Tack-Free Time of Elastomeric Sealants: Test Method & Applications
What Is the Test for Tack-Free Time of Elastomeric Sealants?
The test for tack-free time (TFT) of elastomeric sealants is a standardised procedure that determines the elapsed time from sealant application to the point at which its surface no longer adheres to a lightly pressed polyethylene film. This test, governed primarily by ASTM C679, is a fundamental property determination for all moisture-curing, solvent-releasing, and chemically curing elastomeric sealants used in architectural, structural, aerospace, and industrial sealing applications.
The Significance of ASTM C679
ASTM C679 — Standard Test Method for Tack-Free Time of Elastomeric Sealants — was developed specifically to address the need for a reproducible, quantitative measurement of sealant surface cure progression under defined environmental conditions. Unlike informal touch tests, ASTM C679 provides a defined:
- Test specimen geometry (bead cross-section)
- Substrate (aluminium Q-panel or similar)
- Environmental conditions (23°C ± 2°C, 50% ± 5% RH)
- Test probe material (polyethylene film)
- Load and contact time (100 g weight, 5 seconds contact)
- Pass/fail criterion (no sealant pick-up on film, no film deformation in sealant)
This standardisation enables meaningful comparison of tack-free times between different sealant products and between lots of the same product.
Sealant Types Evaluated by ASTM C679
ASTM C679 applies to a wide range of elastomeric sealant chemistries:
- Silicone sealants (one-part and two-part): Acetoxy-cure, neutral-cure (oxime, alkoxy, amine)
- Polyurethane sealants: One-part moisture-cure and two-part reactive systems
- Polysulfide sealants: Used in aircraft integral fuel tanks and architectural joints
- Modified silicone (MS polymer / SPUR): Silyl-modified polyether or polyurethane hybrids
- Butyl and EPDM sealants: Used in roofing and glazing applications
Each sealant chemistry has a characteristic TFT behaviour depending on its cure mechanism and the prevailing temperature and humidity conditions.
Relationship Between TFT and Other Cure Properties
Tack-free time is related to but distinct from other sealant cure properties:
- Skin-over time: Similar to TFT but sometimes used interchangeably in industry practice
- Full cure depth: The time for cure to progress through the full joint cross-section, which can take days to weeks for deep one-part sealant beads
- Open time (for two-part sealants): Time available for tooling and finishing after application before premature surface skin formation
Understanding these relationships helps sealant manufacturers and applicators optimise formulation and application procedures for their specific construction or manufacturing context.
Test Procedure Summary
A standard sealant bead of defined cross-sectional profile (approximately 12 mm × 6 mm) is applied to a clean substrate and placed in the standard test atmosphere. At regular time intervals (typically every 10–15 minutes initially, then every 30–60 minutes as the cure progresses), a 25 mm × 25 mm polyethylene film square is placed on the sealant surface under a 100 g weight for 5 seconds and then removed. The tack-free time is the first time interval at which the film can be removed cleanly, without lifting sealant or leaving an impression.
Conclusion
The tack-free time (TFT) test for elastomeric sealants is a critical quality and performance evaluation used to determine when a freshly applied sealant surface has cured sufficiently to resist adhesion upon light contact. Governed primarily by ASTM C679, this method provides a standardised and reproducible way to assess early-stage curing behaviour under controlled environmental conditions.
TFT is especially important for silicone, polyurethane, polysulfide, and hybrid sealants, where surface cure directly affects handling, tooling, dust pick-up resistance, and readiness for service. Since curing performance is strongly influenced by temperature, humidity, bead thickness, and sealant chemistry, ASTM C679 enables reliable comparison across products and production batches.
Why Choose Infinita Lab for ASTM C679 Tack-Free Time Testing?
Infinita Lab provides ASTM C679 tack-free time testing for elastomeric sealants through our nationwide accredited laboratory network, with controlled environmental testing chambers ensuring compliance with the standard’s temperature and humidity requirements.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
What is tack-free time in sealants? Tack-free time is the time required for the sealant surface to lose its stickiness after application, such that a polyethylene film placed on it does not adhere or pick up material.
How is the test performed? A standard sealant bead is applied to a clean substrate and kept under controlled laboratory conditions. At regular intervals, a polyethylene film is lightly pressed onto the surface under a 100 g weight for 5 seconds. The tack-free time is recorded when the film lifts off cleanly.
Is tack-free time the same as full cure time? No. Tack-free time refers only to surface cure, while full cure time is the time required for the sealant to cure through its entire depth, which may take several days depending on bead thickness.
Can the ASTM C679 method be used for two-part sealants? Yes. ASTM C679 is applicable to two-part sealants as well as one-part systems. For two-part sealants, the TFT clock starts from the time of mixing the two components, as this is when the curing reaction begins.
Why is polyethylene film used as the probe material in ASTM C679? Polyethylene's very low surface energy (approximately 30 mJ/m²) means it does not inherently bond to most sealant formulations when they are genuinely tack-free, providing a clear pass/fail discrimination. Its consistent, defined mechanical properties also ensure reproducible contact geometry and force distribution across the sealant surface.
