In-line, coated, biaxially oriented polypropylene film and method for the production thereof

Abstract

The invention relates to a biaxially oriented polypropylene film (BOPP) comprising layers (A) to (D), wherein layers (B) to (D) contain biaxially oriented polypropylene and layer (A) contains polyurethane and nanoparticles and has a layer thickness of 25 to 300 nm, layer (B) contains polymers with functional groups that are capable of forming 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 film, and layer (D) represents an outer layer of the film containing an anti-blocking agent. The invention also relates to a method for producing films of this type.

Claims

1. A polypropylene film comprising 4 layers (A) to (D), which are arranged in order from (A) to (D), wherein layers (B) to (D) comprise biaxially oriented polypropylene and wherein layer (A) contains polyurethane and nanoparticles and has a layer thickness of 25 to 300 nm, layer (B) further contains polymers with functional groups that are capable of forming 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 film, and layer (D) represents an outer layer of the film and comprises an anti-blocking agent.

2. The polypropylene film according to claim 1, wherein layer (A) has a layer thickness in the range of 50 to 150 nm.

3. The polypropylene film according to claim 1, wherein layer (A) has nanoparticles of amorphous silicon dioxide.

4. The polypropylene film according to claim 1, wherein layer (A) has nanoparticles of which the average particle size is in the range of 20 to 150 nm.

5. The polypropylene film according to claim 1, wherein layer (A) contains 0.5 to 20% by weight nanoparticles.

6. The polypropylene film according to claim 1, wherein layer (B) comprises a maleic-acid-anhydride-modified polypropylene.

7. The polypropylene film according to claim 1, wherein layer (B) does not contain any anti-blocking agent.

8. The polypropylene film according to claim 1, further comprises a metal layer or metal oxide layer that is directly connected to the surface of layer (A).

9. The polypropylene film according to claim 1, wherein it has an aluminium oxide layer or silicon dioxide layer that is directly connected to the surface of layer (A).

10. A foodstuff packaging comprising a film according to claim 1.

11. A method for producing a film according to claim 1, comprising the steps of: providing a sheet that is monoaxially stretched in the longitudinal direction and comprises layers (B) to (D), wherein layers (B) to (D) comprise biaxially oriented polypropylene and wherein layer (A) contains polyurethane and nanoparticles and has a layer thickness of 25 to 300 nm, layer (B) further contains polymers with functional groups that are capable of forming 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 film, layer (D) represents an outer layer of the film and comprises an anti-blocking agent, and wherein layers (B) and (D) are outer layers of the sheet, applying a layer of a liquid dispersion containing polyurethane and nanoparticles to layer (B), drying the liquid dispersion to produce layer (A), stretching the sheet in the transverse direction to produce a biaxially oriented film.

12. The method for producing a film according to claim 11, comprising the steps of: providing a sheet comprising layers (B) to (D) which are non-oriented and layers (B) and (D) are outer layers of the sheet, applying a liquid dispersion containing polyurethane and nanoparticles to layer (B), drying the liquid dispersion by heating to produce layer (A), simultaneously stretching the sheet in the longitudinal direction and transverse direction to produce a biaxially oriented film.

13. The method according to claim 11, wherein the surface of layer (B) is subjected to a corona treatment, before the liquid dispersion is applied.

14. The method according to claim 11, wherein the liquid dispersion is applied to layer (B) by reverse gravure kiss coating.

15. The method according to claim 11, wherein a wet layer formed by the liquid dispersion on layer (B) has a mass of 3 to 20 g/m.sup.2 and particularly preferably 6 to 10 g/m.sup.2.

16. The method according to claim 11, wherein the liquid dispersion is an aqueous dispersion.

17. The method according to claim 11, wherein the stretching ratio in the longitudinal direction is in the range of 2 to 8.

18. The method according to claim 11, wherein the stretching ratio in the transverse direction is in the range of 4 to 14.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: FIG. 1 shows the construction of the films according to the invention having layers (A) to (D).

(2) FIG. 2A: FIG. 2A shows the construction of a conventional polypropylene film having a carrier layer and two outer layers (skin layers), one of which contains anti-blocking agent.

(3) FIG. 2B: FIG. 2B shows the construction of a film according to the invention, having the additional layer (A) containing nanoparticles. The particles in the layers are marked.

(4) FIG. 3 FIG. 3 shows a comparison of the oxygen and water vapour barrier properties of conventional polypropylene films (BOPP), metallised BOPP films (met. BOPP) and films according to the invention having a polyurethane coating produced in-line and a metal layer produced off-line having particularly good barrier properties (met. BOPP+ILC). The entire left column is shown shortened (see Table 1).

