Use of an Infrared Camera for Accurate Determination of the

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Use of an Infrared Camera for Accurate Determination of the Temperature of Polymer Filaments Vishnu T. Marla, Robert L. Shambaugh,* and Dimitrios V. Papavassiliou School of Chemical, Biological, and Materials Engineering, The UniVersity of Oklahoma, 100 East Boyd Street, SEC T335, Norman, Oklahoma 73019

An infrared (IR) camera was used to measure the temperature of polymer filaments held in a stream of hot air. Filament diameters of 0.1-4 mm were tested. The air stream velocity and temperature fields were independently measured with a Pitot tube and a thermocouple. Under certain conditions, the air temperatures determined by the thermocouple closely approximate the temperatures of the polymer filaments. These airtemperature measurements were compared to the temperature readings given by the IR camera. This procedure allowed the development of calibration curves for the infrared camera. The calibration curves were found to depend on (a) the diameter of the filament and (b) the image size of the filament upon the pixel array in the IR camera. These calibration curves can be used with the IR camera to allow online measurement of fiber temperature during melt spinning, melt blowing, and other fiber production processes. Introduction Measurement and prediction of the temperature of fine polymer filaments is important in many commercial fiber production processes. Such processes include melt spinning, melt blowing, and wet spinning. The techniques for measuring the polymer temperature can be divided into two categories: contact techniques and noncontact techniques.1 The usual contact technique involves use of a small metal plate with a fine thermocouple probe embedded in the plate. However, this technique disturbs the fiber, and this disturbance can affect the fiber temperature (or can even break the fiber). A common noncontact technique involves the use of infrared thermography. In infrared thermography, the radiant energy that is emitted by the target is received by the detector elements of an infrared (IR) camera. The energy is then converted to a temperature reading. This paper addresses two difficulties relating to the use of infrared thermography to measure fiber temperature. First, the emissivity of the polymer filament must be accurately known. Second, the effect of the small target size (of the small filament) relative to the detector array must be considered. Emissivity varies with many factors such as the material composition, surface roughness, material geometry, and viewing angle.2 Fujikura et al.3 measured the emissivities of various chlorinated polyethylene sheets. Pure polyethylene (no chlorination) had emissivities ranging from 0.1 to 0.25 for sample thickness from near zero to 100 microns. Polyvinyl chloride had emissivities from 0.1 to 0.7 for the same thickness range. Cao et al.4 measured the emissivities of polyethylene (PE) film. Their measured emissivities ranged from 0.15 to 0.95 for PE thicknesses of 10-500 microns. Fujikura5 also measured the emissivities of polymer films. He determined that, for film thicknesses of 20, 100, and 170 microns, the emissivities of polypropylene films were, respectively, 0.16, 0.39, and 0.48. For polybutylene (PB-1) films of the same thicknesses, the emissivities were, respectively, 0.17, 0.40, and 0.49. For polyethylene films, the respective emissivities were 0.13, 0.30, and 0.37. In summary, the thinner the polymer film, the lower the emissivity. Since fine fibers are thin (and round), they would * To whom correspondence should be addressed. Tel.: (405) 3256070. Fax: (405) 325-5813. E-mail: [email protected].

also be expected to have emissivities that decrease as the fiber becomes finer. Also, the curvature of the fiber affects the viewing angle and this can change the emissivity. Polymers are nonconductors and as such may show a decrease in emissivity for viewing angles other than zero degrees.2 To the best of our knowledge, there has been no detailed study on the emissivity of fine polymeric filaments of circular cross section. Without accurate estimation of the emissivity of the material, an infrared image of an object is merely qualitative, not quantitative. Several investigators have used noncontact techniques to measure the temperature of fibers during the spinning process. Zieminski6 measured the temperature of a moving threadline in melt spinning by placing a heated plate behind the fiber. When the plate temperature was adjusted so that the infrared signature of the fiber matched that of the plate, the fiber temperature was assumed to be equal to that of the plate. With an infrared camera, Bansal and Shambaugh7-9 measured the temperature of (a) polyester fibers during melt spinning and (b) polypropylene fibers during melt spinning and melt blowing. In their work, they corrected their temperature readings by using a slit response factor, or SRF, provided by the camera manufacturer. (This SRF will be discussed below.) More recently, Golzar et al.10 developed techniques for using an IR camera for online temperature measurements during melt spinning. They used auxiliary experiments to determine the effect of fiber diameter on emissivity for polyetheretherketone (PEEK) fibers. They also showed how to use the IR measurements to measure fiber diameter. However, their technique did not take into account the effect of camera pixel size that comes into play when fiber diameters are small or the distance from the camera lens to the target fiber is large. Bendada11 developed a two-color optical system for online temperature measurements during melt blowing. His two-color pyrometer used narrow band filters to separate out background effects. With this technique, fiber diameter and fiber emissivities did not need to be determined. The latest infrared cameras use a grid of microbolometers as thermal radiation detectors. A typical size of the detector array is 320 × 240 elements. A limitation of an array of this size comes into play when viewing small objects (or objects far from the camera). When, as viewed through the camera, a target has the same size or is smaller than the area covered by a single

