In pleated fuel, hydraulic, and process filtration elements manufactured with glass-fibre or melt-blown media, adhesive migration into the filtration matrix begins when a molten hot-melt bead contacts the pleat apex and transient capillary pressure exceeds the local viscous resistance at the fibre surface. Adhesive migration control in these assemblies is therefore a function of the pore-size distribution of the filtration medium, the surface energy of the fibre batt, the melt viscosity of the adhesive, the temperature differential across the bond line, and the presence of a continuous or semi-continuous barrier at the point of adhesive impingement. A sheath-core copolyester filament introduced as a pleat-tip stabilizer or as an integrated spacer provides a controlled lower-melting outer layer that can be fused into the medium before the adhesive bead is applied, while the higher-melting polyester core prevents filament collapse and preserves the pleat spacing function. The sheath is typically a copolyester with a melting endotherm between 110 °C and 180 °C as measured by ISO 11357-3, while the core is a homopolyester with a melting point above 250 °C and a crystallinity of 35 % to 55 % depending on draw ratio and heat-setting. The core retains an elastic modulus at the hot-melt application temperature, limiting the filament from yielding under the mechanical load of the pleat pack. Because the adhesive melts at or slightly above the sheath melting range, the filament sheath may be partially remelted during adhesive application. This partial remelting is not a defect if the sheath layer has already been fused into a continuous film along the pleat tip before the hot-melt bead is deposited. Published data for the specific capillary penetration behavior of copolyester-sheathed filament-in-media configurations under production-scale hot-melt application is limited; however, the governing variables are well documented in textile fibre standards, polymer rheology methods, and filter performance protocols.

Thermomechanical Differentiation Between Copolyester Sheath and High-Tenacity Polyester Core

Differential scanning calorimetry according to ISO 11357-3 distinguishes the sheath and core by separate melting endotherms. The sheath melts within the range of 110 °C to 180 °C, while the core melts between 250 °C and 265 °C. The temperature gap between the two melting endotherms must be at least 60 °C to allow sheath fusion without core orientation loss. The copolyester sheath is typically modified with a second glycol such as cyclohexanedimethanol or diethylene glycol in the range of 10 mol% to 30 mol% to reduce the crystalline melting point and lower the degree of crystallinity. The melting enthalpy of the sheath is generally 20 J/g to 35 J/g, compared with 45 J/g to 65 J/g for the core, meaning that the energy input required to melt the sheath is lower and the molten sheath can wet the medium under light calendering pressure. Because the core remains solid, the filament does not lose its circular cross-section during bonding; this preserves the pleat spacer function while the sheath forms a meniscus between fibre bundles. The glass transition temperature of the sheath is generally 55 °C to 80 °C, while the core glass transition temperature is 70 °C to 80 °C. This difference in thermal transitions is essential because a sheath with too low a glass transition can become tacky at storage temperatures commonly encountered in non-climate-controlled warehouses, while a sheath with too high a glass transition may require excessive activation temperature and cause media scorching. The core polymer is usually a polyethylene terephthalate with an intrinsic viscosity of 0.60 dL/g to 0.72 dL/g before spinning, which provides sufficient tenacity after drawing to resist the compressive forces of the pleated pack and the hydraulic load during service. The tensile strength of the core after drawing and heat-setting ranges from 4.0 cN/dtex to 6.0 cN/dtex as measured by ISO 2062, while the sheath contributes less than 35 % of the total filament strength due to its lower crystallinity. The flexural stiffness of the filament is set by the core diameter; a core diameter of 0.15 mm to 0.45 mm is typical for pleat spacer applications, with the sheath adding an additional 30 µm to 80 µm to the overall filament radius.

The melt viscosity mismatch between sheath and core at the hot-melt application temperature establishes the filament’s behavior during adhesive deposition. The table below reflects typical supplier-reported ranges rather than a single formulation; lot-to-lot variation in sheath melt flow index may drift by ±3 g/10 min when reground sheath material is incorporated above 10 % by weight. In production, this drift shifts the thermal fusion window and must be controlled by melt flow index testing at the extruder hopper according to ISO 1133-1. When the sheath melt flow index is too high, the filament sheath becomes excessively fluid during activation and can saturate the medium, reducing permeability. When the sheath melt flow index is too low, the sheath cannot fill the surface pores of the filter medium, and the adhesive bead penetrates the unfilled voids. The core melt flow index is less critical for barrier function but controls the extrusion stability and the draw resonance behavior during spinning.

