Mixers are used to achieve homogeneity between different components, specially in the production of varnishes or water-based paints. Mixers are used in the following operations:
- Mixing oils or resins.
- Mixing pigments and fillers with coating materials.
- Decreasing the viscosity of resins, and varnishes.
- Mixing additives with paints or varnishes.
- Adding solvents or diluting agents (thinner) to paints, to adjust the viscosity.
- Preparing emulsion (water-based) paints.
There are many types of mixers used in paint industry, they differ in their suitability for different applications. Choice of mixer type depends on the following:
- Viscosity: mixers types used in preparing pastes differ from those used in the production of low viscosity paints.
- Density difference between components: achieving the desired homogeneity depends on the type of impeller, blades design, mixing speed, and inclination of impeller axis with respect to mixing tank axis.
- Solid particle size: Some components, such as pigments agglomerates, have relatively large particle size compared to other components. Also volatility of solvents affects the design of mixers and the need for cooling.
- Manual mixers.
- Automatic mixers.
- Colloid mills.
- Rotary churns.
- Mixing by air streams.
Paints industry uses different types of mills such as roller mills or ball mills, figure below shows three-roller mills in which each roller rotates in the opposite direction of the others and with different speeds with ratio 1:3:9. The clearance between each two rollers must be controlled accurately to maintain the desired finesse of dyes.
This type of mills leads to the desired homogeneity as the dye is dispersed into its particles. This type of mills is open and therefore cannot be used in grinding of paints which contain high volatility solvents as solvent emissions to the atmosphere could occur.
The grinding efficiency and fineness of particle depend on the dimensions of the cylinder, speed of rotation, balls size and balls density. In some mills the length of the cylinder is equal to its diameter, but to maintain higher degree of fineness mills with a length larger than diameter are used. There are other types in which the grinding operation is made in steps inside the mill, as the cylinder is separated into sections with screens with suitable sizes separating the sections.
The initial grinding is done in the first section and the final grinding is done in the final section. In some types of theses mills bars are used instead of balls in order to obtain particles with slightly different sizes. This type of mills is suitable for dry grinding or grinding of colloidal particles. The roller mill and ball mill are used in small factories. Presently, the most common used mills, in large modern factories, are sand mills (vertical or horizontal) and dyno mills.
During the manufacturing steps in paints or varnishes industry or during the oil heating process the liquids are contaminated by foreign matters that fall into them. Moreover the paint may contain particles that were not ground to the required size or some polymers that didn’t dissolve. Some surface hardness may also exist. For all the previous reasons, paints and varnished liquids must be purified by one of the following methods:
- Single cylinder mill: It can work as a screen as all large pigments particles and foreign particles will be separated in the mill hopper.
- Fine screens.
- Filter press.
- Centrifugal separator for varnishes purification.
- Settling for varnishes purification.
The packing may be manual, semi-automatic, or automatic according to the size of production. There is a number of packing machines differing in speed and packs handling.