Rheological Comparison of Chemical and Physical Blowing Agents

Apparatus. To study the shear viscosity of a gas-laden polymer solution, an instrumented capillary die nozzle was equipped on a 55-ton Arburg Allround...
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Rheological Comparison of Chemical and Physical Blowing Agents in a Thermoplastic Polyolefin X. Qin, M. R. Thompson,* and A. N. Hrymak MMRI/ CAPPA-D, Department of Chemical Engineering, McMaster UniVersity, Hamilton, Ontario, Canada L8S 4L7

A. Torres Applications Group, INDESCA, Maracaibo, Venezuela

The influences of popular blowing agents (BAs), both chemical and physical types, in solution with a thermoplastic polyolefin (TPO) were investigated with an in-line capillary rheometer nozzle attached to a conventional reciprocating 55-ton injection molding machine. In the experiments, two types of masterbatch chemical BAs (endothermic and exothermic type) were dry mixed with the TPO resin and compared against two types of physical BAs (carbon dioxide and nitrogen) directly injected into a specially designed injection nozzle containing static mixer elements (SMX type). The effects of the main processing conditions (injection speed, pressure, temperature, and BA type and concentration) on the TPO melt rheology were studied. The viscosity reduction behaviors of chemical BAs and physical BAs were compared. The type of BA had a strong influence on the viscosity reduction behavior of the TPO melt, with the maximum viscosity reduction of 47% with 3.2 wt % CO2 at 250 °C. Both physical and chemical BAs were found to successfully fit a previously developed viscosity model for single-phase gas-polymer solutions. Introduction Thermoplastic polyolefins (TPOs) are being increasingly used to replace thermosets in exterior applications by the automotive industry. These materials are immiscible blends containing a polyolefin elastomer and a polypropylene (PP). A thermoplastic polyolefin can be received as either compounded or reactor grade, and the polypropylene could be a homopolymer or copolymer in the major or minor phase. In other words, TPOs can vary widely in composition to offer a broad range of mechanical properties to the user. In comparison to isotactic PP, a key advantage of TPO is its improved impact properties, particularly at low temperature. However, foamed TPO has not experienced mainstream use because the technology for foaming a TPO using extrusion or injection molding has not yet been well-developed. As well, the absence of detailed material property knowledge for TPOs, due to the complexity of their formulations, is a factor in successful foaming of TPO. There is an increasing interest in developing foamed TPO products to reduce material costs and fuel usage in automobiles. Microcellular foaming serves as an effective means to reduce material consumption with minimal deterioration of the physical and mechanical properties. Complete dissolution of the blowing agent (BA) in the molten polymer is a critical step in the microcellular foam processing and is strongly dependent on the solubility of the BA , the saturation pressure, the degree of mixing, and the dissolution time.1 The dissolution of a BA has a significant impact on the rheological properties of a polymer melt, most notably a reduction in viscosity. While the innovations of polymer resins provide impetus to foam products, there has been motivation to use BAs that are more environmentally benign, such as certain inert physical gases and chemical BAs. Due to concerns of worker safety and the environment, inert physical gases, most notably carbon * Corresponding author. Tel: (905)525-9140, ext. 23213. Fax: (905)521-1350. E-mail: [email protected].

dioxide and nitrogen, have become the most used physical blowing agents (PBAs) nowadays. Compared to hydrocarbon PBA, carbon dioxide and nitrogen both exhibit lower solubility and higher diffusivity in most polymer melts, thus requiring a special high-pressure apparatus for gas addition and modifications to the processing machinery. In comparing these two physical gases, carbon dioxide has a comparatively higher solubility and lower diffusivity in most commodity polymers than nitrogen, and carbon dioxide is more easily processed in its supercritical state. Supercritical fluids have unique sorption characteristics that result in better morphology in foam processing.2 Therefore, carbon dioxide has received more attention than nitrogen in polymer foam production. In recent years, several research groups have studied the foaming process using carbon dioxide as a BA in various polyolefin matrixes3-7 as well as polyolefin composites.2,8-10 Significant work4,10-12 has also been reported in foaming studies with nitrogen, and at least one investigation has examined mixtures of carbon dioxide and nitrogen.13 The impact of carbon dioxide and nitrogen on the rheological properties of polyolefins during the foaming process has been sparsely examined in the literature, and such knowledge is required for optimizing processing conditions. One of the major interests of this paper lies in understanding how the introduction of these gases (i.e., from chemical decomposition or direct injection) affects rheology. Chemical blowing agents (CBAs) evolve gas for foaming by undergoing a thermal decomposition reaction, which provides one or more gas species for polymer expansion. Traditionally, CBAs can be classified into two major categories: endothermic and exothermic. Sodium bicarbonate or alkali carbonates, including both citric acid and citric acid esters, are the key constituents of an endothermic-type BA, primarily liberating carbon dioxide gas (and water) during decomposition. Azodicarbonamide and its modified forms are the most widely used constituents of an exothermic BA, releasing nitrogen gas upon thermal reaction. A CBA normally comes in powder or masterbatch form and thus can be blended with the feedstock

