1. Air-lay Technology
This technology generally differs from other dry-laid webs in its use of very short fibers, mainly woodpulp. As a consequence, most products obtained with this method offer high absorbency as their prominent characteristic. They are also inexpensive and offer the great advantage of being biodegradable.

1.1 Process Description The process starts with defibration of woodpulp supplied in rolls to one or more hammer mills. When bonding is done by thermofusion, the melt fibers are supplied through bale opener and weight metering system to the raw material flow. Each forming head is usually connected to two pre-openers and thus allows the addition of up to two various melt fiber types apart from woodpulp.

Airlaid forming head with Fleissner Design

Fine opening of synthetic fibers is done in the metering tower. Conveyor fans transport the fibers from the hammer mills and the fiber openers to the forming head drums.

Web formation (fig. 1) takes place by means of two rotating drums provided with a perforation that depends on the final product.

The fibers are sucked off through the perforations of the forming drums and are transported with the vertical air flow produced by the vacuum inside the suction box to the web formation belt where they are deposited. The movement of this belt in production direction forms a uniform web with a thickness depending on the speed of the web formation belt.

Once the web is formed, it has a very high volume, but no strength whatsoever. It is, therefore, passed through three bonding stages.

The number of fiber opening and metering systems depends on the number of different melt fiber and woodpulp types. A standard line can process up to 2 different melt fiber types plus woodpulp in each forming head.

In the same way, the line can process 4 different woodpulp types into a web at the same time. Line capacity mainly depends on working width and number of forming heads, while working widths of 600 mm (for laboratory plants) up

to 5400 mm are normally used and speeds of up to 300 m/min can be reached. Product weights can range from 10 g/m² to 600 g/m².

The line capacity is influenced by the blending ratio between melt fibers and woodpulp and the physical properties of the melt fibers. Staple length, fiber structure, fiber titer, fiber conductivity and fiber surface properties are the decisive factors.

1.2 Bonding of Webs Latex bonding is the most common bonding process. First the binder is sprayed onto the top surface of the web and dried. Afterwards the web is sucked up by a top belt and the bottom side of the web is subjected to the same treatment. An alternative is offered by

the thermobonding process with melt fibers where the synthetic fibers are heated by a flow of hot air in a belt oven until they start to melt and bond with the loose cellulose fibers. Thermobonding is a clean and energy-saving process, but often requires binder bonding at the surface to avoid dust formation during make and use of the web. This can be achieved by surface

impregnation with a foam padder. With the application of latex as a light-weight foam the surface is bonded and the bulky and absorbent inner layer is maintained. On the other hand, very little energy is required for evaporation of the water contained in the binder. Another bonding method consists in placing air-lay products onto carded webs which are subsequently hydroentangled.

1.3 Typical Products
As mentioned at the beginning, air-lay webs are mainly produced from short fibers. Most products therefore consist of woodpulp and blends of woodpulp with short staple synthetic fibers.

Although there is a wide range of applications for these products, they have one thing in common: their good absorbency. Most products can be found in the field of wiping cloths where production lines with a capacity of almost 150,000 tons per year are installed. The majority of wiping cloths are produced for industrial purposes, baby wipes rank second.
Another important sector is covered by hygiene products and incontinence interlining webs. In this connection it is of very great importance that the absorbing web layer with super-absorbent powder or fiber is very thin and placed inside the composite. This sector covers about one quarter of the entire air-lay production.

There is also a number of products once called niche products, but now coming into the market in large quantities. They comprise hydroentangled composite webs which actually could be assigned mainly to the field of wiping cloths. They are followed by towels, napkins and table mats, sanitary napkins and panty liners. About 20 % of the entire air-lay production are used in other fields of application as, for example, in the food and beverages industry.

The highest growth rates for air-laid products can be found in the field of diaper production.
In the manufacturing process, the absorbing layer of the diapers and the acquisition fabric can be produced at the same time and wound into a roll together. This results in a very good bond between both webs and the process is more cost-efficient.

A clear development can also be detected in the field of filter media. This development also uses the particularly good and uniform distribution of the individual fiber components which are decisive for a specific filtration purpose.

Renowned producers of air-lay products are currently working on developments that often result in multi-layer webs. These composite webs consist of the most different types of material and are intended for a variety of applications. This applies above all to composites of fiber layers with air-laid layers that are hydroentangled together.

The total production of air-laid products has increased to currently more than 300,000 tons/year. The decisive factor for the choice of machinery and equipment or processes for production of air-laid goods is the customers' demands on the final products. This can be illustrated by the following table.

Apart from the physical properties in the above table, health and skin tolerance of the products play an ever more important role based on their main application in the fields of cosmetics and hygiene. In addition, the requested properties are sometimes contradictory such as e.g. softness and tensile strength. The raw materials used and the appropriate production process are the essential factors for meeting the above mentioned demands.

