Elvax 3170 EVA resina de copolímero, grau fotovoltaico

DuPont™ Elvax® 3170 EVA copolymer resin is an ethylene-vinyl acetate feedstock controlled for photovoltaic encapsulant film manufacture. The nominal vinyl acetate comonomer content is 33 wt% when determined by Fourier-transform infrared spectroscopy per ASTM D5594 or ISO 8985. Melt mass-flow rate is 15 g/10 min as measured by ISO 1133-1:2022 at 190 °C under a 2.16 kg load. Density is 0.96 g/cm³ per ASTM D792. Differential scanning calorimetry in accordance with ISO 11357-3 produces a peak melting temperature of approximately 62 °C. Vicat softening temperature is approximately 36 °C when tested per ASTM D1525. These are typical property values, not contractual specification limits.

The high vinyl acetate content increases the amorphous phase fraction relative to EVA resins with lower comonomer levels. This shift lowers crystallinity and melting point while improving light transmission and interfacial adhesion to glass and polyvinyl fluoride backsheet. It also increases moisture uptake and compounded sheet tack. The resin is therefore formulated with anti-blocking agents, processing stabilizers, and controlled cooling conditions to permit roll-to-roll handling after slot-die extrusion.

The grade is intended for compounding with peroxide crosslinking agents, silane coupling agents, ultraviolet absorbers, and hindered amine light stabilizers before film extrusion. Because photovoltaic encapsulant performance depends on gel content and optical uniformity, lot selection emphasizes low gel count, controlled ionic residue, and consistent vinyl acetate distribution. Particulate gels in the raw resin are minimized to limit pinhole defects in thin encapsulant sheet. Sodium and chloride residues are controlled because they influence volume resistivity and potential-induced degradation. Published numeric gel-count specifications for this specific configuration are limited; incoming resin is typically released against melt-flow and compositional tolerances rather than a universal gel metric.

What Processing Window Governs Slot-Die Extrusion of Photovoltaic Encapsulant Sheet?

On a cast film line equipped with a single-screw extruder with 30:1 L/D and a barrier screw with Maddock mixing sections, the feed throat is maintained below 50 °C to prevent pellet bridging. Barrel zone set points are typically distributed between 80 °C and 110 °C. Melt temperature at the die inlet is limited to 115–120 °C when a peroxide masterbatch is present. This upper limit is imposed because common peroxide initiators used in EVA encapsulation, such as tert-butyl peroxy-2-ethylhexyl carbonate with a 10 h half-life at 97 °C, can initiate premature crosslinking in hot spots. Screw speed is adjusted to hold melt pressure before the screen pack at 8–15 MPa; pressure fluctuation greater than ±1 MPa frequently indicates gel accumulation on the screens or inconsistent feed.

If the resin has been exposed to relative humidity above 60%, pre-drying in a desiccant dryer at 50–60 °C for 4 h is required to reduce moisture below 0.05 wt%. Residual moisture hydrolyzes silane coupling agents during compounding and generates microvoids in the cast sheet. A screen stack of 60/80/100 mesh is typical; optional gear pump delivery stabilizes die pressure but increases shear heating, so the pump inlet melt temperature is monitored. The slot-die gap is commonly held at 0.8–1.2 mm to produce sheet in the 0.40–0.60 mm thickness range. Draw ratio is kept below 2:1 to limit residual stress and thickness variation. Chill rolls are held at 10–20 °C. Lower chill roll temperatures reduce blocking but can condense moisture; higher temperatures increase tack and may cause roll blocking in storage. Edge trim and recycled film are reincorporated at 5–15 wt%; higher recycle fractions can raise gel count and increase optical haze.

The melt is shear-thinning; increasing screw speed reduces apparent viscosity but raises melt temperature. Therefore, shear heating rather than barrel set points often defines the practical maximum output. Processors monitor specific energy input and keep it below approximately 0.12 kWh/kg when peroxide-containing masterbatch is present. If specific energy exceeds this value, the melt temperature can enter the peroxide decomposition region and produce localized crosslink gels that appear as fisheyes in the cured encapsulant.

Vacuum Lamination Cure Kinetics and Long-Term Module Qualification

The compounded EVA sheet is laminated between glass, cell strings, and backsheet in a vacuum laminator. A typical cure profile uses heated platens at 150 °C, evacuation at 50–100 Pa for 5 min, followed by pressing at 0.08–0.10 MPa for 12–15 min. The peroxide decomposes to form radicals that abstract hydrogen from the ethylene sequences of the EVA backbone, producing crosslinks. Gel content after lamination is commonly specified between 75% and 85% when measured by solvent extraction per ASTM D2765. Gel content below 70% is associated with creep and cell movement during field service; gel content above 90% can reduce elongation and increase brittleness.

