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On-line viscosity testing can cut costs in sheet extrusion: the use of online viscosity measurement to optimise production to maintain quality standards is demonstrated in this description by Gneuss of its on-line viscometer used to monitor the viscosity of PET in thermoforming sheet production.

For polymers which degrade due to hydrolysis, shear stress, or thermal and/or oxidisation influences, monitoring the polymer viscosity on-line can help keep production parameters within the boundaries needed to maintain product quality.

The example described here is the extrusion of PET into sheet for thermoforming. Under temperature and in the presence of moisture, the material's molecular weight will decrease. Figure 1 shows the influence of moisture on the molecular weight of PET (shown by plotting its intrinsic viscosity) with differing moisture levels over the same time and at the same temperature.

If the viscosity of the polymer drops, this has a serious, negative effect on the mechanical properties of the final product. The lower molecular weight will lead to a reduced tensile strength and when the sheet is eventually thermoformed to deep, thin walled containers, the container wall is prone to splitting. A further problem is maintaining uniform sheet thickness tolerances if the polymer viscosity fluctuates as the material will then flow at different rates through the die.

One of the main causes for fluctuating residual moisture content is variations in residence time in the drying equipment, the drying temperature, the dew point temperature of the drying gas and the impact of contact with the ambient air. Even if all these factors can be kept stable, the residual moisture content of the input material can still vary (for example seasonally) which will affect the melt viscosity and molecular weight of the film. It is well known that problems occur, for example, when switching from one drying unit to another.

In order to reduce the impact of viscosity variations, manufacturers of PET thermoforming sheet typically make the sheet thicker than necessary, thereby using more material than necessary.

By monitoring the viscosity, it is possible for the sheet producer to take corrective measures. Measurement of the molecular weight or intrinsic viscosity are typically only possible with complicated laboratory equipment and not in real time. So a simple and reliable real-time viscosity measurement which can be correlated to the intrinsic viscosity offers valuable opportunities.

THE ONLINE VISCOMETER

Gneuss has developed an on-line viscometer which permits continuous monitoring of the melt viscosity and the corresponding processing conditions.

The design brief for this unit was to provide a device which is compact and could be retrofitted without negatively affecting the process (with no measurable increase in residence time), be simple to use, able to withstand rough production conditions and which provides results comparable in their accuracy to laboratory results.

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Typically, the Online Viscometer is flanged in between, for example, a screen changer and melt pump in the extrusion line. The melt channel diameter can be matched to the customer's existing equipment (between 20 and 110 mm diameter) and--within certain constraints--the hole pattern for the connections can be varied to permit the most compact installation. The equipment includes a melt (metering) pump with drive motor, reduction gearbox and the necessary melt pressure and temperature sensors together with the visualisation and control hard and software.

In operation the melt pump diverts a small proportion of the melt flow (no more than 3 kg/h) through a capillary slot. The elastic properties of the melt which could potentially disturb the measurement are filtered out by length of the capillary which evens out the laminar flow (figure 3).

From the volumetric flow through the capillary, the exact dimension of the capillary and the differential pressure in the capillary it is possible to calculate the shear stress and the dynamic viscosity.

The Online Viscometer measures the shear stress and the corresponding viscosity. By varying the speed (RPM) of the gear pump, it is possible to set different shear stresses and to measure the corresponding viscosities. With this data and with some correction factors (Weissenberg/Rabinowitsch, Schummer, Dodge/ Metzner or Reiner/Philipoff) a viscosity curve across a range of shear stresses can be measured and translated into real results.

The unit is heated electrically or by a liquid/vapour heat transfer medium. The user interface is a touch panel display which shows all the running conditions and has a menu structure which enables the operator to adjust the parameter settings.

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By selecting the appropriate capillary slot depth (the capillary can be exchanged during production) viscosities within a range of 1 to 20,000 Pa.s can be measured. The measurement range can also be adjusted.

The unit was designed to avoid dead spots or edges where shear- and temperature-sensitive polymers could hang up or the flow could stagnate. One innovative feature is the hinged design of the capillary which enables it to be removed and cleaned without stopping production.

APPLICATION EXAMPLE

This is a brief description of the experience of a customer operating the Gneuss Online Viscometer in a line for the production of A PET sheet for thermoforming.

The company runs a large number of products, comprising different proportions of bottle flake (reground PET bottles) and virgin material. The more virgin material is used, the higher the viscosity and vice versa.

In some cases, a viscosity drop could be observed when changing from one dryer silo to another. This is typically caused by moisture which has gathered at the base of the silo. After a few hours, the viscosity will return to normal as the moisture content is reduced down through the silo. It is also possible to see changes in the viscosity when changing from (virgin) raw material from one manufacturer to another.

Due to the ability to monitor the viscosity, the company was able to introduce counter-measures to keep the value within the optimum range. By doing this, it was possible to halve the quantity of scrap produced. At the same time, it was possible to reduce the average film thickness by approximately 10 per cent. The investment in this technology was recovered in only 6 months.

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Title Annotation:testing & inspection
Comment:On-line viscosity testing can cut costs in sheet extrusion: the use of online viscosity measurement to optimise production to maintain quality standards is demonstrated in this description by Gneuss of its on-line viscometer used to monitor the viscosity of PET in thermoforming sheet production.(testing & inspection)
Publication:British Plastics & Rubber
Date:Feb 1, 2009
Words:991
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