Impact resistant biodegradable compositions and methods for the production thereof

Field of invention

The present invention relates to the field of biodegradable materials. More precisely, it is related to melt-processible biodegradable plastic compositions comprising plasticized starch ester. Such compositions are especially desirable in packaging industry, wherein they formed into packages as such or as raw material providing additional features for product performance. Description of the related art

Biodegradable materials are presently in high demand for applications in packaging materials. Commonly used polypropene, polyethene, polystyrene, and other non-biodegradable plastic-containing packaging materials are considered contrary to sustainable development. Biodegradable materials that are flexible, pliable and non-brittle are needed in a variety of applications.

For such applications, the biodegradable material must have mechanical properties that allow it to be formed into and hold the desired container shape, being resistant to collapse, tearing or breaking. Further, biodegradable sheet materials having properties comparable to polypropylene and polyethylene materials are needed, for example, in laminating packaging materials.

Starch is an abundant, inexpensive biodegradable polymer. It belongs to the class of bioplastics having a polysaccharide skeleton. A variety of biodegradable starch- based materials have been proposed for use in packaging applications. A composition containing plasticized native starch together with PLA or similar is known e.g. from application US 6506824. Conventional extrusion of starch produces expanded products that are brittle, sensitive to water and unsuitable for preparation of packaging materials. Consequently, there have been studies to find starch- based products with flexibility, pliability or resiliency and other mechanical properties acceptable for various biodegradable packaging applications. Such attempts have generally focused on chemical or physico-chemical modification of starch, the use of expensive high amylose starch or mixing starch with synthetic polymers to achieve the desired properties while retaining biodegradability. Results related to destructuhsed starch have been published e.g. in EP0409782 A2.

However, there still remain problems related to inferior or low impact strength properties of biodegradable plastics, while simultaneously maintaining a high level of stiffness. It is known that by reducing the amount of the plasticizer in a melt- processible plastic, relatively high stiffness can be achieved. As a result, the im- pact resistance of the material is typically destroyed, rendering the material extremely brittle and even preventing the use of some processing methods such as injection molding due to exposure to damage already in the production phase. A further drawback is poor stability of impact strength. Known bioplastics have tendency to develop brittleness over time. These problems are especially related to melt-processible, externally plasticized starch esters.

Other materials such as the commercially most abundant polylactide have inherently good stiffness and strength, but suffer from brittleness in pure form, limiting their scope of applications or requiring petrochemical-based impact modifiers, which in turn subsequently degenerate the stiffness and strength of the said mate- rial.

Reflecting the state of the art, document WO 97/03121 , discloses melt-processible starch compositions wherein continuous matrix of plasticized starch derivative is mixed with biodegradable fibres. Examples of such fibres include pine pulp, cotton fibres, lactic acid based polymer fibres, and biodegradable glass fibres. Said fibres provide discontinuous interfaces which are believed to contribute to water absorption properties. To obtain said discontinuities, the composition consists of two phases, the starch matrix to which distinct fibres mixed into. To obtain fibres, they must be isolated, wowen, spun, drawn or degraded from raw materials, which requires energy and appropriate equipment.

To solve the abovementioned problems the object of the present invention is to provide melt-processible compositions having improved impact resistance also fulfilling requirements for biodegradability. Another object is to provide a novel method for producing melt processible and biodegradable plasticized starch ester based compositions having improved impact resistance. Summary of the invention

The invention is based on the idea that by melt processing is prepared a blend, which comprises a plasticized starch ester and at least one other biodegradable plastic, and which blend has an immiscible phase structure.

Hence, as the first aspect of the invention the composition contains from 50 to 90 weight % plasticized starch ester, which forms a continuous phase and from 10 to 50 weight % of at least one other biodegradable plastic, which forms a second, fi- brillated phase. As a result, the biodegradable plastic according to invention, has shown impact strengths that are multifold compared to biodegradable plastics represented in the prior art. Another benefit of the present invention is the stability of this strength during storage. The inventors surprisingly found that the dumbbell- shaped specimen exhibited even better impact strength after storage for a few months than readily after blending and processing. This is contrary to characteris- tics observed for conventional biodegradable plastics.

More specifically, the composition according to the invention is characterized by what is stated in claim 1.