10.1021/ie0510932 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/22/2006

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directly at the hopper, usually requiring that no modifications are necessary to the processing machinery. Most CBAs leave some residues in the polymer matrix, which can affect the rheological and mechanical properties of the foam product. Several research groups have studied endothermic14-17 and exothermic14,18-21 CBAs, though more recently the direction of research has involved combining both types to control the foam structure.22,23 A comparison of the individual effects of endothermic and exothermic CBAs on the rheology of a polymer melt only has been done recently.24 The rheological properties of a heterogeneous TPO melt and its gas-laden solution have not been compared in the literature, especially under high shear rate. No results have been published comparing the influence of chemical versus physical BAs on the melt rheology of a polyolefin. An in-line capillary rheometer can provide meaningful data relevant to processing conditions, allowing researchers to explore the rheological behavior of a gas-polymer solution. Thus, measurements collected from an in-line rheometer are used for the present study to explore the rheological behavior of a TPO melt with four popular BAs used for foam manufacture: endothermic CBA, exothermic CBA, carbon dioxide, and nitrogen (injected as a PBA). The effects of temperature and BA type and concentration on the viscosity of TPO melt are investigated in this work. In addition, we extend previous results of the rheology of low-density polyethylene (LDPE)/BA system24 to TPO systems. Introduction of the Model. As described in our previous work,24 the shear thinning behavior of a gas-laden polymer solution can be described by a simplified Cross-Carreau model for viscosity:

(γτ˘ )

η(γ˘ ) = η0n

n-1

(Bf)

the slope of the isothermal curve at 230 °C in the PVT plot, it was determined that βP ) 2.10 × 10-3 (1/MPa). The viscoelastic scaling method employed by this model is used to obtain a master viscosity curve in which the effects of temperature, pressure, and BA concentration on the shear viscosity may be expressed by a scaling factor, R ) RTRCRP:

ln RT ) ln

( (

ln RC ) ln

(1)

ln RP ) ln

where η0 is the zero-shear viscosity, and n and τ are constants in the model. The zero-shear viscosity of a polymer can be explained as a function of the free volume fraction (f ):25

η0 ) A exp

Figure 1. PVT curve for TPO.

(2)

where A and B are constants for the polymer. It assumes that the viscosities of the polymer melt change only through the zeroshear viscosity while the processing conditions are varied:

f ) fr + (1 - fr)βT(T - Tr) - (1 - fr)βP(P - Pr) + φwg (3) where wg, βT, βP, and φ are the weight fraction of BA used, thermal expansion coefficient, isothermal compressibility coefficient, and BA expansion coefficient, respectively. In this work, the reference temperature (Tr) and the reference pressure (Pr) were selected to be 230 °C and 0.5 MPa, respectively. fr is the reference free volume fraction of the pure polymer at Tr and Pr. The thermal expansion coefficient (βT) and isothermal compressibility coefficient (βP) used in the viscosity model were determined from pressure-volume-temperature (PVT) measurement. The PVT behavior of the pure TPO (shown in Figure 1) was measured with an isothermal cooling procedure in the range of temperatures from 170 to 255 °C and pressures from atmospheric to 60 MPa (see ref 24 for details). For the interested reader, the PVT data for the TPO may be represented by the characteristic parameters, P* ) 294 MPa, T* ) 637 K, and F* ) 0.9351 g/cm3 according to the Sanchez-Lacombe equation of state.26 From the isobaric line of 0.5 MPa in the PVT plot, the slope of the specific volume at the reference pressure was determined to be βT ) 2.75 × 10 - 3 (1/°C). Similarly, from

(

) ( ) ( ) (

η0(T, Pr)

η0(Tr, Pr)

