You are here:
Electrophoresis is the phenomenon of a charged particle moving towards an electrode opposite to its electrical properties under the action of an electric field.
Under defined conditions, the distance travelled per unit time by a charged particle under the action of a unit electric field strength (i.e. the mobility) is a constant and is the characteristic physical and chemical constant of that charged particle.
Different charged particles with different charges, or the same charge but different charge to mass ratio, in the same electric field electrophoresis, after a certain period of time, due to the movement of different distances and separated from each other. The distance of separation is proportional to the voltage of the applied electric field and the electrophoresis time.
Under the action of an applied DC power supply, colloidal particles move in a dispersion medium in a directional manner towards the cathode or anode, a phenomenon called electrophoresis.
The technique of using this phenomenon to deposit substances on the surface of a conductor is known as electrophoresis or electrophoresis (coating), paint deposition and electrodeposition.
Electrophoresis equipment is machinery and equipment that controls and uses the electrophoretic phenomenon of charged particles moving in an electric field to achieve the application of benefits
such as separation and precipitation in the electrophoretic process.
The benefits of electrodeposition in electrophoresis technology can be applied to the painting of metal surfaces, and the machinery and equipment that applies the electrodeposition effect to the painting of paint is called electrophoretic coating equipment.
Electrophoretic coating is a
special coating method in which the electrically conductive material is dipped
in a tank filled with water diluted with a relatively low concentration of
electrophoretic paint as the anode (or cathode), and another cathode (or anode)
is set up in the tank with its counterpart, and after a period of direct
current is passed between the two poles, a uniform, fine, non-water-dissolved
coating film is deposited on the surface of the material being coated.
In the colloidal
solution, the charged colloidal ions dispersed in the medium, under the action
of DC electric field, move towards the direction of the electrode with
heterogeneous charge. As the colloidal ions are subject to the resistance of
the dispersed medium during the movement, the resistance to ion migration in
the electric field is much greater compared to that of the real solution, and
the moving speed is slower, as if swimming in the dispersed medium, so it is called
electrophoresis. The speed of electrophoresis of colloidal particles depends on
the strength of the electric field and the characteristics of the double
electric layer structure of the water-soluble resin when dispersed.
When a direct current electric field is applied to an aqueous
solution containing an electrolyte, the water is electrolysed in the electric
field, precipitating oxygen in the anodic region and hydrogen in the cathodic
region.
Anodic
reaction: 2OHˉ→ ↑ O2 + 2H+ 4e
Cathodic
reaction: H2O + e → ↑1/2H2 + OH
Generally, the
higher the content of impurity ions in the electrophoresis body, i.e. the
greater the conductivity of the system, the more violent the electrolysis of
water, so that due to the escape of a large amount of gas at the electrode, the
resin deposition will be interspersed with pores, resulting in coating pinholes
and roughness and other ills. Therefore, impurity ions should be prevented from
being brought into the electrophoresis fluid during the electrophoretic coating
process as much as possible to preserve the quality of the coating.
During the electrodeposition coating, the charged particles
(resins and pigments) reach the electrode of opposite charge under the action
of electric field and are neutralized by H (anodic electrophoresis) and OH
(cathodic electrophoresis), turning into a coating film insoluble in water,
which is stable and dense and homogeneous. This process is called
electrodeposition.
As in anodic
electrophoresis, when the negatively charged water-soluble resin particles
reach the anode (the coated workpiece) under the action of a direct current
electric field, an electrodeposition reaction occurs, starting with the
discharge of OH
2OHˉ → ↑O2 + 2H+ 4e
OH + H = H2O
The result of this reaction is a build-up of H around the anode area, i.e. a
decrease in the local pH value, at which point the excess H reacts with the
RCOO resin anion, causing the resin to precipitate and deposit on the surface
of the anode (coated workpiece)
RCOO + H → ↓ROOOH
.
This is a phenomenon
where the dispersed medium moves in the opposite direction to the
electrophoretic particles.
The electroosmotic effect in the electrophoretic
coating process is due to the hydrated positive ions adsorbed in the coating on
the anode, which are subjected to an electric field to produce an internal
osmosis force moving towards the negative electrode, thus passing through the
deposited coating and causing a significant reduction in the water content of
the deposited coating, which is about 5-15% and can be directly baked to obtain
a dense, flat and smooth structured coating.
III. Principle of
electrodeposition coating.
1、Electrophoresis is one of the most
effective methods of painting metal workpieces.
2、The electrophoretic coating process is
accompanied by four chemical-physical changes, namely electrolysis,
electrophoresis, electrodeposition and electroosmosis.
