TITLE

Biodegradable composition for film blowing

TECHNICAL FIELD

The present invention relates to the composition of a starch based material, consisting of starch and a plasticizer with a regulated moisture level that enables first a thermoplastic compound in the form of pellets, then using the pellets for thermoplastic extrusion, like in film blowing at specific processing conditions, resulting in biodegradable films made of starch from cultivated renewable sources. The invention makes a rational production process possible for starch materials with moisture related functionality and low environmental impact.

BACKGROUND OF THE INVENTION

The demand for biodegradable polymers is increasing and a variety of material mixtures and processes are used to produce products that exhibit various properties. Films from starch can be cast from aqueous solutions or suspensions, which have the consequence that significant amounts of water have to be evaporated in order to obtain the film. This can be relatively time and energy consuming. When converting starch in the molten state different shapes of the product may be attained, including extruded films. The evaporation step is then avoided and replaced by a cooling phase. In order to achieve thin films the melt has to be shaped after the extruder die. This can be done by pulling and squeezing the melt through calendaring nips or by stretching the melt in two directions as in film blowing.

Film blowing is a commonly used method for producing plastic films. A hollow tube is extruded and then expanded by increasing the pressure inside the tube. The tensile properties of the melt are of great importance for the final result in the melt, the calendaring and the film blowing. Poor melt tenacity is a potential limitation when extruding or processing thermoplastic starch, i.e. starch blended with plasticizer and water, at elevated temperatures. Higher melt tenacity increases the possibility to produce thinner films and also lead to higher production rates. The process of film blowing starch materials has been shown to be difficult to control, particularly difficult to avoid rupture of the film bubble and to avoid surface stickiness at the up-take of the blown film. Different methods to avoid such processing problems include using blends of starch with

substantial amounts of polyvinyl alcohol), poly(caprolactone) or polyester. Such materials are also based on fossil fuels. This new composition is solely based on cultivated renewable polymers.

US 5,362,777A describes a thermoplastically processable starch. Native starch-was mixed with preferably 5 to 35 % additive or plasticizer and melted at 150 to 300 degrees Celsius. It is said to be possible to have it processed further directly in accordance with the ordinary known plastic processing methods. However, the patent also describes that the method steps should be performed under substantially anhydrous conditions.

It is therefore an object of the present invention to provide a method of preparing a thermoplastic processable starch at lower temperatures with solely starch and a plasticizer. By using a modified starch and lower temperatures the disadvantages with water vaporization described in above mentioned patent is removed. Moreover, a more detailed processing method for film preparation is described.

Similar compositions have previously been used for creating sheets in calendaring production, similar to rolling mills. Production through film blowing on the other hand enables thinner films and a more rational way of producing it. Another production method for producing films is through flat film extrusion, yet it has been found that this production method is much demanding on the viscosity of the material. Starch based materials are shown to have a limited viscosity performance, making film blowing more suitable. Finally, film forming is another method used for producing films of various compositions. The film is in this case formed from diluted material and the solvent is evaporated. One such starch-based film, created by film-forming, is described in patent WO0192401 A3. However, this film is not produced by film blowing and the composition comprises synthetic polymers, gum and plasticizers, wherein the plasticizer content preferably is in the region of 33-75 %. The advantages of using film blowing as in the present invention are substantially higher production levels, making mass production convenient.

The invention defines a composition and its production into a pellet, which is a prerequisite for producing a biodegradable starch based film by thermoplastic extrusion without having to add additional polymers as in previous cases. One type of extrusion method that can be used is film blowing.

SUMMARY OF THE INVENTION

The present invention relates to a homogeneous biodegradable polymeric composition solely consisting of a) a starch having a viscosity in the range of 20 mPa s to 70 mPa s as measured with a Brookfield LVDV visco-meter at 50 ⁰ C and 100rpm, and b) a plasticizer wherein the moisture level is less than 20 % (w/w).

In a preferred embodiment of the invention the content of starch is 100 parts per weight and between 10 and 49 parts per weight, more preferably 15 and 40 parts per weight and most preferably between 20 and 30 parts per weight of plasticizer

In another preferred embodiment of the invention the starch has a viscosity in the range of 30 mPa s to 60 mPa s, and most preferably the viscosity should be in the range of 40 mPa s to 50 mPa s as measured with a Brookfield LVDV visco-meter at 50 ⁰ C and 100rpm.

