Fibres & Filaments

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  • 8/3/2019 Fibres & Filaments


  • 8/3/2019 Fibres & Filaments





    Polypropylene (PP) is a popular material for fibre manufacture due to its properties like

    low density, easy processability, excellent orientation characteristics, superior tensileproperties, good chemical resistance, hydrophobicity and resistance to micro-organisms.

    These properties have resulted in use of PP fibre in carpets, rugs, apparel, homefurnishings, geotextiles, strappings, personal hygiene products, medical drapes andvarious other applications.

    Polypropylene filaments are produced by extrusion spinning process. Usually the

    product obtained from the spinning process is not directly suitable for commercial use.

    The post spinning operations involved are stretching and/or texturising and crimping. Instretching process, the spun filaments are stretched or drawn to several times its original

    length to increase molecular orientation and thus the tensile strength. These filamentsmay then be subjected to further operations, according to the final usage. Thermal setting

    and thermal relaxation processes provide dimensional stability; twisting and interlacingor intermingling provide interfilament cohesion; texturising and crimping provides avoluminuos yarn and cutting of continuous filaments into small pieces provide staple


    Melt Spinning Process

    The principle method of manufacturing PP fibres is melt spinning. The melt spinningprocess is shown schematically in Fig 1. In this process, the polymer alongwith

    stabilizers, rheological modifiers or colourants is fed into an extruder through a hopper.The extruder is usually of single screw type. The molten polymer existing from theextruder is distributed through a manifold to various spinning positions under constant

    pressure. The design of manifold system must avoid dead spaces as well as provide equalresidence time for the melt to reach at each spinning position. A spinning position

    usually consist of a melt pump eg. gear pump, a filter pack and set of spinnerets. A gearpump feeds metered quantity of melt to the filter pack. The polymer melt is then forced

    through a fine filtering assembly consisting of sand particles of 20-80 m size, a series of

    stainless steel wire gauges of different mesh sizes and a distributor plate. Filtration of themolten polymer before it enters the spinneret capillaries, homogenizes the melt and

    removes gels, and also eliminate gaseous bubbles. Efficient filtration brings down thebreakage rate to almost below six breaks per 1000 kg filament production and also

    reduces the frequency of thick and thin places in the spun filaments. After filtration, themelt forced through spinneret capillaries. Spinnerets are metal plates containing holes.Each spinneret contains few to thousands of holes depending on the process or product

    type. It ranges from few for monofilaments, upto thirty to ninety for filament yarn and afew thousands for production of staple fibres. The capillary dia of holes usually ranges

    from 0.2 - 0.3 mm and their length ranges from 1 to 3 times the diameter. The cross-

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    section of spinneret capillary may be round, trilobal or of the shape that needs to be

    imparted to the filaments.

    Fig 1. Schematic diagram of melt spinning process

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    The molten polymer filaments coming out of the spinneret in the form of fine stream of

    fluid are cooled by quench air to solidify over the first 80-100 centimeters of its lengthwhile being drawn off by the take-up device from the bottom of the spin line. The

    cooling of molten filaments is performed using purified chilled air at around 8 -14C andan air velocity of 0.6-0.8 m/sec. The filaments are then made to converge into a bundle

    with the help of ceramic guides. A spin finish is applied before the bundles are wound upon a package tube roll, which is often friction driven by a roller. Generally the yarnpasses through two godets before reaching the package winder. In order to lay the

    filaments satisfactorily around the package, it is grasped by the forked thread guide andmoved equally to the edges of the package by a traverse guide.

    F&F Spinning Equipments Used in India

    Neumag, Germany Extrusion Systems Ltd., UK Plasticizers Engg, UK Toray Engg, Japan Fare, Italy Mackey, US Reifenhauser, Germany Starlinger, Austria Bukwang, Korea Tae Sung, Korea Dong Kwang, KoreaResin Selection

    For polymer to perform well in filament spinning process, it has to meet certain basic

    requirements. These are :-

    Capability of being produced easily into fine filaments Good processing stability and melt strength to eliminate melt flow breaks and thus

    deteriorating physical properties of filaments

    Polymer cleanliness (very low filter index) to eliminate filter pack blockage Narrow molecular weight distribution resin (polydispersity between 4-7) to ensure

    consistent melt flow characteristics

    The capability of orient readily to eliminate filament breakages during post spinningoperations

    Good end use properties, ie. dimensional stability, UV radiation and colour stability,gas fading stability, particularly for outdoor use products

