1.5D*38mm Polypropylene Staple Fibe

The designation 1.5 D × 38 mm polypropylene staple fiber identifies a melt-spun, drawn, crimped, and cut fiber produced from fiber-grade isotactic polypropylene homopolymer. The model string 1.5D*38mm encodes nominal linear density and cut length; supplier suffixes may identify spin finish chemistry, cross-section, optical-brightener content, crimp configuration, and color. The nominal linear density is 1.5 denier, equivalent to 1.67 dtex under the conversion factor 1.111 dtex/D. The cut length is 38 mm, with a common commercial tolerance of ±1.5 mm. The fiber is supplied in compressed bales with a spin finish applied at 0.10 %–0.30 % by mass. The standard spin finish is hydrophobic; a durable hydrophilic finish is specified separately for hygiene acquisition layers and wet-laid processes.

Feedstock melt mass-flow rate is typically controlled at 18 g/10 min–25 g/10 min at 230 °C and 2.16 kg load per ISO 1133-1:2022. The fiber is available in raw white, optical white, semi-dull, and solution-dyed grades; round cross-section is standard, while trilobal and hollow cross-sections are separate commercial variants. During staple production, melt-spun filaments are hot-drawn and stuffer-box crimped before cutting to 38 mm. Crimp frequency is typically 10–16 crimps per 25 mm when assessed by ASTM D3937. Lot-to-lot variation in crimp frequency outside this range influences carding cohesion and web cloudiness.

The 1.5D*38mm designation is a dimensional specification rather than a chemical identifier. Two suppliers may supply fiber with the same denier and cut length but different melt flow rate, spin finish chemistry, crimp geometry, antioxidant package, and optical brightener. These secondary properties determine carding speed, thermal bonding temperature, web tensile strength, and outdoor durability. Therefore, the dimensional designation alone does not establish fitness for a given nonwoven process.

End-use routes include carded thermal-bonded hygiene coverstock, needlepunched geotextiles, automotive interior substrates, air and liquid filtration media, acoustic insulation, and dry-laid wipes. The 38 mm cut length is compatible with roller-top carding, cross-lapping, air-laying, and wet-laying systems; the 1.5 D fineness increases fiber count per unit mass and supports uniform web formation at low basis weights.

Table 1 lists lot-level physical data. The values are supplier-lot typical ranges and must be confirmed against a certificate of analysis; they are not universal specification limits for all grades.

What Thermal Bonding Parameters Govern Web Formation and Fiber Fusion?

Thermal bonding of 1.5 D × 38 mm polypropylene staple occurs in a narrow temperature band because mono-component PP has a single melting endotherm. Differential scanning calorimetry per ISO 11357-3:2018 places the melting peak at 160 °C–170 °C. Calender and through-air ovens are operated with surface or air temperatures from 145 °C to 165 °C. Below 145 °C, surface melting is incomplete and web tensile strength is low; above 165 °C, the fiber forms film, web loft collapses, and bond-point shrinkage appears. The effective processing window can be as narrow as 5 °C. This is the principal difference from PE/PP bicomponent staple, which bonds at 130 °C–135 °C and tolerates broader temperature variation because the sheath polymer carries the adhesion.

On production calender lines, bond area is set by engraved roll pattern, nip pressure, and line speed. For a 20 g/m² coverstock, the process is typically limited by thermal transfer through the web thickness rather than by carding output. Residence time must be reduced when web bulk exceeds 50 g/m² to avoid core melting and surface film formation. Spin finish migration is a batch-sensitive variable: finish content above 0.30 % by mass can reduce calender bond-point adhesion and produce peeling during tensile testing according to ISO 9073-3. Reported production-scale carding experience indicates that doffer wrapper formation increases when spin finish falls below 0.10 % by mass. In needlepunch lines, the 38 mm staple is opened, carded, cross-lapped, and needled before thermal setting. Needle density and depth are set lower than for polyester because polypropylene has lower bending stiffness and a lower melting point; high needle speeds generate local frictional heating and surface fusion if needle parameters are not controlled.

Through-air bonding requires uniform air distribution. A temperature variation across the oven width greater than ±3 °C can produce regional web strength variation, especially at low basis weights. Calender roll surfaces are usually release-coated to prevent fiber sticking; PTFE and similar non-stick roll treatments are common. The polymer does not require pre-drying before carding because moisture regain is below 0.1 % per ASTM D2654, but static charging at relative humidity below 40 % can increase card web breaks unless ionizing bars or humidification are installed.

Polypropylene Staple Fiber Versus Polyester and Bicomponent Staple at Equivalent Cut Length

Polypropylene differs from polyethylene terephthalate staple in density, melting point, moisture regain, and degradation chemistry. PP density is 0.90 g/cm³–0.91 g/cm³, compared with 1.38 g/cm³ for PET. At equal denier and cut length, the lower density yields approximately 50 % more fiber per unit mass. The melting peak for PP is 160 °C–170 °C; PET requires processing or setting above 230 °C. Moisture regain for PP is below 0.1 % by ASTM D2654, while PET regain is approximately 0.4 %. PP is resistant to hydrolysis and to ambient aqueous acids and alkalis; PET undergoes hydrolytic degradation in wet heat above 70 °C in closed systems. Conversely, PP is more sensitive to thermo-oxidative degradation and UV exposure, and unstabilized grades embrittle after extended air aging above 90 °C.

