DESCRIPTION

The present invention relates to a biodegradable film with improved tensile strength at break while maintaining good tear strength properties, obtained by a process of uniaxial or biaxial cold stretching of bubble film.

Biodegradable starch-based films for manufacturing products such as bags for differentiated waste collection, shopping bags, food packaging, mulching films, nappies and hygiene products are widely used on the market. In fact, biodegradable films which are able to degrade once their primary use is over without causing an accumulation of waste in the environment are used in these application sectors. However these biodegradable films must be thin and have superior mechanical properties.

Stretching processes applied to biodegradable films can be used to obtain small thicknesses and superior mechanical properties.

For example, EP 1 755 864 describes a stretching process through which biodegradable films having a thickness of less than 70 pm and characterised by high tensile strength at break can be obtained. However, the films described do not have high tear strength and high renewability content - necessary properties for the use of these films in certain application areas such as shopping bags or food packaging bags.

The above disadvantages have now surprisingly been overcome according to the present invention through a biodegradable film obtained by a cold stretching process having a starch- based composition containing at least one biodegradable aliphatic-aromatic polymer comprising at least one aliphatic long-chain dicarboxylic acid of renewable origin. Surprisingly it has been observed that when stretched e.g. by more than 15% the film having this composition has high tear strength properties.

Biodegradable films according to this invention are particularly suitable for use in a variety of applications such as shopping bags, hygiene product films, mulching films and food packaging films such as fruit and vegetable bags.

In particular, films comprising compositions with at least one polysaccharide derivative and at least one biodegradable polymer, in particular a biodegradable aliphatic-aromatic dicarboxylic acid/diol polymer, are suitable for cold stretching.

Films comprising a composition containing starch and at least one biodegradable aliphatic- aromatic dicarboxylic acid/diol polymer are particularly preferred.

The biodegradable polymer of the biodegradable film according to the present invention is preferably a biodegradable aliphatic-aromatic dicarboxylic acid/diol polymer obtained from at least one diol, at least one aromatic acid with multiple functional groups and at least one long- chain aliphatic dicarboxylic acid of renewable origin.

The content of aromatic acid with multiple functional groups is between 40 and 80% by mole and preferably 45 and 70% by mole in relation to the total dicarboxylic acid content.

The content of long-chain aliphatic dicarboxylic acid of renewable origin is from 5 to 100%, advantageously from 5 to 90% and more advantageously from 5 to 80% in relation to the total aliphatic dicarboxylic acid content by mole. Preferably, the content of long-chain aliphatic dicarboxylic acid of renewable origin is from 10 to 70%, more preferably from 10 to 60% by mole, and even more preferably from 10 to 50% in relation to the total aliphatic dicarboxylic acid content by mole, the content from 12 to 35% by mole being particularly preferred.

In fact, the biodegradable films based on commonly used aliphatic-aromatic polyesters (for example based on adipic acid) generally undergo a deterioration of the tear strength when stretched to reach higher breaking loads. Unexpectedly, the biodegradable films based on aliphatic-aromatic copolyesters comprising the above amounts of long-chain aliphatic acids of renewable origin show, together with the improvement of the breaking load, also an increase in the tear resistance in the machine direction with respect to the non-stretched films of the same thickness.

For instance, a biodegradable composition comprising an aliphatic-aromatic copolyester containing 15% of long chain renewable aliphatic dicarboxylic acid (e.g. poly butylene adipate- co-butylene azelate-co-butylene terephthalate with 15% of moles of azelaic acid with respect to the total molar content of aliphatic dicarboxylic acids) allowed to obtain a cold stretched film whose tear resistance in the machine direction was unpredictably about 25% higher (with a stretch of 20%) or even 35% higher (with a stretch of 50%), than the non-stretched film having the same thickness. At the same time, the load at break respectively increased of 5% or 50%. To provide a comparison, a 20% cold stretched film comprising an aliphatic-aromatic copolyester (e.g. poly butylene adipate-co-butylene terephthalate, PBTA) -based composition without long-chain aliphatic dicarboxylic acid of renewable origin exhibits a halving of the tear strength value compared to the unstretched film.