On a filament spinning line with a core extruder of 45 mm to 75 mm screw diameter and a sheath extruder of 25 mm to 45 mm screw diameter, both having L/D ratios from 24:1 to 36:1, the sheath polymer is metered through a melt pump to a crosshead die with an annular distribution gallery. The core polymer is introduced through a separate melt pump and the combined melt flow is drawn from a spinneret capillary at 280 °C to 295 °C. The sheath percentage is controlled by gravimetric feeders and checked by selective dissolution of the sheath in a solvent or by differential scanning calorimetry on cut cross-sections. A sheath percentage of 15 % to 35 % of the total filament cross-sectional area is typically specified for adhesive barrier applications; lower sheath percentages are cheaper but reduce the continuous film-forming capacity at the pleat apex. Higher sheath percentages above 45 % may create excessive melt drips and bonding to neighboring pleats. The spun filament is drawn between heated godets at draw ratios from 2.5:1 to 5.5:1, with the first godet held at 80 °C to 120 °C and the second godet held at 140 °C to 180 °C. The drawn filament is then heat-set in a hot-air or heated-plate zone at 160 °C to 220 °C to stabilize the core orientation and relax sheath stresses. A critical processing boundary is the pre-drying requirement: the sheath copolyester must be dried to below 50 ppm moisture before extrusion, and at ambient relative humidity above 60 %, undried material can absorb more than 0.3 % moisture in 30 min, producing hydrolysis during extrusion and localized film defects.

What Melt Viscosity Mismatch Is Required to Block Hot-Melt Capillary Wicking?

The penetration of a hot-melt adhesive into a nonwoven or glass-fibre filter medium can be modeled by the Washburn relationship for laminar capillary flow. The migration distance from the adhesive bead into the medium is proportional to the square root of the pore radius, the adhesive surface tension, the cosine of the contact angle, and the time of molten contact, divided by the adhesive viscosity. Because the penetration distance is proportional to the square root of time, the first few tenths of a second after adhesive application are the most critical for migration control. In a porous medium with a mean flow pore diameter of 10 µm to 40 µm, capillary pressure can exceed 0.1 bar when the contact angle is below 60°, which is common for polyamide hot melts on polyester fibre surfaces. The only quantitative prediction possible on a production line is the ratio of the adhesive melt viscosity to the sheath melt viscosity at the same shear rate. To block wicking, the sheath must flow into the medium at a lower viscosity than the adhesive during its own activation step, filling the surface pores before the adhesive arrives. The adhesive may then remelt only the outermost surface of the sheath film because the fused sheath and the core remain below the adhesive application temperature. The barrier function therefore does not depend on the sheath being more viscous than the adhesive at the moment of adhesive application; it depends on the sheath having already formed a solid or semi-solid continuous film after cooling from its own fusion step.

Typical sheath melt viscosities of 80 Pa·s to 250 Pa·s at 100 s⁻¹ and 200 °C are appropriate for media with mean flow pore diameters from 10 µm to 40 µm, while the adhesive viscosity at those same shear rates remains 40 Pa·s to 120 Pa·s for polyamide and 30 Pa·s to 100 Pa·s for polyolefin. That places the adhesive lower in viscosity than the sheath at the moment of contact, which is contrary to the often-assumed requirement that a more viscous adhesive would migrate less. The important variable is not the instantaneous adhesive viscosity but the permeability of the surface layer that the sheath has left behind. A continuous fused film across the pleat apex reduces the effective pore radius below the critical value at which capillary pressure can overcome viscous resistance. However, if the sheath film is discontinuous or cracked, the adhesive will find low-resistance pathways through the cracks. This is why filament eccentricity must be controlled. A sheath layer with a thickness variation of more than 15 % of the filament diameter can create thin sections that fail to fill the medium surface, leaving channel openings. In production, the target viscosity mismatch between the sheath and the adhesive is often set so that the sheath viscosity during activation is 2 to 5 times higher than the adhesive viscosity during application, but the fused film rather than the instantaneous viscosity is the dominant barrier mechanism.