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detector element (pixel), then that detector element will not give the same response as it would when viewing a larger target of the same material at the same temperature. This relative size problem is generally referred to as the spatial resolution. The spatial resolution may be defined as the ability of the thermal imaging radiometer to accurately measure the temperature of objects whose size is of the same order as the detector element. The manufacturers of infrared cameras sometimes provide a slit response function curve that characterizes their thermal imaging system by the relative response to a vertical slit target of various widths (e.g., Inframetrics Operator’s Manual12). This type of curve helps to quantify the error in the measurement of “line” target temperature as a function of the width of the target. However, the manufacturers provide little information on the technique used to generate such correction factors or the geometry (e.g., cylinder, flat ribbon, or circle) of the target. One goal of our work is to develop a simple technique to calibrate an IR camera and thereby use the camera for accurate measurement of the temperature of small cylindrical targets. Basic Thermographic Terms and Technical Specifications of the IR Camera The IR camera used in the present study was a ThermaCAM S60 manufactured by FLIR Systems (Portland, OR).13 The spectral range for this camera is 7.5-13 µm (i.e., it is a longwave camera). The thermal sensitivity of this camera is 0.06 °C at 30 °C. Thermal imaging radiometers with about 3-5 µm (shortwave) or about 3-14 µm (broad band) spectral response are also commercially available. The S60 camera’s IR detector is 320 elements × 240 elements (76 800 elements). The camera comes equipped with a “primary” IR lens with a field of view (FOV) of 24° × 18° and with a minimum focus distance of 0.3 m (11.81 in.). The field of view can also be interpreted in terms of the actual linear dimensions (height and width) of the viewed object in the viewfinder of the camera. These linear dimensions are a function of the distance L between the camera lens and the viewed object. For the camera’s lens, the horizontal field of view (HFOV) and the vertical field of view (VFOV) are defined as

HFOV ) 2 × L × TAN (12°)

(1)

VFOV ) 2 × L × TAN (9°)

(2)

Of course, for cameras other than the S60, the angles in eqs 1 and 2 would be different. The instantaneous field of view (IFOV) at a certain distance from the target is defined as the ratio of the HFOV at that distance to the number of thermal detector elements in the horizontal direction (for the S60 there are a ) 320 elements or pixels). Alternatively, the IFOV is the ratio of the VFOV to the number of detector elements in the vertical direction (for the S60 there are b ) 240 elements or pixels). Thus,

IFOV )

HFOV VFOV or a b

(3)

In this equation, a is the number of pixels in the horizontal direction and b is the number of pixels in the vertical direction. The alternative definitions of IFOV in eq 3 give values (for the S60) that are only 1% different. (In terms of subtended angle, the IFOV for the S60 is 1.3 mrad in both the horizontal and vertical directions.) The terms in eq 3 are illustrated in Figure 1. As can be seen, the HFOV, VFOV, and the IFOV increase as the distance from the IR lens to the target (L) increases. Also,

Figure 1. Horizontal field of view (HFOV), vertical field of view (VFOV), and instantaneous field of view (IFOV) for the primary IR lens. L is the distance from the front of the lens to the target.

the response of the IR camera would be maximum when the IFOV is minimum (i.e., when the IR camera is closest to a given target). Table 1 shows the HFOV, VFOV, and the IFOV for several working distances. Other features of this S60 camera can be found on the manufacturer’s website and in the camera’s user manual (FLIR ThermaCAM S60 Operator’s Manual13). As just pointed out, for the S60 camera, the HFOV and VFOV are essentially equal. If an infrared camera is used wherein the HFOV and VFOV are not equal, then a different definition of IFOV should be used. A good choice would be a normalized diagonal defined as

IFOV )

x

VFOV +( (HFOV a ) b ) 2

2

2

(4)

Our experiments also involved the testing of a “100 µm” close-up lens for which the focusing range was 80-110 mm. For this lens, the IFOV values for the minimum and maximum focus distances are approximately 80 µm and 108 µm, respectively. The close-up lens fits over the primary lens of the IR camera, and eqs 1-3 also apply to the close-up lens. The most significant advantage of the close-up lens is that, since the IFOV of the close-up lens is so small compared to the IFOV of the primary lens, the signal from very small targets is far stronger than with the primary lens. However, the close-up lens is susceptible to damage from the target, since the working distance is so small. Also, even if contact is avoided, overheating of the lens is possible when the target temperature is high. Experimental Section Figure 2 is a schematic of the test unit that was built to calibrate the IR camera. Compressed air regulated at 70 psi was passed through a thermal mass flow meter. The flowrate was set at 50 slpm of air at standard conditions of 21 °C and 1 atm pressure. The air then passed through a heater with two 750 W cartridge heaters. The heated air was then directed through a 0.277 in. ID and 0.375 in. OD stainless steel tube; this tube had three heated zones, each zone had a 420 W tape heater, and each of these zones had a thermocouple that was used to monitor and control the temperature. To further maintain uniform temperature, this tube was wrapped in two layers of insulation: an inner layer of fiberglass foam insulation and an outer layer of fiberglass cloth that was tightly wrapped around the inner layer. The air from the 0.952 cm (0.375 in.) tube was fed to the top of a vertically mounted, 304 stainless steel pipe with an ID of 1.25 cm (0.493 in.) and an OD of 1.71 cm (0.675 in.). The pipe length was 76.2 cm (30 in.). Hence, the pipe L/D was 60, which was long enough to establish a well-developed

