Release force stability in silicone-coated coextruded biaxially oriented poly(ethylene terephthalate) facestock is evaluated as the change in 180° peel force required to separate a pressure-sensitive adhesive laminate from the silicone surface after defined ageing intervals. Coextruded BOPET facestock is manufactured by coextrusion of a poly(ethylene terephthalate) core with skin layers comprising copolyester, amorphous poly(ethylene terephthalate), or particle-modified poly(ethylene terephthalate) compositions; total film thickness typically ranges from 12 µm to 75 µm, while skin layers account for 1 µm to 5 µm per side. The skin layer facing the silicone coating is engineered for anchorage, surface roughness, and oligomer control, but its composition also creates an interphase in which low-molar-mass cyclic and linear oligomers, humidity-driven hydrolysis products, and migrated antiblock particles can alter crosslink density and release force. Release force stability is therefore not a single coating property but a system response that includes substrate surface chemistry, silicone cure kinetics, coat weight uniformity, adhesive chemistry, ageing history, and mechanical handling.

Test values are most commonly obtained under FINAT FTM 4 using a TESA 7475 acrylic test tape applied at 20 °C and 50 % RH, conditioned for 24 h and 1 week, and pulled at 180° with a constant separation speed of 300 mm/min. For cross-validation with North American specifications, ASTM D3330/D3330M-04(2018) Test Method A is referenced when a 180° peel geometry is required; ISO 8510-2:2007 provides a flexible-to-flexible 180° peel method for comparative failure-mode analysis. Acceptance bands are application-specific: medical processing liners commonly require release force in the range of 8 cN/25 mm to 30 cN/25 mm, while precision electronics die-cut liners may require 3 cN/25 mm to 15 cN/25 mm. Stability is assessed as the absolute change and percentage drift between 24 h and designated ageing endpoints at 40 °C, 50 °C, or 50 °C/85 % RH.

Why Does Post-Cure Release Force Drift Divergence Between Tin-Catalyzed Condensation and Platinum-Catalyzed Addition Systems on Coextruded BOPET Exceed 25 cN/25 mm After 30 Days?

The divergence arises from differences in siloxane network formation and residual reactive species. Tin-catalyzed condensation systems cure through hydrolysis of acetoxy, alkoxy, or oxime-functional silanes, releasing condensation by-products and retaining tin catalyst residues in the crosslinked film. On a coextruded BOPET skin, the retained catalyst can accelerate post-cure condensation when ambient moisture diffuses through the film or through the adhesive laminate. This produces a progressive increase in crosslink density and a corresponding release force climb from an initial value of 6 cN/25 mm to values above 30 cN/25 mm after 30 days at 40 °C. Platinum-catalyzed addition-cure systems proceed through hydrosilylation of vinyl-functional siloxanes with hydride-functional siloxanes and generate no volatile condensation by-products; the post-cure network approaches equilibrium rapidly when the Si-H conversion is complete. The residual platinum catalyst does not significantly reshuffle the network unless unsaturated adhesive components or moisture-sensitive moieties migrate into the coating.

On coextruded BOPET, interfacial inhibition is a separate source of drift. Platinum-catalyzed systems are sensitive to nitrogen-containing slip additives, sulfur-containing stabilizers, and residual amine-functional primers that may be present in the skin layer or may bloom from the core. When inhibition occurs, the Si-H conversion is incomplete, the extractable fraction increases, and the initial release force may be low but rises as uncured siloxane fractions migrate into the adhesive or as the adhesive plasticizes the under-cured network. The test response is a bimodal release profile: peel force is high at very low peel speeds due to cohesive deformation of the under-cured silicone, then falls at high speed, and after ageing the failure mode shifts to adhesive slip-stick. Published data for the exact drift magnitude in inhibited layers is limited because commercial formulations vary; however, quality-control measurements of extractable silicone fraction by solvent extraction and X-ray fluorescence coat weight before and after extraction are used to detect under-cure.

The cure window for solventless platinum systems is narrow on coextruded BOPET. Web temperatures must remain between 120 °C and 150 °C for 10 s to 30 s, depending on oven length and line speed. At temperatures below 120 °C, hydrosilylation conversion is incomplete and release force drift increases; above 150 °C, coextruded BOPET undergoes thermal shrinkage and the skin layer can re-crystallize, reducing anchorage. The processing window is therefore ≤ ±5 °C for many 23 µm and 36 µm facestocks. On production-scale multi-roll offset gravure coaters with 5 m to 7 m hot-air ovens and line speeds of 150 m/min to 400 m/min, web temperature is monitored by infrared pyrometry and calibrated against a thermocouple-in-contact reference. A temperature deviation of 5 °C across the web can generate release force differences of 5 cN/25 mm to 12 cN/25 mm between edge and center positions after 1 week at 50 °C.

