Permeable sheet material

Abstract: The invention relates to layer sheet material which is useful in a wide variety of applications as a barrier to liquids and particulate material, but which will allow the passage of air. An example of such a material, suitable for use in clothing for industrial environments and the like, comprises a first layer (2) comprising a sheet of impervious non-linting, homogenous material, such as plastics sheet, having a multiplicity of micropores (5) to allow the passage of air, bonded at discrete positions (3) to a second layer (1) of fibrous material which by its constructions is air permeable but impervious to liquids and particles, so as to leave interstices (4).

Description: The invention concerns improvements in layered sheet material, which is permeable to air but which is generally impervious to particles and liquids, and is particularly although not exclusively suitable as a material for industrial use and from which work clothing can be made. It is a general requirement of many industries and medicine to provide clothing, especially disposable clothing, which protects the wearer against undesirable particles or liquids, or work carried out in a clean room, for example, becoming contaminated with undesirable particles from the worker, while at the same time remaining permeable to air, i.e. "breathable", in order to be comfortable to wear. Examples of such applications include paint spraying, mining, removal of asbestos or other harmful particulate materials, electronics, chemical industries, surgery. In addition, for many of these applications the material used for making such clothing must often fulfil various further requirements. For instance, where vulnerable electronic components or inflammable gases are involved, as in paint spraying or some chemical applications, the material must have antistatic properties or be treatable to this end. For applications where cleanliness is important, as in surgery, paint spraying or electronics, the material should not itself produce any contamination by linting. Various materials are known which fulfil some of these requirements, in particular spun-bonded olefin material such as "Tyvek" (trade mark) and spun-bonded and thermal-bonded polypropylenes, but none satisfies all the requirements of a material for work clothing. For instance, "Tyvek" has antistatic properties, is impervious to liquid and particles and is largely non-linting, but displays only limited air permeability and is therefore uncomfortable to wear. Polypropylene films can be made which are breathable and can be liquid- and particle-proof, depending on their manufacture, but are prone to linting and tend to lose any liquid-barrier properties when anti-static treated. The invention provides in one of its aspects, a layered sheet material formed of first and second layers, the first layer comprising a sheet of non-linting, homogenous impermeable polypropylene material having provided therein at a predetermined density/metre² micropores of a size in the range 0.1 to 0.4 mm to provide said layer with a controlled degree of porosity whereby the layer is permeable to air through said pores, and the second layer being comprised of spun-bonded and/or melt-blown polypropylene fibres and being by its fibrous construction air-permeable and substantially impervious to liquids and particles, the two layers being joined to form a laminated construction by bonding at a plurality of localised points or regions in a pattern, said pattern extending over at least substantially the entire area of the layered sheet material, and arranged to provide areas or interstices where no bond is present. Preferably, the first layer, which would advantageously constitute the outer layer for clothing, is a microperforated polypropylene film of continuous polymer or possibly of fibrous composition compressed to a smooth homogenous layer, or a blend of such constituents, pierced by a large number of microperforations so as to make it breathable; and liquids or particles which might otherwise pass through the perforations are stopped by the impervious second, or inner layer. Liquids are normally prevented from breaking through the microperforated film because of the surface tension across the hole diameter. A further liquid barrier is provided by the effect of surface tension across the fibre spacings of the other layer. The second, or inner, layer is composed of a fibre substrate of polypropylene fibres, which layer provides strength to the combination of layers and is easily bondable to the outer layer. The substrate is a spun-bonded and/or melt-blown fibre material. Sheet material formed of two layers according to the invention may combine the advantages and obviate the disadvantages of the two layers individually and be thus suited to provide comfortable and effective industrial clothing. The outer layer prevents any lint from the fibrous inner layer reaching the outside of the garment and further can be treated against static electricity without affecting the liquid-barrier property of the inner layer. The inner layer provides strength to the sheet and captures any liquids or particles passing through the outer layer. Because the two layers are not bonded completely, i.e. not continuously over their whole area but only at a plurality of spaced, localised points or small regions distributed uniformly or randomly over the extent of the layers, the lamination is still breathable through those microperforations which do not coincide with bonding points. This is not the case with standard bonding methods which involve an entire, co-extensive or overall lamination of adhesive or heat bonding. The localised bonding of the purposes of the invention may be effected by pressing the layers between embossing points on a thermo-bonding roller or by any other suitable method. Conceivably the two layers could be glued together by local application of adhesive to give the required effect; for example, a hot melt glue system by which it is possible to apply a localised glue pattern. For a better understanding of the invention, embodiments of it will now be described, by way of example, with reference to the accompanying figure, which shows a two-layer sheet in accordance with the invention. In Figure 1, which is not to scale, a substrate 1, of polypropylene fibre, is bonded to a homogenous film 2, likewise of polypropylene by a mesh-shaped pattern of points or very small regions of bonding 3. The bonding between film 2 and substrate 1 thus does not extend in an unbroken, co-extensive area over the entire extent of the sheet formed by the two bonded layers but leaves free spaces of interstices 4. Prior to bonding, the film 2 is penetrated mechanically to form a predetermined number of microperforations 5 allowing air to permeate in a controlled manner through the otherwise continuous impermeable film 2. The film may be white, transparent or coloured, and has a gloss finish. Although some of these perforations 5 may be blocked by the bonding process at bonding points 3, those coinciding with the free spaces 4 remain open so that the sheet as a whole is air-permeable since the substrate 1, although not itself perforate, is air-permeable through its fibrous structure by virtue of its composition. The bonding is preferably made by passing the two layers between embossed rollers and thermally bonding them together over the mesh-shaped pattern 3, though sonic or other point- or localised bonding methods can be used. The substrate 1 is fibrous and is usually a compacted non-woven sheet. The substrate gives mechanical strength to the sheet, so that the strength-to-weight ratio of the sheet can be higher than that of a comparable sheet made of a single material, and is itself impervious to particles and liquids, so that any such which are allowed through the perforations 5 in the film layer 2 cannot penetrate the substrate 1, and likewise no particles from the inside can escape through the sheet. The substrate is of a random non-woven construction as produced by melt-blowing and/or spin-bonding techniques. The film 2 and/or the substrate 1 can be anti-static treated separately on one or both sides, before bonding, in a known manner without affecting the barrier properties, and the sheet comprising the bonded layers can be sterilised for surgical purposes. The sheet in accordance with the invention is therefore capable of meeting many more of the requirements for work clothing than single-layer material such as has been used previously. The size and spacing of the bonding points or regions can be varied as required: increasing their size or decreasing their spacing improves the bonding of the two layers to each other but, because more of the pores are occluded by the process, decreases the air-permeability of the finished product. Decreasing the bonding point size or increasing the spacing has the converse effect. Typically the pore spacing will be of the order of 1 mm. The pore size of the microperforations is 0.1-0.4 mm so as to give a hole coverage of roughly 2 to 10%. Reducing the hole coverage may not directly enhance the barrier effect, according to tests, so that a 10% hole coverage giving adequate breathability may also be acceptable as a liquid and particle barrier. The bonding point dimensions will be about 0.5-1.0 mm with a spacing of perhaps 1-2 mm and the bonded area might be around 20% of the total, e.g. using a bonding roller with a 20% embossed area. The pores and/or the bonding regions may be randomly arranged or in a regular array, such as the rectangular array shown in the Figure. If both are in similar arrays the arrays may be arranged coherently so that the bonding points consistently do not overlap the pores. Although the material of the invention is particularly suitable for making clothing and to this end can be processed in the normal way, no special methods being necessary, other applications are envisaged. For instance, the material could be used in filters or to make covers for machines where air circulation is necessary but contamination is to be prevented. The dimensions of the pores and the fibres of the substrate can vary depending on the use to which the material is to be put. For instance, in electronic environments the particle sizes to be excluded would be of the order of a micron and the fibre substrate would have to be constructed accordingly. Similar considerations would apply for surgical uses. On the other hand, for use in mining for example the fabric could be much coarser.

