Metallised fabric

METALLISED FABRIC

This invention relates to metallised fabrics, and in particular is concerned with a so-called microporous metallised fabric suitable for use where thermal-insulating properties are required.

It is known to metallise at least one surface of a fabric in order to enhance the thermalinsulating properties of that fabric. For the case of a porous fabric, provided that the metallising is performed in accordance with known procedures, the metallising does not significantly affect the porous nature of the fabric, nor does the metallising significantly reduce the durability of the fabric; moreover, the metallising often increases the flexibility of the fabric. As a result, metallised fabrics of this kind have been used in the manufacture of apparel intended to be worn in extreme climatic conditions, and also in the manufacture of articles required to have excellent thermal-insulating characteristics coupled with light weight, such as blankets and sleeping bags.

More recently, such metallised fabrics have been used in the manufacture of blinds for the screening of glass-houses: by drawing out a blind of such fabric over and around the crop-growing area of a glass-house when the external ambient temperature is below that within the glass-house, the heat loss from the glass-house can greatly be reduced, leading to much lower heating costs.

The substrate fabric for use in the manufacture of a metallised fabric of the kind described above may be woven from natural fibres, such as cotton fibres, or may be woven from blended natural and synthetic fibres or even just from synthetic fibres. More commonly however the substrate fabric is manufactured from continuous, relatively fine fibres of a synthetic resin (polymer) such as a polyethylene or a polyester, which fibres are spread with a random orientation into a thin layer, and then united by the application of heat and pressure; such a manufacturing method can be performed in manner known per se so that the finished fabric has a

i.micro-porous structure. A synthetic microporous fabric of this kind is sold by E.I. du Pont de Nemours, Inc., under the Trade Mark TYVEK, Style 1621C or 1622E.

A substrate fabric of the kind just described above may be metallised, conventionally with aluminium, by means of a vacuum deposition technique. This metal has excellent thermal reflective properties which greatly enhance the 10 thermal insulative characteristic of the finished metallised fabric, and also aluminium particularly lends itself to deposition in this way. Moreover, it is possible to deposit a sufficiently small amount of aluminium so as not significantly to 15 affect the porous nature of the fabric whilst still imparting to the fabric the required heat reflective properties. The porous nature of the metallised fabric is most important for many of the uses of the fabric, where the fabric must be able to breathe 20 that is to say, moisture laden air may pass through the fabric.

As mentioned above, metallised fabrics of the just-described kind have been used for the manufacture of blinds for the thermal insulation of glass-houses. When metallised, the aluminium 5 metallising is directed outwardly, and the blind relies on the so called 'emissivity' characteristic of the metallised fabric - that is to say, the ability of the fabric to radiate heat inwardly of the glass-house from the non10 metallised surface. However, experience has shown that a glass-house blind made of this material may have a very limited life, in that the aluminium metallising relatively quickly starts detaching from the substrate fabric. Though the rate of detachment 15 might be greatest where the fabric is subjected to the greatest mechanical stresses - for instance by friction or abrasion on fixed components, or by flexing or crumpling of the fabric - nevertheless sometimes the metallising does detach even where the 20 mechanical stresses are quite small. The reasons for this are not fully understood but are thought to be connected with the high humidity environment which often prevails in a glass-house, or they may possibly be connected with the precise chemical composition of such liquids as may contact or collect on the top surface of the glass-house blind. A somewhat similar problem has been noted when metallised fabrics are used in the presence of moisture, for instance in the case of clothing.

In an attempt to prolong the life of a metallised fabric subjected to a hostile environment, recently various attempts have been made to enhance the adhesion of the aluminium metallising to the substrate fabric, but up to the present time these attempts have met with limited success. Increasing the thickness of the metallising can reduce the flexibility of the fabric, leading to yet more rapid detachment of the metallising if the fabric is crushed or crumpled, and in any event an increased metallising thickness tends to block the pores of the fabric. On the other hand, a protective post-treatment such as the application of a lacquer, varnish or other siccative paint-like coating also tends to block the pores either completely or to an unacceptable extent, if that post-treatment is to have any effect.

This invention stems from trials and tests aimed at improving a microporous metallised fabric, in an attempt to decrease the rate of detachment of the aluminium metallising, whilst not significantly affecting the microporous structure of the finished fabric.

Accordingly, this invention provides a microporous metallised fabric comprising a microporous flexible fabric substrate having printed on to one face thereof a thin film of a carbonloaded ink which is substantially opaque, a layer of metal deposited on the film of carbon-loaded ink, and a thin film of a substantially translucent polyamide-based ink printed on to the deposited metal, the two ink films being applied at such rates as not significantly to affect the microporous nature of the fabric substrate.

It has been found that the combined use of the carbon-loaded ink film and the polyamide-based ink film, respectively under and over the metallising on the fabric, most significantly prolongs the life of that metallising. By employing a conventional printing process to apply the inks, the ink may be 5 deposited on the metallising in an amount per unit area which is sufficiently small not significantly to affect the microporous structure of the substrate fabric.

The carbon-loaded ink is preferably an 10 acryllic-based ink and contains from 10% to 20% of carbon particles, when a liquid. Most preferably, the carbon particle content is substantially 15% for the ink in a liquid state, which corresponds to about 25% for the ink when dry.

Polyamide-based printing inks are known per se, and comprise a suspension of a polymerisable amide resin in a solvent, the resin polymerising as the solvent evaporates after the ink has been printed on a surface. Examples of such inks are supplied by Porth Textiles.

