DE1221014B - Process for the production of paraformaldehyde - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

C08g

German class: 39 c-18

Number: 1221014

File number: M 57501IV d / 39 c

Filing date: July 15, 1963

Opening day: 14th Wed 1966

It is known that the polymerization of formaldehyde by suitable alteration of the pH value in a sufficiently concentrated ordinary maintained at 5O ⁰ C aqueous formaldehyde solution can be set in motion.

The formation of polyoxymethylenes and their precipitation from solution is just like the ratio from the amount of precipitated formaldehyde to the initial amount of formaldehyde, through various Working conditions influenced, in particular by:

1. the initial formaldehyde concentration of the

aqueous solution,
. 2. the pH value,

3. the crystallization temperature.

It is also known that

1. The higher the initial concentration of the aqueous formaldehyde solution, the higher it is Conversion of formaldehyde to polymer. This is of course limited because of the difficulty Concentrate formaldehyde solutions beyond a given level.

2. A polymerization can either be achieved by lowering the pH value below 2 or by increasing it over 5 can be achieved.

In practice, the pH is appropriately regulated between 8 and 10. Higher pH levels are effective, but not beneficial, as they promote the Cannizzaro response.

3. The lower the crystallization temperature, the higher the polymer yield. Man must, however, bear in mind that polymers which are formed at temperatures below 25 ° C, contain a high proportion of low molecular weight products, their ointment-like consistency centrifugation and any subsequent treatments are very difficult. To products with interesting properties, it is stated in the literature that the following Working conditions are required:

a) presence of large amounts of methanol;

b) Crystallization temperatures between 25 and 40 ° C in the presence of a solid phase (Polyoxymethylene), which in the ratio of 0.25 to 0.75 with respect to the formaldehyde is added to the solution;

c) Crystallization at a higher temperature (e.g. 60 to 70 ° C).

It has now been found that readily water-soluble paraformaldehyde can be obtained by using an aqueous process for the production of paraformaldehyde

Applicant:

MONTECATINI Societä Generale per l'Industria Mineraria e Chimica, Milan (Italy)

Representative:

Dr. Dr. J. Reitstötter and Dr.-Ing. W. Bunte,
Patent Attorneys, Munich 15, Haydnstr. 5

Named as inventor:

Ibrahim Dakli,

Nazareno Lupi,

Valentino Zamboni,

Avio D'Emilio, Milan (Italy)

Claimed priority:
Italy of July 17, 1962 (14 410),
dated June 28, 1963 (36 627)

igen formaldehyde solution can be prepared by means of alkali to a pH value of 8 to 10, cooling the alkaline formaldehyde solution and isolating the solid product obtained, if the aqueous formaldehyde solution, if necessary after concentration under vacuum to a CH ₂ O content above 45%, Compounds of the general formula

R ₁ -C

^S C

Ra

added, in which R _{1 is} a hydrogen atom, an alkyl, aryl, partially or fully hydrogenated aryl or alkenylamino radical, R ₂ and R _{3 are} identical or different and are amino or alkenylamino radicals.

Known measures can be used for isolation, either by centrifugation or dries on a chilled metal surface.

The paraformaldehyde produced according to the invention

can either be in the form of a crystalline powder

609 589/329

that allows easy centrifugation and drying (even at working temperatures below 25 ° C), or in the form of flakes that are easy to handle and to are transporting.

These two forms are identical from a chemical and physicochemical point of view.

The only difference lies in the macroscopic physical form of the compounds: The Flakes are coherent crystalline aggregates.

The paraformaldehyde produced according to the invention shows particularly interesting properties, such as Complete solubility in water immediately after its production (formaldehyde content over 80%) and even after prolonged storage at room temperature (at least for a few months) or even after drying (formaldehyde content over 96%). This property is related to the proportional low molecular weight (average degree of polymerization = 15 to 70).

The stabilizers used in the process of the present invention are believed to act as a Molecular weight regulators by reducing the kinetic energies of chain growth. Using benzoguanamine presumably grows at least four polyoxymethylene chains on the benzoguanamine molecule. Analog growth was used for the others according to the invention Stabilizers proven.

