1. The process of flame retardancy of polyester fibers
The flame retardancy of polyester fibers is usually divided into two main processes. Firstly, high-performance flame retardants are selected and developed. Then, physical or chemical methods are used to add flame retardants to the fiber spinning raw materials, so that the flame retardants are firmly and evenly distributed in the fibers. Moreover, the introduction of flame retardants has minimal impact on the physical and mechanical properties of the fibers.
2. Methods of fiber modification
There are currently four main methods for modifying flame-retardant fibers:
2.1 Co polymer flame retardant modification
Small molecule flame retardants containing flame retardant elements, mainly phosphorus, halogens (I, Br, Cl, F), sulfur, etc., are used as comonomers to participate in the polymerization process of fiber polymers, allowing the flame retardants to bind to the polymer's macromolecular chains and achieve long-lasting flame retardant effects. Co polymer flame retardants must be suitable for high-temperature polymerization conditions, have good stability, not decompose, and have no side reactions.  
2.2 Blended flame retardant modification
The method of spinning flame-retardant fibers by adding flame retardants to the spinning melt or solution. The flame retardants used include low molecular weight compounds, high molecular weight compounds, inorganic compounds, and their mixtures. For blending with melt, flame retardants are required to withstand high temperature spinning and have good compatibility with the main polymer, without affecting post spinning treatment. They also have no significant impact on the physical and mechanical properties of fibers, and have good flame retardancy and durability.  
2.3 Grafting flame retardant modification
Fiber graft copolymerization is an effective and durable flame retardant modification method, such as using high-energy radiation for graft flame retardant modification.  
2.4 Post flame retardant finishing
The method of modifying fibers or fabrics by immersing them in a solution of water or other solvents containing flame retardants, and then subjecting them to processes such as pressing and drying to give them flame retardant properties.  
3. List the production methods of flame-retardant polyester filament
3.1 Slice drying
The melting point and maximum crystallization temperature of flame-retardant polyester chips are lower than those of ordinary polyester chips, but the apparent viscosity is higher than that of ordinary polyester chips. If ordinary polyester chip drying process is used, flame retardant chips are prone to adhesion, agglomeration, yellowing, and cannot be produced normally. Therefore, the drying process temperature of flame retardant slices should be controlled lower, the duration should be longer, and the air volume should be higher. At the same time, increasing the vibration intensity enables the flame retardant slices to have a good boiling effect on the boiling bed, in order to disperse adhesive particles. Actual process control: pre crystallization temperature of 148 ℃, drying temperature of 155 ℃, air flow rate of 8.5 m3/h, drying time of more than 12 hours, good drying effect of flame retardant slices, viscosity of 0.621, moisture content of 1.8 × 10-5. Attention should also be paid in production: during the initial feeding, the pre crystallization temperature should be controlled to be lower, below 140 ℃. Pay attention to observing the boiling situation of the pre crystallized slices, adjust the reasonable dosage, strictly control the heating rate, and reduce slice agglomeration.
3.2 Spinning temperature
The melting point of flame-retardant chips is lower than that of ordinary polyester chips, so the spinning temperature control should also be lower than that of PET. However, due to the inclusion of flame retardants, the activity of large molecular chains is weakened and the apparent viscosity of the melt is increased, resulting in poor melt flowability. Low spinning temperature can easily subject the spinneret assembly to excessive melt pressure, but excessive temperature can cause rapid thermal degradation of flame retardant polyester with poor heat resistance.
3.3 Cooling Forming
Compared with ordinary fibers, the addition of flame retardants to flame-retardant fibers significantly accelerates the crystallization rate of the fibers. Therefore, it is necessary to strengthen the cooling conditions appropriately, which is beneficial for improving the mechanical properties of flame-retardant fibers. However, the wind speed should not be too high, as excessive wind speed can cause oscillation of the filament and increase the unevenness of the filament.
4、 The development trend of flame-retardant polyester fiber
1. Functional compounding
Functional composite is a new trend in the development of functional fibers today, aimed at expanding the application areas of existing single functional fibers, increasing the added value of products, and enhancing their market competitiveness. Its varieties include flame retardant+cationic, flame retardant+antibacterial, flame retardant+hygroscopic, etc.
2. High tech
Flame retardant polyester/inorganic nanocomposite materials can not only meet the flame retardant levels required in many applications, but also maintain or even improve the original excellent properties of polyester, making this composite flame retardant polyester and fiber have broad and attractive development prospects.
3. Greening
The greening of flame-retardant fibers refers to reducing the toxic effects of the production process on the environment and operators, and preventing the fibers from having adverse effects on the wearer. Because the flame retardants used in flame-retardant fibers generally contain elements such as halogen, phosphorus, and sulfur, which are highly toxic. When a fire occurs, there will be no "secondary toxicity". At present, the production processes of flame retardant fibers that are beneficial to the environment and daily use include skin core composite spinning method and flame retardant microcapsule method.