EVATHENE UE2828 EVA Copolymer Resin,Solar Cell Encapsulation Grade

Can a 150 °C Curing Step Raise Gel Fraction Above 75 % Without Pre-Cure Bubble Trapping?

The cure step in a laminator is evaluated by gel fraction according to ASTM D2765-16, using xylene extraction at 130 °C for 12 h. For EVA encapsulant films based on a 28 wt% vinyl acetate copolymer, the target gel fraction after lamination is typically 75–90 %. Cure at 150 °C for 18 min yields a gel fraction near 75 %, while 150 °C for 25 min pushes the gel fraction above 85 % in thick film sections below the cell. Bubble trapping occurs when the laminator platen pressure is released before the peroxide decomposition exotherm has peaked. The peroxide system, commonly tert-butyl peroxy-2-ethylhexyl carbonate (TBEC) or 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, decomposes rapidly above 160 °C. Platen temperature is set at 150–160 °C with chamber pressure of 0.03–0.06 MPa above atmospheric. Pre-press time of 2–3 min at 60–70 °C allows the encapsulant to flow around cell edges. The degree of cure is confirmed by differential scanning calorimetry using ASTM D3418-21, observing residual exotherm below 5 J/g.

Laminator venting cycles must be synchronized with the peroxide decomposition temperature. If the chamber is opened above 80 °C before the crosslink network has developed, dissolved blowing agents and peroxide decomposition gases form bubbles at the glass interface. The operating boundary is a minimum 15 min under full pressure before venting. In dual-chamber laminators, the second chamber is held at 165 °C for 8 min to complete cure. The terminal product is a cured glass-EVA-cell-EVA-backsheet laminate with a gel fraction suitable for IEC 61215-1:2021 thermal cycling and damp heat durability.

Glass-Side Peel Strength After 1000 h Damp-Heat Exposure

Glass-side adhesion depends on silane coupling agent type and loading. Methacryloxypropyltrimethoxysilane is favored over vinylsilane in high-humidity service because the methacryloxy group participates in the peroxide crosslink network. Loading ranges from 0.2 wt% to 0.5 wt% in the encapsulant compound. Addition below 0.2 wt% produces peel strength below 40 N/cm after 1000 h of 85 °C/85 % RH damp heat when tested according to ASTM D903-98(2017). Addition above 0.5 wt% can cause film blocking on the chill roll and a tacky surface. Clean float glass is used without antireflective coating for peel specimens. Peel strength before damp heat is typically 60–100 N/cm. Cohesive failure within the encapsulant is accepted; adhesive failure at the glass interface is rejected. The test is performed on a universal tensile machine with a jaw speed of 100 mm/min.

Glass incoming humidity and cleanliness are critical. Glass washed with deionized water and dried at 60 °C has a water contact angle below 10°. If the contact angle exceeds 15°, the adhesion promoter migration is reduced. On production lines, glass surface contamination from cutting oils or vacuum cup residues creates localized adhesion loss at module corners. Modules with corner adhesion below 30 N/cm after damp heat are rejected. The terminal product is a glass-side adhesive bond layer that maintains optical transmittance above 90 % measured per ASTM E903-20 and peel strength above the specified threshold after environmental testing.

Fluoropolymer backsheet bonding requires a different surface preparation sequence than glass adhesion. Polyvinyl fluoride and polyvinylidene fluoride backsheets are corona-treated in-line to a surface energy of 48–56 dyn/cm before layup. The EVA encapsulant layer adjacent to the backsheet may contain a slightly higher silane loading of 0.4–0.6 wt% to compensate for lower surface energy. Lamination pressure is raised to 0.08–0.10 MPa during the last 5 min of cure. This increases backsheet adhesion. Peel testing is performed after a 180° peel test per ASTM D1876-08(2015). The failure mode is cohesive in EVA at 60–90 N/cm. Bottlenecks in production occur when the corona treatment decays below 44 dyn/cm within 4 h after treatment; this causes backsheet delamination at module corners after damp heat. In coextruded or non-fluoropolymer backsheets, corona is not used. A polyurethane-based primer may be used instead.

