Journal of Textile Research ›› 2025, Vol. 46 ›› Issue (10): 1-10.doi: 10.13475/j.fzxb.20241202201

• Fiber Materials •     Next Articles

Simulation of devolatilization and viscosity increase reaction of polyamide 6 falling film for direct melt spinning

CHEN Shichang1,2(), LOU Shunyue1,2, CHEN Wenxing1   

  1. 1. National Engineering Laboratory for Textile Fiber Materials Processing Technology, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, China
    2. Zhejiang Provincial Innovation Center of Advanced Textile Technology, Shaoxing, Zhejiang 312000, China
  • Received:2024-12-10 Revised:2025-04-19 Online:2025-10-15 Published:2025-10-15

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Abstract:

Objective In view of the current situation of long chip spinning production process and high energy consumption in the production of polyamide 6(PA6), a process simulation analysis was conducted on the preparation of low-volatile polyamide 6 melt. By adding a falling film devolatilization reactor with vacuum facility after two-stage polymerization, multiple systems such as cooling granulation, hot water extraction, drying, solid phase adhesion and extrusion melting could be eliminated to realize the direct spinning of polyamide 6 fiber.

Method The outlet data of polyamide 6 two-stage VK tube established in Polymer Plus is taken as the inlet parameter of the falling film devolatilization reactor. The mathematical model established according to reaction kinetics, mass transfer and material balance is essentially a set of partial differential equations. MatLab is used to calculate the devolatilization and viscosity increase process of the falling film devolatilization reactor. When the reactor operation reaches a steady state, the concentration of each component in the reactor no longer changes with the change of time, and the concentration of the main component at the liquid phase outlet of the devolatilizing reactor and related technical indicators are obtained.

Results The model investigated the effects of different melt feed parameters, reaction temperature, pressure and other factors on the number average molecular weight (Mn), extractable content and end group concentration during the viscosity increasing process of falling film reactor. The results showed that high temperature and low pressure improved the performance of the product, but too low pressure caused costs increase for industrial production, making it more difficult to control. The by-products in high-temperature products tended to increase, which is not conducive to subsequent spinning. The results suggested that when the relative viscosity (ηr) of inlet melt was increased, the ηr of polymer increased slowly with the increase of reactor length, and the contents of monomer caprolactam (mCPL) and oligomer (CO) decreased gradually. When the reaction temperature of the falling film reactor was increased, the Mn of polyamide 6 melt gradually increased, while mCPL and CO gradually decreased. When the pressure of the reactor was reduced, the Mn gradually increased, and the mCPL and CO gradually decreased. Therefore, the process parameters of 260 ℃ and 200 Pa were selected to obtain a high viscosity polymer with a Mn of 21 709 g/mol, monomer content (mCPL) of 0.230%, and oligomer content (CO) of 0.115%.

Conclusion When the ηr of inlet material increases, the ηr of melt decreases with the increase of falling film flow distance, and the mCPL and CO decrease gradually. After the melt with a ηr of 2.35 passes through the falling film devolatilization reactor, its ηr can be increased to 3.1. The effects of reaction temperature, pressure and other conditions on polymer number, Mn, ηr, mCPL, CO and end group concentration in falling film devolatilization reactor were studied. With the increase of reaction temperature, the Mn of polymers in a certain range increased linearly, the mCPL decreased from 7.062% to 0.230%, the CO decreased from 0.710% to 0.115%, and the end group concentration([NH2] and [COOH]) also decreased gradually. By controlling the pressure of the falling film devolatilization reactor, it is shown that the relative viscosity of the polymer is improved at low pressure. Reduce the mCPL and CO in melt. By adjusting and controlling the inlet melt parameters of the falling film devolatilization reactor, high quality PA6 melt can be obtained under suitable devolatilization reaction temperature and vacuum degree. The research results are conducive to the development of direct spinning technology of PA6 melt.

