This article outlines the development of
the textile coating industry and the fundamental principles behind today’s
advanced coating processes. With the increase of lightweight construction,
fiber reinforced materials are becoming increasingly important, and therefore
represents a growth area for textile coating.
Humans are the only primates without fur to protect themselves from the
elements and first used animal skins and furs to shield themselves from either
the cold or from UV radiation, depending on where they were in the world.
Over 5,000 years ago, fabrics woven from plant fibres and wool were then
developed, bringing many advantages such as their warmth, softness and
breathability, as well as UV-shielding, and the development of dyeing gradually
gave rise to the concept of fashion.
There remained, however, a problem – protection against rain for those
in wet climates, and later, for seafarers. Furs and leather were still widely
used for this purpose until very recently.
Waterproofing
It took until the 19th century for a workable solution to
finally be developed by the Scottish textile manufacturer and inventor Charles
Macintosh, although waterproofing garments with rubber was not a new idea,
having been practiced by the Aztecs in pre-Columbian times.
Later, French scientists made balloons gas-tight and impermeable by
impregnating fabric with rubber dissolved in turpentine, but this solvent was
not satisfactory for making apparel.
Macintosh too, first impregnated a thick woollen fabric with a solution
of natural rubber. The result was waterproof but stank of petroleum and was
sticky due to the wool grease.
Only when a method was developed for coating the fabric on one side and
heating the rubber in a dryer with the addition of sulphur – the process of
vulcanization – was the Macintosh coat fabric ready for commercialisation.
How the fabric was coated and in which drying oven it was vulcanized is
unfortunately not known, but this development formed the basis for textile
coating as we know it today.
Air knife coating
There are two fundamentally different basic coating processes – air
knife coating and roller knife coating.
In air knife coating, a squeegee blade brushes over the surface of the
textile, pressing the highly viscous coating paste into the spaces between the
material. It is impossible to coat low-viscosity chemicals with this method or the
paste will drip through the meshes/weave interstices.
Air knife coating, however, is used firstly wherever sealing of the
fabric is required, for example on umbrellas to prevent spray mist getting
through to the inside. Other examples include shower curtains, rainwear, bag and rucksack fabrics, tents etc.
Air knife coating is also used for mattress tickings and upholstery
fabrics. In this case a back coating is applied which has a double function –
the material is made liquid-tight and in addition it is fixed. In the case of
upholstery fabrics, this fixes the pile, but can also be used to achieve
technical effects such as flame protection.
In fashion and decorative articles, air knife coatings are also used for
one-sided colouring, while textile materials for shoes are coated to make them
waterproof.
Technical textiles
The areas of application with the air knife coating of technical
textiles are extremely diverse, ranging from filter fabrics to textile seals
and to carbon fibre impregnation.
In addition to coating with a thickened paste, there is also air knife
coating with foam. In this case, physical foam is produced in a special foam
machine (similar to whipped cream) and placed in front of the coating knife.
The foam is pressed into the fabric by the knife and the foam is destroyed.
This so-called unstable foam coating is used, for example, for
over-dyeing jeans. In a coloured/colourless version, nonwoven fabrics are also
fixed and overdyed in this way.
The term “unstable” does not mean this is bad foam. Unstable foams
remain stable below room temperature for at least five minutes and do not
decompose, but the air bubbles then burst under the knife, or at the latest
when the foam is subsequently heated in the dryer.
Foam coating with the air knife has many advantages – by diluting the
coating chemical with air, less drying power is required, and the penetration
depth is lower, while the breathability of the textile is maintained.
Roller knife coating
In roller knife coating – also called roller nip coating – the
application with the knife is practically flying, without touching the upper
side of the textile.
This has various effects on the final product. The application in the
nip, for example, covers the surface of the textile with the coating compound
to give this side of the fabric a plastic-like surface, which is determined by
the chemistry used.
Well-known examples of roller knife coated fabrics are tarpaulins, life
jackets, carpet backing, upholstery fabrics, trunk covers, sealing materials
and many others.
Roller knife coating places very high demands on the precision of the
machine, in contrast to air knife coating. Nevertheless, combinations of these
two coating types are mainly offered today. For this purpose, the coating knife bar is designed to be horizontally
adjustable and the precision achieved depends on the supplier of the coating
machine.
