Determination of Weight Change in Sulfur Corrosion Testing of Foam Materials

Foam sulfur corrosion weight change testing is a gravimetric method that quantifies the corrosive volatile sulfur compounds emitted by polymeric foam materials by measuring the mass gain of exposed silver or copper coupons. This test is critical for validating foams, foam tapes, and gaskets used in display modules, electronic assemblies, and automotive systems, where sulfide formation can tarnish silver-plated contacts and degrade electrical performance. This article explains the test principle, sample types, procedures, evaluation criteria, and typical applications as performed by accredited third-party laboratories.

Technical Overview

Polymeric foam materials such as EVA, PE, PU, EPDM, neoprene, silicone, and adhesive-coated foam tapes may contain sulfur-bearing substances, including vulcanization agents, accelerators, antioxidants, pigments such as ultramarine blue, flame retardants, tackifying resins, and adhesive residues. When these foams are subjected to elevated temperature and humidity inside sealed electronic enclosures, they can release volatile sulfur species such as hydrogen sulfide, carbonyl sulfide, carbon disulfide, sulfur dioxide, mercaptans, and elemental sulfur vapor.

These volatile sulfur compounds react readily with silver and copper surfaces. The typical reaction is 2Ag + H2S + 1/2 O2 → Ag2S + H2O, which produces a silver sulfide film that progresses from light yellow to brown and finally to black discoloration. Because sulfide formation deposits sulfur atoms onto the metal surface, the coupon gains measurable mass, and this mass gain is directly proportional to the amount of corrosive sulfur delivered by the foam sample.

The weight change determination therefore exposes a pre-weighed, high-purity silver or copper coupon to the foam specimen in a sealed glass vessel under controlled temperature and humidity, then re-weighs the coupon after a defined duration. The mass change per unit area, typically expressed in mg/dm² or μg/cm², provides an objective, quantitative index of the sulfur corrosivity of the foam. This gravimetric approach is more rigorous than visual discoloration rating alone, and it complements related environmental test concepts described in ASTM B809 for humid sulfur vapor exposure and IEC 60068-2-60 for flowing mixed-gas corrosion testing, adapted to a static sealed-system assessment of material outgassing.

Testing Samples

EVA and PE Foams

Closed-cell ethylene-vinyl acetate and polyethylene foams are widely used as cushioning pads, backlight spacers, and packaging inserts in display and consumer electronics. Potential sulfur sources include processing additives, colorants, and laminated adhesive layers, making them a primary category for sulfur corrosion screening.

Polyurethane (PU) Foams

Open-cell and closed-cell PU foams used for sealing, shock absorption, and acoustic damping may contain amine-based catalysts, antioxidants, and flame retardants that can liberate sulfur-containing decomposition products during thermal aging.

EPDM and Neoprene Rubber Foams

Sulfur-vulcanized rubber foams such as EPDM and chloroprene (neoprene) are among the most significant sulfur emission sources because residual curing agents and accelerators can continue to outgas after fabrication. They are commonly tested for automotive gaskets, battery module sealing, and industrial enclosure applications.

Foam Tapes and Adhesive Laminates

Double-sided acrylic foam tapes, PET core foam tapes, and adhesive-laminated foam pads used in module assembly are frequently implicated in silver electrode tarnish cases. Both the foam core and the adhesive layers are evaluated together, since adhesives can contribute their own sulfur-containing residuals.

Silicone Foams and Gaskets

Peroxide-cured silicone foams used in lighting, telecom, and aerospace sealing applications are tested to verify that curing by-products and filler additives do not generate corrosive sulfur emissions under service conditions.

Testing Items

The scope of a foam sulfur corrosion weight change determination typically includes the following measurable parameters:

• Mass change of a silver coupon after exposure to the foam specimen, expressed as mg/dm² or μg/cm²
• Mass change of a copper coupon as an alternative or supplementary sensing metal
• Mass change of the foam specimen itself, indicating the loss of volatile emissions during aging
• Visual tarnish rating of the coupon surface on a defined 0 to 4 discoloration scale
• Blank control coupon mass change to subtract background environmental contribution
• Supplementary surface analysis of coupon deposits by SEM-EDS or XPS to confirm sulfide formation
• Optional total sulfur content of the foam by combustion ion chromatography or XRF screening

Testing Methods

Apparatus and Materials

The test requires clean borosilicate glass jars with gas-tight seals, a constant-temperature chamber or oven with control of ±2 °C, humidity generation capability for humid test variants, high-purity silver coupons of at least 99.9% Ag and optionally copper coupons of at least 99.9% Cu, an analytical balance with 0.1 mg readability, inert tweezers, powder-free gloves, analytical-grade ethanol or acetone for coupon cleaning, and a desiccator for controlled cooling. Coupon dimensions are documented so that the exposed surface area can be calculated precisely.

