Stock temperature measurement

The measurement of the temperature of the polymer melt is of major importance. Unfortunately, several factors complicate the stock temperature measurement substantially. It is very important to be aware of these complications in order to properly appreciate the measured value. 

Numerous detailed studies have been devoted to the measurement of temperature profiles in polymer melts flowing through channels. One of the most comprehensive studies on the theoretical and experimental aspects of temperature measurement of polymer melts was carried out by van Leeuwen [15-18]. Other studies on melt temperature measurement are listed in the following references [19-23]. When a polymer melt flows through a channel, a certain temperature profile will establish itself in the polymer melt. The temperature profile in a steady-state process after some time will become constant with respect to time this is the so-called fully developed temperature profile. 

The temperature at any point can be predicted from the equations of mass, momentum, and energy. When a temperature sensor, such as a thermocouple, is inserted into the polymer melt stream to measure the temperature of the melt, the steady-state flow is disturbed, and a new steady state will develop after a short time. Therefore, the measured temperature will be different from the true, undisturbed melt temperature. Thus, in order to determine the true melt temperature, certain corrections have to be made to the measured (disturbed) melt temperature. The factors that have to be taken into account are  

The design of the temperature sensor should be such that the above-mentioned errors are minimized. Various designs of temperature sensors for stock temperature measurement are shown in Fig. 4.16. 

The depth adjustment capability has another use besides avoiding damage. It allows the measurement of the temperature profile across the depth of the flow channel using only one probe. Adjustable upstream temperature probes are currently commercially available, e.g., by Goettfert. However, their application in commercial extruders is still rather limited. 

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An important advantage of the IR stock temperature measurement is the rapid response time, which is about ten milliseconds. The response of conventional melt thermocouples is several orders of magnitude slower. This means that rapid temperature fluctuations can be made visible with IR allowing a more detailed study of the dynamic behavior of extruders and injection molding machines [83, 84]. 

A built-in thermocouple for stock temperature measurement which was in a thermocouple well located above the rotors in the middle of the chamber and extending about 25 mm in a 45 degree downward direction from the right-hand wall. 

Figure 10.2 Method of estimating correct temperature of the compound for Experiment II, Run 4 (see Table 10.3 for experimental details). Solid curve, temperature recorded by thermocouple, 2 broken curve, estimated stock temperature, tp, O stock temperature measured by probe, t A stock temperature estimated from the temperature difference, - 2, of other runs.

Calibration is time consuming when performed correctly. It may require 1 or 2 days to perform all the necessary steps (i.e., prepare stocks, filter, measure absorbance, check purity, dilute, mix, and inject calibrants). Once the stock solutions and mixed calibration solutions have been prepared, a calibration check can be performed in -4 hr. Sample preparation, depending on the matrix, may require a few minutes or a few hours. If an autosampler is unavailable for overnight injection the extracts are typically stable overnight, refrigerated at - 20° to 4°C. It is prudent to maintain the autosampler tray temperature from 4° to 15°C to reduce sample degradation. HPLC analysis of the extracted sample requires 20 to 60 min. Typically one technician can extract 12 to 24 samples per day to be analyzed overnight or the next day. 

Preparation of Blends. The blends were prepared by milling the desired quantity of polyether additive with a portion of a masterbatch chosen from those described in Table II. Both the temperature and procedure used for the milling operation were critical. As is discussed in a subsequent section, temperatures that can be attained on a steam-heated mill were generally too low to attain good properties. Consequently, most milling was carried out on a 4-inch oil-heated mill at the desired temperature. The temperatures given are the mill roll temperatures. However, when the stock temperature was measured, it was about the same as the mill roll temperature. 

Repeated Extrusions. 0.25% Tris(nonylphenyl) phosphite and 0.25% polymeric phenolic phosphite were added to unstabilized polypropylene, and the resins were run through the laboratory extruder four times. The barrel temperature was 450°F., and the stock temperature of the extrudate was approximately 500°F. Melt flow rate of samples taken after each pass was measured according to ASTM 1238-62T. 

