Hot Setting Phenolic Glues

The hot setting phenolic glues have their important place in making plywood. They are usually made with a molar ratio of about 1.5 - 2.25 formaldehyde to 1 of phenol. If prepared under weakly alkaline conditions, as formation of the resin proceeds, separation into two layers takes place an aqueous layer above. Under more strongly alkaline conditions, above pH of about 10 but depending on molecular ratio and degree of condensation, a homogeneous solution is formed, this makes the resin appear water soluble, whereas it is in fact soluble in the aqueous alkali. By comparison with pure phenol resins, cresol resins require higher concentrations of alkali to ensure solubility. Nowadays, phenolic resin plywood glues are applied mainly as aqueous alkaline solution, and to a small extent as film glues.

Of the phenolic substances that may be used to replace ordinary phenol in adhesives, mixed-isomer cresol, as distinct from meta-cresol-and mixed isomer xylenols are most used. In subsequent polycondensation of the resultant methylol phenols (the hardening reaction), further shown that the rates of polycondensation although lower are of the same relative order.

The curing of the solution form of phenolic wood glues at a high temperature say 130oC, is attended by certain disadvantages; one is the evaporation of moisture from the wood, another is the absorption of the resin by the wood, resulting in excessive penetration. The latter is influenced by the porosity of the wood and by its moisture content. So far as the glue itself is concerned there are three factors that contribute to penetration; low initial viscosity on heating, a rather slow rate of gelatin (compared with urea glue), and the fact that the resin is of low average molecular weight compared with many other polymers. The like hood of penetration is decreased as the curing temperature is lowered. It is not easy to overcome all these defects in one resin, and at the same time ensure it having satisfactory storage life.

Conversion of Resol to Thermoset Resin (Resite)

If the A-stage resin is heated, it passes to the "Resitol" or "B-Stage. In the B-stage the resin is swollen by solvents and instead of melting becomes rubbery on being heated, due to molecular entanglement by branching and some cross linking. On further heating the resitol is converted into the more completely cross linked "Resite" or C-stage resin, which is substantially insoluble and infusible.

An A-stage resol that is made by reacting a novolak with additional formaldehyde (two-stage resin) follows a similar route to the C-stage.

The thermosetting reaction normally takes place rapidly at elevated temperature but extremely slowly at room temperature. However, by addition of strong acid catalyst the C-stage can be reached rapidly at room temperature and this is the method used to cure cold-setting phenolic adhesive.

Since an alkaline catalyst favors the formation of the highly reactive methylol compounds and an acid catalyst the formation of the relatively unreactive phenol-methylene chains, the reaction of phenol and formaldehyde in equimolecular proportions can produce either a resol or a novolak, depending on the catalyst used.

Types of resin used as adhesives

Completely novolak forms of phenolic resins are not themselves important as adhesives. On the other hand the two stage resin (the novolak that has been converted to a resol by addition of formaldehyde) and also the single stage resin, which is potentially thermosetting from the outset, form the two basic chemical types of phenolic resins used as adhesives.

When used as a hot setting adhesives in, for example, plywood manufacture, the resin is made, stored and finally applied in its A stage, and in the hot press passes through the B stage to the C stage.

The average user may not be greatly concerned due to:

Ø Hot setting phenolic resins which are either strongly alkaline and water soluble and commonly used as adhesive for plywood, or weekly alkaline and alcohol-soluble as used in the Two-Polymer adhesives.

Resins which are cold setting on addition of strong acids and therefore of importance as assembly glues.

Novolak Formation

Novolaks are normally made under acid conditions, when the formation of methylol compounds is slow, whereas the conversion of methylol compounds to diphenylmethanes is rapid. Thus, with a molar excess of phenol, methylol group do not accumulate in the reaction mixture and the resulting molecules are linear (II and V), giving a permanently fusible resin, a novolak. The ratio of formaldehyde to phenol is usually 0.6 - 1,0 and strong acids, for example para-toluene slphonic acid, oxalic and sulfuric acid are common catalysts.

