GB2239252A - Catalytic process for preparing phenolic resins or novolac resin - Google Patents

Abstract

A catalytic process for preparing phenolic resins or of novolac resins, comprises the condensation of aromatic hydroxy compounds and compounds containing aldehyde functions, in particular condensation of phenol and formaldehyde, in the presence of an organic acidic catalyst having the general formula FCXY - COOH wherein X and Y, which can be either the same as or different from each other, represent an H or F atom, preferably trifluoroacetic acid. Particular resins prepared include bisphenol F resins.

Description

1 1 CATALYTIC PROCESS FOR PREPARING PHENOLIC RESINS OR NOVOLAC RESINS The

present invention relates to a catalytic process for preparing phenolic resins or novolac resins.

In particular, the present invention relates to a catalytic process for preparing phenolic resins of novolac type by means of the catalytic reaction of a phenol or an aromatic hydroxy compound in general, with an aldehyde or a compound containing aldehyde groups.

Still more precisely, the object of the present invention is the preparation of low-molecular-weight novolac resins, in particular F bisphenol, or 4,41methylene-bis-phenol resins.

The so obtained products can be defined in general as "novolacs",, and constitute one of the most important classes of phenolic resins, which have a large number of important applications at the industrial level. Among these, the following can be cited: their use as basic intermediates for the synthesis of epoxy systems, their application inthe field of metal casting, as thermoplastic moulding powders, in abrasive and

2 refractory systems, for the production of sound damping materials, and so forth.

Other applications, well-known to those skilled in the art, are in the fields of wood composite materials (plywood, and the like), friction materials for brakes, adhesives, phenolic fibres, and so on.

There are a large number of literature references on the production of phenolic resins, either of resolic type or of novolac type. As regards the latter, production of e.g. phenolic resins of novolac type by reaction of phenol with formaldehyde, catalysed by a strong acid of organic or inorganic nature, in an aqueous medium, is known.

Inasmuch as the reaction rate is substantially a function of the hydrogen ion potential in the aqueous medium, strong acids are particularly suitable as catalytic systems. Usually, strong acids such as hydrochloric acid, phosphoric acid, sulphuric acid, oxalic acid or p-toluenesulphonic acid are used; the reactions are carried out at temperatures within the range from room temperature to 100C.

In this way, novolac resins are obtained, the molecular weigl-kt of which varies as a function of the 0 Z1 F 3 operating parameters, such as the molar ratio of formaldehyde to phenol in the starting reaction mixture (U.S. pat. No. 4,400,554), the reaction time, the temperature, and so forth.

However, although such catalytic systems are effective from a kinetic viewpoint, their use involves a number of drawbacks due to nature of the catalytic system, which are often quite difficult to resolve.

For example, although hydrogen chloride is easily available due to its low cost, it can only be used in equipment which is suitably resistant to its high corrosivity index. Furthermore, if formaldehyde and HCl are both present in the gas phase at concentrations higher than 100 ppm, they can generate 1,1-dichloromethyl-ether, which is known to be a highly toxic compound.

Sulphuric acid and phosphoric acid are rather effective, however they easily lead to the formation of very intensely coloured compounds which reduce the quality of the end product. Furthermore, if even small amounts of them remain in the resin, they will catalyze a disproportionation reaction, generating large amounts of free phenol, which is undesirable in the end resin [see Hoyt H. et al "Paint Varnish Production,' 48, 13 -.h 4 Due to its-considerable acidity combined with low cost and an easy availability on the market, oxalic acid is one of the most widely used catalysts. The data reported in technical literature demonstrates that it tends to sublime easily at about 10bC under reduced pressure, with no decomposition. However, laboratory test results show that the removal of the oxalic acid from the reaction medium is not really simple, and can cause problems of cloudiness in the final epoxy resin. Furthermore, as with the other catalysts listed above, oxalic acid catalyzes disproportionation reactions leading to the generation of free phenol in the end resin. Furthermore, the reaction of phenol and formaldehyde in the presence of atmospheric oxygen can generate small amounts of coloured products belonging to the class of aurins, which have demonstrated to be critical in subsequent processing steps.

From the result of these observations it derives that, as the acidic catalyst systems cannot be easily removed from the reaction medium after the catalysis step, the use of these systems is nearly always combined with a neutralization step using alkali, or other agents, such as ionexchange resins, basic absorbing earths, and so on. Such a step is carried out at the end of the condensation reaction and before starting the distillation of the excess phenol, in order to prevent 1 0 1 dealkylation problems due to the presence of acid traces in the end novolac product.

