High-shear dispersion process and emulsion stability control in the production of high-performance heavy-duty anti-corrosion coatings

Introduction: Dispersion Process—The Core Procedure of Heavy-Duty Anticorrosive Coatings

High-shear dispersion is the first critical process that determines the final performance in the production of heavy-duty anti-corrosion coatings. Insufficient dispersion leads to pigment flocculation and settling, while excessive dispersion damages the resin molecular chains. Core objective: complete pigment deaggregation, establishment of a stable resin-pigment interfacial layer, and no hard settling during storage.

I. Fluid Dynamics and Process Parameters of High-Shear Dispersion

Disc peripheral line speed directly determines shear force. For epoxy systems, 18-25 m/s is recommended. Below 18 m/s, pigment agglomerates cannot be broken up; above 25 m/s, shear heating (3-5°C/min) occurs, accelerating pre-reaction. Line speed formula: V=π×D×N/60. Pigment dispersion follows the three stages of wetting → deagglomeration → stabilization. For epoxy zinc-rich systems, high-molecular block copolymer dispersants containing acidic anchoring groups (2%-5%) are recommended.

II. Selection of Anti-Settling Agents and Stability Strategy

For epoxy zinc-rich primer (zinc dust density 7.14 g/cm³), a combination of organophilic bentonite and polyamide wax (each at 1.0%) is recommended to achieve no hard settling at 50°C for 30 days.

III. Process Adjustment and Equipment Maintenance for Winter and Summer Seasons

Winter (35°C): reduce linear speed to 18–22 m/s, control jacket cooling water temperature at ≤55°C. Aqueous systems: high water surface tension (72.8 vs 25–30 mN/m) requires efficient wetting agents; large latent heat of evaporation (2260 vs 350–500 kJ/kg) results in small temperature rise, allowing speed increase to 28 m/s. Equipment maintenance: tooth wear (>2000 h) reduces efficiency by 30%–50%; eccentricity (>0.5 mm) creates dead zones. Recommend inspection every 500 h and replacement of dispersing disc every 1000 h.

Technical Deepening: Engineering Economics and Parameter Science of Dispersion Processes

Dispersion process is not merely a technical issue—it is a core cost driver in manufacturing. Taking a factory with an annual output of 3,000 tons of heavy-duty anti-corrosion coatings as an example: the energy consumption of the dispersion process accounts for 35%–45% of total energy consumption. Optimization strategies—(1) Adopt variable frequency speed regulation instead of constant speed—automatically adjust the disperser disc speed according to coating viscosity—can save 20%–30% energy; (2) The jacket cooling water of the dispersion tank is preheated for the next batch of raw materials via a heat exchanger—heat recovery rate >60%—saves over 100,000 yuan in natural gas annually; (3) Arrange products of the same color series for continuous production in the same dispersion tank—reduce cleaning frequency—reduce cleaning solvent waste by over 20 tons per year. The “optimal economic point” of the dispersion process is not technical optimality—but the lowest comprehensive cost of technology + energy consumption + cleaning + labor.

The “over-grinding” of dispersion quality is a common waste—after the dispersion time exceeds the optimal dispersion point—every additional minute of dispersion—increases energy consumption but does not improve fineness—and the coating temperature keeps rising—which may cause resin pre-reaction and additive degradation. During the dispersion process, the factory should take samples every 5 minutes to test fineness, plot the “fineness-time curve”, and stop dispersion immediately when the curve enters the plateau region—rather than following a fixed time schedule.

Industry Case: A Million-Dollar Rework Lesson from Poor Dispersion

A steel structure painting project – after construction of epoxy zinc-rich primer, dense pinholes (>50 per m²) appeared on site. Investigation of spraying parameters showed all were normal – sampling inspection revealed coating fineness >50μm (standard <30μm) – root cause tracing – the dispersion time for this batch was reduced to 20min (standard 25min / to catch up with schedule) + dispersion disc linear speed was only 18m/s (standard 22m/s / worn dispersion disc not replaced) – the combination of these two factors led to insufficient pigment dispersion. Ultimately, the project required full re-sandblasting + repainting, with rework cost 3 times the original painting cost – exceeding 2 million yuan.