The terminal product is a laminated module with backsheet adhesion adequate for IEC 61215-1:2021 damp heat and thermal cycling. Production lines with high relative humidity during layup experience condensation on chilled backsheet rolls; this is prevented by conditioning rolls at 25 °C for 12 h before use. Backsheet surface moisture above 0.15 g/m² suppresses the silane reaction at the interface. In such cases, the laminate is subjected to a post-cure at 85 °C for 4 h to recover adhesion. The operational boundary for primer-coated backsheets is a 10 °C minimum layup temperature because primer viscosity below that point becomes too high for uniform transfer.

When Coextruded Polyolefin Backsheets Replace Laminated Fluoropolymer

In coextruded polyolefin backsheet constructions, the encapsulant layer and the polypropylene or polyethylene backsheet are joined without an adhesive tie layer. The rheology of EVATHENE UE2828 must be matched to the backsheet layer to avoid interfacial instability. Melt flow rate at 190 °C/2.16 kg is measured per ISO 1133-1:2022. A difference in apparent viscosity above 15 % between the EVA layer and the adjacent polyolefin layer causes transverse wave marks at line speeds above 18 m/min. In cast coextrusion, the EVA layer is processed at 160–175 °C while the polyolefin backsheet layer is processed at 200–230 °C. The melt streams are joined in a feedblock with a temperature gap of less than 25 °C at the merging mandrel. Interfacial adhesion before lamination is not a module performance requirement; adhesion develops during the laminator cure.

The lower processing temperature of EVA limits backsheet line speed because the coextrusion line cannot exceed a die temperature of 180 °C. At higher die temperatures, peroxide masterbatch in EVA decomposes and forms gel spots. The extruder barrel zones are set to 70–110–140–160–170 °C. The use of a barrier screw with a 28:1 L/D ratio is required for homogeneous melt temperature. Terminal product is a coextruded backsheet-encapsulant roll, which reduces layup operations and eliminates a separate backsheet lamination step. The roll must be stored at 5–15 °C to prevent blocking and premature silane migration.

Split-Feed Peroxide Masterbatch Compounding Without Premature Crosslink Formation

Compounding of EVA encapsulant formulations with peroxide and silane requires a twin-screw extruder with a 44:1 L/D ratio and a side feeder at barrel zone 8. The base resin is fed into the main hopper at barrel 1. Peroxide masterbatch is side-fed after the melting and mixing zones to limit residence time at high temperature. Barrel temperatures from zone 1 to zone 11 are 60–130–150–160–160–150–140–120–100–100–100 °C. Screw speed is held between 350 rpm and 450 rpm. Specific mechanical energy input is controlled below 0.18 kWh/kg. The die head pressure is maintained below 3 MPa. Under these conditions, gel count measured by press-plate gel analysis remains below 10 particles per 100 cm² at 200 µm particle size. The concentrate is then pelletized and frozen at -20 °C to prevent peroxide decomposition during storage.

The main bottleneck is the low melt temperature tolerance of the peroxide before decomposition; a temperature overshoot above 170 °C at the die plate can produce crosslinked agglomerates that plug the strand die. EVATHENE UE2828 with high VA content has high compatibility with the peroxide masterbatch but also high moisture absorption; pre-drying at 65 °C for 4 h at -60 kPa vacuum is used. Avoid combination with amine-based antioxidants or metal stearate acid scavengers that deactivate the silane coupling agent. Terminal product is a silane-peroxide masterbatch concentrate for cast film extrusion.

Low-pressure lamination gaps around string-interconnect ribbons become flow-limited when melt viscosity is too high at the initial platen temperature. At 70 °C, the EVA encapsulant has not reached the zero-shear viscosity plateau; its complex viscosity may still be above 10^5 Pa·s. The module layup is placed in a laminator at a chamber pressure of 0.02–0.05 MPa. During pre-press, the EVA is heated to 70–80 °C for 3–4 min and flows into gaps between glass, cell, and interconnect ribbons. Void-free filling around 0.2 mm-thick copper ribbons depends on the resin melt flow index and silane masterbatch dilution ratio. At a melt flow index below 20 g/10 min, edge voids remain around string ends. At a melt flow index above 35 g/10 min, the encapsulant may squeeze out beyond the glass edge and contaminate the laminator belt. Encapsulant-grade EVA with 28 % VA from this resin is typically in the 25–35 g/10 min range. The lamination recipe uses a two-stage pressure profile: 0.03 MPa for 3 min, then 0.09 MPa for 12 min.

