GB2130964A - Plastics film and bags and sacks therefrom - Google Patents

Abstract

A thermoplastics film includes first and second layers formed by melt extrusion from synthetic thermoplastics material, one of said layers comprising a blend of linear low density ethylene polymer and at least one of (a) a second ethylene polymer which has a higher density and is selected from ethylene homopolymers and copolymers of ethylene and at least one other alpha -olefin and (b) polypropylene, and the other of said layers comprising linear low density ethylene polymer. At least one of said layers 32 provides a face of the film and at least a part of the surface of the outer face 12 of the film has a higher coefficient of friction than the inner face. Such a multilayerfilm is suitable for use in the manufacture of bags or sacks. <IMAGE>

Description

SPECIFICATION Plastics film and bags and sacks therefrom This invention relates to plastics film and bags and sacks made therefrom.

Plastics bags and sacks, especially those made from polyolefins such as polyethylene, have been known for a long time and because of their physical properties and low cost have largely replaced other materials in a wide variety of end uses, especially as dustbin liners and in the packaging of materials such as fertiliser, peat, etc.

However, despite much research, there have been problems in developing a plastics alternative for heavy duty paper sacks, especially the multi-ply paper sacks, pillow sacks or valve sacks e. g. of the kind used for packaging some agricultural and horticultural products, which has an acceptable performance and yet is sufficiently cheap.

According to the present invention, there is provided a thermoplastics bag or sack at least one of the walls of which inclues first and second layers each formed by melt extrusion from a synthetic thermoplastics material, and wherein one of said layers comprises a blend of linear low density ethylene polymer and at least one of (a) a second ethylene polymer which has a higher density and is selected from ethylene homopolymers and copolymers of ethylene and at least one other a-olefin and (b) polypropylene, and the other of said layers comprises linear low density ethylene polymer, at least one of said layers provides a face of the wall and at least a part of the surface of the outer face of the wall has a higher coefficient of friction than the inner face.

In the simplest form of the bag or sack, the wall will have just the two layers but it may consist of three or more layers in which case the specified two layers preferably provide the two faces of the wall.

In general both of the walls of the bag or sack will have said multi-layer construction but this is not essential because the benefit of the invention can still be obtained when only one of the walls is so constructed provided that all the sacks (or bags) in a stack are arranged with said wall facing in the same direction i. e. either with said wall being the upper wall or with said wall being the underneath wall of all the sacks.

Although the layers of the wall may be separate if desired, they are preferably fused together in which case the wall may be formed in a single operation e. g. by a process such as coextrusion.

Thus, the invention also provides a multi-layer film comprising a laminate having at least two layers each of which is formed by melt extrusion from a synthetic thermoplastics material, and wherein a first one of said layers comprises a blend of linear low density ethylene polymer and at least one of (a) a second ethylene polymer which has a higher density and is selected from ethylene homopolymers and copolymers of ethylene and at least one other a-olefin and (b) polypropylene and a second one of said layers comprises linear low density ethylene polymer, at least one of said layers provides a face of said film and at least a part of the surface of one face has a higher surface coefficient of friction than the other face of the film.

The layers forming the laminate may be thermally fused together while at least one is still in the heat softened state as a result of being melted for extrusion, and preferably the film is made of coextrusion. Coextrusion involves extruding two or more different plastics layers through a common die to produce a multi-layer product in a single step. The plastics materials forming the layers may be extruded through different orifices in the same die, whereby the layers are extruded as separate entities and fuse together downstream of the die, or they may be extruded through the same orifice.

In its simplest form, the film of the present invention will have two layers which will provide the respective inner and outer faces of the wall of a bag or sack made from the film. However, it may contain more than two layers, thus offering the opportunity of further varying the physical properties of the film and of bags or sacks made therefrom.

The film may be flat film extruded from a slot die but preferably it is tubular film, e. g. produced by the blow extrusion process. It is preferred that the surface of the film having the higher surface coefficient of friction is provided by one of the two specified layers. This layer may be discontinuous if desired, e. g. in the form of one or more bands or strips. For example, where the film is tubular film, the or each band or strip may be produced in the machine direction or at an angle thereto whereby a helical pattern is formed.

Bags and sacks according to the invention may be made from the film with the face having the higher surface coefficient of friction providing the outer face of the wall of the bag or sack. The bags and sacks may be formed from the film in known manner and may be of simple sack form or, for example, in the form of pillow sacks or valve sacks.

