Electrophoretic deposition
The electrophoretic deposition (EPD) is a widely used industrial process in which colloidal particles under the influence of an electric field to be deposited on an electrode. Other names that are used for the electrophoretic deposition process are cathodic dip painting (KTL) or anodic dip painting (ATL).
Cathodic dip painting, also known as "cataphoresis", is an electrochemical process in which the workpiece is coated in an immersion bath. It is well suited for painting complex structures and large quantities. KTL is a standard method for ensuring corrosion protection in vehicle bodies .
background
Suspending colloidal particles in aqueous media leads to the dissociation of ions and the formation of an electrochemical double layer around the particle. When an electric field is applied, the charged colloidal particles are attracted to the electrode with the opposite charge and are deposited on it. During the migration of the colloidal particles to the electrode, the diffuse layer, the electrochemical double layer, shears off due to the movement of the particle and the friction of the solvent. As a result, the actual electrochemical double layer becomes thinner and the particles are deposited by overcoming the repulsive forces and increased particle-particle collisions on the electrode. Conductive electrodes (substrates) are a prerequisite for the deposition process.
Important parameters for the separation process are:
- Zeta potential of the colloids
- Electrophoretic mobility of the colloids
- Conductivity of the substrate (separation electrode)
- Conductivity of the solvent
- Viscosity of the solvent
- Dielectric constant of the solvent
Electrochemical dip painting
A basic distinction is made between anodic and cathodic electrodeposition coating. Both dip painting processes are electrochemical painting processes. The item to be painted is immersed in an electrically conductive, aqueous dip paint and a direct voltage field is applied between the item to be painted and a counter electrode . The basic principle of electrodeposition coating is to precipitate water-soluble binders on the surface of the item to be coated, which is used as an electrode , and thus to create a closed, adhesive coating film. Through capillary processes, the water in the process bath is almost completely "pressed out" of the paint film.
Cathodic dip painting
In cathodic dip painting, the paint is deposited as a result of chemical reactions ( coagulation ) of the binder. This is implemented with direct voltages of 200 to 260 volts by means of an electrical current flow from an external electrode ( anode ) via the conductive paint to the item to be painted ( cathode ). The current applied creates hydroxide ions on the cathode (body), which neutralize the binding agent and thus cause it to coagulate. This proceeds according to the following reaction equation:
Cathodic dip painting is well suited for automated coating. It is a very environmentally friendly method, since demineralized water is the predominant solvent used today. The paint yield is up to 98.5%, so only 1.5% of the paint used is discharged. The result of the KTL is a very even coating of metal surfaces and cavities with even layer thicknesses and good surface quality. The ability of the dip coating to coat cavities well is known as the ability to grip .
technology
The main components of the KTL basin are a rectifier to supply the system with direct voltage, several dialysis cells, a circulation system for constant, even mixing of the paint (avoidance of pigment sedimentation ), temperature control, filter systems to remove dirt and ultrafiltration systems for supply of the system with flushing medium to rinse off the adhering paint.
Dialysis cells are usually used as anodes in the KTL bath . They are attached to both sides of the basin and, as anode, form the corresponding counterpart to the cathodic body. The anodes are covered by special anion exchange membranes . Anolyte is pumped through the cells by an external system . During the painting process, disruptive anions form in the paint , which are transported from the paint through the membrane into the anolyte and then disposed of. The neutralizing agent that has become free is thus removed through the anolyte . This means that the pH value in the KTL basin remains within the process parameters. In addition to discharging excess ions, the anolyte also serves to cool the anodes, as a lot of heat is generated during the process. The cooling of the entire KTL bath, however, is ensured by the circulation circuits of the paint material.
Chemicals
The KTL basin contains binders, pigment paste, water-miscible organic solvents and water. Epoxy resin - amine adducts and blocked isocyanates are often essential components of binders and pigment paste . The binder and pigment paste make up the majority of the approx. 20% solids content of the paint. The electrodeposition paint also consists of around 80% deionized water. In addition, there is a small proportion of water-soluble solvents (1 to 2%), acids (0.4%) and additives .
The epoxy-amine adduct is brought into a water- dispersible form by adding a neutralizing agent . An organic acid (usually formic acid ) is used for this. Often only some of the functional groups are reacted with neutralizing agents. This process runs exemplarily according to the following equation:
The molar ratio of acid to functional group is called the degree of neutralization. A degree of neutralization of about 30% is sufficient to achieve the desired water dispersibility. An organic acid is also used to adjust the slightly acidic character ( pH value ) in the KTL basin. During the process, an approximately 15-20 µm thick primer layer forms on the surface of the components, which serves to protect against corrosion and stone chips.
Harden
During the curing process after the KTL, linear chain molecules are formed in the epoxy resin-containing paint together with the blocked isocyanate , which are three-dimensionally networked with one another and thereby form a stable structure. The crosslinking takes place through high temperatures (approx. 185 ° C).
Light-controlled electrophoretic deposition / 3D printing
Scientists at Lawrence Livermore National Laboratory in California "have controlled electrophoretic deposition by directional exposure" and "have both the ability to pattern different materials (in the plane of the electrode) and the ability - one material over another high quality material to deposit - proven ". This means that several layers can be created and thus also three-dimensional models can be created.
literature
- L. Besra, M. Liu: A review on fundamentals and applications of electrophoretic deposition (EPD). In: Progress in materials science. 52 (1), 2007, pp. 1-61.
- S. Vogel: Process development for the electrophoretic deposition of ceramic layers and microstructures. Dissertation . Freiburg University Library, DNB 1004171641 .
Individual evidence
- ^ A b c d e Hans-Joachim Streitberger, Artur Goldschmidt: BASF Handbook Painting Technology . 2nd ed., Rev. Vincentz Network, Hannover 2014, ISBN 978-3-86630-892-3 .
- ↑ a b c d Bernd Strehmel, Peter Mischke, Michael Groteklaes, Thomas Brock: Textbook of lacquer technology; 4. revised edition. Vincentz Network, [Place of publication not identified], ISBN 3-86630-815-9 .
- Jump up ↑ Andrew J. Pascall, Fang Qian, Gongming Wang, Marcus A. Worsley, Yat Li, Joshua D. Kuntz: Light-Directed Electrophoretic Deposition: A New Additive Manufacturing Technique for Arbitrarily Patterned 3D Composites . In: Advanced Materials . tape 26 . Wiley-VCH Verlag, Weinheim 2014, p. 2252–2256 , doi : 10.1002 / adma.201304953 .
- ↑ David Gotsch: Light-Controlled Electrophoretic Deposition: A New 3D Printing Technology . In: 3druck.com . April 15, 2014 ( 3druck.com [accessed January 11, 2015]).