WO1993006013A2

WO1993006013A2 - Biodegradable packaging film

Info

Publication number: WO1993006013A2
Application number: PCT/CA1992/000407
Authority: WO
Inventors: David Aung
Original assignee: David Aung
Priority date: 1991-09-19
Filing date: 1992-09-18
Publication date: 1993-04-01
Also published as: WO1993006013A3; AU2578192A

Abstract

The invention relates to a biodegradable film comprised of starch and water. The film is impermeable and flexible and of sufficient tensile strength to withstand stretching by a factor of 50 % or greater. The film may also contain flour and additives. The starch may be corn, tapioca and wheat and the flour may be corn, rice and wheat. Suitable additives that may be used include softening agents, firming agents, anti-caking agents, preservatives, colouring agents, flavouring agents and anti-oxidant agents. The film may be used for wrapping food or forming bags. The invention also relates to a process for preparing the film.

Description

Title: Biodegradable Packaging Film

FIELD OF THE INVENTION

The invention relates to a water repellent biodegradable film and a process for preparing said film. BACKGROUND OF THE INVENTION

Flexible films made from plastic or paper are widely used for packaging a variety of products. Thin plastic film is used to wrap take-out food in commercial outlets and food left-overs in the home. Plastic-based films are also commonly formed into bags for carrying all manner of products, including food. Disposable plastic film is convenient, easy to use, inexpensive, hygienic, requires no washing and can be discarded after use. However, these oil-based plastic products are not biodegradable and contribute to litter problems. They are generally disposed of in landfill sites where they accumulate and persist indefinitely as environmental contaminants.

Wood-based wrapping materials, such as waxed paper, are more environmentally friendly but their uses are limited by the properties of paper. Paper products are opaque and can not be stretched like plastic films.

Paper products are generally folded rather than stretched.

Biodegradable polymers, such as starch, have been incorporated into oil-based plastics. A corn starch- based additive is often used at a concentration of between 6 and 15% of the final product. In the appropriate environment, such as a landfill site, microorganisms digest the starch. Bulk biodegradation occurs at concentrates approaching 50% starch but, as the percentage of starch increases there is a concomitant loss of physical properties of the plastic (See Modern Plastics Encyclopedia Mid-October 1990 issue p. 178). United Kingdom patent application No. 2,0219,836, discloses a method and composition of materials for preforming starch with a lubricating fluid into pellets for use in the extrusion of biodegradable plastic. However, so called biodegradable plastics are comprised of traditional oil- based plastic polymers loaded with starch or other rapidly decomposing material as a binder. The binding material breaks down rapidly but leaves small particles of the plastic polymer which are not biodegradable. The oil- based plastics blended with biodegradable materials such as starch have not gained commercial acceptance because they are not fully biodegradable and they are expensive to produce.

Natural polymers which can be processed by conventional plastics technology have been obtained as fermentation products from single cell microorganisms. Biocycle, March 1989, p. 58 discloses the isolation of a biodegradable polymer poly(3-hydroxybutyrate-3 hydroxyvalerate). However, the polymer is expensive to produce and can not compete commercially with oil-based plastics.

Water-soluble films for food packaging have been produced from methyl cellulose, hydroxy-propyl cellulose, and hydroxypropylated high-amylose corn starch. United States Patent No. 3,549,619 to Mark and Mehltretter teaches a method for the preparation of amylose acetate dispersions capable of yielding edible transparent films suitable for packaging of food. The water-soluble food packaging films are produced from high amylose corn starch acetylated with acetic anhydride. The resulting corn starch acetate granules are cooked by steam jets at 177°C to disintegrate the granules. Water-soluble food packaging films are then cast from the resulting amylose acetate aqueous dispersions. It is disclosed that the product is a water-soluble, edible, flexible film which is especially suited to package dry foods intended to be added to liquid prior to use, such as coffee or soup.

There is a need for a biodegradable flexible film which is stretchable, environmentally friendly and prepared by a commercially useful process. In particular there is a need for a water repellent biodegradable film suitable for wrapping food or forming into bags. SUMMARY OF THE INVENTION

The present invention provides a biodegradable film comprising starch and water, whereby said film is impermeable and flexible and of sufficient tensile strength to withstand stretching by a factor of 50% or greater. Preferably the film is comprised of starch, flour, water and additives, is 0.025 to 0.125 mm thick and has a stretch factor of from 50-100% and the starch is corn, tapioca or wheat starch; the flour is corn, rice or wheat flour; and the additives are softening agents, firming agents, anti-caking agents, preservatives, colouring agents, flavouring agents and anti-oxidant agents.

