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How to Determine the Exact Ratio of Epoxy Resin to Curing Agent in Waterborne Epoxy Paint in Practical Operations
2025-08-25

How to Determine the Exact Ratio of Epoxy Resin to Curing Agent in Waterborne Epoxy Paint in Practical Operations

In practical operations, the exact ratio of epoxy resin to curing agent in waterborne epoxy paint needs to be determined comprehensively based on theoretical calculations, product characteristics, and actual working conditions. The core is to ensure complete reaction between the two and match performance requirements. Here are the specific steps and methods:

I. Core Theoretical Basis: Matching Epoxy Equivalent Weight and Active Hydrogen Equivalent Weight

Obtain Key Parameters

  • Request the "epoxy equivalent weight (EEW)" of the epoxy resin from the supplier: the number of millimoles of epoxy groups per gram of resin (unit: mmol/g). For example, the EEW of E-51 waterborne epoxy is approximately 0.5-0.55 mmol/g.
  • Obtain the "active hydrogen equivalent weight (AHEW)" of the curing agent: the number of millimoles of active hydrogen per gram of curing agent (unit: mmol/g). For example, the AHEW of waterborne polyamide 650 is approximately 0.3-0.35 mmol/g.

Theoretical Ratio Calculation

Following the principle of "total epoxy groups = total active hydrogen," the formula is:
Curing agent mass = Epoxy resin mass × (Epoxy resin EEW ÷ Curing agent AHEW)

 

Example: For 100g of E-51 waterborne epoxy (EEW = 0.5 mmol/g) paired with a waterborne polyamide with AHEW = 0.33 mmol/g, the theoretically required curing agent mass = 100 × (0.5 ÷ 0.33) ≈ 151.5g (but in practice, it will be lower due to the influence of solvents in the waterborne system).

II. Practical Operation Steps: From Reference to Verification

Step 1: Start with the Supplier's Recommended Ratio

Almost all suppliers of waterborne epoxy and curing agents provide a clear recommended ratio (e.g., "epoxy resin: curing agent = 100:30"). This is the optimal initial value based on product characteristics (already considering the impact of solvents and additives in the waterborne system on the reaction).

 

Example: A certain brand of E-44 waterborne dispersion recommends a ratio of 100:25-30 with waterborne T31 curing agent, which can be directly used as the starting point for testing.

Step 2: Verify the Rationality of the Ratio Through Small-Sample Testing

Prepare small samples with different ratios (adjusted within ±5% of the recommended range) and test key performances:

 

Test Item Test Method Qualification Criteria (Example)
Curing completeness After curing at 25°C for 7 days, wipe the paint film surface with acetone for 30 seconds No stickiness, no dissolution marks
Mechanical properties Test film hardness (pencil hardness tester) and adhesion (cross-cut test) Hardness ≥ 2H, adhesion ≤ Grade 1
Pot life Observe the system state after mixing and record the time from mixing to thickening/stratification Pot life ≥ 2 hours (meeting construction needs)
Water resistance Immerse the paint film in distilled water for 24 hours No blistering, no peeling, no significant discoloration

 

  • If curing is incomplete (sticky) under a certain ratio, it indicates insufficient curing agent, and the ratio needs to be increased;
  • If the paint film is too brittle (cracks in bending tests), it indicates excessive curing agent, and the ratio needs to be reduced.

Step 3: Adjust the Ratio According to Working Conditions

  1. Construction temperature:
    • Low-temperature environment (<15°C): Curing reaction slows down. The amount of curing agent can be appropriately increased (e.g., +5% of the upper limit of the recommended ratio), or 0.5%-2% of waterborne accelerator (such as waterborne DMP-30) can be added;
    • High-temperature environment (>30°C): Reaction accelerates. The amount of curing agent needs to be reduced (e.g., -5% of the lower limit of the recommended ratio) to avoid brittle paint film.
  2. Substrate characteristics:
    • Porous substrates (e.g., concrete, wood): Curing agent may be absorbed by the substrate, requiring an increase in dosage (+5%);
    • Smooth metal substrates: The recommended ratio can be used to avoid reduced adhesion due to excess.
  3. Performance requirements:
    • Pursuing high corrosion resistance (e.g., industrial anti-corrosion): Slightly excessive curing agent (+3% of the recommended ratio) to ensure sufficient cross-linking;
    • Pursuing flexibility (e.g., wood coating): Slightly insufficient curing agent (-3% of the recommended ratio) to retain a small amount of unreacted epoxy groups for improved elasticity.

III. Precautions

  • Mixing uniformity: Waterborne systems are prone to demulsification due to ratio deviations. First, stir the epoxy resin evenly, then slowly add the curing agent and stir at high speed for 2-3 minutes (rotational speed 300-500 rpm).
  • Batch consistency: Epoxy equivalent weight/active hydrogen equivalent weight may vary slightly between different batches of products. Small-sample testing should be repeated for each new batch of materials.
  • Recording and iteration: Record each ratio and corresponding performance to form an "optimal ratio database" suitable for your own working conditions (e.g., in a workshop at 20°C, the ratio of E-51 waterborne epoxy + 650 waterborne polyamide at 100:32 achieves the best results).

 

Through the four steps of "theoretical calculation → supplier reference → small-sample testing → working condition adjustment," the ratio in practical operations can be accurately determined, ensuring a balance between paint film performance and construction efficiency.

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