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Solution for white spots in coating

by:dcfpower     2021-03-13

In the lithium battery pack production industry, the least contact and the most inexplicable pits (white spots) appear during coating and black spots after rolling. What is the matter?

Phenomena: In the battery production coating process, it often appears as a bubble-like white dot-like pit, or a foil-like problem; many people like to judge this as a bubble, but it is not the case, but Shrinkage. Combined with the process recipe, the problem may lie in the VGCF. Experimental program: As a process, keeping the formula unchanged to improve the slurry is the basic principle.

Adding NMP-type high-boiling organic solvents to aid the dispersion of water-based slurry and improve the coating and drying effect is a common practice, and the results also verify the feasibility of the process.

VGCF agglomerates are used as low surface tension points, leading to shrinkage. However, in experiments, the 50% solid content is 2500mpa's viscosity, but the problem has not been solved: is it really limited by the equipment's capacity (line speed is also Just like 15m/min)?

VGCF+Water+PVP+CMC, these few kinds of scraping fineness will see shrinkage holes, then the problem is clear. Is it absolutely the shrinkage caused by the reunion of VGCf?

Obviously, the problem is not that simple. The colleagues in the slurry process basically have a habit. The viscosity of the water-based slurry is basically controlled below 3000mpa's. However, here, in order to reduce the shrinkage, the viscosity must be increased; Are you completely confused?

Under certain conditions, the shrinkage cavity is determined by the following formula:

Q (flow rate per unit time) u003d h (wet film thickness) 2×△v (surface tension gradient)/2n (paint viscosity) This formula is sufficient to prove the reason why NMP with high viscosity and low surface tension works. However, as a technician, you should know that viscosity and surface tension gradient are just influencing factors, not the most fundamental factors.

The influencing factors are basically determined: viscosity, drying, surface tension, VGCF, rubber particles (CMC).

Conjecture of other factors: Sodium carboxymethyl cellulose and PVP are surfactants, one end is hydrophilic and the other end is hydrophobic. When the concentration is too high, micelles are formed with hydrophilic groups facing outward ( Water solvent slurry), hydrophobic group included, combined with this slurry formula (VGCF, Super-P ratio is higher, CMC1.8%), VGCF Super-P itself is hydrophobic, so it is easier to affinity with micelles , Combined with the CMC+water+VGCF+PVP observed on the micrometer, there is a phenomenon of shrinkage, which reduces the ratio of polymer CMC and PVP. The design principle is to maintain the material unchanged and reduce the ratio of CMC and PVP. The essence is to reduce the concentration of colloids.

For the first time in the experiment, 3500mpa’s viscosity, 80um shrinkage holes can be seen on the fineness meter; coating is improved.

The second time in the experiment, 2500mpa’s viscosity, 65um fineness, and shrinkage; the coating effect is consistent. If it is the original ratio and the viscosity is 2500, then the coating shrinkage is impossible to see.

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