KENGO ST EVA Film

KENGO ST EVA Film is a peroxide-curable ethylene-vinyl acetate encapsulant supplied as a continuous cast film for photovoltaic module lamination. The model suffix ST identifies the fast-cure grade within the KENGO EVA Film portfolio and is specified for both glass/backsheet and glass/glass crystalline silicon architectures. Commercial supply formats include nominal thicknesses of 0.45 mm, 0.50 mm, and 0.65 mm, with roll widths up to 2200 mm, a thickness tolerance of ±10%, and a width tolerance of ±2 mm. The uncured film is single-side embossed with an average surface roughness of Ra 3–8 µm measured by ISO 4287:1997, which provides interlaminar air evacuation during vacuum lamination. The resin composition is formulated to a vinyl acetate content of 28–33 wt% with a peroxide crosslinking package; after lamination, the cured encapsulant develops a gel content above 80% when measured by ASTM D2765-16. Melt flow rate of the uncured film is typically 20–35 g/10 min at 190 °C and 2.16 kg per ISO 1133-1:2022. The film is positioned between the glass, cell string, and backsheet, where silane coupling chemistry provides adhesion to glass and hot-melt bonding provides adhesion to the backsheet. The ST formulation is selected when a laminator must operate with shorter dwell times than a standard-cure EVA grade without switching to a fundamentally different encapsulant chemistry.

What Processing Window Does the ST Formulation Require on a Vacuum Laminator?

On a three-chamber vacuum laminator with a usable platen area of 2.2 m × 1.1 m and silicone membrane pressure maintained at 100–110 kPa, the fast-cure formulation permits a total cycle of 10–12 min for a 0.45 mm film in a glass/backsheet stack when the platen setpoint is 145–155 °C. The same cycle extends to 12–15 min for a 0.65 mm film because the thicker encapsulant acts as a thermal resistor between the glass and cell plane. Crosslinking accelerates with platen temperature; below 140 °C, gel content determined by ASTM D2765-16 is frequently below 70%, which is insufficient for creep resistance and damp-heat adhesion retention in IEC 61215-2:2021 qualification. Above 160 °C, the peroxide decomposes too rapidly, producing volatile decomposition products faster than the vacuum step can remove them, with bubble formation at cell ribbon crossings and edge regions. The practical processing window is therefore ±5 °C around the recommended platen setpoint, and platen uniformity should be verified with a multi-point contact thermocouple array to maintain a temperature spread below ±2 °C. Degassing time before membrane pressurization should be no less than 5 min; insufficient degassing produces void clustering at the busbar interface.

The vulcanization kinetics of the ST grade are shifted to reduce time-at-temperature above the peroxide decomposition onset. During cure at 150 °C, oscillatory rheometry at 1 Hz shows an initial storage modulus below 1×10^5 Pa for the molten film; after 6–8 min, the modulus crosses 1×10^6 Pa and reaches a plateau when gel content reaches 80%. The gel point, defined by the crossover of storage and loss modulus, occurs at approximately 50–60% of the total cure time under the recommended platen temperature. This viscoelastic transition is process-relevant because cell string movement is prevented only after gelation; if membrane pressure is applied too early, cell shifting and ribbon deformation occur. Peak cure exotherm by differential scanning calorimetry at 10 °C/min is 15–25 W/g, measured under ASTM E2160-04. Published data for a specific laminator model and backsheet combination is limited; final cycle time must be confirmed by gel content and peel testing on the target line.

In cast film production, the compounded EVA resin is plastified in a single-screw extruder with a 75–90 mm screw diameter and L/D 30:1, using barrel and adaptor zones held between 80 °C and 110 °C and a slot die maintained at 90–120 °C. Compounding upstream of the film line may use a co-rotating twin-screw extruder with L/D 44:1 and a screw speed of 200–400 rpm to disperse the peroxide masterbatch and stabilizers without local hot spots that would trigger premature decomposition. The melt exits the die into a chilled roll stack at 15–25 °C; rewind tension is maintained at 10–25 N/m. Winding tension above 30 N/m has been observed to create roll blocking and surface imprinting that later appear as laminate voids under ASTM D1003-21 transmitted-light inspection. Cross-web thickness variation on a 2200 mm web should remain below ±5%; larger variation produces edge squeeze-out during lamination. Slitting in a cleanroom converting step uses rotary blade gaps of 0.05–0.10 mm; edge burrs produced at larger gaps are a recurring source of delamination initiation because the jagged film edge prevents complete vacuum withdrawal at the laminate perimeter.

For glass/glass modules, the same ST film is used in either a two-step vacuum bag process or a single-chamber laminator with edge clamps. The absence of a flexible backsheet changes the pressure distribution; membrane pressure must be reduced to 80–95 kPa for the first 3–4 min until the edges are sealed, then raised to 100–110 kPa for the curing plateau. If full pressure is applied too early, the encapsulant can be squeezed out at the perimeter and cell strings can shift by more than 1 mm. Edge-squeeze-out weights above 3 g/m are associated with visible edge voids and reduced insulation resistance in the module frame after IEC 61730-1:2021 wet insulation testing.

