Process for the Manufacture of Porous Solids Consisting of Crosslinked Urea-Formaldehyde Polycondensation Products

The present invention provides a process for the manufacture of porous solids having an internal surface area greater than 10 m.sup.2 /g of crosslinked urea-formaldehyde polycondensation products, which is characterised in that an aqueous solution or urea and formaldehyde, or of a precondensate of urea and formaldehyde, is adjusted to a pH value of 0 to 4 by adding a Bronsted acid having a dissociation constant of not less than 1.times.10.sup.-.sup.6, so that an insoluble and infusible precipitate consisting of polymeric condensation product is formed, with the molar ratio of formaldehyde:urea being at least 0.5 and at most 1.2, preferably at least 0.5 and at most 1.0, not later than at the time of precipitation, and that the resulting precipitate is separated off and dried.

As examples of Bronsted acids having a dissociation constant .gtoreq.10.sup.-.sup.6 there may be mentioned: formic acid, acetic acid, chloracetic acids, acrylic acid, oxalic acid, malonic acid, maleic acid, tartaric acid, citric acid, sulphurous acid, nitric acid, phosphoric acid, sulphuric acid and hydrochloric acid. The strong acids, such as nitric acid, sulphuric acid and hydrochloric acid can also be employed in the form of their ammonium salts or amine salts or in the form of mixtures of these salts with the free acids.

Particularly suitable substances are sulphamic acid (amindosulphonic acid H.sub.2 N--SO.sub.3 H), and also water-soluble ammonium hydrogen sulphates of general formula

wherein R denotes a hydrogen atom or an organic radical which does not impair the solubility in water, such as especially an alkyl, cycloalkyl, hydroxyalkyl, aralkyl or aryl radical.

Examples of such optionally substituted ammonium hydrogen sulphates of formula (I) are, in addition to NH.sub.4 .sup.+. HSO.sub.4 .sup.-, CH.sub.3 --NH.sub.3 .sup.+. HSO.sub.4 .sup.-; C.sub.2 H.sub.5 NH.sub.3 .sup.+. HSO.sub.4 .sup.-; HO--CH.sub.2 CH.sub.2 --NH.sub.3 .sup.+. HSO.sub.4 ; ##SPC1##

Where the basicity of the amine component is too great, some excess of strong acid e.g. sulphuric acid is appropriately added to the ammonium salt solution. The formation of the insoluble precipitate of polymeric formaldehyde-urea condensation product can be carried out in the presence of surface-active substances (detergents) or of protective colloids. In some cases these result in an increase in the specific surface area. The surface-active substance can be either ionic or non-ionic. As ionic representatives of this class of substances there may for example be mentioned: cationic quaternary ammonium bases or anionic fatty alcohol sulphates or sulphonates, alkylarylsulphonates, salts of sulphosuccinic acid esters, and monoglyceride sulphates, and as non-ionic representatives polyglycol ethers of fatty alcohols or alkylphenols may for example be mentioned. Protective colloids are, in this context, understood as water-soluble macromolecular substances which greatly increase the viscosity of aqueous solutions. Typical representatives of this class of compound are the sodium salt of carboxymethylcellulose, methyl-, ethyl- and .beta.-hydroxyethyl-cellulose, polyvinyl alcohol, water-soluble polymers and copolymers of acrylic acid or of methacrylic acid. The concentrations at which surface-active substances and/or protective colloids display their best effect depend on the chemical structure and molecular weight of the substances. They are generally active in amounts of between 0.1 and 10% by weight, preferably between 0.5 and 5% by weight, relative to the weight of urea and formaldehyde.

Numerous embodiments of the process according to the invention can be carried out: appropriately, an aqueous solution which contains urea and formaldehyde in the requisite molar ratio or contains a precondensate of the two components, is mixed with the aqueous acid solution at temperatures of between room temperature and 100.degree.C. The aqueous solution of the precondensate is as a rule prepared in the pH range of between 6 and 9 and in the temperature range of between 40.degree. and 100.degree.C. The reaction time should appropriately be so long that the bulk of the formaldehyde (about 90%) has the opportunity to react with the urea, but not so long that the compatibility with water of the precondensate becomes so low that it can no longer be homogeneously mixed with the acid solution. Relatively high temperatures and relatively low pH values lead to the desired degree of polycondensation within a shorter time. If desired, a precondensate of urea and less than the total amount of CH.sub.2 O employed can first be manufactured, and the remaining quantity of formaldehyde only added during the subsequent precipitation.

The surface-active substance and/or the protective colloid can be present in one of the two aqueous solutions, or in both aqueous solutions. Acid-sensitive substances are better added to the urea-formaldehyde solution. The two solutions can be mixed by adding one of the solutions dropwise to the other whilst stirring. It is however, also possible rapidly to mix the two solutions with one another and to leave them standing at an elevated temperature to continue reacting.

For most end uses it is advantageous, but not essential, to neutralize the polymer precipitate. NaOH, KOH, Ca(OH).sub.2 and aqueous ammonia are suitable for this purpose. Washing of the polymeric precipitate is in most cases not necessary. The drying is best carried out in a hot stream of air. In the case of short contact times, very high drying temperatures (up to 500.degree.C) can be used. For some end uses it is advantageous to comminute the dried process product. Pin mills, hammer mills, cross-beater mills, air jet mills and ball mills are suitable for this purpose. The products of the process do not consist of non-porous but of porous more or less agglomerated individual particles. The size of these agglomerates is determined by the mechanical comminution process. The size of the individual particles is not significantly less than 1 micron. Since the specific surface area is up to 300 m.sup.2 /g the bulk part of the surface is the internal surface area, which exists in the form of pores, grooves and internal cavities. The formaldehyde to urea ratio in the final products will vary with the ratio employed in the process. When the ratio employed in the process is 0.5 the ratio in the final product is 0.8. As the ratio utilized in the process increases up to 1.0 the ratio in the final product increases up to 1.0.

