Variation of Comonomer Content in LLDPE Particles with Different

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Variation of Comonomer Content in LLDPE Particles with Different Sizes from Industrial Fluidized Bed Reactor Reza Rashedi, and Farhad Sharif Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02186 • Publication Date (Web): 07 Sep 2015 Downloaded from http://pubs.acs.org on September 12, 2015

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Industrial & Engineering Chemistry Research

Variation of Comonomer Content in LLDPE Particles with Different Sizes from Industrial Fluidized Bed Reactor Reza Rashedi, Farhad Sharif * Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, 15875-4413, Iran. Corresponding author E mail: [email protected] (Farhad Sharif). Tel:+98(21)64542400 , Fax: +98(21)66468243

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ABSTRACT: Properties of linear low density polyethylene (LLDPE) are highly influenced by molecular weight distribution and comonomer content. Our study quantify comonomer content variation for LLDPE particles with different sizes. Powder from the fluidized bed reactor was sieved to subsets with different sizes. FTIR showed a meaningful difference in average comonomer content among subsets. To study comonomer content distribution in each subset, successive self-nucleation and annealing (SSA) thermal fractionation via differential scanning calorimetry (DSC) was performed. This study showed each subset not only differs in the average comonomer content but also in comonomer content distribution. Particles with median diameter of 1500 micron had highest average comonomer content, which was different from the final product. Minimum average comonomer content and narrowest comonomer distribution was observed in the smallest particles with median diameter of 750 micron and high molecular weight.


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 INTRODUCTION

particle diameters cause a difference in access of monomer and comonomer to the active sites inside particles pores resulting in different comonomer distribution in the particles with different sizes.

Linear low density polyethylene (LLDPE) is a copolymer made of ethylene and a higher α-olefin such as 1-butene, 1-hexene and 1-octene. Comonomer is fed with ethylene in polymerization reactor to produce a copolymer which has better properties such as better impact and film tear properties, compared with the homopolymer.

Kiparissides et al. have shown that polymer particles with different crystallinities exhibit different growth rate, which dramatically influences the development of the PSD and MWD in the bed.4 Experimental evaluation for dependency of particle size and their properties for different grades of an industrial fluidized bed reactor showed more variation in grades having comonomers including HDPE-copolymer and LLDPE.3

LLDPE is commonly produced by Ziegler–Natta or metallocene catalysts in different types of polymerization reactors including: gas phase, high pressure loops etc. When conventional Ziegler–Natta catalysts are used in fluidized bed gas phase reactor (FBR), copolymerization does not occur uniformly in all particles. Heterogeneities in the properties have been reported and attributed to comonomer distribution with significant effect on the end - use properties of the material, such as puncture resistance.1,2

Different thermal fractionation methods have been used to quantify short chain branching and its distribution in LLDPE's. These methods are based on a special heat treatment (annealing) of the samples and the subsequent analysis of melting behavior by DSC. In these methods a particular thermal treatment on the samples, such as stepwise crystallization (SC), and self-nucleation annealing (SSA) were applied.11-15 SSA exhibited better separation than SC, and provided results in shorter time for LLDPE samples.16-18 In all previous works, LLDPE's resulting from different catalyst systems and different structures have been studied well, using thermal analysis methods.19,20 In this work we have studied LLDPE particles from reactor in powder form (e.g. before compounding), for the first time to analyze the effect of particles size on the comonomer content.

Recently, we studied polyethylene powder from an industrial reactor and showed that there was a meaningful difference in the properties of powders with different sizes.3 In our study, we realized that the difference was more significant when a comonomer had been present. Therefore, this work has focused on LLDPE to study carefully comonomer distribution in powders with different sizes to explain the difference in properties opening the way to find appropriate methods to minimize variation in final product properties. In an ethylene copolymer such as LLDPE with no long–chain branching two molecular parameters have major effect on the final product properties; those are molecular weight distribution (MWD) and comonomer distribution, determining physical, mechanical and optical properties of LLDPE. Therefore it is highly desirable to be able to analyze comonomer distribution in the particles that work as granular reactor during fluidized bed polymerization.

