Thermoplastic starch

Abstract

The invention relates to a method for producing thermoplastic starch, in which a mixture comprising starch and a polyol, preferably selected from the group comprising polyethylene glycol, monosaccharides, sugar alcohols such as glycerol, sorbitol, erythritol, xylitol or mannitol and mixtures thereof, in a quantity of between 10 and 25 wt. % of the mixture, and an epoxide selected from the group comprising epoxidised plant oils such as soybean oil, linseed oil, sunflower oil, rapeseed oil and mixtures thereof, in a quantity of between 0.1 and 6, preferably between 2.5 and 3.5 wt. % of the mixture, is extruded, the mixture also containing an acid, preferably a carboxylic acid selected from the group consisting of citric acid, malic acid or tartaric acid, in a quantity of between 0 and 1, preferably between 0.1 and 0.5 wt. % of the mixture. The invention also relates to such a produced thermoplastic starch, to a compound produced by means of the thermoplastic starch, and to a film produced from such a compound.

Claims

1. A method for producing a thermoplastic starch, the method comprising extruding a mixture including: starch, a polyol in a quantity of between 10 and 25 wt. % of the mixture, an epoxidized plant oil selected from a group consisting of soybean oil, linseed oil, sunflower oil, rapeseed oil and mixtures thereof, in a quantity of between 0.1 and 6 wt. % of the mixture, and a carboxylic acid selected from a group consisting of citric acid, malic acid and tartaric acid in a quantity of between 0.1 and 1 wt. % of the mixture.

2. The method according to claim 1, wherein the polyol is one selected from a group consisting of polyethylene glycol, monosaccharides, sugar alcohols and mixtures thereof.

3. The method according to claim 1, wherein the quantity of the epoxidized plant oil in the mixture is between 1 and 4.5 wt. %.

4. The method according to claim 1, wherein the quantity of the carboxylic acid in the mixture is between 0.1 and 0.5 wt. %.

5. The method according to claim 1, wherein the polyol is one selected from a group consisting of sorbitol, erythritol, xylitol, mannitol and mixtures thereof, and the quantity of the polyol in the mixture is between 10 to 15 wt. %.

6. The method according to claim 5, wherein the polyol is one selected from a group consisting of sorbitol and erythritol, and the quantity of the polyol in the mixture is between 10 to 15 wt. %.

7. The method according to claim 1, wherein the quantity of the polyol is between 13 to 15 wt. %.

8. The method according to claim 1, wherein the mixture has an epoxide to polyol ratio of between 1:2 to 1:8.

9. The method according to claim 1, wherein the mixture further includes additional lactic acid.

10. The method according to claim 1, wherein the mixture is extruded at a temperature of 100 to 175° C.

11. A thermoplastic starch obtainable by a method according to claim 1.

12. The thermoplastic starch according to claim 11, which has a bulk density of 70 to 85 g/100 ml.

13. A compound containing the thermoplastic starch according to claim 11, extruded with at least one thermoplastic polymer.

14. The compound according to claim 13, wherein the thermoplastic polymer is one selected from a group consisting of polyolefins, polyamides, polyurethanes, polyesters and mixtures thereof.

15. A film produced by at least one of blown extrusion and flat film extrusion of the compound according to claim 13.

16. The method according to claim 10, wherein the extruding occurs in a twin-screw extruder with a separate vacuum zone in which degassing takes place by application of a vacuum.

17. The method according to claim 9, wherein the quantity of the epoxidized plant oil in the mixture is between 2.5 and 3.5 wt. %.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1: TPS with addition of acid and a) 0%, b) 2%, c) 4%.

(2) FIG. 2: Polarisation for 3, 5 and 6% ELO—magnified 100 times.

(3) FIG. 3: Polarisation for 3, 5 and 6% ESBO—magnified 100 times.

(4) FIG. 4: Smoking behaviours of the films.

(5) FIG. 5: Bulk density of the TPS according to the invention depending on the amount of ELO.

DETAILED DESCRIPTION

(6) The present invention will now be explained in greater detail by means of the following examples.

(7) TABLE-US-00001 TABLE 1 Material properties of film materials based on TPS and the polyester Ecoflex from BASF, DE (compounded 1:1), wherein in the case of TPS, different plasticisers were used in comparable proportions (in each case 13 wt. % of the substance listed in the table in combination with 4 wt. % solid sorbitol) Tensile strength Extensibility Tear resistance Plasticiser (MPa) (%) (N/mm) Glycerol 10.5 540.0 87 Xylitol 14.1 155.0 78 Sorbitol 12.0 56.0 62 Isosorbide 9.2 31.0 59

(8) Both in the comparative examples mentioned in Table 1 and in the following examples according to the invention, native starch (native maize starch, Maisita 21000) was mixed with a plasticiser (10-25 wt. %), acid (0.1-1 wt. %) and, of course only in the examples according to the invention, an epoxidised plant oil (0.1-6 wt. %) in a one-step extrusion process, broken down and plasticised. For this purpose, the TPS was produced in a twin-screw extruder with vacuum degassing; all additives are added directly to the extrusion method via appropriate metering units. Processing took place in a temperature range between 100 and 160° C. (a strong brown colouring may be seen above 160° C.).

