COMPOUND OR FILM CONTAINING THERMOPLASTIC STARCH AND A THERMOPLASTIC POLYMER

Description

[0001] The present invention relates to a method for producing a compound containing thermoplastic starch, and a film produced from this compound.

[0002] According to the common definition, thermoplastic starch (hereinafter also referred to as TPS) is an amorphous or semi-crystalline material consisting of expanded or destructured starch and one or more plasticizers. TPS can be repeatedly converted to a plastic state and re-cured, allowing it to be shaped under the influence of heat and shear, thus enabling processing using techniques from the plastics industry. TPS as a material typically has hydrophilic properties, which means its material properties are highly dependent on the surrounding climate. For this reason, TPS is rarely used directly or exclusively for the production of bioplastics.The use of finely dispersed TPS (dispersed phase) in a polymer matrix (continuous phase), on the other hand, offers the possibility of a) significantly increasing the bio-based content in plastic formulations and b) integrating a biodegradable component, depending on the choice of matrix polymer. Materials requiring complete biodegradability or compostability necessitate the use of a polymer matrix that can be degraded or metabolized in biological media and under the influence of water by microorganisms.

[0003] Thermoplastic polymers can be repeatedly melted by increasing the temperature. After cooling, they exist in a predominantly crystalline or amorphous structure. This property is used for shaping purposes. Examples of biodegradable thermoplastic polymers include polybutylene adipate co-terephthalate, polycaprolactone, polylactic acid, and polybutylene succinate. Native starch does not exhibit this behavior. When processed into a thermoplastic material, the originally semicrystalline, granular structure is broken down to create a continuous amorphous phase, thus making starch malleable using conventional plastics processing methods. When heated above its gelatinization temperature, starch begins to swell in the presence of water. During this process, liquid diffuses into the interior of the granules and ultimately interacts with the free hydroxyl groups of the starch molecules.This breaks the hydrogen bonds, the material loses its crystallinity, and eventually amorphous regions begin to dissolve. The process is essentially determined by the temperature profile.

[0004] Up to a threshold of approximately 50 °C, the process is largely reversible. With further heating, irreversible and significant swelling sets in. The loss of crystallinity causes the starch granules to lose their onion-skin structure and the birefringence visible under a microscope, and the viscosity of the suspension increases rapidly. In an extrusion process, plasticizers are added as an alternative to water to achieve starch breakdown under these water-limited conditions. The use of plasticizers such as glycerin, sorbitol, erythritol, polyethylene glycol, various mono- and disaccharides, or sugar alcohols reduces intermolecular interactions by breaking the hydrogen bonds between the starch molecules, similar to the effect of water.The process in the extruder is accompanied by a splitting of the polymer chains and thus a partial depolymerization, which causes both the melting and glass transition temperatures to drop below the degradation temperature.

[0005] US patent 5,362,777 discloses the production of thermoplastic starch (TPS) by adding plasticizers, e.g. sorbitol or sorbitol; vegetable fats can also be added to improve flow properties.

[0006] WO 99 / 61524 concerns a film made of a thermoplastic polymer mixture containing TPS, at least one polyester urethane, a plasticizer such as sorbitol, and oils containing epoxy groups as lubricants, in particular epoxidized linseed oil.

[0007] DE 198 24 968 A1 also discloses a film made of a thermoplastic polymer mixture containing TPS, with a polymer obtainable by polycondensation or polyaddition, containing plasticizers, e.g. sorbitol, and vegetable fats or oils as lubricants.

[0008] According to WO 2012 / 162085 A1, TPS, oil and / or wax (epoxidized vegetable oil or linseed oil) are disclosed. TPS is a starting material; the presence of another thermoplastic polymer is essential for the processing of thermoplastic starch.

[0009] WO 2009 / 118665 A2 discloses a melt-extruded substrate containing a thermoplastic starch formed from a starch, plasticizers and over 10 wt. % lactic acid,

[0010] EP 0 596 437 A2 describes a biodegradable polymer mixture consisting essentially of starch and at least one hydrophobic polymer. The hydrophobic polymer is to be at least largely biodegradable and thermoplastic, and the mixture with the starch is to comprise a polymeric phase mediator or a macromolecular dispersant, such that the starch exists as a dispersed phase and the hydrophobic polymer as a continuous phase, with the phase mediator or dispersant being responsible for the molecular coupling of the two phases. Preferably, thermoplastic starch is used, which is produced essentially in the absence of water using sorbitol or glycerol.

[0011] WO 2006 / 042364 A1 finally discloses a mixture of sorbitol and other plasticizers, e.g., epoxidized linseed oil. Starch is a starting material; apart from the starch, a water-soluble polymer is also present, e.g., polyvinyl alcohol, polyvinyl acetate, or copolymers of ethylene and vinyl alcohol.

[0012] Despite the addition of plasticizers, TPS, already known from the aforementioned prior art, is inherently brittle and hydrophilic. Therefore, the high requirements (strength, water resistance) placed on technical products in film extrusion cannot be met when using pure TPS.