The object of the present invention is therefore a biodegradable film obtained by a cold stretching process with a stretch of above 10%, preferably above 15%, said starch-based film containing at least one biodegradable aliphatic-aromatic dicarboxylic acid/diol polymer obtained from at least one diol, at least one aromatic acid with multiple functional groups and at least one aliphatic dicarboxylic acid characterised in that the latter comprises 5 to 100% by mole of at least one long chain dicarboxylic acid of renewable origin having more than 6 carbon atoms in the main chain and in that the aromatic acid content is between 40 and 70% by mole in relation to the total dicarboxylic acid content by mole.

In this invention with “long chain dicarboxylic acid of renewable origin” are meant dicarboxylic acids of renewable origin having more than 6 carbon atoms in the main chain. Of these, those preferred are dicarboxylic acids selected from saturated C7-C24, preferably C7-C18, dicarboxylic acids, their esters, salts and mixtures. Particularly preferred are dicarboxylic acids selected from the group comprising suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, brassylic acid, 1,16-hexadecanedioic acid, 1,18-octadecanedioic acid and their esters and mixtures. Azelaic acid is particularly preferred.

Products obtained from sources which because of their intrinsic characteristics are regenerated or not exhaustible on the scale of a human lifetime and, by extension, those whose use does not jeopardise natural resources for future generations, are to be considered of renewable origin. The use of products of renewable origin also helps to reduce CO2 in the atmosphere and decrease the use of non-renewable resources. A typical example of a renewable source is plant crops.

For the purposes of this invention,“renewability” means percentage renewability measured according to standard EN 16640:2017.

Preferably, the renewability of the biodegradable film according to the present invention is greater than 20%, more preferably greater than 30%, and even more preferably greater than 40%. Advantageously, the renewability of said film is over 50%.

The aromatic acids with multiple functional groups in the biodegradable film polymers according to the present invention are preferably selected from aromatic dicarboxylic acids of the phthalic acid type, preferably terephthalic acid or isophthalic acid, more preferably terephthalic acid, and heterocyclic aromatic dicarboxylic compounds, preferably 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, 2,3-furandicarboxylic acid, 3,4-furandicarboxylic acid, more preferably 2,5-furandicarboxylic acid, and their esters, salts and mixtures. Terephthalic acid and 2,5-furandicarboxylic acid are particularly preferred.

The diol component of the biodegradable polymer in the biodegradable films according to this invention may include saturated aliphatic and unsaturated aliphatic diols, aromatic diols and mixtures thereof.

The saturated aliphatic diols are selected from the group comprising 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,11-undecanediol, 1,12-dodecanediol, 1,13-tridecanediol, 1,4-cy cl ohexanedi ethanol, neopentylglycol, 2-methyl- 1,3 -propanediol, dianhydrosorbitol, dianhydromannitol, dianhydroiditol, cyclohexanediol, cyclohexanmethanediol, dialkylene glycols and polyalkylene glycols of molecular weight 100- 4000 such as polyethylene glycol, polypropylene glycol and mixtures thereof.

The unsaturated aliphatic diols are more preferably selected from the group comprising cis 2-butene- 1,4-diol, trans 2-butene- 1,4-diol, 2-butyne-l,4-diol, cis 2-pentene-l,5-diol, trans 2-pentene-l,5-diol, 2-pentyne-l,5-diol, cis 2-hexene-l,6-diol, trans 2-hexene- 1,6-diol, 2-hexyne-l,6-diol, cis 3 -hexene- 1,6-diol, trans 3 -hexene- 1,6-diol, 3 -hexyne- 1,6-diol and their mixtures.

On the other hand the aromatic diols are preferably selected from the group comprising 2,5-furandimethanol, 2,4-furandimethanol, 2,3-furandimethanol, 3,4-furandimethanol, more preferably 2,5-furandimethanol and their mixtures.

In a preferred embodiment the diol component is selected from 1,2-ethanediol, 1,4-butanediol and their mixtures. In an even more preferred embodiment, the diol component comprises, or consists of, 1,4-butanediol.

In another preferred embodiment the diol component is of renewable origin.

In an even more preferred embodiment, the diol component is of renewable origin and comprises, or consists of, 1,4-butanediol.

