Analysis of Function and Mechanism of Solvents in Electrophoretic Coatings

Analysis of Function and Mechanism of Solvents in Electrophoretic Coatings

I. Functions of Solvents in Electrophoretic Coatings

Primarily composed of alcohols and ether‑alcohol co‑solvents, solvents in electrophoretic coatings serve as key regulating media for bath stability, film appearance, anti‑corrosion performance and processing window. Their specific functions are as follows:

1. Stabilizing the bath solution and solubilizing/dispersing resins
They dissolve polymer resins and facilitate uniform dispersion of pigments and fillers. They lower the critical micelle concentration of resins, ensuring long‑term stable emulsification of resins in aqueous systems and preventing resin precipitation, phase separation as well as pigment flocculation and sedimentation.

2. Improving deposition performance, film thickness and throwing power
They reduce the impedance of wet films, enabling charged colloidal particles to continuously migrate to inner cavities and narrow gaps of workpieces. This effectively increases film thickness and enhances coating coverage on blind/dead corners of workpieces.

3. Optimizing film levelling and eliminating appearance defects
They decrease the surface tension of bath solution and improve substrate wetting capacity. They exert a plasticizing effect on wet films and enhance coating fluidity, which mitigates surface defects such as orange peel, pitting, craters and pinholes.

4. Widening the processing window
They stabilize processing voltage and current parameters. The bath solution can maintain stable production even under fluctuating temperature and voltage conditions, reducing defect rates caused by process variations.

5. Assisting baking degassing and preventing pinholes and blistering
Their boiling points lie between that of water and the softening point of resins. In the early baking stage, they prop open microporous channels in the coating, allowing internal moisture to escape slowly and orderly. This avoids premature sealing of the film and pinholes/blisters induced by trapped water vapor pressure.

6. Acting as alcohol‑based blocking agents
In electrophoretic coatings (especially cathodic electrophoretic coatings), alcohols are mainly used as blocking agents for blocked isocyanate curing agents. They enable low‑temperature curing of coatings and improve comprehensive film properties.

II. Microscopic Action Mechanism of Solvents

1. Mechanism for solubilization and bath stabilization

The main‑stream resin for cathodic electrophoresis is aminated epoxy resin, which acquires hydrophilicity through salt‑formation by acid neutralization. Emulsification stability relying solely on hydrophilic groups is limited. As weakly‑polar media, ether‑alcohol solvents insert between hydrophobic segments of resin molecules, disrupting intermolecular hydrogen bonds and van der Waals cohesion. They reduce the overall cohesive energy of resins and build composite hydration layers of “resin‑solvent‑water”, greatly lowering interfacial tension and realizing long‑term bath stability.

2. Electrodeposition film‑forming mechanism

Water electrolysis occurs on the cathode workpiece. The pH at the workpiece interface rises rapidly, and cationic resins lose solubility via de‑salting and precipitate to form gelled wet films.

Solvents swell wet‑film structures and reduce film resistance. This ensures continuous migration and deposition of charged particles after initial film formation, increasing both film thickness and throwing power for inner cavities. Meanwhile, they lower the critical pH for resin precipitation to achieve uniform and slow deposition and avoid rough films caused by rapid deposition.

3. Mechanism for levelling, defoaming and wetting

• Reduce bath surface tension, wet micro‑uneven surfaces and minor oil‑contaminated spots on substrates to prevent craters.

• Plasticize wet‑film resins, decrease glass‑transition temperature (Tg), enhance molecular fluidity and realize self‑levelling before baking.

• Expand the network structure of wet films to reserve escape channels for electrolytic bubbles and mitigate bubble entrapment.

4. Anti‑blistering mechanism during baking

Pure water is highly polar and cannot swell epoxy resins. In solvent‑free systems, precipitated resins curl and compact rapidly with instant closure of micropores. Trapped internal water vapor expands at high temperature and leads to blisters and pinholes.

Solvents prop open resin chain segments in advance to form connected microporous channels. During low‑temperature pre‑baking, solvents evaporate gradually and moisture escapes orderly through such channels, producing dense and pore‑free coatings.

