IE44587B1 - Process for the manufacture of urea-formaldehyde polycondensates - Google Patents

Abstract

SHORT DESCRIPTION The invention relates to a manufacturing method decondensats urea-formaldehyde. It consists of a) treating the Ebul-lition and at a pH between 4 and 5.5 for 10 to 60 minutesune aqueous combination of formaldehyde and urea whose rapportmolaire F / U is between 4 and 5.5 and the content matièresactives between 60 and 85% and b) after neutralization of mélangeréactionnel, to add urea to obtain an F / Ucompris between 2 and 3.5, c) then treating the reaction mixture-nel to boiling and has a pH between 4 and 6 until thatthe viscosity at 20 ° C of a test sample adjusted to 60% extraitsec is between 8 and 20 poises, and d) after coo-dissement neutralization and, optionally bringing the condensate to a solids value at most equal to 65%. The condensatsainsi obtained have a very high storage stability, even at high ambient temperatures.

Description

The present invention relates to the manufacture of urea-formaldehyde polycondensates intended for gluing and impregnating lignocellulpsic materials.

The search for a sufficiently long period of preser5 vation for condensation products of urea and formaldehyde is one of the major problems posed by the use of these thermosensitive resins.

There already exist very stable urea-formaldehyde precondensates, of which the processes of manufacture generally depending on the absorption of gaseous formaldehyde in aqueous solutions of urea are described, for example in French Patent No. 1,214,996 and German Patent No. 1,239,290. These precondensates are stable at ambient temperature for several months and can be easily stored.

Other processes of manufacture (see, for example German Patent No. 1,168,882) and numerous modifications of these products (see, for example French Patent No. 1,487,980 and German Patent No. 2,334,280) have been described.

As their name indicates, these precondensates are only one stage in the production of glues and binders, since a later condensation in acid medium and in the presence of a complement of urea is necessary in all cases in order to attain a sufficient degree of condensation and thus obtain adequate reactivity. In fact, since these urea-formalde25 hyde precondensates o f commerce usually have a high formaldehyde/urea molar ratio (from 2.5 : 1 to 10 : 1, mostly 5:1), a certain quantity of urea must be added to them before the condensation in acid medium. This acid conden44587 - 3 sation may be followed, after neutralisation, by a second addition of urea so as to obtain the desired formaldehyde/ urea molar ratio.

If the stability of the precondensates of urea and formaldehyde is excellent, that of the condensates, i.e. of the final resins (ready for use), is generally very poor having regard to the requirement of reactivity which the resins must satisfy before ev ’rything. The stability of the condensates does not exceed 4 months at ambient temperature (22-28°C), 25 days at 40°C and 8 days at 50°C. When the condensates are made in countries with a temperate climate such as those of Northern Europe, their period preservation is generally sufficient to permit them to be supplied safely to market geographically near. The same cannot be said however for the hot countries.

With the conventional processes of synthesis, an appreciable increase in the stability of the resins can in general only be attained (for example, by the diminution of the degree of condensation or the addition of buffer salts) at the cost of an appreciable reduction in reactivity. Only drying by spraying of the aqueous solutions of urea-formaldehyde resins and their presentation in pulverulent form confers on the glues a distinctly increased period of preservation. However, the particular technology of this process leads to much more costly products than the glues in the form of solutions.

By means of the present invention it is possible to obtain urea-formaldehyde condensates with a formaldehyde/ urea molar ratio from 2:1 to 3:5:1 which show a very great stability on storage, even at particularly high ambient o temperatures e.g. at 40-50 C. 587 - 4 According to the present invention therefore a process for the manufacture of a urea formaldehyde condensate having a formaldehyde/urea molar ratio of 2:1 to 3:5:1 is provided which comprises: (a) refluxing at a pH from 4 to 5.5, preferably from 4.5 to 5.2, for a period from 10 minutes to one hour, preferably from 20 to 40 minutes, an aqueous combination of formaldehyde and urea of which the molar ratio of formaldehyde/urea is from 4.1 to 5.5:1 and the content of IO active materials (urea + formaldehyde) is from 60% to 85% by weight, preferably from 70% to 85%, then (b) after neutralisation of the reaction mixture, adding urea thereto so as to obtain a formaldehyde/urea molar ratio from 2:1 to 3.5:1 preferably from 2:1 to 3:1, (c) refluxing the reaction mixture at a pH from 4 to 6, preferably from 4.5 to 5.5, until the viscosity at 20°C of the reaction mixture, determined on a sample adjusted to 60% of solids content, is from 8 to 20 poises, preferably from 10 to 15 poises, and finally (d) cooling and neutralising the product and, if its solid content exceeds 65%, bringing it to a value at most equal to 65%, and preferably less than or equal to 60%.