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Table 1. HFOV, VFOV, and IFOV for Various Target Distances (L) for the Primary IR Lensa L (inches)

L (cm)

VFOV (inches)

VFOV (cm)

HFOV (inches)

HFOV (cm)

IFOV (inches)

IFOV (mm)

12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 64 66 68 70

30.48 35.56 40.64 45.72 50.8 55.88 60.96 66.04 71.12 76.2 81.28 86.36 91.44 96.52 101.6 106.68 111.76 116.84 121.92 127 132.08 137.16 142.24 147.32 162.56 167.64 172.72 177.8

3.802 4.435 5.069 5.703 6.336 6.970 7.603 8.237 8.871 9.504 10.138 10.772 11.405 12.039 12.672 13.306 13.940 14.573 15.207 15.841 16.474 17.108 17.741 18.375 20.276 20.910 21.543 22.177

9.656 11.266 12.875 14.485 16.094 17.703 19.313 20.922 22.532 24.141 25.750 27.360 28.969 30.579 32.188 33.797 35.407 37.016 38.626 40.235 41.844 43.454 45.063 46.673 51.501 53.110 54.720 56.329

5.102 5.952 6.803 7.653 8.503 9.354 10.204 11.054 11.905 12.755 13.605 14.456 15.306 16.156 17.007 17.857 18.707 19.558 20.408 21.258 22.109 22.959 23.810 24.660 27.211 28.061 28.912 29.762

12.959 15.119 17.279 19.439 21.599 23.758 25.918 28.078 30.238 32.398 34.558 36.718 38.878 41.037 43.197 45.357 47.517 49.677 51.837 53.997 56.156 58.316 60.476 62.636 69.116 71.275 73.435 75.595

0.016 0.019 0.021 0.024 0.027 0.029 0.032 0.035 0.037 0.040 0.043 0.045 0.048 0.050 0.053 0.056 0.058 0.061 0.064 0.066 0.069 0.072 0.074 0.077 0.085 0.088 0.090 0.093

0.405 0.472 0.540 0.607 0.675 0.742 0.810 0.877 0.945 1.012 1.080 1.147 1.215 1.282 1.350 1.417 1.485 1.552 1.620 1.687 1.755 1.822 1.890 1.957 2.160 2.227 2.295 2.362

a

The IFOV is based on HFOV.

Figure 2. Test unit built for calibration of the IR camera. The r and z directions are shown for the cylindrical coordinate system on the basis of the orientation of the 0.493-in. stainless steel pipe. The z-axis coincides with the longitudinal axis of this discharge pipe.

velocity profile in the pipe.12 For the experiments, the gas in the vertical pipe was kept in the range of 105-141 °C, which corresponds (at 50 slpm) to a Reynolds number of 4300-4600. Mean air velocity measurements below the pipe discharge were taken using a Pitot tube with an outer diameter of 0.7 mm, an inner diameter of 0.4 mm, and a 30° bevel (60° included angle). The measurements were taken with the cylindrical coordinate system shown in Figure 2. A two-way valve enabled us to measure the stagnation pressure from either of these two instruments: (1) an Ashcroft digital industrial pressure gauge (Model 2074; range 0-30 psi with 0.25% accuracy) supplied by Cole Parmer or (2) a Dwyer Model 25 liquid manometer. The Model 25 manometer had a range of 1.27-76.2 mm of water. The Pitot tube was attached to a three-dimensional Velmex manual traverse that could position the Pitot tube precisely within the flow field. The traverse has a 0.001 in. (25.4

