Plastic UV coatings common problems

One. Poor adhesion

Mechanism: There is no chemical cross-linking between the single-liquid primer and the UV topcoat, essentially the intermolecular forces between the primer and the topcoat (including intermolecular van der Waals forces and hydrogen bonding forces), as well as the primer and The mutual dissolution between the topcoats and the physical anchoring brought about by the penetration require a matching between the primer and the topcoat. The primer is too hard to paint, and the UV topcoat is too weak to form a good interlayer bond. The primer is too soft to bite the bottom, hair, and fog. The same topcoat is too thick. Too much volume shrinkage also has an effect on adhesion.

A. Possible causes of adhesion:

1 The primer has high hardness; the coating time is too long after painting, and it is too dry; the high aluminum powder content of aluminum powder paint is an important factor affecting the adhesion between layers.

UV topcoat causes: The adhesion between the resin and the monomer itself is not high; or the high functionality in the formulation design causes too much volume shrinkage during radical polymerization (especially when the coating film is too thick, the volume shrinkage affects the adhesion) More obvious); or UV curing is not complete, does not form a molecular chain of sufficient length. The solvent is too weak or too volatile, resulting in insufficient secondary dissolution of the primer and plastic substrate. The surface tension is too high to adequately wet and level the primer, and the formulation design is unreasonable.

B. Vacuum plating UV primer solution:

1. Adjust the hardness of the primer to increase the baking temperature of the primer to extend the baking time. The aluminum powder content should be controlled within a reasonable range, using a primer system compatible with UV.

2. Control the volume shrinkage of the UV coating film, adjust the solvent's solubility and evaporation rate, adjust the film thickness, initiator dosage and lamp power, re-select raw materials and adjust the formula.

2. Hardness

A. Factors affecting hardness and solutions

1. Definition of hardness: The hardness of the coating generally refers to the hardness of the pencil test on the surface of the coating. The test results mainly depend on the hardness of the coating itself, and also on the toughness and surface slip of the coating.

2. Possible reasons for affecting hardness:

3. Primer hardness and film thickness;

The structure and functionality of the selected main UV resin, the higher the monomer functionality, the higher the hardness, the larger the volume shrinkage, and the poor adhesion during thick coating; the final UV curing conversion rate of the formulation requires UV energy to be no less than 800mj/cm2. . The crosslink density of the acrylic double bond in the coating;

4. UV line infrared leveling temperature and time, UV exposure energy and dry film thickness.

B. Possible solutions:

1. Adjust the hardness of the primer;

2. Adjust the topcoat formula to increase the crosslink density, but not too high, and the hardness is better than H;

3. Increase the film thickness, controlled at 30-40um;

4. Increase the exposure intensity, but not too high, otherwise there are too many functional groups, too many cross-linking points, too much volume shrinkage, and the adhesion will be poor when thick coating;

5. Adjust the infrared leveling temperature and time so that the solvent in the UV paint should be fully released after the infrared leveling, and then the UV curing, but the infrared leveling temperature is too high or the time is too long, which will cause the UV paint to bite the primer and lower. The hardness of the entire coating.

3. Pinholes and pitting

A. Causes and solutions External factors:

1. There is too much dust in the environment; the surface of the object is not cleaned (dust, oil, static electricity, etc.);

2. The primer coating process is defective, not fine enough or rough.

B. Painting factors:

1. The filtration in the formulation system is not clean or unreasonable, the volume shrinkage is too large, the wettability of the coating film is not good, and local agglomeration is easily formed, resulting in pitting;

2. The dissolving power of the diluent is not enough, and the incompatible matter is precipitated after the solvent is volatilized;

3. The viscosity of the spray is too high, the pinhole caused by the too thick coating film, or the improper use of the additive, the microbubbles on the bottom surface rise without cracking.

C. Possible solutions:

1. Improve the dust-free level of the paint shop;

2. Degreasing and removing wax from the material, using an electrostatic gun to remove dust;

3. To ensure the fineness of the primer, the color paint is controlled below 15 microns, the silver paint is controlled below 30 microns, and the film thickness, flatness and hiding power of the primer are balanced;

4. Adjust the compatibility, curing rate and volume shrinkage of the formulation, and introduce a resin with a large molecular weight and a small volume shrinkage;

Control the solvency and volatilization gradient of the diluent to achieve a three-way balance of solvency, volatilization rate and bite bottom;

5. Adjust the reasonable combination of additives to achieve the balance of wetting, leveling and defoaming performance.

4. Causes and analysis of whitening of coating film

A. Possible reasons

1. The humidity of the light curing coating environment is too high, and the surface is condensed with water when exposed;

2. The temperature of the primer line is too high, and the primer line thinner evaporates too quickly. When volatilized, the water falls on the surface of the primer and becomes white.

