Crystallization Elution Fractionation Research Articles

Quantifying the content of various types of polypropylene in high density polyethylene blends

To enhance the effectiveness of mechanical plastic recycling, it is best to separate different types of plastic during collection and recycling. This ensures the integrity and quality of recycled materials. For instance, the presence of polypropylene (PP) in recycled polyethylene (PE) can result in inconsistent and undesirable material, impacting the quality and performance of the recycled PE. This underscores the need for an analytical technique to accurately detect the presence of PP in recycled PE materials. In this study, we propose a quantitative analysis using a solution-based crystallization elution fractionation (CEF) technique to assess the polypropylene (PP) content in high-density polyethylene (HDPE) blends. The proposed methodology inherently distinguishes between the semi-crystalline PP matrix, including Homo-PP and Random copolymer PP, as well as the non-crystalline copolymer PP, and then quantifies each segment. A series of commercially available Ziegler–Natta catalyzed polymers were used to obtain the calibration curves per different type of PP materials, i.e., Homo-PP and Random copolymer PP. These calibration curves were then utilized to quantify Homo PP and copolymer PP (either semi-crystalline ethylene-propylene or amorphous ethylene-propylene) in different virgin polymer blends. The proposed methodology provides a comprehensive approach to characterizing the polypropylene content within polyethylene systems, especially in the context of qualifying polyolefin recyclate or polymer blends.

Effect of solvent type on chemical composition distribution analysis of polyolefins using crystallization elution fractionation (CEF)

For microstructure characterization of polyolefin by separation-based techniques with infrared detector, either 1,2-dichlorobenzene (o-DCB) or 1,2,4-trichlorobenzene (TCB) is commonly selected as a dissolution solvent and mobile phase. Herein, we studied the effects of o-DCB and TCB on the comonomer detection and chemical composition distribution (CCD) by crystallization elution fractionation (CEF) with a filter-based IR detector. We first compared the detector sensitivity on methyl group content or short chain branching (SCB) in polyethylene-co-butene with varied sample concentrations in o-DCB and TCB. TCB demonstrated superior comonomer detection in the IR detector with a single linear calibration across a wide range of sample concentrations. In contrast, o-DCB displayed stronger concentration dependent methyl group response, requiring a more complex equation in the calibration. Furthermore, o-DCB provided better separation of comonomer distribution with less peak broadening and co-crystallization effects, as observed with various polyolefins, including high density polyethylene (HDPE), linear low-density polyethylene (LLDPE), polypropylene (PP), and ethylene-propylene copolymer (EP).

Influence of the longest ethylene and isotactic propylene sequences on crystallization elution fractionation of ethylene/propylene copolymers

AbstractCrystallization elution fractionation (CEF) is the newest crystallization‐based technique for estimating the chemical composition distribution of ethylene/1‐olefin copolymers. Understanding the separation mechanism of CEF for ethylene/propylene copolymers over their full compositional range is challenging because the crystallizabilities of the copolymer chains depend on the longest ethylene sequence and on longest isotactic propylene sequence. We developed a mathematical model to describe the CEF mechanism for ethylene/propylene copolymers over the entire compositional range using population balances for the crystallization and dissolution stages. The joint distribution of longest ethylene and isotactic propylene sequences determines how the copolymer populations crystallize and dissolve. The model was validated with experimental CEF profiles of ethylene/propylene copolymers varying from pure ethylene to propylene homopolymers.

Composition analysis of post‐consumer recycled blends of linear low‐ and low‐density polyethylene using a solution‐based technique

AbstractBlends of linear low‐density polyethylene (LLDPE) and low‐density polyethylene (LDPE) are commonly used in film applications, including those made with post‐consumer recycled (PCR) film resins. The ratio of LLDPE/LDPE in PCR resins can vary based on the complexity of the waste stream, making it essential to determine their composition before incorporating them into virgin resins through mechanical recycling. In this study, we propose a quantitative analysis using a solution‐based crystallization elution fractionation (CEF) technique to determine the composition of PCR LLDPE/LDPE blends. The results were compared with those obtained by a standard differential scanning calorimetry (DSC) technique. A series of Ziegler–Natta catalyzed LLDPE resins were blended with LDPE to establish the CEF calibration curves. The CEF calibration curves were found to be similar for 1‐butene LLDPE/LDPE and 1‐hexene LLDPE/LDPE blends. Fourier transform infrared spectroscopy and nuclear magnetic resonance were used as needed to determine the type of comonomer in the LLDPE component for the DSC methodology, which varies the calibration curve. We found that the CEF and DSC techniques provide comparable results in terms of LDPE content for neat polymer blends, depending on the density of each component. However, the proposed CEF technique provides more accurate and reliable results than previously published techniques in estimating LDPE content in PCR LLDPE/LDPE blends that may contain inorganic compounds.