[0013] Due to significant viscosity differences, fine dispersion of TPS in a polymer matrix is ​​only efficient under high shear stress (the TPS has a very high viscosity, while the polymer tends to have a low viscosity). This can lead to mechanical damage of the TPS phase and a consequent browning of the compound material. Furthermore, the high viscosity of the untreated TPS makes it more difficult to process, resulting in increased torque and pressure conditions in the extruder.

[0014] Furthermore, compatibility at the interfaces between the hydrophilic TPS and the hydrophobic polymer is limited. This leads to an impairment of the mechanical material properties (tensile strength, ductility) as well as to optical losses (decreased transparency and thus increased opacity) in the final product. To date, no practical solution for this latter problem has been found in the literature.

[0015] CN 107 955 212 relates to a fully biodegradable plastic film containing a thermoplastic starch, a biodegradable polymer such as polylactic acid (referred to as poly(lactic acid) in the document), and other ingredients. The composition used to produce blown films contains 20-80% by weight of such poly(lactic acid), preferably the weight ratio of thermoplastic starch to poly(lactic acid) is approximately 20:80 to 80:20. The potential transparency of the blown film produced is not mentioned.

[0016] A similar situation applies to CN 103 159 984, which also discloses the use of poly(lactic acid) together with thermoplastic starch, with the poly(lactic acid) present in an amount of 8-51% by weight. CN 103 159 984 also does not disclose any potential transparency of the manufactured product, nor does it mention film or blown film.

[0017] Due to insufficient compatibility, the TPS grades currently available on the market generally do not allow for use in proportions exceeding 30-40 wt.% in the compound or film without significantly impairing the mechanical properties of the end products (films). However, the production of films with a higher proportion (>40 wt.%) of renewable raw materials such as TPS is desirable. The opacity associated with increasing starch content is an additional limiting factor. Particularly in the packaging industry, the switch to bio-based and biodegradable materials is essential for sustainability reasons and to reduce the generation of durable plastic waste. This sector also places specific demands on the transparency (or opacity) of film materials, as the transparency of packaging is a crucial criterion for meeting customer expectations in a wide range of applications (e.g.,transparent plastic outer packaging, fruit and vegetable bags).

[0018] Various publications address the problem of opacity when adding TPS to biopolymer compounds. These publications discuss different factors, such as the amylose / amylopectin ratio, the type and content of plasticizer, and the influence of fillers on transparency.

[0019] The object of the present invention is to overcome the aforementioned disadvantages of the prior art and to provide a method for producing a compound or film containing thermoplastic starch and a thermoplastic polymer, which compound can be used for the production of transparent films by blown or flat film extrusion.

[0020] A film or foil is a flat, thin material with a thickness in the range of 2-500 µm, whereby the achievable film flexibility depends essentially on the type of raw material used and the film thickness.

[0021] The object of the invention is achieved by a process for producing a compound or film containing thermoplastic starch, an alpha-hydroxycarboxylic acid (ROHCOOH), where R represents CH₂ or CH₃CH, in an amount of 0.1 to 5, preferably 0.1 to 3, particularly preferably 0.1 to 1 wt.% based on the thermoplastic starch, and a thermoplastic polymer, in which process the compound or film is subjected to an additional heating step to 100–140 °C after its extrusion. Surprisingly, it has been found that a transparent film can be obtained by the additional heating step to 100–140 °C of a compound containing thermoplastic starch and a thermoplastic polymer in a subsequent processing step, as required by the invention. However, the heating step, which is essential according to the invention, can also be carried out directly on the film after further processing of the compound.Regarding the addition of an alpha-hydroxycarboxylic acid ROHCOOH according to the invention, it has been shown that exceeding the upper limit of 5 wt. % (based on the thermoplastic strength) leads to a reduction in the service life of the produced compound / film due to decomposition and generally a deterioration of the physical properties.

[0022] It is also preferred if the additional heating step after extrusion lasts at least 15 minutes, preferably at least 30 minutes, particularly preferably at least 60 minutes for the compound and at least 2 minutes, preferably at least 5 minutes, particularly preferably at least 60 minutes for the film. Surprisingly, it has been found that by adding an alpha-hydroxy carboxylic acid, preferably lactic acid, compounds can be produced which, when processed according to the prior art, optionally (especially if the additional heating step has not already taken place during the production of the compound) with a subsequent heating step to 100-140 °C, preferably to 120-140 °C, for at least 15 minutes, preferably at least 30 minutes, particularly preferably at least 60 minutes (for the compound), yield a transparent film.The additional heating step, which is essential according to the invention, can, as mentioned, also be carried out directly on the film after further processing of the compound. Heating the described films to 100-140 °C, preferably to 120-140 °C, for a duration of at least 2 minutes, preferably at least 5 minutes, and particularly preferably at least 60 minutes, surprisingly also results in a transparent film. When the term "transparent" is used in connection with the present invention, it refers in each case to a comparison with the untreated film material (or to a film material produced from untreated compounds), whereby "transparent" is understood as an increase in transparency compared to the reference material. The measurement or calculation of transparency or opacity (cloudiness) has been discussed in various publications. An increase in transparency or opacity is defined as...A reduction in opacity is defined as a reduction in absorption detectable by spectroscopy (measured, for example, at a wavelength of 550 nm) compared to the respective reference material.