FAQ

Q1: Why does a large amount of foam form during the dispersion process?The dispersing disc is positioned too high and draws in air, the linear velocity >25m/s causes a cavitation effect, and the surfactant reduces surface tension. Solution: Adjust the immersion depth to 1/3 below the liquid surface, reduce the rotation speed, and add 0.1%-0.3% defoamer.

Q2: How to determine if the pigment is sufficiently dispersed?The grindometer is the most direct method. A Hegman grind gauge (0-8 scale) or wet film gloss measurement by drawdown—when the gloss no longer increases with further dispersion, that is the endpoint.

Q3: Causes of “particle coarsening” in epoxy zinc-rich primer?(1) Insufficient dispersant or poor matching of anchoring groups; (2) Storage temperature >40°C intensifies Brownian motion; (3) Trace moisture triggers oxidation on the zinc powder surface.

Q4: How to adjust the dispersion process in winter and summer?Winter: Preheat resin to 25-30°C, increase wire speed to 22-25 m/s, extend by 5-10 min. Summer: Reduce wire speed to 18-22 m/s, control jacket cooling water temperature at ≤55°C.

Q5: What are the differences between water-based heavy-duty anti-corrosion dispersions and solvent-based ones? Water has high surface tension and requires efficient wetting agents; its large latent heat of vaporization and small temperature rise allow for increased rotation speed; the anti-settling system differs (it is recommended to use fumed silica or PU thickener instead of organic bentonite).

Q6: Why defoam at low speed?Reduce to 200-500 rpm for 5-10 min, bubbles escape under low shear, and subsequently add solvent to dilute.

Q7: Will zinc dust react with epoxy resin during dispersion?Under normal temperature (80°C and in the presence of water or acidic substances, it may oxidize or release hydrogen gas. Strictly control temperature ≤55°C.

Q8: Single-shaft vs dual-shaft disperser selection?Single-shaft is suitable for low viscosity (85% VS). Three-shaft planetary is recommended.

Q9: How to predict storage stability by sedimentation test?Take 100mL in a graduated cylinder, 50°C/7 days. Criteria: upper clear layer <5%, bottom sediment re-dispersible, top-bottom fineness difference <5μm. All three must pass for release.

Q10: Impact of disperser disc wear? Tooth tip wear (>2000h) reduces efficiency by 30%-50%; eccentricity (>0.5mm) creates dead zones. It is recommended to inspect every 500h, replace every 1000h, and calibrate concentricity monthly.

FAQ: In-Depth Technical Q&A Supplement

Q11: How do the differences in domestic and international standards for this technology affect product export?Domestic standards (GB) differ from ISO/ASTM standards in test methods and acceptance criteria. For example, salt spray testing—GB/T 1771 (equivalent to ISO 7253) has test conditions basically consistent with ASTM B117—but the rating systems (ISO 4628 vs ASTM D610/D714) differ—export products must also indicate the corresponding international standards when providing test reports, otherwise overseas customers cannot make a comparative assessment. It is recommended to list both GB and ISO/ASTM dual-standard indicators in the TDS (Technical Data Sheet) of export products—to enhance the trust of international customers.

Q12: How to verify the long-term service performance of this technology in actual engineering?Laboratory accelerated testing (salt spray/QUV/cyclic corrosion) provides comparative data—but cannot fully replace actual outdoor exposure testing. Recommendations—(1) Set up outdoor exposure racks at both the factory location and typical customer locations (e.g., coastal C5-M/industrial C4)—conduct annual inspections of coating appearance/adhesion/film thickness changes—establish a company-owned outdoor service database; (2) Collaborate with universities/research institutes—combine enterprise data with academic research—enhance data credibility.

