Weathering tests should not be interrupted while the samples are in the middle or end of a wetting phase. Handling or evaluating wet samples is a source of error which must be avoided.
Spray and Condensation Cycles in Standards
How long does a weathering test run? Typically, they are planned to run for 1,000 hours, seven days, two weeks, 100 cycles or other durations. And what do most weathering cycles have in common? They end with a spray or condensation phase and wet test specimens.
Such is the case in most xenon-arc and fluorescent-UV standards, e.g.
• ISO 4892-2 xenon for plastics
• ISO 16474-2 xenon for coatings
• ISO 16474-3 Fl/UV for coatings
• ASTM G155 xenon weathering
This is not an issue if testing continues and the samples will dry in the next dry phase. However, if testing ends or is interrupted, the samples stay wet.
It is important to note when preprogrammed tests will end to avoid having them stop at night or over a weekend, because the samples will stay wet and in an undefined state. Even if the test is interrupted manually, the samples will still be wet.
ASTM G147 recommends avoiding handling of wet test specimens, since wet surfaces are more sensitive compared to dry surfaces. This creates a dilemma since there is no solution to abide by this guidance unless the test specimens are somehow dried in a defined manner or interrupting a test in a wet phase is avoided.
BST in a Xenotest Beta+ during ISO 4892-2 Method A Cycle 1. Cycle ends in wet phase.
Weathering researchers who are aware of this problem typically avoid interrupting tests for examination in a wet phase. They accomplish this by not running a full number of cycles. For example, they will run tests for 100.5 cycles, or for one week minus thirty minutes, instead of ending with a complete cycle.
BST in a Xenotest Beta+ during ISO 4892-2 Method A Cycle 1. Test ends at the end of a dry phase.
Standard Test Methods Ending in a Dry Phase
Some standards already use test cycles which end in a dry phase. The most important are:
• SAE J2527 (3-hour cycle)
• ASTM D7869 (24-hour cycle)
By this simple measure, it is very unlikely that intermediate evaluations and total test durations are planned ending in a wet phase.
Standards Giving Guidance
EN 927-6 on fluorescent-UV weathering of wood coatings has a one-week test cycle ending with a 30-minute phase of water spray at the end. A statement in an informative annex is easy to be overlooked. It says that intermediate evaluations should be done “at the end of the cycle (driest point of cycle)”
which is before “the last step of spraying” or 30 minutes before the end of the cycle (“1 week minus 30 minutes”).
Testing and evaluations are still planned for full weeks and the total test duration is 12 weeks (2016 hours) which is in contradiction with the evaluation requirements. This needs to be clarified in future revisions.
ISO 11507, now withdrawn, was the original standard for fluorescent UV testing of coatings, stating: “…examination shall be carried out towards the end of the dry phase of the test cycle.” When this standard was harmonized with ISO 4892-3 (FL/UV for plastics) and became replaced by ISO 16474-3 (Fl/UV for coatings), this statement unfortunately disappeared.
A similar statement will soon be reintroduced in current revisions of ISO 4892-1 (general requirements for weathering of plastics) and ISO 4892-3: “It is recommended to plan the exposure in a manner that the sampling takes place at the end of a dry phase or that the samples are allowed to dry before evaluations”. Harmonization with other standards, e.g. the ISO 16474-series, is also planned.
The 102/18 Cycle Should Become an 18/102 Cycle
Going forward, it would be beneficial to update all wet/dry cycles in weathering to always end in a dry phase. This change in the test sequence is only a small technical change of the cycle. And the impact on typical test durations can be neglected.
The huge benefit is reducing the potential risk of inconsistent test results, caused by handling wet specimens or keeping specimen in a wet state for a long time.
Whenever you have the chance to adopt your test cycles, we recommend adjusting your instrument programming to avoid this potential source of error for all future testing.
More Information
You find more information on this topic in our recorded online seminars on weathering standards and test methods, ASTM D7869 for automotive exterior, and setting up a weathering tests.