Sorption of Liquid Diluents in Polyethylene: Comprehensive

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Kinetics, Catalysis, and Reaction Engineering

Sorption of liquid diluents in polyethylene: comprehensive experimental data for slurry polymerization Lenka Krajáková, Martina Lásková, Josef Chmelar, Klára Jindrová, and Juraj Kosek Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00377 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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Industrial & Engineering Chemistry Research

Sorption of liquid diluents in polyethylene: comprehensive experimental data for slurry polymerization Lenka Krajáková, Martina Lásková, Josef Chmelař, Klára Jindrová, Juraj Kosek* Department of Chemical Engineering, University of Chemistry and Technology Prague, Technická 5, Prague 6, 166 28, Czech republic Correspondence to: J. Kosek (E-mail: [email protected]) Abstract The thermodynamics of polyolefins in reaction mixtures is of considerable interest because sorption and diffusion in growing polymer particles strongly affect polymerization rate, copolymer composition and subsequent product degassing. The thermodynamics of polyethylene (PE)/hydrocarbon systems related to gas-phase polymerization was extensively studied and described in the literature. However, such data are not yet available for PE-liquid penetrant systems related to slurry polymerization. Hence, we developed a simple methodology for investigating the solubility of liquid penetrants in polymer samples. Common diluents (n-octane, n-hexane, n-pentane) were sorbed in PE samples of varying density from 902 to 967 kg m-3. Equilibrium solubilities were obtained by extrapolation from the drying data using the initial slope method. The dependence of liquid n-hexane solubility in PE on temperature (in the range 25 – 68 °C) is the principal outcome of our measurements. Comprehensive measurements provide a unique set of thermodynamic data useful especially for slurry olefin polymerizations. Keywords: polyethylene; liquid sorption; thermodynamics Introduction The majority of industrial polyolefin production is nowadays based on catalytic polymerization. Three main types of catalytic polymerization processes are used commercially: (i) gas-phase polymerization in fluidized or stirred bed reactor, (ii) solution polymerization, and (iii) slurry polymerization in liquid dispersion.1 In the case of gas-phase polymerization, a gaseous diluent and monomer (or monomers) are present in the reactor and the polymer is created on solid catalyst particles dispersed in the gaseous reaction mixture. In solution and slurry polymerization, a liquid diluent is present in the reactor and a gaseous monomer is transferred through the liquid diluent and a growing polymer to react on the active catalyst sites. In solution polymerization the produced polymer is dissolved in the reaction mixture, while slurry polymerization involves a dispersion of nascent polymer particles in a liquid.

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During all types of catalytic polymerization, transport processes in the reaction mixture significantly influence the reaction kinetics, which is conditioned by the monomer concentration at active catalyst sites. This concentration is given by the solubility and diffusivity of the monomer(s) in the solvent (or diluent) and in the nascent polymer. The solvent or the diluent added to the reaction mixture improves the monomer transport to active catalyst sites, as well as removal of polymerization heat.2 On the other hand, the solvent sorbed in the polymer affects its mechanical properties (e.g., softening) and can even cause the adhesion of the polymer to reactor wall.3 The knowledge of low-molecular weight species sorbed in the polymer is thus important for the prevention of the reactor fouling by polymer adhesion. Moreover, this knowledge is needed to optimize the degassing process, in which the solvent/diluent and the unreacted monomer(s) are removed from the produced polymer after the polymerization.4 Thermodynamic data for the solubilities of ethylene and other gaseous monomers (e.g., propene, 1-butene, 1-hexene) in polyethylene (PE) are commonly available. 5, 6, 7, 8, 9, 10, 11 Up to now, however, there have been only a few studies of phase equilibria for PE/liquid solvent system, 12, 13, 14, 15 although the slurry phase catalytic polymerization of olefins constitutes a large part of the polyolefin industry. At first, Richards 12 measured the solubility curves of paraffinic wax and PEs of different molecular weight in nitrobenzene and xylene, but these solvents are not commonly used in ethylene polymerization. Schnell et al.15 determined cloud point temperatures and pressures as a function of PE/n-hexane mixture compositions at pressures of up to 150 bar and temperatures up to 500 K. In paper de Loos et al.14 the authors experimentally evaluated LST (lower solution temperatures) in LLDPE/liquid n-hexane system as a function of pressure at temperatures 400-600 K and pressures up to 130 bar. All these studies provide interesting thermodynamic data, but at pressures and temperatures far from the conditions of commercial slurry polymerizations, which are typically 10-30 bar and 80-110 °C. Some previous studies were focusing on sorption equilibria of liquids in low density polyethylene (LDPE),16, 17 but LDPE is a product of free-radical polymerization and thus such sorption equilibria are not relevant for catalytic olefin polymerization. The objective of this work was to study sorption equilibria of liquid hydrocarbon diluents in PE produced by catalytic polymerization. We measured the solubilities of liquid n-hexane, n-pentane and n-octane in PE grades with densities 902-967 kg m-3. Moreover, we measured the temperature dependence of phase equilibria in PE/liquid diluent systems. Experimental Experimental procedure Since the separation of the wet polymer powder from the liquid cannot be done as quickly and effectively as needed, we decided to determine the sorption of liquids in polymer plate instead of particles. For this purpose, the nascent polymer powder (particles of diameter < 2mm) was hotpressed (at temperature 180-250 °C) for about 1 hour to produce a thin plate (ca. 30×50 mm with thickness of ca. 1 mm). After cooling, the plate weight was measured on an analytical balance. The plate was then inserted into the liquid diluent at a specified temperature and left for 48 hours 2 ACS Paragon Plus Environment

