Spinning of Manmade Fibre
The polymer processing from the solid to the fluid state can take place with two methods:
- by melting:
This method can be applied on thermoplastic polymers which show stable performances at the processing temperatures (this method is used by 70% of the manmade fibres).
- by solution:
The polymer is dissolved in variable concentrations according to the kind of polymer and of solvent, anyhow such as to produce a sufficiently viscous liquid (dope) (this method is used by 30% of the manmade fibres).
Spinning via melting is definitely preferable as it entails a simple transformation of the physical state, however, it can be applied only to a polymer having a melting temperature (PA 6 a Pa 6.6, PES, PP).
This last method is evidently more complicated than melt spinning, owing on one hand to the necessity of dissolving the polymer in a proper solvent, and on the other to the necessity of removing and recovering the polymer after extrusion.
In the case of melt spinning, the extruded polymer, owing to its fast cooling, is transformed directly into a filament while keeping substantially unchanged the form of the cross-section resulting from the filament geometry.
On the contrary, in the case of solution spinning the extruded filaments are subject to considerable structural changes brought about by the process for solvent extraction from the polymer mass.
Solvent removal can take place in two ways:
Dry Spinning of Manmade Fibres
The solvent is removed through flows of warm gas suitably directed to the extruded filaments; the gas temperature should be higher than the boiling temperature of the solvent, which will be extracted from the filaments, recovered and recycled.
Filament solidification proceeds according to the extent of solvent evaporation; it takes place faster on the external yarn layers (thus creating a crust or skin), and successively slows down while proceeding towards the interior.
As a consequence of the mass exchange, the original (round) cross-section of the filament undergoes a contraction, thus generating cross-sections which characterize the various kinds of fibres and spinning processes.
Wet Spinning of Manmade Fibres
This spinning method is based on the introduction of an extruded polymeric viscose into coagulation baths where the liquor, usually water, behaves as a solvent towards the polymer solvent and as a non-solvent towards the polymer mass.
Practically the solvent which is contained in the fibre in the amorphous state (gel) is spread towards the liquor and at the same time, the liquid of the bath is spread towards the interior of the fibre.
The processing speeds are dependent on several parameters, as type and concentration of the polymeric solvent and of the liquor which bring about structural variations in the fibre.
In particular the formation of an outer gardened and more compact cortex (skin), similarly to what happens in dry spinning slows down the coagulation mechanism of the inner filament portion (core), thus creating unevenness with a more or less porous structure (voids formation).
The fibre cross-sections result more or less modified, from the original round form to a lobated form, with a wrinkled surface.
General Spinning Process of Manmade Fibres
The fluid polymer mass (melted or solution mass) is guided through distribution lines, to the metering pumps (gear system), which guarantee a constant flow rate to the spinning positions composed of a series of filters which purify and distribute the polymer.
These are coupled with perforated plates of variable thickness and size, which are usually circular and made of special stainless steel (for melt spinning), but also of precious metals or of vitreous material (for solution spinning).
The holes (capillaries), the number of which on the plate varies depending on the kind of fibre and can reach several thousand can have circular or special cross-sections (shaped or hollow sections)
The filaments extruded from the spinnerets, after being converted back to their original state of solid polymer, are interrupted and taken up in suitable packages (bobbins, cans) or conveyed directly to subsequent processing phases.
In the case of melt spinning, if the polymer does not derive already in the melted state from polymerization, the fluid polymer mass is obtained through melting of the solid polymer grains (chips).
This operation was originally carried out inside containers (pipes) which were electrically heated and equipped with grids to separate solid grains from the polymer during melting (grid melting device)
The use of such a system is at present limited only to few applications and has been replaced by more reliable and efficient devices (screw extruder).
The relations which connect some spinning parameters one another (and are calculated for melted polymers) are the following:
Polymer Flow Rate
Mf= VF Tsp/1 0,000
mf= polymer quantity for each yarn (g/min)
VF= take-up speed (m/min)
Tsp= linear mass of taken-up yarn (dtex)
If we know the linear mass of the drawn yarn (Td) and the draw ratio R, the relation becomes:
mF= VF TdR/10,000
Extrusion speed of the melted polymer
VB = 4m/ 3.142d2p
VB = extrusion speed at spinneret hole (m/min)
MB=polymer quantity per spinneret hole (g/min)
d = hole diameter (mm)
p= density of melted polymer (g/cms)
Drawing of Manmade Filaments
The polymer extruded by the spinnerets in the form of filaments has not yet the properties which are typical of a textile fibre.
In fact, the polymer mass (solidified through cooling or solvent removal) is characterized by a mass of disorderly placed molecular chains (in the amorphous state).
which provides the material with poor thermal and chemical stability, low resistance to ageing, high plasticity and deformability and consequently insufficient physical/textile properties.
If we take natural fibres as models, we need to orientate the molecular chains (orientation phase) in the direction of the fibre axis and at the same time or successively activate or increase the ordered arrangement of the intermolecular structure (crystallization phase).
This process can be partly activated during spinning by increasing the ratio between the take-up speed and the extrusion speed (spinning ratio).
But, except the case of high-speed spinning of continuous filament yarns, the process needs to be completed by an additional operation of mechanical drawing
The process entails winding the yarns on rollers or cylinders running at high speed and can be carried out continuously on filaments coming from the spinning room (single-phase process)or on filaments coming from a phase subsequent to spinning (two-phase process).
The speed ratio between the delivery or drawing rollers and the feeding rollers is the draft ratio R.
The mechanical configuration of the rotating devices and the filament path are designed in order to ensure the equivalence of fibre-speed with the speed of contact organs.
Draft ratio levels are variable and depend on the fibre typology on the production process and on the end-use characteristics, they can fluctuate between values slightly higher than 1 (1,2 for traditional cellulose fibres) and max. 10 (for acrylic fibres).
Usual ratios for thermoplastic fibres are situated between 3 and 5; higher values identify fibres for technical applications.
Optimal conditions for fibre drawing are attained when the molecular chains show high mobility and creep.
This result is in practice attained by increasing temperature to levels higher than glass transition and by introducing plasticizers which can make the structure more deformable and can reduce glass transition temperature (generally by acting upon the system water/humidity or using spinning solvents).
From an operational point of view, the draft zone can be operated at room temperature (cold drawing) or at heated conditions (warm drawing) and consists of rollers, contact plates, heated air chambers or steam chambers and of immersion baths.
In order to provide the drawn fibres with thermal stability, usually, these fibres undergo also treatment at a temperature higher than drawing temperature, under controlled tensions or in a free state, with the objective of eliminating internal tensions through a readjustment of intermolecular chemical links and of the crystallization degree.