Nonwoven technologies: A critical analysis
Nonwoven fabrics have quietly revolutionised consumer, medical, and industrial market places throughout the world, aver Laga SK, Vignesh Dhanabalan and Daniel Karthik. Nonwoven is an engineered fabric structure made directly from fibres, to provide specific function to ensure fitness for purpose. The term ?nonwoven? is often used as a generic description of a fabric that is not produced by process of weaving or knitting, more broadly, a fabric that is different from a traditional textile fabric. Like textile fabrics, nonwoven is a planar structure that is produced with varying degrees of integrity, surface texture, thickness, flexibility, and porosity that involves low cost and production process.
Nonwoven fabrics have quietly revolutionised consumer, medical, and industrial market places throughout the world, aver Laga SK, Vignesh Dhanabalan and Daniel Karthik.
Nonwoven is an engineered fabric structure made directly from fibres, to provide specific function to ensure fitness for purpose. The term ?nonwoven? is often used as a generic description of a fabric that is not produced by process of weaving or knitting, more broadly, a fabric that is different from a traditional textile fabric. Like textile fabrics, nonwoven is a planar structure that is produced with varying degrees of integrity, surface texture, thickness, flexibility, and porosity that involves low cost and production process. In fact, the technologies used to make nonwoven fabrics are based on fundamental principles used to produce textiles, papers, and plastics. In this regard, nonwovens are fabrics that are made by mechanically, chemically, or thermally interlocking layers or networks of fibres or filaments or yarns.
In spite of this mass-production approach, the nonwovens industry produces wide range of fabric properties from open wadding suitable for insulation containing only 2-3 per cent fibres by volume to stiff reinforcing fabrics where the fibre content may be over 80 per cent by volume. One of the major advantages of nonwoven manufacturing is that, it is generally done in one continuous process directly from the raw material to the finished fabric. When compared to other fabric manufacturing techniques non woven technique is found to the best in terms of production ratio.
Raw materials and their properties
The end use of materials has been the driving force for the development of products and technology. Raw material is the key factor in designing of material, the selection of raw material affects the function and quality of the product. The manufacturing processes for nonwoven fabrics invariably consist of two distinct phases, web formation and followed by subsequent bonding:
General production steps for non woven manufacturing
Dry laid process: In dry laid process, web is produced from staple fibres. These fibres are directly laid from a carding machine to form the matt ranging from 10-2500 gsm. The carding machine used is not the regular ones they are equipped with worker and stripper rollers. There are three methods for dry lying of web.
Parallel lying: The mass per unit area of card web is normally too low to be used directly in a nonwoven. They are increased by laying several card webs one over the other to form the matt. The simplest and cheapest way of doing this is by parallel lying. Figure 1 show five cards raised slightly above the floor to allow a long conveyor lattice to pass underneath. The webs from each card fall onto the lattice forming a matt with five times the mass per unit area.
Parallel laying of carded webs
Cross lying: In cross lying, the cards (or cards) are placed at right angles to the main conveyor as shown in Fig.2. In this case the card web is traversed backwards and forwards across the main conveyor resulting is a zig-zag motion as shown in Fig 2
Air lying: The air-laying method produces the final matt in one stage without first making the intermediate lighter weight web. It is capable of running at high production speeds but is similar to the parallel-lay method. The width of the final matt is the same as the width of the air-laying machine, usually in the range of 3?4m.
Opened fibre from the opening/blending section is fed into the back of hopper (A), which delivers a uniform sheet of fibres to the feed rollers. The fibre is then taken by the toothed roller (B), which is revolving at high speed. The worker and stripper rollers set to the roller (B) to improve the opening power. A strong air stream (C) dislodges the fibres from the surface of roller B and carries them onto the permeable conveyor on which the matt is formed. The stripping rail E prevents fibre from reticulating round the cylinder B. The air flow at D helps the fibre to stabilise in the formation zone.
Wet lying: The wet-laid process was mainly developed from papermaking. This was undertaken because the production speeds of papermaking are very high compared with textile production.
Textile fibres are cut very short by textile standards (6?20mm), but at the same time these are very long in comparison with wood pulp. The fibres are then dispersed into water and the rate of dilution has to be great enough to prevent the fibres aggregating.
Wet-laid nonwovens represent about 10 per cent of the total market, but this percentage is tending to decline. They are used widely in disposable products, for example in hospitals as drapes, gowns, sometimes as sheets, as one-use filters, and as cover stock in disposable nappies etc.