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under constant conditions. During this time the sorption equilibrium was established. Increasing the sorption time (up to 96 hours) had no effect on the results. In the next step, the plate was pulled out of the liquid, its surface was quickly dried by a filtration paper and placed on the analytical balance. The mass decrease during the desorption was automatically recorded by a computer connected to the balance. For correct evaluation, it is crucial to set the zero time exactly at the moment when the plate was removed from the liquid. Processing of measured data The solubility S (in grams of penetrant per gram of polymer) is usually calculated as: 𝑆=

𝑚(𝑡 = 0) ― 𝑚𝑃𝐸 𝑚𝑃𝐸

(1)

where mPE is the mass of the dry polymer plate measured before the immersion into the liquid and m(t = 0) is the mass of swollen polymer extrapolated from experimental data to zero time. In the experimental procedure described above it is not possible to determine the solubility of the penetrant in the polymer sample directly at the moment when the polymer plate was taken out from the liquid, because the plate is first dried and then transferred to the balance. A significant amount of the penetrant has already desorbed from the plate during the manipulation time, thus the first measured value cannot be used to evaluate the solubility. Therefore, the solubility is evaluated from the measured data by the extrapolation to the zero time by the initial slope method as described below in Supporting Information. In most experiments, the dry polymer mass after the experiment (𝑚𝑃𝐸∞) was found to be lower than 𝑚𝑃𝐸 before the experiment. The reason is that a fraction of the polymer was partially dissolved in the diluent Thus we calculated the solubilities according to the following equation instead of Eq. (1): 𝑆=

𝑚(𝑡 = 0) ― 𝑚PE∞ 𝑚PE∞

(2)

To determine the mass at zero time m(t = 0) we need to find a suitable function for the timedependence of the sorbed penetrant mass. Therefore we employed the initial slope method to extrapolate m(t) to m(t = 0) as described in Supporting Information. We found the dependence of the swollen polymer weight on time for the early stage of desorption in the following form: (3) 𝑚(𝑡) = ― 𝑎 𝑡 + 𝑏 where 𝑎 = 2𝐴𝑐eq

𝐷 𝜋

(4)



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When we extrapolate the mass of the sample to zero time we obtain: 𝑚(𝑡 = 0) = 𝑏

(5)