The roller knife can be used in the same way as the air knife with
paste, for example in the coating of PVC tarpaulins, emergency slides,
inflatable boats and sealing mats.
Both unstable and stable foams are used in roll knife coating. If a
layer of unstable foam is applied, it decomposes in the first zones of the
dryer.
The roller knife coating of unstable foams (also referred to as
“metastable foams”) is used in the production of jeans to dye over the denim
material on one side, for example. By applying the coloured foam on the
surface, a good over-dyeing is achieved, which can be washed out easily in
industrial laundering to achieve the desired “stonewash” effects.
Stable foams survive the drying process in the dryer (under very mild
drying conditions) and leave the dryer as a foam layer.
Black-out fabrics
A good example of an application for roller knife coating with stable
foams is in the production of black-out fabrics for blinds or curtains. These
products require special treatment in order to retain the softness of the
fabrics and to ensure that it is still possible to wind blinds up and down.
A special coating called Black-Out has been established to achieve this,
involving a three-stage series of stable foam coatings with the roller knife.
The first coating is usually white, followed by a black layer and then a
white layer again. These three layers are dried and are with a crush calendar
after each layer is applied. A fourth dryer passage then cure all three layers
together with the possible addition of a last topcoat to improve the grip.
This process is complex and expensive, and mistakes can result in the
entire production run being rejected, so experienced and trained personnel are
required.
A similar process is used in the production of advertising banners,
which is called ‘block-out’. This is a multi-layer foam coating to prevent the
image/text of the banner from showing through on the back side of the material.
Rubber coating
Let’s return here to the Macintosh and coating with rubber, as a rather
amazing application for roller knife coating.
The applied rubber layer is so waterproof and airproof that such
materials can also be used for lightweight boats, life rafts, life jackets and
emergency slides in aircraft. However, such basic waterproof fabrics have a problem in apparel, in not
allowing the moisture generated by the wearer to escape.
Consequently, the textile industry was challenged to develop a material
that would repel rain, but at the same time be breathable for the wearer.
Probably the first product to meet this challenge was (and still is)
marketed as Gore-Tex for outdoor clothing. Gore-Tex, however, is not a coated
fabric, but a waterproof, breathable membrane that has been laminated.
The availability of water vapor permeable polyurethane dispersions also
allowed direct coating on the inside of the fabric. This is where roller knife
coatings are applied. Depending on the required stress, stable foam coatings
and also paste coatings are used.
Lamination
Lamination is generally understood to be the joining together of two or
more layers of textile, film, membrane or fleece and to keep the layers
together an adhesive is needed, which can be applied by either coating or
screen printing.
A distinction can be made here between dry or wet lamination. In wet lamination, the adhesive is initially applied to the first layer
and the second layer is then placed in the wet application before the two
materials are dried and fixed together. The disadvantage of this process is the hardness/rigidity of the
laminated end product.
In dry lamination, the adhesive is applied to the first material and
dried and the second layer is then applied to it via high pressures, usually by
a calender.
A special case is that of stable foam lamination. In this process, a layer of foam is applied by a roller doctor blade and
carefully dried. The second layer is then placed into the dry foam by a crush
calender. Afterwards, however, this laminate must still be thermally fixed.
Foam lamination has the softest touch and in the case of polyurethane foam the
laminate is also thermally resistant, as the adhesive is not thermoplastic
after fixing.
Conclusion
In this article I have tried to provide an overview of the technology of
textile coating and would like to conclude by listing just some of the coated
materials that are to be encountered in daily routines.
We can start with the mattress cover, slippers, the shower curtain and
the bathmat and move through to the dining table with its coated tablecloth,
then out to the hallway for a rain jacket and umbrella. In the car, countless
more coated fabrics are to be found, from the seat cover to the trunk, and just
as many coated materials will be encountered by commuters using trains or
buses.
Textile coating is still
a technology of the future with which money can still be made. With the
increase of lightweight construction, just as one example, fibre reinforced
materials are becoming increasingly important. Here, textiles or fibre scrims
are only used to reinforce the plastic matrix, but the technology of production
is similar and therefore represents another growth area for textile coating.
*Jürgen Hanel is the Head of Technical
Textiles, A. Monforts Textilmaschinen GmbH & Co. KG.
