1.Introduction
2.Bonding and Finishing with Chemical Binders and Chemicals
2.1Binding Agents and Binder-dependent Nonwovens Characteristics
2.2Application Methods
2.3Liquid Binders and Foamed Binders
2.3.1Application of liquid binders
2.3.1.1Wet-in-wet application
2.3.2Application of foamed binders
2.3.2.1Wet-in-wet application
2.3.3Curing of binders
2.3.4Binder migration
2.4Printing, Printbonding
2.4.1Gravure printing
2.4.2Round screens
2.5Chemical Finishing/Dyeing
2.5.1Dyeing
2.5.2Chemical finishing
3.Bonding of Spunlace Nonwovens by Hot Air
3.1Thermofusion with Hot Air
3.2Heatsetting
4.Conclusion
There are 2 types of machine operating on the through-air drum principle:
1-drum line for light-weight webs with dwelling conveyor incorporated in the drying section above the drum.
multi-drum line for higher web weights, e.g. bitumen carrier webs.
After completion of drying, the web is heated abruptly to air temperature by the hot air sucked through and dwells at this curing temperature for the required curing period.
2.3.4Binder migration
Thermal migration of the binder during drying can have quite a negative effect on the nonwovens quality. Water and binder tend to migrate during the drying process from the inside of the web to the warmer web surfaces where the water can evaporate. Dissolved or dispersed binder particles have the same tendency and consequently concentrate at the surfaces.
Migration is subject to various influences (see Fig.15)
Trials have shown that binder application by the wet-in-wet method does not differ a great deal from the dry-in-wet process. Moreover, this method does not require the complicated addition of liquor required for the wet-in-wet liquor process. The foam quantity supplied to the rollers in usually consumed quickly. There is no formation of liquor volume with excess binder so that no interaction between bath foam volume and wet nonwoven can occur. There is a typical difference between addition application of wet-in-wet foam technology and more or less complete saturation application of wet-in-wet liquor application.
Regardless of liquid or foam impregnation of spunlace nonwovens, a clearly higher strength for identical fiber weights is achieved for spunlaced nonwovens compared with a mechanically bonded web (Fig. 13). This entails that less binder has to be applied in subsequent binder bonding processes. This means cost savings both for binders and for drying energy.
2.3.3Curing of binders
When high demands are made on resistance of nonwovens to water and solvents as well as for high dimensional stability (e.g. for roofing membranes), binders are preferred that contain curing agents because these properties can be achieved with cured binder.
Binders can both be self-crosslinking (e.g. n-methyl acrylamide group) or can be cured through their own curing agent (e.g. melamine formaldehyde). Curing of binders can take place both in liquid and in foamed binder dispersions. Curing requires a certain curing period. This period mainly depends on temperature and flow speed of air through the web.
The higher the temperature, the shorter the dwelling period. However, the temperature is limited by the fiber type used which must not be damaged. Through-air dryers always offer an optimum air speed; this is the reason why the TAD requires a shorter curing zone than belt dryers or can dryers (Fig. 14).
The following serious disadvantages are to be expected from migration:
irregular distribution of binder and of dyestuff and chemicals in the web
reduced strength inside the web causing delamination of the product
harder touch
face-to-back variations of physical properties such as e.g. wear resistance
decreasing wettability and absorbing power at the surface
increased drying period as a result of binder film formation at the surface
The following possibilities for control and minimization of migration exist:
reduced drying temperature; however, this is not desirable because it also reduces the drying rate
high air speed in order to achieve quick drying and limit the time of migration
This also is the reason why through-air dryers with high through-air speeds have the lowest migration rate compared with other drying systems.
Low wet pick-up method: the less water is applied, the less water is evaporated and the lower the risk of migration.The foam impregnation process is perfectly suited for this method.
Chemical methods:
- use of thickeners which reduce migration due to higher viscosity
- coagulation agents or heat sensitizing agents.These auxiliaries are added to the binder dispersion
and cause agglomeration of latex particles into bigger units at a specific temperature.
This temperature must be lower than the temperature at which the water in the web starts to migrate towards the surface as a result of the evaporation processes. (See Fig. 16) shows the temperature processes during evaporation and the typical binder migration zone during the adiabatic wet bulb temperature.
