Mitsui TAFMER for PV encapsulants EVA Film

Anti-PID Resistance in Monofacial PERC Front Encapsulant Films

On a 2,200 mm wide cast-film line, EVA resin with vinyl acetate content 28–33 wt% is dry-blended with ethylene–butene TAFMER pellets at a let-down ratio of 10–25 wt% of total polymer mass. Compounding is carried out on a twin-screw extruder with an L/D ratio of 44:1 and eleven barrel sections, using distributive mixing elements rather than high-shear kneading blocks to avoid premature peroxide decomposition. The compound is pelletized underwater at 40°C water temperature. The front-side encapsulant must maintain volume resistivity above 5×10¹⁴ Ω·cm after damp heat because the p-type PERC cell is biased at −1000 V during PID validation. When TAFMER replaces part of the EVA matrix, acetic acid evolution during 85°C/85% RH exposure decreases because the ethylene–butene backbone contains no vinyl acetate side groups. The compound typically incorporates 0.5–0.8 phr of methacryloxy silane adhesion promoter, 0.6–1.0 phr of tert-alkyl peroxyester crosslinking agent, and 0.5–1.0 phr of triallyl isocyanurate co-agent. TAFMER must be pre-dried at 60°C for 4 h when warehouse relative humidity exceeds 60%; residual moisture above 300 ppm produces bubble streaks in the cast film. The film is extruded at melt temperature 190–210°C, calendered through a three-roll stack, and wound with controlled cooling to avoid crystallinity haze. Gel fraction after lamination is assessed per ASTM D2765-16 and is held in the 70–80% band for front-side modules. PID resistance is confirmed through 96 h of −1000 V bias at 85°C/85% RH per IEC TS 62804-1:2015, with a maximum power loss limit below 5%. On production-scale lamination lines, the largest batch-to-batch variance is not the raw resin but the peroxide masterbatch dispersion; film lots with poorly dispersed peroxide show gel-fraction spikes of 8–12% across a single master roll.

Bifacial rear-side films operate under a different bonding priority. For a glass/glass module, the rear encapsulant is typically co-extruded at 0.50–0.65 mm thickness and compounded at a higher TAFMER fraction of 20–35 wt% because the rear layer is not the primary optical aperture. Water vapour transmission through the cured film is measured per ASTM F1249-20 at 38°C/90% RH; the TAFMER-rich layer reduces equilibrium moisture uptake from roughly 0.35 wt% for unfilled EVA to 0.15–0.22 wt% after 1,000 h of damp heat. The practical effect appears in edge delamination during thermal cycling. Modules are cycled 200 times from −40°C to +85°C per IEC 61215-2:2021, and rear-side adhesion to structured glass remains above the required minimum when the silane coupling agent is held at 0.6–0.8 phr. However, TAFMER by itself is not an edge sealant; polyisobutylene edge tape is still required to limit water ingress. Film-edge puckering on 1,300 mm wide unwinds is controlled by maintaining a taper tension profile of 120–180 N and by post-annealing the master roll at 28°C for 48 h before slitting. On a laminator with a 2.4 m wide entry bed, the rear film must be handled with ionizing bars because static discharge can attract airborne particulates that create optical hot spots after lamination.

What Changes When the EVA Peroxide Cure Rheometer Torque Curve Is Shifted by TAFMER?

The moving-die rheometer curve at 150°C, tested per ASTM D5289-19a, shows two effects when TAFMER replaces EVA. Minimum torque ML decreases by 0.2–0.6 dN·m because the lower vinyl acetate content reduces melt elasticity. Scorch time t_s2 increases by 20–45 s, which widens the processing window before 10% peroxide conversion and reduces the risk of pre-crosslinking inside the extruder die. The cure time t_90 typically moves from 8–10 min for standard EVA to 12–15 min for a 20 wt% TAFMER blend unless the triallyl isocyanurate content is raised by 0.2–0.4 phr. On a lamination line with a 12 min chamber dwell, this shift is compensated by raising the hold section temperature from 150°C to 154°C or by selecting a less hindered peroxyester with a shorter half-life. Production-scale measurement on a 1,400 mm wide film line shows that web uniformity, not press capacity, is the main bottleneck. A heated platen gradient of more than ±2°C across the module can generate a 4–6% gel-fraction difference from center to edge. The TAFMER-containing film is more sensitive to this gradient because its lower crosslink density pushes the cooler edge below the 65% gel-fraction threshold for wet adhesion. Lamination recipes therefore use a nine-point gel-fraction map rather than a single center-point test. The temperature gradient is measured with 9 embedded thermocouples in the platen and accepted only if the spread remains within 2°C.

