In fact, the addition of methylol groups might enhance the elasticity of the cured resin and improve shrinkage properties 138,391. The chemical shift in the region 171.0- 172.0 ppm in the LMWPF resin is assigned to the carbon of formic acid, the formation of which might be caused by the oxidation of formaldehyde l40]. However, no chemical shift was observed for PUF resin in Fig. 2.