Handbook of Adhesives and Sealants

Standard Test Methods 169Figure 4.22 Typical creep curve for sealant. 20Ozone and ultraviolet (UV) radiation are factors in environmentalexposure of many sealants. Major cities now exhibit 50 pphm of ozonein their atmospheres, which is the level set in standard ozone environmentaltests. Ozone at this level will cause surface cracking afterreasonable exposure. UV will similarly cause some hardening of certainsealants and will cause some to lose adhesion. UV is of specialconcern for sealants that are used in glass applications. Resistance tosurface degradation after several hours in a standard weatherometeris indicative of good performance for a sealant.Resistance to solvents or chemicals is not a standard requirementfor sealants. However, the tests are easy to run, and most sealantmanufacturers have such data. The time for immersion varies considerably,and it should be recognized that the absorption of chemicalinto the sealant is generally not very fast.The effect of heat aging at moderate temperatures on sealants isgenerally through the loss of volatile plasticizers. Higher temperaturesresult in increased crosslinking, oxidation, and pyrolysis of thesealant. This, in turn, results in increased hardness and reduction ofmovement capability. ASTM C 920 specification requires a maximumpercentage of weight loss of 10–12% after heat aging for two weeks at158F. The conditioning generally specified is the application of accumulatedtime at temperature expected in service. For example, thetotal hours of service expected at 200F is estimated over the sealant’slife and incorporated into the testing methodology. It is unwise to attemptto accelerate testing by increasing temperature above the actualservice temperature without knowledge of the material characteristicsof the sealant. At higher temperatures, additional reaction mechanismsmay take place related to the thermal movement of moleculeswithin the sealant.