The present invention also provides a method of forming a biodegradable film comprising preparing a mixture of starch and water, heating and mixing the mixture to form a viscous liquid; casting said viscous liquid into a film, wherein the relative amounts of the ingredients and the process conditions are selected such that the film is impermeable and flexible and of sufficient tensile strength to withstand stretching by a factor of 50% or greater.

The film may be further processed to form a sheet of a predetermined thickness and a water repellent may be coated on the surface of the film by powder or liquid spraying.

The mixture of starch and water used in the method of the invention may be in pellet form. Additives such as softening agents, firming agents, anti-caking agents, preservatives, colouring agents, flavouring agents and anti-oxidant agents may also be added to the mixture of starch and water.

The water repellent used in the method of the invention may be casein, zein, ethyl cellulose, gluten, glutenin or alginate.

In an embodiment of the method of the invention, a mixture is prepared using starch in an amount of 2-30% by weight and water in an amount of 65-85% by weight. In a second embodiment of the method of the invention, the mixture of ingredients is heated at a temperature of from 175 to 195°F and the viscous liquid has a viscosity of from 50-1,000 cps.

In a preferred method of the invention, a biodegradable film is formed by mixing starch in an amount of 2-30% by weight, flour in an amount of 0-10% by weight, water in an amount of 65-85% by weight and additives in an amount of 0-15% by weight; feeding the mixture into a cooking reactor; cooking and mixing the mixture in a cooking reactor to form a viscous liquid; feeding said liquid into a casting and pressing mold; casting and pressing said viscous liquid into a film in said casting and pressing mold.

The film may be further processed by the additional steps of feeding the film through a rolling machine, and coating water repellent on the surface of the film by powder or liquid spraying, and cooling and drying the film. Preferably, the mixture is cooked in the cooking reactor at a temperature of from 175 to 195°F and the viscous liquid has a viscosity of from 50-1,000 cps. The casting and pressing mold has a die temperature of 104-140°F and a pressure of 4,000-30,000 psi. The water repellent may be selected from the group comprising casein, zein, ethyl cellulose, gluten, glutenin or alginate.

The properties of the film of the invention make it suitable for wrapping foodstuffs. Sheets of the thin film may be used in a similar manner to thin plastic films or waxed paper. The thin film may also be formed into bags suitable for a variety of uses including containing foods and groceries. In particular, the film of the invention may be entirely composed of natural edible ingredients such as starch and flour and accordingly is biodegradable and edible. It will be appreciated that small amounts of additives will not affect the biodegradable nature of the film. Preferably, natural biodegradable and edible additives should be employed. The film is also flexible and in a preferred embodiment has a thickness of from 0.025 to 0.125 mm, a stretch factor of 50-100%, and is translucent and is water repellent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with reference to the drawings which show a preferred embodiment of the present invention and in which:

Figure 1 is a schematic drawing showing a method for producing a biodegradable thin film;

Figure 2 is a schematic drawing of a casting and pressing mold;

Figure 3 is a schematic side view of another apparatus according to the present invention;

Figure 4a and b are an end view and sectional view of an outer part of a slide nozzle; and

Figures 5a and b are an end view and a sectional view of an inner, nozzle part of the slide nozzle. DETAILED DESCRIPTION OF INVENTION

The present invention provides a biodegradable film comprised of starch, water and optionally, additives. The film is sufficiently impermeable, flexible and strong to be used for wrapping food or other organic material. The film may be translucent so that the contents are readily visible. The film may be used in a similar fashion to thin plastic food wraps or waxed paper to retain the freshness of foodstuffs. It is also contemplated that the thin film could be formed into bags for a range of uses such as fast-food and food wrap, sandwich and lunch bags and grocery bags.

In a preferred embodiment the film has a moisture content of from 10 to 30%, a thickness of from 1 to 5 mil, a density of from 0.01 to 0.3 g/cc, a stretch factor of at least 50% and is stable over a temperature range of from -20 up to 140°C. The film is preferably comprised of starch, most preferably corn, tapioca or wheat; flour, most preferably corn, rice or wheat; additives; and, water.

