In high-speed disposable hygiene manufacturing, the acquisition layer is positioned between the topsheet and the absorbent core to accept an insult of 5 mL synthetic urine and distribute it laterally before the core swells. The fiber selection for a thermally bonded acquisition layer is determined by the differential melting range between the sheath and core, linear density distribution, crimp geometry, surface finish, and the thermal-hydraulic profile of the bonding equipment. On a production card line with a working width of 2,500 mm operating at 120–220 m/min, the residence time in a through-air oven may be 4–8 s; the sheath polymer must therefore reach a viscosity low enough to form autogenous point bonds without causing the core to lose its oriented crystalline structure. Supplier technical data sheets for polyethylene/polyethylene terephthalate bicomponent staple fibers commonly specify a sheath melting peak of 128–132°C and a core melting peak above 245°C according to ASTM D3418. The bonded web is tested for mass per unit area by ISO 9073-1, caliper by ISO 9073-2, tensile properties by ISO 9073-3, and liquid strike-through by NWSP 70.7 or ISO 9073-13. Thermally bonded acquisition layers are produced predominantly from sheath/core staple fibers, because the sheath can be melted in a controlled manner while the core remains structurally intact. Three polymer families dominate: polyethylene sheath/polyethylene terephthalate core, polypropylene sheath/polypropylene core, and co-polyester sheath/polyester core, with polylactic acid bicomponents offered where renewable carbon content is specified. The choice is not interchangeable; each polymer pair has a different thermal gap, melt viscosity ratio, wet collapse response, and compatibility with through-air ovens and calender rolls.

What Limits the Processing Window in Through-Air Ovens?

The upper processing limit in a through-air oven is set by the onset of sheath melt flow and the possibility of molten polymer transfer to the conveyor belt. For a polyethylene sheath with a melting peak of 128–132°C, forced convection ovens are typically operated with an air set point of 132–138°C because the sheath requires a local temperature at least 3–5°C above its peak melting endotherm to achieve bond point formation within 4–8 s. On equipment with slot nozzle air velocities of 1.5–2.5 m/s and a cross-machine temperature uniformity of ±2.5°C, the practical window becomes ±3°C around the optimum set point; below this range the bond points are incomplete and the cross-direction tensile strength falls below 5 N/5 cm as measured by ISO 9073-3, while above this range the sheath polymer drains from the fiber surface and deposits on the oven belt, producing a web with intermittent holes and a belt-cleaning shutdown. The lower limit is controlled by the melting onset of the sheath, not by the core, and it is influenced by the heating rate used in differential scanning calorimetry. A heating rate of 10°C/min under ASTM D3418 may report a peak melting point 2–4°C higher than the melting onset observed under industrial through-air heating rates above 100°C/min; this difference must be accounted for when transferring laboratory thermal data to production settings. Polypropylene sheath/polypropylene core fibers have a higher melting sheath peak of 160–165°C, which shifts the oven set point to 165–170°C and increases energy input, but the thermal gap between sheath and core is only 20–30°C compared with 115–125°C for PE/PET, so over-temperature excursions are more likely to cause core deformation and web shrinkage exceeding 4% in the cross-machine direction. The oven profile is also a factor: a three-zone through-air oven with increasing temperature from 130°C to 138°C and a final cooling zone below 40°C is used to prevent post-bonding tackiness on winders, and any deviation in the cooling zone above 45°C has been associated with blocking in roll form. Thus fiber selection for through-air bonding is a narrow-window problem in which the sheath melting range, core melting range, and oven air temperature uniformity must be specified together.

On calender bonding equipment with engraved steel rolls, the fiber selection problem shifts from air residence time to nip pressure and surface temperature. A typical calender for 30–50 g/m² acquisition layer webs uses a bond area of 12–18%, a line speed of 80–150 m/min, a nip pressure of 40–70 N/mm, and an oil-heated roll surface temperature of 135–142°C for polyethylene sheath fibers. The sheath polymer must flow into the engraved points within a nip dwell time of 0.5–2.0 ms, which requires a melt flow rate between 25 g/10 min and 40 g/10 min at 190°C and 2.16 kg according to ISO 1133-1. If the melt flow rate falls below 20 g/10 min, the bond points remain superficial and the nonwoven fails in peel mode under 0.5 N per bond point; if it rises above 45 g/10 min, the sheath polymer is squeezed out of the nip and forms film-like edges around the calendar points, which increases flexural rigidity above 40 mJ and reduces liquid pass-through. The core polymer must resist deformation at the roll surface temperature; polyethylene terephthalate core with a melting peak above 245°C and a degree of crystallinity above 30% maintains filament diameter under compression, while a polypropylene core with a melting peak of 160–165°C may soften at roll temperatures near 145°C and collapse the web caliper by 20–30%. Calender bonding creates a flatter, denser acquisition layer than through-air bonding, with measured caliper under 0.5 kPa of 0.6–1.0 mm for a 40 g/m² web compared with 1.2–1.8 mm for a through-air bonded web of the same basis weight. The selection of fiber crimp also interacts with calendering: fibers with 10–14 crimps per 25 mm and crimp amplitude between 0.15 mm and 0.25 mm produce a more open carded web that is partially preserved after point bonding, whereas straight fibers below 6 crimps per 25 mm produce a dense web that may fail acquisition time tests. The roll temperature must be held within ±2°C across the face width because polymer flow at the nip is exponentially sensitive to temperature, and a 5°C rise can increase bond point thickness variation by 35% and create a visible bar mark at the roll edge.