[0023] Preferably, the compound according to the invention comprises a thermoplastic polymer selected from the group consisting of polyolefins, polyamides, polyurethanes, polyesters, and mixtures thereof. Preferably, the compound contains polyesters as the thermoplastic polymer, which are readily miscible with the TPS due to their viscosities. The polymers used can be biodegradable or non-biodegradable, with the former being preferred. The compound properties, such as strength, can be adjusted via the polymer mixture. When using a thermoplastic starch produced according to the invention, it is even possible to provide a TPS content in the compound of up to 65 wt.%.

[0024] According to the present invention, the described compound can be produced in a) separate steps (1. starch plasticization and 2. subsequent compounding with a thermoplastic polymer in a separate apparatus), or b) in a single-step process (starch plasticization and compounding in one step in one apparatus). Transparent films can be obtained according to the invention based on both the compound produced in a) and the compound produced in b).

[0025] While any thermoplastic starch can be used according to the invention, a thermoplastic starch produced by a special process is particularly preferred in which a mixture of starch is extruded with a polyol, preferably selected from the group comprising polyethylene glycol, mono- and disaccharides, sugar alcohols such as glycerol, sorbitol, erythritol, xylitol or mannitol and mixtures thereof, in an amount of 10 to 25 wt. % of the mixture and an epoxide, selected from the group comprising epoxidized vegetable oils such as soybean oil, linseed oil, sunflower oil, rapeseed oil and mixtures thereof, in an amount of 0.1 to 6, preferably 1 to 4.5, particularly preferably 2.5 to 3.5 wt. % of the mixture.The formulation for producing thermoplastic starch (TPS) is, from both a processing and materials engineering perspective, the production of a thermoplastic starch with an optimized property profile. The starting materials are starch, a plasticizer (10-25 wt%), and an epoxidized vegetable oil (0.1-6 wt%). The final product is cold-water swellable to cold-water soluble. For the production of thin-walled film materials (in the range of, for example, 10-50 µm thickness), it is important to disperse the TPS as finely as possible within the compound matrix. Surprisingly, it has been shown that with such a thermoplastic starch, a TPS particle size of < 5 µm can be achieved in the polymer matrix, thus preventing the formation of micro-roughness (film surface) and the associated mechanical weaknesses.The use of these TPS in the form of a finely dispersed compound phase, in combination with, for example, biodegradable thermoplastic polyesters (the continuous phase), offers a simple way to increase moisture resistance and optimize the properties of the final product. This also allows for the adjustment of the biodegradability of the final product. The sustainability of the final product can be enhanced by the increased proportion of TPS made possible by this method. For the epoxy, it has been shown that the melt's absorption capacity is exhausted at 6 wt%; a higher dosage leads to oily deposits on the product or equipment.

[0026] It is important to include an additional heating step to 100-140 °C, preferably 120-140 °C, for at least 15 minutes, preferably at least 30 minutes, particularly preferably at least 60 minutes (compound), or to 100-140 °C, preferably 120-140 °C, for at least 2 minutes, preferably at least 5 minutes, particularly preferably at least 60 minutes (film), either during or after the compound production process, or, if no heating step is used during / after the compound production process, after the blown film has been produced. Only this additional heating step makes it possible to modify the compound in such a way that a surprisingly transparent film is obtained. As demonstrated in Table 3, this, along with appropriate additives (preferably lactic acid), makes it possible to achieve transparency.to achieve an opacity that approximates that of the starch-free pure polymer (e.g., PBAT). Strength:

[0027] The starch used for the production of thermoplastic starch can be any conventional tuber, cereal, or legume starch, e.g., pea starch, corn starch (including waxy corn starch), potato starch (including waxy potato starch), amaranth starch, rice starch (including waxy rice starch), wheat starch (including waxy wheat starch), barley starch (including waxy barley starch), tapioca starch (including waxy tapioca starch), and sago starch. Starches of natural origin generally have an amylose content of 20 to 30% by weight, depending on the plant species from which they are obtained. According to the invention, this also includes amylopectin-rich starches, which have a significantly increased amylopectin content, or products containing an increased amylose content. In addition to naturally occurring amylopectin-rich starch types and high-amylose starches obtained through breeding, starches obtained through chemical and / or physical fractionation can also be used.Amylopectin-rich or highly amylose starches produced from genetically modified plants may be used. Functionalized starches, defined as follows, may also be used. Functionalized strength:

[0028] The starch used for the production of thermoplastic starch can also be a functionalized starch. Whenever the term "starch" is used in this description and in the claims, it also includes functionalized starch. Functionalization includes, for example, etherification or esterification. The following describes several derivatization processes that can be used, alone or in combination, for the further derivatization of starch derivatives. The type of derivatization and the raw material base of the starch used are closely related to the specific application of the respective product. The methods for this are generally known. Specifically, the focus here will be on functionalization in slurry, paste, (semi-)dry processes, and functionalization via reactive extrusion.

[0029] In general, starch derivatives are distinguished between starch ethers and starch esters. Furthermore, a distinction can be made between nonionic, anionic, cationic, and amphoteric as well as hydrophobic starch derivatives, which can be produced via slurry, paste, semi-dry, or dry derivatization, as well as via derivatization in organic solvents.

