Resorcinol Formaldehyde Adhesive

Resorcinol Adhesive have a greater importance than the phenolic (resol) glue that cures on addition of a strong acid, is the resorcinol (novolak) glue that cures on addition of formaldehyde. Like the less expensive acid-setting phenolic, the resorcinol glue also sets without heat, but unlike the phenolic glue it cures under neutral or near neutral conditions. This behavior is not appreciably affected if a substantial part of the recorcinol is replaced by ordinary phenol.

The reaction in relation to the preparation of resorcinol formaldehyde glues.
Resorcinol, like ordinary phenol is trifunctional in its reactions with formaldehyde, but having a second hydoxyl group in the meta position it is more powerfully directing in the para- and ortho- positions. The reactivity is such that equimolar proportions of resorcinol and formaldehyde will react by them selves at room temperature to produce a gel. Such one stage resins are, however, of small practical worth as adhesives and so commercial resorcinol glues are two stages resins; the first stage is carried out by the manufacturer in forming the novolak, and the second stage is carried out by the user in converting the novolak to reside by adding formaldehyde.




In the preparation of a novolak having good stability, the ratio of formaldehyde must be kept well below the point at which there is no likehood of gelation occuring, for example, 1.0-1.5 moles formaldehyde to 2.0 moles resorcinol. The reaction (a condensation reaction in which water is split off) is carried out with heating, and under acid or alkaline conditions; strongly acid conditions may be used even as low as pH 1, but strongly alkaline conditions should be avoided, the limit being about pH 9. Whether condensed under acid or alkaline conditions the resulting novolak resins are completely soluble in water. Acid condensed novolaks, especially those made under strongly acid conditions, generally have superior film forming properties. Provided the ratio of reactants and the pH of the system are within the limits needed to produce novolak, and equally important, provided all the factor together allow reaction to be satisfactorily controlled, then a fairly wide tolerance is permissible.

The properties of phenol formaldehyde

Other adhesive as alternative of urea formaldehyde use is phenol formaldehyde. Phenolic glues are resistant to cold cured ones have a lower resistance to strong alkalies than the alkali cured glues, and quite naturally the converse applies; both type are resistant to all the common organic solvents. The properties of phenol formaldehyde...

Although phenolic glues are comparatively brittle, this is not a property of significant importance in many applications, for example, in plywood manufacture; nor in most wood gluing operations provided thick glue lines can be avoided. Dark color is often a disadvantage and the addition of a white pigment decreases it to only a limited extent.

Phenolic glues can be considered tolerant to most fungicidal chemicals; particular examples are sodium pentachlorophenate and isosafrole. In gluing wood, the moisture content of the timber is generally considered to be a more important factor than with urea glues. In this respect, greater latitude is claimed by dispersing, in a normal alkali condensed glue, a powdered phenolic resin condensate that is insoluble at room temperature but capable of swelling as the pressing temperature is reached.





The behavior of any type of adhesive depends in part on the material of which the adherents are made, sometimes to an appreciable extent. Durability is an important example of this. For example, phenolic metal adhesives in their operation at high temperatures are more durable on aluminum than on stainable steel, an aspect of durability that is discussed more fully in connection with the phenolic/epoxy adhesives. The durability of phenolic wood glues varies with different timbers; but factors such as glueline thickness can also have an effect. Investigations on the durability of phenolic glued wood joints have been made in England and in America.

Throughout the world the highest specifications for plywood, even if phenolic resins are not mentioned by name, are designed to make their use almost mandatory.

Phenolic Adhesive from Vegetable Tannins

Research in connection with the development of adhesives from vegetable tannin has been reported from various parts of the world, and it is possible that these phenol containing natural substances will one day figure prominently in the synthesis of phenolic wood glues, especially hot curing ones, or be used in association with them.

A variety of tannin extracts, containing different types of highly reactive phenols, is available commercially. They are derived mainly from trees, especially from barks and woods, but many represent a cheap source of mixed phenols containing nuclei of recorcinol and phlorogluecinol types. Most extracts are now available as spray dried, water soluble powder and the difficulty soluble extracts dissolve readily at a slightly alkaline pH.