A variety of additives may be used in the film depending on the desired results. Examples of suitable additives are provided in Table I. For further lists of suitable additives see, for example, R.S. Igoe, Dictionary of Food Ingredients, Van Nostrand Reinhold, New York 2nd Ed., 1989; and R.J. Lewis Sr. Food Additives Handbook, Van Nostrand Reinhold, New York, 1989.

Most preferably, the film has a water repellent coating. The water repellent may be applied by powder or liquid spraying techniques as generally described in R.F. Bunshah, Deposition Technologies for Film and Coatings; and Finishing Guidebook, Metal Finishing Magazine, 1988. Examples of suitable water repellents include casein, zein, ethyl cellulose, glutenin and alginate.

A preferred embodiment of the present invention also relates to a method for producing the above-described film from the ingredients comprising 2-20% starch

(particle size 150-250 mesh) 0-15% additives, 0-10% flour and 65-85% water. The ingredients are mixed, sieved and cooked in a cooking reactor to form a viscous mixture. The viscous mixture is formed into a film by the techniques of casting and pressing, and a water repellent may be applied on one or both sides of the film by powder or liquid spraying techniques. The film may be further processed by cooling, cutting to the desired shape and packaging.

Various formulations of natural ingredients may be used in the process as feed materials. The feed materials comprise starch, water and additives. Flour may also be included. The flour and starch are obtained from natural cereal sources, such as corn, tapioca or wheat starch and corn, rice or wheat flour. The particle size of the starch and flour is preferably in the range of 150- 250 mesh and the moisture content is in the range of 8.0- 15% for flour and 10-23% for starch. As discussed above, a variety of additives may be used depending on the nature of the film to be produced.

The flavouring agent may be one or more of a natural flavour or an artificial flavour, or a combination of natural and artificial flavours. Examples of suitable natural flavouring agents include grapefruit oil, jasmine oil, lemon oil, lime oil, orange oil and rose oil. The desired result as well as the nature of the flavouring agent will determine the actual amount used in any particular incident.

The colouring agent may be a natural or artificial colouring agent or a combination of both. The amount of colouring agent to be added can be determined by visual requirements. Natural colouring agents such as saffron, paprika, beetroot, crocein and carotene are preferably used as colouring agents.

Flavouring and colouring agents are most preferably selected from the relevant list of approved agents, for example those approved by Health and Welfare, Canada and the Federal Drug Agency, United States.

A preferred process is shown in Figure 1 and described below. The feed materials are fed into a weigh mixer in the following proportions, 2-30% by weight starch, 0-10% by weight flour, 65-85% by weight water and 0-15% by weight additives. The mixture is sifted in a sifter to provide particles of uniform size. The mixture, water, flavouring and colouring agents are held in storage tanks and are fed into an electrically heated cooking reactor 1 (for example, Chester Jenson Co., model 70N5).

The feed materials may be in the form of a pellet, which is formed by cooking the starch and flour in a cooking reactor as detailed below and feeding the resulting viscous liquid into a conventional pellet mill. Preferably, the pellet has a moisture content of 12 to 15%.

In the cooking phase, the ingredients are mixed by stirring at 50-200 rpm and cooked at 175-195°F, until a viscous liquid mixture results (50-1,000 cps). Most preferably the mixture is cooked at 180°F with a stirring speed of 90 r.p.m. to yield a mixture having a viscosity of 200cps.

The viscous mixture is fed to a casting and pressing mold 2 which is water cooled to 104-140°F. The casting and pressing mold is operated with a mold pressure of 4,000 to 30,000 psi and a liquid pressure of 14-50 psi. Most preferably the casting and pressing mold is operated with a mold pressure of 10,000 psi and a die temperature of 110°F, and the mold force is completed in 10 seconds. In the casting and pressing mold the viscous mixture spreads over the countersurfaces of the mold and pressure is applied for 2-30 seconds. During compression the mold is water cooled to cool the film to approximately 110°F. The warmed water effluent may be recycled for heat exchange.

