Multilayer films

Abstract

There is provided a multilayer film comprising a starch layer and at least one other layer. The multilayer film has excellent barrier properties. The starch layer comprises a modified starch having a degree of substitution less than 1.5. Suitable other layers include polyolefins. The multilayer film finds use in packaging, particularly in packaging foodstuffs.

Claims

1. A multilayer film comprising: (a) at least one starch layer comprising a modified starch; and (b) at least one other layer having a water vapour permeability coefficient less than 1 g.mm/m.sup.2.24hr.atm measured at 38° C. and 90% relative humidity; and wherein the total thickness of the at least one starch layer is in the range of from about 50 to about 150 microns and is greater than 20% of the total thickness of the multilayer film, wherein the modified starch has a degree of substitution less than 1.5; wherein the multilayer film has an oxygen permeability coefficient (OPC) that remains below 0.2 cm.sup.3 mm/m.sup.2.24h.atm at 75% relative humidity for at least 10 days, wherein the OPC is normalised to 1 mm sample thickness and is based on starch layer thickness.

2. The multilayer film according to claim 1, wherein the multilayer film has an oxygen permeability coefficient (OPC) that remain below 0.1 cm.sup.3 mm/m.sup.2.24h.atm at 75% relative humidity for at least 10 days.

3. The multilayer film according to claim 1, wherein the multilayer film has an oxygen permeability coefficient (OPC) less than 0.6 cm.sup.3 mm/m.sup.2.24h.atm at 50% RH.

4. The multilayer film according to claim 1, wherein the water vapour permeability coefficient of the at least one other layer is less than 0.5 g.mm/m.sup.2.24hr.atm.

5. The multilayer film according to claim 1, wherein the total thickness of the at least one starch layer is at least about 100 microns.

6. The multilayer film according to claim 1, wherein the at least one other layer comprises a polyolefin, polyethylene terephthalate, nylon, polyvinylchloride and polyvinylidene dichloride or mixtures thereof.

7. The multilayer film according to claim 6, wherein the polyolefin is selected from the group consisting of high density polyethylene, low density polyethylene, linear low density polyethylene, polypropylene, biaxially orientated polypropylene and mixtures thereof.

8. The multilayer film according to claim 1, wherein the multilayer film comprises an inner starch layer and two outer layers.

9. The multilayer film according to claim 1, wherein the at least one starch layer is fixed to the at least one other layer by an adhesive.

10. The multilayer film according to claim 1, wherein the modified starch comprises a high amylose starch.

11. The multilayer film according to claim 1, wherein the modified starch is chemically modified to include a hydroxyalkyl C.sub.2-6 group or modified by reaction with an anhydride of a carboxylic acid.

12. The multilayer film according to claim 1, wherein the at least one starch layer further comprises at least one water soluble polymer.

13. The multilayer film according to claim 12, wherein the at least one water soluble polymer is selected from the group consisting of polyvinyl alcohol and polyvinyl acetate and mixtures thereof.

14. The multilayer film according to claim 1, wherein the at least one starch layer further comprises at least one plasticiser.

15. The multilayer film according to claim 14, wherein the at least one plasticiser comprises one or more polyols.

16. The multilayer film according to claim 1, wherein the at least one starch layer comprises one or more nanomaterials.

17. The multilayer film according to claim 16, wherein the nanomaterials comprise clay or modified clay or both.

18. A method of making the multilayer film according to claim 1, wherein the method comprises at least one of the steps of co-extrusion, coating, casting or film blowing.

19. Packaging comprising the multilayer film according to claim 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) It will now be convenient to describe the invention with reference to particular embodiments and examples. These embodiments and examples are illustrative only and should not be construed as limiting upon the scope of the invention. It will be understood that variations upon the described invention as would be apparent to the skilled addressee are within the scope of the invention. Similarly, the present invention is capable of finding application in areas that are not explicitly recited in this document and the fact that some applications are not specifically described should not be considered as a limitation on the overall applicability of the invention.