When now coagulation below this wet bulb temperature is made possible due to the selected heat sensitivity, migration is avoided.Typical coagulation temperatures for through-air dryer and can dryer range between 40 and 55C and between about 50 and 60C for infrared pre-dryers.
2.4Printing, Printbonding
Partial bonding and dyeing with certain patterns is desired for many nonwovens applications instead of full-surface bonding with binders. This applies, for example, to household wiping cloths, but also to nonwovens for medical and sanitary purposes. Nonwovens bonded by the printbonding system have a lower strength, but they also have a textile hand and better absorbency due to binder-free areas.
Light-weight fiber webs are partially bonded with dispersions of self-crosslinking and thermoplastic acrylic resins by
gravure printing or
screen printing.
2.4.1Gravure printing
An engraved roller in this process supplies the binder liquid in points onto the web length. As the liquid transfer also requires a certain pressure, a rubber-coated counter-roller is needed.
The gravure printing process makes higher demands on nonwoven wettability. This property, however, is perfectly ensured by wet spunlaced nonwovens. This system therefore also allows high production speeds to be achieved.
The printing system consists of an engraved bottom roller (the bottom application system can also consist of 2 rollers) and a rubber-coated counter-roller. A doctor blade removes excess dyestuff. This machine can also be used for various other finishing processes as, for example, for impregnation of a web with chemicals or for hydrophilic or hydrophobic treatment. For this purpose the engraved roller has a fine structure. In case of higher web weights, however, the printbonding method results in 2-sidedness.
To lay out a multi-purpose finishing line for lighter spunlace nonwovens, foam padder and printing unit together are also combined with a dryer for in-line processes (Fig. 17).
The printing substance can be dyed with pigment dyestuff and a low share of binder as used for textile printing.
Liquor viscosity is a critical factor as migration into the surrounding binder-free zones shall be avoided.
Customary concentrations range from 30-36 % for in-line processes with humid material. For off-line print bonding the concentration used is only about 20 %.
To achieve a defined print, the web can be dried after spunlacing and before printing also to low residual moisture contents.
2.4.2Round screens
Instead of an engraved roller, a round screen usually employed for screen printing is used in this process. The paste-like binder liquid is fed into the rotating screen through a pressure pipe; from there it is passed by a doctor blade through perforations onto the web. The concentration of the printing mass lies between 20-40 % depending on a dry or moist web.
2.5Chemical Finishing/Dyeing
2.5.1Dyeing
Where spunlace nonwovens have to assume decorative tasks, e.g. for floor coverings, as table linen and bed linen or for manufacture of clothes, dyestuff application is needed.
As long as these nonwovens also have to be chemically bonded with binders in whole or in part, the dyestuff is evenly added to the binder bath. In this case, liquid impregnation is preferred to foam impregnation where dark shades are used. For excellent dyeing results the off-line process should be employed. The processes for dyeing are similar to the methods already described for binder application.
When dyeing nonwovens already bonded, the nonwoven is treated practically like a woven material and dyed by the method and using the dyestuff of classical dye shops for the individual fiber types or fiber blends (e.g. thermosol immersion bath process with dispersion dyestuff for PES webs). Fleissner supplies suitable continuous dyeing lines for this purpose.
2.5.2Chemical finishing
Where binder-bonded nonwovens are used, all properties desired for the end product will be tried to be obtained by means of the binder liquor.
Pure spunlace nonwovens not bonded by binder that are to be provided with special properties by finishing will have to be submitted to an additional after-treatment. These chemical finishing processes are realized by full bath impregnation or foam impregnation.
The following finishing operations are possible:
Stiffening finish
e.g. for sunshades and blinds
Finishing of toe cap nonwovens
Softening
Subsequent softening with silicon products, e.g. for interlinings, is particularly effective.
Antistatic finishing
Especially important for home textiles, wall coverings, wallpapers, furniture and mattress upholstery cloths, dust-bonding wiping cloths
Dirt-repelling finishing
e.g. with fluorcarbon resin chemicals
Sanitary finishing
Antibacterial or fungicidal finishing is required in the hygiene sector and for hospital use. Auxiliary agents for achieving these properties can be applied onto and into the fibers both before web formation and subsequently onto the completed spunlace web.