Figure 2 shows a schematic drawing of a suitable casting and pressing mold. The mold is constructed from polished and tempered stainless steel. The base, top and sides of the mold are at least h inch thick and the surface may be ground to the desired texture (smooth or rough). To produce the film of the invention, the viscous mixture is pumped, for example at 5,000 psi into the die through apertures such as 5. The mixture spreads out over surface 7. The die is cooled by cool water circulated, for example, at 2 gallons per minute, through inlets 1 and 3. During the pressing phase, hydraulic or pneumatic powered compression is applied to the mold in the direction shown by arrow 6 for approximately 10 seconds. The formed film may be further processed by feeding through rolling machine 3 to produce a film of even thickness. The sheet may be fed through a rolling machine set at 1-5 mil, at a temperature of from 70-90°F, preferably 80°F, with teflon or rubber coated steel rollers rotating at approximately 9-150, preferably 100 rpm. The resulting film is fed through cooling chamber 4 for approximately 3 minutes and cooled by cooled forced air at 40-70°F.

Preferably, a water repellent is coated onto one or both surfaces of the film. Suitable water repellents include casein, zein, methyl cellulose, glutenin, gluten and alginates such as sodium and calcium alginate and in particular water repellents sold under the trade marks Algin (Prescott Company), Ethocel (Dow Chemicals), Prolait (Charles Tennant), Modglut (Ogilvie Mill) and Zein (Freeman Industries). The water repellent may be applied with a spray pressure of from 50-200 psi by powder or liquid spray coating techniques. For example, the water repellent may be applied using a Christy Machine Company tube/core coater, using a nozzle pressure of 100 psi. The film may be coated on one or both sides and the coating may have a thickness of from 0.25-0.75 mil, preferably 0.5 mil.

The film may be cut into the desired shape and size by cutting machine 5. Alternatively the cooling and cutting steps may be carried out in one machine, for example a Greerco multi-zone cooling/cutting machine with an 8 foot capacity. The film may be packed and stacked for shipping, for example in an Eagle Packaging, Dyna-Pak Stacking/Packing Machine with a 240 bag per minute capacity.

It will be appreciated that modifications can be made to the embodiment described in connection with Figure 1, and that such modifications are within the contemplation of the present invention. As an example, the cooking reactor may be an extrusion cooker with a single screw. The extrusion cooker may be used to mix and cook the sifted mixture of feed materials and water and any desired colouring and flavouring agents. In particular, the sifted mixture, water and any colouring and flavouring agents are introduced by means of a volumetric screw feeder (at a temperature of about 131 to 176, preferably 158 to 176) into an extrusion cooker with a single screw rotating at a speed of between about 20-480 rpm. An example of such an extrusion cooker is an extruder typically used for plastics. Blowing agents such as ascorbic acid and sodium lactate may be used in the extrusion process. In the extrusion cooker, the material is mixed and kneaded at a temperature of 176 to 266, preferably 212 to 266°F and cooked at a temperature of 176 to 266, preferably 212 to 266°F. The resultant material can then be injected at a pressure of 1,000 to 4,500, preferably 1,300 to 4,000psi, into a flat die where the film is set and cut. The die may be about 6-48 inches and may have a single blade which is operated at a speed of 60-120 rpm. The die pressure is generally in the range of 10,000 to 35,000 psi. The die may be adjusted to obtain a desired thickness for the film. After the film is set and cut it may be transferred to a coating machine after which it may be dried, preferably in an oven. A circular die can be attached by means of a die-adapter to the extrusion cooker to produce blown film using an air ring cooled oscillator system (die pressure 2,500 to 4,500psi).

The flexible film may also be formed into containers, packaging blocks, lids, etc in a compression mold using high temperature and pressure.

The following examples are illustrative of the present invention:

Example 1

A range of formulations as .shown in Table II were tested, employing the seven basic ingredients of corn starch, tapioca starch, wheat starch, corn flour, potato flour, rice flour and wheat flour to determine the preferred formulations for preparing the flexible film. The preferred formulations are shown in Table III

The particular formulations shown in Table IV were processed as generally described above to obtain a mixture with particles having a mesh size of 200 and a moisture content of 12% by weight. The ingredients were cooked at 180°F in an electrically heated cooking reactor (model 70 N5, Chester-Jensen Co.) with a stirring speed off 90 rpm. The ingredients were stirred and cooked to yield a mixture with a viscosity of 200 cps.

