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Canlin Classroom: coating materials - characteristic 2
  • Release date:2020-03-09
  • Browse times:4928



  You can reduce the viscosity of the coating by diluting it with a solvent, heating it, or both. You can also reduce the viscosity of the paint by cutting. Shearing is a collective term for various methods of agitating, compressing, rubbing, or moving a liquid.


We classify fluids into two types, one is Newtonian fluid and the other is non-Newtonian fluid.

Newtonian fluid

Refers to fluids that have no effect on fluid viscosity at a given temperature. Agitation also has no effect on its physical properties. Such fluids include water, certain oils, and nitrocellulose paints. What affects viscosity is temperature.

Non-Newtonian fluid

Most fluids used in the coating industry, at a given temperature, their viscosity changes under shear. Non-Newtonian fluids can be divided into three categories:

 -Pseudoplastic fluid (viscosity decreases with increasing shear force)

-Dilatant fluid (viscosity increases with increasing shear force)

-Plastic fluid (viscosity decreases with increasing shear). It has an initial shear value, which is laborious at the beginning of agitation, and once agitated, it takes less effort.


  According to the coating's response to shear and stirring, the coating is often regarded as colloid → solution → colloid. The coatings just bought from the supplier are colloidal, such as latex paint. When it is agitated, it is called a thin liquid, and when it stops stirring, it returns to a colloidal state. The conversion capability between the two states is determined by the molecular structure of the coating. Coatings with this characteristic do not settle. It also has advantages when spraying. Once the coating is stirred, it will easily flow to the spray gun and be atomized, and it will become colloidal when sprayed on the surface of the object, which reduces sagging.


  The abrasiveness of coatings must be considered. Because it is related to the life of the equipment. Oils and greases are very abrasive and zinc-rich primers are very abrasive. All fluids are somewhat abrasive when pressurized. Because the factors that affect abrasiveness vary widely, it is difficult to predict equipment life. These factors include: the flow rate, pressure, flow rate of the fluid in the system, and the physical characteristics of the fluid. When using highly abrasive paints, such as zinc-rich paints, enamel glazes, and paints containing sharp and hard particles, be sure to choose the equipment that is matched with them. Tungsten carbide carbide flow nozzles and pump valve seats should be used. Newly developed plastic and fluororesin materials are widely used in pumps, which are designed for abrasive coatings. When using highly abrasive coatings for airless spraying, tungsten carbide nozzles should be used. Low-pressure spraying equipment cannot be used for high-pressure airless spraying. Doing so will not only damage the equipment quickly but is also unsafe.


  Precipitation of pigments and other solid particles dispersed in water or suspended in solution is common, and most coatings require frequent stirring to maintain uniformity. Some paints, such as metallic paint, must be continuously circulated between the mixing station and the spraying station and stirred in the storage tank to prevent precipitation (color difference) of the paint. Sometimes it is better to use a small-bore nozzle and a small internal warp pipe in the system. It speeds up the fluid and reduces the amount of sediment. The principle of "first in-first out" should be followed when storing paint. Various stirring equipment can be used, but the lowest stirring speed should be used if the required requirements are met.

 12. Application period

  The pot life is used to describe two-component coatings. Two-component coatings generally include a resin and a curing agent. The two are mixed in a certain ratio before spraying. Once mixed, a chemical reaction occurs and begins to solidify. The pot life is the time that the coating can be prevented after mixing. For different coatings, the pot life ranges from seconds to hours.

  Ordinary spray equipment is not suitable for coatings with short pot life. It is necessary to apply special equipment for precise calculation and coating of two-component coatings. Special equipment is required to clean equipment using two-component coatings.

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