Linear low density ethylene polymer is ethylene polymer having a density corresponding to that of conventional low density polyethylene, i. e. up to about 0.925 g/cm3 but which has been manufactured by a low pressure process employing a metal-based catalyst such as a Ziegler, Phillips, Union Carbide, Dow or Standard catalyst, and has fewer CH3-groups per 1000 carbon atoms than conventional low density polyethylene made by the high pressure process using a peroxy catalyst. In general, linear low density ethylene polymers contain a small percentage of a second a-olefin as comonomer.

Said second a-olefin normally has at least 4 carbon atoms.

In one of the layers, preferably the layer which does not provide the outer face of the bag or sack, linear low density ethylene polymer is employed in admixture with a second polymer. The linear low density polymer is preferably a copolymer of ethylene and an a-olefin having more than four, preferably at least six and most preferably more than seven carbon atoms. Particular examples of such a-olefins are hexene-1, octene-1, and decene-1. The second polymer, which is preferably polyethylene or a copolymer of ethylene with another a-olefin, has a higher density than the linear low density polymer with which it is mixed. In general, said second ethylene polymer will have a density of at least 0.926 g/cm3 and may be selected from so-called medium density polymers, having a density in the range 0.926-0.940, and high density polymers, having a density in the range 0.941-0.965 or possibly even higher. Preferably it has a density in the range 0.930-0.945 g/cm3.

To obtain the optimum combination of properties in the bag or sack, especially of puncture resistance, tear strength, elasticity and tensile strength, it is preferred that the blend of the two polymers has a density in the range about 0.914 to about 0.937, more preferably about 0.920 to about 0.935 and most preferably about 0.923 to about 0.930 g/cm3, when determined from the following equation: p = ApA + BpB where p is the density of the blend and pA and pB are the densities of the linear low density polymer and the other polymer respectively, and A and B are the fractions of each of these polymers employed in the blend. Thus, the higher the density of the second polymer, the less of it should be used in the blend.

Alternatively, the second polymer in the blend may comprise polypropylene or a mixture of polypropylene and a polyethylene or ethylene/ a-olefin copolymer having a density greater than that of the linear low density ethylene polymer.

Where polypropylene or said mixture is present and the layers are produced in fused form by coextrusion, it is preferred that the linear low density ethylene polymer forms at least 25% by weight of the blend and the polypropylene is preferred to form not more than 25%, and more preferably not more than 10% by weight, of the blend.

If desired, the blend may include minor amounts of other compatible polymeric materials e. g. low density polyethylene or ethylene copolymer, e. g. ethylene/vinyl acetate copolymer, although in general the inclusion of such materials is not believed to provide any benefit and in amounts greater than about 20-25% of the blend, by weight, they may cause curling when the layers are produced in fused form by coextrusion.

In any event, any such additional polymeric material must be capable of being processed at the temperatures required for extruding the blends when coextrusion is employed.

The other layer, which is preferably that which provides the outer face of the bag or sack, comprises linear low density ethylene polymer. For best results it is preferred that the density thereof is in the range about 0.916 to about 0.925, more preferably about 0.918 to about 0.922 g/cm3. It may also include a minor amount of other compatible plastics material, e. g. low or high density ethylene polymer or copolymer ; however no advantage seems to be gained thereby.

The increased coefficient of friction of the outer face of the sack or bag may be achieved by mechanical and/or electrical treatment of the surface, by applying a friction-increasing additive such as a tackifier and/or a rubbery polymer to the surface or, preferably, by incorporating such an additive in the polymer composition employed in the production of the layer providing said outer face. However, where an additive is applied or incorporated it is important that the surface thus obtained is not sticky to the touch since otherwise it will interfere with the subsequent processing of the layer (or of a multi-layer e. g. coextruded film including said layer) e. g. on sack-making machinery, will be unpleasant to manhandle and will attract dirt and dust. It is also necessary, where the additive is incorporated in the composition, for said additive to be processable at the temperature required for extruding the thermoplastics layer in which it is incorporated.

Preferably, the required increase in friction coefficient is achieved by including in the composition from which the layer providing the surface is formed a polymeric component which has blocking properties. Examples of preferred such polymers are generally amorphous polymers of olefins having at least three carbon atoms, especially isobutylene polymers such as polyisobutylene.

Surprisingly, having regard to the syruppy nature of such polymers, which would be expected to result in a surface which would be sticky to the touch, attract dust, and impossible to handle on automatic machinery, particularly good results are obtained using isobutylene polymers having a molecular weight Mw (polyethylene equivalent) in the range of about 500 to a few thousand, e. g. up to about 3000 or 5000.

