Q. How should I approach rheological testing with a new polymer?
A. The optimization of rheological tests is iterative; the best way to determine the most ideal testing conditions is to experiment with the material. That said, there are some basic questions that we can ask ourselves about the polymer in order to speed up the optimization process:
1. Is it thermoplastic or thermosetting? I would have serious reservations about performing a test on a thermoset; great care should be taken to discover the conditions that bring on thermosetting and then to meticulously avoid those conditions while testing.
2. Do we know the optimum temperature range for processing of the polymer? Many materials are listed in tables of Melt Flow Rate data, and temperature information from these tables is transferrable for use in a Capillary Rheometer. The key is to enable stable flow while imposing the intended processing conditions. If the material flows in an unstable manner, the temperature should be raised; if the material shows degradation, the temperature should be lowered. Temperature change should be of the order of 5-10 degrees per attempt and the selection of optimum processing temperature should be made on the basis of satisfactory performance of the extrudate. This may take several attempts.
3. How viscous is the material? Highly viscous polymers will need a large diameter capillary so that the test is not confined to a very low shear rate by the limitations of the pressure transducer. Conversely, low viscosity polymers, such as PVA, require the use of a much smaller die so that the barrel is not emptied before the test is complete. Often MFR data can help in making these decisions.
4. What pressure transducer shall I use? Until the first test is complete, it won’t be clear how much pressure the fluid will exert under test conditions. The most highly rated pressure transducer should be fitted; if the chosen transducer is too delicate it may be damaged. The results of this test will be of very low resolution, but will be accurate enough to determine a more appropriate choice.
5. What if I want to do a Bagley correction? A Bagley correction is a test using two barrels that calibrates the machine against the effects of barrel-die geometry. Because the Bagley correction is specific to the testing parameters under which it is performed, it is worth waiting until appropriate testing conditions are agreed upon before performing one. Additionally, initial tests should be run on a single bore. This in case the testing conditions prove to be inappropriate and the test must be aborted. Having filled only one barrel, only one barrel of material is wasted and less cleaning is required.
A. The optimization of rheological tests is iterative; the best way to determine the most ideal testing conditions is to experiment with the material. That said, there are some basic questions that we can ask ourselves about the polymer in order to speed up the optimization process:
1. Is it thermoplastic or thermosetting? I would have serious reservations about performing a test on a thermoset; great care should be taken to discover the conditions that bring on thermosetting and then to meticulously avoid those conditions while testing.
2. Do we know the optimum temperature range for processing of the polymer? Many materials are listed in tables of Melt Flow Rate data, and temperature information from these tables is transferrable for use in a Capillary Rheometer. The key is to enable stable flow while imposing the intended processing conditions. If the material flows in an unstable manner, the temperature should be raised; if the material shows degradation, the temperature should be lowered. Temperature change should be of the order of 5-10 degrees per attempt and the selection of optimum processing temperature should be made on the basis of satisfactory performance of the extrudate. This may take several attempts.
3. How viscous is the material? Highly viscous polymers will need a large diameter capillary so that the test is not confined to a very low shear rate by the limitations of the pressure transducer. Conversely, low viscosity polymers, such as PVA, require the use of a much smaller die so that the barrel is not emptied before the test is complete. Often MFR data can help in making these decisions.
4. What pressure transducer shall I use? Until the first test is complete, it won’t be clear how much pressure the fluid will exert under test conditions. The most highly rated pressure transducer should be fitted; if the chosen transducer is too delicate it may be damaged. The results of this test will be of very low resolution, but will be accurate enough to determine a more appropriate choice.
5. What if I want to do a Bagley correction? A Bagley correction is a test using two barrels that calibrates the machine against the effects of barrel-die geometry. Because the Bagley correction is specific to the testing parameters under which it is performed, it is worth waiting until appropriate testing conditions are agreed upon before performing one. Additionally, initial tests should be run on a single bore. This in case the testing conditions prove to be inappropriate and the test must be aborted. Having filled only one barrel, only one barrel of material is wasted and less cleaning is required.
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