How to Choose Pigments for Rotational Molding Plastic Products?

How to Choose Pigments for Rotational Molding Plastic Products?

Besides knowing the types of pigments used in plastics, it is also important to know the key factors and technical performance to be considered when choosing the the best pigments and colorants for rotational molding. Major performance properties of pigments in plastics are decided by their weather resistance or aging, heat resistance, pigment fluidity, the final application , manufacturing process and environmental factors. When determining how to color rotomoulded product, there are many factors to consider in order to achieve high quality levels.

• ​Weather resistance or aging

Due to the environmental factors such as sunlight, temperature, rain, and etc,  rotational molding plastic products will fade, or discolor. Among them, ultraviolet radiation is a major factor result in the aging of rotational molding plastic products. In practice, weather resistance can be measured in a simulated climatic box, or by accelerated weather resistance instrument.  

Exposure to the ultraviolet ray has adverse effects on the rotomoulded plastic products. It takes some time in testing the damage. The commonly used method is by accelerated testing. The effect of exposure to the UV in a simulated climatic box for few days or weeks are equal to the exposure of a few months and years in practice. Pigment selection for plastics used in indoor and outdoor applications require UV protection rating 5 and rating 8 respectively.

• Heat resistance

Heat resistance refers to the properties of a product that can maintain its excellent physical properties under heated conditions, including resistance to dry heat, damp heat, long-term high temperature, and short-term high temperature. Plastic products are generally measured by Martin heat-resistant temperature. For example, when the heating rate is 50℃/h, the warping of tested sample is 50kg/cm2, it drops 6mm in the reading indicator.

The thermal stability of rotomoulded plastic product is defined by the  environmental temperatures, as well as and the manufacturing process temperature. It requires specific discoloring tolerance in manufacturing process and color fastness in the final application.

• Pigment fluidity

Two methods are commonly used in coloring rotomoulded plastic products

1.Masterbatch

Based on predetermined formula, the measured amount of colorants such as pigments are added to the base resin and premixed evenly in in pellet (grain) form, which is called master batch. The masterbatch are blended to the rotational molding resin, and then are grounded into power.

2.Dry Pigment Mixing

Powdered colorants are added to rotational molding resin powders and then dry blended at high speed or low speed. The colorants are adhere to the surface of the rotational molding resin powers.

However there are disadvantages in using the premixed dry pigment resin. Negative effects are liable to occur at the process of resin melting. The viscosity increase, while melt flow rate decreases, which makes the manufacturing difficult, such as  scattering easily, easily dirtying machinery. Defects are caused in the final rotomoulded products, such as pores, inconsistent thickness, and uneven inner surface, and etc.

Therefore, The colouring of a plastic product is not a simple job. To achieve high level quality, the pigment and colourant selection for rotational molding depends on the major factors of compatibility,pigment levels, color properties, coloring methods and the application of final production.