The particle size distribution of plastic masterbatch has a direct and critical impact on the uniformity of its melt mixing in the matrix plastic. This impact runs through the entire process of melting, dispersion, and fusion. By changing the contact area between the masterbatch and the matrix plastic, the melting speed, and the kinetic state during mixing, it ultimately determines the uniformity of the mixing system.
When the particle size distribution of the masterbatch is relatively uniform and the particle size is moderate, the masterbatch can form a good contact with the matrix plastic during the melt mixing process. The uniform particle size allows each plastic masterbatch to occupy a relatively consistent space in the matrix, with uniform spacing between each other. The matrix plastic can be evenly wrapped around the masterbatch, and the heat transfer is more balanced. During the heating and melting stage, masterbatches of similar sizes will begin to soften and melt at a similar time to avoid the inability to evenly disperse the components of the masterbatch after melting due to the complete melting of some masterbatches while the other part remains solid. This synchronous melting state lays the foundation for subsequent uniform mixing.
If there are too many large particles in the masterbatch, it will significantly affect the uniformity of melt mixing. The contact area between the masterbatch of large particles and the matrix plastic is relatively small, and the path for heat to be transferred from the matrix to the inside of the masterbatch is longer, resulting in a slower melting rate. During the mixing process, when most of the small particle masterbatch has been melted and dispersed, the large particles may not be completely melted, forming a solid "hard core". These hard cores are difficult to fully fuse with the matrix plastic, and will form local component enrichment in the final product, destroying the overall uniformity. At the same time, large particles are less susceptible to shear force during mixing, difficult to be broken and dispersed, and easy to form local agglomerations in the matrix, further aggravating the uneven mixing.
Too small particles account for too high a proportion in the masterbatch, which will also have an adverse effect on the mixing uniformity. Small particles have a large surface area and high surface energy, and are easy to adsorb each other to form agglomerates. If these agglomerates are not fully broken up before melting, they will be difficult to melt like large particles, resulting in local uneven mixing. Even if the agglomerates are broken up, small particles may form local areas with excessively high concentrations in the matrix due to excessive diffusion after melting, especially when the mixing time is short. It is difficult to achieve uniform distribution through diffusion, resulting in too much masterbatch in some areas of the matrix plastic and too little in other areas.
Too wide a particle size distribution range, that is, the presence of a large number of large particles, medium particles and small particles at the same time, will make the melt mixing process complicated and difficult to control. The melting speeds of particles of different sizes vary greatly. Small particles melt quickly and then begin to diffuse, medium particles are in the melting process, and large particles melt slowly. This asynchronous state leads to the simultaneous presence of solid, semi-molten and molten masterbatches in the mixed system, increasing the difficulty of mixing. Under the action of shear force, plastic masterbatches in different states will be affected by different forces, making it difficult to disperse evenly in the matrix, which ultimately leads to composition fluctuations in the product and affects the performance consistency of the product.
Reasonable particle size distribution can promote the interface fusion between masterbatch and matrix plastic. When the masterbatch particle size matches the flow characteristics of the matrix plastic, the masterbatch components after melting can be fully combined with the matrix plastic through interface diffusion to form a uniform transition layer. For example, the distribution of medium particles as the main and a small amount of small particles as the auxiliary can not only ensure the consistency of the melting speed, but also fill the gaps between the medium particles with small particles, increase the contact points between the masterbatch and the matrix, promote interface fusion, and allow the plastic masterbatch components to evenly penetrate into every corner of the matrix to avoid component differences at the interface.
Particle size distribution also affects the shear force transmission during mixing. Under the shear action of the mixing equipment, masterbatches with uniform particle size can evenly withstand shear force and be effectively dispersed into the matrix. If the particle size distribution is uneven, large particles will hinder the transmission of shear force, resulting in greater shear force on the surrounding small particles, while the shear force in the area far away from the large particles is insufficient, resulting in uneven shear force distribution, which in turn leads to uneven masterbatch dispersion. Uniform particle size distribution allows shear force to be evenly transmitted in the mixing system, ensuring that each plastic masterbatch can be properly sheared and evenly dispersed.
In addition, the particle size distribution will also affect the sedimentation and distribution of the masterbatch in the matrix plastic. In the initial stage of melt mixing, if the masterbatch particle size difference is too large, the large particles may have different sedimentation speeds in the matrix due to gravity, resulting in a higher masterbatch concentration at the bottom than at the top. Even after subsequent mixing, it is difficult to completely eliminate this uneven initial distribution. The masterbatch with uniform particle size has a similar sedimentation speed in the matrix, and the initial distribution is more uniform, which provides a good foundation for subsequent melt mixing and helps to achieve overall uniformity in the end.
In short, the particle size distribution of plastic masterbatch directly determines the uniformity of its melt mixing in the matrix plastic by affecting multiple links such as melting speed, contact area, shear force transmission, and interface fusion. Reasonable particle size distribution should be determined according to the characteristics of the matrix plastic, the performance of the mixing equipment, and the requirements of the product to achieve uniform distribution of the masterbatch components in the matrix and ensure stable performance of the product.
