Finishing Technology For Spunlace Nonwovens
Adding value by binder bonding, thermofusion and finishing
By Alfred Watzl
Although many spunlace nonwovens receive their final properties through spunlacing technology, there are others that are further finished or additionally bonded. For more than 30 years, Fleissner delivered more than 120 finishing lines for spunlace nonwovens for all kinds of applications. These included both inline systems with AquaJet machines and offline systems with widths of up to 5,000 millimeters (mm). Fleissner can now offer complete spunlace lines from fiber preparation to completely finished nonwoven rolls as a result of its acquisition by the Germany-based Trützschler Group.
These finishing processes include: impregnation with chemical binders; finishing with chemicals; dyeing or printing; and thermofusion or heat setting. These additional finishing steps can be realized inline or offline.
Many nonwovens producers prefer separate finishing lines installed after a spunlace line. Figure 2 (below, top) shows a spunlace line with two cards for staple fiber webs ranging from low to very high web weights, and Figure 3 (below, bottom) shows a spunlace line for spunbond webs for high production speeds. In these two cases, the webs are dried before they are finished further. For many finishing processes and high web weights, this is a necessary step.
There also are lines in operation that further bond the web inline after spunlacing and comprise an intermediate drying stage to achieve optimum finishing results.
By using different binder recipes, different nonwovens can be produced from the same web. The major binder classes are listed in Table 1.
In addition to monomers and comonomers, functional groups such as cross-linking agents are incorporated. These groups influence the properties of the polymer and consequently those of the nonwoven including mechanical properties and resistance to solvents.
The finished polymer emulsion is obtained finally by the incorporation of various additives. These additives are used to influence coagulation temperature (thermal sensitivity of binder liquid), foamability, wettability, migration behavior and printability.
Consequently, substances in various concentrations found in dispersions include: binding agents; wetting agents; thickeners; catalysts; antifoaming agents; water; thermal sensitizing agents; filling materials; dye pastes; and flame-protection agents.
The binder liquid properties allow the requested nonwovens properties to be obtained within wide limits. Many demands are made on the binding agents used, and properties such as improved ecological and toxicological safety, and reduced flammability are important.
Binding points are concentrated at the surface in surface binding. This is typically achieved with spray, doctoring and surface foam applications.
Partial binding locally bonds the nonwovens surface in the shape of mostly regular patterns. Processes used for this purpose are print bonding and printing.
In most cases, one of the following processes is used when either liquid or foamed binders are applied:
• full bath impregnation — in padder trough or inside gap (liquid);
• one-sided metered binder application, which includes doctoring (spreading), kiss roller application, small-surface application via engraved rollers, small-surface application via round screen or spraying; and
• foam application.
Heavy nonwovens, such as substrates to be coated, require offline impregnation because it is very difficult to achieve proper through-impregnation in a wet-in-wet inline process with the usual immersion padders. Foam impregnation also is out of the question for an inline process, while an offline process offers economical advantages because of reduced moisture input.
In case of lightweight nonwovens — up to 80 to 90 grams per square meter (g/m2) — foam impregnation always should be preferred to liquid impregnation because considerable technical efforts often are required to pass the web through the liquor without tension in order to avoid drafts.
For lightweight nonwovens, inline impregnation with foam also should be preferred as a wet-in-wet process for cost-saving reasons (See Figure 4).
The liquor for liquid impregnation is contained either in a trough arranged before the padder or directly in the padder gap. Gap application offers advantages from reduced liquor volume, but makes very high demands on fiber wettability because the web must be completely soaked with binder liquid in a very short time.
The moisture content following suction removal after the AquaJet in turn mainly depends on vacuum inside the suction slot, fiber type or fiber blend, and speed.
The wet-in-wet impregnation process must be controlled accordingly, which results in greater operation skills, recipe know-how and process knowledge.
During the wet-in-wet application, a more or less intense exchange between the water carried along by the web and the impregnation liquor takes place on the web. To ensure a defined binder application, the liquor volume removed with the contained binder solids must be added, and constant thinning of the liquor by water carried in by the web as a result of liquor exchange must be compensated.
It is generally known that two parameters, liquor application and binder concentration, are enough for dry-in-wet application to calculate binder application. For the wet-in-wet application method, however, these two parameters are not a sufficient indication for binder application. Apart from binder concentration, the water content of the incoming web and the liquor content of the outgoing web — the differential liquor application — must be known.
Binder metering consequently is of great importance because there may be a drop of binder concentration in the impregnation bath as a result of the water exchanged in the incoming web.
In traditional addition metering, the consumed binder is replenished, and constant thinning of the liquor using water is compensated for by adding a higher concentration of binder liquor to the impregnation bath.
Naturally, liquor application and binder application depend on the nip pressure set for the rollers.
The advantages of foam binders include:
• wide range of application quantities down to minimum application — for example application of various auxiliaries;
• increased uniform binder distribution in the surface;
• surface impregnation and through-impregnation;
• reduced risk of migration during drying;
• good strength with reduced flexural strength due to punctual bonds;
• improved nonwoven volume; and
• good textile hand.
For web bonding with foam, basically the same binder liquids are used as for bonding with liquid binders. Foamability in the foam mixer is achieved by addition of foaming agents and foam stabilizers. Foam is characterized by the foam weight per liter and the foam stability. The foam stability influences the disintegration speed of the foam bubbles and thus the processing behavior.
Depending on the desired effect, the liquor is beaten into foam of 5, 10 or 20 times its volume — with a weight per liter of foam between 30 and 300 grams per liter — and then applied onto the web between two rollers.
The foam impregnation process is suitable both for one-sided binder, or kiss roller, application and for through-impregnation. Mere surface bonding also is possible. The foam binder is supplied from the mixer and distributed onto the rollers or into the roller gap by means of oscillating foam distribution devices. For two-sided foam application, two distributing devices are mounted above the rollers. The penetration depth can be controlled by setting the gap between the two rollers.
Generally, binder application by the foam application method is determined by three influencing factors: concentration of binder liquor; foam weight per liter; and roller gap.
One example for finishing nonwovens with foam binders is the production of bitumen carrier webs. These production lines are supplied in widths of up to 5,400 mm for mechanically needled and spunlaced nonwovens. The acrylate binder provides the bonded web with sufficient strength and dimensional stability for passage through the hot bitumen bath.
Another example is foam impregnation and drying of lightweight webs for interlinings, medical webs, and webs for sanitary purposes and wiping cloths.
Editor's Note: Alfred Watzl is director of sales and marketing, Fleissner GmbH and Co. KG, Germany.