When It Comes To Flame-retardant PET Filament Spinning, The Biggest Headache Isn’t Failing The Flame-retardant Test—it’s The Filament Breaking.

Engineers who work on flame-retardant PET fibers have all encountered this situation: once the flame retardant is added, the LOI meets the standard and the V-0 rating is achieved, but during the spinning process, there are frequent fiber breaks, the spinnerets become clogged, the filtration pressure skyrockets, and the fiber strength drops significantly. After going through a cycle of modifying the formulation, adjusting the process, and changing raw materials—all while maintaining flame retardancy—spinnability is lost.

Where does the problem lie?

The core conflict in flame-retardant PET filament spinning lies in the “incompatibility” between flame retardants and the spinning process. The spinning process places extremely high demands on material purity, thermal stability, and rheological properties, whereas the addition of flame retardants often compromises spinnability in the following ways:

Poor dispersion leads to clogged spinnerets. If flame retardant particles are not evenly dispersed, they can agglomerate into large clusters, blocking filters and spinnerets during the spinning process. At best, this results in frequent fiber breaks; at worst, the entire production line must be shut down for cleaning.

Insufficient thermal stability leads to gas generation. PET spinning temperatures range from 280 to 290°C. If flame retardants decompose at this temperature and produce gas, bubbles will form in the melt, causing the spun fibers to have voids and reduced strength; in severe cases, the fibers will break immediately.

Particle morphology affects crystallization. The blending process distributes the flame-retardant material in a granular state within the fiber; this granular distribution directly affects the fiber’s crystallinity and uniformity, resulting in reduced fiber strength. This explains why many flame-retardant PET fibers suffer from “excessive flame retardancy at the expense of strength.”

Poor compatibility leads to delamination between the masterbatch and the chips. When blending flame-retardant masterbatch with PET chips, delamination may occur if the components are incompatible, resulting in unstable spinning and an increased fiber breakage rate.

How can this be resolved? Start with formulation design.

Step 1: Select the Right Flame Retardant System

Flame retardants for spinning must meet three criteria: resistance to high-temperature decomposition, good compatibility with PET, and fine particle size. Currently, halogen-free phosphorus-based systems are the mainstream recommendation. Take ZDP as an example—a melt-type organic hypophosphite halogen-free flame retardant with a high thermal decomposition temperature that can melt and disperse at PET processing temperatures, making it less likely to clog spinnerets. Compared to ADP, the zinc ions in ZDP promote the formation of a denser char layer during combustion, which helps improve resistance to melt dripping and the flame retardancy durability of the fibers. Its melt-dispersible nature ensures more uniform dispersion during the spinning process, reducing fiber breakage and increased filtration pressure caused by agglomeration of solid particles. It is suitable for high-temperature processing applications—such as PET spinning and engineering plastics—that require both spinnability and flame retardancy efficiency.

Although traditional brominated solutions (decabromodiphenylethane + antimony trioxide) are highly effective, they may affect color and face environmental regulatory pressures in certain industries.

Step 2: Synergist Selection

The choice of synergists directly affects melt-drip resistance and spinnability. Phosphorus-silicon synergism can significantly improve resistance to melt dripping; phosphorus-nitrogen synergism forms a denser expanded carbon layer when heated; inorganic nano-synergists (such as nano-montmorillonite) catalyze carbonization at high temperatures and utilize their layered structure to form a physical barrier. However, synergists must be specially selected organic-modified varieties; otherwise, they themselves will become the source of dispersion issues.

Step 3: Establishing the Additive System

Dispersants prevent flame retardant agglomeration and improve processing flowability. High-molecular-weight polyester-based superdispersants, polyethylene wax, or EVA are all viable options. Coupling agents enhance the interfacial adhesion between the flame retardant and the PET resin. Antioxidants prevent the thermal-oxidative degradation of PET during high-temperature processing; a blended system of hindered phenols (e.g., 1010) and phosphite esters (e.g., 168) is recommended.

Step 4: Control Addition Levels and Processing Temperatures

During downstream spinning, 5%–10% flame-retardant masterbatch is typically blended with PET chips. If the spinning temperature for pure PET is approximately 280–290°C, it is recommended to lower the temperature by 5–10°C after adding the flame retardant to maintain stable melt viscosity.

Three Key Metrics to Monitor Closely During Production:

Filter Test: Evaluates the content and dispersion of non-meltable substances in the masterbatch through a melt filtration test. A rapid increase in filtration pressure typically indicates poor dispersion or thermal stability. This is the first critical step in determining whether the masterbatch is suitable for the spinning line.

Thermogravimetric Analysis (TGA): Ensures that the masterbatch exhibits an extremely low rate of thermal weight loss at PET spinning processing temperatures. High thermal weight loss indicates that the flame retardant will decompose during processing, directly leading to fiber breakage and voids.

Fiber Strength Retention Rate: The extent to which fiber breaking strength decreases after flame retardant modification is key to determining the success of a formulation. A good formulation should maintain an LOI of ≥30% while keeping the strength retention rate within a reasonable range.

YinSu Flame Retardant Accumulated Solutions in the PET Flame-Retardant Spinning Field

Yinsu Flame Retardants has specialized in flame retardant modification for many years. To address the challenge of fiber breakage in PET spinning applications, we offer support through the following product lines:

WADP-10: A modified organophosphorus flame retardant with a pH close to neutral, a thermal decomposition temperature >350°C, and a weight loss rate ≤1.8% at 350°C. It is suitable for high-temperature PET spinning and exhibits significantly improved dispersion after modification.

ZDP, a melt-type organic hypophosphite flame retardant, melts and disperses at processing temperatures, reducing the risk of particle agglomeration and pore blockage. Zinc ions promote the formation of a dense carbon layer, balancing flame retardancy efficiency with spinnability.

K100 is a nano-organic montmorillonite carbonization synergist with a layer spacing of 3.0 nm. It works synergistically with the primary flame retardant to form a dense, barrier-like carbon layer, improving mechanical and processing properties while helping to balance flame retardancy and spinnability.

For flame-retardant PET filament spinning, flame retardancy and spinnability are not mutually exclusive. By selecting the right flame-retardant system, properly matching synergists and additives, and controlling the processing window, filament breakage issues can be resolved at the formulation stage.

If you need samples or technical specifications, please feel free to contact the YinSu Flame Retardant Technology Team.