Quantification of Chemical Composition Distribution of Polyolefin Materials for Improved Accuracy and Speed

Crystallization-based separation techniques are widely used for chemical composition distribution (CCD, also often referred as short chain branching distribution SCBD) measurement in the polyolefin industry, in which CCD is among the most critical structural parameters to define their properties and applications. The desire to improve the sample throughput rate by shortening the analysis time in the current CCD techniques would normally sacrifice measurement accuracy due to the cocrystallization phenomena, in which the polymer chains with similar molecular architectures will cocrystallize or coelute enigmatically in the CCD analysis. Herein, we reported the investigation by exploring various separation packing materials on the cocrystallization phenomena. In comparison with the commercially available Crystallization Elution Fractionation (CEF) technique, an improved technique for CCD measurement (iCCD) was developed using the columns packed with gold-coated nickel substrate or spherical gold particles with or without the dynamic cooling process. With iCCD, it was observed with equivalent cocrystallization degree to the industrial gold standard benchmark Wild-TREF method but with over 70+ times improved sample throughput rate (62 vs 4500 min). The experimental CCD chromatogram for a polyolefin blend similarly assembled to the mathematically constructed result further demonstrated the minimized cocrystallization and measurement accuracy in the iCCD method. In addition, the superior mechanical properties of gold-coated nickel particles enabled the long-term robustness and enhanced measurement precision of the iCCD technique during the repeated thermal treatment cycles and pressure variations.

Experimental study on the effect of molecular weight and chemical composition distribution on the mechanical response of high‐density polyethylene

AbstractThis article aims to appraise the effect of microstructure comprising molecular weight distribution and chemical composition distribution on the mechanical properties of high‐density polyethylene (HDPE). HDPE resins were synthesized using several titanium–magnesium‐supported Ziegler–Natta catalysts in the industrial gas phase reactor under the same polymerization condition. Gel permeation chromatography and crystallization elution fractionation (CEF) were conducted on the resins to characterize the molecular weight and comonomer distribution. Crystallization, thermal and rheological behavior were evaluated following differential scanning calorimetry, polarization light microscopy, and rheometric mechanical spectrometry. The resins with higher soluble fraction in trichlorobenzene below 80°C (highly branched low molecular weight chains) exhibited longer crystallization time based on the crystallization kinetic obtained from the Avrami model. Rheological determination of the molecular weight between entanglements (Me) and the average lamella thickness based on the Gibbs–Thomson equation revealed that the entanglement density and impact strength decreased, and the average lamella thickness increased with an increase in the ratio of CEF eluted fraction below 80°C to the crystallizable fraction in the range of 80–90°C.

Determination of Short Chain Branching in LLDPE by Rheology

AbstractThe relationship between the structure and the final properties of materials is an important issue in polymer science. In particular, the properties of polyolefins are impacted by their branching contents. A simple rheological method is presented to estimate the degree of short chain branching (SCB) in linear low‐density polyethylene (LLDPE). A range of ethylene‐1‐hexene copolymers is synthetized using a metallocene catalyst to obtain homogenous LLDPE models with various comonomer contents. The polymers are analyzed by size exclusion chromatography, 13C NMR, and crystallization elution fractionation to determine both their molar masses and their chemical composition distributions. Subsequently, they are measured by rheology to assess the impact of SCB. This work provides calibration curves for rheology and proves that it is possible to predict the SCB content in LLDPE from the melt viscosity analysis.

