Chapter IV.

Mechanical technology of nonwovensDepartment of nonwovens

Wet laid, Air laid, and Random laid

Wet laid - Principle

Mechanical technology of nonwovensDepartment of nonwovens

This topic is disskused in subject „ Thermal and chemical technology of nonwovens“ because wetlaid fabricks are usually chemical bonded.

Wet laid nonwovens are made by a modified papermaking process. That is, the fibers to be used are suspended in water, which is subsequently taken out.

In terms of wet laid technology compared with dry laid technology is possible to process short and smooth fibers (for example glass microfibers).

The typical features of airlaid and wetlaid fabrics are:

•Random orientation of fibers on the fabric surface

•Wider range of area density compared with dry laid technology

Wetlaid – used material

Mechanical technology of nonwovensDepartment of nonwovens

The wet laid fibers must be miscible in water so their main feature are:

• low surface tension (which is possible to adjust by surfactants

•small lenght or low ratio between the fiber length and diameter – it is possible to process short fibers (2-50 mm)

•low crimpiness – it is possible to proces smooth fibers

A major objective of wetlaid nonwovens is to produce structures with textile properties (flexibility, strenght etc...) at speeds similar to papermaking process. Textile fibers tend to be longer, stronger and relatively inert when compared to papermaking fibers, which are short, fine and are able to pack together into a dense structure. The structure, which contains textile fibers, is more open and extensible but preserves the basic strenght and stability of the fabric. Thus wet laid textile is stronger, softer, bulkier, more drapeable, less smooth and more porous than paper.

Wet laid – principle of technology

Mechanical technology of nonwovensDepartment of nonwovens

There are three characteristic stages in the manufacture of nonwoven bonded fabrics by the wet-laid method .

•Swelling and dispersion of the fiber in water; transport of the suspension on a continuous traveling screen

•Continuous web formation on the screen as a result of filtration

•Drying and bonding of the web

Fiber swelling and dispersion

Suspension transport

Web formation

Water recycling

Wet laid – examples of end uses of wet laids

Mechanical technology of nonwovensDepartment of nonwovens

•Special papers:

synthetic fiber paper, dust filters, liquid filters, overlay paper, stencil paper, tea bag paper, paper for wrapping susage and cooked meats

•Industrial nonwovens for:

waterproof sheeting for roofs, shingling, separators, filters, reinforcement material for plastics, backing material, shoe uppers, decoration, interlinings, insulation

•Nonwovens similar to textiles:

surgical clothing, bed-linen, table cloths, servittes, towes, household cloths, face cloths, nappy, sanitary articles

Air laid - principle

Mechanical technology of nonwovensDepartment of nonwovens

The fiber material is at first opened by rotating cylinder named lickerin. Then single fibers are dispersed into the air stream and condensed on the perforated cylinder or belt.

Air laid fabric compared with carding technology has these features:

• The fibers are oriented randomly on the fabric surface – isotropic structure.

• Voluminious webs can be produced

• The range of the area weight is wider (15 – 250 g/m2) but the mass uniformity of light air laid (up to 30 g/m2) is bad.

• Wide variety of processable fibers

Air laid – production problems

Mechanical technology of nonwovensDepartment of nonwovens

• Low level of opening fiber material by lickerin rollerThus is suitable to use pre-opened fibers or combine air laid with card machine – Random card machine

• Various structures of web in width of layer due to irregular air flow close to walls of duct

This problem requires high quality design of duct.

• Possible entangling of fibers in air streamThis problem can be reduced by increasing the ratio air/fibers which nevertheless means decrease in performance and increase of energy consumption due to high volume of flowing air. The relation between air flow and performance of device shows the importance of fiber lenght and fiber diameter. QA is air flow, K is device constant, P is performance of device (kg/hour), L is lenght of fiber staple (m) and D is fiber fineness (dtex).

D

LPKQA

2.. Thus is suitable to use short fibers for this

technology.

Example of air laid machine

Mechanical technology of nonwovensDepartment of nonwovens

Production up to 3 000 kg/hour

Scheme of airlaid line dust controll

Random cards – combination of air laid and carding technology

Mechanical technology of nonwovensDepartment of nonwovens

A major objective of this combination is isotropic textile fabric (random orientation of fibers) with good mass uniformity of light fabrics and with high production speed.

• The first part – card machine opens perfectly fibrous material so single fibers are as a output.

• The second part – air laid system uses the centrifugal force to strip the fibers off a roller and. put them down on an air controlled scrim belt.

Main variations of random cards I.

Mechanical technology of nonwovensDepartment of nonwovens

Airlaid function of random card:

1) Random roller between main cylinder and doffer, which rotate in the opposite direction of the main cylinder.

Main cylinder

Random roller

Mechanical technology of nonwovensDepartment of nonwovens

2) Centrifugal force of mean cylinder strips the fibers off

Main variations of random cards II.

Random card Fehrer K12

Example of random card line

Fehrer

Mechanical technology of nonwovensDepartment of nonwovens

V 21/R - K 12

K 12 "HIGH-LOFT"

V 12/R Working widths (m)

1.2 - 5.4 1.2 - 5.4

Production speeds (m/min)

up to 30 * 1 - 10 *

Weight range (g/m2)

40 - 3000 * in special

configuration up to 6000

500 - 6000*

Capacity (kg/h/m)

up to 450 * up to 1500 *

Air laid and random cards: used fibers

Mechanical technology of nonwovensDepartment of nonwovens

synthetic fibres, viscose, cotton and blends thereof; natural fibres such as flax, hemp, sisal etc.; reclaimed textile waste and shoddy,1.7 - 2000dtex

max. 120 mm staple length

Air laid and random cards: end products

interlinings, shoe linings, „high loft" products for the garment and furniture industries; base material for coating substrates and synthetic leather; waddings;geotextiles, filter materials; needle blankets; carpets and wall coverings; technical felts insulation felts; mattress felts, waddings for the upholstery and automotive industry; undercarpets