Spun lying: Spun lying includes extrusion of the filaments from the polymer raw material, drawing the filaments and laying them into a mat. As laying and bonding are normally continuous, this process represents the shortest possible textile route from polymer to fabric in one stage. When first introduced only large, very expensive machines with large production capabilities were available, but later much smaller and relatively inexpensive machines have been developed. Further developments have made it possible to produce microfibres on spun-laid machines giving better filament distribution. Smaller pores present between the fibres for better filtration, softer feel and also the possibility of making lighter-weight fabrics. For these reasons spun-laid production are increasing more rapidly than any other nonwoven process.
Melt blown: Melt blowing is another method of producing very fine fibres at high production rates without the use of fine spinnerets. Figure 6 shows that the polymer is melted and extruded in the normal way but through relatively large holes. As the polymer leaves the extrusion holes it is hit by a high speed stream of hot air at or above its melting point, which breaks up the flow and stretches the many filaments until they are very fine. At a later stage, cold air mixes with the hot and the polymer solidifies. Then the jet of fibre are collected on the collector in the foam of membranes.
Bonding is the sequential process in manufacturing of nonwoven product. The web that is formed in the above mentioned process is not sufficient enough to hold them together they have to bonded to ensure no loss of material is found during usage.
Needle punching technology: The needle punching system is used to bond dry laid and spun laid webs. The needle punched fabrics are produced when barbed needles are pushed through the fibrous web, forcing fibres to form self interlock. This action occurs in needle punching around 2,000 times a minute.
In needle punching the bonding of the fibre web is the result of intertwining of the fibres and of the inter fibre friction caused by the compression of the web.
Product characteristics: Needle felts have a high breaking tenacity and also high tear strength but the modulus is low and the recovery from extension is also poor. Their Unique physical properties like elongation in all (x, y,& z) directions for mould able applications is good. High strength makes them an overwhelming choice of geo-textiles. The principal advantage is that the nonwoven is practically homogeneous in comparison with a woven fabric so that the whole area of a nonwoven filter can be used for filtration, whereas in a woven fabric the yarns effectively stop the flow, leaving only the spaces between the yarns for filtration.
Hydro entanglement/Spun lace technology: The process of producing an entanglement by means of heavy water jets at very high pressures through jet orifices with very small diameters is spun lace technique. This is similar to a needle loom, but uses lighter weight matt. A very fine jet of this sort is liable to break up into droplets, particularly if there is any turbulence in the water passing through the orifice. If droplets are formed the energy in the jet will spread over a much larger area of matt so that, the energy per unit area will be much less. Consequently the design of the jet to avoid turbulence and to produce a needle-like stream of water is critical.
Product characteristics: Products made from spun lace technique have very good textile drape (low stiffness) and very soft to handle. No chemical or melt binder is required therby making its product possible to prepare 100 per cent natural fibres suitable for sanitary products. Uniform surface can be obtained due to more fine interlacing of fibres (compared to needling). Very high textile production: up to 300 m/min for carded airlaid webs and of up to 500 m/min for wetlaid and spunbond (meltblown) can be made. Faric width up to 6000 mm can be produced. So wide range of textile structure (depending especially on the perforated belt structure) can be designed and manufactured. Stitch bonding technique: It is a technique in which fibres in a web are bonded together by stitches sewn or knitted through the web to form a fabric. The finished fabric usually resembles corduroy.The fibres are bonded into the loops but the thread does not contribute to loop formation. The basic types of structure are pillar-stitch and tricot-stitch (Fig.9). It is also possible to use two systems of threads, so both the types of structure can be simultaneously applied.
Formation of stitch bonding
Product characteristics: They are highly voluminosity, softness and have good absorbent behavior, good elasticity and air-permeability. Finishing the raw web-knitted material by means of raising, cropping or tumbling, an even raised pile is achievable in heights ranging from 2 to 17 mm. Such pile web-knitted fabrics with base material are suitable for the manufacture of blankets, shoe lining, soft toy material and lining for winter garments.
Chemical bonding involves treating either the complete matt or alternatively isolated portions of the matt with a bonding agent with the intention of producing cohesion between fibre layers. Although many different bonding agents could be used, the modern industry uses only synthetic lattices, of which acrylic latex represents at least half, and styrene?butadiene latex and vinyl acetate latex roughly a quarter each. When the bonding agent is applied it is essential that it wets the fibres, otherwise poor adhesion might result. Most lattices inherently contain a surfactant to disperse the polymer particles, but in some cases additional surfactant may be needed to aid wetting. Followed by drying of latex by evaporating the aqueous component and leaving the polymer particles together with the additives. During this stage the surface tension of the water pulls the binder particles together forming a film over the fibres and a rather thicker film over the fibre intersections. Smaller binder particles will form more effective film than larger particles, other things being equal. The final stage is curing and in this phase the matt is brought up to a higher temperature than drying for the fixation to take place.
