Can Biopolymers Power the Future of Air Filtration?
Passive electrostatic filtration media mainly rely on fossil-based polymeric electret fibers, which have refined through decades of research. However, the increased use of fossil-based materials is contributing to higher carbon emissions, global warming, and climate changes. The growing concern over the environmental impact of non-biodegradable plastics arises from the fact that most end up in landfills or oceans, while only a small portion is ever recycled. So, these pressing environmental issues have intensified a push for renewable energy and sustainable resources. To replace traditional fossil fuel-based insulation materials, biobased, biodegradable, and compostable alternatives must achieve or closely match the electrical, mechanical, and thermal properties of their synthetic counterparts.
Importance of Electrostatic Filtration Media
Electrostatic fibers offer two major benefits:
- Higher efficiency at the same resistance to airflow – They provide superior filtration without increasing pressure drop.
- Enhanced submicron particle capture – Electrostatic forces are particularly effective for particles below 1 micron, as illustrated in Figure 1.
Table 1 summarizes how some important filtration parameters effect key filtration mechanisms, revealing that while a reduction in fiber diameter enhances efficiency across all four mechanisms, a decrease in filtration velocity produces more nuanced effects: diffusion and electrostatic attraction increase, interception remains unchanged, and impaction declines1, 2, 3. Understanding these interactions is critical for optimizing filter design, particularly because their combined effects typically result in a most penetrating particle size (MPPS) between 100–300 nm, representing the most challenging particles to capture efficiently.
Approach
To evaluate the potential of biopolymers for electrostatic fiber applications, their electrical, mechanical, thermal, and structural properties must be examined, as electret performance depends on both intrinsic polymer characteristics and fiber manufacturing conditions (Figure 2).
Key considerations include:
- Dielectric constant (affecting charge storage),
- Dielectric tangent loss/dissipation factor (representing charge retaining)
Additionally, polymer polarity is important in determining how materials interact with electric fields, while volume resistivity measures the material’s resistance to electrical flow. Other factors such as hydrogen bonding, surface energy, and hydrophobicity also play significant roles in determining the material’s adaptability in electrostatic filtration applications.
Dielectric Properties
Dielectric materials are electrical insulators with a permanent dipole moment, allowing them to store energy through polarization. Most polymers function as dielectrics, accumulating charge when polarized.
The dielectric constant (ε) indicates a material’s ability to store electrical energy, defined as the ratio of charge storage in the material to that in air or vacuum. Measured via ASTM D150, it uses parallel
plate capacitance to evaluate energy storage performance. Several factors influence dielectric measurements, including frequency, temperature, and relative humidity. While rigid materials like films facilitate straightforward testing, fibrous materials present measurement challenges due to air interference between fibers4, 5, 6, 7. According to the literature, biopolymers such as PLA, PHA, and PHB exhibit higher dielectric constants than polypropylene (PP).
- The dielectric constant value holds particular importance across multiple applications. Polymers with low dielectric constants are especially valuable for:
- Electrostatic filter media
- High-frequency circuit design
- Power transmission systems
- High-speed network infrastructure
The dissipation factor (tan δ) serves as a critical indicator of energy loss in electrical insulation and capacitor dielectrics, significantly affecting both performance and operational lifespan. Maintaining a low dissipation factor is critical for preserving insulation integrity, as it minimizes undesirable energy dissipation.
Key factors influencing the dissipation factor include:
- Moisture absorption, which elevates the dissipation factor and consequently degrades insulation properties
- Material composition, where higher dielectric constants typically correlate with increased dissipation
- Contaminant removal, as extracting oils or impurities can improve insulation efficiency by reducing energy loss
Understanding and controlling the dissipation factor is crucial for optimizing the charge retention performance of electrostatic articles1, 3, 6. While dissipation factors vary widely, biopolymers such as PLA and PHB typically exhibit higher dissipation factors than polypropylene (PP).
Another key factor to consider is electrical resistivity. Although polymers are generally showing insulating behavior, they can still exhibit a conduction under a DC field. A material with a volume resistivity above 10⁹ Ω·m is considered a good insulator, with resistivity being inversely proportional to the dielectric constant. Even trace impurities can significantly alter electrical resistance. Several factors influence resistivity, including:
- The type and concentration of additives
- Moisture content
- Material degradation due to prolonged use.