[0030] Anionic and nonionic functionalization of starch encompasses those derivatives in which the free hydroxyl groups of the starch are substituted by anionic or nonionic groups, respectively. Starch can also be anionically functionalized through oxidative processes such as treatment with hydrogen peroxide or hypochlorite, or through a laccase / mediator system.

[0031] Anionic and nonionic derivatization can, in principle, be carried out in two ways: a) Functionalization achieves esterification of the starch. Inorganic or organic acids of varying oxidation states, usually divalent, or their salts, esters, or anhydrides serve as functionalizing agents. Mixed esters or anhydrides can also be used. The esterification of the starch can be carried out multiple times, so that, for example, distarch phosphoric acid esters can be produced. Preferably, the starch used according to the invention is the result of esterification with mono-, di-, or tricarboxylic acids having an alkyl chain with 1 to 30 carbon atoms or a carbamate, particularly preferably acylated, such as succinylated, octenylsuccinylated, dodecylsuccinylated, or acetylated. b) During functionalization, the starch undergoes etherification. Methyl, ethyl, hydroxyethyl, hydroxypropyl, hydroxybutyl, carboxymethyl, cyanoethyl, or carbamoylethyl ether starches, or mixtures thereof, can be used.

[0032] Cationic functionalization of starches encompasses those derivatives in which a positive charge is introduced into the starch through substitution. The cationization processes involve amino, imino, ammonium, sulfonium, or phosphonium groups. Such cationic derivatives preferably contain nitrogen-containing groups, in particular primary, secondary, tertiary, and quaternary amines or sulfonium and phosphonium groups, which are linked via ether or ester bonds.

[0033] Another group consists of amphoteric starches. These contain both anionic and cationic groups, making their applications very specific. Most are cationic starches that are further functionalized either by phosphate groups or by xanthates.

[0034] Esters are classified as either simple starch esters or mixed starch esters, whereby the substituent(s) of the ester can be of various types: In the ester residue RCOO-, the R group can be an alkyl, aryl, alkenyl, alkaryl, or aralkyl group with 1 to 20 carbon atoms, preferably 1 to 17 carbon atoms, and preferably 1 to 6 carbon atoms. These products include the derivatives acetate (prepared from vinyl acetate or acetic anhydride), propionate, butyrate, stearate, phthalate, succinate, oleate, maleate, fumarate, and benzoate.

[0035] Etherification is largely achieved by reaction with alkylene oxides (hydroxyalkylation) containing 1 to 20 carbon atoms, preferably 2 to 6 carbon atoms, and particularly 2 to 4 carbon atoms, especially by using ethylene and propylene oxides. Methyl, carboxymethyl, cyanoethyl, and carbamoyl ethers can also be prepared and used. The reaction of starch with monochloroacetic acid or its salts serves as an example of carboxyalkylation. Furthermore, hydrophobic etherification reagents such as glycidyl ethers or epoxides should be specifically mentioned. The alkyl chain length of these reagents ranges from 1 to 20 carbon atoms; aromatic glycidyl ethers are also possible.

[0036] Examples of derivatization with glycidyl ethers include o-cresol glycidyl ether, polypropylene diglycol glycidyl ether, tert-butylphenyl glycidyl ether, ethylhexyl glycidyl ether, hexanediol glycidyl ether and neodecanoic acid glycidyl ester.

[0037] Another possibility for alkylation is alkylation via alkyl halides, for example via methyl chloride, dialkyl carbonates, e.g. dimethyl carbonate (DMC) or dialkyl sulfate, e.g. dimethyl sulfate.

[0038] The starches used for esterification, etherification, and crosslinking, as well as chemically non-functionalized starches, can also be tempered (in slurry) or inhibited (dry or semi-dry reaction) via thermophysical modifications. Starches can also be functionalized using hydrophobizing reagents. Etherified hydrophobic starches are obtained when the hydrophobic reagents contain a halide, an epoxide, a glycidyl, a halohydrin, a carboxylic acid, or a quaternary ammonium group as a functional group. For esterified hydrophobic starches, the hydrophobic reagent usually contains an anhydride. Starch can also be hydrophobized by mixing a starch or a starch derivative with fatty acid esters.

[0039] All the aforementioned functionalizations of starch can be achieved not only through the conversion of native starch, but also through the use of degraded forms. These degradation processes can be hydrolytic (acid-catalyzed), oxidative, mechanical, thermal, thermochemical, or enzymatic. This allows not only the structural modification of starch, but also the rendering of the starch products solubilizable or swellable in cold water.