There are different ways to carry out the chemical bonding process. Some of the commonly used ones are:
- Saturation bonding: saturation bonding wets the whole matt with bonding agents, so that all fibres are covered in a film of binder.
- Foam bonding: Application of chemicals as foam. The binder solution and measured volume of air are passed continuously through a driven turbine which beats the two components into consistent foam.
- Spray bonding: Similar latex binders may also be applied by spraying, using spray guns similar to those used in painting, which may be either operated by compressed air or be airless.
- Print bonding: Print bonding involves applying the some types of binder to the matt but the application is to limited areas and sets a pattern
- Powder bonding: the bonding agent in the powder form is sprinkled.
Product characteristics: The fabric property is governed by the elastic nature of the fibre and the resin. Hence the fabric modulus is of the order of the fibre modulus that is extremely high. A high modulus in a spatially uniform material means that it will be stiff, which explains why saturation-bonded fabrics are very stiff relative to conventional textiles. At the same time tensile strength is low, because the bonds tend to break before most fibres break. Print-bonded fabrics are much softer in feel and also much more flexible owing to strong effect of the free fibres in the unbounded areas. They are also significantly weaker than saturation-bonded fabrics owing to the fibres slipping in unbounded areas, but knowing the fibre length and the fibre orientation distribution it is possible to design a print pattern which will minimise the strength loss.
Each spray application alters the thickness of the matt slightly, but it is still left substantially lofty, the drying and curing stage also causes some small dimensional changes. The final product is a thick, open and lofty fabric used widely as the filling in quilted fabrics, for duvets, for some upholstery and also for some types of filter media.
Thermal bonding: Thermal bonding is increasingly used at the expense of chemical bonding for a number of reasons. Thermal bonding can run at high speed, whereas the speed production of chemical bonding is limited by the drying and curing stage. Thermal bonding takes up little space compared with drying and curing ovens. Also thermal bonding requires less heat compared with the heat required to evaporate water from thebinder, so it is more energy efficient.
Thermal bonding requires a thermoplastic component to be present in the form of a homophile fibre, powder, film, web, hot melt ores a sheath as part of a bicomponent fibre. Heat is applied until the thermoplastic component becomes viscous or melts. The polymer flows by surface tension and capillary action takes place at fibre-to fibre crossover points where bonding regions are formed.
Methods of thermal bonding
- Hot calendaring
- Belt calendaring
- Through-air thermal bonding
- Ultrasonic bonding
- Radiant-heat bonding, etc.
Product characteristics: Products can be relatively soft and textile-like depending on blend composition and bond area. The material production does not involve any chemical use making it environmentally friendly and 100 per cent recycling of fibre components can be achieved. High bulk products can be bonded uniformly throughout the web cross section.
Non woven are considered finished products when they are out of the production line no external chemical or mechanical finishing is required t maintain its property. Special treatments like flame retardant, antistatic agents, antimicrobial, coloration might be applied to increase their functional property.
Needle felts have a high breaking tenacity and also high tear strength but the modulus is low and the recovery from extension is also poor. For this reasons, needle felts subjected to load has to have some form of reinforcement to control the extension to give better dimensional stability and increases the resistance to wear.
The wipes produced by hydro entanglement are guaranteed lint free, because it is argued that if a fibre is loose it will be washed away by the jetting process. It is interesting to note that the hydro entanglement process came into being as a process for entangling matts too light for a needle loom, but that the most recent developments are to use higher water pressures (400 bar) and to process heavier fabrics at the lower end of the needle loom range. Fabric uses include wipes, surgeon?s gowns, disposable protective clothing and backing fabrics for coating.
Thermal bonding is much less energy intensive, kinder to the environment and more economical. The bonding method has a significant effect on product properties. Depending on the bonding method, product properties vary from nonporous, thin, and non extensible, and nonabsorbent to open, bulk, extensible and absorbent.
All thermal bonding methods provide strong bond points that are resistant to hostile environment and to many solvents too.
Bond strength increases up to a maximum and then decreases with increase in bonding temperature for both staple fibre thermal bonding webs. Bond strength increases with increase in bond area. Bond strength increases with increase in bond size. Effect of bonding temperature, bond area, and bond size on fibre morphology in the unbonded region is negligible.
Nonwoven fabrics have quietly revolutionised consumer, medical, and industrial Market places throughout the world. They have been the ingredient through which many traditional products have been made better and the means by which many new products have been made possible.
Nonwovens are the fabrics that we don?t see, but are there where we need them; they are the fabrics that we don?t recognise, but are performing in ways that others can?t. Each of the basic technology systems has its specific advantages and limitation. Researches and experimental works are being carried out to explore the best possible use of nonwoven.
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The authors are with the D.K.T.E.S Textile & Engineering Institute, Ichalkaranji-416115(M.H), India, Email: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org