The definition of fabric coating is usually accepted as the application of a polymer or resin to one side of a piece, of fabric. A simple analogy is spreading butter onto toast.
The modern coating industry dates from the early nineteenth century, when Charles Macintosh made the first rubber-coated fabric. His name became synonymous with the raincoat. Automotive fabrics are coated for a number of reasons, the two most important being to improve abrasion resistance and secondly to confer some flame-retardancy (FR) properties. Early heavy knitted automotive fabrics were coated to control fabric stretch. Other properties, which can be imparted by coating include high frequency (HF) weldability, by application of a PVC latex, and barrier properties to liquids. The higher the amount of coating applied the better the barrier properties.
However, fabric handle can be stiffened significantly by coating especially if the coating resin applied is not chosen carefully. Fabrics are also sometimes coated to modify stretch and to control porosity. In general only woven fabrics can be easily coated by the usual methods - knitted fabrics are generally too stretchy and dimensionally unstable. Having stated this, it is believed some heavy-duty knitted fabrics sometimes back-coated to reduce excessive stretch. Woven velvet fabrics must be coated to lock in the pile.
PoIymers applied are generally watered acrylic, polyurethane or PVC lattices. Acrylics are probably the most versatile and are used the most.
Polyurethanes are a little more expensive but generally have better stretch properties. The polymers are mixed with water and other ingredients such as thickening agents, foaming agents, fillers for economy and when necessary, FR chemicals. The whole mixture is referred to as a compounded resin.
Sometimes extra cross-linking agents, wetting agents and other specialist additives are also included. The compound is mechanically foamed by high speed agitation and air pumped in to give a foam of a predetermined density usually about O.2g/cm3. This compound is pumped on top of the fabric reverse side up, in front of a doctor blade in front of a stenter. This particular method of fabric coating is referred to as the 'direct method' and there are a number of variations. When the doctor blade or 'knife' actually touches the unsupported fabric, it is referred to as a 'floating' knife and the method 'knife on air'. When higher levels of polymer are applied, the fabric is supported by a table or roller and a finite gap between the blade and the supported fabric is set using a feeler gauge. The size of the gap is another factor which determines add-on. This method is referred to as 'knife over roller' or 'knife over table'.
The same factors that already are known to act in foam processing (as mentioned above) control compound add-on. Motion of the fabric forwards into the stenter oven spreads the foamed coating evenly onto the surface of the fabric. On drying under the action of the stenter the foam collapses and an even coating is obtained on the back of the fabric. Foaming is necessary to prevent the compound wetting and sinking into the fabric and penetrating to the face side.
This method is excellent for applying relatively low add-ons of resin, say up to about 30-40g/m2. When much heavier weights need to be applied, the compound is not foamed but thickened with a thickening agent. This has the same effect as foaming, allowing the resin to sit on the surface of the fabric without sinking in, and penetrating to the face side.
Resin penetration can lead to fabric stiffening and chalk marking or other appearance problems. Both foam processing and foam coating can also be carried out using rotary-screen techniques, which have certain benefits such as allowing certain knitted fabrics to be coated by the direct method, but which entail more expensive plant.
Originally published in New Cloth Market: December 2009