Phenol-formaldehyde reaction kinetics

Base-catalyzed phenol-formaldehyde reactions exhibit second-order kinetics [Eq. (5)]. Several alkylphenols such as cresols also follow this rate equation ... 

The reaction conditions, formaldehyde-to-phenol ratios, and concentration and type of catalyst govern the mechanisms and kinetics of resole syntheses. Higher formaldehyde-to-phenol ratios accelerate the reaction rates. This is to be expected since phenol-formaldehyde reactions follow second-order kinetics. Increased hydroxymethyl substitution on phenols due to higher formaldehyde compositions also leads to more condensation products.55... 

Although the condensation of phenol with formaldehyde has been known for more than 100 years, it is only recently that the reaction could be studied in detail. Recent developments in analytical instrumentation like GC, GPC, HPLC, IR spectroscopy and NMR spectroscopy have made it possible for the intermediates involved in such reactions to be characterized and determined (1.-6). In addition, high speed computers can now be used to simulate the complicated multi-component, multi-path kinetic schemes involved in phenol-formaldehyde reactions (6-27) and optimization routines can be used in conjunction with computer-based models for phenol-formaldehyde reactions to estimate, from experimental data, reaction rates for the various processes involved. The combined use of precise analytical data and of computer-based techniques to analyze such data has been very fruitful. 

The chapter ends with a case study. Four different reduced kinetic models are derived from the detailed kinetic model of the phenol-formaldehyde reaction presented in the previous chapter, by lumping the components and the reactions. The best estimates of the relevant kinetic parameters (preexponential factors, activation energies, and heats of reaction) are computed by comparing those models with a wide set of simulated isothermal experimental data, obtained via the detailed model. Finally, the reduced models are validated and compared by using a different set of simulated nonisothermal data. 

In this section, the phenol-formaldehyde reaction is introduced as a case study. This reaction has been chosen because of its kinetic complexity and its high exothermic-ity, which poses a strong challenge for modeling and control practice. The kinetic model presented here is adopted to simulate a realistic batch chemical process the identification, control, and diagnosis approaches developed in the next chapters are validated by resorting to this model. 

The kinetic model developed in Sect. 2.4 for the phenol-formaldehyde reaction belongs to a wider class of kinetic networks made up of irreversible nonchain reactions. In this section, a general form of the mathematical model for this class of reactive systems is presented moreover, it is shown that the temperature attainable in the reactor is bounded and the lower and upper bounds are computed. 

According to the approach described in [11], two alternative reduced kinetic models are proposed here to describe the phenol-formaldehyde reaction network introduced in Sect. 2.4. This approach includes, first, the selection of a general class... 

For a safe operation, the runaway boundaries of the phenol-formaldehyde reaction must be determined. This is done here with reference to an isoperibolic batch reactor (while the temperature-controlled case is addressed in Sect. 5.8). As shown in Sect. 2.4, the complex kinetics of this system is described by 89 reactions involving 13 different chemical species. The model of the system consists of the already introduced mass (2.27) and energy (2.30) balances in the reactor. Given the system complexity, dimensionless variables are not introduced. 

The model-based controller-observer scheme requires to solve online the system of differential equations of the observer. The phenol-formaldehyde reaction model is characterized by 15 differential equations, and it is, thus, unsuitable for online computations. To overcome this problem, one of the reduced models developed in Sect. 3.8.1 can be adopted. In order to be consistent with the general form of nonchain reactions (2.27) adopted to develop the controller-observer scheme, the reduced model (3.57) with first-order kinetics has been used to design the observer. The mass balances of the reduced model are given by... 

Other recent references to phenol—formaldehyde condensations include those of Yeddanapalli et al. [200, 201]. The complications of melamine/ formaldehyde and urea/formaldehyde reactions kinetics are analogous, and have been examined in two recent papers [202, 203]. 

Resoles are usually mixtures of a number of methylol phenols, with small amounts of higher condensation products involving methylene and benzylic ether linkages. The kinetics of base-catalyzed phenol-formaldehyde reactions have been extensively researched. At high dilution and at a pH not exceeding 10, the rate equation can be expressed by Eq. (13), where [PhO ] denotes the concentration of phenoxide anion and [F] the concentration of unreacted formaldehyde, determined titrimetrically. " ... 

Freeman and Lewis [23] published one of the first complete kinetic investigations of phenol with formaldehyde. The hydroxymethylation at 30°C was followed by analysis using quantitative paper chromatography with detection of five products (2-hydroxymethylphenol, 4-hydroxymethyl-phenol, 2,4-dihydroxymethylphenol, 2,6-dihydroxymethylphenol, and 2,4,6-trihydroxymethylphenol). More recently, Zavitsas and Beaulieu [24] used GLC to investigate the kinetics of the phenol-formaldehyde reaction using only catalytic amounts of base and at pH ranges where the second-order rate expression was shown to be valid. 

In general, the reaction between a phenol and an aldehyde is classified as an electrophilic aromatic substitution, though some researchers have classed it as a nucleophilic substitution (Sn2) on aldehyde [84]. These mechanisms are probably indistinguishable on the basis of kinetics, though the charge-dispersed sp carbon structure of phenate does not fit our normal concept of a good nucleophile. In phenol-formaldehyde resins, the observed hydroxymethylation kinetics are second-order, first-order in phenol and first-order in formaldehyde. 