Q13: What should SMEs pay attention to when purchasing related raw materials/equipment?(1) The batch stability of suppliers is more important than unit price—it is recommended to require suppliers to provide COA data for >10 batches—and evaluate batch variation (CpK); (2) For equipment procurement, visit peers who have used the equipment for >2 years to understand the long-term reliability and after-sales service quality of the equipment—rather than relying only on the demonstration data from the equipment supplier; (3) For key raw materials (resin/curing agent)—maintain at least 2 qualified suppliers to guard against single-supply risk.

Q14: What is the current state and trend of digital transformation in this field?The digital transformation of the coatings industry is evolving from “point-based applications” (automation of individual equipment/processes) to ”system integration” (full-chain ERP+MES+PMS). Currently, the digitalization of small and medium-sized coatings factories has the ”highest ROI investment” in automatic batching systems + digitalization of quality control data — with a payback period of 1-3 years — which is the prioritized recommended direction. Future trend — AI + sensors enabling real-time optimization of process parameters — further reducing quality fluctuations between batches.

Q15: How can a newly entered coating engineer quickly master this technology?(1)Combine theory and practiceDo not only read literature without touching actual production—nor rely solely on experience without studying theory;(2)Build a “failure case archive”Every customer complaint/production anomaly/coating failure—record the root cause and resolution process—this is the most effective learning material;(3)Learn from suppliersTechnical personnel from resin/additive/pigment suppliers are the carriers of “tacit knowledge” in this field—communicate more with them about solutions to specific problems.

Engineering Application and Implementation Recommendations

Pre-construction preparation and risk assessment

Before formal construction, the three prerequisite tasks must be completed: (1) Substrate condition confirmation — inspect the substrate moisture content (concrete <4% / steel no visible water film), surface preparation grade (abrasive blast Sa2.5 / manual St3) and salt contamination (chlorides dew point +3°C) — construction may proceed only when all three are satisfied — any exceedance will cause irreversible defects during coating curing; (3) Coating batch verification — verify the coating batch number, production date and COA test report — confirm the coating is within shelf life and key indicators (viscosity / fineness / curing time) meet requirements.

Key control points during the construction process

During construction, it is necessary to continuously monitor and record the following parameters: (1) Wet film thickness (WFT) of each coat (wet film thickness gauge / at least 5 points per 10m²) — the conversion relationship between WFT and target dry film thickness (DFT) is DFT = WFT × volume solids (%) — if WFT deviation is found, immediately adjust spraying parameters; (2) Drying/curing time of each coat — epoxy system requires surface dry (2-4h/23°C) → hard dry (6-12h) → full cure (7 days) — the application of the next coat must be within the optimal recoat window of the previous coat (usually 4-24h after surface dry) — recoating too early → interlayer solvent penetration and lifting/Recoating too late → reduced interlayer adhesion; (3) Continuous recording of construction environmental conditions — record temperature/humidity/dew point every 2h — archived as part of the completion document.

Quality Acceptance and Completion Documentation

The final acceptance of the coating system shall be based on the acceptance criteria specified in the contract (e.g., ISO 12944 / SSPC-PA 2 / GB 50205) — key acceptance items include: (1) Dry film thickness (DFT / ≥5 points per 10m² / any single point ≥80% of nominal value / average within 100–120% of nominal value); (2) Holidays detection (wet sponge method for DFT 500μm / zero holidays); (3) Adhesion (pull-off method ISO 4624 / ≥ design value / failure mode preferably cohesive failure); (4) Visual inspection (no sagging / no orange peel / no particles / uniform gloss). All acceptance test data shall be compiled into as-built documentation including test reports + construction records + paint batch numbers + environmental records — serving as the data baseline for the 25-year warranty period of the coating system — with an archival period of ≥5 years.

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Summary

High-shear dispersion is a core process at the intersection of fluid mechanics, colloid chemistry, and equipment engineering. Kexin New Materials’ coating plant operates automated dispersion production lines equipped with online temperature control and variable-frequency speed regulation systems, with every batch of coating strictly following process standards and quality control procedures.