BIAXIALLY-ORIENTED POLYETHYLENE FILMS FOR THERMOFORMING, PROCESS FOR THE PRODUCTION THEREOF, THEIR USE, A PROCESS FOR THERMOFORMING AND ITS PRODUCTS

Abstract

A biaxially-oriented film for thermoforming is disclosed including at least 75% by weight polyethylene and at least 95% by weight polyolefins, based on the total mass of the film. The polyethylene has an elongation at break of at least 7 in the stress-strain diagram, wherein the stress-strain diagram is measured at a temperature of 10° C. below the melting point of the polyethylene and the melting point is determined by differential scanning calorimetry using a heating rate of 10° C. per minute. A process for producing such films and to the use thereof are disclosed. A process for producing shaped bodies from such films and to the shaped bodies themselves are disclosed.

Claims

1. A biaxially-oriented film for thermoforming, wherein the film contains at least 75% by weight polyethylene and at least 95% by weight polyolefins, based on the total mass of the film, wherein the polyethylene has an elongation at break of at least 7 in the stress-strain diagram, wherein the stress-strain diagram is measured at a temperature of 10° C. below the melting point of the polyethylene and the melting point is determined by differential scanning calorimetry using a heating rate of 10° C. per minute.

2. The film according to claim 1, wherein the stress-strain diagram of the polyethylene has an upper yield point and the yield stress is the highest stress in the stress-strain diagram.

3. The film according to claim 2, wherein the stress-strain diagram of the polyethylene additionally has a lower yield point and all stress values in the case of strain values higher than the strain at the lower yield point are lower than the mean value of the yield stress and the stress at the lower yield point.

4. The film according to claim 3, wherein the stress-strain diagram of the polyethylene exhibits no stress-strain hardening.

5. The film according to claim 1, wherein the polyethylene is a polyethylene chosen from the group consisting of HDPE, MDPE and LLDPE.

6. The film according to claim 1, wherein the modulus of elasticity of the film is at least 800 MPa.

7. The film according to claim 1, wherein the shrinkage of the film is at most 5% in the longitudinal direction and at most 5% in the transverse direction.

8. The film according to claim 1, wherein the film has a thickness in the range from 25 μm to 2000 μm.

9. The film according to claim 1, wherein the film has a 2-sigma thickness distribution value of 15% or less.

10. The film according to claim 1, wherein the stretch ratio in the longitudinal and in the transverse direction lies in the range from 1.2 to 5.5.

11. A process for producing the film according to claim 1, wherein the film is produced according to a process comprising the following steps: extruding a film comprising at least one layer comprising polyethylene composed of at least one polymer melt, cooling the film to form a film, elongating the film in the longitudinal direction and in the transverse direction, wherein the stretch ratio in the longitudinal and in the transverse direction lies in the range from 1.2 to 9.

12. The process according to claim 11, further comprising a heat treatment which is subsequently carried out, wherein during the heat treatment relaxation is performed in the longitudinal direction and in the transverse direction.

13. The process according to claim 11, wherein the polyethylene used has a density in the range from 0.89-0.98 g/cm.sup.3.

14. The process according to claim 11, wherein the polyethylene used has an MFI (190° C./2.16 kg) of 0.3-8 g/10 min.

15. The process according to claim 12, wherein the extent of the relaxation is in the range from 2% to 12%.

16. (canceled)

17. A process for producing a shaped body, wherein the film according to claim 1 is deformed by thermoforming.

18. The process according to claim 17, wherein the film thus obtained is deformed by thermoforming.

19. The process according to claim 17, wherein the film is formed with a degree of deformation in the range from 2 to 6.

20. A shaped body formed fully or partly by thermoforming from a biaxially-oriented film, wherein the body comprises at least 75 wt. % polyethylene and at least 95 wt. % polyolefins, based on the total mass of the shaped body.

21. The shaped body according to claim 20, wherein the film contains at least 75% by weight polyethylene and at least 95% by weight polyolefins, based on the total mass of the film, wherein the polyethylene has an elongation at break of at least 7 in the stress-strain diagram, wherein the stress-strain diagram is measured at a temperature of 10° C. below the melting point of the polyethylene and the melting point is determined by differential scanning calorimetry using a heating rate of 10° C. per minute.

22. The shaped body according to claim 20, wherein the part of the shaped body that was formed by thermoforming, the ratio of the greatest thickness of the formed film to the smallest thickness of the formed film is not more than two.

23. The shaped body according to claim 20, wherein the water permeability measured to ASTM E 96 at 30° C. and 90% relative humidity is not more than 8 g/m2/day.

24. The shaped body according to claim 20, wherein the oxygen permeability measured to ISO 15105-2 at 23° C. and 0% relative humidity is not more than 4000 cm3/m2/day.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0108] FIGS. 1 and 2 show shaped bodies according to the invention which have been produced from films according to the invention.

