Bi-axially stretched article

Abstract

The present invention relates to a biaxially stretched article obtained by stretching a thermoplastic composition in a machine direction and a transverse direction at elevated temperature, said thermoplastic composition comprising a polyolefin phase containing at least one polyolefin, a starch phase containing thermoplastic starch, and at least one compatibiliser, wherein the article has a layered morphology with alternating layers of starch phase and polyolefin phase, said layers of starch phase and polyolefin phase extending in machine direction and transverse direction.

Claims

1. A biaxially stretched article comprising: a thermoplastic composition comprising a polyolefin phase containing a polyolefin, a starch phase containing thermoplastic starch, and a compatibiliser, wherein the article is obtained by stretching the thermoplastic composition in a machine direction and a transverse direction at elevated temperature to provide a layered morphology with layers of starch phase alternating with layers of polyolefin phase, said layers of starch phase and polyolefin phase extending substantially continuously in the machine direction and the transverse direction, said layers of starch phase comprising less than 5 wt % polyolefin and said layers of polyolefin phase comprising less than 5 wt % starch.

2. The article according to claim 1 wherein the thermoplastic composition comprises from 10-70 wt % of the polyolefin, from 10-70 wt % of thermoplastic starch, and from 5-40 wt % of the compatibiliser the weight percentages being based on the weight of the thermoplastic composition.

3. The article according to claim 1 wherein the stretch ratio in machine direction is at most 20 and/or wherein the stretch ratio in transverse direction is at most 4.

4. The article according to claim 1 wherein the compatibiliser is selected from the group consisting of ethylene vinyl acetate copolymers, polyolefins having at least 1 wt % maleic anhydride grafted thereon, ethylene vinyl alcohol copolymers, ethylene acrylic acid copolymers, partially hydrolised and saponified polyvinylacetate, random terpolymers of ethylene, butylacrylate and maleic anhydride or mixtures of at least two of these compatibilisers.

5. The article according to claim 1 wherein the starch phase has an MFI of from 2-20 g/10 min measured at 130 C. under a load of 10 kg as measured in accordance with ISO 1133.

6. The article according to claim 1 wherein the polyolefin phase has an MFI of from 2-20 g/10 min measured at 130 C. under a load of 10 kg as measured in accordance with ISO 1133.

7. The article according to claim 1 wherein the ratio of the MFI of the polyolefin phase and the MFI of the starch phase, both measured at 130 C. under a load of 10 kg as measured in accordance with ISO 1133, is from 0.5-1.5.

8. The article according to claim 1 wherein the compatibiliser is a partially hydrolised and saponified polyvinylacetate.

9. The article according to claim 1 wherein the thermoplastic composition comprises a further compatibiliser being a polyolefin having at least 1 wt % maleic anhydride grafted thereon.

10. The article according to claim 1 wherein the article is a film having a thickness of from 2-250 m.

11. A multilayer film comprising the film according to claim 10 wherein said film is provided with at least one further synthetic film of a synthetic polymer extending in machine direction and transverse direction.

12. A method for producing an article according to claim 1 comprising providing a thermoplastic composition comprising a polyolefin, thermoplastic polyester, thermoplastic starch and a compatibiliser and feeding said thermoplastic composition to an extruder, extruding the thermoplastic composition through an extrusion die and stretching the thermoplastic composition upon exiting the extrusion die at elevated temperature in machine direction and transverse direction, or feeding a polyolefin or a mixture of two or more polyolefins, thermoplastic polyester, starch, a processing aid for making thermoplastic starch and a compatibiliser to an extruder, extruding these components under such conditions that a thermoplastic composition comprising a polyolefin, thermoplastic polyester thermoplastic, starch and a compatibiliser is formed in the extruder and stretching the thermoplastic composition upon exiting the extruder via an extrusion die at elevated temperature in machine direction and transverse direction.

13. The article according to claim 1, wherein the article is an oxygen and water vapour diffusion barrier layer.

14. The article according to claim 1 wherein the thermoplastic polyester is present in an amount of from 20 to 60 weight %.

15. The article according to claim 10 wherein the film has a modulus of at least 50 MPa as measured according to ASTM 882 and an elongation at break of at least 200% as measured according to ISO 527-3.

