3. Principal Physical Test Methods
3.4 Some Special Features of General Physical Tests
3.4.3 Set
3.4.3.1 Permanent Set
Rubbers deform under load and rarely return completely to their original dimensions when the load is removed. The difference between the original and final dimensions, expressed in various ways, is known as permanent set and can be measured in tension, compression, or shear. However, permanent set in shear is not often required, and there are no recommended standards for its measurement.
In practice, the measurement of permanent set depends upon carefully defining the con- ditions of the test, the time for which the test is conducted, and the time that is allowed for the test piece to recover. Although standardized, these conditions are arbitrary, and it is sometimes difficult to obtain reproducible results and to relate them to how the rubber will perform under service conditions.
3.4.3.2 Tension Set
The method for measuring tension set consists of stretching a standard strip or dumbbell test piece to a constant strain, holding the elongation for a standard time, removing the
load, and allowing the test piece to recover for 30 minutes. The increase in length between the reference marks or narrow section of the test piece, expressed as a percentage of the original length of the narrow section, is the permanent set in tension.
= −
− × L L L L
Tension Set 100
s
1 0
0 (3.3)
where:
L0 = the original unstrained reference length Ls = the strained reference length
L1 = the reference length after recovery
The percentage strain value should be selected in accordance with the final application of the vulcanizate and with reference to its breaking elongation and the test temperature. A value of 100% strain is preferred, but 25, 50, 200, and 300% are acceptable alternatives. The test can be made at a temperature of 23, 70, 85, 100, 125, or 150°C; and the duration of the test can be 24, 72, or 168 h, the test period commencing 30 minutes after specified strain has been reached.
The measurement of tension set under constant stress and the use of ring-type test pieces are no longer recommended in some countries in Europe, although two sizes of rings are standardized in ISO 2285.
3.4.3.3 Compression Set
The practice in Europe is to measure compression set after constant strain at ambient or high temperatures following ISO 815. The measurement of compression set after constant stress is no longer recommended.
Small cylindrical disks of known dimensions are compressed to a fixed height in a sim- ple jig. The jig consists of two or more flat, parallel metal plates that are sufficiently rigid to withstand the stress without bending and are of adequate size to hold the test piece or pieces within the area of the plates. The plates are clamped together by nuts and bolts.
Steel spacers of the appropriate thickness, in the form of rings around each bolt, are placed between the plates to control the thickness of the test pieces while compressed.
The recommended sizes for the disks are either 13 mm diameter by 6.3 mm thick or 29 mm by 12.5 mm. The two sizes do not necessarily give the same values for compression set, and comparisons should always be made with similar test pieces. The use of lubricant on the con- tact surfaces of the plates is optional and may, in some cases, give more reproducible results.
However, the lubricant may also affect the compression set values obtained, and again, com- parison should only be made when the test conditions are similar.
It is recommended that three test pieces be used, either separately or as a set, for each determination and the results averaged. The bolts are tightened so that the plates are drawn together uniformly until they are in contact with the appropriate spacers, generally sized to give a compression of 25%. The apparatus containing the test pieces is introduced without delay into the central part of an oven that is maintained at test temperature. The recommended temperatures are 23°C or one of nine temperatures between 70 and 250°C.
The duration of the test is 24 h for tests at elevated temperatures or 72 h at 23°C.
At the end of the specified time, the test pieces are removed from the jig and allowed to recover at 23°C for 30 minutes before the thickness is re-measured. The compression set
is the difference between the original thickness of the test piece and that after recovery expressed as a percentage of the initially applied compression.
− ×
− t t
t t Compression Set at constant strain = r 100
s 0
0
(3.4)
where:
t0 = the original thickness of the test piece tr = the thickness of the test piece after recovery ts = the thickness of the spacer
Thus if there is no recovery, the compression set is 100%, and if the test piece fully recovers to its original thickness the compression set is 0%.
3.4.3.4 Interpretation
Permanent set measurements can be useful for production control because they provide an indication of the degree of vulcanization that has taken place. They can also be helpful when selecting a compound from a number of alternatives. Unsuitable compounds can often be identified if a permanent set falls in the high range; differences in values will probably not give a true indication of performance in service.
Probably the most amazing property of rubber is not its elasticity but the degree to which it recovers after being subjected to high strains. Both permanent set and compression set are important laboratory tests that are worthy of close attention. Let us consider the gen- eral deformation curve. Over the first infinitesimal strains, the deformation is spontane- ously elastic. The next part of the deformation curve is composed of a highly elastic and a plastic deformation, and plastic deformation may not be wholly recoverable.
On removal of the stress, immediate recovery of the spontaneous elastic deformation occurs, followed by the slower recovery of the highly elastic components. The plastic com- ponent may be irreversible, and a permanent deformation due to this factor remains. If the temperature is now raised, some further recovery usually occurs but in general, it remains incomplete.
To evaluate the state of cure, better discrimination is obtained when the test is carried out at high elongations, say 75% of the elongation at break. On the other hand, for inner tubes, this test is too severe and it is better to carry out the test at an elongation of 100% or 50%.
The normal compression set test consists of compressing samples by 25% of their height and maintaining them under this degree of compression for 22 h at 70°C.
The conditions have been chosen so as to provide a basis for a good routine test, but where the service conditions (e.g., limits of deflection or load, temperature range, etc.) are known, this test can easily be modified. Too often, compounds that have a high compres- sion set in the 70°C test are rejected for an application involving lower temperatures where the compound might operate satisfactorily.
The term creep is applied to the slow overall change in strain that occurs after the com- pletion of the initial elastic deformation, when the rubber is subjected to continued stress.
Creep continues more or less indefinitely. As the flow does not take place exactly according to
a known mathematical law, it is inadvisable to extrapolate the results; therefore, long-term creep tests are essential.