phenolic resin adhesives

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Phenolic Resin Adhesives

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26Phenolic Resin AdhesivesA. PizziEcole Nationale Suprieure des Technologies et Industries du Bois,

Universit de Nancy I, Epinal, France

I. INTRODUCTIONPhenolic resins are the polycondensation products of the reaction of phenol with formal-dehyde. Phenolic resins were the first true synthetic polymers to be developed commer-cially. Notwithstanding this, even now their structure is far from completely clear, becausethe polymers derived from the reaction of phenol with formaldehyde differ in one impor-tant aspect from other polycondensation products. Polyfunctional phenols may react withformaldehyde in both the ortho and para positions to the hydroxyl group. This means thatthe condensation products exist as numerous positional isomerides for any chain length.

This makes the organic chemistry of the reaction particularly complex and tedious tounravel. The result has been that although phenolic resins were developed commerciallyas early as 1908, were the first completely synthetic resins ever to be developed, and havevast and differentiated industrial uses today, and great strides have been made in both theunderstanding of their structure and their technology and application, several aspects oftheir chemistry are still only partially understood.It may be argued with some justification that such a state of affairs is immaterial,because satisfactory resins for many uses have been developed on purely empirical groundsduring the past 90 years. However, it cannot be denied that the gradual understanding of the

chemical structure and mechanism of reaction of these resins has helped considerably inintroducing commercial phenolic resins designed for certain applications and capable ofperformances undreamed of in formulations developed earlier by the empirical rather thanthe scientific approach. Knowledge of phenolic resin chemistry, structure, characteristicreactions, and kinetic behavior remains an invaluable asset to the adhesive formulator indesigning resins with specific physical properties. The characteristic that renders these resins

invaluable as adhesives is their capability to deliver water, weather, and high-temperatureresistance to the cured glue line of the joint bonded with phenolic adhesives, at relativelylow cosII. CHEMISTRYPhenols condense initially with formaldehyde in the presence of either acid or alkali toform a methylolphenol or phenolic alcohol, and then dimethylolphenol. The initial attackCopyright 2003 by Taylor & Francis Group, LLC

Page 2may be at the 2-, 4-, or 6-position. The second stage of the reaction involves methylolgroups with other available phenol or methylolphenol, leading first to the formation oflinear polymers [1] and then to the formation of hard-cured, highly branched structures.Novolak resins are obtained with acid catalysis, with a deficiency of formaldehyde. Anovolak resin has no reactive methylol groups in its molecules and therefore without

hardening agents is incapable of condensing with other novolak molecules on heating.To complete resinification, further formaldehyde is added to cross-link the novolak resin.Phenolic rings are considerably less active as necleophilic centers at an acid pH, due tohydroxyl and ring protonation.However, the aldehyde is activated by protonation, which compensates for thisreduction in potential reactivity. The protonated aldehyde is a more effective electrophile.The substitution reaction proceeds slowly and condensation follows as a result offurther protonation and the creation of a benzylcarbonium ion that acts as a nucleophile.Resols are obtained as a result of alkaline catalysis and an excess of formaldehyde.A resol molecule contains reactive methylol groups. Heating causes the reactive resol

molecules to condense to form large molecules, without the addition of a hardener. The


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function of phenols as nucleophiles is strengthened by ionization of the phenol, withoutaffecting the activity of the aldehyde.Megson [2] states that reaction II (in which resols are formed by the reaction ofquinone methides with dimethylolphenols or other quinone methides) is favored duringalkaline catalysis. A carbonium ion mechanism is, however, more likely to occur. Megson

[2] also states that phenolic nuclei can be linked not only by simple methylene bridges butalso by methylene ether bridges. The latter generally revert to methylene bridges if heated

during curing with the elimination of formaldehyde.Copyright 2003 by Taylor & Francis Group, LLC

Page 3The differences between acid-catalyzed and base-catalyzed process are (1) in the rateof aldehyde attack on the phenol, (2) in the subsequent condensation of the phenolicalcohols, and (3) to some extent in the nature of the condensation reaction. With acidcatalysis, phenolic alcohol formation is relatively slow. Therefore, this is the step thatdetermines the rate of the total reaction. The condensation of phenolic alcohols andphenols forming compounds of the dihydroxydiphenylmethane type is, instead, rapid.The latter are therefore predominant intermediates in novolak resins.

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26Phenolic Resin AdhesivesA. PizziEcole Nationale Suprieure des Technologies et Industries du Bois,Universit de Nancy I, Epinal, France

I. INTRODUCTIONPhenolic resins are the polycondensation products of the reaction of phenol with formal-dehyde. Phenolic resins were the first true synthetic polymers to be developed commer-cially. Notwithstanding this, even now their structure is far from completely clear, becausethe polymers derived from the reaction of phenol with formaldehyde differ in one impor-tant aspect from other polycondensation products. Polyfunctional phenols may react withformaldehyde in both the ortho and para positions to the hydroxyl group. This means that

the condensation products exist as numerous positional isomerides for any chain length.This makes the organic chemistry of the reaction particularly complex and tedious tounravel. The result has been that although phenolic resins were developed commerciallyas early as 1908, were the first completely synthetic resins ever to be developed, and havevast and differentiated industrial uses today, and great strides have been made in both theunderstanding of their structure and their technology and application, several aspects of

their chemistry are still only partially understood.It may be argued with some justification that such a state of affairs is immaterial,because satisfactory resins for many uses have been developed on purely empirical groundsduring the past 90 years. However, it cannot be denied that the gradual understanding of thechemical structure and mechanism of reaction of these resins has helped considerably inintroducing commercial phenolic resins designed for certain applications and capable ofperformances undreamed of in formulations developed earlier by the empirical rather thanthe scientific approach. Knowledge of phenolic resin chemistry, structure, characteristic

reactions, and kinetic behavior remains an invaluable asset to the adhesive formulator in

designing resins with specific physical properties. The characteristic that renders these resinsinvaluable as adhesives is their capability to deliver water, weather, and high-temperatureresistance to the cured glue line of the joint bonded with phenolic adhesives, at relativelylow cost.II. CHEMISTRYPhenols condense initially with formaldehyde in the presence of either acid or alkali toform a methylolphenol or phenolic alcohol, and then dimethylolphenol. The initial attackCopyright 2003 by Taylor & Francis Group, LLC


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Page 2may be at the 2-, 4-, or 6-position. The second stage of the reaction involves methylolgroups with other available phenol or methylolphenol, leading first to the formation oflinear polymers [1] and then to the formation of hard-cured, highly branched structures.

Novolak resins are obtained with acid catalysis, with a deficiency of formaldehyde. Anovolak resin has no reactive methylol groups in its molecules and therefore withouthardening agents is incapable of condensing with other novolak molecules on heating.To complete resinification, further formaldehyde is added to cross-link the novolak resin.Phenolic rings are considerably less active as necleophilic centers at an acid pH, due tohydroxyl and ring protonation.However, the aldehyde is activated by protonation, which compensates for thisreduction in potential reactivity. The protonated aldehyde is a more effective electrophile.

The substitution reaction proceeds slowly and condensation follows as a result offurther protonation and the creation of a benzylcarbonium ion that acts as a nucleophile.Resols are obtained as a result of alkaline catalysis and an excess of formaldehyde.A resol molecule contains reactive methylol groups. Heating causes the reactive resolmolecules to condense to form large molecules, without the addition of a hardener. Thefunction of phenols as nucleophiles is strengthened by ionization of the phenol, withoutaffecting the activity of the aldehyde.Megson [2] states that reaction II (in which resols are formed by the reaction of

quinone methides with dimethylolphenols or other quinone methides) is favored during

alkaline catalysis. A carbonium ion mechanism is, however, more likely to occur. Megson[2] also states that phenolic nuclei can be linked not only by simple methylene bridges butalso by methylene ether bridges. The latter generally revert to methylene bridges if heatedduring curing with the elimination of formaldehyde.Copyright 2003 by Taylor & Francis Group, LLC

Page 3The differences between acid-catalyzed and base-catalyzed process are (1) in the rateof aldehyde attack on the phenol, (2) in the subsequent condensation of the phenolicalcohols, and (3) to some extent in the nature of the condensation reaction. With acidcatalysis, phenolic alcohol formation is relatively slow. Therefore, this is the step thatdetermines the rate of the total reaction. The condensation of phenolic alcohols andphenols forming compounds of the dihydroxydiphenylmethane type is, instead, rapid.

The latter are therefore predominant intermediates in novolak resins.

Copyright 2003 by Taylor & Francis Group, LLC

Page 4Novolaks are mixtures of isomeric polynuclear phenols of various chain lengths withan average of five to six phenolic nuclei per molecule. They contain no reactive methylol

groups and consequently cross-link and harden to form infusible and insoluble resins onlywhen mixed with compounds that can release formaldehyde and form methylene bridges(such as paraformaldehyde or hexamethylenetetramine).In the condensation of phenols and formaldehyde using basic catalysts, theinitial substitution reaction (i.e., the formaldehyde attack on the phenol) is fasterthan the subsequent condensation reaction. Consequently, phenolic alcohols are initi-ally the predominant intermediate compounds. These phenolic alcohols, which containreactive methylol groups, condense either with other methylol groups to form

ether links, or more commonly, with reactive positions in the phenolic ring (ortho

or para to the hydroxyl group) to form methylene bridges. In both cases water iseliminated.Mildly condensed liquid resols, which are the more important of the two types ofphenolic resins in the formulation of wood adhesives, have an average of fewer than twophenolic nuclei in the molecule. The solid resols average three to four phenolic nucleibut with a wider distribution of molecular size. Small amounts of simple phenol, phe-nolic alcohols, formaldehyde, and water are also present in resols. Heating or acidifica-tion of these resins causes cross-linking through uncondensed phenolic alcohol groups,and possibly also through reaction of formaldehyde liberated by the breakdown of theether links.