µm) resolution, which is particularly useful in flow fields with steep velocity gradients. The air temperature measurements below the pipe discharge were made using a J-type 0.01 in. diameter thermocouple connected to an Omega microprocessor thermometer (model HH21) that digitally displayed the measured temperature. The accuracy of this thermocouple device was (1°C. Polymer fibers of different diameters (0.1-4 mm) were prepared in our laboratory using an extruder and gear pump assembly, the details of which can be found in Marla and Shambaugh.14 These fiber diameters are typically used in products such as spunbonded geotextiles, hollow fibers,15 and glue applicators.16 (The techniques described in this paper can also be used to correlate data for fibers of finer diameters. The ratio of target size, in the camera viewfinder, to pixel size is the dominant factor, which is the same whether or not a fine fiber is observed close up or a larger fiber is observed from a greater distance.) The fibers had circular cross sections, and the diameter of the fibers was measured using an optical microscope. The polymer used in most of the experiments was 88 MFR Fina Dypro isotactic polypropylene. This polymer had an Mw of 165 000 g/mol and an Mn of 41 500 g/mol. Some experiments with polybutylene fibers were also done. This resin was Grade 0400 manufactured by Basell Polyolefins and had an MFR of 20. The fiber samples were positioned horizontally at a position 8 mm below the exit of the pipe. At this distance, the air flow is very nearly “plug” flow for small values of “r” (i.e., near the center of the fiber sample; see Figure 4). As shown in Figure 3, a wire holder was used to mount the fiber; this holder was positioned such that the “bow” of the holder was out of the flow stream of the air. Using the traverse, the fiber was positioned exactly below the center of the discharge tube. The IR camera was placed in the same horizontal plane as was the fiber (i.e., at the same z value), and the axis of the camera’s lens was arranged perpendicular to the axis of the fiber. As described in Chandos and Chandos,17 the viewing angle is very


Figure 3. Experimental setup for holding a fiber in the center of the airstream. A C-shaped aluminum wire was used to mount the fiber horizontally in the airfield, and a 3-D traverse unit was used to position the wire-fiber assembly. As in Figure 2, the coordinates r and z are based on the stainless steel tube. A second cylindrical coordinate system with symbols r′ and z′ (this system is not shown in this figure) is based on the orientation of the fiber with the z′ axis running down the center of the fiber. For all IR measurements, the camera was located in the horizontal plane defined by the fiber location. Also, the major axis of the camera lens was perpendicular to the axis of the fiber. The C-shaped aluminum wire was not in the horizontal plane; this orientation of the wire prevented the wire from obscuring the camera’s view of the fiber.

Figure 4. Velocity profile of air at different downstream (z) locations as a function of radial distance (r). The nominal air velocity (the volumetric flow divided by the cross-sectional area of the discharge pipe) is 9.56 m/s for the temperature of 141 °C and the volumetric flowrate of 50 slpm.

important in radiometric measurements. For various distances (r values) from the polymer fiber, the real-time IR image of the fiber could be seen either on a 4 in. LCD screen attached to the IR camera or on a monitor by means of a Firewire (IEEE 1394) cable connecting the IR camera with a personal computer. For all IR measurements, the background for the fiber was the laboratory wall, which was at room temperature and was about 1 m beyond the fiber. Results and Discussion Air Velocity and Temperature Fields near the Center of the Fiber. While the air flowrate was kept constant at 50 slpm, the heaters were adjusted to obtain air temperatures of 105 °C, 121 °C, and 141 °C at 8 mm below the discharge (z ) 8 mm). The standard deviation of these temperatures was less than (1°C. The polypropylene fibers, with a melting point of 165

Figure 5. Temperature profile of air at different downstream (z) locations as a function of radial distance (r).

°C, were run at all three of these temperatures. The polybutylene fibers, with a melting point of 124 °C, were run primarily at 105 °C (though a few data points were taken at 121 °C). For a discharge temperature of 141 °C, the radial velocity and temperature profiles of air at various z values are shown in Figures 4 and 5, respectively. It can be seen that the air flowfield at z ) 8 mm displays a flat velocity and temperature profile for r values of several mm or less. Thus, if a fine polymer filament is placed at the center of the flow field at z ) 8 mm, then the center of this filament (over a distance of several mm) would be exposed to uniform gas temperature and gas velocity. In Figures 4 and 5, the flat-topped (plug flow) profiles at low z values gradually change into bell-shaped profiles as z increases; this type of behavior is typical for round jets. Now, in a fine fiber, the heat transfer in the fiber radial direction is rapid compared to heat transfer along the fiber axis. Because of this, and because of the flatness of the air velocity and temperature profiles at z ) 8 mm, it can be safely assumed that (at steady state) the temperature of the fiber near the fiber’s center is equal to the air temperature at this r and z position (r ) 0 and z ) 8 mm). (Conductive heat transfer, as described by Fourier’s law, is dependent on the gradient dT/dx. For radial heat transfer, the dx can be approximated as half the radius. Because this dx is so small, the gradient is very large, which makes radial heat transfer much greater than axial heat transfer.) Radial and Axial Temperature Profiles in the Fibers. As described earlier, the FLIR camera’s sensor records about 78 000 individual temperature measurements. Also, the camera software permits the display of the temperature along a line. For example, the radial temperature profile in a fiber can be determined by taking the temperature along a vertical line (i.e., a line perpendicular to the fiber axis). Figure 6 shows the radial (r′) fiber temperature profile for a 0.855-mm polypropylene fiber. As can be seen, the profile is almost flat near the center (near r′ ) 0, where r′ is the radial position within the fiber). For the experimental conditions used to develop this figure, the fiber diameter of 0.855 mm spans eight of the pixels in the camera’s sensor. (As described above, the fiber diameter, lens-to-fiber distance, and the detector array size all interact to determine this pixel number.) As filaments get smaller or the camera is positioned farther from the filament, a situation is reached wherein one or less than one pixel is involved in sensing the filament temperature. For these finer filaments, we assumed that

