Improvement of color value of bio-based polyamide 56 fibers

2.1 Materials

Bio-based adipic acid and bio-based pentamethylene diamine were purchased from DuPont (Wilmington, DE, USA). Fluorescent whitening agent OB-1 was purchased from Shanghai Maigao Chemical Co., Ltd. (Shanghai, China). Purple degrees agent (RSD) and blue degrees agent (Rls) were purchased from Shanghai Hengcai Chemical Industry Co., Ltd. (Shanghai, China). Cobalt acetate was purchased from Nanjing Anyang Chemical Industry Co., Ltd. (Nanjing, China). PA56 and PTT (polytrimethylene-tereph-thalate) was purchased from DuPont.

2.2 Synthesis and spinning of PA56

PA56 was prepared via the in-situ condensation polymerization reaction by firstly using bio-based adipic acid and bio-based pentamethylene diamine as monomers, then adding RSD and R1s, fluorescent whitening agent OB-1 and cobalt acetate as modifiers. The additive amounts of modifiers were 0.1%, 0.15%, 0.20%, 0.25% and 0.30% to the polymer mass. The polymerization temperature was between 290°C and 300°C, polymerization time was 5–6 h, the vacuum degree was −0.10 to −0.03 MPa.

The modified masterbatch was prepared by melt blending with a twin screw based on the mass ratio of 10% modifier, 10% PTT and 80% PA56. All materials were dried before blending. PA56 chips were dried at 165°C for 8 h with nitrogen protection. The prepared masterbatch was also dried in a vacuum oven at 100–110°C for 5–6 h. Then the spinning experiment was carried out in a TMT spinning and drawing machine. The temperatures of the screw are 282°C, 286°C, 288°C, 291°C, 293°C and 296°C, respectively; the screw’s speed was 40–50 rpm, the extrusion process takes 5–8 min, the side cooling blowing was adopted with the air temperature 22–26°C, wind humidity 90–96% and wind speed 0.45 m/s. The spinning speed was 4500 m/min, total drawing ratio was 3 and the hot roller temperature was 160°C.

2.3 Characterization

Intrinsic viscosity: The intrinsic viscosity of PA56 was measured by using an ubbelohde viscometer according to GB/T1632-93 (determination of viscosity number and intrinsic viscosity of polymer dilute solution). A mixing solvent of phenol and cellon (3/2, v/v) was used, and the test temperature was 25°C. The test was repeated at least three times and an average test result was obtained. Standard deviation is controlled at ±0.1 dl/g.

Strength test: The strength of PA56 fiber was tested according to GB/T14344 (test methods for tensile properties of chemical filament yarns). The test was repeated for five specimens and the test result obtained was based on the average. Standard deviation is controlled at ±0.5 cN/dtex.

Linear density test: The linear density of PA56 fiber was tested according to GB/T14343 (test method for linear density of chemical filament yarns). The test was repeated for five specimens and the result obtained was based on the average. Standard deviation is controlled at ±0.5 dtex.

Chromaticity test: The chromaticity of PA56 fiber was tested according to GB/T 17644-2008 (test method for whiteness and chromaticity of textile fibers) by using 6801 colorimeter (BYK Chemie GMBH, Wesel, Germany). The test was repeated on the three specimens and the result obtained was based on the average. The standard deviation was controlled at ±0.5.

3.1 Raw material analysis

Polyamide is susceptible to becoming yellow when exposed to the light or sunshine. As for bio-based PA56, the raw materials including adipic acid and pentanediamine are made by biological fermentation products. Thus the residual lignin impurities are easily oxidized in the air and it produces a chromophore which further deepens the yellow color (3), (8), (10). In addition, as the melting point of bio-based PA56 is about 258°C, the drying temperature and spinning temperature for PA56 are really high. Therefore, the crystallization and drying process as well as the spinning process make this situation worse.

Table 1 shows color values for the biological basis PA56 chip before and after drying. It suggests that before the drying processes, L and YI of PA56 chips are 63.0 and 46.3. After drying at 165°C for 8 h, the YI value increases to 70.3, indicating that the chips turn yellow in this drying process. Meanwhile, the L value reduces and B value increases.

3.2 Modification in the polymerization process

During the polymerization process of bio-based PA56, we added the appropriate amounts of the different modifiers separately, including fluorescent whitening agent OB-1, blue degree agent Rls, purple degree agent RSD and cobalt acetate. Figure 1 shows the effects of different modifiers on L, B and YI of PA56 fibers. The analysis indicates that purple and blue light waves cannot suppress yellow and gray light waves. Therefore, the addition of purple degree agent and blue degree agent has no effect on the color values of the bio-based PA56 fibers. Moreover, it can see that the addition of the cobalt acetate also has no effect on the chromaticity of PA56 fiber.

Figure 1:

Effects of different modifiers on the color values of PA56 fibers.

In addition, the fluorescent whitening agent OB-1 shows a positive effect on the improvement of the color values for PA56 fiber. With the addition of OB-1 gradually increased the L increases gradually. Meanwhile, both B and YI values. In this work, the optimum additive amount of OB-1 is 0.2–0.25%. The fluorescent whitening agent in the fiber matrix emits blue fluorescence by absorbing ultraviolet light, which is complementary to yellow light and plays the role of fluorescent whitening.

The effect of the additive amount of OB-1 on the intrinsic viscosity of PA56 is shown in Figure 2. As the additive amount of OB-1 increased, the intrinsic viscosity of PA56 decreased, implying that the addition of OB-1 hindered the polymerization reaction to some extent. When the additive amount of OB-1 is 0.2%, the intrinsic viscosity of PA56 reduces to 1.32 dl/g. The decrease of molecular weight has a negative effect on the mechanical properties of fibers. As shown in Figure 3, the breaking strength of PA56 fiber reduced with the increase of the additive amount. However, the fiber strength in this work is consistently higher than 3.0 cN/dtex, it can meet the requirement of the finished fabric.

Figure 2:

Effect of the additive amount of OB-1 on the intrinsic viscosity of PA56.

Figure 3:

Effect of the additive amount of OB-1 on the fiber strength.

The above results show that addition of blue degree agent, purple degree agent or cobalt acetate during the polymerization process provides no strongly manifested improvement of the whiteness and color values of PA56 fibers. However, for the fiber containing fluorescent whitening agent OB-1, L value increased to 86, the B value dropped to 6.3 and the YI value decreased to 12.9. The improvement of fiber color is obvious. However, fluorescent whitening agent has a negative influence on the polymerization reaction. The conditions of polymerization reaction such as temperature and reaction time should be further adjusted for obtaining the appropriate molecular weight.

3.3 Melt blending modification in the spinning process

For melt spinning of PA56, we adopted two methods to add the masterbatch. They are: (1) injecting masterbatches through a chip down the pipeline in front of the screw, then melting the mixing masterbatches and PA56 evenly in the screw spinning after the screw; (2) injecting masterbatches into the melt on-line by the addition of a device in by-pass form on the backside of the melting pipeline, leading the molten liquid to flow out the main pipeline and then mixing it with modified master-batches; this is then spun after blending twice using a dynamic mixer and a static mixer. In this paper, we analyzed the fluorescent whitening modification in the spinning process for a bio-based polyamide FDY fiber (77 dtex/48 f) as an example.