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Focus on Materials : Elastomers & Thermoplastics

Materials

Properties of an elastomer

Basic principles

Techné uses elastomers and thermoplastics for manufacturing seals and technical parts. They are charaterised by specific properties that determine theirsealing capabilities and life time.

Description-Elastomers-ISO-1629-Techne

Techné mainly uses thermoplastics for its machining workshop. Fore more information on those materials, see Machined parts catalogue:

Machined parts

Vendredi 30 Mai 2014Read onlineDownload

Hardness of an elastomer

The hardness measures the resistance of a sample to permanent plastic deformation due to a constant compression load from a sharp object.


The hardness measurement unit of rubber called IRHD (International Rubber Hardness Degree), is defined by the ISO 48. IRHD is a synonym for DIDC (Degré International de Dureté des Caoutchoucs in French).

Techné measures its parts in IRHD.

Durometre-IRHD-Techne

General tolerances

Whatever the chosen unit is, the tolerances are typically within +/- 5 points to take into account the variability of the compound preparation process and of the production process, but also the potential inaccuracy of the measurement method.

The advantages of an IRHD measurement

The measurement on finished parts is more accurate, and closer to the customer's need and his application.

Moreover the IRHD measurement guarantees the curing of the finished part, whereas the measurements done on a hardness button won't: same compound vut different curing parameters.

Finally the production of different parts with the same compound (or mixture) often requires different curing parameters. For example, an O-ring with cross section Ø5.33 will require a longer curing time than an O-ring with cross section Ø1.78. It is therefore important to check the hardness on the finished part.

Differences in results

When hardness measurements are done according to the ISO 48 and ASTM D2240 (which is usually only possible on hardness buttons, not on finished parts), results with IRHD scale and the Shore A scale are fairly similar, especially for low hardness.

However, they are not strictly the same. Indeed, depending on the shape of the part, we can regularly notice a 5 points hardness difference between Shore A and IRHD measurement done on the part. This difference can even be extented to 10 points difference in extreme cases.

In general, IRHD results are inferior to Shore A results. That is why it is common to use "75 Shore A compounds" to obtain "70 IRHD parts". For example, parts moulded with a "70 Shore A compound" can give finished parts with hardness from 62 to 65 IRHD, and therefore out of the 70+/- IRHD tolerance.

IRHD scale :

IRHD-Scale-Techne

Specific gravity of an elastomer

The density of specific gravity is the ratio in between the weight of a volume of the measured material and the weight of an equal volume of a reference substance, usually pure water.

Techné uses a densitometer to check the specific gravity of its materials (Cf. photo).

If one is not equipped with a densitometer, it is however possible to measure the volume of a part by emmersing it in water and measuring the weight of the volume of water displaces (Archimedes' principle).

Densimetre-Techne
Most-Widely-Used-Densities-Techne

With a simple density measurement, it is sometimes difficult to identify a material. Indeed, some rubber densities are similar to others. By knowing how and where the seals were uses - temerature, pressure, environment (water, oils, air, etc.) - it is possible to identify the material that was used.

For a sure rubber identification, Techné uses a TGA (Thermo-Gravimetric Analysis) as well as DSC (Differencial Scanning Calorimetry) to determine their degree of vulcanisation.

Temperature resistance of an elastomer

Elastomers are characterised by their temperature use range. Some may be available with an extended temperature range. For example NBR is usually down to -25°C, however Techné can provide low-temperature NBR that will work in temperature as low as -40°C or even -50°C.

Temperature resistance of elastomers - Comparison table:

Comparison-Table-Temperature-Resistance-Elastomers-Techne

*This information is on a guidance basis only. Do not tend simultaneously to the limit of all the properties of the material. For very high or low working temperatures, consult our technical department.

Compression set of an elastomer

When compressed, rubber parts show a plastic and an elastic deformation range. To determine the elastic recovery of the rubber, Techné measures the compression set.

This is also used to evaluate the quality of the vulcanisation.

Compression-Set-Techne

The comrpession set test is performed according to the ASTM D395 B. Principle: the specimen is compressed by 25% in a special mounting temperature. The specimen is therefore aged for 24, 48 hours or more.

The lower the result, the better the compression set is:

  • The material has a good shape memory if the copression set is close to 0%
  • The material has a bad shape memory if the compression set is close to 100%

Tear strength of an elastomer

Tear strength according to ASTM D624/B determines the sensivity of elastomers to stress concentrations due to cuts and tears (measured in N/mm). This feature has no link with tensile strength or elongation at break. Tear strength if often useful to characterise materials used for membranes or diaphragms.

Tensile strength of an elastomer

The aim is to measure the force needed to tear a standard slab (according to ASTM D412/C dumbbell shaped slabs) during a regular traction. The tensile strength (called "T") is measured in MPa whereas the elongation (called "A") is measured in % of the initial size of the test slab.

For tests on O-rings or slabs, Techné uses a dynanometer which is equipped with motorised wheels in order to distribute the constraint.

The elongation at break measured on O-rings will not be as good as the elongation measured on test slab (50% of what is indicated on the material data sheet).

Material hardness has a big influence on elongation at break and tensile strength.

  • For a low hardness: A high, T low
  • For a high hardness: A low, T high
Dynamometre-Techne

Ageing resistance of an elastomer

To choose the appropriate compound, fluids in contact with seals must be analysed. It is therefore important to specify the environment in which the rubber will be working in.

A material that is not appropriates will have direct influence on the efficiency of the sealing: hardness modifications, elongation reduction, loss of tensile strength, swelling or retraction of the rubber part.

According to the ASTM D471 standard, test slabs are placed in a fluid for a given amount of time (24, 48, 72, ..., hours) at a given temperature. After ageing we measure variations of:

  • Hardness
  • Volume
  • Tensile strength
  • Elongation
  • Weight

In order to avoid testing a lot of different fluids, reference fluids have been created:

  • Air, Water, Seam,
  • ASTM 1, 2, 3 oils
  • Fuel A, B, C, etc.

The table bellow gives the indicative resistance to these fluids:

Resistance-Table-Usual-Fluids-Elastomers-Techne

For specific fluids (alcohols, chemicals, solvents, ...) contact our technical department or refer to the chemical compatibility table our our "Rubber seals" catlog (page 120). Thse chemicals compatibility indicators are given on guidance bases only. Techné recommends to all its customers to make trials with the chosen material and fluid used.

Rubber seals

Vendredi 30 Mai 2014Read onlineDownload

General rubber and thermoplastics description

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