Curing Kinetics of Phenol-Formaldehyde Resins

Introduction 

DSC and TGA are well-known techniques commonly used to perform kinetic studies of materials in thermal analysis (TA). Knowledge of the influence of temperature on the reaction rate of phenol-formaldehyde resins (PF resols) allows the polymerization rate to be predicted. This is of great practical importance for example in the manufacture of wood composite materials or for the storage of the resins.

Most of the resol cure kinetics reported in the literature have been obtained using the Borchardt-Daniels method. Here only one run at one heating rate (dynamic measurement) is necessary to obtain the desired kinetic parameters

The kinetic parameters are, however, usually heating rate dependent and care must be taken when evaluating and interpreting the results. Likewise, the activation energy is often assumed to be constant throughout the reaction. This is, however, not true for complex curing reactions such as occur in phenolic resins. A phenol-formaldehyde (PF) resin may be of low viscosity at the start of the run and fit the assumptions for dilute solutions. But as the curing reaction proceeds, the material undergoes gelation, i.e. changes from a liquid to a rubbery state, and possibly vitrifies (transition from a rubbery to a glassy state). 

The cross-linking reduces molecular mobility and results in the process changing from being kinetically controlled to diffusion-controlled [1]. The model-free kinetics (MFK) method, developed by Vyazovkin [3-5] is based on the assumption that the activation energy, Ea, is dependent on the conversion (α). At a particular conversion, the activation energy, Ea, is independent of the heating rate. Evaluation by MFK requires at least three dynamic measurements performed at different heating rates. Since the reaction of phenol and formaldehyde under alkaline conditions to a PF resin is complex, MFK evaluation seems a suitable method to describe the curing behavior.

Experimental Details 

A liquid PF resol consisting of low and high molecular weight fractions (1:1 by weight) was synthesized in the laboratory. The high molecular weight fractions were prepared with a formaldehyde-tophenol ratio of 2.2 and a ratio of NaOH to phenol of 0.35; the low molecular weight fractions had ratios of 2.2 and 0.17. The resulting resin had a solid content of 46% («pan method»).

A METTLER TOLEDO DSC 20 with STAR^e software was used for all the thermoanalytical measurements. Since such adhesive formulations contain water, reusable 30-µl high-pressure steel crucibles capable of withstanding vapor pressures up to 10 Mpa were used. The dynamic DSC measurements were performed at heating rates of 5, 10 and 20 K/min in the temperature range 25 to 250 °C. The DSC curves were evaluated with different kinetic methods (STAR^e software options).

Measurements and Results 

Evaluation According to Borchardt-Daniels

Figure 1 shows the DSC curves of the PF resol measured at heating rates of 5, 10 and 20 K/min. The exothermic reaction peaks were shifted to higher temperatures at increasing heating rates due to kinetic effects. The beginning of an endothermic decomposition reaction can be seen from 200 °C onward especially in the curve measured at 20 K/min.

Conclusions 

The reaction behavior of phenol-formaldehyde resins can be studied with different kinetic evaluation methods. From the results, the conversion-temperature-time behavior can be predicted and answers given to technological questions. The activation energy is often the key quantity. A comparison of the methods has shown that only model free kinetics (MFK) is able to adequately describe the complex curing reaction in most respects. 

The Borchardt-Daniels method can hardly determine the activation energy properly and the ASTM E698 method yields the activation energy only at the highest reaction rate (DSC peak temperature). MFK however shows the activation energy curve as a function of conversion. 

This can provide useful information for the interpretation of the curing mechanisms besides yielding trustworthy simulation data. It has for example been shown for PF resins in wood composites (He et. al. [1]) that the degree of cure of the resin in the wood is not so high as when the resin is used in the pure form. Diffusion of the resin in the wood, which has a porous structure, is hindered or even prevented through gelation of the adhesive.

_{Curing Kinetics of Phenol-Formaldehyde Resins | Thermal Analysis Application No. UC 193 | Application published in METTLER TOLEDO Thermal Analysis UserCom 19}