Sourcing Equivalent Plastic Materials in the Global Economy

By Michael Sepe, Materials Analyst

How can you tell what plastic material was used by an overseas molder without the name, supplier, or data sheet? You have to take a test.

One of the notations on a part drawing that produces the greatest amount of controversy and anxiety for all involved is the phrase “or equivalent.” It typically appears after the callout for the primary material, such as “Lexan 141R or equivalent.”

The purported intent of this broadened specification is to provide sourcing flexibility for the molder. The expectation on the part of the end user is that the molder will shop the marketplace for lower-cost materials that will provide the same level of performance. This is an expectation that is often met on the cost side but sometimes presents long-term problems on the performance end.

This problem becomes even more of a challenge when the material used to produce first-article parts is an unknown. This may sound like an unusual occurrence; however, it is happening with greater frequency because of an interesting trend in plastic part sourcing. It turns out that much of the cost advantage in sourcing molded parts in Asia comes not from differences in the piece part cost but in the cost and timeline differences for tooling.

Depending upon whom you talk to, you will hear stories of mold prices that are 25-50% of those quoted domestically and, often just as important, delivery times that are reduced by 40-70%. Once the mold is built and first parts have been evaluated, the logistics of managing an ongoing supply chain from such a great distance tend to erode what small cost advantage may exist between an Asian supplier and a North American one. Consequently, the mold is then shipped to the United States and placed with a supplier here for the purpose of supporting ongoing production requirements.

Shooting In The Dark
This is where it gets interesting. As much as we like to talk about the global economy, the reality is that there are still many plastic raw materials that are not available everywhere in the world. In addition, there are a lot of commodity materials like polystyrene and polypropylene that are commonly used in other parts of the world but are unfamiliar to domestic buyers and engineers. (Anyone ever heard of Nizhnekamsknefekhim polystyrene?) So when an Asian or an Eastern European supplier runs off the initial sampling of parts from the new mold, it may make a selection based on the “or equivalent” notation that leaves the OEM and its prospective domestic supplier at a loss.

Language barriers and differences in units of property measurement aside, there are a lot of performance characteristics that simply cannot be captured in a data sheet. Therefore, even if the product information for the original material can be obtained and interpreted, there is no guarantee that this will result in the selection of a suitable substitute.

There are also concerns regarding regulatory agency approvals such as UL, FDA, and NSF. Most domestic material suppliers are familiar with the test requirements associated with flammability or relative thermal index, or they at least understand the importance of having their products evaluated for these properties and then listed by the appropriate agency. Foreign suppliers may not have this focus and are often playing catch up on getting their products registered, just as we in the United States are in a largely reactive mode when it comes to the RoHS requirements.

If the mold transfer is not an amicable one, the problem becomes even greater since no information on the composition of the original material may be provided. In this case the domestic supplier is told to source a “similar” material based on scant information about the current material and the application. The guidance may be something like, “It’s a PP and it feels kind of flexible so it’s probably a copolymer of some kind. Call your distributor and see what they have in stock.”

This strategy is almost certainly doomed to failure because the supplier is aiming at a target it cannot see. And even if the short-term property profile of the substitute material is similar, there are multiple routes to these properties. The composition details often have a greater influence over midterm to long-term properties than one might suspect.

A Wrong Guess
This case involves a recreational product made from a polypropylene. The initial parts were produced from a material that was described only as a copolymer. No data sheet, no specifications, and no identification of a supplier were provided. However, the parts, which needed to exhibit a particular balance of stiffness and impact resistance, did function satisfactorily. When the mold was moved, any information that may have existed on the material used did not travel with the mold.

The scenario of sourcing against an unknown product resulted in using a material that molded well and produced parts of satisfactory dimensional consistency. However, the parts failed in two modes. Some cracking was observed around attachment points, and where the parts did not crack they tended to stretch too much. From a material property standpoint this translates into a material that lacks both stiffness and toughness.

Polypropylene has been defined by the industry as a commodity material, and this leads many to think of it as a simple product. However, the performance range of polypropylene is broad and the capabilities of a particular material are dependent upon many subtle details of chemistry and structure. This becomes apparent when materials are compared for differences in composition.

The two most important determining factors in the final performance of a polypropylene product are the structure (homopolymer, random copolymer, impact copolymer, and so forth) and the average molecular weight of the compound. The former can be probed to significant degree by examining the manner in which the material melts and recrystallizes and therefore is an ideal candidate for differential scanning calorimetry (DSC). The latter can be quickly assessed using melt-flow-rate (MFR) measurements, a property that appears on almost every data sheet for a PP product.

The MFR results showed that the two sets of parts were likely not made from the same grade of material. The good parts had an MFR just below 4 g/10 min while the failing parts came in a little below 7 g/10 min. However, these are not large differences provided that the composition of the products does not differ significantly.

A Revealing Peak
But the DSC work showed considerable differences. Figures 1 and 2 show the first heat melting process for the material from the good part and the failed part. It doesn’t require a polymer chemist to see that there is a difference in composition. The material in the good part exhibits a single strong melting point that appears to be consistent with a PP homopolymer. However, the material in the failed part shows two distinct melting peaks—one associated with polypropylene and the other in the temperature range consistent with polyethylene.

While it is difficult to determine the exact structural relationship of the ethylene and propylene constituents, it is obvious that the ethylene represents a significant portion of the structure in the failed material. This
material may be a specialty copolymer or a physical mixture blended by the molder, but it will have different properties from the other material. The addition of ethylene blocks to a PP backbone typically results in a softer, more flexible compound. This accounts for the increased stretching and deflection. However, this modification also provides impact resistance. An increased tendency for cracking would not be expected.

\However, cooling the sample under controlled conditions in the DSC showed that the good material recrystallized at a lower temperature than the failed product. The higher recrystallization temperature of the failed material was consistent with a significant degree of nucleation. Nucleated PP materials provide several benefits, including reduced shrinkage and faster cycles. But another consequence of nucleation is reduced toughness.

A Clear Second Heat
Finally, it is always important when conducting DSC tests to heat the material through its melting point a second time. In semicrystalline materials like polypropylene, the first heating process provides a view of the material in its as-produced state. But the rapid cooling processes associated with molding almost always result in a structure that is not representative of the inherent nature of the material.

The slow cooling process in the DSC allows the material to structurally rearrange into its ideal state. Usually, this results in a slight shift in the material’s melting point and an increase in the heat of fusion, a sign that the material is more crystalline after slow cooling than it was after rapid cooling during the molding cycle. However, sometimes the second heat also reveals some secrets regarding composition that were not previously apparent.

Figures 3 and 4 show the second heat results for the two polypropylenes. The second heat for the failed part does not produce any new information; it simply confirms what we already know. However, the good material reveals some new details in the form of a distinct second melting point that was not present during first heat.

This new melting point occurs near 150°C, which is a signature temperature for the melting of random copolymer polypropylenes. This suggests that the original supplier was using a material that was more complex than originally supposed. The addition of the random copolymer may even have been a press-side modification, which could explain the lack of a data sheet for the material.

However the original supplier arrived at the formulation used to make the initial successful parts, it is certain that duplication of the key properties could be difficult even with property information. Without any guidance in material selection, the new supplier is left to sample from among the thousands of possible PP grades, an exercise that exhausts a lot of resources and still may not produce a satisfactory product. A little investigation into the composition and properties of the original material can pay back 100 times over when the costs of delays in product introduction and the possibilities of product failure are considered.

November 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.