(2) Polyolefins

(3) Suitable LLDPE, HDPE and polypropylene can be produced by a Ziegler, single-site, or any other olefin polymerization catalyst. Ziegler catalysts and co-catalysts are well known in the art. Metallocene single-site catalysts are transition metal compounds that contain cyclopentadienyl (Cp) or Cp derivative ligands. For example, U.S. Pat. No. 4,542,199, teaches the preparation of metallocene catalysts. Non-metallocene single-site catalysts containing heteroatomic ligands, e.g., boraaryl, pyrrolyl, azaborolinyl or quinolinyl are also well known in the art.

(4) The HDPE can also be multimodal. By “multimodal” it is meant that the polymer comprises at least two components, one of which has a relatively low molecular weight, the other a relatively high molecular weight. The multimodal polyethylene can be produced by polymerization using conditions that create a multimodal polymer product. This can be accomplished by using a catalyst system with two or more different catalytic sites or by using two or multi-stage polymerization processes with different process conditions in the different stages (e.g. different temperatures, pressures, polymerization media, hydrogen partial pressures, etc). Multimodal HDPE may be produced by a multistage ethylene polymerization, using a series of reactors, with comonomer addition in only one of the reactors.

(5) Modified Starch

(6) A preferred modified starch component is hydroxypropylated amylose starch. Other substituents may be hydroxyethyl or hydroxybutyl to form hydroxyether substitutions, or anhydrides such as maleic phthalic or octenyl succinic anhydride can be used to produce ester derivatives. The degree of substitution (the average number of hydroxyl groups in a unit that are substituted) is preferably 0.05 to 1.5. A preferred starch is a high amylose maize starch. Another preferred starch is a high amylose tapioca starch. A preferred modified starch component is a hydroxypropylated high amylose starch (for example ECOFILM® marketed by National Starch and Chemical Company, or Gelose® A939 marketed by Penford).

(7) The other starch component, if utilised, is any commercially available starch. This may be derived from, for example, wheat, maize, tapioca, potato, rice, oat, arrowroot, and pea sources. These starches may also be chemically modified.

(8) Water Soluble Polymer

(9) The water soluble polymer component of the starch layer is preferably compatible with starch, water soluble, biodegradable and has a low melting point compatible with the processing temperatures for starch. Polyvinyl alcohol is a preferred polymer but polymers of ethylene-vinyl alcohol, ethylene vinyl acetate or blends with polyvinyl alcohol may be used. A preferred concentration range 4 to 12% by weight, more preferably 8%-12%.

(10) Plasticiser

(11) A range of plasticisers and humectants are useful additions to the starch layer, in order to aid processing and control and stabilize the mechanical properties of the barrier material, in particular in reducing dependency on moisture content and relative humidity. The desired plasticiser content depends primarily on the required processing behaviour during a (co)-extrusion process and subsequent blowing or stretching processes as well as on the required mechanical properties of the end product.

(12) Cost and food contact are important issues in choosing the appropriate plasticizer. The preferred plasticizer is a mixture of polyols, in particular sorbitol, and one or more other polyols particularly glycerol, maltitol, mannitol and xylitol, although erythritol, ethylene glycol and diethylene glycol are also suitable. The plasticizer plays a triple role:

(13) 1. it provides suitable rheology for the extrusion compounding process and for the lamination process,

(14) 2. it positively affects the mechanical properties of the product and,

(15) 3. it may act as an anti-retrogradation or anti-crystallizing agent.

(16) The preferred plasticizer content is up to 20% by weight of the starch layer depending on the particular application and co-extrusion or lamination process.

(17) Sorbitol, glycerol and maltitol blends are particularly suitable for modifying the mechanical properties of the formulation, as is xylitol and blends of xylitol with sorbitol and glycerol. The larger the number of OH groups, the more effective the plasticiser is in reducing crystallisation. Sorbitol, maltitol and xylitol are particularly good humectants. Glycerol helps dissolve polyvinylalcohol during processing. Crystallisation is observed when sorbitol is used on its own. Some polyols (sorbitol and glycerol in particular) may exhibit migration to the surface, where either an opaque crystalline film may form in the case of sorbitol, or an oily film in the case of glycerol. Blending various polyols inhibits this effect to varying degrees. Stabilisation may be enhanced with the addition of glycerol monostearate and sodium stearoyl lactylate as emulsifiers. Furthermore, synergistic effects with salt result in stronger effects on mechanical properties.