The biodegradable polymer of the biodegradable film according to the present invention may include repetitive units derived from at least one hydroxyacid in amounts between 0 and 49% by mole, preferably between 0 and 30% by mole, in comparison with the total moles of dicarboxylic component. Examples of convenient hydroxyacids are glycolic acid, hydroxybutyric acid, hydroxycaproic acid, hydroxyvaleric acid, 7-hydroxyheptanoic acid, 8-hydroxyheptanoic acid, 9-hydroxynonanoic acid, lactic acid or lactides. Hydroxyacids may be inserted in the chain as such or as prepolymers/oligomers, or they may also be reacted with dicarboxylic acids or diols in advance.

The molecular weight Mn of said biodegradable polymer is preferably > 20000, more preferably > 40000. With regard to the polydispersity index of the molecular weights Mw/Mn, this is instead preferably between 1.5 and 10, more preferably between 1.6 and 5 and even more preferably between 1.8 and 2.7.

The molecular weights Mn and Mw may be measured using Gel Permeation Chromatography (GPC). The determination may be carried out with the chromatography system maintained at 40 °C, using a set of two columns in series (particle diameter 5 pm and 3 pm with mixed porosity), a refractive index detector, chloroform as eluent (flow 0.5 ml/min) and using polystyrene as reference standard. The terminal acid groups content of said biodegradable polymer is preferably less than 100 meq/kg, preferably less than 60 meq/kg and even more preferably less than 45 meq/kg. The terminal acid groups content may be measured as follows: 1.5 - 3 g of polyester are placed in a 100 ml flask together with 60 ml of chloroform. After complete dissolution of the polyester, 25 ml of 2-propanol is added, together with, just before the analysis, 1 ml of deionised water. The solution thus obtained is titrated with a previously standardised solution of NaOH in ethanol. An appropriate indicator, such as a glass electrode for acid-base titration in non- aqueous solvents, is used to determine the titration end point. The terminal acid groups content is calculated on the basis of the consumption of NaOH solution in ethanol according to the following equation:

KK, -v rj-iooo

Terminal acid groups content (meq /kg polymer)— where: Veq = ml of NaOH solution in ethanol at the end point of the titration of the sample;

Vb = ml of NaOH solution in ethanol needed to reach pH = 9.5 during the blank titration; T = concentration of NaOH solution in ethanol expressed by mole/litre;

P = weight of the sample in grams.

Preferably, the polyester in the composition according to the present invention has an inherent viscosity (measured with an 0b Ubbelohde viscometer in 1 : 1 v/v dichloromethane- trifluoroacetic acid solution of concentration 0.5 g/dl at 25 °C) greater than 0.3 dl/g, preferably between 0.3 and 2 dl/g, more preferably between 0.4 and-1.4-dl/g.

In the meaning of the present invention a biodegradable polymer is a biodegradable polymer according to standard EN 13432.

Said biodegradable polymer can be synthesised using any of the processes known in the prior art. In particular, it can be advantageously accomplished through a polycondensation reaction. The synthesis process can advantageously be conducted in the presence of a suitable catalyst. Examples of suitable catalysts include organometallic tin compounds such as stannoic acid derivatives, titanium compounds such as orthobutyl titanate, aluminium compounds such as Al-triisopropyl, Antimony and Zinc and Zirconium and mixtures thereof.

As regards the starch component of the biodegradable films according to the present invention, the term starch relates to all types of starch, in particular the following: flour, native starch, hydrolysed starch, destructured starch, gelatinised starch, plasticised starch, thermoplastic starch, biofillers comprising complexed starch or mixtures thereof. Particularly suitable according to the invention are starches such as potato, maize, tapioca and pea starch. Starches that can be easily destructured and have high initial molecular weights, such as potato and maize starch, are particularly advantageous. Starch and cellulose may be present either as such or in chemically modified form, such as in the form of starch or cellulose esters with a level of substitution of between 0.2 and 2.5, hydroxypropylated starch, fatty chain modified starch, or cellophane.

By destructured starch we refer here to the teachings included in Patents EP 0 118 240 and EP 0 327 505, meaning as such the starch processed in such a way that it does not substantially show the so-called "maltese crosses" under a polarised light microscope and the so-called "ghosts" under a phase contrast optical microscope.