Accelerated ageing at 50 °C and 85 % RH of a solventless platinum-cured silicone coating on a chemically primed coextruded BOPET facestock with 0.12 % by mass silica antiblock in the skin layer is used to separate hydrolysis-driven release force drift from purely thermal drift. Water vapour permeates the film from the uncoated side and accumulates at the silicone–PET interphase, where it attacks ester linkages and generates oligomeric terephthalic acid-hydroxyethyl ester species. These hydrolysis products plasticize the primer and lower interfacial adhesion to the silicone network, causing mixed adhesive–cohesive failure and measurable silicon transfer to the pressure-sensitive adhesive. Dry-heat ageing at 50 °C does not reproduce this mechanism; humidity ageing is therefore mandatory for release liners stored in non-breathable pouches. Published data for the exact drift magnitude in this specific configuration is limited, but industrial control limits for medical-grade laminates commonly restrict release force drift to ±8 cN/25 mm relative to the 24 h value over 14 days at 50 °C/85 % RH. Rolls stored at an ambient relative humidity above 60 % are pre-dried at 120 °C for 10 min before coating to reduce hydrolytic degradation during the cure oven.

Oligomer Bloom and Surface Energy Decay at the Coextruded Skin–Silicone Interphase

Cyclic trimer, tetramer, and pentamer oligomers are intrinsic to poly(ethylene terephthalate). Coextruded BOPET facestock can exhibit oligomer bloom when thermal history, stretching, and skin layer composition create a thermodynamic driving force for low-molar-mass species to migrate to the film surface. A skin layer with reduced crystallinity or a higher copolyester content may have greater oligomer mobility than the core. When a silicone release coating is applied, bloomed oligomers are trapped at the coating interface. Over time, the oligomers can re-enter the crosslinked silicone matrix, diffuse along grain boundaries, or concentrate at the silicone–adhesive interface. The result is a release force increase because the oligomer-rich interlayer behaves as a weak boundary layer that fails cohesively at low peel energies. Surface energy measurements before coating are carried out according to ISO 19403-2:2017; the target wetting tension for a chemically primed coextruded BOPET surface before silicone coating is generally between 38 mN/m and 46 mN/m. Values below this range may indicate excessive oligomer bloom or inadequate corona treatment; values above 50 mN/m may indicate over-treatment that increases surface carboxyl species and can interfere with platinum-catalyzed cure.

Release force stability is also affected by surface energy decay after coating. A well-cured polydimethylsiloxane surface has a dispersive surface energy near 20 mN/m to 22 mN/m. If oligomer bloom continues after coating, the surface becomes contaminated with PET-like domains, increasing the polar component and raising the release force. The magnitude of the change can be monitored by contact angle hysteresis; an increase in hysteresis from 5° to 15° or greater often corresponds to a release force drift above 10 cN/25 mm under FINAT FTM 4. Routine quality-control methods include extraction of surface oligomers with xylene or chloroform followed by gravimetric analysis; values above 0.5 % by mass of facestock are considered high for critical die-cut release liner applications, although published data for this specific configuration is limited.

When Solventless Silicone Coat Weights Fall Below 1.0 g/m² on High-Slip Coextruded Skin Layers

Solventless platinum-catalyzed silicone systems on high-slip coextruded BOPET skin layers can show a discontinuous loss of release force stability when the dry coat weight falls below 1.0 g/m². At coat weights below this threshold, the silicone layer may not cover peaks in the surface roughness profile created by antiblock particles or may fail to isolate low-molar-mass slip additives that bloom from the high-slip skin. The resulting surface consists of micro-domains of exposed or thinly covered poly(ethylene terephthalate), which produce high local peel forces and variable release. Peel testing under FINAT FTM 4 then shows a high standard deviation: release force values may range from 8 cN/25 mm to 45 cN/25 mm within the same roll. Above 1.2 g/m², a continuous crosslinked silicone network typically forms, and the release force becomes uniform within ±2 cN/25 mm. The exact threshold depends on skin roughness; high-slip facestocks with a coefficient of friction below 0.30 as measured by ISO 8295:1995 often contain sufficient migrating slip additive to require a coat weight at the upper end of the 1.0 g/m² to 1.4 g/m² range.

The interfacial failure mode changes at low coat weight. Instead of clean adhesive release from the silicone, the peel front oscillates between adhesive–silicone separation and silicone–facestock delamination. Silicone transfer to the adhesive increases, and subsequent adhesion values measured by FINAT FTM 11 may fall below 80 % of the control value. On a production-scale five-roll offset gravure coater, maintaining a coat weight of 1.1 g/m² to 1.3 g/m² is therefore standard for high-slip coextruded BOPET facestocks intended for stable release. Lower coat weights may be acceptable on smooth, chemically primed surfaces with a skin roughness Rz below 1 µm, but the process window becomes narrow and requires X-ray fluorescence coat weight monitoring at a frequency of at least one measurement per 500 linear metres.

