unit 10-fundamentals of fibre spinning.pdf

Unit 10Fundamentals of

man made fibre spinning

Claudio Tonin [email protected] 2013-14

CNR ISMAC Institute for Macromolecular Studies Biella www.bi.ismac.cnr.it

Politecnico of Turin www.polito.it

Course of Textile Fibres and Technology

1marted 20 maggio 2014

Most synthetic and artificial manufactured fibres are made (spun) by extrusion forcing a thick, viscous liquid polymer (about the consistency of cold honey) through the tiny holes of a device called a spinneret to form continuous filaments of semi-solid polymer.

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The process of extrusion and solidification of endless filaments is called spinning, not to be confused with the textile operation of the same name, where short pieces of staple fibre are twisted into yarn.

The spinnerets used in the production of most manufactured fibers are similar, in principle, to a bathroom shower head.

As the filaments emerge from the holes in the spinneret, the liquid polymer is converted first to a rubbery state and then solidified.

It consists of a small, thimble-shaped, metal nozzle having fine holes through which a spinning solution is forced to form the filaments.


In their initial state, the fiber-forming polymers are solids and therefore must be first converted into a fluid state for extrusion. This is usually achieved by: - melting, if the polymers are thermoplastic synthetics (i.e., they soften and melt when heated), or by

- dissolving them in a suitable solvent if they are non-thermoplastic (such as cellulosics or proteins). If they cannot be dissolved or melted directly, they must be chemically treated to form soluble or thermoplastic derivatives.

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There are three (main) methods of spinning filaments of manufactured fibers:

melt, wet, dry, solution spinning}

The viscous solution, prepared by melting or chemically dissolving raw material, emerges from the spinneret as long fibres that are then solidified by cooling, evaporation, or coagulation.

... plus a number of modern methods (dry-jet wet spinning, gel spinning, melt blown spinning, electrospinning ...)


4Melt spinning Solution spinningSolution spinning

PolyamidePolyester

PolyethylenePolypropilene

Dry spinning Wet spinning

PolyamidePolyester

PolyethylenePolypropilene

Rayon acetateRayon triacetate

AcrylicModacrylicAramidic

PVC

Viscose rayon Rayon cupro

AcrylicModacrylicAramidic

PVC

Methods of spinning filaments of manufactured fibers


5Melt spinning is the preferred method of manufacture for polymeric fibers. The polymer is melted and pumped through a spinneret with numerous holes (one to thousands).

The molten fibers are cooled, solidified, and collected on a take-up wheel.

Thermoplastic polymers such as poly(ethylene terephthalate) and polyamide (nylon) are melt spun in high volumes.


6Filament formingThe action of pumping the molten polymer through small holes in the spinnerette results in threads of polymer which are then cooled by a stream of cool air.

As streams of viscous polymer exit the spinneret into cool air and solidify, the individual polymer chains tend to align in the fiber because of viscous flow.


7Melt spinning


8Melt spinning plants layout.

Spinning plants are developed in height more than 10 m.


9 Melt spinning plant


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Single screw extrusion is one of the core operations in melt polymer fibre spinning.The foremost goal of a single screw extrusion process is to build pressure in a polymer melt so that it can be extruded through a die and fed to the spinnerets. Most machines are plasticating: they bring in solids in pellet or powder form and melt them as well as building pressure.


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Thermoplastic polymer chips are melted and fed to the spinnerets under very high pressure,up to 20 MPa (1 MPa = 10 bar). A single screw extruder feed many spinnerets by through a multiple way pipelinecalled spider.


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The action of pumping the molten polymer from the screw extruder to the spinnerets (through the spider) is made by gear metering pumps (volumetric pumps).

The metering pump is a positive-displacement and constant-volume device for uniform melt delivery to the die assembly.

It ensures consistent flow of clean polymer mix under process variations in viscosity, pressure, and temperature.

The metering pump also provides polymer metering and the required process pressure. The metering pump typically has two intermeshing and counter-rotating toothed gears.

The positive displacement is accomplished by filling each gear tooth with polymer on the suction side of the pump and carrying the polymer around to the pump discharge, as shown in figure. The molten polymer from the gear pump goes to the feed distribution system to provide uniform flow to the spinnerets in the spider assembly (or fiber forming assembly).


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A spinneret may have from one to several thousands holes depending on the polymer to be spun and the spinning method.

Spinnerets for melt spinning may have from some ten to two hundred holes(there is the need of cooling even the inside filaments by heat transfer to the airstream)


The spinneret is regarded as the heart of the spinning operation. Here is where the spinning liquid starts to turn into a filament; here is where the shape of the filament is defined (the shape, not the final count).

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Manufacturing of spinnerets is a fundamental operation carried out by precision machining (such as laser drilling). A precision spinneret made of precious metal, provides holes that are worth more than the surrounding gold and platinum.The hole diameters are from 100 to 800 m, while the thickness of the spinneret for melt spinning is up to 10 mm because of the high pressure it is submitted.