Starch destructuring is advantageously carried out by an extrusion process at a temperature of between 110°C and 250°C, preferably between 130°C and 220°C, preferably at pressures of between 0.1 MPa and 7 MPa, preferably between 0.3 MPa and 6 MPa, preferably providing a specific energy greater than 0.1 kWh/kg during this extrusion.

Starch destructuring is preferably carried out in the presence of 1 - 40% by weight, in relation to the weight of starch, of one or more plasticisers selected from water and polyols having from 2 to 22 carbon atoms. As far as the water is concerned, this may also be the water naturally present in starch. Of the polyols, polyols with 1 to 20 hydroxyl groups containing 2 to 6 carbon atoms, their ethers, thioethers and organic and inorganic esters are preferred. Examples of polyols are glycerine, diglycerol, polyglycerol, pentaerythritol, polyglycerol ethoxylate, ethylene glycol, polyethylene glycol, 1,2-propanediol, 1,3 -propanediol, 1,4-butanediol, neopentylglycol, sorbitol, sorbitol monoacetate, sorbitol diacetate, sorbitol monoethoxylate, sorbitol diethoxylate, and mixtures thereof. In a preferred embodiment starch is destructured in the presence of glycerol or a mixture of plasticisers comprising glycerol, preferably comprising 1 - 90% by weight of glycerol. Preferably the destructured starch comprises 1 - 40% by weight of plasticisers selected from those listed above in relation to the weight of starch. Compositions comprising destructured starch are particularly preferred. Preferably the starch in the mixture is present in the form of particles having a circular, elliptical or otherwise ellipse-like cross-section having an arithmetic mean diameter of less than 1 pm and more preferably less than 0.5 pm mean diameter measured along the major axis of the particle.

The composition of the biodegradable film according to the present invention is typically produced by mixing, preferably in an extruder, at a temperature of between 150°C and 250°C. The composition of the biodegradable film according to the present invention also optionally includes one or more polymers, typically in quantities of 1 - 50% by weight, preferably 2 - 30% by weight of the biodegradable polymer. Said polymers are generally selected from the group comprising hydroxyacid polyesters, polyolefins, aromatic polyesters, polyester- and polyether- urethanes, polyurethanes, polyamides, polyamino acids, polyethers, polyureas and polycarbonates.

The composition of the biodegradable film according to the invention also optionally includes one or more additives selected from fillers, plasticisers, UV stabilisers, lubricants, nucleating agents, surfactants, antistatic agents, pigments, flame retardants, compatibility agents, polyphenols, reinforcing fillers, coupling agents, antioxidants, anti-mould agents, waxes and process coadjuvants.

Among the hydroxyacid polyesters, those preferred are: lactic acid polyesters, poly-e- caprolactone, polyhydroxy butyrate, polyhydroxybutyrate valerate, polyhydroxybutyrate propanoate, polyhydroxybutyrate hexanoate, polyhydroxybutyrate decanoate, polyhydroxybutyrate dodecanoate, polyhydroxybutyrate hexadecanoate, polyhydroxybutyrate octadecanoate, poly-3 -hydroxybutyrate-4-hydroxybutyrate.

Preferably, the hydroxyacid polyesters comprise at least 80% by weight of one or more lactic acid polyesters in relation to the total weight of hydroxyacid polyesters. The lactic acid polyesters are preferably selected from the group comprising poly-L-lactic acid, poly-D-lactic acid, poly-D, L-lactic acid stereo complex, copolymers comprising more than 50% by mole of said lactic acid polyesters or mixtures thereof. Particularly preferred are lactic acid polyesters containing at least 95% by weight of repetitive units derived from L-lactic or D-lactic acid or combinations thereof, typically having a molecular weight (Mw) greater than 50,000 and a dynamic viscosity of between 50 and 700 Pa-s, preferably between 80 and 500 Pa-s (measured according to ASTM D3835 at T 190°C, velocity gradient of 1000 s ¹ , D of 1 mm and L/D of 10), such as Ingeo™ Biopolymer 4043D, 325 ID and 6202D branded products.

