Lateral Stress and Density Measurements for PC, ABS, HIPS, and PP Bulk Resin Feed Stock
Modified on Friday, 01 May 2015 11:01 PM by mpieler — Categorized as: Paper of the Month
Lateral Stress and Density Measurements for PC, ABS, HIPS, and PP Bulk Resin Feed Stock
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Stephen J. Derezinski, Extruder Tech, Inc., Penfield, NY 14526
Abstract
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A special test device [1-3] was used to make laboratory measurements of the lateral stress and bulk density of four resin feed polymers: polycarbonate (PS), acrylonitrile butadiene styrene (ABS), high impact polystyrene (HIPS), and polypropylene (PP). The data are all presented as a function of primary compressive stress on the bulk pellet resin feed stock. Data for lateral stress, lateral stress ratio, and density are shown, and regression functions are provided. Data for the bulk temperature rise during compression are also provided.
Introduction
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Lateral stress has been shown to be a factor in solids conveying [2] as it is the driving force that creates the frictional force on the barrel and screw that governs solids conveying. The same frictional force is also key to the onset of melting where higher lateral stress gives higher rubbing friction on the barrel to initiate melting sooner. More time (or extruder length) for melting results in more complete melting which is key to stable flow and pressure.
Materials
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PC (DOW Icalibre 300-6), ABS (DOW Magnum 342EZ), HIPS (DOW Styron 1170), and PP (DOW DX5E66) resin feed stocks in pellet form were analyzed. Sample sizes of about 4 grams were used, and two samples were tested for each resin. All temperatures were close to 27
^{o}
C. Figure 1 pictures the four resin feed stocks used for this study.
Experimental Equipment
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Figure 2 shows the test cell from previous work [2,3] used to measure the stresses and bulk density of the different resins. The instrumentation consists of two load cells (primary load and lateral load), a linear displacement transducer (specimen length), and a thermocouple (resin bulk temperature). The dimensions of the test chamber and the specimen length provide data to calculate the bulk density of the sample as a function of stress. The two load cells and the dimensions of the test chamber give the needed data to calculate the axial and lateral stresses developed during compression.
The experimental technique used here establishes the starting point of compression by connecting the displacement transducer directly to the top cylinder as shown in Figure 2. The ram of the press (not shown in Figure 2) is not connected to the top cylinder. Therefore, the origin of the data of each load cell data is precisely set when the displacement transducer records compressive strain as the press ram is lowered.
Temperature of the resin in the sample is monitored during the test, and this has been added since the last report [3]. A thermocouple made of 30 gauge wire was made by twisting the wire pair to approximate the bulk temperature. The thermocouple extended half of the distance (~6 mm) across the diameter (~12 mm) of the cell cavity. The small size of wire minimized the effect of its presence on the compressive strain and had optimized response time. The initial value and maximum values of temperature are recorded. All resins had an initial bulk temperature of about 27
^{o}
C.
Lateral Stress vs. Primary Stress Data
Figures 3-6 show the raw data for lateral stress versus primary stress for the four polymers, PS, ABS, HIPS, and PP. The data are repeated to provide two data sets (1 and 2) for each polymer. As can be seen, the variation in lateral stress versus primary stress is slight for PC, HIPS, and PP, but is noticeable for ABS in Figure 4. This suggests that the variation in lateral stress for ABS may be unique to it.
Lateral Stress Regression Functions
Regression curves for the data of lateral stress versus primary stress shown in Figures 3-6 are given in Figure 7. The data for each of the two sample runs of each polymer shown in Figures 3-6 are combined and fitted with the regression function (trend line). A (0,0) intercept is assumed (zero primary stress = zero lateral stress). The trend line consistently used is a 6
^{th}
order polynomial. Four trend functions for lateral stress (y) versus primary stress (x) are given below as follows:
Figure 7 clearly shows the relationship between the lateral stresses for the four polymers. PP has the greatest value of lateral stress while PC has the lowest lateral stress function. ABS and HIPS appear to be very comparable, which is logical since they are both styrene. However, it is not generally the case that the same general polymer has similar lateral stress as was shown for lateral stress measurement of three polyethylene resins; LLDPE, LDPE, and HDPE [2].
Equations 1-4 are only applicable within the shown specified limits of primary stress. Extrapolation above the upper limit will likely result in large error. Limited extrapolation above the upper limit of primary stress should be done with a linear trend line, if need be.
