● Phase separation impact: Incompatible residual solvents (e.g., DMF) change coagulation rate—oversized pores or defective skin—lowering flux/selectivity.
● Material contamination: Residual corrosives (strong acids/bases) react with membrane (e.g., PA hydrolysis); metal fines can embed and reduce rejection.
● Biocompatibility: Medical membranes require very low residuals (e.g., ethanol ≤10 ppm) to avoid adverse reactions.
● Corrosion acceleration: Residual Cl⁻ (e.g., from HCl cleaning) induces pitting in stainless steel.
● Dedicated: Tailored to a specific material—e.g., solvent-resistant coatings for PVDF; high-strength materials for PA; ceramic coatings for CTA; titanium for PA to prevent hydrolysis. Higher precision/stability but requires frequent changeover in multi-product plants.
● Recommendation: For high-volume, choose universal to reduce cost; for high-precision (e.g., hemodialysis), choose dedicated to ensure performance.
● Rheology impact: Adjust temperature/pressure accordingly.
● Thermal sensitivity: Some additives decompose at high T, forming gels in dead zones—leading to clogs.
● Insoluble additives/nanofillers (TiO₂, CNTs, activated carbon):
● High sensitivity—primary risk is clogging. Particle size ≤1/5 of orifice (e.g., ≤20 μm for 0.1 mm rifice), with excellent dispersion (no agglomeration).
● Abrasion: Hard fillers wear orifices and capillaries, shifting critical dimensions.
● Agglomeration: Accumulates at contractions/orifices, causing blockages.
● Sedimentation: Dense fillers settle in low-shear dead zones.
● Safety factor:
● Industry practice: 3–4 for continuous precision chemical equipment.
● Formula (simplified): Allowable working pressure ≈ yield strength / safety factor.
● Example: 304 SS yield at temperature ~240 MPa; with SF = 4, allowable ≈ 60 MPa. Actual design set well below for safety margin.
● Nitriding (e.g., ion nitriding): Raises surface hardness (HV ≥ 1000) and corrosion resistance; suitable for high temperature TIPS and chlorinated media; good for PSf/PES dopes. Less resistant to fluorinated solvents; hydrophobicity moderate; hang-up reduction less than DLC; may slightly increase roughness.
● General principle: For strong solvents (e.g., DMF), prefer DLC; for high temperature/pressure, prefer nitriding. Always test coating compatibility to avoid chemical failure.
● Material-first: Prefer SUS304/316L; only consider coatings for extreme conditions (high solids/high wear) after long-term validation.
● Not strictly “the lower the better”:
● Diminishing returns below Ra ~0.4 μm with sharply rising cost.
● Exceptions: Ultra-smooth walls can alter wall slip of highly elastic melts; high-solids (>30%) systems may show slip–stick pulsation. Some systems exhibit an optimal roughness window.
● Over-polishing can reduce beneficial lubricating films; in viscose-like charged colloids, adsorption can worsen spinnability.
● Hang-up: Rough surfaces increase friction, leaving residual dope that forms gels and contaminates subsequent runs.
● Optimization: Aim for low roughness (Ra ≤ 0.8 μm) via polishing to reduce fouling and hang-up.
● Eccentricity: Damage can induce uneven clamping loads, generating small bending moments that distort internal flow alignment and cause eccentric fibers.
● Wear effects: Orifice enlargement, edge rounding, microcracks cause:
● Higher flow → thinner walls.
● Lower shear → looser skin.
● Higher eccentricity → lower strength.
● Batch variability: Even identical drawings yield micro-variance in orifice/gap, concentricity, and finish—changing spinning dynamics and membrane performance (flux, rejection, strength).
● Quality management: Treat hollow fiber spinnerets as critical assets; log use history against product performance; scrap/refurbish when beyond tolerance.
● Pressure sensing: Use high speed transducers at the hollow fiber spinneret inlet; look for pump-synchronous ripple.
● Correlation test: Change pump speed; if defect period tracks, strong coupling is indicated.
● Rule out pump wear: Ensure ripple is not solely due to pump mechanical issues (gear mesh, wear).
● Root cause: System compliance (line stiffness/length), dope compressibility, and pump ripple form a resonance that amplifies pressure waves.
● SCC resistance: Chlorides drive SCC in stainless steels. If chloride exposure is possible (raws, cleaning water), prefer 316L; for extremes, use Hastelloy C-276.
● Basis: Choose materials with proven resistance to the anticipated solvents, oxidants, and SCC environment.
● Too high temperature: Thermal degradation; low viscosity; breaks in the air gap; cracks/voids in cross-section.
● Uniform temperature control: Produces dense, crack-free cross-sections; improves diameter uniformity and mechanics.
● Viscosity gradients: Hotter zones → lower μ → faster flow → smaller swell; cooler zones → higher μ → slower flow → larger swell; leads to hole-to-hole diameter differences.
● Phase-separation kinetics: Temperature directly affects solvent–non-solvent exchange; nonuniform temperature yields different pore sizes/porosities across the plate.
● Local hot spots: Lower μ → faster flow → thinner walls; coarser finger-like pores.
● Local cold spots: Delayed phase separation → thicker skin or sponge-like structure; lower flux.
● Circumferential ΔT: Causes wall-thickness nonuniformity, eccentricity, even helical fibers.
● Axial ΔT (TIPS): Premature solidification before exit causes unstable extrusion.
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