GB2039788A - Breathable Fabric and Sack - Google Patents

Abstract

A breathable fabric is disclosed, which comprises a structure woven from thermoplastic polymeric tapes, wherein the woven structure 11 is coated with a foamed permeable coating 12. The tapes are preferably made from polyethylene having a density between 0.940 and 0.970 g/cm<3>. The coating is preferably a lace coating 12 (thus leaving holes 13 in the coating) made from polyethylene having a density between 0.910 and 0.960 g/cm<3> prior to coating the structure. The fabric is useful for making breathable sacks. <IMAGE>

Description

SPECIFICATION Breathable Fabric and Sack The present invention relates to a breathable woven fabric of thermoplastic polymeric tapes and sacks made therefrom.

Breathable woven fabrics are useful in packaging applications, particularly for example for making sacks intended to contain finely divided solid agricultural products e.g. seed.

The term "breathable" refers to the ability of the fabric to allow gases and moisture to pass freely through the fabric.

The manufacture of woven tape structures is known. Such structures may be formed from oriented tapes of polypropylene or polyethylene.

Structures of this type may be subject to a tendency to unravel or the weave may be subject to a tendency to distort which may affect the utility of the woven structure in some end uses.

Techniques for the stabilization of woven structures are known, for example the use of coatings as described in U.K. Patent 1,185,553.

For some applications it is desired to retain the breathable nature of an uncoated woven fabric, e.g. for agricultural sacks and tarpaulins. One method for obtaining a breathable sack disclosed in Canadian Patent 909,726 involves inserting a plastic liner punctured with a plurality of small holes, e.g. 0.3 mm in diameter into a sack woven from oriented plastic tapes. In general, however, methods involving perforation of plastic liners and insertion of liners into sacks tend to be expensive.

For packaging granular or finely divided solids, e.g. fertilizer or seed, in sacks of woven structures it is desirable not only to provide a sack which is breathable but also to ensure that the weave of the structures do not exhibit a tendency to open i.e. to ensure that the weave is stable.

Weave stability is also important for making tubes of woven structures, made from polyethylene tapes, coated with a thin layer of polyethylene by the method disclosed in U.S.

patent 3,951,050. Such tubes are useful for making industrial sacks.

Techniques for stabilization of woven structures which retain the breathable nature of the structure are known, for example by use of an adhesive as is described in U.S. Patent 2,521,055 and by heating as is described in Canadian Patent 995,100. Such techniques tend to be expensive or require facilities in addition to the weaving and coating facilities normally available to manufacturers of fabric woven from thermoplastic polymeric tapes.

A breathable fabric having a stable weave has now been found.

Accordingly the present invention provides a fabric comprising a structure woven from thermoplastic polymeric tapes, said structure having a foamed permeable coating bonded to at least one side thereof.

in a preferred embodiment the coating is a lace coating.

In another embodiment the thermoplastic polymeric tapes are selected from the group consisting of polypropylene and polyethylene, .especially polyethylene having a density between 0.940 and 0.970 g/cm3.

In yet another embodiment the lace coating is made from a synthetic thermoplastic polymer capable of meit extrusion.

In another embodiment the fabric comprises a structure woven from tapes of polyethylene having a density between 0.940 and 0.970 g/cm3, said structure having a foamed permeable coating of polyethylene bonded to at least one side thereof. The density of the polyethylene used for the coating is preferably 0.910 to 0.960 g/cm3, especially 0.910 to 0.940 g/cm3, prior to coating the structure.

In a further embodiment the material comprising the lace coating covers between about 10% and 90% and especially between about 30% to 70% of the side of the woven structure to which the coating is bonded.

The present invention also provides a process for making a breathable fabric comprising extruding a coating of a synthetic thermoplastic polymer onto and bonding the coating to a woven structure of synthetic thermoplastic polymeric tapes, wherein the coating material contains a blowing agent in an amount sufficient to make the coating porous.

In one embodiment the coating process comprises extruding a web of the coating material from a stationary slot die onto a web of the woven structure travelling beneath and transversely to the direction of extrusion of the web of coating material.

In a preferred embodiment the temperature of extrusion, the amount of blowing agent, the distance between the die and the woven web, rate of travel of the woven web are adjusted to such an extent that the coating thus formed is a lace coating having a coating weight between about 12 and 70 g/cm2 of woven structure.

In a further embodiment the coating material is polyethylene having a density between 0.910 and 0.960 g/cm3, especially between 0.910 and 0.940 g/cm3, prior to extrusion containing a blowing agent selected from the group consisting of zinc carbonate, sodium bicarbonate, hydrated alumina and azodicarbonamide and the woven structure is made from tapes of polyethylene.

In yet another embodiment the lace coating is formed using zinc carbonate as the blowing agent in a concentration of about 2% by weight of the coating material.

The present invention also provides a breathable sack made from a fabric of the present invention.

The choice of polyethylene for the porous coating in the fabric for use in stitched or heat sealed breathable sacks may depend on factors such as the slip resistance of the outer surface of the sack, the heat sealability of the fabric and ease of coating the woven structure, among other things.

It will be appreciated by those skilled in the art that polyethylenes having a density in the range of 0.910 to 0.940 g/cm3 are somewhat easier to apply as a coating than polyethylene having higher densities. However, some polyethylenes having a density in the range of 0.940 to 0.960 g/cm3 have recently been developed which are equaily suitable for coating e.g. may be used at production rates approaching those of the lower density polyethylenes.

In the embodiment where the woven structure is made from polypropylene, the coating polymer is selected from homopolymers and copolymers of propylene.