The chemistry of the vegetable tannins has recently been surveyed in detail, both of surveyor whom have provided exhaustive bibliographies. Tannin extracts are described as of two kinds, hydrolysable tannins and condense tannins. The former are built up form a variety of phenolic acids (related structurally to gallic acid) which are joined by ester linkages to a central sugar residue. They are of little interest in the formation of phenolic adhesives. The latter type contain mixtures of "polyphenol" (i.e. polynuclear polyhydric phenol) that react rapidly with formaldehyde. The structure of the actual tannin molecules are uncertain but they are clearly related to various C15 compound.

The important of leather tannin extracts, has covered the synthesis of adhesives made wholly from quebracho or mimosa extract and formaldehyde, as well as the partial replacement of ordinary phenol in P.F. glues. Utilizing a tannin extract as the only phenolic compound could be marketed. Certain unpublished work has indicated two main reason for this. Firstly the pot-life is short; associated with this, the ratio of pot-life to curing time (at temperatures used in plywood manufacture) is relatively low. The probable explanation is that the resin contains molecules of a wide range of size and structure, a small percentage of highly reactive molecules being sufficient to cause gelatin and termination of the pot-life, whereas satisfactory curing requires also the cross linking of less reactive molecules. Secondly, the resins lack good spreading and wetting properties, especially when fillers are added. The adhesive have excellent resistance to water, including boiling water.

Thickening Agents

In a different class to filters but in some ways having the same effect are thickening agents, soluble substances added to increase viscosity. These also are used mainly in the hot setting plywood glues. One effect of increasing the viscosity is to enable more water to be added, so making the glue cheaper, similar to adding fillers. Indeed, it is obvious that both methods can be used together; and there are probably some applications where a little of each is desirable.

Among the earliest thickening agents were the soluble and dextrins. In more recent years boric acid, polyvinyl alcohol, and methyl cellulose and polyethylene glycol have been used or suggested. Quite small amounts only are required, usually less than two percent. Less than one percent of a hydroxy-substituted alkyl cellulose is recommended, usually hydroxyethyl cellulose since it has better water solubility than methyl cellulose. The disadvantage of methyl cellulose are an inverse water solubility temperature curve, and a liability to be salted out by the strong alkali in the modern medium temperature plywood adhesives. Not quite in the same class as either the filter or thickening agent is the addition of sugar (sucrose); this probably has certain advantages, although being soluble it would be expected to reduce water resistance more than an insoluble substance if used in substantial amounts.

Over the years a large amount of chemical research has been done on phenolic resins generally, but in the field of adhesives no revolutionary advance has been made. Many users of phenolic adhesives have no doubt speculated on the advantages, particularly in plywood manufacture, to grained by adding urea-resins. This can only be done in certain cases, and the results are usually disappointing. The urea resins do not make very satisfactory polymers when cured under alkaline conditions, they may a more useful place a resinous adulterants in the acid setting phenolic glues; and there is also the question of incompatibility.

Fillers and Extenders

The subject of fillers and extenders is concerned mainly with the use of phenolic glues in plywood manufacture. By applying a relatively thick layer of extended (or filled) glue, compared with a thin layer of an unextended glue, fewer glue starved areas result when coating uneven veneers in roller type glue, spreaders. This is a matter of considerable technical importance. Another is the effect of fillers in reducing absorption and penetration in the gluing of porous veneers. One of the recognized shortcomings of liquid phenolic glues cured at high temperatures is penetration. Fillers may be added to impart special rheological properties; the incorporation of filler can also facilitate spreading. Finally, there is the advantage of cheapening the flue mixture.





It has been the practice to use the terms "filler" and "extender" rather loosely and often synonymously. In considering the use and definition of these terms suggest the following: "filler" being the non-reactive component of the mixed adhesive, and usually added on the maker's instructions (presumably also by the maker), and "extender" being that added by the glue user (not necessarily with the maker's knowledge or consent). There is much to be said in favor of this distinction.

The commonly used filler or extenders are wood flour, walnut, coconut, and pecan shell flour, Soya bean flour and such cheap inorganic fillers as gypsum, powdered chalk, clays and a number of other oxides and silicates including talc. The reside from oat-hulls and corn cobs after extracting furfural is recommended, and of special interest to eastern countries is the use of ground rice husk as filler. There really is no limit to the list of insoluble substances that could be pulverized and used. One of the latest is ground plum stones.

