Double Crystal Melting Peak Generation for Expanded Polypropylene

Jan 17, 2013 - Microcellular Plastics Manufacturing Laboratory, Department of Mechanical ... Improved cell morphology and thermal properties of expand...
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Double Crystal Melting Peak Generation for Expanded Polypropylene Bead Foam Manufacturing Mohammadreza Nofar,† Yanting Guo,†,‡ and Chul B. Park*,†,‡ †

Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3 G8 ‡ National Engineering Research Center of Novel Equipment for Polymer Processing, The Key Laboratory of Polymer Processing Engineering, Ministry of Education of China, South China University of Technology, China ABSTRACT: A high-pressure differential scanning calorimeter chamber was used to investigate double crystal melting peak generation in polypropylene. The double crystal melting peak forms during the batch-based, expanded polypropylene (EPP) bead foaming process. This double peak structure is required for good sintering and the desired geometric shape of the final bead foam products. To form the double peak structure, isothermal treatment over a certain period of time is necessary at a high temperature and saturated pressure during the batch foaming process. This study investigated the influences of various saturation temperatures, time, and pressure on double crystal melting peak behavior. It was seen that the temperature was the most sensitive parameter on double peak generation. Also, the longer saturation time increased the amount of perfect crystals with a high melting temperature. Furthermore, as the saturation pressure increased, the required saturation temperature for generating the second peak with perfected crystals decreased through the plasticization effect of CO2.

1. INTRODUCTION Expanded polypropylene (EPP) beads are foamed polypropylene (PP) pellets used in steam-chest molding, which is a sintering process that produces three-dimensional EPP foam products. EPP products are used mainly in automotive parts, thermal insulation, high-end packaging, construction materials, and toys. The properties needed for such applications as packaging materials usually require high dimensional stability and exceptional elasticity, which do not exist in expanded polystyrene (EPS) and expanded polyethylene (EPE) foams. Also, unlike EPS foams, EPP bead foams have excellent impact resistance, energy absorption, and chemical and water resistance.1−6 Furthermore, EPP bead foams do not hold any blowing agents within the beads like EPS.7 On the other hand, EPP beads are almost three times as expensive as EPS beads, mainly due to high transportation costs.5,8 For EPP bead foams, copolymers with a base of propylene monomers are generally used.9−14 PP copolymer is more appropriate than homo-PP for EPP purposes because the latter generally has an inferior impact property at low temperatures.15,16 These copolymers can be binary copolymers, such as a propylene−ethylene copolymer or a propylene−butene copolymer, or a ternary copolymer, such as propylene− ethylene−butene copolymer.14,17 Batch foaming is a process used to manufacture EPP bead foams. It is more costly than extrusion bead foaming because of the long processing time. But while batch foaming is a more expensive process, it can produce beads with a higher cell density and greater closed cell content, because of the better control than the continuous processes. Furthermore, a desirable double crystal melting peak structure can also be developed from the batch process.18,19 The manufacture of EPP bead foams with two peak crystal structures has been well established.18−22 First, unfoamed PP © 2013 American Chemical Society

micropellets can be produced using an extruder. Subsequently, the PP micropellets are impregnated with a physical blowing agent in a batch foaming process at elevated pressures and temperatures around PP’s melting point over a certain period of time.19,22 After sufficient gas-loading time, during which the blowing agent impregnates and saturates the pellets, the pressure is released, and it creates expanded bead foams.18−20,23 During the gas impregnation stage, a new crystal melting peak is created at a higher temperature, Tmhigh. The phenomenon of creating multiple crystal melting peaks can be observed in most semicrystalline polymers. 24 The appearance of a new peak can be due to various crystal structures,25−27 different crystal sizes,28 and their rearrangement and perfection during the heating process or isothermal treatments.29 The newly generated crystal peak during the isothermal gas-impregnation stage of the EPP beads stems from the perfection of α crystal phase out of unmelted crystals, which has a higher orientation and a higher melting temperature than the original peak and is called α2.14,29 The melting temperature of this peak is typically above the annealing temperature. The first melting peak, at Tmlow, is created during the rapid cooling process in batch foaming and is called α1. Some of the literature reports the creation of α1 and α2, which are α forms of crystals with various degrees of perfection.30−37 Choi et al.14 have shown that when the temperature is increased to the annealing point, the less perfect (less close-packed) crystals melt and the more perfect (more close-packed) crystals that exist above the annealing temperature remain unmelted. Then, during the isothermal treatment, the Tmhigh from unmelted crystals Received: Revised: Accepted: Published: 2297

