Hollow Fibre Membrane Spinning: The Parameters That Control Fibre Structure

At a glance. In dry-jet wet spinning, hollow fibre structure is set by five groups of variables: dope composition and viscosity, bore fluid composition and flow, the air gap and its humidity, take-up speed relative to extrusion speed, and coagulation bath conditions. Flow rates and draw set the fibre's dimensions, coagulant strength sets where the skin forms, and shear and draw set orientation and pore size. Change one variable at a time and log every run.

How does hollow fibre spinning work?

A polymer dope, typically PES, PSf, PVDF or PAN dissolved in NMP, DMAc or DMF with additives such as PVP or PEG, is pumped through the annulus of a tube-in-orifice spinneret. A bore fluid flows through the central needle to hold the lumen open. The nascent fibre passes through an air gap, or straight into the bath in wet spinning, and phase separation proceeds from both surfaces at once: from the inside by the bore fluid and from the outside by the coagulant. A take-up wheel draws the fibre away, after which it is washed and post-treated.

The thermodynamics are the same non-solvent induced phase separation that governs flat sheets, covered in our phase inversion casting guide. What hollow fibres add is geometry, two coagulating surfaces and the mechanics of a moving thread. The reviews by Peng and co-workers and by Wojciechowski and co-workers summarise decades of results; the points below are the ones that matter on the machine.

Which spinning parameters matter most?

Parameter	What it controls	Typical direction of effect
Polymer concentration and dope viscosity	Macrovoids, skin density, strength	Higher concentration gives a denser, more sponge-like wall and lower flux
Additives (PVP, PEG, salts)	Pore formation, hydrophilicity, viscosity	Pore formers raise porosity up to a point; LiCl can suppress macrovoids
Bore fluid composition	Inner skin	Water forms a dense inner skin; solvent-rich bore fluid leaves the lumen surface open
Bore fluid flow, relative to dope flow	Inner diameter and wall thickness	Higher bore flow gives a larger lumen and a thinner wall
Dope extrusion rate and spinneret shear	Chain orientation and outer skin pores	Higher shear gives smaller pores, lower flux, higher rejection and strength
Air gap length	Fibre size and outer surface	A longer gap thins the fibre; the effect on outer pores depends on the dope
Air gap humidity and temperature	Outer surface porosity	Humid air starts phase separation early and opens the outer surface
Take-up speed (draw ratio)	Diameter, wall thickness, pore shape	Faster take-up stretches and thins the fibre and can elongate pores
Bath temperature and composition	Demixing rate and pore size	A warmer or weaker coagulant changes demixing speed and pore size
Post-treatment	Pore stability on drying	Washing removes solvent; glycerol prevents pore collapse when dried

Two of these deserve evidence. Chung, Qin and Gu found that raising the shear rate in the spinneret for PES ultrafiltration fibres lowered pure water flux and raised separation, tensile strength and modulus, which they attributed to greater molecular orientation in the outer skin. For the air gap, Khayet and co-workers found that PVC fibres became thinner as the gap increased, while outer pore size and water flux rose and solute separation fell. Other polymers respond differently, which is why the air gap is screened experimentally for each dope.

How do dope and bore flows set fibre dimensions?

At steady state the dope leaves the spinneret annulus at a mean velocity equal to its volumetric flow divided by the annulus area. The draw ratio is the take-up speed divided by that velocity.

Take a spinneret with a 1.2 mm outer and 0.6 mm inner annulus diameter. The annulus area is π/4 × (1.2² − 0.6²) ≈ 0.85 mm². At a dope flow of 3 mL/min (3,000 mm³/min), the extrusion velocity is about 3.5 m/min. A take-up speed of 7 m/min therefore gives a draw ratio of about 2, and the fibre leaves thinner than the spinneret. Die swell, solvent loss and shrinkage move the final dimensions, so treat the calculation as a set point, not a prediction.

The bore-to-dope flow ratio sets the lumen. Too little bore flow, or a bore fluid that coagulates the inner wall too slowly, lets the inner wall buckle into an irregular, lobed lumen. Too much flow stretches the wall until it thins unevenly or breaks. The Wojciechowski review reports that warming the coagulation bath to 40 °C prevented irregular internal contours in PVDF-HFP fibres.

Wet spinning or dry-jet wet spinning?

Wet spinning places the spinneret in or at the bath, so both surfaces coagulate immediately. It suits low-viscosity dopes that cannot hold together across a gap, and it gives less elongational orientation. Dry-jet wet spinning leaves an air gap, typically a few millimetres to tens of centimetres, during which gravity and draw stretch the fibre and humid air can begin phase separation at the outer surface. It is the usual route for ultrafiltration and gas separation fibres.

