BE1003498A3 - CATALYTIC PROCESS FOR THE PREPARATION OF "NOVOLAK" PHENOLIC RESINS. - Google Patents

Abstract

A catalytic process for the preparation of phenolic resins of the novolak type is described by condensation of hydroxylated aromatic compounds and of compounds containing aldehyde functions and, in particular, by condensation of phenol and formaldehyde in the presence of an organic acid catalyst having the general formula FCXY-COOH in which X and Y, which can be either equal or different from each other, represent H or F.

Description

CATALYTIC PROCESS FOR THE PREPARATION OF "NOVOLÄK" PHENOLIC RESINS

DESCRIPTION OF THE INVENTION

The present invention relates to a catalytic process for the preparation of phenolic resins of the novolak type.

More particularly, the present invention relates to a catalytic process for the preparation of phenolic resins of the novolak type by the catalytic reaction of a phenol or * more generally, of an aromatic hydroxyl compound with an aldehyde or a compound comprising aldehyde functions.

More specifically, the present invention relates to the preparation of novolak resins with low molecular weight and * in particular, those of the bisphenol F or 4,41-methylene-bis-phenol type.

The products thus obtained can generally be called "novolaks" and constitute one of the most important classes of phenolic resins which find a large number of important applications on an industrial level. Among these, the following may be mentioned: their use as basic intermediates for the synthesis of epoxy systems, their application in the metal casting sector, as thermoplastic molding powders, in abrasive and refractory systems, for the production of noise absorbing materials, etc.

Other applications well known to specialists in this technique are those which are in the field of wood composite materials (plywood and others) and friction materials for brakes, adhesives, phenolic fibers, etc.

There are a large number of references in the literature relating to the production of phenolic resins, either of the resolving type or of the novolak type.

As regards the latter, which constitute the object of the present invention, the production for example of phenolic resins of the novolak type by the process of the reaction of phenol with formaldehyde catalyzed by a strong acid of organic or inorganic nature in medium aqueous, is well known.

Since the reaction rate depends essentially on the potential of hydrogen ions in the aqueous medium, strong acids are particularly suitable as catalytic systems. In general, strong acids such as hydrochloric acid, phosphoric acid, sulfuric acid, oxalic acid, p-toluenesulfonic acid are used; the reactions are carried out at temperatures in the range from room temperature to 100 ° C.

Novolak resins are obtained in this way, the molecular weight of which varies according to operating parameters such as the molar ratio of formaldehyde to phenol in the starting reaction mixture (patent Ü.S. n® 4,400,554), the duration of the reaction. , temperature, etc.

However, although catalytic systems of this kind are kinetically efficient, their use involves a number of drawbacks deriving from the very nature of the catalytic system and which are often difficult to overcome.

Thus, for example, although hydrogen chloride is readily available thanks to its low price, it can only be used with pieces of equipment that can withstand its high corosivity index. In addition, if HCl and formaldehyde are both present in the gas phase at concentrations above 100 ppm, they can produce 1,1-dichloromethyl ether which is a compound whose highly toxic character is well known.

Sulfuric acid and phosphoric acid are quite effective, but they tend to easily form compounds with intense coloring, which decreases the quality of the final product. Furthermore, even if only small quantities of these products remain in the resin, they are sufficient to catalyze the disproportionation reaction which would produce large quantities of undesirable free phenol in the final resin (see Hoyt H. et al. in "Paint Varnish Production" 413, 13 (1957 Given its high acidity combined with a low price and good availability on the market, oxalic acid is one of the most commonly used catalysts. However, even if the data cited in the technical literature demonstrate that it tends to sublimate easily at around 100 ° C under reduced pressure and without decomposition, the results of laboratory tests have shown that the elimination of oxalic acid from the reaction medium is not not really simple and can lead to problems with the epoxy resin, and just like the other catalysts mentioned above, oxalic acid catalyzes the reacts disproportionation resulting in the formation of free phenol in the resin obtained. On the other hand, the reaction of phenol and formaldehyde in the presence of atmospheric oxygen can cause the production of small quantities of colored products belonging to the class of aurines, which have been found to be critical in the later stages of treatment.

It can be concluded from these observations that, since the acid catalyst cannot be easily removed from the reaction medium after the catalysis step, the use of these systems is almost always combined with a phase of neutralization by alkalis or with other agents such as ion exchange resins, basic absorbent earths, etc.

