Polybutylene terephthalate (PBT) is a semicrystalline thermoplastic polyester produced by transesterification of dimethyl terephthalate with 1,4-butanediol, followed by polycondensation and, for fibre-grade resins, solid-state polymerisation. Lash and beauty fibre applications use resin with intrinsic viscosity in the range of 0.80–1.05 dL/g when tested according to ISO 1628-5 because the molecular weight must support melt strength during precision spinneret drawdown without producing excessive die swell. Melt spinning is carried out at 240–260°C through spinnerets with hole diameters of 0.30–0.80 mm, followed by water quenching, two-stage drawing at a total ratio of 2.5:1–4.0:1, and heat stabilisation at 140–180°C. The resulting monofilament is cut and tapered for lash arrays. PBT does not require external plasticiser to achieve lash-compatible flexural behaviour; its glass transition temperature of 45–50°C under ISO 11357-2:2020 places the material above the brittle limit at skin temperature, while its melting temperature of 222–232°C under ISO 11357-3:2018 allows melt processing without degrading the polyester backbone.

In production-scale PBT lash fibre lines, diameter control is an extrusion discipline rather than a single measurement. A single-screw extruder with an L/D ratio of 24:1–30:1 and a melt pump is used to smooth pressure pulsation. A 20–25 µm screen pack removes gel particles that would otherwise generate diameter spikes at the spinneret. Online laser diameter gauges with a resolution of 0.001 mm are placed downstream of the quench bath and before the draw stand. For a nominal 0.10 mm PBT lash fibre, commercial acceptance specifications commonly require a mean diameter within ±0.01 mm and a coefficient of variation below 2.0% across a production lot. When lot-to-lot melt flow rate deviates by more than 5 g/10 min at 250°C/2.16 kg under ISO 1133-1:2022, the pressure drop across the screen pack changes and the diameter distribution widens unless melt pump speed is adjusted. Addition of 0.5–2.0 wt% carbon black masterbatch for black lash fibre increases melt viscosity sufficiently to require a melt-temperature trim; high-shear dispersion in a twin-screw masterbatch line with 44:1 L/D reduces this effect but cannot eliminate it. Moisture content is checked by Karl Fischer coulometry using ISO 15512:2016; a moisture level above 0.02% is cause for batch rejection or extended drying. Direct diameter verification uses calibrated optical microscopy with image analysis. When gravimetric linear density is used, diameter is back-calculated from ISO 1973:1995 assuming circular cross-section; this derivation is invalid after taper grinding and must be replaced by direct optical or scanning electron microscopy.

Measurement uncertainty for direct microscopy is controlled by calibration against a stage micrometer with traceability to a national metrology institute. A minimum of 30 fibre segments per lot is measured at body locations separated by at least 100 mm to avoid within-spool bias. For laser diameter gauges, calibration verification at shift start and after any polymer lot change is standard; gauge drift of ±0.002 mm is detectable only if a reference filament of known diameter is run daily. Batch-to-batch intrinsic viscosity drift of ±0.02 dL/g changes melt pressure on a 25 mm single-screw extruder and can increase diameter coefficient of variation from 1.8% to 3.2% if downstream draw speed is not trimmed. This is a central processing conflict because excessive draw-speed correction moves the taper grinding set point and can shift final tip radius outside cosmetic tolerance.

Why Does PBT Resist Humidity-Induced Diameter Drift Better Than PA6 in Lash Wear?

The lash environment is transiently humid: skin perspiration at the lid margin, adhesive solvents, and ambient relative humidity cycles from 30% to 90% in normal use. PBT has an equilibrium moisture uptake of 0.07–0.10% after 24 h immersion at 23°C under ISO 62:2008, whereas PA6 absorbs 1.8–2.2% at the same condition. Because water plasticises polyamides, a PA6 monofilament can lose a large fraction of its dry-state flexural modulus when conditioned to saturation, while PBT retains most of its 2.0–2.6 GPa flexural modulus measured by ISO 178:2019. Moisture-driven swelling in PA6 can change measured diameter by several percent; PBT dimensional change at equilibrium relative humidity is below the resolution of ordinary shop-floor micrometres. The comparative data are summarised in the table.

The table values are typical published ranges for unfilled base resin; oriented fibre properties vary with draw ratio and heat-setting history. The dimensional stability of PBT under humidity is a direct processing advantage for lash fibre because the diameter and taper measured at the production line are retained more closely during warehousing, carding, and end-use exposure to skin moisture.

Taper Profiling, Tip Radius and Bending Compliance in PBT Lash Fibre

Taper geometry is generated after drawing by centreless abrasive cutting or laser-based tip shaping. The taper converts a constant-diameter monofilament into a graded beam. Bending stiffness follows the relationship EI = Eπd⁴/64; a tip reduced from 100 µm to 25 µm has a theoretical local bending stiffness of 0.39% of the body value. This steep dependence explains why small differences in tip radius dominate skin-contact feel. Production equipment for PBT lash fibre commonly specifies a taper length of 2–4 mm and a tip radius below 50 µm, though published data for specific grinding parameters on cosmetic lash lines is limited. Scanning electron microscopy of tapered PBT monofilaments shows that abrasive cutting can leave surface striations with roughness average values up to 0.4 µm; these striations are reduced by a post-grind thermal rounding step at 160–180°C for 2–5 s. Thermal rounding exploits PBT's rapid crystallisation to smooth the tip without blunting it. If the taper transition is too steep, flexural stress concentrates at the transition zone and cyclic bending during brush insertion can produce stress whitening and eventual fracture. If the tip is too blunt, the lash end behaves as a stiff point and can irritate the eyelid margin during application. Therefore, taper profile tolerances are typically held at ±0.2 mm on length and ±10 µm on tip radius in controlled production. The relevant modulus for bending is the axial tensile modulus of the oriented fibre, not the isotropic resin flexural modulus; draw ratios of 2.5:1–4.0:1 can produce axial tensile moduli from 2.5 GPa to 5.0 GPa depending on heat setting, but published orientation-modulus curves for cosmetic PBT monofilament remain limited.