η0(T, Pr, wg) η0(T, Pr)

η0(T, P, wg)

η0(T, Pr, wg)

)

)

) ) )

B B f(T, Pr) f (Tr, Pr)

(4a)

B B f (T, Pr, wg) f (T, Pr)

(4b)

)

B B f (T, P, wg) f (T, Pr, wg)

(4c)

Thus, substituting eq 3 into eq 4a-c, we obtain

ln R ) ln(RTRCRP) )

(

)

B B fr + (1 - fr)βT(T - Tr) - (1 - fr)βP(P - Pr) + φwg fr (5)

A master flow curve is constructed by plotting the scaled viscosity η(wg, γ˘ )/R versus scaled shear rate Rγ˘ , as wg is varied. The model has only three fitting parameters (i.e., βT, βP, and φ). Experimental Section Materials. The TPO composite used for the experiments was Dexflex 940 (Solvay Engineered Polymers) with a melt index of 18 g/10 min (ASTM D1238, 230 °C/2.16 kg) and a density of 0.97 g/cm3. Two CBAs (supplied by Clariant) were used in this study: endothermic type masterbatch, Hydrocerol HK40E (major constituents being an alkali carbonate, citric acid, and citric acid esters in a carrier resin), and an exothermic type masterbatch, Hydrocerol 1191 (major constituent being azodicarbonamide in a carrier resin). The carrier resin was a polyethylene with a measured melt index of 35 g/10 min (ASTM D1238). The active species make up approximately 40 wt % of the CBA masterbatch for the endothermic species and 25 wt % for the exothermic species, as described by the supplier. The decomposition behaviors of the two CBAs have been previously reported.24 The evolved gas analysis (EGA) technique was used to obtain the quantity of evolved gas from each of the two CBAs. For Hydrocerol HK40E, the decomposition product of

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carbon dioxide was determined to be approximately 34.3 mL (STP)/g of CBA while the simultaneously evolved water vapor was separated through condensation and was not measured. The amount of gas yield for Hydrocerol 1191 was approximately 48.8 mL (STP)/g, which was considered to be mostly nitrogen based on the supplier data. The PBAs used in the work were carbon dioxide (Vital Air commercial grade) and nitrogen (Vital Air HP+ grade), used as received. Apparatus. To study the shear viscosity of a gas-laden polymer solution, an instrumented capillary die nozzle was equipped on a 55-ton Arburg Allrounder 320S reciprocating injection molding machine.24 Two pressure transducer ports and two thermocouple ports located respectively before and after the capillary die in the in-line rheometer nozzle allowed us to monitor the pressure and temperature changes across the die. With regards to trials using CBA, the injection molding machine was operated at the highest possible speed to obtain shear rates comparable to industrial conditions. For physical gas addition, a special gas injection system was required. The injection system involves a high-pressure gas regulator, a syringe pump (Isco D series, model 260D), and a gas injector (a 20-µm porous metal plug) (see Figure 2 in ref 12 for details). The introduced gas was mixed with the melt in the nozzle through a set of static mixing elements (SMX type). Rheological Measurements. Viscosity measurements were conducted on the TPO/endothermic and exothermicCBA mixtures ranging from 0 to 5 wt % masterbatch CBA (5 wt % typically being the highest concentration used industrially in polymer foams.). The steady shear viscosity of the TPO with CBAs was measured for shear rates up to 200 000 s-1 and pressures up to 34 MPa. For the PBA-based solutions, carbon dioxide was added up to 3.1 wt %, and nitrogen was injected up to 2.2 wt %. The physical gas addition was chosen at these levels to ensure complete dissolution within the melt at our processing conditions.27,28 The gas injection pressure must be higher than the injection pressure of the polymer melt, while the shear rate range used for this study was determined by the injection pressure of the injection molding machine. Due to the pressure limit of the syringe pump (41 MPa), the highest shear rate (i.e., injection speed) allowable in the rheological study was 27 000 s-1 for pressure conditions up to 11 MPa. All experiments (both for CBA and PBA trials) were performed at a nozzle temperature of 230 and 250 °C. Shear viscosity was determined from the pressure difference between the two pressure transducers before and after the capillary die. The viscosity of the developed single-phase polymer/BA solution was calculated using the pressure drop between the two pressure transducers and corrected using the Rabinowitsch equation and Bagley method. Two dies of the same diameter yet different lengths (i.e., 1 and 12 L/D) were employed for the latter correction. The data presented are based on averaged pressure measurements taken from five replicate runs. Results and Discussion Validation of In-Line Rheometer. Similar to our validation of the measured viscosity by the in-line rheometer in the previous work,24 the data collected for the TPO were compared with results from an off-line rheometer (ROSAND dual-bore capillary rheometer) for processing temperatures of 230 and 250 °C. The results from the two rheometers showed excellent agreement with one another at each temperature, as shown in Figure 2. The isothermal assumption necessary to the conventional method for calculating steady shear viscosity was considered to be valid since the difference in melt temperature determined from thermocouples before and after the capillary die was small between 0.7 and 2.2 °C.