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As with novolaks, the methylolphenols formed condense with more phenols to formmethylene-bridged polyphenols. The latter, however, quickly react in an alkaline systemwith more formaldehyde to produce methylol derivatives of the polyphenols. In additionto this method of growth in molecular size, methylol groups may interact with oneanother, liberating water and forming dimethylene ether links (CH2OCH2). This is

particularly evident if the ratio of formaldehyde to phenol is high. The average molecularweight of the resins obtained by acid condensation of phenol and formaldehyde decreases

hyperbolically from over 1000 to 200, with increases in the molar ratio of phenol toformaldehyde from 1.25:1 to 10:1.Thermomechanical analysis (TMA) on wood joints bonded with phenolform-aldehyde (PF) adhesives has shown that, frequently, the joint increase in modulusdoes not proceed in a single step but in two steps, yielding an increase in the modulusfirst derivative curve presenting two major peaks rather than the single peak obtained

for mathematically smoothed modulus increase curves [3]. This behavior has beenfound to be due to the initial growth of the polycondensation polymer leading firstto linear polymers of critical length for the formation of entanglement networks. Thereaching of this critical length is greatly facilitated by the marked increase in concen-tration of the PF polymer due to the loss of water on absorbent substrates such aswood, coupled to the linear increase in the average length of the polymer due to theinitial phase of the polycondensation reaction. The combination of these two effectslowers markedly the level of the critical length needed for entanglement. Two modulus

steps and two first derivative major peaks then occur, with the first peak due to the

formation of linearPF oligomer entanglement networks, and the second one due to theformation of the final covalent cross-linked network. The faster the reaction ofphenolic monomers with formaldehyde, or the higher the reactivity of a PF resin,the earlier and at lower temperature the entanglement network occurs, and thehigher is its modulus value in relation to the joint modulus obtained with the final,covalently cross-linked resin (Fig. 1).Copyright 2003 by Taylor & Francis Group, LLC

Page 5A. Acid CatalysisConsideration must be given to the possibility of direct intervention by the catalyst in thereaction. Hydrochloric acid is the most interesting case of an acid catalyst, as is ammoniaof an alkaline catalyst. When the PF reaction is catalyzed by hydrochloric acid, two

mechanisms may come into operation. Vorozhtov has proposed a reaction route that

passes through the formation of bischloromethyl ether (ClCH2OCH2Cl) [4]. Zieglerhas suggested a route through the formation of a chloromethyl alcohol (ClCH2OH) asintermediate [5,6]. The second route appears to be the more probable. Both hypothesesagree that chloromethylphenols are the principal intermediates. The chloromethylphenolshave been prepared and isolated by various means. They are highly reactive compoundswhich, with phenols, form dihydroxydiphenylmethanes and complex methylene-linkedmultiring polyphenols. Reaction is highly selective and takes place in the para position.

B. Alkaline CatalysisDifferent mechanisms of alkaline catalysis have been suggested according to the alkaliused. When caustic soda is used as the catalyst, the type of mechanism which seems themost likely is that which involves the formation of a chelate ring similar to that suggestedby Caesar and Sachanen [7]. The chelating mechanism was thought to initially cause theformation of a sodiumformaldehyde complex or of a formaldehydesodium phenateFigure 1 Thermomechanical analysis (TMA) of the hardening of a PF resin in situ in a wood joint.

Increase of modulus of elasticity (MOE) of the joint as a function of temperature at a 10

C/minconstant heating rate (); first derivative (4).Copyright 2003 by Taylor & Francis Group, LLC

Page 6complex and is similar in concept to the mechanisms advanced for metal ion catalysis ofphenolic resins in the pH range 3 to 7. However, while the cyclic metallic ion catalysis ringcomplexes have even been isolated [8], this is not the case for the sodium ring complex,evidence for its existence being rather controversial, the predominant indication being that


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it does not form [9].When ammonia is used as a catalyst, the resins formed are very different in someof their characteristics from other alkali-catalyzed resins: the reaction mechanismappears to be quite different from the of sodium hydroxide-catalyzed resins. An obviousdeduction is that intermediates containing nitrogen are formed. Several such intermedi-

ates have been isolated from ammonia-catalyzed PF reactions [1012] and hexamineprepared resins [1316] by various researchers. Similar types of intermediates are

formed when amines or hexamethylenetetramine (hexamine) are used instead of ammo-nia. In the case of ammonia the main intermediates are dihydroxybenzylamines andtrihydroxybenzylamines, such benzylamine bridges having been shown to be muchmore temperature stable than previously thought and to impart particular characteristicsto the resin [1316].These intermediates contain nitrogen and have polybenzylamine chains. They react

further with more phenol causing splitting and elimination of the nitrogen as ammonia orproducing eventually nitrogen-free resins. However, as benzylamine bridges have beenshown to be much more temperature stable than previously thought, this requires a con-siderable excess of phenol and a high temperature, or heating for a rather long time. Withphenolhexamethylenetetramine resins of molar ratio 3:1, the nitrogen content of the resincannot be reduced to less than 7% when heated at 210C. When the ratio is increased to7:1, the nitrogen content on heating at 210

C can be reduced to less than 1%. Contrary to

what was widely believed it has been clearly demonstrated that in the preparation ofPFresins starting from hexamethylenetetramine the di- and trihydroxybenzylamine bridgeswhich are initially formed are very stable and are able to tolerate for a considerable lengthof time a temperature as high as 100C [13] yielding in certain aspects (only) resins ofupgraded characteristics. This behavior is closely tied to the reactions characteristic ofhexamethylenetetramine to form iminomethylene bases [1416], which are discussed in the

melamine resins chapter in this volume (Chap. 32).Ammonia-, ammine-, and amide-catalyzed phenolic resins are characterized bygreater insolubility in water than that of sodium hydroxide-catalyzed phenolic resins.The more ammonia that is used, the higher the molecular weight and melting point thatare obtained without cross-linking. This is probably due to the inhibiting effect of thenitrogen-carrying groups (i.e., CH2NHCH3 or CH2NH2), which is caused by theirslow rate of subsequent condensation and loss of ammonia. Ammonia, amines, andamides are sometimes used as accelerators during the curing of phenolic adhesives for

wood productsC. Metallic Ion Catalysis and Reaction OrientationIn the pH range 3 to 7 the higher rate of curing of phenolic resins prepared by metallic ioncatalysis is due to preferential ortho methylolation [17] and therefore also to the highproportion of orthoortho links of the uncured phenolic resins prepared by metallic ioncatalysis. The faster curing rates of phenolic resins prepared by metallic ion catalysis isthen due to the higher proportion of the free higher-reactive para positions available forfurther reaction during curing of the resin. The mechanism of the reaction [8] involves theformation of chelate rings between metal, formaldehyde, and phenols or phenol nuclei in aresin.The rate of metal exchange is solution [8,18] and the instability of the complexformed determine the accelerating or inhibiting effect of the metal in the reaction ofphenol with formaldehyde. The more stable complex II is, the slower the reaction pro-ceeds, to the formation of resin III. A completely stable complex II should stop thereaction from proceeding to resin III. If complex II is not stable, the reaction will proceedto form PF resins of type III. The rate of reaction is directly proportional to the instabilityor the rate of metal exchange in solution of complex II. The acid catalysis due to the metal

ion differs only in degree from that of the hydrogen ion [19].The effect of the metal is stronger than that of hydrogen ions, because of highercharge and greater covalence, since its interaction with donor groups is often much greater[19]. This allows phenolic resin adhesives to set in milder acid conditions. Most covalentmetals ions accelerate the PF reaction. The extent of acceleration depends on the type ofmetal ion and the amount of it that is present. The capability of acceleration in order ofdecreasing acceleration effectiveness has been reported to be [11] PbII, ZnII, CdII,NiII > MnII, MgII, CuII, CoII, CoIII > MnIII, FeIII ) BeII, AlIII > CrIII, CoII. The most


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important conclusion to be drawn is that the accelerating effect is indeed present in boththe manufacture ofPF resin and its curing. Therefore, the fast rate of curing of high-orthophenolic resins can be ascribed only partially to the high proportion of para positionsavailable. The other reason for the fast rate of curing is that the metallic ion catalyst is stillpresent, and free to act, in the resin at the time of curing. In such a resin, a considerable

number of ortho positions (especially of methylol groups in ortho positions to the phenolichydroxyls) are still available for reaction and capable of complexing.Copyright 2003 by Taylor & Francis Group, LLC

Page 8III. CHEMISTRY AND TECHNOLOGY OF APPLICATION OF PHENOLICRESIN ADHESIVES FOR WOODA. General Principles of ManufactureA typical phenolic resin is made in batches, in a jacketed, stainless steel reactor equippedwith an anchor-type or turbine-blade agitator, a reflux condenser, vaccum equipment, andheating and cooling facilities. Molten phenol and formalin (containing 37 to 42% for-maldehyde or paraformaldehyde), in molar proportions between 1:1.1 and 1:2, along withwater, and methanol are charged into the reactor and mechanical stirring is begun. Tomake a resol-type resin (such as those used in wood adhesives manufacture), an alkaline

catalyst such as sodium hydroxide is added to the batch, which is then heated to 80 to100

C. Reaction temperatures are kept under 95 to 100C by applying vacuum to thereactor, or by cooling water in the reactor jacket. Reaction times vary between 1 and 8 haccording to the pH, the phenol/formaldehyde ratio, the presence or absence of reactionretarders (such as alcohols), and the temperature of the reaction.Since a resol can gel in the reactor, dehydration temperatures are kept well below

100C, by applying vacuum. Tests have to be done to determine first, the degree ofadvancement of the resin, and second, when the batch should be discharged. Examplesof methods of such tests are the measurement of the gel time of a resin in a 150C hot plateor at 100C in a water bath. Another method is measuring the turbidity point, that is,precipitating the resin in water or solutions of a certain concentration.

