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Plastic Test Standards: Back to Basics

By Ben Howe

If you've spent much time looking at plastics 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 most likely to encounter are ASTM, ISO, IEC, and UL standards. There are, of course, other organizations such as DIN, BS, NF, and JIS. A wide variety of other test standards dot the datasheet landscape. With so many different test standards appearing on datasheets 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.

A good place to start is the origin of the data on the datasheet. Each data point on a datasheet represents an average of several tests and is usually reported as being 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 datasheet says it should; it could do a little better, it could do a little worse. Don't forget that processing conditions play a role in how well a material performs too.

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, he reports an estimate of $15,000 to test a single material for a complete datasheet. This is one reason why many resin suppliers only provide datasheets with one plastic test standard set. This may also be the reason behind the practice of double-listing data; displaying data as representing both the ASTM and ISO values despite only being tested with one standard.

As an example, take the two following Polycarbonate materials: Dow Plastics CALIBRE™ 201-10 and Vamp Tech VAMPCARB 0023 V0. Both have ASTM and ISO data, but only the Dow datasheet has differences between the corresponding ASTM and ISO data points (see below). 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:

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 datasheet is not necessary. I've gotten requests from people for an ASTM datasheet for a resin that had a perfectly acceptable ISO datasheet. This is a good example of placing too much importance on the test standard. When viewed through the lens of data uncertainty, these datasheet requests are pointless. Are you looking for a flexural modulus value or ASTM D790 (the test standard defining flexural testing of plastics)? I'm not saying that the test standards themselves are not important, what I am saying is that the differences between values generated by different plastic 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:

•  Compression set data for ASTM D395 can only be directly compared to ISO 815 if the ASTM
value was obtained using Method B.
•  Density, ISO 1183 Method A compares to ASTM D792, while ISO 1183 Method D compares
to ASTM D1505.
•  Any Rockwell hardness value above 130 is impossible.
•  Any Durometer/Shore hardness value above 100 is impossible.

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 datasheets. 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 (I get requests for this one all the time).
If strange test standards have confused you in the past, 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 is telling you. Roughly 15% of the 86329 materials in the IDES database report only ISO data, while many others offer a blend of ASTM and ISO data to form a complete datasheet. Now that you can battle datasheet confusion, that's even more data you can utilize. Next month I'll focus on finding alternative plastics and how to search the IDES database using the principles I've outlined in this article.

If you're new to the plastics industry, need a quick reminder, or you haven't considered the difference between different test standards, you're in luck. IDES maintains a variety of documents that are overviews of various test standards. These documents are intended to be summaries, if you need to know exactly what a test standard requires, consult the test standard itself. Details on how to access these summary documents are outlined below.

Prospector

You can access these documents by clicking on "Resources" at the top of the Prospector and clicking on "Test Method Descriptions". Simply find the test standard in question and click on the link. You can also click on a property on the datasheet to open a new window containing the description of the test method. Mouse-over a property name on a datasheet. If it turns into a link, there's a help file available.

You can access these documents by placing your mouse over "Tools" and select "Property Descriptions". This will open a new browser window. Simply find the property you're looking for in the property tree and click on it. While conducting a property search you can click on the property name in the search window to open a new window containing the description of the test method. You can also click on a property on the datasheet to open a new window containing the description of the test method. Mouse-over a property name on a datasheet. If it turns into a link, there's a help file available.

Related links: Plastics Properties and Test Methods | ASTM | ISO | UL | Plastics Unit Converter