Preparation And Application Of Phenol-Formaldehyde Oligomers

Phenol-formaldehyde oligomers (PFO) are synthesized using phenol and formaldehyde in the presence of various catalysts, which compose of trihydroxymethlated phenol, dihydroxymethylated phenol, hydroxymethylated phenol and a small amount of low-molecular-weight components. Compared with normal phenol-formaldehyde resin, PFO has lower molecular weight, higher reactivity, lower viscosity, and better solubility in water. PFO can not only further polymerize to form phenol-formaldehyde resin, but also be used intermediates for reacting with other reagents. PFO can be used in wood industry, oil exploitation, the enrichment and segregation of metallic ions, etc.Sodium hydroxide and ammonia are commonly used as catalyst for the reaction between phenol and formaldehyde. Phenol/formaldehyde molar ratio, reaction temperature and reaction time have various effects on PFO compositions. Experimental results showed that the PFO prepared under gentle conditions in the presence of sodium hydroxide as catalyst have better performances.When PFO was used as modifier for low F/U molar ratio urea-formaldehyde (UF) resin, it was the best method to adding PFO in to UF at the end of synthesis when temperature was about 45℃. Compared with UF without modification, the resulted modified UF resin had improved bond strengths with dry strength up to 1.86 MPa and wet strength up to 1.82 MPa (Soaking in 63℃water for 3h), and lower formaldehyde emission (30.8% reduced). However the color of the cured modified resin became a bit darker, close to the color of birch wood.When PFO was used to modify poplar wood by impregnation, MORs of impregnated samples increased and then decreased with ammonia content decreased, and were all higher than that of untreated samples. The impregnated wood using 0.5 mol ammonia have better mechanical properties.Based on the comparisons of various synthetic conditions of PFO, solid content of impregnation solution, and pH value, the modified poplar wood with PFO (P:F:NaOH 1:2.5:0.1, molar ratio) reacted at 60℃for 10 hours, adding 0.5mol ammonia, solid content 30%, pH value 10.0, and cured at 180℃for 45min had best mechanical properties and dimensional stability. The MOR and MOE increased 37%and 22%, respectively; while volumetric swelling (soaking in 20℃water for 24h) decreased about 80%. And scanning electron microscope (SEM) indicated that PFO mainly existed in cell walls of modified wood.