In a preferred embodiment of the invention the starch is a modified starch derived from potato, maize, rice or wheat, or a mixture thereof.

In a further preferred embodiment of the invention the plasticizer is selected from the group consisting of naturally occurring biodegradable compounds such as glycerol, sorbitol or urea.

In a further preferred embodiment of the invention the glycerol is used in combination with water.

In a further preferred embodiment of the invention the modified starch is a hydroxypropylated and/or oxidized normal potato starch.

In a further preferred embodiment of the invention the modified starch is a hydroxypropylated and/or acid hydryolysed normal potato starch.

In a further preferred embodiment of the invention starch is a hydroxypropylated and oxidised normal potato starch (HONPS) with an average number of carboxylic acid groups per anhydroglucose units of 0.04 and a degree of substitution with regard to the hydroxypropyl groups of 0.11.

In a further preferred embodiment of the invention the composition has been shaped into a pellet having a volume in the range from 0,5 mm ³ to 1000 mm ³ , more preferably from 1 ,5 mm ³ to 500 mm ³ and most preferably from 10 mm ³ to 75 mm ³

In a further preferred embodiment of the invention the moisture content in the pellet after a conditioning is in the range of 0.01 % (w/w) to 20 % (w/w) of the total weight of the pellet, more preferably 5 % (w/w) to 15 % (w/w) of the total weight of the pellet and most preferably 7 % (w/w) to 11 % (w/w) of the total weight of the pellet.

The present invention also relates to a process for producing a bio-degradable film composed of a starch and a plasticizer by film blowing, characterized in that it comprises the following steps:

Mixing 100 parts per weight of starch with 10 to 49 parts per weight of plasticizer, more preferably 15 parts and 40 parts per weight of plasticizer and most preferably between 20 and 30 parts per weight of plasticizer and shaping said composition into pellets having a volume of 0.5 mm ³ to 1000 mm ³ , more preferably from 1 ,5 mm ³ to 500 mm ³ and most preferably from 10 mm ³ to 75 mm ³

Conditioning the pellets until they have a moisture content of 0.01 % (w/w) to 20 % (w/w) of the total weight of the pellet, more preferably 5 % (w/w) to 15 % (w/w) of the total weight of the pellet and most preferably 7 % (w/w) to 11 % (w/w) of the total weight of the pellet

- Drying the pellet just before film blowing in temperatures in the range of 80 ⁰ C to 140 ⁰ C during 0.5 minutes to 6 minutes, more preferably in temperatures of 90 ⁰ C to 120 ⁰ C for 1 minute to 5 minutes, and most preferably the pellets are dried in temperatures of 100 ⁰ C to 110 ⁰ C for 2 minutes to 4 minutes

- Feeding pellets into a extruder through a hopper

Melting said pellets in the extruder

Feeding said melted pellets through a die

Blowing a bubble by using pressured air

Cooling of the film in a bubble tower

- Pressing the double layers of blown film together using a calendaring nip, and

- Rolling up of the films on nip rolls

In a further preferred embodiment of the invention the pellets are conditioned in a conditioning chamber having a relative humidity of 30% to 70% for at least 1 day in a surrounding temperature of 10 ⁰ C to 35 ⁰ C, and more preferably for at least 2 days in a surrounding temperature of 15°C to 3O ⁰ C and most preferably for at least 3 days in a surrounding temperature of 20°C to 25 ⁰ C.

In a further preferred embodiment of the invention the temperature during the processing is in the range of 60 ⁰ C to 150 ⁰ C, more preferably should the processing temperature be in the range of 80 ⁰ C to 14O ⁰ C, and most preferably should the processing temperature be in the range of 90°C to 12O ⁰ C.

The invention also relates to a biodegradable film solely consisting of a) a starch having a viscosity in the range of 20 mPa s to 70 mPa s as measured with a Brookfield LVDV visco-meter at 50°C and 100rpm, and b) a plasticizer wherein the moisture level is less than 20 % (w/w).