September 26, 2012 January 10, 2013 January 17, 2013 January 17, 2013 dx.doi.org/10.1021/ie302625e | Ind. Eng. Chem. Res. 2013, 52, 2297−2303

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resultant crystallization behaviors were investigated. In our study, a high-pressure differential scanning calorimeter (HPDSC) chamber was used to control the processing temperature and time exposure accurately.

increases with a higher perfection, whereas the portion of the first peak that forms during the cooling process decreases since more crystals are formed to the higher melting peak. As previously noted, a steam-chest molding machine can be used to produce low-density foam products with threedimensional shapes by inducing sintering of the bead foams. Recently, Zhai et al.38,39 and other researchers40−47 conducted research on steam-chest molding and the processing simulation for EPP beads. Good sintering requires a desirable double crystal melting peak structure during the manufacture of the final product through this process. When the steam temperature selected during molding is between the low melting temperature, Tmlow, and the newly created crystal peak temperature, Tmhigh, the crystals with the low melting temperature, Tmlow, will melt, but the crystals with the high melting temperature, Tmhigh, will not. Therefore, the crystals with Tmlow contribute to good bead sintering, and the crystals with T mhigh maintain the overall shape of the foam product.18,20,48−50 Consequently, excellent bead foam products can be produced with good sintering and desirable shapes. Figure 1 shows a typical double peak melting behavior of EPP foamed beads38 and the steam temperature range being used during the steam-chest molding.

2. EXPERIMENTAL PROCEDURES 2.1. Materials. A random PP copolymer (SEP-550) from Honam Petrochemicals Co. was selected to explore the double crystal melting peak generation. The density and melt flow index (MFI) of the PP resin were 0.9 g/cm3 and 7.5 g/10 min (ASTM D-1238, at 230 °C and 2.16 kg), respectively. These pellets were melted in a regular DSC (DSC2000 TA Instruments, New Castle, DE) at 200 °C for 10 min followed by a cooling process at a rate of 30 °C/min to remove the thermal and stress histories of the pellets. Figure 2 shows the DSC heating thermogram of the cooled PP pellets. The pellets showed a single melting peak at 144.6 °C with total crystallinity of 43% .

Figure 2. DSC heating thermogram of the cooled PP pellets after thermal and stress history removal.

CO2 with a 99% purity produced by Linde Gas was used as the impregnation gas in the high-pressure DSC chamber. 2.2. Experimental Setup. The PP pellets cooled in the DSC after stress and thermal removal were loaded into the aluminum pan in a high-pressure DSC chamber (NETZSCH DSC 204 HP, Germany). After loading of the specimens into the high-pressure chamber, the CO2 saturation pressure was applied. The specimens were then heated to the saturation temperature at a rate of 50 °C/min. Various saturation temperatures (Ts) and pressures (Ps) were applied to observe their effects on the generation of the double crystal melting peak. The effect of the saturation time on the evolution of the double crystal melting peak was also investigated. After isothermal saturation, the specimens were cooled by liquid nitrogen to room temperature at a cooling rate of 30 °C/min (the maximum achievable cooling rate of the equipment). The chamber was simultaneously depressurized at a very slow rate of 45 bar/min. The slow depressurization rate was chosen to minimize the effect of foaming on crystallization.51 The samples were then degassed at 80 °C for more than 24 h. Zhai et al.38 have shown that annealing at 80 °C does not affect the double crystal melting peak structure. Subsequently, the samples were heated to 200 °C at a rate of 10 °C/min to explore the effect of the saturation parameters on the double crystal melting peak behavior. Figure 3 shows a schematic of batch process simulation in the HP-DSC chamber and the changes in pressure and temperature versus time.