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The tiny openings are very sensitive to impurities and corrosion and, in some cases, the spinneret must be made from very expensive, corrosion-resistant metals.

The liquid feeding them must be carefully filtered (not an easy task with very viscous materials)Spinneret-Filters consist of several layers of stainless-steel woven wire cloth with square mesh weaves up to aperture widths of 25 micrometers (= 500 Mesh)

FilterDrainageSpinneret


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Filters and spinneret pack.


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Melt spun fibers can be extruded from the spinneret in different cross-sectional shapes (round, trilobal, pentagonal, octagonal, and others). Trilobal-shaped fibers reflect light differently and give an attractive sparkle to textiles.

Pentagonal-shaped and hollow fibers, when used in carpet, show less soil and dirt. Hollow fibers trap air, creating insulation and provide loft characteristics equal to, or better than, down.


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Profiled hole spinnerets


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Spinneret shapeand related cross-sectionalfibre shape

die swell(Barrus effect)


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Bi-component Melt blown spinning Flow simulation of bi-component fiber spinneret.

The figure shows one channel in the bi-component fiber spinneret. The red and blue represent the two polymers and the green represents the interface between the two. The real life photograph shows validity of the results.


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Maintenance is also critical, and spinnerets must be removed and cleaned on a regular basis to prevent clogging.

LENZING ULTRASONIC SPINNERET CLEANING UNIT


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Big rectangular spinneret for melt spinning


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Drawing


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Continuous drawing plant


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Drawing pulls the molecular chains together and orients them along the fiber axis, creating a considerably stronger yarn.

Stretching of the fibers in both the molten and solid states provides for orientation of the polymer chains along the fiber axis.

While extruded fibers are solidifying, or in some cases even after they have hardened, the filaments may be drawn to impart orientation and strength.


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Fibre forming filament during solidification:


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Crystallization during filament solidification: the influence of take-up velocity (winding speed)


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LOY MOY POY HOY FOY FOY(SHS)

Final speed (winding) (m/min) 1 200 2 000 3 500 4 500 5 000 7 000

Tenacity (cN/dTex) 1,09 1,44 2,61 3,07 3,8 3,44

Elongation (%) 420 271 130 85 32 32

Further processing FDY FDY DTY ATY ATY ATY

Example: Properties of a polyester filament with final count 76 dTex

Legenda:LOY Low Oriented Yarn MOY Medium Oriented YarnPOY Partially Oriented YarnHOY Highly Oriented Yarn

FOY Fully Oriented YarnSHS Super High SpinningFDY Fully Drawn Yarn (by separate drawing)DTY Drawn Texturized YarnATY Air Jet Texturized Yarn


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Tensile properties of a polyester filament at different winding speed

Tenacity

Elongation

7 000 m/min (FOY SHS)

5 000 m/min (FOY)

4 500 m/min (HOY)

3 500 m/min (POY)

2 000 m/min (MOY)


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Forces on a fibre forming filament during solidification:

Fsurf surface tensionFrheo rheological force (viscosity)Ffric friction forceFinert inertial force

Fext external force (winding, drawing)Fgrav gravity force

die swell(Barrus effect)

Xs = solidification length


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Profiles on a fibre forming filament during solidification:


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Spinnability is the ability of a fluid to form continuous filaments of unlimited length

The following parameters influence spinnability:

X/V0 = R /

where:

X = solidification lengthV0 = estrusion linear speed R = capillary (hole) radius = viscosity (laminar flow) = surface tension

Solidification length is normally from 1 to 5 m.Solidification time is from 0.001 to 0.25 min.

Braking of the filament occurs because of the external or internal forces on the filament.


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Two kinds of failure (breaking) may occur: - cohesive failure, when the external forces (winding & drawing overcome the filament tenacity;- capillary failure, with drop formation (function of the ratio /). (L = breaking length; = viscosity; = surface tension).

Breaking occurs according to the mechanism providing the minimum length( supports capillary failure; supports cohesive failure).

Braking of the filament occurs because of the external or internal forces on the filament.


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Count control of a forming filament:

Linear mass: Tex = Mass in grams of 1000 m filament

Tex = 1000 M/L = 1000 M t /L t = 1000 W/V

where:W = mass rateV = final speed (winding)

W = A x Wp x x V0

where:A = hole areaWp = polymer mass concentration (=1 for melt spinning) = polymer densityV0 = estrusion linear speed

then:

Tex = 1000 A Wp V0 / V

Drawing ratio

V

V0

cooling air


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Melt spinning plants layout:

a - winding (taking up) & drawingb - c - separate drawingd - wet finishing (softening, sizing, ...)e - dryingf - texturizingg - packagingh - cut (staple)i - packaging

A small amount of finishing oil may be applied to the yarns before wind-up.


