Determination of Volatile Content in Thermal Conductive Gels for Photovoltaic Modules

Volatile content determination is a critical quality control test for thermal conductive gels used in photovoltaic modules, ensuring long-term reliability and optical clarity over a 25-year service life. This article provides a comprehensive technical overview of the gravimetric weight-loss method, relevant international standards, sampling procedures, calculation formulas, and typical acceptance criteria for silicone-based thermal interface gels in PV applications. Engineers, quality managers, and material scientists will find practical guidance on specimen preparation, testing conditions, and result interpretation.

Technical Overview

Thermal conductive gels are low-modulus, silicone-based interface materials widely used in photovoltaic modules to enhance heat transfer between solar cells, backsheets, junction boxes, and heat-dissipating structures. These gels typically consist of polysiloxane matrices heavily loaded with thermally conductive ceramic fillers such as alumina, aluminum nitride, zinc oxide, or boron nitride. During formulation, curing, and service, residual low-molecular-weight siloxanes, process solvents, moisture, and unreacted oligomers can migrate out of the gel as volatile matter.

Because photovoltaic modules are hermetically sealed laminates expected to operate for more than 25 years, even small quantities of volatiles can cause serious reliability problems. Condensed volatiles may fog the inner surface of the front glass, reduce optical transmittance, contaminate cell metallization, promote encapsulant degradation, and degrade electrical insulation performance. Therefore, the determination of volatile content—usually expressed as the percentage of mass loss after thermal conditioning—is an essential incoming-material inspection and material qualification test for thermal conductive gels in the PV industry.

Testing Samples

Single-Component Thermal Conductive Gel

Ready-to-use, moisture-cure or thermally cured single-component gels supplied in syringes, cartridges, or pails. Specimens are dispensed directly into pre-conditioned weighing dishes and tested either uncured or after a defined pre-cure, depending on the agreed specification.

Two-Component Thermal Conductive Gel

Platinum-catalyzed addition-cure or tin-catalyzed condensation-cure systems supplied as Part A and Part B. The components must be mixed at the manufacturer-specified ratio by weight, de-aired under vacuum to remove entrained bubbles, and cured under defined conditions before volatile content measurement.

Cured Gel Specimens and Module Sub-Assemblies

Fully cured gel sheets, beads, or potted junction box assemblies taken from production lines or field-aged modules. These samples are tested to evaluate post-cure volatile release, long-term thermal stability, and consistency between material lots.

Testing Items

The core deliverable of this test is the volatile content, defined as the mass fraction lost by a specimen after heating under specified temperature and time conditions. Related measurands frequently requested by PV manufacturers include:

• Volatile matter content at 105 °C, 150 °C, or 200 °C (percentage of mass loss)
• Mass loss after full cure versus partial cure (degree-of-cure verification)
• Total Mass Loss (TML) and Collected Volatile Condensable Materials (CVCM) under vacuum outgassing conditions
• Thermal weight-loss stability for reliability correlation with damp heat and thermal cycling exposure
• Lot-to-lot consistency and incoming material conformity assessment

Testing Methods

Gravimetric Oven Weight-Loss Method

The standard approach is a gravimetric procedure adapted from ISO 248 and ASTM D5669 for volatile matter in rubber-like polymeric materials. A typical protocol is as follows:

• Condition clean aluminum or glass weighing dishes to constant mass in a desiccator and weigh each dish to 0.1 mg on an analytical balance.
• Dispense approximately 1 g to 3 g of gel sample into each dish, spreading it into a thin, uniform layer of no more than 2 mm to ensure complete volatilization.
• For two-component systems, mix Parts A and B at the specified ratio, de-air under vacuum, dispense, and cure according to the supplier recommendation, for example 150 °C for 30 minutes.
• Heat the loaded dishes in a forced-convection oven at the specified temperature, commonly 150 °C ± 2 °C for 1 hour, or another agreed condition such as 105 °C for 3 hours or 200 °C for 1 hour.
• Transfer the dishes to a desiccator, cool for 30 minutes, and reweigh to 0.1 mg. Repeat the heating-cooling-weighing cycle until constant mass is achieved, defined as a change of less than 0.1 mg between consecutive cycles.
• Test a minimum of three specimens per sample and report individual and mean values.

Calculation Formula

The volatile content is calculated as: Volatile content (%) = (m0 − m1) / m0 × 100, where m0 is the initial mass of the specimen and m1 is the mass after thermal conditioning to constant weight. Results are reported together with the curing condition, test temperature, dwell time, and any deviations from the standard protocol.

Vacuum Outgassing Method (TML and CVCM)

For applications where condensable contamination is the primary concern, the outgassing method described in ASTM E595 may be applied. Pre-weighed specimens are held at 125 °C under vacuum (approximately 5 × 10⁻⁵ torr) for 24 hours. The Total Mass Loss is determined gravimetrically, while Collected Volatile Condensable Materials are quantified on a cooled collector plate maintained at 25 °C. Typical acceptance thresholds in the space and electronics industries, often referenced by PV engineers, are TML below 1.0% and CVCM below 0.10%.

Influencing Factors and Precautions

• Specimen thickness and surface area significantly affect volatilization rate; thin, uniform layers must be maintained.
• Incomplete cure of addition-cure gels can inflate apparent volatile content; cure verification is recommended.
• Oven ventilation rate, temperature uniformity, and calibration directly influence reproducibility.
• Hygroscopic fillers may introduce moisture-related mass loss; a separate low-temperature drying step can help distinguish moisture from siloxane volatiles.

Applications

• Incoming quality control of thermal conductive gels and gap-filler materials at photovoltaic module manufacturers and junction box suppliers.
• Material qualification and supplier comparison during new product introduction for cell-level, backsheet, and frame-based thermal management designs.
• Research and development support for formulation optimization of low-volatility silicone systems with reduced cyclic siloxane content.
• Reliability engineering studies correlating volatile content with damp heat endurance, thermal cycling performance, and PID resistance in accordance with IEC 61215 and IEC 61730 test sequences.
• Failure analysis of fogging, yellowing, delamination, and insulation degradation observed in field-aged or chamber-exposed modules.
• Independent verification testing at accredited third-party laboratories for product certification and customer audits.

FAQ

Why is volatile content so critical for photovoltaic modules?

PV modules are sealed laminates with design lifetimes exceeding 25 years. Volatiles released from thermal gels cannot escape and instead condense on the inner glass surface or cell surfaces, reducing light transmission, promoting encapsulant discoloration, and potentially compromising dielectric insulation. A low volatile content is therefore a key reliability indicator.

What test temperature and time should be selected?

The condition should reflect the worst-case service temperature without inducing thermal decomposition. Common industry conditions include 150 °C for 1 hour for general screening, or 200 °C for accelerated assessment. ASTM E595 uses 125 °C for 24 hours under vacuum when condensable contamination is the concern. The chosen condition must always be stated in the test report.

What is a typical acceptance criterion?

Most PV specifications limit mass loss to 1.0% or below, with stricter limits of 0.5% or less for materials located in the optical path or in direct contact with encapsulants. For vacuum outgassing testing, TML below 1.0% and CVCM below 0.10% are commonly referenced thresholds.

Can the test be performed on cured gel instead of the uncured product?

Yes. Testing cured specimens isolates the volatiles released after crosslinking is complete, which better represents long-term in-module behavior. Both uncured and cured measurements are often requested together to fully characterize a material.

How much sample is required and what is the typical turnaround?

Approximately 10 g to 20 g of gel per lot is generally sufficient for three replicate specimens plus retained sample. Gravimetric volatile content testing is typically completed within three to five working days, including curing and constant-weight verification.