Although it is a rather delicate operation, particularly when the reaction medium is of aqueous-organic character, the neutralization step removes the acidity from the system but may endanger the same quality of the novolac resin, due to the formation of insoluble inorganic salts which form a precipitate and are responsible for the phenolic resin not having a perfectly glossy appearance and for an increase of viscosity which can be observed in the same product over time.

Furthermore, the presence of these inorganic salts in a purely organic matrix can cause technical drawbacks during the processing step, because the so formed salt can not always be easily removed from the reaction medium by means of a simple filtration step.

A further problem derives from the neutralization operation which, if not carefully carried out, may cause alcalic colour changing of the novolac resin and consequently the formation of coloured species and chemical modifications of the product itself.

This problem has also been partially solved by using solvents to extract the resin from the reaction medium, 0 6 t but this process has proved to be a very complex one, with regard particularly to the large reaction volumes, and the recovery of the solvent from the extracted resin.

The Applicant has now discovered that the above reported drawbacks and disadvantages which affect the prior art technology can be overcome if the reaction between the phenolic compounds or aromatic mono-hydroxy or poly-hydroxy compounds in general and formaldehyde or the compounds containing one or more aldehyde groups in general is carried out in the presence of a catalyst comprising an organic, strong, volatile acid which can be removed from the reaction medium without undergoing decomposition and which can therefore be recycled.

The process according to the present invention is therefore highly ecologically acceptable and makes it possible to prepare phenolic resins of novolac type which are free from any traces of acid in the system and from undesired inorganic salts, avoiding thereby the neutralization and the solvent extraction steps.

The present invention provides a process for preparing phenolic resins or novolac resins from a mono-hydroxy or poly-hydroxy aromatic compound, in particular a phenol or a higher homologue thereof, and a compound containing at least one aldehyde group, in 0 7 7 7 particular a lower aldehyde, preferably formaldehyde, in the presence of an acidic catalyst, wherein said catalyst is an organic acid having the general formula (I):

FUY-COOH (I) in which: X and Y, which can either be the same as or different from each other, represent an H or F atom.

Acids having the general formula (I) used as catalysts according to the present invention have been shown to be particularly suitable because besides meeting the necessary conditions for effectively catalysing the phenol-formaldehyde condensation reaction, they display the following further physical and chemical features:

(a) their boiling points under atmospheric pressure are equal to, or lower than, 170'C, which values allow them to be easily and completely removed from the reaction medium by direct evaporation-volatilization; (b) they have complete chemical insertness towards the reaction system, so that secondary reactions do not take place; (c) they show a considerable chemical insertness towards oxidising or reducing agents, and therefore do not give rise to degradation phenomena.

0 8 Acids falling within the scope of the above defined formula (I), include the following for example:

monofluoroacetic acid: CH 2 FCOOH; difluoroacetic acid: CHF 2 COOH; trifluoroacetic acid: W 3 COOH; the boiling points of which are within the range from 60 to 160C and the pKa values of which, at 25C are within the range from 0.1 to 3.

Due to the particularly high stability and volatility, trifluoroacetic acid is preferred for both economic and operative reasons.

Trifluoroacetic is liquid under normal conditions, and is totally miscible in water.

For exemplifying purposes, in the following table the main physical and chemical characteristics of trifluoroacetic acid are given.

Appearance at 25C: Specific gravity at 25'C: Boiling point at 760 mm Hg: Melting point: Vapour presIsure at 25C:

clear liquid 1.484 g/cm 3 71.8C -15. 4C 107 mm Hg 0 I-L 9 Viscosity at 25'C: pKa value at 25C: pH, in solution at a concentration of 11.4 g/litre of H 2 0 and at 20C: Boiling point of the azeotrople, mixture with H, at 760 mm Hg:

0. 813 cP 0. 23 1 105. 5 C (20. 6% of H 2 0) The acid catalyst according to the present invention is used in such amounts as to secure the usual acid catalytic conditions necessary for the condensation reaction to take place, normally giving pH values within the range from about 0.2 to about 2, according to the acids used. Compositions having from 0.001 to 10% by weight of acid having the formula (I), relative to the reaction mass, will be sufficient.

When trifluoroacetic acid is used, the amount thereof is preferably within the range from 0.005% to 4% by weight relative to the reaction mass; larger amounts do not give any additional advantages as regards the characteristics of the product, or the reaction speed.

When operating within said ranges, no corrosion phenomena are observed as regards the materials customarily used for building the industrial operating facilities.

