Resin Spec Check: Test standards—back to the basics

Knowing what the numbers mean on a data sheet and how they can help you saves worry over minor deviations.

If you’ve spent much time looking at materials technical data, you’ve seen a lot of test standards. Perhaps you just ignore them, or only give them brief attention. Maybe you fuss over every footnote and can tell the difference between ASTM D1505 and ASTM D792 (or maybe that’s just me).

The most common test standards you’re likely to encounter are ASTM, ISO, IEC, and UL. There are, of course, other organizations, such as DIN, BS, NF, and JIS. A wide variety of other test standards dot the data sheet landscape. With so many different test standards appearing on data sheets, it’s easy to get confused. If you find yourself in this situation, it’s time to step back for a moment and get back to basics.

What a data sheet really tells you

A good place to start is the origin of the data on the data sheet. Each data point on a data sheet represents an average of several tests and is usually reported as a “typical value.” With this in mind, you can interpret a tensile strength value of 1000 psi as being around 1000 psi. If you test a sample of the material and get a tensile strength of 990 psi, that’s pretty good—it’s within 1% of the reported value. Don’t expect a material to behave exactly as its data sheet says it will. It could do a little better, it could do a little worse. Don’t forget that processing conditions also play a role in how well a material performs.

Another thing to remember is that it costs a lot of money to have a material tested. In Michael Sepe’s article, “Getting the Most Out of Your Data” from the forward to Pocket Performance Specs for Thermoplastics, 3rd ed. (www.ides.com/articles/plastics_data.asp), he reports an estimate of $15,000 to test a single material for a complete data sheet. This is one reason why many resin suppliers provide data sheets with only one set of test standards. This may also be the reason behind the practice of double-listing data, or displaying data as representing both the ASTM and ISO values, despite being tested with just one standard.

As an example, take the two following polycarbonate materials: Dow Plastics’ Calibre 201-10 and Vamp Tech’s Vampcarb 0023 V0. Both have ASTM and ISO data, but only the Dow data sheet lists differences between the corresponding ASTM and ISO data points (see table above). Most likely Dow Plastics has the resources to run all the tests, while Vamp Tech probably doesn’t, but wants to reach as broad an audience as possible.

Common sense is king

We can lump data ranges and potentially misrepresented data into the term “data uncertainty.” Armed with this knowledge, we can conclude that placing a great deal of importance on the test standards reported on a data sheet is not necessary. I’ve gotten requests from people for an ASTM data sheet for a resin that had a perfectly acceptable ISO data sheet. This is a good example of placing too much importance on a test standard.

When viewed through the lens of data uncertainty, these data sheet requests are pointless. Are you looking for a flexural modulus value or ASTM D790 (the test standard defining flexural testing of plastics)? This does not mean that the test standards themselves are not important, but the differences between values generated by different test standards are small enough that data uncertainty fills the gap.

There are cases where knowing the differences between standards can be beneficial. For example, if you are looking for a rigid material and you see flexural modulus data listed under ASTM D747 instead of ASTM D790, you’d realize that the material is too flexible to be tested with ASTM D790. This simple fact should raise a warning flag that you could be looking at the wrong material. Here are a few other examples where test standard knowledge might come in handy:

Bet you didn’t know

Units are indirectly related to test standards and deserve some attention, because they are a source of some confusion. The primary confusion comes from Izod and Charpy impact units differing between the ASTM and ISO standards. ASTM specifies units of energy per unit length (J/m), while ISO specifies units of energy per unit area (kJ/m²). No, that’s not a typo. ASTM specifies SI units as the standard for many of its test methods. If you were expecting units of ft-lb/in for the ASTM Izod unit, then you have fallen victim to another cause for confusion: unit system/test standard grouping.

ASTM values are reported in English units and ISO values are reported in SI units so often that we’ve been conditioned to expect these combinations. If you’re not a fan of SI units, you’re in luck, because you can select the displayed unit system on IDES data sheets. If you still don’t see a unit you’re comfortable with, you can always perform a unit conversion, provided the units are of the same class. You can’t however, convert J/m to kJ/m² (we get requests for this one all the time).

If strange test standards have confused you, just remember a few simple ideas to combat the confusion: The data itself is important, the values reported give you an idea of what the performance should be (not what they will be), and in most cases test standards don’t lead to drastically different results. With these things in mind you’ll be able to draw upon a larger selection of materials (maybe even less expensive ones) because you’ll be focused on what the data are telling you. Roughly 15% of the 60,000-plus materials in the IDES database report only ISO data, while many others offer a blend of ASTM and ISO data to form a complete data sheet.

Next time we’ll focus on finding alternative resins and how to search the IDES database using the principles outlined in this article.

Ben Howe is an engineer at IDES, a plastic materials information firm based in Laramie, WY. IDES’s primary product is The Plastics Web, a database of materials that provides vertical search capabilities on a variety of resins and materials technical data. Howe can be reached at [email protected], or (307) 742-9227. Visit IDES online at www.ides.com.