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    Repol and competitor's grades for fibres and filaments

    Property Repol










    Solvay Eltex-

    P FHY681(D)

    MFI (g/10min) 20 20 20 35 40 35

    Xylene solubles (%) 3.0-3.2 5.0 4.7 3.0-3.2 5.0 4.2

    Tensile yield strength


    34 30 33 34 32 32

    Elongation at yield(%)

    11 12 11 10 10 10

    Flexural modulus


    1500 1400 1500 1600 1450 1500

    Izod impact (J/m) 27 29 38 22 24 28

    Effect of Resin Variables on Fibre Properties

    There are mainly four resin variables that can significantly affect the properties of PPfibres

    1. Molecular weight and molecular weight distribution2. Additives3. CatalystMolecular weight and molecular weight distribution

    Molecular weight (MW) and molecular weight distribution (MWD) are important andcan have a significant effect on processing in the melt and solid state and on the tensile

    properties of the fibre. The polydispersity (mol. wt. distribution index) of commerciallyavailable polymer ranges from 2 to 12. The narrower the molecular weight distribution,

    the easier the control of spinning; a molecular weight distribution of around 3-5 isconsidered ideal for high speed spinning. In general, changes in molecular weight havethe following effects :

    The degree of crystallinity in fibre increases as molecular weight decreases The nucleation rate increases as the molecular weight decreases The crystallisation rate increases with increase in polydispersity Optimum spinning temperature increases with increase in molecular weight and with

    the breadth of the molecular weight distribution. However, the effect of molecular

    weight distribution is less

    Broad molecular weight distribution demonstrates higher draw resonance and poorspinnability

    Polypropylene with a narrow molecular weight distribution shows higherbirefringence (molecular orientation index) at higher spinning speeds, andconsequently higher tenacity and lower elongation

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    The trend on influence of polymer characteristics on spinning parameters and mechanicalproperties of PP fibres are given in Table 1.

    Influence of polymer characteristics on spinning parameters and

    properties of PP fibres

    Property Resins with sameshape MWD. Trend

    as MW increases

    Resin with sameMW. Trend asMWD narrows

    Maximum obtainable draw ratio Increases Decreases Maximum obtainable melt draw down


    Increases Increases *

    Draw resonance Decreases Decreases Ease of orientation Decreases -- Extrudability at the spinnerette Increases -- Fibre strength Decreases Increases Elongation at a given draw ratio Increases Decreases Tenacity at a given draw ratio Decreases Increases Modulus at a given draw ratio Decreases Decreases Orientation at a given draw ratio Decreases Increases Processing speed Increases Increases* It should be noted that in fibre spinning, elongational viscosity is more important thanshear viscosity. Narrow MWD polymers show less shear thinning and therefore

    elongational viscosity increases at higher extrusion rates, leading to higher meltorientation. This results in higher orientation in the spun fibre leading to higher tenacityand lower extensibility. Whereas, broad MWD polypropylene tends to be more shear

    thinning. Therefore, necking can result and ultimately melt fracture occurs at highspinning speeds. The fibre is less oriented and shows higher elongation.


    Additives are incorporated in PP, both to ease processing as well as to impart certaindesired properties in the fibre with respect to end product. The types and amount of

    additives used determine how well the resin processes. If the additive level isinsufficient, it can lead to melt flow break during extrusion, leading to deterioration inphysical properties of the final product. Inappropriate addition of additives can lead to

    problems like yellowing, gas fading and pinking.

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    The type of catalyst used during PP manufacturing process has a direct bearing on the

    resin characteristics. It affects the xylene soluble level, the molecular weight distributionand the level and type of catalyst residues.

    Catalyst residues can affect the colour of the resin and also have an effect on the finaltextile produced. The effect of oligomer level (xylene solubles) is reflected during

    processing as smoke or residue build up. It is also believed that a certain amount ofoligomers content is beneficial during processing but their effect on textile properties isnot fully understood.

    Other minor resin variables which affect either processing or final product properties are

    gels, filterability, pellet size and shape, pigmentation / masterbatches

    Effects of Processing Variables on Fibre Properties

    Besides the effect of resin properties, processing conditions like temperature and spinning

    speed also affect fibre properties. Some of the important processing variables arediscussed below

    Throughput rate

    Throughput has a direct bearing on the phenomenon of melt fracture. The shear rate (orshear stress) that a polymer melt experiences during processing is determined by the

    throughput. Beyond a certain critical value of shear rate, the diameter of the extrudatevaries considerably. This phenomenon is called melt fracture. Melt fracture is a diephenomenon and cannot be eliminated by decreasing spinning speed. It can be

    eliminated either by decreasing the throughput or by increasing the melt temperature.