Polyamide hot-melt adhesives based on dimer fatty acids and aliphatic diamines exhibit an amine value between 0.5 mg KOH/g and 5 mg KOH/g and are applied from 180 °C to 210 °C. Polyolefin hot-melt adhesives, which are typically metallocene-catalyzed polyethylene or polypropylene copolymers, are applied from 150 °C to 180 °C and possess lower surface energy but can be formulated with tackifying resins that raise the acid number. The copolyester sheath has a surface energy between 40 mN/m and 45 mN/m, closer to the polyamide adhesive than to the polyolefin system. This promotes interdiffusion across the sheath-adhesive interface but does not guarantee chemical interlocking. Incompatible systems containing high amine concentrations and residual moisture can generate voiding at the interface when the adhesive is held above 200 °C for more than 30 min. Therefore, when polyamide adhesives are used, the filament sheath should not be treated with amine-based antistats or lubricants; stearate-based or ethoxylated ester finishes are preferred. The filament finish level must be below 0.5 % by weight to avoid forming a low-molecular-weight interlayer that reduces sheath-to-medium bonding. The selection of the adhesive and the filament sheath should include a compatibility check against the supplier’s technical data sheet for amine value, acid number, and melt viscosity at the application shear rate.

When Copolyester Sheath Thickness Drops Below 20 µm, Adhesive Strike-Through Escalates

The continuous film that forms at the pleat apex depends on sheath thickness, pleat apex geometry, calender pressure, and dwell time. For a filament diameter of 0.25 mm to 0.60 mm, a sheath thickness below 20 µm is insufficient to fill the surface pores of a glass-fibre medium with a mean flow pore size of 25 µm to 35 µm. A sheath thickness of 30 µm to 80 µm is generally required to produce a continuous fused band across the pleat tip without over-saturating the medium. The sheath thickness is not directly controlled on the spinning line unless the crosshead die is fitted with independent melt pumps and annular gap sensors. When the sheath percentage is calculated only from total filament weight and core weight, a variation of ±4 % in sheath content can remain undetected and still produce a critical change in barrier performance. This is because the sheath layer thickness is disproportionately affected by small changes in outer diameter when the core diameter is fixed. A reduction of 10 µm in outer diameter at a constant core diameter can reduce the sheath thickness by more than 25 %, depending on the geometry. Such dimensional drift is often caused by spinneret hole wear, melt pressure fluctuation, or changes in draw ratio due to godet temperature drift. The resulting thin sheath layer fractures under the calender nip or fails to fill the medium pores, and the hot-melt adhesive then penetrates through the void spaces. In extreme cases, adhesive strike-through can extend 3 mm to 8 mm into the pleat flank, covering a significant fraction of the effective filtration area.

On production-scale rotary pleating lines running at 20 m/min to 60 m/min, the filament is registered to the pleat apex by a profiled roller before the hot-melt adhesive nozzle. Dwell time under the heated calender or hot-air slot is typically 0.2 s to 1.2 s. If the sheath does not flow sufficiently during this period, the subsequent adhesive bead penetrates the unfilled media pores and can travel along the fibre bundles. This penetration is not visible in a post-filter bubble point test unless the adhesive covers more than 15 % of the pleat area. Batch-to-batch variance in filament sheath percentage can be as high as ±4 % absolute when the sheath extruder screw speed control has an unstable melt pressure due to regrind content. When the sheath percentage drops, the barrier disappears and the pressure drop at rated flow increases by 10 % to 20 % without any change in the clean element weight. The operational boundary for pre-drying is also critical: at a relative humidity above 60 %, undried sheath copolyester can absorb more than 0.3 % moisture in 30 min, producing hydrolysis during extrusion and localized film defects. The sheath film must also be free of low-molecular-weight oligomer bloom, which can create weak boundary layers. Oligomer extraction tests using xylene or tetrahydrofuran according to the fibre supplier’s method should show less than 0.5 % extractable matter after spinning.