The viscous mixture was fed into the casting and pressing mold described above and shown in Figure 2. The viscous mixture was pumped into the casting and pressing mold at a pressure of 5,000 psi and allowed to spread over the countersurfaces of the mold. The extruded material may be introduced into a mold which is shaped and sized to form for example wrapping films, cups lids, boxes, plates, trays, straws and eating utensils. 10,000 psi pressure was applied to the mold for 10 seconds. During compression, cooled water (2 gallons per minute) was pumped into the mold to cool the forming film to about 110°F. The warmed water effluent was recycled for heat exchange. The formed film was fed through a rolling machine equipped with teflon coated steel rollers turning at a speed of 100 rpm and a temperature of 80°F.

Water repellent was coated on the formed film by powder or liquid spray coating techniques. Ethylcellulose was sprayed on the film to a depth of 0.5 mil using a nozzle pressure of 100 psi in a water resistance coating machine (Christy Machine Company, tube/core coater, equipped with powder and liquid dispenser). The film was cooled at 75°F for 3 minutes in a Greerco multi-zone cooling/cutting machine with 8 feet capacity.

The films were tested for moisture content (AACC

4416, Sartorius MA 30), thickness (ASTM D-1005-84, Fowler micrometer), density (ASTM D-792-86, Sartorius B120 S), elongation strength (ASTM D-638, Chatillon/BG 100), stability (Thermal Conductivity Tester, K. Matic/Rapid K, Holometrix) and water resistance (ASTM D-870-87) and the results are shown in Table V.

To compare the flexible films of the invention with flexible films of the prior art a variety of flexible films known in the art were tested as described above. The following commercially available films were tested: aluminum foil (Alcan), paper bags (Dow Chemical), plastic wrap (First Brand) and plastic bags (First Brand), and the results are shown in Table VI.

Reference will now be made to Figures 3 - 5, which show an alternative embodiment of the invention.

In Figure 4, there is shown a platform 40 for supporting apparatus. Above this is supported a mixer 42, which is connected to a volumetric feeder 44. The volumetric feeder 44 in turn supplies a hopper 46. A control panel 48 includes the necessary control instrumentation devices.

An extruder 50 has a barrel and screw in known manner, and is driven by a motor 52. The extruder 50 is a single screw extruder, and here is an Engel Model ES55 Injection Molding Machine, as produced by Engel Canada Inc.

The screw and barrel configuration should be chosen to give desired characteristics. Here, these are: mixing; cooking; pressurising; and expansion. To ensure adequate mixing, a modified barrel profile was used including channels.

The end of the extruder barrel is connected to a mold 54, which is shown, for molding a single article; as detailed below a different mold configuration would be employed for molding sheet or film in accordance with the present invention.

Turning to Figures 4 and 5, these show details of the sliding nozzle seal. This comprises a first nozzle part or nozzle body 82, which is threaded at one end, as indicated at 84, for securing in the end of the barrel body 50. The nozzle body 82 defines a bore 86. As shown in Figure 3, this bore 86 comprises a relatively wide inlet portion 86a with a diameter of 0.75 inches, an immediate portion 86b of slightly narrower diameter, and an upper or outlet end portion 86c of yet narrower diameter. The portion 86c has a diameter 0.5 inches. The overall bore 86 and nozzle body 82 have a length of 3.25 inches.

The nozzle or second part of the nozzle assembly is shown at 88 in Figure 4, and comprises an upper or outlet portion 90, having a hemispherical abutment surface 92 for abutting a corresponding surface of a mold inlet plate. A bore 94 extends from the hemispherical surface 92 through to a short cross bore 95.

The inlet part 88 has an inlet end portion 96, into which the bore 94 extends. At the lower or inlet end of the inlet portion 96, a screw 98 secures an end member 100. This end member 100 has a diameter of 0.74 inches, and hence is sized to fit within the bore part 86a with a clearance of 0.010 inches. In use, the nozzle part 88 is slidingly mounted in the bore 86, and the end member 100 retains the two elements together.

Although not shown, the inlet plate is provided with a hemispherical recess, corresponding to the end surface 92, and a short bore leading from this into the mold cavity.