The amount of filler that can be added -with wisdom- varies considerably it depends on density, practical size and shape; a small particle size is necessary in order to ensure a smoothly spreading mix, and a mesh size of 200 (British standard 410) is common. In the case of wood and shell flour, unlike wood flours absorb water very slowly and therefore do not cause a marked increase in viscosity during the period of the pot-life. Dried blood and starches are extenders of a different class because they are not insoluble and they are adhesives in their own right.

Objectionable as excessive additions of fillers are, the use of fillers is often technically justified –economic considerations apart. The maximum amount that may be added cannot be defined, it depends on the strength, resistance to various influences and durability required. In the manufacture of plywood, a weight of coconut shell flour to the weight of resin solids is nowadays not uncommon.

Sulphonated Phenolic Resin Adhesives

The only object of sulphonated is to convert a water insoluble resin to one that is soluble. Sulphonated resins may be prepared by a number of different methods, on of which is to react a novolak with concentrated sulfuric acid. The sulphonated novolaks may be cured at room temperature by reaction with additional formaldehyde, or they may be converted to resols that are water soluble by reacting with formalddhyde under alkaline conditions. In sulphonating a novolak it is important that is should have a sufficiently low molecular weight; otherwise the relatively large molar ratio of sulfuric acid that is required will cure the resin.

A better method is to use a sulphonated phenol, such as phenol para-sulphonic acid or its sodium salt. The whole amount of the phenol need not be sulphonated in order to give a resin with satisfactory water solubility; it therefore follows that it is possible to mix together a sulphonated novolak and a resol.

When sulphonated are made entirely under alkaline conditions, precipitation of the resin does not occur at any stage and so, in the preparation of the resin, the reaction cannot be followed by diluting samples with water. It can, however, be followed, and an end-point established by diluting with a selected organic solvent (perhaps mixed with water), or by measuring viscosity.

The sulphonated resols may be spry-dried to produce powder glue which can be easily re-dissolved in water. Curing can be effected by adding a suitable acid such as dilute hydrochloric acid.

Curing of Cold Setting Phenolic Glues

In the curing of the cold setting phenolic adhesives is that they are capable of giving good strength on curing at low temperatures, down to zero centigrade in fact. The setting of many other synthetic adhesives -even if they cure at all at low temperatures- is protracted and may result in a poor end product.

In the curing of the cold setting phenolic glues, the addition of an aqueous solution of an acid such as sulfuric, hydrochloric or acetic, causes the resin to be precipitated from solution. The acid commonly used as hardeners are therefore sulphonic acids derived from aromatic hydrocarbons, such as benzene or toluene sulphonic acid. These, or similar acids of certain aromatic compounds are compatible with the resin chiefly owing to the solubility effect of the sulphonic acid group. Alternatively, however, a phenolic resin that has itself been unsulphonated resins, para-toluene sulphonic acid is most often used it is a strong acid, its normal solution having a pH of less than 0.5.

In a study of the effect of pH on cold setting resin prepared from formal 4-dehyde and monohydric phenols, it has been shown that a "barrier pH" exists, above which practically speaking, curing does not take place at room temperature. The barrier pH is roughly between 1 and 3 depending on the type of phenol; with ordinary phenol it is about pH 1.

While most acid set phenolic glues appear to give joints of good durability over long periods, both at high and low temperatures, there is evidence that a higher proportion fall below a specified minimum strength after a number of years than with resorcinol glues and that the acid hardeners used definitely attack the wood". A method of eliminating some of the danger of acid attracts on wood has been described. It evolves the addition of a "protective solution" to the acid catalyzed glue. This protective solution is designed to liberate an alkaline substance when the wood is subjected to an elevated temperature either during fabrication or in subsequent use. The composition of the solution is not stated.

Cold Setting Phenolic Glues

The cold setting acid hardening phenolic glues have a smaller but more varied use than the hot setting alkaline ones, being confined almost entirely to the assembly gluing of wood. In the preparation of the resins, phenol itself is normally used, generally with a molecular ratio between 1.5 and 2.2 of formaldehyde to one of phenol. After heating under alkaline conditions, the resin is neutralized or made weakly acid, to obtain a homogenous solution in a non-alkaline medium it is necessary to remove most of the water and re-dissolved the resin in an organic solvent such as ethanol, iso-propanol or acetone.