In a further embodiment the invention the biodegradable film has a thickness of between 15 μm to 1000 μm, more preferably a thickness of 20 μm to 500 μm and most preferably a thickness of between 20 μm and 100 μm.

In a further embodiment the invention the biodegradable film has a glycerol content of 22 parts per 100 parts of starch, a tensile modulus of 0.5 GPa, a tensile strength of 10.5 MPa and a strain at break of 7 % in the flow direction

In a further embodiment the invention the film will shrink in the flow and/or perpendicular direction of the film depending on the surrounding temperature and/or moisture content.

The present invention also relates to the use of the pellet having the properties described above for film blowing.

The present invention also relates to the use of the pellet having the properties described above for thermoplastic extrusion.

The present invention also relates to the use of the film having the properties described above for film blowing.

The present invention also relates to the use of the biodegradable film for composting.

The present invention also relates to the use of the biodegradable film as a gas barrier.

The present invention also relates to the use of the biodegradable film as an adhesive.

The present invention also relates to the use of the biodegradable film as a water soluble packaging material.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates the limiting process parameters as identified for film blowing of biodegradable film made from thermoplastic starch.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention relates to a biodegradable compound comprising starch, a plasticizer and a moisture content that enables the production of film through film blowing. Generally, the invention relates to a composition, a pellet and a biodegradable film, the process for making the film using the composition and the pellet and the use of a film..

A bio-degradable composition suitable for film blowing

The first part of the invention relates to a composition comprising a starch, a plasticizer and water. It has now been found that the precise composition of the starch and plasticizer is very important for the possibility of film manufacturing and for the product properties. The water content is also important since it amplifies the plasticizer's effect. If the amount of plasticizer content is too high related to the modified starch content the blown film will have a sticky surface and the film will be impossible to separate after the passage of the calendaring nip. If on the other hand the plasticizer content is too low the material will have insufficient flow properties for stretching during the blowing, and at the nip the material will fail into a brittle matter. Preferably a composition comprising 100 weight-parts of starch contains between 10 and 49, more preferably 15 and 40 and most preferably between 20 and 30 parts per weight of plasticizer. Preferably the plasticizer is a naturally occurring and biodegradable compound or derivate thereof. Most preferably the

plasticizer is glycerol. Glycerol is readily produced and is cost efficient. Other plasticizers commonly used together with starch are sorbitol and urea. What is important is to get a plasticizer that controls the glass transition temperature, which at the same time affects the gas barrier function. The glass transition temperature helps determine the softness of the material at a given temperature.

Moreover, it has been found that starch, having lower molecular weight than the native normal starch, in this invention referred to as a modified starch, works better in melt processing than a normal starch. The modified starch can be derived from potato, maize, rice and wheat starch, or a mixture thereof. It is known that the modified starch has significantly lower melt viscosity and exhibited a less pronounced shear-thinning behavior than the normal starch. Examples of modified starches which can be used in the present invention are those which have been hydroxypropylated and/or acid hydryolysed or hydroxypropylated and/or oxidized. An example of a hydroxypropylated and oxidised normal potato starch (HONPS) which can be used for the present invention has an average number of carboxylic acid groups per anhydroglucose units of 0.04 and a degree of substitution with regard to the hydroxypropyl groups of 0.11. The modified starch can be characterized in terms of viscosity after jet-cooking a 10% aqueous solution at 120 ⁰ C, then evaluating the viscosity of the solution with a Brookfield LVDV visco-meter at 50 ^c C and 100rpm. Preferably the viscosity should be in the range of 20 mPa s (milli Pascal second) to 70 mPa s, more preferably the viscosity should be in the range of 30 mPa s to 60 mPa s, and most preferably should the viscosity be in the range of 40 mPa s to 50 mPa s. However, the invention is not limited to the use of a modified starch, but any starch having the above properties will fall within the scope of the invention. Although the moisture content of the composition affects the results as well, it will be more important at a later stage of the processing. No extra water needs to be added if the initial moisture content of the starch is about 10 % (w/w) of the starch-glycerol mixtures. Below this moisture content a small amount of water may be needed to obtain a workable composition which can be shaped into pellets. Reduced moisture content lowers the problem with a sticky surface. However, the required torque from the extruder and the die pressure increases significantly due to the lower moisture content, giving a higher viscosity. The moisture content of the compound may be changed at a later stage to suit the intended use, as discussed in the conditioning section below.

