Sealing Strip CVCM Testing: Volatile Condensable Materials Analysis for Elastomeric Seals

CVCM (Collected Volatile Condensable Materials) testing evaluates the outgassing behavior of sealing strips under high vacuum and elevated temperature in accordance with ASTM E595. Because elastomeric seals are one of the dominant sources of molecular contamination in spacecraft, optical instruments, and high-vacuum processing equipment, CVCM screening is a critical step in material qualification and contamination control. This article explains the technical principle, test methodology, acceptance criteria, and typical applications of CVCM testing for sealing strips.

Technical Overview

Sealing strips manufactured from silicone, EPDM, fluorocarbon, and other elastomeric compounds inevitably contain low-molecular-weight species such as plasticizers, siloxane oligomers, residual cure by-products, processing oils, and absorbed moisture. When these seals are exposed to the vacuum environment of space or high-vacuum industrial equipment, these volatile species are released from the material—a phenomenon known as outgassing. A fraction of the outgassed molecules will migrate and re-condense onto cooler adjacent surfaces, forming contaminant films that degrade optical transmittance, scatter light, increase solar absorptance, and impair electrical contacts and sensitive sensors.

CVCM testing quantifies this risk. CVCM (Collected Volatile Condensable Materials) is defined as the mass of volatiles that condense on a collector plate held at 25°C divided by the original specimen mass, expressed as a percentage. The test is performed together with TML (Total Mass Loss) determination, and both parameters were institutionalized as a standard screening method in ASTM E595, which originated from spacecraft material contamination control programs. For space applications, the widely applied acceptance thresholds are CVCM ≤ 0.10% and TML ≤ 1.00%.

For sealing strips specifically, CVCM data provide the basis for:

• Qualification of seal materials for spacecraft hatches, docking mechanisms, and instrument enclosures
• Screening of commercial elastomer compounds before use in vacuum chambers and semiconductor tooling
• Comparison of competing compounds and verification of low-outgassing material grades
• Assessment of vacuum-baking or post-curing processes intended to reduce volatile content
• Support of contamination control plans and material declaration documentation

Testing Samples

Silicone Rubber Sealing Strips (VMQ)

Silicone is the most extensively used sealing material in space-grade and vacuum-grade seals due to its wide temperature range and compression set resistance; however, standard silicone compounds often contain volatile siloxane cyclics that condense readily on optical surfaces. CVCM testing distinguishes standard grades from specially formulated low-outgassing silicone compounds and validates post-curing or vacuum-baking treatments.

Fluorosilicone Sealing Strips (FVMQ)

Fluorosilicone strips used in fuel-resistant and low-temperature sealing applications are tested to confirm that fuel-resistant plasticizers and process additives do not exceed condensable limits.

EPDM Sealing Strips

EPDM compounds used for door seals, panel seals, and outdoor weather-sealing are evaluated for waxes, process oils, and residue from peroxide or sulfur cure systems that may outgas in vacuum or elevated-temperature service.

Fluoroelastomer Sealing Strips (FKM)

FKM strips specified for high-temperature and chemical-resistant sealing are tested for volatile cure residues, plasticizers, and decomposition by-products generated during post-vulcanization.

Foam and TPE Sealing Strips

Silicone foam, polyurethane foam, and thermoplastic elastomer (TPE/TPV) sealing strips contain blowing agent residues, cell-opening surfactants, and extension oils, all of which are significant potential sources of condensable volatiles.

Sample Preparation Requirements

Specimens are cut from finished sealing strips into small pieces that fit the sample boats, with a total mass per specimen of approximately 200–250 mg as required by the test method. Replicate specimens are recommended so that average values and scatter can be reported. Surface coatings, release agents, and cleaning residues must be documented, as they can strongly influence the measured CVCM value.

Testing Items

The core measured parameters in a CVCM test campaign for sealing strips are:

• CVCM (Collected Volatile Condensable Materials) – the percentage of the original specimen mass that outgassed during the test and re-condensed on the 25°C collector plate; the primary contamination-control indicator
• TML (Total Mass Loss) – the total percentage of mass lost by the specimen during the 24-hour vacuum exposure at 125°C
• WVR (Water Vapor Regained) – the percentage of water reabsorbed by the specimen after reconditioning at 23°C and 50% relative humidity for 24 hours; useful for separating moisture loss from true material loss
• RML (Recovered Mass Loss) – the calculated non-water mass loss (TML minus WVR), which reflects the irreversible loss of organic volatiles

Testing Methods

The reference method is ASTM E595, Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment. The European space standard ECSS-Q-ST-70-02 defines an equivalent thermal-vacuum outgassing procedure, and many spacecraft material specifications reference these methods directly.

