Chapter 11

Which polymers are ideal for moulding?

Choosing the right plastic for moulded parts which are exposed to chemicals

The properties and behaviour of plastics can change under the influence of chemicals. For this reason, it is important to choose the right material for moulded parts that are exposed to chemicals.

The choice should be based on resistance tests carried out on materials and moulded parts. Very complex processes take place when plastics are exposed to chemicals. The most important factors influencing these processes are:

• The composition of the plastic material (the resin, fillers and reinforcements, additives) and its structure (crystallinity);
• What kind of chemical is it in contact with (acid, base, salt solution, solvent or cleaning agent, fuel, oil or grease);
• The conditions of exposure (time, temperature, pressure).

As a result of this complexity, damage to the plastic or moulded part can be due to a variety of mechanisms (see diagram).

Diagram 1: Diagram showing the mechanisms through which chemicals can damage plastics.

Resistance requirements

The requirements for chemical resistance of plastic parts can be met only if one knows exactly to what stresses the parts will be exposed in practice. Chemical resistance tables, based on tests of plastic materials with typical chemicals, are the first important criterion of choice. However, tests on moulded parts in real-life conditions are essential to reach firm conclusions. Such tests can be carried out at elevated temperatures, so as to be able to predict the long-term behaviour of parts with a high degree of reliability within an economically and technically justifiable test period.

Testing resistance

When testing for chemical resistance it is important to distinguish between tests on test bars made of the various plastics resins and tests on moulded parts.

Tests on moulded parts are important because a part’s processing and its actual operating conditions can have a major influence on its resistance to chemicals.

In both cases standard chemicals are used, in which test parts are immersed for a certain time at a fixed temperature.

Following removal of the part from the chemical, changes in its weight and dimensions, its mechanical, electrical or optical characteristics and surface quality are measured and compared to the pre-test values. The summary results of such tests, usually qualitative, can be readily found in chemical resistance tables.

When testing moulded parts, test conditions should match the part’s future real-life operating conditions as closely as possible.

Raising the testing temperature can substantially shorten the time that a test takes; as a rule (also theoretically-based), raising the test temperature by 10ºC cuts the test time by half, provided no additional damage mechanisms due to temperature enter the picture.

In practical terms, this means the effects of a one-year chemical exposure at 20ºC can be determined in just one week if the testing temperature is raised to 80ºC.

Stress-cracks

When considering chemical resistance, stress-cracks are a special case. They develop in moulded parts when they are exposed to a combination of mechanical tensile stresses and a surfactant medium; this term also includes humidity. They lead to premature failure of parts through brittle fracture and they are a treacherous form of damage, because stress-cracks often show up only a long time after moulding, although their cause is faulty part design or internal stresses after processing.

A uniform quantification of the susceptibility of various plastics to stress-cracking is not possible at present. In general, detergents (wetting agents) generate stress-cracking.

Within any one type of plastic resin, resistance to stress-cracking rises with molar mass, i.e with chain length and branching. This knowledge enables producers to develop types with better stress-crack resistance.

For a qualitative determination of stress-crack formation, a moulded part is immersed in a stress-crack generating medium; raising the test temperature accelerates this process, too. Then, an optical examination is made for micro-cracks. The tensile creep test and the bent strip test are suitable for a more precise determination of stress-crack resistance.

In both cases the test specimen, usually a standard bar according to DIN EN ISO 3167, is immersed in the test medium under stress and at a raised temperature. In the tensile creep test, the specimen is under a constant tensile stress and the time to fracture is measured.

A whole series of such tests serves to determine how high the tensile stress can be, in order to reach a certain desired lifetime.

In the bent strip test according to DIN EN ISO 4599, test bars are clamped across steel templates of different radii, so different outer fibre strains are produced in the specimen.

On completion of the desired test period, the specimens are removed and optically examined. After that, the reduction in mechanical properties, compared to the original values, is measured with the usual methods.

Working conditions

Figure 1: The shift plate of the 7G-Tronic automatic gearbox, located in the oil sump, carries all the components of the fully integrated gear change control system. Made of a glass reinforced DuPont™ Zytel® nylon, it withstands the aggressive additives in the gear oil, which can reach 140ºC.

Figure 2: More than 50 separate parts of this extremely compact water softener are made of DuPont™ Delrin® acetal resin, which offers the required combination of good resistance to water and regenerating salt, even at high temperatures, as well as high-strength and good weldability.

Parts for automotive applications must have particularly good resistance to oils and fuels, even at the high temperatures found near the motor.

One example is the shift plate, which carries the fully integrated gear change control system of DaimlerChrysler’s 7G-Tronic seven-speed automatic gearbox.

The complex part, around 40cm long and moulded to close tolerances, maintains its vital functional properties, working fully immersed in gear oil at up to 140ºC. The heat stabilised DuPont™ Zytel® LM70G35 HSLX nylon 66, which has outstanding resistance to hot gear oil and all its aggressive additives, is optimal for this part. Reinforcement with 35% glass fibre gives the shift plate the necessary dimensional stability.

The plastic material used for a new resource-saving water softener made by Delta Water Engineering of Belgium has to withstand water, regenerating salt in granular and dissolved form, and high temperatures.

In addition, it has to have high tensile strength and good weldability. DuPont™ Delrin® acetal resin meets these requirements, and DuPont provided help with material selection, mould-fill simulation for complex shaped parts and with optimising processing conditions.

Fluoropolymers occupy a special position among plastics: they are resistant to virtually all chemicals, are practically insoluble and are thermally stable at temperatures up to more than 300ºC ^[1]. The reason for this, is the polymer’s extremely stable fluorine carbon bond. Teflon® polytetrafluoroethylene, discovered almost 70 years ago, is the “granddaddy” of fluoropolymers. It does not melt and therefore has to be processed by special techniques.

Teflon® FEP and Teflon® PFA, which can be readily extruded and injection moulded, are of considerable and growing significance. Their most important applications are hoses, linings and functional parts equipment used in the chemical and pharmaceutical industries. Literature:

^[1] Brück, M: Beständigkeit ist alles [Resistance is everything]. Chemie Technik 12/2006, pp.54-55

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

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