By Owais AliReviewed by Lexie CornerSep 18 2024This article highlights the significance of polyolefin characterization in improving polymer performance and sustainability, driven by advanced analytical techniques and recent technological innovations.
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Polyolefins are thermoplastics synthesized from ethylene and propylene, primarily sourced from oil, natural gas, and sugar cane. They constitute about 50 % of plastic consumption in Europe and represent half of the global annual polymer production, exceeding 150 million metric tons. Their widespread use is attributed to their versatility, cost-effectiveness, and ease of polymerization, with ongoing advancements in new grades and applications.
However, these materials can also be recycled into high-value, sustainable polymers through advanced recycling technologies, contributing to a more circular and eco-friendly plastic economy.1
Significance of Polyolefin Characterization
Polyolefins, such as polyethylene and polypropylene, exhibit complex molecular structures that vary depending on their type and synthesis methods.
For example, low-density polyethylene (LDPE) is characterized by a range of short and long chain branches, while linear low-density polyethylene (LLDPE) displays a distribution of chain lengths and comonomer content. These structural variations influence crystallinity and reactivity in depolymerization.
Accurate characterization, including information on average chain length, chain length distribution, branch type, and frequency, is crucial for predicting polymer behavior and improving the reproducibility of upcycling processes.1,2
Techniques Used for Polyolefin Characterization
Chromatographic Techniques
Key factors for polyolefin characterization include molar mass and chemical composition, such as comonomer content and branching. Liquid chromatography (LC) techniques, including high-temperature gel permeation chromatography (HT-GPC) and solvent gradient interaction chromatography (SGIC), are effective for analyzing these metrics.
HT-GPC assesses molar mass distribution by separating molecules based on size, while SGIC evaluates chemical composition distribution using a stationary phase.
Temperature gradient interaction chromatography (TGIC) and simultaneous solvent and temperature gradient chromatography are newer methods for separating polyolefins based on temperature-induced interactions with stationary phases. These techniques offer improved separation efficiency for complex blends and have been enhanced by incorporating dual-column setups with varying temperature zones.
Combining these methods in two-dimensional liquid chromatography (2D-LC) helps understand the interrelationship between molar mass distribution and chemical composition distribution, providing a detailed polymer fingerprint and composition-dependent variations that cannot be observed through GPC alone.3
Differential Scanning Calorimetry
Understanding the relation between thermal behavior and chemical structure is crucial for optimizing processing and reducing production cycle time in complex olefin copolymers.
In this context, differential scanning calorimetry (DSC) is widely used to examine polyolefins' melting and crystallization behavior, providing valuable insights into their chemical structure. However, its full potential is realized when combined with preparative temperature rising elution fractionation (pTREF) to link chemical composition with thermal properties.
Recently, high-performance DSC (HyperDSC) with heating rates up to 500 °C/min has been used to measure small samples and improve the detection of weak transitions, such as cold crystallization and glass transitions.
When combined with size exclusion chromatography (SEC), SEC-HyperDSC provides valuable insights into the relationship between polymer chain structure and thermal properties, offering a more detailed analysis of polyolefin microstructures.1
Crystallization-Based Fractionation Techniques
Crystallization-based fractionation methods are used for characterizing semicrystalline polyolefins with melting temperatures ranging from ambient to over 160 °C, depending on their composition and tacticity.
Key techniques, such as temperature-rising elution fractionation (TREF), crystallization elution fractionation (CEF), and crystallization analysis fractionation (CRYSTAF), utilize differences in crystallization behavior to separate polyolefin fractions.
TREF measures crystallizability by cooling a polyolefin solution and observing the precipitation temperatures of different crystalline phases. Although traditionally a time-consuming process (taking up to 100 hours), recent advancements have reduced its duration to 3-4 hours.
CRYSTAF provides faster results, typically within 100 minutes, while CEF enhances separation efficiency by incorporating dynamic crystallization with the continuous solvent flow, reducing analysis time to under 30 minutes.
These techniques are often combined, such as in TREF-SEC, which integrates SEC for molecular size (molar mass) analysis with TREF for copolymer composition, offering a more comprehensive understanding of polyolefins with complex molecular distributions.1
Equipment Used for Polyolefin Characterization
Recent advancements in polyolefin characterization equipment have significantly enhanced the accuracy and efficiency of polymer analysis.
Polymer Char's HT-HPLC and SGIC 2D
Polymer Char's HT-HPLC instruments enable one-dimensional and two-dimensional separations. These systems are equipped with an evaporative light scattering detector (ELSD) for solvent gradient mode, and it supports the use of refractive index (RI) or infrared (IR) detectors in isocratic mode. This flexibility enhances its utility in precise polymer analysis and fractionation.1
Using a dual-column setup, the SGIC 2D system further refines polyolefin analysis by separating polymers based on chemical composition and molar mass.
It automates the entire process, including sample dissolution, filtration, and injection, while providing a comprehensive two-dimensional distribution of the polymer's characteristics. This instrument is particularly effective for low crystallinity or complex polarity samples, reducing analysis time to hours and minimizing solvent consumption.4
Agilent PL-GPC 220
The Agilent PL-GPC 220 Integrated GPC/SEC System stands out for its wide temperature range of 30 to 220 °C, which allows for the analysis of various polymers in different solvents. Its advanced features include a high-precision isocratic pump, a dual-zone-heated autosampler, and an improved RI detector, all of which contribute to superior reproducibility and sensitivity.