The terminal product is a void-free module before the final cure plateau. Operators monitor the flow front through the glass by infrared thermography during pre-press. Voids larger than 0.5 mm² around busbars are rejected because they reduce insulation resistance under IEC 61215-2:2021 wet leakage current tests. The nominal cell gap width of 2–3 mm must be filled without trapping gas. If the laminator chamber evacuation rate exceeds 100 Pa/s, air bubbles are drawn out of the encapsulant too rapidly and later collapse as gel content increases. The boundary for low-pressure filling is therefore a maximum evacuation rate of 80 Pa/s.

Double-Glass Edge Cure and Moisture Ingress Under Cyclic Mechanical Load

Double-glass modules without a permeable backsheet cure differently at the edge than at the center. The glass edges conduct heat faster, so the edge reaches 150 °C earlier. The center of the module may still be at 130 °C when the edge has already exceeded 160 °C. This thermal gradient produces edge cure above 90 % gel content while the center remains below 80 %. Residual peroxide in the center migrates toward the edge during long-term operation. Moisture ingress from the edge seal is tested by damp heat at 85 °C/85 % RH for 1000 h according to IEC 61215-2:2021 MQT 13. The edge encapsulant is inspected for acetic acid generation; EVA with high VA content releases acetic acid at a rate dependent on residual catalyst and silane protection. A glass fiber-reinforced edge tape or butyl edge seal is used to reduce moisture ingress. Mechanical load testing per IEC 61215-1:2021 MQT 16, with 2400 Pa static load, is used to verify edge crack resistance. The terminal product is a double-glass module for utility-scale installations.

Published data for this specific configuration using EVATHENE UE2828 is limited; the above cure inhomogeneity is known from EVA encapsulants with similar VA content. Process validation is required per module design. The edge seal must be compatible with peroxide-cured EVA; butyl seals containing phenolic tackifiers can retard edge cure. The laminator program for double-glass modules includes a 5-min edge-cooling step before venting. The maximum edge temperature during lamination is held below 175 °C to avoid acetic acid formation that corrodes cell fingers. The terminal module is subjected to 85 °C/85 % RH damp heat for 2000 h in extended reliability testing.

At a cast-film encapsulation line with a 90 mm single-screw extruder having an L/D ratio of 30:1, EVATHENE UE2828 is processed with a barrel temperature profile of 80–120–140–160–170 °C from feed to adapter. The melt temperature at the die lip is held below 180 °C to prevent premature peroxide decomposition in masterbatch-containing compounds. The cast film is drawn onto a polished chrome chill roll at 15–25 °C, followed by a second cooling roll at 10–20 °C. Film thickness for standard single-glass modules is controlled between 0.45 mm and 0.55 mm; thickness deviation across a 1,300 mm web is maintained within ±0.03 mm using an automatic thickness gauge feedback loop. Melt flow index measured according to ASTM D1238-23 at 190 °C/2.16 kg is used as incoming resin lot control. High shear mixing in the screw is limited to a maximum screw speed of 120 rpm unless the formulation contains a free-radical scavenger. The resulting film is wound into rolls with interleaving polyethylene release liner to prevent blocking.

Production-scale cast-film lines with this resin typically reject gel-particle defects larger than 200 µm because they become visible in glass-side adhesion after lamination and fail optical inspection under IEC 61215-2:2021 MQT 13 visual checks. Die pressure above 8 MPa indicates gel accumulation at the die lip, requiring screw speed reduction or zone temperature correction. The resin is pre-dried at 65 °C for 4 h when storage RH exceeds 60 %; moisture above 300 ppm in the hopper promotes acetic acid release during lamination. The terminal product is a pre-cure encapsulant roll stock ready for glass-EVA-cell-EVA-backsheet layup.