Therefore, another aspect of the present invention is a method for processing a thermoplastic, biodegradable starch composition, comprising

a) Optional pre-plasticizing and plasticizing of starch ester;

b) Admixing from 50 to 90 wt-% of plasticized starch ester with from 10 to 50 wt-% of at least one other biodegradable plastic;

c) Melt-processing mixture obtained from step b.

Without confining to a specific model or mechanism, it can be noted, that the in- corporation of a plasticized starch ester with a suitable amount of plasticizer into inherently brittle biodegradable plastic such as polylactide, can under selected processing conditions and ratio of the blend components result into apparently viscosity-driven microstructural fibrillation of the starch ester and production of continuous, highly impact resistant composition.

More specifically, the method according to the invention is characterized by what is stated in claim 15.

Third aspect of the invention is to provide a biodegradable starch composition for use in single unit production. More specifically, the use is characterized by what is stated in claim 20.

Substantial benefits are obtained with the invention. The raw materials of the starch composition according to the invention are primarily based on renewable

resources and they are fully biodegradable and some also compostable. The starch component can be derived from any natural starch; it does not have to be starch rich in amylose. The composition is melt processible with ordinary plastic processing equipment. The material has good strength and stiffness properties, adjustable by changing the ratio and quality of the components of the composition. The impact strength of the material is, in comparison with the material that contains only one component, at least 100% better.

Short description of the drawings

The composition and the method of the invention will next be explained in more detail by referring to the enclosed figures, in which

Figure 1 illustrates a SEM image of a fracture surface of a composition with 20% PLA in plasticized starch acetate.

Figure 2 illustrates the effect of plasticizer concentration on the impact strength of plasticized starch acetate blend with 20% PLA.

Figure 3 illustrates the stiffness and impact strength of the blends of the plasticized starch acetate with 20% of various biodegradable plastics.

Figure 4 illustrates the impact strength as measured after storage of about 2, 4 and 6 months of specimens manufactured according to the method of the present invention of a blend of plasticized starch acetate with 20% of PLA.

Figure 5 illustrates the effect of the amount of other biodegradable plastic on the impact strengths, measured for various the blends of the plasticized starch acetate with PLA, namely 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 wt-% of PLA, the rest being plasticized starch acetate .

Detailed description of the invention

The present invention provides novel impact resistant bio-based and biodegradable material compositions based on blends of starch ester with one or more other biodegradable plastics.

Plasticized starch ester forms the continuous phase and the other biodegradable plastic forms the second fibhllated phase. The latter contributes to the self- reinforced structure, formed by the combination of the processing and the polymer properties, and enabling the stress transfer from the matrix to the fibrillar phase

consisting of the other biodegradable plastic. The synergism created by this blend results as unique structure which is illustrated in Figure 1. The biodegradable plastic according to invention, has demonstrated impact strengths that are at least double compared to biodegradable plastics representing prior art.

The biodegradable starch composition comprises a plasticized starch ester from 50 to 90 wt-%. Preferably the amount of plasticized starch ester is from 65 to 85 wt-% of the total composition. Here the plasticized starch ester refers to processed mixture, which is prepared by plasticizing starch ester with plasticizing agent (plas- ticizer). In other words it is a sum of the weights of starch ester and plasticizing agent in relation to the total composition weight. In the composition of the present invention, the amount of the plasticizing agent ranges from 5 to 70 wt-%, preferably from 30 to 35 wt-% of the total composition. The amount of starch ester can be calculated accordingly. One preferred plasticizing agent is triethycitrate (TEC). It has been found that compositions comprising from 15 to 35 wt-% of the total com- position of TEC, show especially good impact resistance (Figure 2).

The preparation of suitable starch esters and the applicable plasticizers have been described in great detail in, e.g., International Publication No. WO97I03121.

Polymer products, such as boards and films, can be manufactured from the new compositions with methods known per se in polymer technology. The composi- tions according to the invention are especially applicable to single unit production. The single unit production may be selected from preparation of films or sheets, injection moulding, thermoforming, blow moulding, coating of paper or board and compression moulding.

The material also fulfils the requirements for food packaging by migration simula- tion tests.

