Module Backsheet (PET / Fluorinated): UV Resistance, Hydrolysis Resistance and the Fluorine-Free Route

Cross-section of a multi-layer PV backsheet structure

The backsheet is the outermost layer facing UV, moisture and mechanical stress; its stability sets the module's long-term reliability class

The backsheet accounts for less than 0.5% of module BOM cost, yet it decides whether the module passes TUV certification and the IEC 61215 / IEC 61730 aging tests. This is a classic low-cost, high-responsibility component - a cracked or delaminated backsheet usually means failure of the entire module.

I. Backsheet architecture and material interfaces

A mainstream backsheet is a 3- or 5-layer co-extrusion, and the outer fluoropolymer layer carries the UV load.

Mainstream backsheets are three- or five-layer co-extruded composite structures. Understanding each layer's material characteristics is the precondition for choosing the right additives:

LayerTypical materialMain functionAging concerns
Outer layerFluoropolymer resin (PVDF) / anti-UV co-extrusion layerUV resistance, weather resistance, chemical-media resistanceLong-term outdoor UV aging, chalking, cracking
Adhesive layerCopolyamide-type adhesivesInter-layer bondingInterfacial hydrolysis, compatibility with adjacent layers
Core layerPET (polyester film)Primary mechanical support, moisture barrierHydrolysis resistance, strength retention
Inner layerPOE / EVA co-extrusion layerBonding interface with the encapsulant filmIon content, compatibility with the encapsulant
The easily overlooked point:PET is the most misjudged material in the polyester family. Many customers assume the PET core layer only needs to meet mechanical strength, but polyester hydrolyzes and loses strength far faster than expected, directly affecting the backsheet's load capacity and corner tear resistance. Tiangang® AO-1010 and AO-1076 have long been used in polyester systems to slow hydrolytic degradation - a frequently overlooked but high-value application point in PV backsheet formulation.

II. Additive differences between the fluorinated and fluorine-free routes

Driven by both regulation and cost, fluorinated backsheets are evolving toward fluorine-free and low-fluorine routes. The two routes differ markedly in additive requirements:

DimensionFluorinated route (PVDF face layer)Fluorine-free / low-fluorine route (anti-UV co-extrusion)
Face-layer weather resistanceCarried mainly by the PVDF itselfHighly dependent on the additive system
Anti-UV requirementSupplementary - slows face-layer agingCritical - UVA carries the primary protection
HALS requirementModerate - suppresses deep oxidationHigh - carries long-cycle protection
Antioxidant requirementPET core-layer hydrolysis dominates; antioxidants play a supporting roleHigh - antioxidants determine core-layer life

In short: the fluorinated route assigns weather resistance to the face-layer material; the fluorine-free route must close that gap with the additive system. This is also the root cause of why additive loading and technical importance rise together in the fluorine-free transition.

III. Tiangang's recommended formulations

The anti-UV face layer takes UV-326 or UV-328 at 0.5-1.5 phr, covering the full 290-400 nm band.

Application layerRecommended productReference loadingNotes
Anti-UV face layer / co-extrusion layerUV-326 / UV-3280.5 – 1.5 phrBenzotriazole, full 290-400 nm coverage
UV-234 (triazine)0.5 – 1.5 phrAcid- and alkali-resistant; suits coexistence with the adhesive layer
PET core-layer hydrolysis resistanceAO-10100.3 – 1.0 phrSlows polyester hydrolysis, maintains strength retention
AO-10760.2 – 0.6 phrLiquid, easy to disperse; suits impregnation or coating processes
Long-cycle weather resistanceHS-944 / BW-10LD0.2 – 0.8 phrHigh-molecular-weight HALS, low migration, no blooming
Functional masterbatch optionMB570-20PETCalculated on the masterbatchDesigned specifically for PET substrates, with excellent dispersibility
On loadings: the loading ranges given are common engineering reference ranges. Final dosage must be confirmed against backsheet layer thickness, resin grade, inter-layer adhesive system and target climate zone (especially high-UV desert plants) through accelerated-aging validation. The Tiangang technical team can provide a matching validation plan.

IV. TUV backsheet certification focus points

Tiangang® MB570-20PET is an anti-UV functional masterbatch developed specifically for PET substrates.

For TUV backsheet certification, the following items are directly tied to the additive formulation and worth preparing in advance:

Test itemWhat is assessedFormulation-related risk points
UV preconditioning (IEC 61215 MQT 10)Material performance retention after UV exposureInsufficient UVA - strength retention and transmittance miss the target
damp heat 85/85 (MQT 13)Insulation and mechanical retention under damp heatInsufficient antioxidant - accelerated PET hydrolysis, declining insulation performance
Mechanical load test (MQT 01)Integrity after installation and wind loadEmbrittlement after aging leads to cracking; insufficient HALS accelerates degradation

Q: Does Tiangang offer a functional masterbatch specifically for PET?

A: Yes.adapted to PET's processing temperature and dispersion characteristics. It markedly reduces the problem of uneven dispersion in high-viscosity PET systems, which causes insufficient effective additive concentration.

Q: Do fluorinated backsheets still need a UV absorber?

A: Yes, but the role differs. The PVDF face layer carries the primary protection; additives serve to protect the PET core layer and inter-layer interfaces and to slow the aging of the face layer itself. They are supplementary rather than critical, but still need to be included in long-term reliability demonstrations.

Q: Can backsheet formulations use liquid additives?

A: Liquid grades such as AO-1076 disperse better than powders in coating and impregnation processes and suit enhancement processes where the PET surface is impregnated. Tiangang also offers liquid HALS grades.

If you are upgrading a backsheet formulation - especially moving from fluorinated to fluorine-free, or addressing the UV and damp-heat items for TUV certification - send us the layer structure, layer materials and film thicknesses. The Tiangang technical team will provide targeted additive recommendations.

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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Certifications

  • 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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