Stock solution. Measure 1 ml. of phenylhydrazine by pipet into a 10-ml. volumetric flask, add 3 ml. of acetic acid and swirl under the tap to cool to room temperature, and dilute with water to 10 ml. The solution contains 1 mmole of phenylhydrazine acetate per milliliter. If a substance to be tested for the presence of a carbonyl function is soluble in water to the extent of 8-10 microdrops in 1 ml. of water, dissolve 1 mmole of sample in 1 ml. of water and add 1 ml. of the stock solution. Separation of an oil or solid indicates the presence of a carbonyl function. If the sample is soluble to the extent of only about 4 microdrops in 1 ml. of water (diethyl ketone), treat 1 mmole of material with 1 ml. of water and add a few drops of methanol until solution is complete then add 1 mole of stock solution. A sample insoluble in water can be dissolved in methanol, ethanol, or dioxaiie and treated with 1 ml. of stock solution. 

Trying to measure the melt temperature could be deceiving. As an example, an extruded extrudate with a room temperature pyrometer probe will often give a false reading because when the cold probe is inserted, it becomes sheathed with the plastic that has been cooled by the probe. A more effective method is by using what some call the 30/30 method. One simple raises the temperature of the probe about 30F (ISC) above the melt temperature and then keep the probe surrounded with hot melt for 30 s. The easiest way to preheat the probe is to place the probe on, near, or in a hole in the die. By preheating above the anticipated temperature, just prior to inserting it into the melt, then it requires the probe to actually be cooled by the melt. The lowest temperature reached will be the stock temperature. It... 

Stock temperature n. In plastics processing, the temperature of the plastic (as opposed to temperatures of metal parts of the equipment) at any point. Often taken to mean, if not otherwise qualified, the temperature of the melt within an extruder had or leaving the nozzle of an injection molder. Stock temperatures may be measured by sturdy thermocouples or thermistors inserted into the plastic stock, or by infrared instruments pointed at emerging extrudates, sheets being thermo-formed, etc. 

Commercial applications of UTT measurement started in 1976 however, the method has not found widespread use. An important application in extrusion is the noncontacting temperature measurement of heat-sensitive materials. Protruding temperature sensors can easily cause degradation with such materials. The UTT measurement, pressure compensated, can be used for stock temperature control in a similar fashion as with the conventional melt temperature sensor (see Section 4.6). 

IR probes that can be mounted in an extruder barrel or die are commercially available [6]. These probes are used to measure a more or less average stock temperature over a certain depth of the polymer, about 1 to 5 mm for most unfilled polymers. The actual depth of the measurement is determined by the optical properties of the polymer melt, in particular the transmittance. The measurement is affected by variations in the consistency of the polymer melt. Thus, when fillers, additives, or other polymeric components are added, the temperature readings will be affected. 

It is difficult to measure the actual polymer melt temperatures inside an extruder. The reason is that an immersion probe cannot be used because the rotating screw will shear it off. Flush-mounted temperature sensors do not give a good indication of the actual stock temperature. Thermochromic materials can be added to the feedstock to determine whether the stock temperatures in the extruder exceed the color transition temperature of the material. 

The heating and cooling system is used to achieve a certain degree of control of the polymer melt temperature. However, stock temperature deviations do not necessarily indicate a heating or cooling problem because heat is added directly to (or removed from) the barrel and only indirectly to (from) the polymer. The local barrel temperature as measured with a temperature sensor determines the amount of barrel heating or cooling. The temperature that is controlled is actually a barrel temperature. 

The predicted pressure profile is obviously a direct result of the assumptions made in the calculations. Winter assumed isothermal conditions at the barrel wall and adiabatic conditions at the flight tip. With stock temperature increases in the order of 100°C and more, it is unlikely that the isothermal boundary condition is valid for the barrel. For the same reason, it is unlikely that the adiabatic boundary condition is valid for the flight tip, particularly since the rest of the screw will be at much lower temperature. Unfortunately, it is difficult to measure actual temperature and pressure profiles. Thus, the predicted temperature and pressure profiles have not been compared to experimental results. 

Thus, bulk temperature measurements may not properly reflect actual stock temperatures. This is the case in screw extruders where very high local temperatures can occur. The same holds true for high intensity internal mixers. In such devices, pure mechanical degradation is unlikely to occur. Therefore, degradation processes in polymer melts involving mechanical stresses tend to be rather complex. 

