How to Avoid Pitfalls in Color Matching and Processing with 8 Different Types of Special Color Masterbatches?

To formulate a bright white product, the primary choice should be titanium dioxide with a bluish undertone (preferably produced by the chloride process, as it has a smaller particle size and a uniform molecular weight distribution). Using titanium dioxide with a bluish undertone to formulate white imparts a sense of freshness. Conversely, if a titanium dioxide with a larger particle size and a yellowish undertone is chosen, no matter what methods are used to adjust the color tone, it will be impossible to achieve a bright pure white.

Due to insufficient whiteness of titanium dioxide, additives are often added during plastic coloring to enhance whiteness. A very small amount of blue and purple pigments or fluorescent whitening agents can be added to increase whiteness. The whitening effects of titanium dioxide are as follows: the commonly used and simplest method is to use phthalocyanine white blocks, whose whitening effect is known in the market as magnetic white; the best whitening effect is achieved with fluorescent whitening agents, but their cost is the highest.

When formulating special black products with carbon black, attention should be paid to the color tone of the carbon black. Under incident light, carbon blacks with smaller particle sizes generally exhibit a bluer hue compared to those with larger particle sizes. However, under transmitted light (transparent coloring) and when blending gray shades, they appear brownish. To achieve a jet-black and glossy plastic product, choose low-structure carbon black with small particle size. This is because the blackness of carbon black coloring mainly depends on light absorption; the smaller the particle size, the higher the light absorption and the weaker the light reflection, resulting in greater blackness. After selecting the above type of carbon black, special attention must be paid to the dispersibility of the carbon black to achieve a satisfactory coloring effect. Only by resolving the dispersibility issue can the highest tinting strength of carbon black be attained.

After oxidation, carbon black introduces polar groups such as —OH and —COOH, which can improve the dispersibility of carbon black. Therefore, in the performance indicators of carbon black, there is a volatile matter index. The higher the value, the higher the degree of oxidation.

No matter what, carbon black always carries some yellowish tint, so blue can be used for color adjustment, with a dosage of 8% to 10%. In this way, the blackness of the prepared black color is greatly enhanced. Pigment Blue 15 is generally used. If, after using a blue colorant for adjustment, purple or red is further used for color adjustment, the resulting black may appear even deeper.

When preparing gray with carbon black, attention should be paid to the color undertone of the carbon black. If the carbon black has a small particle size, adding titanium dioxide will result in a gray with a yellowish undertone. If the carbon black has a large particle size, adding titanium dioxide will produce a gray with a bluish undertone. The particle size and particle size distribution of titanium dioxide also have a significant influence on its color; larger particles tend to yield a yellowish tint, while smaller particles give a bluish tint. Therefore, when preparing gray, it is essential to understand the relationship between the particle size and color undertone of both carbon black and titanium dioxide. Otherwise, if the undertone is misjudged and further pigments are used to adjust it, the process will only become increasingly complicated.

After determining the carbon black and titanium dioxide for gray product color matching, other colors may sometimes need to be added to achieve the user's requirements. Since the amount of pigment added is small, pigments with low tinting strength should be selected, such as iron oxides (Pigment Yellow 119, Pigment Red 101) and metal oxides (Pigment Yellow 53, Pigment Brown 24). This approach helps to improve the color stability of the color matching system.

(1) Dark-colored outdoor plastic products.

Outdoor plastic products such as stadium seats, bridge cable sheaths, construction materials, advertising boxes, turnover boxes, and plastic automobile parts are used outdoors for long periods and therefore must have good lightfastness and weather resistance. Inorganic pigments generally possess good light stability and can be used for coloring outdoor plastic products, but their application is limited due to their low tinting strength.

When formulating dark-colored outdoor products, the first step is to select colorants with good lightfastness and weather resistance. Secondly, titanium dioxide should be used as little as possible or not at all in the product, ensuring the pigment concentration is as high as possible. Additionally, the coloring formulation should contain an adequate amount of ultraviolet absorbers and antioxidants to prevent discoloration of the resin.

(2) Light-colored outdoor plastic products.

Light-colored plastic products are formulated by adding a large amount of titanium dioxide and a small amount of pigments. Since the weather resistance and heat resistance of most organic pigments decrease to varying degrees after being mixed with titanium dioxide, this impact is even greater for light-colored varieties due to the small amount of pigment added. Many varieties suffer from poor weather resistance, which leads to fading of plastic products, customer complaints, and a very negative impact. In addition, since organic pigments have high tinting strength, only a small amount is needed for light-colored products; however, the transmission of errors can cause color differences during production, resulting in color deviations during use.

For light-colored varieties, inorganic pigments should be used, mainly taking advantage of their good heat resistance and low tinting strength. The selectable inorganic pigments include Pigment Yellow 119 (iron oxide), Pigment Yellow 53 (metal oxide), Pigment Brown 24 (metal oxide), Pigment Brown 184 (bismuth vanadate), Pigment Red 01 (iron oxide), Pigment Blue 29 (ultramarine), Pigment Green 8 (cobalt blue), and Pigment Brown 50 (cobalt green).

