Cross-linking Elastomers
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Cross-linking elastomers
Definition and types of cross-linking
In the field of polymeric materials, cross-linking refers to the chemical reaction that leads to the interlinking of macromolecular chains and thus to the formation of networks [1].
Rubbers can be cross-linked in various ways to form an elastomer. A distinction is made in [2] between:
- Sulphur cross-linking
- Peroxide cross-linking
- Cross-linking with reactive resins
- Cross-linking with quinone dioxime compounds (only in combination with an oxidising agent)
- Cross-linking with isocyanates
- Radiation cross-linking
- Photochemical reaction
- Electron beam
- Gamma rays
Cross-linking systems and acceleration of the cross-linking reaction
The choice of cross-linking system depends on the rubber and the process, but also, for example, on the intended application of the material, although cross-linking using sulphur-accelerator systems remains the most important method. As early as 1839, CHARLES NELSON GOODYEAR discovered by chance that natural rubber (NR) mixed with sulphur and white lead changed colour from grey to black when heated, lost its stickiness and became insoluble. The resulting properties far exceeded those of raw rubber. THOMAS HANCOCK later led the way in putting GOODYEAR’s discovery to commercial use. It was subsequently found that the addition of other metal oxides, such as CaO and ZnO, had a positive effect on the reaction time of sulphur vulcanisation. It was not until the beginning of the 20th century that the accelerating effect of basic compounds was recognised; these can significantly speed up the cross-linking process and influence efficiency through increased formation of cross-linking nodes [1].
The possibility of cross-linking molecular chains exists whenever reactive groups or double or triple bonds are present at the chain ends or within the chain.
An advantage of sulphur cross-linking is that the cross-linking reaction is insensitive to most components of the compound, as well as to water and oxygen. Basic components in the rubber compound act as accelerators, whilst acidic components act as retarders [1].
The resulting properties of the vulcanised product are determined by the cross-linking density (see also: entropy elasticity) and by the chemical structure of the cross-linking sites. The cross-linking density, in turn, depends on the vulcanisation time and the vulcanisation temperature.
Mechanism of the cross-linking reaction
Cross-linking transforms rubbers and rubber compounds from a viscous (plastic) state into a rubber-elastic state. In the process, intermolecular cross-linking bridges are formed, which prevent flow. The resulting rubber elasticity is based on changes in the entropy of the molecular chains during deformation and recovery of the elastomer (see Fig. 1) [2].
| Fig. 1: | Mechanism of entropic-elastic deformation [3] |
During the deformation of viscoelastic materials, both elastic and viscous mechanisms are at work. If the material is deformed in a purely elastic manner, the energy is stored and released in full once the load is removed. In this case, the deformation is reversible. In viscous processes (see: viscosity), on the other hand, the work done is converted into heat. The deformation is partly irreversible [2].
The elastic processes during deformation are time-independent, whilst the viscous processes are time-dependent. Consequently, viscoelastic motion takes a long time to reach a steady state. The time- and temperature-dependent behaviour of elastomers is particularly evident when determining the shear modulus G as a function of temperature T (see Fig. 2).
| Fig. 2: | Shear modulus as a function of temperature for highly cross-linked, weakly cross-linked and uncross-linked elastomers [5] |
It can be observed that elastomers remain glass-like up to their glass transition temperature, which, depending on the polymer, lies at temperatures well below 0 °C; they do not flow viscously even at high temperatures, but instead behave elastically within the temperature range from the glass transition temperature to the decomposition temperature.
In the energy-elastic region (< Tg), the polymer molecules are frozen in a glass-like state. As the temperature rises, the mobility of the polymer chain segments increases. The transition region to the entropic-elastic region is referred to as the transition region or softening region. In this region, the temperature dependence is particularly pronounced. The inflection point is referred to as the glass transition temperature (Tg).
Entropic elasticity (rubbery elastic region) is characterised by relatively low shear modulus values with comparatively low temperature dependence and a low loss modulus. In this state, the chain segments possess a high degree of mobility. If the flow processes are inhibited by coarse-meshed chemical cross-linking or by entanglement of the polymer chains, deformations are accompanied by a decrease in entropy. In uncross-linked polymers, the extent of this region depends on the molecular weight. Only once a critical chain length has been exceeded can a physical network form through chain cross-linking. In cross-linked polymers, the entropy-elastic region extends up to the decomposition temperature. The region in which the polymer exhibits entropy-elastic behaviour is the application range for elastomers [4].
See also
References
| [1] | Briehl, H.: Chemie der Werkstoffe. 3rd revised and expanded Edition, Springer Vieweg (2014) (ISBN 978-3-658-06224-8) |
| [2] | Röthemeyer, F., Sommer, F.: Kautschuktechnologie. Carl Hanser, Munich Vienna, 3rd Edition (2013) (ISBN 978-3-446-43776-0) |
| [3] | Bergmann, W.: Werkstofftechnik Teil 1: Grundlagen. 7th Edition, Carl Hanser, Munich Vienna (2013) (ISBN 978-3-446-43536-0; see AMK-Library under L 9-1) |
| [4] | Schnetger, J.: Lexikon Kautschuktechnik. 3rd Edition, Hüthig Publishing, Heidelberg (2004) (ISBN 978-3-410-21468-7; see AMK-Library under K 7) |
| [5] | Menges, G., Haberstroh, E., Michaeli, W., Schmachtenberg, E.: Werkstoffkunde Kunststoffe, 5th completely revised Edition, Carl Hanser, Munich Vienna (2002) (ISBN 978-3-446-21257-2; see AMK-Library under L 38); 6th Edition (2011) ISBN 978-3-446-42762-4 |