| Sample No. | Acid catalyst | Formaldehyde content (%) | Solids content (%) |
| S1 | phthalic anhydride | 4.53 | 42.0 |
| S2 | 5% H2SO4 | 3.31 | 44.0 |
| S3 | 10% HAc | 3.92 | 44.0 |
| S4 | 15% HCOOH | 4.42 | 46.8 |