The "plasticized starch ester" means a product, which can be produced from native starch and which can be molded at moderate temperature, typically between room temperature and 200 ⁰ C, preferably ca. 30-180 ⁰ C. The starch component can be plasticized by introducing a plasticizing agent, in the presence of which the starch component typically dissolves and swells.

The plasticized starch ester of the composition according to the invention, in the following also called the starch component, can be based on natural starch having an amylose concentration of 0-100% and an amylopectin concentration of 100-

0%. Thus, the starch component can be derived from barley, potato, wheat, oat, peas, maize, tapioca, sago, rice, and similar bulb or cereal plants. It can be based also on starch derivatives prepared from said natural starch by oxidation, hydro- lysation, cross-linking, cationisation, grafting, ethehfication, or estehfication.

It has been found preferential to use a starch-based component, which is plasti- cized from an ester formed by starch and one or several aliphatic C2-C2 ₄ - carboxylic acids. The carboxylic acid component of an ester of this kind can then be derived from a lower alkane acid, such as acetic acid, propionic acid, butyric acid, or a mixture thereof. The carboxylic acid component can, however, also be derived from natural saturated or unsaturated fatty acids. Palmitinic acid, stearic acid and mixtures thereof are examples of the fatty acids. The ester can also comprise both long and short-chained carboxylic acid components. The mixed esters of acetate and stearate are examples of the latter ester.

The preparation of fatty acid esters of starch can be carried out for example as disclosed in the prior art publication by Wolff, I. A, Olds, D.W., and Hubert, G. E.

Starch acetate can alternatively be prepared by reacting starch with acetic acid anhydride in the presence of a catalyst. The catalyst used comprises, for example, 50% sodium hydroxide. Other preparation processes known in the art for preparation of acetates are suitable for the preparation of starch acetate. By varying the amount of acetic anhydride, the amount of the alkali used as a catalyst, and the reaction time, it is possible to prepare starch acetates having different amounts of degrees of substitution. Suitable anhydrides include e.g. those having number of carbons from C ₄ to Ci ₆ .

The carboxylic acid and/or the anhydride used in the estehfication of the starch is selected such that the starch ester formed belongs to a group of form C2 to C22 esters, and preferably from C2to C10 esters.

For the purpose of the present invention, "starch ester" denotes starch estehfied with a degree of substitution (DS) in the range of 0.5-3.0, preferably 1.0-3.0, and in particular 1.3-2.0. The preferred starch component is esterified starch, prefera- bly starch acetate, having a DS from 0.5 to 3.0, more preferably from 1.0 to 3.0, and in particular from 1.3 to 2.0.

The second polymer in the composition according to the invention, is a biodegradable plastic, which can also be a bioplastic. By bioplastics is here referred to a

form of plastics derived from renewable biomass sources, such as hemp oil, soy bean oil, corn starch, pea starch, or sugar canes, optionally exploiting fermentation. When compared to traditional plastics which are derived from petroleum, bio- plastics are characterised by the fact that the petrochemical resin is replaced by a vegetable or animal resin, and/or the bolsters (glass or carbon fibre or talc) are replaced by natural fibre (wood fibres, hemp, flax, sisal, jute) . In the embodiments of the present invention, the bioplastics chosen are also biodegradable, hence they are broken down into CO2 and water by microorganisms. Even more preferably, some of the products obtainable from the biodegradable bioplastics of the inventi- on are compostable. Compostability here means that they can be put into an industrial composting process and will break down by 90% within 6 months. Biopo- lymers that do this can be marked with a 'compostable' symbol, under European Standard EN 13432 (2000). Packaging marked with this symbol can be put into industrial composting processes and will break down within 6 months (or less). An example of a compostable polymer is PLA film under 20μm thick: films which are thicker than that do not qualify as compostable, even though they are biodegradable. Thus, the compostability is naturally dependent on the product formulation, i.e. physical characteristics such as surface to weight ratio, porosity, etc.

The use of suitable biodegradable plastic in the composition greatly improves strength and hydrophobicity properties of the composition. Biodegradable plastics in the present blends are in particular selected from polylactate (PLA), poly(hydroxybutyrate) (PHB), polycaprolactone (PCL) and polyhydroxy butyrate valerate (PHB-V), cellulose-based ester derivatives or a mixture thereof. An example of the effect of selection on impact strength can be seen in Figure 3. Par- ticular examples of such cellulose derivatives include the cellulose acetate, cellulose propionate or cellulose butyrate or mixtures or mixed esters thereof. The amount of the other biodegradable plastic(s) in the composition of the invention ranges from 10 to 50 wt-%, and preferably from 15 to 35 wt-%.