[0109] FIG. 3 shows the stress-strain diagrams of three different polyethylenes.

LIST OF ABBREVIATIONS

[0110] ABS acrylonitrile-butadiene-styrene copolymer [0111] BO biaxially-oriented [0112] BOPE biaxially-oriented polyethylene [0113] BOPP biaxially-oriented polypropylene [0114] BOPS biaxially-oriented polystyrene [0115] CPP cast polypropylene (cast PP) [0116] Cast PET cast polyethylene terephthalate [0117] EVOH ethyl vinyl alcohol copolymers [0118] HDPE high-density polyethylene [0119] LLDPE linear low-density polyethylene [0120] MDPE medium-density polyethylene [0121] MD longitudinal direction, machine direction [0122] PA polyamide [0123] PE polyethylene [0124] PET polyethylene terephthalate [0125] PP polypropylene [0126] PS polystyrene [0127] PU polyurethane [0128] PVC polyvinyl chloride [0129] TD transverse direction, transverse to the machine direction

EXAMPLES

Measurement Methods

[0130] Film thicknesses: DIN 53370; layer thicknesses: DIN EN ISO 3146; tensile strength: ASTM D 882; elongation at break: ASTM D 882; modulus of elasticity: ASTM D 882; coefficient of friction: DIN EN ISO 8295, U/U; heat shrinkage: BMS TT 0.9; 100° C./5 min and BMS TT 0.2; 120° C./5 min; coefficient of friction: DIN EN ISO 8295; haze: ASTM 1003; gloss: ASTM 2457; corona treatment: ASTM 2457; puncture resistance: DIN EN 14477 (1.0 mm rounded-tip pen); oxygen permeability ISO 15105-2; at 23° C. and 0% relative humidity; water permeability: ASTM E 96, at 30° C. and 90% relative humidity.

Example 1

Production of a Film According to the Invention

[0131] A three-layer film was produced. High-density polyethylene (HDPE) having a density of 0.953 g/cm.sup.3 and an MFI of 0.9 g/10 min (190° C.; 16 kg), which was produced by suspension polymerisation with a Ziegler-Natta catalyst (“Hostalen GD 9555” from LyondellBasell) was added to three extruders. A twin-screw extruder having a vacuum and a 200-μm filter was used as main extruder for the middle layer (base layer), which extruder was operated at a temperature of 200 to 260° C. in all zones. A single-screw extruder having a 200-μm filter and no vacuum was used as co-extruder for each of the two outer layers, which extruder was operated at a temperature of 230 to 250° C. in all zones. A 3-layer die was used as extrusion die. The melts were extruded through the die and applied to a chill roll. A standard high-pressure air knife was used to assist in pressing the film against the chill roll. The temperature of the chill roll was 70-100° C. The surface speed of the chill roll was 2 m/min. A water bath was not used. The film was then wound up.

[0132] The cast film was produced in a thickness of 1850 μm and a width of 35 cm. The middle layer makes up 80% of the film thickness, the other two layers each make up 10% of the film thickness. The modulus of elasticity of the cast film thus obtained was 609 MPa in the longitudinal direction.

[0133] The film was then stretched on a pilot plant. A 90×90 mm square was cut from the centre of the film. This was clamped in the pilot plant. Stretching was effected by simultaneous stretching in the longitudinal and transverse directions. The temperature during stretching was 125° C. for all tests. The pre-heating time was 300 seconds. Degree of stretch and the extent of relaxation for the various films according to the invention that have been produced are shown in Table 1 below. The films were stretched in the longitudinal and transverse directions at the same speed and with the same degree of stretch. The stretching lasted two seconds. Heat treatment was carried out on some of the films produced (see Table 1). This took place at 125° C. for five seconds. The films were then relaxed. The extent of relaxation is shown in Table 1. The speed was 12% relaxation per second. Seven films according to the invention were manufactured.

TABLE-US-00001 TABLE 1 Film No. 1 2 3 4 5 6 7 Degree of stretch 3 × 3 3 × 3 3.5 × 3.5* 3.5 × 3.5** 4 × 4 6 × 6 6 × 6 MD × TD Heat treatment Yes Yes Yes Yes Yes No Yes Relaxation [%] 15 10 8 8 — — — Tensile strength MD 55 68 73 88 81 135 123 [MPa] Elongation at break 456 310 289 245 251 80 134 [%] Modulus of 1264 1212 1203 1209 1171 1523 1247 elasticity [MPa] Film thickness [μm] 210 210 152 150 120 55 55 *Stretching speed 50% to 100% per second **Stretching speed greater than 100% per second

[0134] As can be seen, the modulus of elasticity is approximately doubled by the stretching.