Production of pellets from the composition

Advantageously the composition (described above) used for the film blowing process is in the form of pellets. This does not only enable using the composition in a conventional film blowing equipment, it also facilitates the distribution and the handling process of the composition. One way of producing pellets, but the production of pellets is not limited to this method, would be to cut strands extruded from a compounding extruder into small pieces. The size of the pellets are preferably in the range from 0,5 mm ³ to 1000 mm ³ in volume, even more preferred are pellets of the size in the range from 1,5 mm ³ to 500 mm ³ in volume, and most preferred are pellets of the size in the range of 10 mm ³ to 75 mm ³ in volume.

In order to optimize the pellets for the film blowing process they should be conditioned prior to processing. By conditioning at a specific humidity it enables the pellets to obtain the necessary moisture content. The moisture content of the mixture after conditioning are preferably in the range of 0.01 % (w/w) to 20 % (w/w), more preferably the moisture content of the mixture after conditioning are in the range of 5 % (w/w) to 15 % (w/w) and the most preferred moisture content of the mixture after conditioning is in the range of 7 % (w/w) to 11 % (w/w). One possible way to conduct this conditioning preparation is in a closed conditioning chamber. Preferably the pellets should be conditioned in the range of 30% to 70% relative humidity for at least 1 day if the conditioning is carried out in a surrounding temperature that is in the range of 10 ⁰ C to 35°C in order to prepare it for a successful film blowing process, and more preferably for at least 2 days if the conditioning is carried out in a surrounding temperature that is in the range of 15°C to 30 ⁰ C and most preferably the pellets should be conditioned for at least 3 days if the conditioning is carried out in a surrounding temperature that is in the range of 20 ⁰ C to 25 ⁰ C. Moreover, the pellets should be dried just before film blowing to give the pellet a reduced moisture level only at the surface and make it less sticky. This will reduce the problems related to bridging in the hopper of the extruder. Bridging leads to an uneven feed of the material and thus instabilities in the flow through the die, resulting in different manufacturing problems, such as collapse of the blown bubble. Preferably the pellets are dried in temperatures in the range of 80 ⁰ C to 140 ⁰ C for a time in the range of 0.5 minutes to 6 minutes, more preferably the pellets are dried in temperatures in the range of 90 ⁰ C to 120 ⁰ C for a time in the range of 1 minute to 5 minutes, and most preferably the pellets are dried in temperatures in the range of 100°C to 11O ⁰ C for a time in the range of 2 minutes to 4 minutes.

Film blowing

The pellets are processed into a film by the conventional method of film blowing:

• Feeding pellets in the extruder through a hopper

• Melting of the pellets in the extruder • Feeding the melt through a die

• Blowing a bubble by using pressured air

• Cooling of the film in the bubble tower

• Pressing the double layers together with a calendaring nip

• Roll up of the films on nip rolls If the processing temperature (the temperature at the die), used is too high, the glycerol will start to evaporate and this can lead to processing disturbances. Moreover, if the processing temperature is too low the composition will not melt and be impossible to blow. In order to avoid these problems the processing temperature should preferably be in the range of 60°C to 150 ⁰ C, more preferably should the processing temperature be in the range of 80 ⁰ C to 140°C, and most preferably should the processing temperature be in the range of 9O ⁰ C to 120 ⁰ C. Optimal processing and composition conditions for film blowing of a biodegradable film made from thermoplastic starch can be seen In Figure 1. What has been found is that starch based films need more time for cooling compared to polyethylene films in order to avoid sticky surface before roll up of the films. A polyethylene film has the ability to change state more or less instantly at solidification, while starch based films has more of a gradual shift of state. Therefore, large scale film blowing facilities with tall bubble towers could make use of a composition with higher plasticizer content, as a tall tower enables a longer cooling time making the film less sticky.