Resins that are watersoluble and have a low molecular weight are finished at as low

a temperature as possible, usually around 40 to 60C. It is important that the liquid, water-soluble resols retain their ability to mix with water easily when they are used as woodadhesives. Resols based on phenol are considered to be stable for 3 to 9 months. Propertiesof a typical resin are a viscosity of 100 to 200 cP at 20C, a solids content of 55 to 60%, awater mixibility of a minimum of 2500%, and a pH of 7 to 13, according to the application

for which the resin is destined.Phenolformaldehyde (PF) resins present lower reactivity at a pH of about 4. Theaccepted effect of the pH and of the phenol/formaldehyde molar ratio on the rate ofpolymerization and rate of hardening of phenolic resols is shown in Fig. 2. Recently,however [9], the concepts expressed in the graph have been found to be only partiallycorrect, at least with regard to the dependence of the PF adhesive rate of curing as afunction of pH. The expected asympthotic acceleration expected over pH 7 to 8 and due tothe formation of phenate ions has been proven not to be the only effect present. At firstacceleration occurs, but after a pH of approximately 8 to 9, the rate of hardening of theresin slows down considerably [9], as shown in Fig. 3, contrary to accepted wisdom. Thereare several reasons for this behavior [9], the easier of these to accept being the formation ofa ring involving phenol, the methylol group, and Na ions, which was postulated already 50years ago [7]. The existence of this ring has been shown to be untrue [9] and the persistenceof the concept is due to the ease with which the behavior shown in Fig. 3 can be explained.The reason for the acceleration, however, was ascribed to and proved to be due to theexistence of and equilibria pertaining to quinone methides [9,20]. The structure of theelusive oligomeric quinone methides in PF resins has also been elucidated [21] (see page549).


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The probable reason why the behavior in Fig. 3 was not noticed earlier appears to bedue to the slow gel times ofPF resins, which makes it very tedious to check reactivityeffectivelyB. Curing Acceleration Under Alkaline Conditions1. - and-Set Acceleration

The so-called - and -set acceleration of curing for very alkaline PF resins for foundrycore binders was pioneered in the early 1970s [22], although it had been discovered in the

early 1950s [22]. In this application the addition of considerable amounts of esters or otherchemicals in liquid form (-set) or as a gas (-set), such as propylene carbonate, methylformate, glycerol triacetate, and others, was found to accelerate resin curing to extremelyshort times. This technique is now used extensively around the world for foundry core PFbinders [22] and is being considered for wood adhesives [9] and rigid alkaline PF foams.The technique is applicable in the approximate pH range 7 to 14. The mechanism that

makes PF curing acceleration possible has only been explained recently [9] and differentexplanations exist (see below); it is based on the carbanion behavior of the aromatic nucleiFigure 2 Rate of polymerization as a function of pH for phenolic resols of different molar ratios at

120C (old concept).Copyright 2003 by Taylor & Francis Group, LLC

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of phenate ions, leading to a more complex variant of the KolbeSchmitt reaction. Theester, or residue of its decomposition, attacks the negatively charged phenolic nuclei, andits reaction is not limited to the ortho and para sites, transforming the phenolic nuclei in atemporary condensation reagent of functionality higher than 3, leading to much earliergelling. Furthermore, temporary condensation occurs not only according to the PFmechanism but also according to a second reaction superimposed on it [9,23] (Fig. 4).Other explanations and mechanisms for this occurrence have also been advanced:determination by TMA of the average number of freedom of polymer segments betweencross-linking nodes ofPF resin hardened networks indicate that additive accelerated P

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I. INTRODUCTIONPhenolic resins are the polycondensation products of the reaction of phenol with formal-dehyde. Phenolic resins were the first true synthetic polymers to be developed commer-cially. Notwithstanding this, even now their structure is far from completely clear, becausethe polymers derived from the reaction of phenol with formaldehyde differ in one impor-tant aspect from other polycondensation products. Polyfunctional phenols may react withformaldehyde in both the ortho and para positions to the hydroxyl group. This means thatthe condensation products exist as numerous positional isomerides for any chain length.This makes the organic chemistry of the reaction particularly complex and tedious tounravel. The result has been that although phenolic resins were developed commercially

as early as 1908, were the first completely synthetic resins ever to be developed, and havevast and differentiated industrial uses today, and great strides have been made in both theunderstanding of their structure and their technology and application, several aspects oftheir chemistry are still only partially understood.It may be argued with some justification that such a state of affairs is immaterial,because satisfactory resins for many uses have been developed on purely empirical groundsduring the past 90 years. However, it cannot be denied that the gradual understanding of thechemical structure and mechanism of reaction of these resins has helped considerably inintroducing commercial phenolic resins designed for certain applications and capable of

performances undreamed of in formulations developed earlier by the empirical rather than


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the scientific approach. Knowledge of phenolic resin chemistry, structure, characteristicreactions, and kinetic behavior remains an invaluable asset to the adhesive formulator indesigning resins with specific physical properties. The characteristic that renders these resinsinvaluable as adhesives is their capability to deliver water, weather, and high-temperatureresistance to the cured glue line of the joint bonded with phenolic adhesives, at relatively

low cost.II. CHEMISTRY

Phenols condense initially with formaldehyde in the presence of either acid or alkali toform a methylolphenol or phenolic alcohol, and then dimethylolphenol. The initial attackCopyright 2003 by Taylor & Francis Group, LLC

Page 2may be at the 2-, 4-, or 6-position. The second stage of the reaction involves methylolgroups with other available phenol or methylolphenol, leading first to the formation oflinear polymers [1] and then to the formation of hard-cured, highly branched structures.Novolak resins are obtained with acid catalysis, with a deficiency of formaldehyde. Anovolak resin has no reactive methylol groups in its molecules and therefore withouthardening agents is incapable of condensing with other novolak molecules on heating.To complete resinification, further formaldehyde is added to cross-link the novolak resin.

Phenolic rings are considerably less active as necleophilic centers at an acid pH, due tohydroxyl and ring protonation.

However, the aldehyde is activated by protonation, which compensates for thisreduction in potential reactivity. The protonated aldehyde is a more effective electrophile.The substitution reaction proceeds slowly and condensation follows as a result offurther protonation and the creation of a benzylcarbonium ion that acts as a nucleophile.Resols are obtained as a result of alkaline catalysis and an excess of formaldehyde.A resol molecule contains reactive methylol groups. Heating causes the reactive resol

molecules to condense to form large molecules, without the addition of a hardener. Thefunction of phenols as nucleophiles is strengthened by ionization of the phenol, withoutaffecting the activity of the aldehyde.Megson [2] states that reaction II (in which resols are formed by the reaction ofquinone methides with dimethylolphenols or other quinone methides) is favored duringalkaline catalysis. A carbonium ion mechanism is, however, more likely to occur. Megson[2] also states that phenolic nuclei can be linked not only by simple methylene bridges butalso by methylene ether bridges. The latter generally revert to methylene bridges if heated

during curing with the elimination of formaldehyde.


