TAFMER DF&A Series EVA Modification Film

TAFMER DF&A Series EVA Modification Film is supplied as a coextrudable modifier layer for ethylene-vinyl acetate encapsulation systems used in photovoltaic module assembly. The film is manufactured from Mitsui Chemicals TAFMER DF ethylene-vinyl acetate copolymer resins, combined with TAFMER A ethylene-α-olefin copolymer resins where lower glass transition temperature and reduced water vapor transmission are required. Rollstock thickness is normally specified between 0.10 mm and 0.20 mm, with slit widths controlled to ±1 mm and corona-treated surfaces held above 38 mN/m wetting tension per ISO 8296:2003. The product is positioned for lamination between glass and cell or cell and backsheet, where it modifies melt flow, adhesion, and electrical resistance of a conventional EVA encapsulant without replacing the full encapsulant layer.

What Distinguishes DF&A Film from Conventional High-VA EVA Encapsulants?

The principal difference is reduced vinyl acetate loading in the bulk matrix. Conventional EVA encapsulant grades contain 28 wt% to 33 wt% vinyl acetate to provide low-temperature flexibility and silane-based adhesion. The DF&A modification film achieves flexible behavior through ethylene-α-olefin copolymer blending, so overall vinyl acetate content can be lower. Peak melting temperature measured by ASTM D3418-15 shifts upward toward the 55 °C to 88 °C range, while glass transition temperature determined by dynamic mechanical analysis falls between -40 °C and -25 °C depending on the A-series ratio. The result is a film with lower deacetylation tendency above 200 °C, lower moisture uptake, and improved dimensional stability after lamination. Compared with a peroxide-cured EVA encapsulant, the DF&A modification film is not a standalone crosslinkable encapsulation layer; gel content after lamination is lower when the film is used alone, and the material is specified as a modification layer adjacent to a crosslinkable EVA core.

On production-scale cast-film lines equipped with 65 mm single-screw extruders at 30:1 L/D ratio, melt temperatures below 170 °C produce edge bead instability and irregular gauge bands. Melt temperatures above 230 °C generate die-lip deposits associated with acetic acid evolution from vinyl acetate repeating units. Typical barrel profiles hold 150 °C to 210 °C, adapter zones at 210 °C, and die zones at 195 °C to 215 °C. Chill roll temperature is maintained at 18 °C to 30 °C for cast film; blown-film variants require a blow-up ratio of 2.0:1 to 3.0:1 and a frost line height below 300 mm to control blocking. Gear pump inlet pressure is held between 4 MPa and 8 MPa to damp screw surge. Resin moisture above 0.05 wt% before extrusion increases bubble formation and should be corrected by drying at 60 °C to 70 °C for 4 h to 6 h.

Grade Selection and the Role of Melt Flow Rate, Density, and Vinyl Acetate Content

Grade selection is controlled by three direct measurements: melt mass-flow rate at 190 °C and 2.16 kg per ISO 1133-1:2022, density per ASTM D792-20, and vinyl acetate content by Fourier transform infrared spectroscopy or thermogravimetric analysis. DF-series resins used in the modification film typically cover a melt flow range from 2 g/10 min to 25 g/10 min; the lower end is selected for blown-film lines, while cast-film lines accept higher flow to reduce motor load and improve gauge uniformity. Density ranges from 0.88 g/cm³ to 0.94 g/cm³. Increasing vinyl acetate content raises adhesion to glass measured by ASTM D1876-08 in T-peel mode, but lowers volume resistivity measured by ASTM D257-14. A VA variation of ±0.5 wt% can shift T-peel adhesion by approximately 10% on a production laminator; incoming resin lots are therefore blended to a certified VA window. Film thickness is verified across the web by ISO 4593:1993, with a 0.20 mm nominal film held between 0.18 mm and 0.22 mm.

Published data for this specific DF&A film configuration are limited; the ranges above are compiled from supplier technical data sheets and encapsulant qualification reports, not from a single production lot.

Rheological characterization using capillary viscometry at 190 °C shows shear thinning from approximately 1,200 Pa·s at 100 s⁻¹ to 400 Pa·s at 1,000 s⁻¹ for a DF-grade film compound. A high-VA EVA encapsulant typically displays lower melt viscosity under the same conditions, which explains why coextrusion of DF&A with high-VA EVA requires matching average residence time rather than matching melt temperature alone. In a coextrusion feedblock, the viscosity ratio at the skin/core interface should remain below 2:1 at 100 s⁻¹ to prevent the skin layer from being encapsulated by the higher-flow EVA core.

