Textile coating

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A textile coating is the application of a visible layer of plastic to a textile substrate (carrier material). The aim of this process is to change the physical properties of the carrier material, for example water tightness, mechanical strength, chemical resistance, lightfastness, abrasion resistance, gas and moisture permeability, design, appearance, haptics , surface design and volume. The textile coating is a sub-term of textile finishing .

The textile coating differs from the so-called textile finishing in that the latter changes the physical properties, but not the visual impression of the carrier material, for example by impregnating it with water-repellent chemicals. A textile finish only envelops the individual fibers, the pores of the textiles usually not being closed and gas permeability being maintained.

Applications

The areas of application for textile coating are very diverse and cover almost all areas of life. Frequently encountered products are medical articles such as plaster material and washable covers, microporous membranes, artificial leather of all kinds, truck tarpaulins, tents, awnings, rainwear, sportswear and articles, fashion articles, home textiles , military clothing and equipment, conveyor belts , filter membranes in the industrial sector and a variety of technical products.

Procedure

The coating can be applied as a solid or a liquid. In the first case, prefabricated foils, including foam foils, are permanently connected to the textile substrate by means of glue or by melting them. This process is called lamination or lamination ; when using gas flames to melt the film surface, one speaks of flame lamination. Another possibility is to sprinkle meltable, pulverized plastics ( thermoplastics ) onto the textile backing and then melting and compacting using hot rollers that press the plastic against the backing material. Such thermoplastics can also be melted by means of extrusion or via calender units and applied and pressed directly onto the textile. The use of PVC and thermoplastic polyurethanes (TPU) is widespread in these processes.

Dried open-cell whipped foam made of polyurethane dispersion on cotton fabric, foam density 300 g · l −1 , layer thickness 4 mm

Application from the liquid phase is also widely used. These can be solutions of plastics in organic solvents , solvent-free, reactive compounds ( high solids polyurethanes ), aqueous dispersions or plastisols , which are bonded to the carrier material by suitable application processes. It is applied by dipping, knife coating , roller application , printing or spraying. In any case, drying at higher temperatures in the range 150 to 190 ° C for several minutes is necessary in order to solidify the coating accordingly and to make it ready for use.

In order to prevent unwanted sagging of the liquid coating agent into the textile substrate during application and to be able to use suitable amounts, the viscosity and, in the case of aqueous whipped foams, the foam liter weight is adjusted so that a paste-like consistency is present. In addition to the substrate, the type of squeegee , which determines the application height, and the dwell time until entry into the drying tunnel also have an influence on the penetration depth .

In addition to the direct coating of the textile material, there is also reverse coating or transfer coating . Here the coating is first applied to a release paper , cured and then laminated to the textile carrier material in a second step. Then, after the coating has dried and bonded firmly to the textile, the release paper can be removed and reused several times. The advantage of this process is, on the one hand, the possibility of giving the coating a defined surface structure, such as grain , by pre- embossing the release paper ; on the other hand, the bond to the carrier can be better controlled than with direct coating . The articles produced in this way are therefore often much softer.

Example of the manufacturing steps for the typical polyurethane synthetic leather: 1) cotton fabric, 2) coagulation from DMF, 3) + 4) application of the polyurethane coating and finish in the reverse process

A special feature is the coagulation process, also known as DMF coagulation or wet process . In this process, the coating agent is a pasty solution of polyurethane resin in dimethylformamide (DMF) and is applied to the textile carrier material either by dipping or by knife coating. The textile is then passed through several subsequent countercurrent water baths. The DMF solvent dissolves in the aqueous phase and the polyurethane resin, as it is insoluble in water, remains on the textile as a sponge-like structure. Quite soft and voluminous coatings can thus be produced, as they are particularly suitable for the production of artificial leather. The coagulation of the polyurethane is almost complete in the first water bath. The subsequent baths are used to thoroughly wash out the solvent. For this reason, the highest DMF content can be found in the first bath. Modern systems recover almost all of the solvent through purification steps and distillation. The subsequent drying does not serve to harden, but only to evaporate the water. The technical design of a coagulation system is therefore fundamentally different from that of the coating systems. The flat structure thus obtained in combination with the textile carrier is referred to as coagulate .

The textile coating is carried out on continuous webs in a width usually between 1.50 to 4.50 m. Since several coatings with different layer thickness and structure are often applied one on top of the other, up to four coating systems are connected in series, in which a textile is coated several times. As textile support can fabric , knitted fabric (knitted goods), nonwovens ( nonvowens ), but also coagulates , s. o., are used. In order to prevent excessive linear expansion and deformation, especially with elastic carriers, for example knitted fabrics, under the tensile stress in the coating machine, tenter frames are used, which use claws or needles to stretch the carrier perpendicular to the direction of coating and move along accordingly.

The production speed of the coating systems depends on the length of the drying ovens and the maximum drying speed of the coating agent for a given layer thickness. Modern coating systems can travel over 20 meters / minute; Coagulation systems are slower at around 10 meters / minute.

literature

  • Andreas Giessmann: Substrate and textile coating. Springer, Berlin 2003, ISBN 3-540-43426-7 .
  • Marc Van Parys: Coating. Eurotex Industriele Hogeschol Van Het, Belgium 1994, OCLC 41909583 . (engl.)
  • Harro Träubel: New Materials permeable to Water Vapor. Springer Verlag, Berlin / Heidelberg / New York 1999, ISBN 3-540-64946-8 . (engl.)
  • Ulrich Meier-Westhues: Polyurethanes / lacquers, adhesives and sealants Vincentz Network Verlag, Hanover 2007, ISBN 978-3-86630-896-1 , p. 199 ff.
  • Ulrike Luckmann: touch book. Volume 2: ABC of technical terms. Weyarn 1992, OCLC 246211709 . ( online at: touchbuch.com )