Resol Formation

In the preparation of the potentially thermosetting (resol) adhesives, a trifuctional phenol is required in order to produce breached polymers capable of forming a three dimensional cross linked network. Thus, phenol itself is most frequently used; mixed cresol (chiefly: meta and para isomers) and mixed xylenols may be used to replace part of or all of the more expensive phenol, with some loss of functionally. The narrowing of the price gap over recent years has made the use of mixed cresols less attractive.

In the production of resol adhesives the ratio of formaldehyde to phenol ranges from about 1:1 to 3:1. The commonest catalyst are strong alkalis such as sodium hydroxide, ammonium is also employed. Under alkaline conditions, the formation of methylol compound is rapid, but the condensation reaction to give diphenylmethanes is comparatively slow. Thus with a molar excess of formaldehyde, mono-, di-, and trimethylol compounds tend to accumulate in the reaction mixture, together with polynuclear compounds (eg. III and VI) resulting from further condensation.

At this point, the resol is described as being in in the "A" stage, being easily melted and soluble in a number of organic liquid, in particular the lower alcohols and ketones.

Phenol Formaldehyde Adhesive

Phenol Formaldehyde resin commonly called a phenolic glue or more simply a "P.F." glue, means a condensation product of formaldehyde and a monohydrate phenol including phenol itself, cresol and xylenols.

Although the phenolic tannins of vegetable origin consist of both monohydridic and polyhidric molecular structure, it is more appropriate to discuss them in this section than under resorcinol formaldehyde "R.F." adhesive. The later and also resorcinol/phenol adhesive are discussed in the next section.


Condensation products of phenol and formaldehyde can be either potentially thermosetting and known as "Resols," or thermoplastic and known as "Novolaks."



A resol is the type of product formed when formaldehyde is used in molar excess under (normally) alkaline condition, while a novolak is the type of product formed when phenol is used in molar excess under (normally) acid conditions. By reacting with sufficient additional formaldehyde under alkaline conditions, it is possible to convert a novolak to a resol. The two-stage resol prepared in this way differ in certain physical properties, such as intrinsic viscosity, from a resol made by direct reaction of phenol and formaldehyde under alkaline conditions. The Novolak and Resol reactions in relation to the preparation of phenolic resin glues

The reaction of phenol and formaldehyde

The product of the first reaction between phenol and formaldehyde is either ortho- or para-monomethylol phenol (I), which can subsequently react with more formaldehyde to yield di- and tri-methylol phenol. In the ensuing reaction, a methylol group of one molecule may react either with the nucleus of a second phenolic molecule splitting of water to form a dihydroxydiphenylmethane type of compound (II and III) or it may react with a methylol group attached to another phenolic molecule to form a dihydroxydibenzyl ether (IV), water again being eliminated. Under both acid and alkaline condition, this ether in turn splits off formaldehyde to form a methylene linked compound, the formaldehyde becoming available for further reaction.

CH-R---CH2OH or OH-R-CH2OH (I)

OH-R---CH2OH---CH2R-OH (II)

OH-R---CH2OH---CH2-OH-R-CH2OH (III)
OH-R—CH2OCH2-R-OH (IV)

OH-R—CH2-R-OH)n-R-OH (V)

The extent to which these reactions take place depends on the ratio of phenol to formaldehyde, the temperature, the pH, and the catalyst.

Hardening of Melamine Adhesive and The Properties

The fundamental difference in the curing of melamine resins and urea resins is that, whereas the latter can be cured at room temperature, melamine resins required a minimum temperature of about 65 ^oC. A melamine resin can in fact be hardened (imperfectly) at a much lower temperature by lowering the pH sufficiently, but the cured product has poor cohesive and adhesive melamine resins since this requirement inevitably limits their usefulness as adhesives.