Particles were separated to different subsets and their comonomer content were evaluated by the SSA fractionation technique in a differential scanning calorimeter. Thermal behavior of the particles is the consequence of their structural heterogeneity, and it was examined to correlate the endothermic curve profile to LLDPE heterogeneity in comonomer content.

It has been shown that MWD is affected by the particles size distribution in the gas phase fluidized bed reactors.4 Particles size distribution is one of the morphological distributed properties that plays a major role in the product final properties. Particle growth, average particle size and particle size distribution in fluidized bed polymerization reactors have also been the focus of several researches.5-10 They have pointed out that PSD is affected by the following parameters: catalyst PSD, catalyst residence time in prepolymerization stage, particles fragmentation and growth pattern, particles attrition and elutriation and also reaction condition.

Polyethylene powder samples used in this study were taken from an industrial plant with two continuous gas phase reactor in series.21 Fourth generation commercial Ziegler-Natta catalyst was used to produce linear low density polyethylene contained 1butene as comonomer with characteristics listed in Table 1. Polymer powder from the plant was separated by a sieve shaker according to ASTM 1921. The particles were divided into seven subsets, with median sizes given in the Table 2. Four subsets with higher population (C, D, E and F) were selected for this study. Scanning electron microscopy (SEM) was used to investigate particles cross-sectional porosity. The equipment was a Vega II from TESCAN Co. with high resolution and field emission. For cross-section evaluation, particles were embedded in low viscosity

EXPERIMENTAL SECTION

It's also important to understand the interdependence of these distributed properties on each other in order to properly assess the effect of operating variables on the performance of fluidized bed reactors. Different



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epoxy resin and cut with a razor blade. The crosssectional samples were gold coated at a high vacuum.

temperature for 2 minutes. Samples were then examined from 50 °C to 160 °C at a rate of 10 °C/min. Degrees of crystallinity and melting temperatures were obtained from the second heating thermogram. Degree of crystallinity was calculated from:

Table 1. Characteristics of the polyethylene grade studied Mw produ (g/mol ct

(g/10mi

)

(g/cm3) n)

1.0

0.922

0

Table 2.Subsets of sieved particles from reactor powder subset A B C D E F G

size range (µm) 4000

(1)

Successive self-nucleation/annealing (SSA) method used for the LLDPE particles thermal fractionation was based on that proposed by Muller and Arnal.16 The optimum self-nucleation temperature (Ts-opt) was determined according to Porcello and coworkers method.18 The first self-nucleation temperature (Ts1) was 129 oC and the other eleven Ts were between 124 o C and 74 oC with a spacing of 5 oC, producing, respectively, twelve exothermic and endothermic curves. The LLDPE particles thermal fractionation was performed after the determination of the T s-opt of each subset. The LLDPE subsets were fractionated using a fractionation window of 5oC following the procedure displayed in Figure 1.

10500 3.7

E

∆H𝑓 ∆H𝑓𝑐

X%, ∆Hf and ∆Hf,c are degree of crystallinity, enthalpy of fusion and enthalpy of fusion of the 100% crystalline polyethylene, respectively. Enthalpy of fusion of 100% crystalline polyethylene was considered 293.6 KJ/Kg.23

Density

/Mn

LLDP

𝑋% =

MFI Mw

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particle median diameter (µm) 31.5 281.5 750 1500 2400 3400 >4000

