Ind. Eng. Chem. Res. 2005, 44, 6685-6691
`
`6685
`
`Effect of Supercritical Gas on Crystallization of Linear and
`Branched Polypropylene Resins with Foaming Additives
`
`Hani E. Naguib,*,† Chul B. Park,† and Seung-Won Song‡
`Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering,
`University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8, and Maytag Appliance
`Division, 2500 Dr. F. E. Wright Road, Jackson, Tennessee 38305
`
`The thermal behaviors of linear and branched polypropylene (PP) with foaming additives were
`investigated using a normal and a high-pressure differential scanning calorimeter (DSC).
`Specifically, the effects of material branching, dispersed additives, cooling rates, and dissolved
`blowing agents on the crystallization temperature of PP resins were elucidated. Introducing
`branches between polymer chains increased the crystallization temperature of PP significantly.
`Foaming additives such as talc and glycerol monostearate (GMS) as well as processing parameters
`such as cooling rate also played major roles during the crystallization process. The experimental
`results indicate that the crystallization temperature increases up to 30 °C by introducing
`branches and/or adding additives to the PP materials. However, the crystallization temperature
`was lowered as the supercritical fluid (such as nitrogen or carbon dioxide) was dissolved in the
`branched PP. The effect of hydraulic pressure was identified by performing DSC study by
`employing helium as an inert gas, which has a very limited solubility in the polymer matrix.
`
`Introduction
`
`Due to its outstanding functional characteristics and
`low material costs, PP has been considered as a sub-
`stitute for other thermoplastic materials in industrial
`applications. PP is a member of the semicrystalline
`polyolefin family, which is resistant to chemicals and
`abrasion. PP has a number of advantages over poly-
`styrene and polyethylene.1 PP has a higher rigidity as
`compared to other polyolefins; PP offers higher strength
`as compared to polyethylene and better impact strength
`as compared to polystyrene, and PP provides a higher
`service temperature range and good temperature stabil-
`ity as compared to other polyolefins. However, only
`limited research has been conducted on the production
`of PP foams because of their weak melt strength and
`melts elasticity, which makes comparison of the foam
`with that of other plastics difficult.2
`In this context, long-chain branched PP materials
`with high melt strength have been developed as a
`foamable grade.3-7 Few studies have investigated the
`effect of branching on the crystallization behaviors of
`PP resins.8-10 It was observed that the introduction of
`branching in the PP resins increases crystal nuclei
`density. This promotes faster crystallization, and, hence,
`higher crystallization temperatures were observed for
`branched materials as compared to linear materials.
`It is certain that the nucleating agent used in the
`foam processing plays an important role in determining
`the cellular structure of thermoplastic foams. The final
`foam structure is usually unacceptable without the
`addition of a nucleating agent unless a thermodynamic
`instability via a rapid solubility drop is utilized to
`promote a large number of nuclei.11,12 In other words,
`
`* To whom correspondence should be addressed. Tel./Fax:
`(416) 978-7054. E-mail: naguib@mie.utoronto.ca.
`† University of Toronto.
`‡ Maytag Appliance Division.
`
`if the nucleating agent is not added, the cell density
`becomes too low, and the bubble size becomes too large.
`Moreover, it has been discovered that even a small
`amount of nucleating agent could significantly lower the
`processing pressure and pressure drop rates which were
`required for the production of foam with a large cell
`density.11,12 It is beneficial if a high nuclei density can
`be obtained at a lower pressure and with a smaller
`pressure drop rate by adding a small amount of nucle-
`ating agent/additive.
`The understanding of the thermal behaviors of the
`polymer/additive system is critical in the processing of
`semicrystalline polymers as crystallization is strongly
`affected by the foaming additives dispersed in the
`polymer melt matrix. The effects of heterogeneous
`nucleating agents including talc and calcium carbonate
`on the crystallization of PP have been previously
`studied.13-19
`In the extrusion foaming process, two kinds of blowing
`agents can be used: chemical and physical blowing
`agents. Chemical blowing agents can decompose at
`processing temperatures and produce gases necessary
`for foaming, while directly injected physical blowing
`agents can dissolve into the polymer melt in the
`extruder. Because the dissolved gas under high pressure
`in the foaming process would vary the crystallization
`temperature of the polymer, the effect of gas on the
`thermal behavior needs to be investigated. Dey et al.