Claims: A layered sheet material formed of first (2) and second (1) layers, the first layer (2) comprising a sheet of non-linting, homogenous impermeable polypropylene material having provided therein at a predetermined density/metre²micropores (5) of a size in the range 0.1 to 0.4 mm to provide said layer (2) with a controlled degree of porosity whereby the layer (2) is permeable to air through said pores, the second layer (1) being comprised of spun-bonded and/or melt-blown polypropylene fibres and being by its fibrous construction air-permeable and substantially impervious to liquids and particles, the two layers being joined to form a laminated construction by bonding at a plurality of localised points or regions (3) in a pattern, said pattern extending over at least substantially the entire area of the layered sheet material, and arranged to provide areas or interstices (4) where no bond is present. Layered sheet material as claimed in claim 1 wherein the layers of the material are bonded by heat and pressure in a thermo-bonding technique applied in said pattern. Layered sheet material as claimed in either one of claims 1 to 2, wherein the layers of the material are bonded by adhesive material applied in said pattern. Layered sheet material as claimed in any one of the preceding claims wherein said micropores (5) of the first layer (2) are formed with a diameter selected from said size range according to the surface tension of said liquids.

Inventors: WOODBRIDGE T J, WOODBRIDGE, T.J.
Applicants: DON & LOW NONWOVENS LTD, DON & LOW NONWOVENS LIMITED
Application year: 1990
Main classification: B32B3
IPC codes: B32B5, A41D31