Cover film, method of making cover film, and method of using cover film for enclosing compressed material

Abstract

A cover film for the compressing of bales, characterized in that the cover film has been stretched and comprises an inner layer (1) and outer layers (2), wherein the inner layer (1) of the film comprises a copolymer of an ethylene and an -olefin, wherein the -olefin is selected from the group consisting of 1-butene, 1-hexene, 1-heptene, and 1-octene, and is used for copolymerization with the ethylene, wherein the copolymer is present in an amount of more than 5% by weight and less than 50% by weight of the inner layer (1) of the film, wherein the inner layer (1) further comprises a linear polyethylene, and wherein the outer layers (2) of the film fail to comprise any significant proportion of the copolymer.

Claims

1. A cover film for enclosing bales of compressed material, the cover film having been stretched, and the cover film comprising: an inner layer and an outer layer; the inner layer comprises: a copolymer of an ethylene and an -olefin, wherein the -olefin is selected from the group consisting of 1-butene, 1-hexene, 1-heptene, and 1-octene, and is used for copolymerization with the ethylene; a proportion of said copolymer is more than 5% by weight and less than 50% by weight of the inner layer; and a linear polyethylene; and the outer layer does not have any significant proportion of said copolymer.

2. The cover film according to claim 1, wherein a proportion of the copolymer is less than 40% by weight of the inner layer.

3. The cover film according to claim 1, wherein the copolymer is a plastomer.

4. The cover film according to claim 1, wherein the copolymer is a thermoplastic elastomer.

5. The cover film according to claim 1, wherein the copolymer is at least one of a copolymer of ethylene with at least one of an (-1)-methyl-branched -olefin or an isomeric linear -olefin.

6. The cover film according to claim 1, wherein the copolymer is a copolymer of ethene and octene.

7. The cover film according to claim 1, wherein the copolymer is a metallocene-catalyzed copolymer.

8. The cover film according to claim 1, wherein the copolymer is a metallocene polyethylene (mPE-LLD) with a density of 0.886 to 0.935 g/cm.sup.3.

9. The cover film according to claim 1, wherein the inner layer of the film comprises a proportion of the copolymer of more than 8% by weight of the inner layer.

10. The cover film according to claim 9, wherein the inner layer comprises a proportion of the copolymer of less than 40% by weight of the inner layer.

11. The cover film according to claim 1, wherein the film (20) is stretched at a temperature of more than 80 C.

12. The cover film according to claim 11, wherein the cover film (20) is stretched at a temperature of less than 150 C.

13. The cover film according to claim 1, wherein the cover film is stretched at a temperature at which the copolymer is in molten form.

14. The cover film according to claim 13, wherein the cover film is stretched at a temperature at which all components other than the copolymer are in non-molten form.

15. The cover film according to claim 13, wherein the cover film is stretched at between 5 and 20 C. below the crystallite melting point of the film.

16. A combination of a bale of compressed material wrapped with the cover film of claim 1, wherein the cover film has restoring forces for further compression of the wrapped bale.

17. The combination according to claim 16, wherein the further compression is at least 0.5% of a diameter of the bale.

18. The cover film according to claim 1, wherein the film comprises at least three layers.

19. The cover film according to claim 1, wherein a thickness of the film is less than 20 m.

20. A method for enclosing bales of compressed material, comprising the following steps: providing the cover film of claim 1; stretching the cover film; compressing material in a chamber to form a bale of compressed material having a bale diameter; wrapping the cover film around the compressed bale in the chamber; and removing the wrapped bale from the chamber, and the cover film further compressing the bale by at least 0.5% of the diameter of the bale compared to the diameter of the bale on removal from the chamber.

21. A method for producing a cover film, comprising the following steps: blow extruding a copolymer and other components to form a film tube; dividing the film tube into film sheets, the film sheets each including an outer layer and an inner layer; the inner layer comprises: said copolymer, and said copolymer comprises an ethylene and an -olefin, wherein the -olefin is selected from the group consisting of 1-butene, 1-hexene, 1-heptene, and 1-octene, and is used for copolymerization with the ethylene; a proportion of said copolymer is more than 5% by weight and less than 50% by weight of the inner layer; and a linear polyethylene; and the outer layer does not have any significant proportion of said copolymer, and stretching the film sheets in an MDO unit in a longitudinal direction at a temperature at which the copolymer is in molten form and the remaining components are in non-molten form.