Mechanical irritation is a skin-contact safety endpoint as much as chemical irritation. A tip radius below 20 µm can generate point pressure sufficient to disrupt the stratified epithelium at the lid margin, even if cytotoxicity and sensitisation tests are negative. Measurement of tip radius by scanning electron microscopy is therefore relevant to safety, not only to aesthetics. Centreless abrasive cutting of PBT also creates a directional surface pattern; if the post-grind thermal rounding step is omitted, the resulting micro-ridges can abrade the skin during repeated placement and removal.

When Skin-Contact Safety Data Must Support a Cosmetic Product Information File

Although PBT lash fibre is not a medical device component by default, many cosmetic manufacturers apply ISO 10993 testing to support the toxicological risk assessment required in the EU by Regulation (EC) No 1223/2009, Annex I. The polymer itself is regarded as biologically inert, but the finished fibre is tested because processing aids, colourants, and surface treatments alter the toxicological profile. Cytotoxicity is assessed with L929 mouse fibroblast cells under ISO 10993-5:2009; a viable cell count of at least 70% relative to negative controls is the conventional acceptance threshold. Skin sensitisation endpoints are addressed by ISO 10993-10:2021 or the local lymph node assay under OECD 429. In vitro irritation is evaluated with reconstructed human epidermis under ISO 10993-23:2021 or OECD 439. Extractables testing follows ISO 10993-12:2021 using polar and non-polar solvents. For PBT, the primary leachable species are low-molecular-weight cyclic oligomers and trace titanium or antimony catalyst residues. Overall migration data for PBT in food-contact conditions may be used as a reference point, but it is not a substitute for cosmetic skin-contact testing. The compliance matrix is shown below.

PBT is not listed as a prohibited or restricted substance in Annex II or Annex III of the EU Cosmetic Regulation. However, the safety assessment must still address the colourant system, surface treatment, residual monomer, and any process aid that may migrate from the finished fibre. In coloured PBT lash fibre, the colourant is a separate toxicological variable: disperse dyes used in carrier dyeing can include substances classified as skin sensitisers under REACH Annex XVII, and carbon black is generally preferred for black lash fibre because it is not soluble in sebum. The migration kinetics of cyclic oligomers in PBT are diffusion-controlled; at skin temperature the effective diffusion coefficient is orders of magnitude below that in plasticised vinyl, which supports the observation that extractables from PBT are surface-limited rather than bulk-mobilised.

Extraction Resistance Is Not a Cytotoxicity Pass in PBT Lash Fibre

Low extractables do not automatically establish skin-contact safety. A PBT compound can meet low overall migration limits yet still contain a migrating surface finishing agent or an incompatible black pigment dispersion that triggers local irritation. PBT lash fibre is generally free of plasticisers, formaldehyde donors, and amine-based process aids; amine-based additives are specifically avoided because residual primary and secondary amines can catalyse ester hydrolysis at melt-processing temperature and generate free 1,4-butanediol in the final fibre. Moisture control is an operational boundary: resin exposed to ambient relative humidity above 60% must be pre-dried at 120°C for 4–6 h in a desiccant dryer with a dew point below −30°C, or intrinsic viscosity drops during extrusion and produces oligomer-rich fibre surfaces. For black lash fibre, carbon black is typically used at 0.1–0.5 wt%; the pigment must be bound within the polymer matrix because free carbon black particles can transfer to skin during handling, although they are not classified as skin sensitisers. A supplier certificate of analysis for PBT lash fibre should list residual 1,4-butanediol, titanium and antimony content by inductively coupled plasma mass spectrometry, and confirm that the fibre is not a nanomaterial as defined in Article 2(1)(k) of the EU Cosmetic Regulation (EC) No 1223/2009. PBT as a solid polymer is not classified as hazardous under the CLP Regulation (EC) No 1272/2008, but this classification does not remove the need for finished-article testing.

On lash carding and tape conversion lines, PBT monofilaments below 0.08 mm show higher breakage rates because carding needles can tear the taper zone; this is a production observation rather than a published standard. Corona or plasma treatment can raise PBT surface energy from its intrinsic range of 35–40 mN/m under DIN 55660-2:2011 to improve adhesive wetting, but over-treatment creates polar carboxyl groups that increase water retention after cleaning and can promote dust pickup during storage. High-speed lash packaging lines therefore specify a maximum water contact angle drift of 5° from the plasma set point; larger drift indicates surface ageing and may require re-treatment or lot quarantine. Published data for this specific configuration is limited, and these limits are normally established by site-specific process qualification rather than by a harmonised standard. The fibre also interacts with cyanoacrylate and latex-based adhesives: PBT's low moisture uptake limits adhesive cure inhibition compared with PA6, but some primer solvents can swell the amorphous regions of PBT and temporarily lower its surface hardness during application.