`studied the effect of physical blowing agents on the
`crystallization temperature of polymer melts using a
`specially designed high-pressure reactor.20 The crystal-
`lization temperature of LDPE was measured as a
`function of gas pressure by monitoring the sample
`temperature using a thermocouple attached to the lid
`of the reactor. A variety of techniques have been
`employed to investigate the thermal behavior of polymer
`and compressed gas systems.21 A differential scanning
`
`10.1021/ie0489608 CCC: $30.25 © 2005 American Chemical Society
`Published on Web 07/08/2005
`
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`BOREALIS EXHIBIT 1043
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`6686 Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005
`
`calorimeter (DSC) was employed to scan polymer sample
`which was presaturated with gas. However, a loss of
`gas was unavoidable during the sample handling and
`scanning.22 A sealed high-pressure DSC pan was also
`used to scan a polymer-gas system.23 Handa et al.
`developed a high-pressure calorimetric technique to
`investigate the glass transition depression characteris-
`tics of PMMA with compressed gas.24 They also studied
`the effect of supercritical CO2 on the polymorphism in
`syndiotactic polystyrene25,26 and the plasticization ef-
`fects of polymers with high-pressure gases.21,24
`He and Zoller studied the crystallization kinetics of
`PP, polyamide, and poly(ethylene terephthalate) (PET)
`using a pressure dilatometer to follow the volume
`changes associated with the crystallization process.27 In
`addition, Zhang and Handa reported the CO2-assisted
`melting of semicrystalline polymers such as polystyrene
`and poly(ethylene terephthalate) using a high-pressure
`DSC.28 Crystallization of amorphous polymers induced
`by supercritical CO2 has been investigated and com-
`pared with thermal crystallization.22,29,30 Amorphous
`poly(ethylene terephthalate), poly(vinylidenefluoride)/
`poly(methyl methacrylate) blends, and bisphenol A
`polycarbonate have been used for those studies. It can
`be concluded that the plasticization of CO2 can facilitate
`crystallization in certain polymers to an extent compa-
`rable to that achieved using an organic liquid or vapor.
`Takada et al. have investigated the effect of dissolved
`CO2 on the crystallization behaviors of semicrystalline
`polymers such as PP, poly(ethylene terephthalate)
`(PET), and poly (L Lactide) (PLLA).31-33 They concluded
`that the dissolved CO2 decreased the overall crystal-
`lization rate of PP within the nucleation dominated
`temperature region. They suggest that the dissolved
`CO2 decreases the melting and the glass transition
`temperatures and prevents formation of critical size
`nuclei.31 They also show that the presence of CO2 in the
`PET increased its overall crystallization rate. CO2 also
`decreased the glass transition temperature and the
`melting temperature of PET.32 Finally, they indicated
`that CO2 has also decreased the glass transition tem-
`perature and the melting temperature of PLLA.33
`The crystallization behavior of semicrystalline mate-
`rials is a critical factor in plastic foam processing. In a
`typical extrusion foaming process, the temperature of
`the melt decreases due to external cooling outside the
`foaming die and due to the cooling effect resulting from
`isentropic expansion of the blowing gases. Thus, the
`processing temperature at the final stage determines
`the time required for the polymer melt to solidify. For
`semicrystalline polymers, it was observed that the
`polymer melt solidifies at the moment of crystallization.
`Therefore, the foam structure “freezes” at the crystal-
`lization temperature which will affect the cell growth
`mechanism to a point where the foam cannot be fully
`expanded.34,35 In addition to the effects of the processing
`parameters on the crystallization, the materials param-
`eters and foaming additives can also contribute to
`changes in crystallization temperatures. The crystal-
`lization behaviors of linear and branched PP resins with
`the dispersed foaming additives and supercritical gases
`are investigated in this paper.
