Research and summary of coating of power 9V carbon battery

Coating process is a process based on the study of fluid properties, in which one or more layers of liquid are coated on a substrate. The substrate is usually a flexible film or backing paper, and then the coated liquid coating is dried in an oven or cured to form a film layer with special functions. The main methods of coating lithium-ion battery poles are comma roller transfer coating and slit extrusion coating.

1. The influence of slurry properties on coating

1.1. The influence of viscoelasticity of coating solution on coating results

The properties of the coating solution itself have a great influence on the coating results. Before you coat, the shear rate-viscosity data of the coating solution is a very important consideration. When the coating solution has very strong viscoelasticity, it usually brings greater difficulty to the coating process.

Viscoelasticity is the reaction of viscosity and elasticity after fluid deformation. Some fluids only have viscosity changes under shear force, while more complex fluids have strong rebound properties after shear deformation. How to judge the viscoelastic properties of a fluid? It can be judged by the shear force and viscosity curve, as shown in the following figure. When subjected to shear force, the viscosity has a hysteresis phenomenon, which is a viscoelastic.

After the pressure is released, this change in the fluid will greatly affect the coating effect.

So what is the impact?

In slit coating, if the solution is squeezed out from the lip of the coating head with a large shear force, or coated at a faster speed, because the solution will shrink after the coating is completed and the pressure is released, it will eventually lead to the formation of vertical stripes. In severe cases, the shrinkage of the solution will cause wrinkles and curling of the film.

So how to avoid these defects?

In the process of slit coating, we need to carefully control the flow of the fluid inside the coating head, let the solution flow out of the lip very evenly, and try to minimize the shear force of the solution.

Generally speaking, general coating solutions have the ability to self-level, and viscoelastics are not easy to flow, so the flatness of the film will be lower.

When you want to coat a viscoelastic body, you must first fully understand the reaction of this polymer with a higher molecular weight under stress, so that you can better offset the defects caused by it through process control during the coating process.

Generally speaking, coating this type of fluid under lower shear force is the best way to reduce the probability of defects!

2. Basic issues of coating process

2.1. The importance of vacuum box in SlotDie coating

In the SlotDie coating process, the fluid will involve pressure, viscoelastic force, inertial force, surface tension, etc. Under the action of various forces, the entire fluid system reaches a balance to form a stable coating process.

When analyzing the force and flow rate of the fluid in the die slit and gap, we will approximate the model based on the lubrication theory:

Where Pc is the die slit or the inner cavity pressure, PD is the die upper lip pressure, and PE and PU are the pressures in different areas of the lower lip

Then based on the Couette fluid and Poiseuille flow pattern, according to the Stokes equation, it can be calculated:

The flow rate of the liquid in the die slit:

The flow rate of the liquid at the lower lip of the die:

In the stable coating process, the flow rate at the lower lip of the die should be 0, and only when it is 0 is the droplet position at the lower lip stable.

Based on the above derivation, it can be calculated that PU is less than PE.

Then let's look at the flow rate at the die lip, which is relatively more complicated.

According to the diagram above, the flow rate at the die lip can be calculated:

According to the above formula, it can be obtained:

If you want to get a thinner coating, then you want t to be less than 1/2 of HD, so if that's the case, you must do this:

That is,

The conclusion is that Po is less than atmospheric pressure. How to achieve less than atmospheric pressure is to add a vacuum box. In other words, if you want to get a relatively thin coating thickness, the use of a vacuum box is very necessary. The above is the theoretical basis for the necessity of using a vacuum box in the entire die coating.

2.2. Die coating problem of particle-containing solution

The general simple research method is to first regard the solution as a Newtonian fluid and analyze the relationship between the average particle concentration and viscosity. However, experimental data show that the actual situation of the particle-containing solution when coated with a slot die is much more complicated than imagined. In 1987, Leighton and Acrivos found in the experiment that when the solution is subjected to a certain shear force, the particle distribution in the solution will be uneven. For many functional coatings, the microstructure of the coating is very important to the functional performance of the coating.

Therefore, it is very important to understand and predict the distribution of coating particles in the coating process.

Mathematical model

I have always believed that the fluid model in the die coating process is a relatively complex model because it involves the analysis of more different interfaces. As shown below:

According to the law of conservation of momentum:

Mass conservation:

Particle migration mechanism in solution:

The above formula assumes:

-Particles are hard spheres and incompressible

-Ignore Brownian diffusion, and the particle diameter is less than 0.5 microns

Based on the above formula and assumptions, the following conclusions are drawn:

The situation of particle-containing solution in the coating slit

*The concentration of particles is relatively low near the slit wall due to high shear force

*The particle concentration in the middle of the slit is the highest, and agglomeration may occur

2. The situation of particle-containing solution near the coating lip

*The concentration of liquid particles is low near the lip due to shear force

*The particles move to the low shear area and accumulate near the substrate surface

The above article content mainly comes from Carvalho Slot Coating of Particle Suspension

2.3. Multilayer Co-coating Technology

Coating multiple layers on a substrate at the same time, these multiple layers do not mix, and can be dried. This process is very powerful. This article talks about some things that need to be paid attention to when using slot coating (Slot Die) to coat 2-3 layers at the same time. If you have to coat more than 3 layers at a time, you can only use slope coating (Slide Die), as shown in the cover picture. Multilayer co-coating was mainly used in the production of photosensitive film before, so many related patents were also Kodak's at the time.