Compared with PE/PP bicomponent staple of the same cut length, mono-component 1.5 D × 38 mm PP staple has a higher bonding temperature and a narrower window. Bicomponent fiber can be through-air bonded at 130 °C–135 °C because the PE sheath melts before the PP core. Mono-component PP is selected when a single melting endotherm is required for downstream thermal stability or when sheath-core morphology is not acceptable. Table 2 compares the product with same-denier PET staple and coarser PP staple.

Published data for this specific configuration is limited in open technical literature; the comparative values in Table 2 are typical supplier technical-bulletin data and must be verified for each lot.

When a dry-laid line switches from 6 D × 64 mm polypropylene staple to 1.5 D × 38 mm polypropylene staple, fiber count per unit mass increases by approximately 4 times at equal web density. The finer staple reduces mean pore diameter and increases specific surface area, which is exploited in air and liquid filtration media and in hygiene acquisition layers. In production carding trials, 38 mm staple generates fewer doffer wrappers than 51 mm or 64 mm staple on high-speed roller-top cards, but fine denier increases the number of fibers per unit area and may require lower lickerin and cylinder speeds to prevent static charging and fusion. Reported line data indicate that reducing carding speed by 10 %–20 % relative to coarse-denier settings is a common starting condition when switching to 1.5 D staple; the actual setting must be determined from web uniformity and fiber temperature, not from denier alone.

In needlepunched filtration substrates, the smaller cross-section of 1.5 D fiber yields tighter pore structures. Clean pressure drop increases if web density is held constant; to keep air permeability constant, basis weight or needling intensity can be reduced. Particle capture efficiency for 2 µm–10 µm particles is reported to improve with finer fiber in nonwoven filter media, but the effect is dependent on web density, pore-size distribution, and surface finish. Batch-to-batch crimp variation of ±2 crimps per 25 mm can shift web density and pressure drop; incoming crimp measurement is therefore recommended. For liquid filtration and hygiene acquisition layers, a durable hydrophilic finish changes strike-through and re-wet behavior; these properties should be tested by EDANA/INDA methods because standard spin finish is hydrophobic. Published data for this specific configuration is limited.

When 1.5D×38mm PP Staple Is Specified for Sterile or Outdoor Nonwovens

For sterile or outdoor nonwoven applications, polypropylene staple fiber is intrinsically hydrophobic, with moisture regain below 0.1 % by ASTM D2654. This eliminates pre-drying before carding and reduces biological growth risk in stored rolls, but it limits liquid absorption unless a hydrophilic finish or bicomponent fiber is incorporated. The fiber withstands aqueous acids and alkalis at ambient temperature and is not subject to hydrolytic degradation; however, it softens in chlorinated solvents and in some hot aliphatic solvents. Thermo-oxidative degradation is the main processing constraint. Continuous exposure above 90 °C in air consumes antioxidant additives and causes discoloration or embrittlement unless the polymer is stabilized for the intended service temperature.

Because the melting peak is 160 °C–170 °C, steam sterilization at 121 °C can produce shrinkage or bond-point creep in load-bearing nonwovens. Polypropylene is therefore specified less often for reusable medical devices requiring repeated autoclaving. For single-use nonwovens sterilized by ethylene oxide or gamma radiation, PP can be specified, but radiation doses above 25 kGy may initiate chain scission in unstabilized grades; lot-specific antioxidant documentation is required. Outdoor geotextiles require carbon-black or UV-stabilized grades; unstabilized PP loses tensile strength rapidly under ASTM D4355 xenon-arc exposure. Carbon-black content of 2 %–3 % by mass is common in UV-stabilized geotextile grades. Unfilled polypropylene has a limiting oxygen index near 17 %–18 % per ISO 4589-2, so automotive interior composites must be tested to FMVSS 302 or ISO 3795 and usually require flame-retardant treatment or lamination. Food-contact grades may reference 21 CFR 177.1520 or Regulation (EU) No 10/2011, but compliance is grade-specific and not implied by denier or cut length.

Material documentation should include a certificate of analysis covering linear density, cut length, tenacity, elongation, spin finish content, and DSC melting peak. Regulatory compliance to EU REACH Regulation (EC) No 1907/2006 and RoHS Directive 2011/65/EU is commonly declared in supplier documentation, but lot-specific verification is required because additive packages vary by grade. If the product is used in through-air bonded high-loft webs, the oven air temperature should not exceed 165 °C, and residence time must be reduced when web bulk exceeds 50 g/m² to avoid core melting and surface film formation.