Melamine resins, being more heat-reactive than urea resins, will cure in a reasonable time at a high temperature, for example, 120 – 130 ^oC without addition of a hardener, but for curing at say 100 ^oC hardeners are generally added. Acid or ammonium salt of strong acids are used. The properties of melamine adhesives cured at high temperatures without a hardener are not inferior to those obtained by curing at a lower temperature with a hardener.

The properties Melamine Adhesive

In the field of adhesives the principle use of melamine resins is in upgrading urea resins as described before.

As glue by themselves they have a limited use in making plywood. The wet strength falls between that of plywood made with phenolic and with urea glues, although flying much nearer to the former. Melamine adhesives are, for example resistant to boiling water, but if the immersion period is prolonged the retention of strength of glued joints is inferior to that of joints made with phenolic adhesives. In spite of this, the opinion has been expressed in the USA that under severe conditions of exposure to heat and moisture, the durability of melamine adhesives is similar to that of moderately alkaline phenol and resorsinol glues, but result of durability test made in England tend not to confirm this.

Stability of Melamine Formaldehyde Resin

Aqueous solution of melamine resin adhesive are much less stable than those of urea resins; they are therefore normally spray-dried soon after manufacture producing a powder glue with a storage life of least a year. The user re-dissolves the powder in water to give solution that remain stable for a few days depending on temperature and resin concentration. Instability result in the solution becoming cloudy and later semi-solid. In its early stages this state can be reversed by heating the resin to 40o to 50 ^oC provided no hardener has been added.

Partial etherification of low molecular weight resins with one of the lower alcohols, in particular methanol, improves stability in the liquid state. These resin, which can be produced with infinite solubility in water, are valuable in textile applications but have not found much use as adhesives. A method of related to melamine and also reacts with formaldehyde as well as methanol formaldehyde resins is inferior to melamine resins it may be expected that a co-condensate would have downgraded properties according to the ratio of dicyamide to melamine.

Melamine Formaldehyde Adhesive

A melamine formaldehyde adhesive (also commonly called a melamine glue or an MF) is a condensation product of unsubstituted melamine and formaldehyde.

Melamine formaldehyde resin have much in common with urea formaldehyde resin; they were, however, introduced many years after urea resin and have not attained great importance as adhesives. Melamine resin is, however, widely used as textile finished and in the manufacture of decorative laminates and plastic molding.

The melamine formaldehyde reaction in its relation to adhesives.

The reaction between melamine and formaldehyde has been studied by Kohler, and by Gams, Widmer and Fisch. The process of resinification are analogous to those of urea formaldehyde, that is related to urea resins therefore applied broadly to melamine resins, the reaction being similarly governed by the five factor; molecular ratio, pH value, temperature, concentration and time.

In the preparation of adhesives, melamine and formaldehyde in a molecular ratio 1 : 2.5 to 1 : 3.5, are reacted at a pH of between 8.0 to 9.0 and a temperature at or near to the boiling point. As with the urea reaction, condensations follows the formation of methylol compounds, and as resinification proceeds the solution become more and more hydrophobic. The reaction is continued until water toleration at room temperature is such that at least an equal volume of water can be added without precipitation occurring.

Addition of Polyvinyl Acetate to Urea Formaldehyde Adhesives

Many people concerned with gluing wood and other cellulose materials and having at hand both urea glues and polyvinyl acetate glues have mixed them together and used them in this way. A large number of test with such mixtures have no doubt been made in laboratories throughout the world, but with the exception of the report of Raknes on work done in Norway, little has been published, Raknes investigation was extensive but brought to light no unexpected effect, the properties of the adhesives being governed largely by the ratio of the thermosetting to the thermoplastic component.

Water soluble U.F. glues are compatible with aqueous dispersion of polyvinyl acetate, although mixture of the two may not be stable over long periods of time. The acidity of PVA dispersion may reduce the pot-life of UF adhesive, and in extreme cases the addition of conventional hardener may not be required. Where the amount of U.F. component is for example PVA would be further reduced, but where the U.F. component is for example only 10 % of the total, there may little or no advantage in adding a hardener.