2. Air-laid Composite Lines with Hydroentanglement
Today chemical bonding still provides the main share of air-laid products. However, the importance of chemical bonding has decreased during the past years because thermal bonding and especially hydroentanglement allow to produce more user-friendly products (i.e. hygiene or cosmetics products without chemical additives, hence offering skin tolerance) and softer products of identical or higher strength. At the same time, the production methods themselves have become more environmentally friendly.

Hydroentanglement has gained considerable importance lately as the developments in the field of hydroentanglement also benefit the air-laid products. A reduced energy consumption per kg of raw material used, the reduction of material loss, the reduction of water consumption through the use of optimized filter systems as well as reliability and minimum maintenance requirements of the lines are decisive factors for the use of hydroentanglement with air-laid products.

Especially worth mentioning is the next to ideal possibility of producing so-called composites by hydroentanglement of various raw materials. In this process, the individual layers are assigned certain characteristics such as moisture absorption, moisture barrier, strength or softness. One example of many products are baby wipes.

In some cases, the lines for 2-layer and 3-layer composites are supplied with or also without pre-bonding stage. In the latter process, all web layers are placed one by one on top of each other and are then jointly hydroentangled. The process used depends on the application of the respective product.

Fleissner has been delivering complete lines, i.e. spunlacing including filtration, high-pressure and low-pressure components as well as complete process control systems since 1995. Continuous advancement and the experience gained with 40 production lines so far make it possible to run the Fleissner spunlacing line at production speeds of more than 300 m/min for working widths of up to 6 m.

Nonwovens produced by the SBAL process (air-laid/spunlace combination) offer products adapted in an optimum manner to the properties demanded at low raw material cost.
A carding web is pre-bonded by hydroentanglement in a first stage. Then woodpulp is spread on top by means of a forming head according to the air-lay technology (see section 2). The fiber flow is divided in two before reaching the drums and supplied from two sides. The two drums rotate in opposite directions above a web formation belt under which a vacuum is created inside a suction chamber in order to suck off the air. The fibers are deposited on the belt or in the case described on a carded PES web on the snowfall principle.

The quantity of biers supplied to the web formation belt and the speed of the belt determine the uniformity and thickness of the air-laid web.

After the carding web passed through the air-laid unit, the 2-layer composite (carded PES/air-laid pulp) is fed to the subsequent hydroentanglement system where the pulp layer is bonded with the PES web.

By addition of another fiber layer from a second card located before the above mentioned hydroentanglement system, also 3-layer composites (carded PES/air-laid pulp/carded PES) can be produced. The cellulose fiber layer can also be supplied through tissue rolls instead of air-laid forming heads.

In addition, Fleissner has started a highly interesting development, allowing to produce new product generations by combining the spunlace process with an air-lay machine.

Since all 3 processes (spunbond, air-laid and spunlace) can operate at high speeds (500 m/min), wiping cloths and hygiene products can be produced of various multi-layer composites in a highly cost-efficient manner. Naturally it is also possible to use viscose or PP fibers instead of PES.

3. Economic Efficiency of Spunlace Air-laid Composites
Both with chemically bonded air-laid products and with spunlaced carded/air-laid products wiping cloths play an important role. Therefore, the following will be a comparison of energy and raw material cost for a 60 g/m² web consisting of 50% PES/50% pulp or 70% PES/30% viscose. Investment and labor cost will not be taken into consideration, because only the yearly savings of energy and raw material cost are of interest here. The PES/viscose line consists of two cards with fiber opening, a spunlace unit, a dryer and a winder. The PES/pulp line comprises two cards with fiber opening, an air-laid unit, one spunlace unit each before and after the air-laid unit, a dryer and a winder.

The line speed is assumed to be identical for both lines although the PES/pulp line with one air-laid layer could be operated at a 1.5 times higher speed. The assumed production rate at 7000 hours and 3600 mm working width is about 9000 tons/year in both cases. Calculations show that the specific energy cost consumption for the air-laid composite product is about 2.84 kWh/kg fiber compared with 2.0 kWh/kg fiber with pure carding webs.

However, the fiber cost clearly differ (PES approx. 1.5 US$/kg, viscose approx. 2.2 US$/kg and wood pulp approx. 0.58 US$/kg). A comparison, therefore, shows that the cost for energy and fibers are 50 % higher for a pure carding web than for a carded/air-laid composite.
The higher energy cost of the air-laid line are more than compensated by the considerably lower raw material cost. However, depending on the working width and the number of required air-laid forming heads, the investment cost for the air-laid composite are higher so that an economic decision will have to be made from case to case.

As the raw material cost always represent the biggest share in a product cost analysis, however, the air-laid composite always has an advantage with respect to the production cost.