Inline coated biaxially oriented polyethylene foil and method for the production thereof

Abstract

The present invention relates to a biaxially oriented polyethylene foil (BOPE) comprising layers (A) to (D), wherein layers (B) to (D) contain biaxially oriented polyethylene and layer (A) contains polyurethane and nanoparticles and has a layer thickness of 25 to 300 nm, layer (B) comprises polymers having functional groups which are capable of forming covalent bonds with polyurethane and are directly connected to layer (A), layer (C) has at least a layer thickness of 50% of the total thickness of the foil and layer (D) represents an outer layer of the foil, which contains antiblocking agents. The invention further relates to methods for producing such foils.

Claims

1. A polyethylene foil comprising 4 layers (A) to (D) arranged in the order of (A) to (D), wherein layers (B) to (D) comprise biaxially oriented polyethylene and wherein layer (A) comprises polyurethane and nanoparticles and has a layer thickness of 25 to 300 nm, layer (B) further comprises polymers having functional groups, which can form covalent bonds with polyurethane and is directly connected to layer (A), layer (C) has at least a layer thickness of 50% of the total thickness of the foil, and layer (D) represents an outer layer of the foil and comprises antiblocking agents.

2. The polyethylene foil according to claim 1, wherein layers (B) to (D) each contain at least 70 wt. % of polyethylene, in each case based on the total mass of the layer.

3. The polyethylene foil according to claim 1, wherein layer (A) has a layer thickness in the range of from 50 to 250 nm.

4. The polyethylene foil according to claim 1, wherein layer (A) comprises nanoparticles consisting of amorphous silicon dioxide.

5. The polyethylene foil according to claim 1, wherein (A) comprises nanoparticles of which the average particle size is in the range of from 20 to 150 nm.

6. F The polyethylene foil according to claim 1, wherein layer (A) contains 0.5 to 20 wt. % of nanoparticles.

7. The polyethylene foil according to claim 1, wherein layer (B) comprises a maleic anhydride-modified polyethylene.

8. The polyethylene foil according to claim 1, wherein layer (B) contains no antiblocking agents.

9. The polyethylene foil according to claim 1, wherein it comprises a metal layer or a metal oxide layer which is directly bonded to the surface of layer (A).

10. The polyethylene foil according to claim 1, wherein it comprises an aluminium oxide layer or a silicon dioxide layer which is directly bonded to the surface of layer (A).

11. A food packaging comprising a foil according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the structure of the foils according to the invention having layers (A) to (D).

(2) FIG. 2A shows the structure of a conventional polyethylene foil again with a base layer and two outer layers (skin layer), one of which contains antiblocking agents.

(3) FIG. 2B shows the structure of a foil according to the invention, with the additional layer (A), which contains nanoparticles. The particles in the layers are marked.

(4) FIG. 3 shows a comparison of the oxygen and water vapour barrier properties of conventional polyethylene foils (BOPE), metallised BOPE foils (met. BOPE) and foils according to the invention with inline-generated polyurethane coating and offline-generated metal layer having particularly good barrier properties (met. BOPE+ILC). The leftmost column is shown in abbreviated form (see Table 1).

(5) FIG. 4 is a schematic representation of a reverse gravure kiss coating process as used in the present method. The dotted regions on the foil show the part of the foil coated on one side. The liquid dispersion in the reservoir is also shown as dots.

EXAMPLES

(6) Materials:

(7) Takelac WPB 341 is an aqueous dispersion from Mitsui Chemicals having 30% polyurethane content. Levasil CT3 PL (also called “PL Levasil 30/50”) is an aqueous dispersion of colloidal silicon dioxide from Akzonobel having a colloidal silicon dioxide content of 50 wt. % (particle size 80-100 nm; spec. surface 35 m.sup.2/g; BET surface: 40 m.sup.2/g). Admer NF-408E is a polyethylene homopolymer with grafted maleic anhydride groups having an MFR of 3 g/10 min at 190° C. and a weight of 2.16 kg (ASTM D 1238), a density of 0.92 g/cm.sup.3, a Vicat softening temperature of 100° C. (D1525) and a melting point of 120° C. “Constab AB 06001 LD” is a low-density polyethylene having a proportion of 4 wt. % of a synthetic silicon dioxide based on the total weight of the “Constab AB 06001 LD”. The particles of silicon dioxide have a particle size of 4 μm. The product is available from “CONSTAB Polyolefin Additives GmbH”, Rüthen, Germany.

(8) Particle sizes are preferably measured by particle size analysis by laser diffraction.