Silane coupling agents, typically vinyltrimethoxysilane at 0.3–0.5 phr, are added to promote adhesion to glass and cell surfaces. The alkoxy groups react with surface silanols, while the vinyl group participates in the peroxide cure. Peel adhesion to glass after lamination is often specified by module manufacturers at not less than 40 N/cm when measured by a 180° peel test. Ultraviolet absorber packages commonly include benzotriazole at 0.1–0.3 phr and hindered amine light stabilizer at 0.1–0.2 phr to control yellowing under xenon-arc weathering. Luminous transmittance of cured 0.45 mm sheet is typically greater than 91% per ASTM D1003; yellowness index is typically below 1.5 per ASTM E313.

Bubble formation during lamination is a primary failure mode. If vacuum evacuation is too short or if moisture in the sheet exceeds 0.05 wt%, bubbles nucleate at cell gridlines and busbar edges. These bubbles reduce optical coupling and can delaminate under thermal cycling. Press pressure must be high enough to flow the melt into the cell spacing but low enough to avoid squeezing the melt away from cell edges. A two-step pressure ramp from 0.03 MPa to 0.10 MPa is used to balance fill and squeeze-out.

For module qualification, the encapsulated laminate is subjected to damp heat at 85 °C and 85% RH for 1000 h and thermal cycling between -40 °C and 85 °C for 200 cycles under IEC 61215-1:2021. The encapsulant must maintain adhesion and optical clarity through these exposures. Acetic acid generated by hydrolysis of vinyl acetate units during damp heat can corrode busbars and gridlines; therefore, acid scavengers and low-acid resin variants are used. Volume resistivity of cured encapsulant is typically specified above 1×10¹⁴ Ω·cm per IEC 60093 to limit potential-induced degradation. Amine-based processing aids or colorants should be avoided because they can scavenge radicals, discolor the sheet, and destabilize the silane coupling reaction.

Incoming resin quality control for photovoltaic encapsulant lines typically includes verification of vinyl acetate content by FTIR within ±1 wt% of nominal and melt mass-flow rate within a lot-to-lot control window of approximately ±2 g/10 min. A shift of 1 wt% vinyl acetate alters the peak melting temperature and peel adhesion behavior; therefore, blending of lots with divergent VA content is avoided unless pilot-scale lamination confirms equivalent cure and adhesion. Film thickness is measured by beta gauge or laser profilometry across the web. Thickness variation is typically limited to ±5% of nominal because low spots can leave cells unencapsulated and high spots can increase lamination cycle time.

Automated optical inspection systems classify gels and black specks in the cast sheet. Acceptable gel levels are set by module manufacturers and are not universally standardized; published data for this specific configuration is limited. The resin should not be dry-blended with ionomers, acid-modified ethylene copolymers, or amine-containing masterbatches. Such contamination shifts pH, creates optical haze, and can interfere with peroxide cure or silane adhesion. Metal detectors with ferrous sensitivity of 1.0 mm are installed after granulation of recycled edge trim and before the feed hopper to protect the extruder screw from metal fines.

Storage of bulk pellets is recommended below 25 °C and below 60% RH to limit moisture uptake and blocking. If silo storage exceeds 30 days, re-verification of moisture content and melt-flow stability is common practice.

When an 18 wt% or 28 wt% Vinyl Acetate Grade Is Substituted in Photovoltaic Encapsulant Lines

Substitution of a lower-vinyl-acetate EVA into a photovoltaic encapsulant line changes the melting, adhesion, and optical profile. A 28 wt% VA grade with melt flow of 6 g/10 min melts at approximately 73 °C; its lower melt flow increases head pressure and shear heating at equal screw speed, so barrel temperatures may need to be reduced by 5–10 °C to hold the die-inlet melt below the peroxide half-life threshold. Its lower VA content raises crystallinity and reduces equilibrium moisture uptake but also lowers glass and backsheet adhesion after cure. An 18 wt% VA grade melts near 85 °C and exhibits further reduced adhesion and transparency below 400 nm, making it unsuitable for most glass-glass photovoltaic laminates without additional adhesion promoters.

Compared with a high-flow 33 wt% VA grade with melt flow of 43 g/10 min, Elvax 3170 has greater melt strength and improved gauge stability in cast film, but it develops higher head pressure and may require a wider die gap for thin-gauge encapsulation. The melt flow of 15 g/10 min balances extrusion output, web stability, and rapid flow into the cell gaps during lamination. In multi-layer encapsulant structures, Elvax 3170 is often placed adjacent to the glass to maximize adhesion, while a lower-flow or higher-melting EVA or polyolefin elastomer forms the backside layer to reduce blocking and improve dimensional stability.

The choice of photovoltaic-grade resin also affects lamination cycle time. Lower-melting grades such as Elvax 3170 flow and wet the cell interconnects at lower platen temperature, but they require controlled lamination pressure to avoid bubble entrapment. If platen temperature exceeds 155 °C, volatile additives or moisture can form bubbles before the cure network sets. Conversely, if platen temperature is below 140 °C, the peroxide half-life extends and full gel content may not be reached within the standard 15 min cycle. These boundaries define the operational window for encapsulant production and module assembly.