Page 3The differences between acid-catalyzed and base-catalyzed process are (1) in the rateof aldehyde attack on the phenol, (2) in the subsequent condensation of the phenolic

alcohols, and (3) to some extent in the nature of the condensation reaction. With acidcatalysis, phenolic alcohol formation is relatively slow. Therefore, this is the step thatdetermines the rate of the total reaction. The condensation of phenolic alcohols andphenols forming compounds of the dihydroxydiphenylmethane type is, instead, rapid.The latter are therefore predominant intermediates in novolak resins.Copyright 2003 by Taylor & Francis Group, LLC

Page 4

Novolaks are mixtures of isomeric polynuclear phenols of various chain lengths withan average of five to six phenolic nuclei per molecule. They contain no reactive methylolgroups and consequently cross-link and harden to form infusible and insoluble resins onlywhen mixed with compounds that can release formaldehyde and form methylene bridges(such as paraformaldehyde or hexamethylenetetramine).In the condensation of phenols and formaldehyde using basic catalysts, the

initial substitution reaction (i.e., the formaldehyde attack on the phenol) is fasterthan the subsequent condensation reaction. Consequently, phenolic alcohols are initi-ally the predominant intermediate compounds. These phenolic alcohols, which containreactive methylol groups, condense either with other methylol groups to form


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ether links, or more commonly, with reactive positions in the phenolic ring (orthoor para to the hydroxyl group) to form methylene bridges. In both cases water iseliminated.Mildly condensed liquid resols, which are the more important of the two types ofphenolic resins in the formulation of wood adhesives, have an average of fewer than two

phenolic nuclei in the molecule. The solid resols average three to four phenolic nucleibut with a wider distribution of molecular size. Small amounts of simple phenol, phe-

nolic alcohols, formaldehyde, and water are also present in resols. Heating or acidifica-tion of these resins causes cross-linking through uncondensed phenolic alcohol groups,and possibly also through reaction of formaldehyde liberated by the breakdown of theether links.As with novolaks, the methylolphenols formed condense with more phenols to formmethylene-bridged polyphenols. The latter, however, quickly react in an alkaline system

with more formaldehyde to produce methylol derivatives of the polyphenols. In additionto this method of growth in molecular size, methylol groups may interact with oneanother, liberating water and forming dimethylene ether links (CH2OCH2). This isparticularly evident if the ratio of formaldehyde to phenol is high. The average molecularweight of the resins obtained by acid condensation of phenol and formaldehyde decreaseshyperbolically from over 1000 to 200, with increases in the molar ratio of phenol toformaldehyde from 1.25:1 to 10:1.Thermomechanical analysis (TMA) on wood joints bonded with phenolform-

aldehyde (PF) adhesives has shown that, frequently, the joint increase in modulus

does not proceed in a single step but in two steps, yielding an increase in the modulusfirst derivative curve presenting two major peaks rather than the single peak obtainedfor mathematically smoothed modulus increase curves [3]. This behavior has beenfound to be due to the initial growth of the polycondensation polymer leading firstto linear polymers of critical length for the formation of entanglement networks. Thereaching of this critical length is greatly facilitated by the marked increase in concen-tration of the PF polymer due to the loss of water on absorbent substrates such as

wood, coupled to the linear increase in the average length of the polymer due to theinitial phase of the polycondensation reaction. The combination of these two effectslowers markedly the level of the critical length needed for entanglement. Two modulussteps and two first derivative major peaks then occur, with the first peak due to theformation of linearPF oligomer entanglement networks, and the second one due to theformation of the final covalent cross-linked network. The faster the reaction ofphenolic monomers with formaldehyde, or the higher the reactivity of a PF resin,the earlier and at lower temperature the entanglement network occurs, and the

higher is its modulus value in relation to the joint modulus obtained with the final,covalently cross-linked resin (Fig. 1).Copyright 2003 by Taylor & Francis Group, LLC

Page 5A. Acid Catalysis

Consideration must be given to the possibility of direct intervention by the catalyst in thereaction. Hydrochloric acid is the most interesting case of an acid catalyst, as is ammoniaof an alkaline catalyst. When the PF reaction is catalyzed by hydrochloric acid, twomechanisms may come into operation. Vorozhtov has proposed a reaction route thatpasses through the formation of bischloromethyl ether (ClCH2OCH2Cl) [4]. Zieglerhas suggested a route through the formation of a chloromethyl alcohol (ClCH2OH) asintermediate [5,6]. The second route appears to be the more probable. Both hypothesesagree that chloromethylphenols are the principal intermediates. The chloromethylphenolshave been prepared and isolated by various means. They are highly reactive compoundswhich, with phenols, form dihydroxydiphenylmethanes and complex methylene-linkedmultiring polyphenols. Reaction is highly selective and takes place in the para position.B. Alkaline CatalysisDifferent mechanisms of alkaline catalysis have been suggested according to the alkaliused. When caustic soda is used as the catalyst, the type of mechanism which seems themost likely is that which involves the formation of a chelate ring similar to that suggestedby Caesar and Sachanen [7]. The chelating mechanism was thought to initially cause theformation of a sodiumformaldehyde complex or of a formaldehydesodium phenateFigure 1 Thermomechanical analysis (TMA) of the hardening of a PF resin in situ in a wood joint.


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Increase of modulus of elasticity (MOE) of the joint as a function of temperature at a 10

C/minconstant heating rate (); first derivative (4).Copyright 2003 by Taylor & Francis Group, LLC

Page 6complex and is similar in concept to the mechanisms advanced for metal ion catalysis ofphenolic resins in the pH range 3 to 7. However, while the cyclic metallic ion catalysis ringcomplexes have even been isolated [8], this is not the case for the sodium ring complex,evidence for its existence being rather controversial, the predominant indication being thatit does not form [9].When ammonia is used as a catalyst, the resins formed are very different in some

of their characteristics from other alkali-catalyzed resins: the reaction mechanismappears to be quite different from the of sodium hydroxide-catalyzed resins. An obviousdeduction is that intermediates containing nitrogen are formed. Several such intermedi-ates have been isolated from ammonia-catalyzed PF reactions [1012] and hexamineprepared resins [1316] by various researchers. Similar types of intermediates areformed when amines or hexamethylenetetramine (hexamine) are used instead of ammo-nia. In the case of ammonia the main intermediates are dihydroxybenzylamines andtrihydroxybenzylamines, such benzylamine bridges having been shown to be much

more temperature stable than previously thought and to impart particular characteristics

to the resin [1316].These intermediates contain nitrogen and have polybenzylamine chains. They reactfurther with more phenol causing splitting and elimination of the nitrogen as ammonia orproducing eventually nitrogen-free resins. However, as benzylamine bridges have beenshown to be much more temperature stable than previously thought, this requires a con-siderable excess of phenol and a high temperature, or heating for a rather long time. Withphenolhexamethylenetetramine resins of molar ratio 3:1, the nitrogen content of the resincannot be reduced to less than 7% when heated at 210C. When the ratio is increased to7:1, the nitrogen content on heating at 210C can be reduced to less than 1%. Contrary towhat was widely believed it has been clearly demonstrated that in the preparation ofPFresins starting from hexamethylenetetramine the di- and trihydroxybenzylamine bridgeswhich are initially formed are very stable and are able to tolerate for a considerable lengthof time a temperature as high as 100

C [13] yielding in certain aspects (only) resins ofupgraded characteristics. This behavior is closely tied to the reactions characteristic ofhexamethylenetetramine to form iminomethylene bases [1416], which are discussed in themelamine resins chapter in this volume (Chap. 32).Ammonia-, ammine-, and amide-catalyzed phenolic resins are characterized bygreater insolubility in water than that of sodium hydroxide-catalyzed phenolic resins.The more ammonia that is used, the higher the molecular weight and melting point thatare obtained without cross-linking. This is probably due to the inhibiting effect of thenitrogen-carrying groups (i.e., CH2NHCH3 or CH2NH2), which is caused by their

slow rate of subsequent condensation and loss of ammonia. Ammonia, amines, andamides are sometimes used as accelerators during the curing of phenolic adhesives forwood products.Copyright 2003 by Taylor & Francis Group, LLC

Page 7C. Metallic Ion Catalysis and Reaction OrientationIn the pH range 3 to 7 the higher rate of curing of phenolic resins prepared by metallic ioncatalysis is due to preferential ortho methylolation [17] and therefore also to the highproportion of orthoortho links of the uncured phenolic resins prepared by metallic ioncatalysis. The faster curing rates of phenolic resins prepared by metallic ion catalysis isthen due to the higher proportion of the free higher-reactive para positions available forfurther reaction during curing of the resin. The mechanism of the reaction [8] involves theformation of chelate rings between metal, formaldehyde, and phenols or phenol nuclei in a

resin.


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The rate of metal exchange is solution [8,18] and the instability of the complexformed determine the accelerating or inhibiting effect of the metal in the reaction ofphenol with formaldehyde. The more stable complex II is, the slower the reaction pro-ceeds, to the formation of resin III. A completely stable complex II should stop thereaction from proceeding to resin III. If complex II is not stable, the reaction will proceed

to form PF resins of type III. The rate of reaction is directly proportional to the instabilityor the rate of metal exchange in solution of complex II. The acid catalysis due to the metal

ion differs only in degree from that of the hydrogen ion [19].The effect of the metal is stronger than that of hydrogen ions, because of highercharge and greater covalence, since its interaction with donor groups is often much greater[19]. This allows phenolic resin adhesives to set in milder acid conditions. Most covalentmetals ions accelerate the PF reaction. The extent of acceleration depends on the type ofmetal ion and the amount of it that is present. The capability of acceleration in order of

decreasing acceleration effectiveness has been reported to be [11] PbII, ZnII, CdII,NiII > MnII, MgII, CuII, CoII, CoIII > MnIII, FeIII ) BeII, AlIII > CrIII, CoII. The mostimportant conclusion to be drawn is that the accelerating effect is indeed present in boththe manufacture ofPF resin and its curing. Therefore, the fast rate of curing of high-orthophenolic resins can be ascribed only partially to the high proportion of para positionsavailable. The other reason for the fast rate of curing is that the metallic ion catalyst is stillpresent, and free to act, in the resin at the time of curing. In such a resin, a considerablenumber of ortho positions (especially of methylol groups in ortho positions to the phenolic

hydroxyls) are still available for reaction and capable of complexing.