Flame-proof finishing
Although this property is mainly determined by the fibers and binders used, additional effects can frequently be achieved by supplementary finishing.
Coating
Hydrophilic, hydrophobic, oil-repellent treatment
In the water-repellent finishing process (e.g. for surgical drapes and gowns), fluoric chemicals are applied to the web. This can be done by full bath impregnation, by foam application or with kiss rollers. Any of these methods have their advantages and disadvantages. The foam application method offers advantages with respect to energy saving and good oil, water and alcohol repellency, while the water impact penetration value due to the necessary use of wetting and foaming agents could be better, provided that this property is demanded. Wet application with foam is 50 %, with kiss rollers 85-110 % and with immersion 130 %.
For hydrophobic treatment of PES/pulp spunlace sandwich nonwovens that side of the nonwoven is treated where pulp is concentrated because that side is denser than the PES fiber side. The applied web surface then forms the outside of surgical gowns for protection of the surgeon. The spunlace web thus treated then serves as barrier against the transfer of liquids such as sweat, blood, alcohol and oils that might contain bacteria.
3.Bonding of Spunlace Nonwovens by Hot Air
Also by subsequent thermal bonding with melt fibers in the spunlace web or by heat setting of fiber webs, product-specific properties can be optimized in the same way as by binder bonding.
3.1Thermofusion with Hot Air
It might be useful to provide spunlaced nonwovens with properties of increased strength and reduced pilling tendency in addition to mechanical fiber entanglement. In these cases, an impregnation is often not desirable because it affects the touch of the material. The fiber blend is then mixed with a share of bicomponent fibers which are automatically activated in the dryer after drying is completed and melted with the other fibers.
Bonding between fibers by thermofusion can be cohesive or adhesive. Cohesive bonding is achieved when intermolecular interactions between fibers of the same polymer take place (e.g. bicomponent fibers).
Thermofusion can take place in-line without integration of another machine in the existing dryer (Fig. 18). For drying or combined drying and thermofusion, one-drum or multi-drum lines are used (Fig. 19). Generally multi-drum dryers are preferable because they ensure:
automatic material transportation and
an alternating air flow through the nonwovens
which is a great advantage especially for higher web weights.
When nonwovens contain a shrinking fiber component or in case of heat setting processes, shrinking of the nonwoven must be avoided. This is achieved by two needle rings which are mounted on the drum and maintain the width of the material. Such a device can be used, however, on a single-drum dryer only.
Fleissner can also supply belt dryers with through-air flow or with air jetting for drying and thermofusion, if requested.
3.2Heatsetting
Heatsetting is a process achieving high dimensional stability of nonwovens made of synthetic fibers at high temperatures. Dimensional stability can be characterized by stability of shape and resistance to shrinkage. This method is mainly applied to nonwovens which are subsequently submitted to coating processes (e.g. roofing felts, coating substrates for PU and PVC, shoes and floor coverings).
Heatsetting is mostly done by means of hot air and in the majority of cases, the fibers employed are PES fibers.
The heatsetting effect not only depends on the temperature, but also on the dwelling period. The maximum heatsetting temperatures for the best-known man-made fibers are as follows:
PES230C
PP130C
PA190-225C
These temperatures approach the softening area and lie about 20-40C below the melting point.
Normally, in case of highly air-permeable spunlace nonwovens, the material is heated to heatsetting temperature within 1-2 seconds. The entire heatsetting process is completed within 5-12 seconds, depending on the web weight.
Heatsetting is done for
setting of fibers
reduction of tensions in the web
achieving dimensional stability
avoiding width shrinkage during subsequent finishing processes such as coating etc.
The Fleissner perforated drum through-air principle allows very quick heating of the web to heatsetting temperature; this means a very compact machine with all the advantages of low energy consumption. Shock-like cooling is realized on the through-air cooling drum. In some cases, the material passes through a calibrating unit before cooling in order to influence the thickness of the web.
Most heatsetting processes of nonwovens require the material width to be controlled and shrinkage to be avoided while under the temperature influence. For this purpose, the Fleissner one-drum dryer with adjustable or stationary needle rings is used.
