🎯 Objective
To synthesize urea-formaldehyde (UF) thermosetting resin through step-growth condensation polymerization and study its properties.
📖 Principle / Theory
Urea-formaldehyde resin is a thermosetting polymer formed by the condensation reaction of urea and formaldehyde in two stages:
- Stage 1 (Addition, Alkaline): Urea reacts with formaldehyde to form methylol urea (mono- and di-methylol urea) at pH 8–9.
- Stage 2 (Condensation, Acidic): Methylol groups condense at pH 4–5 to form linear and cross-linked polymer network (–NH–CH₂–NH–).
The final cured resin is hard, transparent, resistant to heat and solvents — widely used in plywood adhesives, laminates, and moulding compounds.
🧰 Apparatus Required
Round bottom flask (250 mL), reflux condenser, thermometer, heating mantle, pH meter, glass rod.
🧪 Chemicals Required
Urea, 37% Formaldehyde solution (formalin), NaOH solution (10%), dilute HCl (10%), pH paper.
⚗️ Procedure
- Take 6 g of urea in a 250 mL round bottom flask. Add 10 mL of formalin (37% HCHO).
- Adjust pH to 8.0–8.5 with NaOH solution. Fit reflux condenser.
- Heat the mixture at 70–80°C for 30 minutes with stirring. Observe solution become clear.
- Cool to room temperature. Now slowly add dilute HCl to adjust pH to 4.5–5.0.
- Continue heating at 70°C under reflux until the resin starts to appear turbid (cloud point).
- Pour a few drops on a glass plate. When it turns white/cloudy on standing, the resin is ready.
- Cool and pour into a mould or petri dish. Allow to cure for 30 minutes.
- Observe the properties of the cured resin (hardness, colour, solubility in water).
📊 Observations & Calculations
ℹ️
Note the pH at each stage, the appearance of the solution at each stage, and the properties of the final cured resin (hardness, colour, transparency).
| Parameter | Stage 1 (Alkaline) | Stage 2 (Acidic) | Final Cured Resin |
|---|---|---|---|
| pH | ______ | ______ | — |
| Temperature (°C) | 70–80°C | 70°C | Room temp. |
| Reaction time (min) | 30 min | Until cloud pt. | 30 min (curing) |
| Appearance of solution | Clear / colourless | Turbid / cloudy | Hard white solid |
| Weight of urea taken (g) | ______ | ||
| Weight of formalin taken (mL) | ______ | ||
| Yield of resin (g) | ______ | ||
Properties of cured resin:
Colour: ______ | Hardness: ______ | Solubility in water: ______
% Yield = (Weight of resin / Theoretical weight) × 100 = ______%
Colour: ______ | Hardness: ______ | Solubility in water: ______
% Yield = (Weight of resin / Theoretical weight) × 100 = ______%
✅ Result
Urea-formaldehyde resin has been successfully synthesized. The resin is hard, white/opaque, and insoluble in water after curing. Yield = ______ g.
⚠️ Precautions
- Formalin is toxic — work in a well-ventilated area or fume hood.
- Monitor pH carefully at each stage using a pH meter.
- Do not allow the temperature to exceed 85°C during reflux.
- Wear gloves, lab coat, and safety goggles throughout the experiment.
❓ Viva-Voce Questions
1. What is a thermosetting polymer? How does it differ from a thermoplastic?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
2. Explain the mechanism of UF resin synthesis.
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
3. What are the industrial applications of urea-formaldehyde resin?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
4. What are the environmental concerns related to formaldehyde emissions from UF resins?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
5. What are bio-based alternatives to UF resins?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
6. What is the role of pH in the synthesis — why is alkaline stage followed by acidic stage?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
📚 References
- Odian, G. – Principles of Polymerization, 4th Ed.
- Stevens, M. P. – Polymer Chemistry: An Introduction