Abstract:

Objective The improvement in polyamide 6(PA6)-based matrix properties by filling functional nanomaterials and application of PA6-based fiber have attracted much attention due to the recent rapid development in nanotechnology. Among all these filling nanomaterials, carbon black(CB)has been widely used as an ideal filling material due to its relatively small particle size and good compatibility with PA6-based matrix. In order to fabricate dope-dyed polyamide 6 filament with deep blackness, a series of PA6/CB composites and fibers were prepared via in situ polymerization and subsequently melt spinning.

Method PA6/CB composites with carbon black content of 1%-3% were fabricated via in-situ polymerization by using highly dispersed aqueous carbon black slurry as modifiers, and melt-spinning were employed to achieve dope-dyed PA6/CB composite fibers. Microscopic structure, melting crystallization parameters and crystallization behavior and thermal stability of composites and/or fibers were determined using scanning electron micro-scope (SEM), differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA) respectively. Mechanical properties, orientation factor, chroma value and color fastness were then used to characterize the resultant structure and properties of fibers.

Results Quenching the surface of PA6/CB composites resulted in tougher surface than pure PA6 composite, without significant agglomerations, indicating that the carbon black was dispersed reasonably well (Fig. 1). A mostly smooth quenching surface was observed with relatively lower carbon black content, while staircase structure was formed when content of carbon black reached 3.0%. Similar results were witnessed on the surface of PA6/CB fibers when a smooth surface without aggregation of carbon black (Fig. 2). Combined with the curves obtained by DSC and crystallinity parameters calculated with DSC data(Tab. 1), carbon black had hardly any influence on the melting peak of PA6/CB composites, as indicated by a single melting peak observed on these two second heating curves with no difference in melting point (T_m) between samples. Crystalline temperature (T_c) reached as high as 190.30 ℃, 13.66 ℃ higher than that of pure PA6, which was 176.64 ℃. Obvious improvement in crystallinity of PA6/CB composites came when carbon black content was set at 3.0%, which was 31.41% higher than that of the pure PA6 (Fig. 3 and Tab. 1). Degree of undercooling (ΔT) temperature reached as low as 28.78 ℃ in PA6/CB composites, which is about 15% less than pure PA6 when carbon black content is 2.5%. Weight loss of PA6/CB composites in the temperature range 100-600 ℃ were less than that of pure PA6, and all initial decomposition temperatures (temperatures when 5% weight had been lost) surpassed 390 ℃ (Fig. 4). In pure PA6, a characteristic diffraction peak around 21.5° indicated the presence of γ crystal formation. However, this peak disappeared upon the addition of carbon black. Instead, two characteristic diffraction peaks at around 20.1° and 23.3°-23.4° emerged, representing a more stable PA6 α crystal formation (Fig. 5). When using the same draw ratio in fiber spinning, the tensile strength reached its highest value at a carbon black content of 1.0%. However, as the loading content of carbon black increased, the tensile strength gradually declined, although it remained significantly higher than that of pure PA6. Notably, at a draw ratio of 3.2 and a carbon black content of 3.0%, the tensile strength increased by 20%. At the same time, the elongation at break decreased continuously(Tab. 3). Tensile strength and orientation factor went up with the increase of draw ratio in melt spinning procedure in the same carbon black content while elongation at break went down. The L value, representing blackness of PA/CB fibers, dropped from 91.57 in pure PA6 to as low as 17.13 with added carbon black. Higher linear density caused lower L value. Rubbing, washing and light color fastness of PA6/CB fibers all reached level 4-5 in fastness tests.

Conclusion Carbon black disperses well in the matrix and on the surface of PA6/CB fibers which promoted a more stable PA6 α crystal formation, and it improves thermal stability of PA6/CB composites. In addition, carbon black provides a reinforcing effect of PA6/CB fibers, and the tensile strength is higher than the pure PA6 ones. Moreover, the blackness of composite fibers is getting deeper with the increase of carbon black content while remaining excellent color fastness. As a result, highly uniform dispersion of carbon black particles in PA6 matrix can be achieved via in-situ polymerization, resulting in high blackness and good color fastness of composite fibers, while maintaining great mechanical properties. All these characteristics are well-suited for the production of fine denier fibers. In general, in comparison to traditional dyeing processes, in-situ polymerization method has significant advantages in energy saving, color fastness, color uniformity, and fabric resilience. It is a simple production process with low cost and high efficiency, making it suitable for large-scale production of single-color fibers. However, there are challenges such as high polymerization reaction temperature, high requirements for long-term heat resistance of colorants, and difficulties in equipment cleaning during production switching, which need to be addressed in the future.

Key words: in-situ polymerization, polyamide 6, carbon black, melt spinning, dope-dyed, polyamide 6/carbon black composite fiber