Coatings

Flow Test of Resins and Varnishes

October 3, 2025
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Here is a Step-by-Step SOP (Standard Operating Procedure) for checking the flow of an alkyd resin on a glass plate.

SOP: Flow Test of Alkyd Resin on Glass Plate

1. Objective

To evaluate the flow and leveling characteristics of an alkyd resin by observing its behavior on a clean glass substrate.

2. Apparatus & Materials

  • Glass plate (100 × 150 mm, 3–4 mm thickness, grease-free, polished edges)
  • Applicator (Bird applicator / doctor blade) or disposable pipette (for drop test)
  • Spirit / acetone / xylene (for cleaning)
  • Stopwatch / timer
  • Vernier scale / ruler (for flow distance measurement)
  • Controlled environment (25 ± 2 °C, 50–60% RH)

3. Sample Preparation

  1. Bring the alkyd resin sample to room temperature.
  2. Adjust viscosity, if required, to application consistency with xylene/mineral spirits (note solvent and % added).
  3. Record initial viscosity (ASTM D1200 / Ford Cup No.4, sec).

4. Procedure

A. Film Applicator Method (preferred)

  1. Clean the glass plate thoroughly with acetone/spirit, then dry.
  2. Place the glass plate on a perfectly level table.
  3. Pour sufficient quantity (3–5 ml) of resin on the top edge of the plate.
  4. Draw down the film using a Bird applicator at 100–150 µm wet film thickness.
  5. Start the timer immediately.
  6. Allow the film to flow and level naturally for 30 minutes.

B. Drop Flow Method (alternate)

  1. Place the glass plate horizontally.
  2. Using a pipette, drop 0.5–1.0 ml resin at the center.
  3. Allow the drop to spread without disturbance.
  4. Measure diameter / flow length at fixed intervals (5, 10, 20, 30 min).

5. Observations

  • Flow distance (mm or diameter, as applicable).
  • Surface leveling quality:
    • Excellent – perfectly smooth, no streaks/brush marks.
    • Good – minor marks, but film mostly levels out.
    • Fair – visible streaks, orange peel, or partial leveling.
    • Poor – severe cratering, edge piling, brush marks retained.
  • Note presence of pinholes, craters, orange peel, edge beading.

6. Evaluation Criteria

  • Compare against control resin or product specification.
  • Good flow = uniform spreading, smooth surface, gloss development.
  • Poor flow = irregular edges, surface defects, lack of gloss.
  • Record film thickness, temperature, humidity, viscosity along with results.

7. Reporting Format

Sample CodeDateTemp/°CRH %Film Thickness (µm)Flow Distance (mm)Leveling GradeRemarks

8. References

  • ASTM D4062: Standard Test Method for Leveling of Paints
  • ISO 2431: Paints and Varnishes – Determination of Flow Time

For alkyd resin application and flow tests, the resin-to-solvent ratio depends on whether you want to evaluate the resin in its as-supplied state or in a ready-to-apply paint/varnish form.

General Guidelines (SaitechAI)

1. Flow & Leveling Test (QC Lab)

  • To evaluate flow on glass, the resin is usually thinned to a standard viscosity rather than fixed % ratio.
  • Target viscosity: 20–30 sec in Ford Cup No.4 at 25 °C.
  • This typically corresponds to:
    • Resin solution (alkyd at 60–70% solids) : Solvent mix (xylene / mineral spirits / butanol blend) = 3:1 to 4:1 by weight.
    • In other words: 20–30% solvent on resin solids basis.

2. For Clear Alkyd Varnishes

  • Commercial application viscosity: 18–22 sec FC4 @ 25 °C.
  • Resin : solvent mixture ≈ 70:30 (w/w).
  • Solvent blend:
    • Aromatic solvent (xylene/toluene) – for solvency and flow.
    • Aliphatic solvent (mineral turpentine, white spirit) – for balance & cost.
    • Alcohol (butanol, IPA, etc.) – small %, improves flow, prevents cratering.

3. For Pigmented Alkyd Paints

  • Grind stage: high resin content, minimal solvent (for pigment wetting).
  • Let-down stage: dilute to spray/brush viscosity.
  • Final ratio: resin solution : solvent = 65–75 : 25–35.
  • Adjust depending on application method:
    • Brushing – slightly higher viscosity (less solvent).
    • Spraying – lower viscosity (more solvent, up to 40%).

4. Key Notes

  • Always adjust viscosity, not just % ratio — since resin solids vary by grade (50–70%).
  • Avoid over-thinning: excess solvent causes poor build, sagging, low gloss.
  • Too little solvent: poor leveling, brush marks, orange peel.

✅ Recommended starting ratio for lab flow test: Resin solution : solvent mix = 75 : 25 (w/w), adjust to 20–30 sec FC4 viscosity before application.

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