[0040] Finally, the starch can also be present as a graft polymer or as a graft copolymer, such as with products from the group of polyvinyl alcohols or polyesters. Epoxidized vegetable oils:

[0041] From a chemical perspective, the epoxides used for the production of TPS according to a preferred embodiment of the present invention are cyclic ethers. Epoxides can interact with the hydroxyl groups of starch. The group of substances classified as epoxides includes, among others, the epoxidized oils used according to the invention, particularly vegetable oils. Due to their chemical structure, epoxides are unstable; that is, the ring structure opens and can react with the starch or, in combination with, for example, water, to form a diol. The opening of the epoxide ring can be catalyzed by acids (e.g., carboxylic acids). Preferably, epoxidized vegetable oils such as soybean or linseed oil (ESBO, ELO) are used. Epoxidized linseed oil has a viscosity of approximately 900 mPas at 25 °C and an epoxide oxygen content of at least 8.5 wt.%.Epoxidized soybean oil, on the other hand, has a viscosity of approximately 300–450 mPas (also at 25 °C) and an epoxy oxygen content of 6.5–7.5 wt.%. The viscosity measurements carried out for the purposes of the present invention were each performed in a viscometer according to EN ISO 3219. Polyols:

[0042] According to a preferred embodiment of the present invention, the mixture used for the production of the TPS in the compounds contains a polyol selected from the group consisting of sorbitol, erythritol, xylitol, mannitol, and mixtures thereof, in an amount of 10 to 25 wt.%. These polyols are so efficient as plasticizers in the TPS (interaction with hydroxyl groups) that processing can take place within the process window (low pressure, low torque). The polyols can also be added to the TPS as a syrup (solution in water), which improves their miscibility into the melt, resulting in more homogeneous TPS, more homogeneous compounds, and smoother films. Furthermore, these polyols have the advantage over glycerin that they are solid at room temperature but exist as a melt during processing and can thus act as plasticizers.

[0043] Preferably, the mixture for the production of the TPS contains sorbitol or erythritol as a polyol in the compound in an amount of 10 to 15 wt. % of the mixture.

[0044] It is also advantageous if the mixture used to produce the TPS contains polyol in a quantity of 13 to 15 wt.% of the mixture. It has been shown that the proportion of polyol as a plasticizer in the TPS should not be too high, as this can otherwise lead to potential problems in food contact. For example, the plasticizer could leach out if it is present in excess; on the other hand, a certain percentage of plasticizer is necessary to a) be able to process within the process window (pressure, torque) and b) ultimately achieve the required film properties (elongation, tensile strength).

[0045] According to a further preferred embodiment of the present invention, the mixture for producing the TPS contains epoxy to polyol in a ratio of 1:2 to 1:8, preferably 1:4 to 1:6, and particularly preferably 1:5. In the range of 1:2 to 1:8, TPS processing is good (pressure, torque, and cutability of the melt for the production of granules), and an increase in bulk density is noticeable. A ratio of 1:5 ultimately fulfills all the required properties of the film, namely a tensile strength > 10 MPa and an elongation > 300%.

[0046] It is particularly preferred if the mixture for the production of the TPS further contains an acid, preferably a carboxylic acid selected from the group consisting of citric acid, malic acid, acetic acid, or tartaric acid, in an amount of 0.1 to 1, preferably 0.1 to 0.5 wt.% of the mixture. Such an acid acts both as an activating agent for the epoxy and as a processing aid, since it a) cleaves the branched chains of amylopectin and thus increases the proportion of linear molecules. The behavior of the polymer thus becomes more similar to that of classical thermoplastic materials. b) Furthermore, the addition of the acid causes depolymerization of the molecules at the glycosidic bond. The effect of changes in process conditions such as temperature, pressure, and residence time can thus be better predicted. Carboxylic acids such as citric acid, malic acid, acetic acid, or tartaric acid have proven suitable for this purpose.

[0047] In the process according to the invention, it is preferably provided that the mixture for the production of the TPS in the compound is extruded at a temperature of 100-175 °C, preferably in a twin-screw extruder and at reduced pressure in the final section of the extruder. Within the specified temperature range, the raw material is thermally stable during continuous processing. The twin-screw extruder enables the efficient destructuring of the starch (breaking down the crystallinity of the native starch) by forced conveying. Reduced pressure in the final section of the extruder is important for adjusting the water content of the TPS product; this affects further processing and should ideally be between 4-6 wt.%.

[0048] A thermoplastic starch obtainable by one of the methods disclosed above preferably has a bulk density of 70 to 85 g / 100 ml. This thermoplastic starch produced in this way is therefore significantly denser than a TPS produced without the use of an epoxy resin; see also the enclosed [document / reference]. Figure 1Reference is made to figures from which these differences are clearly visible. The determined bulk densities of the produced thermoplastic starches are also shown in the accompanying Figure 2. The invention also provides a compound containing either a conventional thermoplastic starch or a thermoplastic starch produced as described above, extruded with at least one thermoplastic polymer and an alpha-hydroxycarboxylic acid (ROHCOOH), where R represents CH₂ or CH₃CH, in an amount of 0.15 to 5, preferably 0.1 to 3, particularly preferably 0.1 to 1 wt.% based on the thermoplastic starch. If these compounds are subjected to a heating step to 100-140 °C, preferably 120-140 °C, for at least 15 minutes, preferably at least 30 minutes, particularly preferably at least 60 minutes during or after their production, they can be directly used for further processing, e.g.The compound can be used on the film production line, resulting in transparent films. Alternatively, such a compound, containing either a conventional thermoplastic starch or a thermoplastic starch produced as described above, extruded with at least one thermoplastic polymer and an alpha-hydroxycarboxylic acid (ROHCOOH), where R represents CH₂ or CH₃CH, in an amount of 0.1 to 5, preferably 0.1 to 3, particularly preferably 0.1 to 1 wt.% based on the thermoplastic starch, can also be used on a film production line without a heating step. In such a case, however, the blown film produced from such compounds must then be subjected to the aforementioned heating step at 100–140 °C, preferably at 120–140 °C, for at least 2 minutes, preferably at least 5 minutes, particularly preferably at least 60 minutes. Only through this heating step is a transparent film obtained.

