Thermal conductive gels used in photovoltaic modules must sustain adhesion, thermal transfer, and electrical insulation despite long-term contact with acids, alkalis, salt solutions, solvents, and cleaning agents. Chemical resistance testing based on ISO 175, ASTM D543, ASTM D471, and IEC 62788 quantifies post-exposure changes in mass, volume, hardness, thermal conductivity, and dielectric performance. This article outlines sample preparation, reagent selection, immersion and spot-test protocols, acceptance criteria, and typical applications for PV module thermal gel compatibility evaluation.
Technical Overview
Thermal conductive gels—typically silicone or polyurethane-based, heavily filled with thermally conductive ceramic or metal oxide particles—are dispensed in photovoltaic modules to bridge air gaps between cells or ribbons and heat-dissipating structures such as backsheets, frames, junction box housings, and heat sinks. Their soft, compliant nature maintains intimate interfacial contact through thousands of thermal cycles while providing electrical insulation and stress relief.
In service, these gels are exposed to a chemically active environment. Encapsulant degradation releases acetic acid; atmospheric deposition delivers acid rain and salt mist; installation and maintenance involve cleaning agents, alcohols, and sealant solvents; and emerging applications such as building-integrated PV, floating PV, and automotive roof modules add contact with concrete alkalis, water treatments, fuels, and coolants. Chemical attack can cause swelling, plasticizer or siloxane extraction, filler-matrix debonding, adhesion loss, and drift in thermal or dielectric properties. Chemical resistance testing therefore forms an essential part of material qualification and incoming inspection for PV module manufacturers and material suppliers, and is frequently executed by accredited third-party laboratories under recognized international frameworks.
Testing Samples
Sample configuration determines which properties can be measured after exposure. A complete evaluation program typically includes all of the following specimen types.
Cured Free-Standing Gel Plaques
Two-part gels are mixed, degassed, and cast into flat sheets—commonly 50 mm × 50 mm × 2 mm—and cured according to the manufacturer's specified schedule (for example, 25 °C for 24 h followed by 80 °C for 1 h). Plaques enable accurate measurement of mass change, volume change, hardness, thermal conductivity, and dielectric properties without substrate interference. A minimum of three to five specimens per reagent and per time interval is recommended for statistical validity.
Gel-on-Substrate Assemblies
Gel is dispensed and cured onto representative module surfaces, including tempered glass, anodized aluminum, tinned or bare copper, and fluoropolymer-based backsheet films such as PVF or PVDF laminates. These assemblies permit evaluation of adhesion retention, interfacial corrosion, and substrate staining after chemical exposure, reflecting the true bonded state rather than bulk behavior alone.
Module-Level Coupons and Mini-Modules
For final qualification, small laminates or cut module sections containing dispensed gel, or complete junction boxes potted or bonded with the gel, are exposed to reagents and subsequently assessed visually and, where applicable, through functional checks such as diode heat dissipation performance or insulation resistance.
Control and Reference Specimens
Unexposed control specimens, conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 24 h, are retained for each batch so that all post-exposure changes are referenced to a time-matched baseline.
Testing Items
The evaluation matrix covers appearance, physical, functional, and interfacial properties measured before and after exposure:
- Visual appearance: discoloration, turbidity, blistering, cracking, surface tack, dissolution, phase separation, or bleeding of filler or oil
- Mass change: percentage mass gain (absorption and swelling) or mass loss (extraction of oligomers, plasticizers, or siloxanes)
- Dimensional and volume change: swelling ratio and linear expansion
- Hardness change: Shore 00, Shore A, or Asker-C depending on gel softness, per ISO 868 or ASTM D2240
- Thermal conductivity retention: measured by heat-flow meter (ASTM C518 or ISO 22007-2) or laser flash method (ASTM E1461 or ISO 22007-4)
- Volume resistivity: per IEC 62631-3-1 or ASTM D257
- Dielectric strength: per IEC 60243-1 or ASTM D149
- Adhesion retention: 90° or 180° peel strength (ASTM D903 or ASTM D3330) or lap-shear strength (ASTM D1002) on substrate assemblies
- Substrate interaction: corrosion, staining, or migration products on adjacent metal or glass surfaces
Testing Methods
Applicable Standards
Immersion testing is principally governed by ISO 175 and ISO 1817 (rubber and elastomeric materials), ASTM D543 (plastic resistance to chemical reagents), and ASTM D471 (rubber property—effect of liquids). Chinese national equivalents include GB/T 11547 and GB/T 1690. Polymeric material compatibility screening may also reference UL 746A, while IEC 62788 series procedures and IEC 61730-2 safety requirements frame material measurement and module-level validation within the PV industry.
Reagent Selection and Test Conditions
Reagent lists are tailored to the module service environment. Typical selections include 5 % sulfuric acid (acid rain simulation), 5 % acetic acid (encapsulant degradation product), 10 % hydrochloric acid, 10 % sodium hydroxide or potassium hydroxide solution, 3 % to 10 % sodium chloride solution (coastal salt mist), isopropyl alcohol and ethanol (module cleaning), diluted neutral and alkaline detergents, toluene or xylene (sealant and coating solvents), 50 % ethylene glycol coolant solution (hybrid thermal-PV systems), and deionized water as a baseline. Exposure temperatures are commonly 23 ± 2 °C for standard conditioning and 40 °C, 60 °C, or 85 °C for accelerated evaluation, with durations of 24 h, 72 h, 168 h, and 672 h (28 days) as standard checkpoints.
