WO1992009739A1

WO1992009739A1 - Coated fabric for weather protection

Info

Publication number: WO1992009739A1
Application number: PCT/AU1991/000555
Authority: WO
Inventors: Peter Richards Terry; Julian R. Egerton
Original assignee: Rheem Australia Limited
Priority date: 1990-11-29
Filing date: 1991-11-28
Publication date: 1992-06-11

Abstract

A flexible fabric suitable for weather protection purposes particularly adapted for example to covering agricultural grains in outdoor storage including a woven synthetic substrate with a melt extruded coating where the coating is a blend of an olefin copolymer and a high loading of an anhydrous, high density, insoluble, low toxicity inorganic filler. The high density filler is selected from abundantly available naturally occurring minerals having a specific gravity of 3.5 or more. Particularly suitable minerals may be non-hydrated forms of oxides, carbonates, sulphates and silicates of metals including aluminium, barium, iron, manganese, magnesium, titanium or zinc. A particularly suitable filler is barium sulphate. The high loading of inorganic filler involves inclusion of 10-20 % of the filler with respect to the mass of the coating. A suitable olefin copolymer is ethylene methyl acrylate copolymer or ethylene butyl acrylate copolymer.

Description

COATED FABRIC FOR WEATHER PROTECTION

BACKGROUND

This invention relates to a flexible fabric suitable for weather protection purposes. It more specifically relates to such a fabric comprising a woven synthetic substrate with a melt extruded coating over the substrate.

Although not a limitation an application for which the invention is particularly suited is as a temporary on-ground agricultural grain storage cover tarpaulin. It can be used also in other weather protection cover applications such as in the building industry.

Cover tarpaulins made from synthetic resins are already well known. However, very large tarpaulins suitable for covering large objects or areas out of doors have specific requirements. Firstly, their mass per unit area needs to be appropriate for the way in which they are handled and the size of the object to be covered. It has been found through long experience that a so-called medium weight fabric having a mass per unit area of 400-500 grams per square metre is particularly suited. In grain storage cover applications, at this mass per unit area the covers sit neatly on the stack and yet are not too heavy for the manual handling involving the fitting and removing of the cover.

It is also highly desirable in such applications that these fabrics have a high surface co-efficient of friction. They must not be slippery so preventing walking upon when covering a sloping sided object. The fabric must have good resistance against cracking, particularly cracking relating to repeated flexing. These requirements derive from the fact that the covers are folded and stored when not in use and that in the outdoors they are subjected to repeated flexing due to flapping caused by the wind.

The fabrics must also have good weather resistance, particularly in respect of the ultraviolet component of sunlight and also to rain, hail and wind. They require strength classified as medium, namely tensile strength of 1000 Newtons per 50mm and 80 Newtons for wing tear strength. Being required generally in the form of extremely large tarpaulins, these fabrics require excellent weldability to enable multiple widths and lengths to be joined. They also require excellent adhesion of the coating to the substrate since this underlies the required weld strength and wind flap resistance to delamination. Particularly when used in an agricultural environment an economical cost is most important, since production of staple farm commodities generally demands lowest possible costs.

In the application relating to grain coverage, another important property is resistance to fumigants used to treat the stored grain and the ability for the fabric as a whole to be produced in formulations approvable for food contact. This applies at least in respect of the side of the fabric which is to make contact with the grain.

Known fabrics already in use for this purpose fall into two categories, those based on polyvϊnyl chloride (PVC) coated fabrics or those based on low density polyethylene (LDPE) coated fabrics. The PVC coatings are generally applied to substrates of polyester, polyvinyl alcohol and nylon. The LDPE family of fabrics generally involve a low density polyethylene (LDPE) coating a woven high density polyethylene (HDPE) substrate.

The P C coated fabrics generally meet mechanical strength requirements well but have only fair weatherability and are of relatively high cost, particularly when formulated for food contact purposes. On the other hand, the LDPE coated fabrics are relatively low in cost (including food contact formulations) and can be formulated to have good weatherability. However, they have only fair crack and flex crack resistance and are often too low in mass per unit area to be easily handled in the application involving very large covers over grain storage.

Attempts to provide a heavier weight more crack and flex crack resistant fabric based on polyolefins have met with only very limited success hitherto. Any attempt to increase the mass per unit area by increasing either the woven substrate mass or the coating mass have led to fabrics which tend to be too stiff and have poor crack resistance. However, the present invention discloses a fabric based on a filled ethylene copolymer coated woven HDPE (although a woven polypropylene substrate would also be satisfactory) which meets all the application requirements by combining the best characteristics of both the PVC coated and HDPE coated fabric types already known, to a surprisingly effective extent.

SUMMARY OF THE INVENTION.

The invention consists of a flexible fabric suitable for weather protection purposes comprising a woven synthetic substrate and a melt extruded coating where the coating comprises a blend of an olefin copolymer and a high loading of an anhydrous, high density, insoluble, low toxicity inorganic filler.