Heatsetting of spunlace nonwovens can be done in-line or off-line. In both cases, the material must be dry first. In lines operating at low production speed with in-line heatsetting process, drying and heatsetting can be done one after the other on a 1-drum line (Fig 20). In lines with high production speeds, drying is realized on a multi-drum dryer which is followed by a 1-drum heatsetting oven. Fig. 21 shows an off-line heatsetting line with a width of 6.8 m.
4.Conclusion
All wet and dry finishing processes shown, including compact web formation from fiber opening, card and AquaJet to dryer and winder for finished nonwoven rolls can be carried out at the Fleissner technical center in Egelsbach.
Together with our customers, we can optimize all nonwovens qualities by in-line trials with:
fiber opening/blending
web formation with card
spunlacing with AquaJet
Spunlace System
airlay process
through-air
Drying/heatsetting
automatic winder
and off-line trials for:
all liquid and foam impregnation methods and
heatsetting and thermobonding processes.
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1.Introduction
The worldwide production of spunlace nonwovens has practically doubled between 1995 and 2000. When comparing the figures of 1985 and 2000, the increase in production has actually quadrupled (See Fig. 1) Also in the period between 2000 and 2006, the installed capacity worldwide has doubled. Thus spunlace products have the biggest growth rate of all nonwovens.
Although many spunlace nonwovens receive their final properties through spunlacing technology, there are other spunlace nonwovens that are further finished or additionally bonded. For more than 30 years, namely since the beginnings of spunlace technology, Fleissner delivered more than 120 finishing lines for spunlaced nonwovens for all kinds of applications. These included both inline systems with AquaJet machine and offline systems with widths of up to 5,000 mm. Having been acquired by Tretzschler group, Fleissner can now offer complete spunlace lines from fiber preparation to completely finished nonwoven rolls.
These finishing processes include:
Impregnation with chemical binders
Finishing with chemicals
Dyeing or printing
Thermo fusion or heat setting
Many nonwovens producers prefer separate finishing lines that are installed after a spunlace line.
The following 2 examples show a spunlace line with 2 cards for staple fiber webs ranging from low to very high web weights (See Fig. 3) and a spunlace line for spunbond webs for high production speeds (See Fig. 4). In these cases, the webs are dried before they are finished further. For many finishing processes and web weights (e.g. high web weights) this is a requirement.
Taking into consideration that the nonwovens are already wet after the spunlacing treatment, it makes sense to realize the additional finishing processes in-line, wherever possible, in order to save cost for drying energy.
Such a production line is illustrated schematically in Fig. 5. Certainly the installation of complete lines with in-line finishing process is the most compact design.
It should also be mentioned here that there are lines in operation which further bond the web in-line after spun lacing and comprise an intermediate drying stage to achieve optimum finishing results.
2.Bonding and Finishing with Chemical Binders and Chemicals
Bonding by means of chemicals usually comprises at least 2 steps: first the binder is applied and then the bonding process is triggered by means of thermal treatment. The nonwoven bonded by adhesion comprises a binding agent which 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 by binder 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.
2.1 Binding Agents and Binder-dependent Nonwovens Characteristics
Liquid binders allow their formula to be custom-made so that it 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 a table (Fig. 6).
Figure 6: Important Binder Liquid Classes
In addition to monomers and co-monomers, functional groups such as e.g. cross-linking agents are incorporated. They influence the properties of the polymer and consequently those of the nonwoven such as, for example, mechanical properties and resistance to solvents.
The finished polymer emulsion is finally obtained by addition of various additives. These additives are used to influence coagulation temperature (thermal sensitivity of binder liquid), foamability, wettability, migration behavior, printability etc. Consequently, the substances listed below are contained in various concentrations in dispersion:
binding agent
wetting agent
thickener
catalyst
antifoaming agent
water
thermal sensitizing agent
filling material
dye pastes
flame protection agent
The binder liquid properties allow the requested nonwovens properties to be obtained within wide limits (Table Fig. 7).
Characteristic applications of binder-finished nonwovens are listed in a table (Fig. 8).
Apart from many demands made on the binding agents used, it is above all properties such as
improved ecological harmlessness
toxicological safety
reduced flammability
which are decisive.