Page 8III. CHEMISTRY AND TECHNOLOGY OF APPLICATION OF PHENOLICRESIN ADHESIVES FOR WOOD

A. General Principles of ManufactureA typical phenolic resin is made in batches, in a jacketed, stainless steel reactor equippedwith an anchor-type or turbine-blade agitator, a reflux condenser, vaccum equipment, andheating and cooling facilities. Molten phenol and formalin (containing 37 to 42% for-maldehyde or paraformaldehyde), in molar proportions between 1:1.1 and 1:2, along withwater, and methanol are charged into the reactor and mechanical stirring is begun. Tomake a resol-type resin (such as those used in wood adhesives manufacture), an alkalinecatalyst such as sodium hydroxide is added to the batch, which is then heated to 80 to

100

C. Reaction temperatures are kept under 95 to 100C by applying vacuum to thereactor, or by cooling water in the reactor jacket. Reaction times vary between 1 and 8 haccording to the pH, the phenol/formaldehyde ratio, the presence or absence of reactionretarders (such as alcohols), and the temperature of the reaction.Since a resol can gel in the reactor, dehydration temperatures are kept well below100

C, by applying vacuum. Tests have to be done to determine first, the degree ofadvancement of the resin, and second, when the batch should be discharged. Examplesof methods of such tests are the measurement of the gel time of a resin in a 150C hot plateor at 100C in a water bath. Another method is measuring the turbidity point, that is,precipitating the resin in water or solutions of a certain concentration.Resins that are watersoluble and have a low molecular weight are finished at as lowa temperature as possible, usually around 40 to 60C. It is important that the liquid, water-soluble resols retain their ability to mix with water easily when they are used as woodadhesives. Resols based on phenol are considered to be stable for 3 to 9 months. Propertiesof a typical resin are a viscosity of 100 to 200 cP at 20C, a solids content of 55 to 60%, awater mixibility of a minimum of 2500%, and a pH of 7 to 13, according to the applicationfor which the resin is destined.Phenolformaldehyde (PF) resins present lower reactivity at a pH of about 4. Theaccepted effect of the pH and of the phenol/formaldehyde molar ratio on the rate of


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polymerization and rate of hardening of phenolic resols is shown in Fig. 2. Recently,however [9], the concepts expressed in the graph have been found to be only partiallycorrect, at least with regard to the dependence of the PF adhesive rate of curing as afunction of pH. The expected asympthotic acceleration expected over pH 7 to 8 and due tothe formation of phenate ions has been proven not to be the only effect present. At first

acceleration occurs, but after a pH of approximately 8 to 9, the rate of hardening of theresin slows down considerably [9], as shown in Fig. 3, contrary to accepted wisdom. There

are several reasons for this behavior [9], the easier of these to accept being the formation ofa ring involving phenol, the methylol group, and Na ions, which was postulated already 50years ago [7]. The existence of this ring has been shown to be untrue [9] and the persistenceof the concept is due to the ease with which the behavior shown in Fig. 3 can be explained.The reason for the acceleration, however, was ascribed to and proved to be due to theexistence of and equilibria pertaining to quinone methides [9,20]. The structure of the

elusive oligomeric quinone methides in PF resins has also been elucidated [21] (see page549).The probable reason why the behavior in Fig. 3 was not noticed earlier appears to bedue to the slow gel times ofPF resins, which makes it very tedious to check reactivityeffectively.Copyright 2003 by Taylor & Francis Group, LLC

Page 9

B. Curing Acceleration Under Alkaline Conditions1. - and-Set AccelerationThe so-called - and -set acceleration of curing for very alkaline PF resins for foundrycore binders was pioneered in the early 1970s [22], although it had been discovered in theearly 1950s [22]. In this application the addition of considerable amounts of esters or otherchemicals in liquid form (-set) or as a gas (-set), such as propylene carbonate, methyl

formate, glycerol triacetate, and others, was found to accelerate resin curing to extremelyshort times. This technique is now used extensively around the world for foundry core PFbinders [22] and is being considered for wood adhesives [9] and rigid alkaline PF foams.The technique is applicable in the approximate pH range 7 to 14. The mechanism thatmakes PF curing acceleration possible has only been explained recently [9] and differentexplanations exist (see below); it is based on the carbanion behavior of the aromatic nucleiFigure 2 Rate of polymerization as a function of pH for phenolic resols of different molar ratios at

120

C (old concept).


Page 10of phenate ions, leading to a more complex variant of the KolbeSchmitt reaction. Theester, or residue of its decomposition, attacks the negatively charged phenolic nuclei, andits reaction is not limited to the ortho and para sites, transforming the phenolic nuclei in atemporary condensation reagent of functionality higher than 3, leading to much earliergelling. Furthermore, temporary condensation occurs not only according to the PFmechanism but also according to a second reaction superimposed on it [9,23] (Fig. 4).Other explanations and mechanisms for this occurrence have also been advanced:determination by TMA of the average number of freedom of polymer segments betweencross-linking nodes ofPF resin hardened networks indicate that additive accelerated PFFigure 4 Cure retarding at high pH and ester acceleration effect of NaOH and KOH-catalysed PF

resins (ester propylene carbonate). Note curve 5, the effect of 4 months aging of the PF resin of

curve 1 on the extent and starting pH of the retardation effect. Compare the start of acceleration forcurves 4 and 6, showing the differences between propylene carbonate and triacetin esters, and

compare the starting point of acceleration at pH 5.5 and pH 7.1. The bumps on the curves at

pH 811 are caused by methylene ether formation, decomposition, and rearrangement [9].

Figure 3 Schematic relationship of gel time to pH for phenolic resols (new concept).Copyright 2003 by Taylor & Francis Group, LLC

Page 11resin polycondensations and hardening present several different acceleration mechanisms.[23]. Some additives such as sodium carbonate appear to present a purely catalytic effect


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on the polycondensation reaction [23]. Other additives such as propylene carbonate pre-sent both a catalytic effect as well as including an increase in the average functionality ofthe system, due to further cross-linking, or alternative reactions in which the acceleratoritself participates, leading to a tighter final network [23]. These alternative cross-linkingreactions could be of a different nature, such as the propylene carbonate case in which the

reaction appears to be related to a KolbeSchmitt reaction, or they could be similar to theaccelerating effect due to the hydrolysis of formamide to formic acid and ammonia with

the subsequent rapid reaction of the latter with two or more hydroxybenzylalcohol

groupsofPF resols [23]. The rapid reaction of the -NH2 group of formamide with two hydro-xybenzyl alcohol groups ofPF resols, a reaction which is also characteristic of urea andmethylamine, also appears likely to occur. In some cases such as in formamide none of thetwo acceleration mechanisms detected appear to be due to catalytic action only, but bothappear to be related to additional cross-linking reactions. Both liquid and solid phase 13C

nuclear magnetic resonance (NMR) supporting evidence of the mechanisms proposed hasbeen presented [23].Further proof of complex reactions between propylene carbonate and phenolic nucleileading to compounds in which the carbonic acid has attacked the phenolic ring has beenpresented [23] based on the 13C NMR spectrum of the product of the reaction of resorcinolwith propylene carbonate, in the absence of formaldehyde. Resorcinol was chosen as itsaromatic ring is a stronger nucleophile than that of phenol and thus if any reaction hadoccurred this would be less elusive and much more easily observed [23]. The reaction pro-

ducts which appeared to be formed were carboxylic and dicarboxylic species. That they

might be present was also derived by NMR [23]. It must be pointed out that such structuresneed to be only transitory and not permanent to obtain the same effects noted experimen-tally. Such a subsequent lability could be the reason why it is difficult to observe suchlinkages in the hardened resin except for faster reacting phenols where they can be observeddue to early immobilization of the network which surely occurs.It must also be remembered that in hot temperature curing of phenolic resins,their polycondensation is accelerated particularly on a wood substrate surface, first by

heterogeneous catalysis effect by the cellulose [24], and secondly by the substratessubtracting water from the system and thus increasing its effective concentration,always a very important effect in polycondensation reactions [3,25]. Under these con-ditions the existence of the additional cross-linking mechanism will then be even moremarked. It is also clear that if the anhydride exists it might decompose at highertemperature curing, with what type of further reactions it is not possible to say withthe data available.Once defined the nature of the accelerating mechanism induced by increased cross-

linking and its existence through the determination of the increased tightness of the PFnetworks formed [9,23], it is necessary to address the nature of the other acceleratingmechanism that appears to be common to both sodium carbonate and propylene carbo-nate. The apparent failure by different analyses [26] such 13C NMR to find any trace ofC O after purification of sodium carbonate accelerated PF resins indicates quite clearlythat the sodium carbonate effect may well be purely catalytic and that the C O istransformed during the reaction to another group, or even more likely that the C Odisappears from the system as CO2 or precipitates completely away as sodium hydrogencarbonate. The presence of the C O has been clearly noticed in non purified samples ofacceleratedPF resins [9,23,27] but strictly speaking this in only proof of the additionalCopyright 2003 by Taylor & Francis Group, LLC