A triple-orifice spinneret adds a third channel for co-extrusion, either a second dope for dual-layer fibres or a solvent stream that keeps the outer surface open. Bernardo and co-workers used one to produce polyimide gas separation fibres with a porous outer layer and a thin inner skin, and found that, at moderate dope flow rates, a saline bore fluid improved gas selectivity more than a solvent-containing one.

What changes from laboratory to pilot spinning?

A laboratory machine with syringe pumps is ideal for screening dopes, because each run uses little polymer. Pilot spinning needs continuous dope supply from gear pumps, degassed and filtered dope to prevent bubbles and pinholes, temperature control of the dope line and spinneret, longer baths and washing, and tension-controlled winding so the draw ratio stays constant along kilometres of fibre. Above all, it needs data logging, because a spinning campaign is only reproducible if every set point is recorded against every bobbin.

Step-by-step checklist for a spinning campaign

1. Characterise the dope: viscosity at spinning temperature, cloud point with the chosen non-solvent, and freedom from bubbles and gels after degassing and filtration.

2. Record the spinneret geometry: annulus outer and inner diameters, and bore needle diameters.

3. Set dope and bore flows from the target dimensions and calculate the extrusion velocity.

4. Choose the bore fluid composition for where you want the selective skin.

5. Set the air gap and measure humidity and temperature in it, enclosing the gap if the room varies.

6. Start take-up near free fall, with no tension, then raise the draw ratio in steps.

7. Control bath temperature and replace the coagulant before its solvent content builds up.

8. Wash in fresh water, changed regularly, until residual solvent is removed.

9. Post-treat with glycerol or another humectant before drying, then pot test modules.

10. Characterise each condition: SEM cross-sections, pure water permeability, rejection or MWCO, pore size, tensile strength and burst pressure.

11. Change one variable at a time and keep a run log that links every set point to every sample.

Frequently asked questions

What is the air gap in hollow fibre spinning?

It is the distance between the spinneret face and the surface of the coagulation bath. A zero gap is wet spinning; a positive gap is dry-jet wet spinning. The gap length and the humidity in it affect fibre dimensions, chain orientation and the structure of the outer surface.

Which bore fluid should I use?

Water is a strong coagulant and forms a dense inner skin, which suits fibres meant to be fed from the lumen. Mixtures of solvent and water coagulate more slowly and leave a more open inner surface, which suits outside-in operation. Salts or other additives in the bore fluid are a further way to tune the inner skin.

Why do my fibres have large macrovoids?

Finger-like macrovoids form when demixing is very fast and the dope is not viscous enough to resist the intruding non-solvent. Raising polymer concentration or dope viscosity, adding a salt such as LiCl to the dope, or adding solvent to the coagulant or bore fluid to delay demixing all help to suppress them.

Why is the lumen not round?

An irregular or lobed lumen usually means the inner wall buckled while it was still soft. Increase the coagulating strength of the bore fluid so the inner wall sets sooner, make sure the bore flow is high enough to support the lumen, and check whether a warmer bath helps.

How do I stop pores collapsing when the fibres dry?

Capillary forces during drying can collapse the pores of a wet-spun fibre. Soaking the washed fibres in a glycerol solution before drying keeps the pores open; alternatively, store the fibres wet until they are potted and tested.

What is the draw ratio?

It is the take-up speed divided by the mean velocity of the dope leaving the spinneret. A ratio above 1 stretches the fibre, reducing its diameter and wall thickness and orienting the polymer chains, which can change pore size and mechanical strength.

How Tech Inc. builds hollow fibre spinning machines

Our laboratory hollow fibre membrane casting machine is a complete spinning line: a precision spinneret with interchangeable tips, independent dope and bore fluid pumps, a temperature-controlled coagulation bath and a take-up system that sets the draw ratio. It runs both wet and dry-jet wet spinning with PVDF, PES, PSf, PAN and other fibre-forming polymers. For scale-up, our automatic pilot hollow fibre casting machine adds PLC control of bore and dope flow, air gap, temperature and speed, with real-time monitoring and storage of process data. The full range is on our hollow fibre membrane making page.

The rest of the workflow comes from the same supplier: a hollow fibre potting machine for test modules, capillary flow porometers for pore size, a tensile strength tester with DAQ for mechanical properties, and membrane testing systems for flux and rejection. Each machine is configured to the client's polymers, solvents and fibre sizes, then designed, built and factory-tested in our own facilities. Tech Inc. is part of an engineering group founded in 1990, and our membrane-making equipment is trusted by universities and R&D laboratories worldwide. For how logged spinning parameters become training data, see AI in membrane research.

Send your polymer system, target fibre dimensions and throughput to mail@techincresearch.com or call +1 (289) 622-1032 to discuss the spinning line your programme needs.