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The Materials Analyst, Part 120: The specification for unobtainium

If material specs are on an official-looking document, are they right? Let reason and logic prevail, and your part will not suffer.

If material specs are on an official-looking document, are they right? Let reason and logic prevail, and your part will not suffer.

While not exactly part of the official material science vernacular, the word “unobtainium” has become a common way of referring to those elusive materials that everyone wants but no one has figured out how to make. The liquid crystal polymer that you can buy for $2.00/lb, the polymer that can resist every chemical known to man, and the polypropylene (PP) with a melting point of 200°C all fall under the heading of this type of wishful thinking.



This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure. Michael Sepe, our analyst and author, is an independent materials and processing consultant based in Sedona, AZ. Mike has provided analytical services to material suppliers, molders, and end users for 20-plus years. You can reach him at [email protected].
(The melting point of PP is between 150°C and 170°C, depending on whether the material is a high-crystallinity homopolymer or a random copolymer. This did not prevent an automotive engineer from once insisting that I produce samples for a part that was to be used under the hood where the temperature was known to reach 190°C. The addition of 50% glass fiber did not change the melting point, thus eliminating a fairy-tale cost-reduction strategy).

The engineers, procurement folks, and consultants who seek these miraculous materials are either deluded or prescient depending on how long they’re willing to wait for the hard science and economics to catch up with the dream.

However, in working on one recent failure analysis, I discovered that at least one specification for unobtainium has already been written, and given the source of this specification, it is very likely that there are more. The project had all the elements that make a failure investigation interesting: molding problems, part design issues, and elaborate quality systems that were not followed even when the documentation made it obvious that there were deviations in procedures. But most revealing was the fact that the material specified on the print was not being used and, more importantly, did not exist.

Find the focus
The part is molded from acrylonitrile-styrene-acrylate (ASA), a variant on the better-known and more common ABS, but the rubbery modifier, butadiene, has been replaced by an acrylic-based elastomer. Butadiene in its polymerized form contains what chemists refer to as unsaturated sites, double bonds between carbon atoms that are particularly susceptible to attack by any number of agents, including ultraviolet (UV) radiation.

The acrylic rubber does not contain these double bonds and for this and other reasons related to chemistry, acrylic polymers have very good UV stability. So where the mechanical property profile of ABS would otherwise be desirable, ASA offers an excellent alternative for products that spend a lot of time outdoors.

This part was molded in one location but used in two different facilities, where it was part of a larger assembly. It had been in use for about five years and a sudden increase in field complaints related to the ASA part made it the subject of a lengthy and detailed investigation that focused on several areas, including material composition, molding process, part design, and assembly process.
All of these areas are candidates for investigation whenever failures occur. And in the search for a root cause it is common to find that there are multiple causes that intersect to produce the problem. Often not all of these causes need to be addressed, and in these cases it is vital to understand the relative importance of each contributing factor and fix the ones that have the greatest impact on the product.

In this case, some tests showed that the molding process was at least part of the problem. However, addressing the deficiencies in the process did not halt the flow of product complaints. An evaluation of the part design indicated that there were opportunities for improvements, but the tool changes involved were potentially costly and there was no guarantee that even after these were addressed, the improvements would be sufficient to provide a lasting remedy. Aspects of the assembly process that involved exposure to chemicals were a clear contributor as well and were a key factor in the reluctance to make design changes.

However, there was a lingering concern that changes had occurred over time in the raw material and that these were a principal factor in the problem. Some of the analytical tests did suggest that there may have been some adjustments to the formulation of the raw material. One group within the problem-solving team homed in on this as a factor that may have increased the sensitivity of the material to the variations in the molding and assembly process and accentuated the shortcomings in the design. This led to an examination of the raw material specification, and this is where it became really interesting.

Too much variation
The raw material specification was governed by an official-looking 16-page document that laid out in great detail the various specifications for ASA molding and extrusion materials, along with two related types of polymers. The document had originally been issued as a British Standard and had been adopted as an ISO standard.

The part drawing, rather than referring to a specific grade of ASA from a particular manufacturer, referenced a series of callouts that fell within the framework of the ISO specification. These callouts focused on four properties: Vicat softening point, melt volume flow rate, Charpy notched impact strength, and tensile modulus.

The first revelation that came out of an examination of this specification was that the language allowed for a certain degree of variation in the material composition. This was of considerable concern to those in the material-is-the-root-cause camp because variation in the raw material was being sought as a key part of the problem. In particular, there was an allowance for as much as 5% of the compound to consist of substances other than ASA.

The second finding was that the compound did not meet at least one of the four specified properties. The print spec called for a material with a melt volume flow rate of 5-10 cm3/10 min. The nominal value for the grade being used to mold the part was 21 cm3/10 min!

This deviation was of concern because the melt volume flow rate is related to the average molecular weight of the polymer. In materials like ASA, there are other factors that can contribute to a change in flow rate, but in general, higher flow rates are associated with lower average molecular weight. Additionally, susceptibility to stress cracking, which was the failure mechanism, increases as molecular weight declines. While there was no guarantee that simply increasing the molecular weight of the polymer would solve the stress cracking problem, it certainly was a question that needed to be investigated.

An exercise in futility
But it gets better. The callout on the drawing also showed, perhaps not surprisingly, that the Charpy notched impact strength was also below what was being specified, while the Vicat softening point and the tensile modulus were within spec. A search began for an ASA that embodied all of the properties in the specification. Interestingly, it was found that when the Charpy impact strength of the materials in the various ASA product lines increased, the Vicat softening point and the tensile modulus declined to a point where they fell below the specified range.

This is not entirely surprising, since the most efficient method of improving the impact resistance of a rubber-modified material is to increase the rubber content in the compound. When the concentration of the soft, flexible rubber increases, the stiffness and heat resistance of the material decline. But what made this case particularly fascinating is that such a lot of work had gone into creating a specification that contained a combination of properties that ASA cannot fulfill.

This method of material specification certainly is not unique to ISO. It has a long tradition with ASTM for thermoplastics and an even longer one involving crosslinked rubber compounds under SAE J 200. These get their inspiration from the metals industry, where designations for various alloys are tied to clear guidelines for composition, hardness, and key physical properties. In each case an attempt has been made to get away from specifying materials according to a particular supplier’s market offerings and tie the spec to a set of performance-based properties.

Unfortunately, polymer formulations, whether they are thermoplastics or thermosets, are not as simple and straightforward as metals. And the notion that the full range of performance criteria for a durable goods product that must last for years under a range of rigorous application environments can be captured by four short-term properties is ludicrous. But this is the brave new world of document-driven engineering, where simply writing something into a specification removes the need for thoughtful practice and a complete understanding of material properties and their interaction with part design, processing, and the world in which the product must function.

Specifications are important and no one is suggesting otherwise. Ideally, however, specifications embody historical knowledge about what works and what doesn’t. They are a reflection of this knowledge codified in a way that allows others to benefit from that hard-won knowledge. Writing specifications that call for the use of nonexistent materials, and then pretending that such incomplete descriptions satisfy the requirements of good engineering, do a great disservice to the plastics industry in general. The fact that they are presented in official-looking documents that bear the names of international organizations may provide some comfort to the users of these specifications, but they do little to guarantee success.
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