0 PC As disclosed hereinabove, the catalysts according to the present invention are used in a process for the preparation of phenolic resins of novolac type, by means of a condensation reaction of a phenol compound with an aldehyde compound. The above said tondensation reaction is itself known, and need not be particularly illustrated herein, in that it is widely described in relevant technical literature references.

0 Phenolic resins, or novolac resins are generally obtained by means of the reaction of an aromatic monohydroxy or poly-hydroxy compound, preferably a phenol or a higher homologue thereof, with an aldehyde, particularly a lower (i.e., C 1-C 4) aldehyde, preferably formaldehyde, or with a compound containing one or more aldehyde groups.

The reaction concerned is the well-known condensation reaction, which proceeds according to the mechanism of electrophilic substitution on the phenolic ring, widely described in technical literature, and which can be schematically represented as follows:

t 11 1 H-CHO + H(+) H2 -C() OH OH OH H2-C(+)OH + H 0 H CH2 OH OH 0 OH CH2 OH + H(+) OH OH 0 + CH2 CH20H (bisphenol F) The so obtained bisphenol F can optionally be reacted with one or more molecules of methylol-derivative in order to form novolacs having higher moleculaIr weight values, according to the following reaction scheme:

0 12 0 CH2 + n 0 > 6-_6 OH OH CH2 OH > 0 CH2 - 6- OH OH OH (III) -n wherein in n is a number within the range from 1 to 10.

Among these compounds, bisphenol F novolac is preferred.

The molar ratio of the compound containing at least one aldehyde group to the aromatic hydroxy compound is generally lower than or equal to 1, and is preferably w'thin the range from 1:1 to 0.01:1,. according to the i operating conditions of temperature, starting materials, 13 1 and in particular according to the desired type of end compound.

The reaction temperature is usually within the range from 10C to 100'C, and is preferably within the range from 20C to about 90'C, under refluxing conditions.

When the process is carried out within the above said condition ranges, reaction times within the range from 0.5 to 10 hours are generally sufficient. The reaction is carried out under an insert gas atmosphere, for example nitrogen, argon, and so forth, and under atmospheric or superatmospheric pressure.

At the end of the reaction, the catalyst according to the present invention can be easily separated from the reaction mass by distillation, rectification, and so forth, according to conventional techniques; it can be then recovered and recycled together with the unreacted phenolic compound. Accordingly, the present invention has ecological advantages.

According to the present invention condensation reactions of aromatic mono-hydroxy or polyhydroxy compounds, in particular phenols or higher homologues thereof, with aldehydes or compounds containing one or more aldehyde functional groups, can be catalyzed in 0 v 14 1 order to obtain the corresponding phenolic resins or novolacs.

Among the aromatic mono-hydroxy or poly-hydroxy compounds, the following can be cited: phenols and their upper homologues comprising, besides phenol, such compounds as cresols, xylenols, possibly substituted with other alkyl groups, such as p-tert.-butyl-phenol, nonyl-phenol, and so forth; the diphenols, resorcinols, bisphenol A (also 'Ibis-All), i. e., [2, 2-bis(4-hydroxyphenyl)-propane], p-phenylphenol, and so forth.

Among the aldehydes, or compounds containing aldehyde groups, those which contain from 1 to 4 carbon atoms are preferred; for example formaldehyde, acetaldehyde, furfuryl aldehyde, paraformaldehyde, polyaldehydes, such as glyoxal, and so forth.

According to the preferred operating procedure, the process according to the present invention is carried out by reacting phenol or polyphenolsand formaldehyde or polyaldehydes in a molar ratio of aldehyde to phenol within the range from 1:1 to 0.01:1, in an aqueous medium, in the presence of the acidic catalyst of formula (I), by operating at a temperature within the range from 20 to 100'C, under nitrogen at atmospheric pressure with the reaction system being kept undet 0 A'- 1 1 1 refluxing conditions, in order to reduce any possible losses of volatile substances.

The system is maintained at constant temperature throughout the condensation with the concentration of free aldehyde in the reaction system being periodically checked.. When concentration of aldehyde approaches zero, the condensation reaction can be regarded as complete.

At the end of the reaction, the distillation of the catalyst is carried out in aqueous-organic solution, which is then rectified, so as to concentrate the solution of the acid in the aqueous phase.

The latter operation-of rectification can be carried out under reduced pressures, and at the required temperature for the separation of acidwater from the reaction mixture.

The distillation residue contains the phenolic resin and a small amount of free phenolic compound, which can be suitably removed by means of a steam stripping.