CLOEREN MultiLayer Slot Die

Because in the multilayer co-coating process, multiple layers of solution need to be coated at the same time, so each layer of solution to be coated needs to have a unique set of The pumping system is independent, and each pumping system is also a metering system. Of course, the type and size of the pump needs to be selected according to the characteristics of each solution.

The key to multi-layer co-coating is that the solutions cannot mix when they come out of the coating head and are applied to the substrate. How to do it?

Surface energy...

The surface energy of the upper solution must be lower than that of the lower solution, which can be achieved by adjusting the surfactant. There must be enough surface energy difference between each layer of solution to achieve stratification between the solutions

σtop<σbottom

The challenges of multi-layer co-coating are mainly the following:

How to avoid mixing of the coatings in the coating gap at the contact interface between the coatings.

How to adjust the separation point of the solution at the coating head lip so that the solution is at the coating head lip The difference in viscosity will affect whether the layers will be cross-mixed. During the coating and curing process, the release of solution stress needs to be considered, which will cause layer mixing.

Difference in surface energy In addition to the above, the wetting of the solution between different layers and the wetting between the solution and the substrate need to be considered.

There is also a very critical factor to consider, that is, when the coating thickness is less than 1/3 of the coating gap, the coating will easily mix.

Now, for multi-layer co-coating, we consider more about the characteristics of the solution itself, and discuss less about the details of the process. With the development of technology, in the near future, multi-layer co-coating will become the only choice for cost saving.

The main content of this part is translated from CoatingTechSlotDies.

III. Coating defect analysis

3.1. Formation and solution of "gel" defect

"Gel" defect is common in coating process. The defect appears as a small round or oval dot. Usually, you can see a gelatinous particle in the center of the defect dot with the naked eye or under a microscope.

Gel is mainly formed during the preparation of glue. Some polymer particles are not dissolved in the solvent, but only swell and eventually form soft gel particles. These particles are highly shrinkable and easy to compress, so it is difficult to filter through the filter. When there is a certain pressure in the filtration system, the gel particles will deform and eventually pass through the filter screen. A 15-micron gel particle can be compressed and pass through a 5-micron filter.

The best way to solve gel is of course to make the polymer completely dissolved in the solvent as much as possible during the preparation of glue. The full dissolution of the polymer usually requires special treatment. For example, in some cases, it is necessary to premix the solvent and the polymer. The two-component solvent is sometimes helpful for the dissolution of the polymer. In addition, the order of adding the components in the preparation of glue must be strictly followed. Optimizing the temperature curve of the reaction is also the key to controlling gel. Finally, during the coating process, the pump glue system should keep a low pressure as much as possible, so that the filter can effectively filter out as much gel as possible.

3.2. Defects in surface tension

The main problem of the coating process is to study the properties of various surfaces/interfaces between the solid substrate and the coating liquid. Today, we will briefly introduce some coating defects or phenomena related to surface tension:

(1) Orange peel phenomenon

During the coating process, due to the evaporation of the solvent in the coating solution, temperature differences are generated in different areas. At the same time, due to the different evaporation rates of the solvent on the surface and the bottom of the solution, the solvent content also has a concentration difference, resulting in a surface tension gradient and natural convection. The coating solution flows from the area with low surface tension to the area with high surface tension, which ultimately causes the uneven coating surface or orange peel phenomenon. If the drying rate of the oven is too fast, or the hot air speed of the oven is too fast, the solution will solidify prematurely before leveling, forming this defect.

Real orange peel and orange peel on the coating film

Methods to reduce this type of defect:

Reduce the drying rate so that the solution has enough time to level

Try a different solvent

Add some low-volatile solvents to the solution

Add surfactants

(2) Thick edge

After coating, the solvent at the edge of the coating solution evaporates faster than the internal area, so the coating solution in the low surface tension area flows to the edge and accumulates, making the edge too thick.

(3) Pinholes and craters

During the coating process, the coating substrate is contaminated by objects with lower surface tension, such as oil droplets, dust, etc., and the coating solution around the contaminants flows in the direction of higher surface tension, forming defects like craters. If the contaminants already exist on the substrate before coating, defects similar to pinholes will be formed.

3.3. Regular vertical stripe defects

Usually, the "regular vertical stripe" (Ribbing) defect refers to parallel stripes that appear along the machine direction and have this defect on the entire coating width. If you use a comb or a rake to scratch the rubber surface just out of the coating head along the machine direction, you will artificially scratch out the same appearance defect. (As shown below)

The most disgusting thing about this defect is that once it occurs, your product yield will drop to 0% at once, because you can't find a place you can use, and other things such as protrusions and lines can be removed when cutting.