A step of this kind is carried out at the end of the condensation reaction and before starting the distillation of the excess phenol, in order to avoid the P ^ ®klemes of desalkylation due to the presence of traces of acid in the novolak product. final.

On the other hand, although this is a fairly delicate operation, especially when the reaction medium has an aqueous-organic character, the neutralization step eliminates the acidity of the system but can harm the very quality of the novolak resin due to the formation of insoluble inorganic salts which form a precipitate and are responsible for the non-perfectly shiny appearance of the phenolic resin and for the increase in viscosity which can be observed in this product over time.

In addition, the presence of inorganic salts in a purely organic matrix can give rise to technical drawbacks during the treatment stage, because the salt thus formed cannot always be easily removed from the reaction mass by a simple filtration operation. .

! Another problem is due to the fact that this neutralization operation can give rise, if not carried out with care, to an alkaline change in color of the novolak resin and, consequently, to the formation of colored compounds and to chemical changes to the product itself.

This problem was partially solved by using solvents for the extraction of the resin from the reaction medium, but this process proved to be very complex, especially in the case of large reaction volumes as well as the recovery of the solvent from the extracted resin. .

The applicant has found that the abovementioned drawbacks and disadvantages which affect the technologies specific to the prior art can be overcome if the reaction between the phenolic compounds or, in general, between the aromatic monohydroxylated or polyhydroxylated compounds and formaldehyde or, d 'Generally, the products containing one or more aldehyde functions, is carried out in the presence of a catalyst consisting of a strong volatile organic acid which can be removed from the reaction medium without undergoing decomposition phenomenon and which can therefore be recycled.

The process of the present invention is therefore very ecological and makes it possible to prepare phenolic resins of the novolak type which are free of all traces of acid in the system as well as undesirable inorganic salts, which therefore avoids neutralization and extraction by solvents.

Consequently, the subject of the present invention is a process for the preparation of phenolic resins or novolak resins starting from a monohydroxylated or polyhydroxylated aromatic compound and, in particular, a phenol or a higher homolog thereof and of a compound containing at least one aldehyde function and, in particular, a lower aldehyde (that is to say C 1 -C 4) and, preferably, formaldehyde in the presence of an acid catalyst, said process being characterized in that said catalyst is an organic acid having the general formula (I) î FCXY-COOH (I)

in which X and Y, which can be either equal or different from each other, represent H or F

The acids having the general formula (I) used as catalysts according to the present invention are known to be particularly suitable because not only do they meet the conditions necessary to effectively catalyze the phenol-formaldehyde condensation reaction but also because they have the characteristics physical and chemical following: (a) their boiling points under atmospheric pressure are equal to or lower than 170 ° C, these values allow them to be completely and easily eliminated from the reaction medium by evaporation - direct volatilization; (b) they exhibit complete chemical inertness to the reaction system, so that side reactions do not occur; (c) they exhibit considerable chemical inertness with regard to reducing or oxidizing agents and therefore do not give rise to degradation phenomena.

Among the acids falling within the scope of the above formula (I), the following may be mentioned by way of example: monofluoroacetic acid: C ^ FCOOH; difluoroacetic acid: CHF2COOH; trifluoroacetic acid: CF ^ COOH whose boiling points are in the range of 60 to 160 ° C and whose pKa values at 25 ° C are in the range of 0.1 to 3.

Owing to its particularly high stability and volatility, trifluoroacetic acid is preferred from both an economic and a functional point of view.

Trifluoroacetic acid is liquid under normal conditions and is completely miscible with water.

By way of example, the tables below reproduce the physical and chemical characteristics of trifluoroacetic acid.

Appearance at 25 ° C transparent liquid

Specific weight at 25 ° C 1.484 g / cm3

Boiling point (760 mmHg) 71.8 ° C

Melting point -15.4 ° C

Vapor pressure (25 ° C) 107 mmHg

Viscosity at 25eC 0.813 cP

PKa value at 25 ° C 0.23 pH in solution at a concentration of 11.4 g / liter of H2O (20 ° C) 1 1

Boiling point of the mixture 105.5 ° C (20.6% azeotropic with H20 (750 mmHg) of H20)

The acid catalyst according to the present invention is used in amounts making it possible to ensure the usual acid catalysis conditions necessary for the condensation reaction to take place, these conditions normally being comprised in the pH range from 0.2 to approximately 2 depending on the acids used. Therefore, amounts in the range of 0.001 to 10% by weight of acid having the formula (I) based on the reaction mass may suffice.