According to the experimental evidence, a large part of the polyoxymethylene chains contain the benzoguanamine chemically bound.

This was due to the change in the infrared spectrum and the solubility values of benzoguanamine and its methylol derivative and further demonstrated by the fact that the concentration of the benzoguanamine groups in paraformaldehyde after purification and crystallization not changed.

During centrifugation to separate the paraformaldehyde obtained as a crystalline powder an aqueous solution with a formaldehyde content of 36 percent by weight is used as the mother liquor, which is also noteworthy that this mother liquor even when stored at low temperature seems to be stable (e.g. at -10 ° C for some Days). It can therefore be used directly for various technical applications or after Removal of the alkali can be fed back into the process.

To carry out the process in practice, it is advisable to proceed as follows:

The concentrated formaldehyde solution is made alkaline by adding a 30% NaOH solution up to a pH value between 8.0 and 10.0.

Slow and gradual cooling of the alkaline formaldehyde solution with a CH ₂ O content = 45% and crystallization while stirring at a temperature between 10 and 40 ° C (preferably at about 20 ° C). The solution is held in the crystallization stage for a period of 30 minutes to 20 hours or even longer, depending on the temperature used.

Filtering the resulting pulp in a centrifuge in order to reduce the residual water content of the crystals to a minimum value, preferably below 20%, while treating the pulp or the centrifuged crystals with dilute solutions of weak organic acids to neutralize the NaOH excess.
Treatment of the deposited in the centrifuge. Mother liquor with a cation exchange resin to remove the amount of NaOH present.

If you want the paraformaldehyde in the form of flakes from the aqueous formaldehyde solutions obtained using the stabilizers which can be used according to the invention, the following procedure is expediently carried out before:

Use of a formaldehyde solution (e.g. up to 65 percent by weight) as the starting product.
Concentrate at a reduced pressure of preferably 50 to 200 mm Hg to a formaldehyde content of at least 80 percent by weight.

The concentrated formaldehyde solution is made alkaline by adding a 30% NaOH solution up to a pH value between 8.0 and 10.0.

Cooling the solution on a chilled metal surface (chilled with water or other Media) over a different period of time, preferably for 1 to 3 minutes. the The layer thickness of the product can vary, preferably it is 0.5 to 2 mm. The cooling one Surface can consist of a rotating drum or a metal belt, which is moved at variable speeds or from another apparatus which is in the causes a cooling in the manner described above.

Scrape off the cooled product and flake it with a suitable knife.
You can work continuously, semicontinuously or batchwise according to the two procedures described above; you always get a paraformaldehyde in finely divided crystalline powder or flake form, free of ointment-like products and easily completely soluble in water at temperatures above 50 ° C, with an extremely low number of average molecular weights of the polymoxymethylene chains, calculated on the basis of the number of terminal hydroxyl groups (between 450 and 2100), and a formaldehyde content above 75% and preferably 80%.

The excellent solubility properties of the products are also retained when they are subjected to a drying process up to a content of non-chemically bound water below 1 to 2% (CH ₂ O content greater than 96%).

The paraformaldehyde produced according to the invention can also be used to produce alcoholic formaldehyde solutions can be used for products for which lower water content is preferred (Varnishes, finishes, etc.) are used.

The properties of the paraformaldehyde prepared according to the present invention were determined by measuring the following Checked parameters:

1. Formaldehyde content (determined using the sulfite method on dissolved crystal). The crystals always show a formaldehyde content of over 80% after centrifugation.

2. Average degree of polymerization (average number of monomeric units that form the polyoxymethylene chains).

This is determined either from the content of terminal hydroxyl groups according to the infrared spectrographic Procedure or determined from the chemical formaldehyde titer.

These determinations are made on the paraformaldehyde dehydrated by washing with acetone carried out.

The determination of the terminal hydroxyl groups is based on the determination of the infrared absorption at 2.92 μ due to the CH ₂ OH groups, which absorption is measured on a suspension of the crystals in paraffin oil.

The formaldehyde content of the washed crystals is determined by iodometric analysis.

The calculation of the average degree of polymerization based on the formaldehyde content is carried out according to Staudinger, whereby, apart from the presence of chemically bound Water, the presence of bound benzoguanamine, determined by means of the UV spectrophotometric, is taken into account.