To prepare light-colored transparent products, the resin itself must be transparent and colorless, such as polypropylene and transparent polystyrene. It is necessary to select colorants with good transparency and consistent color tone. Generally, inorganic pigments have poor transparency and cannot be used. Among organic pigments, those with good transparency include: Pigment Yellow 139 (Isoindoline), Pigment Yellow 199 (Anthraquinone), Pigment Red 149 (Perylene), Pigment Red 254 (Diketopyrrolopyrrole), Pigment Blue 15:1 (Phthalocyanine), Pigment Green 36 (Phthalocyanine), and Pigment Brown 41 (Condensed Azo).

The addition ratio of pearlescent pigments in plastic injection products is generally 1%, and in extruded film products it ranges from 4% to 8%, depending on the thickness of the plastic film. Similarly, in co-extrusion composites, the pearlescent content in the pearlescent layer should also be increased accordingly. Depending on the thickness of the plastic pearlescent layer, the addition amount is 5% to 10%.

(1) Pay attention to the selection of pearl pigments and organic pigments.

In the process of plastic processing, the following points should generally be noted when using pearlescent pigments: The colored resin should have good transparency; pigments with good transparency (such as organic pigments) or solvent dyes should be selected as much as possible; pearlescent pigments should not be mixed with titanium dioxide. To achieve a certain covering power, small particle size pearlescent pigments can be used simultaneously, and pigments with high covering power should be avoided as much as possible; when pearlescent pigments are used in outdoor plastic products, their weather resistance needs to be considered.

(2) Pay attention to the processing and molding techniques of pearlescent pigments.

During injection molding, increasing the back pressure can enhance the screw's mixing ability, thereby improving the dispersion of pearlescent pigments. The processing temperature during injection molding is generally selected at the upper limit of the resin's recommended usage temperature range to ensure the dispersion of the pearlescent powder. During molding, the flow of the melt drives the automatic orientation of the pearlescent pigment flakes, achieving a good pearlescent effect.

The surface finish of the mold is very important. The smoother the mold, the more uniform direction alignment and smooth pearlescent luster can be obtained.

The design of the mold gate is also very important. Choosing a single gate rather than multiple gates can better reduce weld lines at the gate. The gate should usually be located at a thick section, away from flow obstacles. The distance between the end of the gate and the runner system should be as small as possible to minimize uneven and random distribution of pearlescent pigments caused by differences in fluid resistance.

Since pearlescent pigments exist in flake form, their particle size may decrease during plastic processing due to the influence of shear force, leading to a reduction in pearlescent effect. By using a larger aspect ratio and appropriately fine filter screen to increase the pressure at the machine head, the damage to the pearlescent pigments from shear forces during processing can be minimized.

When using pearlescent pigments in PMMA, PC, and PA systems, pre-drying treatment is required. In PVC plastics, when using gold and bronze series pearlescent pigments, special attention must be paid because the presence of free iron ions can accelerate the decomposition of PVC resin. Some silver-white pearlescent pigments may yellow in certain pigment products, so it is recommended to use anti-yellowing series products.

The pigment used in silver plastic products is actually aluminum powder. Because the surface of aluminum can strongly reflect the entire visible spectrum, including blue light, aluminum pigment can produce a very bright blue-white mirror-like reflection.

Aluminum powder is available in different varieties for use in plastics. The particles with an average diameter of 5 μm have excellent coloring power and hiding ability; particles with an average diameter of 20–30 μm can be used together with colored pigments; particles with an average diameter of 330 μm provide a coarse sparkling effect.

The melting point of aluminum powder is 660°C, but when it is directly exposed to air at high temperatures, its surface is oxidized to a gray-white color. Therefore, surface treatment is applied to aluminum powder to form a protective silica coating, which provides heat resistance, weather resistance, and acid resistance.

The pigment used in golden plastic products is copper powder, which is prone to oxidation at high temperatures and also has poor weather resistance, becoming dull after prolonged outdoor exposure. By applying a treatment technique to coat the surface of the copper powder with a silicon oxide protective layer, its heat resistance, weather resistance, and acid resistance can be greatly improved. After silicon oxide treatment, the color masterbatch produced does not exhibit dullness during high-temperature injection molding.

To achieve special metallic effects, a small amount of highly transparent organic pigments with similar color tones may be appropriately added. However, it should be noted that adding excessive amounts of titanium dioxide and pearlescent powder to the formula can also make the product appear dull.

Due to the generally flaky structure of metallic pigments, the use of metallic and pearlescent pigments in injection molding can easily lead to flow lines, affecting the appearance of plastic products. The following methods can be used to reduce or mitigate this situation: use large particle-sized metallic pigments, increase the amount of metallic pigment added, choose high-viscosity resins, enlarge the injection aperture, and increase the injection speed.