`
`Experimental Section
`Materials. Linear and branched PP materials with
`MFRs (ISO 1133, 230 °C/2.16 kg) of 11 and 2.3 dg/min,
`respectively, were chosen as the polymer materials. The
`
`degree of long chain branching of the branched PP is
`0.21 per 1000 carbon atoms. They were supplied by
`Borealis AG Austria, and they are denoted in this paper
`as Linear P1 and Branched P1, respectively. Talc (A7
`with a top-cut of 7 microns, Naintsch) and glycerol
`monostearate (GMS, Pationic 909, PATCO Polymer
`Additives) were used as foaming additives in this study.
`Helium (BOC Gas 99.9% purity), nitrogen (BOC Gas
`99.9% purity), and carbon dioxide gases (BOC Gas,
`99.5% purity) were used in the high-pressure experi-
`ments.
`DSC Measurements. The crystallization experi-
`ments were performed with a DSC (TA Instruments,
`DSC 2910). In particular, a regular DSC cell and a high-
`pressure DSC cell were used in the experiments. The
`regular DSC cell was used for investigating the effects
`of branching, additives, and cooling rate on the crystal-
`lization behavior of the PP materials. The high-pressure
`DSC cell was used for investigating the effects of
`dissolved gases on the crystallization behavior of PP
`materials. The calibration was conducted using indium.
`The purging gas used was nitrogen at a flow rate of 50
`mL/min. Samples for the DSC experiments (typical
`weight 3-4 mg) were taken from the extrudate in the
`form of a very thin disk (typical thickness 150-200
`(cid:237)m).36 For the nonisothermal experiments, the samples
`were pressurized to 1.37, 2.75, 4.13, and 5.51 MPa and
`then heated to 220 °C and kept at this condition for 30
`min to erase the thermal history.37 Following this step,
`the samples were cooled at 10 °C/min (if not specified)
`to 50 °C. Next, the samples were heated at 10 °C/min
`up to 200 °C. During the cooling and heating processes,
`the crystallization and melting patterns of the samples
`were recorded. The degree of crystallinity was deter-
`mined based on the heating cycle in the DSC thermo-
`gram. The percent crystallinity is measured as the ratio
`of the heat of fusion for PP materials (the area of the
`melting endotherm) and the heat of fusion of 100%
`crystalline polymers. It should be noted that efforts were
`made to conduct isothermal experiments for PP and PP
`materials with foaming agents; however, the crystal-
`lization was so quick and the shift in the exothermic
`heat flow curves31,32 could not be observed. Further
`studies need to be conducted to verify the isothermal
`behavior for PP resins with foaming additives.
`
`Results and Discussion
`Experiments with the Regular DSC Cell. Effect
`of Branching. Figure 1 shows the cooling sections of
`the DSC thermograms obtained from linear and branched
`PP without any additives. The cooling rate was fixed to
`be 10 °C/min. It was observed that the branching of PP
`chains significantly promoted the crystallization kinetics
`of the PP resins by increasing the crystallization tem-
`perature about 20 °C. This result supports the findings
`of previous studies.8,9 Figure 1 also illustrates that the
`peak of linear PP materials shows a shoulder (or double
`peak) in the peak caused by two-stage crystalliza-
`tion.18,38,39 It can be understood that the interactive
`motion of the PP matrix layer at the particle surface
`changes the crystallization speed of the PP matrix which
`results in the double peaks crystallization on the DSC
`thermogram.18
`Effects of Foaming Additives. The thermal behav-
`iors of linear and branched PP resins with foaming
`additives such as talc and GMS were also investigated
`in this study. The concentrations of talc and GMS were
`
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`Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6687
`
`Figure 1. DSC thermograms of linear and branched PP.
`
`Figure 3. Effects of talc on the degree of crystallinity of linear
`and branched PP.
`
`Figure 2. Effects of talc on the crystallization behaviors of linear
`and branched PP.