With each component having advantages or disadvantages not possessed by the other, mixtures can be formulated that, as it were, upgrade one component while inevitably downgrading the other. But desirable intermediate properties can be obtained, for example increase impact strength compare with a straight U.F. adhesive, and reduced creep under stress compared with a straight PVA adhesive. Other effects created by the PVA component include an increase in tack or grab, and a reduction of wet strength and heat resistance.

Although there is little evidence of recommendation by adhesive manufacturers it is clear that certain desirable properties can be obtained with mixtures varying from about 20 % of one component to 20 % of the other.

That there is some interest among adhesive manufactures is, however, shown by the granting of a patent on the spray-drying of a mixture of a U.F. resin and a PVA dispersion, from which patent specification it may inferred that the dried mixture can be easily re-dissolved or re-dispersed in water.

Incorporation of the Hardener in the Resin

Pre-catalyst urea glue has been used to describe glues which are mixture of powder urea formaldehyde hardener and a powder resin. Such a mixture can have an adequate storage-like provide the moisture content is low. Among hardening agents claimed to give satisfactory storage-life are ammonium salts of non-volatile acids, and aluminum sulfate. Although these glues lack the flexibility of usage which separate hardener bestow, they have the advantage that they can be made ready for use simply by adding water. When they have been dissolved in water, the pre-catalyst urea glues have the same pot-life as if the hardener were added by the user, there is no latent hardening effect.

The idea of providing a latent hardening effect has always appeared attractive, (by latent hardener is meant one that is without effect until the mixture is heated). A satisfactory latent hardener would constitute an important development because it would remove the difficulties associated with a limited pot-life. The problem is, however a difficult one solve with a U.F. glue, but easier with a U.F. molding powder. In a Japanese study it has been reported that "pyridine-mono acetic chloride" (presumably the pyridine salt of mono chloroacetic acid), and also a mixture of pyridine and di-ammonium imidosulfamate, prolong the pot-life and cause rapid curing at high temperature.

This implies a high ratio of pot-life to curing time –perhaps above the average- but not necessarily truly latent hardening. A somewhat cumbersome but probably effective approach is that of coating the hardener particles with a wax that melts just before the curing temperature is reached. The complete coating of solid particles of a hardener is however not easy, a recently published method describes the use of a solution of an acid amide which crystallizes on to the hardener particles.

Curing Agents (hardeners) for UF Adhesive

The curing agents, also called hardeners, accelerators, or catalysts, that are added to the resin by the user, have one thing in common. They are either acidic substances by themselves, or they are capable of liberating acid when mixed with the resin. The later class comprises ammonium salts of strong acids. Ammonium salts are more widely used than acids; they are cheap, convenient to handle, and give a high ratio of pot-life to setting time. As hardeners for use at both normal and elevated temperatures the ammonium salts of strong acids are in many respects ideal. They function as hardeners by reacting with the free formaldehyde in the resin, and/or the formaldehyde liberated under the conditions of curing, to give the corresponding acid, hexamethylene tetramine (hexamine) and water. The most commonly used ammonium salt is ammonium chloride, which liberates hydrochloric acid.

4 NH4CL + 6 CH2O ====> 4 HCl + (CH2)6N4 + 6 H2O

The immediate decrease in pH that takes place on addition of the salt is followed by progressive further decrease as formaldehyde is liberated from methylol groups. Incidentally the pH at the time of gelatin is not as low as that of the synergistic liquid that is expelled at a later stage of the hardening reaction.

The rate of liberation of formaldehyde and consequently the rate of fall in pH is sharply increased by an increase in temperature, which is one of the reaction why ammonium salt are excellent hot hardeners. Apart from the more common ammonium salts of strong inorganic acids, ammonium thiocyanate is a very efficient hardener, but is poisonous.