In a particularly preferred embodiment the other biodegradable polymer comprises PLA. Presently, it is commercially available with reasonably low price. Experimental data obtained within the present embodiment shows maintained or even increased impact strength during storage, rendering said embodiment especially in- tresting. Such storage characteristics are contrary to traditional experiences with biodegradable polymers.

Preferred compositions are listed in the following:

A composition comprising

- 50-90% starch acetate

- 5 -30% plasticizer

- 10-50% at least one other biodegradable plastic. The composition can be prepared according to a method known per se by mixing the components of the composition, i.e., the starch based component and the biodegradable plastic, with each other. Preferably, the plasticizing of starch acetate is done prior to blending.

The starch compositions according to the invention are plastic and melt- processible. Typically, the viscosity of the melt polymer is 10-5000 Pa s, preferably 50-2000 Pa s, measured with capillary rheometry at a temperature in the range of 140 to 200 ⁰ C and at a shear rate of 200 1/s.

Such compositions have exhibited impact strengths (IS) of at least 10 kJ/m ² . However, some embodiments (32.5% TEC, 20% PLA) showed IS of at least 35 kJ/m ² . Composition (30% TEC, 20% PHB-V) yielded IS at least 90 kJ/m ² . The best performing compositions according to invention had IS outside the measuring range of the equipment, hence more than 120 kJ/m ² . These comprised PCL (20% or 50%) or PHB (20%) as the other biodegradable plastic.

Semi-manufactured and final products can be manufactured from the composi- tions according to the invention. The products can be solid or at least partly hollow. Films and boards are examples of the products. However, the compositions can also be used for the coating of paper and cardboard. They can be used to produce specimens by injection molding and products such as packages, sacks and bottles by compression molding, thermoforming and blow molding.

The present invention is applicable to both filled and unfilled compositions. The present starch compositions can further be mixed with other polymers, e.g. by melt blending, in order to produce polymer mixtures of more than to polymer components. Filling agents, known per se, can further be added to the mixture. Suitable filling agents are starch and modified starch components. In one embodiment of the invention, the composition is filled with a suitable filler selected from the following: an inorganic filler such as CaCO3, talc, glass fibre, organic particulate such as wood dust, or bio-based fibrous material such as wood fibre, argo fibre such as bast fibre, or any combination thereof.

To produce a thermoplastic, biodegradable starch composition, the method requires at least mixing and melt-processing steps. The starch ester needs to be plasticized. However, it could be performed directly in relation to the melt processing but optionally even as a separate process. In other words, the plasticizing step can be the first step of the method but a skilled person understands that if plasticized starch ester is available, e.g. commercially, the step of plasticizing is not necessarily performed immediately before admixing. The same applies to pre- plasticizing. Plasticizing comprises addition of a plasticizing agent to starch ester. Yet another option is adding plasticizing agent to the admixing step together with starch ester and other biodegradable polymer and additives if any. Then plasticizing and mixing steps are combined.

When plasticized starch ester is available either from another process or from previous step, it is next mixed with at least one other biodegradable plastic. Optionally there could be other additives, such as fillers, added at this step. The proportions of main components range from 50 to 90 wt-% of plasticized starch ester, and from 10 to 50 wt-% of at least one other biodegradable plastic.

Then, this mixture, obtained from previous step, is melt-processed. The use of a twin-screw extruder for melt processing has provided excellent results. During said processing the polymers soften and melt, and form a structure visualised in Figure 1 , providing strength properties characteristic for the present invention. Without being bound to a theory, it is believed that the use of shear strengths contribute to the orientation of other biodegradable plastic in the melt mixture during the melt- processing. This structure is believed to contribute to the measured, unexpectedly beneficial properties observed during experiments conducted. The product obtain- able by said method demonstrates impact strength, stiffness and storage stability characteristics non-observed for conventional biodegradable plastics.

