Электрофоретикалық тұнбалау: Технология мен қолданылуы

Electrophoretic deposition (EPD), is a term for a broad range of industrial processes which includes electrocoating, cathodic electrodeposition, anodic electrodeposition, and electrophoretic coating, or electrophoretic painting. A characteristic feature of this process is that colloidal particles suspended in a liquid medium migrate under the influence of an electric field (electrophoresis) and are deposited onto an electrode. All colloidal particles that can be used to form stable suspensions and that can carry a charge can be used in electrophoretic deposition. This includes materials such as polymers, pigments, dyes, ceramics and metals. The process is useful for applying materials to any electrically conductive surface. The materials which are being deposited are the major determining factor in the actual processing conditions and equipment which may be used. Due to the wide utilization of electrophoretic painting processes in many industries, aqueous EPD is the most common commercially used EPD process. However, non aqueous electrophoretic deposition applications are known. Applications of non aqueous EPD are currently being explored for use in the fabrication of electronic components and the production of ceramic coatings. Non aqueous processes have the advantage of avoiding the electrolysis of water and the oxygen evolution which accompanies electrolysis.

The first patent for the use of electrophoretic painting was awarded in 1917 to Davey and General Electric. Since the 1920s, the process has been used for the deposition of rubber latex. In the 1930s the first patents were issued which described base neutralized, water dispersible resins specifically designed for EPD. Electrophoretic coating began to take its current shape in the late 1950s, when Dr. George E. F. Brewer and the Ford Motor Company team began working on developing the process for the coating of automobiles. The first commercial anodic automotive system began operations in 1963. The first patent for a cathodic EPD product was issued in 1965 and assigned to BASF AG. PPG Industries, Inc. was the first to introduce commercially cathodic EPD in 1970. The first cathodic EPD use in the automotive industry was in 1975. Today, around 70% of the volume of EPD in use in the world today is the cathodic EPD type, largely due to the high usage of the technology in the automotive industry. It is probably the best system ever developed and has resulted in great extension of body life in the automotive industry

There are thousands of patents which have been issued relating to various EPD compositions, EPD processes, and articles coated with EPD. Although patents have been issued by various government patent offices, virtually all of the significant developments can be followed by reviewing the patents issued by the U. S. Patent and Trademark Office.

During the aqueous deposition process, gas is being formed at both electrodes. Hydrogen gas is being formed at the cathode, and oxygen gas at the anode. For a given amount of charge transfer, exactly twice as much hydrogen is generated compared to oxygen on a molecular basis. This has some significant effects on the coating process. The most obvious is in the appearance of the deposited film prior to the baking process. The cathodic process results in considerably more gas being trapped within the film than the anodic process. Since the gas has a higher electrical resistance than either depositing film or the bath itself, the amount of gas has a significant effect on the current at a given applied voltage. This is why cathodic processes are often able to be operated at significantly higher voltages than the corresponding anodic processes. The deposited coating has significantly higher resistance than the object which is being coated. As the deposited film precipitates, the resistance increases. The increase in resistance is proportional to the thickness of the deposited film, and thus, at a given voltage, the electric current decreases as the film gets thicker until it finally reaches a point where deposition has slowed or stopped occurring (self limiting). Thus the applied voltage is the primary control for the amount of film applied. The ability for the EPD coating to coat interior recesses of a part is called the "throwpower". In many applications, it is desirable to use coating materials with a high throwpower. The throwpower of a coating is dependent on a number of variables, but generally, it can be stated that the higher the coating voltage, the further a given coating will "throw" into recesses. High throwpower electrophoretic paints typically use application voltages in excess of 300 volts DC. The coating temperature is also an important variable affecting the EPD process. The coating temperature has an effect on the bath conductivity and deposited film conductivity, which increases as temperature increases. Temperature also has an effect on the viscosity of the deposited film, which in turn affects the ability of the deposited film to release the gas bubbles being formed. The coalescence temperature of the coating system is also an important variable for the coating designer. It can be determined by plotting the film build of a given system versus coating temperature keeping the coating time and voltage application profile constant. At temperatures below the coalescence temperature, film growth behavior and rupturing behavior is quite different from the usual practice as a result of porous deposition. The coating time also is an important variable in determining the film thickness, the quality of the deposited film, and the throwpower. Depending on the type of object being coated, coating times of several seconds up to several minutes may be appropriate. The maximum voltage which can be utilized depends on the type of coating system and a number of other factors. As already stated, film thickness and throwpower are dependent on the application voltage. However, at excessively high voltages, a phenomenon called "rupture" can occur. The voltage where this phenomenon occurs is called the "rupture voltage". The result of rupture is a film that is usually very thick and porous. Normally this is not an acceptable film cosmetically or functionally. The causes and mechanisms for rupturing are not completely understood, however, the following is known:

Commercially available anodic EPD coating chemistries typically exhibit rupturing at voltages significantly lower than their commercially available cathodic counterparts. For a given EPD chemistry, the higher the bath conductivity, the lower the rupture voltage. For a given EPD chemistry, the rupture voltages normally decrease as the temperature is increased (for temperatures above the coalescence temperature). Additions to a given bath composition of organic solvents and plasticizers which reduce the deposited film's viscosity will often produce higher film thicknesses at a given voltage, but will generally also reduce the throwpower and the rupture voltage. The type and preparation of the substrate (material used to make the object being coated) can also have a significant effect on rupturing phenomenon.

In certain applications, such as the deposition of ceramic materials, voltages above 3–4V cannot be applied in aqueous EPD if it is necessary to avoid the electrolysis of water. However, higher application voltages may be desirable in order to achieve higher coating thicknesses or to increase the rate of deposition. In such applications, organic solvents are used instead of water as the liquid medium. The organic solvents used are generally polar solvents such as alcohols and ketones. Ethanol, acetone, and methyl ethyl ketone are examples of solvents which have been reported as suitable candidates for use in electrophoretic deposition.