Dry spinning is used to form polymeric fibers from solutions. The polymer is dissolved in a volatile solvent and the solution (dope) is pumped through a spinneret with numerous holes (up to two thousands). As the fibers exit the spinneret, air is used to evaporate the solvent so that the fibers solidify and can be collected on a take-up wheel. Stretching of the fibers provides for orientation of the polymer chains along the fiber axis. Cellulose acetate (acetone solvent) is an example of a polymer which is dry spun commercially in large volumes. Due to safety and environmental concerns associated with solvent handling this technique is used only for polymers which cannot be melt spun.

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Process Schematic


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air

solvent recovery

polymer solution

gear pump

Dry spinning plants layout


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Dry spinning head


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Diacetate dry spinning plant


Wet spinning is the oldest process used for fiber-forming substances that have been dissolved in a solvent. A polymer solution (dope) is "spun" through a spinneret into a liquid that has no dissolving power for the polymer, resulting in precipitation or coagulation of the polymer in fibrous form. Spinnerets are submerged in a chemical bath and as the filaments emerge they precipitate from solution and solidify.

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In some cases (viscose rayon), the spinneret must be made from very expensive, corrosion-resistant metals.

Process Schematic

coagulation spin-bath


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Spinneret immersed inthe coagulation chemical spin-bath

filamentspolymer solution feed


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Wet spinning into the coagulation spin-bath


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In wet spinning, the filament speed must be low because of the liquid surrounding the forming filament, but the spinnerets have numerous holes, up to many thousands.

Spinning method Melt Dry Wet

Spinneret holes (n) up to 200 up to 2 000 up to 200 000

Speed (m/min) 1 500 - 7 000 100 - 1 000 5 - 150


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Wet spinning plants layout


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Wet spinning is the most complex and critical method of spinning fibres, both for engineering and environmental aspects.

Viscose rayon staple fibre process: spinning


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Viscose rayon staple fibre process: hot stretching the tows


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Viscose rayon staple fibre process: traction unit after hot stretching


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Dry-jet wet spinning (gel spinning) is a combination of dry and wet spinning.The fibres are formed by spinning into an air gap and then coagulating in a spin bath.

They are then washed and dried.

a) dry spinning b) wet spinning c) melt spinning d) gel spinning


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Melt blown spinning

Melt blowing (MB) is a process for producing fibrous webs or articles directly from polymers or resins using high-velocity air or another appropriate force to attenuate the filaments. The MB process is one of the newer and least developed nonwoven processes. This process is unique because it is used almost exclusively to produce microfibers rather than fibers the size of normal textile fibers. MB microfibers generally have diameters in the range of 2 to 4 m, although they may be as small as 0.1 m and as large as 10 to 15 m. Differences between MB nonwoven fabrics and other nonwoven fabrics, such as degree of softness, cover or opacity, and porosity can generally be traced to differences in filament size.

A typical MB process consists of the following elements: extruder, metering pumps, die assembly, web formation, and winding.


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Die Nosepiece

From the feed distribution channel the polymer melt goes directly to the die nosepiece. The die nosepiece is a wide, hollow, and tapered piece of metal having several hundred orifices or holes across the width. The polymer melt is extruded from these holes to form filament strands which are subsequently attenuated by hot air to form fine fibers. In a die's nosepiece, smaller orifices are usually employed compared to those generally used in either fiber spinning or spunbond processes. A typical die nosepiece has approximately 0.4-mm diameter orifices spaced at 1 to 4 per millimeters (25 to 100 per inch).

The air manifolds supply the high velocity hot air (also called as primary air) through the slots on the top and bottom sides of the die nosepiece, as shown in Figure.

The high velocity air is generated using an air compressor.

The compressed air is passed through a heat exchange unit such as an electrical or gas heated furnace, to heat the air to desired processing temperatures.

Typical air temperatures range from 230C to 360C at velocities of 0.5 to 0.8 the speed of sound.


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There are two types of die nosepiece used: capillary type and drilled hole type. For the capillary type, the individual orifices are actually slots that are milled into a flat surface and then matched with identical slots milled into a mating surface.

The two halves are then matched and carefully aligned to form a row of openings or holes as shown in Figure. By using the capillary type, the problems associated with precise drilling of very small holes are avoided. In addition, the capillary tubes can be precisely aligned so that the holes follow a straight line accurately.

The drilled-hole type has very small holes drilled by mechanical drilling or electric discharge matching (EDM) in a single block of metal, as shown in Figure .