[0049] As previously explained, a TPS produced as described above is particularly advantageous in the compound for manufacturing a transparent film by blown or flat film extrusion. Surprisingly, it has been found that the smoking that is practically unavoidable when using a TPS known from the prior art no longer occurs during the production of such a film.

[0050] The above-mentioned mixtures with their individual components are processed into a thermoplastic melt in the extruder under temperature and shear stress. EXAMPLES:

[0051] The present invention will now be explained in more detail with reference to the following examples and figures. Unless otherwise stated, percentages and ratios are always relative to mass.

[0052] Figure 1The improvement in transparency is shown using a film made of glycerin-TPS / PBAT 1:1, from left to right: untreated, untreated but with the addition of lactic acid, with the addition of lactic acid, and after the heating step provided according to the invention.

[0053] In the following trials, corn starch was fed into an extruder as the starting material using a solid dosing system. Stearic acid (1 wt%) was added to improve processability (reduce torque). The mixture was processed in a twin-screw extruder at a temperature profile of 100–130 °C and a rotational speed of 250 rpm, and granulated at the die plate by hot blowing. The resulting material is water-soluble and can be incorporated as a finely dispersed TPS (dispersed phase) into, for example, polyester melts (continuous phase) via a separate extrusion step. The thermoplastic starch is compounded with polybutylene adipate terephthalate (PBAT) to form a polyester in a 1:1 ratio in a twin-screw extruder. Suppliers:

[0054] Sorbitol, Glycerin, Stearic Acid - Brenntag, AT DL-Lactic Acid - Sigma Aldrich PBAT - BASF ESBO - Hobum, AT Citric Acid - Jungbunzlauer, AT Machine types:

[0055] Extrusion (TPS and Compound): Theysson TSK 30, 28D, 7-zone blown film line: OCS BFT400V3

[0056] The opacity of the pure carrier polymer, such as pure polyester (as a continuous compound phase), is used as the threshold for the increase in transparency provided for in the invention. To explain this, Table 1 below shows an opacity comparison of a film consisting of pure polybutylene adipate terephthalate (PBAT, Ecoflex) with a film made of a mixture of PBAT and glycerol-plasticized TPS (mixture 1:1) and with a film made of a mixture of PBAT and lactic acid-additized glycerol-plasticized TPS (mixture 1:1): Table 1: Comparison of opacity of film materials without thermal treatment Comparison PBAT film (65 µm thick) reference Film PBAT / Glycerin-TPS 1:1 (40 µm) PBAT / Glycerin-TPS1:1 film, TPS 5% with lactic acid (50 µm) Absorption (wavelength 550 nm) 0,30 1,02 0,36 Conversion with reference to film thickness*) OPACITY 4,60 25,38 7,20 *) The relationship between absorption and layer thickness via the Lambert-Peer extinction coefficient was verified in the following experiment: A = log 10 I 0 I = ε ⋅ l ⋅ c (where ε = extinction coefficient, I = layer thickness, c = concentration → the opacity-causing factor in this case is the starch - since the starch content was kept constant in the experiments, the factor c is neglected or not considered separately). Table 2: Influence of film thickness on ε·c Sample thickness of film PBAT / Glycerin-TPS 1:1 with lactic acid (mm) = I Absorption = A Comparison ε·c 0,050 0,360 7,2 0,100 0,720 7,2 0,150 1,080 7,2 0,200 1,420 7,1

[0057] Table 3 below shows the respective results after the thermal treatment of the films according to the invention at 130 °C for a duration of 15 minutes: Table 3: Comparison after thermal treatment of films at 130 °C for 15 minutes Comparison PBAT film (65 µm thick) reference Film PBAT / Glycerin-TPS 1:1 (40 µm) PBAT / Glycerin-TPS film 1:1, TPS with 5% lactic acid (50 µm) Absorption (wavelength 550 nm) 0,16 0,46 0,12 Conversion with reference to film thickness*) OPACITY 2,42 11,38 2,46

[0058] Table 4 shows the properties of film materials (before and after thermal treatment) based on compounds consisting of PBAT and various thermoplastic thicknesses, the difference being in the plasticizer used for TPS production. Table 4: Material properties of film materials based on TPS and BASF's Ecoflex polyester (compounded 1:1), whereby different plasticizers in comparable proportions (13 wt. % of the substance listed in the table in combination with 4 wt. % solid sorbitol) were used in the production of the TPS - the described "treatment" refers to heating the produced films at 130 °C for a duration of 15 minutes Plasticizers Opacity before treatment Opacity after treatment Glycerin 25,38 11,38 Xylitol 27,08 10,43 Sorbitol 21,17 16,72