The effect of adding hardeners which are themselves acidic is to reduce the pH is one step to its final value. It therefore follows that with the progressive fall that occurs with an ammonium salt the rate of hardening is a more gradual process giving a higher ratio of pot-life time especially at normal temperature.

Water Toleration of Urea Formaldehyde Adhesive

The methylol urea compounds referred to earlier are truly, if only slightly water soluble, that is to say, the more solvent (water) that is added, the greater the weight of solute dissolved. On the other hand, the resinous liquids used as adhesives are not true solutions but colloidal dispersion, and although it is possible to produce dispersion that are infinitely compatible with water, the reaction products used as adhesives are not generally of this type.

The extreme case of a resin having infinite compatibility with water at all temperatures is an indication that the resin having infinite compatibility with a high ratio of formaldehyde (and probably smells strongly of it), or that it has been relatively under reacted.

Practically all U.F. adhesives having limited water-toleration at normal temperature can be diluted at such temperature with at least an equal volume of water before showing any sign of opalescence due to incipient precipitation. Water-toleration always increases with an increase in temperature but individual behavior varies widely; some resins precipitate readily at say 20 ^oC but are infinitely water-tolerant at 40 ^oC. The physical nature of the precipitate also varies; it may be fluffy and bulky, gummy or powdery.

"Water-solubility" is a term that cannot accurately be applied to resins that precipitate (even though low molecular-weigh fraction may remain in solution): as more water is added, less resin remains in solution. The resin may, however be considered as a solvent for water, because if more resin is added to a system containing a water-precipitated fraction, the precipitate re-dissolves and homogeneity restored, but with a consequent increase in resin concentration.

Reaction of Urea Formaldehyde

The addition of limited amount of urea to resin to the resin solution just before use, either directly or in a hardener is a well-known and often recommended way of reducing the smell of formaldehyde. But it is not easy to determine quantitatively what type of product this contributes, whether resin or insoluble methylene urea compound; it depends on the molar ratio, the pH on addition, the rate at which the pH falls during curing, and the temperature.

It will be noticed that the term “water tolerant” is used. This expression, for reasons that are given later, is often more accurate and therefore is preferable to “water-solubility.” Here, the term is used to apply to the behaviour of the uncured resin, and should not be confused with “water-resistance,” a term that is applied to the cured adhesive.




Formaldehyde is used in the form of an aqueous solution (formalin), and the reaction with urea normally proceeds from liquid aqueous reactants to a solid end-product, but the manufacturer stops the reaction at an intermediate stage while the product is still liquid; the user initiates the final stage of the reaction to convert the liquid to the solid end-product. The final stage-the curing reaction is a continuation of the condensation reaction started by the manufacturer. Unlike some phenolic resins, a urea resin normally requires the addition of a hardening agent to initiate the final stage. As received by the user, therefore, a U.F. glue is generally, but not always, a two package adhesive.

The simplest reaction products of formaldehyde and urea are methylol compounds. Monomethylol and dimethylol urea are the primary products of the reaction, and their realative proportions are determined by the molecular ratio of formaldehyde to urea. The methylol urea are not resinous materials but crystalline compounds of simple molecular structure formed by the addition of a molecule of formaldehyde to an amine group; they are not themselves adhesives. The methylol ureas are produced under neutral or weakly alkaline conditions (heat may not be applied), and in the preparation of a resin they are not separated from the reaction mixture. After an appropriate period of time, the reaction mixture is made weekly acid to promote the condensation reaction leading to formation of the resin. At this stage the three parameters, pH, temperature and time, have to be considered together in determining the desired end-point. An alternative method entails determining the temperature at which a clear resin becomes cloudy on cooling; or the two methods can be combined.

After arresting the reaction by neutralization, the solution can be evaporated to increase the resin concentration and viscosity, or it can be spray-dried to produce a powder that is re-constituted with water by the user of the glue.