Scope:

ASTM C679 is used to test the quality of the curing sealants by measuring Tack-free time. The tack-free time is a measure of the surface cure time, the time at which the coating properly adheres and is capable of providing maximum protection. Different locations have different temperatures and humidity. The sealant is tested for all the possible temperature and humidity conditions in which the sealant might be used.

The tack-free time is correlated to a variety of useful parameters such as the time interval before the sealant:

(1) resists damage due to light surface contact,
(2) resists picking up dirt,
(3) resists impinging rainfall.

Test procedure:

In ASTM C679, a polythene film is placed on a sealant and a weight of 30 g is placed on the polythene film. After 30 seconds, both the weight and the polythene film are lifted, and the polythene strip is examined for any sealant. With some sealants, a very thin film of oil or sealant always gets pasted on the polythene film. In such situations, the thin film is ignored and the data is recorded regardless. 

Specimen Size:  

For ASTM C679, the test specimen should be a factory-sealed can of compound (minimum contents, 450 mL) with an appropriate curing agent, or a standard cartridge, factory filled and sealed.

Data: 

Following things are reported in ASTM C679 testing: 

• Tack-free time, the time until no sealant was picked up on the polyethylene film. 
• Whether the film was clean or the film contained a thin layer of plasticizers. 
• The temperature and humidity conditions, which are different from the standard conditions.

Conclusion:

ASTM C679 determines the Tack-Free Time of Elastomeric Sealants, a type of mending glue used in construction.