Accelerated Ageing Test Matrix and Release Force Acceptance Bands for Medical and Electronics Die-Cut Liners

Qualification of a coextruded BOPET release liner for medical or electronics die-cut applications requires a matrix of ambient, dry-heat, humid-heat, and mechanical stress tests. Release force is recorded at 24 h after lamination and after each ageing interval. The test methods are selected to capture different drift mechanisms: ambient ageing detects slow post-cure network equilibration; dry-heat ageing at 50 °C detects thermal oligomer bloom and adhesive component migration; humid-heat ageing at 50 °C/85 % RH detects hydrolysis-driven anchorage loss; and rotary die-cutting simulation detects mechanical under-cure or silica transfer. The table below summarizes the test matrix.

Acceptance bands are application-specific and are not prescribed globally; therefore, the following bands are compiled from industrial qualification protocols rather than a single standard. Medical processing liners commonly require a 24 h release force of 8 cN/25 mm to 30 cN/25 mm, with a drift limit of ±8 cN/25 mm after 14 days at 50 °C. Electronics die-cut liners may require 3 cN/25 mm to 15 cN/25 mm, with a drift limit of ±4 cN/25 mm after 14 days at 50 °C/85 % RH. A subsequent adhesion ratio below 80 % or a silicone extractables value above 0.5 % by mass is typically cause for rejection. Published data for a specific customer laminate is limited; qualification always uses the production adhesive and liner combination because adhesive rheology and tackifier migration modify release force stability.

High-speed rotary die-cutting of 23 µm and 36 µm coextruded BOPET release liners running at 150 m/min generates repeated bending, compressive stress, and frictional heating at the cutting station. The silicone coating is subjected to micro-cracking if the facestock is too brittle or if the coating is over-cured and glassy. Micro-cracks provide pathways for adhesive migration to the poly(ethylene terephthalate) surface and create local release force spikes. Release force stability after die-cutting is evaluated by comparing peel force before and after a defined number of die-cutting cycles; a stable liner typically shows a drift of less than 3 cN/25 mm after 10,000 cycles. No harmonized standard governs this die-cut cycle test; the method is derived from FINAT FTM 4 release force measurement before and after the die-cutting operation. The coating must also withstand sheet-fed laser die-cutting in electronics applications where localized thermal exposure can exceed 120 °C for 5 s; a coextruded BOPET skin containing a low-melting copolyester component may soften at these temperatures and allow the silicone to delaminate, causing a sharp rise in release force and silicone transfer. Therefore, the selection of the coextruded skin layer must balance corona adhesion, oligomer control, and thermal resistance.

Gamma Sterilization Shifts Release Force Stability Only When the Skin Layer Contains Radical-Sensitive Species

Batch-to-batch variation in coextruded BOPET skin layer formulation is a field-observed source of release force instability in medical device release liners. The skin layer may contain different concentrations of silica, calcium carbonate, or crosslinked polymeric antiblock particles depending on the film manufacturer and production campaign. These particles alter the topography of the silicone coating and can nucleate micro-failures during peel. Gamma sterilization at a typical dose of 25 kGy to 40 kGy induces free-radical reactions in both poly(ethylene terephthalate) and silicone. In the silicone network, gamma irradiation can cause crosslinking and chain scission; the net effect on release force depends on dose and the presence of antioxidants or radical scavengers in the facestock and adhesive. On coextruded BOPET, the skin layer may preferentially generate oxygenated species that increase surface energy and raise release force. Post-sterilization release force drift can be as high as 15 cN/25 mm when the liner is stored for 12 months at ambient temperature, but published data for this specific configuration is limited because commercial sterilization validation data are confidential.

To control batch-to-batch variation, incoming film rolls are tested for surface energy, roughness, coefficient of friction, and oligomer extractables before coating. A skin layer with a wetting tension below 38 mN/m or an oligomer extractables value above 0.5 % by mass is rejected for critical medical applications. Silicone coat weight is maintained at 1.1 g/m² to 1.3 g/m² for gamma-stable release liners. The cured silicone is then evaluated under FINAT FTM 4 before and after gamma sterilization; a drift of more than 10 cN/25 mm or a subsequent adhesion ratio below 85 % triggers a full failure analysis. The absence of a single harmonized standard for release force stability means that the test matrix must be agreed between film supplier, silicone supplier, converter, and device manufacturer.