3. Solubility in water.

The surprising water solubility of the product prepared according to the invention is evident from the

ίο values shown in the table when compared with the values determined on commercially available paraformaldehyde samples. A paraformaldehyde (designated PF. F , a dry product with more than 93% CH ₂ O) and a paraformaldehyde (designated

with PF. S containing about 80% CH ₂ O) is used.

The values refer to solutions that go through

Add 20 parts by weight of the product to 100 parts of deionized water at neutral pH without Buffer addition can be obtained.

sample

Average
Degree of polymerization

50 ⁰ C 60 minutes 30 minutes 98 88 98 90 99 94 99 95 57 38 10 6th

Dissolved percentage

60 ° C. 30 minutes 60 minutes 92 99 95 99 97 99 98 99 56 90 n.b. n.b.

8O ⁰ C

30 minutes I 60 minutes

n. 1
n.4
n.12
n.30
PF. S.

PF. F.

n. b. = not determined

50
30th
20th

25 to 30
160 to 180

The solubility of the samples is virtually complete: A 20% solution of the untreated product has at neutral pH and at 5O ⁰ C in equilibrium no more than 0.5% insoluble (in relation to the product) residue.

Even after several months of storage of the product at room temperature, the insoluble residue of solutions under the same conditions is not greater than 1 ⁰ Io. If you work at higher temperatures or pH values, the insoluble residue disappears completely.

In Fig. 1 is the solubility of the various paraformaldehyde samples in deionized water at 50 ° C reproduced with a concentration of 15 percent by weight. The time in minutes is open the abscissa and the percentage of undissolved formaldehyde on the ordinate in logarithmic Plotted on a scale (where currently zero is the percentage of undissolved paraformaldehyde 100% was set).

The paraformaldehyde samples prepared with PF. F and PF. S are designated, are commercial products, which have already been described with reference to the table, while those with PFI and PF samples designated 12 represent those of paraformaldehyde, which were obtained in Examples 1 and 4 of the present description.

In Fig. 2 shows the solubility of the same samples of paraformaldehyde in a buffer at pH 5.75 at 50 ° C. with a concentration of 15 percent by weight. In the coordination system, those are already included in F i g. 1 listed values are entered. The dotted line (d) shows the zone of soluble-98
99
99
99
65

approximately
33

Paraformaldehyde according to the present invention.

Since the dissolution of paraformaldehyde in water, in the case of dilute solutions, a hydrolysis reaction the hemiacetal bond according to the following scheme:

HO - (CH ₂ - O) _n - CH ₂ OH + H ₂ O

-> CH ₂ (OH) ₂ + HO - (CH ₂ O) _n _! - CH ₂ OH

is, this can be viewed as a first-order reaction, albeit in a heterogeneous phase.

In fact, FIG. 1 that the solubility values, namely the proportion of hydrolyzed paraformaldehyde, is a logarithmic function of contact time.

The constant of this first-order reaction therefore becomes a function for the same concentration be the number of moles, d. i.e., it becomes with a decrease in the average molecular weight (the degree of polymerization) decrease, as is confirmed by the values in FIG. 1 "(Lobering, Ber., 69, p. 1844, 1936).

It is obvious that the products obtained according to the invention always fall within the area defined by the following limits: (c = 100 Vo, t = 00, (C = 5%, t = 600 that means that they always have a surprising rate of dissolution even without Have alkalization.

The rate of dissolution depends on the pH of the solution; at a pH of about 4.5 the acetal bond has the greatest stability against hydrolysis, so that it is expedient to use the product to dissolve at least at neutral pH.

The product according to the invention shows better dissolution properties than commercially available products, even at pH values at which the hydrolysis constant is very low (pH 5.75), as shown in FIG. 2 emerges.

The paraformaldehyde prepared according to the invention contains no or only minimal amounts of low molecular weight polyoxymethylene glycol and therefore differs from the products obtained in the absence of stabilizers. The amount of such unstable low molecular weight polyoxymethylene glycols of low melting point was determined by extraction with acetone at 50 to 60 ^° C .; it is less than 1 to 2% (Staudinger, aad, p. 248).

