KENGO WHITE EVA Film (thermo set )

When a crystalline silicon photovoltaic module is assembled with a polymer backsheet, the rear encapsulant layer controls interfacial adhesion, moisture ingress rate, and the optical energy recovered from the inactive spaces between cells. The product designated KENGO WHITE EVA Film (thermoset) is a peroxide-cured ethylene-vinyl acetate encapsulant loaded with a white inorganic pigment system. It is supplied in nominal thickness grades of 0.45 mm and 0.50 mm with a thickness tolerance of ±0.03 mm, in slit widths up to 2100 mm and a width tolerance of ±2 mm. Model codes are assigned by nominal thickness and slit width; exact alphanumeric codes must be read from the manufacturer’s roll label. The uncured film has a density of 0.95 g/cm³ to 0.98 g/cm³ when measured per ISO 1183-1:2019, and a melt mass-flow rate of 20 g/10 min to 40 g/10 min at 190 °C under 2.16 kg load per ISO 1133-1:2022. The vinyl acetate comonomer content is typically in the range 28 wt% to 33 wt%, determined by attenuated total reflectance infrared spectroscopy calibrated against certified reference copolymers. The product is intended for rear-side lamination in monofacial modules where the white layer increases rear internal reflectance and reduces the light absorption loss at the backsheet interface.

How Does Peroxide-Initiated Thermoset Cure Separate This Film from Thermoplastic Encapsulant Webs?

Before lamination the film is thermoplastic and can be slit, wound, and positioned as a continuous web. The thermal cure in the vacuum laminator introduces covalent crosslinks through decomposition of an organic peroxide initiator. The critical processing window is governed by the peroxide half-life: the initiator used in this class of encapsulant typically exhibits a 1-minute half-life temperature near 150 °C and a 10-hour half-life temperature below 100 °C, so storage must remain below 30 °C to limit premature scission of the peroxide. After cure, gel content measured by xylene extraction per ASTM D2765-16 rises from near zero in the uncured state to 70% to 85% in the laminated module. This crosslinked network removes the melt-flow behaviour: the cured material cannot be reprocessed by extrusion or hot-melt welding, which distinguishes it from thermoplastic polyolefin encapsulants and from thermoplastic ethylene-vinyl acetate copolymer films used in non-PV laminating. The cure reaction is not a drying process; it is a network-forming reaction that consumes the peroxide and creates a three-dimensional polymer architecture.

The white pigment loading applied to this film increases the low-shear melt viscosity of the uncured web relative to transparent EVA. Oscillatory rheometry at 150 °C and 1 rad/s typically shows a complex viscosity increase of 20% to 40% at pigment loadings between 5 wt% and 10 wt%. The pigment is dispersed as a fine inorganic phase with a median particle size below 1 µm; agglomerates above 10 µm are associated with optical hot spots and reduced peel adhesion at the backsheet interface. Incoming-roll quality control should include a 25 µm grind gauge or a particle-size analyser to monitor dispersion quality. Production-scale extrusion of similar EVA encapsulant films typically occurs on a twin-screw compounding line with a length-to-diameter ratio of 40:1 to 48:1, using a melt filtration system with mesh size 100 µm to 150 µm to remove gels and pigment agglomerates. These conditions are typical of EVA encapsulant manufacture and are not a substitute for the manufacturer’s process validation.

On a 3-chamber vacuum laminator with a 2.2 m × 4.2 m pin-type heated platen and a silicone diaphragm, the white thermoset film is processed within a platen-temperature band of 148 °C to 155 °C. A typical cycle holds vacuum at −0.1 MPa gauge for 3 min to 4 min before diaphragm pressure of 0.6 bar to 0.8 bar is applied for 8 min to 12 min. Batch-to-batch variation in gel content observed on production-scale lines is approximately ±3% absolute, which is within the tolerance required for repeatable peel-adhesion results. Film exposed to relative humidity above 60% for more than 24 h must be predried in a dehumidified oven at 40 °C for at least 12 h with a dew point below −20 °C before lamination, because retained moisture reduces void-free bubble clearance and can hydrolyse the silane coupling system at the glass interface. Rolls should be stored in the original moisture-barrier packaging at 0 °C to 25 °C and protected from direct UV exposure; the shelf life typically does not exceed 6 months from date of packaging.

Because the peroxide cure is exothermic, differential scanning calorimetry per ISO 11357-5:2020 on the uncured film shows a cure exotherm onset near 110 °C to 120 °C and a peak exotherm between 140 °C and 150 °C at a heating rate of 10 °C/min. The exotherm is the basis for selecting the lamination platen setpoint. If the platen temperature exceeds 160 °C, the peroxide decomposes too rapidly and the film may gel before complete void removal; if the platen temperature falls below 145 °C, the cure may not reach the target gel content within the available cycle time. This processing window of approximately ±5 °C around the optimum setpoint is one of the critical threshold risks for this product class, and it is the reason that thermocouple profiling of the glass–encapsulant–backsheet stack is required before high-volume lamination.