The molecular weight distribution of the subsets were determined by high temperature gel permeation chromatography (GPC) on a PL-GPC 220 instrument. It was operated at 160°C using 1,2,4-trichlorobenzene (TCB) as solvent stabilized with 0.1% wt. of BHT. Columns were calibrated with narrow molecularweight-distribution polystyrene samples at 145oC with 0.1% polystyrene in TCB. The FTIR spectra were recorded from 4000 to 400 cm-1 with a Bruker Tensor 27 spectrometer. A minimum of 32 scans are added with a resolution of 0.5 cm-1. The 1-butene content was determined by using the methyl group absorption band at 769 cm-1 and 1378 cm-1.22 Calorimetric measurements were performed by using a differential scanning calorimeter Mettler Toledo DSC 823e. The instrument was calibrated with indium and tests were carried out under nitrogen blanket. Weighed samples (10 mg) were sealed into aluminum pans, heated from 50 °C to 160 °C at a rate of 10 °C/min, then held at 160 °C for 5 minutes to allow complete melting of all the crystallites, and cooled down at a rate of 10 °C/min to 50 °C and held at this

Figure 1. Schematic representation of the applied thermal program during SSA Endothermic curves were analyzed using the software STARe SW 11 and quantitative data on the peak area and heat of fusions (ΔH) were obtained. Lamellar thickness of different lamellae was

calculated from SSA curves using the Thomson-Gibbs equation, given below: Tm=Tom(1-2 σ /ΔHlc)


(1)

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Where lc is the lamellae thickness, Tm is the apex temperature of each melt peak of the SSA endothermic curves, Tom is the equilibrium melting point of an infinite polyethylene crystal (418 K), ΔH is the heat of fusion of the crystal with infinite lamellar thickness (288 x 106 J/m3), and σ is the free energy of the lamellae surface (70 x 10-3 J/m2).

size distribution of the product collected from dryer section is broad, however its mean size is 2004 micron. Different properties of small particles is the main reason for removing them from the main product in order to minimize final product properties deviation in industrial plants.

Arithmetic mean (Ln) and weighted mean (Lw) lamellae thickness of the subsets, were calculated according to the following formulas: 24,25 𝐿𝑛 = ∑𝑛𝑖=1 𝐿𝑖 𝑥𝑖

(2)

𝐿𝑤 = ∑𝑛𝑖=1(𝐿2𝑖 𝑥𝑖 )⁄∑𝑛𝑖=1(𝐿𝑖 𝑥𝑖 )

(3)

Where Ln is the arithmetical mean lamellae thickness, Lw is the weighted mean lamellae thickness, Li is the lamellae thickness corresponding to i-th peak in the SSA curve, and xi is the segment of the i-th peak in the SSA curve. Melting enthalpy of each peak in the SSA curve has been determined and its contribution in total area was calculated to obtain individual peak segment. The broadness index of lamellae thickness distribution is defined as the ratio of Lw/Ln.

Figure 2. Particle size distribution of LLDPE granules produced in gas phase reactor

 RESULTS AND DISCUSSION (a)

(b)

(c) (

C

(d)

Figure 2 displays the particle size distribution of LLDPE product reactor powder. It can be seen that the Figure 3. SEM pictures of the cross sectional morphology of LLDPE particles subsets: (a) subset C, (b) subset D, (c) subset E and (d) subset F

As studied and reported earlier, there are property variation including density in polyethylene particles with different sizes. Subset C has the highest density, and the density decrease by increasing particle size.3 SEM images in Figure 3 show the cross-sectional morphology of the particles including the reason for lower density in large particles. The images were obtained from a number of particles from each set, and selected in a way to represent a subset. . If we compare the morphologies of small particles and larger ones, we can see that these particles exhibit a continuous and compact structure, with little apparent macroporosity. It can be seen that by increasing the particles size the more open structure with wider pores have been

obtained. For subset F, there are pores as large as one quarter of the particle diameter. The particles were industrial samples and they have been polymerized for at least 1.5 to 2 hours in each gas phase reactor, so most of channels for monomer diffusion have been filled with polymers, especially in small particles where polymerization rate is significantly higher in presence of 1-butene as comonomer. This is probably the main reason for more compact structure in small particles of LLDPE particles (subsets C, D). Figures 4 and 5 show molecular weight and molecular weight distribution for studied LLDPE grade subsets. According to Figure 4, the weight average molecular



