Hydrophobic PP Staple Fiber for Battery Separators & Industrial Specialty Paper

Grade HPP-BS-1.5/6 is a hydrophobic polypropylene staple fibre produced from controlled-rheology homopolymer resin with a melt mass-flow rate of 18–25 g/10 min measured under ISO 1133-1:2022 at 230 °C/2.16 kg. The fibre is cut to a nominal length of 6 mm at a linear density of 1.5 dtex, with a water contact angle greater than 95° by the Wilhelmy plate method and a non-ionic antistatic finish level of 0.10–0.25 wt%. Total ash after ignition at 600 °C is controlled below 0.05 wt% under ISO 3451-1:2019, and n-heptane extractables are limited to 0.5 wt% to reduce contamination in electrical separator substrates.

The grade is supplied in compressed bales with a bulk density of 120–180 kg/m³, conditioned to 20–25 °C and 40–60 % RH. It is intended for wetlaid nonwoven lines using inclined wire formers, rotary drum thickeners, and through-air driers. Because the fibre is intrinsically hydrophobic, aqueous dispersion requires a temporary wetting agent in the pulper; residual surfactant concentration in the finished web must be verified by conductivity or surface tension measurement to avoid interference with electrolyte wetting.

Fibre dispersion and headbox stability in wetlaid nonwoven production

Stock preparation for hydrophobic polypropylene staple fibre in wetlaid processes requires controlled shear and surfactant dosing. A low-consistency cylindrical pulper operating at 300 rpm with a deflaker gap of 0.2–0.5 mm is used to separate fibre bundles without generating fines below 0.5 mm. Non-ionic glycol ether dispersant is added at 0.02–0.10 wt% of fibre mass. Dosages above 0.5 wt% produce stable foam, increase white-water conductivity above 10 µS/cm, and leave residual surfactant on the dried web that reduces electrolyte uptake in lithium-ion separator grades.

Headbox consistency is maintained between 0.02 wt% and 0.08 wt%. Above 0.10 wt%, hydrophobic fibres flocculate because the contact angle exceeds 95°, and fibre-fibre attraction in the boundary layer increases. Formation quality is monitored on-line with a beta-formation tester; a formation index below 70 at 15 g/m² basis weight is considered inadequate for separator substrate. White-water pH is held between 5.5 and 7.5. Alkaline pH above 9.0 can hydrolyze the non-ionic finish and alter fibre surface charge, while acidic pH below 4.5 accelerates corrosion of stainless-steel machine chests and increases metal ion release into the furnish.

The 6 mm cut length with a coefficient of variation below 5 % is selected because shorter fibre below 3 mm can migrate to the felt side during dewatering, creating two-sidedness; longer fibre above 10 mm forms ropes in headbox recirculation. Furnish temperature is not raised above 45 °C because finish solubility changes and foam stability increases. Retention aid addition with anionic polyacrylamide at 0.02–0.05 wt% dry polymer on fibre mass is used on some lines to improve first-pass retention of the hydrophobic fibre without over-flocculating the sheet.

For lithium-ion battery separator base web, the fibre is metered at 10–40 wt% into fibrillated cellulose or synthetic pulp. Wetlaid sheet is formed at 15–50 g/m² on an inclined wire, dewatered by vacuum boxes at 0.2–0.6 bar, and dried on a through-air drier at 110–135 °C. Calendering follows at roll temperatures of 90–130 °C and linear pressures of 40–120 N/mm. The calendering step reduces sheet thickness from approximately 80 µm to 25–45 µm and sets the Gurley air resistance in the range 200–800 s/100 cm³ when tested under ISO 5636-5. Mean flow pore diameter, measured by capillary flow porometry, is typically controlled below 0.2 µm for cathode-to-anode separation layers. Published data for exact pore diameter distributions when this specific grade is blended with commercial nanofibrillated cellulose is limited; pilot trials with a 20 wt% addition have shown the need to raise headbox vacuum by 0.1 bar to maintain target formation.

The hydrophobic surface lowers water uptake and reduces heat shrinkage relative to rayon or polyester staple in the separator. Hot-air shrinkage at 120 °C for 10 min remains below 2.5 %, while polyethylene sheath bicomponent fibres may shrink more than 5 % under the same conditions. The product does not contain titanium dioxide delustrant, so residual ash is below 0.05 wt%, minimizing the risk of ionic extraction when exposed to LiPF₆-based electrolytes. The moisture regain of polypropylene is below 0.1 %, whereas rayon separator fibre typically retains 12–14 % moisture at equilibrium, making the PP grade advantageous in dry-room battery assembly.

