As industrial users, all of us have certain expectations of paints. Paints must adhere to the substrate we are painting. The paint should have’ good consistency and should provide a smooth covering without craters or other surface irregularities. Colour must be the same in each drum we use.
Paint Manufacturing Process
Simply put, paints are mixtures of resins, solvents and pigments. Ingredients are added to these major components to provide the characteristics needed for the specific application. The ingredients are weighed and mixed in a mixing reactor with controlled conditions. The entire process can take as little as 30 minutes to make the paint. Once mixed the paint is continuously circulated through filters to remove impurities that would affect the surface characteristics of the paint. Samples of the paint are sent to laboratory where it must pass several quality control checks before it is released. A few of the selected checks are as below.
Quality Control Checks
- Draw Down Test: In this test, the paint is spread on a glass plate with a wire-wrapped rod. After drying the surface is examined for irregularities such as craters, gels and particles of grit. Gloss and adhesion properties can also be checked. Surfaces are categorized as acceptable or unacceptable.
- Hegman Grind Test: A Hegman gauge is a rectangular block of stainless steel with a wide, inclined groove machined in the top surface. The gauge is graduated along with its depth. Paint is placed at the deep end of the groove and spread toward the shallow end with a scraper bar. As contaminants larger than the space between the groove and scraper bar are encountered, scratches are formed along the groove surface. Each paint formulation has a specification for the position and number of scratches. If the paint fails either of these tests, circulation through the filters continues until subsequent tests are positive. Only then can the paint be packaged.
Types of Filters Used
Generally, bags and resin bonded cartridges are used as paint filters. Following are the properties which a Filtration media should have:
- Must be silicone-free
- Must remove gels effectively
- Should minimize the circulation time required before packaging
- Should be capable of filtering a complete batch
- Should not collapse or allow fibre material to migrate into the paint
- Should not begin pigment particles removal with an increase in efficiency during filtration
Silicone Free Filter Media is Necessity
Silicone is a release agent and even minute amounts in paint can adversely affect the adhesion and appearance of paint. Silicone can cause paint to peel and can cause craters in otherwise smooth painted surfaces. Filters can be a source of silicone. Threads on filter bags or fibre additives and dyes on filter media may contain silicones. Even lubricants on the filter media production equipment and cleaning materials may contain silicone. Filter manufacturers must take precautions to eliminate sources of silicone.
Filtration of Gels
Gels are highly cross-linked polymer webs caused by the presence of multi-functional monomers during polymerization. They have high molecular weight and do not dissolve in the solvent. They deform under stress and cause problems during filtration. Gels could work through if the media deforms as the pressure increases. Therefore, rigid, resin bonded filters are preferred because they provide a stable fibre matrix. To increase the efficiency of gel removal lower than normal flow rates are recommended.
Maintaining Consistent color
Filters should remove large particles and agglomerates, not dispersed pigments. Classification is separation of particles as a function of size: large particles are captured and small particles, like pigments, are allowed to pass through the cartridge. Clarifying filters remove all (or most) particles without regard to size. As filters begin to plug by forming a cake, they tend to become more like a clarifying filter. Good classifying filters will have uniform fibre structures throughout the efficiency control layer and a controlled graded density construction.
Cartridges with low resin content tend to compress as the pressure drop increases. This increases efficiency and may contribute to pigment removal. Wound type filters tend to form filter cakes rapidly, leading to frequent changes and batch process disruption. The low resin content filter act like a classifier until about 25 psid when the media begins to compress and the efficiency increases. The wound cartridge construction has lower initial efficiency than the others and shows an increase in efficiency.
Filtration media in paint application need to have necessary properties to produce quality paints, like silicone-free, good efficiency, long life, cleanliness, rigidity, ability to classify particles. In the past, the use of resin bonded cartridges has been highly prevalent for the industry due to its rugged nature and low migration of fibre. However, in recent times the resin bonded filter manufacturing has been discontinued due to the environmental impact of such manufacturing process. New alternate methods are being adapted like, the Stainless steel metal cartridges and Bag filters. The stainless steel metal elements have a unique advantage in that they can be cleaned and re-used.
We at filter concept can support paint manufacturers in offering a complete solution to paint filtration. Please get in touch with application support teams to know more on our solutions for the paint manufacturing Industry, firstname.lastname@example.org