16. The article according to claim 14, wherein the thermoplastic polyester is poly(butylene terephthatate-co-adipate).

17. The article according to claim 1 having a stretch ratio in transverse direction of at least 1.5 the stretch ratio in transverse direction being defined as: ${\mathrm{SR}}_{\mathrm{td}}=\frac{W_1}{W_0}$ and a stretch ratio in machine direction of at least 2, the stretch ratio in machine direction being defined as: ${\mathrm{SR}}_{\mathrm{md}}=\frac{T_0}{T_1{\mathrm{SR}}_{\mathrm{td}}}$ wherein SR.sub.md =Stretch ratio in machine direction, SR.sub.td =Stretch ratio in transverse direction, W.sub.0 =Width of the thermoplastic composition before stretching in transverse direction, W.sub.1 =Width of the biaxially stretched article, T.sub.0 =Thickness of the thermoplastic composition before stretching in machine and transverse direction, and T.sub.1 =Thickness of the biaxially stretched article.

Description

(1) The present invention will now be further explained by the following non limiting Figures and Examples.

(2) FIG. 1 shows a model of the layered morphology of the article according to the present invention.

(3) FIG. 2 schematically shows a method for producing the article according to the present invention.

(4) FIGS. 3a to 3h shows SEM photographs of examples I to VIII respectively both in machine direction and transverse direction.

(5) FIG. 1 is a schematic representation of a biaxially stretched article 1 according to the present invention. The machine direction is indicated as MD, the transverse direction is indicated as TD and the thickness direction is indicated as Z. From FIG. 1 it is clear that the layers 2,3 extend in machine direction and transverse direction. The layers indicated with reference numeral 2 represent the polyolefin phase whereas the layers indicated with reference numeral 3 represent the starch phase. As can be clearly seen layers 2 and layers 3 alternate, i.e. the polyolefin phase layers 2 are stacked with the starch phase layers 3 in an alternating manner. The number of layers 2,3 predominantly depends on stretch ratios, thickness of the article and the thermoplastic composition. Article 1, not to scale, shows five layers 2,3 but the skilled person will understand that article 1 according to the present invention is not limited to such number of layers. In FIG. 1 the outer layers (i.e. the layers on the top and on the bottom of the stack) are shown to be layers of polyolefin. However, the present inventors have established that such outer layers may also be of thermoplastic starch.

(6) By means of electron microscopy the present inventors have established that the layer thickness (i.e. the thickness in Z direction) of the starch phase may be from 0.1-50 m, and the thickness of the polyolefin layers may be from 0.1-50 m. Preferably the layers are at most 20 m, more preferably at most 10 m.

(7) FIG. 2 schematically shows the stretching in machine and transverse direction for producing the article according to the present invention. Volume element 4 having a certain width in transverse direction, a certain length in machine direction (MD) and thickness in thickness direction is bi-axially stretched, meaning it is stretched in machine direction and transverse direction. As a consequence of the stretching in machine direction and transverse direction the thickness in thickness direction Z will decrease. Article 1 of the present invention is not a foam and consequently the density of volume element 4 remains substantially unchanged upon the stretching in machine and transverse direction. In the embodiment where the article of the present invention is manufactured by means of film blowing the stretch ratio in machine direction is equal to the ratio of take up speed of the blown film and the speed of the extrudate leaving the extrusion die. Such has been defined as ratio for draw down in U.S. Pat. No. 5,082,616 for example.

EXAMPLES I-XII

(8) An overview of the examples I-XII can be found in Table 1 below.

(9) FIGS. 3a to 3h show SEM photographs of Examples I to VIII in both machine direction (MD) and in transverse direction (TD).

(10) The polyester was Ecoflex F blend C1200, commercially available from BASF, having an MFI at 190 C. and 2.16 kg of between 2.4 and 4.5 g/10 min. The LDPE was Nexcoat 5, commercially available from SABIC having an MFI of about 5 g/10 min as measured according to ISO 1133 at 190 C. and 2.16 kg and having a density of about 919 kg/m.sup.3. The compatibiliser was a partially (+/40%) hydrolysed and saponified polyvinylacetate. Native starch was purchased from Emsland Group.

(11) Further additives are summarised under other.

(12) The thermoplastic composition forming the basis of the article according to the present invention was prepared by feeding the components to a first zone of a twin screw co-rotating extruder. The temperature profile of the extruder was 30-60-110-160-160-110 C. at a screw speed of 80 rpm and a torque of 60-110 Nm. The thermoplastic starch was formed in the first zones of the extruder before the polyolefin started to melt. To avoid degradation and or yellowing of the starch the temperature of the last zone of the extruder including the extrusion die was reduced to about 110 C. The starch was used as such, i.e. it was not dried or otherwise treated before feeding to the extruder.