Mechanism for solvents to broaden resin voids:

• Like‑dissolves‑like effect and chain‑segment expansion: Hydrophobic resin skeletons (benzene rings, carbon chains, etc.) show good compatibility with ether‑alcohol solvents. Small solvent molecules penetrate gaps between polymer chains, weaken intermolecular forces, increase spacing between resin chains and build stable microporous structures, into which water molecules cannot enter.

• Plasticization and shrinkage inhibition: Deposited wet films are water‑containing gel networks. Without solvents, resins tend to shrink spontaneously and pores collapse. Solvents plasticize resins, improve chain‑segment flexibility and suppress collapse and closure of micropores.

• Volatilization leaving channels: During baking, solvents evaporate progressively. Spaces previously occupied by solvents become continuous capillary channels, serving as the exclusive pathways for moisture escape.

5. Mechanism as blocking agents

• Blocking reaction at ambient temperature:
Isocyanate groups (‑NCO) are highly reactive and will react with hydroxyl groups to cause gelation at room temperature. Hydroxyl groups of alcohols react with isocyanate groups to generate carbamates and block ‑NCO groups. The reaction is shown below:

1. Blocked ‑NCO groups lose chemical activity.

2. Premature cross‑linking is avoided during paint storage and bath operation to maintain bath stability.

3. Blocked isocyanates are electrodeposited onto workpieces together with base epoxy resins and remain in wet electrophoretic films.

• De‑blocking reaction at high temperature:
When the curing temperature reaches the de‑blocking temperature of curing agents, carbamate bonds cleave upon heating and the above reaction reverses.

1. Active ‑NCO groups are regenerated.

2. Free alcohol blocking agents are heated and volatilize out of coatings.

3. Released ‑NCO groups cross‑link with hydroxyl groups (‑OH) on electrophoretic resins.

III. Impacts of Solvent Content on Electrophoretic Coatings and Routine Maintenance

1. Impacts of solvent content on electrophoretic films

• Insufficient solvent: Closed voids hinder moisture escape → excessive pinholes, reduced film thickness and poor levelling.

• Appropriate solvent: Connected voids allow smooth degassing → good levelling appearance and stable film thickness.

• Excessive solvent: Over‑large voids and loose structures → soft films, poor compactness and significant deterioration of salt‑spray resistance and other properties.

2. Main causes of solvent loss

• Ultrafiltration discharge (largest loss source): Ultrafiltration membranes only retain high‑molecular‑weight resins, while small‑molecule solvents pass through completely. Thus, solvent concentration in discharged ultrafiltrate is nearly equivalent to that in the bath.

• Natural volatilization induced by high ambient temperature, high bath temperature and strong exhaust ventilation.

• Carry‑out loss via workpieces and hangers.

3. Routine solvent replenishment and precautions

3.1 Calculation formula

3.2 Scheduled replenishment in mass production

Solvents shall be replenished daily in mass production. The daily replenishment rate is generally controlled at 0.2‰‑0.5‰ of bath volume. The initial addition amount shall be determined by laboratory testing and tracking. After batch production starts, solvent content shall be tested weekly to revise empirical volatilization coefficients.

3.3 Precautions for solvent replenishment

3.3.1 Feeding requirements

• Pouring stock solvent directly into the main bath is strictly forbidden, as it may trigger local demulsification, flocculation and particle defects.

• Dilute solvents by 5‑10‑fold with water and add slowly to the auxiliary tank under circulating conditions.

• Maintain circulation for ≥30 min after feeding.

• Adopt small‑quantity multiple‑time addition.

3.3.2 Feeding timing

• Preferred: replenish during heat‑preservation shutdown or before startup.

• Prohibited: large‑batch replenishment during energized mass production.

• Replenish solvent deficits according to process requirements upon completion of ultrafiltration discharge.

3.3.3 Seasonal and temperature adjustment

• Summer (high temperature): frequent small‑dose replenishment.

• Winter (low temperature): reduce replenishment volume to prevent excessive solvent accumulation.

Solvents are core media for bath stability, deposition, levelling, degassing and anti‑corrosion of electrophoretic systems. On‑site operation and maintenance principles: maintain moderate concentration; add in small multiple batches; keep balanced component ratios; discharge excess solvent instead of simple dilution when over‑limit.