During the storage process, the phenomenon that the solid component sinks to the bottom of the container is called precipitation, which is also called sedimentation. When the precipitation phenomenon is severe, particles such as pigments in the coating are precipitated into a dense mass which is not easily redispersed by stirring, and is called agglomeration.



01 phenomenon



When the paint is opened, it is inserted into the paint bucket with a stir bar. When it is lifted, if the color of the adhered paint is one, there is no precipitation. If it is thick and thick, the bottom has a feeling of difficulty in agitation, that is, precipitation occurs. When the stir bar cannot be inserted into the bottom of the bucket, the bottom precipitates tough and cannot be stirred evenly, and agglomeration occurs. Common types of precipitation coatings are: red lacquer, antifouling paint, antirust paint (such as micaceous iron oxide antirust paint), low performance latex paint.



02 reasons



(1) The pigment used is not finely ground or poorly dispersed; in the formulation, the pigment base ratio is too large, and the density of the pigment filler is large, which is easy to cause precipitation.



(2) Due to too much diluent added, the viscosity of the coating is low, the coating loses its normal surface suspension state, and the pigment filler precipitates.



(3) A certain chemical polymerization reaction or mutual adsorption occurs between the pigment and the resin to cause gelation and precipitation.



(4) The storage time is too long, exceeding the retention period of the paint, especially for a long time. The color paint storage time is too long, and the pigment filler has a large density, and a precipitate forms at the bottom of the sauce, eventually forming an induration. Some resins also deteriorate.



(5) The storage temperature is too high, and the storage temperature of the general paint is preferably 15~25°C. Above 30 ° C, the suspension properties of certain pigments and resins in the paint are destroyed, causing the viscosity of the coating to decrease and precipitate agglomerates.



03 Prevention measures



(1) When designing the coating formulation, consider the appropriate pigment base ratio. Generally, the ratio of pigment to base should not exceed 2:1, except for special varieties. The pigments should be selected to have a lower density. Some pigments such as red powder, chrome yellow, micaceous iron oxide, iron red, and precipitated barium sulfate are all dense, and are used less or as appropriate with low-density pigments.



(2) The viscosity of the coating should not be too low, and the selected pigments and fillers should not react with each other. Such as zinc powder, aluminum powder, zinc oxide, etc. are more active, in the paint with poor storage stability, try not to pack or can be packaged. When the paint is produced, use suitable equipment, and grind and disperse as much as possible to achieve the specified viscosity and fineness. For example, chlorinated rubber coatings have poor stability and are easily reacted with iron-based materials and are produced using stainless steel or stone-lined equipment.



(3) Add anti-settling agent to the coating, and make a thixotropic coating with special properties of a small amount of additives to change the suspension state of the coating. Commonly used anti-settling agents are stearic acid series, fumed silica, modified bentonite, hydrogenated castor oil and the like. There are many kinds of imported anti-settling agents. Generally, 0.1%-0.5% can be added to prevent precipitation. When 2%-3% is added, the coating can be made into thixotropic coating to form a higher film thickness.



(4) The paint should be stored in a cool and ventilated place, and the paint bucket should be placed horizontally, inverted or shaken several times. Pay attention to the storage period of the paint, first use it first. Special care should be taken for some coatings with low shelf life, such as chlorinated rubber, vinyl chloride, and polyurethanes.



(5) For the deposited paint, it can be used after being stirred by hand or mechanically. For the agglomeration phenomenon, the flowable part should be poured out first, and the sediment should be scraped from the bottom of the container with a spatula. After grinding, the flow medium is poured back into the original barrel and mixed thoroughly. If it is still unable to mix according to this method, there is still dry precipitate, and the paint can only be scrapped or degraded.