1.5-25D*28-76mm PET staple fibers for thermal and filling industry

The polyester staple fiber product designated 1.5-25D*28-76mm is a dimensional family rather than a single proprietary grade. It covers linear densities from 1.5 D to 25 D and cut lengths from 28 mm to 76 mm. Within thermal bonding and filling operations, the product serves as the bulk, load-bearing fibre in carded webs that are subsequently bonded with a separate low-melt binder fibre. It is supplied for nonwoven padding, synthetic fill for quilts, mattress toppers, automotive cushions and high-loft apparel insulation. In a typical through-air bonded line, the base fibre is blended with 5–30% by mass of low-melt PET binder fibre, then carded and bonded at a temperature selected for the binder sheath rather than for this fibre.

The product is available from virgin PET resin with an intrinsic viscosity of 0.60–0.68 dL/g determined under ISO 1628-1:2021. Recycled bottle-grade feedstock may be supplied when the downstream specification allows wider colour scatter and slightly lower tenacity stability. Surface finishing is specified separately: raw, hydrophilic and siliconised fill finishes are common. Siliconised filling grades carry a finish deposit of 0.10–0.30% by mass, which lowers fibre-to-fibre friction and improves loft recovery after compression packaging. The polymer is not designed to fuse during thermal bonding; its primary melting endotherm in differential scanning calorimetry under ISO 11357-1:2016 is near 250–260°C.

What does the 1.5-25D × 28-76mm marking specify on a technical datasheet?

Denier and cut length are selected as a pair because carding behaviour is controlled by both. A 1.5 D round solid PET fibre has an approximate filament diameter of 12 μm, while a 25 D round solid fibre approaches 50 μm; hollow cross-sections reduce apparent density by 10–25% and raise the loft-to-mass ratio for filling grades. The 28–38 mm segment is used primarily for fine-denier hygiene and filtration webs where uniform coverage matters more than high bulk. The 38–51 mm segment is the standard carding range for general nonwoven padding. The 51–76 mm segment is reserved for garnetting, air-lay and coarse filling applications where longer staple increases entanglement and recovered thickness.

Denier tolerance is narrower for fine counts because a 0.1 D shift in a 1.5 D fibre represents a 6.7% mass variation, sufficient to alter web basis weight in lightweight hygiene webs. In 25 D fibre, the same absolute shift is less critical but influences card feed uniformity. Batch release records therefore commonly segregate tolerance bands by denier group instead of applying a single absolute tolerance across the full family.

Carding behaviour changes sharply across the range. Fine 1.5 D fibre increases web uniformity and bonding-point density but raises card loading and static accumulation when ambient humidity falls below 40% RH. Coarse 15 D to 25 D fibre opens more readily but requires wider worker-stripper gauges and lower doffer speeds to prevent nep formation and fibre wrapping. Production-scale cards commonly run at cylinder speeds of 600–1200 m/min for fine denier and reduce to 300–700 m/min for coarse filling grades. These settings are equipment-specific and cannot be transferred between different card widths or wire geometries without preliminary trials.

Card loading is not purely a function of denier. Finish type, ambient humidity and fibre crimp interact. In a production trial on a 1.5 m wide card with metallic wire, changing from a hydrophilic finish to a siliconised finish increased doffer static and required a reduction in cylinder speed of approximately 15% to maintain web transfer. Longer cut lengths within the same denier can produce higher tensile strength in bonded webs but also increase the probability of fibre wrapping around carding cylinders. A 76 mm fibre in a card designed for 38–51 mm is considered out of specification for the machine even if the fibre itself is within commercial tolerance.

Thermal bonding window and the effect of cut length on web uniformity

Thermal bonding of this PET staple fibre is indirect. The base fibre remains solid while a low-melt PET binder fibre with sheath melting near 110–130°C or 180–220°C flows across junction points. The process window is therefore limited by two competing risks. Oven set points below 140°C produce incomplete binder-sheath flow and low web tensile strength measured under ISO 9073-3:1989. Set points above 175°C accelerate dry-heat shrinkage of the PET base fibre, causing web edge pull-back, density gradients and boardiness. A low-shrinkage grade with hot-air shrinkage below 3% at 180°C for 15 min is required for through-air bonded technical padding. The accepted oven set-point band is commonly no wider than ±5°C around the target when line speed fluctuates.

Air velocity in the through-air oven is commonly reported between 0.5–1.5 m/s; high air velocity can disturb web formation, while low velocity reduces heat transfer into the web. For a web basis weight of 200–600 g/m², required residence time in continuous ovens is generally in the range of 15–60 s. The product itself contributes conductive heating mass without flowing; therefore the limiting thermal parameter is the base fibre dry-heat shrinkage rather than fibre melting.