How are regular vertical stripes (Ribbing) produced:

From the perspective of fluid dynamics, this defect is caused by the unstable force of the coating liquid at different positions. This instability will present a lateral sinusoidal distribution of the coating thickness on the coated substrate.

The difference in the force of the fluid causes the uneven distribution of the coating thickness. The difference in force is caused by the superposition of the viscoelastic force, inertia force, surface tension and other forces of the coating liquid itself in different directions. Generally, it is difficult to eliminate the stripes by leveling during the drying process. Although the difference in coating thickness may be relatively small, in many cases or at a certain angle, this defect can still be easily observed by the naked eye. Of course, if you understand its generation mechanism, you can still avoid the occurrence of this defect through process adjustment. From another perspective, it must have a certain process operation window. Take the slit coating method as an example:

If you use a roller with grooves such as a wire rod to coat, when this defect occurs, your first feeling is that the grooves of the wire rod cause this defect, but when you measure the gap width of the stripes and compare it with the groove width of the wire rod, you find that The two are not the same, and it is not entirely the fault of the groove!

How to effectively avoid this defect:

The first step to solve the problem is to determine whether your current process is within a reasonable process window (process window), because only when you know the process window can you know how to adjust it. You can't just adjust it blindly and take your chances! During coating, many process parameters are adjustable, and you can achieve the effect you want by adjusting them.

The second step, if you really don't know the process window, there are some general methods.

Dilute your coating solution to reduce the viscosity of the solution (actually, reduce the Ca of the solution)

See if you can add some surfactants

If it is roller coating, reducing the diameter of the roller is also an effective method

The third step, if you find a better operating condition, you must make this operating file into a standard operating procedure, so that you can make good products sustainably and repeatably!

The original translation is from Edward Cohen How can the ribbing defect be iminated in web coating".

3.4. Horizontal stripe defect

We talked about vertical stripes before. Today, let's talk about horizontal stripes. Stripes have always been a popular shape in the coating industry, and they always show off in different ways.

Usually, the horizontal stripes we see often appear regularly, or appear with frequency, and run horizontally through the entire film surface.

Why do we have to talk about this defect alone? Because for many years, everyone has blamed the coating head for this defect, but in many cases, this defect has nothing to do with the coating head. I really can't stand it, so I'm going to talk about it.

Because it gives people the illusion that you watch the solution come out of the coating head and then apply it on the substrate. Once it is applied, the horizontal stripes appear, so the biggest suspect must be the coating head! In fact, there are two main reasons for the generation of horizontal stripes, but these two main reasons are well hidden and not easy to be discovered by you.

The first possibility is the pump. If the pump itself has pulses, the solution pumped outIf the flow rate is not stable and continuous, the solution coming out of the coating head will not be stable.

Therefore, when choosing a metering pump, it is best to choose a pulse-free gear pump, pressure tank, screw pump, etc. to connect to the coating head. These stable pumping systems can ensure that the flow rate coming out of the coating head is also stable. If the above pulse-free pump is not suitable for your process, at least install a buffer device for the pump to minimize the pulse.

In addition, the compressible pipe connecting the pump and the coating head may also be the cause of coating fluctuations. In short, a good coating solution delivery system is your first step in coating.

The second possibility is that the interference of your coating equipment caused this defect.

The simplest verification method is to compare the frequency of the horizontal stripes with the vibration frequency of the equipment to see if they are consistent. If they are exactly the same, then if possible, stop the equipment to see if the defect disappears. If so, find a way to move or replace the equipment.

When the coating roller is not well connected to the motor, the probability of this defect is very high. For example, when the back roller on the slot die head equipment has a large jump, you can easily have this defect regularly during coating.

Now there are many devices that can test the roller jump, so if you decide to buy a good coating die head, it is recommended that you also buy a good back roller behind the die head. A good supplier will help you adjust the roller jump before leaving the factory, not just the high precision of the die head.

How to solve the horizontal stripe defect?

Well, we don’t just give fish, but also teach them how to fish. If it is caused by the equipment, then replace the roller and motor. If it is caused by the pump, then replace the pulse-free pump.

However, sometimes you find that after making the above efforts, the horizontal stripes are still there. It is recommended to install a vacuum box on the coating head (slotdie) to try. If there is a negative pressure at the moment when the coating solution is applied to the substrate, it can stabilize your coating droplets well, which can greatly reduce fluctuations.

The vacuum box is not a panacea. If it is not used properly, it will become the culprit of horizontal stripes. Imagine when the vacuum negative pressure is unstable or pulsating.

Sometimes if you do some treatment on the surface of the coated substrate to make the coating solution wet better on the substrate, or in other words, have better leveling, this will also help alleviate this defect.

Having said so much, what I want to emphasize most is that the coating process is actually a process composed of many unit operations and is the result of teamwork, so the blame for the horizontal stripe defect cannot be put on the coating head alone.

So many times when analyzing coating defects, you can't focus on the most likely cause you think. Sometimes it is more complicated than you think, or the cause is not as obvious as you think.

Read recommendations:

6F22

Dilute lithium silicate battery.3.7v 2200mah 18650 lithium battery

Lithium battery safety

601848 battery Factory

18650 lithium ion battery cell