Example 2

Production of Shaped Bodies

[0135] Shaped bodies were produced from Film 1 of Example 1 by thermoforming. For this purpose, the film was heated to 125° C. and shaped bodies were formed therefrom in a thermoforming machine. The thermoforming was carried out by vacuum thermoforming at a temperature of 125° C. The plugging speed was 200 mm/s, the plug-assist material was HYTAC-WFT (epoxy resin with hollow glass spheres and Teflon).

[0136] FIG. 1 shows two shaped bodies produced in this way. As can be seen, these have a regular shape. The round mould has a diameter of 150 mm and a height of 20 mm at the upper vertex. The film thickness in the centre of the shaped body is 100 μm. The film thicknesses of the square moulds are in the range from 190 to 210 μm. Therefore, stretching is uniform, and the shaped bodies formed are strong enough to be able to be used as packaging materials. It can also be seen on the grid of the square shaped body that the stretching is very uniform and without large irregularities. This leads to very regular wall thicknesses of the shaped bodies, which is a prerequisite for their dimensional stability and their barrier effect. The water permeability is less than 6 g/m.sup.2/day and the oxygen permeability is less than 3500 cm.sup.3/m.sup.2/day.

[0137] FIG. 2 shows a shaped body where the side walls are perpendicular to the original plane of the film. The deformation (the stretching) is very high at the side walls in such a mould. The mould could nevertheless be produced successfully. The side walls have a film thickness of 120 μm and the base (or lid) in the centre of the moulding has a film thickness of 210 μm. The side walls are noticeably more transparent than the base of the moulding.

Example 3

Production of a Film According to the Invention

[0138] Material, layer construction and layer thickness of the cast film correspond to the details in Example 1. A BT-55-32D double-vacuum main extruder with a 400-μm filter is used as the extruder for the middle base layer. The temperature of the extruded melt was 258° C. A BT-43-30D single-vacuum co-extruder with a 200-μm filter was used for each of the two outer layers at a temperature of 263° C. An EDI 3-layer die was used. The chill roll had a temperature of 80° C. and a high-pressure air knife was used to assist with the application of the film to the chill roll. A water bath was not used.

[0139] The film thus produced was biaxially stretched in a simultaneous process. The film first passes through 3 pre-heating zones in the stretching oven that have temperatures in the range from 136 to 144° C. The film then passes through 2 stretching zones that have temperatures in the range from 116 to 130. This is followed by 2 further-heating zones in which heat treatment is effected. These further-heating zones are operated at a temperature of 105 to 110° C. Finally, the film passes through a cooling zone having a temperature of 80° C.

[0140] The film thus produced has a thickness of 51.99 μm and a 2-sigma film thickness value of 8.32%. The 2-sigma value is important for the film according to the invention because films having a large variation in thickness result in defective products during thermoforming. If parts of the film that are very thin are located in places that are highly mechanically stressed during the thermoforming, the thermoforming products will leak and/or have mechanical weak points that impair their properties.

[0141] A total of 3 films were produced. The following table gives further data about this test.

TABLE-US-00002 TABLE 2 Film No. Properties Unit 8 9 10 Degree of stretch MD × TD 3.2 × 2.9 4.2 × 3.8 5 × 4.2 Thickness - nominal value μm 52.0 30.0 25.0 Thickness MD centre μm 51.99 30.47 24.54 TD μm 54.21 31.27 25.16 Tensile strength MD MPa 76 116 130 TD MPa 122 144 149 Elogation at break MD % 330 192 152 TD % 242 148 131 Modulus of elasticity MD MPa 1340 1221 1296 TD MPa 1399 1178 1194 Coefficient of u/u* static 0.20 0.19 0.14 friction u/u sliding 0.21 0.20 0.16 u/d* static 0.26 0.22 0.18 u/d sliding 0.27 0.24 0.20 d/d static 0.19 0.21 0.19 d/d sliding 0.21 0.21 0.18 Haze % 20.60 10.70 8.09 Gloss 45° 40 65 72 Heat shrinkage 100/5 MD % 3.17 3.73 3.43 TD % 2.33 3.87 0.93 Heat shrinkage 120/5 MD % 12.50 14.67 14.20 TD % 13.67 16.43 12.83 Puncture Force N 9.49 9.31 7.81 resistance Force g 968.2 949.5 796.2 (1.0 mm Force/area N/mm 175.4 280.8 311.8 rounded-tip Work until break mJ 10.35 8.31 6.44 pen) Elongation at break mm 2.12 2.00 1.91

[0142] Remarks: Thickness, tensile strength, elongation at break, modulus of elasticity, haze and heat shrinkage were measured directly after production. All other measurements were made a day later. Abbreviations: u: up; d: down.