This versatile system integrates additional detectors and techniques such as TREF, FTIR, and ELSD for comprehensive polymer characterization.5
The TA Instruments Q2000
The TA Instruments Q2000 DSC is noted for its high baseline stability, precision, and sensitivity performance. Its Tzero® technology and advanced modulated DSC provide detailed thermal analysis of polymers.
The system can measure wide temperature ranges and conduct quantitative assessments of polymer blends, such as polyethylene and polypropylene, which are essential for understanding and recycling post-consumer plastics.
A recent study used this instrument to develop methods for quantifying polyethylene content in recycled polyolefin blends, demonstrating its effectiveness in analyzing complex polymer mixtures and correlating results with other analytical techniques like cross-fractionation chromatography (CFC) and nuclear magnetic resonance (NMR) spectroscopy.6,7
Recent Advances in Upcycling Polyolefins into High-Performance Sustainable Products
Sustainable Conversion into Liquid Alkanes via Hydrogenolysis
A study published in JACS Au demonstrated that hydrogenolysis of polyethylene using ruthenium nanoparticles offers a sustainable approach to converting plastic waste into valuable liquid alkanes (C7–C45) and methane (CH4) under mild conditions. These liquid alkanes can be used as fuels or chemical feedstocks, while methane can serve as a natural gas component or a raw material for chemicals like methanol.
The proposed method broke C–C bonds at temperatures between 200 and 250 °C, achieving up to 45 % yield of liquid n-alkanes from polyethylene and producing nearly pure methane at higher temperatures. In addition, the Ru/C catalyst displayed high activity and selectivity and was successfully applied to both model and post-consumer polyethylene, including LDPE and plastic bottles.
This process provides a more controlled and energy-efficient alternative to traditional methods, highlighting its potential for developing high-performance, sustainable polymers and advancing waste plastic valorization.8
Deg-Up Strategy for Extracting High-Value Chemicals from Polyolefins
A study published in Proceedings of the National Academy of Sciences proposed a new tandem degradation-upcycling (Deg-Up) strategy to extract high-value chemicals, such as diphenylmethane (DPM) and benzophenone, from polystyrene (PS) waste with high selectivity.
This method involves degrading PS waste to aromatic compounds using ultraviolet (UV) light and then converting these intermediates into DPM using AlCl3 as a catalyst at ambient temperatures and atmospheric pressure. In addition, it uses low-cost, readily available catalysts and creates a self-sustainable process where the degraded intermediates act as solvents.
Laboratory tests produced high-value chemicals like DPM, benzophenone, and 1,2-diphenylethane, which are extensively used in fragrance and pharmaceuticals.9
Conclusion
As polyolefin characterization technologies advance, they will be essential in addressing global plastic waste and environmental sustainability challenges while enabling the development of next-generation polymers with superior properties and performance.
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References and Further Reading
- Pasch, H., Ndiripo, A., Eselem Bungu, PS. (2022). Multidimensional analytical protocols for the fractionation and analysis of complex polyolefins. Journal of Polymer Science. https://doi.org/10.1002/pol.20210236
- Sun, J., Dong, J., Gao, L., Zhao, YQ., Moon, H., Scott, SL. (2024). Catalytic Upcycling of Polyolefins. Chemical Reviews. https://doi.org/10.1021/acs.chemrev.3c00943
- Brüll, R., Arndt, JH. (2021). Modern polyolefin analysis - Developments in the area of liquid chromatography. [Online] Wiley Analytical Science. Available at: https://analyticalscience.wiley.com/content/article-do/modern-polyolefin-analysis
- Polymer Characterization. (2024). SGIC 2D - Comprehensive Two-dimensional Liquid Chromatography at High Temperature. [Online] PolymerCahr. Available at: https://polymerchar.com/products/analytical-instruments/sgic2d
- Saunders, G., MacCreath, B. (2011). Analysis of polyolefins by GPC/SEC. [Online] Agilent Technologies. Available at: https://www.agilent.com/Library/applications/5990-6971EN%20GPC-SEC%20Polyolefins%20app%20compend.pdf
- TA Instruments. (2012). Thermal Analysis. [Online] TA Instruments. Available at: https://www.tainstruments.com/pdf/brochure/2012%20DSC%20Brochure%20r1.pdf
- Scoppio, A., Cavallo, D., Müller, AJ., Tranchida, D. (2022). Temperature-modulated DSC for composition analysis of recycled polyolefin blends. Polymer Testing. https://doi.org/10.1016/j.polymertesting.2022.107656
- Rorrer, JE., Beckham, GT., Román-Leshkov, Y. (2020). Conversion of polyolefin waste to liquid alkanes with Ru-based catalysts under mild conditions. Jacs Au. https://doi.org/10.1021/jacsau.0c00041
- Xu, Z., et al. (2022). Cascade degradation and upcycling of polystyrene waste to high-value chemicals. Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.2203346119
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