Melt Index Rheomerter – OP5
The function of the OP5 rheometer from Optical Control Systems (OCS) is to make certified measurements of the Melt Index and/or Polydispersity of small solid polymer samples. The primary duty of these measurements is overall control of many types of polymerisation processes. This ensures that the product can be made to specific formulations. The secondary duty is Quality Control in final product selling specification and in batch control. The OP5 is logically situated in the plant analysis laboratory, which ensures best reliability and maintainability for these calibrated, precision measurements.
Scope of Application:
• Powder or pellets from polymer processes
• Powder ex-reaction
• Pellets post extrusion
• Samples transported to Laboratory via OCS-Porpoise systems
Representative samples are transported from various locations of the polymer manufacturing plant at the call of each analyser. Process Control and QC are full time activities, which require, as a minimum, one sampling point for each analyser. Representative samples are extracted from each reactor stage and from the finished product. In simple plant configurations, such as LDPE, PS, PET and Nylon this usually means one OP5 but in complex plants such as PP, HDPE and LLDPE two or more OP5 analysers would be required. The complex plants have powder samplers, which have degassing and catalyst deactivation stages close the analyser. At the plant end, these samplers can extract at the main discharge valve(s) of the reactor or at the primary de-gassing vessel, etc. according to the plant configuration. In the case of the pellet samples, these would normally be supplied from central (OCS) source, and in such cases the supply to the rheometers carries a priority to minimise the sampling delay time.
Melt flow measurements are performed after the solid sample is melted and conditioned to the appropriate test temperature. In the OP5 the melting process minimises any changes to the structure of the polymer but making a very rapid transition from solid to liquid, which substantially obviates shear damage, cross-linking, thermal degradation and other degradation processes. This sample preparation removes the last traces of any trapped air or gas and overall makes an important contribution to making the sample at the point of measurement fully representative of the process. In this unique way the OP5 makes control through rheology as reality.
The melt flow is regulated by a sealed gear pump, which transports a metered quantity through a shaped die which has the normal MI die L/D. The shaping of the die is to minimise the delay in the slow moving polymer near the walls, without deviating the actual rheology relationships too far from the standard MI die.
Measurements of pressure and flow rate are used to derive the standard Melt Index. Many features of the apparatus for measurements are covered in the patents. The patent, Melt Flow determination in Polymer process, has the following grants, EU 989 45440, GB 233 4958 and US 09/622558. These patents also have integrated process control application. Systematic correlations are applied to compensate for the relationship between the OP5 die and the lab test die (both ASTM D1283 and ISO 1133 tests use the same die form). The corrections derived from these correlations are specific to the actual polymer process and the local test methodology used to make the test. This is because each plant produces subtly different polymer structures and the manual test method result varies with operator and procedures. Once established the corrections do require regular verification, which is supplied through the OP5 operating software. The IPR of the operations are described in the EU CTM, Registration Certificate No 002729309. The methodology of calibration and measurement gives the OP5 class leading accuracy, which can be used to certify the finished product and thus completely replace routine testing using the Lab Melt Indexers. It is normal to certify these calibrations through a third party specialist. SGS, who are respected globally for such duties, have contract approval with major polymer producers for such onerous tasks.
The OP5 measurement of Melt Index is performed in a batch process, termed a cycle. The MI result is based on a tiny part of the sample, which gives the OP5 a pin point accuracy. The result shows every small variation in the polymer product and which can be used to steer polymer reactions in a way not possible by long term averaging or less than adequate sample preparation. The MI result can of course be used for Quality Control but the finely resolved and accurate measurements will bring a better precision and thus maximise the added value of the production.
Typically the OP5 samples every 5 minutes to fit in with the cleaning – measurement-sequence. The delay of measurement is slightly greater than cycle time of the sequence. Although the cycle has been optimised for accuracy but by using the necessary cleaning part of the cycle, the delay is made nearly independent of sample MI value. The real time delay (7-15mins) between the reaction and the measurement, which includes any sampling and sample preparation delay, is comfortably below process requirements whether it is used for reaction control and QC. In fact provided the delay is adequate, process control and QC places a very strong requirement on full-time calibration and best accuracy.
The unique features of the OP5 equipment open up the huge opportunity of process control through rheology.
Performance Features:
• Powder or pellets from polymer processes
• Controls – Menu-driven Windows interface and easy customisation
• Access – Password protected to 5 levels
• Optimum location – Use of laboratory environment to ensure high run time and accessibility for calibration and any maintenance
• Low delay time – Fast sampling, sample preparation and measurement results in delay time more than adequate for control
• Accuracy – Calibration is established by Porpoise Transfer Standard ™ Optional full-time statistical process control packages
• Full-time calibration – A statistical calibration system that conforms OP5 to the ISO 1133 or ASTM D1283 standards full time
• Tables – Tabular display according to individual cycle measurements of temperatures, Pressure, flow rate, consumed weight of material, test results
• Set points – Tabular display of all set points
• Real time display – Continuous trend display of MI with tramlines and product name overlays. Additional trend displays, temperatures, pressures, mass flow, total mass flow
• Recipe control of cycle and conditions – Operator control via menu of parameters, sequences and user Product Names
• Sample size control – Adaptive sample sizing to minimise delay of measurement
• Statistical analysis packages – User friendly calculations of averages, standard deviation, Cpk and other process variables. Export facilities
• Alarms – External alarm interface
• Open database – All records can be converted into any standard file format
Benefits:
• Powder or pellets from polymer processes
• Integrated process control through Porpoise measurement technology
• Improvement of process efficiency
• Improvement of quality at source – All product made to recipe
• Improvement of quality selection facility
• Measurements certified to ISO 1133 or ASTM D1283 standards
• High availability, high reliability
• Low maintenance budget, low time to repair
• Third party certification of product via SGS
• Elimination of customer complaints