Lateral Stress Ratio Functions
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The lateral stress ratio is defined as the ratio of the lateral stress divided by the primary stress. The lateral stress ratio function of primary stress is useful in modeling solids conveying as it provides a convenient mathematical method for including lateral stress as the product of stress ratio and primary stress. The lateral stress ratio also provides more detail about the lateral stress that is not obvious from the lateral stress versus primary stress data (Figures 3-6). This is particularly true at low stress levels (inlet to solids conveying situation), so it is important to the analysis of every solids conveying process. Figures 8- 11 show the lateral stress ratio functions calculated from the raw data of lateral stress shown by Figures 3-6.
The resulting equations for stress ratio are the same as Equations 1-4 but with the exponential power of each term reduced by a value of one. Equations 1-4 are 6
^{th}
order polynomials, so lateral stress ratio functions are then 5
^{th}
order polynomials. Figure 12 plots the resulting 5
^{th}
order equations for each of the four resins tested, and comparison to the raw data of Figures 7-10 shows that the 5
^{th}
order polynomial approximates the data well. Figure 12 demonstrates more detail about the lateral stress than does the raw data of lateral stress in Figures 3-6.
The lateral stress ratio as defined cannot be calculated from the raw data at zero primary stress (0/0 has no mathematical result). This is obvious from Figures 8- 11 by the sporadic nature of the curves near zero primary stress. However, the 5
^{th}
order function for lateral stress ratio has a constant term that is the limit for lateral stress at zero primary stress. This limit provides a single value unique for each resin that indicates lateral stress at the entrance to solids conveying. Table 1 summarizes the limit of stress ratio at zero primary stress.
Table 1 Stress Ratio Limit at Zero Primary Stress
Bulk Density
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Data:
The bulk density of each of the four resins was obtained from the measurement of compressive strain and measured mass of the sample. Data for the bulk density are plotted as a function of primary stress in Figures 13-16. The data show the full cycle of compression to the maximum value of primary stress and then the reverse to zero primary stress. Excellent agreement between each of the two trials of all four samples is evident from the data of Figures 13-16.
Regression of Density Data:
Trend curves were obtained for the bulk density data of Figures 13-16. The trend curves are for only the compressive leg of the density data. Figure 17 shows the regression curves so obtained, and extrapolation to 30 MPa of primary stress is illustrated. The regression equations for density (y) versus primary stress (x) in Figure 16 are given as follows:
Energy Balance
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Plastic Deformation:
Mechanical energy used for the compression of the resin sample by the test cell is calculated by integration of the density versus primary stress data shown in Figures 13-16 according to a method previously given [1]. Integration for compression to maximum stress provides the total energy. Integration during the reverse of the load to zero provides elastic energy of the specimen. The plastic energy is the total energy less the elastic energy. The results shown in Table 2 give the ratio of plastic energy to total energy. This is then used to characterize the ability of the polymer “solid plug” to elastically expand during melting to maintain solid contact (high rubbing friction) with the barrel for efficient melting. The high plasticity (low elasticity) calculated here is detrimental to maintaining solids contact with the barrel when the solids plug is being eroded by the removal of melt. Low plasticity (high elasticity) would provide better conditions for efficient melting, and extrusion process data for this effect were previously cited [1]. Good screw design with proper compression section length can alleviate this issue.
Table 2 Temperatures, Maximum Stress, and Plasticity Ratio
Heat Generation:
Temperature measurement of the polymer during the test was used to investigate the temperature change that may accompany heat generation during compression of the resin. A thermocouple was located (see Figure 2) in the center of the polymer sample to register the initial and maximum bulk temperature during the test, and the results are shown in Table 2. The temperature was found to increase, and the level of increase was from about 1 to 3
^{o}
C for all four resins.
Summary
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Lateral stress and bulk density were measured for four feed resins, PC, ABS, HIPS, and PP. Data for primary stress up to about 30 MPa show that the lateral stress of the bulk feed does depend on the polymer. The stress ratio is used as a function of primary stress to provide details of the lateral stress and provide an efficient mathematical method for incorporating lateral stress into computational models. Regression equations are provided for such use in computational applications. The bulk density of the resins was accurately measured, and regression equations for it are given. The plasticity of the resin feed is calculated from the compressive energy based on the density versus primary stress data and shown to be over 90% for all resins tested.