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Figure 6. Radial (r′) temperature profile for the center (z′ ) 0) of a PP fiber of diameter 0.855 mm. These temperatures were taken along a vertical line that was drawn on the IR image of the fiber. In terms of the coordinate system shown in Figures 2 and 3, the line was drawn in the z direction with r ) 0 mm. The data in this figure were taken with the 100 µm close-up lens, the lens to fiber distance was 80 mm, and the emissivity was assumed to be 0.82. No corrections were applied to the temperature readings.

the maximum measured temperature (i.e, the maximum along a vertical line) is the temperature that is closest to the correct temperature. Even this temperature may need to be corrected, and this correction will be discussed below. From this point onward, unless otherwise mentioned, fiber temperatures were assumed to be the maximum values along vertical lines. With a fine filament, it is reasonable to assume that, at steady state, the actual, rather than measured, fiber temperature is uniform in the fiber’s radial direction. (The assumption is made that the response of a single sensor element is independent of the neighboring elements. Certainly, the bell-shaped curve on Figure 6 suggests that the center pixels are responding differently and fairly independently of the pixels that record temperature farther from the fiber’s center. The manufacturer did not provide any proprietary information as to how accurate this assumption is, nor did the manufacturer provide information as to whether the camera software does automatic correction or image smoothing. However, even if there are some effects of neighboring elements or software corrections, the empirical correlations described herein are still accurate for the camera being tested.) While Figure 6 shows a fiber’s radial (r′) temperature profile, Figure 7 shows axial (z′) fiber temperature profiles for several PB fibers; these temperatures are temperatures along a horizontal line where r′ ) 0. (Observe that, in the plane of Figure 3, the z′ direction for the fiber corresponds to the r direction shown on Figure 3.) The profiles are flat at positions near the center of the filament. This means that the conduction of heat energy in the axial direction is essentially zero for positions near the center of a fiber. Thus, since both radial and axial conduction are zero for the center segment of the fiber, it is safe to assume that the center segment of the fiber has reached the temperature of the air impacting the fiber. Since we have a good estimate of the actual temperature of the fiber from the measured air temperature (taken with a thermocouple), we can calibrate the temperature readings (of the fiber) taken with the thermal imaging camera. One additional complication is that, as can be seen in Figure 7, the final (uncorrected) temperature of the filaments is constant for fibers of 2-mm diameter and greater, but the temperature decreases for fiber diameters less than 2

Figure 7. Axial (z′) temperature profiles of PB fibers of various diameters. These profiles are from IR readings along a horizontal line that overlays the fiber axis at r′ ) 0. The emissivity setting in the IR camera was 0.85 for all measurements. The lens and camera position are the same as for Figure 6. No temperature corrections were applied to the temperature readings.

mm. As will be discussed below, these lower temperature readings will be attributed to the curvature of the fine filaments. The first step in calibration was to establish the appropriate emissivity for use with the camera. Emissivity is a user-defined input to the camera software. As suggested in the above discussions of Figures 6 and 7, the center temperature (r′ ) 0 and z′ ) 0) of the fiber is assumed to reach the same temperature as the surrounding air. Using the close-up lens at a working distance of 90 mm, the temperature of a 1.9-mm polypropylene (PP) fiber placed in the flowfield with a discharge temperature of 141 °C was measured. It was assumed that the IR temperature reading of this fiber was not affected by curvature effects since the diameter is almost 2 mm (see the discussion of Figure 7). Since the fiber was assumed to achieve the same temperature as the air (141 °C), the emissivity of the fiber was adjusted (the IR camera has adjustable emissivity) until the center temperature reading of the fiber was 141 °C. From this procedure, the emissivity was found to be 0.82. This procedure was repeated for discharge temperatures of 105 °C and 121 °C, and the emissivity was found to be the same. Using a similar procedure for 2.5-mm and 4-mm diameter PB fibers, it was determined that the emissivity of PB is 0.85. A 1.9-mm fiber, when viewed with the close-up lens at a distance of 90 mm, spans 19 detector elements (or pixels) on the IR image. Hence, the (IR camera) response of at least 17 of these pixels should be near 100%. A 1.33-mm fiber was placed in the flowfield, and it was observed that, with a user-specified emissivity of 0.82, the recorded temperatures were 138 °C, 118 °C, and 101 °C ((1 °C) for thermocouple-measured discharge temperatures of 141 °C, 121 °C, and 105 °C ((1 °C), respectively. Now, this 1.33-mm fiber, when viewed with a close-up lens at 90 mm, occupies around 13 pixels, and hence one would expect that the response would be near 100% (for at least the center 11 pixels) and that the recorded temperatures for these pixels would be the same as for a 1.9-mm fiber. The lower temperature readings from the smaller diameter were assumed to be related to curvature effects (see Figure 7 for evidence of these effects for PB fibers). An empirical correction for these curvature effects will be developed in the discussion below.