Development of a robust calibration method for improving the long-term precision of polyethylene comonomer content distribution using crystallization elution fractionation

Comonomer content distribution (CCD), also commonly known as chemical composition distribution (CCD) and short chain branching distribution (SCBD), describes the variation of short chain branching composition between individual polymer chains in polyolefin materials. It is of particular importance for controlling polyolefin performance. Crystallization-based separation methods have evolved over the past four decades aiming at resolution, speed, precision, and accuracy. Two of the commonly used techniques are Crystallization Elution Fractionation (CEF) and Temperature Rising Elution Fractionation (TREF), where polymer chains are physically separated along the column or on the surface of the support based on their crystallinity, respectively. CEF analysis takes much less time than TREF. There is a critical need for precise temperature calibrations for data repeatability. This report demonstrates a novel CEF methodology using a two-point technique to consistently and conveniently calibrate the comonomer composition and column temperature. This column temperature calibration methodology was adopted in a study by tracking the reproducibility over a period of 8 years, using multiple instruments located in different laboratories and in different geographies. The results exhibited superior repeatability, with less than 0.3% of the relative error calculated from 3000 data points of the eluting peak temperature, thus demonstrating this as a robust method for industrial labs that require good quality controls.

Analyses of Short Chain Branches in Polyolefins with Improved 1H NMR Spectroscopy.

Polyolefin microstructures, for example, short chain branching (SCB) and short chain branch distribution (SCBD), have a direct impact on properties and thus ultimately influence end-use applications. The 1H NMR approach to analyze SCB and SCBD is particularly useful when only a limited amount of sample is available, for example, polyolefin film layers or the fractions from polyolefin separation techniques, such as gel permeation chromatography (GPC), crystallization elution fractionation (CEF), high temperature liquid chromatography (HTLC), and thermal gradient interaction chromatography (TGIC). In this paper, we discuss the best approach to find a good decoupling frequency and propose an improved 1H pulse sequence with homonuclear decoupling for better measuring SCB. With this new pulse it is possible to reach a S/N of 10 (level of quantification) for the methyl signal from SCB in an ethylene-hexene copolymer (EH, 3.6 mol % H) in 3.5 min with 0.5 μg of sample. We also show an easy method to calculate SCB/1000C and demonstrate the proper use of heteronuclear single quantum coherence (HSQC) to measure SCB in a complicated system. A very quick approach to examine the presence of a small amount of LDPE in a polyolefin sample is also suggested, which can reduce NMR acquisition time from a couple of days to a few minutes.

Atypical Multiple Site Behavior of Hafnocene Catalysts in Ethylene/1-Hexene Copolymerization Using Trioctylaluminum and Borate

Solution copolymerizations of ethylene and 1-hexene were performed using the dichloride (HfCl2) and dimethyl (HfMe2) analogues of the bis(n-propylcyclopentadienyl)hafnium precatalyst. Tetrakis(pentafluorophenyl) borate dimethylanilinium salt ([B(C6F5)4]−[Me2NHPh]+) (B) or methylaluminoxane (MAO) was employed as activator, and tri-n-octylaluminum (TOA) was used as scavenger. Crystallization analysis fractionation (CRYSTAF) or crystallization elution fractionation (CEF) was used to measure the chemical composition distributions (CCD). Above a minimum 1-hexene threshold concentration, the CCD profiles of all copolymers were bimodal, and the areas under the peaks depended on the B/HfCl2 and B/Al ratios. Decreasing the TOA concentration reduced the weight fraction of the higher crystallinity copolymer. Copolymerizations using MAO also produced copolymers with bimodal CCDs. Copolymerizations using HfMe2/B without TOA produced copolymers with unimodal CCD. These results suggest that TOA reacts with either the catalyst or the activator, generating a new active site with a lower propensity for incorporating comonomer than the original catalyst system.

Mathematical Modeling of Multiple High Temperature Thermal Gradient Interaction Chromatography (m‐HT‐TGIC) for Ethylene/1‐Olefin Copolymer Blends

High temperature thermal gradient interaction chromatography (HT‐TGIC) have been developed to measure the chemical composition distribution (CCD) of ethylene/1‐olefin copolymers over a wide range of compositions. Multiple high temperature thermal gradient interaction chromatography (m‐HT‐TGIC) is a concept developed to further enhance the physical separation of copolymer components by performing multiple adsorption/desorption cycles, similarly to the operation of multiple crystallization elution fractionation (m‐CEF) developed earlier. In this work, a mathematical model for describing m‐HT‐TGIC is developed based on population balances in multiple non‐equilibrium adsorption/desorption stages for ethylene/1‐olefin copolymer blends. The effects of number of m‐HT‐TGIC cycles, section length, and column length are also reported and discussed.

