Evaluating materials in a lab can be a good way to predict their performance in the field. It can, that is, as long as the materials in question have been around for a while. Newer materials can lack the long-term history needed to validate lab predictions. The situation worsens when the newer materials behave differently than their more established counterparts. Thermoplastic vulcanizates (TPVs) in the past have suffered on both scores. Santoprene, the leading TPV, has been around for just 22 years, compared to about 200 for the thermoset rubbers it often competes against. And in terms of its mechanical properties and processing, it behaves like an oft-misunderstood cross between a thermoset rubber and a thermoplastic.
Advanced Elastomer Systems, the maker of Santoprene (www.santoprene.com), has a crew of its own design engineers who already understand these materials and know how to predict their performance, according to Mike Bednarik, technical manager for application development. But nothing boosts design confidence like some feedback from the real world. "And we now have enough real parts to take an analytical look at long-term performance," he says.
And he's not kidding when he says "real." For a recent study of Santoprene's durability, Santoprene Senior Chemist Joseph Pfeiffer and Senior Technical Specialist Andre Van Meerbeek combed through recycling plants and junk yards to procure used parts suitable for mechanical testing. "We compared the parts taken out of service to new parts where possible," reports Pfeiffer. In cases where new parts were no longer available, Pfeiffer and Van Meerbeek did the next best thing: They ground up old Santoprene components and used that regrind to injection mold sample plaques. These they compared to samples molded from a virgin material. Here's what they found:
Wear Doesn't Get the Boot
The protective boots for automotive rack and pinion gears represent an early success for TPVs. Like the thermoset rubbers it replaced, TPV had the right balance of flex and compression performance as well as chemical and thermal resistance. And it had something else going for it: TPV can be blow molded into boots with thinner wall sections than thermoset rubber. "There are significant cost and weight reductions," says Van Meerbeek.
So how do the boots hold up in this application? Pfeiffer says lab tests and modeling had previously done a good job mimicking the conditions under the hood, demonstrating that TPV material would work in this demanding application. But with commercial parts in production since 1982, an opportunity arose to test some parts that had seen some action. Pfeiffer and Van Meerbeek obtained used boots from a plant that re-manufactured steering gears. "The number of years the boots had been in use could not be determined," Pfeiffer acknowledges. But sources at the plant estimated that the gears sent in for reconditioning tend to be at least five years old on average.
After cleaning the boots with high pressure hot water and steam (110C at 100 bars), Pfeiffer and Van Meerbeek ground them and injection molded the reclaimed material into standard test plaques. They then compared the physical and mechanical properties in both an unaged condition and after aging in air and in oil (see Table 1). "The data shows that in absolute terms, the old TPV has slightly lower properties than the new material," Pfeiffer says. Yet some of the key indicators of the material's ability to hold up in this dynamic application did not change significantly. Pfeiffer points out, for example, that the new and old TPVs had nearly the same properties after air and oil aging. Compression set also remained virtually identical. "These observations demonstrate that the TPV material is capable of retaining its engineering property performance," Pfeiffer concludes.
TPVs have increasingly seen use as a replacement in all sorts of automotive seals. One early breakthrough for these materials was the development in 1992 of secondary roof profiles for Volkswagen's Seat subsidiary. For its Toledo model, Seat replaced a thermoset rubber (EPDM) with TPV in the roof profiles that run from the lower corner of the front windscreen along the roof line to the top of the rear windscreen. The design uses two kinds of TPV materials: Two sections made from a soft 58 Shore A grade seal against the door and car body, respectively. These soft TPV sections are hidden from solar radiation, sometimes by flock, but usually by a co-extrusion of a harder 50 Shore D TPV. "Roof profiles are more exposed to solar radiation than any other rubber profile on a car," says Van Meerbeek, explaining the need for a top protective section. The same construction principle saw use again in 1994 on the roof profiles of the Seat Ibiza. This time, the top protective section was made using an 87 Shore A TPV.
Seal of approval: Stress-stain curves for TPVs reclaimed from used glass-run channels suggest good retention sealing properties.