(5) FIG. 4: FIG. 4 is a schematic representation of a reverse gravure kiss coating process such as is used in the present method. The dotted regions on the film show the part of the film coated on one face. Further, the liquid dispersion in the reservoir is shown dotted.

(6) FIG. 5: Cross section through the film according to the invention in accordance with Example 2.

EXAMPLES

(7) Materials:

(8) Takelac WPB 341 is an aqueous dispersion from Mitsui Chemicals having a 30% polyurethane content. Levasil 30/50 is an aqueous dispersion of colloidal silicon dioxide from Akzonobel having a colloidal silicon dioxide content of 50% by weight (particle size 80-100 nm; specific surface area 35 m.sup.2/g; BET surface area 40 m.sup.2/g). Admer AT 3177E is a polypropylene homopolymer having grafted-on maleic acid anhydride groups having a Vicat softening temperature of 135° C. (D1525). Moplen HP522 is an isotactic polypropylene homopolymer “Moplen®” HP 522 H from Basell having a Vicat softening temperature of 155° C. (ISO 306). Adsyl 3C30F is an isotactic polypropylene homopolymer from Basell having a Vicat softening temperature of 122° C. (ISO 306). POLYBATCH® ABVT22SC is an anti-blocking agent from Polybatch.

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

Example 1

(10) To produce a film according to the invention, Adsyl 3C30F and 5% by weight POLYBATCH® ABVT22SC based on the weight of the mixture were mixed. Admer AT 3177E for layer (B), Moplen HP522 for layer (C) and the aforementioned mixture for layer (D) were melted separately, in one extruder each, at 240 to 265° C. and extruded using a slot die. Layer C is extruded using a twin-screw extruder, and layers (B) and (D) are likewise each extruded using a twin-screw extruder. The melt extruded through the slot die is cooled using a cooling roll, and a cast sheet is thus obtained. This cast sheet is stretched in the longitudinal direction, with a stretching ratio of 5, in a stretching system at a sheet speed of 10 m/min. In this context, the preheating rolls have temperatures in the range of 80 to 104° C., the stretching rolls have temperatures in the range of 95 to 105° C., and the annealing rolls have temperatures in the range of 95 to 100° C. After stretching in the longitudinal direction, the sheet has a speed of 50 m/min. After the resulting monoaxially oriented sheet (MOPP sheet) is cooled, the surface of layer (B) of the sheet is initially subjected to a conventional corona treatment. This improves the wetting of the film surface with the polyurethane.

(11) Subsequently, an aqueous polyurethane dispersion is applied to the MOPP sheet on layer (B) as a coating material by reverse gravure kiss coating, using a reverse gravure coater. For this purpose, engraved rolls are used, which have depressions that are filled with the dispersion on each rotation. In this context, the temperatures of the sheet and the polyurethane dispersion are lower than 100° C. During contact with the sheet web, part of the liquid is transferred to the sheet. An opposed rotation of the roll counter to the movement direction of the sheet results in a uniform wetting region on the sheet web. In this context, a 12 g/m.sup.2 gravure roll is used, which produces a wet layer having 12 g aqueous dispersion per square metre sheet surface. The dispersion used for coating consists of 48% by weight Takelac WPB 341, 1.2% by weight Levasil 30/50 and 50.8% by weight water, each based on the total mass of the dispersion.

(12) Subsequently, the sheet is stretched in the transverse direction, with a stretching ratio of 9, in a stretching oven, the preheating zone having a temperature in the range of 175 to 190° C. and the aqueous polyurethane dispersion thus being dried during the preheating in the stretching oven before the stretching to form a polyurethane layer (A). The stretching zones have temperatures of 160 to 170° C., and the annealing zone has a temperature in the range of 160 to 168° C. All temperatures set out herein are the temperature of the air in the corresponding zones of the stretching oven. Subsequently, the film is metallised in a further off-line method. Aluminium is used as the metal, and is applied by standard vacuum metallisation (PVD). The layer thickness is 45 nm, and the optical density of the layer is 2.7.