The study of PF polymerization is far more difficult than that of methylolation due to the increased complexity of the reactions, the intractability of the material, and a resulting lack of adequate analytical methods. When dealing with methylolation, we saw that every reactive ring position had its own reaction rate with formaldehyde that varied with the extent of prior reaction of the ring. Despite this rate sensitivity and complexity, all reactions kinetics were second-order overall, first-order in phenol reactive sites and first-order in formaldehyde. This is not the case with the condensation reactions. 

Alkaline co-condensation to yield commercial resins and the products of reaction obtained thereof [93,94] as well as the kinetics of the co-condensation of mono methylol phenols and urea [104,105] have also been reported [17]. Model reactions in order to prove an urea-phenol-formaldehyde co-condensation (reaction of urea with methylolphenols) are described by Tomita and Hse [98,102, 106] and by Pizzi et al. [93,104] (Fig. 1). 

Only a few studies have tackled the problem of deriving a detailed kinetic model of the phenol-formaldehyde reactive system, mainly because of its complexity. In recent years, a generalized procedure has been reported in [11,14] that allows one to build a detailed model for the synthesis of resol-type phenolic resins. This procedure is based on a group contribution method and virtually allows one to estimate the kinetic parameters of every possible reaction taking place in the system. 

In order to estimate the kinetic parameters for the addition and condensation reactions, the procedure proposed in [11, 14] has been used, where the rate constant kc of each reaction at a fixed temperature of 80°C is computed by referring it to the rate constant k° at 80°C of a reference reaction, experimentally obtained. The ratio kc/k°, assumed to be temperature independent, can be computed by applying suitable correction coefficients, which take into account the different reactivity of the -ortho and -para positions of the phenol ring, the different reactivity due to the presence or absence of methylol groups and a frequency factor. In detail, the values in [11] for the resin RT84, obtained in the presence of an alkaline catalyst and with an initial molar ratio phenol/formaldehyde of 1 1.8, have been adopted. Once the rate constants at 80°C and the activation energies are known, it is possible to compute the preexponential factors ko of each reaction using the Arrhenius law (2.2). 

In this section, the phenol-formaldehyde reactive system is considered as an example of identification of reduced kinetic models. The kinetic model containing 13 components and 89 reactions, developed in Sect. 2.4 to study the production of 1,3,5-methylolphenol, is too detailed and complex for control and monitoring purposes. Thus, in this section this model is referred to as detailed model, while four reduced kinetic models, based on lumped components and reactions, are developed. 

Plots of (absorbance)-1 versus time were linear for greater than 90% of the reaction for each system studied. These observations may be interpreted in terms of the reaction obeying second-order kinetics where the reactants are equimolar (40). Second-order or more complex kinetics are typical of phenol-formaldehyde condensations under alkaline conditions (7). 

Lignin fillers decreased the cure rate of phenol-formaldehyde resin. Here, the filler acts as a diluent and does not have the ability to affect the reaction kinetics by interaction with the polymer. Glass fibers also decreased the rate of cure of a phenolic resin in another study. 

It is interesting to note that while -CH2-O-CH2- ether bridged compounds have been isolated for the phenol-formaldehyde [24] reaction, their existence for fast-reacting phenols such as resorcinol and phloroglucinol has been postulated, but they have not been isolated, as these two phenols have always been considered too reactive with formaldehyde. They are detected by a surge in the concentration of formaldehyde observed in kinetic curves due to methylene ether bridge decomposition [19]. 

The primary causes of accidents in the chemical industry are technical failures, human failures and the chemical reaction itself (due to lack of knowledge of the thermochemistry and the reaction kinetics) [156]. As discussed previously, polymerization reactions are subject to thermal runaway, so that it is not surprising to learn that polymerization reactions (64 from 134 cases) are more prone than other processes to serious accidents [157]. Among the polymerization processes, the phenol-formaldehyde resin production seems to be the worst case, although incidents have been reported for vinyl chloride, vinyl acetate and polyester resins polymerization processes. 

Using DMA the curing reactions of phenol-formaldehyde resins have been followed [1]. The evolution of various rheological parameters was recorded for samples of the resins on cloth. A third-order phenomenological equation described the curing reaction. The influences of the structure, composition, and physical treatment on the curing kinetics were evaluated. 

MS has been widely used to identify the products and their formation kinetics in the degradation of filled reaction layers, e.g., phenol-formaldehyde resins [53, 54], epoxide resins, polyesters and polyacrylates [55]. 

The reaction mechanism for the basic catalysis of phenolics is not completely understood. However, the reaction kinetics are known to be first order in methylene glycol and in phenol. A complete comparison of the relative reaction rates of producing the seven possible methylol phenols is given by Knop and Pizzi. Table 3 shows the rate constants,the relative rate constants compared to 4-hydroxymethyl phenol (4-HMP) and the relative rate constants after allowing for the statistical weighting of the two ortho positions in both phenol and 4-hydroxy-methylphenol. Comparison of kinetic rate constants for formaldehyde addition in the ortho positions relative to the kinetic rate constant for the addition of formaldehyde to phenol in the para position shows that the formation of 2-hydroxybenzyl alcohol is favored over 4-hy-droxybenzyl alcohol 1.7 1. The formation of the 2,4-dihydroxymethylphenol (2,4-HMP) is 1.2 1 compared to the formation of 2,6-dihydroxy methylphenol at 1.40 1. Interestingly, the 2,4,6-trihydroxy methylphenol (2,4,6-... 

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