Experimental Study on the Relationship between Particles Size and

Gas phase production of polyethylene (PE) results in a powder with particles of different sizes. This experimental work on the samples from an industr...
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Experimental Study on the Relationship between Particles Size and Properties of Polyethylene Powder from an Industrial Fluidized Bed Reactor Reza Rashedi and Farhad Sharif*

Ind. Eng. Chem. Res. 2014.53:13543-13549. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/27/18. For personal use only.

Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran 15875-4413, Iran ABSTRACT: Gas phase production of polyethylene (PE) results in a powder with particles of different sizes. This experimental work on the samples from an industrial reactor shows that there is a difference between Mw and properties of the PE particles as a function of their sizes. The results showed that for each type of polyethylene there is a critical particle size for which Mw reduces to a minimum. Particle size was found to have strong influence on the rheological behavior of high-density polyethylene (HDPE) homopolymer. The smallest homopolymer HDPE particles showed the highest Mw with distinctly different flow behavior. Differences in properties for HDPE copolymer and linear low density polyethylene (LLDPE) samples were less profound.

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INTRODUCTION Polyethylene (PE) is commonly produced in low pressure, catalytic bulk, slurry, and gas phase reactors in the presence of Phillips-Chromium, Ziegler−Natta (ZN), and metallocene catalysts. Gas phase polyethylene production takes place in continuous fluidized bed reactors (FBRs). During this process, small catalyst particles (ranging from 20 to 80 μm) are continuously fed into the reactor to react with the incoming fluidizing monomer gas to form polymer particles ranging from 100 to 5000 μm. Catalyst particles are encapsulated by the growing polymer and form final polymer particles during polymerization in the fluidized bed.1−3 In industrial polymerization reactors, there are catalyst particles with different sizes which behave differently. Nemeth et al. reported that properties of final polymer powder are the average of what produced by catalyst particles of different activities.4 In a fluidized bed polymerization reactor, it is assumed that polymer particles are well mixed, the reaction is nearly isothermal, and reaction heat is removed effectively by the fluidizing gas. Although, some heterogeneity in polymer properties is observed in the particles with different sizes when a highly active catalyst is used in a gas phase fluidized bed reactor.5 Kiparissides et al. modeled polymerization and predicted the rate of polymerization at the particle level as a function of the particle diameter, distribution of catalyst active sites, and the postulated kinetic mechanisms. They showed that at the particle level, external and internal mass and heat transfer limitations become significant, especially for highly active catalysts. They indicate that molecular weight and molecular weight distribution depends on the particle size.6 In gas phase reactors, resistance to the heat and mass transfer within particles and within the bed is extremely important.7,8 Several modeling studies evaluated and confirmed the effect of mass and heat transfer resistances on the growth of single particle in the ethylene gas phase polymerization reactor.9−13 © 2014 American Chemical Society

Previous studies showed that there are two mechanisms for monomer transfer from the bulk of the gas phase to the active metal sites. These mechanisms are the penetration of monomer molecules through open catalyst/particle pores and the diffusion through the amorphous domains of the semicrystalline polymer particle.14 On the other hand catalyst particle fragmentation results in easy access of monomer molecules to the active sites, keeping the polymerization rate high. Other studies highlighted that the reaction rate is more sensitive to the temperature rather than monomer concentration.15 The polymerization rate at the particle level, depends on the particle temperature, active metal accessibility by monomer, and monomer solubility.16,17 While theoretical studies hint to the difference in property as a function of particle size, there has not been any experimental work to verify this difference and its extent in the open literature. Therefore, this experimental study attempts to quantify the degree of heterogeneity in an industrial reactor and its impact on properties as particle size varies. This work specifically addresses the following questions, using industrially produced polymer particles: (a) Is there any meaningful difference between properties of polymer particles with different sizes in different grades? (b) Which properties are different and to what extent? (c) Is there any significant difference in molecular weight and molecular weight distribution of particles with different sizes? We report our evaluation of density, rheological characteristics, and molecular weight distribution of the particles with different sizes for three different PE grades from an industrial gas phase reactor, to answer the above questions. Received: Revised: Accepted: Published: 13543

April 3, 2014 August 2, 2014 August 7, 2014 August 7, 2014 dx.doi.org/10.1021/ie501398y | Ind. Eng. Chem. Res. 2014, 53, 13543−13549

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distribution polystyrene samples at 145 °C with 0.1% polystyrene in TCB. Specimens for rheological testing were prepared from meltblended resins by molding discs in a Carver hydraulic press. Samples were about 2 mm thick, to match the plate diameters of the rheometer. A MCR-302 rheometer (AnttonPaar) was used to study rheological properties. Shear strain amplitude (γ′) of 1% was used, since strain sweep showed it was sufficiently small to produce dynamic properties independent of γ′. Frequency sweep was carried out in the range 10−1−103 rad/ s. A parallel plate geometry of 25 mm diameter was employed for the frequency sweep tests. All measurements were conducted in a nitrogen atmosphere, supplied from a nitrogen cylinder, as the convecting fluid to prevent possible oxidative degradation during tests.

