Reaction with phenol-formaldehyde prepolymers

If polymeric procyanidins extractable from conifer tree barks are to be used in adhesive formulations requiring condensation with phenol-formaldehyde prepolymers, these reactions must be performed at acidic pH conditions, and because of solubility limitations, this will probably require the use of sulfonate derivatives. 

A-ring of the tannin (194). Resin synthesis conditions varied in the approach to addition of formaldehyde. For example, the tannins could be crosslinked by reaction of a phenol-resorcinol-formaldehyde resin carrying little or no methylol functionality by addition of paraformaldehyde or by reaction of a phenol-formaldehyde prepolymer carrying a comparatively high methylol functionality. Adhesive formulations were similar to those described above. Typically, 100 parts of a 55% solids content water solution of wattle tannin were combined with 0.25 parts of defoamer, 7 parts of paraformaldehyde and 9 to 10 parts of coconut shell powder the pH was adjusted to 6.5 to 7.4 by addition of 40% NaOH. These adhesives also provided exterior quality glue lines. They are exceptional in their tolerance to high moisture content veneer and permit fast curing rates, subjects of particular interest in the plywood industry today. 

Applications for cold-setting, wood-laminating adhesives initially followed the same approach (47) used for laminating resins from western hemlock (38) (i.e., reaction of tannin with phenol-resorcinol-formaldehyde prepolymers). Improvements resulted through the application of Kreibich s Honeymoon technique (48) wherein one side of the material to be bonded is treated with resin and the other with catalyst. One of the preferred systems (49) was phenol-resorcinol-formaldehyde or tannin-resorcinol-formaldehyde at pH 8 with extra paraformaldehyde on the A-side and tannin at 53% solids or tannin-resorcinol-formaldehyde at pH 12 on the B-side. Such resin systems are currently used to laminate eucalyptus or pine in most South African timber-laminating plants. 

In contrast to the linear thermoplastic polymers, which are soluble and fusible, the cross-linked network polymers are insoluble and infusible. They are formed from polymerizing systems containing monomers or prepolymers with a functionality of three or more. A good example is the phenol-formaldehyde resin systems. The cross-linking reaction takes place in the bond under applied pressure and heat, and the whole adhesive bond might consist of only one super giant molecule. Such resins are, therefore, called thermosetting resins. 

The route to crosslinked phenol-formaldehyde resins via resoles corre.sponds to that used by Baekeland in his original commercial technique, They now tend to be used for adhesives, binders, and laminates. The resole prepolymers are made typically in batch processes, using a trace of ammonia (about 2% on phenol) as the alkaline catalyst. Care has to be taken with this process since, despite the molar excess of formaldehyde, there is sufficient of each component present in the prepolymer to permit the formation of a highly crosslinked product. Indeed, such a product will form if the resole is heated excessively, but the problem can be avoided by careful attention to the conditions of reaction and by ensuring that polymerisation is not allowed to proceed for too long. 

Novolacs are low-molecular-weight, fusible but insoluble prepolymers (18) prepared by reaction of formaldehyde with molar excess of phenol. Novolacs, unlike resoles, do not contain residual methylol groups. A high-molecular-weight network polymer similar to that of resoles is formed by heating novolac with additional formaldehyde, paraformaldehyde, or hexamethylenetetramine. 

Phenol-formaldehyde Random prepolymers are prepared by reacting phenol (/ = 3 for the ortho and para positions in the ring) with bifunctional formaldehyde. The base-catalyzed reaction produces a mixture of methylol phenols. 

Synonyms Resorcinol-formaldehyde latex Resorcinol resin Classification Phenol-formaldehyde resin Definition Thermosetting prepolymer cured by reaction with paraformaldehyde... 

Step growth condensation reactions involving multifunctional reactants. More or less densely crosslinked products such as epoxy and phenol-formaldehyde resins may be prepared by the reaction of multifunctional monomers or prepolymers with each other or with other appropriate molecules. 

The most commonly known phenolic composite group is phenol formaldehyde polymers (phenoplasts). They are produced by polycondensation of a phenol and a mixture of phenols (phenol and phenol derivatives like cresol-resorcinol or para tertiary butyl phenol) with an aldehyde, usually formaldehyde and hexamethylene tetramine. Reaction of formaldehyde with phenol (up to 3 moles of formaldehyde can react with one mole of phenol - phenol acts as a three functional monomer) yields methylol groups in the ortho and para positions of the phenol molecule. In a further reaction, the methylol groups condenses with another molecule of phenol to form a methylene bridge. In practice, a prepolymer (usually a powder) is prepared first which is then cured later to the shape of the article in the mould. 

