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What is compression set?

‘Set’ is a term that is used to describe something that has been deformed by a force and when released does not return fully or at all to its original shape – it is said to have “set” in this position. Caravan tyres are a good example. When the owner lays up the caravan for the winter, he needs to ensure the tyres are properly inflated. If this is not done and the tyres are left underinflated, due to the weight of the caravan the bottom of the tyre that is in contact with the road will squash flatter than it should. After being left in this position for 5 months the tyres develops a “set”. Even when subsequently properly inflated, the tyre will retain a set and will be misshapen and useless. When a customer closes a door onto a Silicone Engineering sponge gasket, they want to know that it will spring back when they open the door so that on closing again, the seal is maintained. Good compression set properties are mostly to do with good rubber choice and fully chemically cross linked and post cured products. The compression set test is a common test in our laboratory.

A standard test specimen is measured then compressed in a jig to a certain percentage of its original thickness. The jig is placed in an oven at high temperature for a specified time then released and left to recover whereupon it is then re-measured. A calculation is done and a compression set value is derived.

Note: It is important to be aware that there are two ways to represent compression set.

As an expression of the amount compressed.

% = ((t0 – t1) / (t0 – ts)) x 100

As an expression of the overall thickness of the test specimen.

% = (t0 – t1) / t0 x 100

Where,

t0 = original thickness

t1 = thickness of specimen after recovery

ts = thickness of the spacer bar used

The difference is that the “Numbers” will be different, people just look at numbers without necessarily understanding the significance. At 1 above, the percentage set is a percent of the thickness compressed whereas at 2 above, the percentage set is a percent of the overall thickness of the test piece. Let’s give an example;

A 20mm specimen is compressed by 50%

(20mm – 15mm) / (20mm – 10mm) x 100 = 50% compression set.

(20mm – 15mm) / 20mm x 100 = 25% compression set

Silicone Engineering can see from the above that if the customer considers the number only i.e. 25% or 50%, then they think that number 2 is better yet both of these are the same, they are only representing them as different functions, number 1 as a function of the amount compressed and 2 as a function of the overall thickness of the specimen. It is therefore, important to know which standard is being used when looking at compression set values. This is because also, the pass/fail criteria is different for each of methods 1 & 2. Take a look at the table below.

Standard Maximum Compression Set %  Calculation (from above)
SAE AMS 3195/6 60 1
ASTMD 1056 60 1
EN ISO 1856 15 *** 2

*** EN ISO 1856 (or its predecessor) is the standard that Silicone Engineering have used throughout its 55+ year history. The standard tells you how to conduct the test and how to calculate the results, unlike the other example of ASTMD 1056 where this is both a standard and specification, EN ISO 1856 being only a standard does not stipulate a maximum compression set value, this is left to the individual to decide. In this example, SE decided to use this standard and to specify its own value at 15% maximum compression set.

Compression set in Silicone Sponge – Open and Closed cell:

It is also important that the customer knows whether the cell structure is open or closed as this has a bearing on the test itself and can make a huge difference to the value. Here are all the parameters that are needed to perform the test.

– How much to compress the sample

– How long to go in the oven

– Temperature of the test

– Recovery time before re-measuring

– Calculation to use.

Compression set more than virtually any other rubber test, needs to be qualified by reporting all the above parameters. It is useless to quote a number without the accompanying data which shows how it was performed and under what conditions. This is because all of these items will and do affect the values quoted. Squash it more – the result will be worse, leave it longer, turn up the heat – the result will be worse. When the compression jig is opened and the samples released, it takes time for the sponge to return to normal. ASTMD 1056 points out that open cell sponges require 30 minutes recovery before re-measuring whereas closed cell varieties require 24 hours recovery before re-measuring.

Why is this? When open cell sponges are compressed, the air within is immediately expelled and depending on the amount of compression, the rubber content is compressed some. On releasing from this compression, the air immediately fills the cells returning the sponge very quickly to near normal. Closed cell sponge works a bit different.

When the sponge is compressed, the air inside each cell is compressed and depending on the amount of compression the rubber content is compressed some. With the open cell sponge the air has gone straight away but with the closed cell sponge the air cannot escape but is compressed. As the temperature increases within the ageing oven the air will become even more pressurised. Silicone rubber is a permeable elastomer. This means that gases including air will penetrate then permeate the very thin membrane like walls of the sponge cells. Over the next 24 hours – the duration of the test – some air will escape due to the high temperature and pressure exerted by the test jig. On releasing from this compression, the partially airless cells want to hold the sponge in the compressed state – a bit like sucking it in! The rubber portion of the sponge (the other portion is air) will try to exert pressure through its natural “memory” however it is initially fighting against the fact that each partially evacuated balloon is resisting. Atmospheric pressure and silicone’s inherent permeability will, over time, recover the sponge to “near” normal.

Rather a lengthy explanation hopefully by now you can see that there is a vital difference between the two types of sponge that needs to be recognised and acknowledged. Herein potentially there is a problem. Customers who have historically dealt with open cell sponges and so are used to very fast recovery times, expect the same from our closed cell sponge.

To recap: ASTMD 1056 points out that open cell sponges require 30 minutes recovery before re-measuring whereas closed cell varieties require 24 hours recovery before re-measuring.

There is good reason for this as explained above, and those who have written the standards have done so from a point of knowledge on the subject however, even after having had this explained and pointed out, some can be adamant that they need 30 minute recovery.

To find out more, check out our silicone products page!

About the author

Simon Holmes
 

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