The test sequence is as follows:

• Conditioning: specimens are conditioned at 23 ± 2°C and 50 ± 5% relative humidity for 24 hours, then weighed to obtain the initial mass using a microbalance
• Loading: each weighed specimen is placed in an individual sample boat within the heated copper bar of the outgassing apparatus, with a chromium-plated collector plate positioned 12.7 mm above the specimen and maintained at 25°C
• Vacuum exposure: the chamber is evacuated to a pressure of 6.7 × 10⁻³ Pa (5 × 10⁻⁵ torr) or lower while the specimens are held at 125 ± 1°C for 24 hours; volatiles leaving the specimen escape through a vent and can condense only on the chilled collector
• Post-test weighing: after venting with dry nitrogen, the specimens are reweighed to determine TML, and the collector plates are reweighed to determine CVCM; specimens intended for WVR determination are reconditioned at 23°C and 50% RH for 24 hours and weighed again

The results are calculated as: TML = (initial mass − final mass) / initial mass × 100%, and CVCM = mass condensed on collector / initial specimen mass × 100%. Blank collectors and certified reference materials are run in parallel for quality control, and the microbalance provides microgram-level resolution to ensure the required measurement uncertainty.

The commonly applied acceptance criteria for spacecraft materials, derived from long-standing space program requirements, are:

• CVCM ≤ 0.10%
• TML ≤ 1.00%

Sealing strips intended only for terrestrial vacuum service may be assessed against application-specific limits agreed between supplier and end user, but the space thresholds are widely used as a default screening benchmark.

Applications

CVCM testing of sealing strips is applied across aerospace and high-technology industries:

• Spacecraft hatch seals, docking and berthing mechanism seals, and module penetration seals where contaminant films would degrade seal-adjacent mechanisms
• Satellite optical payloads, telescopes, and sensor windows, where even nanometer-scale condensate layers increase scatter and reduce throughput
• Solar array and radiator assemblies, where condensed organics raise solar absorptance and raise operating temperatures
• Ground-based thermal vacuum chambers and space simulation facilities, in which seals are a persistent internal contamination source
• Semiconductor, display, and vacuum deposition equipment, where elastomeric door and fixture seals can contaminate wafers, masks, and chamber optics
• Material research and development programs comparing formulation, post-cure, and vacuum-bake strategies for low-outgassing elastomer compounds

FAQ

What CVCM value is considered acceptable for a sealing strip?

The universally applied spacecraft criterion is CVCM ≤ 0.10% together with TML ≤ 1.00%. Materials exceeding these thresholds are normally rejected for unrestricted use near optics, detectors, or thermal control surfaces, although waivers may be granted based on distance, shielding, and thermal analysis.

Why is the test performed at 125°C?

The 125°C exposure provides an accelerated, conservative simulation of the thermal environment experienced by materials during spacecraft bakeout and early mission phases. It drives off volatiles aggressively within a practical 24-hour test window while remaining below the degradation temperature of most common elastomers.

How much sample is required and in what form?

Approximately 200–250 mg of material per specimen is needed, cut into small pieces from the finished sealing strip. Running three replicate specimens is recommended so that average values and variability can be reported.

Can a sealing strip that fails CVCM testing still be used?

In many cases yes. Extended post-curing or vacuum baking at elevated temperature can remove a large fraction of volatiles, and retesting will confirm whether the treated material now meets the limits. Alternatively, a certified low-outgassing compound grade should be selected.

What is the difference between TML and CVCM?

TML measures everything the specimen loses, including water vapor that never condenses on cool surfaces, while CVCM measures only the fraction that actually re-condenses on a 25°C surface. CVCM is therefore the more direct indicator of contamination risk to downstream optics and equipment.

How long does CVCM testing take?

The conditioning, 24-hour vacuum exposure, and weighing sequence requires approximately three to four working days per test, with several additional days if replicate conditions or pre-treatment evaluations are included.