(9) The plasticiser may be presented in both solid and liquid form, and it is also possible to split the addition (i.e. addition partly in solid and partly in liquid form). The oil component is added untreated in liquid/pumpable form.

(10) Table 1 above shows that the use of plasticisers other than glycerol without the addition of epoxidised plant oil leads to a deterioration of mechanical properties. The exclusive substitution of glycerol by plasticisers such as sorbitol, isosorbide or xylitol in a TPS is therefore not appropriate and, in the case of film materials based on TPS and polymer, has been shown to lead to losses in terms of the achievable mechanical material properties.

(11) Only the addition of epoxidised plant oils (for example epoxidised linseed oil (ELO), epoxidised sunflower oil, epoxidised rapeseed oil or epoxidised soybean oil (ESBO) and mixtures thereof), as provided for in accordance with the invention, leads to the incorporation/mixing of the plasticiser in the TPS, even when sorbitol is used for example. An improvement of the phase compatibility in the compound, through the integration of the oil component in the TPS, subsequently leads to an increase in the mechanical material properties.

(12) The activation of the epoxide functionality in the epoxidised plant oils is promoted by the addition of acids. Preferably carboxylic acids (which ideally may be produced on a sustainable basis) such as citric acid, tartaric acid, acetic acid, itaconic acid, malic acid or lactic acid are used.

(13) Furthermore, the addition of epoxidised plant oil reduces the die pressure and thus inhibits the described expansion (see the attached drawings, from which the advantages of the method according to the invention can be clearly seen).

(14) As a side effect of the optional acid addition, the reduction of the relative molar mass of the TPS is observed (hydrolysis), which results in easier flowability (lower viscosity) and improves processability and miscibility with the polymer. The addition of process additives to optimise processability is permitted up to a proportion of 3 wt. % (for example fatty acids such as palmitic acid, myristic acid, stearic acid or behenic acid). The starch types, by definition, may be used as starting raw materials, for example with a dry substance of ˜90 wt. %.

(15) The extrudates produced in accordance with the invention are suitable for further processing into compounds (for example in combination with polyesters). Only on the basis of the compounds is it possible to produce end products such as film materials.

(16) In the following examples A, the production of the TPS or compound will be described first. This is followed in examples B1 to B4 by an explanation of further processing into blown films based on TPS and polyester.

Example A

(17) Corn starch is fed into the extruder as the starting raw material by means of solids metering. Sorbitol (10-15 wt. %) is used as the plasticiser component. Stearic acid (1 wt. %) is used to improve processability (torque reduction). Acid and epoxidised plant oils include citric acid (0.1-0.5 weight %) and epoxidised oil (3, 5 and >6 weight %). The mixture is processed using a temperature profile in the range 100-130° C. and at a speed of 250 rpm in a twin-screw extruder and pelletised at the die plate by means of hot die-face pelletising. The resulting material is water-soluble and may be incorporated as finely distributed TPS (disperse phase) via a separate extrusion step, for example in polyester melts (continuous phase).

(18) The thermoplastic starch from Example A1 is compounded together with polybutylene adipate terephthalate (PBAT) as polyester in a ratio of 1:1 in a twin screw extruder.

(19) Suppliers:

(20) Sorbitol, glycerol, stearic acid—Brenntag, AT

(21) ELO, ESBO—Hobum, AT

(22) Citric acid—Jungbunzlauer, AT

(23) Machine types:

(24) Extrusion (TPS and Compound): Theysson TSK 30, 28D, 7 zones

(25) Blown film line: OCS BFT400V3

(26) TABLE-US-00002 TABLE 2 A1 - Influence of the added quantity of ELO for a product according to the invention with a plasticiser content of 13 wt. % (sorbitol) with 0.1 wt. % citric acid ELO Polarisation Molar mass Particle (wt. %) cross (kDa) (μm) 0 0 1850 0.8 3 0 1070 1.4 5 7 1270 5.4 >6 8 1310 11.7

(27) It is shown that the breakdown is impaired at higher ELO concentrations. At an ELO content of 6 wt. % and above, a saturation of the system was observed (oily film on the extrudates). A reduction in the shear effect due to the ELO addition increases the size of the dispersed TPS particles.

