared and tested according to the procedures utilized in connections with Examples 9 through 27 above.

EXAMPLE TWENTY-EIGHT

This sample was prepared using 60% Chemplex 3404B LDPE, 39% cornstarch and 1% Dow voranol 235-044 (3800 MW) . This sample exhibited an average modulus of 8931, an average yield stress of 442, an average ultimate stress of 600, an average elongation to break of 1.47 and an average fracture energy of 801.

EXAMPLE TWENTY-NINE

This sample was prepared using 60% Chemplex 3404B

LDPE, 37% cornstarch and 3% Dow voranol 235-044 (3800 MW). This sample exhibited an average modulus of 6858, an average yield stress of 409, an average ultimate stress of 577, an average elongation to break of 1.39 and an average fracture energy of 715.

EXAMPLE THIRTY

This sample was prepared using 60% Chemplex 3404B LDPE, 39% cornstarch and 1% Dow voranol 222-056(2000 MW). This sample exhibited an average modulus of 8652, an average yield stress of 433, an average ultimate stress of 605, an average elongation to break of 1.47 and an average fracture energy of 801.

EXAMPLE THIRTY-ONE

This sample was prepared using 60% Chemplex 3404B LDPE, 37% cornstarch and 3% Dow voranol 222-056(2000 MW). This sample exhibited an average modulus of 6712, an average yield stress of 407, an average ultimate stress of 576, an average elongation to break of 1.42 and an average fracture energy of 728.

EXAMPLE THIRTY-TWO

This sample was prepared using 60% Chemplex 3404B LDPE, 39% cornstarch and 1% Dow voranol 232-023(7200 MW). This sample exhibited an average modulus of 10116, an average yield stress of 497, an average ultimate stress of 626, an average elongation to break of 1.58 and an average fracture energy of 911.

EXAMPLE THIRTY-THREE

This sample was prepared using 60% Chemplex 3404B LDPE, 37% cornstarch and 3% Dow voranol 232-023(720 MW). This sample exhibited an average modulus of 7380, an average yield stress of 375, an average ultimate stress of 570, an average elongation to break of 1.50 and an average fracture energy of 761.

Replacement of LDPE with HDPE,

LLDPE or PP

The beneficial affects of PPG as an additive have also been noted in connection with compounds containing starch in combination with HDPE, LLDPE or PP. Except where specifically noted, the samples considered in the following examples were prepared and tested according to the procedures utilized in connection with examples 9 through 27 above. The results of these studies are shown in FIGS. 7 through 11.

EXAMPLE THIRTY-FOUR

This sample was prepared usiny 40% high density polyethelyene (Olin 506 supplied by Dow Chemical) in combination with 60% cornstarch. A mixing temperature of 160°C was used to compensate for the higher melting point of the HDPE relative to the LDPE used in the earlier examples. This sample, which contained no PPG and hencer served as a control exhibited an unmeasurable modulus, a yield stress of 850, an ultimate stress of 906, an

elongation to break of 0.11 and a fracture energy of 55.

EXAMPLE THIRTY-FIVE

This sample was prepared using 40% HDPE (Olin 506), 58% cornstarch and 2% aldrich 20,230-4 PPG having a molecular weight of 425. The mixing temperature was 160°C, and the resulting sample exhibited a modulus of 10584, a yield stress of 487, an ultimate stress of 653, an elongation to break of 5.2 and a fracture energy of 3003.

EXAMPLE THIRTY-SIX

This sample was prepared using 40% linear low density polyeththylene (Norlin GA 502-010 M8NN26) and 60% cornstarch. The sample was mixed at 160°C to compensate for the higher melting point of LLDPE relative to LDPE. The resulting sample, which served as a control, exhibited a modulus of 19680, a yield stress of 596, an ultimate stress of 631, an elongation to break of 2.97 and a fracture energy of 1396.

EXAMPLE THIRTY-SEVEN

This sample was prepared using 40% LLDPE (Norlin

GA 502-010 M8NN26), 58% cornstarch and 2% Aldrich 20,230-4 PPG having a molecular weight of 425. The sample was mixed at 160°C and exhibited a modulus of 4116, a yield stress of 238, an ultimate stress of 557, an elongation to break of 6.69 and a fracture energy of 2748.