The film

Preferably the film produced by film blowing of above mentioned composition has a thickness at time after production of between 15 μm to 1000 μm, more preferably the thickness is between 20 μm and 500 μm and most preferably the thickness is between 20 μm and 100 μm .The film is anisotropic, having a higher tensile strength and modulus in the flow direction. This kind of anisotropy is well-known in the art of film-blown synthetic polymers and is not unexpected. A test of a film with a glycerol content of 22 parts per 100 parts of modified starch showed a tensile modulus of 0.5 GPa, tensile strength of about 10.5 MPa and strain at break of 7 % in the flow direction. The film will have a constant volume after storage in room temperature, even though shrinkage could be experienced in the flow direction and/or the perpendicular direction. This shrinkage gives the film interesting features that can be utilized in various applications, as the shrinkage is affected by the surrounding moisture levels and temperatures. From the inventors perspective this gives the film novel and unique performance, compared to films containing synthetic polymers.

The film is biodegradable and the moisture levels affect the rate of degradability. Thus, the film is environmentally friendly with a vast possibility to use in a common composting system. The film has adhesive characteristics, when the moisture level is increased, that can be used in different applications. The film can also work as a gas barrier, for example against oxygen. Moreover, a novelty with this film is that it only consists of cultivated renewable sources compared to other films containing fossil oil products in forms of synthetic polymers. Thus, the film can be used in the manufacture of disposable plastic film, bags or shopping bags, where there exists a need for biodegradable products. The film as such or in the form of a bag can be used for storage of food products including refrigerated or deep-frozen packaged food. Also, the cultivated sources makes it possible to eat the film and make the package an integrated part of the food product.

An additional feature to the film is that it may dissolve, or degrade rapidly, in hot water. Thus, the film can be used as a soluble packaging material for food in the form of e.g. a bag.

EXAMPLES

The following examples are included to demonstrate one embodiment of the invention. It should be appreciated by those skilled in the art that the techniques disclosed in the

example which follows are given for the purpose of illustration only and are not intended to limit the scope of the invention.

EXAMPLE 1

An example of use is to create films using film blowing equipment. The starch used in this 5 example is a hydroxypropylated and oxidised normal potato starch (HONPS) with an average number of carboxylic acid groups per anhydroglucose units of 0.04 and a degree of substitution with regard the hydroxypropyl of 0.11. The plasticizer used is glycerol together with water. The starch and the glycerol is premixed and fed into a compounding extruder, in this case with a diameter of D=46 mm and length L=11D. The screw speed is

10 24 rpm and the barrel and die temperature is set to 90 ^° C. No extra water needs to be added if the initial moisture content of the starch is about 10 % (w/w) of the starch-glycerol mixtures. In this example, the mixture of HONPS and glycerol is 22 parts per weight of glycerol per 100 parts per weight of dry starch. Extruded strands from the extruder are cut into pellets with a size of about 3 mm in diameter and 5 mm in length. The pellets are then

15 conditioned in a closed chamber at 53% relative humidity and 23 ⁰ C for at least three days prior to processing. The conditioning results in a water content of 9 % (w/w). To overcome problems related to bridging in the hopper of the extruder the pellets are dried at 105 ^° C in a ventilated oven for 2 minutes prior to the extrusion.

The film blowing is done using a compact extruder, screw diameter D=19 mm, screw 20 length 25D and compression ratio of 4: 1. In order to measure pressure loss over the die a melt pressure transducer is positioned at the entrance of the die and connected to a pressure indicator. The extruder is equipped with a conventional temperature controlled film blowing die having a diameter of 24 mm and a film blowing tower with a calendaring nip and take up rolls. The velocity of the take up rolls and the pressure inside the film 25 bubble can be adjusted to achieve a blow-up ratio greater than two times the die diameter to achieve as good results as possible. The extruder temperature profile is preferably 90, 120, 155 and 95 ^° C.

By following these instructions a film consisting solely of modified starch, plasticizer and water is produced.

30 EXAMPLE 2

The following simple example describes the applicability of the film. A film, produced as described in Example 1, having a length of 25 mm and width of 25 mm, was put in water. The thickness of the film was about 100 μm and the temperature of the water was 96

degrees Celsius. After submerging the film and stirring, the film dissolved completely in less than 10 seconds.