Chapter 9

How can you boost elasticity?

Examining the most elasticated polymers on the market

Materials used to produce drive shaft boots, air conduits, brake hose and boot uppers have a common feature: they must be elastic and yet able to withstand considerable stress. Injection mouldable thermoplastic elastomers deliver this combination.

Plastic parts are usually expected to keep their shape, i.e. they should not change shape (or only minimally) under load. In some cases, however, they are expected to show a high degree of resilience as well as shape retention, even under moderate stress.

The classic example for this is the bellows and the classic material for bellows is rubber. For some time now, thermoplastic elastomers (TPE) have been available for such applications.

Through continuous development, TPEs have today reached a high technological level, so they combine high flexibility, i.e. low hardness, with good mechanical properties and good thermal, chemical and aging resistance.

From the processor’s point of view, they have a big advantage in that they can be processed like thermoplastics, i.e. they can be injection moulded, extruded, blow moulded and they are recyclable.

Testing hardness

The hardness of a material can be described as the resistance it offers against indentation by another, harder body. Hardness testing methods are derived from this definition.

A harder body is placed vertically on the surface of the part being tested and then impacted with a certain load. After a given period – under constant load when testing soft and elastic plastics – the depth of indentation into the test part is measured and the hardness calculated accordingly. All the methods require relatively little in the way of apparatus, they are rapid, simple and reliable. As the tests leave only a minimal mark on the surface, they are non-destructive – and for this reason they are among the most frequently used tests.

Table 1: Generally used tests for hardness of plastics and elastomers.

The most widely used standard methods for TPEs are the Shore A and Shore D tests, while for plastics, the Ball Hardness and Rockwell Hardness tests are used. There is also scratch hardness as a measure of wear and abrasion, and rebound hardness as a dynamic property of the plastic under test; we shall not discuss these two further here.

Shore A hardness tests can be used for soft elastomers and for very soft TPEs. The testing body is a steel rod of 0.79mm diameter, having a truncated 35º cone. This rod is loaded with a mass of 1kg,

i.e. a force of 9.81N, and the depth of indentation is read off after 15 seconds. In the Shore D test, the steel rod has a 30º conical point with a tip, having an 0.1mm radius.

The rod is loaded with a mass of 5kg (49N) and the depth of indentation is read off after 15 seconds. In both Shore tests, the maximum indentation depth is 2.5mm, the hardness values range from 100 (no indentation) to 0 (2.5mm indentation).

In other words, the lower the numerical value, the less the material’s hardness. For materials in the 0-50 Shore range, the test part should be at least 9mm thick, for higher Shore values, it should be at least 6mm.

An advantage of the Shore methods is they are done with simple equipment that can be carried around, so tests can be done anywhere.

The steel ball used for the Ball Hardness test has a diameter of 5mm. Depending on the hardness of the plastic material being tested, it is placed under a load of 49, 132, 358 or 961N. These different loads take account of the fact that for geometrical reasons, the indentation depth does not increase in linear proportion to the load. The load should be chosen so the depth of indentation after 30 seconds lies between 0.15 and 0.35mm. The test piece must be at least 3mm thick.

While the hardness tests described above measure both the elastic and permanent (plastic) deformation, the Rockwell test considers only the plastic portion.

The testing body, a steel ball, is placed on the test piece with a preliminary minor load; then an additional test load (major load) is applied for 15 seconds. The major load is then removed but the minor load remains in place on the ball; the indentation depth is then measured after 15 seconds.

There are different scales, so the test can cover plastics with a broader hardness range. The most widely used are the M scale (ball diameter 6.35mm, test load 980N) and the R scale (12.7mm, 588N).

Instrumented hardness tests enable further conclusions to be drawn about the behaviour of plastics. In a computer controlled test set-up, the progression of forces during the whole test is measured.

It is composed of the increasing portion, while the testing body penetrates the test piece, the gradual decrease during the holding period and the drop when the load is removed.

The interpretation of the values provides inter alia information about the elastic and plastic portions of the total deformation. With TPEs, processing costs can be cut and quality boosted.

Figure 1: With this swirl generator made of Hytrel® thermoplastic polyester elastomer, the material’s high flexibility and very good resilience ensure combustion gases strike the turbocharger blades at the right speed and direction.

Figure 2: DuPont™ ETPV thermoplastic vulcanizates withstand continuous exposure to 160ºC, so they are suitable for demanding automotive applications such as spark-plug covers (top) or brake and hydraulic hoses.

Parts made of thermoplastic elastomers have secured a firm role in cars and commercial vehicles, in sports equipment, as well as in domestic equipment and white goods. Their use is particularly advantageous if the design succeeds in exploiting the material’s functionalities.

A swirl generator (see Figure 1) made of DuPont™ Hytrel® thermoplastic polyester elastomer provides an innovative example. Its task is to create a swirling motion in the combustion gases for certain diesel motors before they enter the turbo-charger.

In this part, the position of the blades self-adjusts automatically to the gas flow, without any external regulating elements, thanks to the high elasticity of the Hytrel® 5555 HS. The good resilience and high flex-fatigue resistance of the material are also important in this application.

The hardness of Hytrel® is in the range of 30-70 Shore D. The DuPont™ ETPV thermoplastic vulcanizates are much more flexible: their hardness lies in the 60-90 Shore A range.

The ETPVs (see Figure 2) combine many of the useful properties of cross-linked elastomers and TPEs. They can be fabricated like thermoplastics in a stable, rugged process with short cycle times, producing parts of constant quality.

These high-performance thermoplastics are especially suitable for demanding applications in the motor compartment, such as various seals, covers for spark-plugs, ignition coils and for thermally stressed brake hose and hydraulic tubing.

Elasticity

High elasticity, good recovery, high strength and tear strength, as well as good heat-resistance, resistance to aging, chemicals and fluids are the principal features of the high-performance thermoplastic elastomers available today.

They offer the additional advantage that they can be processed simply and efficiently by all the conventional processes for thermoplastic plastic materials.

# # #

The examples in this series of articles are intended to illustrate underlying principles and to explain the main influencing factors.

top of page