Biodegradable and waterproof shaped articles based on thermoplastic starch with lower retrogradation and improved mechanical properties

Abstract

The invention relates to a method for producing a biodegradable thermoplastic starch-based article intended to come into contact with foodstuffs. In this method, the semi-crystalline starch granules are transformed into a homogeneous and almost amorphous material, called thermoplastic starch, by the addition of plasticizers at high temperatures and under shear. Thermoplastic starch is modified with organic acid during melt processing to prevent the retrogradation of starch. Moreover, cellulose derivatives are used as the reinforcement filler of thermoplastic starch. The article is produced using hot-pressing and then coating by immersion in a waterproofing solution.

Claims

1. A method for producing a biodegradable thermoplastic starch-based article, comprising: (a) dissolving of an organic acid in water, and then mixing with polyol plasticizer and starch to obtain a homogeneous material; storing the mixture in a sealed plastic bag overnight so that starch could be soaked with plasticizer; adding a reinforcement material and second plasticizer to the premixed starch blend and then manual feeding of this mixture into a co-rotating twin-screw extruder (screw diameter (d)=20 mm, length: diameter (L/D) ratio=40:1) at a screw speed of 70 rpm; (b) hot-pressing of the extruded mixture at 140 degree C. to obtain a shaped article; (c) coating of the shaped article by immersion in a waterproofing solution.

2. The method of claim 1, wherein the biodegradable thermoplastic starch-based article comprises 55-65 parts by weight of the starch, 9-12 parts by weight of the polyol plasticizer, 6.5-13 parts by weight of water, 2-4 parts by weight of the organic acid, 3-10 parts by weight of the reinforcement material, 12-18 parts by weight of the second plasticizer.

3. The method of claim 1, wherein the starch is cornstarch.

4. The method of claim 1, wherein the organic acid is citric acid, acetic acid, ascorbic acid or a combination thereof.

5. The method of claim 1, wherein the polyol plasticizer is glycerol, sorbitol or a combination thereof.

6. The method of claim 1, wherein the reinforcement material is carboxymethyl cellulose, microcrystalline cellulose, or a combination thereof.

7. The method of claim 1, wherein the second plasticizer is a mixture of sodium alginate and glycerol.

8. The method of claim 1, wherein the starch is dried for 12 h at 80 degree C. to have water content less than 1%.

9. The method of claim 1, wherein the extrusion processing is performed under the following temperature profile along the extruder barrel (from feed zone to die): 85-95-110-115-105 degree C.

10. The method of claim 1, wherein the thermoplastic starch-based article is produced by the twin-screw extruder equipped with an efficient vacuum venting system to remove the water vapor.

11. The method of claim 1, the hot-pressing is performed at 140 degree C. and 2 MPa for 5 min.

12. The method of claim 1, wherein the waterproofing solution comprises a film forming substance in a mixture of solvents.

13. The method of claim 12, wherein the film-forming substance is nitrocellulose or polylactic acid.

14. The method of claim 12, wherein the solvents used in nitrocellulose coating are acetone and ethyl acetate.

15. The method of claim 12, wherein the solvents used in polylactic acid coating are dichloromethane and chloroform.

16. The method of claim 1, wherein the coating layer has a thickness of 15 to 50 micrometers.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows the effect of aging on the tensile strength of TPS and TPS composite films of Example 1.

(2) FIG. 2 shows the effect of aging on the elongation at break of TPS and TPS composite films of Example 1.

DETAILED DESCRIPTION OF THE INVENTION

(3) The first objective of the disclosure is to provide a thermoplastic starch-based article that has a low retrogradation rate during the storage time. This object is achieved by chemical modification of TPS with organic acids, and by the addition of retrogradation inhibitors such as sodium alginate, carboxymethyl cellulose and microcrystalline cellulose.

(4) The organic acid promotes the fragmentation of starch granules and destroys its intermolecular and intramolecular hydrogen bonds, thereby improving the interaction between starch and the plasticizer. As a result, the stronger hydrogen bonds of starch with the plasticizer make it more difficult to re-crystallize during the storage time of TPS. Moreover, the TPS processing in the presence of organic acid can decrease its melt viscosity, glass transition temperature (T.sub.g) and melting point (T.sub.m) and improve its fluidity.

