Phenol-formaldehyde ratio

This reaction is carried out under base-catalyzed conditions and with a formaldehyde/phenol ratio greater than unity. The resulting product is called a C state resin or resite. 

Foam. PhenoHc resin foam is a cured system composed of open and closed ceUs with an overall density of 16—800 g/cm. Principal appHcations are in the areas of insulation and sponge-like floral foam. The resins are aqueous resoles cataly2ed by NaOH at a formaldehyde phenol ratio of ca 2 1. Free phenol and formaldehyde content should be low, although urea may be used as a formaldehyde scavenger. 

Experiment 2. Effect of Molar Ratio of Sodium Hydroxide to Phenol of Phenolic Resin on Strength Properties of Lignin-Phenolic Resin Adhesives. Sodium hydroxide has been the predominant chemical used as a catalyst in resol resin technology. Through variation in the amounts of the catalyst and the method of catalyst addition, a wide variety of resin systems can be formulated. This experiment examined the properties of phenolic resins formulated with various sodium hydroxide/phenol ratios and their effects on the bond properties of structural flakeboards made with lignin-phenolic resin adhesive systems. Variables for resin preparation were four molar ratios of sodium hydroxide/phenol (i.e., 0.2, 0.45,0.7, and 0.95). The formaldehyde/phenol ratio and solids content were fixed at 3/1 and 42%, respectively. 

The most popular is the 40% solids content family, since it seems to give a little better cost/performance relationship. This experiment examined the effect of phenolic resin solids on the properties of structural flakeboards bonded with the lignin-phenolic resin system. The variables were three levels of phenolic resin solids content - 39%, 46%, and 54%. The formaldehyde/phenol ratio and NaOH/phenol ratio were 3/1 and 0.7/1, respectively. 

As expected, the gel time decreased consistently as formaldehyde/phenol ratio increased to 3.0/1 thereafter, further increase in formaldehyde/phenol ratio had little effect on the gel time. The effects of formaldehyde/phenol ratio on pH, viscosity, solid content, and specific gravity were not significant. 

Figure 1 shows the GPC chromatograms of phenolic resins reacted at different formaldehyde/phenol ratios. In general, the chromatogram is divided into... 

Formaldehyde to Phenol Ratio. The effect of formaldehyde/phenol ratio on pH, viscosity, and gel time of the lignin-phenolic resin system is summarized as follows ... 

It is noted that the gel time decreased as the formaldehyde/phenol ratio of the phenolic resin in the system increased. Furthermore, the gel time curve of the lignin-phenolic system was, in general, similar to that of phenolic resins measured in the previous section. The similarity of the gel time curves (Figure 3) may indicate that the phenolic resin plays the major role in affecting the cure speed of the lignin-phenolic resin system even though the phenolic resin consisted of only 25% by weight in the system. 

Figure 2. Effect of formaldehyde/phenol ratio on number (Mn) and weight (Mw) average molecular weight of phenolic resins.

Figure 3. Gel time curves of phenolic and lignin-phenolic resins as affected by formaldehyde/phenol ratio.

The resins are either phenol-formaldehyde (PF), or urea-formaldehyde/phenol-formaldehyde copolymers (UF/PF), both being "resols", with a formaldehyde/phenol ratio higher than one. 

The resin used for pre-coating is a phenolic "novolac", with a formaldehyde/phenol ratio lower than 1. Hexamethylenetetramine is added to the resin as a hardener. Hexa decomposes at 160 C into its 2 basic components formaldehyde and ammonia. When pre-coated sand contacts the heated pattern, the hexamethylenetetramine decomposes and the formaldehyde cross-links the resin binder to form the characteristic strong bond. 

Several extensive kinetic studies on the polymethylolation of phenol have been reported (22,24,25). For the reaction scheme shown in Fig. 1, seven different rate constants must be determined. Despite different solution concentration, temperatures, and methods of analysis, comparing reaction rates (26-28) from each study using an NaOH catalyst gave fairly close agreement that rate constants increase with methylol substitution. In fact, dimethylol-substituted phenols react with formaldehyde two to four times faster than phenol. As a result, unreacted phenol remains high in resole resins (5-15%) even though the formaldehyde/phenol ratio is as high as 3 1. 

High Ortho Novolaks. The process for high ortho novolaks is similar to the one used for those catalyzed by strong acid. Zinc acetate is used at concentrations higher than the acids, typically 2% or more. The formaldehyde/phenol ratio is similar (0.75-0.85) but yields are 5-10% lower than those produced with strong acids, and reaction times are longer. Problems with gel particles and bulk gelation occur more frequently because small amounts of reactive dibenzyl ether... 

NOTES (a) Formaldehyde phenol ratio = 0.794. (b) The melt viscosity is checked by placing a 1-gm sample on a grooved polished hot plate that is... 

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