13C-NMR study on cure-accelerated phenol-formaldehyde resins with carbonates

Abstract

Both liquid-and solid-state 13 C-NMR spectroscopies were employed to investigate the cure-acceleration effects of three carbonates [propylene carbonate (PC), sodium carbonate (NC), and potassium carbonate (KC)] on liquid and cured phenolformaldehyde (PF) resins. The liquid-phase 13 C-NMR spectra showed that the cureacceleration mechanism in the PC-added PF resin seemed to be involved in increasing reactivity of the phenol rings, while the addition of both NC and KC into PF resin apparently resulted in the presence of ortho-ortho methylene linkages. Proton spinlattice rotating frame relaxation time (T 1H ) measured by solid-state 13 C-CP/MAS-NMR spectroscopy was smaller for the cure-accelerated PF resins than for that of the control PF resin. The result indicated that cure-accelerated PF resins are less rigid than the control PF resin.

Synthesis and Mechanism of Metal-Mediated Polymerization of Phenolic Resins

“…The 13 C NMR spectra are shown in Figure 4, and their corresponding assignments of groups' signals are shown in Figure 5 [12,[18][19][20]. The chemical shift of 150.0-158.0 ppm was assigned to phenoxy carbons (C1-OH), which was used as an integral standard and analytical standpoint.…”

Thermal Degradation and Stability of Accelerated-curing Phenol-formaldehyde Resin

“…It has been reported that the esters or residue of their decomposition could attack the negatively charged phenolic nuclei in a polycondensation reaction, resulting in a higher functionality to activate methylol groups and hence increasing the curing speed and reducing the gel time of PF resin (Park and Riedl 2000;Pizzi and Stephanou 1994). Moreover, the bivalent metallic salts can accelerate both the action of phenolic nuclei with formaldehyde and the condensation of methylolphenols with other phenolic nuclei by forming metallic ion/phenol/formaldehyde complexes (Pizzi 1979a,b), resulting in an accelerating effect of Mg 2+ on gelation of PF resin.…”

“…However, the curing accelerators are consumed during the reaction, indicating that they act not only as true catalysts but that they are also involved in self-condensation (Conner et al 2002). Therefore, the application of propylene carbonate (PC) in accelerated-curing PF resins with this mechanism not only causes the reduction of resin consume, but also can reduce the hot-pressing time for the manufacture of panel products such as medium density fiberboard (MDF) (Park and Riedl 2000). Other studies on the effect of urea addition to PF resins after condensation found that urea addition could lower the free formaldehyde content and increase the degree of polymerization, while the curing rates at first increased and then decreased due to an increase in the amount of low-molecular weight methylolureas (Fan 2009;He and Riedl 2003).…”

Reactivity, chemical structure, and molecular mobility of urea–formaldehyde adhesives synthesized under different conditions using FTIR and solid‐state 13C CP/MAS NMR spectroscopy

“…T 1H measurements have been made for PF resole resins. 21,22 However, the 13 C-NMR with cross-polarization (CP) and magic angle spinning (MAS) method was not previously used to monitor the structural characterization of UF resins prepared under different reaction pH conditions. Even though many investigators studied the chemical structure of UF resins prepared under different reaction pH conditions, the work done to study the influence of hardener types on the reactivity and molecular mobility based on chemical structure of the resins has been limited.…”