Figure 1. Typical double peak melting behavior of foamed beads.38

The characteristics of the double crystal melting peaks determine the surface quality and the mechanical properties of the bead foam products. If the low temperature peak is dominant, then the foam product may not have the same geometry as the mold. In contrast, if the high temperature peak is dominant, then the sintering may be weak, resulting in poor mechanical properties. The manufacturing technology for EPP beads is well-known, but the mechanisms of double crystal melting peak generation with the presence of dissolved blowing agent have not been fully understood. The earlier work done by Choi et al.14 clarified the mechanisms of double crystal melting peak generation without any dissolved gas. They showed the effects of annealing temperature and annealing time at atmospheric pressure on the crystal perfection that cause the second peak creation of PP terpolymers. However, the actual EPP bead manufacturing process is conducted at high pressure with a blowing agent, so the gas is dissolved in the beads, and the dissolved gas would affect the crystallization (i.e., double crystal melting peak) behaviors. In this context, we studied the effect of the dissolved blowing agent on the double crystal melting behavior. Specifically, the processing pressure, the saturation temperature, and the saturation time were varied and the 2298

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Figure 3. Schematic of batch process simulation and changes in pressure and temperature versus time.

Figure 4. Heating thermographs of samples saturated (a) at temperatures between 80 and 145 °C and (b) at a more narrow temperature range between 135 and 143 °C, at 45 bar of CO2 for 60 min.

2.3. Crystallization Structure Analysis Using X-ray Diffraction. X-ray diffraction analysis was used to analyze the effect of saturation parameters on the crystal structure after the saturation process. The X-ray diffraction was run on a Siemens D5000 Diffractometer System operating at 50 kV/35 mA. A high power, line-focus Cu Kα source was used in combination with a solid state Kevex detector. The experimental data were collected in a slow step scan mode (ss 0.02 deg/s, 1.3 s/step) within the range 10−30° (2θ). The obtained diffraction patterns were processed by Diffrac Plus data processing software including Eva 8.0.

temperature shifted to a higher temperature (above Tm) due to the rearrangement of the crystals with more closed-packed structure (higher perfection) and increased thickness of crystal lamellas.14,29 The rearranged crystal melting peak was at 157 °C, being 27 and 13 °C above the saturation temperature and the original melting peak, respectively. By increasing the saturation temperature to 135 °C, which is close to the Tm, some portion of crystals melted during the saturation time and some portion remained unmelted. Furthermore, while saturating at 140 °C (much closer to Tm), the portion of melted crystals increased while the portion of unmelted crystals decreased. During the isothermal saturation at 135 and 140 °C, the mobility of the unmelted crystal molecules was enhanced. Hence the retraction and rearrangement of these molecules were facilitated to form crystals with a higher degree of perfection. These perfected crystals had melting temperatures at 157.7 and 160.9 °C, respectively, for saturation temperatures of 135 and 140 °C.14,29 This shows that higher crystal perfection was achieved when the saturation temperature was 140 °C. This was due to the increased mobility of the unmelted crystal molecules during saturation, which required lower dissipation energy for rearrangement to become more closepacked and more perfect. During the saturation period, besides perfection of unmelted crystals, further retraction and folding of molecules will occur, resulting in further growth of these perfected crystals with higher melting temperature.14 As discussed, the peak with lower crystal melting temperature (Tmlow) forms during the cooling process (after saturation). After saturation at 135 and 140 °C, the Tmlow started to appear more clearly. In fact, the molten crystals during these saturation temperatures formed the low melting temperature crystals

3. RESULTS AND DISCUSSION 3.1. Effect of Saturation Temperature on Double Crystal Melting Peak Generation. A number of saturation temperatures, ranging from 80 to 145 °C, were chosen to investigate their effects on the PP crystallization behavior and to find out the critical temperature range that generates the double crystal melting peak structure. Figure 4a shows the crystallization behavior of the PP specimens during the heating cycle, after being saturated at different temperatures (80−145 °C) for 60 min and at a saturation pressure of 45 bar of CO2. When the samples were saturated at 80, 100, 110, and 120 °C, there was only one melting peak around 150 °C and the broadness of the crystal melting peak was narrowed a little because of the melting of the small amount of imperfect crystals around the saturation temperature and the formation of more crystals at ∼20 °C above the saturation temperature. At the saturation temperature of 130 °C, still the majority of the crystals were unmelted even after the saturation time of 1 h. However, during this saturation time, the crystal melting 2299

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Figure 5. (a) Melting peaks and (b) total crystallinity and second peak/total crystallinity of samples saturated at various temperatures and 45 bar of CO2 for 60 min.