One specific embodiment is a product obtainable by the method described above. Preferably, it has been prepared by melt-processing which applies in a twin-screw extruder. After compounding, the blends are cooled and cut into granulates in a continuous process. The granulates are dried before injection moulding. The composition processed this way comprises from 50 to 90 wt-% of a plasticized starch ester, which includes a plasticizing agent in an amount from 5 to 40 wt-% (total composition) and from 10 to 50 wt-% of at least one other biodegradable plastic as starting materials. Such a composition is characterized by the combination of the processing and the polymer properties which together contribute to an impact

strength superior to either of the components alone. However, despite of this, no fibre weaving processes need to be incorporated and the composition consists of 100% biodegradable material. Unlike impact modifiers, all the components of the composition are abundant and inexpensive.

In the examples and figures, it will be shown that the mechanical properties are improved far more than would be expected from applying the rule of mixture.

EXPERIMENTAL

Materials and methods

Amylose rich maize starch was supplied either by National Starch, USA (Hylon VII: 62% amylose: 38% amylopectin) or by Gargill, USA (Cerestar Amylogel 03003: 65% amylose, 35% amylopectin). The plasticizer triethyl citrate (TEC: Citroflex2) was obtained from OneMed (Finland). The polylactide used (HM 1011 ) was produced by Tate&Lyle, PHB from Biomer P 226; Biomer, Krailling, Germany, PCL (CAPA 6800 Polycaprolactone) from Solvay Caprolactones, Solvay Interox Ltd., Cheshire, United Kingdom, and the PHB-V (Enmat 601 ) from Tianan Biologic Material Co., Ltd. Ningbo, Zhejiang Province, China.

Impact strength

Impact strengths of the blends were determined using Charpy Ceast Resil 5.5 Impact Strength Machine and ISO 179 standard at a temperature of 23°C and humid- ity of 50%. Tests were performed on unnotched specimens. Length, width and thickness of the specimens were 80, 10 and 4 millimetres, respectively. Width and thickness of each specimen were determined exactly by using a slide gauge. Depending on the impact energy needed, different pendulum values (1 J, 2 J or 5 J) were used. Results were reported as impact energy (J) by the impact strength ma- chine. Impact strength was then calculated using the information of impact energies and specimen dimensions.

Impact strength (kJ/m ) = Impact energy (J) * 1000 Width (mm) * Thickness (mm)

Storability

A number of specimens as defined above, of starch acetate, comprising 29.8% of TEC and blended with 20% of PLA, were stored. The specimen were stored at

23°C, 50% RH." Follow-up was performed after approximately 2, 4 and 6 months after impact moulding by measuring impact strengths. Results were reported as impact energy (J) by the impact strength machine. Impact strength was then calculated using the information of impact energies and specimen dimensions. Tensile tests

Tensile properties were measured on an lnstron 4505 Universal Tensile Tester with 10 kN load cell and a 5 mm/min tensile speed at standard conditions (23°C, 50%), using modified ISO 527 standard. Dumbbell-shaped samples with dimensions of about 4 mm thickness and 10 mm width were tested with a gauge length of 50 mm, grip distance of 115 mm and extensometer gauge of 50 mm. Dimensions of every specimen were double-checked with a slide gauge. Values of modulus, tensile strength, yield strength and strain to break were determined from the average of five samples with constant crosshead speed of 5 mm/min.

Melt flow index

The melt flow index values of the blends were determined using a RAY-RAN Melt Flow Indexer, Model 3A Digital Auto. The method was based on ISO 1 133 and SFS 3150 standards with weight of 2.16 kg, die bore of 2 mm and piston travel of 2.54 cm. Tested materials were granulated after the extrusion process. Different temperatures were used depending on the melt flow of the materials. Temperature was decreased when viscosity of the material became too low. When changing the temperature, same material was remeasured with the lower temperature. Xylene was used for cleaning the machine after each measurement. The results represent calculated averages of three successive measurements. For each measurement two different methods were used. In both methods the material was first preheated for six minutes with a piston and without the weight.

Method 1:

Melted material was cut with a sharp tool and the timer was turned on manually at the same time. After decided time, flowing material was cut again and the timer was turned off. The dribbled material was weighted using accuracy of +/- 1 mg. Melt flow index was then calculated as:

Sx m MFI (T, M) = t g / 10 min,

where MFI is the melt flow index, T is temperature ( ⁰ C), M is weight (kg), S is reference time (600s), m is mass of the material (g) and t is drainage period (s).