How does this grade differ from conventional textile PP staple fibre in wetlaid electrical separator furnish?

Conventional textile polypropylene staple is not controlled for wetlaid dispersion, low ash, or fine cutting tolerance. Textile grades commonly carry a spin finish of 0.30–0.80 wt%, may contain 0.05–0.30 wt% titanium dioxide, and exhibit cut-length coefficients of variation above 8 %. In contrast, HPP-BS-1.5/6 is specified with finish below 0.25 wt%, ash below 0.05 wt%, and cut-length coefficient of variation below 5 %. Coarse fused fibre count is limited to fewer than 5 mg/kg; textile staple typically has no such pass-fail criterion.

In industrial specialty paper, the fibre is added at 20–40 wt% of furnish to reduce Cobb water absorption and improve dimensional stability after wetting. Applications include battery pasting paper, automotive filter base stock, gasket sheet, and synthetic printing paper. A wetlaid sheet containing 30 wt% of this fibre and 70 wt% softwood kraft, bonded with an acrylic latex at 15 wt% solids on dry fibre, typically exhibits a Cobb 60 value below 20 g/m² under ISO 535 and a wet tensile retention above 35 % of dry tensile measured by ISO 3781. The fibre’s density of 0.90–0.92 g/cm³ lowers sheet density relative to polyester staple at 1.38 g/cm³, which is advantageous for filtration media where thickness and air permeability are specified at 200 Pa according to ISO 5636-3.

For filter base stock, the hydrophobic fibre reduces water retention and increases wet strength after resination. A furnish containing 75 wt% softwood kraft, 25 wt% PP fibre, and 20 wt% phenolic resin on total dry mass yields a cured filter paper with burst strength above 150 kPa under ISO 2758 and a water filtration rate controlled by the fibre’s non-swelling character. Because polypropylene does not hydrate, pore channels remain stable during wet filtration cycles; cellulose-only sheets can lose 10–25 % of initial air permeability after the first wet-dry cycle due to fibre swelling and hydrogen-bond rearrangement.

In electrical insulation paper, the low moisture absorption and low dielectric constant of polypropylene are critical. The fibre is used with mica or aramid pulp in high-voltage cable papers where moisture content after 24 h at 50 % RH shall remain below 0.5 wt%. The dielectric constant of polypropylene is 2.2–2.6 at 1 kHz, lower than polyester’s 3.0–3.5; published data for finished sheet dielectric strength is limited and depends on density and resin saturation. The product is not a thermal binder. It melts at 160–170 °C and should not be used as the sole bonding fibre in through-air bonded webs unless a sheath-core bicomponent fibre or latex binder is included. In heat-seal applications above 160 °C, the fibre softens and can block through-air drier drums or press rolls; line speeds should be limited and roll release coatings verified before sustained operation.

When electrolyte wettability and thermal stability are both required below 30 µm thickness, the process window narrows

Polypropylene homopolymer has an upper continuous service temperature of approximately 105 °C and a short-term peak tolerance near 120 °C before oxidative chain scission becomes significant. For separator substrates thinner than 30 µm, this thermal boundary interacts with calendering conditions. If roll temperature exceeds 130 °C, the fibre surface may form film-like domains that reduce porosity and raise Gurley values; if roll temperature is below 90 °C, thickness reduction may require excessive linear pressure above 150 N/mm, causing fibre damage and pinhole formation. The acceptable calendering window for this product in thin separator base webs is therefore narrow, and published data for commercial lines using ceramic-coated steel rolls is limited. Operators should map air permeability and pinhole count through a full roll-temperature sweep from 80 °C to 140 °C before setting production parameters.

Oxidative stability is monitored by oxidation induction time under ISO 11357-6 at 200 °C; the formulated stabilizer package is selected to maintain an OIT above 20 min. This does not imply suitability for continuous exposure to hot concentrated sulfuric acid, boiling alkaline solutions, or strong oxidizing agents. The fibre should not be used in applications with continuous operating temperatures above 105 °C or in contact with copper-based pigments at elevated temperature, because copper ions catalyze polypropylene degradation. Bales stored at high humidity should be conditioned at 20–25 °C and 40–60 % RH for 24 h before opening to prevent static discharge and feed throat bridging on bale openers.

Batch release testing includes fibre length distribution on an Almeter instrument, fibre fineness by vibroscope under ISO 1973, and residual finish by solvent extraction. White-water conductivity after pilot dispersion at 0.05 wt% consistency is recorded as a batch-to-batch control; values above 15 µS/cm indicate excessive finish migration or surfactant carryover. The bale packaging is dated and lot-coded to allow traceability from resin lot to finished separator reel.