Paint Defects-Chalking

Paint Defects-Chalking

1．Description

This is a typical coating defect that occurs when the paint film degrades and gradually decomposes from the object or building being coated over time, typically on the surface of the coating (as shown in Figure 1). This phenomenon is usually caused by weathering, exposure to ultraviolet radiation, or other environmental factors that lead to the degradation of coatings over time. Epoxy coating is a typical type of coating that is prone to powdering.

Figure 1.Surface powdering diagram of coating

2．Probable Causes:

a. Exposure to ultraviolet radiation: Ultraviolet radiation is the main cause of paint powdering. When the paint is exposed to sunlight and heat for a long time, due to photo oxidation reaction ①, thermal decomposition reaction ② and other reasons, the resin polymer chains in the paint decompose and gradually become small particles, causing paint powdering. This may cause the surface of the coating to decompose, resulting in powdery or chalky residues on the surface.

b. Low quality coatings: Using low-quality coatings without appropriate additives to resist weathering and UV rays can also lead to paint powdering.

c. Temperature changes: Frequent temperature changes cause thermal expansion and contraction stresses in the coating, leading to the appearance of small cracks inside the coating and accelerating the powdering process.

d. Damp environment: High humidity or long-term exposure to moisture can cause the coating to absorb water and expand, leading to internal structural damage. At the same time, due to hydrolysis reactions and other reasons, the performance of the coating decreases, resulting in powdering.

e. Incorrect construction steps, such as insufficient drying time between two constructions or failure to follow the manufacturer's instructions, can also lead to paint powdering.

3．Prevention:

a. Add a layer of weather resistant topcoat: Add a layer of strong UV resistant topcoat to achieve the best effect;

b. Control environmental conditions: Whenever possible, avoid exposing coatings to environments with strong ultraviolet radiation, high humidity, or drastic temperature changes;

c. Choose higher quality coatings: Choose coatings that are more suitable for different service environments, use coatings in areas that require long-term exposure to ultraviolet radiation, and try to choose coatings with better weather resistance and water resistance, such as fluorocarbon resin;

d. Correct construction according to product instructions: Leave sufficient drying time between two constructions and follow the manufacturer's instructions for construction.

4．Repair:

Use cleaning tools to remove the powdered part, using a hard bristled brush or high-pressure water gun. If the water itself is not enough to remove the residual powdered coating, soapy water can be used as cleaning water. If necessary, it is necessary to reapply the coating, and an additional layer of UV resistant topcoat can be chosen to achieve the best results.

5．Interpretation of professional terms:

① Photooxidation reaction: Under ultraviolet irradiation, chemical bonds such as aromatic ether bonds in epoxy coatings absorb photon energy and break, producing free radicals. The generated free radicals will react with oxygen in the air to form peroxide free radicals. These peroxide radicals are very reactive and can further trigger a series of oxidation reactions, causing polymer molecular chains to break and crosslink. As the oxidation reaction continues, the polymer molecular chains are gradually destroyed, the molecular weight decreases, the cross-linking structure of the coating disintegrates, and some small molecular fragments are formed. These small molecular fragments gradually accumulate on the surface, ultimately leading to coating pulverization.

② Thermal decomposition reaction: Under high temperature conditions, chemical bonds in epoxy coatings absorb thermal energy and undergo vibration and fracture. Especially some weaker bonds, such as C-O bonds and C-N bonds on the side chains, are more prone to breakage, resulting in small molecule compounds and free radicals. The free radicals generated by thermal decomposition will further trigger the degradation reaction of molecular chains, causing the length of polymer molecular chains to shorten and the molecular weight to decrease. At the same time, small molecule compounds produced by thermal decomposition will form pores and defects inside the coating, weaken the structural strength of the coating, and ultimately lead to surface pulverization of the coating.

③ Hydrolysis reaction: Water, as a nucleophilic reagent, can attack certain chemical bonds in epoxy coatings. For example, under alkaline or acidic conditions, oxygen atoms in water molecules can launch nucleophilic attacks on ester bonds, amide bonds, etc. in epoxy groups or curing agents, leading to chemical bond breakage and the formation of corresponding alcohols, acids, and other products. In this process, the decomposition of the curing agent will reduce the cross-linking density of the coating, while the hydrolysis of the epoxy resin will cause the molecular chains to break, and the integrity of the coating will be damaged, leading to powdering.

(Note: The above professional terminology analysis is based on epoxy resin as an example.)