Page 1 of 5 DIFFERENCE IN THERMOFORMING PROCESSABILITY OBSERVED FOR THREE HIGH IMPACT POLYSTYRENES Caroline Woelfle, Kurt Koppi, Stephane Costeux, Todd Hogan, Joe Dooley, Ronald Van Daele, Alexander De Bokx, Alexander Droste, Sjoerd De Vries, Jerome Claracq, Berend Hoek, and Rudi Salmang The Dow Chemical Company, Freeport, TX-77541 Abstract The difference in thermoforming processability of three STYRON high-impact polystyrene (HIPS) resins was investigated. Experiments were performed on a moldbottle thermoformer as well as a step-case tool thermoformer. Disparities observed were analyzed in terms of differences in rheological properties of the resins. Conclusions were drawn as to which rheological characteristics the resins should possess in order to give the largest temperature processing window coupled with high quality thermoformed parts. Introduction Understanding the effects of a material s rheological properties and its processing parameters on thermoforming processes is essential for optimizing processing conditions, as well as developing new materials. In the past, studies have evaluated the influence of rheological parameters on the quality of thermoformed parts by both experimentation and simulation [1-3]. The objective of our study was to analyze the thermoforming processability of three highimpact polystyrene (HIPS) resins, investigate the influence of the resins rheological properties on the quality of the resulting thermoformed parts, to develop a better fundamental understanding of the thermoformability of HIPS resins. HIPS resins are widely used commercially for fabricating thermoformed parts (fridge parts, plastic cups ). The first part of our study analyzed the processability of thermoformed cups fabricated on a plug-assisted vacuum MTS thermoformer while the second part compared the sheet sag observed for parts processed on an AVT thermoformer The sagging of the sheet was measured as a function of the time spent by the extruded sheet in the oven. A comparison of this sag to the sag obtained through simulations conducted on FormSim a software program developed by the Industrial Materials Institute of the National Research Council of Canada (IMI/NRC) was made [3]. This software uses a K-BKZ [4] constitutive equation to describe the nonlinear viscoelastic behavior of the resins coupled with the WLF equation describing the temperature dependence of the resins. The K-BKZ parameters, the moduli, and corresponding relaxation times were obtained by fitting the constitutive equation to the dynamic mechanical and extensional measurements obtained for the three resins. Experimental Procedure Three HIPS resins possessing distinct rheological properties were utilized in this study: STYRON 1200, 1170, and 484. These resins have melt flow rates (MFR) of 5.0, 2.1, and 2.8 g/10 min (200 C/5kg) and will be referred to in the paper as,, and, respectively. The dynamic mechanical spectroscopy measurements (viscosity vs. frequency and temperature, loss and storage modulus vs. frequency and temperature) were performed on a TA Instruments ARES rheometer. The measurements were obtained at three different temperatures: 170, 190, 210, and 230 C. The uniaxial extensional viscosity measurements were performed at three Hencky rates: 0.1, and 10s -1 on a SER (Sentmanat Extensional Rheometer, Xpansion Instruments) at 170 C. Thermoforming experiments were performed on two distinct thermoforming tools. For a general evaluation of the resin processability, the Dow MTS thermoformer designed and built in Dow Benelux was utilized to generate a series of thermoformed cups for each resin and for a window of sheet temperatures (negative/female tooling). This set-up was developed as a tool for conducting experiments where all processing conditions are easily retrievable. A statistical design of experiment represented in Figure 1 was developed in order to maximize the information obtained while minimizing the number of experiments. It included three plug speeds (40, 80, and 120 mm/s), and three maximum plug heights (85, Trademark of The Dow Chemical Company Trademark of the Industrial Materials Institute Trademark of TA Instruments