Heterojunction cell stacks require lamination temperatures below 145°C to protect the transparent conductive oxide and amorphous silicon passivation layers. The encapsulant compound is formulated with a higher melt-flow TAFMER grade selected at 3.0–5.0 g/10 min under 190°C/2.16 kg per ISO 1133-1:2022. This permits a lamination platen set-point of 135–140°C while still achieving melt coverage over the busbar and ribbon. Because the cure temperature is below the usual EVA peroxide activation range, the formulation uses a fast-curing peroxyester and 1.2–1.8 phr triallyl isocyanurate to raise gel fraction into the 62–70% range measured by ASTM D2765-16. The practical trade-off is a shorter open time: compounded film stored at 10–20°C shows only 72–96 h of ambient lay-up stability before the peroxide begins to decompose. DSC analysis per ISO 11357-3:2018 shows a broad melting endotherm from 42°C to 78°C for the TAFMER-modified compound, which avoids residual crystallites that would scatter light after rapid cooling. Production experience with HJT modules indicates that edge voiding is most common when the glass enters the laminator below 60°C. Preheating the glass layup to 65–70°C before the vacuum ramp reduces the peak melt viscosity at the cell edges and prevents trapped air. The compound also uses a lower silane load, 0.4–0.6 phr, because the low lamination temperature reduces silane condensation to the glass surface; higher silane levels cause glass-side haze after damp heat.

When TAFMER Replaces a Portion of the EVA Matrix in Flexible Lightweight Modules

When TAFMER replaces a portion of the EVA matrix in flexible lightweight modules, the encapsulant film is extruded at 0.28–0.40 mm thickness and the lower zero-shear viscosity improves wetting of textured cells without requiring high vinyl acetate content that would reduce tensile elongation. Tensile tests per ASTM D882-18 on free films show elongation at break of 550–650% and elastic recovery after 100% strain above 90%. Bending-fatigue testing is not fully standardized for this configuration; published data for this specific flexible-module configuration is limited, and qualification frequently relies on custom mandrel-bend fixtures. The same low modulus that enables flexibility creates a separation risk at the solder ribbon stitch points if the methacryloxy silane adhesion promoter is below 0.3 phr. The practical formulation boundary is 0.5–0.7 phr silane and a post-lamination cure that is extended by 2–4 min to allow siloxane network formation at the backsheet interface. In vehicle-integrated photovoltaic installations, the dynamic mechanical tan delta per ISO 6721-4 measured at 1 Hz stays above 0.1 from −20°C to 40°C, which reduces acoustic noise and microcrack propagation. On a 1,200 mm wide cast film line, edge dimpling is reduced when the TAFMER fraction reaches 25 wt% and the melt temperature is held at 195°C. The film is also processed with a lower chill roll temperature of 15–20°C to control blocking, but this increases surface haze slightly and must be balanced against the optical specification for the front side.

High-Transmittance TOPCon Front Encapsulant Chemistry and Anti-UV Yellowing Limits

TOPCon front films require initial light transmittance above 91% over the 380–1100 nm range with a haze contribution below 2% after lamination. The optical specification is tested per ASTM D1003-21, and yellowness index is tracked per ASTM E313-20. In standard EVA, UV stabilizers and antioxidants are needed to prevent chromophore formation after UV preconditioning, but the TAFMER portion reduces the concentration of vinyl acetate sequences that generate acetic acid under UV and thermal exposure. A typical TOPCon formulation uses 10–15 wt% TAFMER, 0.3–0.5 phr hindered amine light stabilizer, 0.1–0.3 phr UV absorber with a cut-off near 350 nm, and 0.05–0.15 phr phosphite antioxidant. The TAFMER fraction is kept at the lower end because higher polyolefin content can raise haze slightly through phase-separated domains that scatter short-wavelength light. The real production limitation is yellowing after damp heat rather than initial transmittance: modules laminated with 15 wt% TAFMER show a yellowness index increase of 1.2–2.0 units after 1,000 h at 85°C/85% RH, compared with 2.5–3.5 units for standard EVA controls, provided that the film is not over-cured. If gel fraction exceeds 85%, residual free radicals can continue to attack the antioxidant package and the yellowing advantage is lost. The front film is therefore validated by cross-section adhesion and transmittance after UV preconditioning specified in IEC 61215-2:2021 followed by 1,000 h damp heat. For TOPCon, the UV absorber cut-off must not shift below 350 nm, otherwise the short-wavelength external quantum efficiency of the cell is reduced and the module current drops in the 340–380 nm region.