Materials and chemicals All polyethylene samples were supplied by LyondellBasell in a form of nascent reactor powders from gas-phase polymerization with Ziegler–Natta catalysts and without any further treatment (by annealing or pelletization). All polyethylene samples were co-polymers of ethylene and 1-hexene with various co-monomer content. Samples densities and crystallinities (weight fraction of the crystalline phase) are summarized in Table 1. The crystallinities of the samples can slightly differ for the hot-pressed plates than for the original polymer powders, but this fact is neglected here. All PE samples have the weight-average molecular weights (Mw) above 50 kg mol-1, thus the solubilities of penetrants per gram of polymer are independent of Mw. 18, 19 The used diluents were n-hexane (p.a. grade, PENTA), n-pentane (p.a. grade, Acros Organics) and n-octane (Merck Eurolab). Table 1: Sample densities, crystallinities. Density* Crystallinity * [wt.%] [kg m-3] VLLDPE A 902 0.359 VLLDPE B 908 0.403 LLDPE A 918 0.473 LLDPE B 923 0.508 MDPE 938 0.610 HDPE A 947 0.670 HDPE B 950 0.690 HDPE C 967 0.799 * Densities and crystallinities for polyethylene powders at room temperature Sample

Results and discussion To determine the measurement error, we carried out several sorption experiments three times at the same conditions (temperature, three identical PE plates). We found out the results to be well repeatable with the relative standard deviation less than 5 %. Sorption of n-hexane in polyethylene at room temperature Eight polyethylene samples covering the density range of industrially produced grades (Table 1) were measured. Experiments were performed at room temperature (≈ 25 °C). The solubilities of n-hexane evaluated according to Eq.(2) (S) or Eq. (6) (Sam) are summarized in Table 2.


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Table 2: Evaluated solubilities of n-hexane in different PE samples at room temperature. Sam [ghexane/gPE] [ghexane/gPE-am.ph.] VLLDPE A 0.436 0.680 VLLDPE B 0.229 0.384 LLDPE A 0.178 0.338 LLDPE B 0.126 0.256 MDPE 0.062 0.159 HDPE A 0.058 0.176 HDPE B 0.053 0.171 HDPE C 0.044 0.219 Figure 1a shows experimental data on n-hexane desorption fitted by Eq. (3) for MDPE. The parameters a, b in Eq. (3) were found by linear regression in the data processing software Origin. Because the fitting function (3) was derived only for the thin surface layer of the semi-infinite slab, the model fits the experimental data only for about initial twenty minutes of desorption. Fortunately, using this model, we need to extrapolate the data back to time t = 0 s, at which the sorption equilibrium of the liquid n-hexane in the polymer sample was established. At t = 0, the polymer plate was still immersed in the liquid n-hexane. As expected, the mass of the sorbed n-hexane per unit mass of PE sample decreases nonlinearly with time in Figure 1a. The desorption data can be alternatively plotted as the dependence of sample weight on the square root of time. The mass decrease is then linear (at t → 0) and the linear fitting function according to Eq. (3) can be used to estimate the variables a and b. Figure 1b shows the linear dependence on 𝑡 and the extrapolation to the zero time is better visible than in Figure 1a. Sample

S

Figure 1: Experimental data and fitting function for MDPE (ρ = 938 kg m-3). The mass of the sorbed n-hexane is plotted as the dependence on a) time, b) square root of time. Based on the data from Table 1 and Table 2, we can conclude that the solubility of n-hexane decreases with increasing polymer density and crystallinity, as depicted in Figure 2. This trend can be expected, because n-hexane can penetrate only into the amorphous phase of semi-crystalline polymer structure and with the increasing polymer density the fraction of the amorphous phase decreases. The fact that only the amorphous phase can sorb the penetrant leads us to relate the 5 ACS Paragon Plus Environment


solubility to the unit mass of the amorphous phase computed from the solubility S and the crystallinity 𝒘𝒄𝒓 (i.e., the weight fraction of the crystalline phase) as: 1 (6) (1 ― 𝑤𝑐𝑟) Re-calculated solubilities Sam related only to the amorphous phase of PE are shown in the last column of Table 2 and plotted in Figure 2, where the solubilities related either to semicrystalline PE or only to the amorphous part can be compared. Without available robust data about crystallinity changes due to swelling, all solubilities Sam in this paper are scaled per unit mass of amorphous phase of dry sample. 𝑆𝑎𝑚 = 𝑆