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As soon as the molten polymer is extruded from the die holes, high velocity hot air streams (exiting from the top and bottom sides of the die nosepiece) attenuate the polymer streams to form microfibers. As the hot air stream containing the microfibers progresses toward the collector screen, it draws in a large amount of surrounding air (also called secondary air) that cools and solidifies the fibers, as shown in figure. The solidified fibers subsequently get laid randomly onto the collecting screen, forming a self-bonded nonwoven web. The fibers are generally laid randomly (and also highly entangled) because of the turbulence in the air stream, but there is a small bias in the machine direction due to some directionality imparted by the moving collector.


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The collector speed and the collector distance from the die nosepiece can be varied to produce a variety of melt-blown webs. Usually, a vacuum is applied to the inside of the collector screen to withdraw the hot air and enhance the fiber laying process.


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Product Characteristics

Melt-blown webs usually have a wide range of product characteristics. The main characteristics and properties of melt-blown webs are as follows:

1. Random fiber orientation.

2. Lower to moderate web strength, deriving strength from mechanical entanglement and frictional forces.

3. Generally high opacity (having a high cover factor).

4. Fiber diameter ranges from 0.5 to 30 m, but typically from 2-7 m.

5. Basis weight ranges from 8-350 g/m2, but typically 20-200 g/m2.

6. Microfibers provide a high surface area for good insulation and filtration characteristics.

7. Fibers have a smooth surface texture ranging from circular to flat in cross-section.

8. Most melt-blown webs are layered in structure, the number of layers increases with basis weight.

The fiber length in a melt-blown web is variable; it can be produced in the range from a few millimeters to several hundred centimeters in length and usually exists over a broad range.


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Polymer type

The type of polymer or resin used will define the elasticity, softness, wettability, dyeability, chemical resistance and other related properties of formed fibers. One of the advantages of melt-blown technology is to handle many different polymers as well as mixture of polymers. Some polymers, which can be melt-blown, are listed below. However, the list is not complete.

1. Polypropylene is easy to process and makes good web.

2. Polyethylene is more difficult to melt-blow into fine fibrous webs because of its melt elasticity.

3. PBT processes easily and produces very soft, fine-fibered webs.

4. Nylon 6 is easy to process and makes good webs.

5. Nylon 11 melt-blows well into webs that have very unusual leather like feel.

6. Polycarbonate produces very soft-fiber webs.

7. Polystyrene produces an extremely soft, fluffy material with essentially no defects.

The most widely used polymer is polypropylene. Polypropylene with its low viscosity has a low melting point and is easy to draw into fibers. It comprises 70-80% of the total North American production.


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APPLICATIONS

The melt-blown system is unique because the process generates a fine fiber not available to the other nonwoven processes. Micro-denier fiber (less than 0.1 denier per filament) is not really available as a nonwoven fibrous raw material. Hence, the melt-blown process, which can produce such a fiber, opens new vistas of products and applications. At the present time, the following market segments are successfully served by melt-blown products:

1) Filtration media

This market segment continues to be the largest single application. The best known application is the surgical face mask filter media. The applications include both liquid filtration and gaseous filtration. Some of them are found in cartridge filters, clean room filters and others.

Filtration and workclothes


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2) Medical fabrics

The second largest MB market is in medical/surgical applications. The major segments are disposable gown and drape market and sterilization wrap segment.

Surgical drapes and gown and sterilization wrap


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3) Sanitary products

MB products are used in three types of sanitary protection products - feminine sanitary napkin, Spunbond-MB diaper top sheet, and disposable adult incontinence absorbent products.

4) Oil adsorbents

The best known application is the use of sorbents to pick up oil from the surface of water, such as encountered in an accidental oil spill and for mats in machine shops and in industrial plants.

5) Apparel

The apparel applications of melt-blown products fall into three market segments: thermal insulation, disposable industrial apparel and substrate for synthetic leather. The thermal insulation applications takes advantage of microvoids in the structure filled with quiescent air, resulting in excellent thermal insulation.

6) Hot-melt adhesives

The MB process has a special feature: it can handle almost any type of thermoplastic material. Thus, the task of formulating a hot-melt adhesive to provide specific properties can be greatly simplified by using the melt blown system to form the final uniform adhesive web.

7) Electronic specialties

Two major applications exist in the electronics specialties market for melt blown webs. One is as the liner fabric in computer floppy disks and the other as battery separators and as insulation in capacitors.

8) Miscellaneous applications

Interesting applications in this segment are manufacture of tents and nonwoven fabrics for agriculture.


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MBT extruded sorbents are available in various surface textures, colors, thicknesses, and dimensions. Chemicals and additives, may change their absorbent properties and sorption rates. Meltblown grades offer good filtration and barrier properties beside oil sorbency. Besides PP PBT, PE and elastomeric grades can be found in the market.

Filtration and workclothes


unit 10-fundamentals of fibre spinning.pdf

Documents

gel spinning

blown spinning

melt spinning plants

jet wet spinning

molten polymer

viscous liquid polymer

threads of polymer

streams of viscous polymer