Figure 8. The variation of signal strength as a function of the ratio of IFOV to fiber diameter. The signal strength in this figure is defined as ratio of the measured temperature of the fiber (using an emissivity of 0.82 in the IR camera) to the actual fiber temperature (in °C). The data for the three discharge temperatures (105 °C, 121 °C, and 141 °C) and for several fiber diameters are plotted. These data were collected using the primary lens.

Effects of IFOV and Fiber Diameter. After the emissivities of our polymers were established, we then proceeded to investigate the effects of target size and shape. The close-up lens attachment for the IR camera was removed and measurements were made with the primary lens. This primary lens has a minimum focusing distance of 30 cm, a relatively large value when the small size of our fibers is considered. Hence, the target size was about one pixel or less. This is a situation for which target size calibration (correction) of the temperature reading is necessary. Temperature measurements on numerous PP fibers were made at several target distances ranging from the minimum focusing distance (30 cm) to a maximum distance of 305 cm. These data are plotted in Figure 8. With an assumed emissivity of 0.82, the ordinate is the ratio of the recorded (camera) fiber temperature in °C to the “actual” (convected) temperature in °C (depending on the heater settings, this would be 141 °C, 121 °C, or 105 °C). The abscissa is the ratio of the instantaneous field of view (IFOV) to the fiber diameter. As explained in eqs 1-3, the IFOV increases with target distance. The ordinate in Figure 8 can be construed as the signal strength. For the larger fibers and for small IFOV/diameter, the signal strength is unity (as stated in the above discussion of the determination of emissivity). Figure 8 shows that the signal strength decreases substantially when the target distance increases or the fiber size decreases. Also, for a fiber of certain diameter, the signal strength is almost independent of temperature (i.e, whether the data are taken at 105, 121, or 141 °C). Now, for data with the same abscissa value, a fiber of diameter 0.86 mm would occupy the same area on the IR sensor array as a fiber of 0.69 mm or 0.48 mm or any other diameter. (Simply put, the camera image does not know the difference between something big that is far away versus something small that is close up.) Hence, it might seem logical that the signal strength at the same abscissa should be the same. However, the signal strengths at the same abscissa values do vary, and this variation appears to be related to fiber diameter. Figure 9 shows additional data taken under the same conditions as were used to produce Figure 8. However, the data of Figure 9 were taken with the close-up lens. (If the data of Figure 9 were plotted on Figure 8, the close-up lens data would be difficult to decipher because the points would all lie in the

Figure 9. The same type of plot as shown in Figure 8, except that the close-up lens was used on the camera.

Figure 10. Signal strength as a function of IFOV/diameter for when only data with the largest fiber diameter (1.9 mm) are used. This figure includes data from both the primary lens and the close-up lens. The equation on the figure is an empirical fit to the data.

abscissa range of 0-1.) The data of Figure 9 lie in the same envelope range covered by Figure 8, and everything said about Figure 8 also applies to Figure 9. We can address the problem of diameter by assuming that emissivity may depend on the diameter of the fiber. We also assume that, for large enough fiber diameters, the signal strength as observed with the close-up lens of the IR camera is almost 100%. As described in our discussion of Figures 6 and 7, fiber diameters of 1.9 mm and 3 mm were large enough to show flat temperature profiles (and these diameters had the same center temperature). We hypothesize that, for diameters less than 1.9 mm, the temperature reading is affected by both the fiber diameter and the ratio of IFOV to the fiber diameter. To separate the effects of these two variables, we examined the effect of diameter on thermal response when IFOV/diameter was held constant. An examination of Figure 8 shows that, for fixed values (or narrow ranges) of IFOV/diameter, the thermal responses approach an upper “envelope” limit as fiber diameter increases. To determine this limit, only data for the largest fiber diameter (1.9 mm) were plotted in a graph of signal strength as a function of IFOV/diameter; see Figure 10. We know from earlier measurements (see Figure 7) that 1.9 mm is a large enough diameter to minimize the loss in signal caused by fine diameters. So, Figure 10 is essentially a plot of the upper limit

342


Figure 11. The variation of signal strength with fiber diameter. Signal strength data from Figures 8 and 9 were normalized by multiplying by the inverse of the ordinate values on Figure 10. The equation on the figure is an empirical fit to the data.

Figure 13. Emissivity of polybutylene (PB) fibers as a function of fiber diameter. This plot was produced in a manner similar to that used to produce Figure 12. The close-up lens was used for the measurements. IFOV/diameter ranged from 0.05 to 0.95. Discharge temperatures of 105 and 121 °C were used.

Figure 12. Emissivity of polypropylene (PP) fibers as a function of fiber diameter. These emissivities are the emissivities necessary to shift all the signal strength data (from Figure 11) to unity. Data from both the close-up lens and primary lens are shown. The equation in the figure is an empirical fit to the data. (The plot in Figure 11 was produced using an assumed constant emissivity of 0.82.)