Figure 2: Solubilities of n-hexane in 8 different PE samples, related either to the unit mass of PE (blue squares) or to the unit mass of amorphous PE (red circles). The non-linear trend observed for solubilities S (blue squares in Figure 2) is maintained after the conversion to Sam (red circles in Figure 2), i.e., the amorphous phase does not sorb equally for all measured samples. A significant increase of solubility for crystallinities lower than 60 wt.% (samples with densities below approx. 937 kg m-3) is seen. For samples with crystallinities higher than 60 wt.%, we can consider the solubility to be approximately constant. This fact supports the new morphological model of free and constrained amorphous phase in polyethylene introduced by Chmelař et al.20 Their comprehensive research studied the phase composition and morphology of PE with crystallinities from 34.5 to 81.8 wt.%. According to their results, polyethylene consists of the crystalline phase (rigid), constrained amorphous phase inside the lamellar stacks (semi-rigid), and free amorphous phase outside the stacks (mobile). The free amorphous phase is not present in PE samples with crystallinities above 60 wt.% (HDPEs). In contrary, 25 wt.% of the free 6 ACS Paragon Plus Environment

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amorphous phase was found in VLLDPE with density 902 kg m-3. The free amorphous phase can sorb more penetrants than the constrained amorphous phase, which is greatly affected by the surrounding crystalline lamellar stacks. Thus the solubility Sam in HDPEs is significantly lower than in PEs with lower density and remains approximately constant with increasing fraction of the crystalline phase. On the other hand, the solubility Sam in the lower density PEs (below 930 kg m-3) increases approx. linearly with decreasing crystallinity due to the increasing fraction of the free amorphous phase.

Sorption of n-hexane in polyethylene at elevated temperatures We measured the temperature dependence of n-hexane solubility in 4 polyethylene samples with densities in the range 908 – 967 kg m-3. Each PE sample was measured at 47 °C, 60 °C and at the boiling temperature of n-hexane (68 °C). The evaluated n-hexane solubilities (using Eq. (2) and Eq. (6)) at the three temperatures are summarized in Table 3. Both solubilities (S and Sam) are reported and plotted in Figure 3. For the calculation of Sam, temperature dependent crystallinities were used.19 Previous experimental data at room temperature are also included in Figure 3. Table 3: n-hexane solubilities in 4 PE samples at temperatures 47, 60 and 68 °C. 47 °C Sample

60 °C

68 °C

S Sam S Sam S Sam [ghexane/ [ghexane/ [ghexane/ [ghexane/ [ghexane/ [ghexane/ gPE] gPE-am.ph.] gPE] gPE-am.ph.] gPE] gPE-am.ph.]

VLLDPE B

0.495

0.803

0.983

1.550

1.276

1.963

LLDPE B

0.184

0.363

0.283

0.544

0.399

0.750

HDPE A

0.076

0.226

0.087

0.254

0.098

0.282

HDPE C

0.055

0.271

0.061

0.298

0.066

0.319

Figure 3a and b show that n-hexane solubility increases with temperature for all 4 PE samples. According to our previous research, the solubility of gaseous hydrocarbons (ethylene, propylene) in PE decreases with increasing temperature due to the negative value of sorption heat. 10 On the other hand, Randová et al.21 reported in their measurements the increasing mass of the liquid diluent sorbed in polymers with increasing temperature and Richards 12 also presented the same trend, in agreement with our data for liquid n-hexane. The detailed discussion of crystallinity change due to sorption would be appropriate at this point, however studies of PE crystallinity change due to sorption at different temperatures are to the best of our knowledge only rarely discussed22 and the comprehensive study of this topic is missing. Thus the temperature dependent crystallinity (𝒘𝒄𝒓) in solubilities Sam was considered the same in both dry and swollen samples. 7 ACS Paragon Plus Environment


