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New Die-Mounted Polymer Melt Viscometer Retrofits In Minutes
Viscosity is an essential quality control parameter in the manufacture of polyethylene terephthalate (PET). It is directly related to the degree of polymerization and hence average molecular weight, which in turn determines the physical properties of the polymer. However, monitoring the viscosity of melts under process conditions presents numerous challenges. A new in-line viscometer is now available that provides continuous real-time viscosity measurement directly from the extruder die head yet requires minimal maintenance, no calibration and no modification to existing plant.
Aside from its other uses (beverage and food containers, for example), PET is one of the most important raw materials for man-made fibres. It is used alone or blended with cotton or wool to impart better wash/wear and crease-resistant properties to textiles. It now represents over 50% of world synthetic fibre manufacture.
PET is a long-chain polyester, made by the reaction of bifunctional acids and alcohols in the presence of a metal catalyst. The key polymerisation step is a condensation reaction. In the manufacture of PET, ethylene glycol, catalyst and stabilizers are mixed with terephthalic acid or dimethyl terephthalate in an esterification reactor to form a monomeric “prepolymer”, bis-hydroxyethyl terepthalate (“bis-HET” or “BHET”) mixed with low molecular weight polymers (oligomers).
High molecular weight PET is manufactured by removing ethylene glycol under vacuum, when the bis-HET polymerizes. The reaction temperature must be above the melting point of the polymer (260-265°C) but below the temperature at which decomposition occurs too rapidly (300°C). At 290°C, reaction time is at least two hours, depending upon the type of reactor used. Near the end of the reaction the diffusion rate of the glycol from the molten polymer is rate-limiting, so it must be separated as quickly as possible. This is accomplished by the use of high vacuum and by mixing the melt continuously in order to expose the maximum amount of surface. The partial pressure of glycol over the polymer melt must be reduced to less than 6 mm Hg if useful polymers are to be obtained.
Finally, the molten PET is “melt spun”, i.e. the melt is extruded through a spinneret and water-quenched to form a glasslike amorphous material. Process conditions, spinning speed and melt rheology all affect the flow through the spinneret and the resulting physical properties of the spun fibres.
As the polymer grows in length and the average molecular weight increases, the viscosity of the reaction mixture increases. The increase eventually stops when thermal degradation of the polymer sets in. It is important to monitor the molecular weight because it significantly affects the properties of the resulting resin. High molecular weight is essential for good mechanical properties, providing stiffness, toughness and creep resistance while still giving sufficient flexibility to resist bursting and breaking under pressure. When very high molecular weights are required, as is the case for bottle-grade PET resins, the polymerization may be carried out in stages with different reaction conditions at each stage. When the polymer is to be used for fibre, its molecular weight should be between 14,000 and 20,000.
Since viscosity is a good indicator of average molecular weight, significant improvements in product quality can be achieved by continuous viscosity monitoring. A compact on-line viscometer specially designed for polymer melts has now been introduced by Hydramotion Ltd of York, England. The new ViscoMelt 5000 Extruder Viscometer is a drop-in device intended for direct installation on the extruder die head with no need to divert any part of the melt flow. Hydramotion pioneered high-performance process viscometry, and this die-mounted instrument complements the XL7-900 range of contoured-probe viscometers the company already supplies for installation on polymer melt pipelines upstream of the extruder.
The slim-line probe of the ViscoMelt 5000 is designed for maximum compatibility with existing plant. It has a sensing tip just 7mm in diameter, which makes it easy to use with the industry standard ½"-20 UNF fitting. There is no need for a bulky high-pressure flange, so it will often be possible to retrofit the viscometer on an existing port without having to add or modify any pipework. The result is that installation is very simple and can usually be completed in minutes.
The heart of the ViscoMelt 5000 system is the single sensor probe that extends into the process stream. The probe is made to vibrate at a high frequency, and these vibrations are dampened by the viscous drag of the fluid. Viscosity can be derived directly from the extent of the damping. This is an ideal technique for viscosity measurement since it is practically independent of other process variables.
Viscosity is measured continuously in real time at pressures over 200 bar (2900 psi) and at temperatures up to 400 °C (752 °F) without requiring any special cooling. Sensitivity is high and repeatability is excellent. Outputs include a 4-20mA analogue viscosity signal and, optionally, RS232 or RS422/485 serial data.
For the first time, polymer manufacturers will be able to monitor melt viscosity at the heart of the die head with minimum fuss. The tip of the ViscoMelt probe is designed to reach the centre of the extruder pipe, providing continuous real-time melt viscosity measurement with minimal impact on flow. Because the instrument installs directly into the melt flow, variations in the polymer are detected instantaneously in real time.
The wetted parts of the sensor are all 316 stainless steel, making the probe extremely rugged. It is an all-welded construction with no moving parts, seals or bearings which could wear out or fail in service. The instrument requires no on-site calibration and maintenance is virtually zero. Also, there are no nooks or crannies where material can collect, so gel formation cannot occur.
ViscoMelt probes are available for viscosity ranges from 0 to 10,000 pascal-seconds (100,000 poise). All models are intrinsically safe, certified for use in Zone 0 Hazardous Areas (explosive atmosphere continuously present), equivalent to Class I, Division 1, Group A.
The ViscoMelt 5000 viscometer is quick and easy to install and provides an accurate, reliable and durable solution for the continuous real-time measurement of melt viscosity. It is ideal for the immediate detection of viscosity changes resulting from switches in compounding formulation or variations in the degree of polymerisation.
The on-line viscometer provides much more frequent and rapid monitoring than is possible using side-stream or off-line methods. Real-time viscosity measurement allows manufacturers to respond instantly to off-spec conditions, which reduces wastage and delays in production while improving product quality. At a production facility in Germany, for example, results are already showing through in improved process control and enhanced product quality. Such continuous in-process monitoring of melt viscosity cuts costs and leads to greater efficiency. The ViscoMelt 5000 viscometer makes these benefits available to all.