Air-laid absorbent core production on horizontal forming lines uses a mixture of cellulose fluff pulp, superabsorbent polymer granules, and bicomponent staple fibers that is dispersed into a forming air stream and deposited under vacuum. The sheath/core ratio of the bicomponent binder fiber controls the amount of low-melting polymer available to create load-bearing intersections during subsequent thermal bonding. In calender bonding, the engraved roll applies nip pressure while heating the web surface to temperatures above the sheath melting onset; in through-air bonding, the web passes through a convection oven where heated air fuses the sheath polymer at fiber crossings. The core polymer remains solid under normal processing conditions, so the sheath fraction dictates the thickness of the molten polymer film that forms at each bond point. Commercial PE/PP bicomponent fibers are available with sheath/core ratios of 20/80, 25/75, 30/70, 40/60, and 50/50 by weight; co-PET/PET grades are supplied over a narrower high-sheath range for applications requiring higher thermal stability. Dry tensile strength of the bonded core is commonly measured according to ISO 9073-3:2023 using a 200 mm/min crosshead speed and a 50 mm gauge length, while wet tensile after 1 h immersion in 0.9% saline is used to evaluate durability under urine insult because SAP swelling imposes internal shear on the bonded network.

Why Does Bond Point Coalescence Fail When Sheath Thickness Falls Below 0.35 µm?

The sheath thickness of a bicomponent fiber is not directly proportional to the sheath/core ratio because the cross-sectional area fraction depends on the square of the radius. For a 2.2 dtex PE/PP staple with a density of 0.95 g/cm³, the overall fiber radius is approximately 2.7 µm; the sheath layer thickness at 20 wt% sheath is approximately 0.3 µm, whereas at 30 wt% it increases to approximately 0.5 µm. During calender bonding, the fiber cross-section is compressed in the nip, and the sheath polymer must flow into the wedge-shaped space where two or more fibers overlap. If the available sheath volume is too low, the center of the bond point remains resin-starved, and the measured tensile failure changes from cohesive ductile rupture of the sheath film to adhesive failure at the core-sheath boundary or complete fiber pull-out. Manufacturer processing guides for PE/PP bicomponent staple typically specify a minimum sheath fraction of 20 wt% for air-laid calender lines; below this level, the calender window narrows to 3–5°C and edge dusting during slitting becomes a production-scale defect. The relationship between sheath fraction and bond point strength is therefore a threshold function rather than a linear dilution effect, and published data for this specific configuration is limited to general supplier bulletins and nonwoven laboratory reports.

Wet integrity shifts the failure mechanics in absorbent cores containing superabsorbent polymer. When SAP expands in 0.9% saline, individual SAP particles exert localized volumetric strain on adjacent bond points; bonded regions with high sheath concentration undergo ductile yielding, whereas low-sheath networks separate by brittle adhesive failure at the fiber-pulp interface. Wet tensile retention according to ISO 9073-3:2023 for a 100 g/m² air-laid core containing 15 wt% bicomponent fiber is typically 40–60% of dry tensile for 30/70 PE/PP grades, but retention can fall below 30% when the sheath fraction is 20 wt% or lower because the sheath film is not continuous at every fiber intersection. Capillary wicking along the exposed core-sheath boundary is observed in low-sheath grades; this interfacial channel accelerates saline penetration and can initiate premature bond failure before SAP swelling reaches equilibrium. In diaper core end-use testing, this failure mode appears as cracking along the longitudinal machine direction and localized core separation during fluff pad integrity testing.

Calender Bonding Window Narrowing at Sheath Fractions Below 20 wt%

On a pilot air-laid line equipped with a heated engraved steel roll and a smooth rubber back-up roll, the usable bonding window for a 20/80 PE/PP bicomponent is approximately 3–5°C wide, compared with 8–12°C for a 30/70 grade at equivalent basis weight and forming conditions. At roll surface temperatures below 130°C, the sheath polymer at the outermost fiber intersections remains below its melting plateau and insufficient film formation occurs, generating dusting and edge shedding during slitting. Above 136°C, the thin sheath film overheats rapidly, and the core begins to soften at the bond point crown; this causes point collapse, visible strike-through of molten polymer onto the forming wire, and local reduction in core caliper. Process engineers compensate by reducing nip load from 70 N/mm to 45 N/mm, but lower nip pressure further reduces bond point contact area and tensile strength. The practical consequence is that a 20/80 grade cannot be processed at line speeds above 80 m/min on conventional calender lines without web breaks, while a 30/70 grade maintains bond integrity to 120 m/min. These thresholds are consistent with equipment manufacturer technical bulletins for heated calender roll surface temperatures and nip load capacities.