Another compound which has been found to somewhat imitate the active site of peroxidases is the commercially available Fe(II)-salen catalyst. This catalyst was used successfully to produce phenol polymers, which could be of interest for industrial production [153,154]. For example, cardanol can be polymerized by the Fe(II)-salen catalyst [155]. Due to the unsaturated bonds in the side chain of the cardanol components, the resulting polymers could be thermally cured, or cured by use of cobalt naphthenate to give brilliant films with a high-gloss surface. This reaction proves that reactive prepolymers can be synthesized from renewable resources (cardanol is the main component obtained by thermal treatment of cashew nutshell liquid). This process could be a true alternative to conventional phenol-formaldehyde resins (Scheme 25) [ 155]. Other non-heme iron complexes have been foimd to... 

The amino resins or plastics, closely related to the phenolics in both synthesis and applications, are obtained by the polymerization of formaldehyde with urea (XXXVII) (/ = 4) or melamine (XXXVIII) (f — 6). Synthesis of the amino plastics can be carried out either in alkaline or acidic conditions [Drumm and LeBlanc, 1972 Nair and Francis, 1983 Updegraff, 1985]. Control of the extent of reaction is achieved by pH and temperature control. The prepolymer can be made at various pH levels depending on the reaction temperature chosen. Polymerization is stopped by cooling and bringing the pH close to neutral. Curing of the prepolymer involves heating, usually in the presence of an added acid catalyst. 

The functionality may vary with reaction conditions. For example, in base-catalyzed copolymerization of phenol and formaldehyde, both monomers are bifunctional at ambient temperature, but phenol becomes trifunctional if the temperature is raised sufficiently. Copolymerization at ambient temperature can produce essentially linear, liquid, resole-type "prepolymers" of low molecular weight. Upon acidification and heat-curing, methylene and ether crosslinks formed by the now trifunctional phenol units transform the polymer into an insoluble resin [7] (see next page). The original Bakelite was such a "thermosetting" product. 

Resorcinol Resins. The reactivity of phenol with formaldehyde is greatly increased with two hydroxyl groups on its nucleus (resorcinol VIII). Room temperature polymerization is observed without the need for any catalyst. The rate of reaction goes through a minimum at a pH of 3.5 and increases at lower or higher pH values. To make a useful adhesive, prepolymers, similar to novolaks, are pre-... 

Formation of novolak involves an acid-catalyzed reaction of formaldehyde with excess phenol (i.e., formaldehyde-to-phenol mole ratio less than 1). The initial methylol phenols condense with the excess phenol to form dihydroxydiphenyl methane, which undergoes further condensation yielding low-molecular-weight prepolymer or novolak. Unlike resoles, novolaks do not contain residual methylol groups. They are fusible and insoluble. 

Phenolics are made by the polymerization of phenol and formaldehyde, as shown in Fig. 4.10. The reaction is carried out to a low degree of polymerization, making what is called a prepolymer. This material may be diluted with a solvent or more monomer to make a workable liquid. The cross-linking reaction is carried out by the addition of a catalyst and heat. 

The PF prepolymers may be of two types, the novolaks and resols. The novolaks are prepared from excess phenol and formaldehyde (in the molar ratio 5 4) under acidic conditions. The monomers react slowly to form o- and p-methylol phenols (Reaction 1), which then condense rapidly with further phenol to give dihydroxyl diphenyl methane types (Reaction 2) ... 

The novolac-type phenolic prepolymer was synthesized in a 1.0 L glass reactor equipped with a thermometer, reflux condenser, and stirrer. The reagents, 188 g phenol (2 mol) and 135 g formaldehyde (in 37wt.% water solution, 1.67 mol), were fed into a flask reactor. The reaction was catalyzed by adding dilute sulfuric acid solution (2g of sulfuric acid dissolved in 10 ml of water) and reacted at 100°C for 7 hr in the reactor. The reaction was continued until the prepolymer with the desired viscosity (500-2000 cps at 25°C) was obtained. At the end of the reaction, the calculated amount of NaOH (1.63g) was dispersed in 10 ml of water and added to neutralize the sulfuric acid catalyst and then stirred for an additional 30 min. The mixture was dehydrated under a pressure of 100 mm H2O at 120-130°C until the resin was clear. 

Commercially, two grades of prepolymers (novolacs and resoles) are made through the polymerization of phenol and formaldehyde. Novolacs are linear polymer chains with httle branching and are formed when the pH of the reaction mass is low (2 to 3). Resole prepolymers are manufactured at high pH (9 to 11) and are highly branched. The characteristics of the prepolymer formation are complex some of these are given in Table 3.5. The important feature of the polymerization, as can be seen from the table, is the different reactivities of the sites. 

---