FIRST Normal Series EVA Film F406PS (UV transmittance)

F406PS is an ethylene-vinyl acetate film supplied in the FIRST Normal Series with a specified UV transmittance parameter. The numerical transmittance value is measured at incoming inspection on 0.45 mm raw film under ASTM E903 across the 300–400 nm band. Because the film is a thermoset encapsulant, its optical path length and refractive index change after peroxide crosslinking. UV transmittance therefore must be verified on cured laminates as well as raw rolls. The following downstream scenarios differ in glass type, lamination temperature, gel content target, and terminal certification.

Lamination Stack Position and Crosslinking Baseline in Crystalline Silicon Modules

Conventional backsheet-based crystalline silicon modules are built in the layup sequence low-iron tempered front glass / F406PS / cell strings / F406PS / backsheet. Front-side F406PS is typically supplied at 0.45 mm or 0.50 mm; rear-side film may be the same caliper. The film mass per square metre is determined by density, which is close to 0.95 g/cm³ after melt processing, giving approximately 428–475 g/m² for a nominal 0.45 mm film. Lamination is carried out in a vacuum diaphragm laminator with heated platens. The platen setpoint is held between 145°C and 155°C, and the vacuum chamber is evacuated to 0.1–1.0 mbar before diaphragm pressure is applied at 800–1000 mbar relative to atmosphere. Total cycle time for this film class is 15–22 min; the exact dwell time is adjusted to the glass thickness and the number of modules loaded per laminator opening. Crosslinking is initiated by thermal decomposition of a peroxide curing agent. After lamination, gel content is checked by solvent extraction under ASTM D2765-16; a typical acceptance window is 75–85%. Undercure below 75% produces poor shear creep resistance and increased acetic acid release after damp heat. Overcure above 85% embrittles the encapsulant and raises the risk of cell microcrack propagation during thermal cycling.

The UV transmittance reading of F406PS is relevant because a high UV-A flux passing through the front glass and encapsulant can reach the cell surface, the backsheet, and the rear encapsulant layer. Modules using UV-transparent F406PS must therefore pass the UV preconditioning sequence of IEC 61215-2:2021, which applies 15 kWh/m² of UV-A at 60±5°C. Low-iron PV glass attenuates heavily below 320 nm, so the measurable UV transmittance through a full laminate is dominated by the 320–400 nm band. Incoming QC should not overlook lot-to-lot variation in UV transmittance because stabilizer package adjustments can shift the 380 nm value by several percentage points. Terminal products are utility, commercial, and residential monocrystalline and multicrystalline modules certified to IEC 61215 and IEC 61730. For North American markets, UL 1703 listing is the relevant safety certification. A common manufacturing bottleneck is platen temperature non-uniformity; a variation of more than ±2°C across the platen causes edge gel content to differ from centre gel content, producing visible flow marks and peel adhesion differences after damp-heat exposure. Incoming film with a melt flow index in the class range of 20–30 g/10 min at 190°C/2.16 kg under ISO 1133-1:2022 flows readily into string gaps, but excessive flow at high temperature can push encapsulant beyond the module edge and contaminate the laminator belt.

In glass-glass bifacial construction, F406PS is used on both sides of the cell strings with transparent rear glass instead of a polymer backsheet. The stack is anti-reflective tempered front glass / F406PS / cell string / F406PS / rear glass, with each F406PS layer at 0.40–0.50 mm and glass panes commonly at 2.0 mm or 2.5 mm. Lamination uses a vacuum diaphragm laminator at 148–155°C for 18–22 min. Because the rear glass is impermeable to moisture, acetic acid produced by EVA hydrolysis cannot diffuse out as easily as in backsheet modules. This makes UV transmittance an important degradation variable: if both the front glass and F406PS allow high UV-A transmission, UV reaches the rear EVA layer and the cell rear side. Gel content in glass-glass builds is therefore controlled toward the upper end of the cure window, commonly 80–88%, to limit chain mobility and corrosion by acetic acid. Edge sealants such as butyl are used to prevent moisture ingress at the module perimeter. The terminal product is a bifacial utility module rated under IEC 61215 and measured for rear-side current gain under IEC 60904-1-2. Potential-induced degradation is evaluated under IEC 62804. A process conflict specific to glass-glass lamination is trapped gas between the glass sheets. If vacuum level is not held below 1 mbar before membrane pressure is applied, edge voids form near the cell busbars and remain visible after cure.

What Changes When F406PS Replaces Standard UV-Cut Encapsulant in Thin-Film Lamination?

