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Views: 39 Author: Yinsu Flame Retardant Publish Time: 2026-10-13 Origin: www.flameretardantys.com
Why Is It Difficult to Make PET Film Flame-Retardant? — An In-Depth Analysis from Molecular Structure to Industrial Applications
PET inherently possesses a certain degree of heat resistance and mechanical strength, making it widely used in packaging, electronics, electrical insulation, protective films, and other fields. However, when applied to electronic and electrical appliances, architectural interiors, automotive interiors, and specialized protective films, the flame-retardant properties of standard PET film are often insufficient.
I. PET's “Flammability Gene”
The molecular chain of PET is formed by the polycondensation of terephthalic acid and ethylene glycol, and its main chain contains a large number of ester bonds (-COO-) and methylene groups (-CH₂-). Ester bonds are prone to thermal decomposition at high temperatures, while methylene groups provide an abundant supply of hydrocarbon fuel. In terms of elemental composition, PET contains as much as 62.5% carbon and 4.2% hydrogen; this high carbon-to-hydrogen ratio means that a large amount of heat is released during combustion. More critically, PET exhibits severe melt-dripping during combustion—as the material melts under heat, it drips, not only carrying the flame with it but also igniting combustible materials below, leading to “secondary fires.”
II. The Three Major Technical Bottlenecks in Flame Retardancy
1. Difficulty in Suppressing Heat Release Rate
PET combustion is a typical example of melt combustion. Test data show that the limiting oxygen index (LOI) of pure PET is only 20%–22%, far below the requirement of 26% or higher for flame-retardant materials. In cone calorimeter testing, the peak heat release rate (pHRR) of pure PET can reach as high as 800–1,200 kW/m², meaning that once ignited, the fire will spread rapidly. Flame retardants must take effect before the material decomposes, but PET’s processing temperature typically ranges from 270–290°C, while its decomposition temperature is only 30–50°C higher than the processing temperature. This places extremely high demands on the heat resistance of flame retardants.
2. The Conflict Between Melt Dripping and Charring
An ideal flame-retardant PET should be able to both suppress melt dripping and promote surface charring. However, these two objectives are often contradictory:
Modification to prevent melt dripping: Polytetrafluoroethylene (PTFE) or silicone resins are typically added to suppress dripping by increasing melt viscosity. However, these substances are hydrocarbons themselves and may actually intensify combustion.
Char Formation Promotion: Phosphorus- or nitrogen-based flame retardants must be introduced to form a dense char layer on the material’s surface. However, PET itself has an extremely low char formation rate (<5%), and the char layer has poor adhesion to the matrix, making it prone to cracking during melt flow.
The industry has long faced the dilemma that “adding anti-dripping agents reduces flame retardancy, while adding flame retardants leads to more severe dripping.”
3. The Trade-off Between Transparency and Flame Retardancy
One of the major advantages of PET film is its high transparency (visible light transmittance > 85%). However, most flame retardants—whether traditional brominated, phosphorus-based, or inorganic hydroxides—form scattering centers or chromophores in the matrix. For example, adding 10% aluminum hydroxide reduces the film’s transparency to below 40%; meanwhile, phosphorus-based flame retardants tend to cause PET to yellow at high temperatures. For optical-grade PET film, flame retardant modification almost inevitably means sacrificing this core property.
III. Mainstream Flame-Retardant Technologies and Their Limitations
Brominated Flame Retardants: Highly Effective but Limited by Environmental Concerns
Traditional brominated flame retardants, such as decabromodiphenylethane, are extremely effective for PET; an addition of just 8%–12% is sufficient to achieve UL-94 V-0 rating. However, the combustion of brominated compounds produces toxic substances such as dioxins, and their use is restricted under the EU RoHS Directive. Furthermore, brominated flame retardants have poor compatibility with the PET matrix, which can easily lead to film embrittlement.
Phosphorus-Based Flame Retardants: Currently the Mainstream Choice but Prone to Hydrolysis
Phosphate esters (such as BDP and RDP) are currently the preferred choice for PET flame retardancy, functioning through a dual mechanism involving both the gas phase and the condensed phase. However, phosphorus-based flame retardants suffer from poor hydrolytic stability—when PET films are used in humid environments, these flame retardants slowly hydrolyze, leading to a decline in flame-retardant performance; simultaneously, the acidic substances produced catalyze the degradation of PET.
Nano-composite technology: Promising but costly
Layered silicates (such as montmorillonite) or graphene nanoplates can create a “maze effect” within PET, slowing down heat and gas transfer. However, nanomaterials are difficult to disperse in the PET matrix, and when added at levels exceeding 3%, they cause a sharp increase in melt viscosity, which affects film processing. Currently, the cost of nano-reinforced PET is 2–3 times that of virgin material, limiting its large-scale application.
IV. Areas for Breakthroughs: Reactive Flame Retardants and Synergistic Systems
Faced with the challenges described above, the industry is transitioning from “physical additives” to “chemical modification.” Reactive phosphorus-based flame retardants (such as phosphorus-containing diols) can directly participate in the polycondensation reaction of PET, embedding flame-retardant elements into the molecular chain. This approach not only solves the problem of migration and leaching but also achieves V-0-level flame retardancy while maintaining transparency.
Another trend is synergistic flame-retardant systems. For example, phosphorus-nitrogen synergy can function simultaneously in both the vapor phase and the condensed phase; phosphorus-silicon synergy can both promote char formation and enhance the thermal stability of the char layer. Recent research indicates that microencapsulation technology—which encapsulates flame retardants within a polymer shell—can achieve “smart release”: the flame-retardant components are released only under the high temperatures of a fire, without affecting the material’s performance under normal conditions.
V. Conclusion
The challenge of flame retardancy in PET films is, at its core, a game of multi-objective optimization: it must suppress combustion while maintaining transparency; prevent melt dripping while preserving mechanical properties; and provide highly effective flame retardancy while remaining environmentally friendly and non-toxic. With the advancement of reactive flame retardants and nano-synergistic technologies, we can look forward to seeing safe, high-performance flame-retardant PET films widely used in new energy vehicles, 5G equipment, and the aerospace sector.