(13) A film having the following properties is obtained:

(14) Layer thicknesses: entire film 18.1 μm (according to DIN 53370), layer (A) 100 nm, layers (B) and (D) 1 μm, layer (C) 16 μm, metal layer (M) 45 nm; tear resistance (ASTM D 882) MD: 157 N/mm.sup.2, TD: 291 N/mm.sup.2; elongation at break (ASTM D 882) MD: 250%, TD: 69%; modulus of elasticity (ASTM D 882) MD: 1231 N/mm.sup.2, TD: 2475 N/mm.sup.2; coefficient of friction (DIN EN ISO 8295, U/U) MD: 0.58 μS, TD: 0.53 μk; heat shrink (BMS TT 0.2; 120° C./5 min) MD: 4%, TD 0.8%; oxygen permeability (ISO 15105-2) <7 cm.sup.3 (m.sup.2dbar) (at 23° C. and 0% relative air humidity); water permeability (ASTM E 96) 0.1 g/(m.sup.2d) (at 38° C. and 90% relative air humidity); metal adhesion (adhesive tape test, TP-104-87) 5/5, EAA sealing, AIMCAL Process TP-105-92 for the adhesion of metallised films, N/15 mm) >3, no stripping of the metal.

(15) As can be seen from these values, the mechanical properties of the films according to the invention are similar to those of conventional commercial barrier films. However, the oxygen permeability and water permeability are greatly reduced. In use, this leads to an increased shelf life of foodstuffs packed using them.

(16) FIG. 3 shows the oxygen permeability (OTR, oxygen transmission rate) and the permeability to water vapour (WVTR, water vapour transmission rate) for a metallised film according to Example 1 (met. BOPP+ILC, metallised biaxially oriented polypropylene film with in-line coating, far right), for a conventional commercial non-metallised biaxially oriented polypropylene film of the same thickness (BOPP, far left, the entire left column being show shortened (see Table 1; see also FIG. 2A for the schematic drawing of this film)) and for the same film with metallisation (met. BOPP, in the centre). As can be seen, by way of the film according to the invention, the oxygen permeability can again be dramatically reduced by comparison with the conventional commercial metallised BOPP film, without major additional expense being accrued during production. The oxygen permeability is reduced by almost a factor of 10. The permeability to water vapour is likewise heavily reduced.

(17) Table 1 shows the numerical data for the films:

(18) TABLE-US-00001 Oxygen permeability Permeability to water vapour [cm.sup.3/m.sup.2 * day] [g/m.sup.2 * day] BOPP 1500*  5 met. BOPP 60 0.3 met. BOPP + ILC  7 0.1 *Shown shortened in FIG. 3

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

Example 2

(20) This example is carried out in accordance with Example 1, but with the following modifications:

(21) The film contains 6 layers with a layer construction A-B-Z.sub.1-C-Z.sub.2-D, Z.sub.1 and Z.sub.2 representing intermediate layers. Layer C is the base layer, and consists of polypropylene homopolymer from Repol (in Almassora, Castellon, Spain). Layers B, Z1, Z2 and D contain polypropylene copolymer. The materials for layers B, C, D, Z1 and Z2 are all extruded separately from one another using twin-screw extruders. The thicknesses of the layers are as follows: A: 100 nm, B: 0.77 μm, Z.sub.1: 2.52 μm, C: 11.04 μm, Z.sub.2: 2.53 μm, D: 1.18.

(22) During the stretching in the longitudinal direction, the preheating rolls have temperatures in the range of 100 to 120° C., the stretching rolls have temperatures in the range of 65 to 75° C., and the annealing rolls have temperatures in the range of 120 to 130° C. During stretching in the transverse direction, the preheating zone has a temperature in the range of 180 to 160° C., the stretching zone has temperatures of 160 to 170° C., and the annealing zone has a temperature in the range of 160 to 170° C. The sheet speed upon leaving the slot die is 87 m/min. The sheet speed after stretching in the longitudinal direction is 400 m/min. When the polyurethane dispersion is applied, a 6 g/m.sup.2 gravure roll is used, causing an approximately 6 μm thick liquid layer to be applied. During heating for the stretching in the transverse direction, this layer is dried, and the remaining layer thickness is 1 μm. Subsequently, the layer thickness is reduced further by the stretching.

(23) The sheet width after the stretching in the longitudinal direction is 1100 mm. The width of the coating (the width of layer A) was 1000 mm.

(24) The oxygen permeability of the film is 5 cm.sup.3/m.sup.2*day, and the permeability to water vapour is 0.1 g/m.sup.2*day.

(25) FIG. 5 is a cross section through the film according to the invention according to Example 2. The magnification is ×40,000. The dark, thin uppermost layer is the aluminium layer. The light layer below the aluminium layer is layer A. The two dark dots are nanoparticles. The thick, dark grey layer below is layer B. It is clearly visible that the nanoparticles are capable of increasing the surface roughness of the aluminium layer without leading to an opening in the layer. The nanoparticles therefore have no or very little negative effect on the barrier effect of the aluminium layer, but ensure the required surface roughness, which enables transport on rolls during processing and prevents interlocking when the film is wound and unwound.