Resoles curing reactions

The kinetics of resole cure reactions monitored via FTIR suggest that a diffusion mechanism dominates below 140°C. The cure above 140°C exhibits a homogeneous first-order reaction rate. The activation energy of the cure reaction was -"-49.6 kJ/mole.66... 

The phenolic resol curing reaction and dimethacrylate polymerization were conducted at 170°C for 90 minutes, yielding vinyl compound/phenolic IPN. (See Scheme 3.)... 

Properties of vinyl compound/phenolic IPN were discussed and the following conclusions drawn. MMA radical polymerization proceeded rapidly in the presence of phenolic resol. Poly (ethylene glycol) dimethacrylate, 23G, and phenolic resol IPNs were synthesized by simultaneous radical polymerization and phenolic resol curing reaction. These IPNs had a structure of poly 23G chains and cured phenolic resol chains so well entangled with each other that the whole acted as a single phase. This type of IPN is considered to... 

Cure Rate of the Phenolated SEL Resins. 13C NMR spectra of the phenolated SEL formaldehyde-treated resins revealed the formation of methylol groups. A similar cure reaction to resole type phenolic resins is expected to occur with the phenolated lignin-based resins. Since cure rate normally determines production capacity of a board mill, it is important that new types of adhesives have at least the same cure rate as the conventional phenolic adhesives. Cure analysis of resins has usually been examined by... 

Phenol-formaldehyde adhesives are produced by a condensation polymerization reaction between phenol and formaldehyde. The phenolics used for exterior particleboard are made at a formalde-hyde/phenol ratio greater than 1.0 i.e., they are classified as resoles and additional formaldehyde is not required to complete the curing reaction to a highly cross-linked network structure. Many characteristics can be incorporated into the adhesives by changes in the F/P ratio, condensation pH, and condensation time. The reactive solids content is normally between kO and 50 percent since the stability and viscosity are adversely affected at higher solids. 

Method 1 Poly (ethylene glycol) dimethacrylate (23G), and phenolic resol (PR) were mixed at 50°C for 10 minutes and dicumyl peroxide (DCP) was added. The polymerization and phenolic curing reactions then took place simultaneously at 170°C for 90 minutes in 200mm x 10mm x 1.5mm stainless steel mold. This product was called IPN 1. 

Next, pol3nnerization and curing reactions were conducted simultaneously. Table I shows the results of the reactions. By using various vinyl compounds and initiators, poljnnerization of over 90% and condensation conversion over 80% was obtained (as measured by IR spectrum of the cured materials). Figure 3 shows typical IR spectra of dimethacrylate and phenolic resol IPN. Reaction conversion was calculated from the peak of C-C double bond, methylol, and carbonyl groups. 

Polymerization of 23G proceeds in a way different from the curing reaction of phenolic resol. A cross-linking reaction between 23G and phenolic resol cannot be considered because there... 

Resol synthesizing, foaming and curing reaction— when resol synthesizing (addition, condensation)... 

A PRF similar to adhesive 2 is prepared and a PF resol of the same type as adhesive 4 is used as a hardener. The schematic curing reaction is shown in the Formula 9. 

Curing reactions applied to epoxy prepoiymers, unsaturated polyesters, resoles, and novolacs make use of three general classes of prepoiymers which are distinguished by the number and location of sites of functional groups available for subsequent crosslinking reactions. These three general classes have been defined as discussed in the following sections. 

Abe I, Akimoto N 1976 [The inhibitory effect of kapur wood extractives on the curing reaction of the resol.] Mokuzai Gakkaishi 22 191-196... 

Phenolic Resins. Phenohc resins (qv) are formed by the reaction of phenol [108-95-2] C H O, and formaldehyde [50-00-0] CH2O. If basic conditions and an excess of formaldehyde are used, the result is a resole phenohc resin, which will cure by itself Hberating water. If an acid catalyst and an excess of phenol are used, the result is a novolac phenohc resin, which is not self-curing. Novolac phenohc resins are typically formulated to contain a curing agent which is most often a material known as hexamethylenetetraamine [100-97-0] C H22N4. Phenohc resin adhesives are found in film or solution... 

Resoles. The advancement and cure of resole resins foUow reaction steps similar to those used for resin preparation the pH is 9 or higher and reaction temperature should not exceed 180°C. Methylol groups condense with other methylols to give dibenzyl ethers and react at the ortho and para positions on the phenol to give diphenyknethylenes. In addition, dibenzyl ethers eliminate formaldehyde to give diphenyknethanes. 

In some resole appHcations, such as foam and foundry binders, a rapid cure of a Hquid resin is obtained at RT with strong acid. The reactions proceed in the same manner as those of novolak resin formation. Methylol groups react at ortho and para phenoHc hydrogen to give diphenyknethane units (41). 

The importance of the nature of the catalyst on the hardening reaction must also be stressed. Strong acids will sufficiently catalyse a resol to cure thin films at room temperature, but as the pH rises there will be a reduction in activity which passes through a minimum at about pH 7. Under alkaline conditions the rate of reaction is related to the type of catalyst and to its concentration. The effect of pH value on the gelling time of a casting resin (phenol-formaldehyde ratio 1 2.25) is shown in Figure 23.15. 

The mechanism of this reaction has been studied by several groups [133,174-177]. The consensus is that interaction of ester with the phenolic resole leads to a quinone methide at relatively low temperature. The quinone methide then reacts rapidly leading to cure. Scheme 11 shows the mechanism that we believe is operative. This mechanism is also supported by the work of Lemon, Murray, and Conner. It is challenged by Pizzi et al. Murray has made the most complete study available in the literature [133]. Ester accelerators include cyclic esters (such as y-butyrolactone and propylene carbonate), aliphatic esters (especially methyl formate and triacetin), aromatic esters (phthalates) and phenolic-resin esters [178]. Carbamates give analogous results but may raise toxicity concerns not usually seen with esters. 

The PVF is made by acidic reaction between poly(vinyl alcohol) (PVA) and formaldehyde. The poly(vinyl alcohol) is, in turn, made by hydrolysis of poly(vinyl acetate) or transesterification of poly(vinyl acetate). Thus, residual alcohol and ester functionality is usually present. Cure reportedly occurs through reaction of phenolic polymer hydroxyls with the residual hydroxyls of the PVA [199]. The ester residues are observed to reduce bond strength in PVF-based systems [199]. This does not necessarily extend to PVF-P adhesives. PVF is stable in strong alkali, so participation of the acetals in curing is probably unimportant in most situations involving resoles. PVF is physically compatible with many phenolic resins. 

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