The definition of fabric coating is usuallyaccepted as the application of a polymer or resin to one side of a piece, offabric. A simple analogy is spreading butter onto toast.
The modern coating industry dates from the earlynineteenth century, when Charles Macintosh made the first rubber-coatedfabric. His name became synonymous with the raincoat. Automotive fabrics arecoated for a number of reasons, the two most important being to improveabrasion resistance and secondly to confer some flame-retardancy (FR)properties. Early heavy knitted automotive fabrics were coated to controlfabric stretch. Other properties, which can be imparted by coating include highfrequency (HF) weldability, by application of a PVC latex, and barrierproperties to liquids. The higher the amount of coating applied the better thebarrier properties.
However, fabric handle can be stiffenedsignificantly by coating especially if the coating resin applied is not chosencarefully. 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. Havingstated this, it is believed some heavy-duty knitted fabrics sometimesback-coated to reduce excessive stretch. Woven velvet fabrics must be coated tolock in the pile.
PoIymers applied are generally watered acrylic,polyurethane or PVC lattices. Acrylics are probably the most versatile and areused the most.
Polyurethanes are a little more expensive butgenerally have better stretch properties. The polymers are mixed with water andother ingredients such as thickening agents, foaming agents, fillers foreconomy and when necessary, FR chemicals. The whole mixture is referred to as acompounded resin.
Sometimes extra cross-linking agents, wettingagents and other specialist additives are also included. The compound ismechanically foamed by high speed agitation and air pumped in to give a foam ofa predetermined density usually about O.2g/cm3. This compound is pumped ontop of the fabric reverse side up, in front of a doctor blade in front of astenter. 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 polymerare applied, the fabric is supported by a table or roller and a finite gap betweenthe blade and the supported fabric is set using a feeler gauge. The size ofthe gap is another factor which determines add-on. This method is referred toas 'knife over roller' or 'knife over table'.
The same factors that already are known to actin foam processing (as mentioned above) control compound add-on. Motion of thefabric forwards into the stenter oven spreads the foamed coating evenly ontothe surface of the fabric. On drying under the action of the stenter the foamcollapses and an even coating is obtained on the back of the fabric. Foaming isnecessary to prevent the compound wetting and sinking into the fabric andpenetrating to the face side.
This method is excellent for applying relativelylow add-ons of resin, say up to about 30-40g/m2. When much heavierweights need to be applied, the compound is not foamed but thickened with athickening agent. This has the same effect as foaming, allowing the resin tosit on the surface of the fabric without sinking in, and penetrating to theface side.
Resin penetration can lead to fabric stiffeningand chalk marking or other appearance problems. Both foam processing and foamcoating can also be carried out using rotary-screen techniques, which havecertain benefits such as allowing certain knitted fabrics to be coated by thedirect method, but which entail more expensive plant.
Originally published in New Cloth Market:December 2009
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