Figure 14. The data of Figure 8 replotted using the empirical emissivity relation from Figure 12. The equation in the figure is an empirical fit to the data.

of the data fields shown in Figures 8 and 9. Figure 10 allows us to separate out the effect of IFOV/diameter on the signal strength. Signal strength data from Figures 8 and 9 were normalized by dividing with the ordinate value on Figure 10. (The empiricalfit equation on Figure 10 was used for this procedure.) This process shifted the data to approximately a single curve; see Figure 11. Next, the effect of fiber diameter on emissivity was determined from the data in Figure 11. These emissivity values were calculated by finding the emissivities necessary to shift the signal strength of each of the data points (from Figure 11) to unity. The infrared camera software was used to make these calculations. (Keep in mind that, even after the experiment has been completed, the stored temperature data can be modified by changing the emissivity.) Figure 12 shows the results of this procedure. Thus, we have accomplished our goal of separating the effect of fiber diameter from the effect of IFOV/diameter.

A procedure similar to what has just been described for polypropylene fibers was also applied to thermal readings of polybutylene fibers. Figure 13 shows the emissivity versus fiber diameter for polybutylene. Both Figure 12 and Figure 13 show that the emissivity increases with increasing fiber diameter. Furthermore, the emissivities approach asymptotic values of 0.82 and 0.85, respectively, for PP and PB fibers. Golzar et al.10 used a similar approach for polyethyletherketone (PEEK) fibers and observed the same trend of increasing emissivity with increasing size. Also, Fujikura5 observed this type of trend while measuring the emissivity of polymer films of different thicknesses. Fujikura also found that, as with our data, the emissivity of polybutylene is slightly higher than the emissivity of polypropylene. By using the modified emissivity shown in Figure 12, the actual temperature of a PP fiber can be determined from a single variable (the ratio of IFOV/diameter). To test the efficacy of Figure 12, the data in Figure 8 were replotted using this modified emissivity; the result is Figure 14. As can be seen, the data fall essentially along a single curve in Figure 14. A power law fit to the data is shown on this figure; this fit has an R2 of 0.9695.


A Procedure for Measuring Fiber Temperature with an IR Camera. Using Figures 12 and 14, the following four-step procedure can be used to estimate the actual temperature of a PP fiber: 1. Determine the diameter of the polymer fiber and use this diameter to adjust the emissivity using Figure 12. Insert this value into the settings of the IR camera. The fiber diameter can be determined from auxiliary measurements with techniques such as high-speed photography. 2. Measure the distance between the target and the IR camera. Calculate the IFOV with eq 3. 3. Using the power law fit in Figure 14, find the ordinate corresponding to the calculated IFOV/diameter. 4. Measure the temperature of the target with the IR camera. Divide this value by the ordinate obtained in step 3 to obtain the actual temperature of the polymer. As an example, consider a PP fiber of 0.475-mm diameter with the IR camera at a distance of 101.6 cm from the fiber (with the camera oriented as described in Figure 3). The HFOV, VFOV, and the IFOV for these conditions are 43.19 cm, 32.18 cm, and 1.35 mm, respectively. From Figure 12, the emissivity for this fiber diameter is 0.542. The ratio of IFOV to the fiber diameter is 2.84. From Figure 14, this value of 2.84 corresponds to an ordinate value (signal strength) of 0.561. Hence, the measured temperature of the fiber (using an emissivity of 0.542 in the IR camera) should be divided by 0.561 to obtain the actual temperature of the polymer. The calibration curves in Figures 12-14 make it possible to use the IR camera far from the target in situations where it is not safe or practical to get too close to the target. While the calibration curves were developed primarily for PP and PB fibers in the range of 100 µm to 2 mm, this technique can be adapted to fibers of any polymeric material and for diameters beyond the range that were tested. With appropriate caution, these calibration curves can help determine the online temperature of fibers during actual spinning operations. Except at the die head, the change in fiber diameter (during a spinning process) is small when compared to the axial distance along the fiber, that is, the fiber diameter is nearly constant for short distances along the fiber, so correlations based on diameter can be applied. Another issue is the motion of the fiber during spinning; static test conditions were used in the experiments described herein. The reaction time (exposure time) of a camera pixel relative to the fiber motion is important for this issue. Conclusions When using an IR camera, there are errors in the temperature readings when the target dimensions are of the same order as that of each element of the thermal sensor. Furthermore, the diameter of a small fiber also affects the temperature reading. To overcome these problems, a procedure was developed to calibrate an IR camera and thus enable a user to obtain accurate temperature measurements of polymer fibers of diameters in the range 100 µm to 2 mm at different target distances. It was found that, by introducing an emissivity that varied as a function of the fiber diameter, calibration curves based on signal strength as a function of both fiber diameter and target distance can be obtained. The data produced on our test stand were well fit with these calibration curves. These data involved temperature readings produced with PP and PB fibers of various diameters and at various temperatures. On the basis of these calibration curves, a four-step procedure was proposed to accurately determine the temperature of polymer fibers of known diameter