Page 12cross-linking mechanism just discussed above or it could just be due to any carbonic acidsalts still present in the system. That this mechanism exists is proven by the acceleration ofhardening being marked for high molar ratio (formaldehyde/phenol ! 2.5) PF resins inwhich all available ortho and para sites on the phenol are blocked by methylene ormethylol groups. In this case a soft gel, and no subsequent rapid hardening is obtained.The mechanism involved could then be one of the two proposed up to now, namely thehydrogen carbonate ion intermediate activated complex and derived mechanisms [26,28]which present inherent disadvantages that have been outlined [23], which have been pro-posed without any evidence, and for which direct evidence would be rather difficult togather, and the mechanism [23] based on rapid transesterification reactions of the hydro-xybenzyl alcohol group of a PF resol. This latter mechanism is based on the very facile


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transesterification of propylene carbonate with methanol through which dimethyl carbo-nate is rapidly obtained [23].It is interesting to remark that other reactive materials which will readily undergotransesterification analogous to that of propylene carbonate with methanol are trialkylborates, tetraalkyl titanates, and trialkyl phosphates in an alkaline environment. Also gas

injection of methyl borate (and carbon dioxide) has been found to enhance the results ofwood composites bonded with formaldehyde-based resins [29], just as the addition of

propylene carbonate and glycerol triacetate have been shown to do in wood compositesbonded with phenolic resins.In the case of wood adhesives, first glycerol triacetate (triacetin) and secondly gua-nidine carbonate are the accelerating esters of choice yielding long pot-lives at ambienttemperature and fast cure times at higher temperature, and are used in proportions vari-able between 3% and 10% of adhesive resin solids [3032]. Propylene carbonate is unsui-

table for wood adhesives application as it yields far too short pot-lives at ambienttemperature. Methyl formate and other esters, including propylene carbonate, are usedinstead in foundry core binders where sometimes the proportion of ester accelerator usedis up to an equal amount of the resin solids; hence the accelerator application technology israther different from one field to another. Most other esters are much less effective accel-erators at higher temperature, or they shorten the ambient temperature life of the resin tosuch an extent that in practice the resin cannot be used [3032]. Triacetin gives long pot-lives and short cure times instead due, among other reasons to its lower rate of hydrolysis

at ambient temperature. Another series of compounds, some of which where finally found

to yield sufficiently rapid acceleration at higher temperatures still coupled with increasedstrength of the cured resin as well as sufficiently long shelf-life at ambient temperature,were the salts of guanidine. Guanidine carbonate, guanidine hydrochloride, and guanidinesulphate were tried with positive results [32]. Guanidine carbonate appeared to be the bestPF accelerator; both its accelerating capability remained acceptable, while the shelf life atambient temperature of the PF and phenolureaformaldehyde (PUF) resins to which ithad been added in different proportions was much longer and the performance in particle-

board preparation was the same as triacetin [32]. Even in the case of some industrial highercondensation resins, their pot-life was as long as three weeks with the guanidine carbonatealready incorporated in the resin [32].It has repeatedly been established that the energy of activation of the reactionof polycondensation ofPF resins, and also of ureaformaldehyde (UF), melamineformaldehyde (MF), and other resins, is markedly influenced by the presence of wood[24,3339]. In the presence of wood as a substrate, the energy of activation of the poly-condensation reaction, and hence of the hardening ofPF and other resins is considerably

lowered. This implies that resin polymerization and cross-linking proceeds at a muchCopyright 2003 by Taylor & Francis Group, LLC

Page 13faster rate when the resin is in molecular contact with one or more of the wood constitu-ents [24,33]. It was indeed shown that catalytic activation of the hardening and advance-

ment of a PF and other polycondensation resins induced by the wood substrate did existand was a rather marked effect. The reason for the effect has been found to be due to themass of secondary attraction forces binding the resin to the substrate [24,33]. These causevariations in the strength of bonds and intensity of reactive sites within the PF oligomerconsidered, an effect well known in heterogeneous catalysis for a variety of other chemicalsystems [40], bond cleavage and formation within a molecule being greatly facilitated bychemisorption onto a catalyst surface. This work indicated also which bonds in the PFresin were involves and what was the extent of the acceleration of the hardening reactioncaused by such an effect [33].2.Addition of Acetals and Transacetalization ReactionsAnother recent approach which has shown considerable promise in markedly decreasingthe percentage ofPF adhesive solids on a board has been found almost by chance. It isbased on the addition to the resin of certain additives capable of decreasing the percentageof any PF resin needed for bonding while still conserving the same adhesive and jointperformance. These additives work best for melamineureaformaldehyde (MUF) adhe-sives, but give acceptable results forPF resins too. These additives are the acetals [41,42],methylal and ethylal being the two most suitable due to their cost to performance ratio,which do not release formaldehyde at pHs higher than 1 [43]. Methylal has according to


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results reported by the Enviromental Protection Agency (EPA) an LD50 value of 10,000against that of 100 for formaldehyde, and is thus classed as nontoxic. The addition of thesematerials to the glue mix of a formaldehyde-based resin improves considerably its mechan-ical resistance and the performance of the bonded joint, mainly through its solvent actionon the higher molecular weight colloidal fraction of the resin. Molecular, colloidal inter-

actions causing diffusional hindrance problems are thus overcome and minimized bybringing the resin system to a homogeneous phase and as a consequence higher resin

strengths are obtained. It has also been shown that the supposed existence of transaceti-lization reactions forming bridges in the resin hardened network which are better able tooptimize the glue line and bonded joint viscoelastic dissipation of energy did not occur[44]. This is in general valid for MUFs, some UFs, and PFs, but the effect is particularlyevident and particularly marked for the MUF resins [44]. However, still respectable (butmore modest than MUFs) increases in PF resin strength of up to 25%, or alternatively

decreases in PF resin solids content of as much as 20% while conserving the sameperformance, have been obtained in the case of wood particleboard [44].3. Urea Acceleration and Phenol-Urea-Formaldehyde Exterior-Grade ResinsLow condensation PF resins have been coreacted under alkaline conditions with up to42% molar urea on phenol during resin preparation to yield PUF resins capable offaster hardening times and presenting better performance than equivalent pure PF resinsprepared under identical conditions [3134]. The reason that urea reacts with relativeease with PF resols under alkaline reaction conditions can be ascribed to the relative

reactivities toward methylol groups of urea and phenolic nuclei. A study has shown that

there are definite pH ranges in which the reaction of urea unreacted NH 2 and NHgroups with formaldehyde in competition with phenol or with the methylol groupscarried by a PF resin, is more favorable than is autocondensation of the PF resinitself [31,33,45] (Fig. 5)

Page 1



I. INTRODUCTIONPhenolic resins are the polycondensation products of the reaction of phenol with formal-dehyde. Phenolic resins were the first true synthetic polymers to be developed commer-cially. Notwithstanding this, even now their structure is far from completely clear, becausethe polymers derived from the reaction of phenol with formaldehyde differ in one impor-tant aspect from other polycondensation products. Polyfunctional phenols may react withformaldehyde in both the ortho and para positions to the hydroxyl group. This means that

the condensation products exist as numerous positional isomerides for any chain length.This makes the organic chemistry of the reaction particularly complex and tedious tounravel. The result has been that although phenolic resins were developed commerciallyas early as 1908, were the first completely synthetic resins ever to be developed, and havevast and differentiated industrial uses today, and great strides have been made in both theunderstanding of their structure and their technology and application, several aspects oftheir chemistry are still only partially understood.It may be argued with some justification that such a state of affairs is immaterial,because satisfactory resins for many uses have been developed on purely empirical groundsduring the past 90 years. However, it cannot be denied that the gradual understanding of thechemical structure and mechanism of reaction of these resins has helped considerably inintroducing commercial phenolic resins designed for certain applications and capable ofperformances undreamed of in formulations developed earlier by the empirical rather thanthe scientific approach. Knowledge of phenolic resin chemistry, structure, characteristicreactions, and kinetic behavior remains an invaluable asset to the adhesive formulator indesigning resins with specific physical properties. The characteristic that renders these resinsinvaluable as adhesives is their capability to deliver water, weather, and high-temperatureresistance to the cured glue line of the joint bonded with phenolic adhesives, at relatively


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low cost.II. CHEMISTRYPhenols condense initially with formaldehyde in the presence of either acid or alkali toform a methylolphenol or phenolic alcohol, and then dimethylolphenol. The initial attackCopyright 2003 by Taylor & Francis Group, LLC

Page 2may be at the 2-, 4-, or 6-position. The second stage of the reaction involves methylolgroups with other available phenol or methylolphenol, leading first to the formation oflinear polymers [1] and then to the formation of hard-cured, highly branched structures.

Novolak resins are obtained with acid catalysis, with a deficiency of formaldehyde. Anovolak resin has no reactive methylol groups in its molecules and therefore withouthardening agents is incapable of condensing with other novolak molecules on heating.To complete resinification, further formaldehyde is added to cross-link the novolak resin.Phenolic rings are considerably less active as necleophilic centers at an acid pH, due tohydroxyl and ring protonation.However, the aldehyde is activated by protonation, which compensates for thisreduction in potential reactivity. The protonated aldehyde is a more effective electrophile.The substitution reaction proceeds slowly and condensation follows as a result of

further protonation and the creation of a benzylcarbonium ion that acts as a nucleophile.Resols are obtained as a result of alkaline catalysis and an excess of formaldehyde.