Satisfactory results are obtained from the polymer at concentrations of no more than a few per cent by weight. For example, the preferred isobutylene polymer may be employed in amounts of 0.5 to 10%, preferably 1 to 8%, more preferably 2 to 6% and most preferably 2.5 to 5%, by weight based on the thermoplastics material forming the layer in which the additive polymer is incorporated.

Preferably, the additive is employed in an amount to obtain a coefficient of friction, determined by ASTM test D-1894 (static test), of at least 0.8, preferably at least 0.85 and most preferably at least 0.87.

While the polymer compositions employed to form the two layers may also contain other additives conventionally employed with ethylene polymers, e. g. pigments, antioxidants and u. v. stabilisers, it is particularly advantageous where one of said layers provides the inner face of the bag or sack for the composition employed for that layer to include a slip additive so as to reduce the surface coefficient of friction, as measured by ASTM D-1894 (static test), preferably to less than 0.5, more preferably less than 0.3. The bags and sacks thereby obtained are easier to fill.

In one embodiment of the invention, which is particularly desirable where the bags or sacks are to hold living horticultural produce, e. g. seeds, bulbs or corms, at least one of the layers of the wall contains an infra-red and/or ultra-violet absorber, e. g. carbon black, to inhibit premature sprouting. Preferably, the inner layer (or, where the bag or sack comprises more than two layers, an inner layer) is thus modified, thereby permitting the outer layer to be distinctively coloured e. g. for identification purposes or for appearance.

While it has been found that the bag or sack of the invention may be stitched using essentially the same automatic equipment as is conventionally used with paper sacks it may be found advantageous to coat the area of the outer face of the wall to be contacted by stitching or automatic bag handling machinery e. g. the shoe of a sewing machine, with a band of material having a lower coefficient of friction than said outer face. The coating may be applied to the bag or sack or to the appropriate face of a film from which the bag or sack is to be formed. One convenient method of achieving this is to coat the area in question with a band of printing ink since this may be done simultaneously with the printing of any desired legend on the face of the bag, sack or film.

Bags and sacks according to the invention exhibit a combination of puncture resistance, tear strength, burst strength, tensile strength and lowslip outer surface that enables them to be employed as acceptable substitutes for heavy duty paper sacks including multi-ply sacks and provides them with a stacking capability which can exceed that of the paper sacks. Particularly good results are obtained when the bags or sacks are made from the multi-layer film of the invention, especially when the film has been produced by coextrusion. Such bags and sacks stack better than paper sacks and have higher resistance to water penetration, improved printability and better inhibition of the germination of seed products packed in them; can be stitched like paper sacks or heated sealed, and can be stored outside.

The bags and sacks or the films employed in their production may be microperforated in known manner e. g. for applications where otherwise the materials stored in them would sweat. They may also be provided with liners, if desired, and the liners may be plastics or some other material e. g. paper.

The overall thickness of the wall of the bag or sack and the ratios of the thicknesses of the individual layers may be chosen in accordance with the intended end-use. In general, the overall wall thickness will be in the range 25 to 200 microns with thicknesses of at least 140-150 microns generally being required for heavy duty use. The ratios of thicknesses of the layers formed from the linear low density polymer and the layer formed from the blend will generally lie in the range 25: 75 to 50: 50 with the higher values for the first mentioned layer within this range being preferred for the thinner overall wall thicknesses.

By way of particular Example, black sacks were made from tubular film having an overall thickness of about 150 microns and formed by the blow co-extrusion of a first composition which contained a slip agent and carbon black and the polymeric component of which comprised a blend having a density (calculated as described herein) of 0.9245 and comprising 25% by weight of an ethylene copolymer containing about 1.2 mole % hexene-1, having a density of 0.938 and marketed by Petrofina as FINA HR 501 and 75% by weight of a linear low density ethylene copolymer containing about 3 mole % octene-1, having a density of 0.920 and marketed by Dow Chemicals as DOWLEX 2045, which composition formed the inner layer of the tube; and a second composition the polymeric component of which was marketed by UNIFOS as UNIFOS 8006 and comprised a blend of (a) a linear low density ethylene copolymer containing about 3 mole % of butene-1 and (b) about 4% by weight of isobutylene polymer having a molecular weight Mw (polyethylene equivalent) in the region of about 1000 to 2000 and a broad molecular weight spread of from about 300 to 3000, which composition formed the outer layer with a thickness of about 38 microns. The sacks were found to be suitable as replacements for multi-ply paper sacks for the packaging of e. g. fertiliser and seeds in 25 kg loads.