4. Stabilizer content.

The stabilizers used in the process according to the invention are used in amounts from 0.01 to 5 percent by weight added.

5. Thermal stability.

The paraformaldehyde obtained according to the invention has, despite its low number average molecular weight, a stability against thermal decomposition at 100 ° C, which is similar to that of high molecular weight Products is.

The products produced according to the invention have after dehydration by washing with acetone erne depolymerization rate at 100 ° C in a stream of air of about 1 to 1.5% per minute.

The following examples illustrate the invention.

Unless otherwise stated, the parts given in the examples are parts by weight.

example 1

2000 g of a 36% aqueous formaldehyde solution were placed in a 3-1 flask fitted with a stirrer, thermometer and reflux condenser filled.

The solution was gradually heated to 50 ° C. with stirring.

Thereupon 3.0 g of benzoguanamine crystals were added and the above working conditions during sustained for about 16 hours.

Thereafter, the solution was concentrated under reduced pressure (50 to 100 mm Hg) to give about 1200 g of a stabilized aqueous formaldehyde solution having a CH ₂ O content of about 55%.

At this point in time, the pH was raised by suitable addition of NaOH (30% solution) 9.0 set.

The solution was then stirred slowly, so that a temperature drop of 55 ° C to 20 ° C was obtained within about 90 minutes, cooled and then the resulting broth during kept at a temperature of 20 ° C for a further 4 or 5 hours.

The crystallized paraformaldehyde was then deposited; 445 g were obtained in this way a solid with a formaldehyde content of about 83% and 775 g of a mother liquor with a Formaldehyde content of about 38%.

The paraformaldehyde obtained had a number average molecular weight of about 800 and rapid and complete water solubility (95% of the product was in. at pH 7.0 and 30 ° C 30 minutes solved).

Example 2

2000 g of a 36% aqueous formaldehyde solution were under reduced pressure (50 to 100 mm Hg) concentrated, whereby about 1200 g of a 55% formaldehyde solution were obtained, kept at 55 ° C and with 3.0 g of benzoguanamine crystals was moved.

The solution treated in this way was then kept at 60 ° C. for 1 hour. After adjusting the pH with NaOH, cooling and centrifuging as described in Example 1, 400 g of paraformaldehyde with a CH ₂ O content of about 83% and 800 g of mother liquors with a CH ₂ O content of about 38.4 were obtained % obtain.

The paraformaldehyde obtained confirms the molecular weight values and the properties of solubility in water, as found in the crystalline product of had been found in the above example.

Example 3

2000 g of a 36% strength aqueous formaldehyde solution were used in those described in Example 1 Filled in the flask and heated to 50 ° C. with stirring.

Then 3.0 g of benzoguanamine crystals were added and the working conditions described above maintained for about 16 hours.

The solution was then concentrated under reduced pressure (50 to 100 mm Hg) to give about 1200 g of a stabilized aqueous formaldehyde solution having a CH ₂ O content of about 55%.

The pH of the solution was adjusted to about 9.0 by the appropriate addition of NaOH (30% solution) set.

The solution was then cooled to 20 ° C. with stirring and held at this temperature for 1 hour. The crystallized paraformaldehyde was then separated off by centrifugation: 309 g of a solid with a CH ₂ O content of about 86% and 861 g of mother liquors with a CH ₂ O content of about 41% were obtained in this way.

The paraformaldehyde thus obtained had a number average molecular weight of about 1100 and a water solubility similar to that of the crystals of Examples 1 and 2.

Example 4

2000 kg of a 36% strength aqueous formaldehyde solution were introduced into a 3 m ³ autoclave equipped with an anchor stirrer, temperature control and reflux condenser. The solution was gradually heated to 50 ° C. and then, after adding 3 kg of benzoguanamine crystals, left to stand at this temperature for 12 hours.

The solution was then under reduced pressure to a formaldehyde content of about 56% concentrated.

The pH was adjusted to 9.0 by adding a 30% strength NaOH solution and the solution as described in Example 1, cooled.

Centrifugation of the pulp gave: 465 kg of paraformaldehyde with a CH ₂ O content

of 84.2%, 755 kg mother liquors with a CH ₂ O content of 38.9 ^fl / o.