Preferably the film according to the present invention comprises a mixture of at least one biodegradable polymer according to the present invention and at least one hydroxyacid polyester comprising between 1% and 80% by weight, more preferably between 2% and 70% by weight of said hydroxyacid polyesters, in relation to the sum of the weights of biodegradable polymer and the latter respectively.

Of the polyolefins, those preferred are: polyethylene, polypropylene, their copolymers, polyvinyl alcohol, polyvinyl acetate, poly ethyl vinyl acetate and polyethylene vinyl alcohol.

Of the aromatic polyesters, those preferred are: PET, PBT, PTT in particular with a renewable content of more than 30% and polyalkylenefuran dicarboxylates. Of the latter, those particularly preferred are poly(l,2-ethylene-2,5-furandicarboxylate), poly(l,3-propylene-2,5- furandicarboxylate), poly(l,4-butylene-2,5-furandicarboxylate) and their mixtures. Examples of polyamides are: polyamide 6 and 6.6, polyamide 9 and 9.9, polyamide 10 and 10.10, polyamide 11 and 11.11, polyamide 12 and 12.12 and their combinations of the 6/9, 6/10, 6/11, 6/12 type.

The polycarbonates may be selected from the group comprising polyethylene carbonates, polypropylene carbonates, polybutylene carbonates, their mixtures and copolymers.

The polyethers may be selected from the group comprising polyethylene glycols, polypropylene glycols, polybutylene glycols, their copolymers and mixtures with molecular weights from 70,000 to 500,000.

The fillers are preferably selected from the group comprising kaolin, barytes, clay, talc, calcium and magnesium, iron and lead carbonates, aluminium hydroxide, diatomaceous earth, aluminium sulfate, barium sulfate, silica, mica, titanium dioxide, wollastonite, starch, cellulose, chitin, chitosan, alginates, proteins such as gluten, zein, casein, collagen, gelatin, natural gums, rosinic acid and its derivatives and their mixtures. With regard to cellulose, it may for example be present in the form of cellulose fibres or as wood flour.

Advantageously more than one filling agent may be used. Particularly preferred are mixtures containing starch and at least one other filler. As regards plasticisers, in addition to any plasticisers preferably used for the preparation of destructured starch described above, there may be present one or more plasticisers from the group comprising phthalates, such as diisononyl phthalate, trimellitates, such as trimellitic acid esters with C4-C20 mono-alcohols preferably selected from the group comprising n-octanol and n-decanol, and aliphatic esters having the following structure:

Ri-O- C(0)-R4-C(0)-[-0-R ₂ -0-C(0)-R ₅ -C(0)-]m-0-R3

where

Ri is selected from one or more of the groups comprising H, linear and branched, saturated and unsaturated alkyl residues of the C1-C24 type, residues of polyols esterified with C1-C24 monocarboxylic acids;

R2 comprises -CH2-C(CH3)2-CH2- groups and C2-C8 alkylenes, in which said -CH2-C(CH3)2- CH2- groups comprise at least 50% by mole;

R3 is selected from one or more of the groups comprising H, linear and branched, saturated and unsaturated alkyl residues of the C1-C24 type, polyol residues esterified with C1-C24 monocarboxylic acids;

R4 and R5, which are the same or different, comprise one or more C2-C22, preferably C2-C11, more preferably C4-C9 alkylenes, and comprise at least 50% of C7 alkylenes by mole; and m is an integer between 1 and 20, preferably between 2 and 10, more preferably between 3 and 7. Preferably, in these esters at least one of the Ri and/or R3 groups will include polyol residues esterified with at least one C1-C24 monocarboxylic acid selected from the group comprising stearic acid, palmitic acid, 9-ketostearic acid, 10-ketostearic acid and mixtures thereof, preferably in quantities greater than or equal to 10% by mole, more preferably greater than or equal to 20% by mole, even more preferably greater than or equal to 25% by mole, in relation to the total quantity of Ri and/or R3 groups.

Examples of such aliphatic esters are described in Italian patent application MI2014A000030 and applications PCT/EP2015/050336, PCT/EP2015/050338.