Specimen Preparation and Conditioning

Foam specimens are cut to defined dimensions, commonly 50 mm × 50 mm at natural thickness or strips totaling a specified surface area, with a minimum of three replicates per condition. Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours in accordance with ISO 291 conditioning practice, then weighed to record the initial mass. Metal coupons are polished, degreased by solvent wiping, rinsed, dried, and weighed to obtain the initial coupon mass m0. All handling is performed with gloves and tweezers to avoid contamination.

Exposure Procedure

The foam specimen is placed in the sealed glass jar together with the pre-weighed coupon positioned on an inert support so that the coupon and foam are in close proximity but not in direct contact. A parallel blank jar containing only a coupon is prepared as the control. Typical exposure conditions are 60 ± 2 °C for 24 to 96 hours for the dry sulfur vapor method, while humid variants maintain 90% relative humidity or above using a water reservoir within the vessel, consistent with the humid sulfur exposure concept of ASTM B809. More severe schedules such as 85 °C for 168 hours may be applied for accelerated qualification.

Weighing and Calculation

After exposure, jars are opened in a low-contamination area, coupons are transferred to a desiccator for controlled cooling, subjected to the specified post-cleaning protocol or weighed as-received to retain the sulfide film depending on the agreed specification, and re-weighed to obtain the final mass m1. The mass change per unit area is calculated as Δm = (m1 − m0) / A, where A is the exposed coupon area, and the blank-corrected value is reported. The foam specimen is also re-weighed to determine its percentage mass loss attributable to volatile emission.

Evaluation Criteria

Acceptance limits are defined by the customer specification or product standard, and typical material requirements specify a coupon mass gain of no more than 0.1 to 0.3 mg/dm² combined with no visible yellowing or tarnishing compared with the blank control. Results are commonly classified into negligible, light, moderate, and severe corrosivity tiers based on both the gravimetric value and the visual rating. Borderline or disputed results are confirmed by microscopic examination and elemental analysis of the coupon surface, and the overall behavior may be benchmarked against mixed flowing gas corrosion classifications referenced in IEC 60068-2-60 or GB/T 2423.51 for the end-product environment.

Applications

Foam sulfur corrosion weight change determination supports material selection, incoming inspection, and failure analysis across multiple industries:

• Validation of foam pads, tapes, and gaskets used in LCD, OLED, and mini-LED display modules with silver bus electrodes and silver paste circuits
• Touch panel and cover lens assembly materials in consumer electronics and tablets
• Backlight unit cushions, reflector spacers, and light guide attachment foams
• LED packaging, lighting fixtures, and luminaire sealing foams
• Automotive electronics including silver-plated connectors, sensor modules, camera modules, and battery pack gaskets
• Telecommunication enclosures, server hardware, and aerospace cabin electronics
• Root-cause investigation of silver tarnish and contact resistance failures in assembled products
• Supplier qualification and periodic quality monitoring programs

FAQ

What sample size and how many replicates are required?

A typical submission includes at least three foam specimens of approximately 50 mm × 50 mm at natural thickness per test condition, along with any adhesive laminates in their end-use configuration. Additional material is useful if multiple temperatures, humidities, or durations are requested.

What result is considered a pass?

Most enterprise specifications require a blank-corrected coupon mass gain of no more than 0.1 to 0.3 mg/dm² with no visible discoloration of the silver surface. The exact limit depends on the sensitivity of the silver components in the end product and should be agreed before testing.

Why measure weight change instead of only evaluating color change?

Visual tarnish rating is subjective and cannot detect early-stage or low-level sulfur attack, whereas gravimetric mass change provides a quantitative and reproducible value. The weight change method can detect corrosivity even before visible yellowing appears, enabling tighter incoming material control.

What is the difference between the dry and humid test variants?

The dry sulfur vapor method isolates the thermal outgassing behavior of the foam at controlled temperature, while the humid variant adds moisture that accelerates sulfide formation kinetics and better simulates humid service environments. Both variants are available, and humid exposure is generally the more conservative screen.

Can the test quantify the total sulfur content of the foam?

The coupon weight change reflects the volatile, corrosion-active sulfur emitted under the test conditions rather than total sulfur. If total sulfur content is needed, supplementary combustion ion chromatography or XRF screening can be performed on the same material to give a complete picture.

How long does the test take?

Typical exposure durations range from 24 to 96 hours plus specimen conditioning, coupon preparation, and weighing, so a standard report is usually issued within five to ten working days. Extended 168-hour accelerated schedules and supplementary surface analysis require additional time.