Finally, the phenomenon of bulk temperature increase during compression of the resin feed is measured to be up to 3
^{o}
C for the resins tested here.
Conclusions
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1. The lateral stress for PP was measured to be the greatest. The lateral stress ratio was close to 0.4 over the entire primary stress range (0-20 MPa).
2. The lateral stress for the PC was the lowest, and the lateral stress ratio was about 0.3 over most of its range. However, it had a lateral stress ratio of up to about 0.5 at initial primary stress levels. This relatively high initial stress ratio was unique among the four resins tested.
3. The lateral stress for ABS and HIPS (both styrene resin) were measured to be very similar and between that of PC and PP.
4. The lateral stress ratio for ABS and HIPS was just above 0.3 over most of the measured range. Low initial values of about 0.23 greatly differentiate these resins from PC and PP with initial stress ratios of about 0.4.
5. Heat is generated during the compression of bulk resin feed stock as measured by an increase in bulk temperature of 1 to 3
^{o}
C.
6. The energy of plastic deformation for all resins was above 90% of the total energy of compression as calculated by integration of the density versus pressure data.
Acknowledgements
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Very special thanks to son, Stephen J. Derezinski, III of Platform Technology Ventures, Inc., for specifying the temperature measurement equipment appropriate for this experimental work.
References
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1. S. J. Derezinski 2009. “Laboratory Measurements of Resin Bulk Specific Gravity of PET and LDPE,” ANTEC 2009-Proceedings of the 67th Annual Conference & Exhibition, Chicago, IL, June 22-24, 2009, Society of Plastic Engineers, pp. 136-141.
2. S. J. Derezinski 2010. “Measurements of Biaxial Stresses During the Compression of Bulk Resin Feed,” ANTEC 2010-Proceedings of the 68th Annual Conference & Exhibition, Orlando, Fl, May 16-20, 2010, Society of Plastic Engineers, pp. 617-622.
3. S. J. Derezinski, “Lateral Stress and Bulk Density of PET Resin with Recycle”, ANTEC 2011- Proceedings of the 69th Annual Conference and Exhibition, Boston, Ma, May 1-3, 2011, Society of Plastic Engineers, pp 100-100.
Key Words
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Stress, bulk density, solids conveying, single screw, extrusion, pellets, biaxial stress, plastic resin, bulk properties
Figure 1. Four Resins Tested
Figure 2. The Test Cell for Measuring the Primary and Lateral Forces During Compression of Bulk Resin Feed as a function of bulk density [1,2]. Diameter of the cylinder is 12.7 mm. The linear displacement transducer records the height of the sample to establish the evolving bulk density value and lateral area of force. A thermocouple is centered in the resin to obtain the starting and maximum temperatures during compression.
Figure 3. Polycarbonate Lateral Stress versus Primary Stress Raw Data
Figure 4 Lateral Stress Raw Data for ABS Resin
Figure 5. Lateral Stress Raw Data for HIPS
Figure 6. Raw Data for Polypropylene Lateral Stress
Figure 7. Lateral Stess Function for Four Polymers. Curves obtained by regression analsysis of two data sets for each of the four polymers (Figures 3-6).
Figure 8. Polycarbonate Stress Ratio Data versus Primary Stress. Curves calculated from data of Figure 3.
Figure 9. Stress Ratio for ABS Resin. Raw data of Figure 4 used to calculate the stress ratios.
Figure 10. Lateral Stress Ratio for HIPS. Curves calculated directly from raw data of Figure 5.
Figure 11. Stress Ratio for Polypropylene Curves calculated directly from raw data of Figure 6.
Figure 12. Lateral Stress Ratio Functions for Four Resins The curves are based on the regression equations, Equations 1-4, represented in Figure 7. The lateral stress ratio functions are obtained from Equations 1-4 by merely lowering the exponent in each term by a value of 1.
Figure 13. Raw Data for Polycarbonate Density versus Primary Stress. Two samples shown. Arrows indicate compression and release.
Figure 14. Raw Data for Density of Two ABS Samples. Arrows indicate compression and release.
Figure 15, Raw Density Data for HIPS Resin Density Arrows indicate compression and release.
Figure 16. Raw Data for Bulk Density of Polypropylene Arrows indicate compression and release.
Figure 17. Regression Curves for Bulk Density of Four Resins Only the density during the compression of each resin is shown See Equations 5 to 8.
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