Obviously, the residence time and its distribution only partially determine the chance of degradation in an extruder. The other factors that play an important role are the actual stock temperatures and the strain rates to which the polymer is exposed. The actual stock temperatures and strain rates are closely related. In the extruder, there are two major areas of concern the screw channel and the flight clearance. Janssen, Noomen, and Smith [65] studied temperature distribution of the polymer melt in the screw channel. Temperature distribution of the polymer right after the end of the screw was measured, for instance, by Anders, Brunner, and Pan-haus [66]. The temperature variations in the screw channel at the end of the screw were reported to be less than 5 to 10°C and relatively close to the barrel temperature. More recently, Noriega et al. [145] measured melt temperature distribution with a thermocomb and found temperature variations as high as 20 to 30°C. 

Batches, each weighing 70.00 g, were mixed in an electrically heated Brander mixer-measuring head (with a capacity of 60 to 85 ml, a rotor speed ratio drive-to-driven of 3 2, air cooled, with stock temperature thermocouple, fitted with cam-type rotors) at 40 rpm, with the temperature set points adjusted to 100°C. The mixer-measuring head was coupled to a Brabender electronic torque rheometer. 

The larger batches, each weighing 275.35 g, were mixed in a Brabender Prep Mixer measuring head, electrically heated, air cooled, with stock temperature thermocouple, fitted with Banbury-type rotors) at 40 rpm. The start-up procedure was similar to that used with the smaller mixing head however, the amount of masticated rubber temporarily removed from the mixer (to make room for the addition of the bulky carbon black) was scaled up from 10 g (taken from the 70-g batch) to 40 g. Fatigue resistance was measured by the Monsanto Fatigue to Failure Tester (ASTM D 4482). 

In this part of the work, the compositions were not vulcanized prior to the evaluations of dispersion quality (assignment of values of DR). The compositions, prepared according to the recipe for uncured samples of Table 1, were mixed at the various speeds. In the case of each run, small (about 0.1 g) samples were withdrawn for dispersion-quality evaluation after the various mixing times listed in Table 1. The samples were withdrawn by using the modified pliers described above. Attempts were made to keep the measured stock temperatures nearly constant by manually changing the temperature set points of the mixer during each run. 

Therefore, the discharge temperature should always be checked against the actual temperature for a specific rotor speed/compound formulation/machine temperature and then the temperature indicator on the mixer should be used as an indication of temperature change and not of the actual temperature of the stock. The same comment applies to infra-red temperature measurement as the infra-red emis-sivity of the stock depends on the colour, texture, etc., of the stock. 

A portable pyrometer for direct measurement of the stock temperature and temperatures of various parts of the machine. 

The final stock temperature was measured immediately after the door was opened at the corresponding spot where the rotor temperature was measured. However, because the stock was still banded on the rotor, it was not possible to see whether or nor the spot was exactly at the centre of the broad side of the blade. 

Direct measurements of the stock temperature indicated that there was a large run-to-run variation. This was interpreted to represent temperature non-uniformity within one batch, which was as much as 13.5 °C. Also, the direct measurements indicated a 12-39 °C higher temperature than that indicated by the built-in thermocouple. The latter, being placed in a rather large well, might have been cooled by the large heat sink of the machine body. 

A method for estimating the true compound temperature is illustrated in Figure 10.2, which shows the temperature recorded by thermocouple, the stock temperature, J, measured directly by a probe and the stock temperatures estimated from the temperature difference, - 2> of other runs. These - 2 values were added to the t 2-curve of this run to obtain the J-curve. It was assumed that the temperature difference between the curve and the 2-curve increased smoothly with the rise of the stock temperature. This is based on the previously stated interpretation that the temperature difference, - 2> is caused by the heat-sink effect on the thermocouple. Such an effect is expected to increase with rising stock temperature. The fact that the pattern of 2-curves is very reproducible means that the temperature non-uniformity within the compound is not registered by the thermocouple. This is probably due to the time lag of the thermocouple, which averages out the temperature non-uniformity of the very rapidly sweeping compound. 

The actual temperature measurement of the stocks gave temperature differences of as much as 13.5 °C, indicating temperature non-uniformity within the stock. This is very plausible because rubber is a poor conductor, the shear field within the mixing chamber is exceedingly non-uniform, and thus, the viscous heating of the material is very non-uniform. 

As solution gas is liberated, the oil shrinks. A particularly important relationship exists between the volume of oil at a given pressure and temperature and the volume of the oil at stock tank conditions. This is the oil formation volume factor (B, measured in rb/stb or rm /stm ). 

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