Coarse particles and impurities in carbon black have a significant impact on the smoothness of the appearance of plastic products, the processing performance in conductive applications, and the performance of the final product. The requirements are stricter than carbon black for rubber.
Carbon black serves two purposes in plastics as a colorant. The use of carbon black to create black plastics and the use of carbon black to modify colors imparted by other colors rely on different aspects of carbon black's ability to interact with light.
The primary attribute of carbon black in coloring applications is the blackness of its dispersed phase, which is a complex function of particle size, aggregate size and degree of dispersion. The smaller the particle size, the larger the surface area, the stronger the ability to absorb light, and the more effective the forward scattering of light when it is irradiated by light. A second important property is its undertone, which is a complex function of scattering and absorption efficiencies as a function of light wavelength. For carbon black for plastics, the base color should be between blue and blue-brown.
A second attribute of carbon black in coloring applications is its ability to improve the intensity of other colors when pigmented. The effect of tinting can be measured by tinting strength, which measures the ability of carbon black to change the appearance of white pigments. The tinting strength depends mainly on the aggregate size and its distribution. The hue of the tint is related to the undertone imparted by the carbon black, which is generally less blue due to the increased preferential absorption of blue light as the particle size of the carbon black decreases.
Carbon black selection for coloring Various pigment carbon blacks can be used for plastic coloring. High-pigment carbon black or medium-pigment carbon black with the finest particle size (13-19nm) is suitable for plastic products that require extremely high blackness. For plastics with obvious natural color such as ABS, the amount of carbon black added is about 1%-2%, and other plastics are added 0.5%-1%. For black plastic products that do not require high blackness and gloss, ordinary pigment carbon black with medium particle size (20-25nm) is used. When plastics are colored, such as red, light brown, grass green, sky blue and various darker colors, a small amount of carbon black needs to be added. Therefore, low-pigment carbon black with a particle size larger than 50nm and poor tinting strength is used. The amount of such carbon black can be slightly larger, and the weighing error is relatively small.
The ultraviolet protection function of carbon black in plastics Generally, when exposed to sunlight, plastics will undergo photochemical degradation due to ultraviolet radiation, which is called photoaging, resulting in surface cracking, brittleness and reduced mechanical properties. Among polyolefin plastics, high-density polyethylene has the best light resistance, and low-density polyethylene is resigned. Tests have proved that the addition of carbon black can significantly improve the weather resistance of polyethylene plastics. The outdoor durability of polyethylene plastics without pigments is only 9 months, while that of polyethylene plastics with carbon black can reach more than 25 months. When the particle size of carbon black is small, due to the increase of the surface area, its light absorption or light-shielding ability increases, so the ultraviolet protection ability is enhanced, but when the particle size is smaller than 20-25nm, its protective effect tends to be uniform. The reason is that when the particle size is too large After hours, backscattering decreases, and light that continues forward threatens the stability of the polymer.