    There is also a relationship between the throughput and spinning speed which can be

    explained as under. In the absence of a take-up force and for a given throughput, the meltexhibits swell as it exits the die. Polymer molecules are randomly coiled. When they

    traverse through the die, they are stretched and oriented which impart them with someenergy. As the polymer molecules exit the die, this stored energy is released andmanifests as die swell, where the diameter of the extrudate is larger than the diameter of

    the die exit. When throughput increases, die swell increases. Therefore, the polymermolecules require a longer process time to achieve a similar state of order as the

    molecules at a lower throughput. The relationship between throughput and take-up speedis illustrated in Figure 2.

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    For combinations of operating conditions of throughput and take-up speed below the

    curve in Figure 2, continuous fibre spinning will occur and for combinations above the

    curve, continuous spinning will not occur. Either the fibre will fail due to melt fracture orthe take-up force will be higher than the capabilities of the fibre resulting in fibrebreakage. It can therefore be said that for the fibre spinning the maximum spinning speeddecreases with increasing throughput.

    Spinning speed

    Spinning speed affects the drawdown ratio. Drawdown ratio is the ratio of the fibrevelocity at the take-up rolls to that at the die exit. Drawdown ratio is an expression of

    deformation that the melt undergoes before being taken up on cold rolls. For a giventhroughput, the drawdown ratio increases with increasing spinning speed.

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    Draw ratio

    Draw ratio is an expression of the amount of deformation a fibre undergoes in the solid

    state. It is expressed as the ratio of the speed of the draw rolls to the speed of the feedrolls. Orientation of the molecules occurs in both the melt and solid state processes.Draw ratio determines the solid state orientation. As draw ratio increases, tenacity

    increases and percent elongation decreases. Increasing the draw ratio increases theorientation of the chains and decreases the amount of unoriented amorphous material.

    The effect of draw ratio on the physical properties of the fibre are shown in Figure 3.

    Percent relaxation

    It is defined as the ratio of the difference in the draw and relax roll speeds to the draw rollspeed. This affects the residual shrinkage of a textile and decreases percent elongation.

    Relaxation is equivalent to an annealing process where the chains are allowed to formmore perfect crystalline arrangements that are thermodynamically stable


    Extrusion temperature, drying air temperature, draw temperature and relaxationtemperature are all important parameters that eventually determine the physical properties

    of the fibre. The extrusion temperature affects the rheological behaviour of the melt andthroughput. Temperature of the air for cooling determines the crystalline arrangements

    and the morphology of the fibre which ultimately affect the properties of the fibre. The

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    draw or orientation temperature and relaxation temperature decide the extent of

    orientation of the chains in the fibre axis direction and hence have a bearing on physicalproperties like tensile strength of the fibre. Generally the tenacity increases with

    increasing orientation.

    Colouration of Polypropylene Filaments

    Coloured PP filaments constitutes more than 55% share of the total PP fibre and filament

    production of India. The coloured PP filaments are usually made by mass colouration orpigmentation. The process involves intermixing of colourants with the polymer prior toextrusion. In order for the pigmentation of polypropylene to be successful, several

    important criteria have to be met.

    1. The pigment must be stable to the extrusion process2. The pigment must be dispersed evenly in the meltThe pigments differ in their ease of dispersion. Pigments such as titanium dioxide andcadmium or chrome pigments normally are among the easiest to disperse. Others, such

    as carbon black and iron oxides are the most difficult. Hydrophilic pigments aggregateseverely whereas organophilic pigments have less tendency towards aggregation.

    Problems in Filament Spinning and Possible Causes

    1. Non-uniformity of the spun filaments(i) Heterogenity of spinning fluid (polymer melt)(ii) High melt temperature than suitable for spinning(iii) Throughput variation(iv) Dirty and choked spinnerette capillaries(v) Variation in quench air velocity(vi) Variation in godet / take-up winder speeds(vii) Non-uniform dispersion of colourants in the melt

    2. Low tensile strength of spun filaments(i) Throughput variation(ii) Heterogenity of polymer melt(iii) Lower melt viscosity due to high MFI polymer(iv) High quench air velocity(v) Very low quench air temperature(vi) Low take-up velocity

    3. Low elongation in spun filaments(i) Heterogenity of polymer melt(ii) Lower melt temperature(iii) Low quench air velocity(iv) Higher quench air temperature(v) Higher take-up velocity

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    Testing of Fibres, Filaments and Fabrics

    Depending on the end use, there is a whole range of properties that needs to be tested. A

    comprehensive list of tests for fibre and fabric is given below which applies for bothwovens and non wovens.