After removing the last traces of phenol, the pH of the reaction product, measured in a 50% ethyl alcohol solution is within the range from 5.9 to 6.5.

1 0 fli 1 rL- i i 1 i i 16 The acidic catalyst of formula (I) is totally recovered from the distillation and can be recycled for subsequent processing steps, or it can be removed from the system by means of the use of ion-exchange resins or by means of a distillation step with the proper separation effectiveness.

The invention will be further described by way of example with reference to the following examples. In no way shall said examples be construed as being limitative of the invention.

Exampl e 1 4,700 g (50 mol) of phenol and 305 g of H 2 0 are charged into a reaction vessel; the temperature is then increased up to WC, with the mass being kept under nitrogen at atmospheric pressure.

When a homogeneous solution is obtained, 1.5 g of W 3 COOH at 99% is added.

The pH of the solution is 0.3. Then, 60 g of a 50% solution of formaldehyde in H 2 0 (i.e. about 1 mol of formaldehyde) is added. The molar ratio of phenol:formaldehyde in the reaction volume is 1: 0.02. At time zero of, the condensation, the composition of the reaction mixture is as follows:

0 X 17 1 phenol:

formaldehyde H 2 0:

W 3 COOH 92.22 % by weight 0.59 % by weight 6.55 % by weight 0.029% by weight The system is refluxed at 95-100C. After approximately 50 minutes, the concentration of formaldehyde is lower than 0.02% and the condensation is regarded as terminated.

The distillation process is then started. Said distillation is carried out at first at the same reaction temperature; then the pressure is gradually. decreased down to a residual value of 1 torr, and the temperature is increased to 160C. In this way, CP 3 COOH, H 2 0 and the excess of phenol are removed.

The distillation residue is submitted to steam stripping in order to remove any residual traces of free phenol.

g of novolac is obtained, which contains 93.2% of bisphenol F, and displays the following physical and chemical characteristics:

1 1J9 18 1 Melting point (capillary) pH (50% solution in ethanol) Gardner colour (50% solution in ethanol) Appearance (50% solution in ethanol) Colour H 2 0 Free phenol = 80-85C = 5. 9 1 = glossy liquid = white = 0.7% by weight = 0.5% by weight The distilled fraction is submitted to a rectification using a tray column. The composition of the overhead fraction still contains 1.5 g of CP 3-COOH, corresponding to the amount charged into the reaction vessel.

Example 2

4,000 g (50 mol) of phenol, 365 g of H 2 0, 18 g of W 3 COOH at 99% and 116.9 g of a 36% solution of formaldehyde in H 2 0 are charged into a reaction vessel.

The molar ratio of phenol:formaldehyde is = 1 0.033; and the content of W 3 COOH = 0.4% by weight The process is carried out in the same way as in Example 1. 225 g of novolac is obtained, which contains 0 fk, 19 1 91.7% of bisphenol F having the following physical and chemical characteristics:

Melting point (capillary); pH (50% solution in ethanol) Gardner colour (50% solution in ethanol): Appearance (50% solution in ethanol) colour H 2 0 Free phenol = 78-82C = 5. 9 = 1 = glossy liquid = white = 0.6% by weight = 0.4% by weight From the analysis of the distilled fraction, after rectification according to Example 1, W 3 -COOH quantitatively recovered.

Example 3

1980 g of phenol, 2780 g of H 2 0, 25 g of W 3 COOH (at 98%) and 140 g of a 36% aqueous formaldehyde solution are charged into a reaction vessel.

Molar ratio of phenol:formaldehyde % by weight of W 3 COOH = 0.50 0. 08 The process is carried out in the same way as.in Example 1. Aftr a 90-minute reaction time, 260 g of 0 P4-1 A-S-- novolac is obtained, which contains 86.1% of bisphenol F having the following physical and chemical characteristics:

pH (50% solution in ethanol) 5.8 Gardner colour (50% solution in ethanol) Appearance (50% solution in ethanol) H 2 0 Free phenol = glossy liquid = 0.8% by weight = 0.3% by weight At the end of condensation, the system is cooled down to room temperature.

Following cooling, the reaction mixture separates into two well-distinct phases: the underlying organic phase, which is discharged, and the aqueous supernatant phase which, besides a certain amount of phenol, contains 21 g of W 3 COOH, which correspond to 84% of the total amount initially charged into the reaction.

The previously discharged organic phase is distilled, as described in Example 1. The distillate obtained has two well separated phases, an organic phase and an aqueous phase, in which distillate the residual 16% of W 3 COOH charged into the reactor is distributed as f ol l ows.