Where it is possible to produce phenolic glues that are quite strongly alkaline yet stable-that is in terms of both pot-life and shelf life- the addition of a strong acid results in a short pot-life, which of course is the reason why the one is hot setting and the other cold setting. The later is therefore supplied as two part adhesive, a resin and a hardener.

The addition of the acid hardener required to effect setting at room temperature generally causes an exothermic reaction; the lower the pH and the higher the resin concentration, the higher the rise in temperature. The increase in temperature may be enough to shorten the pot-life considerably; on the other hand, the quantity of heat generated in a glue line of normal thickness is unfortunately not enough to accelerate setting of the adhesive in the joint. The exothermic reaction makes it necessary to observe accuracy in weighing or measuring-two much hardener and pot life is short, too little and the rate of hardening is disproportionately increased.

One advantage of cold setting phenolic adhesive is that they are capable of giving good strength on curing at low temperatures, down to zero centigrade in fact. The setting of many other synthetic adhesive-even if they cure at all at low temperatures is protracted and may result in a poor end-product.

Phenolic Epoxy Adhesive for Metal

The adhesion to metal of unmondified phenolic resins can increased by adding an epoxy resin, in particular a bisphenol a resin, and furthermore, since this addition reduces brittleness, band strength is increased. It, however, hot strength is nor to be seriously reduced, the amount of bisphenolic, it follows therefore that the cross linking of the epoxy resin (assuming no other curing agent is added) is provided for by the large excess of phenolic resin.

Interest in phenolic epoxy adhesives first came about through the demand for adhesives with improved hot strength capable of withstanding the dynamic heat generated by fast flying aircraft. The early reports published on this type of adhesive quoted hot strengths that were clearly an advance on values attainable at that time with other adhesive systems. But the excellent result obtained on stainless steel was good, the strength on prolonged heating deteriorated much more rapidly than on aluminum alloy. Some experienced conclude that the iron catalyzed the thermal decomposition of the adhesive, the decrease in joint strength is a consequence of this.

Some observation with phenolic epoxy adhesives, bonds made on titanium, brass, copper and low carbon steel also bad low resistance to heat ageing compared with aluminum. Some improvement in the behavior of stainless steel was brought about by specially treating the metal surface or by adding 1% manganese dioxide to the resin. An increase in the heat stability of stainless steel joints could be affected by adding certain metal chelates to the adhesive.

The ratio of phenolic to epoxy resin can be varied within wide limits, but the copositions that give the best hot strength (although probably not the best heat ageing) contain at least foru times as much phenolic as epoxy resin. In some formulations that have published, dicyandiamide is also added as well as a large amount of aluminum powder.

Curing Temperature Phenolic Glue

During recent years the curing temperature of the phenolic plywood flues has come down from the region of 130 - 140OC and 105 - 115oC, this is a decrease of considerable importance and has justified such glues being classified separately as medium temperature plywood glues.

The decrease in temperature has been made possible by a production technique that produces a resin of higher molecular weight than hitherto. In effect, the resin temperature advances the reaction and carried out in his reaction vessel a large part of the "curing," which would otherwise take place in the hot press. The molecular weights of "medium temperature" plywood adhesive are relatively high, and therefore the resins that can be isolated from them by neutralizing the solution are less easy to dissolve in common phenolic resin solvent such as ethyl alcohol and acetone, they may be considered to be approaching an incipient B stage. For solution to have good storage-life, a dilute (with respect to resin), but rather strongly alkaline solution is necessary; a resin content of 40-50% and a pH of 12-13 are therefore common. If the pH is of the order of 9.5-11 the glue will cured fairly but the storage-life, even at a resin concentration of only 40-50%, will be short. In raising the pH, by adding further strong alkali, an increase in storage life is obtained. Another effect of increase pH is reduced viscosity, which is probably the result only of improved solubility.

To rise the pH of a phenolic glue to 12-13, a relative large addition of strong alkali, usually sodium or potassium hydroxide, is required, much larger than that required to give the same pH in water alone. This doe to what may be described as the buffering effect that is characteristic of many resinous polymer. The addition of a small proportion of an alkylene polyamine such as ethylene diamine is claimed to improve the stability of aqueous solution of P.F. resins.