Key words: polyamide 6, relative viscosity, falling film reactor, devolatilization, process simulation, direct melt spinning

CLC Number: 

  • TQ342.1

Fig.1

Production process of polyamide 6 fiber"

Tab.1

Simulation results of polyamide 6 two-stage polymerization"

参数 目标值 模拟值
数均分子量Mn 12 000~15 000 g/mol 14 538 g/mol
相对黏度ηr 2.100~2.400 2.350
端胺基初始浓度[NH2] — 45.634 mmol/kg
端羧基初始浓度[COOH] — 62.228 mmol/kg
水初始浓度[H2O] — 60.088 mmol/kg
己内酰胺质量分数mCPL 7.000%~9.000% 7.060%
低聚物O质量分数CO 0.080%~2.000% 0.710%

Fig.2

Plug flow model of falling film reactor flow"

Tab.2

Mechanism of devolatilization of polyamide 6"

反应编号 反应方程式
S1 $\mathrm{P}_{1}+\mathrm{P}_{1} \underset{k_{1} / K_{1}}{\stackrel{k_{1}}{\rightleftharpoons}} \mathrm{COOH}-\mathrm{NH}_{2}+\mathrm{H}_{2} \mathrm{O}$
S2 $\mathrm{NH}_{2}+\mathrm{COOH} \underset{k_{1} / K_{1}}{\stackrel{k_{1}}{\rightleftharpoons}} \mathrm{~B} \_\mathrm{ACA}-\mathrm{B} \_\mathrm{ACA}+\mathrm{H}_{2} \mathrm{O}$
S3 $\mathrm{P}_{1}+\mathrm{COOH} \underset{k_{1} / K_{1}}{\stackrel{k_{1}}{\rightleftharpoons}} \mathrm{COOH}-\mathrm{B} \_\mathrm{ACA}+\mathrm{H}_{2} \mathrm{O}$
S4 $\mathrm{P}_{1}+\mathrm{NH}_{2} \underset{\overline{k_{1} / K_{1}}}{\stackrel{k_{1}}{\rightleftharpoons}} \mathrm{NH}_{2}-\mathrm{B} \_\mathrm{ACA}+\mathrm{H}_{2} \mathrm{O}$
S5 $\mathrm{CPL}+\mathrm{H}_{2} \mathrm{O} \underset{k_{2} / K_{2}}{\stackrel{k_{2}}{\rightleftharpoons}} \mathrm{P}_{1}$
S6 $\mathrm{P}_{1}+\mathrm{CPL} \underset{k_{3} / K_{3}}{\stackrel{k_{3}}{\rightleftharpoons}} \mathrm{NH}_{2}-\mathrm{COOH}$
S7 $\mathrm{NH}_{2}+\mathrm{CPL} \underset{k_{3} / K_{3}}{\stackrel{k_{3}}{\rightleftharpoons}} \mathrm{NH}_{2}-\mathrm{B} \_\mathrm{ACA}$
S8 $\mathrm{O}+\mathrm{H}_{2} \mathrm{O} \underset{\overline{k_{4} / K_{4}}}{\stackrel{k_{4}}{\rightleftharpoons}} \mathrm{COOH}-\mathrm{NH}_{2}$
S9 $\mathrm{O}+\mathrm{P}_{1} \underset{k_{5} / K_{5}}{\stackrel{k_{5}}{\rightleftharpoons}} \mathrm{COOH}-\mathrm{B} \_\mathrm{ACA}-\mathrm{NH}_{2}$
S10 $\mathrm{O}+\mathrm{NH}_{2} \underset{k_{5} / K_{5}}{\stackrel{k_{5}}{\rightleftharpoons}} \mathrm{~B} \_\mathrm{ACA}-\mathrm{B} \_\mathrm{ACA}-\mathrm{NH}_{2}$

Fig.3

Distribution of components outside tube of falling film devolatilization reactor. (a)ηr and Mn; (b)[NH2] and [COOH]; (c)QCPL and Q H 2 O"

Fig.4

Variation of melt parameters along reactor axis at different feed initial relative viscosities"

Fig.5

Variation of Mn along reactor axis at different temperatures"

Fig.6

Component concentration along reactor axis at different temperatures"

Fig.7

Variation of Mn along reactor axis at different pressure"

Fig.8

Component concentration along reactor axis at different pressure"

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