If the trifluoroacetic acid is used, the amount of it is preferably in the range of 0.005% to 4% by weight relative to the reaction mass; larger amounts do not provide additional benefits in terms of product characteristics or reaction rate.

When operating in the said ranges, no corrosion phenomenon is observed for the materials generally used for the construction of industrial installations.

As mentioned above, the catalysts of the present invention are used in a process for the preparation of phenolic resins of the novolak type by the condensation reaction of a phenolic compound with an aldehyde compound. Said condensation reaction is known in itself and need not be described in detail, since it has been fully described in the corresponding references in the technical literature.

Phenolic resins or novolak resins are generally obtained by the reaction of a monohydroxylated or polyhydroxylated aromatic compound which is preferably a phenol or a higher homologue thereof with an aldehyde which is preferably formaldehyde or with a compound containing one or more aldehyde functions.

The reaction used is the well known condensation reaction which. takes place according to the mechanism of the electrophilic substitution of the phenolic cycle, widely described in the technical literature, and which can be represented schematically as follows:

Bisphenol F thus obtained can optionally react with one or more molecules of iriethylol derivatives, in order to form novolaks having higher molecular weights according to the reaction scheme according to

where "n" is a number in the range of 1 to 10.

Among these compounds, the bisphenol F novolak is preferred.

The molar ratios of the aldehyde compound to the hydroxylated aromatic compound are generally less than or equal to 1 and are preferably in the range of 1: 1 to 0 * 01ï1 depending on the operating conditions of the temperature * the raw materials and, in particular * depending on the type of compound desired.

The reaction temperature is generally in the range of 10 ° C to 100 ° C and is preferably in the range of 20 ° C to about 90 ° C under reflux conditions.

If the process is carried out within the above ranges of conditions, reaction times in the range of 0.5 to 10 hours are generally sufficient. The reaction is carried out under an atmosphere of inert gas such as, for example, nitrogen, argon, etc. and at atmospheric or supra-atmospheric pressures.

At the end of the reaction, the catalyst of the present invention can easily be separated from the reaction mass by distillation, rectification, etc. following conventional techniques; it can then be recovered and recycled with the unreacted phenolic compound, this offering an additional advantage of ecological character obtained thanks to the present invention.

As mentioned above, the condensation reactions of monohydroxylated or polyhydroxylated aromatic compounds and, in particular, phenols and higher homologues thereof with aldehydes or compounds containing one or more aldehyde functional groups can be catalyzed according to the present invention in order to obtain the corresponding phenolic resins or novolaks.

Among the monohydroxylated or polyhydroxylated aromatic compounds, the following may be mentioned: phenols and their superior counterparts comprising, in addition to phenol, compounds such as cresols, xylenols optionally substituted by other alkyl groups such as p-tert-butylphenol , nonylphenol ,. etc., diphenols, resorcinols, bisphenol A (also called "bis-A"), that is to say f2,2-bis- (4-hydroxyphenyl) propane} - p-phenylphenol, etc.

Among the aldehydes or compounds containing aldehyde functions, those containing from 1 to 4 carbon atoms are preferred, among which mention may be made, in addition to formaldehyde, acetaldehyde, furfurylaldehyde, paraformaldehyde, polyaldehydes such as glyoxal, etc.

According to the preferred procedure, the process of the present invention is carried out by reacting phenol or polyphenols and formaldehyde or polyaldehydes with a molar ratio of aldehyde to phenol in the range of 1: 1 to 0 , 01: 1 in an aqueous medium, in the presence of the acid catalyst of formula (I), operating at a temperature in the range of 20 to 100 ° C under atmospheric nitrogen pressure, while the reaction system is maintained in reflux conditions in order to reduce the possible losses of volatile substances.

The system is kept at constant temperature throughout the condensation, with the value of the concentration of free aldehyde in the reaction system checked periodically. When the values of the concentration of aldehyde tend towards zero, it can be considered that the condensation reaction is complete; At the end of the reaction, the distillation of the catalyst is carried out in aqueous-organic solution, then the solution is rectified so as to concentrate the solution of the acid in the aqueous phase.

This last rectification operation can be carried out under reduced pressure and at the temperature required for the separation of the acid and the water from the reaction mixture.

