Environmental performance testing of thermal conductive gels for photovoltaic modules verifies that these silicone-based thermal interface materials comply with global chemical restrictions such as RoHS, REACH SVHC screening, and halogen-free requirements before they are integrated into solar products. As module warranties extend to 25–30 years and export markets tighten chemical regulations, accredited third-party laboratories play a critical role in confirming that thermal gels contain no hazardous substances and release no harmful volatiles. This article explains the sample requirements, test items, analytical methods, applicable standards, and common questions related to the environmental evaluation of thermal conductive gels used in PV modules.
Technical Overview
Thermal conductive gels are soft, curable thermal interface materials widely used in photovoltaic modules to bridge air gaps between solar cells, backsheets, junction boxes, and heat-dissipating structures. They are typically formulated from silicone (or, less commonly, polyurethane) polymers loaded with thermally conductive ceramic fillers such as alumina, aluminum nitride, boron nitride, or zinc oxide. After curing, the gel forms a compliant elastomeric layer that transfers heat efficiently while absorbing thermomechanical stress during decades of outdoor service.
Why Environmental Performance Matters
Although thermal conductive gels are sealed inside the module laminate or junction box, their chemical composition directly affects environmental compliance, long-term reliability, and end-of-life handling. Potential concerns include restricted heavy metals and flame retardants introduced via fillers or additives, halogenated residues, residual low-molecular-weight cyclic siloxanes that can migrate and deposit on glass or encapsulant interfaces, and volatile organic compounds emitted during high-temperature operation. Environmental performance testing quantifies these risks and supports product declarations, customer audits, and market access.
Regulatory and Standards Framework
Evaluations are typically structured around the EU RoHS Directive (2011/65/EU) and its testing standard series IEC 62321, the REACH Regulation (EC) No. 1907/2006 for Substances of Very High Concern (SVHC), China RoHS requirements described in GB/T 26572 and the GB/T 39560 series, Japanese marking requirements aligned with JIS C 0950, and halogen-free material criteria defined in IEC 61249-2-21. Emission and outgassing behavior is assessed using the ISO 16000 series and ASTM E595, while leaching behavior can be evaluated under EN 12457 or ASTM D3987 extraction protocols.
Testing Samples
Test programs are designed around the physical state of the material, because the analytical approach differs for liquid components, cured elastomers, and finished assemblies. Typical sample types include:
Uncured Gel Components
For two-component products, the base (Part A) and curing agent (Part B) are submitted separately in quantities of approximately 10–20 g each. Testing the uncured components allows direct digestion and solvent extraction for formulation-level restricted substance analysis and detection of residual monomers, catalysts, and process additives.
Cured Gel Specimens
Coupons cured according to the supplier's recommended schedule (for example, 1–4 hours at elevated temperature or 24 hours at room temperature) are used for outgassing, emission, and leaching studies. A minimum of 50–100 g of cured material is generally required to complete a comprehensive test package.
In-Situ Gel Assemblies
Gel layers applied to backsheet films, aluminum substrates, or harvested from finished modules can be separated and homogenized for analysis. This verifies the environmental status of the material as it actually exists in the product and captures any contamination introduced during application.
Aged and Stressed Samples
For research and qualification programs, samples exposed to damp heat (85 °C / 85% RH for 1000 hours) or thermal cycling can be retested to determine whether aging generates new extractables or increases volatile release over the module service life.
Testing Items
A complete environmental assessment typically covers the following categories, each with defined acceptance limits:
- RoHS restricted substances: lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyls (PBB), polybrominated diphenyl ethers (PBDE), and the four phthalates DEHP, BBP, DBP, and DIBP, each limited to 1000 ppm (cadmium limited to 100 ppm).
- REACH SVHC screening against the current Candidate List, plus Annex XVII restricted substances such as organotin compounds and short-chain chlorinated paraffins.
- Halogen content: total chlorine, bromine, fluorine, and iodine, with halogen-free guidance of chlorine below 900 ppm, bromine below 900 ppm, and combined chlorine plus bromine below 1500 ppm per IEC 61249-2-21.
- Cyclic siloxanes D4, D5, and D6, together with total volatile siloxanes, to control outgassing-related adhesion and optical risks.