(1) Electrophoresis:
(2)
Electrolysis:
(3)
Electrodeposition:
(4) Electroosmosis:
The electrophoresis tank is the core part of the electrophoresis system, according to the principle of electrophoresis, the electrophoresis support is placed between two buffers, the electric field connects the two buffers through the electrophoresis support, different electrophoresis uses different electrophoresis tanks. The commonly used electrophoresis baths are.
There are two electrophoresis tanks, upper and lower, and a cover with platinum electrodes. The upper tank has a number of holes, which are plugged with silicone plugs when not in use. The hole to be used is equipped with a silicon rubber plug that can be inserted into the electrophoresis tube (glass tube). The inner diameter of the electrophoresis tube was 5-7mm in the early days, but now it is getting thinner and thinner to ensure cooling and micronisation.
The basic principle and structure of the vertical plate electrophoresis tank is basically the same as that of the disc electrophoresis tank. The only difference is that the gel and electrophoresis are not in the electrophoresis tube, but in the middle of a parallel glass plate placed vertically.
The shape of the horizontal electrophoresis tank varies, but the structure is more or less the same. They generally consist of an electrophoresis tank base, a cooling plate and electrodes.
For charged biological macromolecules to swim in an electric field, an electric field must be added and the resolution and speed of electrophoresis is closely related to the electrical parameters used during electrophoresis. Different electrophoresis techniques require different voltages, currents and power ranges, so the choice of power supply is primarily based on the needs of the electrophoresis technique.
For example, polyacrylamide gel electrophoresis and SDS electrophoresis require voltages of 200 to 600 V.
(1) Tank body (2) Agitation and circulation
system (3) Electrode device (4) Paint temperature regulation device (5) Paint
replenishment device (6) Ultra-filtration device (7) Ventilation device (8)
Power supply device (9) Post-swim washing device (10) Paint storage device
Electrophoretic painting can generally be divided into two categories: continuous production through-painting and intermittent fixed-painting.
For continuous production, the workpieces are suspended from a conveyor and other processes (pre-treatment - drying) to form a continuous production line, which is suitable for mass production.
For intermittent production stationary electrophoretic coating equipment, the workpiece is formed into intermittent coating production line with the help of monorail electric hoist or other forms of conveyors and other processes (pre-treatment - drying), which is suitable for medium batch coating production.
In the painting project electrophoresis voltage, time, temperature, cathode to anode area ratio, paint solids and paint PH value influence the quality of the coating, in practice the above parameters are determined according to the customer's process parameters.
The cathodic electrophoresis coating metal is less susceptible to oxidation at the cathode, the coating is more universal compared to anodic electrophoresis, the tank solution is not polluted by the ultrafiltration system, the tank solution is easy to maintain and the coating is more corrosion resistant.
The coating is cured in a curing oven. The curing ovens can be of two types: continuous and intermittent.
The temperature profile of the curing process has a significant impact on the quality. The temperature profile of the baking process generally needs to be checked regularly with an oven temperature tracker. This ensures that the temperature profile meets the process requirements.
Electrophoretic paint film has the advantages of plump, even, flat and smooth coating, and the hardness, adhesion, corrosion resistance, impact performance and penetration performance of electrophoretic paint film are obviously better than other coating processes.
(1) Using water-soluble paint, using water as the dissolving medium, saving a lot of organic solvents, greatly reducing atmospheric pollution and environmental hazards, safety and health, while avoiding the hidden danger of fire.
(2) High painting efficiency, small loss of paint, the utilisation rate of paint can reach 90% to 95%.
(3) Uniform coating film thickness, strong adhesion, good coating quality, various parts of the workpiece such as the inner layer, depressions, weld seams, etc. can obtain uniform and smooth paint film, solving the coating problems of other coating methods for complex-shaped workpieces.
(4) High production efficiency, the construction can realize automatic continuous production, greatly improving labor efficiency.
(5) Complex equipment, high investment cost, high power consumption, high temperature required for drying and curing, complex management of paint and coating, strict construction conditions, and wastewater treatment.
(6) Only water-soluble paint can be used, the colour cannot be changed during the painting process, and the stability of the paint is not easily controlled when stored for too long.
(7) Electrophoretic coating equipment is complex, with a high technological content, and is suitable for the production of fixed colours
Michael Zhang
Sales
Manager
Metal Surface Treatment Technology
M +86 150 8119 5156 (wechat/whatsapp)
E sales06@coating-equipment.com
W www.coating-equipment.com
+86-186 3213 8668