The theoretical explanation of temperature dependence of liquid penetrant solubility in PE can be found in the increasing vapour pressure of a penetrant with temperature. We can consider the solubility of a liquid to be similar to the solubility of gas at saturated vapour pressure conditions (temperature and solvent activity is the same). We carried out PC-SAFT simulations of n-hexane (vapour) sorption isotherms in PE up to 99% of the n-hexane saturated vapour pressure at the considered temperatures. Binary interaction parameter khexane,PE and pure component parameters used in PC-SAFT simulations are summarized in Table 4 and their meanings are as follows: (i) the number of segments per molecule m; (ii) the temperature-independent segment diameter σ; and (iii) the segment energy parameter ε/k, where k is the Boltzmann constant. The obtained isotherms are depicted in Figure 4 as an illustrative explanation of this phenomenon. The trend in Figure 4 is clear – vapour pressure of n-hexane is increasing with temperature more strongly than is the decrease of n-hexane solubility in polymer (at isobaric conditions). The black rectangles in Figure 4 approximate the solubility of liquid n-hexane (at n-hexane activity 0.99 at each temperature). The dashed line between the rectangles represents the increase of solubility with temperature observed in our liquid sorption measurements. The results presented in Figure 3a confirm the trend of the increasing n-hexane solubility (Eq. (2)) with decreasing PE density. But if the solubilities are related only to the amorphous phase of PE (Figure 3b), the solubilities in HDPEs (HDPE A and C) do not follow this trend. Solubilities in HDPE A are lower at each temperature than for HDPE C with higher density. However, this difference is not significant and we can consider the solubilities Sam in HDPEs approximately constant taking into account two facts: (i) after the hot-pressing of PE powder, the density (and crystallinity) of the resulting PE sample can slightly differ and thus introduce an error into the calculation of Sam, and (ii) the relative standard deviation of our measurements is about 5 % for HDPEs.


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Figure 3: Temperature dependence of n-hexane solubilities in different PE samples. Solubilities are related to a) entire PE sample, b) amorphous phase of PE sample. Table 4: Pure component parameters and binary interaction parameter used in PC-SAFT simulations. Compound m (-) n-hexane 3.0576 polyethylene 1316.9 khexane,PE

σ (Å) 3.7983 3.9876

ε/k (K) 236.77 246.00 0.012

reference Gross and Sadowski (2001)23 Novak et al., (2006)8 Alizadeh et al., (2017)24

Figure 4: PC-SAFT isotherms of n-hexane vapour sorption in PE and an illustrative temperature dependence of n-hexane solubility. 9 ACS Paragon Plus Environment


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Evaluation of Flory-Huggins interaction parameters One of the simplest theories describing the thermodynamics of polymer-solvent system is the Flory-Huggins equation:25 𝜇1 ― 𝜇01 = 𝑅𝑇[𝑙𝑛 (1 ― 𝜙P) + 𝜙𝑃 + 𝜒𝜙P2]

(7)

where µ1 is the chemical potential of the penetrant (n-hexane), µ10 is the standard chemical potential, R is the universal gas constant, T is temperature, ϕP is the volume fraction of the polymer and χ is the Flory-Huggins interaction parameter. Eq. (7) is considered for a very long polymer chain length. From the equality of chemical potentials of the component in each phase at equilibrium conditions, the chemical potential of the penetrant in the polymer phase is the same as in the pure penetrant. Two assumptions are made: (i) consideration of the pure liquid at normal pressure as a standard state; (ii) neglect a small amount of polymer dissolved in the liquid phase. Then the left hand side of Eq. (7) is zero for liquid sorption in polymer, because n-hexane is in the form of a pure liquid at normal pressure and its chemical potential µ1 equals µ10. Thus we get the relationship for the Flory-Huggins interaction parameter χ:

𝜒=

―[ln (1 ― 𝜙P) + 𝜙𝑃] 𝜙P2

(8)

where the volume fraction of the polymer ϕP (in the mixture of amorphous and sorbed liquid phase) can be approximated as: 1 𝜌am (9) 𝜙P = 𝑆𝑎𝑚 1 + 𝜌am 𝜌hexane The symbol Sam represents the experimentally determined solubility of n-hexane in PE per unit mass of amorphous phase, because the Flory-Huggins theory does not consider any crystalline phase in the polymer. The density of the amorphous phase of PE is 0.855 g cm-3. 26, 27 The Flory-Huggins parameter χ characterizes the interaction between the molecules of the polymer and penetrant. Values of χ greater than 2 indicate weak interactions, while χ values between 0.5 and 2 are characteristic for polymer-solvent systems with more intense interactions. And a χ value lower than 0.5 signifies the total dissolution of the polymer in the solvent.28 The Flory-Huggins interaction parameter χ was computed from the sorption experiments with n-hexane and four PE samples at four different temperatures. The χ value was always above 0.5, which corresponds to our experiments, in which the PE film was never totally dissolved in nhexane. Figure 5 shows the dependence of the Flory-Huggins parameter χ on PE density for all 10 ACS Paragon Plus Environment

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measured temperatures. The χ value for LDPE membrane evaluated in Randová et al.21 was added to the same figure for comparison. Figure 5 also shows the decrease of χ with increasing temperature for all samples. The inverse relationship between the parameter χ and temperature is commonly proposed in the literature, e.g., Rubinstein and Colby 29 or Schuld and Wolf 30, and is most often presented in the simple form: 𝜒= 𝛼+

𝛽 𝑇

(10)

where α and β are experimentally evaluated parameters and T is temperature. Parameters α and β in Eq. (10) were evaluated from our experimental data and are summarized in Table 5.