B. Solution:

1. The humidity of the coating environment should be controlled as high as 80%; 40%-70%, temperature 20°C-30 °C

2. Reduce the evaporation rate of the solvent and reduce the thickness of the coating film.

Fives. The reasons and analysis of the formation of fingerprints

The fingerprints are derived from the combination of sweat and small molecule migration in the human body, mainly in the following categories:

1. Small molecule acrylate monomer remaining in the UV coating curing process;

2. Small molecules produced by decomposition of the initiator;

3. The role of silicone additives in migrating to the surface;

4, in general, human sweat can not be solved, can only think of ways from the formula;

The choice of monomers is important, especially for monofunctional monomers; note the degree of cure and conversion of the coating exposure;

The dosage of the initiator is reasonable to select an auxiliary agent with low surface tension and good stain resistance.

six. shrinkage cavity

A. Causes and analysis of the formation of shrinkage cavities

1. The surface of the object to be coated is oily or has wax and dust on the surface. When the surface tension is too low, it is easy to produce shrinkage holes;

2. When the air compressor is painted, oil or water is mixed into the paint to cause shrinkage holes;

3. The surface tension of the coating is too high, and the surface energy of the coated object is too low to cause shrinkage at the time of coating;

4. The surface of the object to be coated is contaminated with silicone or an excessive amount of short-wave silicone oil used in the coating is also likely to cause shrinkage.

B. Solution:

1. Check if the oil and water in the air compressor is not put for a long time; check if the oil water filtration system is working properly, it is better to install an air drying filter;

2. Clean the surface of the object with white oil or IPA before painting to remove surface oil, wax and other stains;

3. The surface tension of the coating should be adjusted as close as possible to the surface of the coated object. If the coating is not wet well, the solvent can be adjusted. After the coating, the shrinkage cavity can be adjusted with a low surface tension additive;

4. Objects whose surface has been contaminated with silicone should be specially cleaned. The additives used in the coating should be carefully screened and evaluated.

Seven. Not resistant to boiling

A. Cause analysis

1. The coating is thin or the coating is not cured thoroughly;

2. The main resin or monomer selected in the formulation contains a hydrophilic group;

3. The crosslink density of the coating is low

4. Small molecules remain after curing;

5. The adhesion of the bottom surface is poor.

B. Solution:

1. Selecting monomers and resins with better hydrophobicity;

2, the degree of curing of the coating and film thickness should be properly controlled;

3. The higher the crosslink density and conversion rate, the better;

4. Control the curing rate and reduce the residual of small molecules;

5. Adjust the adhesion of the bottom surface to achieve the stress balance of the material and the coating.

Eight. Leveling

A. Analysis of the causes of poor leveling

1. The diluent has insufficient solubility or the volatilization rate is too fast;

2. The oil ratio is low,

3. The coating viscosity is too high, resulting in poor leveling;

4. Short-wave additives are used in the formulation with high molecular weight or low coating solid content;

5. Poor resin compatibility.

B. Solution

1. Adjust the dissolving power or volatilization rate; control the oil opening ratio and coating viscosity;

2. The molecular weight of the formula and the solid content of the coating should be controlled reasonably; (short-wave additives are used less or not; choose a resin with good compatibility

nine. Cracking

A. Cause analysis

1. The supporting primer does not have enough drying time, the topcoat thinner is too strong or the volatilization is too slow

2. The exposure energy is too high, the cure shrinkage is too high, the crosslink density is high or the conversion rate is too low.

3. The coating film is too thick or the temperature during the exposure process is too high.

B. Solution:

1. The primer should have sufficient temperature and drying time after spraying;

2. The topcoat thinner should not be too strong and the volatilization is too slow;

3. Control the exposure energy, add low-functionality resin and monomer to adjust volume shrinkage;

4. Reduce the crosslink density to increase the conversion rate; control the film thickness and the temperature of the exposure process.

ten. Wrinkle

A. The main cause of wrinkling of the paint film after being covered

1. The baking time of the primer is too short, and the paint film is too thick;

2, the primer drying speed is too slow;

3, light curing solubility is too strong.

B. Solution:

1. Increase the baking temperature or extend the baking time to control the film thickness of the paint film;

2, improve the drying speed of the primer; 3, reduce the light curing solubility.