- Volatile organic compound (VOC) and semi-volatile organic compound (SVOC) emissions, including formaldehyde and aldehyde-ketone profiles.
- Per- and polyfluoroalkyl substances (PFAS): total organic fluorine screening followed by targeted quantification of PFOA, PFOS, and related compounds.
- Heavy metal leachability under water-based extraction to assess release potential during service or recycling.
- Outgassing characteristics: total mass loss (TML) and collected volatile condensable materials (CVCM).
- Supporting material characterization: polymer identification by FTIR and filler loading by thermogravimetric analysis for quality control purposes.
Testing Methods
Accredited third-party laboratories apply standards-based analytical methods tailored to the polymer matrix and the target substance class:
- X-ray fluorescence (XRF) screening per IEC 62321-3-1 provides rapid non-destructive preliminary results for metals and bromine.
- Acid or microwave-assisted digestion followed by ICP-OES or ICP-MS, per IEC 62321-4 and IEC 62321-5 and aligned with ISO 11885 and ISO 17294-2, quantifies lead, cadmium, mercury, and elemental heavy metals.
- Hexavalent chromium in polymers is determined by boiling-water extraction and colorimetric UV-Vis spectrophotometry per IEC 62321-7-1.
- PBB and PBDE flame retardants are quantified by GC-MS per IEC 62321-6, and phthalates by GC-MS per IEC 62321-8.
- Total halogens are measured by combustion ion chromatography per IEC 62321-3-2, EN 14582, and ASTM D7359.
- VOC and SVOC emissions are evaluated in emission test chambers per ISO 16000-9, ISO 16000-10, and ISO 16000-11, with GC-MS quantification per ISO 16000-6 and formaldehyde determination per ISO 16000-3.
- Cyclic and linear siloxanes are measured by headspace GC-MS or GC-FID, complemented by volatile content determination via thermogravimetric analysis per ASTM E1131 and ISO 11358, and vacuum outgassing per ASTM E595.
- PFAS screening uses combustion ion chromatography for total fluorine with confirmatory targeted LC-MS/MS analysis of individual compounds.
- Leaching studies follow EN 12457 or ASTM D3987 extraction, with eluates analyzed by ion chromatography and ICP-MS.
Applications
Environmental performance testing of thermal conductive gels supports multiple points in the photovoltaic value chain:
- Incoming material control by module manufacturers, ensuring that every thermal interface material entering production meets internal and customer chemical specifications.
- Qualification of new gel formulations or replacement suppliers, providing documented equivalence during engineering change management.
- Compliance dossiers supporting module certification under IEC 61215 and IEC 61730, customer declarations, and market access in the EU, China, Japan, and other regulated regions.
- Building-integrated photovoltaics (BIPV), where low-emission and low-VOC documentation is requested as part of green building documentation.
- Research and development, including filler purity studies, volatile-content optimization, and aging stability assessment.
- End-of-life and recycling programs, where leaching data assists in waste classification of decommissioned modules.
FAQ
Why does an internal gel need environmental testing if it is sealed inside the module?
Sealed materials still fall within the scope of chemical regulations, can release volatiles that migrate through the laminate over time, and influence waste classification at end of life. Testing documents that the material is safe across the entire product lifecycle, not only during use.
Are silicone gels automatically halogen-free?
No. While the silicone polymer itself contains no halogens, fillers, flame retardants, adhesion promoters, and process residues may introduce chlorine or bromine. Combustion ion chromatography is required to confirm halogen-free status.
What is the difference between RoHS testing and REACH SVHC screening?
RoHS testing targets a defined list of ten restricted substances with fixed concentration limits. REACH SVHC screening covers a much larger and regularly updated candidate list of substances, reporting any presence above 0.1% weight-by-weight for notification obligations.
How much sample and time are required?
A full test package generally requires 50–100 g of material including both components of two-part systems. Standard turnaround is typically 7–15 working days, with XRF screening available in as little as 2–3 working days.
Can testing be performed on gel already applied in a finished module?
Yes. Gel can be excised from finished modules or laminates, separated from substrates, and homogenized before analysis, allowing verification of the exact material configuration delivered to customers.