A resol molecule contains reactive methylol groups. Heating causes the reactive resolmolecules to condense to form large molecules, without the addition of a hardener. Thefunction of phenols as nucleophiles is strengthened by ionization of the phenol, withoutaffecting the activity of the aldehyde.Megson [2] states that reaction II (in which resols are formed by the reaction ofquinone methides with dimethylolphenols or other quinone methides) is favored during

alkaline catalysis. A carbonium ion mechanism is, however, more likely to occur. Megson[2] also states that phenolic nuclei can be linked not only by simple methylene bridges butalso by methylene ether bridges. The latter generally revert to methylene bridges if heatedduring curing with the elimination of formaldehyde.Copyright 2003 by Taylor & Francis Group, LLC

Page 3The differences between acid-catalyzed and base-catalyzed process are (1) in the rate

of aldehyde attack on the phenol, (2) in the subsequent condensation of the phenolicalcohols, and (3) to some extent in the nature of the condensation reaction. With acidcatalysis, phenolic alcohol formation is relatively slow. Therefore, this is the step thatdetermines the rate of the total reaction. The condensation of phenolic alcohols andphenols forming compounds of the dihydroxydiphenylmethane type is, instead, rapid.The latter are therefore predominant intermediates in novolak resins.Copyright 2003 by Taylor & Francis Group, LLC

Page 4Novolaks are mixtures of isomeric polynuclear phenols of various chain lengths withan average of five to six phenolic nuclei per molecule. They contain no reactive methylolgroups and consequently cross-link and harden to form infusible and insoluble resins onlywhen mixed with compounds that can release formaldehyde and form methylene bridges(such as paraformaldehyde or hexamethylenetetramine).

In the condensation of phenols and formaldehyde using basic catalysts, theinitial substitution reaction (i.e., the formaldehyde attack on the phenol) is fasterthan the subsequent condensation reaction. Consequently, phenolic alcohols are initi-ally the predominant intermediate compounds. These phenolic alcohols, which containreactive methylol groups, condense either with other methylol groups to formether links, or more commonly, with reactive positions in the phenolic ring (ortho

or para to the hydroxyl group) to form methylene bridges. In both cases water iseliminated.Mildly condensed liquid resols, which are the more important of the two types ofphenolic resins in the formulation of wood adhesives, have an average of fewer than two


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phenolic nuclei in the molecule. The solid resols average three to four phenolic nucleibut with a wider distribution of molecular size. Small amounts of simple phenol, phe-nolic alcohols, formaldehyde, and water are also present in resols. Heating or acidifica-tion of these resins causes cross-linking through uncondensed phenolic alcohol groups,and possibly also through reaction of formaldehyde liberated by the breakdown of the

ether links.As with novolaks, the methylolphenols formed condense with more phenols to form

methylene-bridged polyphenols. The latter, however, quickly react in an alkaline systemwith more formaldehyde to produce methylol derivatives of the polyphenols. In additionto this method of growth in molecular size, methylol groups may interact with oneanother, liberating water and forming dimethylene ether links (CH2OCH2). This isparticularly evident if the ratio of formaldehyde to phenol is high. The average molecularweight of the resins obtained by acid condensation of phenol and formaldehyde decreases

hyperbolically from over 1000 to 200, with increases in the molar ratio of phenol toformaldehyde from 1.25:1 to 10:1.Thermomechanical analysis (TMA) on wood joints bonded with phenolform-aldehyde (PF) adhesives has shown that, frequently, the joint increase in modulusdoes not proceed in a single step but in two steps, yielding an increase in the modulusfirst derivative curve presenting two major peaks rather than the single peak obtainedfor mathematically smoothed modulus increase curves [3]. This behavior has beenfound to be due to the initial growth of the polycondensation polymer leading first

to linear polymers of critical length for the formation of entanglement networks. The

reaching of this critical length is greatly facilitated by the marked increase in concen-tration of the PF polymer due to the loss of water on absorbent substrates such aswood, coupled to the linear increase in the average length of the polymer due to theinitial phase of the polycondensation reaction. The combination of these two effectslowers markedly the level of the critical length needed for entanglement. Two modulussteps and two first derivative major peaks then occur, with the first peak due to theformation of linearPF oligomer entanglement networks, and the second one due to the

formation of the final covalent cross-linked network. The faster the reaction ofphenolic monomers with formaldehyde, or the higher the reactivity of a PF resin,the earlier and at lower temperature the entanglement network occurs, and thehigher is its modulus value in relation to the joint modulus obtained with the final,covalently cross-linked resin (Fig. 1).Copyright 2003 by Taylor & Francis Group, LLC

Page 5A. Acid CatalysisConsideration must be given to the possibility of direct intervention by the catalyst in thereaction. Hydrochloric acid is the most interesting case of an acid catalyst, as is ammoniaof an alkaline catalyst. When the PF reaction is catalyzed by hydrochloric acid, twomechanisms may come into operation. Vorozhtov has proposed a reaction route thatpasses through the formation of bischloromethyl ether (ClCH2OCH2Cl) [4]. Ziegler

has suggested a route through the formation of a chloromethyl alcohol (ClCH2OH) asintermediate [5,6]. The second route appears to be the more probable. Both hypothesesagree that chloromethylphenols are the principal intermediates. The chloromethylphenolshave been prepared and isolated by various means. They are highly reactive compoundswhich, with phenols, form dihydroxydiphenylmethanes and complex methylene-linkedmultiring polyphenols. Reaction is highly selective and takes place in the para position.B. Alkaline CatalysisDifferent mechanisms of alkaline catalysis have been suggested according to the alkaliused. When caustic soda is used as the catalyst, the type of mechanism which seems themost likely is that which involves the formation of a chelate ring similar to that suggestedby Caesar and Sachanen [7]. The chelating mechanism was thought to initially cause theformation of a sodiumformaldehyde complex or of a formaldehydesodium phenateFigure 1 Thermomechanical analysis (TMA) of the hardening of a PF resin in situ in a wood joint.

Increase of modulus of elasticity (MOE) of the joint as a function of temperature at a 10C/min

constant heating rate (); first derivative (4).Copyright 2003 by Taylor & Francis Group, LLC


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Page 6complex and is similar in concept to the mechanisms advanced for metal ion catalysis ofphenolic resins in the pH range 3 to 7. However, while the cyclic metallic ion catalysis ringcomplexes have even been isolated [8], this is not the case for the sodium ring complex,

evidence for its existence being rather controversial, the predominant indication being thatit does not form [9].When ammonia is used as a catalyst, the resins formed are very different in someof their characteristics from other alkali-catalyzed resins: the reaction mechanismappears to be quite different from the of sodium hydroxide-catalyzed resins. An obviousdeduction is that intermediates containing nitrogen are formed. Several such intermedi-ates have been isolated from ammonia-catalyzed PF reactions [1012] and hexamineprepared resins [1316] by various researchers. Similar types of intermediates are

formed when amines or hexamethylenetetramine (hexamine) are used instead of ammo-nia. In the case of ammonia the main intermediates are dihydroxybenzylamines andtrihydroxybenzylamines, such benzylamine bridges having been shown to be muchmore temperature stable than previously thought and to impart particular characteristicsto the resin [1316].These intermediates contain nitrogen and have polybenzylamine chains. They reactfurther with more phenol causing splitting and elimination of the nitrogen as ammonia orproducing eventually nitrogen-free resins. However, as benzylamine bridges have been

shown to be much more temperature stable than previously thought, this requires a con-

siderable excess of phenol and a high temperature, or heating for a rather long time. Withphenolhexamethylenetetramine resins of molar ratio 3:1, the nitrogen content of the resincannot be reduced to less than 7% when heated at 210C. When the ratio is increased to7:1, the nitrogen content on heating at 210C can be reduced to less than 1%. Contrary towhat was widely believed it has been clearly demonstrated that in the preparation ofPFresins starting from hexamethylenetetramine the di- and trihydroxybenzylamine bridgeswhich are initially formed are very stable and are able to tolerate for a considerable lengthof time a temperature as high as 100C [13] yielding in certain aspects (only) resins ofupgraded characteristics. This behavior is closely tied to the reactions characteristic ofhexamethylenetetramine to form iminomethylene bases [1416], which are discussed in themelamine resins chapter in this volume (Chap. 32).Ammonia-, ammine-, and amide-catalyzed phenolic resins are characterized by

greater insolubility in water than that of sodium hydroxide-catalyzed phenolic resins.The more ammonia that is used, the higher the molecular weight and melting point thatare obtained without cross-linking. This is probably due to the inhibiting effect of thenitrogen-carrying groups (i.e., CH2NHCH3 or CH2NH2), which is caused by theirslow rate of subsequent condensation and loss of ammonia. Ammonia, amines, andamides are sometimes used as accelerators during the curing of phenolic adhesives forwood products.Copyright 2003 by Taylor & Francis Group, LLC

Page 7C. Metallic Ion Catalysis and Reaction OrientationIn the pH range 3 to 7 the higher rate of curing of phenolic resins prepared by metallic ioncatalysis is due to preferential ortho methylolation [17] and therefore also to the highproportion of orthoortho links of the uncured phenolic resins prepared by metallic ion

catalysis. The faster curing rates of phenolic resins prepared by metallic ion catalysis isthen due to the higher proportion of the free higher-reactive para positions available forfurther reaction during curing of the resin. The mechanism of the reaction [8] involves theformation of chelate rings between metal, formaldehyde, and phenols or phenol nuclei in aresin.The rate of metal exchange is solution [8,18] and the instability of the complexformed determine the accelerating or inhibiting effect of the metal in the reaction ofphenol with formaldehyde. The more stable complex II is, the slower the reaction pro-ceeds, to the formation of resin III. A completely stable complex II should stop the

reaction from proceeding to resin III. If complex II is not stable, the reaction will proceed