at any working distance. This approach can be used for measurement of filament temperature in fiber-making processes such as melt spinning and melt blowing. Acknowledgment This work was supported by an NSF GOALI grant (DMII0245324). The support of 3M and Procter & Gamble is also gratefully acknowledged. Nomenclature a ) number of pixels in the horizontal direction for the detector array b ) number of pixels in the vertical direction for the detector array HFOV ) horizontal field of view (cm) VFOV ) vertical field of view (cm) IFOV ) instantaneous field of view (cm or mm) L ) distance between front of lens and the target object (cm) Mn ) number average molecular weight (g/mol) Mw ) weight average molecular weight (g/mol) MFR ) melt flow rate of polymer r ) radial coordinate position defined relative to the air discharge tube; see Figures 2 and 3 (mm) r′ ) radial coordinate position defined relative to the fiber; note that r′* r (mm) Tactual ) the temperature of the air stream as measured with a thermocouple; as described in the text, this temperature is assumed to be very close to the actual temperature of the center of a fiber placed in the air stream (°C) Tmeasured ) the uncorrected temperature of the fiber as measured with the IR camera (°C) z ) axial coordinate position defined relative to the air discharge tube; see Figures 2 and 3 (mm) z′ ) axial coordinate position defined relative to the fiber; note that z′ * z (mm) Literature Cited (1) Ziabicki, A. Fundamentals of Fiber Formation; John Wiley and Sons: London, 1976; pp 165-168. (2) Welty, J. R.; Wicks, C. E.; Wilson, R. E.; Rorrer, G. Fundamentals of Momentum, Heat, and Mass Transfer, 4th ed.; Wiley: New York, 2001; pp 195 and 379-393. (3) Fujikura, Y.; Suzuki, T.; Matsumoto, M. Emissivity of Chlorinated Polyethylene. J. Appl. Polym. Sci. 1982, 27 (4), 1293-1300. (4) Cao, B.; Sweeney, P.; Campbell, G. A. Infrared Characteristics of Thin Polymer Film: Temperature Measurement of Polyethylene, Annual Technical Conference, Society of Plastics Engineers, New York, May 1-4, 1989. (5) Fujikura, Y. Emissivity of Polymers. Yamagata Daigaku Kiyo, Kogaku 1999, 25 (2), 55-68. (6) Zieminski, K. F. Development and Applicability of a Mathematical Model for the High Speed Melt Spinning of Crystallizable Polymers. Ph.D. Dissertation, The University of Tennessee, Knoxville, TN, 1986; pp 7577. (7) Bansal, V.; Shambaugh, R. L. On-line Density and Crystallinity of Polyethylene Terephthalate During Melt Spinning. Polym. Eng. Sci. 1998, 38 (12), 1959-1968. (8) Bansal, V.; Shambaugh, R. L. On-line Determination of Diameter and Temperature During Melt Blowing of Polypropylene. Ind. Eng. Chem. Res. 1998, 37 (5), 1799-1806. (9) Bansal, V.; Shambaugh, R. L. On-Line Determination of Density and Crystallinity During Melt Spinning. Polym. Eng. Sci. 1996, 36 (22), 2785-2798. (10) Golzar, M.; Beyreuther, R.; Brunig, H.; Tandler, B.; Vogel, R. Online Temperature Measurement and Simultaneous Diameter Estimation of Fibers by Thermography of the Spinline in the Melt Spinning Process. AdV. Polym. Technol. 2004, 23 (3), 176-185.

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(11) Bendada, A. H. A Two-color Optical System for the On-line Characterization of Extruded Blown Polymer Microfibers. J. Opt. A: Pure Appl. Opt. 2004, 6, 180-186. (12) Model 600L Operator’s Manual; Inframetrics Inc.: North Billerica, MA, 1989. (13) ThermaCAM S60 Operator’s Manual; FLIR Systems: Boston, MA, 2004. (14) Marla, V. T.; Shambaugh, R. L. Modeling of the Melt Blowing Performance of Slot Dies. Ind. Eng. Chem. Res. 2004, 43 (11), 27892797. (15) de Rovere, A.; Shambaugh, R. L. Melt-Spun Hollow Fibers for Use in Nonwoven Structures. Ind. Eng. Chem. Res. 2001, 40 (1), 176187.

(16) Marla, V. T.; Shambaugh, R. L.; Papavassiliou, D. V. Using Swirl Dies to Spin Solid and Hollow Fibers. Ind. Eng. Chem. Res. 2006, 45 (7), 2331-2340. (17) Chandos, R. J.; Chandos, R. E. Radiometric Properties of Isothermal, Diffuse Wall Cavity Sources. Appl. Opt. 1974, 13 (9), 2142.

ReceiVed for reView June 2, 2006 ReVised manuscript receiVed September 1, 2006 Accepted October 18, 2006 IE060703A

Use of an Infrared Camera for Accurate Determination of the

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