August 24, 2015

Behind the Arc Rating: Dimensional Change

Have you ever bought a new pair of jeans, wore them once, washed them, and when you try to wear them again come to find… they’ve shrunk!? If your clothing vendor of choice was required to conform to the ASTM F1506 specifications of AATCC 135 Dimensional Change of Fabrics after Home Laundering, that might not have happened! ASTM F1506 requires shrinkage for all woven fabrics to be less than 3%.  The committee has been discussing revising this to 5% for some time, but the change has not occurred to date. This explains the standard we test to when a client fully tests to ASTM F1506 and the requirements a certification body like SEI (Safety Equipment Institute) or UL would review when third-party certifying to ASTM F1506. Most companies self-certify to ASTM F1506.

What is AATCC 135 Dimensional Change of Fabrics after Home Laundering?

“This test method is intended for the determination of dimensional changes of fabrics when subjected to home laundering procedures used by consumers” – Source: AATCC 135 (editorial edits added)

The AATCC 135 test method designates how to measure the growth and shrinkage of fabrics after laundering. The wash specifications are spelled out in the method; for ASTM F1506 we use Machine Cycle 3, Wash Cycle IV, & Drying Procedure A iii for five wash & dry cycles. In actual home laundering terms that would be: permanent press with hot water and tumble dry on permanent press. Additionally, the wash requires a standardized detergent, in either 66g for a 4 lb. wash load, or 80g for an 8 lb. wash load.

After washing, we determine the percent change of the material: Dimensional Change (%) = 100 x (n’n)/n where n represents the original measurement and n’ is the measurement after laundering.

To prepare our specimens per the method, we first cut three 15”x15” squares from the fabric sample with diagonal spacing between the selvages and following the machine direction (Figure 1 from AATCC 135-2012).

We then mark each specimen with a set of benchmarks in a 10” square (Figure 2 from AATCC 135-2012)diagram for cutbench marks

With our marked specimens we measure both the length & width at three different benchmarks; and record and average these as the initial measurements (n). We now wash the squaresper the standard, and measure the benchmarks again to record and average the post laundering measurements (n’).  Using the averages we calculate percentage change in the length and width directions. Shrinkage is reported with a (-) sign, and growth is reported with a (+) sign. Finally, we determine the absolute average of both directions to determine the overall change.

We specify change because fabrics don’t always shrink!

Just like shrinking jeans, you might also have had a t-shirt that stretched out after a wash. Knits are more likely to shrink or grow than woven fabrics because of their construction, so ASTM F1506 doesn’t set a requirement for dimensional change on knitted materials; but, it does require the laboratory to report the result to the garment manufacturer for design considerations and to adjust for shrinkage. Woven fabrics, though, have a more uniform construction, and are not permitted to change more than 3% in the test protocol of the standard. Even then, sometimes fabrics shrink in one direction, but grow in another. Thus, after reporting the change by direction (shrinkage or growth), we determine change based on the absolute average, to account for OVERALL change, not just the average of positive and negative integers.

Why is all of this important? First, shrinkage slightly affects the arc rating.  Some manufacturers have allowed shrinkage for testing in arc, and then controlled it for testing in dimensional change in order to meet the ASTM F1506 specification.  This can give an unfair arc rating to the fabric.  This is why ArcWear reports Actual Aerial Density (AAD) on all fabrics in arc testing.  When we work with SEI to certify a fabric, we perform arc testing and dimensional change from the same sample to assure that the fabric gives a realistic arc rating.

Additionally, if an electrical worker’s clothing has shrunk, the worker might either wear the gear improperly, or make modifications to the gear, both of which can be hazardous considering their work environment. So the answer is: to keep them safe, get tested.

That’s all from us at ArcWear, see you next time at Behind the Arc Rating!

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