Polymerization shrinkage
Polymerization shrinkage describes the shrinkage that occurs during the setting reaction of plastics (e.g. composites in dentistry ). Depending on the material used, there is shrinkage in the range between about 1.5-3%, and in the case of so-called flowable plastics also by 4% of the initial volume , with the shrinkage occurring within about 5 minutes and the volume remaining constant thereafter .
The reason for this shrinkage is to be found in the fact that the strong crosslinking of the polymers allows the chains to move closer together and thus take up less space.
The aim is to keep the degree of shrinkage as low as possible; more modern plastics have a lower shrinkage rate than conventional plastics, with materials with a high proportion of filler components achieving the lowest values.
A distinction is made between these fillers:
- Macro fillers (relatively large grain size)
- Micro fillers (relatively small grain size - easy to polish)
- and hybrid fillers (filler with different grain sizes - greatest packing density possible)
The shrinkage is observed both during the polymerization and during the crosslinking (curing) of monomers. This volume shrinkage is caused by various factors. The main cause is that the monomers move from the van der Waals distance to the covalent distance when a covalent bond is formed during polymerization. This can be shown using the example of ethene polymerization .
Change in distance during polymerization. This results in the shrinkage. The distance between the monomers from the van der Waals distance (3.40 Å) varies to the covalent distance of a single bond (1.54 Å), resulting in a net change of −1.86 Å. The change from the double bond (1.34 Å) to a single bond (again 1.54 Å) leads to a slight expansion (+0.2 Å). Both effects add up to a remarkable shrinkage.
Individual evidence
- ↑ Reinhard Marxkors, Hermann Meiners: Pocket book of dental materials science . Deutscher Ärzteverlag, 2005, ISBN 978-3-934280-85-4 , p. 275 ( google.com ).
- ↑ Sadhir, Rajender K. (1992). Expanding Monomers: Synthesis, Characterization and Applications. CRC Press. ISBN 0-8493-5156-1 .