Preferably the high density filler is chosen from the group consisting of abundantly available naturally occurring minerals having a specific gravity of 3.5 or more.

Preferably those minerals include the non-hydrated forms of oxides, carbonates, sulphates and silicates of metals including aluminium, barium, iron, manganese, magnesium, titanium or zinc.

Preferably the olefin copolymer is selected from the group consisting of ethylene methyl acrylate copolymer and ethylene butyl acrylate copolymer.

Preferably the high loading of inorganic filler involves inclusion of 10-20% of the filler with respect to the mass of the coating.

DETAILED DESCRIPTION.

The invention is further described with respect to a particularly preferred example of a formulation suitable for the application as above described and possible variations thereof.

Two examples were produced comprising a base fabric woven HDPE substrate having masses of 150 grams per square metre and 190 grams per square metre. If such a base fabric were to be coated with extrusion coated LDPE on each side in order to reach a total mass of 400 grams per square metre for the coated fabric, this would require thicknesses of typically 114 micrometres (urn) for LDPE. This would be much too thick to make an effective cover for the purpose as the overall fabric would be too stiff and very prone to cracking and flex cracking associated with higher forces required for bending. Using an ethylene copolymer as the coating resin, a required thickness of perhaps 112 urn would be more flexible than the LDPE but would be much stiffer than, for example, a PVC coated fabric. Using what is referred to herein as a high loading of dense inorganic filler, namely barium sulphate, at 15% concentration, blended in with the extrusion coating resin which is ethylene methyl acrylate copolymer (EMAC) a coating of 105 gsm on both sides of the substrate as referred to in the previous paragraph at a thickness of 80 urn was found to provide a fabric which was not excessively stiff. The fabric so formulated has excellent resistance to delamination of the coating and possesses the required medium mass per unit area of around 410 gsm. It also has very good crack and flex crack resistance, much better than LDPE although perhaps not quite as good as the PVC coated fabrics. It has excellent weatherability, superior to - LDPE coated fabrics, particularly with the inclusion of ultraviolet stabilisers such as HALS. The fabric is also economical in price, between that of LDPE and PVC coated fabrics and is able to be welded very readily. It also has good strength and excellent cut and abrasion resistance. In overall properties it compares extremely favourably with PVC coated fabrics at a substantially lower price. The inclusion of HALS as UV stabiliser needs to be balanced in terms of the degree of protection improvement available with increasing concentration (against sunlight) versus the reduction in food contact acceptability attendant with increasing concentration of HALS. The use of an opaque dense filler increases opacity and adds to the UV protection.

Further experiments using a different ethylene copolymer, namely ethylene butyl acrylate copolymer (EBAC) show that this is probably satisfactory for the purpose. The inclusion of up to 30% of a dense filler, namely barium sulphate with EBAC, showed a substantial deterioration of coating adhesion strength to a substrate of woven HDPE. Thus a 20% loading was thought to represent an indication of the upper limit of a "high" loading of dense filler.

Other dense fillers which may be equally satisfactory to barium sulphate include barium oxides, iron oxides, manganese dioxide, magnesium oxide, aluminium oxide, titanium dioxide or zinc carbonate or mixtures of these. Of these, magnesium oxide with a specific gravity of about 3.5 is considered to be the least dense of the possibly suitable fillers which would be satisfactory in providing the required mass per unit area and flexibility properties of the coated fabric. Subject to ready availability at a low enough cost, it is believed that the oxides, carbonates, sulphates and silicates of metals abundantly available in the earth's crust would be satisfactory for the purpose without entailing excessive cost of the resultant fabric. Thus, in this context a dense filler is regarded as one which has a specific gravity of 3.5 or more. Also a high loading of this filler which would not cause deterioration of the properties of the resultant coated fabric would be regarded as not more than 20% on a weight basis.

It is to be noted that the inclusion of 3-4% of a dense filler in the HDPE or polypropylene of the substrate woven tapes causes unacceptable loss of strength together with poor adhesion to coatings of other olefins.

Claims

1. A flexible fabric suitable for weather protection purposes comprising a woven synthetic substrate and a melt extruded coating where the coating comprises a blend of an olefin copolymer and a high loading of an anhydrous, high density, insoluble, low toxicity inorganic filler.

2. A fabric as claimed in claim 1 in which the high density filler is chosen from the group consisting of abundantly available naturally occurring minerals having a specific gravity of 3.5 or more.

3. A fabric as claimed in claim 2 in which the minerals are selected from the group consisting of the non-hydrated forms of oxides, carbonates, sulphates and silicates of metals including aluminium, barium, iron, manganese, magnesium, titanium or zinc.

4. A fabric as claimed in any one of the previous claims in which the olefin copolymer is selected from the group consisting of ethylene methyl acrylate copolymer and ethylene butyl acrylate copolymer.

5. A fabric as claimed in any one of the previous claims in which the high loading of inorganic filler involves inclusion of 10-20% of the filler with respect to the mass of the coating.