Page 2 of 5 90, and of the mold). This design gave five sets of conditions per sheet temperature utilized. As this MTS thermoforming set-up did not involve sheet sagging, additional experiments were performed on an AVT thermoformer (positive tooling) to compare the differences in sagging for the three resins. Sheet sag was measured as a function of the time spent by the extruded sheet in the oven. Three times were chosen: 117, 127, and 135s. The average of five experiments was taken for each time and each resin. Results and Discussion MTS Thermoformer Figure 2, 3, and 4 show the different thermoformed parts obtained on the MTS thermoformer for the design of experiments represented in Figure 1. It should be noted that the cups are formed by pushing the mold cavity down onto the plug. It can be seen that the temperature window of processability was different for each of the three resins. For, cups could be formed for a sheet temperature as low as 128 C, for the two plug speeds of 40 and 120 mm/s, and the two plug heights of 85 and. Well-formed cups were obtained for sheet temperatures of 133, and, for the five processing conditions of the design: (85%, 40 mm/s), (, 40 mm/s), (, 80 mm/s), (85%, 120 mm/s), and (, 120 mm/s). As the sheet temperature reached, the cups obtained for a plug speed of 40 mm/s and plug heights of 85 and were not completely formed, which was attributed to the high sheet temperature coupled with the low plug speed utilized. As represented in Figure 5, possesses the lowest viscosity of the three resins. At elevated temperatures, the viscosity of this resin diminishes even more which reduces the thermoforming window. This phenomenon was not observed for the highest plug speeds (80, and 120 mm/s) as a higher plug speed gives less time for the sheet to collapse before reaching the top of the cavity mold. For, cups were formed with a sheet temperature higher than for (starting at 136 C) which may be explained by the higher viscosity of (Figure 5). The higher viscosity of made the formation of well-shaped cups more difficult at lower temperature, as the sheet offered more resistance to deformation. For the lowest sheet temperature of 136 C, only two cups could be properly formed, corresponding to the processing conditions (85%, 40 mm/s) and (, 80 mm/s). Well-shaped cups could be formed for all the processing conditions starting at a sheet temperature of. The higher viscosity of also permitted the fabrication of well-formed parts at a higher temperature (147 C) than for for the highest plug speeds of 80 and 120 mm/s. This may also be explained by the difference in viscosity wherein a higher viscosity provides more resistance to deformation at higher temperature for the resin sheet. For, the window of temperature processability was in between the windows of processability of and. This behavior correlates with the shear viscosity of the three resins. The lowest temperature where cups could be formed from was 133 C which is intermediate between that of and 2. The highest processability temperature for was which corresponded to the higher processability temperature observed for. The cups obtained for the lowest plug speed of 40 mm/s, however, were almost completely collapsed for, while for, only the top of the cups was slightly waved for the same processing conditions The pronounced extensional flow occurring during thermoforming made it necessary to characterize the extensional viscosities of the three resins. Figure 6(a) and 6(b) represent the extensional viscosities of the three resins for a Hencky extensional strain rate of 0.1 and 10s -1, and a temperature of 170 C. Similarly to what was observed for the shear viscosities, presented a higher extensional viscosity than, which in turn presented a higher extensional viscosity than. This trend matched the correlations made between thermoforming processability of the resins and their respective shear viscosities. Münstedt et al. [5] demonstrated that strain-hardening had an influence on the uniformity of thermoformed parts fabricated from polypropylene and polyethylene. It may be inferred from Figure 6(a) and 6(b), by comparing the extensional viscosity at 0.1 and 10s -1, that all three resins presented strain hardening at the Hencky extensional strain rate of 10s -1. was found to have the highest degree of strain hardening among the three HIPS resins. Sheet Sag Sheet sag is a determining factor influencing the quality of the resulting part. It was measured for the three resins for a representative heating time in the oven of 127s. The average sheet sags obtained