(5) The increase in the degree of crystallinity of TPS with increasing storage time results in a decrease in the elongation at break and an increase in the tensile strength of the product (FIGS. 1 and 2). After storage for three months, the samples show higher tensile strength and lower elongation at break values as a result of the retrogradation process. This effect is much less pronounced in the case of the TPS composites containing sodium alginate and carboxymethyl cellulose (or microcrystalline cellulose). Reducing retrogradation by the addition of sodium alginate can be explained by the fact that it acts as a strong water binder, effectively depriving the starch chain of usable water for recrystallization and preventing retrogradation. In the presence of microcrystalline cellulose, the decreased recrystallization of TPS can be attributed to the strong starch-cellulose interactions and the limited mobility of starch chains. In the case of carboxymethyl cellulose, both mentioned conceptions can be regarded as being true.

(6) The present formulations utilize sodium alginate as a plasticizer and retrogradation inhibitor, and carboxymethyl cellulose and microcrystalline cellulose as a reinforcement and retrogradation inhibitor.

(7) The second objective of the disclosure is to provide a waterproof thermoplastic starch-based article.

(8) Regarding the TPS solubility in water, the coating technique is an effective and economic method to prevent contact of water with the starch. In the present invention, PLA and nitrocellulose are used as a coating due to their hydrophobicity and biodegradability. The results show that nitrocellulose coating has higher adhesion strength compared to PLA coating. The high interfacial adhesion between nitrocellulose and TPS which is attributed to excellent compatibility and interaction between the nitrocellulose and TPS surface, improves the water resistance of TPS. Moreover, this coating provides a smoother, glossier, or scuff-resistant surface.

(9) The present disclosure provides a method for producing the biodegradable articles made from natural substances instead of petroleum-derived plastics used in food packaging. Overall migration tests are performed to assess whether the TPS articles coated with nitrocellulose can be in contact with food. The total amount of migrated substances is then compared to the limit given in Regulation (EU) No 10/2011 and FDA Regulation 21 CFR175.300. The test conditions selected for the experiments comply with regulations of single-use plastic tableware. All the results from these biodegradable samples are below the overall migration limit (Table 1). Therefore, it can be concluded that this material is allowed for food contact applications.

(10) The shaped articles provided herein meet the requirements of Commission Regulation (EU) No 10/2011 (plastic materials and articles intended to come into contact with food) and FDA Regulation 21 CFR175.300 (Indirect Food Additives: Adhesives and Components of Coatings). Furthermore, these products are non-toxic and eco-friendly with a low retrogradation rate during storage and good biodegradation performance.

(11) The biodegradable thermoplastic starch-based article can comprise of 55-65 parts by weight of the starch, 9-12 parts by weight of the polyol plasticizer, 6.5-13 parts by weight of water, 2-4 parts by weight of the organic acid, 3-10 parts by weight of the reinforcement material, 12-18 parts by weight of the second plasticizer.

EXAMPLES

Example 1: A Method for Producing the TPS Composite Includes the Following Steps

(12) The citric acid (2 parts by weight) was firstly dissolved in water (7 parts by weight) and then mixed with one-half of the glycerol (10 parts by weight). Next, the cornstarch (59 parts by weight), which was previously dried for 12 h at 80 degree C. to have water content less than 1%, was gradually added and thoroughly mixed by using a high-speed mixer. The mixture was sealed in a plastic bag and stored overnight so that starch could be soaked with plasticizer. Sodium alginate (4 parts by weight) and the remainder of the glycerol (10 parts by weight) were mixed (named second plasticizer), and then blended with the premixed starch and microcrystalline cellulose (8 parts by weight). Afterward, reactive extrusion processing was performed by manual feeding of this mixture into a co-rotating twin-screw extruder (screw diameter (d)=20 mm, length: diameter (L/D) ratio=40:1) equipped with an efficient vacuum venting system to remove the water vapor. The screw speed was adjusted to 70 rpm. The temperature profile along the extruder barrel was 85-95-110-115-105 degree C. (from feed zone to die). The extruded filaments were air cooled and granulated using a blade grinder equipped with a nominal internal diameter of 2 mm. Finally, the TPS composite granules were hot-pressed for 5 min at 140 degree C. under a load of 2 MPa in order to produce TPS composite films with the approximate thickness of 0.8 mm. Subsequently, these films were immersed in a waterproofing solution, and then put into a hot-air oven at 40 degree C. for 4 h to evaporate the residual solvent. The waterproofing solution was comprised of a film-forming substance in a mixture of solvents. The solvent used in nitrocellulose coating was a mixture of acetone and ethyl acetate (70/30, v/v). The solvent used in PLA coating was a mixture of chloroform and dichloromethane (50/50, v/v). The concentration of the coating solution was fixed at 5% w/v to investigate the interfacial adhesion of the coating. The thickness of the coating layer was 15 to 50 micrometers. The overall migration values of TPS composite coated with nitrocellulose are presented in Table 1. The values were in the range of 0.41-4.87 mg/dm.sup.2.