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to form PF resins of type III. The rate of reaction is directly proportional to the instabilityor the rate of metal exchange in solution of complex II. The acid catalysis due to the metalion differs only in degree from that of the hydrogen ion [19].The effect of the metal is stronger than that of hydrogen ions, because of highercharge and greater covalence, since its interaction with donor groups is often much greater

[19]. This allows phenolic resin adhesives to set in milder acid conditions. Most covalentmetals ions accelerate the PF reaction. The extent of acceleration depends on the type of

metal ion and the amount of it that is present. The capability of acceleration in order ofdecreasing acceleration effectiveness has been reported to be [11] PbII, ZnII, CdII,NiII > MnII, MgII, CuII, CoII, CoIII > MnIII, FeIII ) BeII, AlIII > CrIII, CoII. The mostimportant conclusion to be drawn is that the accelerating effect is indeed present in boththe manufacture ofPF resin and its curing. Therefore, the fast rate of curing of high-orthophenolic resins can be ascribed only partially to the high proportion of para positions

available. The other reason for the fast rate of curing is that the metallic ion catalyst is stillpresent, and free to act, in the resin at the time of curing. In such a resin, a considerablenumber of ortho positions (especially of methylol groups in ortho positions to the phenolichydroxyls) are still available for reaction and capable of complexing.Copyright 2003 by Taylor & Francis Group, LLC

Page 8III. CHEMISTRY AND TECHNOLOGY OF APPLICATION OF PHENOLIC

RESIN ADHESIVES FOR WOODA. General Principles of ManufactureA typical phenolic resin is made in batches, in a jacketed, stainless steel reactor equippedwith an anchor-type or turbine-blade agitator, a reflux condenser, vaccum equipment, andheating and cooling facilities. Molten phenol and formalin (containing 37 to 42% for-maldehyde or paraformaldehyde), in molar proportions between 1:1.1 and 1:2, along with

water, and methanol are charged into the reactor and mechanical stirring is begun. Tomake a resol-type resin (such as those used in wood adhesives manufacture), an alkalinecatalyst such as sodium hydroxide is added to the batch, which is then heated to 80 to100C. Reaction temperatures are kept under 95 to 100C by applying vacuum to thereactor, or by cooling water in the reactor jacket. Reaction times vary between 1 and 8 haccording to the pH, the phenol/formaldehyde ratio, the presence or absence of reaction

retarders (such as alcohols), and the temperature of the reaction.

Since a resol can gel in the reactor, dehydration temperatures are kept well below100C, by applying vacuum. Tests have to be done to determine first, the degree ofadvancement of the resin, and second, when the batch should be discharged. Examplesof methods of such tests are the measurement of the gel time of a resin in a 150C hot plateor at 100

C in a water bath. Another method is measuring the turbidity point, that is,precipitating the resin in water or solutions of a certain concentration.Resins that are watersoluble and have a low molecular weight are finished at as lowa temperature as possible, usually around 40 to 60C. It is important that the liquid, water-soluble resols retain their ability to mix with water easily when they are used as woodadhesives. Resols based on phenol are considered to be stable for 3 to 9 months. Propertiesof a typical resin are a viscosity of 100 to 200 cP at 20C, a solids content of 55 to 60%, awater mixibility of a minimum of 2500%, and a pH of 7 to 13, according to the applicationfor which the resin is destined.Phenolformaldehyde (PF) resins present lower reactivity at a pH of about 4. Theaccepted effect of the pH and of the phenol/formaldehyde molar ratio on the rate ofpolymerization and rate of hardening of phenolic resols is shown in Fig. 2. Recently,however [9], the concepts expressed in the graph have been found to be only partiallycorrect, at least with regard to the dependence of the PF adhesive rate of curing as afunction of pH. The expected asympthotic acceleration expected over pH 7 to 8 and due tothe formation of phenate ions has been proven not to be the only effect present. At first


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acceleration occurs, but after a pH of approximately 8 to 9, the rate of hardening of theresin slows down considerably [9], as shown in Fig. 3, contrary to accepted wisdom. Thereare several reasons for this behavior [9], the easier of these to accept being the formation ofa ring involving phenol, the methylol group, and Na ions, which was postulated already 50years ago [7]. The existence of this ring has been shown to be untrue [9] and the persistence

of the concept is due to the ease with which the behavior shown in Fig. 3 can be explained.The reason for the acceleration, however, was ascribed to and proved to be due to the

existence of and equilibria pertaining to quinone methides [9,20]. The structure of theelusive oligomeric quinone methides in PF resins has also been elucidated [21] (see page549).The probable reason why the behavior in Fig. 3 was not noticed earlier appears to bedue to the slow gel times ofPF resins, which makes it very tedious to check reactivityeffectively.Copyright 2003 by Taylor & Francis Group, LLC

Page 9B. Curing Acceleration Under Alkaline Conditions1. - and-Set AccelerationThe so-called - and -set acceleration of curing for very alkaline PF resins for foundry

core binders was pioneered in the early 1970s [22], although it had been discovered in theearly 1950s [22]. In this application the addition of considerable amounts of esters or other

chemicals in liquid form (-set) or as a gas (-set), such as propylene carbonate, methylformate, glycerol triacetate, and others, was found to accelerate resin curing to extremelyshort times. This technique is now used extensively around the world for foundry core PFbinders [22] and is being considered for wood adhesives [9] and rigid alkaline PF foams.The technique is applicable in the approximate pH range 7 to 14. The mechanism thatmakes PF curing acceleration possible has only been explained recently [9] and different

explanations exist (see below); it is based on the carbanion behavior of the aromatic nucleiFigure 2 Rate of polymerization as a function of pH for phenolic resols of different molar ratios at120

C (old concept).Copyright 2003 by Taylor & Francis Group, LLC

Page 10of phenate ions, leading to a more complex variant of the KolbeSchmitt reaction. The

ester, or residue of its decomposition, attacks the negatively charged phenolic nuclei, andits reaction is not limited to the ortho and para sites, transforming the phenolic nuclei in atemporary condensation reagent of functionality higher than 3, leading to much earliergelling. Furthermore, temporary condensation occurs not only according to the PFmechanism but also according to a second reaction superimposed on it [9,23] (Fig. 4).Other explanations and mechanisms for this occurrence have also been advanced:determination by TMA of the average number of freedom of polymer segments betweencross-linking nodes ofPF resin hardened networks indicate that additive accelerated PFFigure 4 Cure retarding at high pH and ester acceleration effect of NaOH and KOH-catalysed PFresins (ester propylene carbonate). Note curve 5, the effect of 4 months aging of the PF resin of

curve 1 on the extent and starting pH of the retardation effect. Compare the start of acceleration for

curves 4 and 6, showing the differences between propylene carbonate and triacetin esters, and

compare the starting point of acceleration at pH 5.5 and pH 7.1. The bumps on the curves atpH 811 are caused by methylene ether formation, decomposition, and rearrangement [9].

Figure 3 Schematic relationship of gel time to pH for phenolic resols (new concept).


Page 11resin polycondensations and hardening present several different acceleration mechanisms.

[23]. Some additives such as sodium carbonate appear to present a purely catalytic effecton the polycondensation reaction [23]. Other additives such as propylene carbonate pre-sent both a catalytic effect as well as including an increase in the average functionality ofthe system, due to further cross-linking, or alternative reactions in which the acceleratoritself participates, leading to a tighter final network [23]. These alternative cross-linkingreactions could be of a different nature, such as the propylene carbonate case in which the


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reaction appears to be related to a KolbeSchmitt reaction, or they could be similar to theaccelerating effect due to the hydrolysis of formamide to formic acid and ammonia withthe subsequent rapid reaction of the latter with two or more hydroxybenzyl alcohol groupsofPF resols [23]. The rapid reaction of the -NH2 group of formamide with two hydro-xybenzyl alcohol groups ofPF resols, a reaction which is also characteristic of urea and

methylamine, also appears likely to occur. In some cases such as in formamide none of thetwo acceleration mechanisms detected appear to be due to catalytic action only, but both

appear to be related to additional cross-linking reactions. Both liquid and solid phase 13Cnuclear magnetic resonance (NMR) supporting evidence of the mechanisms proposed hasbeen presented [23].Further proof of complex reactions between propylene carbonate and phenolic nucleileading to compounds in which the carbonic acid has attacked the phenolic ring has beenpresented [23] based on the 13C NMR spectrum of the product of the reaction of resorcinol

with propylene carbonate, in the absence of formaldehyde. Resorcinol was chosen as itsaromatic ring is a stronger nucleophile than that of phenol and thus if any reaction hadoccurred this would be less elusive and much more easily observed [23]. The reaction pro-ducts which appeared to be formed were carboxylic and dicarboxylic species. That theymight be present was also derived by NMR [23]. It must be pointed out that such structuresneed to be only transitory and not permanent to obtain the same effects

phenolic resin adhesives

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