Encapsulant Film (EVA / POE): Yellowing, PID and UVA+HALS Synergy

Encapsulant film accounts for roughly 2-4% of module BOM cost, yet it determines the only physical barrier between the cells and the outside environment. Over its 25-year life it must simultaneously deliver: low water-vapor transmission (WVTR), no yellowing, no delamination and no conductivity. And once module operating voltage rises above 1500 V, the film must also take on resistance to PID.

I. Three failure paths of the encapsulant film

Three failure paths dominate: yellowing, delamination and PID, all driven by 290-400 nm UV plus residual VA groups.

Failure mode	Mechanism	Appearance and consequence
Yellowing (rising ΔYI)	Photo-oxidation of EVA generates yellow quinonoid products; residual VA and acidic by-products further catalyze degradation under UV	The film yellows overall, transmittance drops and module power degrades; in severe cases the cells themselves are stained
PID (potential-induced degradation)	The high electric field between the positive and negative poles (DC 1500 V+) drives carriers to the passivation-layer interface of the cell, weakening passivation	Fill factor and open-circuit voltage drop, with non-linear power decay; humidity accelerates the process
Delamination / bubbles	Residual acetic acid and degradation products accumulate at the interface and become chemically incompatible with the adhesive layer	Inter-layer peeling and blistering; fails the IEC 61215 damp-heat test

Key insight: these three are not independent of one another. Yellowing is itself a signature of photo-oxidation, and the acidic species produced by degradation in turn accelerate delamination; once delaminated, the interface opens a path for moisture and ions, accelerating PID. Treating them as three separate problems and patching each in isolation therefore tends to deliver limited results.

II. Why UVA and HALS must be combined

A UVA absorbs one photon and is consumed, while HALS regenerates; over 290-400 nm the two must be paired.

This is the most central - and most often oversimplified - principle in encapsulant formulation. To understand it, note that the two additive types act at different positions:

Benzotriazole UVAs: single-pass consumption

After absorbing 290-400 nm UV they form quinonoid intermediates, one molecule absorbs only once, and are then consumed. They intercept photons at the periphery of the system, but have an upper limit on effective lifetime.

Hindered-amine HALS: cyclic regeneration

They terminate excited-state hydrocarbons through quenching and act as hydrogen donors in a regeneration loop, with no net consumption of themselves, so in theory they can protect for the full service life.

The key point: the UV energy absorbed by UVA is released as heat, which in turn protects the HALS chromophore from photolysis. Conversely, the hydroxylamines and nitroxyl radicals generated by HALS can regenerate the consumed UVA chromophore, delaying its failure. The two protect different targets and are consumed differently - one is single-pass, the other cyclic - so they must be combined to cover the full life cycle.

On the synergy factor: the industry often says UVA+HALS synergy multiplies efficiency severalfold, but that multiple depends heavily on the base formulation: too little HALS and the synergy never shows; too much and the alkalinity can gel EVA or aggravate acidic by-product issues. When Tiangang designs formulations for customers we apply synergy-factor assessment - adding a same-order UVA dose on top of the base formulation and checking whether the improvement in HALS efficiency exceeds the baseline, rather than applying a fixed synergy multiple.

III. Tiangang's recommended formulations

Plan A for EVA encapsulant film: UV-326 or UV-328 at 0.3-0.8 phr together with a HALS.

Plan A | EVA encapsulant film (general weather resistance)

Component	Recommended product	Reference loading	Function
UV absorber	UV-326 / UV-328	0.3 – 0.8 phr	Intercepts UV, reduces photoinitiation
High-molecular-weight HALS	Tiangang® HS-944	0.2 – 0.6 phr	Excited-state quenching, long-term protection
	BW-10LD (HALS 622)	0.2 – 0.5 phr	Polymeric, high molecular weight, low migration
Primary antioxidant	Tiangang® AO-1010	0.1 – 0.3 phr	Traps free radicals; the primary line of defense
Secondary antioxidant	Tiangang® AO-168	0.1 – 0.3 phr	Decomposes hydroperoxides
	AO-B215 (1010+168 pre-blend)	0.3 – 0.8 phr	All-in-one, fewer compounding steps

Plan B | POE encapsulant film (ion control first)

POE film performs very well on PID resistance and moisture barrier, but is extremely sensitive to ionic and metal-ion impurities. Additives used in the formulation must therefore be inherently low in ionic and metal-ion content, to avoid introducing new PID drivers.

Component	Recommended product	Why this product
UV absorber	UV-234 (triazine) / UV-360	The triazine structure is more acid- and alkali-resistant and compatible with POE
HALS	HS-944 / BW-10LD	High molecular weight and non-migrating; avoids interfacial blooming that would compromise adhesion
Antioxidant	AO-1010 + AO-168	Controls oxidative degradation introduced by the thermal history
Anti-PID synergy	Pair with low-ion flame-retardant grades	Avoid introducing metal-ion and anionic impurities

Compliance note: UV-327 is a chlorinated grade whose classification under the EU CLP regulation has been tightened, creating import-compliance uncertainty.UV-326 and UV-328are benzotriazoles without chlorine - the standard substitution path for avoiding this risk. Where higher acid resistance is required, the triazines UV-234 and UV-360 can be evaluated further.

IV. The EVA-to-POE shift: industry trend and response

Driven by rising bifacial-module penetration and PID failure cases, POE/EVA hybrid encapsulant systems are becoming the mainstream direction. This brings a key formulation-level shift:

from pursuing weather resistance to pursuing ion purity - the metal-ion content and ash of additives become an entry threshold;
from single components to synergistic systems - UVA, HALS, antioxidants and ion scavengers must all be present;
from short-cycle validation to full-life-cycle demonstration - accelerated tests such as damp heat 85°C/85%RH per IEC 62788 become mandatory;

On P/E hybrid systems: P/E hybrid encapsulant systems (e.g. 30% POE + 70% EVA, or three-layer co-extrusion) strike a compromise between cost and performance, but ratios differ widely between manufacturers. The additive package needs to be validated against the specific ratio and cannot be carried over directly from a pure-EVA or pure-POE formulation.

If you are developing or optimizing a PV encapsulant formulation, send us the resin system (EVA grade / POE supplier), the ratio, the processing temperature and the target validation items (yellowing level, PID damp-heat duration). The Tiangang technical team will provide the matching additive package and validation support.

Contact the Tiangang technical team →

Technical reviewTiangang Technical CenterVersion2026-11-07

Specifications and application notes on this page are compiled from Tiangang's published technical data. Actual values are governed by the COA shipped with each lot and the technical agreement confirmed by both parties.

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National High-Tech Enterprise
Vice Chairman Unit, China Plastics Processing Industry Association (CPPIA)
Vice Chairman Unit, Agricultural Plastics Branch, China Agricultural Plastics Application Technology Society
Member, Ageing Methods Subcommittee, National Technical Committee on Plastics (SAC/TC15/CS5)
Member, Society of the Plastics Industry (SPI), USA

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