Method 2:

Density of the material was assumed to be about 1 g/cm ³ and the auto timer was therefore also used. Right piston travel was chosen and the shaft of the timer was set under the weight. The time was determined automatically by the melt flow index machine. Results were calculated as follows:

427x L (x d ) MFI (T, M) = t cm ³ / 10 min,

where MFI is the melt flow index, T is temperature ( ⁰ C), M is weight (kg), 427 is mean of areas of piston and cylinder x 600, L is piston travel (mm), d is density and t is drainage period (s). Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) measurements were done on a DSC 2920 (TA Instruments) under nitrogen atmosphere. About 5-8 mg samples, weighted accurately and encapsulated in aluminium crucibles, were first cooled to -40°C, then heated up to 200 ⁰ C with a ramp of 5°C /min and cooled again to -40°C to eliminate previous thermal history of melt characterization. Then the sample was heated again 5°C/min to 200 ⁰ C and cooled back to -40°C, resulting endothermic curves. The values of melting temperatures (T _m ), glass transition temperatures (T ₉ ) and heat enthalpy of melting (δH) were obtained using TA Universal Analysis 2000 program.

Morphological studies

The morphologies of the TPS/PLA blends were investigated by a JEOL JSM-T100 Scanning Electron Microscope. Samples were prepared by keeping them in the deep freeze (temperature -70 ⁰ C) and breaking them afterwards. The fractured surfaces were coated with a fine gold layer using a BAL-TEC Balzers SCD 050 Sput- ter Coater.

Processing Acetylation

The acetylation of starch was performed according the method described by Lammers et al and detailed in patent WO9703121. The degree of acetylation (DS _ACET ) was determined by a titration method described by Elomaa et al. Determination of DS _ACET by hydrolysis involved complete basic hydrolysis of the ester linkages. Chemical structure was confirmed with FT-IR spectroscopy. The infrared spectrum of these starch acetates revealed a strong absorption at 1749 cm ^"1 , characteristic of the ester carbonyl band. This absorption is not seen in the spec- tras from the starting material (Cerestar, HylonVII). Approximated molecular weights of starting materials were determined with Size Exclusion Chromatography (SEC). Hylon VII: 470 000 and CereStar: 670 000. Thermal stability increases with acetylation, e.g. degradation temperature of HylonVII is 303 ⁰ C and degradation temperature of corresponding acetate is 336 ⁰ C. Glass transition temperature of corresponding acetate is approximately 156 ⁰ C.

Compounding and injection molding

The starch acetate was dried at 40 ⁰ C in a vacuum furnace at from -60 to -90 kPa for at least one hour before compounding. A twin-screw extruder (ZE25x48D, Ber- storff GmbH, Hannover, Germany) with co-rotating mixing screws designed for op- timal melt mixing were used for compounding. The diameters and lengths of the screws were 25 mm and 1200 mm, respectively. The feeders were single-screw feeders with diameters of 24 mm and 12 mm. Temperatures from 60 ⁰ C (the feeding section) to 180 or 200°C (melting zones and the die) depending on the used materials were used. Screw speed was 300 rpm. Vacuum of -30 - -60 kPa was used during the compounding. After compounding, the blends were air cooled and cut into granulates in a continuous process.

The granulates were dried in a vacuum furnace at from -0.60 to -0.90 bar at 40 ⁰ C for at least one hour before injection moulding. Blends were injection moulded with Engel injection moulder (ES 200/50 HL, Engel Austria GmbH, Schwertberg, Aus- tria) into ISO 3167 tensile test specimen with length of 150 mm, with the center section 10 mm wide by 4 mm thick by 80 mm long. Applied parameters depended on the moulded material, temperatures being approximately from 130°C to 200°C, pressures from 30 to 80 bars and the post pressures from 10 to 70 bars.

Results

Example 1

Preparation of a starch acetate - other biodegradable plastic compositions. The following composition (composition A) was prepared, comprising:

— 64% by weight of starch acetate having a degree of substitution 2.74, and

— 16% by weight of thethyl citrate, and

— 20% by weight of polylactide

The starch acetate was plasticized to 35% by weight plasticizer using Twin-screw extruder type compounder as described above. A blend was then prepared using 80% by weight plasticized starch acetate and 20% by weight of polylactide.

