Why does nylon fade easily when dyed? How to choose colorants for nylon?
Nylon, the professional name of polyamide, is abbreviated as PA. Nylon has high mechanical strength, high rigidity, high toughness, excellent abrasion resistance, and good electrical performance, and is widely used in machinery, automobile, household appliances, aviation, metallurgy and other fields. Although nylon has the above advantages, its dyeing performance is poor because of its high processing temperature and strong acid reducing agent in the molten state, which can make pigments or dyes that are stable at the processing temperature fade by reduction, resulting in problems such as darkening and yellowing; Therefore, nylon colorants need strong chemical stability, heat resistance and weather fastness.
nylon
The characteristics of nylon determine that there are special performance requirements for colorants for nylon:
1. Heat resistance
The color of pigments is caused by conjugated double bonds. Nylon is often reductive at high temperature, which destroys the double bond and causes discoloration; It is worth noting that the heating time of the pigment should be considered as well as its high temperature resistance. For example, when nylon 6 and polyester are polymerized, the heating time is 4 hours. Therefore, the colorant used for nylon must have thermal stability that can withstand 280 ℃ high temperature.
Resin processing temperature and requirements for colouring agent
2. Chemical stability
The colorant and nylon shall not react chemically. As a highly active resin, nylon shows reducibility in its melting state, which is easy to change the color of colorants. Affected by it, the selection range of colorants for nylon is relatively small, which requires that the colorants have good chemical stability.
3. Migration resistance
There are three main types of migration of colorants:
① Solvent extraction, i.e. color bleeding in water and organic solvent; ② Contact and migration, causing pollution to adjacent objects; ③ The surface is sprayed with frost. When heated, the solubility of the colorant in the polymer is large, while at room temperature, the solubility of the colorant is small.
In general, organic pigments are dissolved in polymers and other organic substances to varying degrees, which is easy to migrate, while inorganic pigments do not.
4. Colouring force
The coloring force is related to the characteristics of the colorant itself and its particle size, which generally increases with the decrease of the particle size. When color pigments are mixed with white pigments, the dyeing power can be detected.
5. Dispersibility
In general, the better the dispersion of pigments, the higher their tinting power. Only when colorants are uniformly dispersed as tiny particles in polymer, can they have good coloring effect.
Colorants can be divided into two categories: pigments and dyes, in which pigments are divided into inorganic pigments and organic pigments. How to select colorants for nylon? It is recommended to combine the characteristics of the above colorants for nylon to see:
Dyes have bright colors and complete chromatograms, but their heat resistance, weather resistance and solvent resistance are poor. They are easy to decompose and discolor at plastic processing temperature or during use at a higher temperature, so they are rarely used in nylon coloring.
Organic pigments have high tinting strength and brightness. However, due to the high processing temperature of nylon, it requires high heat stability of the pigment, and the strong reducibility of nylon resin when it melts, making many organic pigments for plastics unsuitable for use in nylon.
Inorganic pigments are superior to organic pigments in light resistance, climate resistance, migration resistance and chemical resistance, and are generally suitable for plastic coloring; However, compared with organic pigments, inorganic pigments generally have the disadvantage of relatively weak coloring strength and brightness. Inorganic pigments have high light reflection index and are commonly used in opaque products.
Inorganic pigments commonly used in nylon include pigment yellow 184 (bismuth yellow), pigment blue 28 (cobalt blue), pigment blue 29 (ultramarine blue), pigment green 50 (cobalt green), pigment brown 24 (titanium chrome brown), pigment yellow 53 (titanium nickel yellow), pigment black 28 (copper chrome black), titanium dioxide, carbon black, etc.