To mold or form film or sheet, a die would be used having a narrow slot opening with a width in the range 0.5 - 3 mil, corresponding to the thickness of sheet to be formed. For plain flat film, a straight slot or nozzle would be provided. A circular slot can be provided, in known manner, where it is desired to make a tube. Such a continuous tube can be further processed, again in known manner, to form bags.

To inject material through the mold in known manner, the mold and barrel 50 would be brought together, and pressed together with sufficient force to form a seal.

This causes the nozzle part 88 to be pressed downwards within the nozzle body 82, as viewed in Figures 3 and 4, bringing the end portion 90 into abutment with the nozzle body 82. The lower face of the cylindrical member 100 is spaced by 2.938 inches from an abutment face 102 of the end portion 90. As this is less than the overall length of the nozzle body 82, this ensures the end member 100 is retained in the bore part 86a and cannot extend beyond it. A spacing of 2.688 inches, between the abutment face 102 and the end member 100 enables the nozzle part 88 to reciprocate in the two bore portions 86b, c between a closed postion and an injection position.

In the closed position, the nozzle member 100 abuts an annular stop between the bore portions 86b, c, and the cross bore 95 is within the bore portion 86c, and hence effectively is closed off. The bore portion 86c and the nozzle end portion 96 have closely similar diameters for this purpose. In the injection position, the abutment surface 102 abuts the top of the nozzle body 82, and the cross bore 95 is located in the bore portion 86b. This enables material to be injected around the end element

100, through the bore parts 86b and c, through the cross bore 95 and bore 94, and through the bore of the inlet plate 70 into the mold cavity 62.

After a predetermined amount of the material has been injected through the mold, the injection is stopped, and the pressure applied by the barrel 50 released. This permits the nozzle part 88 to travel to the closed position, under the influence of the pressure within the nozzle body 82. The cross bore 95 is then located in the narrow bore portion 86c, and hence closed.

To facilitate separation of the various components, and prevent the material used from sticking to the various components, a Teflon coating can be used on surfaces contacting the composition. Additionally, the inlet plate was coated with Teflon, to facilitate separation from the sliding nozzle 82, 88.

A number of tests have been carried out using this apparatus, to form shaped articles. These were carried out using a mixture using one hundred parts dry material, using formulations comparable to those given in Table IV without the water listed, two parts per hundred MAGIC baking soda, and 27 parts per hundred water. The dry ingredients were thoroughly mixed before the water was added. For producing film according to the present invention, the same ingredients as listed in Table IV could be used with the moisture content reduced to 15 - 25%.

Feeding the equipment may require care, since it is designed for free-flowing material. Bridging can occur in the mixer 42 and/or feeder 44.

Although not shown in detail, the mold can either comprise a simple, straight slit, or alternatively, a circular slit, either of which may have a thickness in the range of 0.5 -3 mil. A straight slit will produce a flat ribbon of material, while a circular slit will produce a tube. The tube can be later processed, after cooling, to form bags, in known manner.

Table VII sets out the set, actual and best temperature figures for forming this sheet material.

As indicated, the temperature in the barrel should increase from a range of 158 - 176°F to 212 - 266°F at the nozzle or adaptor 82, 88. The die itself should be maintained at a temperature in the range of 239 - 293°F.

The present invention has been described in detail and with particular reference to the preferred embodiments; however, it will be understood by one having ordinary skill in the art that changes can be made thereto without departing from the spirit and scope thereof. TABLE I

Function of Suitable

Additive Additives

Softening Syrup, sugar, sorbitol, glycerol, glucose, sodium lactate & others

Firming Aluminium & calcium sulphate, calcium phosphate, chloride, gluconate St others

Non-sticking Silicate of sodium, calcium, calcium-aluminum, magnesium oxide & others

Non-staling Sucrose, sucrose acetate isobutyrate & others

Anti-oxidant Citric & benzoic acid derivatives, sorbic acid & sorbic sodium, calcium, potassium salt, ascorbic acid & ascorbic acid sodium, calcium salt & others

Expansion Agent Sodium bicarbonate, ascorbic acid, sodium lactate

TABLE VI

COMPATIBLE FLEXIBLE MATERIAL

vs.