(18) Other Plasticizers

(19) Polyethylene glycol compounds may be used as emulsifying agents, plasticizers or humectants. Polyethylene oxide and polyethylene glycol alternately or together may also provide an increased water resistance, to prevent swelling which may result in delamination in multi-layer structures (MLS).

(20) An alternative plasticiser is epoxidized linseed oil or epoxidized soybean oil. Being hydrophobic these additives may improve moisture sensitivity of the material. These plasticisers, preferably stablilized with an emulsifying system, aid processing but do not result in a significant further reduction in Young's modulus. Other plasticizers more commonly used in the PVC industry may be suitable, including tributyl citrate, 2,2,4 trimethyl-1,3-pentanediol diisobutyrate, and acetyl tri-ethyl citrate.

(21) One may use up to 20% of a humectant or water binding agent or gelling agent which may act as a (co)plasticiser such as carrageenan, xanthan gum, gum arabic, guar gum or gelatine. Other humectants may be used such as sugar or glucose. Biopolymers such as carrageenan, typically used in food products as thickeners and partially soluble in cold water, fully soluble in hot water, are suitable to tailor mechanical properties. By binding water these components may have a significant plasticizing function. Gelatine may be added to improve the mechanical properties and reduce moisture sensitivity. Xanthan Gum has a high water holding capacity and also acts as an emulsifier and in starch compositions has an anti-retrogradation effect. Gum Arabic may also be used as a texturiser and film former, and the hydrophilic carbohydrate and hydrophobic protein enable its hydrocolloid emulsification and stabilization properties. Guar gum has similar anticrystallisation effects in starch compositions. Another suitable humectant is glyceryl triacetate.

(22) Fatty Acid and/or Fatty Acid Salt

(23) Fatty acids and/or fatty acid salts may be used as lubricants. The starch layer preferably comprises between 0.1 to 1.5% by weight of a C.sub.12-22 fatty acid and/or a C.sub.12-22 fatty acid salt. The fatty acid and/or fatty acid salt component is more preferably present in concentrations of 0.6 to 1%. Stearic acid is a particularly preferred component. Sodium and potassium salts of stearic acid may also be used. Cost can be a factor in the choice of this component but lauric, myristic, palmitic, linoleic and behenic acids are all suitable.

(24) Adhesive

(25) Polyurethane based adhesives are particularly suitable for fixing the other layer to the starch layer. The polyurethane adhesive may be prepared in situ through reaction of one or more isocyanates with the starch layer. Through reaction of the surface hydroxyl functions of the starch with isocyanate, urethane functions are formed. Preferred isocyanates are diisocyanates. Those skilled in the art would be able to select suitable isocyanates from the wide range typically employed in the art of polyurethane synthesis.

(26) Alternatively, the polyurethane adhesive may comprise one or more polyols. Such two component systems comprising diisocyanate and polyol are well known in the art.

(27) The adhesives may or may not contain solvent. The solvent may be organic or water based.

(28) Exemplary isocyanates include methylene diphenyl diisocyanate and toluene diisocyanate. Exemplary polyols include polyether polyols such as polyethylene glycol or polypropylene glycol and polyester polyols such as adipate based polyols.

EXAMPLES

(29) OTR was measured using ASTM F 1927-98 and WVTR was measured using ASTM F 1249-01. All component weights are expressed on a dry basis.

Example 1

(30) A starch film was prepared by extrusion processing of a mixture of 88.5% by weight modified starch (ECOFILM®, National Starch and Chemical Company), 9% by weight polyvinylalcohol (Elvanol® 71-30), 2% by weight Cloisite 20A (Southern Clay Products) and 0.5% stearic acid and casting into a 300 μm sheet. This was then adhesively laminated on each side to 100 μm HDPE film using MOR Free PU adhesive (Rohm and Haas). The lamination was performed on a standard laminating machine.