(13) To analyze the effect of storage time on TPS retrogradation, the uncoated samples for mechanical testing were stored in closed containers at 23 degree C. with a relative humidity of 50% for three months. The measurement was repeated three times for each sample, and the results were averaged. In order to compare the results, the glycerol-plasticized starch (with a starch/glycerol ratio of 75/25 (% w/w)) was also produced. The changes in tensile strength and elongation at break with time are shown in FIGS. 1 and 2, respectively.

(14) TABLE-US-00001 TABLE 1 Overall migration of TPS composite coated with nitrocellulose under standardized testing conditions Contact time Overall Migration and migration limit Food simulant temperature Standard (mg/dm.sup.2) (mg/dm.sup.2) Distilled water 30 min at FDA 0.57 7.75 100 degree Regulation 21 C. CFR175.300 Distilled water 2 h at 66 FDA 0.41 7.75 degree C. Regulation 21 CFR175.300 Ethanol 8% 2 h at 66 FDA 4.01 7.75 (v/v) degree C. Regulation 21 CFR175.300 Acetic acid 3% 2 h at 70 Commission 3.42 10 (w/v) degree C. Regulation (EU) No 10/2011 Ethanol 10% 2 h at 70 Commission 4.87 10 (v/v) degree C. Regulation (EU) No 10/2011

Example 2

(15) The acetic acid (3 parts by weight) was firstly dissolved in water (10 parts by weight) and then mixed with one-half of the glycerol (9 parts by weight). Next, the cornstarch (57 parts by weight), which was previously dried for 12 h at 80 degree C. to have water content less than 1%, was gradually added and thoroughly mixed by using a high-speed mixer. The mixture was sealed in a plastic bag and stored overnight so that starch could be soaked with plasticizer. Sodium alginate (3 parts by weight) and the remainder of the glycerol (9 parts by weight) were mixed (named second plasticizer), and then blended with the premixed starch and carboxymethyl cellulose (9 parts by weight). The other steps were similar to Example 1.

Example 3

(16) The citric acid (2 parts by weight) was firstly dissolved in water (11 parts by weight) and then mixed with one-half of the glycerol (9 parts by weight). Next, the cornstarch (61 parts by weight), which was previously dried for 12 h at 80 degree C. to have water content less than 1%, was gradually added and thoroughly mixed by using a high-speed mixer. The mixture was sealed in a plastic bag and stored overnight so that starch could be soaked with plasticizer. Sodium alginate (4 parts by weight) and the remainder of the glycerol (9 parts by weight) were mixed (named second plasticizer), and then blended with the premixed starch and microcrystalline cellulose (4 parts by weight). The other steps were similar to Example 1.

Example 4

(17) The citric acid (3 parts by weight) was firstly dissolved in water (9 parts by weight) and then mixed with sorbitol (6 parts by weight) and one-half of the glycerol (6 parts by weight). Next, the cornstarch (60 parts by weight), which was previously dried for 12 h at 80 degree C. to have water content less than 1%, was gradually added and thoroughly mixed by using a high-speed mixer. The mixture was sealed in a plastic bag and stored overnight so that starch could be soaked with plasticizers. Sodium alginate (6 parts by weight) and the remainder of the glycerol (6 parts by weight) were mixed (named second plasticizer), and then blended with the premixed starch and carboxymethyl cellulose (4 parts by weight). The other steps were similar to Example 1.