Table 3.
Specifications for the factors used in experimental analyses.
| Variables | Codes | Units | Levels | ||
|---|---|---|---|---|---|
| −1 | 0 | 1 | |||
| Molar ratio (urea/ formaldehyde) | X 1 | 1 | 1.2 | 1.35 | 1.5 |
| Reaction temperature | X 2 | °C | 30 | 40 | 50 |
| Reaction time | X 3 | h | 1 | 1.5 | 2 |
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