Quality Applications

Y-501C Differential Viscometer Benefits 50-percent reduction in PET viscosity variation Analyzes 110 samples in 24 hours Easy backflushing maintenance www.viscotek-usa.com

Automated Measurement Methods Reduce Variation

In search of a method for automating intrinsic viscosity measurements, Wellman Inc., the world's largest plastics recycler and a leading manufacturer of fibers, plastic packaging and engineering resins, selected the Y-501C Differential Viscometer from Viscotek Corp., of Houston.

 The Y501 uses a unique process to eliminate manual variability from viscosity measurements. Wellman reduced meas-ured intrinsic viscosity variation in its PET polymer by 50 percent by implementing automated measurement methods. The technique uses a robot, which weighs and dilutes the sample, and a Y-501C viscometer that determines intrinsic viscosity without requiring human intervention. Use of this process has reduced measured intrinsic viscosity variation from 0.008 to 0.003-0.004.

 The new process begins when the operator places the sample in a tube. The robot then weighs the sample, calculates the amount of solvent that needs to be added to the tube to provide the desired concentration and adds the solvent. If the exact sample weight is more or less than the nominal value, the robot compensates by adding more or less solvent to arrive at the target concentration. The robot then places a sampling tube into the polymer solution. The tube leads to the Viscotek instrument and is used to fill a sample injection loop. The sample loop is flushed with a polymer solution before it's injected into the Viscotek viscometer.

 The viscometer includes two matched stainless steel capillaries in series, with a sample introduction valve between them. The pressure generated by solvent flowing through the first capillary is measured and compared to the pressure generated by a solution of the polymer dissolved in the solvent flowing through the second capillary. This leads to a direct measurement of the relative viscosity. Pressure and transducers measure the differential pressure, which (along with the sample concentration) is used to calculate the measured intrinsic viscosity of the polymer solution.

 After the sample is analyzed, approximately 25 mL of solution remains in the dissolution tube. The robot transfers the residual solution into a solvent waste container. The empty tube is discarded into a waste container.

 The automated process operates 24 hours per day and can analyze approximately 110 samples during that time period. It requires only occasional intervention, whereas the old process required nearly constant attention by a technician. But the primary advantage of the new process is the much lower variability in intrinsic viscosity measurements, achieved through the elimination of human error. Automation also reduces operator exposure to phenol/tetrachloroethane solvent.

 The viscometer has now been running for one year without any serious problems. In-line filtering removes solid solvent contaminants that would otherwise plug the capillaries. The small quantities of undissolved polymer particles that do build up inside the capillaries over time do not affect the measurement (as they do with glass tube methods) because the baseline response is calibrated before each injection. Maintenance of the Viscotek viscometer consists primarily of backflushing the capillaries weekly.