    Physical Chemical Aesthetics

    Tensile StrengthElongation

    Burst strengthTear strengthSeam strength

    Surface frictionAir permeability

    Fluid porosityWater repellencyDensity


    Weight / Mass

    Resistance to organicsResistance to inorganics

    UV resistanceThermal behaviourFlammability


    Hydrophobic / Philic natureLiquid repellencySterilizability

    BiodegradabilityStatic generation

    Soft/Stiff feelOdour


    Fluffy/Hard natureBouncy

    ReversibleTextile/Papery appearance

    Applications of Polypropylene Fibre and Filaments

    Fibre Type End Uses Characteristics

    Filter and other industrialfabric. Non wovens fabrics

    for hygiene, medical non-woven geotextiles

    Excellent chemicalresistance and strength, long

    life, lightest fibre

    1-4 Denier

    Backing for pile fabric in

    carpets carpet pile

    Coverage Dimensional

    stability on washingstrength

    Upholstery fabric Absorption resistance

    Excellent soil resistanceExcellent ease of cleaning

    Excellent colour fastnessLow specific gravity

    Blankets High bulk excellent thermalinsulations. Dimensional

    stability on washing softhandle

    5-8 Denier

    Fibre fill and pile lining Excellent thermalinsulations. High bulk

    resilience low moistureabsorption

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    Fibre Type End Uses Characteristics

    Tufted and woven carpets

    Artificial grass

    Excellent soil resistance

    Excellent ease of cleaningLow static build upWear resistance pileHeight retention

    15 Denier

    Non-woven fabrics

    GeotextilesMedical fabrics

    Strength wear resistance

    Stain resistance coverageLong life

    B - Filament Yarn

    Fibre Type End Uses Characteristics15 Denier Women's hosiery Snag resistance excellent

    feel. Easy care

    Knit wear Dimensional stability on

    washing high bulk softhandle. Can be blendedwith rayon, cotton or wool

    Car Upholstery Strength abrasion resistance


    Socks, underwear andouterwear

    Coverage strength wearresistance

    30-420 Denier

    PP/Cotton & PP/Rayon

    Blend Fabrics

    Good abrasion resistance.

    Low moisture absorptionsEasy drying characteristicsLight weight

    Laundry bags Chemical resistance,abrasion resistance,dimensional stability

    Twines, ropes, cordage,

    fishnets, cargo handling,nets, straps for bags and


    High strength wet or dry

    light enough to float,lightest fibre. Low

    moisture pick up Highestresistance to weathering,microorganism etc.

    420-840 Denier

    Backing for pile fabrics Dimensional stability on

    washing strength

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    Fibre Type End Uses Characteristics

    Tufted carpets Coverage, wear resistance,

    pile height retention,excellent soil resistance,excellent ease of cleaning

    Carpet backing Excellent colour fastnesswith pigments. Strength

    Dimensional stability Highcovering fabric

    Bag sewing thread Rot and mildew resistance

    Strength high yieldChemical resistance

    Low moisture pick up

    Window channel fabrics Long wear high bulk

    840-3200 Denier

    Woven geotextiles Low moisture pick upHigh strength, excellentresistance to bacterial attack

    Type of Yarn and Denier Range Produced in India

    # Denier Type

    1 65/24 Single crimped

    2 65/24/2 Crimped double twisted

    3 90/24 Single crimped

    4 90/24/2 Crimped double twisted

    5 150/40 Flat / twisted

    6 210/40 Flat / twisted

    7 210/80 Flat / twisted

    8 300/70 Fully drawn yarn

    9 420/70 Fully drawn yarn

    10 840/80 Flat / twisted

    11 840/140 Flat / twisted

    12 400, 900, 1600, 2300 Air textured

    13 900/90 Flat / twisted14 840/120 Flat / twisted

    15 1260/120 Flat / twisted

    16 1800/240 Textured

    17 2600/240 Textured