Paint Manufacturing Mixing
MIXING FOR PAINT MANUFACTURING
Mixing in the manufacture of paint can have significant effects on both product quality and production time. Particularly paint formulations have become more viscous and complex rheologically as the market has moved from primarily solvent-borne to water-borne formulations. Also the demand for paint concentrates, which are diluted by the customer, has increased the viscosity and shear-thinning behavior of the products.
Pigment is often supplied as a dry powder which must be mixed with a solvent to form a slurry which is fed to the Let Down process. The agitator is required to wet, de-agglomerate and disperse the pigment particles in a liquid. This is usually performed with High-Shear Disperser (HSD) impellers that essentially look like circular saw blades and operate at high rotational and tip speeds.
The main step in the process is Let Down where the ingredients of the paint are mixed to create the product. There are a number of mixing related issues that must be considered for a successful Let Down:
- The batch must be mixed for a sufficient time to ensure that it is homogeneous so that, when a sample is taken for quality control, it is representative of the composition of the whole batch. If the sample is taken too early it may not be representative of the batch composition and small component additions made on the basis of the tests will be in error. In this case several adjustments will need to be made, up to three or four per batch, adding significantly to the cycle time.
- Plants are required to produce paints with different rheological properties in the same equipment sets. These will require different mixing times to reach the required degree of homogeneity dependent on these properties. We have extensive capabilities to measure the rheology of viscous fluids and to size new agitation equipment based on the desired batch time and the correct interpretation of the results.
- Colloidal shock is a phenomenon that occurs when two dissimilar fluids, such as the pigment slurry and latex for example, are mixed. The pigment particles are stable in their original state but, as the two fluids are mixed there is a brief period of time when the pigment particles are unstable and can coagulate to form “grit”. Grit must be filtered from the paint and, in some cases, extra pigment must be added to the batch in order to replace the particles that have been removed. This increases the batch time and adds unnecessarily to raw material costs. We can perform tests at lab-scale that enable this problem to be investigated and make recommendations for operation of plant-scale equipment that will minimize shock.