The Santoprene Durometer Mystery


I have always known that the two-digit numbers in the Santoprene grades referred to the durometer hardness of the grades. For instance,  Santoprene 101-55 is a 55 Shore A, 101-73 is a 73 Shore A, 101-87 is an 87 Shore A etc. Sometimes its good to verify the things that you know to be true.

In talking to someone at a compounder recently, it was pointed out that durometer of Santoprene 101-87 is actually 94 Shore A. I was kind of surprised when hearing this so I decided to check for myself. A quick review of current data sheets for various Santoprene grades shows that something strange has happened.

Grade                                   Durometer Shore A

101-55                                   60

101-64                                   70

101-73                                   78

101-87                                   94

Santoprene 101-55_page_001
Current Data Sheet for Santoprene 101-55

At this point I was questioning my sanity. Were they always different? Did the durometers change? When did this happen and why? Just to verify that I was not insane, I looked up these grades in my old 1988 Plastics Selector and sure enough, the durometers of each of these grades matched the nomenclature, 55A, 64A, 73A and 87A.

Plastic Selector 1

Plastic Selector 2
1988 data sheets for various Santoprene Grades

I was curious if there was any explanation for the changes. Back in 1988, the durometer was tested using ASTM D-2240, the current method listed on the data sheets is ISO 868. These methods are equivalent though so that does not explain the change.

I then thought that maybe ExxonMobil is showing instantaneous test results instead of the traditional 15 second delay. Durometer hardness is tested using a spring-loaded metal point. The tester measures how far the point pierces the surface of a plaque molded from the material. Typically, the tester is lowered onto the plaque, the indicator shoots up high and then slowly drops as the pointer sinks farther into the plaque which yields a lower result. Usually the indicator is read at the 15 second mark. Occasionally 5 second delay testing is used. Instantaneous testing is rarely used but I thought perhaps they were using it now. An instantaneous test would yield a higher test result than a 5s delay test which would in turn yield a higher result than a 15s delay test.

As an aside, data sheets that show durometer hardness should all indicate the delay used. The test result should indicate something similar to this: Hardness, Shore A, 15s delay. Unfortunately, the current Santoprene data sheets incorrectly neglect to show the test delay.

To verify the delay theory, I looked at some recent certificates of analysis that I have. The certificates of analysis from ExxonMobil do properly show the delay. (To not show the delay on the certificate of analysis would be a major infraction in any ISO 17025 audit.) I have a cert for Santoprene 101-73. The property is listed as Durometer Hardness, 15s and the unit of measure is listed as Shore A. The specification is 76-82 and the test result is 79. This would indicate that the nominal 78 Shore A that is listed on the data sheet is indeed a 15s delay test.

Item 816 Santoprene 101-73 lot#PTH2624 and PTG2568_page_001
Recent C of A for Santoprene 101-73

The only conclusion that I can draw is that, at some point, the formulations for Santoprene were changed and all of the hardness’s were raised. I have no idea why this was done and why the nomenclatures were not changed to reflect the new durometers.

Does anyone reading this have any ideas as why the properties of these materials changed? I would love to hear from you.

2 thoughts on “The Santoprene Durometer Mystery”

  1. The old durometer testers were a manual mechanical device that was a 5 second test. Somewhere around 2000, a newer electronic tester was implemented. The durometer number did increase across all grades though the formulation did not change.


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