AES already had reams of data-from Xenon accelerated aging and outdoor exposure tests-showing that these roof-profile materials would retain their mechanical properties and color after long-term exposure to the weather, according to Van Meerbeek. But again, the existence of parts with some history behind them provided an opportunity to bolster these traditional weathering tests. To get samples this time, they obtained Seat roof profiles from a scrap yard in Catalonia, a region of Spain known for its sunny, hot, humid climate. They then measured the color shift (Delta E) by comparing the color of the profile's exterior surface to the inner surface. The results (see Table 2) show that the TPV material experienced "insignificant color changes," Van Meerbeek reports, noting that automotive applications typically look for a Delta E value under three.
Other tests looked at whether the surfaces of the profiles bore the marks of radiation-induced degradation. Here, Pfeiffer and Van Meerbeek took micrographs of the external surface of both profiles. According to Van Meerbeek, these micrographs show that at a magnification of 100, the material surface appears homogeneous. "The only defects that can be seen are micro-scratches that could in no way be seen with the naked eye," Van Meerbeek says.
Double duty: TPV roof profiles, like those used by Volkswagen Seat, combine two grades of elastomers. A soft grade (58 Shore A) makes up the searling sections, while a harder grade (50 Shore D) provides protection against solar radiation.
Glass Channel Doesn't Play Dead
In another commercial sealing application, TPVs replaced thermoset rubber in the glass-run channel for the 1985 Rover 800 model. These glass-run channels were composed of several components: A metal wire reinforcement is covered with two kinds of TPV-a 40 Shore D grade that clamped the profile onto the car body and a 55 Shore A grade that formed small retaining lips. The glass seal itself was made from 67 Shore A TPV to provide the necessary elasticity and compression set. The design also included a slip coat to ensure low friction against the glass. And corner moldings, which joined the profiles at upper corners, also employed a 67 Shore A TPV. "This technically innovative system has been used for the entire life of this model, and provided significant cost advantages due to the short cycle times, low scrap rate, and excellent dimensional tolerances while maintaining adequate sealability," says Van Meerbeek.
Turning again to the junk yard, Pfeiffer and Van Meerbeek found two Rover 800s-one 10 and the other 12 years old-and pulled off the glass-run channels. These they planned to compare to new profiles, which they bought as replacement parts. But Pfeiffer says they found that wall thickness of the replacement part had been changed from original design, so that "a fair comparison of their respective performance in terms of closing force or sealing force was not possible." What was possible, however, was a comparison of relevant sealing properties based on molded samples. Pfeiffer and Van Meerbeek cleaned the old profiles, ground them up and molded them into test plaques. They did the same with the new profiles. They then measured the tensile stress strain properties-a key indicator of sealing performance (see graph). "The properties showed no significant differences," Pfeiffer says.
These three automotive case histories make up just of portion of the validation work AES has been doing lately. Pfeiffer and Van Meerbeek have also looked at construction applications, such as TPV expansion joints and pads for railroad beds. Next up is a study of TPV automotive ductwork, which Pfeiffer expects to complete in a couple of months.
Senior editor Joseph Ogando can be reached at[email protected].
|(Table 1) Physical Properties of Recycled Rack-and-Pinion Boots|
|Hardness, Shore D||41||37||D2240|
|Tensile strength, MPa||16.8||14||D412|
|Elongation at break, %||640||560||D412|
|Stress at 100% Elongation, MPa||9||7.5||D412|
|After heat aging, 168 hrs. at 125C|
|Hardness change, Shore D||+2||+4||D573|
|Tensile strength change, %||+3||+1||D573|
|Elongation at break change, %||-7||-8||D573|
|Stress at 100% Elongation change, %||13||+16||D573|
|After aging in IRM 903 oil for 168 hrs. at 125C|
|Hardness change, Shore D||-23||-18||D471|
|Tensile strength change, %||-27||-28||D471|
|Elongation at break change, %||-42||-40||D471|
|Stress at 100% Elongation, MPa||-25||-26||D471|
|Weight gain, %||+58||+50||D471|
|Compression Set, 22 hrs. at 100C, %||65||65||D395 method B|
|(Table 2) Color Changes of Roof Profiles|
|87 Shore A TPV|
|50 Shore D TPV|