Finishing of non-woven fabric: Details study on printing, softening, coating & laminating

Printing:

Nonwoven fabrics are printed for many applications, particularly in the home furnishing area including wall and floor coverings as well as tablecloths. Flat screen or rotary screen techniques offer wide colour and design range possibilities. The nonwoven fabric is fed continuously along the print table by a moving belt and passes either continuously (in the case of rotary printing) or intermittently (in the case of flat screen printing) below the print screen. A series of screens are used to build up the print design. Adhesive is applied to the belt to keep the fabric in position. This is important to prevent the fabric from lifting during printing, which would affect print definition. A printing  paste  consisting  of  pigment  or  dyestuff.

Improvements: Online print monitoring systems located after the last screen detect and report any repetitive printing faults as they occur during rotary screen printing processes. Faults such as misfits, lint, miss colour and screen blockages can be instrumentally monitored for quality control allowing modifications to be made to reduce waste.

Discharge prints allow the production of light colours on predominantly dark backgrounds using suitable dyestuffs. Generally, the fabric is initially dyed to the required dark shade and then overprinted with a paste containing a discharging agent to discharge some/all of the base dyed colour.  Light and bright shades can be produced on dark grounds with sharp edge and good print definition.

Other print procedures include engraved roller printing and sublimation transfer techniques. With the latter, dyestuffs are transferred by sublimation from a release paper and pre-printed with the design and appropriate dyestuffs onto the nonwoven fabric. The release paper and fabric are brought together and passed around a heated drum, the design being transferred by a combination of heat and pressure. Polyester fabrics are most suitable for printing with sublimable  disperse  type  dyestuffs.

Digital ink-jet printing enables intricate computer design patterns to be transferred  onto  nonwoven   fabric  substrates.

Various conventional printing systems are available, one example is the DOD system which uses thermal or bubble-jet technology whereby the print head ejects a drop of dye at high temperature.

Softening:

In addition to protective garments and clothing, softness is of importance in hygiene products such as sanitary coverstocks and wipes but the chemical composition is an important consideration in skin contact applications. Hydrophilic softeners have the additional effect of increasing wettability so that liquids can be distributed away from areas of high liquid concentration. Hydrophilic  finishes  are often  known as  rewetters.

Coating:

Coating is particularly important in the application of chemical finishes (or coating preparations) to both single use and durable nonwoven fabrics. Coatings are generally aqueous based and may be in the form of solutions or dispersions. A widely used procedure is application by rotating roller directly onto the nonwoven fabric, known as slop padding or kiss roll. An excess of coating application is applied to the fabric and the application is controlled by the bar profile, as surplus is scraped off the fabric surface. Excess is removed by a scraper and end plates act as dams.

In reverse roll coating the coating preparation is metered into the nip between two rotating rollers, a metering roller and an application roller. The nip or gap setting controls the level of application.

A further technique involves passing the fabric over two rollers set a predetermined distance apart, between which a knife or doctor blade is located so as to press onto the fabric. As the fabric passes over the rollers the knife evenly spreads the coating (spreading) over the surface.  This method is mainly applicable to high viscosity coating applications or foams.The Zimmer Magnoroll/Magnoknife is a versatile system, suitable for decorative patterning and printing, finishing and coating, paste dot applications, and adhesive application. The profiled knife is made from magnetisable steel and the pressure it applies to the fabric is adjusted by means of magnetic force, which controls the application level. The magnet can be placed at different positions (Magnetsystem Plus) and is capable of applying coating weights of 40–600 g/m2.

In knife over roller systems the web or fabric passes through a gap between a knife and a support roller. Air knife coating utilises a powerful air jet from the air knife to remove excess preparation.

Hot melt coating is important, for example, in the application of adhesives and bonding agents. The use of melt adhesives requires only cooling zones (drying tunnels are not required) to cool the coating with little if any solvent removal. Thermoplastic melt adhesives include copolyamides, polyesters, copolyesters, polyurethanes, polyurethane, coPVCs and EVA (ethylene vinyl acetate) polymers.

Extrusion (slot or slot die) coating is a technique where a hot melt polymer is extruded through a slot at an angle to the substrate, the speed of the fabric controls the thickness of the applied coating.

In transfer coating, application of the coating is made indirectly to the nonwoven fabric via a release carrier material in a similar manner to transfer dyeing.

In powder dot coating, heat activated thermo-fusible dry powders based on polyamides, polyesters and modified ethylene compounds are applied in coating and lamination processes.

Pressurised foam application systems have emerged in which a ‘closed’ controlled pressure application is used. In solvent coating, the coating polymer is mixed with a suitable solvent providing a coating of suitable viscosity for application to the fabric.

Lamination:

Lamination is obtained by combining two or more pre-formed nonwovens or nonwovens coupled with other products as films, membranes or textile materials, often during the formation process. The lamination of films on nonwovens is often carried out to modify the barrier properties, to lend to the material resistance to the penetration of liquids, particles and microorganisms, or to modify permeability to gas, liquids and biological fluids, or also to improve mechanical properties as abrasion resistance, dimensional stability and elasticity.

Lamination can be wet or dry. In wet lamination the adhesives are applied from a solvent or water dispersion and the adhesive is commonly applied to one substrate. Application is by spraying, slop padding, knife coating or spreading and printing,

Dry lamination uses thermoplastic resins including powders and melt adhesives composed of polyesters, polyamides and co-polymers (coPET), polyolefins, polyurethanes or scrims made from thermoplastic filaments or fibres that are placed between the two substrates to be joined.

Extrusion laminating relies on the application of thermoplastic polymers to the nonwoven fabric, which are melt extruded as an adhesive sheet to laminate two fabrics, one either side of the extruded molten film. A pressure roller nip ensures good adhesive contact.

Flat bed lamination relies on a different principle for combining the substrates and film. The substrate and adhesive film are combined prior to entering a heated, sometimes pressurised section. Usually a plate or belt type system binds the substrates together.

Flame lamination is widely operated in the bonding of filmsand/or nonwoven fabrics to polyurethane foams, mainly for automotiveapplications such as headliners, door panels, seats, sun visors, headrests,carpets and car boot liners. The process involves passing foam over an openflame (for one-sided operation) or between two sets of flame burners forlamination to both sides (3-ply laminate).