When Coextrusion Is Run Below 190°C

When the skin layer is coextruded below 190 °C, interfacial melt strength between the DF&A modification film and a high-VA EVA core is insufficient to prevent non-uniform draw resonance. On a three-layer feedback system with die gap 0.5 mm, draw resonance appears as gauge variation exceeding ±5% at haul-off speeds above 15 m/min. Raising the skin temperature to 200 °C to 215 °C restores stable interfacial flow but increases acetic acid generation if residence time exceeds 60 s. Screw speed should be set so that residence time remains below 45 s in the extruder and adapter; static mixers after the gear pump increase residence time and are avoided when the vinyl acetate content of the layer exceeds 15 wt%. The processing window is narrow because viscosity matching at the skin/core interface is lost when shear heating raises the high-VA core temperature by more than 10 °C above the skin setpoint.

Optical and electrical property testing is not interchangeable. Optical performance is assessed on a glass/DF&A/backsheet laminate after vacuum lamination at 150 °C for 15 min. Total luminous transmittance measured by ASTM D1003-13 is commonly reported above 90.0% for the laminate, while haze is held below 5.0%. Volume resistivity measured by ASTM D257-14 using 500 V DC and a 60 s electrification time is the more critical parameter for potential-induced degradation control; values below 1×10¹⁴ Ω·cm at 25 °C typically indicate excessive ion mobility. Dielectric strength measured by ASTM D149-20 on 0.20 mm film is used as a batch release test at 20 kV/mm or higher.

PID Resistance Depends on Bulk Resistivity and Interfacial Ion Concentration

Published data for this specific configuration is limited, but IEC TS 62804-1:2015 testing on EVA-based laminates indicates that potential-induced degradation is governed primarily by leakage current through the bulk encapsulant and by interfacial ionic concentration at the cell surface. The DF&A film reduces leakage current when volume resistivity remains above 1×10¹⁵ Ω·cm at 85 °C and 85% relative humidity. Module qualification exposures at -1000 V for 96 h typically require less than 5% power loss; films containing high levels of free acetate or sodium ions show higher power loss. The A-series ethylene-α-olefin component reduces moisture ingress because water vapor transmission rate at 38 °C and 100% RH is lower than that of a 0.46 mm high-VA EVA layer.

Mechanical stress in glass-free module designs exposes differences between a DF&A modification film and a standard EVA film. Tensile elongation at break measured by ASTM D882-18 on 0.20 mm film is typically 400% to 700% at 23 °C, but tensile modulus is higher than that of a 33 wt% VA EVA at 85 °C. Creep resistance improves when the A-series content is increased; package-level bending tests use a 90° mandrel at -40 °C to verify that low-temperature flexibility is retained. The film should not be combined with amine-based adhesion promoters or amine-stabilized tie layers, because amine-containing additives accelerate yellowing at lamination temperatures above 150 °C.

Reducing Blocking Force, Shrinkage, and Reel Handling Variation

Film blocking is controlled by adding an anti-block masterbatch with diatomaceous earth or synthetic silica at 500 ppm to 2,000 ppm. Blocking force measured on rollstock by a modified ASTM D3354-21 method should remain below 0.5 N/cm at 23 °C to allow automated unwinding. Machine-direction shrinkage at 85 °C for 15 min is specified below 1.5%, and transverse-direction shrinkage below 0.8%, because higher shrinkage creates cell string misalignment during vacuum lamination. Slitting practice uses shear knives with a side clearance of 1% of film thickness to avoid smearing and edge cracking. Rollstock is supplied on 76 mm or 152 mm internal-diameter fiber cores in splice-free lengths of 300 m to 500 m for 0.15 mm film.

Regulatory compliance for photovoltaic and electronic applications is established through supplier certification rather than film-level declaration alone. The DF&A film is evaluated under EU RoHS 2 Directive 2011/65/EU for cadmium, lead, mercury, hexavalent chromium, PBB, and PBDE. REACH Article 33 declarations cover substances above 0.1% w/w in the article. Food-contact suitability for select grades is assessed under FDA 21 CFR 177.1350 for ethylene-vinyl acetate copolymers and 177.1520 for olefin polymers.

Lamination on a single-chamber vacuum laminator with a 3.2 mm tempered glass front sheet and a 0.35 mm PET-based backsheet uses a three-step pressure profile: 60 kPa for 180 s, 80 kPa for 180 s, and 100 kPa for 240 s at platen temperature 150 °C. The DF&A film is inserted adjacent to the cell string, not against the glass, to preserve glass adhesion from the high-VA EVA layer. After lamination, crosslink density is measured by solvent extraction in xylene at 140 °C for 5 h; gel content above 70% indicates sufficient cure for module handling. This construction is used in roof-integrated photovoltaic systems where reduced moisture ingress and lower acetic acid emission are required by the system supplier.