ASTM International, formerly the American Society of Testing and Materials, is an international standards organization that develops and publishes voluntary consensus standards for a wide range of materials, products, systems, and services. This paper concerns itself with two very similar standards that architects, builders, and engineers rely on for window and door applications.
Why are there two standards for the same application? Which one is better? This article attempts to answer those questions and points out the differences between the two.
In 1974, ASTM published its standard C864, entitled “Standard Specification for Dense Elastomeric Compression Seal Gaskets, Setting Blocks, and Spacers.” The dense elastomer around which C864 was written is created by irreversibly hardening, or curing, viscous liquid resins. The curing process, also known as vulcanizing, is what creates the elastic properties of rubber. Although several elastomers can meet this standard, today, when C864 is specified, it almost always means EPDM1 rubber.
Beginning in the 1960s, thermoplastic rubber (TPR)2 was developed by several companies, a material that processes like a plastic but possesses many of the same properties as rubber. It wasn’t until the late 1970s, however, that Monsanto Chemical introduced a TPR they called Santoprene, and classified it as a thermoplastic vulcanizate (TPV) which was the first time a TPR could seriously be considered in most glazing applications because of its superior compression set. It is a blend of EPDM rubber which is vulcanized in a polypropylene matrix. Unlike EPDM, it could be reground and recycled into new products without losing its initial properties. They marketed their new product under the brand name Santoprene, which is today still the most widely used TPV on the market.
It was for this reason that in 2002 ASTM developed E2203 as a new, alternative glazing standard, called “Dense Thermoplastic Elastomers Used for Compression Seals, Gaskets, Setting Blocks, Spacers and Accessories.” Why ASTM didn’t simply update C864 is an open question, but there are probably several reasons. We leave it to the reader to decide which one makes more sense.
Both standards include references to hardness, tensile strength, elongation, compression set, ozone resistance, heat aging, tear strength, low-temp brittleness. The two standards don’t line up perfectly, however. C864 includes tests for non-staining, and flame propagation, which are not part of E2203. E2203 contains a number of tests not found C864. These are: density, modulus (flexibility), oil resistance, tear resistance, and water absorption. See Table 1 for a full comparison.
The list below includes the most important properties that are tested by both ASTM C864 (EPDM) and E2203 (TPV).
Hardness. The optimal hardness for a seal depends on the application and the design, although 60-70A durometer is a very common range for seals.
Compression set. Compression set3 is the ability of the rubber to recover its original thickness after a load is removed. A sample that recovers fully after the load has been removed is said to have 0% compression set. A sample that doesn’t recover at all has 100% compression set. Compression set is especially important for dynamic seals, such as door seals, where the gasket is under load intermittently. It can be measured using different load time and temperature settings, but in general a compression set of 30% is very good, and 20% is outstanding.4
Heat aging. It is not uncommon to expect a seal to last for 10, 20, or even 30 years. In such cases, the long-term sealing properties can be more important than short-term properties. Because it is impractical to wait 30 years to measure the degradation over time, accelerated aging tests5, also called heat aging, are commonly used to compare different materials. The change in hardness, tensile strength, and ultimate elongation (how far the gasket can be stretched before breaking) are measured after the seal bakes in an autoclave. One comparison of EPDM and TPV after heat aging is shown in the table below.
Weathering. If the seal is an exterior seal, ozone resistance is important.6 Ozone is formed when oxygen is exposed to UV (ultraviolet light), and ozone is known to cause rubber to crack over time. The test accelerates the process by exposing samples to high levels of ozone.
Other properties. Depending on the application, tensile strength, elongation (ability to stretch), UV resistance, oil swell, and high- and low-temperature performance can also be important for a seal.
Benefits of EPDM over TPV
EPDM has been in use since the 1960s and as such is a long-accepted standard. The rubber specification ASTM C864 is widely cited on drawings, and some (but not all) grades of EPDM meet that standard. TPV will not meet C864, owing to the minimum tensile strength requirement and compression set.
Benefits of TPV over EPDM
TPV has superior long-term performance. As you can see in the figure 1 below, after heat aging, In the long term, TPV outperforms EPDM in both compression set and stress relaxation (also known as force retention). Stress relaxation is important in static seals, such as is often the case in inoperable windows.
TPV can be extruded to tighter tolerances than EPDM, which can be important in certain gasket designs.
TPV can be extruded in colors.
We believe that compression set, stress relaxation, heat aging, and ozone resistance are among the most important properties to consider when designing a seal. If you need to meet ASTM C864, EPDM may be the better choice. But be sure that the material offered is certified to C864, because not all grades of EPDM will meet C864.
In the same way, not all TPV grades meet E2203. Those that do offer other benefits worth considering: recyclability, long-term aging, lack of off-gassing volatiles, design flexibility, and colorability.
Table 1
| Test | C864 | E2203 Type III | Santoprene 121-67W175 |
| Hardness (Shore A)7 | 70±5 | 70±3 | 72 |
| Compression Set (22h @ 100C, max)8 | 30% | 35% | 30% |
| Ozone resistance9 | no cracks | no cracks | no cracks |
| Tensile (min)10 | 1800 psi | 870 psi | 1073 psi |
| Elongation break (min)11 | 200% | 350% | 450% |
| Heat aging, max chg in hardness (Shore A)11 | 10 | 3 | 1 |
| Heat aging, max loss in tensile at break12 | 15% | 5% | 4% |
| Heat aging, max loss in elongation at break12 | 40% | 5% | 1% |
| Tear strength, min12 | 100 lb/in | 114 lb/in | 142 lb/in |
| Low-temp brittleness, max temp13 | -40C | -69C | -76C |
| Non-staining14 | Pass | Not specified | unkn |
| Flame propagation15 | Opt I or II | Not specified | Option II |
| Tolerances (RMA) | Precision | As needed | As needed |
| Density16 | Not spec | 0.97±.02 | 0.97 |
| Modulus (100%, min)17 | Not spec | 2.2 | 2.9 |
| Oil resistance (max weight gain)18 | Not spec | 90% | 55% |
| Water absorption (max change)11 | Not spec | 7 | 0 |
Figure 1
Compression set and stress relaxation (force retention) of TPV and EPDM after heat aging.
(Note that some grades of EPDM may perform differently than is shown here.)
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Notes:
1. Ethylene propylene diene monomer.
2. “Thermoplastic rubber” is technically also a dense elastomer, but it does not meet ASTM C864, for reasons explained below.
3. Test method is ASTM D395.
4. Common standards are 22h at 70C and 70 at 125C, but there are others. When comparing two materials, be sure to compare similar test standards.
5. ASTM D573
6. ASTM D1171
7. ASTM D2240
8. D395
9. D1149
10. D412
11. D865
12. D624
13. D746
14. D925 method B
15. C1166. Option I specifies a maximum loss of 4 inches. Option II has no limit.
16. D792
17. D412
18. D471
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