Fiber linear density below 2.2 dtex improves carding uniformity but can produce too many bond points per unit area, leading to a dense web with reduced liquid throughput. Above 6.7 dtex, the web becomes bulky but the number of fiber-to-fiber contacts decreases, and the through-air bond may require a higher oven temperature or longer dwell to maintain tensile strength. Acquisition layer staple fibers are typically specified at 4.4–6.7 dtex with a staple length of 40–60 mm, because this range provides a balance between carding continuity and void volume. Crimp frequency also affects the structural openness of the bonded web. Fibers with 10–14 crimps per 25 mm and a crimp amplitude of 0.15–0.25 mm preserve loft after thermal bonding, while fibers with fewer than 6 crimps per 25 mm produce a flat web with a caliper below 0.8 mm at 0.5 kPa. The carding process itself imposes limits on finish level: too little finish below 0.08% by weight increases card wire wear and static, while too much finish above 0.30% by weight can cause lapping roll buildup and a nonuniform web. Finish selection must also consider migration in storage; polyethylene-based finishes with a viscosity below 20 mPa·s at 25°C may migrate from the fiber surface into the sheath polymer over 60 days at 40°C, altering the sheath melting behavior and reducing the effective bonding temperature. The fiber producer therefore packages the staple with a finish system that is matched to the downstream bonding process, and changing the finish without requalification can shift the through-air oven set point by 2–4°C.

Sheath Viscosity and Bond Point Formation

Melt flow rate governs the balance between bond point integrity and web loft. For bicomponent fiber production, the sheath resin is extruded through a twin-screw extruder with an L/D ratio of 36:1 and a spin pack temperature of 230–250°C for polyethylene or 250–270°C for polypropylene; the melt then enters a spinneret with segmented arc channels to form a uniform sheath thickness of 15–25% of the filament diameter. The sheath melt viscosity at the bonding temperature must be low enough to wet the contact point with another filament, but high enough to avoid capillary wicking away from the bond site. Published supplier data for PE sheath resins used in acquisition layer fibers report a melt flow rate of 25–40 g/10 min at 190°C and 2.16 kg; for PP sheath resins the equivalent range is 20–35 g/10 min at 230°C and 2.16 kg. A batch-to-batch variation of ±5% in sheath melt flow rate can shift the optimum through-air oven temperature by 3°C, and this shift is a recognized cause of periodic tensile strength variation on production lines. The ratio of sheath viscosity to core viscosity also determines whether core filaments remain aligned during bonding. If the sheath viscosity at the processing temperature is 10–15% of the core viscosity, the bond point forms without core deformation; if the ratio drops below 5%, the sheath can migrate along the fiber surface and produce uneven bonding that appears as localized bright spots under transmitted light. Additives in the sheath must be selected for thermal bonding stability: hindered phenol stabilizers are generally compatible, but amine-based antistatic additives are avoided where acid-functionalized sheath resins are used because premature crosslinking can raise melt viscosity and reduce bond point penetration. Fiber finish also contributes to bond formation. A finish with a surface energy above 35 mN/m and a lubricant loading of 0.10–0.25% by weight improves carding uniformity, but excess finish above 0.30% can volatilize in the through-air oven and condense on the nozzles, leading to airflow blockage and a drop in oven heat transfer efficiency of 10–15%. The viscosity data are therefore inseparable from the thermal bonding step, and fiber selection requires a matched sheath MFR range and finish package for the specific oven or calender configuration.

