A too steep angle results in a too high speed of the ball, which leads to turbulent flow conditions and faulty results. The inclination angle of the capillary allows for adjusting the driving force. The time it takes the ball to descend a defined distance within the fluid is directly related to the fluid’s viscosity. A preset angle determines the inclination of the capillary. Gravity acts as the driving force.Ī ball of known dimensions rolls or falls through a closed capillary, which contains the sample liquid. Ī falling-ball / rolling-ball viscometer does not measure a liquid’s flow time, but the rolling or falling time of a ball. For example, pressurized air or an inert gas such as technical nitrogen is employed. The sample under test can be in direct contact with the driving gas, or via a flexible membrane. The dimensions of the capillary show that this kind of apparatus is designed for medium- to low-viscosity substances: mineral oils, paper coatings, and dispersions, and for polymer solutions used to produce synthetic fibers on spinning machines. Shear stress values, however, do not exceed the medium range (a typical value would be 25 kPa). This method can operate in a shear rate range up to 1,000,000 s -1. Gas forces the sample through the capillary at a preset pressure.
The inner diameter lies between 0.2 mm and 1 mm, while the length ranges from 30 mm to 90 mm. These devices feature a glass or steel capillary of exact inner diameter and length. Capillary viscometers utilizing gas pressure Typical applications are highly viscous substances, such as polymer melts, and also PVC plastisols, greases, sealants, adhesives, and ceramic masses. However, the weight is replaced by a driving motor, which realizes high shear stress values (up to 900 kPa) and medium to high shear rates (around 1,500 s -1). Such a viscometer works in the same way as a weight-driven instrument (see the above section).
High-pressure capillary viscometers with electric drive
In such devices, the polymer sample is exposed to medium shear stresses (from 3 kPa to 200 kPa) and medium shear rates (from 2 s -1 to 200 s -1). ISO 1133 states the dimensions of cylinder, piston, and die as well as the approved weight pieces. capillary) at the bottom of the cylinder. The sample then has to pass through an extrusion die (i.e. The steel piston glides down inside a vertical steel cylinder, which contains the sample.
Ī defined weight on top of a piston is pulled down by gravity. Consequently, such an instrument is also named MFR or MVR tester. These parameters help to assess the quality of the melt and to predict its behavior when processing it. The device returns the MFR (melt mass flow rate) in or the MVR (melt volume flow rate) in. This type of instrument plays a major role in checking polymer melts.
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