Figure 2. Shear viscosity curves of TPO at 230 and 250 °C measured by in-line rheometer and ROSAND capillary rheometer.

Figure 3. Pressure drop across the rheometer at 250 °C for TPO, TPO with 5 wt % exothermic CBA, and TPO with 2.2 wt % N2.

In all cases of calculated viscosities, it was assumed that the gas evolved from each type of BA remained in phase with the TPO matrix throughout the rheometer and that in the case of the CBAs, all reactive species had fully decomposed prior to passing through the rheometer. In an injection molding machine, the screw rotation generates mechanical energy to facilitate melting and enhance dispersion and dissolution of the CBA evolved gas into the polymer melt. For the PBA, the gas injector (a 20-µm porous metal plug) was positioned before a set of static mixing elements (SMX type) to aid its dispersion and dissolution into the polymer melt. According to the exit pressure of the capillary die and solubility data of nitrogen and carbon dioxide in molten polypropylene,27,28 the investigated range of gas concentrations was substantially below solubility limits. Thus, the assumption of a single-phase solution appears reasonable when chemical superheat is considered. However, the possibility of bubble nucleation within the capillary die due to high shear rate (i.e., mechanical superheat) remains feasible. In the situation of shear induced nucleation, the onset of nucleation should be observable by a discontinuity in the flow behavior of the gas-polymer matrix as the shear rate was increased. Figure 3 presents the pressure drop data across the rheometer versus apparent shear rate for different TPO solutions containing the highest concentration of either the exothermic masterbatch CBA (nitrogen-based) or nitrogen (PBA) at 250 °C, respectively to ensure that the single-phase assumption is valid. The pressure drop across the in-line rheometer for the highest level of nitrogen-laden TPO melt showed similar linear trends versus apparent shear rate as those found for the non-foamed TPO melt,

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Figure 4. Shear viscosities of TPO with endothermic masterbatch CBA measured by in-line capillary rheometer at 230 °C.

Figure 5. Shear viscosities of TPO with exothermic masterbatch CBA measured by in-line capillary rheometer at 230 °C.

with no discontinuities observed. The nitrogen-based BA were chosen for this validity test since the solubility limit of nitrogen is lower than carbon dioxide in PP melt under similar processing conditions.27,28 Rheology of TPO/CBA Solutions. Figures 4 and 5 show shear viscosity curves of the different TPO solutions at 230 °C with 0-5 wt % masterbatch endothermic CBA and exothermic CBA, respectively. For 250 °C, the viscosity curves did not reveal significant differences from those present beyond the anticipated shift in magnitude attributed to improved chain mobility. The maximum standard error in our calculated viscosities for both temperatures was 13.1% for TPO solutions with the endothermic CBA and 11.5% for the exothermic CBA. The flow curves of all TPO/CBA solutions indicate that the shear viscosity of the TPO decreased with increasing CBA concentration in the melt. The shapes of the curves for the gasladen solutions were similar to that of the TPO melt, except in the high shear region. A slight deviation in the shear thinning region of the TPO melt can be seen when the shear rate was greater than 140 000 s-1; while for TPO/CBA solutions, the viscosity curve still exhibited a steady drop in values. It is possible to speculate that this deviation was a result of the dispersed elastomer phase of TPO, since in the previous work using a homogeneous LDPE melt there was no such deviation in the viscosity curve.24 In comparing the viscosity results of the two masterbatch CBAs at each temperature, the reduction in shear viscosity was greater for the exothermic CBA as compared to the endothermic CBA. Based on the type and concentration of gas evolved during

Figure 6. Shear viscosities of TPO with CO2 measured by in-line capillary rheometer at 230 °C.