This blend was then processed by Engel injection molding machine into specimens suitable for Charpy impact test as described above.

The impact strength was determined by International Standard SFS-ISO 179. The test specimens were of type ISO 179/2D (unnotched). Before the testing, the sam- pies were stored at 23°C and 50% RH for minimum of five days.

The strength obtained from the specimen by the impact testing procedure described above was 6.6 kJ/m ² .

The following composition (composition B) was prepared, comprising:

— 40% by weight of starch acetate having a degree of substitution 2.74, and — 10% by weight of triethyl citrate, and

— 50% by weight of polylactide

As above, The starch acetate was plasticized to 20% by weight plasticizer using twin-screw extruder type compounder as described above. A blend was then prepared using 50% by weight plasticized starch acetate and 50% by weight of poly- lactide.

This blend was then processed by Engel injection molding machine into specimens suitable for Charpy impact test as described above.

The strength obtained from the specimen by the impact testing procedure described above was 13.2 kJ/m ² .

The following composition (composition C) was prepared, comprising:

— 54% by weight of starch acetate having a degree of substitution 2.74, and

— 26% by weight of thethyl citrate, and

— 20% by weight of polylactide The starch acetate was plasticized to 32.5% by weight plasticizer using twin-screw extruder type compounder as described above. A blend was then prepared using 80% by weight plasticized starch acetate and 20% by weight of polylactide.

This blend was then processed by Engel injection molding machine into specimens suitable for Charpy impact test as described above.

The strength obtained from the specimen by the impact testing procedure described above was 35.6 kJ/m ² .

The following composition (composition D) was prepared, comprising:

— 33.75% by weight of starch acetate having a degree of substitution 2.74, and

— 16.25% by weight of triethyl citrate, and — 50% by weight of polylactide

The starch acetate was plasticized to 32.5% by weight plasticizer using twin-screw extruder type compounder as described above. A blend was then prepared using 50% by weight plasticized starch acetate and 50% by weight of polylactide.

This blend was then processed by Engel injection molding machine into speci- mens suitable for Charpy impact test as described above.

The strength obtained from the specimen by the impact testing procedure described above was 17.6 kJ/m ² .

The following composition (composition E) was prepared, comprising:

— 56% by weight of starch acetate having a degree of substitution 2.74, and — 24% by weight of triethyl citrate, and

— 20% by weight of polylactide

The starch acetate was plasticized to 30% by weight plasticizer using twin-screw extruder type compounder as described above. A blend was then prepared using 80% by weight plasticized starch acetate and 20% by weight of polylactide.

This blend was then processed by Engel injection molding machine into specimens suitable for Charpy impact test as described above.

The strength obtained from the specimen by the impact testing procedure described above was 16.6 kJ/m ² . The following composition (composition F) was prepared, comprising:

— 35% by weight of starch acetate having a degree of substitution 2.74, and

— 15% by weight of triethyl citrate, and

— 50% by weight of polylactide

The starch acetate was plasticized to 30% by weight plasticizer using twin-screw extruder type compounder as described above. A blend was then prepared using 50% by weight plasticized starch acetate and 50% by weight of polylactide.

This blend was then processed by Engel injection molding machine into specimens suitable for Charpy impact test as described above.

The strength obtained from the specimen by the impact testing procedure de- scribed above was 13.0 kJ/m ² .

The following composition (composition G) was prepared, comprising:

— 56% by weight of starch acetate having a degree of substitution 2.74, and

— 24% by weight of triethyl citrate, and

— 20% by weight of polycaprolactone The starch acetate was plasticized to 30% by weight plasticizer using twin-screw extruder type compounder as described above. A blend was then prepared using 80% by weight plasticized starch acetate and 20% by weight of polycaprolactone.

This blend was then processed by Engel injection molding machine into specimens suitable for Charpy impact test as described above.

The strength obtained from the specimen by the impact testing procedure described above was >120 kJ/m ² .

