Feature

The importance of ingredient characteristics in master batching

Peter R. Holman, Holman Engineering Inc.

Achieving the right proportion of a minor or micro ingredient in a batch can make or break the final product. But mixing these ingredients uniformly throughout the batch is no simple matter. Using a master batch, also called a premix, can greatly improve your product by increasing production, reducing ingredient handling, reducing worker exposure to ingredients, and providing more uniform mixture quality from batch to batch.

When determining which ingredients to include in the master batch, you need to consider each ingredient’s characteristics in relation to the others. Some important characteristics are chemical compatibility and stability, solubility, particle size, and porosity.

Chemical compatibility and stability. Before creating the master batch, you need to establish the chemical compatibility of the minor and micro ingredients and the carrier. You must also determine whether the ingredients are stable, so they won’t react with one or more other ingredients to produce a toxic compound. However, even safe chemical reactions can have detrimental effects. For instance, some reactions can cause an ingredient to lose its effectiveness over time, reducing the master batch’s shelf life. So it’s typically best to test the master batch’s quality before adding it to a production batch and to use the master batch as quickly as possible.

Solubility. The most desirable ingredients for a master batch are insoluble, because they don’t absorb moisture from environmental humidity. Hygroscopic ingredients create problems because solubility affects the master batch both chemically and physically. Chemically, excess moisture can speed reactions that shorten shelf life. Physically, excess moisture can increase package weight and frequently cause agglomeration and compaction, which can cause problems in discharging the master batch from a bin or hopper or in processing the material.

Particle size. Minor and micro ingredients typically have a small particle size. This compounds solubility effects because the surface area of small particles is relatively greater than that of large particles per given weight. For instance, 1 gram of a micro ingredient with specific gravity of 1.0 and a 44-micron (325-mesh) particle size has an approximate total surface area of 950 square centimeters (147 square inches). As a result, even a mildly hygroscopic ingredient with these characteristics can absorb relatively high percentages of moisture. Particle size differences can also cause ingredients to segregate when the master batch is handled after mixing. This is particularly likely if the master batch’s smallest particles are one-fifth or less the size of the largest particles. Large density variations can also cause ingredients to segregate.

Porosity. The master batch carrier holds, absorbs, and physically stabilizes the minor and micro ingredients. The particle size of the carrier is considerably larger than that of the minor and micro ingredients. During mixing, the carrier’s pores absorb the minor and micro ingredients. To determine whether a carrier can absorb the other ingredients, you need to determine the carrier’s degree of porosity and pore shape. Typically, the carrier can stably accommodate no more than its own weight in minor and micro ingredients. When very high percentages (greater than 50 percent of the master batch’s weight) of minor and micro ingredients are required in a master batch, an inert diluent with a particle size and density similar to those of the carrier can be added to the mix to physically stabilize the ingredients. For instance, in an animal feed master batch, the diluent can be sodium chloride, sucrose, kaolin clay, or ground limestone.

For more information on master batching, you can read the article “How to achieve uniform mixtures with master batching” by Peter R. Holman, Holman Engineering Inc. You can also find articles listed under “Mixing and blending” in PBE's Article Index.