GB1577139A - Plastics sacks - Google Patents

Description

(54) PLASTICS SACKS (71) We IMPERIAL CHEMICAL INDUSTRIES LIMITED Imperial Chemical House, Mill bank, London SW1P 3JF a British Company do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment: This invention relates to plastics sacks and especially to a plastics sack particularly suitable for use in the packaging of pow ders.

In accordance with the invention, we provide a multiwall sack comprising an outer wall of microperforated plastics film and an inner wall of flexible, air-perme able sheet of foamed thermoplastics material.

By the term "microperforated plastics film" we mean plastics film that is per forated by a multiplicity of holes of such size that the film remains impervious to water, but gases can pass through the holes, so that air can escape through the wall.' The film may be perforated by punch ing it with pins against a resilient surface, for example by passing it through a nip formed between'a rotating, resilient, feeding member and a rotating punching roller equipped with projecting pins spaced at the required intervals. Pins formed from pdlished-steel 30 gauge (0.012 inch dia meter) piano wire are yery suitable. Fur 'ther layers may optionally be included, provided that they are air-permeable, be tween the inner and outer walls.

The foamed plastics sheet may be made by the methods described in UK Snecifica tions Nos. 1 220053 and 1 306 373. The materials there described are generally between 0.01 and 0.03 inch (0.25 and 0.76 mm) thick, foamed sheets of a thickness of from 0.005 to 0.15 inch (0.13 to 0.38 mm) being particularly suitable for the present purpose. Foamed sheets having a permeability to air, measured by the method of B S 2925, of from 0.1 to 1.0 ml per sec, per cm head of water, per cm2 of foamed sheet, are preferred.

Suitable thermoplastics for producing the foamed sheet include, for example, polymers and copolymers of olefines such as ethylene, propylene, butene-l; polystyrene and styrene copolymers such as sty renelmaleic anhydride copolymers; and polyvinyl chloride. Because of their cheapness, toughness and ready availability, ethylene polymers and copolymers, or blends thereof, and especially low-density polyethylene, are particularly suitable.

Plastics films especially useful for form- ing the outer wall of the sack include, for example, films of ethylene polymers or copolymers, or of blends thereof, and plasticized polyvinyl chloride films. Films of low-density polyethylene, widely used in non-perforated form for making plastics sacks, are particularly suitable.

Because fine powders are frequently filled into sacks by air-assisted methods, such that air becomes entrained in the powder and passes into the sacks, the sack of the invention is used with particular advantage in the packaging of, powders.

The excess air can then pasts out through the walls of .the sack during and subsequent to filling, and the waste of, snace and difficulties in .stacking the filled sacks that would otherwise occur may thus be avoided. The stack gives a further advantage, in comparison with lined sacks of many types, that the liner is heat-sealable. Moreover, the liner preferably comprises a thermoplastics material which is heat-sealable to the plastics film forming the outer wall, as it may then be joined with the outer wall into heat-seals closing the ends of the sack.

This, of course, is not essential in the construction of the sack, since the liner may be separately inserted and left loose in the sack, or joined, for example, only at the sack mouth, if preferred. However the sealing of the liner into the sack at the end seams is generally more convenient in use, and it allows the sacks to be very easily produced from flat sheets of the microperforated and foamed films by a continuous process. Thus, film and foam webs may be laid one upon the other, their pairs of edges folded over upon a former with the foam inside the film at each fold so that one pair of edges overlaps the other, in which position these edges may be heat-sealed together longitudinally, and sacks may be separated from the length by transversely sealing and severing the web at predetermined intervals.This method may be arranged for the production of valved sacks or of open-mouthed sacks, for example by the methods described in UK Specifications Nos.

1 367 001 and 1 436 126 respectively.

Valved sacks are especially suitable for use in the packaging of powders, since this avoids the possible difficulties of heat-sealing surfaces contaminated by the powder after the sack has been filled. Thus, the invention is very usefully applied to the production of valved sacks.

The sack of the invention may if preferred be made from microperforated tubular film, the sack units being transversely sealed and severed from the web, and the liners inserted through the open ends of the sacks.

A further advantage of the sack of the invention is that it may be economically produced The foamed layer is of the type used for making plastics wallpaper, which for that purpose must be produced to a high specification. Sub-standard and scrap material from such manufacture thus provides a useful and economical source of material for making the sack of this invention, which allows a much greater tolerance in properties of the foamed film. Conversely, the production of sacks constructed in accordance with this invention provides a useful outlet for substandard and scrap material from the production of plastics wallpaper, thus giving economies in that process.

WHAT WE CLAIM IS: 1. A multiwall sack -comprising an outer wall of microperforated plastics film and an inner wall of flexible, air-permeable sheet of foamed thermoplastics material.