(28) FIG. 2 shows the polarisation at 3, 5 and 6% ELO magnified 100 times.

(29) TABLE-US-00003 TABLE 3 A2 - Influence of the quantity of sorbitol added for a product according to the invention with an ELO content of 3 wt. % with 0.1 wt. % citric acid Sorbitol Polarisation Molar mass Particle (wt. %) cross (kDa) (μm) 13 0 1070 1.4 14 0 1340 3.5 15 0 1490 1.4

(30) An increase in the plasticiser content causes an increase in the relative molar mass.

(31) TABLE-US-00004 TABLE 4 A3 - Influence of the quantity of ESBO added for a product according to the invention with a plasticiser content of 15 wt. % (sorbitol) with 0.1% citric acid ESBO Polarisation Molar mass Particle (wt. %) cross (kDa) (μm) 0 0 2040 0.5 3 0 809 1 5 0 832 1.1 6 0 (but spherical 938 1.5 structures visible)

(32) In terms of the relative molecular mass, both the addition of ELO and also of ESBO results in a significant reduction at low oil concentrations. The effect decreases with increasing oil content. The TPS particle size increases with increasing oil concentration. In both cases, an oily surface of the extrudates was observed with 6 wt. % oil added. Thus, the additive could not be incorporated into the matrix—the system showed clear signs of saturation. Therefore, film processing of these TPS variants was abandoned. FIG. 3 shows the polarisation at 3, 5 and 6% ESBO magnified 100 times.

(33) TABLE-US-00005 TABLE 5 A4 - Influence of the quantity of citric acid (ZS) added for a product according to the invention with a plasticiser content of 15 wt. % (sorbitol) with ELO/ESBO (3 wt. %) ZS Polarisation Molar mass Particle (wt. %) cross (kDa) (μm) ELO 0 2 1370 1.3 0.1 0 1490 1.4 0.5 0 578 0.9 ESBO 0 2 798 1.7 0.1 0 809 1 0.5 0 564 1

(34) A significant reduction in the relative molecular mass of the TPS was observed with the addition of 0.5 wt. % acid.

Example B

(35) Based on the “A” test series, blown films were produced at a processing temperature of 170° C. During the course of this, the torque on the film extruder was recorded as an important criterion for processability.

(36) TABLE-US-00006 TABLE 6 Compounding of samples A1 - ALL NON-SMOKING ELO Torque Tensile strength Extensibility (wt. %) (Nm) (MPa) (%) 0 129 7.4 87 3 110 20.8 210 5 106 12.8 246

(37) In conjunction with the use of sorbitol as a plasticiser, it is noted that sorbitol does not generally smoke during blown film processing, however, sorbitol-based materials demonstrate poor mechanical material properties, although this may be significantly improved by adding ELO.

(38) TABLE-US-00007 TABLE 7 Compounding of samples A2 - ALL NON-SMOKING Sorbitol Torque Tensile strength Extensibility (wt. %) (Nm) (MPa) (%) 13 110 20.8 210 14 107 15.2 186 15 114 14.5 405

(39) TABLE-US-00008 TABLE 8 Compounding of samples A3 - ALL NON-SMOKING - 15% sorbitol ESBO Torque Tensile strength Extensibility (wt. %) (Nm) (MPa) (%) 0 138 12.3 78 3 129 11.7 307 5 120 10.6 324

(40) TABLE-US-00009 TABLE 9 Compounding of samples A4 - ALL NON-SMOKING - 15% Sorbitol Torque Tensile strength Extensibility (Nm) (MPa) (%) ZS/ELO (wt. %) 0 175 18.8 250.5 0.1 114 14.5 405.0 0.5 108 17.2 220.7 ZS/ESBO (wt. %) 0 154 13.6 156.0 0.1 129 11.7 307.0 0.5 116 18.8 247.1

(41) Additional tests with glycerol:

(42) TABLE-US-00010 TABLE 10 C1 - Influence of the quantity of ELO added for a product according to the invention with a plasticiser content of 17.0 wt. % (13.0 wt. % glycerol, solid sorbitol 4 wt. %) with 0.1 wt. % citric acid ELO Polarisation Molar mass Particle (wt. %) cross (kDa) (μm) 0 0 419 5.5 3 0 711 3.1 5 1 752 4.0 6 3 (but spherical 767 4.6 structures visible)

(43) As the results of Table 8 above show, the use of glycerol does not lead to the effects described for a combination with pure sorbitol for the product according to the invention. The molar mass already assumes lower values without the addition of ELO. The fineness of the dispersion of the TPS in the polyester matrix is hardly influenced by ELO. The saturation of the system is again also shown by an oily surface of the extrudates with an ELO addition of 6 wt. %.

