Source: New Cloth Market

Introduction:

For the production of medical and hygiene nonwovens and for products in the personal, health care and cosmetics sector mainly cotton, rayon, woodpulp, cotton linters, synthetic fibers and blends of various fibers are used.

Web formation can be done both on. cards and with the air-laid process for short fibers. When arranging several of these machines one after the other, a variety of different multi-layer composites and sandwich structures can be produced.

Bonding of staple fibers can be realized both by thermal bonding, binder bonding and by spunlace technology. The same applies to short woodpulp fibers. The choice of web formation system and bonding method allows obtaining products with custom-made properties. The following description comprises the web formation system of the air-laid technology for short fibers and the spunlace bonding method. Other technologies will naturally be mentioned for comparison.

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.

The process starts with de-fibration of woodpulp supplied in rolls to one or more hammer mills.

When bonding is done by thermo fusion, melt fibers are supplied additionally 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.

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 by the Dan-Web method takes place by means of 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.

The M&J Fiber-tech air-laying process uses a horizontal fiber distribution system above a horizontally moving forming wire to distribute fibers in cross and machine direction before depositing them through a forming screen onto the forming wire, under which a constant vacuum is maintained for regular web formation.

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.

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 300 m/min and more 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

 

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, as already described, is offered by the thermo-bonding 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. Thermo-bonding 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 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 forward-looking bonding method consists in placing air-lay products onto carded webs which are subsequently hydro-entangled together.

As mentioned at the beginning, air-lay webs are mainly produced from short fibers. Most products therefore consist of woodpulp or 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 absort: Jency. Most products can be found in the field of hygiene application.

An 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 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 due to their many advantages. They comprise hydro-entangled 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 productions are used in other fields of application as, for example, in the food and beverages industry.

In the manufacturing process of hydro-entangled composites, the absorbing layer of the diapers and the acquisition fabric can be produced at the same time and wound into a roll together. This result in a very good bond between both webs and the process is more cost-efficient. A 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, same as the R&D departments of machinery manufacturer Fleissner, 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 hydro-entangled 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 this 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.