Example 1

(9) To produce a foil according to the invention, an LLDPE are mixed with an MFR of 2.1 g/10 min at 190° C. (ISO 1133), a density of 0.92 g/cm.sup.3 (ASTM D1505) and a melting point of 126° C. and 5 wt. % “Constab AB 06001 LD” based on the weight of the mixture. LLDPE with an MFR of 2.1 (ISO 1133), a density of 0.92 g/cm.sup.3 (ASTM D1505) and a melting point of 126° C. for layer (C) and the above mixture for layer (D) are melted separately in an extruder at 220 to 250° C. Admer NF-408E is melted for layer (B) at a temperature of 220-250° C. The aforementioned materials are extruded using a wide-slot nozzle. Layers (B) to (D) are each extruded separately using a twin-screw extruder. The melt extruded through the wide-slot nozzle is cooled with the aid of a cooling roll and a cast film is thus obtained. This cast film is stretched at a film speed of 10 m/min in a stretching system in the longitudinal direction with a stretching ratio of 5. The preheating rollers have temperatures in the range of 60 to 92° C., the stretching roller temperatures range from 95 to 105° C. and the annealing roller temperatures range from 78 to 105° C.

(10) After stretching in the longitudinal direction, the film has a speed of 50 m/min. After cooling the resulting monoaxially oriented film (MOPE film), the surface of layer (B) of the film is first subjected to a conventional corona treatment.

(11) The wetting of the film surface is improved by means of the polyurethane. An aqueous polyurethane dispersion is subsequently applied to layer (B) as a coating material by means of reverse gravure kiss coating with the aid of a reverse gravure coater on the MOPE film. For this, engraved rollers are used which have depressions which are filled with the dispersion with each revolution. When it comes into contact with the film web, part of the liquid is transferred to the film. By rotating the roller counter to the direction of motion of the film, a uniform wetting area is created on the film web. A 12 g/m.sup.2 engraving roller is used, which produces a wet layer with 12 g aqueous dispersion per square metre of film surface. The dispersion used for coating consists of 48 wt. % of Takelac WPB 341, 1.2 wt. % of Levasil 30/50 and 50.8 wt. % of water, in each case based on the total mass of the dispersion.

(12) The film is subsequently stretched in a transverse direction in a stretching furnace using a stretching ratio of 8.5, the preheating zone having an air temperature in the range from 144 to 160° C. and so when preheating in the stretching oven the aqueous polyurethane dispersion is dried before stretching to form a polyurethane layer (A). The stretching zones have temperatures of 138 to 156° C. and the annealing zone has a temperature in the range of 128 to 138° C. All temperatures given here are the temperature of the air in the corresponding zones of the stretching furnace. The foil is subsequently metallised in a further offline method. Aluminium is used as the metal, which is applied using physical vapour deposition (PVD). The layer thickness is 45 nm and the optical density of the layer is 2.5.

(13) A foil with the following properties is obtained:

(14) Layer thicknesses: Overall film 20 μm (according to DIN 53370), layer (A) 100 nm, layers (B) and (D) 1 μm, layer (C) 18 μm, metal layer (M) 45 nm; tensile strength (ASTM D 882) MD: 62 N/mm.sup.2, TD: 204 N/mm.sup.2; elongation at break (ASTM D 882) MD: 430%, TD: 56%; modulus of elasticity (ASTM D 882) MD: 280 N/mm.sup.2, TD: 444 N/mm.sup.2; friction coefficient (DIN EN ISO 8295, U/U) MD: 0.58 μS, TD: 0.53 μk; heat shrink (BMS TT 0.2; 100° C./10 min) MD: 3.9%, TD 3.6%; oxygen transmission rate (ISO 15105-2)<13 cm.sup.3/(m.sup.2dbar) (at 23° C. and 0% relative humidity); water transmission rate (ASTM E 96) 0.8 g/(m.sup.2d) (at 38° C. and 90% relative humidity); metal adhesion (tape test, TP-104-87) 5/5, (EAA sealing, AIMCAL process TP-105-92 for the adhesion of metalised films, N/15 mm)>5, no detachment of the metal.

(15) As can be seen from these values, the mechanical properties of the foils according to the invention are similar to those of the commercially available barrier foils. However, the oxygen transmission rate and the water vapour transmission rate are greatly reduced. When used, this results in an extended shelf life of the food packaged with it.

(16) FIG. 3 shows the oxygen transmission rate (OTR) and the water vapour transmission rate (WVTR) for a metallised foil according to example 1 (met. BOPE+ILC, metallised biaxially oriented polyethylene foil with inline coating, far right), for a commercially available non-metallised biaxially oriented polyethylene foil of the same thickness (BOPE, far left, the leftmost column is shown in abbreviated form (see Table 1, see also FIG. 2A for a schematic representation of this foil) and for the same foil with metallisation (met. BOPE, in the centre). As can be seen, the oxygen transmission rate is again dramatically reduced by the foil according to the invention compared to the commercially available metallised BOPE foil, without a great deal of additional effort being involved in the production. The oxygen transmission rate is reduced by a factor of more than 10. The water vapour transmission rate is also reduced.

(17) Table 1 shows the numerical data of the foils:

(18) TABLE-US-00001 Oxygen trans- Water vapour mission rate transmission rate [cm.sup.3/m.sup.2 * day] [g/m.sup.2 * day] BOPE 3000* 2 met. BOPE 193 1 met. BOPE + ILC  13 0.8 *shown in abbreviated form in FIG. 3.

(19) The metal adhesion is also extremely high and almost unique for a metallised biaxially oriented polypropylene film.