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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) also increases. At the particle level, distributed molecular characteristics of the polymer is proportional to the monomer, other reactant concentrations, and their absorption at the active sites and these are affected by particle diameter and reactants radial gradient. In case of LLDPE, due to the higher concentration of comonomer in the reactor, different distribution of molecular properties is expected, especially Mw variation from 56863(d=1500 µm) to 122651(d=750 µm) g/mol.

Figure 5. Molecular weight distribution of final LLDPE and its particle subsets

Results from DSC study of four subsets are exhibited in Figure 6. As the comonomer content increases, the resulting shorter sequences cause the copolymer molecules to crystallize at lower temperatures and in smaller, and less perfect structures. It is well known that short chain branches have a predominant effect on crystallization and melting. Therefore diffusion of higher amounts of comonomer in subset D particles results in more short branches in these particles reducing the length of sequences that are capable to crystallize. Results in Table 3 clearly demonstrate that the particle size affects melting temperature and degree of crystallinity. On the other hand, diffusion and incorporation of comonomer in compact structure of subset C particles caused higher amount of crystallinity in small particles but probably thin shape of these crystals resulted in the lowest melting point. Very similar crystallization and melting behavior was found for subsets E and F which indicate similar ethylene sequence and branch distribution.

Figure 4. Weight average molecular weight (Mw) and comonomer content as a function of the size of LLDPE granules


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Figure 6. DSC melting (a) and crystallization (b) curves of LLDPE particles and final product

Table 3. Crystallization and melting peak values Subset

Tc (oC)

Tm (oC)

∆H (J/g)

X%

C

111.07

124.53

115.13

39.21

D

109.18

126.38

103.23

35.16

E

109.36

126.65

101.38

34.53

F

110.94

126.88

112.81

38.42

Final Product

109.93

126.96

103.56

35.27

SSA is generally employed for the analysis and comparison of commercial samples of ethylene-αolefin copolymers concluding about heterogeneity of copolymers produced with the Ziegler-Natta catalysts.26-28 Although all of these works have been focused on commercial LLDPE but its industrial neat reactor powders had never been studied. Study of final

product comonomer content was compared with particles from reactor in Figure 7. Final product comonomer distribution is closer to subset E. It should be mentioned that this compound and all other reported commercial LLDPE's have passed compounding process which could affect their structure.



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Figure 7. DSC heating scans after SSA fractionation for LLDPE particles subsets and its final product

Table 4. Lamellar thickness (l), enthalpy (ΔH), and melting temperature (T m) relative to the multiple peaks of SSA fractionated LLDPE particles Subsets C, E Subset C (d=750 µm)

Subset E (d=2400 µm)

Peak (no)

Tm(oC)

∆H(J/g)

l (oA)

Peak (no)

Tm(oC)

∆H(J/g)

l (oA)