In order to obtain a sack having similar properties of bursting strength, yield strength and tear-resistance from low density polyethylene, it was found necessary for the sack to have a wall thickness of 200-250 microns; i. e. requiring 33% increase or more in the material required.

Moreover, the surface of the conventional sack had a coefficient of friction of about 0.5 to 0.6 whereas in sacks made in accordance with the present invention, the value for the outer surface was about 0.85 to 0.87, giving much improved stackabiiity. The coefficient of friction of the inner surface, on the other hand, was below 0.5, thereby facilitating filling of the sack.

A 40"x 48"pallet was loaded with 1 tonne of produce packed into 40 sacks stacked in 8 layers of 5 sacks per layer. Using the sacks of the invention, the stack was still stable after tipping the pallet about one of its 48"edges until the opposite edge was more than 20"off the ground.

When low density polyethylene sacks were used, the stack collapsed before a tipping height of 17" and usually substantially less than 17", could be achieved.

There was no evidence of deterioration of the friction properties or of stickiness developing in the surface of the sacks of the invention even after several months storage.

A method of forming a valve bag in accordance with the invention is now described with the aid of the accompanying drawings in which Figure 1 is an elevational view of a bag made from a single ply of film, Figure 2 is a sectional view along line A-A of Figure 1, Figure 3 shows a modification of the bag of Figure 2, Figure 4 is a view corresponding to that of Figure 2 but of a different form of bag, and Figure 5 is a view corresponding to that of Figure 2 of a bag similar in construction to that of Figures 1 and 2 but formed from two separate plies.

Referring to Figures 1 and 2, the bag 2 has a two-ply back wall 10 and a front wall made of two-ply partially overlapping panels 12 and 14 which are integral with the back wall whereby to form therewith a flattened two-ply tube which is closed at both ends by transverse seals 16 and 18.

Panel 14 lies beneath panel 12 in the area of overlap and the free edge of its inner ply 20 projects a short distance beyond the free edge of its outer ply 22. A seal 24 unites both plies of panel 14 with the two plies of panel 12 and a seal 26 unites the projecting end of ply 20 with the two plies of panel 12. The portion left unsealed in the common area of overlap at the top end of the bag forms a tubular valve sleeve which is delineated by the top ends of seals 24 and 26 together with seals 28. The projecting end portion of inner ply 20 acts as a closing flap when the bag has been filled.

It will be understood that in an alternative embodiment, outer ply 22 may project beyond inner ply 20. Furthermore, seal 26 is not essential and seals 28 may be replaced by a continuous seal. In yet another embodiment, illustrated in Figure 4, outer ply 22 can extend over panel 12 instead of beneath it; in other words the portion of panel 12 which overlaps panel 14 lies between plies 20 and 22.

The bag is formed by folding and sealing a flat film or folded slit tubular film only part of the width of which is provided, over at least a part of one surface thereof, with a face having a higher surface coefficient of friction than the other face of the film, said part of the width being that which provides the external faces of the walls of the bag.

That part of the width of the film which provides the inner surfaces of the bag is not provided with such a face. Thus, in the embodiment illustrated in Figures 1 and 2 the bag is formed from a film which comprises a first layer 30 which is one of the two specified layers of this invention, and a second layer 32 which is the other of the two specified layers of the invention and is in the form of a band which covers that part of the width of said first layer which provides the external faces of walls 10,12 and 14 of the bag. This band is formed from a composition containing an additive such as isobutylene polymer which increases the surface coefficient of friction of the layer.

Preferably said first layer 30 is formed from the polymer blend.

Not all of the width of the first layer which provides the external faces of the bag need be provided with said face of higher surface friction coefficient. For example, as illustrated in Figure 3, said face may comprise two separate bands 32A and 32B of the second layer 32, which bands provide the external faces of panels 10 and 12, respectively.

In the embodiment illustrated in Figure 5, the bag of Figures 1 and 2 is formed instead from two pties 40 and 42 at least the outer ply 40 being a multi-layer film of this invention with first layer 30 and second layer 32 with the outwardly facing surface being the surface with the higher coefficient of friction. The inner ply 42 may comprise the same film, in which case the surface having the lower coefficient of friction preferably faces inwards whereby to form the inner face of the bag, or, as shown in Figure 5, it may comprise a different film provided it can be sealed, e. g. by heat sealing, to the outer ply 40.

