How do you determine the surface cure time of a sealant, and how do you evaluate the application life of a multi-component sealant?
Currently, China is proposing two new standards in SC 8 "Sealants" that will help answer these questions. The proposed new standards are under ballot until 7 April 2020.
ISO/NP 4781 Building and civil engineering works - Sealants - Determination of application life is intended to evaluate the application life of multi-component sealants in a manner that is most relevant to the end-user (customer). Application life in this context is to be understood as the period of time during which a sealant, after being mixed with a catalyst or exposed to the atmosphere (at standard conditions), remains suitable for application.
In the literature, this period of time is often also referred to as working life or pot life. However, frequently the definitions used for working life or pot life deviate from above definition of application life. For instance, pot life often is referred to as the time interval during which the viscosity of the mixed system doubles; working life frequently is stated by the sealant supplier to advise the customer how much time they have to work with the material before it reaches such a high state of viscosity that it cannot be properly worked with to produce an acceptable sealed joint. Furthermore, national standards exist which define application life quite differently, when compared to the proposed ISO standard.
Examples of such standards are:
- BS 3712-3: In this standard, samples of the mixed sealant are taken with a spatula and applied on to a mill-finished aluminum alloy surface. The application life is reported as the total time between the completion of mixing and the last test in which the sealant is easily applied to and readily adheres to the aluminum surface.
- ASTM C603: In this standard, the mixed sealant is transferred into a specified cartridge, allowed to rest for 3 h, and then extruded at a given pressure, while measuring the time in seconds required to empty the cartridge. If the value for the total extrusion time exceeds a maximum value deemed by the manufacturer to be acceptable for the application, the end of the application life has been reached.
None of the above-mentioned standards addresses the question most relevant to sealant users, i.e., how long after mixing can the sealant be extruded using a given pressure? Obviously, the concept used by BS 3712-3 addresses another (also important) question, which is: How long after mixing will the sealant provide good adhesion to the substrate? However, since the BS 3712-3 standard utilizes only one substrate and the method of sealant application raises the issue of reproducibility, it is felt that the proposed ISO standard addresses the most relevant issue.
The new standard will build on the concepts disclosed in BS 3712-3 (UK), ASTM C603 (US), and JIS A 1439 (JP). However, it will significantly differ from these standards in that it focuses on a more relevant definition of application life and proposes a unified set of test parameters (representing best practice) and proper terminology.
ISO/NP 4784 Building and civil engineering works - Sealants - Determination of surface cure time specifies a method for the determination of the surface cure of one- and multi-component sealants used in joints in building construction as indicated by the surface cure drying time. Although primarily intended for such applications, the standard may be used in other fields related to sealants and sealed joints where desired.
Current available national and pan-national standards assess skin-over time (SOT) (JIS A 1439) and/or tack-free-time (TFT) (ASTM C679, BS 3712-3, EN 14187-2) as a measure of surface cure. The principle of both test methods is related, i.e., the time is determined when no freshly applied sealant is adhering to a finger or tool (SOT) or a peeled off polyethylene strip (TFT), when touching the surface of the sealant during cure. However, both test methods suffer from considerable shortcomings.
SOT measures are difficult to reproduce (especially when using a finger), while TFT actually is a misnomer as the test does not measure a tack-free time, rather than a transfer time. The test parameters (specimen size, weight, weight dimensions) and actual procedure differ between the various currently existing tests for TFT. Therefore, a global standard using best practice procedure and a correct nomenclature would be desirable. The proposed standard intends to utilize the concept of ASTM C679, BS 3712-3, and EN 14187-2, but propose a unified set of test parameters that allows to achieve global comparability of the test results.
Both the proposed test standards are suitable for room temperature curing sealants and the methods can be used for the substantiation of technical claims in product data sheets, for quality control and for product performance research. The envisioned standards will contribute to the fulfilment of UN Sustainable Development Goal 9 Industry, Innovation, and Infrastructure.
If approved, the two projects will be registered to the work programme of SC 8/WG 22 "Cure properties".