Note that while volume resistivity is essential for assessing bulk insulating properties, surface resistivity is expected to play a more critical role in electret fibers8, 9. Various resistance values have been reported in the literature, with PLA and PHB exhibiting values comparable to those of polypropylene (PP).
Structural Properties
Nonpolar plastics exhibit a low dielectric constant and high resistivity, primarily due to their symmetrical molecular structures and minimal polarity. The extent of their polarity depends on the electronegativity differences between constituent atoms. Common examples include:
- PTFE (Polytetrafluoroethylene)
- PE (Polyethylene)
- PP (Polypropylene)
- PS (Polystyrene)
These materials generally demonstrate high resistivity, low dielectric constants, and balanced dipole arrangements (e.g., PTFE’s alternating – CF₂– groups). Polarity is assessed in Debye units (D) via dipole moment calculations. Key factors influencing polarity include:
- Electronegativity differences between atoms
- Molecular geometry (e.g., symmetry)
- Polar bonds (e.g., C=O, O-H)
- Conformational defects (e.g., helical distortions in polymer chains)
In bioplastics, materials with hydrocarbon chains or weakly polar functional groups tend to display lower polarity. For instance:
- PLA (Polylactic Acid) is relatively low in polarity compared to other biopolymers, owing to its hydrocarbon backbone – though its ester groups introduce slight polarity.
- PHA (Polyhydroxyalkanoates) exhibits moderate polarity due to its ester functionalities.
Another property to consider is hydrogen bonding forming between hydroxyl (-OH) groups, such as those in cellulose, and can significantly influence a material’s dielectric behavior. Increased hydrogen bonding enhances polarization, leading to a higher dissipation factor in certain polymers.
Key Examples:
- Polyamide (Nylon 6): Contains amide groups (–NH–(C=O)–), with one amide per six carbon atoms. These groups attract and retain water molecules, resulting in moisture regain and altered dielectric properties.
- Polylactic Acid (PLA): Features polar carbonyl (C=O) groups, where the electronegative oxygen facilitates hydrogen bonding with water, affecting its dielectric response.
- Polyhydroxybutyrate (PHB): Also contains carbonyl groups that enable hydrogen bonding with water, further influencing dielectric performance.
Semi-Crystalline structure
- Crystalline regions have tightly packed polymer chains, restricting water penetration and hydrogen bond formation.
- Amorphous regions are less densely packed, allowing water molecules to interact more freely and form hydrogen bonds.
This crystalline structure plays a critical role in determining a polymer’s overall dielectric behavior, particularly in hygroscopic environments10, 11, 12.
In electret fiber applications, hydrophobicity proved to be particularly advantageous. When water contacts these fibers, potential hydrogen bond formation can enhance electrical conductivity. However, for optimal electrostatic performance, fibers exhibiting:
- Low surface energy
- High hydrophobicity
- Large contact angles
are preferred, as these properties collectively help preserve crucial insulating characteristics. The combination of these attributes makes such materials exceptionally suitable for electrostatic filtration applications where maintaining insulation integrity is critical.
Thermal Properties
For electrostatic fiber applications, two key thermal transitions – glass transition temperature (Tg) and melting temperature (Tm) – define performance limits. Tg marks the onset of mechanical rigidity; below this point, the polymer remains structurally stable with reduced dielectric losses due to restricted molecular mobility. Tm is the upper thermal boundary; exceeding it leads to crystalline collapse, loss of dimensional and dielectric stability, and potential material failure. Operating within these thermal limits is essential to maintain both mechanical and electret performance.
PLA and PHB exhibit distinct thermal properties compared to polypropylene (PP), a common material in electret filtration. PP has a low Tg (~−10 to 0 °C) and a Tm around 160–165 °C, allowing for flexible handling and stable processing. PLA features a higher Tg (~55–65 °C) and Tm (~150–170 °C), offering better heat resistance but increased brittleness. PHB has a Tg of ~5–10 °C and a Tm of ~170–180 °C, with moderate crystallinity but limited thermal processing stability due to degradation near its melting point. These thermal characteristics are key considerations for electret media performance and manufacturability. The careful selection of polymers with appropriate Tg and Tm values ensures reliable filter performance within specified thermal operating windows, balancing mechanical stability with optimal dielectric characteristics8.