Figure 7. Shear viscosities of TPO with N2 measured by in-line capillary rheometer at 230 °C.

decomposition from each of these CBA species, the results appear to be contradictory to the established literature of carbon dioxide and nitrogen in polyolefins.27,28 This finding is attributed to the combined effects of gas solubility, carrier material, and decomposition residues on the viscosity of our TPO melt. As expected, the viscosity reduction was higher for solutions at higher temperature since the higher mobility of polymer chains and higher solubility of low molecular volatiles in the matrix under these conditions corresponded to greater free volume and thus lower shear viscosity. The viscoelastic shift factor (R) was directly obtained from the fitting procedure of the experimental data. As will be demonstrated in a later section, greater distinction is possible between the effects of CBA type and concentration and the process temperature on viscosity with the shift factor. Rheology of TPO/PBA Solutions. The viscosity of the TPO melt with carbon dioxide or nitrogen gas directly added was measured at various shear rates, temperatures, and gas concentrations. Figure 6 shows the shear viscosity curves of the TPO with 0-3.1 wt % CO2 at 230 °C. Figure 7 shows the shear viscosity curves of the TPO with 0-2.2 wt % N2 at 230 °C. Again, the curves for the higher processing temperature (250 °C) are not displayed since they did not reveal any new observations beyond the anticipated drop in magnitude attributed to increased chain mobility. The maximum standard error for the determined viscosity covering both temperature conditions was 12.8% and 14.5% for the TPO solutions with carbon dioxide and nitrogen, respectively.

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Figure 8. Experimentally determined shift factors for all TPO/BA solutions (lines included for clarity only).

As seen in Figures 6 and 7, the shear viscosity of the TPO melt was reduced with the increased addition of dissolved PBA in the melt. Examination of the data at two processing temperatures showed that the viscosity reduction was higher for solutions at higher temperatures, which was similar to the results with the two CBAs. In comparison of the two gases, the viscosity reduction was larger for the solutions with carbon dioxide than those with nitrogen at both temperatures in terms of equivalent molar concentration. Though these findings differ from our observations with the masterbatch CBAs (based on the species of gas present in the melt), the increased free volume and hence increased solubility of the carbon dioxide in the polymer melt over the nitrogen agreed with published literature results.27,28 According to Bondi,29 carbon dioxide has a larger molecule size and incompressible volume than nitrogen, which provides greater free volume between polymer molecules and thus lower shear viscosity. Comparison of Physical and Chemical Blowing Agents. In foaming processes, the focus has generally been on the nucleation and cell growth of foam with little attention given to the processability of the gas-polymer solution. It is generally assumed in foam processing that the evolved gas of the chosen BA will dictate the rheology based on its solubility in the polymer. The shear viscosity results presented in Figures 4-7 have pointed out that, particularly in the case of CBA, other factors may need to be considered besides the gas alone. To more readily observe the effects of the BA type and concentration as well as processing temperature, the rheological data for each condition were represented by their corresponding shift factor, R. To make a meaningful comparison between chemical versus physical BA species with respect to their influence on rheology, the concentration of the CBA was first converted to reflect the actual quantity of gas dissolved rather than the total weight fraction of the additive. The EGA data previously discussed in this paper were used for this purpose. The shift factors versus gas concentration are shown in Figure 8, which better illustrate the impact of varying the quantity of BA on the rheology of the TPO than the previously shown flow curves in Figures 4-7. In the case of both chemical and physical BAs, the shift factor demonstrates a steady decrease in shear viscosity with increased gas concentration, something not easily discernible from the flow curves in the case of the CBA. The scaling factor values reiterate our earlier observations concerning the greater effect of carbon dioxide on the shear viscosity of the TPO in comparison to nitrogen (for the PBA systems) as well as the effect of process temperature. The two different blowing systems used, chemical versus physical, covered different ranges in gas concentrations, as