Wet collapse resistance is evaluated after multiple insults because the acquisition layer must maintain void volume when saturated. Through-air bonded PE/PET webs with a basis weight of 40 g/m² typically retain 60–70% of their dry caliper after the third 5 mL synthetic urine insult under 0.5 kPa load, while calender-bonded webs retain 35–50%. The difference is a function of bond point morphology: through-air bonding creates three-dimensional point bonds that preserve fiber orientation, whereas calender bonding locally compresses the web and reduces pore volume. Fiber selection affects wet collapse through the core polymer modulus. Polyethylene terephthalate core fibers with a tensile modulus above 2.5 GPa resist bending under load, but polypropylene core fibers with a modulus of 1.2–1.5 GPa may collapse more when the web is saturated, increasing rewet measured by NWSP 80.9 above 0.3 g. This is why PE/PET is preferred for acquisition layers where low rewet is critical, even though its raw material cost is higher than polypropylene. The sheath also contributes to wet performance after bonding: a polyethylene sheath with a moisture regain below 0.1% does not swell in the presence of synthetic urine, while PLA sheath with a moisture regain of 0.4–0.6% may soften slightly after saturation and reduce the web's wet tensile strength. The selection of fiber never ends at the thermal bond; it must include the bonded web's response to liquid loading and compressive recovery.

When PLA-Based Bicomponent Fibers Replace Polyolefin Sheaths

Polylactic acid bicomponent fibers are introduced when a renewable carbon target or compostability claim is attached to the acquisition layer, but the thermal gap between the sheath and core is much smaller than for polyolefin systems. A low-melting PLA sheath may have a melting peak of 125–135°C and a high-melting PLA core may melt at 150–165°C, leaving a gap of only 15–40°C compared with the 117–127°C gap for PE/PET. This narrow gap forces the through-air oven set point to be maintained within ±2°C of the sheath melting peak, because a set point above 140°C can initiate core recrystallization and cause cross-machine shrinkage above 6%. PLA staple fiber also undergoes hydrolytic chain scission during processing if residual moisture is not controlled. Fiber manufacturers dry PLA resin to <0.025% moisture before extrusion and recommend preconditioning baled staple fiber when ambient relative humidity exceeds 60%, because absorbed water can generate steam in the through-air oven and produce surface voids in the sheath. Published data for specific PLA/PLA acquisition layer configurations in high-speed disposable hygiene lines is limited; most published evaluations are based on laboratory carded webs processed at speeds below 30 m/min, so direct transfer to production equipment above 120 m/min requires pilot-scale confirmation. The melt viscosity of PLA is also more shear-thinning than polyolefins, and its viscosity at 135°C may be 2–3 times higher than a polyethylene sheath at the same temperature, which reduces bond point penetration and requires higher nip pressure or longer oven dwell. PLA is incompatible with some conventional spin finishes containing high amine content, because amine groups can accelerate ester hydrolysis and reduce the bonded web tensile strength after aging at 40°C and 75% relative humidity. When PLA is used as a partial replacement, it is often blended with polypropylene or polyethylene terephthalate core fibers at a mass ratio of 10–30% to retain mechanical stability while obtaining renewable carbon content. The processing boundary is therefore not simply a lower oven set point; it involves moisture handling, finish chemistry, dwell-time adjustment, and a reduced tolerance for temperature drift.

Because thermally bonded acquisition layers are used in absorbent hygiene articles that may contact skin, the fiber selection must satisfy both mechanical and migration requirements. Fibers used in European absorbent hygiene products are commonly assessed under REACH Annex XVII restrictions and the EU Ecolabel criteria for absorbent hygiene products, Decision 2014/763/EU; the finished nonwoven may also be certified under OEKO-TEX Standard 100 product class I or II depending on the manufacturer's claim. In the United States, absorbent hygiene articles are not generally subject to 21 CFR 177.1520 unless the same fiber is marketed for food contact, but polymer compliance documentation is still required as part of material qualification. A compliance matrix for a thermally bonded acquisition layer therefore includes mechanical, liquid handling, and chemical safety parameters, each with a specified test method and acceptance window.

However, the acceptance windows in the matrix are not universal; they presuppose a specific fiber diameter and crimp, a through-air or calender bonding route, and a particular test apparatus for strike-through and rewet. A change in synthetic urine composition from 9 g/L sodium chloride to 0.9% saline can alter surface tension and strike-through results, so test fluids must be reported alongside results. Fiber selection must be repeated whenever the basis weight target, bonding equipment, or liquid insult protocol changes; a fiber qualified for a 40 g/m² through-air bonded acquisition layer will not necessarily meet the same acceptance limits when converted on a calender line at 100 m/min.