The present invention may be illustrated by reference to the drawing which is a schematic representation of an embodiment of a woven structure of tapes of polyethylene having a density between 0.940 and 0.970 g/cm3, hereinafter referred to as high density polyethylene, coated with a lace coating of polyethylene having a density between 0.910 and 0.960 g/cm3.

Referring to the drawing the lace coated structure 10 comprises a woven structure 11 of high density polyethylene warp and weft tapes coated with a lace coating 12 of low density polyethylene. The holes 1 3 in the lace coating may give the fabric the appearance of having a fibrillated film coated thereon or the holes may be somewhat elongated and clearly defined. In the embodiment shown in the drawing, the holes are reasonably well defined, but many holes have "strings" of low density polyethylene 14 within the holes.

The lace coated structure may be made by extruding low density polyethylene having about 20 g of blowing agent/kg of polyethylene, at a temperature of between about 2000C and 4000C onto a longitudinally travelling web of woven high density polyethylene tape structure.

It will be understood that desired features of the coating of the fabric e.g. thickness, hole size, stringiness and heat sealability may be controlled by selection of the coating material, blowing agent and by altering processing variables e.g. the relative rates of extrusion of the coating and passage of the base woven structure, distance of extrusion orifice to the base woven structure and temperature of the coating material at extrusion.

The following examples serve to illustrate the present invention.

Example 1 A 33 cm wide web of woven high density polyethylene tapes was transported from a supply beam, past an extrusion coater, having a 0.5 mm wide extrusion die 36 cm in length, and a 1 m wide chill roll and subsequently wound up on a wind-up beam. The woven structure was made from 2.8 mm wide tapes, with 36 ends per 10 centimetres and 28 picks per 10 centimetres. The web was transported past the extrusion coater at a speed of 61 m/min. The coating travelled in an unsupported condition a distance of about 1 5 cm between the die lip and the web.

Low density polyethylene resin having 20 grams of zinc carbonate blowing agent per kilogram of polyethylene was extruded through the die at a rate of 39 kg/hr, and at a temperature of 31 00C. The woven web thus coated was passed over the chill roll which was maintained at a temperature of 1 50C. The woven web so processed had a lace coating about 66 ym in thickness with a standard deviation of about 23 ssm. The coverage of the woven web by the low density polyethylene was about half i.e. about half of the area of the coating had holes therethrough.

The lace coated fabric so formed was breathable, as shown by the following porosity test: Air was introduced into a 9.5 cm internal diameter pipe which was open at one end. The air flow was controlled until the velocity of air exiting from the open end of the pipe was 42.7 m/min. A sample of the coated fabric was clamped over the open end of the pipe and the air flow passing through the fabric was measured. The air flow through the fabric of Example 1 was between about 26.0 and 33.5 m/min. This compares with an air flow of between about 36.5 and 38.0 m/min. through a similar but uncoated structure of woven high density polyethylene tapes.

Example 2 The procedure of Example 1 was repeated except that the speed of transporting the woven web past the extrusion die was increased, and the extrusion rate of the coating was likewise increased to maintain the thickness of the coating to something akin that of Example 1. At a web speed of 122 m/min. the coverage of the woven web by the coating was about three quarters, i.e.

about one quarter of the area of the coating had holes therethrough.

Claims (15)

Claims

1. A fabric comprising a structure woven from thermoplastic polymeric tapes, said structure having a foamed permeable coating bonded to at least one side thereof.

2. A fabric according to Claim 1 wherein the thermoplastic polymeric tapes are made from a polymer selected from the group consisting of polypropylene and polyethylene.

3. A fabric according to Claim 2 wherein the thermoplastic polymeric tapes are made from polyethylene having a density between 0.940 and 0.970 g/cm3.

4. A fabric according to Claim 3 wherein the foamed permeable coating is made from polyethylene.

5. A fabric according to Claim 4 wherein the density of the polyethylene used for coating is between 0.910 and 0.960 g/cm3, prior to coating the structure.

6. A fabric according to Claim 5 wherein the density of the polyethylene used for coating is between 0.910 and 0.940 g/cm3, prior to coating the structure.

7. A fabric according to any one of the preceding claims wherein the coating is a lace coating.

8. A fabric according to claim 7 wherein the lace coating is madefrom a synthetic thermoplastic polymer capable of melt extrusion.

9. A fabric according to Claim 7 or Claim 8 wherein the material comprising the lace coating covers between 10 and 90% of the side of the structure to which the coating is bonded.

10. A fabric coating to Claim 9 wherein the material comprising the lace coating covers between 30 and 70% of the side of the structure to which the coating is bonded.

11. A process for making a breathable fabric comprising extruding a coating of a synthetic thermoplastic polymer onto and bonding the coating to a woven structure of synthetic polymeric tapes, wherein the coating material contains a blowing agent in an amount sufficient to make the coating porous.

12. A process according to Claim 11 wherein a web of the coating material is extruded from a stationary slot die onto a web of the woven structure travelling beneath and transversely to the direction of extrusion of the web of coating material.

13. A process according to Claim 12 wherein the temperature of extrusion, the amount of blowing agent, the distance between the die and the woven structure, and the rate of travel of the woven structure are adjusted to such an extent that the coating thus formed is a lace coating having a coating weight between about 12 and 70 g/cm2 of the woven structure.

14. A process according to claim 13 wherein the coating material has a density between 0.910 and 0.960 g/cm3 prior to extrusion, containing a blowing agent and the woven structure is made from tapes of polyethylene.

15. A process according to Claim 14 wherein the blowing agent is selected from the group consisting of zinc carbonate, sodium bicarbonate, hydrated alumina, and azodicarbonamide.

1 6. A fabric substantially as described with reference to the accompanying drawings.

1 7. A process for making a breathable fabric substantially as described with reference to either of the examples.