Coating and printing refers to the process of applying a coating or
print to one or both sides of a continous web substrate, such as roll of
fabric. The coating or printing is done sometimes for functional purposes
and at other times, for decorative needs.
Coating processes are widely used to enhance and alter the physical
properties and appearance of a fabric. Coating and lamination has bridged
across virtually every product group in the textile industry, including
composites, where its potential is especially wide.
Coating processes - From then to now...
In past, emphasis was laid on coated fabrics, basically on gas or liquid
permeability and its mechanical properties such as flexibility, tensile
strength, tear strength and stretching and shearing properties. Presently,
selection of the maximum combination of textile materials, structures and
coating compounds for anticipated loading condition is usually based on
strength of textile substrate alone.
Coated fabric has its specific fields of application and demand. It has
found extensive application in fields such as medical substracts, protective
clothing, flexible membranes for civil structures, industrial fabrics and
geotextiles. A lack of understanding of interaction between the fiber
structure and coating limits our ability to predict undesirable behaviours,
such as, tear, wrinkling and distortion etc. By choosing the right fabric
substrate and coating material, the fabric can exhibit desired mechanical
properties. We have tried to primarily cover the fundamental understanding
of coating process and its property relationship for coated fabrics.
Most fabrics still utilize nylon, polypropylene, polyester, cotton and wool
fibers and the bulk of polymer films is still based on polypropylene,
polyester and polyethylene. Value added properties such as chemical
resistance, anti-stain (or low surface energy), release, hydrophilicity,
vapor and gas barrier, electrical conductivity and abrasion resistance, may
be added to fabrics, by altering the material surface without affecting its
overall physical and mechanical properties.
The main coating processes have been described here with the help of
illustrative diagrams to help you understand the each process in detail.
Gap Coating -Knife Over
Roll, Kinfe Over Blanket, Floating Knife etc.
This type of coating process relies on a coating being applied to the
substrate which then passes through a split between the knife and a support
roller. As the coating and substrate pass through, the excess is peeled off.
This process can be used for high viscosity coatings and very high coat
weights, such as plastisols and rubber coatings. There are innumerable
variants of the relatively simple process which is rugged, hard-working and
somewhat inaccurate.
Knife Over Roll |
Knife Over Blanket |
Floating Knif
A simple process where the coating is applied to the substrate and the
excess is 'blown off' by a powerful jet from the air knife. This procedure
is typically used for aqueous coatings and is particularly noisy.
Air Knife Coating
In immersion coating process, the substrate is dipped into a bath of the
coating, which is usually of a low viscosity to enable the coating to run
back into the bath as the substrate emerges. This process is frequently used
on porous substrates.
Immersion (Dip) Coating
In the curtain coating process, a bath with an aperture in the base allows
a continuous curtain of the coating to fall into the gap between two
conveyors. The fabric to be coated is passed along the conveyor at a
controlled speed and so receives the coating on its upper surface.
Curtain Coating
A tank of coating solution sits inside a cylindrical screen that is pierced
by holes where it is desired that the coating is extruded. A thatch pushes
the solution through these holes, as the cylinder rotates in contact with
the substrate. Complicated patterns are possible while screens are expensive
and repeat lengths may be problematic.
Reverse Roll
Coating
In reverse roll coating, the coating material is measured onto the
applicator roller by precision setting of the gap between the upper metering
roller and the application roller below it.
The coating is brushed off the application roller by the substrate as it
passes around the support roller at the bottom. The diagram illustrates a
3-roll reverse roll coating process, although 4-roll versions are common. In
Reverse Gravure Coating, the actual coating material is metered by the
engraving on a roller before being wiped off as in a conventional reverse
roll coating process.
L-head Reberse Roll Coater |
Nip-fed Reverse Roll Coater |
Pan-fed Reverse Roll Coater |
Gravure Coating
The gravure coating depends on an engraved roller running in a coating bath
that fills the imprinted dots or lines of the roller with the coating
material. The excess coating on the roller is removed by the doctor blade
and the coating is then deposited onto the substrate as it passes through
the engraved roller and a pressure roller.
Engraved-roll Coater |
Offset Engraved-roll Coater |
Offset gravure is common, where the coating is primarily deposited
on an intermediate roller before transfer to the substrate.
Metering Rod (Meyer
Bar) Coating
In meter road coating, the wire-wound metering rod sometimes known as a
Meyer Bar, allows the desired quantity of the coating to remain on the
substrate. The excess coating is deposited onto the substrate as it passes
over the bath roller.
The quantity is determined by the diameter of the wire used on the rod.
This process is remarkably tolerant of non-precision engineering of the
other components of the coating machine.
Wire Wound Rod Coater
In the Slot Die process, the coating is squeezed out by gravity or under
pressure via a slot and onto the substrate. The process is termed extrusion
when the coating is 100% solid. In the Slot die process, the line speed is
frequently much faster than the speed of the extrusion. This enables
coatings to be considerably thinner than the width of the slot.
Melt Extrusion Coater
In hot melt coating process that is applied commercially, low viscosity is
required to achieve an even coating by solution or dispersion. In a small
number of cases, the desired coating can be melted and applied while still
hot. These are hotmelt processes.
Most of the actual coating techniques still used are hot-melt slot-die,
hot-melt metering-rod etc. Hot-melt wherever possible, brings tremendous
advantages in terms of speed, lack of drying tunnel etc. The main limitation
is the small number of applications for which suitable coatings are
available. Most of these are adhesives.
Coating
Process