Mathematical Modeling of Multiple Crystallization Elution Fractionation (m‐CEF) of Polyethylene and Ethylene/1‐Olefin Copolymer

SummaryMultiple crystallization elution fractionation (m‐CEF) is a new approach for operating CEF using multiple cooling/heating cycles to enhance physical separation of polymers with different chain crystallizabilities. In this work, an m‐CEF model was developed based on the concept of population balance with crystallization and dissolution kinetics. The model was found to describe the previously reported trend of experimental m‐CEF profiles well. Effect of m‐CEF operating cycles and column length was also examined.

Mathematical Modeling of Crystallization Elution Fractionation of Ethylene/1‐Octene Copolymers

Crystallization elution fractionation (CEF) is a relatively new polyolefin characterization technique used to estimate the chemical composition distribution (CCD) of semicrystalline copolymers. CEF is developed to enhance the resolution and reduce the analysis time of temperature rising elution fractionation (TREF) by separating polymer samples in both the crystallization and elution steps. A model based on the concept of population balance, crystallization/dissolution kinetics, and dispersion model is developed to understand the CEF fractionation mechanisms. The proposed CEF model is found to describe well the experimental CEF profiles of a series of ethylene/1‐octene copolymers with different comonomer contents. image

Effect of Solvent Type on High‐Temperature Thermal Gradient Interaction Chromatography of Polyethylene and Ethylene–1‐Octene Copolymers

The effects of the solvent type and operation conditions on the high‐temperature thermal gradient interaction chromatography (HT‐TGIC) of ethylene homopolymers, ethylene–1‐octene copolymers, and their blends are investigated. While the HT‐TGIC profiles of single polymers measured with 1,2,4‐trichlorobenzene (TCB) and chloronaphthalene (CN) are similar, they are always narrower when o‐dichlorobenzene (ODCB) is used, particularly for samples with lower 1‐octene fractions. Significant differences between the experimental and the calculated profiles of binary blends are observed with all three solvents, but better peak separation is seen when the ODCB is used. Having higher fractions of a 1‐octene‐poor component in the blend causes a more significant distortion of the shape expected for the peak from the component with the higher 1‐octene fraction. The effect of the molecular weight on HT‐TGIC profiles is also studied using samples with the same comonomer content and different molecular weights. Samples with low molecular weight have broader distributions and significant lower‐temperature tails, particularly when TCB is used. Chain crystallization after adsorption effects may also play a minor role for low‐comonomer samples. Finally, HT‐TGIC profiles are compared with their equivalent crystallization elution fractionation (CEF) profiles. The HT‐TGIC curves are broader than the equivalent CEF profiles, but these differences decrease as the comonomer content increases. image

Chemical Composition Separation of EP Copolymers by CEF and HT‐SGIC: Crystallization versus Adsorption

Preparative temperature rising elution fractionation (prep TREF) suffers from co‐crystallization effects and, therefore, cannot provide reliable chemical composition distribution (CCD) information. This limitation can be overcome when prep TREF is combined with further fractionation methods such as crystallization elution fractionation (CEF) or high‐temperature solvent‐gradient‐interaction chromatography (HT‐SGIC) as a new approach. By CEF, significant amounts of (co‐crystallizing) amorphous ethylene‐propylene (EP) copolymer are identified in semicrystalline TREF fractions of a heterophasic ethylene/propylene copolymer (HEPC). Complete compositional fractionation with no influence of crystallization effects is accomplished by HT‐SGIC. Prep TREF–HT‐SGIC is found to be the most selective and suitable method for the fast and complete CCD analysis of such complex EP copolymers with CEF providing complementary information. image

Heterogeneous Ethylene and Alpha‐Olefin Copolymerization Using Zirconocene Aluminohydride Complexes

AbstractIn this work we studied the copolymerization of ethylene and different alpha‐olefins (propylene, 1‐butene, 1‐hexene, and 1‐octene) using aluminohydride zirconocene systems supported on modified silica. The copolymerization reactions were carried out in hexanes, at different Al/Zr ratios, using two different comonomer concentrations. The effect of comonomer type on catalyst activity and polymer molecular weight and chemical composition distributions (MWD and CCD) was compared. Average comonomer incorporation was determined by 13C NMR and the CCD by crystallization elution fractionation (CEF). Most of the copolymerizations showed very high catalytic activity (up to 4000 Kg PE/mol Zr), and the molecular weight was higher for the copolymers containing 1‐octene, demonstrating that aluminohydride zirconocene systems can copolymerize ethylene and alpha‐olefins effectively.