EXAMPLE THIRTY-EIGHT

This sample was prepared using 40% polypropylene (T.O. plastics 109-2050-250) and 60% cornstarch. The sample was mixed at 200°C to compensate for the higher melting point of PP relative to LDPE. The resulting sample, which served as a control, exhibited an

unmeasurable modulus, a yield stress of 1302, an ultimate stress of 1512, an elongation to break of 0.04 and a fracture energy of 19.

EXAMPLE THIRTY-NINE

This sample was prepared using 40% PP (T.O.

plastics 109-2050-250), 58% cornstarch and 2% Aldrich 20,230-4 PPG having a molecular weight of 425. The sample was mixed at 200°C and exhibited a modulus of 13673, a yield stress of 605, an ultimate stress of 807, an

elongation to break of 1.68 and a fracture energy of 1251. Soil Degradation Studies

Soils were obtained from farmland top-soil in March before planting. They were sifted with 1/8" screen to remove clumps, plant debris and macroorganisms. The soil, at a depth of three inches, was placed in a plastic box measuring 15"xl0"x6.5", which was lined with stanless steel cloth to allow air circulation. The soils were kept moist but not wet with deionized water and stored in a room at ambient temperature (70-78F) and humidty

(37-90%). This environment provides a broad variety of fungi and bacteria to permit aerobic degradation of anturally biodegradable materials in a reasonalbe time frame, typically less than 1 year.

Thin films (1-3 mils) of plastic material were buried in a 4x5 array at a depth of 2 inches. Samples were removed once per month for evaluation of mass change and Fourier Transform Infra Red (FTIR) spectroscopic analysis of chemical changes.

The samples considered in the investigation were as follows:

A: 36% cornstarch; 4% PPG (Registry No. 25322-G9-4); 60% LDPE by volume

B: 40% cornstarch; 60% LDPE

C: 100% LDPE

Figure 12 shows the mass change as a function of time of burial in soils up to 200 days for each of the samples A, B and C. Rapid degradation occured for the satrch containing samples in the first 50 days when about 40% of the weight was lost and proceeded more slowly thereafter. The PPG containing material exhibited a slightly faster rate initially and degraded at about the same rate as the starch and LDPE material. The control material, pure PE, showed a slight decrease in mass.

Figure 13 shows the change of the C-O-C

stretching vibration in the infrared spectrum near

960-1190cm^-1. This vibration is largely due to starch but also has other contributions. The results are similar to those of Figure 7. The data suggest that the starch is first removed during the initial rapid degradation (up to 50 days) and that the remaining material degrades more slowly.

While particular embodiments of the

invention has been shown and described, it will be obvious of those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Claims

WE CLAIM :

1. A degradable plastic composition comprising by volume starch, low density polyethylene and at least about 0.5% polypropylene glycol.

2. A degradable plastic composition as defined in Claim 1 wherein said polypropylene glycol comprises substantially 0.5% or more by volume of degradable plastic composition.

3. A degradable plastic composition as defined in Claim 1 wherein said polypropylene glycol comprises between substantially 0.5% and 3.0% by volume of said degradable plastic composition.

4. A degradable plastic composition as defined in Claim 1 wherein said polypropylene glycol has a

molecular weight of substantially 425.

5. A degradable plastic composition as defined in Claim 1 wherein said starch comprises substantially 30% or more by volume of said degradable plastic compound.

6. A degradable plastic compound as defined in Claim 5 wherein said degradable plastic compound contains between substantially 0.5% and substantially 10% by volume of said polypropylene glycol.

7. A degradable plastic composition as defined in Claim 1 wherein said starch is cornstarch.

8. A degradable plastic composition as defined in Claim 1 wherein said starch is potato starch.

9. A degradable plastic composition comprising polyethylene, starch and polypropylene glycol, said starch comprising substantially 30% or more by volume of said composition and said polypropylene glycol comprising substantially 0.5% or more by volume of said composition.

10. A degradable plastic composition as defined in Claim 9 wherein said polypropylene glycol has a

molecular weight of substantially 425.

11. A degradable plastic compound as defined in Claim 9 wherein said degradable plastic compound wherein said polypropylene glycol comprises no more than

substantially 3.0% by volume of said degradable plastic compound.

12. A degradable plastic compound as defined in Claim 11 wherein said starch comprises cornstarch.

13. A degradable plastic compound as defined in Claim 11 wherein said starch comprises potato starch.