If Through-Air Bonding Is Substituted for Calender Bonding

Through-air ovens apply convective heat to both surfaces of the air-laid web, and the absence of nip pressure changes the bond formation mechanism from contact melting under compression to surface fusion driven by air temperature and dwell time. For a 50/50 PE/PP bicomponent in a 2.2 dtex staple, through-air bonding at 140°C for 3 s creates bond points around fiber intersections where the sheath polymer coalesces by capillary action and gravity. Reducing the sheath ratio to 20/80 in the same oven yields a lower number of effective bonds per unit volume because the thinner sheath cannot flow sufficiently to bridge the gap at fiber crossings. Increasing the oven temperature by 4–6°C compensates for the lower sheath volume, but this narrows the margin to core softening and can initiate core deformation in the top web layer adjacent to the air nozzles. Through-air bonded cores with 20/80 bicomponent exhibit dry tensile values of approximately 70% of equivalent calender-bonded webs at the same basis weight measured under ISO 9073-3:2023, but they retain better loft and lower stiffness because bond points are not fully consolidated. The selection of through-air versus calender bonding therefore depends on whether the product specification prioritizes tensile strength or absorbent core softness and resilience.

Co-PET/PET Systems Demand a Wider Core-Sheath Temperature Differential

Co-PET/PET bicomponent fibers have a sheath melting onset between 110°C and 180°C depending on the copolymer composition; the PET core remains dimensionally stable to 250°C. The higher core melting point provides a broader processing window than PE/PP, but the lower melt flow of co-PET reduces the rate of polymer coalescence at the bond point. Because co-PET is more shear-sensitive than PE at typical calender nip residence times of 15–40 ms, a sheath fraction below 30 wt% often produces incomplete bond point formation even when the oven temperature is raised to 160°C. Through-air ovens used for co-PET/PET air-laid cores typically operate at 150–170°C with dwell times of 4–6 s; reducing the sheath ratio from 40/60 to 30/70 lowers dry tensile strength by approximately 20% under ISO 9073-3:2023. This system is preferred for applications requiring thermal stability during subsequent lamination or ultrasonic seam welding, because PP cores can soften and lose orientation at lamination temperatures above 150°C.

Core-Sheath Interface Integrity Under Cyclic Humidity Exposure

Cyclic humidity exposure from 20% to 80% RH at 23°C for 48 h cycles induces interfacial stress in air-laid cores because the fluff pulp swells anisotropically while the PE/PP bicomponent remains dimensionally stable. In low-sheath grades with sheath thickness below 0.4 µm, moisture-driven fiber swelling produces microcracks at the sheath-core interface; these cracks propagate along the fiber axis and reduce the effective load-transfer length of the binder fiber. Tensile testing according to ISO 9073-3:2023 after cyclic exposure shows a greater strength loss for 20/80 bicomposite cores than for 30/70 grades, an effect attributed to the absence of a continuous sheath layer that can absorb interfacial strain. This limitation dictates that air-laid cores intended for humid storage or tropical distribution be bonded with sheath fractions of at least 25 wt% or be protected by a nonwoven carrier layer. Incompatibility with high-electrolyte fluid formulations is also recorded: urea-based wetness indicators migrate into the sheath and lower its melting onset below 120°C, causing premature bonding on the forming wire.

Table 1 compares typical sheath/core ratio effects for PE/PP bicomponent fiber in a 100 g/m² air-laid core with 15 wt% binder fiber, bonded on a calender line at 70 N/mm. Values are interpolated from supplier technical bulletins and nonwoven laboratory reports and are not universal specifications.

A compliance and test method matrix for air-laid core bond strength evaluation is given in Table 2. These standards anchor the measured boundaries for process release and incoming fiber control.

Process incompatibility is recorded when PE/PP bicomponent fibers are combined with lotion-treated fluff pulp or surfactant-coated SAP; the sheath film loses interfacial adhesion and bond point tensile strength decreases by more than 15% under ISO 9073-3:2023. Avoid combination with ethylene–vinyl acetate-based binder emulsions because the EVA plasticizer migrates into the PE sheath and lowers its melting onset below 120°C, causing premature bonding on the forming wire. Pre-drying of fluff pulp to 6–8% moisture is required when relative humidity exceeds 60% because residual water vapor at the calender nip acts as a heat sink and prevents sheath polymer coalescence. Published data for specific air-laid core configurations using sheath/core ratios outside 20/80 to 50/50 is limited; outside this range, the core either becomes too brittle or the sheath film occludes the void structure. These operational boundaries apply to PE/PP and co-PET/PET bicomponent systems with fiber linear densities from 1.7 dtex to 6.7 dtex and staple lengths from 3 mm to 12 mm.