Abstract:

Objective The wet coating technology based on the use of waste polyamide fiber is the main means to prepare coated textiles such as trademark webbing, and also an important way to physically recycle polyamide fiber. This technology is characterized by low production cost and excellent product performance. In order to solve the problems associated with calcium carbonate powder in polyamide 6 coating, such as easy accumulation and lack of flame retardant function, calcium carbonate was grafted onto the surface with self-made coupling modifier containing silicon phosphorus flame retardant components to improve its dispersion and facilitate flame retardant function.
Method Calcium carbonate powder was modified by coupling grafting with the reaction product of vinyl trimethylsilane and DOPO as a modifier, which was applied to the preparation of polyamide 6 wet-coated fabric. The effect of calcium carbonate modification degree on its dispersion state in polyamide 6 coating and the surface morphology of the coated fabric was investigated, and the flame retardancy of the coated fabric with different modified degree of calcium carbonate was characterized and compared
Results It was proved by FT-IR and elemental analysis that the coupling modifier was successfully grafted onto the surface of calcium carbonate, and the graft reaction kinetics and hydrophilicity and lipophilicity of the modified calcium carbonate were further studied. The infrared spectrum of calcium carbonate powder before and after the grafting reaction is tested, and the results are shown in Fig. 2. It can be seen that new peaks appear at 1 238 cm^-1 and 765 cm^-1 of the modified calcium carbonate, of which the peak at 765 cm^-1 represents the stretching vibration of Si—O in the coupling modifier molecule, and the peak at 1 238 cm^-1 reflects to the stretching vibration of P=O in the coupling modifier. The results of EDS test of calcium carbonate powder before and after modification are shown in Tab. 1. It can be seen that after the grafting reaction, a small amount of P and Si elements can be detected in the energy spectrum of the cleaned modified calcium carbonate surface. Combined with the infrared spectrum in Fig. 2, it shows that the coupling agent containing silicon and phosphorus is successfully grafted on the surface of calcium carbonate.
The calcium carbonate modified by different dosage of modifier is added to the wet coating slurry to test the limiting oxygen index (LOI) of the polyamide 6 coated fabric. It can be seen that the LOI of the modified calcium carbonate polyamide 6 coated fabric is positively correlated with the mass fraction of the modifier. When the mass fraction of the modifier reaches 40%, the LOI reaches the highest value of 25.1% (Fig. 6). The unmodified calcium carbonate does not have any flame retardancy, while the P and Si components in the modifier can endow calcium carbonate with better flame retardancy. Therefore, with the increase of grafting ratio, the LOI of the modified calcium carbonate polyamide 6 coated fabric will gradually increase, and the flame retardancy will gradually increase. When the grafting ratio is not increased, LOI cannot be further improved. Tab. 2 shows the vertical burning performance of the modified calcium carbonate polyamide 6 coated fabric with different modifier mass fraction. It can be seen that the polyamide 6 coated fabric using unmodified calcium carbonate and low modified calcium carbonate is completely burnt and damaged, with long afterburning time and no smoldering time. 7-8 molten droplets are generated and these molten droplets can ignite the absorbent cotton. When the mass fraction of the modifier exceeds 20%, the vertical damage length gradually decreases, and the afterburning time starts to decrease, with a smoldering time of 2.7 seconds. At this time, the number of melt droplets is still large and the degreased cotton can be ignited, and the flame retardant effect is poor. When the mass fraction of modifier is 40%, the vertical damage length reaches the minimum value of 5.3 cm, the afterburning time is 1.6 s, the smoldering time is 0.7 s, and the number of droplets decreases to 0-1.
Conclusion The effects of calcium carbonate on the morphology and flame retardancy of the polyamide 6 coated fabric before and after modification were compared and characterized. The results showed that when the modification time was 30 h and the mass fraction of the modifier was 40%, the fully modified calcium carbonate with good lipophilicity could be obtained, and the polyamide 6 coated fabric had good flame retardancy with the highest limiting oxygen index of 25.1%, vertical damage length of 5.3 cm, afterburning time of 1.6 s and smoldering time of 0.7 s.

Key words: calcium carbonate, coupling modification, wet coating, polyamide, flame retardant, coated fabric