The paraformaldehyde had properties similar to those of the laboratory products, had a number average molecular weight of about 1,000 and quickly and completely dissolved (95% dissolved in 30 to 40 minutes at 50 ° C).

Example 5

556 g of water with NaOH (0.4 parts per 100 parts of solution) alkalized to pH 9.0 were filled into a 2 l flask which was immersed in a thermostatic bath and was provided with a thermometer, stirrer and condenser.

The thermostatic bath was heated to 80 ° C. and 434 g of paraformaldehyde (obtained according to Example 1 with a CH ₂ O content = 83%) were introduced into the flask with stirring.

Example 6

2000 g of a 36% aqueous formaldehyde solution were placed in a 3-1 flask equipped with a stirrer, thermometer and reflux condenser filled.

The solution was stirred and gradually heated to 50 ° C

While maintaining the working conditions for about 16 hours, 6.0 g of diallyl melamine crystals were obtained added.

The solution was then concentrated under reduced pressure (50 to 100 mm Hg), whereby about 1200 g of a stabilized 55% strength aqueous formaldehyde solution were obtained.

At this point in time, the pH was adjusted by adding NaOH (30% solution) set to about 9.0.

The solution was then cooled with slow stirring so that a uniform temperature decrease took place from 55 to 20 ° C within 90 minutes and the resulting slurry during further Maintained at 20 ° C. for 4 hours.

The crystallized paraformaldehyde was then separated off by centrifugation: 370 g of a solid with a CH ₂ O content of 85% and 790 g of mother liquors with a CH ₂ O content of 36.6% were obtained in this way.

The paraformaldehyde thus obtained had a number average molecular weight of about 1100 and a water solubility similar to that of the product described in Example 1.

Example 7

Under the working conditions of Example 6, 2000 g of a 36% aqueous formaldehyde solution in a flask at 50 ° C heated and kept after addition of 6.0 g Tetrahydrobenzoguanaminkristallen during about 16 hours at 50 ⁰ C.

After concentration under reduced pressure to a CH ₂ O content of 55%, the pH was adjusted with NaOH and the solution was cooled and centrifuged as described in the previous examples.

In this way, 490 g of paraformaldehyde with a CH ₂ O content of about 80% and a CH ₂ O content were obtained

g mother liquors with a
about 36%.

The paraformaldehyde thus obtained, having an average molecular weight of about 1150 confirmed the water solubility properties of those described in the previous examples Products.

Example 8

A 36% formaldehyde solution containing 0.12% benzoguanamine is used in a multi-stage evaporator concentrated to a formaldehyde concentration of 85 percent by weight.

The concentration is carried out with a residual pressure of 100 mm Hg.

To the resulting solution 0.2 0/00 NaOH are added at 90 ⁰ C.

It is then on a metal surface cooled with running water for 100 to 120 Seconds to cool.

The product was 1mm thick and crystallized very quickly, becoming brittle and showing a tendency to peel itself off the cold surface. It was then brought into flake form by means of suitable knives. The flakes contained about 10% water and can be used as such or dried by conventional methods.
The paraformaldehyde flakes thus obtained can be easily transported and used: they have an optimal solubility in that they dissolve in water at 50 ° C in 30 minutes with a concentration of 93% and in 1 hour with a concentration of 100%.

40

45

Claims (1)

Claim: Process for the preparation of paraformaldehyde in solid form by adjusting an aqueous formaldehyde solution by means of alkali to a pH value of 8 to 10, cooling the alkaline formaldehyde solution and isolating the solid product obtained, characterized in that the aqueous formaldehyde solution, optionally after concentration under vacuum to a CH ₂ O content above 45%, compounds of the general formula , N. R ₁ -C CR ₂ * N are added, in which R _{1 is} a hydrogen atom, an alkyl, aryl, partially or fully hydrogenated aryl or alkenylamino radical, R ₂ and R _{3 are} identical or different and are amino or alkyl amino radicals. Considered publications:
French patent specification No. 1228 672. 1 sheet of drawings 609 589/329 7.66 © Bundesdruckerei Berlin