Summary and Discussion
This study explores the potential of bio-based polymers – polylactic acid (PLA) and polyhydroxybutyrate (PHB) – as sustainable alternatives to conventional hydrocarbons and fluorocarbons in electrostatic air filtration applications. Despite exhibiting higher dielectric constants and dielectric losses compared to polyolefins, both PLA and PHB demonstrate properties suitable for electret fiber applications. PLA, in particular, presents a favorable balance of thermal, mechanical, and dielectric characteristics, whereas PHB shows promise but requires further enhancement in thermal stability to meet industrial performance standards.
The structural attributes of filtration media – especially polymer density and fiber network geometry – play a critical role in determining electret filter media performance. For a fixed basis weight and fiber diameter, polypropylene (PP), owing to its lower density, offers greater surface area, thereby improving charge storage capacity. When matching solid volume fractions, PP-based media achieve higher thickness, contributing to enhanced dust holding capacity. Furthermore, if surface areas are equalized at the same basis weight, PLA’s higher density and finer effective fiber diameter result in smaller pore size, leading to increased airflow resistance. These structural differences continue to position PP as advantageous in terms of filter media effectiveness.
A key finding of this investigation is that electret behavior is highly sensitive to the charging method. Fibers optimized for one charging mechanism may exhibit suboptimal performance when subjected to another, and the effectiveness of charge-enhancing additives varies accordingly. This highlights the necessity for further research into the relationships between polymer chemistry, processing conditions, and electret charging mechanisms – ideally supported by advanced simulations and modeling.
In conclusion, while fluorinated and hydrocarbon-based polymers currently dominate the electret filtration space due to their superior dielectric performance, PLA and PHB are the most viable bio-based alternatives identified to date. With appropriate extrusion process control, PLA offers a near-term sustainable option for electret media. PHB, although promising, requires additional material engineering to improve its thermal properties for broader applicability. These findings contribute to the ongoing development of environmentally responsible filtration technologies without sacrificing functional performance.
References:
- V. J. G.-L. O. &. H. L. Hegde, “Overview on thermal and electrical properties of biodegradable polymers,” in IEEE 11th International Conference on the Properties and Applications of Dielectric Materials (ICPADM), 2015.
- T. D and A. B.-M. N. &. A.-C. J. C. Charvet, Aerosol Filtration, Elsevier, 2016.
- 25 8 2024. [Online]. Available: https://members.tm.net/lapointe/Plastics.htm.
- H. S. Nalwa, Handbook of Low and High Dielectric Constant Materials and Their applications, Academic press, 1999.
- V. Hedge, Dielectric Study of Biodegradable and/or Bio-based Polymeric Materials, Universite Grenoble Alpes, 2017.
- J. Hearle and W. Morton, Physical Properties of Textile Fibres, UK: Elsevier Science, 2008.
- D. V. Krevelen and K. T. Nijenhuis, Properties of Polymers: Their Correlation with Chemical Structure; their numerical estimation and prediction from additive group contributions, Elsevier, 2009.
- L. Warnakulasooriya, Bio-polymeric materials for electrical insulation applications, Canada: University of Manitoba, 2022.
- J. F. &. A. W. Shackelford, CRC materials science and engineering handbook, CRC press, 2000.
- 3M, 8 8 2024. [Online]. Available: https://www.3m.com/3M/en_US/bonding-and-assembly-us/resources/science-of-adhesion/categorizing-surface-energy/# .
- M. A. C. A. M. G. A. H. K. M. M. &. V. H. Barletta, “Poly (butylene succinate) (PBS): Materials, processing, and industrial applications,” Progress in Polymer Science, no. 132, p. 101579.
- X. a. M. K. Rong, ”A study of PLA printability with flexography,” in 9th Annual Technical Association of Graphic Arts Technical Conference Proceedings, Pittsburgh, 2007.
