What Causes Poor Color Dispersion of Color Masterbatch?

Color masterbatch is the most common method of plastic coloring. It has a wide range of applications in the plastic processing industry. When using the color masterbatch for coloring, sometimes, the finished products has poor color dispersion. One main reason is that the color masterbatch is not evenly dispersed during the processing.

Dispersibility is one of the most important performance indicators of color masterbatch, which has a significant influence on finished product property and performance. Dispersibility refers to the degree of uniform dispersion of masterbatch in plastic materials. Whether it is an injection molded process, or an extrusion or blown film process, if the dispersion is not good, the final product may show uneven color distribution, dark or shallow irregular lines, or pinhole-like powder spots and other quality abnormalities. 

Dispersibility has a crucial impact on the quality of plastic products. Often, the poor dispersion is not only caused by material but also process. 

In the following, we will discuss what causes poor color dispersion of color masterbatch, from the material and process.

color masterbatch

Material

Poor dispersion of color power

Color masterbatch is a highly concentrated pigment polymer. A large number of powders gathering together will inevitably affect the dispersion of color masterbatch. Many color powders have particularly strong agglomeration, such as the most conventional titanium dioxide, carbon black, etc.They are extremely easy to hold together.  More refined processing results in smaller particle sizes and higher specific surface areas, making it easier to evenly disperse in the resin. According to the customer’s final product and process, choosing the right type of color powder and the right particle size of the color powder has an important influence on the final dispersibility of the masterbatch.

Color Masterbatch

Influence of static electricity

Sometimes, anti-static additives are not added during the production. Directly putting the color masterbatch and raw materials together for mixing and stirring, is very easy to produce static electricity. This static electricity can cause the masterbatch to mix unevenly with the raw material, forming particle agglomeration and thus affecting the dispersion of the masterbatch.

Inappropriate melt index

Very often, when choosing carriers, color masterbatch suppliers will choose resins with higher melt indexes as carriers. This is because low melt index masterbatches struggle to flow and diffuse adequately in high molecular weight resins, impacting their uniform distribution in the base resin. Too low a melt index will easily cause poor dispersion of the masterbatch in the product. However, it is not the case that the higher the melt index, the better. It is necessary to consider the physical properties of the final product, the surface requirements and the processability of the masterbatch.  The melt index of masterbatches needs to be integrated with the condition of carrier and powder. The suitable melt index is very helpful for the dispersion of color masterbatch.

The low addition ratio of color masterbatch

Considering the physical properties of the final product and the total cost of the coloring solution, some masterbatch suppliers will design a low masterbatch addition rate, resulting in poor dispersion of the color masterbatch in the product. 

The inappropriate dispersing system

In order to open the agglomerated powder in the color masterbatch, some dispersants and lubricants are added during the masterbatch processing to assist the dispersion of the powder. If the chosen dispersant has poor interaction with pigment particles or poor compatibility with the carrier resin, the pigment will not disperse evenly. The dispersant’s melting point should be lower than the resin’s and should have good compatibility and affinity with the resin to effectively promote pigment dispersion. Common dispersants include low molecular weight polyethylene wax and stearates, which can effectively wet the pigment particles and prevent re-agglomeration. The amount of dispersant used is also crucial for pigment dispersion; too much or too little can cause the pigment particles to re-agglomerate.

Influence of density of color masterbatch

Color masterbatch is often added with color powder of higher density. For example, in order to realize the strong hiding power of color masterbatch, titanium dioxide is often added in large quantities. Its density is about 4.0g/cm³ (3.8g/cm³~4.4g/cm³). That is larger than the density of many resins. Therefore, the density of color masterbatch may have a big difference with the density of resin. It is easier for the color masterbatch to be deposited at the bottom of the mixer when mixing, which will cause the color inconsistency of the products.

Different colored masterbatches are often mixed together, and if their specific gravities are not precisely controlled, color stratification can occur, further affecting the overall appearance of the product.

Inappropriate choice of carrier

The carrier of color masterbatch is used to carry color powder and other functional additives. The type, quantity, model, melt index and even form of the carrier will greatly affect the processability and dispersibility of the masterbatch. For example, when the same color masterbatch is used for polyolefin products, is the carrier is PP or PE or EVA; is it a kind of carrier or a mixture of several kinds of carriers; which brand is it; how much the melt index is chosen; is it powder or granule, etc., all of them will affect the final performance of the color masterbatch.

If the carrier resin is not compatible with the colored resin, the masterbatch cannot disperse evenly in the product, leading to layering or uneven color. If the carrier resin’s melt index is not suitable, it can result in poor dispersion of the masterbatch in the product, affecting the appearance and performance. Insufficient fluidity of the carrier may also hinder the uniform dispersion of the masterbatch into the base resin when molten, impacting the final product’s quality.

Process of color masterbatch

Influence of process

The plastic molding process has a certain influence on the dispersion of masterbatches. As far as the injection molding process is concerned, temperature, pressure and holding time will all affect the dispersion of masterbatches. For example, improper temperature control of injection molding equipment can prevent the masterbatch from adequately mixing with the resin in the mixing chamber, leading to poor dispersion. Similarly, the extrusion temperature setting can also affect the final product’s dispersion.

Influence of equipment

Plastic molding equipment will also have a certain impact on the dispersion of color masterbatches.

Melt shear processing is a critical step. If parameters such as screw speed and processing temperature are not set correctly, the pigment may not disperse sufficiently.

If the screw is severely worn, the shear force will be greatly reduced, which will weaken the dispersion of masterbatch.

Additionally, improper selection and adjustment of equipment can lead to uneven pigment dispersion, affecting the final product’s color. Unreasonable dispersion methods (such as dispersion media or ultrasonic treatment) can also result in incomplete dispersion of agglomerated particles.

Influence of mold

For injection molding, the mold structure, such as the position of the inlet of the mold, will have a certain impact on the molding of the product and the dispersion of the masterbatch.

The position of the feed port should be determined based on the material’s properties and the structure of the molded part. An improperly positioned feed port can cause uneven distribution of the masterbatch in the melt, affecting the product’s color uniformity and appearance. For instance, if the feed port is placed on the surface of the molded part, it can affect the product’s appearance quality, necessitating adjustments to the feed port position.

For extruder, the runner structure and slope of the mold will affect the dispersion of the product.

The flow channel structure determines the flow state of the plastic melt in the die head. A reasonable flow channel structure can improve the flow and stability of the material flow, thereby enhancing product dispersion. Dead corners in the flow channel can affect the residence time of the melt and prevent the powder from fully mixing during flow, impacting its stability, fluidity, wettability, and uniformity in the solvent.

For example, using linear or arc transition sections can have different effects on the pressure field, velocity field, and shear rate field in the flow channel, thus improving the uniformity and dispersion of the melt.

To sum up, it is not a big deal at all when the product has bad dispersion. Often, the dispersion problem is not a simple material problem or process problem. All relevant parties should promptly brainstorm and analyze the problem, and then solve it one by one. Every party should take part and implement improvements, and ultimately eliminate the relevant problem.

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