1

75.91

0.31

29.47

1

66.83

1.02

26.04

2

77.6

0.36

30.23

2

78.2

0.48

30.50

3

82.6

0.43

32.63

3

83.2

0.53

32.94

4

87.4

0.62

35.39

4

88

0.69

35.78

5

92.4

0.79

38.71

5

92.9

0.91

39.15

6

97.2

1.22

42.68

6

97.9

1.24

43.27

7

102.3

1.66

47.73

7

102.9

1.64

48.44

8

107.4

2.46

54.30

8

108.1

2.22

55.23

9

112.8

3.84

63.48

9

113.4

3.23

64.65

10

118.6

8.94

77.32

10

119.1

5.97

78.97

11

124

13.23

97.55

11

124.7

8.38

100.79

12

129.4

23.78

131.52

12

130.4

13.2

140.72


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The thermally fractionated LLDPE particles show endothermic curves with well separated peaks after the annealing steps but the size of the peaks corresponding to higher lamellae thicknesses is bigger for the D subset. Figure 7 shows that the size of the peaks of subsets E, F containing 8.7 and 7.5% 1-butene as comonomer, are quite close to each other compared with the subset D. The variation in the position and intensity of the peaks are due to the different crystalline fractions present in the particles, as the result of the differences in the particles structures. Particle level comonomer distribution could be attributed to the rate of diffusion of the comonomer towards the catalytic site. For larger particles in subset F the initial polymer layer prevents the comonomer from reaching the site of the catalyst .The small particles in subset D with lower molecular weight showed maximum content of 1-butene. It could be concluded that the low molecular weight species on this particles are more branched and therefore more prone to degradation specially while granulating powders to final product. Measurement of the lamellar thickness (l) in particle samples was carried out using Thomson – Gibbs equation. ∆H value corresponding to each peak was determined from the ∆H of the single fusion peak of the non- fractionated sample. Table 4 shows the Tm, the enthalpy of fusion (∆H) and the lamellar thickness (l) of each melting peak of subsets C (750 µm) and E (2400 μm). Figure 8 shows the lamellar thickness variation extracted from melting peaks for all subsets. The variation in lamellar thickness can be related to the comonomer distribution in the chains with high or low molecular weight in the LLDPE particle samples. An increase in lamellar thickness is directly related to the crystalline fractions of the particles, reflected by the variation in the peak area. It can be seen that most of variation in lamellar thickness happened at the extreme of the melting interval. In Figure 7, focusing on peaks 9 to 12 shows that subset D has clearly different behavior indicating o segments of large lamellae (70-180 A) resulting from low branched chain which are less in this subset compared to the others. The segments of average-sized o lamellae (30-45 A) is almost equal for all subsets.

Figure 8. Lamellar thickness distribution of SSA fractionated LLDPE particles

Since the lamellar thickness increases with temperature, heat of fusion has been used to compare thermal behavior of particles. Figure 9 shows the heat of fusion (∆H) related to different melting peaks of subsets. For subsets C, E and F, the heat of fusion values related to peaks 2 to 9 were higher than the values for the corresponding peak numbers of subset D due to a higher crystalline fraction in these particles. On the other hand a higher heat of fusion for subset F was observed for peak 1. This means that the lamellae in the crystalline fraction corresponding to this peak with a higher heat of fusion are formed by chains with lower comonomer content. Important information can be concluded by analysis of peak 1 and 12, at the extreme of the melting interval .The (∆H) values for the peak denoted as 1 are higher for subsets F. This suggests that subset F has less comonomer in the chain of low molecular weight or lower comonomer content. The peak denoted as number 12 in the C subset corresponds to around 8.1% of the crystallinity, requiring 23.78 J/g to melt. On the other hand, for subsets E and F this peak has almost 4.5 % and 5% of crystallinity and require 13.18 J/g and 14.72 J/g to melt; respectively. Hence, it can be concluded that subset C has less comonomer incorporated in the chains of high molecular weight compared to subsets E and F. Subset C include small particles with average diameter of 750 micron. Results show that these particles absorbs less amount of comonomer while reaction and they contains more amount of



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homopolymer compared to other particles. The thermal behavior of subset D is different of others. It contains more comonomer in its low molecular weight chains and also less comonomer incorporated in its high molecular weight chains compared to subsets E and F.