2.2Application Methods
Generally, the following distinction can be made:
Thorough binding:homogeneous binder distribution over nonwovens thickness and surface.
Binding is achieved by full bath impregnation or with a foam padder.
Surface binding:The binding points are concentrated at the surface. This is typically achieved by spraying, doctoring and surface foam application.
Partial binding:The nonwovens surface is locally bonded in the shape of mostly regular patterns. Processes used for this purpose are printbonding and printing.
In most cases one of the following processes is used where either liquid or foamed binders are applied:
Full bath impregnation in padder trough or inside gap (liquid)
One-sided metered binder application doctoring (spreading) kiss roller application small-surface application by means of engraved rollers small-surface application by means of round screens spraying
Foam application
In the following, we will concentrate on the most common processes such as liquid and foam impregnation; partial application by means of engraved rollers will be described later under print bonding and dyeing.
2.3Liquid Binders and Foamed Binders
2.3.1Application of liquid binders
Binders in liquid form are applied both in-line and off-line onto spunlaced nonwovens. Heavy nonwovens, for coating substrates for example, require off-line impregnation as it is very difficult to achieve proper through-impregnation in a wet-in-wet in-line process with the usual immersion padders. Foam impregnation also is out of the question for an in-line process, while in an off-line process it offers economical advantages due to the reduced moisture input.
In case of light-weight nonwovens (up to 80-90 g/m), foam impregnation should always be preferred to liquid impregnation because often considerable technical efforts are required to pass the web through the liquor without tension in order to avoid drafts.
For light-weight nonwovens, in-line impregnation with foam should also be preferred as a wet-in-wet process for economical reasons, i.e. saving of drying energy (Fig. 9).
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 due to the 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.
2.3.1.1Wet-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 Aqua Jet in turn mainly depends on
vacuum inside suction slot
fiber type or fiber blend
speed
so that the wet-in-wet impregnation process must be controlled accordingly, which results in higher efforts for operation (qualification), 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, namely liquor application and binder concentration, are enough for dry-in-wet application to calculate binder application. For the wet-in-wet application method, however, liquor application and binder concentration 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, i.e. 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 classical addition metering, the consumed binder is replenished and constant thinning of the liquor by water carried in is compensated by adding binder liquor of a higher concentration to the impregnation bath.
Naturally liquor application and binder application depend on the nip pressure set for the rollers.
2.3.2Application of foamed binders
Foamed binders contain less water than corresponding binder liquors because part of the diluting water is replaced by air. The content of solid matter in foam is up to 40-50 %, depending on the binder application quantity, and about 15 % for impregnation liquor. This results in reduced drying cost and consequently reduced energy cost.
The advantages are:
wide range of application quantities down to minimum application (e.g. application of various auxiliaries)
more uniform binder distribution in the surface
surface impregnation and through-impregnation
less risk of migration during drying
good strength with reduced flexural strength due to punctual bonds
improved volume of nonwoven
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 to foam of 5, 10 or 20 times its volume (with a weight per liter of foam between 30 and 300 g/l) and then applied onto the web between 2 rollers (Fig. 10).
The foam impregnation process is suitable both for 1-sided binder application (kiss roller application) and for through-impregnation. Mere surface bonding is possible also (Fig. 11). The foamed 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 2 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 3 influencing factors:
concentration of binder liquor
foam weight per liter
roller gap.
One example for finishing of nonwovens with foamed binders is the production of bitumen carrier webs. These production lines are supplied in widths of up to 5400 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 shows foam impregnation and drying of light-weight webs for interlinings, medical webs and webs for sanitary purposes and for wiping cloths (Fig. 12).
2.3.2.1Wet-in-wet foam application
In case of light-weight webs, in-line impregnation with foam in a spunlace production line offers particular advantages. Since spunlacing already provides the web with sufficient strength, the application of binder not so much serves the purpose of increasing the strength, but rather to improve the surface resistance to wear, reduce the pilling tendency and achieve additional product-improving and product-influencing properties. In these cases the binder solids application usually is low also. The manufacturer will try to operate with minimum water application and a higher binder concentration. It is an advantage here that the web is already wet because this allows quick and uniform wetting of the web structure even at high production speeds despite a low foam weight per liter.
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