`
`changed from 0 to 1.6 wt % and 0 to 1.0 wt %,
`respectively. The experiments were conducted at a
`cooling rate of 10 °C/min.
`Figure 2 shows the effects of talc amounts on the
`crystallization temperature of PP materials. The effects
`of talc were more prominent in the linear PP than in
`the branched PP. After showing a sharp increase in the
`crystallization temperature as the talc concentration
`increased from 0 to 0.2 wt %, the crystallization tem-
`perature did not change much above 0.2 wt %. Figure 2
`shows that the crystallization temperature of the
`branched PP is only 5-10 °C higher than that of the
`linear PP with a talc content ranged 0.8-1.6 wt %. It
`should be noted that the crystallization temperature of
`branched PP is 10-20 °C higher than that of linear PP
`without any talc particles. On the other hand, the degree
`of crystallinity of linear and branched PP resins was
`also measured, and the results are shown in Figure 3.
`It was observed that the degree of crystallinity of
`branched and linear PP increased moderately as the talc
`concentration increased.
`Very similar results were obtained in the GMS case
`(Figures 4 and 5). The GMS, used as an aging modifier,
`also increased the crystallization temperatures and the
`degrees of crystallinity of linear and branched PP resins.
`However, because of its strong tendency of migration
`to the surface of the extrudate,40 the uniformity of the
`GMS in the PP matrixes could not be ensured. It is
`
`Figure 4. Effects of GMS on the crystallization behaviors of linear
`and branched PP.
`
`Figure 5. Effects of GMS on the degree of crystallinity of linear
`and branched PP.
`
`believed that the variations shown in Figure 4 are due
`to the nonuniform distribution of the GMS particles. The
`DSC sample cuts were made perpendicular to the
`direction of the flow from a thin filament extrudate
`(about 4 mm in diameter) to minimize the effect of
`nonuniformity of the GMS particles in the radial direc-
`tion of the filament extrudate.
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`6688 Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005
`
`Figure 6. Effects of cooling rate on the crystallization behaviors
`of linear and branched PP.
`
`Effect of Cooling Rate. The effects of cooling rate
`on the crystallization kinetics of linear and branched
`PP resins were also investigated. The cooling rate was
`varied from to 50 °C/min. Figure 6 shows the depen-
`dence of crystallization temperature on the cooling rate
`for the linear and branched PP materials. It was
`observed that the crystallization temperatures of PP
`resins were very sensitive to the change of cooling rate:
`the crystallization temperatures decreased by 25-30 °C
`as the cooling rate increased from 0.1 to 50 °C/min.
`Experiments with the High-Pressure DSC Cell.
`Design of Cooling System for the High-Pressure
`DSC Cell. To investigate the effects of dissolved gas
`on the crystallization behavior of a polymer, a high-
`pressure DSC cell was used for this experiment. Due to
`the poor cooling capability in the pressure DSC cell, only
`the heating mode has been mainly used in the high-
`pressure experiments.
`A cooling system was developed to provide the exist-
`ing high-pressure cell with a cooling capability. The
`cooling system enables the measurements of the crys-
`tallization behavior of a polymer melt under high
`pressure by mounting a cooling coil connected to a liquid
`nitrogen dispensing unit which supplies liquid-nitrogen
`at a controlled rate. The design for this cooling system
`is shown in Figure 7. Critical experiments were con-
`ducted to verify the pressurizing and cooling functions
`of the modified pressure cell. The pressure was raised
`inside the modified high-pressure cell from atmospheric
`pressure to 5.51 MPa, and at this pressure a uniform
`cooling rate up to 30 °C/min was successfully achieved.
`Additionally, the crystallization behavior of the PP
`materials measured with the high-pressure DSC cell
`was compared to that of a regular cell operating at
`atmospheric pressure. The crystallization thermograms
`of PP materials were determined at atmospheric pres-
`sure using both of the DSC cells, and they were found
`to be nearly identical at the same cooling rates.
`Finally, using this design, the measurements of
`crystallization kinetics under high pressure were suc-
`cessfully carried out using the modified high-pressure
`DSC cell.