On the other hand with increasing comonomer content in subset D the highest value Lw and broadness index is observed, which shows the thickest lamellae and the most heterogeneous branching distribution. This is indicative of the increase of comonomer heterogeneities as the comonomer content increases. Results indicate that in subset C, due to the limited access of comonomer to the internal active sites, there is low comonomer content. On the surface there is an easy and uniform access of comonomer to the active sites which probably results in a homogenous thin lamellae structure. The obtained results showed that the LLDPE particles with different sizes have heterogeneity in molecular weight, molecular weight distribution, comonomer content and comonomer distribution. Results demonstrate that the smallest particles (d=750 µm) behave differently compared to the others. These particles with highest molecular weight, had absorbed lowest amount of comonomer and their final particle showed compact morphology, while next subset particle with mean diameter of 1500 µm with lowest molecular weight incorporate highest amount of comonomer and their final morphology is almost compact and similar to subset C. This finding could be attributed to the rate of diffusion of the comonomer towards the catalytic sites in subset C with high molecular weight chains, because the steric hinderance of the polymer coil prevents the comonomer from reaching the site of the catalyst. High homopolymerization rate in subset C and high copolymerization rate in subset D could be the main reason of particle pores filling and reaching compact morphology.

Figure 9. Heat of fusion relative to the endothermic curve multiple peaks of LLDPE particles SSA fractionated

Zhang and Keating method was used to quantify comonomer distribution in subsets using SSA data.24,25 The arithmetic mean (Ln) and weighted mean (Lw) of lamellae thickness and the broadness index values (Lw/Ln) of different particle subsets are listed in Table 5. Particles in subset C are characterized by lower values Lw and broadness index. This means that particles of this subset contain thinner lamellae and more homogeneous lamellae distribution.

Results show that in larger particles (E, F) comonomer insertion has been reduced but SSA results indicate that comonomer distribution in these subsets are different compared to the subset C. It seems that in larger particles open morphology cause easy access of comonomer to active sites but probably in these subsets the termination process by comonomer (αolefin) which is favored in Ziegler-Natta polymerization systems is predominant and this could be the main reason for the reduction of comonomer content in subsets E and F.

Table 5. Comonomer content and lamellar thickness distribution for LLDPE particles Subset

Comono mer content (wt.%)a

C

6

D

10

E

8.7

F

7.5

Final 7.8 Product a Measured by FTIR

Ln (o A ) 99.9 7 143. 77 95.0 9 94.7 7 101. 05

Lw

Lw/ Ln

(oA)

109. 11 169. 89 110. 23 111. 38 117. 31

 CONCLUSIONS

1.10 2 1.18 2 1.15 9 1.17 5 1.16 1

FTIR and SSA were used to evaluate comonomer distribution in linear low density polyethylene powder from a gas phase reactor. After separation of PE powder to subsets, FTIR study showed the difference in average comonomer content in each subset and final product. For the product with average comonomer content of 7.8%, comonomer total variation in particles was from 6% to 10%. Comonomer absorption was highest in the particles with 1500 µm median diameter with relatively smooth surface and


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lowest molecular weight, compared to the other subsets studied.

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SSA results showed heterogeneity in comonomer distribution of particles. Lamellar thickness of the separated peaks was calculated for different subsets of particles using Thomson-Gibbs equation showing that most of variation in lamellar thickness happened at the extreme of the melting interval. Results indicated that segments of large lamellae with low branched chains were less in the particles with 1500 µm median diameter. Quantitative analysis of SSA data indicated that subset C (750 µm median diameter) had narrow distribution and thinner comonomer structure while subset D (1500 µm median diameter) had thickest lamellae and the most heterogeneous branching distribution. It appears lower molecular weight facilitate comonomer diffusion and addition to the polymer chain. Separation of PE powder to subsets can reduce comonomer variation and consequently final properties variation. Comparison of the final industrial product behavior with the powder subsets shows, despite major fraction of D particles, final product comonomer distribution looks more similar to the particles in E subset. It could be attributed to the compounding process of the powder in industrial plants. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] . Tel.: +98(21)64542400. Fax: +98(21)66468243. Present Address Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, 424 Hafez Ave.,Tehran, Iran. Notes The authors declare no competing financial interest.

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Variation of Comonomer Content in LLDPE Particles with Different

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