EXPERIMENTAL SECTION Polyethylene powder samples used in this study were taken from an industrial plant with two continuous gas phase reactor in series.18 Fourth generation commercial Ziegler−Natta catalysts were used to produce different homopolymer and copolymer grades of polyethylene. Product characteristics are listed in Table 1. Polymer powder from the plant was separated by a sieve shaker according to ASTM 1921. Table 1. Characteristics of the Polyethylene Grades Studied product

Mw (g/mol)

Mw/ Mn

MFI (g/10 min)

density (g/cm3)

HDPE homopolymer HDPE copolymer LLDPE

75100 41400 105000

8.5 4.2 3.7

6 18 1.0

0.960 0.952 0.922

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RESULTS AND DISCUSSION Study of granule density for a homopolymer and copolymers clearly shows significant change in density as a function of particle diameter, as shown in Figure 1. Smaller particles show

The particles were divided into seven subsets, with median sizes given in Table 2. Five subsets with higher population (B, Table 2. Subsets of Sieved Particles from Reactor Powders subset

size range (μm)

particle median diameter (μm)

A B C D E F G

4000

31.5 281.5 750 1500 2400 3400 >4000

C, D, E, and F) were selected for this study. Density of each subset was obtained from density to volume-number average diameter relation.19 For larger particles, subsets D, E, and F, the particles’ density was verified using a density column according to ASTM 1505. Scanning electron microscopy (SEM) was used to investigate the particle surface and cross-sectional porosity. The equipment was a Vega II from TESCAN Co. with high resolution and field emission. The particles were gold coated before analysis at a high vacuum. For cross-section evaluation, particles were embedded in low viscosity epoxy resin and cut with a razor blade. The cross-sectional samples were also gold coated at a high vacuum. For rheological assessments 0.6 wt % Irganox 1010 and 0.5 wt % of Irgafos 168 were added to the powders to prevent degradation during the experiment. Antioxidants were made by Ciba-Geigy and used as received. Powders were compounded with antioxidants using a twinscrew corotating intermeshing extruder, Brabender TSE-20, with a standard compounding configuration. The extruder was equipped with seven heating zones with temperatures ranging along the barrel, from 150 °C for zone 1 (at the hopper) to 200 °C for zone 7 (at the die). The screw speed was 60 rpm with the mass flow rate 0.5 kg/h. Granulated powders were used for the rheological evaluations. Test results for three samples are reported as average values with error bars. A PL-GPC 220 instrument was used to characterize the molecular weight distribution of the samples. It was operated at 160 °C using 1,2,4-trichlorobenzene (TCB) as solvent. Columns were calibrated with narrow molecular-weight-

Figure 1. Particle density versus particle diameter for different polyethylene grades.

higher density, and a significant reduction in particle density is observed as particle diameters increase, although to different extents for different grades. Differences in the density of particles with different sizes might be attributed to different porosity or molecular structure. Reduction in the density in the range of 0.98−0.9 can be the result of the difference in molecular structure, hence; we used GPC for more precise evaluation of molecular weight and molecular weight distribution of the particles. Density reduction to as low as 0.8, however, is a sign of porosity in the particle structure, which was further studied by SEM images of the particles. According to Figure 1, LLDPE and HDPE copolymer particles showed higher densities compared to HDPE homopolymer, indicating formation of more compact structure in the presence of comonomer. It is known that in copolymerization, reaction rate and polymer yield are higher compared to homopolymerization.8 Increasing polymer yield adds more polymers in pores, which results in more compact particles. Figure 2 shows the surface morphology of HDPE homopolymer particles. As widely accepted in literature, all particles exhibited the well-known replica phenomena; that is, the shape of the originally used carrier material is maintained during the polymerization.20−22 It is well-known and studied by several researchers that the final particle morphology is a strong function of the initial fragmentation step and the nature of 13544

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Figure 2. SEM pictures of the surface morphology of HDPE homopolymer particles subsets: (a) subset B, (b) subset C, (c) subset D, (d) subset E, and (e) subset F.

