Nylon colorants are divided into two main categories: pigments and dyes, with pigments further subdivided into inorganic pigments and organic pigments. Due to the unique characteristics of nylon, there are special requirements for colorants. Today, we will primarily focus on how to select colorants for nylon.
1. About Nylon
(2). Characteristics of Nylon
Nylon has high mechanical strength, a high softening point, heat resistance, low friction coefficient, wear resistance, self-lubrication, shock absorption and noise-dampening properties; it is resistant to oil, weak acids, alkalis, and general solvents; has good electrical insulation and is self-extinguishing; it is non-toxic, odorless, and has good weather resistance; however, it has poor dyeability.
(2). Main Applications of Nylon
One of the primary uses of polyamide is in the production of synthetic fibers, where its most notable advantage is its superior abrasion resistance compared to all other fibers. Because polyamide is non-toxic, it can be used as medical sutures.
Due to its safety, lightweight, excellent mechanical strength, wear resistance, and good corrosion resistance, polyamide is increasingly used to replace metals like copper in the manufacture of bearings, gears, pump vanes, and other parts in the mechanical, chemical, instrumentation, automotive, and other industries.
In industry, nylon is extensively used to manufacture curtain threads, industrial fabrics, ropes, conveyor belts, tents, fishing nets, and more.
2. Classification and Performance Comparison of Colorants
Colorants are divided into two main categories: pigments and dyes, among which pigments are further classified into inorganic pigments and organic pigments. The performance comparison between them can be seen in the following table:
| Coloring Agent | Organic Pigments | Inorganic Pigments | Dyes |
| Definition |
The characteristic of organic pigments is that they contain chromophores, which can be excited from the ground state to the excited state by selectively absorbing visible light, thereby causing the molecules to exhibit complementary colors. |
The main components of the pigments are inorganic substances. Almost all inorganic pigments are compounds, often complex mixtures. | Dyes are compounds that can absorb and transmit light of certain wavelengths without scattering any type of light. They are transparent. |
| Thermal stability | Good | Excellent | Good |
| Photostability | Good | Excellent | Good |
| Tinting Strength | Height | Low | Extremely height |
| Relative Density | Low | Larger | Low |
| Solvent Resistance | Good | Excellent | Inferior |
| Opacity | Good | Excellent | / |
| Color Vibrancy | Good | bad | Excellent |
| Toxicity | Small | Larger | / |
| Chromatographic Completeness | Complete | Incomplete | / |
| Price | Height | Low | / |
| Migration Resistance | Good | Height | bad |
3. Properties Required for Dyes Used in Nylon
(1) Heat and Sunlight Resistance
To date, numerous organic pigments, inorganic pigments, and solvent dyes have been suitable for coloring most thermoplastic engineering plastics.
Resin processing temperature and requirements for colorants:
| Resin | Processing Temperature (°C) | Requirements for Powder Formulations |
|
LDPE |
150 - 220 | Durable and non-migratory. |
| HDPE | 160 - 230 | High heat resistance, no migration. |
| PVC | 180 - 250 | Good dispersion and robust, acid-resistant. |
| PP | 230 - 280 | Heat-resistant, good dispersibility. |
| PS | 180 - 240 | Durable, good solubility. |
| ABS | 230 - 310 | Heat-resistant, good dispersibility. |
| PC | 150 - 200 | Heat-resistant, good dispersibility, water-free. |
| PUR | 210 - 270 | Lightfast, acid-resistant, reductive-resistant. |
| PMMA | 200 - 230 | Durable, heat-resistant, non-migratory. |
| PA | 270 - 290 | Heat-resistant, anhydrous, good dispersibility. |
| PET | 270 - 290 | Heat-resistant, non-migratory, and pH-neutral. |
From the table, it is evident that polyamide has a high processing temperature, which requires the use of colorants with high heat resistance. When it comes to organic pigments for plastics, not only is there a certain requirement for their thermal stability, but they also need to have good lightfastness. Many organic pigments can significantly negatively impact the properties of the polyamide melt, hence there are certain restrictions on the organic pigments used for coloring polyamides.
(2) Chemical stability
The colorant and the resin should not undergo any chemical reaction, nor should they promote the decomposition of the resin. For high-activity resins such as polyamide resin (PA), which exhibit reductive properties in their molten state, it is easy for the colorant to change color. Therefore, only a limited number of colorants are available for selection.
(3) Migration Resistance
There are three main types of dye migration:
- Solvent bleeding, which is the seepage of color in water and organic solvents;
- Contact migration, resulting in contamination of adjacent objects;
- Surface blooming, where the solubility of the dye in the polymer is higher when heated, but lower at room temperature.
Generally speaking, the dispersion of inorganic pigments in polymers is heterogeneous and does not result in blooming; whereas organic pigments dissolve to varying degrees in both polymers and other organic substances, making them more prone to migration.
(4) Tinting Strength
The magnitude of tinting strength determines the amount of colorant required to achieve the target color. Generally speaking, the tinting strength increases as the particle size of the colorant decreases. Organic pigments have a higher tinting strength than inorganic pigments, and when colored pigments are used together with white pigments, the tinting strength can be significantly enhanced.
(5) Dispersibility
The colorant must be uniformly dispersed in the polymer as tiny particles in order to achieve a good coloring effect. Pigments contain many agglomerated particles, which must be broken down by high shear force to form smaller agglomerates, thereby meeting the requirements.
4. How to Choose Coloring Agents for Nylon
When considering the characteristics that dyes must satisfy for nylon, and looking at the performance comparison table of several dyes:
Inorganic pigments exhibit superior lightfastness, weather resistance, migration resistance, and chemical resistance compared to organic pigments and are generally suitable for plastics, but they have much lower color strength and brightness. When a good color tone is required, organic pigments are preferred. Additionally, inorganic pigments have a high refractive index and are commonly used in opaque products.
Compared to inorganic pigments, organic pigments display high coloring strength and vividness. The processing temperature of PA (polyamide) is relatively high, which also demands high thermal stability of the pigments. Furthermore, PA resin shows a strong reductive nature when melted, making many organic pigments used in plastics unsuitable for nylon.
At present, organic pigments suitable for nylon, especially light colors, are still somewhat lacking and require further research. Therefore, to achieve more vivid colors, dyes must be chosen.
Dyes are coloring agents that can dissolve in water, oil, or organic solvents, and are generally organic compounds. The advantages of dyes are their bright colors, eye-catching hues, and a complete spectrum of colors. However, the disadvantages are poor heat resistance, weather resistance, and solvent resistance; they can easily decompose and discolor at the processing temperatures of plastics or at higher temperatures. Although dyes have relatively poor temperature resistance, the color will eventually stabilize after high-temperature treatment in nylon, still achieving a certain level of vividness. Additionally, dyes resistant to high temperatures have been developed to meet the requirements.
Dyes are the primary coloring agents for fibers, textiles, and other textile products. They are less commonly used in plastics, mainly in optical plastic products, where they can maintain good transparency in transparent plastics.