The following composition (composition H) was prepared, comprising:

— 35% by weight of starch acetate having a degree of substitution 2.74, and

— 15% by weight of thethyl citrate, and

— 50% by weight of polycaprolactone The starch acetate was plasticized to 30% by weight plasticizer using twin-screw extruder type compounder as described above. A blend was then prepared using 50% by weight plasticized starch acetate and 50% by weight of polycaprolactone.

This blend was then processed by Engel injection molding machine into specimens suitable for Charpy impact test as described above.

The strength obtained from the specimen by the impact testing procedure described above was >120 kJ/m ² .

The following composition (composition I) was prepared, comprising:

— 56% by weight of starch acetate having a degree of substitution 2.74, and

— 24% by weight of thethyl citrate, and — 20% by weight of poly(3-hydroxybutyrate)

The starch acetate was plasticized to 30% by weight plasticizer using twin-screw extruder type compounder as described above. A blend was then prepared using 80% by weight plasticized starch acetate and 20% by weight of poly(3-hydroxy- butyrate).

This blend was then processed by Engel injection molding machine into specimens suitable for Charpy impact test as described above.

The strength obtained from the specimen by the impact testing procedure described above was >120 kJ/m ² .

The following composition (composition J) was prepared, comprising:

— 35% by weight of starch acetate having a degree of substitution 2.74, and

— 15% by weight of thethyl citrate, and

— 50% by weight of poly(3-hydroxybutyrate)

The starch acetate was plasticized to 30% by weight plasticizer using twin-screw extruder type compounder as described above. A blend was then prepared using

50% by weight plasticized starch acetate and 50% by weight of poly(3-hydroxy- butyrate).

This blend was then processed by Engel injection molding machine into specimens suitable for Charpy impact test as described above. The strength obtained from the specimen by the impact testing procedure described above was 15.4 kJ/m ² .

The following composition (composition K) was prepared, comprising:

— 56% by weight of starch acetate having a degree of substitution 2.74, and

— 24% by weight of triethyl citrate, and — 20% by weight of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

The starch acetate was plasticized to 30% by weight plasticizer using twin-screw extruder type compounder as described above. A blend was then prepared using 80% by weight plasticized starch acetate and 20% by weight of poly(3-hydroxy- butyrate-co-3-hydroxyvalerate) .

This blend was then processed by Engel injection molding machine into specimens suitable for Charpy impact test as described above.

The strength obtained from the specimen by the impact testing procedure described above was 92.5 kJ/m ² .

The following composition (composition L) was prepared, comprising:

— 35% by weight of starch acetate having a degree of substitution 2.74, and

— 15% by weight of triethyl citrate, and

— 50% by weight of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

The starch acetate was plasticized to 30% by weight plasticizer using twin-screw extruder type compounder as described above. A blend was then prepared using 50% by weight plasticized starch acetate and 50% by weight of poly(3-hydroxy- butyrate-co-3-hydroxyvalerate).

This blend was then processed by Engel injection molding machine into specimens suitable for Charpy impact test as described above.

The strength obtained from the specimen by the impact testing procedure described above was 43.4 kJ/m ² .

The results of different blends are summarized in Table 1.

Table 1 Compositions and impact strength of the blends.

Composition TEC % Added biodeAdded biodegradable Impact strength kJ/m ² gradable plastic plastic %

A 20 PLA 20 6.6

B 20 PLA 50 13.2

C 32.5 PLA 20 35.6

D 32.5 PLA 50 17.6

E 30 PLA 20 16.6

F 30 PLA 50 13.0

G 30 PCL 20 >120

H 30 PCL 50 >120

I 30 PHB 20 >120

J 30 PHB 50 15.4

K 30 PHB-V 20 92.5

L 30 PHB-V 50 43.4

Effect of the other biodegradable polymer content to the impact strength

Impact strengths of starch acetate/PLA blends with varying PLA-content are summarised in table 2 and illustrated in Figure 5.

Table 1 Compositions and impact strength of the blends.

Storability

Storability results were reported as impact energy (J) by the impact strength machine. Impact strength was then calculated using the information of impact energies and specimen dimensions and presented in Figure 4 as averages calculated. Surprisingly, the samples did not lose their strength as a function of time, but kept and even increased it. It has to be noted that there was some variance between individual specimen. This is very surprising a finding as biodegradable palastics of this type usually have tendency to become brittle and thus lose strength over time.