Synthetic Fibre Spinning Techniques: Melt, Dry, Wet, and Gel Spinning Explained with Applications and Technical Insights Part - 1
Introduction: Why Synthetic Fibre Spinning Matters
The present textile industry has put much importance on synthetic fibres due to durability, performance, strength and functionality. However, they are not naturally developed fibres rather they are engineered by specific spinning methods. How synthetic polymers are converted to filaments or staple fibres is a very important knowledge that any textile technologist, engaged in fibre development, spinning, R&D or a competitor in any competitive exam like GATE, ISRO, or a textile PSU can never neglect.
Spinning of synthetic fibres is not only the forming of a thread. It identifies strength of fibres, orientation of molecules, crystallinity, elongation and thermo-performance. Diversity of polymers needs various forms of spinning processes depending on their chemistry, thermal stability, solubility and processing requirements. This post will discuss four major spinning techniques including melt, dry, wet and gel spinning, their principle, applications and technical features.
1. Melt Spinning-The Bone of Thermoplastic Fibre Manufacture
Melt spinning: Melt spinning has the principle where the thermoplastic polymer is melted and passed through a spinneret to form filaments, which are subsequently cooled and solidified in a very rapid manner.
The common materials utilised are Polyester (PET), Nylon 6, Nylon 6,6, Polypropylene (PP), and Polylactic Acid (PLA)
Application Areas:
- PET in apparel, home textiles and packaging
- Hosiery, sports and technical nylon Nylon for hosiery, sportswear and technical textile
- Ropes, nonwovens and geotextiles polypropylene
- Biodegradable sustainable fabrics PLA
The important Technical Components involve:
- The temperature of the melt should be controlled precisely. To illustrate, PET is cycled between 260 to 290 degrees Celsius whereas Nylon melts at 220 to 265 degrees Celsius.
- Stock extrusion results in high rates of shear, which in turn lower the viscosity of polymer and allow better movement past the fine spinnerets.
- Cooling is by quenching the air at 0.5 to 2 metres per second which influences crystallisation and strength of fibres.
Why Melt Spinning Dominates:
2. Dry Spinning: For Heat-Susceptible and Solvent-Peeling Polymers
Application Areas:
- Sweater acrylic, wool substitutes, and synthetic fur
- Spandex used in stretch fabrics, sportswear, and medical textiles
- Vinyon for filtration fabrics and flame-retardant applications
Key Technical Insights:
- Solvent selection is critical. For example, acrylic uses dimethylformamide (DMF), while spandex uses dimethylacetamide (DMAc).
- Solvent evaporation is governed by mass transfer principles, with diffusion rates typically around 10⁻⁵ square metres per second.
- The final fibre must contain less than 1 percent residual solvent to meet textile safety and regulatory standards.
Use Case Advantage:
3. Wet Spinning - The Most Versatile, and Oldest, of the Soluble Polymers Methods
Principle: The polymer is dissolved in a solvent and is extruded into a coagulation bath which in most cases is a liquid having a nonsolvent, precipitating the polymer into coherent filaments.
Typical Polymers: Viscose Rayon, Modal, Lyocell, Acrylic (other method), Aramid (e.g. Kevlar)
Application Areas:
- Linings, drapery and dresses made of viscose
- High performance and eco-friendly clothes worthy of high performance - Modal and Lyocell
- Bulletproof vests made of Aramid fibre, and heat resistant clothing
- Outdoor acrylic fibres such as awnings and upholstery fibres
Important Process Factors:
- Skin-core morphology and fibre porosity are dependent on composition and temperature of the coagulation bath.
- A typical case in point is viscose rayon where sulphuric acid is the coagulating agent.
- When the coagulation is excessively quick, dense skin tends to form which can limit the ability to uptake dyes or bend.
- Removal of residual chemicals is also important after spinning which involves fibre washing and neutralisation.
Environmental Considerations:
The conventional wet spinning (in particular viscose) uses harmful chemicals such as CS 2. However, new technology such as Lyocell process has led to the closed loop systems which recycle and recapture more than 99 percent of solvent materials which feeds these systems with the effects of polluting the environment drastically decreasing.
4. Gel Spinning: The Process of High Strength Fibres
Principle: Gel spinning is done on the materials of very high molecular weight. Ultra-high crystallinity is attained by drawing (extensively) a semi-dilute polymer solution in filament form.
Typical polymer fighters include: UHMWPE (Ultra-High Molecular Weight Polyethylene), PBO (Polybenzoxazole) and High-strength PAN (carbon fibre precursor)
Application Areas:
- Body armour, cut-resistant gloves and marine ropes in Dyneema and Spectra fibres
- Zylon firefighter equipment and high strength composites
- Carbon fibre carbon PAN fibres in aerospace and automotives
Important Technical Characteristics:
- The concentration of the polymer is very low which can be as low as 0.5 to 2 percent.
- Ratios drawn are as high as 100:1 allowing astonishing molecular alignment.
- The resultant crystallinity is expected to be more than 90 percent resulting in tensile strengths that are immeasurably higher in comparison to conventional fibres.
Performance Metrics:
A particular example is Dyneema fibre which is 15 times stronger than steel per the mass. The gel spinning is however characterized by comparatively low productivity hence can only be used to produce high value textiles that are technical in nature.
5. Students and Professional Technical Takeaways
- Melt spinning is the most widespread and economical technique but it can be applied only to thermoplastic polymers whose melting does not lead to degradation.
- Dry spinning is an option with heat sensitive polymers, although it makes use of volatile solvents and the drying and vapour recovery is large and must be controlled.
- The most chemically demanding is Wet spinning which permits the manufacture of regenerated fibres and ultra-high performance aramids.
- Gel spinning can be used to make extremely strong technical fibres but this is low productive and expensive.
- Fibre morphology, strength, and dyeing behaviour are all influenced by spinning technique, cooling rate, drawing ratio, and spinneret design.
Fibre Cross-Section Shapes and Spinneret Design
While spinning transforms polymer into fibre, the spinneret shape determines the cross-sectional geometry. Round fibres are the most common, but specially designed spinnerets can produce:
- Trilobal cross-sections for improved lustre in polyester
- Hollow fibres for insulation in nonwovens and sportswear
- Octalobal or irregular shapes for higher surface area and better dye uptake