The value of technical data sheets

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01.12.2018

 

Technical data sheets are part of day-to-day business in all industrial sectors. Everyone comes across them and everyone has to deal with them. The purpose of technical data sheets is to provide users with information about the properties, structure and use of products and materials. The situation is no different in rubber processing; they are found most commonly for compounds of rubber, the characteristics of which are described and documented in the technical data sheets (material data sheets).

Businesses often depend heavily on the information in the technical data sheets. Customers inquiring about molded parts (You will find examples of mold parts here) generally request the data sheets for the material compounds they are interested in and then take that information as the basis for their decisions. 

This blog investigates the extent to which this approach makes sense, as well as what the material data sheets really say about rubber and also what they do not say.

What is the purpose of the material data sheets accompanying rubber compounds?

In order to outline the information that can be extracted from material data sheets, we will use as an example the technical data sheet for EF650 made by the Swedish company Trelleborg AB, one of the largest rubber processers in the world. 

Material data sheets are generally very similar in appearance (see for example here or here) no matter which company has provided them. 

Source & Download: Trelleborg AB

Source & Download: Trelleborg AB

As you can see in the upper part of the Trelleborg AB data sheet, the material EF650 is a black EPDM compound with a hardness of 45 + / - 5 Shore A. Unlike Trelleborg AB, other manufacturers often also indicate in this section the type of cross-linking for this material, that is, for instance, whether the EPDM compound is peroxide or sulphur cross-linked.

The sheet then lists the values for the various mechanical properties of the compound, as well as the measurement methods applied to produce those values. The tensile strength (the force required to tear a standard test slab) of the material EF650 is given as 13.3 MPa and elongation at break (longitudinal stretching of a test slab to its breaking point) as 561 percent. The compression set is 20 percent compression for a compression time of over 24 hours at 100 ° C. If you would like more details on how the mechanical properties of rubber are measured, you can read this in the previous blog posting "Elastomer mechanical properties and how they are measured".

In addition to the above measurements, which are carried out according to the standard guidelines at room temperature, Trelleborg AG has also tested the properties of materials at higher temperatures and in contact with water and steam. These values are given on page 2 of the material data sheet. In all three cases, it can be seen that both the tensile strength and the elongation at break are worse in the changed conditions.     

An important point is made at the end of page 1 of the technical data sheet:

“The indicated material properties are average values determined with standard test slabs according to the corresponding specification. These values cannot be used as specification values and may be different from the material properties of finished parts. This is to emphasize that the end user himself is requested to test the material with regard to its suitability in the application. This data sheet is not subject to an automatic update.”

These constraints are of enormous significance. We will have a closer look at this in the section "The problem with material data sheets".

However, technical data sheets for rubber compounds are very useful for the following reasons:

Firstly, they provide valuable references and guidance in assessing the mechanical properties of elastomers.

The standardized test methods mean that a range of materials can be compared to each other. Armed with this information, the user can then make decisions on the suitability of a material for their application.

Material data sheets also often contain information on the cross-linking of the rubber compound and the approvals that are required for some applications (biocompatibility, FDA, etc.).

The problem with material data sheets

In addition to the benefits described above and the information that can be gained from technical data sheets, it is important for the readers and users to be aware of the information the material data sheets do not contain in order to understand their limitations.

In this section, we would like to draw your attention to some of these constraints:

First of all it is important to point out that a material data sheet provides only snapshot data for a given instant. The rubber compound is manufactured and then appropriate tests are performed to determine its mechanical values.

When, at a later date, the compound is made again (e.g. for a customer), there can be quite large deviations from the original values. From batch to batch of material, variations of +/-10% are quite possible. The reason is that a rubber compound consists of various ingredients that are themselves subject to variations that consequently affect the final mechanical properties. As a result, a tolerance range of +/- 5 Shore A is specified for most rubber compounds.

When engineering a molded part and considering the operational stresses it will endure, it is imperative that the user takes into account the fact that materials fluctuate in their mechanical properties indicated on the data sheet.

To some extent, it is possible to check all the values for each delivered batch. But this requires effort, it is time consuming and, in addition to increasing costs, can put back the end delivery date. 

Let’s have another look at the Trelleborg data sheet for the material EF650. It is noted at the bottom of page 1 that the mechanical tests are carried out on standard test slabs and the same results cannot be expected from a finished product. This is generally true for the elastomer material data sheets - and is a very significant point. Mechanical properties depend directly on the geometry of the finished product. For example, the thickness of the part (test slab) plays a big part in its compression set.