The two layers of printing inks are preferably applied at rates of from 2.00 to 4.00 g/m^ and most preferably at rates from about 2.50 to 3.00 g/m^A, so as to leave deposited ink films each having a thickness of the order of about two microns (2 x 10” ^{5 6}m). The lower limit for the ink film thicknesses is defined by the need to protect the metallising from detachment from the fabric, whereas the upper limit is defined by the requirement not to block the pores of the fabric; preliminary trials have shown 10 that if the inks are printed on at rates significantly outside the ranges mentioned above, the resultant film may not be able to impart the desired properties to the metallised fabric.

The precise manner by which the carbon-loaded 15 ink assists the adherence of the metallising to the fabric substrate is not fully understood, but trials and tests in hostile environments have shown significant improvements may be obtained. In part, some of this has been attributed to the ultra-violet 20 screening that the ink may provide to the fabric itself, so preventing changes taking place which could reduce the adhesion of the metallising.

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The carbon-loaded ink film printed on to the substrate fabric is essentially black, and so is substantially opaque. As a result, the fabric when installed in a glasshouse for use as a thermal 5 screen may also serve for photoperiodic control of growing plants, as well as for thermal screening.

The ink film printed on the metallising may be tinted as desired, but not so deeply coloured as substantially to affect the reflective properties 10 of the metallising. For example, the ink may be tinted lightly with a silvery or gold colour. This may however not be particularly important when the fabric is to be used as a glass-house screen, relying on the emissivity characteristic.

It is most preferred for the inks to be printed by means of a photogravure or similar printing process, using an appropriately etched roller so as to obtain the required deposited film thickness. Moreover, tests have shown that optimum 20 properties are obtained if the finished fabric is produced within a relatively short time after commencing production: typically, the metallising should be performed as soon as possible after the carbon-loaded ink film has been applied, and the translucent ink film should be applied before 5 oxidation of the metallising has advanced to a too great an extent. Thus, depending upon the particular conditions, the preliminary indications are that the sooner the process is completed, the more consistent and the better the results are 10 likely to be. Nevertheless, the process should be completed within 48 hours, but more preferably sooner.

The metal layer used in this invention preferably comprises aluminium, deposited by a 15 vacuum deposition technique on the carbon-loaded ink printed fabric substrate, with a thickness lying in the range of from 200 to 300 S (i.e. 20 to 30 nm). As to the fabric substrate itself, it is preferred for this to comprise a so-called spun-bonded 20 oelefin, such as of a polyethylene resin. The manufacture of such a spun-bonded oelefin should be performed in such a way as to give that fabric a microporous structure.

This invention extends to a method of manufacturing a microporous metallised fabric, including the steps of printing a thin film of 5 carbon-loaded ink on to one surface of a microporous fabric, metallising the ink-printed surface, and then printing on the metallised surface a thin film of a polyamide-based ink, the two films being printed at such rates that the microporous structure 10 of the metallised fabric is not significantly affected by the ink films.

Each of the two printing steps of this method is preferably performed by a photogravure or similar process, in such a way as to deposit the ink at a 15 rate of from 2.0 to 4.0 g/m but most advantageously at a rate of from 2.50 to 3.0 g/m . The metallising step preferably is a vacuum deposition process, performed to deposit aluminium on the substrate. The other conditions, materials and so on used in the performance of the method of this invention preferably are as have been described above, with reference to the finished fabric.

In order that this invention may better be understood, one specific example thereof will now be described in detail.

A 3000 yard (2743 metre) roll of Tyvek (Trade

Mark) Style 1621C and sold by E.I. du Pont de Nemours, Inc. was subjected to a printing process, by passing the material through a photogravure printing machine so as to print on the surface of 10 the Tyvek a thin film of a carbon-loaded acryllicbased ink. The actual ink employed was Ref. CIL 1007, Black. The process was performed in such a n way as to deposit approximately 2.50 g/m of the ink (when wet), resulting in a dried film of 15 approximately 2 microns thick.

Then, the printed surface was subjected to a vacuum deposition metallising process, so as to deposit a layer of aluminium the thickness of which fell in the range of from 20 to 30 nm. The 20 metallising process employed is well-known and understood by those skilled in the art and was performed in accordance with accepted procedures; it forms no part of this invention and will not therefore be described in more detail here.

Within two hours of the metallising process, the metallised Tyvek was passed through a 5 Flexographic printing machine so as to print on the metallised surface a film of a polyamide-based ink having a light golden tint. The actual ink employed was Ink Number PT 932, as supplied by Porth Textiles. This printing process was performed in 10 such a way as to deposit approximately 3.0 g/m⁴ of the ink, resulting in a dried ink film of approximately two microns thickness.

After the ink had been allowed sufficient time for the solvent to evaporate, leading to the 15 formation of a polymerised dry film over the metallising, the metallised and printed fabric was rolled, for subsequent use in the manufacture of a thermal-insulating blind for a glass-house.

Preliminary trials on samples of fabric of 20 this invention manufactured as described above showed that when the fabric was subjected to a high humidity environment and then also subjected to mechanical stresses including friction and crumpling, the metallising had a much greater life expectancy, as compared to simple metallised Tyvek not carrying either ink film. Moreover, the microporous nature of the final Tyvek product was apparently not significantly affected by the presence of either polyamide-based ink film.