°C, and the original unsaturated PP samples, were investigated through WAXD to investigate the effect of saturation temperature on the crystal structure when the double crystal melting peak formed. In Figure 6, the original unsaturated PP

during the cooling process. The low melting temperature crystal (first peak) is more pronounced in the case of 140 °C saturation temperature than in the case of 135 °C due to the increased portion of molten crystals at the increased saturation temperature. When the saturation temperature was increased to 145 °C, almost all of the crystals existing in the PP pellet were melted and no unmelted crystals remained to be rearranged and grown with a higher perfection and a higher melting peak. Therefore, all of the crystals re-formed during the cooling process at Tmlow, which is now the same as the original melting peak at 145 °C. Figure 4a shows that, in the range 135−145 °C, the double crystal melting peak structure is changed very sensitively with respect to the temperature while the saturation pressure and time are 45 bar and 60 min, respectively. Figure 4b explores the effect of smaller temperature intervals between 135 and 143 °C at the same saturation pressure and time as in Figure 4a. Figure 5 also shows the Tmhigh and Tmlow, the total final crystallinity, and the ratio of the second peak area over the total crystallinity (i.e., the total peak area) of the saturated samples at different temperatures. As shown in Figures 4b and 5, the increased saturation temperature increased the amount of crystals with low melting peak (the first peak). This was because there were more molten crystals during saturation time at higher saturation temperatures. Consequently, more potential crystals exist to re-form and solidify as the first peak during the cooling process.14 On the other hand, at higher saturation temperatures, the second peak with more perfect crystals became less dominant since fewer unmelted crystals were exposed to rearrangement and perfection during the saturation time.14 Although increasing the saturation temperature decreased the amount of perfected crystals (i.e., the second peak), it created more perfected crystals that had a higher melting temperature due to the increased molecules’ mobility at higher saturation temperatures. The low melting peak was also increased after saturation at higher temperatures possibly due to the fact that the crystallization requires time.52 Figure 5 shows that, after saturation at higher temperatures, the finally achieved crystallinity increased. This can be due to the facilitated retraction of the molecules at higher saturation temperatures that increased the crystal growth with higher perfection and close-packed structure. To investigate various possible crystal types that can form in the PP samples after the saturation and degassing processes, the wide-angle X-ray diffraction (WAXD) test was conducted. The samples cooled in the HP-DSC after saturation at 135 and 140

Figure 6. Wide angle X-ray diffraction for unsaturated PP samples and for PP samples saturated at 135 and 140 °C, 45 bar of CO2, and 60 min, followed by cooling in HP-DSC and degassing at 80 °C.

samples showed only the existence of α phase, which appeared at 2θ = 14, 17, 18.5, 21.5, and 22°.53,54 After saturating the samples at 135 °C, peaks representing α phase became narrower and sharper, especially those at 2θ = 18.5, 21.5, and 22°. This was due to the rearrangement of the crystals and development of highly ordered α phase crystals (perfected α phase crystals) that occurred during the saturation process.55 These peaks became more narrow after saturation at 140 °C due to the further perfection of the crystals that had a higher melting temperature.53,56 3.2. Effect of Saturation Time on Double Crystal Melting Peak Evolution. By choosing 140 °C and 45 bar as fixed saturation temperature and saturation pressure parameters, different saturation times were selected to investigate the dependency of double crystal melting peak evolution on the saturation time. Five different saturation times were selected, and the results are shown in Figures 7 and 8. The results show that 10 min was insufficient for the unmelted crystal molecules to rearrange themselves to form the more perfect crystals with a higher melting temperature. When the samples were saturated for 20 min, the second peak with perfected crystals and thicker lamellas formed clearly due to enough time for molecular rearrangement and the first peak with less perfect crystals also formed.. The double crystal melting peak structure also formed at saturation times of 30, 60, and 90 min but with different peak area ratios. As shown in Figure 8, increasing the saturation time 2300

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Figure 7. Heating thermographs of samples saturated at 140 °C and 45 bar at various saturation times.