Cut length influences web isotropy. Webs made from 28–38 mm fibre display a lower machine-direction-to-cross-direction tensile ratio than webs made from 64–76 mm fibre because longer staple orients along the carding direction. When isotropic loft is required, converters blend two cut lengths rather than relying on a single staple length. Longer staple increases web cohesion but may form ropes on high-speed compact cards designed for 38–51 mm input.

Filling-grade batches are often produced as hollow or high-crimp solid cross-sections. A hollow fibre of 7 D to 15 D and 51–76 mm cut length provides a balance of loft and recovery in compressed cushions. Siliconised finishes are preferred for quilt and pillow filling because the fibre moves under load without breaking. Recovered loft is evaluated after compression to 50% strain for 24 h at 20°C and is more sensitive to crimp retention and finish distribution than to denier alone. Batch-to-batch variation above 5% in recovered thickness often correlates with repeated carding passes that degrade crimp before filling.

The fibre differs from down and feather fill in that it retains loft at 100% relative humidity and is washable without clumping; however, it does not equal the mass-specific warmth-to-weight ratio of high-quality down. This comparison matters in performance apparel where packable loft is weighted against moisture tolerance and ease of care.

When 15-25D filling fibre replaces standard 6D fibre in compressed cushioning

A 15 D hollow fibre delivers higher initial firmness and lower fibre count per gram than a 6 D fibre. The filling mass must therefore be reduced to avoid excessive harness. Compression recovery at 50% strain is measured with a platen applied to a conditioned test pad; published data for this specific configuration is limited, but production trials show that recovered loft improves with longer cut length and higher crimp frequency within the 1.5-25D*28-76mm family. The main operational boundary is carding speed: coarse denier fibre cannot be accelerated through a compact card without visible clumping, and wider card gauge is necessary. Garnetting or air-lay equipment is preferable for 20 D and above.

The change from 6 D to 15 D also alters the air-flow behaviour of the filling mass. Coarser fibre creates larger inter-fibre pores and may reduce thermal resistance per unit thickness if the fill density is too low. Filling line personnel therefore adjust blow-in pressure and density rather than simply substituting mass at equal weight. Static charge during blowing is more manageable with siliconised finish, but the finish deposit must be verified by extraction because excess above 0.30% can reduce web cohesion in downstream quilting.

Where the product family departs from low-melt bicomponent PET, polypropylene and standard virgin polyester

The product cannot replace a binder fibre. Its differential scanning calorimetry curve shows a primary melting endotherm near 250–260°C, whereas low-melt bicomponent PET has a sheath endotherm near 110–130°C. Compared with polypropylene staple fibre, PET has higher thermal stability and better recovered loft above 80°C, but it has higher density and is less resistant to hydrolysis in prolonged moisture at elevated pH. Compared with standard virgin PET filling fibre, the product family is distinguished mainly by the combined width of denier and cut length availability and by finish options, not by a different polymer chemistry.

Recycled PET staple within the same dimensional family can have intrinsic viscosity at the low end of 0.55–0.65 dL/g and wider colour space; it is suitable for non-visible filling but not for white hygiene nonwovens unless a separate whiteness specification is imposed. The functional difference in downstream processing is mostly visible in tenacity scatter and occasional gel particles in carded webs rather than in the nominal melting range.

If batch release documentation is incomplete, which test methods apply?

Batch release for thermal and filling grades generally includes linear density by ISO 1973:2021, cut length by ISO 6989:2019, tenacity and elongation by ISO 5079:2020, finish add-on by solvent extraction, and dry-heat shrinkage at 180°C for 15 min. Nonwoven converters commonly measure bonded-web tensile strength under ISO 9073-3:1989 and recovered thickness using an internal platen recovery method. The finish and feedstock documentation should demonstrate compliance with REACH EC 1907/2006, RoHS 2011/65/EU and OEKO-TEX Standard 100 where required by the final application. Absence of these declarations is treated as a restricted-use review trigger rather than a routine deviation.

Operational limits include a moisture regain below 0.5%, which simplifies drying but does not eliminate static control in dry carding below 35% RH. The fibre should not be exposed to strong alkaline detergents above pH 9 at elevated temperature because PET undergoes hydrolytic chain scission and loses tenacity. For siliconised filling grades, ultrasonic bonding is not recommended unless the fabric is post-washed because the finish can interfere with weld formation. Standard PET staple is combustible and is not inherently flame-retardant; flame-retardant grades are outside the normal 1.5-25D*28-76mm dimensional family unless separately specified. These boundaries define the acceptable converting envelope for the product in thermal and filling operations.