22. The method of claim 21, further comprising installing the cover film as a net replacement for fixing of compressed bulk material in compressing of a bale.

23. A roll of a film produced according to claim 21 adapted for use as a net replacement for fixing material in baling.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention can be derived from the description of an example using drawings and from the drawings themselves.

(2) The figures show the following:

(3) FIG. 1 is a sectional view through a cover film according to the invention,

(4) FIG. 2 is a schematic diagram of blow extrusion,

(5) FIG. 3 is a schematic diagram of a system for stretching of the film (an MDO system),

(6) FIG. 4 is a DSC diagram of a sample of the film, and

(7) FIG. 5 shows a baler with a bale compressed in cover film.

DETAILED DESCRIPTION

(8) FIG. 1 shows a sectional view through a variant of a cover film according to the invention. The film is a coextruded, multilayer blown film. According to the invention, an inner layer 1 comprises a significant content of a copolymer, in particular a plastomer. In the example, the copolymer is a metallocene-catalyzed copolymer of ethene and 1-octene. The ethene/1-octene copolymer has a content of 25 wt % in the inner layer 1 and a density of 0.902 g/cm.sup.3. The layer further comprises a content of 75% of a linear low-density polyethylene (LLDPE) with a density of 0.931 g/cm.sup.3 and a melting point of 127 C. The content of the copolymer based on the entire film is 15 wt %.

(9) The two outer layers 3 contain no copolymer, in particular no plastomer. They are composed of 69 wt % of an LLDPE with a density of 0.923 g/cm.sup.3, 30 wt % of a polyethylene with a density of 0.925 g/cm.sup.3, and 1 wt % of a processing auxiliary master batch.

(10) The thickness of the film in the example is 15 m.

(11) FIG. 2 shows a first stage of the method for producing the cover film. Blow extrusion is carried out in this first stage. The melt exiting the extruder 3 is formed into a tube by means of a ring nozzle 4. The tube is then expanded by blowing in air.

(12) The film tube is withdrawn at a speed that is greater than the discharge speed of the melt. When the temperature drops below the melting temperature of the film, the thickness and circumference of the tube are fixed. In a unit 5, the tube is laid flat and merged between two rollers 6. These squeezing rollers prevent air from penetrating into the film tube that has been laid flat and at the same time withdraw the film tube from the nozzle. The film is then cut, and the film sections obtained in this manner are rolled up in a unit 7. In this first stage of the method, the tube is divided longitudinally into four sections. No interlocking occurs.

(13) FIG. 3 shows a schematic diagram of the second stage of the method for producing the cover film according to the invention. The unit used is an MDO unit with preheating rollers 8, 9, and 10, wherein rollers 8 and 9 have a temperature of 105 C. and roller 10 has a temperature of 108 C. The system further comprises stretching rollers 11 and 12 that rotate at different speeds and thus stretch the film. Stretching roller 11 turns at a lower rotational speed than stretching roller 12, allowing a stretching ratio of approx. 2.5:1 to 3:1 to be achieved. The stretching rollers have a temperature of 105 to 108 C. In this manner, a temperature is applied to the film during stretching that is close to the crystallite melting point.

(14) Fixing rollers 13, 14, 15, and 16 are positioned after the stretching rollers 11, 12 that have a temperature of approx. 107 C. The temperature of the film is then sharply reduced via cooling rollers 17, 18. The cooling rollers have a temperature of approx. 40 C.

(15) FIG. 4 shows an evaluation of a DSC (differential scanning calorimetry) measurement of a sample of the film. The peaks indicate the melting temperature of the film. The lower curve results from the heating phase and the upper curve from the cooling phase. The temperature set in the system for the stretching process is approx. 20 C. below the melting temperature of the film. In this range, the molecular movement is already thermally activated or promoted. The molecular chains can thus be displaced relative to one another without tearing. A further effect of this temperature application is a reduction in constriction due to the tension exerted on the film.

(16) FIG. 5 shows a cylindrical round bale 19 that is wrapped on its outer surface with the film according to the invention 20. The wrapping process takes place in a baler 21, wherein the bale compressed in the cover film falls from the baler onto the field.