Comprehensive filtration is an efficient, low-cost method of reducing coating and drying defects that are the result of foreign material, contaminants, being introduced into the coated layer. In the ideal process these defects could be prevented by not introducing the contaminants at all components of the process. However, practically this is very difficult to accomplish. Therefore, filtration is required in several process locations to reduce these contaminants and prevent them from causing defects. A good analogy for the need of filtration is collision insurance for my car. I do not plan to have an accident. But, if I do have one and it is damaged, the insurance will be appreciated.
There are many potential sources of contaminants in the coating process, which can lead to defects in the web coating process:
- The raw materials for the coated product, substrate, binder, solvents, surfactants, coating aids, solvent, mattes, cross-linkers additives can introduce contaminants.
- The solution mixing process can generate contaminants, by carrying over impurities in the raw materials. Inadequate mixing of the polymer can result in creating contaminants such as polymer gels particles and agglomeration of solid matte particles. Inadequate cleaning of kettles and solution lines can lead to contamination from previous solutions made in same kettles.
- The air used in the dryer can contain contaminants particles, which can be deposited on the coating.
- The air in the coating room can contain, dirt, fibers from clothing and skin debris from the operators, which can be deposited on the substrate or the wet coating.
To minimize these defects, there are two types filters should be used in several components of the overall coating process. Liquid filters to purify the coating solution streams and air filters for the dryer and room air.
Figure 1 is a process diagram that shows key process elements and potential locations for insertion of filters. The filter assembly is relatively inexpensive and it is better to have too many filters then to have an inadequate number.
Filters should be used on all liquid streams to the coating applicator and when transferring solutions from the mixing kettles. There are several types in which can be used, Plate Filters, Cartridge filters, Membrane filters, and Bag Filters.
The dryer and ambient air must be filtered to remove particulate matter. An effective filter for the dryers is the High-Efficiency Particulate Air filters (HEPA). These filters, is allow no more than 3 particles out of 10,000 to penetrate the filtration media. Lower efficiency filters can be used for the ambient air.