Quantifying the Barrier Under Differential Pressure and Thermal Reflow

The barrier efficiency of the fused sheath is evaluated by comparing the initial bubble point and multipass capacity of the pleated element before and after adhesive application. ISO 2942 defines the first bubble point as the pressure at which the first gas bubble emerges through the wetted medium; a reduction of more than 15 % after adhesive application indicates excessive adhesive penetration or media compaction. ISO 2941 specifies collapse/burst resistance; the pleated pack must withstand the rated pressure without cracking the fused sheath or delaminating from the core. ISO 16889 multipass testing provides the most direct evidence of filtration area loss because a migrated adhesive band reduces the effective media area and increases the pressure drop at the same particle loading. The test sequence should include a reference element without adhesive to separate the effect of pleating from the effect of adhesive migration. A capacity retention of at least 90 % relative to the unbonded reference is typically required for hydraulic filtration applications. The test flow rate is set according to the element specification and may range from 0.5 L/min to 5.0 L/min per 100 cm² of media area. Thermal reflow testing after adhesive application can be performed by cycling the element to 100 °C for 1 h and re-measuring bubble point and pressure drop; a shift of more than 5 % suggests that the sheath-adhesive interface is not dimensionally stable under service temperature.

Compliance with food-contact or potable-water requirements introduces additional constraints. The copolyester sheath must meet EU Regulation 10/2011 when used in water contact; for repeat-use components, the worst-case contact time and temperature should be simulated according to the conditions specified in this regulation. In the United States, the polyester resins must comply with 21 CFR 177.1630 or an applicable food-contact notification. When the filter element is used in industrial hydraulic applications, REACH Article 33 requires communication of any candidate list substance above 0.1 % w/w in the filament or adhesive. RoHS restrictions apply only to electrical and electronic equipment and are generally not a direct limitation for filtration media, but lead-free adhesive pigments and cadmium-free sheath colorants are specified in some customer requirement matrices. The finished element must also be tested for extractable particulate matter if it is used in high-purity water or pharmaceutical applications; a rinse volume of at least 10 L per 10-inch cartridge is commonly specified before the filter is released.

Processing Window of ±5 °C During Heated Calender Activation

The temperature control loop on a heated calender must maintain the sheath surface at a setpoint within ±5 °C because the copolyester melting range is broad but the viscosity drop is steep. At a setpoint of 140 °C, a 5 °C upward drift can reduce the sheath viscosity by as much as 30 % to 50 %, depending on the comonomer content; the sheath may over-penetrate the medium and begin to coat the backside. A 5 °C downward drift may leave the sheath below its flow point, permitting the hot-melt adhesive to strike through between the filament and the medium. Temperature uniformity across the nip is measured with a contact thermocouple traversing the roll width; a maximum deviation of ±3 °C is specified for rolls wider than 1.2 m. The calender gap is set to 0.02 mm to 0.08 mm less than the filament diameter to generate sufficient pressure for sheath flow without crushing the core. Line speed, gap, and temperature are interdependent; increasing line speed from 20 m/min to 60 m/min requires an increase in calender setpoint of 3 °C to 8 °C to maintain the same sheath surface temperature because of reduced residence time. On equipment without automatic gap compensation, roll thermal expansion can reduce the effective gap by 0.01 mm to 0.03 mm during warm-up, which must be accounted for before the line is started.

Production-scale failure signatures include a visible adhesive stripe on the downstream side of the pleat pack after the adhesive has cooled, an increase of 0.08 bar to 0.25 bar in clean differential pressure at rated flow, and a reduction of 20 % or more in first bubble point measured according to ISO 2942. When these signatures appear, the sheath fusion profile is interrogated before the adhesive formulation is changed; line audits typically reveal a worn temperature sensor, an incorrect gap setting on the calender, or a change in filament lot with a lower sheath percentage. The failure mode is mechanical and rheological, not adhesive chemistry. Corrective action consists of resampling the filament, verifying sheath percentage by differential scanning calorimetry or selective dissolution, and re-establishing the calender setpoint profile. No corrective action should be based on a single test point because the pleat-pack pressure drop is sensitive to the number of bridged pleats; a minimum of 5 elements per lot should be dissected and inspected at 10× magnification to quantify the adhesive strike-through depth.