Tensile, Reflectance, and Volume Resistivity Response After Damp Heat Exposure

After a standard cure cycle and environmental preconditioning, mechanical and electrical properties are evaluated. Tensile strength and elongation at break are measured on Type IV specimens per ASTM D638-14 at a crosshead speed of 500 mm/min. Typical cured-film tensile strength is 15 MPa to 20 MPa with elongation at break of 400% to 600%. Peel adhesion to glass is tested at 180° per IEC 61215-2:2021 MQT 17; values for this product class are generally reported in the range 40 N/10 mm to 80 N/10 mm, though the exact adhesion value depends on the glass cleaning process and the silane primer. Total hemispherical reflectance of the white layer is measured with an integrating-sphere spectrophotometer per ASTM E1331-15 over 400 nm to 1100 nm; the value after lamination is typically above 90%. Volume resistivity after damp heat at 85 °C and 85% RH for 1000 h per IEC 61215-2:2021 MQT 13 is monitored because moisture ingress and acetic acid generation can reduce the electrical isolation of the encapsulant. For white EVA formulations, published data for this specific product configuration is limited, but EVA-based encapsulants commonly show a decrease in volume resistivity from greater than 10¹⁴ Ω·cm in the dry state to the range of 10¹² Ω·cm to 10¹³ Ω·cm after damp heat when measured per IEC 62631-3-1:2016.

Substitution of a transparent EVA rear layer with the white thermoset film changes the optical architecture of the module rather than simply adding a pigment. The white grade reduces rear-side transmittance to less than 10% across the same 400 nm to 1100 nm band, while transparent EVA transmits more than 90% of the incident light. This loss of rear transparency is acceptable only when the rear layer is positioned behind the cell-facing surface and is not required to transmit light through a glass rear cover. Compared with polyolefin elastomer encapsulants, white EVA exhibits higher equilibrium moisture absorption after 24 h immersion in distilled water at 23 °C per ISO 62:2008; typical values for EVA are 0.3% to 0.5%, while POE encapsulants are usually below 0.1%. The higher moisture uptake correlates with a more pronounced loss of volume resistivity during damp heat and with a greater tendency to generate acetic acid under thermal and humid conditions. These are not processing failures but must be considered when the module is intended for long-duration operation at system voltages above 1000 V or when the bill of materials contains PID-sensitive cell technologies.

When Bifacial Glass–Glass Assemblies Remove the Polymer Backsheet, Which Optical Constraints Apply to the White Layer?

In monofacial modules, the white thermoset film is positioned between the cell string plane and the polymer backsheet. In bifacial glass–glass constructions, a continuous white rear encapsulant is generally not placed directly over the rear glass because it would block light intended for rear-side energy conversion. Instead, white EVA strips may be used selectively in the cell-free regions between bifacial cells or around the array perimeter, where the target is internal light redirection rather than rear-side transparency. The product can be die-cut into intercellular strips with clean edges when processed at 23 °C to 25 °C; edge brittleness increases below 10 °C, requiring conditioning in a temperature-controlled cleanroom before die cutting. The pigment loading in the white film raises the viscosity of the uncured melt and reduces spontaneous bubble clearance during the vacuum stage; therefore, lamination programs for glass–glass modules using white strips require a longer vacuum hold of 5 min to 7 min before diaphragm pressure is applied. Published data for this specific selective-strip configuration is limited, and equipment-specific validation on the actual laminator is required because the effective heat transfer through the glass–glass stack differs from a glass–backsheet stack.

Backsheet adhesion with the white film is influenced by the surface treatment of the rear sheet. Polyamide and fluoropolymer-based backsheets require corona or plasma treatment at the laminator or at the backsheet supplier to achieve peel adhesion above 30 N/10 mm to 40 N/10 mm. The white pigment layer can mask low-level backsheet discoloration but does not compensate for an unprimed interface. Adhesion testing should be performed after a 85 °C/85 % RH, 1000 h damp heat exposure because interfacial hydrolysis is a known failure mode for EVA encapsulants. When the rear substrate is glass, the white film adheres through the same silane coupling chemistry but the lack of a breathable polymer backsheet may increase the concentration of retained acetic acid; published data for this specific configuration is limited, and longer degassing during lamination is often required.

Compliance documentation for the KENGO WHITE EVA Film (thermoset) should include the relevant clauses of IEC 61730-2:2023 for module safety qualification, including the electrical insulation and flammability tests required for the final laminate. The film is typically supplied with a declaration that the formulation is compliant with the restriction of hazardous substances under Directive 2011/65/EU (RoHS) Annex II and that no substance above 0.1% w/w appears on the REACH candidate list as amended. Those statements apply to the film as supplied and do not relieve the module manufacturer from verifying the finished laminate under the final module certification. The product must not be combined with amine-based silane primers or amine-containing anti-oxidants because basic additives can accelerate premature peroxide decomposition and reduce crosslink density at the required lamination temperature. For the same reason, the film should not be exposed to open steam or to cleaning solvents with a pH outside the range of 5 to 8 before lamination. The material is not designed for direct outdoor exposure as an external surface and must be encapsulated between glass and a UV-opaque or UV-stable backsheet or rear cover.