Crystallization Elution Fractionation of LLDPEs Made with Metallocene Catalysts

AbstractSummary: Temperature rising elution fractionation (TREF) and crystallization analysis fractionation (CRYSTAF) fractionate semicrystalline polymers according to their crystallizabilities from dilute solution and have been widely used to measure the CCD of LLDPE. A new fractionation technique, known as crystallization elution fractionation (CEF), has been developed recently. The main difference between CEF and TREF and CRYSTAF is that the crystallization cycle in CEF is performed dynamically under solvent flow in a long column that contains an inert support material. In this paper, several metallocene‐LLDPE resins have been analyzed by CEF to investigate the effect of cooling cycle parameters, comonomer fraction, polymer molecular weight, and blend cocrystallization on the fractionation. This new technique can be used to obtain CCDs with better resolution and in shorter times than TREF and CRYSTAF.

Crystallization Elution Fractionation and Thermal Gradient Interaction Chromatography. Techniques Comparison

AbstractSummary: The chemical composition distribution has been shown to be the most critical and discriminating parameter in understanding the performance of industrial polyolefins with non homogeneous comonomer incorporation. The chemical composition distribution is being analyzed by well known techniques such as temperature rising elution fractionation, TREF, crystallization analysis fractionation, CRYSTAF and crystallization elution fractionation, CEF. These techniques separate according to crystallizability and provide a powerful and predictable separation of components based on the presence of branches, irregularities or tacticity differences, independently of the molar mass. TREF, CRYSTAF and CEF can not be used, however, for the separation of more amorphous resins, and may not always provide the best solution for complex multi‐component resins due to the existence of some co‐crystallization. The application of high temperature interactive HPLC to polyolefins opened a new route to characterize these types of polymers. The use of solvent gradient HPLC for separation of polyethylene and polypropylene and the developments in HPLC on carbon based columns extended further the application of high temperature HPLC in polyolefins. A new approach has been developed recently using the carbon based column but replacing solvent gradient by a thermal gradient which facilitates the analysis of polyethylene copolymers and provides a powerful tool for the analysis of elastomers. Thermal gradient interaction chromatography (TGIC) is being compared with TREF and CEF with the analysis of model samples. The advantages/disadvantages of each technique are being investigated and discussed. The combination of TGIC and TREF/CEF provides an extended range of separation of polyolefins.

Effect of Hydrogen, Electron Donor, and Polymerization Temperature on Poly(propylene) Microstructure

AbstractSummary: Polypropylene made with an industrial heterogeneous Ziegler‐Natta catalyst was characterized using high temperature GPC, 13C NMR, and Crystallization Elution Fractionation (CEF). The effect of hydrogen and electron donor during polymerization on poly(propylene) microstructure was investigated at two temperatures. In addition to the expected electron donor effect on polypropylene tacticity, hydrogen was also found to increase polypropylene tacticity. The effect of changing these polymerization conditions on molecular weight and polydispersity was also investigated. Finally, CEF profiles show how the crystallizability of polypropylene increases by adding hydrogen and electron donor to the reactor.

High Temperature Thermal Gradient Interaction Chromatography (HT‐TGIC) for Microstructure Analysis of Polyolefins

AbstractSummary: High temperature thermal gradient interaction chromatography (HT‐TGIC) is a newly developed technique to analyze comonomer distributions in polyolefins. This paper documents the key differences between crystallization elution fractionation (CEF) and HT‐TGIC, and the advantages of using multiple detectors in HT‐TGIC to provide comprehensive microstructure characterization. A demonstration of the technique using a specifically designed blend, provides comprehensive data in less than 1.5 hours of analysis time by HT‐TGIC. This paper also reports that HT‐TGIC has excellent short term repeatability.