When present, the selected plasticisers are preferably present in quantities between 0.2% and 20% by weight, more preferably between 0.5% and 10% by weight, in relation to the total weight of the mixture. The lubricants are preferably chosen from metal esters and salts of fatty acids such as zinc stearate, calcium stearate, aluminium stearate and acetyl stearate.

Preferably, when used, said lubricants are used in quantities up to 1% by weight of the total weight of the mixture, more preferably up to 0.5% by weight. Examples of nucleating agents include saccharine sodium salt, calcium silicate, sodium benzoate, calcium titanate, boron nitride, talc, zinc stearate and low molecular weight PLA. These additives are preferably added in quantities of up to 10% by weight and more preferably between 2% and 6% by weight with respect to the total weight of the biodegradable polymer. Pigments can also be added if necessary, for example, clays, copper phthalocyanine, titanium dioxide, silicates, iron oxides and hydroxides, carbon black, and magnesium oxide. These additives will preferably be added as up to 10% by weight.

Preferably, the said compatibility agents are selected from compounds having two and/or multiple functional groups including carbodiimide, epoxy, anhydride or divinylether groups or mixtures thereof, carbodiimide and epoxy groups being particularly preferred.

With regard to the polyphenols, these are preferably selected from the group comprising lignin, silibin, silidianine, isosilibin and silicristin and mixtures of these, and are present in quantities preferably between 0.5% and 7% by weight in relation to the total weight of the mixture.

In a preferred embodiment the polyphenol of plant origin advantageously comprises a mixture comprising silibin, silidianin, isosilibin and silicristin. Said mixture can advantageously be obtained by alcoholic extraction from deoiled milk thistle ( Silybum marianum ) seed cake and is also commonly known commercially as silymarin.

After it has been produced by bubble blowing, the biodegradable film according to the present invention is subjected to an uniaxial or biaxial cold stretching process, preferably an uniaxial, cold stretching process in the machine direction, with stretch of above 10%, preferably above 25%, even more preferably above 30%, said process giving rise an increase in the breaking load values while maintaining high tear strength. The stretch in the machine direction is advantageously of below 70% and more advantageously of below 65%. According to a preferred aspect, the biodegradable film according to the present invention is subjected to a stretching of below 60, more preferably of below 55%.

In the context of this invention, stretching means the percentage obtained as follows:

In the context of this invention, cold stretching means as stretching performed on non-melted biodegradable polymer material. More specifically, with reference to films having an initial thickness of less than 50 pm, preferably less than 30 pm, cold stretching is defined as cold stretching performed in a temperature range between 10°C and 80°C, preferably between 20°C and 70°C.

The cold stretching process used to produce biodegradable films according to the present invention can be used on various types of film, for example on single sheet film, single fold film or directly on tubular film. The cold stretching process can in fact take place both discontinuously and in line with the bubble blowing process. If the process is performed in line with blowing of the bubble, it takes place beyond the frost line, i.e. after the point beyond which the bubble has solidified. In this case double bubble blowing processes may also be used.

The cold stretching process used to obtain biodegradable films according to the present invention is preceded by the bubble blowing process, preferably characterised by blow-up ratio (BUR or transversal stretching) values from 2 to 5, and drawdown ratio (DDR or longitudinal stretching) values in the machine direction (MD) from 5 to 60. For the purposes of this invention, DDR is defined as the measurement of the elongation of the molten material extruded from the extruder in the direction of drawing; BUR is defined as the ratio of the bubble diameter to the die diameter. Advantageously, during blown bubble blowing the process parameters are set so that the DDR/BUR values have a ratio from 3 to 15.

The biodegradable film according to the present invention obtained by a cold stretching process is characterised by a tear strength in the machine direction (MD) of more than 100 N/mm, preferably of more than 110 N/mm, determined according to ASTM D1922 (at 23 °C and 55% relative humidity).

The biodegradable film obtained by a cold stretching process according to the present invention is advantageously characterised by a tear strength in the transverse direction (TD) of more than 150 N/mm, determined according to ASTM D1922 (at 23°C and 55% relative humidity).

The biodegradable film obtained by a cold stretching process according to the present invention is characterised by a tensile strength at break of more than 20 MPa, preferably of more than 30 MPa, more preferably of more than 35 MPa and even more preferably of more than 38 MPa, determined according to ASTM D882 (tensile properties at 23°C and 55% relative humidity and Vo=50 mm/min).