shown in Figure 8. The range limits with respect to gas concentration shown in the figure for the two different blowing systems are a result of the mechanism by which the gas is introduced to the polymer melt. PBAs are readily introduced at high gas concentrations, yet at lower quantities they can be quite difficult to regulate due to the standard metering apparatus and normal process fluctuations arising from the injection molding machinery. CBAs tend to be better suited for low gas concentrations due the quantity of gases evolved during decomposition of their chemical species and with consideration of acceptable changes to the rheological and mechanical properties of a polymer product caused by the addition of a carrier resin (if a masterbatch CBA is used) and chemical residues to the matrix. In the overlapping region of gas concentration between the CBA and PBA (i.e., 0.1-0.2 mol/1000 g of TPO), it was observed that the scaling factors of either CO2 or N2 (as PBA) were consistently higher than the corresponding CBA generating a similar gas species at the same temperature. The shift factor values (R) for the endothermic CBA species, which generated carbon dioxide gas, were greater than those determined for the exothermic CBA, which released nitrogen gas. We anticipated the opposite trend based on the solubility differences of the two gases in the polymer matrix; the solubility of CO2 in polypropylene has been found to be almost an order of magnitude greater than nitrogen at typical processing conditions.28 As separate families of BAs, PBA versus CBA, it appears that these blowing techniques incorporate gas into the molten polymer differently. The differences in how the gases from these BAs were dissolved into the matrix, particularly into a polymer blend (such as TPO), may have accounted for the trends shown in Figure 8. The PBA species were mechanically dispersed within the melt and may not have been allowed sufficient time to diffuse preferentially to the polypropylene or elastomeric phases making up the TPO within the time scale of the experiments. The active species of the CBA would have had greater opportunity for localization within one of the phases of the TPO, before gases had been generated, due to preferential segregation of the carrier making up the masterbatch. For the two CBA species, the trends in the shift factor were contrary to our expectations based on the species of gas evolved and contrary to earlier findings for these same two additives processed with a homogeneous polymer (LDPE).24 The heterophasic composition of the TPO was again the most likely cause for the trends shown by the shift factor in Figure 8. Further experimentation is necessary to properly understand the phenomena shown in Figure 8. Viscosity Model in TPO/BA Solutions. The introduced viscosity model and viscoelastic scaling method based on a generalized Cross-Carreau equation, and the free volume concept had been used for LDPE/BA solutions successfully.24 In this work, it was applied for different TPO/BA solutions. The fitting parameters for the different TPO/BA solutions are listed in Tables 1 and 2 for processing temperatures of 230 and 250 °C, respectively. The resulting viscoelastic scaling factors are shown in Figure 8. The advantage of the developed modeling approach over existing methods is the limited use of fitting parameters. It was found from the fitting results that the addition and dissolution of BA influenced the thermal expansion coefficient and isothermal compressibility coefficient in the free volume fraction expression. In other words, the gas dissolution affected the constitutive properties of virgin polymer macromolecules. By applying the concentration-dependent, temperature-dependent, and pressure-dependent scaling factors in eq 4 to scale both viscosity and shear rate, each of the viscosity curves at different temperature, pressure, BA type, and BA concentration

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Figure 9. Experimental data of TPO/BA solutions shifted to a single master curve based on viscoelastic scaling: (a) endothermic CBA, (b) exothermic CBA, (c) CO2, (d) N2. Open symbols correspond to results for 230 °C, and filled symbols correspond to results for 250 °C. Table 1. Model Parameters Used in the Viscoelastic Scaling Analysis at 230 °C TPO/CO2

TPO/N2

parameters

TPO

CBA

PBA

CBA

PBA

A (Pa‚s) B fr Tr (K) Pr (MPa) βT (1/°C) βP (1/MPa) Φ (1/wg) τ (Pa) n

2.54 × 10-4 9.219 0.580 503.2 0.5 2.75 × 10-3 2.10 × 10-3

2.54 × 10-4 9.219 0.580 503.2 0.5

2.54 × 10-4 9.219 0.580 503.2 0.5

2.54 × 10-4 9.219 0.580 503.2 0.5

2.54 × 10-4 9.219 0.580 503.2 0.5

8.23 × 10-4 9.20 × 10-2 5076.1 0.388

1.94 × 10-3 1.40 5076.1 0.388

8.25 × 10-4 0.18 5076.1 0.388

1.43 × 10-3 1.11 5076.1 0.388

5076.1 0.388

Table 2. Model Parameters Used in the Viscoelastic Scaling Analysis at 250 °C TPO/CO2