The residue from the distillation contains the phenolic resin and a small amount of the phenolic compound which can be removed appropriately by steam purification.

After elimination of the last traces of phenol, the pH of the reaction product, measured in a 50% solution of ethyl alcohol, has values ranging from 5.9 to 6.5.

The acid catalyst of formula (I) is completely recovered by distillation and can be recycled for subsequent process steps, or it can be removed from the system using ion exchange resins or by a distillation step with appropriate separation efficiency.

In order to better understand the present invention, certain examples will be described purely by way of illustration, said examples not presenting any limiting character of the present invention.

EXAMPLE 1

4.700 g (50 moles) of phenol and 305 g of H 2 O are introduced into a reaction vessel; the temperature is then brought to 90 ° C. and the mass is maintained under atmospheric nitrogen pressure.

When a homogeneous solution is obtained, 1.5 g of 99% CF3COOH are added.

The pH of the solution is 0.3. 60 g of a 50% formaldehyde solution in H 2 O (1 mol) are added. The phenol / formaldehyde molar ratio in the reaction charge is 0.02. , At time zero of condensation, the composition of the reaction mixture is as follows; * · Phenol: 92.22% by weight - formaldehyde: 0.59% by weight - H2O: 6.55% by weight - CF ^ COOH: 0.029% by weight

The system temperature, which is maintained under reflux, is maintained at 95-100eC. After approximately 50 minutes, the formaldehyde concentration values are less than 0.02% and the condensation is considered to be complete.

The distillation process then begins. This distillation is first carried out at the same reaction temperature, then the pressure is gradually reduced to a residual value of 1 torr and the temperature is brought to 160 ° C. In this way * CF ^ COOH * H2O and the excess phenol are eliminated.

The distillation residue is subjected to steam purification in order to remove all the residual traces of free phenol.

190 g of novolak are obtained, containing 93.2% of bisphenol F and having the following physical and chemical characteristics:

- melting point (capillary) = 80-85eC

- pH (50% solution in ethanol) = 5.9 - Gardner color (50% solution

in ethanol) = O

- appearance (50% solution in ethanol) = bright liquid - color = white - H2O = 0 * 7% by weight - free phenol = 0 * 5% by weight

The distilled fraction is subjected to rectification using a tray column. The composition of the overhead fraction still contains 1.5 g of CFgCOOH, which corresponds to the amount introduced for the reaction.

EXAMPLE 2

4.000 g (50 moles) of phenol * 365 g of HjO, 18 g of 99% CF ^ COOH and 116 * 9 g of 36% formaldehyde in H 2 O are introduced into a reaction vessel.

The phenol / formaldehyde molar ratio is 0.033 and the CF-jCOOH content is 0.4% by weight.

The process is carried out in the same manner as in Example 1. 225 g of novolak containing 91.7% of bisphenol F having the following physical and chemical characteristics are obtained:

- melting point (capillary) = 78-82 ° C

- pH (50% solution in ethanol) = 5.9 - Gardner color (50% solution in 1 ethanol) = ^ 1 - appearance (50% solution in ethanol) = bright liquid - color = white - 1 ^ 0 = 0.6% by weight - free phenol = 0.4% by weight Analysis of the distilled fraction, after rectification as in Example 1, indicates that CF3COOH was recovered quantitatively.

EXAMPLE 3

1980 g of phenol, 2780 g of H 2 O, 25 g of CF 3 COOH (98%) and 140 g of 36% aqueous formaldehyde solution are introduced into a reaction vessel.

The phenol / formaldehyde molar ratio = 0.08. The content of CF ^ COOH = 0.50% by weight.

The process is carried out in the same manner as in Example 1. After a reaction time of 90 minutes, 260 g of novolak containing 86.1% of bisphenol F having the following physical and chemical characteristics are obtained: - pH ( 50% solution in ethanol) * 5.8 - Gardner color (50% solution in ethanol) = 1 - appearance (50% solution in ethanol) = brilliant liquid g 0 s 0.8% by weight 2 - free phenol = 0.3% by weight At the end of the condensation, the system is cooled to ambient temperature.

After cooling, the reaction mixture separates into two very distinct phases: the underlying organic phase which is eliminated and the supernatant aqueous phase which contains, in addition to a certain amount of phenol, 21 g of CF ^ COOH, which corresponds to 84% of the total amount initially introduced into the reaction.