Figure 5: Dependence of the Flory-Huggins interaction parameter χ on PE density for temperatures 25, 47, 60 and 68 °C. Table 5: Parameters α and β for the temperature dependence of χ (Eq. (10)) for the PE/n-hexane system. Sample + n-hexane VLLDPE B LLDPE B HDPE A HDPE C

α -1.9708 -1.7357 -0.6159 -0.2772

β 875.13 858.81 573.05 439.82

Sorption of other liquid hydrocarbons in polyethylene at room temperature



In the following series of sorption experiments, we measured the sorption of n-pentane and n-octane in four PE samples with densities in the range 908 – 967 kg m-3 in order to compare the solubility of different hydrocarbon diluents in PE at room temperature (Table 6). Table 6: Sample characteristics and evaluated n-pentane, n-hexane and n-octane solubilities according to Eq. (2). Sample VLLDPE B LLDPE B HDPE A HDPE C

Density n-pentane solubility n-hexane solubility n-octane solubility [goctane/gPE] [kg m-3] [gpentane/gPE] [ghexane/gPE] 908 0.222 0.229 0.255 0.127 923 0.121 0.126 947 0.059 0.058 0.067 0.051 967 0.043 0.044

The solubilities of n-pentane and n-octane are quite similar to those of n-hexane, but we can conclude that for all four PE samples the solubility of n-pentane is slightly lower than the n-hexane solubility, while the solubility of n-octane is slightly higher than that of n-hexane. Higher hydrocarbons are thus sorbed more than lower hydrocarbons, in qualitative agreement with the results obtained for penetrant sorption from the gas phase.7, 8 This phenomenon can be simply explained by the better miscibility of substances, which have more similar structure. Because noctane has a longer hydrocarbon chain than n-hexane and n-pentane, n-octane is sorbed more into PE with a very long hydrocarbon chain. Dissolution of polymer in liquid As mentioned above (in Section “Processing of measured data”), the partial dissolution of our PE samples in the liquid occurred during our experiments. In some cases (VLLDPE at higher temperature), the mass loss caused by a partial polymer dissolution in the liquid was up to 10 %. To evaluate the composition of the PE soluble part, we employed GPC analysis. For this purpose, the PE plate was inserted into liquid n-hexane in a small vial and left for 48 hour at constant conditions (60 °C, atmospheric pressure), as it was done in the sorption experiments. Then the PE plate was pulled out from the liquid to another vial and the solvent was evaporated overnight in a centrifugal evaporator from both vials (one with the swollen PE plate and the second with the soluble PE parts in n-hexane). Thereafter, the GPC analysis was done for the original PE sample (O), the soluble PE fraction (SF) and the insoluble PE fraction (IF), which was pulled out from n-hexane after the experiment. This procedure and GPC analysis were done for two PE samples (VLLDPE B and LLDPE B) and the characteristics of these samples and the GPC results are summarized in Table 7. Here 𝑀𝑛 is the number average molecular weight, 𝑀𝑤 is the weight average molecular weight and Z is the polydispersity index.


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Table 7: Sample characteristics and evaluated 𝑴𝒏, 𝑴𝒘 and Z. (O)=original sample, (SF)=soluble fraction of the sample, (IF)=insoluble fraction of the sample. Sample VLLDPE B (O) VLLDPE B (SF) VLLDPE B (IF) LLDPE B (O) LLDPE B (SF) LLDPE B (IF)

