Introduction
Solvent evaporation or sample drying can occur in a variety of samples during long term rheological testing or when testing at elevated temperatures. This is particularly problematic for volatile samples containing organic solvents but even aqueous based materials such as ketchup or pastes have been seen to ‘dry out’ quickly when exposed to the atmosphere. Parallel plate or Cone-Plate measuring systems are most susceptible to evaporation and drying since the materials at the plate or cone edge are directly exposed. The issue is further exaggerated by the fact the measured or applied torque (M) varies with the radius (r) to the power of four (r4) for a plate and to the power of three (r3) for a cone, when measuring a Newtonian material. Consequently any change in sample properties at the edge caused by crust formation or loss of sample will have a significant impact on the measurement result. Problems may be encountered with concentric cylinder systems also, albeit to a lesser extent.To overcome such issues it is recommended to use a solvent trap as illustrated in Figure 1. The Kinexus Passive solvent trap system illustrated contains an outer thermal cover (Nylon 66 Glass filled 30%), which is the same material as the standard sample covers that are included with the standard plate and cylinder cartridges on the Kinexus. The inner material is made of Stainless Steel, and has an overlapping feature over the entire seam to give a tight seal. An additional feature is an included purge gas option that can be used to purge gas or vapor into the chamber if required. Inside there are two reservoirs for solvent, one located around the geometry shaft and the other a circular channel located around the circumference of the lower plate. The solvent ring and the plate channel are filled with the sample solvent. When the system is sealed, the vapour creates a saturated system reducing the evaporation of the sample solvent. The solvent trap is compatible with both plate and cylinder cartridges. This application note highlights the problem of solvent evaporation in rheological measurements and shows how it can be overcome by using a solvent trap during rheological testing. To read the full paper, just click here
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