TPO/N2

parameters

TPO

CBA

PBA

CBA

PBA

A (Pa‚s) B fr Tr (K) Pr (MPa) βT (1/°C) βP (1/MPa) Φ (1/wg) τ (Pa) n

2.54 × 10-4 9.219 0.580 503.2 0.5 2.75 × 10-3 2.10 × 10-3

2.54 × 10-4 9.219 0.580 503.2 0.5 5.35 × 10-3 5.52 × 10-3 0.32 5076.1 0.388

2.54 × 10-4 9.219 0.580 503.2 0.5 3.07 × 10-3 2.10 × 10-4 1.59 5076.1 0.388

2.54 × 10-4 9.219 0.580 503.2 0.5 4.71 × 10-3 2.21 × 10-3 0.13 5076.1 0.388

2.54 × 10-4 9.219 0.580 503.2 0.5 7.27 × 10-3 1.52 × 10-2 1.52 5076.1 0.388

5076.1 0.388

were collapsed to a single master curve identical to the viscosity curve for the pure TPO melt at referenced temperature, 230 °C, as shown in Figure 9 for each type of BA. The developed viscosity model and viscoelastic scaling method shows broad applicability to different BAs and appears to be suitable for complex polymer systems, such as TPO as well as LDPE.24

Conclusions The shear viscosity of a TPO melt with dissolved chemical or physical BAs was investigated by an in-line capillary rheometer over a large range of shear rates for two processing temperatures. The viscosity reduction behaviors of CBAs and

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PBAs in TPO melt were compared, and the effect of temperature on viscosity reduction was examined. The viscosity reduction of the TPO in the presence of any BA was increased with processing temperature. It was seen that the exothermic masterbatch CBA (generating nitrogen) had a greater reduction effect on the viscosity of TPO melt than the endothermic masterbatch CBA (generating carbon dioxide), a trend that is opposite to previous results for a homogeneous LDPE and contrary to published solubilities for the generated gases. As to the PBAs, carbon dioxide had a greater viscosity reduction effect on the TPO melt than nitrogen, owing to the difference in physical properties of two gases and their respective solubility in the polymer melt. Comparing the plasticizing influence of the CBA species versus the PBA species, the extent of viscosity reduction was greater for the former additive type based on equivalent gas content in the melt. The differences in plasticization between the CBA and PBA species, and between the CBA species alone, have been attributed to the heterophasic nature of the polymer and the manner by which the BAs incorporate their gases into the matrix. Finally, the viscoelastic scaling scheme for modeling the viscosity of a gas-polymer solution, which was originally used with homogeneous LDPE/BA solutions, was successfully applied to a complex hetero-phasic TPO composite, with all the viscosity data collapsing to a master curve at the chosen reference temperature. Acknowledgment The authors are grateful to the AUTO21 Network Centres of Excellence of Canada and the NSERC for funding this project and to MoldFlow Co. for providing helpful assistance in the design of the in-line rheometer. We thank Solvay and Clariant for their generosity in donations of the resin and BAs. Literature Cited (1) Park, C. B.; Suh, N. P. Filamentary extrusion of microcellular polymers using a rapid decompressive element. Polym. Eng. Sci. 1996, 36, 34. (2) Lee, M.; Tzoganakis, C.; Park, C. B. Effect of supercritical CO2 on the viscosity and morphology of polymer blends. AdV. Polym. Technol. 2000, 19, 300. (3) Dey, S. K.; Natarajan, P.; Xanthos, M.; Braathen, M. D. Use of inert gases in extruded medium-density polypropylene foams. J. Vinyl Addit. Technol. 1996, 2, 339. (4) Park, C. B.; Cheung, L. K. A study of cell nucleation in the extrusion of polypropylene foams. Polym. Eng. Sci. 1997, 37, 1. (5) Royer, J.; Desimone, J.; Khan, S. High-pressure rheology and viscoelastic scaling prediction of polymer melts containing liquid and supercritical carbon dioxide. J. Polym. Sci.: Part B: Polym. Phys. 2001, 39, 3055. (6) Areerat, S.; Nagata, T.; Ohshima, M. Measurement and prediction of LDPE/CO2 solution viscosity. Polym. Eng. Sci. 2002, 42, 2234. (7) Lan, H.-Y.; Tseng, H.-C. Study on the rheological behavior of PP/ supercritical CO2 mixture. J. Polym. Res. 2002, 9, 157. (8) Doroudiani, S.; Park, C. B.; Kortschot, M. T. Processing and characterization of microcellular foamed high-density polyethylene/isotactic polypropylene blends. Polym. Eng. Sci. 1998, 38, 1205.

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ReceiVed for reView September 30, 2005 ReVised manuscript receiVed January 27, 2006 Accepted March 1, 2006 IE0510932