(31) Samples were conditioned for 2 weeks at 50% and 75% RH (for OTR) and 38° C./90% RH (for WVTR) and measured after equilibration.

(32) Tables 1 and 2 collect the results.

(33) TABLE-US-00001 TABLE 1 Nominal % Oxygen of starch Transmission Rate Specimen layer (cm.sup.3/m.sup.2 .Math. 24 h at 23° C., thickness Sample thickness 1 atm pure oxygen) (micron) PE/Starch/PE 60 50% RH <0.05 507 <0.05 470 75% RH <0.05 496 <0.05 468

(34) TABLE-US-00002 TABLE 2 Nominal % Water Vapour of starch Transmission Rate Specimen layer (g/m.sup.2 .Math. 24 h at thickness Sample thickness 38° C., 90% RH) (micron) PE/Starch/PE 60 3.3 507 100/300/100 3.2 470

Examples 2 & 3

(35) A starch film was prepared by extrusion processing of a mixture of 88.5% by weight modified starch (ECOFILM®, National Starch and Chemical Company), 9% by weight polyvinylalcohol (Elvanol® 71-30), 2% by weight Cloisite 20A (Southern Clay Products) and 0.5% stearic acid and casting into a 150 μm sheet. This was then adhesively laminated on each side to 50 μm (Example 2) or 35 μm (Example 3) HDPE using a polyurethane adhesive system from Specialty Adhesives and Coatings. The lamination was performed on a standard laminating machine.

(36) Samples were conditioned for 2 weeks at 50% and 75% RH (for OTR) and 38° C./90% RH (for WVTR) and measured after equilibration.

(37) Table 3 collects the results.

(38) TABLE-US-00003 TABLE 3 Water Vapour Oxygen Transmission Rate Nominal % Transmission Rate (g/m.sup.2 .Math. 24 h at of starch (cm.sup.3/m.sup.2 .Math. 24 h) 38° C., 90% RH) layer 50% RH, Thickness 75% RH, Thickness Thickness Sample thickness 23° C. (micron) 23° C. (micron) WVTR (micron) Example 2 60 0.55 258 0.91 265 3.00 260 PE/Starch/PE 0.46 262 0.98 254 3.22 255 Example 3 68 0.51 222 1.03 228 5.16 225 PE/Starch/PE 0.55 227 1.16 225 5.30 225

Example 4

(39) A starch film was prepared by extrusion processing of a mixture of 90.5% by weight modified starch (ECOFILM®, National Starch and Chemical Company), 9% by weight polyvinylalcohol (Elvanol® 71-30) and 0.5% by weight stearic acid and casting into a 350 μm sheet. This was then adhesively laminated on each side to 50 μm HDPE using a polyurethane adhesive system from Specialty Adhesives and Coatings. The lamination was performed on a standard laminating machine.

(40) Samples were conditioned for 2 weeks at 50% and 75% RH (for OTR) and 38° C./90% RH (for WVTR) and measured after equilibration.

(41) Table 4 collects the results.

(42) TABLE-US-00004 TABLE 4 Water Vapour Oxygen Transmission Rate Nominal % Transmission Rate g/m.sup.2 .Math. 24 h at of starch cm.sup.3/m.sup.2 .Math. 24 h 38° C., 90% RH layer 50% RH, Thickness 75% RH, Thickness Thickness Sample thickness 23° C. (micron) 23° C. (micron) WVTR (micron) PE/Starch/PE 78 0.05 465 0.15 465 3.25 472 0.05 468 0.16 455 3.10 468

Example 5

(43) A starch film was prepared by extrusion processing of a mixture of 90.5% by weight modified starch (ECOFILM®, National Starch and Chemical Company), 9% by weight polyvinylalcohol (Elvanol® 71-30) and 0.5% by weight stearic acid and casting into a 350 μm sheet. This was then adhesively laminated to 50 μm HDPE on one side, and an 80 μm polypropylene film on the other side using a polyurethane adhesive system from Specialty Adhesives and Coatings. The lamination was performed on a standard laminating machine.