Immersion Test Protocol
- Condition all specimens for at least 24 h at 23 ± 2 °C and 50 ± 5 % relative humidity, then record baseline mass, dimensions, hardness, thermal conductivity, and electrical properties
- Immerse specimens fully in the reagent using a liquid-to-specimen volume ratio of at least 8:1, keeping specimens separated and free of contact with container walls
- Maintain the bath at the specified temperature within ± 2 °C; refresh reagents periodically during long-duration tests to preserve concentration
- Remove specimens at each checkpoint, blot surface liquid gently with lint-free cloth within 30 seconds without aggressive wiping, and weigh within 60 seconds because volatile reagents produce time-sensitive readings
- Perform post-exposure hardness, thermal, and electrical measurements immediately and again after a recovery period of 24 h to 48 h at 23 °C (or 40 °C for drying-sensitive materials) to distinguish reversible swelling from permanent degradation
Spot and Wipe Exposure Test
For surfaces that encounter chemicals only transiently—such as frames during cleaning or junction boxes during maintenance—an open-spot test is applied. A droplet or reagent-saturated pad is placed on the cured gel surface for 24 h at 23 °C or 50 °C, covered to limit evaporation, then wiped. The surface is evaluated for softening, tack, gloss loss, or erosion, and residual contamination is wiped with cheesecloth in line with UL 746A-style compatibility screening.
Typical Acceptance Criteria
While project specifications vary, common industry-level limits are:
- No cracking, blistering, dissolution, or loss of cohesive integrity after exposure
- Mass change within +10 % to −5 % for immersion exposure, or tighter limits for spot exposure
- Hardness change not exceeding 10 Shore points
- Thermal conductivity retention of at least 90 % of baseline
- Volume resistivity and dielectric strength retention of at least 80 % to 90 % of baseline
- Adhesion strength retention of at least 70 % to 80 % with cohesive or intact failure mode
- No visible corrosion, staining, or deposit formation on adjacent glass, aluminum, or copper surfaces
Combined-Stress Sequencing
To capture synergistic effects, chemical exposure is frequently sequenced with IEC 61215-2 style thermal cycling (−40 °C to +85 °C) or damp heat (85 °C / 85 % relative humidity) followed by re-measurement of adhesion and thermal performance. This confirms that a chemically plasticized gel does not fail prematurely under subsequent thermomechanical fatigue.
Applications
- Junction box potting and bonding gels, where long-term contact with potting compounds, cleaning solvents, and outdoor pollutants must not degrade diode heat dissipation
- Cell-to-backsheet and cell-to-frame gap filling in high-power modules requiring stable thermal impedance over a 25-to-30-year service life
- Bifacial and glass-glass modules, where rear-side gels face concentrated soiling and cleaning agent exposure
- Building-integrated photovoltaics, involving contact with concrete alkalis, silicone sealants, and their curing byproducts
- Floating and near-shore photovoltaic plants, with salt water, anti-fouling treatments, and humid corrosive atmospheres
- Agrivoltaic installations, where fertilizer and pesticide drift create a chemically diverse exposure environment
- Vehicle-integrated photovoltaic modules, requiring compatibility with fuels, oils, brake fluids, and ethylene glycol coolants
- Material qualification, supplier change management, and incoming quality control for module manufacturers and EPC contractors
FAQ
Which standards govern chemical resistance testing of thermal conductive gels for PV modules?
ISO 175, ASTM D543, ASTM D471, and ISO 1817 define immersion exposure and property-change measurement for plastics and elastomers, with GB/T 11547 and GB/T 1690 as Chinese national equivalents. UL 746A provides polymeric material compatibility screening, and IEC 62788 series procedures support material-level measurement relevant to IEC 61730-2 and IEC 61215-2 module qualification frameworks.
How long should the immersion exposure last?
Screening evaluations commonly use 24 h and 168 h checkpoints, while qualification programs extend to 672 h (28 days) or even 1000 h with periodic reagent refresh. Longer durations with elevated temperature up to 85 °C are used to accelerate diffusion-limited processes when justifying multi-decade field life.
Which reagents are most critical for PV module thermal gels?
Acetic acid reflects encapsulant degradation chemistry and is nearly always included, alongside sulfuric acid for acid rain, sodium chloride solution for coastal sites, isopropyl alcohol and detergent solutions for cleaning compatibility, and application-specific media such as ethylene glycol coolant or concrete alkali extracts for hybrid or building-integrated systems.
Why is thermal conductivity re-measured after exposure rather than assumed unchanged?
Swelling increases inter-particle spacing while extraction of oligomers or plasticizers can alter filler wetting and dispersion, both of which shift effective thermal conductivity. Because the gel exists specifically to transfer heat, conductivity retention—typically required at 90 % or better of baseline—is a decisive pass criterion.
Should testing be performed by an accredited third-party laboratory?
Independent verification by accredited third-party laboratories is strongly recommended for supplier qualification, customer submission, and certification support, because it provides traceable measurement uncertainty, standard-compliant conditioning, and results that are recognized across the international PV supply chain.