Preferably, the biodegradable film obtained by a cold stretching process according to the present invention is characterised by a Modulus of more than 150 MPa, more preferably of more than 200 MPa, determined according to ASTM D882 (tensile properties at 23°C and 55% relative humidity and Vo=50 mm/min).

The cold stretched biodegradable film according to the present invention is advantageously characterised by an increase in the tear strength MD values of above 10%, preferably of above 20%, more preferably of above 30% with respect to the non-stretched film of the same thickness, without decreases in breaking load. The film according to this invention is biodegradable. In the meaning of this invention biodegradable film means a film which is biodegradable in industrial composting according to EN 13432. Preferably the biodegradable film according to this invention is also biodegradable under home composting conditions according to UNI 11355.

The biodegradable film according to the present invention can be either monolayer or multilayer. In the case of multilayer films, such films may comprise at least one layer of starch- based material and at least one layer of biodegradable polymer as such or in a mixture with other polymers.

The biodegradable film according to the present invention is obtained by a cold stretching process through which thin biodegradable films with significant mechanical properties can be produced. These films are therefore useful for the manufacture of products such as bags of all kinds and shapes, in particular bags for differentiated waste collection, shopping bags, food packaging films such as bags for fruit and vegetables, mulching films, nappies and hygiene products. In particular it is possible to produce cold stretched films, obtained by the cold stretching process, of final thicknesses in the range from 5 to 30 pm, preferably from 6 to 25 pm and even more preferably from 7 to 20 pm.

Because of their high tear strength, films obtained by a cold stretching process are particularly advantageous for the production of shopping bags and food packaging.

The present invention will now be illustrated according to a few examples which are not intended to be restrictive. EXAMPLES

Table 1. Compositions.

1 = Poly( 1,4-butylene adipate-co- 1,4-butylene azelate-co- 1,4-butylene terephthalate), with an azelaic acid content of 15% by mole in relation to the sum of adipic acid and azelaic acid, and a terephthalic acid content of 47% by mole in relation to the total dicarboxylic component. MFR 5 g/lOmin (@ 190°C, 2.16 kg) and acidity 40 meq/kg;

ii = Poly(l, 4-butylene adipate-co- 1,4-butylene terephthalate), with a terephthalic acid content of 47% by mole in relation to the total dicarboxylic component. MFR 4.5 g/lOmin (@ 190 °C, 2.16 kg) and acidity 40 meq/kg;

starch = thermoplastic maize starch;

other = 90% by weight PL A - 10% by weight compatibility agent.

The compositions 1 (according to the invention) and 2 (comparison) shown in Table 1 were fed to a bubble film machine, obtaining the thicknesses shown as initial thicknesses in Table 2. The films obtained from the two compositions were compostable in domestic composting according to UNI 11355, reaching a value higher than 90% in comparison with cellulose in 220 days.

Said films were subsequently subjected to a uniaxial cold stretching process in the machine direction (at 65 °C) using different stretching ratios. Table 2 shows the percentage stretch and the final thickness value achieved.

Tests relating to the determination of tear strength were performed according to ASTM D1922 (at 23°C and 55% relative humidity). Tests relating to the determination of tensile strength were performed according to ASTM D882 (tensile properties at 23°C and 55% relative humidity and Vo = 50 mm/min). The tear strength and breaking strength values are shown in Table 2.

Table 2. Process parameters and mechanical properties of films comprising compositions 1 and

2 in Table 1.

Examples 1-2 according to the invention confirmed the improvement in the properties (Breaking load and Tear strength) of 20% and 50% cold stretched films over the non-stretched film of Comparative example 1 having the same composition and the same thickness (8 pm). Additionally, the data provided in Table 2 clearly show a surprising and unpredictable improvement in mechanical properties for cold stretched films in examples 1 - 4 (comprising the composition 1 characterised by the presence of a biodegradable polymer comprising azelaic acid as aliphatic long chain diacid) according to the invention, compared to the properties of the stretched films of comparative example 2 and comparative example 3 (comprising the composition 2 based on adipic acid).