In addition, most tests are conducted at room temperature and with air contact, that is, in accordance with the standard guidelines. On page 2 of the example data sheet, there are also values given for the material at higher temperatures and in contact with water and steam, but extra information like this is rare. As a result, it is often not easy to make an exact statement about the mechanical behavior of a precast component in contact with other media, such as, for example, chemicals. 

The same applies to different temperature conditions. The importance of a change in temperature is frequently underestimated, even though it has a serious effect on the mechanical properties of an elastomer.  The values indicated on a data sheet can worsen dramatically when the temperature decreases or increases. Even when some materials have good heat-resistance, it does not mean that their mechanical properties at high temperatures have the same qualities as described in the data sheet (room temperature).

Figures 1 & 2 show the results for certain mechanical properties of selected FKM or Viton compounds as the temperature changes. Both elongation at break and tensile strength decrease considerably. This is also one of the reasons for the popularity of silicone. In comparison to other materials, silicone retains its mechanical properties for even extreme temperature fluctuations at a very good level.     

Elongation at break vs. temperature, Viton ®

Figure 1: Elongation at break vs. temperature of a selected Viton ® compound

Tensile strength & 100% modulus vs. temperature, Viton ®

Figure 2: Tensile strength & 100% modulus vs. temperature of a selected Viton ® compound

Due to the limits of technical data sheets, the recommendation to the customer is therefore to conduct advance tests on the selected material in the finished part while it is in the intended application and under realistic ambient conditions. Only by doing this can the actual behavior of a rubber compound in the intended application be determined, i.e. the limits of endurance and other information not provided by material data sheets.

However, not all eventualities can be covered, as it is not always possible to predict how and under what conditions a product will be used, for example, by the end user.      

Unexpected cold and the crash of the space shuttle Challenger

The exceptionally tragic, and probably as a consequence very well-known, example of the effect of temperature changes on the properties of elastomers is the crash of the space shuttle Challenger in 1986.

The NASA space shuttle, which made its maiden flight in 1983, had already completed nine successful space missions before it exploded on January 28, 1986, just 73 seconds after take-off of the 10th STS-51-L orbiter mission at a height of 15 kilometers. All seven crew members were killed, including the primary school teacher Christa McAuliffe, who was taking part in a special NASA program. This was the most serious accident in the history of space travel of the United States.

The video shows the live coverage of the US news channel CNN.

An elastomer O-ring was identified as the cause of the disaster. O-rings were used in the construction and installation of solid-propellant rockets (SRB Solid Rocket Booster) on the space shuttle. Among other things, they were meant to prevent the escape of hot combustion gases. For this reason, FKM was chosen as the material for the O-rings, as it is exceptionally resistant to high temperatures and used in environments of 200 °C and higher.  The problem with FKM, however, is that it has low resilience in cold temperatures leading to permanent deformation. 

Temperatures were colder than expected on the night before and during the launch of the Challenger space shuttle on January 28, 1986.

An O-ring usually functions to seal, that is to create initial tension or a positive pressure against a surface by the elasticity of the rubber. The lower the compression set (click here to learn more about compression set) of the material, the better the sealing effect. FKM compounds usually have good (low) compression sets. As described above, the properties of an elastomer change under different temperature conditions.

Exactly this property (low compression set) was lost in the case of the O-rings of the Challenger space shuttle due to the poor cold resilience of FKM rubber. A seal was not created and a leak developed. Due to the leaking O-rings, hot combustion gas escaped from the side of the solid-propellant rockets, which should actually have been exiting through the main nozzle at the rear. The escaping gases damaged one of the connections from the solid-fuel rocket to the hydrogen tank, which ultimately led to the destruction of the tank and explosion of the space shuttle.

The American physicist and Nobel laureate Richard P. Feynman, who was a member of the commission deployed to investigate the accident, pointed out these circumstances, and vividly demonstrated the problem at a press conference: 

SBRs (Solid Rocket Boosters). The engineers made adaptations in line with the findings of the tragic crash of Challenger. They changed the design and construction of the O-rings and integrated a heating system to prevent the temperature decreasing below the critical level for FKM. Since then, there have been no further problems with the O-rings.

If you have any questions or comments on this blog, or in future would like us to focus on a specific aspect of elastomers, feel free to contact us by email at info@hepako.de 

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