Fibres & Filaments

8/3/2019 Fibres & Filaments

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FIBRES AND FILAMENTS

Introduction

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

fibres.

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

H200FG

Montell

VS6500H(A)

Borealis

HF435F(B)

Repol

H350FG

Montell

XS6500H(C)

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

(MPa)

34 30 33 34 32 32

Elongation at yield(%)

11 12 11 10 10 10

Flexural modulus

(MPa)

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

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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Catalyst

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

Temperature

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

AbsorbencySewability

Weight / Mass

Resistance to organicsResistance to inorganics

UV resistanceThermal behaviourFlammability

DurabilityWashability

Hydrophobic / Philic natureLiquid repellencySterilizability

BiodegradabilityStatic generation

Soft/Stiff feelOdour

Smooth/CoarseColourTexture

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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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

coverage

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

ribbons

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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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



8 300/70 Fully drawn yarn

9 420/70 Fully drawn yarn



12 400, 900, 1600, 2300 Air textured

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


16 1800/240 Textured

17 2600/240 Textured

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