(44) TABLE-US-00011 TABLE 11 Compounding of samples C1 - ALL SMOKING ELO Torque Tensile strength Extensibility (wt. %) (Nm) (MPa) (%) 0 97 10.5 540.0 3 91 9.1 411.7 5 89 9.8 410.1

(45) TPS based on glycerol shows a strong smoking effect during blown film processing despite the addition of ELO.

(46) Furthermore, no improvement in material strength was discernible after ELO addition. On the other hand, the extensibility of manufactured films assumed very high values already without ELO.

(47) Analysis methods: Checking the starch breakdown of the TPS (aim: transfer of the granular starch into a homogeneous melt): If an unground sample is available, it must first be ground as finely as possible using a laboratory mill. It is also important to ensure that samples that have already been ground should not be left to stand for too long, as they will quickly attract moisture and thus falsify the result of the dry substance. The dry substance is then determined. Then, the ground TPS is stirred 5.0 wt. % into dry substance. The demineralised water is placed in the beaker and the sample is carefully sprinkled in while stirring. The total weight (sample+demineralised water) is 200 g.

(48) Shearing parameters: 5 minutes at 1000 rpm with a 4 cm toothed disc stirrer.

(49) For analysis, two drops of the well-slurried sample are now applied to a microscope slide by means of a Pasteur pipette and examined under a transmitted light microscope with and without polarisation filter.

(50) Determination of the relative molecular mass by gel permeation chromatography: In the case of starches soluble in cold water, the water is provided and the starch stirred in. 0.75 g starch in dry substance (note exact weight) is stirred into approximately 30 ml ultrapure water. Only when the starch is completely swollen (dissolved) can the process proceed. Next, the pH value is adjusted to pH 7±0.5 using hydrochloric acid (0.1 N/1 N) or NaOH solution (0.1 N/1 N). The neutralised solution is then transferred quantitatively into a 100 ml volumetric flask and made up to the mark with ultrapure water. Subsequently, 2 ml of each sample is placed in a 15 ml Supelco tube; 3 ml DMSO and a magnetic stirring rod are added, and the mixture is heated to approximately 100-105° C. for 60 minutes and then cooled to RT with stirring. After cooling, approximately 1 ml of the samples are filtered through a 1 μm Puradisc 25GD Disposable Filter from Whatman (glass fibre filter) into a 1.5 ml analysis tube and sealed tightly. The solution must be thin, clear and homogeneous, i.e. no “clusters” should be visible. Samples that are still turbid after filtration or that are extremely difficult to filter may be centrifuged for 10 minutes at 13,000 rpm in an Eppendorf centrifuge.

(51) Incorporation of the TPS into the polymer matrix: Some granules of the compound to be examined are briefly dipped in liquid nitrogen, folded in baking paper and carefully broken into fragments with a hammer. The granule fractions are transferred to a glass beaker, coated with 1 M HCL, and stirred for 3 hours on a magnetic stirrer. Hydrochloric acid is then separated out through a folded filter, and the compounds released from starch are rinsed 2-3 times with demineralised water before drying. The moist compound fractions are scattered onto a petri dish and dried in a drying cabinet at 60° C. for one hour. The dried cryogenic fractions may now be examined by electron microscope.

(52) Evaluation of the smoking behaviour: Optical: The evaluation with regard to smoking/non-smoking may be carried out on the blown film line according to FIG. 4 by means of optical inspection. In the case of the evaluation with “smoking”, the development of “wafts of mist” could be seen on the film tube (see FIG. 4). Analytical: The finely ground sample is analysed with regard to the smoking behaviour by means of gas chromatography (headspace technique). During the course of the analysis, the sample is successively heated in and beyond the processing area. The smoking behaviour is evaluated qualitatively and quantitatively by means of mass spectrometry. If a loss of mass of >20 μg/mg sample is detectable after a measuring time of 5 minutes, the evaluation of “smoking” is given.

(53) Determination of the mechanical material properties: According to EN ISO 527-1/-2

(54) Determination of the bulk density: “loose bulk density”, determined by loose filling of a 100 ml measuring cylinder

(55) Specification window: 70-85 g/100 ml

(56) Starch breakdown: <10 polarisation crosses (preferably no polarisation crosses visible)

(57) Molar mass TPS: Mw=500-2000 kDa (preferably between 900 and 1600, better still between 1000 and 1500)

(58) Dispersion in the compound: preferably <5 μm (<20 μm—film-compatible but poor haptics, <10 μm—film-compatible but micro-roughness present).

(59) The superior material properties of films produced from TPS or compound produced in accordance with the invention are demonstrated by an extensibility: >300% at a tensile strength of >10 MPa—A TPS content of 50 wt. % and above may be used (a TPS content of 50 wt. % was used for the tests).