(13) The thermoplastic composition was blown to a film by known method, using a stretch ratio in machine direction of 2 and a stretch ratio in transverse direction of 6.

(14) Examples I, III, IV and VII all show a layered morphology, where the present inventors note that the morphology of Example VII shows some irregularities in comparison with the Examples I, III and IV. The present inventors suspect that the sample was damaged during preparation for SEM analysis.

(15) Examples I and IV both show good values for the oxygen and water vapour permeability. Example III fails the water vapour test despite its layered structure. The present inventors believe that the amount of LDPE is too small in this Example. The predominant layers will therefore be formed by the polyester material, which in itself has a higher water vapour permeability than LDPE.

(16) Example VII shows that even with relatively low amount of compatibiliser it is still possible to obtain a layered morphology with alternating layers of starch phase and polyolefin phase, said layers of starch phase and polyolefin phase extending in machine direction and transverse direction.

(17) TABLE-US-00001 TABLE 1 II V VI XI XII I (comp) III IV (comp) (comp) VII VIII IX X (comp) (comp) Polyester 37.5 53.6 48.2 32.1 54.5 58.5 35.1 32.7 32.1 26.8 37.5 38 LDPE 16.1 0 5.4 21.4 0 0 23.4 21.8 21.4 26.8 25 25.4 Starch 24 24 24 24 24 24 24 24 24 24 24 24 Compatibiliser 8.9 8.9 8.9 8.9 8.9 4.5 4.5 8.9 8.9 8.9 0 0 Glycerol 10.7 10.7 10.7 10.7 10.7 10.7 10.7 10.7 10.7 10.7 10.7 10.7 Other 2.8 2.8 2.8 2.8 1.9 2.3 2.3 1.9 2.8 2.8 2.8 1.9 Morphology Layered Partially Layered Layered Partially Not Layered Not layered layered layered layered O.sub.2 Perm. 5.2 19.2 2.8 2.2 22.4 24 27.5 33 8 44.5 109 123 H.sub.2O Perm. 9.8 Fail Fail 9.7 Fail Fail 7 Fail 8 5.9 15.8

EXAMPLES XIII, COMPARATIVE EXAMPLE XIV AND EXAMPLE XV

(18) Three further experiments were performed with recipes as per Table 2.

(19) The polyester was the same as in Examples I-XII above. The compatibiliser of Example XIII was the same as in Examples I-XII above. The compatibiliser in Example XV was Lotader 3410, commercially available from Arkema. Lotader 3410 is a random terpolymer of ethylene, butyl acrylate and maleic anhydride. The starch was purchased from Avebe.

(20) TABLE-US-00002 TABLE 2 XIII XIV XV Component Weight % Polyester 32.2 37.5 35.1 LDPE 2404TN00 25.1 23.0 LDPE Nexcoat 21.4 Starch (Emsland) Starch (Avebe) 23.9 23.9 23.9 Compatibiliser 8.9 4.5 Glycerol 10.7 10.7 10.7 Other 2.8 2.8 2.8 Impact [g/m] 2.2 0.5 3.6 (Monsanto Dart, ASTM D1709A) Tear strength [kJ/m.sup.2] Perpendicular 121 16 148 Parallel 28 5 69 (Elmendorf method, ASTM D1922/ISO 6383-2) Tensile (23 C., 200 mm/min) Transverse direction Stress @ Yield [MPa] 2.4 Elongation @ Yield [%] 11.9 Stress at break [MPa] 5.7 0.7 8.9 Strain @ break [%] 162 39 419 Tensile (23 C., 200 mm/min) Machine direction Stress @ Yield [MPa] Elongation @ Yield [%] Stress at break [MPa] 10.6 7.6 12.3 Strain @ break [%] 163 103 288

(21) FIGS. 4a to 4c show SEM photographs of Examples XIII, XIV and XV in both machine direction (MD) and in transverse direction (TD).

(22) FIG. 4b, relating to Comparative Example XIV shows an irregular structure wherein the layers delaminate. This is most visible in the picture taken in machine direction (MD). The poor quality of the film of Comparative Example XIV is also deducible from Table 2, which shows that the mechanical properties of the Comparative Example IV film are relatively poor compared to Example XIII and XV.