T
he worldwide production of spunlace nonwovens practically doubled between 1995 and 2000.
When comparing the figures of 1985 and 2000, the increase in production actually quadrupled
(See Figure 1). Also, in the period between 2000 and 2006, the installed capacity
worldwide has doubled. Spunlace products exhibit the largest growth rate of all nonwovens.
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 shows a spunlace line with two
cards for staple fiber webs ranging from low to very high web weights, and figure 3 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.
Figure 3
Taking into consideration that the
nonwovens already are wet after the spunlacing treatment, it makes sense to perform additional
finishing processes inline in order to save on energy. Certainly the installation of complete lines
with inline finishing process is the most compact design.
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.
Bonding And Finishing With Chemical Binders, Chemicals
Bonding by means of chemicals usually
comprises at least two steps: First, the binder is applied; then, the bonding process is triggered
by means of thermal treatment. The nonwoven bonded by adhesion comprises a binding agent that
pastes together the matrix fibers. Spunlace nonwovens are nonwovens bonded by adherence. Binding
agents bond the fibers of a nonwoven by form-fit. A nonwoven reaches its maximum strength when all
fiber-crossing points are bonded in a point-shaped form-fit. This provides binder-bonded nonwovens
with their required application-specific properties of high strength. Wear resistance and stability
against washing and dry-cleaning strain is also reduced.
Binding Agents & Resulting Nonwovens Characteristics
Liquid binder formulas can be
custom-made so they can be adapted to specific production requirements.
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.
Application Methods
Thorough binding involves homogeneous
binder distribution over the nonwovens thickness and surface. Binding is achieved by full bath
impregnation or with a foam padder.
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.
Liquid Binder Application
Binders in liquid form are applied
both inline and offline onto spunlace nonwovens.
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.
Wet-In-Wet Application
The wet-in-wet technology reduces
multi-stage processes by dispensing with an intermediate drying stage. This saves energy, but the
method is said to yield moderately reproducible results because binder application quantity depends
mainly on the effect of squeezing after spunlacing.
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.
Foam Binder Application
In foam binders, part of the diluting
water is replaced by air. The solid matter content in foam is up to 40 to 50 percent, depending on
the binder application quantity, and about 15 percent for impregnation liquors. This results in
reduced drying cost and, consequently, reduced energy cost.
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.
Alfred Watzl is director of sales and marketing at Germany-based Fleissner GmbH & Co.
KG.
March/April 2007