(44) Samples were conditioned for 2 weeks at 50% and 75% RH (for OTR) and 38° C./90% RH (for WVTR) and measured after equilibration.

(45) The results are collected in Table 5.

(46) TABLE-US-00005 TABLE 5 Water Vapour Oxygen Transmission Rate Nominal % Transmission Rate g/m.sup.2 .Math. 24 h at of starch cm.sup.3/m.sup.2 .Math. 24 h 38° C., 90% RH layer 50% RH, Thickness 75% RH, Thickness Thickness Sample thickness 23° C. (micron) 23° C. (micron) WVTR (micron) PE/Starch/PP 73 PP side <0.05 484 0.11 495 2.21 498 facing <0.05 500 0.11 500 2.16 490 permeant PE side Not measured 0.10 500 Not measured facing 0.16 484 permeant

Comparative Example 1

(47) A starch film was prepared by extrusion processing of a mixture of 88.5% by weight modified starch (ECOFILM®, National Starch and Chemical Company), 9% by weight polyvinylalcohol (Elvanol® 71-30), 2% by weight Cloisite 20A (Southern Clay Products) and 0.5% stearic acid and casting into a 290 μm sheet.

(48) Samples were conditioned for 2 weeks at 50% and 75% RH, and OTR measured after equilibration. The results are collected in Table 6.

(49) TABLE-US-00006 TABLE 6 Oxygen Transmission Rate Specimen cm.sup.3/m.sup.2 .Math. 24 h at 23° C., thickness Sample 1 atm pure oxygen (micron) Starch Sheet 50% RH 0.21 283 0.21 289 75% RH 1.48 282 1.30 285

Comparative Example 2

(50) A starch film was prepared by extrusion processing of 100% by weight modified starch (ECOFILM®, National Starch and Chemical Company), and casting into a 300 μm sheet.

(51) Samples were conditioned for 2 weeks at 50% and 75% RH (for OTR) and 38° C./90% RH (for WVTR) and measured after equilibration.

(52) Table 7 shows the results.

(53) TABLE-US-00007 TABLE 7 Water Vapour Oxygen Transmission Rate Transmission Rate g/m.sup.2 .Math. 24 h cm.sup.3/m.sup.2 .Math. 24 h at 38° C., 90% RH 50% RH, Thickness 75% RH, Thickness Thickness Sample 23° C. (micron) 23° C. (micron) WVTR (micron) Starch Sheet 0.50 295 1.30 260 337 290 0.49 320 1.26 295 374 275

Summary of Examples

(54) Table 8 collects the OTR and OPV (oxygen permeation value) for each of the Examples. The OPV are normalised to 1 mm thick samples, based on the core starch layer thickness only.

(55) TABLE-US-00008 TABLE 8 OTR OPV OTR OPV Core Skin (50% RH) (50% RH) (75% RH) (75% RH) Thickness Thickness [cm.sup.3/m.sup.2 .Math. [cm.sup.3 .Math. mm/m.sup.2 .Math. [cm.sup.3/m.sup.2 .Math. [cm.sup.3 .Math. mm/m.sup.2 .Math. Example micron micron 24 h] 24 h .Math. atm] 24 h] 24 h .Math. atm] 1 300 100 <0.05 <0.05 <0.05 <0.05 2 150 50 0.51 0.08 0.95 0.14 3 150 35 0.53 0.08 1.10 0.16 4 350 50 0.05 0.02 0.16 0.06 5 350 50 <0.05 <0.05 0.13 0.05 CE1 290 0 0.21 0.06 1.40 0.41 CE2 300 0 0.50 0.15 1.30 0.40

(56) It is evident from the results that the multilayer films of Examples 1 to 5 show excellent barrier performance. It is noted that where the core starch layer is approximately 300 micron thick, OTR is substantially reduced in samples having outer layers, relative the performance of a starch layer alone. Thinner starch core layers have low OTR at high (75%) RH relative to starch alone. Starch layers alone, in the absence of outer layers, indicate very high WVTR.