Optical Performance, Adhesion Retention, and Electrical Isolation Data

The cured encapsulant is tested as a laminate rather than as a free film. Luminous transmittance through a glass/encapsulant/glass stack is typically above 91% for a 0.45 mm film and above 90% for a 0.65 mm film when measured with an integrating sphere according to ASTM D1003-21; haze after lamination is below 5%. Ultraviolet screening is obtained from the glass and from stabilizer loading in the film; total hemispherical transmittance of the laminate stack per ASTM E903-20 is typically above 89%. The UV cut-on wavelength for 50% transmittance is 360–380 nm. Yellowing index measured by ASTM E313-20 remains below 2.0 after 1000 h damp heat at 85 °C/85 % RH under IEC 61215-2:2021. Peel adhesion to glass, measured by ASTM D903-98(2017) with a 10 mm strip, remains above 60 N/cm after the same damp-heat exposure. Volume resistivity at 500 V DC is above 1×10^14 Ω·cm per IEC 62788-1-2:2020, sufficient for electrical isolation in crystalline silicon modules when combined with the backsheet. The following comparison is based on typical values for commercially available fast-cure EVA, standard-cure EVA, and polyolefin encapsulant films; final qualification values are construction-dependent.

Because the polymer backbone is vinyl acetate, hydrolytic degradation under damp heat releases acetic acid. The rate is reduced in the ST formulation by the lower time-at-temperature during lamination and by the stabilizer package, but the mechanism is not eliminated. In accelerated testing at 85 °C/85 % RH, front-side glass adhesion remains above 60 N/cm for 1000 h, but modules with unsealed edges can show onset of solder-ribbon discoloration after 1500–2000 h depending on backsheet water vapor transmission rate. The migration kinetics of acetic acid in the cured polymer matrix is slow at module operating temperature but accelerates above the glass transition of the cured film; edge-region acetate concentration after 1000 h damp heat is typically below 500 µg/g in properly cured laminates, although published data for the KENGO ST film in this specific configuration is limited. For thin-film devices with transparent conductive oxides, acetic acid release can sensitize the cell edge; in these configurations, an edge seal or non-acetic encapsulant should be evaluated. This constraint is specific to EVA chemistry and is the principal reason a polyolefin encapsulant may be substituted when moisture-sensitive interconnects or transparent conductive oxides dominate the bill of materials.

Before a roll is opened in a cleanroom, the receiving area should verify that the desiccant packaging is intact and that the roll core has not been exposed to condensation. Rolls should be stored in sealed desiccant packaging at 0–25 °C and relative humidity below 50%; storage life from date of manufacture is typically 12 months under these conditions. If the packaging is opened at an ambient relative humidity above 60%, the film absorbs moisture rapidly, and moisture uptake above 0.1 wt% by Karl Fischer titration produces bubbles at the glass interface during lamination. Pre-drying at 60–70 °C for 4–8 h is required before use after high-humidity exposure. The film should not be stored near organic solvents, silicone release liners, or amine-containing cleaning agents; amine species can neutralize the silane adhesion promoter and reduce dry peel strength below 40 N/cm even when cure is otherwise complete. Rolls should be re-wrapped with moisture-barrier foil after partial consumption and returned to 15–25 °C storage within 30 min of exposure to the cleanroom atmosphere.

When Laminator Throughput and Backsheet Thermal History Determine Grade Selection

Selection among KENGO ST EVA Film, a standard-cure EVA, and a polyolefin encapsulant is governed by the interaction of laminator throughput, backsheet thermal stability, edge-seal design, and ion migration risk. The ST grade reduces the dwell below the peroxide decomposition threshold by roughly 30–40% compared with standard-cure EVA; this difference is significant on a three-chamber laminator because the backsheet is not held at 150 °C for the same duration. However, the cured film remains an EVA chemistry, with a water vapor transmission rate higher than a typical polyolefin encapsulant. In glass/backsheet modules with fluoropolymer or polyester backsheets and standard ribbon interconnects, the ST grade is used to increase line speed without changing to a higher-cost encapsulant. In glass/glass modules with moisture-sensitive cell metallization or field exposure in coastal climates, the edge seal and perimeter drainage design become critical; if the edge seal is incomplete, moisture ingress can produce acetic acid accumulation and ion migration along the cell string within 1000–1500 h of damp heat. Polyolefin encapsulants may provide lower water vapor transmission and higher volume resistivity, but their adhesion to glass and their cost profile differ. The KENGO ST EVA Film is therefore positioned for high-throughput crystalline silicon lines where proven silane adhesion, short cure time, and established module qualification history are weighted more heavily than the moisture-barrier advantages of polyolefin alternatives. Before production adoption, the complete module stack should be qualified to IEC 61215-2:2021, IEC 61730-1:2021, and the relevant fire test standard for the target market because encapsulant performance is not separable from glass, backsheet, and edge-seal interactions.

For laminators operated at sites above 1800 m, the vacuum system must compensate for lower ambient pressure. If external barometric pressure falls below 80 kPa, the pump should maintain an absolute chamber pressure below 5 kPa during the degassing step to preserve the same differential pressure across the membrane. Film embossing depth on the ST grade is optimized for standard sea-level vacuum levels; at high-altitude sites, an increase in emboss depth or a longer degassing time may be required to prevent air entrapment at the cell string edges.