`Effect of Supercritical Gases. Figures 8 and 9
`show the effects of pressure related to various gases on
`the crystallization behaviors of linear PP materials
`(Figure 8) and branched PP materials (Figure 9). The
`dependence of crystallization temperature on the pres-
`
`Figure 7. Design of the cooling system for the high-pressure DSC
`cell.
`
`Figure 8. Effects of pressure on the crystallization behavior of
`linear PP.
`
`Figure 9. Effect of pressure on the crystallization behavior of
`branched PP.
`sure was quite different for different gases. It was
`obvious that the gas at high pressure significantly
`affected the crystallization kinetics of the PP materials.
`As a gas permeates into the polymer matrix under high
`pressure, the dissolved gas may change the rate of
`polymeric segmental motion such as rearrangement into
`crystals. Furthermore, because the amount of gas dis-
`
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`Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6689
`
`Figure 10. Effects of hydraulic pressure on the crystallization
`behaviors of linear and branched PP.
`
`solved in the polymer increases with an increase in
`pressure,41 the magnitude of the change in the plasti-
`cization and crystallization will be more pronounced at
`a higher pressure.
`On the other hand, the crystallization kinetics will
`also be affected by the hydraulic pressure applied
`externally by the gas on the polymer melt. When the
`solubility of the gas in the polymer is very low or
`negligible, the kinetics of crystallization will be governed
`by the hydraulic pressure. However, when the solubility
`of the gas in the polymer is considerably high, both the
`hydraulic pressure and the dissolved gas will play their
`roles during the crystallization process in high-pressure
`experiments. The effect of dissolved gas on the crystal-
`lization kinetics can then be extracted by subtracting
`the effect of the hydraulic pressure from the overall
`crystallization behavior of the materials under high
`pressure.
`Effect of Hydraulic Pressure. The effect of hy-
`draulic pressure on the crystallization behaviors of PP
`materials was determined from the high-pressure ex-
`periments with He. Because the solubility of He in a
`polymer is very low, typically 1 order of magnitude lower
`than that of N2 and 2 orders of magnitude lower than
`that of CO2,42 the effect of the dissolved He in a polymer
`melt would be negligible. Therefore, any change in the
`crystallization kinetics under an elevated pressure of
`He can be considered as the effect of hydraulic pressure.
`Figure 10 shows the onset and peak crystallization
`temperatures as a function of the He pressure. It was
`noted that, by increasing the pressure in the DSC cell,
`the onset and peak crystallization temperatures in-
`creased for both the linear and the branched PP resins.
`The crystallization onset and peak temperatures for the
`branched PP increased by about 6 °C when the pressure
`increased from the atmospheric pressure to 5.5 MPa.
`However, for linear PP, the temperatures increased by
`only about 3 °C for the same amount of pressure change.
`As the hydraulic pressure in the DSC cell increases, the
`mobility of the polymer matrix molecules would de-
`crease, and hence would accelerate the crystallization
`process, resulting in a higher crystallization tempera-
`ture. Figure 10 also shows that the increase of the
`crystallization temperature for the branched PP was
`even more pronounced than that of linear PP.
`However, the crystallization temperature shown in
`Figure 10 was not altered much above 2.75 Mpa, and
`
`Figure 11. Effects of dissolved N2 on the crystallization behaviors
`of linear and branched PP.
`
`no further effects of hydraulic pressure were observed
`at higher pressures. It was not clear if the plateau
`region observed was due to the dissolved He in the PP
`materials at elevated pressures, or this showed the
`actual effect of hydraulic pressure on the crystallization
`behavior. This issue will be clarified, once the solubility
`of He in PP resins is measured.
`Effect of Dissolved N2. The effects of dissolved N2
`on the crystallization behaviors of linear and branched
`PP materials were extracted by subtracting the hydrau-
`lic pressure effects (Figure 10) from the overall ther-
`mograms (Figures 8 and 9). Figure 11 shows the onset
`and peak crystallization temperatures as a function of
`the N2 pressure. N2 has a relatively higher solubility
`than He,42 and, therefore, the effects of dissolved gas
`with N2 on the crystallization kinetics of PP materials
`are more pronounced as compared to the case of He.