FLEXIBLE BIODEGRADABLE/EDIBLE PACKAGING MATERIAL TEST DATA

A=Aluminium; P=Paper; Pl=Plastic; B=Bag; W=Wrap; F=Foil

ARAE )

Claims

I Claim:

1. A biodegradable film comprising starch and water, whereby said film is impermeable and flexible and of sufficient tensile strength to withstand stretching by a factor of 50% or greater.

2. A film as claimed in claim 1 further comprising flour.

3. A film as claimed in claim 1 wherein said starch is selected from the group comprising corn, tapioca and wheat.

4. A film as claimed in claim 2 wherein said flour is selected from the group comprising corn, rice and wheat.

5. A film as claimed in claims 1, 2, 3 and 4 further comprising additives selected from the group consisting of softening agents, firming agents, anticaking agents, preservatives, colouring agents, flavouring agents and anti-oxidant agents.

6. A film as claimed in any one of claim 1 - 5, having one or more of the following properties: a moisture content of from 10 to 30%, a thickness of from 1.0 to 5.0 mil, a density of from 0.10 to 0.3 g/cc, a stretch factor of from 50 to 100%, being transparent, being water repellent and being stable over a temperature range of from -10°C to 280°F.

7. A film as claimed in any one of claims 1 to 6, which is formed from an initial composition comprising: 0 - 5 percent by weight flour; 10 - 20% by weight starch; 5 - 10% by weight additives; and 65 - 85% by weight water.

8. A film as claimed in any one of claims 1 - 6, which is formed from an initial composition comprising: a dry mixture, which comprises 0 - 5 parts by weight flour, 10 - 20 parts by weight starch, and 5 - 10 parts by weight additives; and 15 - 25% by weight water, which composition was processed through an extrusion cooker to form the film.

9. A film as claimed in claims 7 and 8, wherein the flour is selected from corn, rice and wheat flour, and the starch is selected from corn, tapioca and wheat starch.

10. A biodegradable film produced by preparing a mixture of starch and water, heating and mixing the mixture to form a viscous liquid; casting said viscous liquid into a film, wherein the relative amounts of the ingredients and the process conditions are selected such that the film is impermeable and flexible and of sufficient tensile strength to withstand stretching by a factor of 50% or greater.

11. A method of forming a biodegradable film comprising preparing a mixture of starch and water, heating and mixing the mixture to form a viscous liquid; casting said viscous liquid into a film, wherein the relative amounts of the ingredients and the process conditions are selected such that the film is impermeable and flexible and of sufficient tensile strength to withstand stretching by a factor of 50% or greater.

12. A method of forming a biodegradable film as claimed in claim 11, wherein the mixture is prepared using starch in an amount of 2-30% by weight and water in an amount of 65-85% by weight.

13. A method as claimed in claim 12 wherein said mixture is heated at a temperature of from 175 to 195°F and said viscous liquid has a viscosity of from 50-1,000 cps.

14. A method as claimed in claim 11, 12 or 13, wherein the starch and including some water is in pellet form, with additional water being provided to give the required proportions.

15. A method as claimed in claim 11, 12, 13 or 14, which further comprises adding additives selected from the group consisting of softening agents, firming agents, anti-caking agents, preservatives, colouring agents, flavouring agents and anti-oxidant agents.

16. A method for forming a biodegradable film from an initial composition comprising: a dry mixture of flour, starch and additives, which respectively comprise 0 - 5 parts, 10 - 20 parts, and 5 - 10 parts of the dry mixture; and water comprising 15 - 25% per weight of the composition; the method comprising mixing and cooking the initial composition in an extrusion cooker under elevated temperature and pressure conditions, and passing the cooked composition through a slot-shaped die to form the film.

17. A method as claimed in claim 16, wherein the flour is selected from corn, rice and wheat flour, and the starch is selected from corn, tapioca and wheat starch.

18. A method as claimed in claim 16 or 17, wherein the die has a slot width of 0.5 - 3 mil, to form a film of corresponding thickness.

19. A method as claimed in claim 16, 17 or 18, wherein the die has a circular slot, for forming a tubular film.

20. A method as claimed in any one of claims 11 - 19, which additionally includes forming a layer of water repellent on the surface of the film by powder or liquid spray.

21. A method as claimed in claim 20, wherein water repellent is selected from the group comprising casein, zein, ethyl cellulose, glutenin or alginate.