As used herein the term "insoluble" means that the products are not soluble in the conventional organic polymer solvents such as for example alcohols, ethers, ketones, hydrocarbons and the like. The products of the invention melt only under conditions of decomposition and are thus designated herein as "infusible".

Because of this structure and the magnitude of the internal surface area, these polymers are suitable for use as adsorbents, for example for off gas and effluent purification. They are furthermore suitable for use as white pigments.

In the examples which follow, parts denote parts by weight and percentages denote percentages by weight.

500 parts of 30% strength aqueous formaldehyde solution are adjusted to pH=7 with dilute sodium hydroxide solution and warmed to 50.degree.C. 300 parts of urea are added and condensation is carried out for 3 hours at pH=7 and 50.degree.C.

The resulting solution of the precondensate is rapidly added dropwise, whilst stirring, to a solution of the following composition, warmed to 70.degree.C:

16 parts of sulphamic acid,

50 parts of aqueous 30% strength formaldehyde solution, and

1705 parts of deionised water.

During the dropwise addition of the solution of the precondensate, a polymer precipitate forms. At the end of the dropwise addition, the pH value is 1.5. The mixture is allowed to continue to react for 6 hours at 70.degree.C whilst stirring and is neutralized with a 10% strength aqueous Na.sub.2 CO.sub.3 solution in a stirred flask, and the product is centrifuged off, washed with water, dried for 24 hours at 80.degree.C and ground for 2 hours in a stoneware ball mill.

353 parts (corresponding to a yield of 1.17 kg per 1 kg of urea employed) of a white free-flowing powder with a specific surface area of 118 m.sup.2 /g are obtained. The mean agglomerate size of the product is about 13 microns, the molar ratio F/U in the product =1.05.

Exactly the same procedure as in Example 1 is followed except that the solution of the precondensate is added dropwise to an aqueous solution which, in addition to 16 parts of sulphamic acid, 50 parts of aqueous 30% strength CH.sub.2 O and 1705 parts of deionized water, also contains 9 parts of one of the surface-active substances or protective colloids indicated in Table 1 below.

Yields and properties of the free-flowing powders obtained as end products are recorded in Table 1 below:

A precondensate solution, which is manufactured according to the data of Example 1, is added dropwise whilst stirring, at 70.degree.C, to a solution of 25 parts of sulphuric acid in 1425 parts of water. A precipitate immediately forms. When all the precondensate has been introduced, the pH value is 1.2. After continuing to react for 6 hours at 70.degree.C, the mixture is cooled to room temperature and rendered neutral, and the product is filtered off, dried for 24 hours at 120.degree. in a stream of air and ground in a high speed pin mill.

352 parts (corresponding to a yield of 1.17 kg per 1 kg of urea employed) of a white, free-flowing powder with the following properties are obtained: Specific surface area 252 m.sup.2 /g Bulk density: 273 g/liter Volatile constituents: 6.9% F/U: 0.98.

A solution of the following composition is prepared:

The pH is adjusted to 7 with aqueous sodium hydroxide solution and condensation is carried out for 3 hours at pH and 70.degree.C.

A solution of 88.5 parts of oxalic acid dihydrate in 160 parts of water is added dropwise whilst stirring, to the resulting precondensate solution. A polymer precipitate forms. The subsequent reaction and working up take place exactly as in Example 7.

418 parts (corresponding to a yield of 0.99 kg per 1 kg of urea employed) of a white, free-flowing powder with the following properties are obtained:

The following series of experiments shows the influence of the molecular ratio of CH.sub.2 O : urea in the operative range according to the invention on the specific surface area of the end products; the batches for the precondensate solutions, the composition of which can be seen from Table 2 below, are in each case condensed for 3 hours at 70.degree.C and pH=7. Each precondensate solution is rapidly mixed with a solution of 14.55 parts of sulphamic acid in 283.5 parts of water at 50.degree.C. In the course thereof, the temperature rises to 65.degree.-70.degree.C. The reaction mixture is left for 3 hours at this temperature, stirred up with water, and neutralized to pH=7.5 with 10% strength aqueous sodium carbonate solution, and the product is filtered off, dried at 120.degree.C and ground on a pin mill (3 passes). The batches, yields and properties of the end products are indicated in Table 2 below:

The following series of experiments shows the influence of the concentration of the polymer-forming agents during the precipitation on the properties of the porous urea-formaldehyde polymers. The batches for the precondensate solutions, the composition of which can be seen from the Table below, are in each case condensed for 3 hours at 70.degree.C and pH=7.

Each individual precondensate is mixed, at 50.degree.C, with a solution of 16 parts of sulphamic acid in 100 parts of H.sub.2 O. The subsequent reaction and working up of the reaction mixture takes place exactly as described for Examples 9 - 14. The material is however, only ground once in the pin mill. The batches, yields and properties of the end products are given in Table 3 below:

This Example describes a test according to the invention. The molar ratio of formaldehyde to urea in the reaction mixture is 1.0. However, the reaction condition does not lead to an optimum of the specific surface area because the product is obtained according to an Aminoplast-R-type process (U.S. Pat. No. 3,553,115). That means monobasic monoethanolamine phosphates are present in the reaction mixture.

A powder with a specific surface area of 22.9 m.sup.2 /g is obtained.

This is an Example according to the invention using a molar ratio F/U of 1.0 and 2 % of Na-carboxymethylcellulose as a protective colloid in the reaction mixture. A powder with a specific surface area of 283 m.sup.2 /g is obtained.