[0059] Table 5 shows film materials containing 30% TPS (glycerin-plasticized and water-based). It can be seen that the transparency effect also occurs with water-based plasticization (an additional plasticizer is not absolutely necessary to achieve the effect). Table 5: Films produced using water-plasticized and glycerin-plasticized TPS (30% TPS in the mixture), treatment at 130°C for 15 minutes. Plasticizers Opacity before treatment Opacity after treatment Glycerin 18,93 10,76 Water 8,09 3,30

[0060] It is evident that the opacity can be reduced by thermal treatment and, depending on the plasticizer used, approaches the opacity or transparency achievable on the reference film (pure PBAT, opacity untreated = 4.6; opacity treated = 2.42).

[0061] In the following examples concerning the production of a preferred TPS, native starch (native corn starch, Maisita 21000) was mixed with a plasticizer (10-25 wt.%), acid (0.1-1 wt.%), and, only in the preferred examples, an epoxidized vegetable oil (0.1-6 wt.%) in a single-stage extrusion process, digested, and plasticized. For this purpose, the TPS was produced in a twin-screw extruder with vacuum degassing; all additives were added directly to the extrusion process via appropriate dosing units. Processing took place in the temperature range between 100 and 160 °C (a strong browning is noticeable above 160 °C).

[0062] The plasticizer can be supplied in either solid or liquid form, and it is also possible to split the addition (i.e., adding it partly in solid and partly in liquid form). The oil component is added untreated in liquid / pumpable form. The extrudates produced are suitable for further processing into compounds according to the invention (e.g., in combination with polyesters). Based on the compounds and the addition of an alpha-hydroxycarboxylic acid (ROHCOOH), where R means CH₂ or CH₃CH (preferably lactic acid), in an amount of 0.1 to 5, preferably 0.1 to 3, particularly preferably 0.1 to 1 wt. % based on the thermoplastic starch, and a heating step to 100–140 °C for at least 15 minutes, preferably at least 30 minutes, particularly preferably at least 60 minutes (compound), respectively, is carried out either during or after the production of the compound or after the production of a blown film from the compound.During at least 2 minutes, preferably at least 5 minutes, particularly preferably at least 60 minutes (film), it is possible to produce transparent end products such as transparent film materials.

[0063] The use of plasticizers other than glycerin without the addition of epoxidized vegetable oil leads to a loss in the mechanical properties of the material. The exclusive substitution of glycerin with plasticizers such as sorbitol, isosorbide, or xylitol in a TPS is therefore not effective and, in the case of film materials based on TPS and polymer, demonstrably leads to a loss in the achievable mechanical properties of the material. Of course, according to the invention, a TPS produced using glycerin and without the addition of epoxidized vegetable oil can also be used; in fact, any TPS can be used as long as the invention includes the addition of an alpha-hydroxycarboxylic acid (ROHCOOH), where R represents CH₂ or CH₃CH (preferably lactic acid), in an amount of 0.1 to 5, preferably 0.1 to 3, and particularly preferably 0.1 to 1 wt.% based on the thermoplastic thickness is provided, and either during or after production of the compound or after production of a blown film from the compound, the heating step to 100-140 °C is carried out for at least 15 minutes, preferably at least 30 minutes, particularly preferably at least 60 minutes (compound), or for at least 2 minutes, preferably at least 5 minutes, particularly preferably at least 60 minutes (film).

[0064] Only with the addition of an alpha-hydroxy carboxylic acid ROHCOOH, where R CH 2 or CH 3 CH (preferably lactic acid), in the specified quantity and with the provision of the mentioned heating step, can blown films with a surprising transparency be produced.

[0065] The table below shows the influence of the alpha-hydroxy acid concentration, in this case the lactic acid concentration, on the opacity of the described films: Table 6: Changes in opacity with increasing lactic acid content - after thermal treatment of the films at 130 °C for 15 minutes Comparison PBAT film (65 µm thick) reference Film PBAT / Glycerin-TPS 1:1 (40 µm) PBAT / Glycerin-TPS film 1:1, TPS with 1% lactic acid (50 µm), required according to... PBAT / Glycerin-TPS film 1:1, TPS with 3% lactic acid (50 µm), required according to... PBAT / Glycerin-TPS film 1:1, TPS with 5% lactic acid (50 µm), required according to... Absorption (wavelength 550 nm) 0,157 0,455 0,232 0,166 0,123 Conversion with reference to film thickness*) OPACITY 2,420 11,375 4,640 3,320 2,460

[0066] According to the present invention, it has surprisingly been shown that the thermal treatment of a compound containing an alpha-hydroxy carboxylic acid (compared to an untreated compound) also results in a reduction in the opacity or an increase in the transparency of a film produced from the compound: Table 7: Transparency values ​​of films whose compounds were only thermally treated before production (130 °C for one hour) Comparison PBAT / sorbitol-TPS film 1:1, TPS with 5% lactic acid, (36 µm) - untreated compound PBAT / sorbitol-TPS 1:1 foil, TPS with 5% lactic acid, (43 µm) - compound treated Absorption (wavelength 550 nm) 0,552 0,427 Conversion with reference to film thickness*) OPACITY 15,333 9,930