0 1 21 Aqueous phase from the distillate = 0.40% of W 3 COOH (2.5 g) Organic phase from the distillate = 0.15% of W 3 COOH (1.0 g) 0 Thus, it can be observed that W 3 COOH is totally extracted from the resin; the quantitative recovery thereof renders it available for subsequent catalysis.

Example 4

1500 g of phenol and 18 g of W 3 COOH are charged into a reaction vessel and the temperature is increased up to 100C. 231 g of a 40% aqueous solution of glyoxal is introduced into the reaction vessel during 60 minutes.

Molar ratio of glyoxal: phenol = 0.1 After 6 hours reaction time, H 2 0, W 3 COOH and free phenol are distilled from the reaction mixture and are charged to a rectification column in which they are submitted to the same treatment as in the preceding Examples. The last traces of free phenol are removed from the reaction mixture by steam stripping. 350 g of resin are obtained and W 3 COOH is totally recovered from the distillation/rectification step, as disclosed in Example 1.

9 22 1 ExamiDl e 5 2650 g of ortho-cresol, 212 9 of H 2 0, 34 g of CP 3 COOH, 986 g of a 36% solution of formaldehyde in H 2 0 and 284 g of paraformaldehyde are charged to a reaction vessel. The"molar ratio of-aldehyde: phenol Is = 0. 87: 1. The reaction system is refluxed at 100C under a nitrogen atmosphere and the condensation reaction is continued until formaldehyde concentration values tending to zero are obtained. The reaction time is 5 hours.

The reaction mixture is distilled under reduced pressure (20 mm Hg) and at a temperature up to 160C, in the same way as Example 1. The distillate, containing H 2 0, W 3 COOH and unreacted ortho-cresol, is charged to a rectification system as in the preeding Examples, in order to recover the catalyst. The catalyst is quantitatively recovered.

The reaction mass is submitted to a further steam stripping until less than 0.5% by weight of free ortho-cresol Is obtained. 2,820 g of novolac resin are obtained. The novolac resin shows the following characteristics:

4 f t 23 Melting point (capillary) Free ortho-cresol: pH (25C) (50% solution in ethanol): Cure time at 150C and 8% of hexamine:

= 65-68C = 0.1% by weight = 6 + 0. 5 0 = 15 minutes The present Invention has been described above purely by way of example, and modifications may be made within the scope of the invention 1 1 f 24 C L A 1 M S 1. A process for preparing phenolic resins or novolac resins from a monohydroxy or poly-hydroxy aromatic compound, in particular a phenol or a higher homologue thereof, and a compound containing at least one aldehyde group, in particular a lower aldehyde, preferably formaldehyde, in the presence of an acidic catalyst, wherein said catalyst in an organic acid having the general formula MXY-COOH i n whi ch: X and Y, which can be either the same as or different from each other, represent an H or P atom.

(I) 2. A process according to claim 1, In which the organic acid has a pKa value at WC within the range from 0.1 to 3 and a boiling point within the range from 60 to 160C.

3. A process according to claim 1 or 2, in which the organic acid Is selected from monofluoroacetic acid; difluoroacetic acid or trifluoroacetic acid.

4. A process according to claim 3, in which the organic acid is trifluoroacetic acid.

0 It 5. A process according to any of the preceding claims, In which from 0. 001 to 10% by weight relative to the reaction mass of the acidic catalyst is used in order to obtain a pH value within t.he range from about 0.2 to about 2.

6. A process according to claim 5, in which from 0.005% to 4% by weight relative to the reaction mass of trifluoroacetic acid is used.

7. A process according to any of the preceding claims, in which the compound containing at least one aldehyde group is a C 1-C 4 aldehyde.

8. A process according to any of the preceding claims, in which the molar ratio of the compound containing at least one aldehyde group to the aromatic hydroxy compound is lower than or equal to 1:1, and is preferably within the range from 1: 1 to 0.01: 1.

9. A process according to any of the preceding claims, In which the reaction is carried out at a temperature within the range from 10C to 100C, and preferably within the range from 20C to WC, under refluxing conditions.

1 0 At, 26 01 i 10. Phenolic and novolac resins, in particular bisphenol F, obtained according to the process according to any of the preceding claims.

11. A process for preparing phenolic or novolac resins substantially as herein described with reference to examples 1 to 5.

Published 1991 at The Patent Office. State House. 66171 High Holborn. UndonWC1R41?. Further copies my be obtained from Sales Branch. Unit 6. Nine Mile PbinL Cwmfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray. Kent.