Elasticity

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Elasticity plastics

Types of deformation

Under certain conditions, most materials, or rather the moulded bodies made from them, exhibit elastic behaviour, i.e. they expand under load and then contract again when the load is removed.

Fig. 1:

Deformation behaviour of plastics: a) elastic, b) linear-viscoelastic and c) nonlinear-viscoelastic

Ideal elasticity

If the deformation is momentarily reversible, it is referred to as ideal elasticity. If stress and strain are proportional, this is HOOKE elasticity (see HOOKE's law), the special case of ideal elastic behaviour.

Ideal elasticity is based on a reversible change in the positions of atoms and groups of atoms. This elastic behaviour is characteristic of ceramics and many metals, as well as thermoplastics, below the glass transition temperature at low elongation.

The cycle of loading and unloading under HOOKE elastic deformation takes place without the exchange of work and heat (∑A = O and ∑Q = O) and is reversible. Theoretically, these illustrated cycles (Fig. 1a) can therefore be repeated infinitely without a change in the modulus of elasticity occurring.

Viscoelasticity

However, plastics generally behave visco-elastically. Viscoelasticity is a delayed elasticity. It is based on relaxation processes (see: relaxation plastics), i.e. on resetting processes of the molecular chains, which take more or less time.

Viscoelastic processes are usually completely reversible in terms of deformation, but not in terms of the deformation path (Fig. 1b). Depending on the plastic and the load, however, irreversible processes can partially prevent the initial length from being fully reached after the load is removed. The cycle of viscoelastic loading and unloading takes place with irreversible conversion of work (∑A > 0) into heat (∑Q < 0) (Fig. 1c).

Rubber elasticity

Rubber-elastic behaviour is exhibited by widely cross-linked polymers, so-called elastomers, above the glass transition temperature, thermoplastics in the quasi-rubber-elastic range and, in some cases, the melts of thermoplastics. Rubber elasticity is based on the endeavour of the chain molecules to adopt the statistically most probable entangled form, which corresponds to the entropically most favourable state.

See also

• Linear-viscoelastic behaviour
• Viscosity
• Relaxation plastics
• Rebound resilience elastomers
• HOOKE's law

References

• Fonds der Chemischen Industrie: Folienserie Neue Werkstoffe. Oehms Druck GmbH, Frankfurt/Main (1992) Folie 27 (ISBN 0174-366X)