[0067] In a preferred embodiment of the present invention, the addition of epoxidized vegetable oils (e.g., epoxidized linseed oil (ELO), epoxidized sunflower oil, epoxidized rapeseed oil or epoxidized soybean oil (ESBO) and mixtures thereof) during the production of the TPS, even when using, for example, sorbitol, leads to the incorporation / mixing of the plasticizer into the TPS. Table 8: Film based on TPS, modified with 3% ESBO, 0.1% citric acid and 3% lactic acid in a 1:1 compound with PBAT - the thermal treatment was carried out at 130 °C for a duration of 15 minutes (film thickness 75 µm) Untreated foil film treated OPACITY 5,61 2,97

[0068] The activation of the epoxide functionality in epoxidized vegetable oils is promoted by the addition of acids. Carboxylic acids (ideally produced sustainably) such as citric acid, tartaric acid, acetic acid, itaconic acid, malic acid, or lactic acid can be used for this activation. Analysis methods: Determining film thickness using a commercially available micrometer

[0069] Apart from the increased transparency (or reduced opacity), the superior material properties of films produced from TPS or compounds manufactured according to the invention are also evident in their elongation: >300% at a tensile strength of >10 MPa. In the process according to the invention, a TPS content of 50 wt.% and above can be used (a TPS content of 50 wt.% was used for the experiments described above). Determination of opacity:

[0070] Direct insertion of the foils into the spectrometer's beam path and measurement in the visible range (wavelength 300-900 nm). Evaluation of the measurement result based on the absorption measured at a wavelength of 550 nm in relation to the foil thickness.

[0071] The improved transparency or reduced opacity is reflected in a reduction of the parameter ε·c to a value of < 10 (see optical comparison in the figures) at a TPS content of 50% in the film (with a minimum content of 35% pure starch).

Claims

1. Method for producing a compound or a film containing thermoplastic starch, an alpha-hydroxycarboxylic acid ROHCOOH, wherein R is CH2 or CH3CH, preferably lactic acid, in an amount of 0.1 to 5, preferably 0.1 to 3, particularly preferably 0.1 to 1 % by weight in relation to the thermoplastic starch, and a thermoplastic polymer, in which method the compound or the film is subjected to an additional heating step to 100-140°C after its extrusion.

2. Method according to claim 1, characterized in that the additional heating step after extrusion lasts at least 15 minutes, preferably at least 30 minutes, particularly preferably at least 60 minutes for the compound, and at least 2 minutes, preferably at least 5 minutes, particularly preferably at least 60 minutes for the film.

3. Method according to claim 1 or 2, characterized in that a polymer selected from the group comprising polyolefins, polyamides, polyurethanes, polyesters and mixtures thereof is used as thermoplastic polymer.

4. Method according to any one of claims 1 to 3, characterized in that for producing the thermoplastic starch, a mixture of starch with a polyol, preferably selected from the group comprising polyethylene glycol, monosaccharides, sugar alcohols such as glycerine, sorbitol, erythritol, xylitol or mannitol and mixtures thereof, in an amount of 10 to 25 % by weight of the mixture, and an epoxide, selected from the group comprising epoxidized plant oils such as soybean oil, linseed oil, sunflower oil, rapeseed oil and mixtures thereof, in an amount of 0.1 to 6, preferably 1 to 4.5, particularly preferably 2.5 to 3.5 % by weight of the mixture, is extruded.

5. Method according to claim 4, characterized in that the mixture for producing the thermoplastic starch contains a polyol selected from the group consisting of sorbitol, erythritol, xylitol, mannitol and mixtures thereof, in an amount of 10 to 15 % by weight of the mixture.

6. Method according to claim 4 or 5, characterized in that the mixture for producing the thermoplastic starch contains, as polyol, sorbitol or erythritol in an amount of 10 to 15 % by weight of the mixture.

7. Method according to any one of claims 4 to 6, characterized in that the mixture for producing the thermoplastic starch contains the polyol in an amount of 13 to 15 % by weight of the mixture.

8. Method according to any one of claims 4 to 7, characterized in that the mixture for producing the thermoplastic starch contains epoxide to polyol in a ratio of 1 to 2 to 1 to 8, preferably 1 to 4 to 1 to 6, particularly preferably 1 to 5.

9. Method according to any one of claims 4 to 8, characterized in that the mixture for producing the thermoplastic starch further contains an acid, preferably a carboxylic acid selected from the group consisting of citric acid, malic acid, acetic acid or tartaric acid, in an amount of 0.1 to 1, preferably 0.1 to 0.5 % by weight of the mixture.

10. Method according to any one of claims 1 to 9, characterized in that the mixture for producing the thermoplastic starch is extruded at a temperature of 100-175°C, preferably in a twinscrew extruder with a separate vacuum zone in which the degassing takes place by applying negative pressure.

11. Compound produced by a method according to any one of claims 1 to 10 for use in the production of a transparent film.

12. Transparent film, produced by blow extrusion or flat film extrusion of a compound according to claim 11.