Effects and Control Measures of Microorganisms (Bacteria, etc.) in Electrophoretic Coating

Electrophoretic paint is an environmentally‑friendly electrophoretic coating with reduced organic‑solvent consumption. It uses water as the dispersion medium, and its aqueous system readily serves as a culture medium for microorganisms. Modern electrophoretic‑coating formulations generally contain little or no heavy‑metal components such as lead and tin, which weakens inhibition against bacteria and other microbes. Operating temperatures for electrophoretic lines normally range from 20‑35 °C with high humidity and poor ventilation — conditions favourable for the survival of many bacterial species.

Adverse Problems Caused by Bacterial Growth in Bath Liquid

Bacterial contamination in electrophoretic baths seldom involves a single species; instead, mixed microbial communities develop. Microbes are introduced via purified‑water systems, tap‑water supplies, ultrafiltration systems, post‑rinsing systems, anode systems, airborne deposition, and incoming coated workpieces. The exact species present depend on ambient conditions. Microbial proliferation destabilizes bath liquid and triggers various coating defects. In severe cases, the entire bath may have to be discarded.

Types of Microorganisms in Bath Liquid

Main microorganisms found in bath liquid include bacteria, moulds and yeasts.

• Bacteria: Invisible to the naked eye and widely distributed. They reproduce rapidly. Heavy contamination first manifests as abnormal bath‑liquid parameters; white flocculent bacterial colonies and sediment appear at later stages.

• Moulds: Fungi widespread in soil, water, air and surfaces of decaying organic matter. They prefer acidic environments and reproduce more slowly than bacteria. They adhere primarily to tank and pipe inner walls, appearing as villous, flocculent white floaters or slippery filamentous deposits.

• Yeasts: Fungi ubiquitous in nature. They thrive in air, soil, water and organisms and can survive both aerobically and anaerobically. They grow within a pH range of 3.0‑7.5 and typically cause abnormal foaming and unpleasant odours in bath liquid.

Impacts of Microbial Growth in Bath Liquid

1. Bath‑liquid parameters: For cathodic electrophoresis, MEQ value and conductivity decrease while pH rises. For anodic electrophoresis or with certain microbial strains, conductivity may increase and pH may drop.

2. Application performance: Bath breakdown voltage declines, and film‑forming performance becomes highly voltage‑sensitive. Bath stability deteriorates, sediment increases, filtration performance worsens, surface foam accumulates and viscosity rises.

3. Coating‑film properties: Finished films exhibit severe orange‑peel texture, roughness, blistering and more particles; adhesion strength decreases.

4. Equipment impacts: Sediment impedes filtration, blocks filter bags and ultrafiltration membranes, reduces ultrafiltrate throughput, increases motor load and shortens service life of filter bags and membranes.

Bacteria‑Content Testing Method (Bacterial Test‑Slide Method)

Immerse the test slide in the sample liquid for several seconds, or rinse the slide surface directly with sample liquid. Place the slide back into its original sealed protective tube and incubate in a constant‑temperature‑humidity chamber for 2‑5 days (3 days typical). Observe microbial growth on the slide.

The detection limit of this method is approximately 1 CFU/cm² or 100 CFU/mL.

• Nutrient agar for bacteria: optimum incubation temperature 30 °C; incubation for 1‑2 days is normally sufficient.

• Rose‑bengal agar for fungi (yeasts and moulds): optimum incubation temperature 25 °C. Yeasts require 2‑3 days incubation; moulds require 5‑7 days.

Microorganism Treatment Methods

Once bacteria have multiplied in bath liquid, biocides shall be applied for disinfection to disrupt microbial cell metabolism. Biocide selection must consider compatibility with bacterial species and electrophoretic‑paint formulations so as not to impair coating‑film performance.

Common biocides include isothiazolinones (Kathon), hydrogen peroxide and silver nitrate. These substances damage cell‑membrane structures and inactivate microbes by breaking protein bonds in bacteria and algae. Continuous exclusive use of one biocide induces microbial resistance. It is recommended to alternate biocides with different working mechanisms every week or each production cycle. A combined dosing strategy of continuous low‑level addition plus intermittent shock dosing ensures effective sterilisation.

The core procedure for microbial elimination in electrophoretic‑coating systems follows the rule: mechanical cleaning first, followed by chemical sterilisation, then filtration to remove microbial debris. Dosing biocides without physical cleaning is strictly prohibited.

Microorganism Inhibition Measures

1. Select electrophoretic coatings with good anti‑microbial properties to improve industrial applicability at the formulation level.

2. Strengthen production‑process management and perform operation and maintenance strictly according to process specifications.

3. Water‑source management: equip purified‑water systems with UV sterilisation, special filtration, chlorination or ozonation units to eliminate bacteria in the water‑supply loop.

4. Take regular samples from electrophoretic bath liquid, rinsing‑tank liquid and purified‑water sources for microbial testing. Bacterial test‑slides offer a convenient, timely and accurate means to quantify bacterial levels in cathodic‑electrophoresis bath liquid and guide timely sterilisation actions.

5. Deploy suitable biocides to kill existing bacteria and suppress new growth, which is an effective measure to maintain normal bath‑liquid performance.

6. Tank draining and cleaning: Pretreatment and rinsing tanks feature many static dead zones. Increase cleaning frequency; replace tank liquor or perform tank draining according to production conditions to remove biofilms adhering to tank surfaces. For tanks fitted with circulation and filtration systems, maintain continuous circulation or adjust circulation intensity to minimise stagnant zones. Periodically clean filters, pipelines and tank walls to remove attached biofilms.