Density [kg m-3] 908

923

Weight [mg] 997.1 77.8 913.7 957.2 7.1 950

𝑴𝒏 [kg mol-1] 29 11 32 26 0.6 29

𝑴𝒘 [kg mol-1] 113 53 117 123 19 123

Z 3.9 5.1 3.7 4.8 34.6 4.2

Comparing the results for the three different samples (O, SF and IF) for VLLDPE B, one can clearly see that the soluble fraction of PE has a lower molecular weight than the original sample and thus the separation of the soluble fraction from the polymer phase slightly increases the average molecular weight of the insoluble fraction (see Table 7). These GPC results are in agreement with theory, because polymers with lower molecular weight generally tend to dissolve better in liquids.31 The partial dissolution of PE samples with respect to the crystallinity change of the original sample and the partitioning of PEs with different molecular weight among solvent-rich phase, swollen amorphous polymer and crystalline polymer would be suitable to analyse here. But this requires the combination of GPC with another techniques.32 The purpose of GPC analysis of our samples was to have better insight into the dissolution of polyethylene samples in liquid solvent and from the analysis we could see that dissolution of PE in liquid depends on its molecular weight, in contrary to independence of penetrant solubility in PE (with 𝑀𝑤 higher than 50 kg mol-1) as published earlier. 18, 19 Conclusion Initial slope method has been applied in this work to the solubility measurement of polymer/liquid solvent systems and tested on eight PE samples of different densities. Our investigation of the influence of temperature and PE density on the equilibrium sorption of the samples revealed a significant increase in n-hexane solubility in PE with temperature and amorphous phase content. The obtained data were used to calculate the Flory-Huggins interaction parameters (χ) for PE/n-hexane and to evaluate the temperature dependence of χ for n-hexane in four PE samples; such data are particularly useful because they are rarely reported in the literature. We also measured the solubilities of n-pentane and n-octane, diluents that are commonly used in PE manufacturing. As expected, longer hydrocarbon diluents showed slightly higher solubility in 13 ACS Paragon Plus Environment


all PE samples. The partial dissolution of some PE samples in the liquid diluent was observed and then by using GPC analysis we determined lower molecular weight for the soluble PE part than for the insoluble part. The data presented here provide the necessary basis for our planned research into co-sorption in the liquid phase and, thereby, represent a step towards the development of a comprehensive database of sorption data for use in industrial PE production. Supporting Information Derivation of time dependence of the sorbed penetrant mass in polymer which is used for extrapolation of experimental desorption data to time zero.

Acknowledgments Authors would like to thank to people from Laboratories of Stereoselective Polymerization leaded by prof. Busico for providing the GPC analyses. Financial support from the Czech Grant Agency (GA-07898S) and Specific University Research (MSMT no. 20/2018) is acknowledged. This work is part of the research programme of DPI, project #804. References (1) Soares, J. B. P.; McKenna, T. F. L., Polyolefin Reaction Engineering. WILEY-VCH Verlag GmbH & Co. KGaA: Weinheim Germany, 2012. (2) Fried, J., Polymer Science and Technology. Pearson Education: 2003; p 608. (3) Chmelař, J.; Matuška, P.; Gregor, T.; Bobák, M.; Fantinel, F.; Kosek, J., Softening of polyethylene powders at reactor conditions. Chem. Eng. J. 2013, 228, 907. (4) Kendrick, J. A. Continous slurry polymerization volatile removal. 2002. (5) Yoon, J.-S.; Yoo, H.-S.; Kang, K.-S., Solubility of α-olefins in linear low density polyethylenes. Eur. Polym. J. 1996, 32, 1333. (6) Lützov, N.; Tihminlioglu, A.; Danner, R. P.; Duda, J. L.; De Haan, A.; Warnier, G.; Zielinski, J. M., Diffusion of toluene and n-heptane in polyethylenes of different crystallinity. Polymer 1999, 40, 2797. (7) Moore, S. J.; Wanke, S. E., Solubility of ethylene, 1-butene and 1-hexene in polyethylenes. Chem. Eng. Sci. 2001, 56, 4121. (8) Novak, A.; Bobak, M.; Kosek, J.; Banaszak, B. J.; Lo, D.; Widya, T.; Harmon Ray, W.; de Pablo, J. J., Ethylene and 1-hexene sorption in LLDPE under typical gas-phase reactor conditions: Experiments. J. Appl. Polym. Sci. 2006, 100, 1124.


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Sorption of Liquid Diluents in Polyethylene: Comprehensive

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