Dry-laid web formation

A.G. Brydon    Garnett Group of Associated Companies, UK (Sections 2.1–2.12)

A. Pourmohammadi    Consultant, Iran (Sections 2.13–2.20)

2.1 Introduction

The dry-laid nonwoven sector utilises carding, garnetting, airlaying and in certain specialist applications, direct feed batt formation processes to convert staple fibres into a web or batt structure that is uniform in weight per unit area.

2.2 Selection of raw materials for carding

Virtually any fibre that can be carded can be, and probably already is, used in nonwovens including both organic and inorganic fibres. As noted in Chapter 1, man-made fibres account for the majority of raw materials used in the nonwovens industry, and in the carding sector, polyester is the most widely used. This is principally because of its suitability for many product applications and comparatively low cost. Polypropylene is also important, particularly in the manufacture of heavyweight needled fabrics for durable products such as floorcoverings and geosynthetics as well as in needlepunched filtration media and lightweight thermal bonded fabrics for hygiene disposables. Viscose rayon is extensively used in the medical and hygiene sectors, principally because of its high moisture regain. The flexibility of the carding process is reflected by the diversity of staple fibre types that are utilised by the industry and includes polymers, glass and ceramic materials. Table 2.1 gives a general overview of the fibres that are carded either alone or in blends. Fundamental to the suitability of a particular fibre for dry-laid processing is its machine compatibility as well as its influence on fabric properties. There are numerous examples of new fibre developments that have been slow to develop because of processing problems, particularly during carding.

Common problems are uncontrolled static electricity, low fibre-to-fibre cohesion and low fibre extension (the minimum required is 2–5%) leading to fibre breakage and poor yield. Whilst natural fibres such as cotton and wool have been carded as long as cards have been in existence, man-made fibres such as polyester have evolved to improve compatibility with high-speed nonwoven carding systems. The applied forces in carding give rise to fibre breakage and permanent fibre elongation, which modifies the original fibre length distribution and in some low-temperature materials such as PVC may be subject to thermal shrinkage during the process.

Whilst exceptions do exist, the general range of fibre dimensions suitable for the carding sector can be given approximately as 1–300 dtex fibre linear density and 15–250 mm mean fibre length. In practice such a range of fibre dimensions could not be satisfactorily processed on one card without modifying the card roller configuration and layout, settings and the card wire. Blending extends the range of fibre lengths and finenesses that can be processed and in certain sectors of the industry carrier fibres are used to aid processing of short, stiff or low surface friction materials. It should also be understood that the mean fibre length and the fibre length distribution as measured before carding is substantially different after carding due to fibre breakage or permanent elongation of fibres in the process. Cotton and other short-staple fibres of < 60 mm fibre length are used in the short-staple spinning industry, where traditionally, a modular sequence of processes has been developed to prepare, card and spin the fibre into yarn. Man-made fibres of similar diameter to cotton are therefore cut to a similar length so that they can be processed on the same machinery, either in 100% or blended form, depending on enduse requirements. Fibres are commonly square-cut to one length prior to processing. This gives a different fibre length distribution from natural fibres, which typically have a trapezoidal-shaped distribution.

Cotton cards are sometimes used to manufacture, for example, feminine hygiene and some absorbent medical products from short-staple fibres of c002E 28–45 mm mean fibre length composed of bleached cotton and viscose rayon. However, the use of short-staple or cotton ‘flat’ cards in the nonwovens industry is not extensive because the revolving flats limit the maximum width of the card to about 1.5 m and the mixing power of the machine is significantly lower than a worker-stripper card. Most carded nonwovens are manufactured from fibres with a mean length typically in the range 45–100 mm, although in some specialist applications fibres outside this range are carded. Accordingly, worker-stripper cards originally developed to process longer fibres are most commonly used by the nonwovens carding industry.

Fibre characteristics not only influence fabric properties but also processing performance. Web cohesion, fibre breakage, nep formation and web weight uniformity are key quality parameters and are influenced by fibre diameter, fibre length, fibre tensile properties, fibre finish and crimp. During the production of man-made fibres, crimp is introduced to increase web cohesion, bulk and sometimes elastic recovery. The crimp shape and frequency as well as its uniformity depend on the manufacturing conditions and in practice are subject to significant variation. The crimp may decay during carding due to the applied forces and temperatures that occur; cellulose fibres are particularly prone to this. Fibre finish modifies both fibre to fibre friction (cohesion) and fibre to metal friction (holding power of the wire) during carding.

Although polymer additives can be introduced before extrusion to influence properties, fibre finish is normally topically applied after extrusion, before the fibre is baled and despatched for carding. Both the static and dynamic friction are important, fibre to fibre and fibre to metal. The ability of a fibre finish to increase fibre cohesion whilst at the same time reducing friction is an example of frictional ‘stick-slip’ behaviour. A useful analogy is to imagine two sheets of glass coated by a thin film of lubricant. Placed together, the glass sheets easily slide over each other but it is not so easy to prise the sheets apart.

In carding, the fibres should readily slide against each other but in a controlled manner. Fibre finishes also contain anti-static agents, the effectiveness of which is particularly important when carding hydrophobic fibres such as polypropylene. Other finish additives may be used either to improve downstream processing efficiency or to meet the end-use requirements of the finished fabric. Accordingly, it is possible to use finish additives to improve wetting out of fibres by modifying surface energy, reduce foaming in processes such as hydroentanglement, meet food contact approval regulations and create biodegradable formulations for disposable fabrics. The moisture content or regain of fibres is also important because of the opportunity to control static electricity during carding as well as the influence of imbibed water on the tensile properties of...