`It was observed that the onset and peak crystalliza-
`tion temperatures did not change much at low pressures
`below 1.4 MPa. Because the solubility of N2 in the
`polymer is very low at a low pressure range, the overall
`crystallization behaviors in the low pressure range were
`mainly governed by the hydraulic pressure effect as
`shown in Figures 8 and 9. Although the effect of
`dissolved N2 on the crystallization was negligible at a
`lower pressure, it was more pronounced at a higher
`pressure. Above 1.4 MPa, the onset and peak crystal-
`lization temperatures decreased moderately all of the
`way to 5.51 MPa.
`It should be noted that these compensated results
`shown in Figure 11 may have some errors due to the
`dissolved He in the PP matrix at high pressures, which
`caused an incorrect hydraulic pressure effect. However,
`it is believed that the margin of error in the crystal-
`lization kinetics is insignificant.
`Effect of Dissolved CO2. As in the case of N2, the
`effects of dissolved CO2 on the crystallization behaviors
`of the linear and branched PP materials were extracted
`by subtracting the hydraulic pressure effect (Figure 10)
`from the overall thermograms (Figures 8 and 9). Figure
`12 shows the onset and peak crystallization tempera-
`tures for linear P1 and branched P1 as a function of
`the CO2 pressure. It was observed that the dissolved
`CO2 suppressed the crystallization of the PP materials
`significantly. The decreased crystallization temperature
`due to the dissolved CO2 was more pronounced in the
`thermogram of the branched PP as compared to linear
`PP. The increased crystallization temperature due to
`the branched structure was lowered back by the dis-
`
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`6690 Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005
`
`Figure 12. Effects of dissolved CO2 on the crystallization
`behaviors of linear and branched PP.
`
`solved CO2, and the crystallization temperature of
`branched PP became almost the same as that of linear
`PP at 5.51 MPa. This indicates that the effect of
`branching on the crystallization kinetics may be ne-
`glected at an elevated CO2 pressure above 5.51 MPa.
`Nevertheless, because of the limitation of the high-
`pressure equipment, experiments at pressure higher
`than 5.51 MPa could not be performed. On the other
`hand, the crystallization temperature of linear PP
`decreased linearly as the pressure increased.
`It is believed that the larger magnitude of change in
`the crystallization of PP resins under high pressure with
`CO2 rather than N2 was due to the difference of
`solubility of gases. The effect of dissolved CO2 was more
`pronounced even at a low pressure, because of the
`higher solubility of CO2 as compared to N2.42 A large
`amount of dissolved CO2 increases the free volume of
`polymer.43 This increased free volume enhances the
`mobility of the polymer chain, and, therefore, the
`crystallization temperature of PP materials is lowered.
`
`Conclusions
`
`In this study, a series of experiments were conducted
`to investigate the effects of material branching, foaming
`additives, cooling rate, hydraulic pressure, and dissolved
`gases on the crystallization behaviors of PP resins.
`Helium was employed to estimate the hydraulic pres-
`sure effects on the crystallization behaviors for high-
`pressure experiments with N2 and CO2 blowing agents.
`The foaming additives considered in this study included
`talc and GMS. The experimental results found in this
`study support the following conclusions:
`(1) Branching in the PP matrix caused a significant
`increase in the crystallization temperature.
`(2) The foaming additives such as talc and GMS
`increased the crystallization temperature of the PP
`materials.
`(3) Crystallization of the PP materials was enhanced
`as the hydraulic pressure increased.
`(4) However, the dissolved N2 and CO2 lowered the
`crystallization temperatures of PP resins. In particular,
`high-pressure CO2 decreased the crystallization tem-
`perature significantly because of its high solubility in
`the polymer matrix.
`
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`Received for review October 27, 2004
`Revised manuscript received May 9, 2005
`Accepted May 19, 2005
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`IE0489608
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