Figure 3. Cross-sectional SEM pictures of HDPE homopolymer, HDPE copolymer, and LLDPE particles.

case of LLDPE, particles in subset F, have pores around onequarter of the particle diameter. Particles have at least 1.5 h residence time in each gas phase reactor, so most of channels for monomer diffusion have been filled with polymers especially in HDPE copolymer and LLDPE particles whose polymerization rate is significantly higher in the presence of butane-1 as a comonomer. This is probably the main reason for the more compact structure in small particles of LLDPE particles (subsets C, D). Molecular weight and molecular weight distribution of three grades of polyethylene were evaluated and reported in three figures. Results for HDPE homopolymer grade are given in Figure 4 as functions of granule sizes. It should be noted that the lines are drawn to guide the eye. Figures 4 and 5 demonstrate that molecular weight and molecular weight distribution varies significantly with the granule size. From the B to C subsets there is a substantial decrease in Mw. Smaller particles had larger polydispersity index and wider molecular weight distribution. Particle temperature and polymerization rate is size dependent at the particle level. Therefore, small particles experience

catalyst fragmentation plays a key role in determining particle morphology.23 It is clear from Figure 2 that the morphology of particles varies according to their sizes. SEM images of the surface reveal that the spherical particles consist of an agglomeration of small fragments, and this phenomenon is more evident for larger particles. It can be seen that a particle in subset B is an almost perfect sphere with a smooth surface; however, it becomes rough as particle size increases. As illustrated in Figure 2, there are few cracks with small gaps (5−15 μm) for subsets B and C, which cause monomer transfer limitation. Increasing particle size to 3400 μm (subset F) results in more and wider surface cracks. SEM images in Figure 3 show the cross-sectional morphology of the particles used in this study. The images were found to be representative of a number of particles from each set, so we can be comfortable drawing conclusions from them. If we compare the morphologies of small particles and larger ones, we can see that these particles exhibit voidless compact structure, with little apparent macroporosity. For all grades especially LLDPE particles, by increasing the particles size, more porous structure with wider pores are obtained. In 13545

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Figure 6. Complex viscosity of HDPE homopolymer subsets.

D, and E. Higher viscosity of the B and F subsets may be attributed to their higher Mw. These significant properties differences could be a sign of different molecular architectures. Very recently some evidence of long chain branching in Ziegler−Natta polyethylene homopolymers has been reported that may explain behavior of the B subset.25 In the next step, molecular weight and polydispersity of high density polyethylene copolymer (with 1-butene concentration less than 2%) which had lower molecular weight than HDPE homopolymer was studied. A higher concentration of hydrogen was used during polymerization, and the amount of small particles (less than 750 μm) significantly increased inside both reactors. Figures 7 and 8 show similar trends for Mw and PDI for HDPE copolymer. Mw and PDI initially drop by particle diameter then increase gradually. For particles larger than 750 μm, Mw increases by particle size, as observed for the HDPE homopolymer. A possible explanation is that the inside structures of large particles are different from those of small ones. Large particles have a higher void fraction or macrocracks (as SEM images show and density measurements confirm) resulting in better mass transfer and controlling mechanisms for polymerization inside the particle. Figure 9 shows rheological behavior of HDPE copolymer particle subsets. As seen in Figure 9, shear thinning behavior of different subsets are very similar. . Figures 10 and 11 show molecular weight, polydispersity index, and molecular weight distribution for LLDPE grade subsets. According to Figure 10, the weight-average molecular weight is significantly higher for small particles (750 μm). Except for fine particles (subset C), as particles diameter increases, the weight-average molecular weight (Mw) increases, while the number-average molecular weight (Mn) and polydispersity index remains roughly constant. At the particle level, distributed molecular characteristics of the polymer are proportional to the monomer, other reactant concentrations, and their absorption at the active sites; these are affected by particle diameter and reactant radial gradient. In case of LLDPE, due to the higher concentration of comonomer in the reactor, different distributions of molecular properties are expected, especially variation in Mw which varies from 56 863 (d = 1500 μm) to 122 651 (d = 750 μm) g/mol. The relationship between the complex viscosity and shear rate of LLDPE particles is presented in Figure 12. Mild shear thinning behavior was observed for particles with 1500 and 2400 diameter while smaller and larger particles (750 and 3400 μm) showed more frequency dependence and their flow curves

Figure 4. Weight-average molecular weight (Mw) and polydispersity of HDPE homopolymer subsets.

Figure 5. Molecular weight distribution for HDPE homopolymer subsets.

higher temperature gradient compared with their surrounding gas while larger ones are very close to the local fluid temperature.24 Small particles, therefore have different rates of polymerization from core to surface resulting in a broader molecular weight distribution. The Mw of the smallest homopolymer particles was significantly higher compared to larger particles and also showed bimodality to some extent. To evaluate the underlying relationship between granule size and molecular characteristics, rheological behavior of resins was studied. The weight-average molecular weight and the molecular weight distribution affect zero shear rate viscosity and the shear rate dependency of viscosity. Figure 6 depicts the flow behavior of HDPE homopolymer granules of different subsets. B and F subsets show higher viscosity compared to C, 13546

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Figure 7. Weight-average molecular weight (Mw) and polydispersity of HDPE copolymer subsets.