were 144, 86, and 102 mm for,, and, respectively. The difference in the amount of sag obtained is directly related to the viscosity of the resins. As demonstrated in Figure 5, the shear viscosities of and are lower than that of, which explains the lower sag obtained for. Similarly, the extensional viscosities of and at low Hencky extensional strain rates are lower than that of, which further confirms the influence of the difference in shear and extensional viscosities on the difference of thermoforming processability of the resins. Simulation of the sheet sag required the determination of the K-BKZ and WLF parameters, as well as the relaxation times and corresponding viscosities. The K-
Page 3 of 5 BKZ parameters,, and, obtained through fitting of the K-BKZ constitutive equation with the rheological experimental data (7, 0.1, and -0.1111, respectively), were found to be similar for the three HIPS resins. The WLF parameters C 1 and C 2 for a reference temperature of 190 C were: 4.95 and 148 for ; 6.12 and 173 for ; and 5.56 and 160 for, respectively. The simulated and experimental sheet sags obtained for the three resins, for a representative heating time of 127 s are represented in Figure 7. It can be seen that there is a strong correlation between the experimental results and the simulated results. The simulated sags obtained were 130, 83, and 108 mm for,, and, respectively. The simulation results confirmed the differences in sheet sag obtained for the three resins. Summary Differences in processability of three STYRON HIPS resins were explored, and explained in terms of the rheological properties of the resins. It was demonstrated that lower extensional and shear viscosities provide a larger window of temperature processability for the thermoformed cup process investigated in this first portion of this study. However, sheet sag resistance was observed to diminish with lower extensional and shear viscosities for the large part thermoforming process investigated in the second portion of this study. Thermoforming simulations performed with the IMI/NRC FormSim software not only correctly predicted sheet sag trends observed for the three HIPS resins but also did a good job of quantitatively predicting the degree of sag measured experimentally. Acknowledgements The authors thank Linda Pecora and Anna Bardetti from the Industrial Materials Institute (Boucherville, Canada) for useful discussions as well as the Dow Chemical Company for the opportunity to present the results of this study. References 1. Lee, J. K., Virkler, T. L., and Scott, C. E., Effects of Rheological Properties and Processing Parameters on ABS Thermoforming, Polymer Engineering and Science, 41, 240-261 (2001). 2. Lee, J. K., Virkler, T. L., and Scott, C. E., Influence of Initial Sheet Temperature on ABS Thermoforming, Polymer Engineering and Science, 41, 1830-1837 (2001). 3. Pecora, L., Bardetti, A., and Laroche, D., Thermoformability of ABS Grades, SPE Tech. Papers, 47, 755 (2003). 4. Bernstein, B., Kearsley, E. A., and Zapas, L. J., Elastic Stress-Strain Relations in Perfect elastic Fluids, Transactions of the Society of Rheology, 27-39 (1965). 5. Münstedt, H., Kurzbeck, S., and Stange, J., Importance of elongational Properties of Polymer Melts for Film Blowing and Thermoforming, Polymer Engineering and Science, 1190-1195 (2006). Key Words thermoforming, high-impact polystyrene, sag, viscosity. Figure 1. Design of experiments
Page 4 of 5 133 C 128 C 85% 85% Figure 2. Cups obtained on the MTS thermoformer with 147 C 136 C 85% 85% Figure 3. Cups obtained on the MTS thermoformer with. 133 C 85% 85% Figure 4. Cups obtained on the MTS thermoformer with.
Page 5 of 5 Shear viscosity (Pa.s) 1.00E+02 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 Freq (rad/s) Figure 5. Shear viscosity of the three resins at 170 C. Extensional viscosity (Pa.s) Extensional viscosity (Pa.s) 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 Time (s) 1.00E+02 1.00E-02 1.00E-01 1.00E+00 Time (s) Figure 6. (a) Extensional viscosity of the three resins at 170 C, and for a Hencky rate of 0.1s -1. (b) Extensional viscosity for the three resins at 170 C, and for a Hencky rate of 10s -1. 200 sag sim = 130 mm sag exp = 144 mm sag sim = 83 mm sag exp = 86 mm sag sim = 108 mm sag exp = 103 mm Figure 7. Simulated and experimental sheet sag obtained for a heating time of 127s.