The organic phase, previously evacuated, is distilled by operating as in Example 1. The distillate obtained consists of two well-separated phases, that is to say an organic phase and an aqueous phase and, in this distillate, the 16% of residual CF ^ COOH introduced into the reactor are distributed as follows: - aqueous phase of the distillate = 0.40% of CFgCOOH (2.5g) - organic phase of the distillate = 0.15% of CFgCOOH (1, 0g)

It can therefore be noted that CF ^ COOfl is completely extracted from the resin. The quantitative recovery of this makes it available again for subsequent catalysts.

EXAMPLE 4

1500 g of phenol and 18 g of CF 2 COOH are introduced into a reaction vessel and the temperature is brought to 100 ° C. A 40% aqueous solution of glyoxal (231 g) is introduced into the reactor for 60 minutes.

The glyoxal / phenol molar ratio = 0.1.

After 6 hours of reaction, the fi ^ O, CF ^ COOH and free phenol are distilled off from the reaction mixture and they are introduced into a rectification column where they are subjected to the same treatment as in the previous examples. The last traces of free phenol are removed from the reaction mixture by steam purification. 350 g of resin are obtained and the CF3C00H is completely recovered in the distillation / rectification stage, as described in Example 1.

EXAMPLE 5

2650 g of ortho-cresol, 212 g of H 2 O, 34 g of CF 2 OOH, 986 of 36% aqueous formaldehyde solution in H 2 O and 284 g of paraformaldehyde are introduced into a reaction vessel. The aldehyde / phenol molar ratio = 0.87. The reaction system is heated to reflux at 100 ° C. under a nitrogen atmosphere and the condensation reaction is continued until the concentration values of formaldehyde tend to zero. The duration of the reaction is 5 hours.

The reaction mixture is distilled under reduced pressure (20 mmHg) up to 160 ° C. with the same operating parameters as in Example 1. The distillate containing H20, CF3COOH and the unreacted ortho-cresol is introduced into a rectification device, as in the previous examples, in order to recover the catalyst. This is recovered quantitatively.

The reaction mass is subjected to another steam purification, until a value of less than 0.5% by weight of free ortho-cresol is obtained. 2820 g of novolak resin are thus obtained. This novolak resin has the following characteristics:

- melting point (capillary) “65-68 ° C

- free ortho-cresol = 0.1% by weight - pH (25 ° C) (50% solution in ethanol) - = 6 ± 0.5 - treatment time at 150 ° C and 8% hexamine = 15 minutes

Claims (9)

1. Process for the preparation of phenolic resins or novolak resins starting from a monohydroxylated or polyhydroxylated aromatic compound and, in particular, from a phenol or a higher homologue thereof, and from a compound containing at least an aldehyde function and, in particular, of a lower aldehyde (that is to say in CJ-C4) and preferably formaldehyde in the presence of an acid catalyst, said process being characterized in that this catalyst is a organic acid having the general formula FCXY-COOH (I) in which X and Y, which may be either equal or different from each other, represent H or F. 2. The method of claim 1, wherein the organic acid has a pKa value at 25 ° C in the range of 0.1 to 3 and a boiling point in the range of 60 to 160 ° C. 3. Method according to any one of the preceding claims, in which the organic acid is chosen from the group consisting of monofluoroacetic acid, difluoroacetic acid and trifluoroacetic acid. 4. The method of claim 3, wherein the organic acid is trifluoroacetic acid. 5. Method according to any one of the preceding claims, in which the acid catalyst is used in amounts ranging from 0.001 to 10% by weight relative to the reaction mass, in order to obtain a pH value included in the range from 0.2 to about 2. 6. Method according to claim 5, characterized in that trifluoroacetic acid is used in amounts ranging from 0.005% to 4% by weight relative to the reaction mass. 7. Method according to any one of the preceding claims, in which the molar ratio of the aldehyde compound to the hydroxylated aromatic compound is less than or equal to 1 and is preferably in the range from 1: 1 to 0.01: 1 . 8. A process according to any one of the preceding claims, wherein the reaction is carried out at a temperature in the range of 10 ° C to 100 ° C and preferably in the range of 20 ° C to 90 ° C under reflux conditions. 9. Phenolic and novolak resins, in particular bisphenol P, obtained according to the process according to any one of the preceding claims 1 to 8.