Claims (30)

1. A thermoplastics bag or sack at least one of the walls of which includes first and second layers each formed by melt extrusion from a synthetic thermoplastics material, and wherein one of said layers comprises a blend of linear low density ethylene polymer and at least one of (a) a second ethylene polymer which has a higher density and is selected from ethylene homopolymers and copolymers of ethylene and at least one other a-olefin and (b) polypropylene, and the other of said layers comprises linear low density ethylene polymer ; at least one of said layers provides a face of the wall and at least a part of the surface of the outer face of the wall has a higher coefficient of friction than the inner face.

2. A bag or sack as claimed in claim 1 in which the layers of the wall are fused together.

3. A bag or sack as claimed in claim 2 in which the wall is formed from coextruded plastics film.

4. A bag or sack as claimed in any one of claims 1 to 3 in which said other of said layers is the outer layer.

5. A bag or sack as claimed in any one of claims 1 to 4 in which said linear low density ethylene polymer of said other of said layers has a density in the range 0.916 to 0.925 g/cm3.

6. A bag or sack as claimed in any one of claims 1 to 4 in which said linear low density ethylene polymer of said other of said layers has a density in the range 0.918 to 0.922 g/cm3.

7. A bag or sack as claimed in any one of claims 1 to 6 in which the coefficient of friction of the outer face of the wall is at least 0.8 as determined by ASTM D-1894 (static test).

8. A bag or sack as claimed in claim 7 in which the coefficient of friction of the outer face of the wall is at least 0.85.

9. A bag or sack as claimed in claim 8 in which the coefficient of friction of the outer face of the wall is at least 0.87.

10. A bag or sack as claimed in any one of claims 1 to 9 in which said one of said layers forms the innermost layer.

11. A bag or sack as claimed in any one of claims 1 to 10 in which the linear low density ethylene polymer employed in the blend is a copolymer of ethylene and an a-olefin having more than 4 carbon atoms.

12. A bag or sack as claimed in any one of claims 1 to 11 in which the blend comprises linear low density ethylene polymer and a second ethylene polymer which has a higher density.

13. A bag or sack as claimed in claim 12 in which said second polymer has a density in the range 0.930 to 0.935 g/cm3.

14. A bag or sack as claimed in claim 11, 12 or 13 in which the blend has a density in the range of about 0.914 to about 0.937 g/cm3.

15. A bag or sack as claimed in claim 14 in which the blend has a density in the range of about 0.920 to about 0.935 g/cm3.

16. A bag or sack as claimed in claim 15 in which the blend has a density in the range of about 0.923 to about 0.930 g/cm3.

17. A bag or sack as claimed in any one of claims 1 to 16 in which the inner face of the wall has a coefficient of friction, as determined by ASTM D-1894 (static test) of less than 0.5.

18. A bag or sack as claimed in any one of claims 1 to 17 which is a valve bag.

19. A bag or sack as claimed in claim 1, substantially as hereinbefore described with particular reference to the Example.

20. A multi-layer film comprising a laminate having at least two layers each of which is formed by melt extrusion from a synthetic thermoplastics material, and wherein a first one of said layers comprises a blend of linear low density ethylene polymer and at least one of (a) a second ethylene polymer which has a higher density and is selected from ethylene homopolymers and copolymers of ethylene and at least one other a-olefin and (b) polypropylene and a second one of said layers comprises linear low density ethylene polymer, at least one of said layers provides a face of said film and at least a part of the surface of one face has a higher surface coefficient of friction than the other face of the film.

21. A multi-layerfilm as claimed in claim 20 which has been formed by coextrusion.

22. A multi-layerfilm as claimed in claim 21 which has been formed by blow-extrusion.

23. A multi-layer film as claimed in any one of claims 20 to 22 wherein said first and second layers provide the faces of the film.

24. A multi-layer film as claimed in any one of claims 20 to 23 wherein in the layer which comprises linear low density ethylene polymer, said polymer has a density as specified in claim 5 or claim 6.

25. A multi-layer film as claimed in any one of claims 20 to 24 in which the layer which comprises linear low density ethylene polymer provides the face of the film having the higher surface coefficient of friction.

26. A multi-layer film as claimed in claim 25 in which said coefficient of friction is as specified in any one of claims 7 to 9.

27. A multi-layer film as claimed in any one of claims 20 to 26 in which the blend is as defined in any one of claims 11 to 16.

28. A multi-layer film as claimed in claim 25, claim 26 or claim 27 in which the layer which comprises the blend forms the other face of the film.

29. A multi-layer film as claimed in any one of claims 20 to 28, substantially as hereinbefore described with particular reference to the Example.

30. A bag or sack formed from a multi-layer film as claimed in any one of claims 20 to 29.