Figure 9. Heating thermographs of the samples saturated at 140 °C for 20 min at various pressures.

from 20 to 90 min enhanced the amount of perfected crystals and the lamellar thickness that had a higher melting peak as a result of a longer saturation time. In fact, a longer saturation time provided more diffusion time for the unmelted crystal molecules to rearrange and grow to form bigger crystals with a close-packed structure. Also, the amount of crystals with a lower melting peak decreased. This was due to the reduction of the available melt that can crystallize during cooling after saturation. As shown in Figure 8, the total crystallinity was also promoted steadily as the saturation time increased, most likely due to the larger amount of induced crystallinity during the saturation. 3.3. Effect of CO2 Saturation Pressure on Double Crystal Melting Peak Behavior. Figure 9 shows the effect of various saturation pressures on double crystal melting peak variation at 140 °C while the saturation time is fixed as 20 min. By increasing the saturation pressure from 15 to 60 bar, a higher amount of gas would dissolve in the polymer,57,58 and the amount of rearranged perfected crystals with a higher melting peak decreased due to the increased plasticization effect of more dissolved CO2. The plasticization effect of dissolved CO2 during the saturation time helped to melt more of the remained unmelted crystals. Therefore, fewer unmelted crystals rearranged to form more perfect crystals and thicker lamellas with a higher melting peak. Hence, the amount of crystals formed during cooling as the first (low temperature melting) peak increased. As shown, at 60 bar saturation pressure, the major portion of the crystals formed during cooling as the original melting peak, i.e., the first peak. However, at lower saturation pressures, the first melting peak was less pronounced. However, more unmelted crystals were exposed

to rearrangement during the saturation time and formed as the second peak with more close-packed crystal structure. The results showed that the plasticization effect of CO2 can compensate for the decreased saturation temperature. Consequently, to generate the desired double crystal melting peak, a lower saturation temperature would be required at an increased saturation pressure and vice versa. In this context, various saturation temperatures and pressures were further explored in section 3.4 to ascertain the effect of variation of saturation temperature versus the saturation pressure for double peak formation. 3.4. Relationship between Saturation Pressure and Temperature for Double Peak Formation. Figure 10 shows the crystal melting behavior of the samples saturated at atmospheric pressure (1 bar) and different CO2 pressures (15, 30, and 60 bar) at various temperatures while fixing the saturation time at 20 min. Figure 7 shows a desired double peak, similar to the reference EPP bead’s double peak (Figure 1), formed by saturating the PP resin at 140 °C and at 45 bar of CO2 pressure. However, Figure 10 shows that, at 15 bar saturation pressure, the required saturation temperature to form a desired double crystal melting peak structure was close to 143 °C. For the samples saturated at 30 bar, this temperature shifted to a lower temperature (somewhere between 141 and 143 °C). Also, at 60 bar saturation pressure, CO2’s high plasticization effect further decreased the required saturation temperature to 137.5 °C. Using a regular DSC (atmospheric pressure, 1 bar), a double crystal melting peak similar to the reference double peak (Figure 1) formed in the absence of dissolved CO2 but at relatively higher temperature (149 °C).

Figure 8. (a) Melting peaks and (b) total crystallinity and second peak/total crystallinity of samples saturated for various times at 140 °C and 45 bar of CO2. 2301

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Figure 10. Heating thermographs of samples saturated at various saturation pressures and temperatures.

peak, temperature was shown to be the most sensitive parameter. The longer annealing time increased the diffusion time for the unmelted crystal molecular rearrangement. Therefore, the increased saturation time increased the perfected crystals and the lamellar thickness that had a high melting temperature. Also, due to the plasticization effect of CO2, the increased saturation pressure decreased the saturation temperature required for generating the higher melting peak with perfected crystals. With the given saturation time, a relationship between the required saturation temperature and the saturation pressure was established for the generation of double crystal melting peaks.

As shown, the required saturation temperature for double crystal melting peak generation decreased by increasing the saturation pressure from 1 to 60 bar as a result of CO2’s plasticization effect. Using Figure 1 as the reference, the relationship between the saturation pressure and temperature is summarized in Figure 11. It is shown that, for the given PP

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 11. Relationship between the required saturation temperature and pressure to create a desired double peak.

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Consortium for Cellular and Microcellular Plastics (CCMCP).

resin and the given time, the saturation pressure required to create the desired double crystal melting peak has an almost linear relationship with the saturation temperature. This linear relationship for the current material is Ts = −0.17Ps + 147.6, where Ts and Ps are the saturation temperature and saturation pressure, respectively. However, further investigation is required to generalize this relationship for other resins and to develop it for different saturation times.

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REFERENCES

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4. CONCLUSION In the context of EPP foam manufacturing, we investigated the dependency of the double crystal melting peak structure on the saturation temperature, saturation time, and saturation pressure using a HP-DSC chamber. In creating a double crystal melting 2302

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