Thin-film CdTe and CIGS sub-modules are laminated on monolithic glass substrates with thermally sensitive layers. The allowed platen temperature is usually lower than for crystalline silicon modules, typically 130–140°C, with dwell extended to 22–30 min. At 135°C, peroxide decomposition in EVA is slower; gel content may plateau at 70–78%. If the module design requires gel content above 80%, lamination time increases beyond 30 min, which can exceed the thermal budget of the thin-film junction. The flow of F406PS at 130–135°C is sufficient to fill laser scribe lines and edge deletion zones, but not excessive. If platen temperature exceeds 140°C, low-viscosity flow can cause encapsulant bleed beyond the glass edge and deposit degraded EVA on the belt. UV transmittance through F406PS is measured before lamination because thin-film modules are subjected to UV preconditioning with 15 kWh/m² UV-A as part of IEC 61215 qualification. A UV-transparent encapsulant combined with UV-transmissive front glass increases the UV dose at the cell layer. The module must therefore pass the sequential UV and damp-heat exposure without developing shunts or delamination at the scribe lines. Published data for F406PS in CdTe/CIGS stacks at the low-temperature end of this window is limited; lamination curves must be verified on actual module coupons before volume release. Terminal applications are CdTe and CIGS building-integrated or ground-mount modules certified to IEC 61215 and IEC 61730.

Building-Integrated Photovoltaic Glazing Requirements Are Not Met by Luminous Transmittance Alone

BIPV modules are laminated as safety glass and must satisfy both PV certification and construction product requirements. A typical stack is tempered or heat-strengthened outer glass / F406PS / PV cells / F406PS / inner glass, with total build thickness between 6.0 mm and 12.0 mm. Lamination is carried out at 140–150°C for 25–35 min in a vacuum bag or fixed-chamber laminator. The longer dwell compared with backsheet PV is needed because the extra glass mass delays heat-up and because edge gaps in the cell layout require uniform flow. UV transmittance is not a substitute for luminous transmittance or colour rendering. A BIPV facade element may transmit 90% visible light but still fail the project UV-control specification if the combined glass and EVA stack transmits too much UV-A below 380 nm. Safety performance is evaluated by impact testing under EN 12600; laminated glass classification follows EN ISO 12543-2. For the North American market, ANSI Z97.1 and ASTM C1172 apply. Weathering durability is checked by exposing the laminate to 1000 h of ASTM G154 cycle 1 and measuring yellowness index under ASTM E313. A stable EVA-glass bond after water immersion is also required. The main processing conflict is controlling adhesion at the cell solder ribbons. Thick glass laminates can retain heat at the centre; if the centre temperature exceeds 155°C, the EVA around the ribbons can overcure and lose flexibility. Terminal products are facades, canopies, skylights, and balustrade elements with embedded PV.

Non-active laminated safety glass fabricated with F406PS as the interlayer is used in interior partitions, skylight glazing, and display panels where UV transmittance is deliberately retained. The build commonly pairs two annealed glass panes of 4 mm or 6 mm with a 0.38 mm F406PS interlayer. Lamination is performed in a vacuum bag oven at 135–145°C with a dwell of 30–40 min after the glass reaches temperature. The finished laminate is measured for luminous transmittance under ISO 9050 or ASTM D1003, with haze typically below 1.5%. UV transmittance in the 320–380 nm band through the full laminate is controlled by the glass and the F406PS stabilizer package. In UV-transparent configurations, the laminate may transmit 80–85% of UV-A when low-iron, UV-A-transmissive glass is used. This is relevant for daylighting studies, museum colour-rendering specifications, and spaces where full-spectrum lighting is required. Safety classification is verified under EN 12600, ANSI Z97.1, and CPSC 16 CFR 1201. The EVA interlayer is not a structural load-bearing material; where post-breakage stiffness is mandated for overhead glazing with high snow load, ionoplast interlayers replace EVA. The principal limitation is moisture sensitivity at the exposed edge. Unprotected edges can reduce peel adhesion below the supplier's control limit after repeated condensation, so a perimeter seal is required for exterior use.

When UV Transmittance Is Deliberately Retained for Agrivoltaic Canopy Modules

Agrivoltaic canopies place PV modules above crops, and the spectral distribution of transmitted light becomes an operational parameter. F406PS is used in the module stack with rear transparent glass or transparent backsheet when the project requires UV-A flux below the array. The layup is low-iron front glass / F406PS / cell strings with controlled spacing / F406PS / transparent cover. Cell pitch is widened to allow light between cells; EVA film thickness is kept at 0.45 mm to control weight. Lamination follows the backsheet or glass-glass process window depending on the rear cover, typically 145–150°C for 18–24 min. UV transmittance through the module is measured with a spectroradiometer at the project level rather than assumed from raw film values. The module must still pass IEC 61215 and IEC 61730. High UV-A transmittance increases the UV dose to the EVA itself, so batch acceptance should include UV preconditioning and damp-heat testing beyond the minimum 15 kWh/m² UV-A exposure. The terminal product is a canopy module for row crops, vineyards, and shade-tolerant agricultural systems. A field-specific failure mode is edge yellowing caused by UV reflection from crop foliage onto the rear side; rear UV transmittance should be modelled with ground reflectance data before specifying a UV-transparent backsheet. If the crop light model does not require UV-A, a UV-cut encapsulant or UV-blocking glass should be used instead.