2. A multiwall sack according to claim 1 in which the foamed sheet has a thickness of less than 0.76 mm.

3. A multiwall sack according to claim 2 in which the foamed sheet has a thickness of from 0.13 to 0.38 mm.

4. A multiwall sack according to any one of the preceding claims in which the foamed sheet has an air permeability of from 0.1 to 1.0 ml per sec, per cm head of water, per cm2 of foamed sheet.

5. A multiwall sack according to any one of the preceding claims in which the foamed sheet comprises a polyolefin, polyvinyl chloride, polystyrene or styrene maleic anhydride copolymer.

6. A multiwall sack according to claim 5 in which the foamed sheet comprises an ethylene polymer, copolymer or blend thereof.

7. A multiwall sack according to any one of the preceding claims in which the microperforated film comprises an ethylene polymer, copolymer or blend thereof, or polyvinyl chloride.

8. A multiwall sack according to any one of the preceding claims in which the inner wall comprises a thermoplastics material which is heat-sealable to the plastics film of the outer wall.

9. A multiwall sack according to claim 8 in which the inner wall is joined with the outer wall into heatseals closing the ends of the sack.

10. A multiwall sack according to any one of the preceding claims which is a valved sack.

11. A method for making a multiwall sack as claimed in any one of the precedinug claims, comprising laying a web of foamed thermoplastics material and a web of microperforated thermoplastics film one upon the other, folding over the pairs of edges with the foam inside the film at each fold so that one pair of edges overlaps the other, longitudinally heat-sealing together the overlapping pairs of edges, and transversely sealing and severing the webs at predetermined intervals to provide the individual sacks.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. construction of the sack, since the liner may be separately inserted and left loose in the sack, or joined, for example, only at the sack mouth, if preferred. However the sealing of the liner into the sack at the end seams is generally more convenient in use, and it allows the sacks to be very easily produced from flat sheets of the microperforated and foamed films by a continuous process. Thus, film and foam webs may be laid one upon the other, their pairs of edges folded over upon a former with the foam inside the film at each fold so that one pair of edges overlaps the other, in which position these edges may be heat-sealed together longitudinally, and sacks may be separated from the length by transversely sealing and severing the web at predetermined intervals.This method may be arranged for the production of valved sacks or of open-mouthed sacks, for example by the methods described in UK Specifications Nos.

1 367 001 and 1 436 126 respectively.

Valved sacks are especially suitable for use in the packaging of powders, since this avoids the possible difficulties of heat-sealing surfaces contaminated by the powder after the sack has been filled. Thus, the invention is very usefully applied to the production of valved sacks.

The sack of the invention may if preferred be made from microperforated tubular film, the sack units being transversely sealed and severed from the web, and the liners inserted through the open ends of the sacks.

A further advantage of the sack of the invention is that it may be economically produced The foamed layer is of the type used for making plastics wallpaper, which for that purpose must be produced to a high specification. Sub-standard and scrap material from such manufacture thus provides a useful and economical source of material for making the sack of this invention, which allows a much greater tolerance in properties of the foamed film. Conversely, the production of sacks constructed in accordance with this invention provides a useful outlet for substandard and scrap material from the production of plastics wallpaper, thus giving economies in that process.

WHAT WE CLAIM IS: 1. A multiwall sack -comprising an outer wall of microperforated plastics film and an inner wall of flexible, air-permeable sheet of foamed thermoplastics material.

2. A multiwall sack according to claim 1 in which the foamed sheet has a thickness of less than 0.76 mm.

3. A multiwall sack according to claim 2 in which the foamed sheet has a thickness of from 0.13 to 0.38 mm.

4. A multiwall sack according to any one of the preceding claims in which the foamed sheet has an air permeability of from 0.1 to 1.0 ml per sec, per cm head of water, per cm2 of foamed sheet.

5. A multiwall sack according to any one of the preceding claims in which the foamed sheet comprises a polyolefin, polyvinyl chloride, polystyrene or styrene maleic anhydride copolymer.

6. A multiwall sack according to claim 5 in which the foamed sheet comprises an ethylene polymer, copolymer or blend thereof.

7. A multiwall sack according to any one of the preceding claims in which the microperforated film comprises an ethylene polymer, copolymer or blend thereof, or polyvinyl chloride.

8. A multiwall sack according to any one of the preceding claims in which the inner wall comprises a thermoplastics material which is heat-sealable to the plastics film of the outer wall.

9. A multiwall sack according to claim 8 in which the inner wall is joined with the outer wall into heatseals closing the ends of the sack.

10. A multiwall sack according to any one of the preceding claims which is a valved sack.

11. A method for making a multiwall sack as claimed in any one of the precedinug claims, comprising laying a web of foamed thermoplastics material and a web of microperforated thermoplastics film one upon the other, folding over the pairs of edges with the foam inside the film at each fold so that one pair of edges overlaps the other, longitudinally heat-sealing together the overlapping pairs of edges, and transversely sealing and severing the webs at predetermined intervals to provide the individual sacks.