5 English Summary

Polymerer i anlæg med CO2 som kølemiddel

Objective of the project

The objective of the project was to clarify which polymer types (for sealing elements and structure elements) are contentslicable in systems with CO₂ as refrigerant and which material parameters are critical in relation to CO₂.

The tested polymer types were selected on the basis of the project participants’ experience in the field. The strength, solubility and diffusion of the polymers were tested with the objective to determine which polymer types are effected the least in the CO₂environment and which parameters (such as pressure and temperature) effect the polymers the most. Although the project group has pointed out that oil (in addition to pressure and temperature) also can influence the physical polymer properties, then it was chosen to disregard that parameter.

Project results

A test rig was built and it was possible to determine the ultimate stress of the polymers in subcritical and supercritical CO₂. Ultimate stress has been determined on the following polymer materials:

EPDM, FKM, HNBR, PTFE, PVDF, PEEK

Solubility and diffusion were measured by means of high pressure weight on the following polymer materials:

EPDM, HNBR

Permeabilty was measured directly by means of 2D permeation cell on the following polymer materials:

PTFE, PTFE + 10% Ekonol, PTFE + PPSO₂ + graphite
FKM
TFM, TFM + 10% glass

Conclusion

Permeabilty

In the light of the permeability measurements it can be said that the polymers have the following order from ”best” to ”worst”, meaning least permeable to most permeable:

TFM

PTFE

TFM + 10% glass

FKM

PTFE + 10% Ekonol

PTFE + PPSO₂ + graphite.

However, there is no great difference between the first five. The last one (PTFE + PPSO₂ + graphite) is much worse than the other five. Besides, the polymers become more permeable when something is added (e.g. glass, ekonol, graphite, PPSO₂).

Solubility and diffusion

General solubility of CO₂ in HNBR is contents. twice as big as in EPDM. On the other hand, diffusion is contents. one order of magnitude smaller. Although solubility is higher in HNBR the diffusion coefficient is substantially smaller and therefore permeability is smaller in HNBR. That goes for all temperature and pressure ranges.

Mechanical properties of polymers

All tested elastomer types (EPDM, FKM, HNBR) seem to be extremely influenced by CO₂. Common for them all is that their ultimate strength depends a lot on CO₂temperature as well as pressure. The fracture elongation of these materials seems to decline and so does the ultimate stress. At supercritical operation an ultimate stress reduction of more than 60% should be expected. The literature study confirms the measurement results. Tests carried out in 1999 at University of Wales in Swansea in Great Britain on fluor elastomers, silicone elastomers and nitrile showed the same tendencies, meaning declining ultimate stress and fracture elongation.

PTFE showed a good tendency as its ultimate stress by and large seemed to be independent of the pressure. On the other hand, the ultimate stress declined when the CO₂temperature increased.

PVDF and PEEK – It has been difficult to obtain good results for the two plastics materials due to very short fracture elongation especially for PEEK. However, it was characteristic for the two materials (PVDF and PEEK) that their properties change at higher pressure and temperature as fracture elongation becomes much longer. The PVDF ultimate stress also seems to be influenced by CO₂ pressure as well as temperature.