Figure 10. Weight-average molecular weight (Mw) and polydispersity of LLDPE subsets.

Figure 8. Molecular weight distribution for HDPE copolymer subsets.

Figure 11. Molecular weight distribution for LLDPE subsets.

Figure 12. Complex viscosity of LLDPE subsets.

Figure 9. Complex viscosity of HDPE copolymer subsets.

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Rheological assessments hints to probable special branching structure in LLDPE for small powder particles (more frequency dependency of their shear thinning behavior) compared to the particles with medium size (d = 1500 μm).

lies higher than midsize particles. As reported in earlier studies, it may be attributed to the particular branching structures.26 The variation of number and weight-average molecular weights as well as rheological properties with respect to the particle size for different grades of polyethylene hints to a significant difference in the molecular structure of PE particles produced in industrial gas phase reactor using commercial ZN catalysts. Our observations on the industrial products are very different from Wanke et al. observations on Mn and Mw reported for laboratory produced PE particles.27 Results show that in HDPE homopolymer and LLDPE grades, small particles (Class B and C, respectively) have higher weight-average molecular weights and broader molecular weight distribution compared to other subsets. There are several possible explanations for the broad molecular weight distribution in the small particles. The most likely cause is the particle higher temperature gradient resulting in heterogeneous polymerization rates.28,29In small particles, monomer access to active sites seems to be restricted, especially to active sites placed in the internal layers. Easy monomer access to the surface active sites, higher monomer concentration at the particle surface, and higher surface temperature result in high polydispersity index in these particles. It may also be the reason for different lengths of polymer chains in B and C subset particles. In these particles, monomer diffusion inside the particle is limited. HDPE copolymer small particles (B subset) have relatively high molecular weight, but not the highest. Their Mw is less than largest particles (subset E). Studies on powder densities and SEM images showed that particles larger than 1500 μm (D, E, F subsets) are more porous. This porosity may have been the result of larger catalyst particles with pores. This porosity makes access and distribution of monomer and other reactants to the active sites on the internal layers much easier resulting in lower polydispersity index. It should be noted that monomer diffusion through open catalyst/polymer pores and also through the formed polymer layer to the active sites is probably the main reason for reaction moderation in large particles. On the other hand, monomer diffusion has a major effect on the polymerization rate and chain growth in larger particles. Results of molecular weight analysis and rheological behavior confirm that there is a meaningful difference in particle properties as a function of their sizes. In each grade the smallest particles showed different molecular weight, molecular weight distribution, and rheological behavior compared to the larger particles. Newly published research on long chain branching (LCB) in Ziegler−Natta polyethylene grades confirms this heterogeneity in polyethylene molecular structure.25 Rheological properties of small particles may be attributed to the formation of LCB on the surface of small particles which needs to be further explored in the future. Small particles of HDPE homopolymer showed more drastic differences with larger ones in the rheological properties compared other grades. This indicates that an increase in final polymer density combined with the absence of comonomer in the reactor results in larger differences in particle properties as a function of their size. By introducing hydrogen and 1-butene as comonomer in the reactor, structural differences in particles were lowered and more homogeneity in rheological properties was observed. For the LLDPE grade where less hydrogen and more 1butene had been used, particle size was still important.

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CONCLUSIONS From the analysis of particles in different subsets for three different PE grades from an industrial gas-phase ethylene polymerization it appeared that there is certain heterogeneity in PE molecular structure and rheological behavior. The heterogeneity in such an industrial scale process was reported for the first time. In HDPE homopolymer subsets, smallest particles showing broader MWD and highest weight-average molecular weight, which is consistent with the results from rheological experiments. For HDPE copolymer, polydispersity index varies between 2.6 and 3.9 and Mw drops by particle diameter to a minimum value and then increases for larger particles. A small amount of 1-butene comonomer results in less deviation in flow behavior for different particle sizes compared to homopolymer. LLDPE particles with 1500 μm diameter showed the lowest molecular weight with narrow molecular weight distribution. LLDPE small particle subsets showed compact particle morphology compared to other grades. While the observations raise many interesting questions about controlling mechanisms for polymerization as the particle size changes, they also offers an opportunity for industry to produce more consistent products by simple separating of particles. While there was similarity between results for particles from homopolymerization and copolymerization, there was also noticeable differences to be further explored.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +98(21)64542400. Fax: +98(21)66468243. Notes

The authors declare no competing financial interest.

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REFERENCES

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