The aqueous combination of formaldehyde and urea, used in the first stage of the process of the invention, may be obtained for example, according to the known processes by simple solution at 90-95°C of urea in formalin Or by heating a mixture of paraformaldehyde, water and urea at the boil and at pH 7-9 until complete solution has taken place. One can also use a commercial urea30 formaldehyde precondensate of which the formaldehyde/urea molar ratio can, if necessary, previously be taken to 4:1 to 5.5:1 by solution of urea at 9O-95°C.

The initial content of active materials (the sum of the theoretical percentages by weight of solid urea and 1OO% formaldehyde) in the aqueous combination of formaldehyde and urea, used in the first stage of the process of the invention, can vary from 60% to 85%. However, compounds of high content (70% to 85%) of active materials are preferred. These high contents may be easily obtained when a commercial precondensate is used or when the initial combination is prepared from paraformaldehyde or from a formalin of high formaldehyde concentration (e.g. equal to or greater than 50%).

The refluxing at a pH from 4 to 6, according to the third stage of the process of the invention, is interrupted when the viscosity at 20°C of a sample adjusted to 60% of solids content is from 8 to 20 poises. The solids content (content of dry materials) is conventionally determined by drying 1 g of product for 3 hours in an oven at 120°C.

By way of indication, the range of 8 to 20 poises for a solids content of 60% corresponds approximately to a range of 20 to 80 poises for a solids content of 65% and of 50 to 150 poises for a solids content of 70%.

Analysis by exclusion chromatography of the condensates obtained according to the process of the invention shows that their degree of condensation is considerably higher than that of the usual commercial condensates.

The mean molecular mass by weight of the condensates according to the invention, determined by exclusion chromatography in dimethylformamide, is 10 to 20 times that of the condensates usually used in the manufacture of plywood and particle boards.

The condensates according to the invention show a very great stability to storage, this stability is from 10 - 6 days to 3 months at 50°C, from 20 days to 6 months at 40°C and more than 6 months at ambient temperature (22-28°C).

One can therefore envisage quite safely transporting the condensates of the invention to places of use far from the place of production. The stability of the condensates of the invention can be increased still further by adding to them a customary stabilising agent such as for example sodium formate, triethanolamine formate, sodium phosphate, magnesium carbonate, or preferably borax, in an amount less than 1% by weight, more particularly from 0.1% to 0.5% by weight, of condensate.

In order to use the condensates obtained by the process of the invention it is sufficient to add to them a variable quantity of urea, either solid or in solution. The binders thus obtained can then be applied with the same facility as the conventional resins in solution or in powder form and confer on the final material excellent mechanical properties, often superior to those obtained with the conventional resins.

According to the type of application envisaged for the condensate (e.g. making particle board, gluing of plywood, foam gluing or impregnation), the usual adjuvants and the quantity of urea necessary to obtain the desired quality (generally from 0% to 20% of urea) are added at the place of use. The urea-formaldehyde condensates of high stability of the invention are universal in the sense that they can be applied directly according to this method to the gluing of plywood, to the production of the particle boards or to the impregnation of paper in place of the aqueous urea-formaldehyde condensates specific to one or other of the applications mentioned.

The invention is illustrated by the following Examples - 7 44587 in which the parts and percentages indicated are by weight.

Example 1 77.1 moles of formaldehyde in the form of 95% paraformaldehyde are progressively introduced at 60°C and at pH 7 into an aqueous 60% solution of urea containing 19.25 moles of urea, and then the mixture is heated under reflux for about 20 minutes.

The clear reaction medium is then acidified to pH 5 by the addition of formic acid and left under reflux for 30 minutes. After neutralisation by means of a 40% solution of sodium hydroxide and cooling, 15.75 moles of urea are added and the pH is taken to 5.8 by the addition of formic acid. The mixture is brought to reflux and maintained at this temperature until the viscosity of the undiluted mixture at 20°C is 115 poises. The mixture is cooled and neutralised with a 40% solution of sodium hydroxide. The solids content of the product thus obtained is equal to 70.8% and it is brought to about 60% by dilution with water.

The physicochemical characteristics of the condensate thus obtained are as follows*Molar ratio formaldehyde/urea 2.2; 1 Solids content 60.1% Viscosity at 20°C 11.5 poises Reactivity at 20°C 34 minutes Stability at 50°C 24 days Stability at 40°C 70 days Stability at ambient temperature (22°C-28°C) more than 6 months By reactivity at 20°C is meant the time taken at 20°C for a mixture containing 100 parts of condensate and 0.7 parts of ammonium chloride to reach the state of a gel. - 8 The time necessary for a sample of condensate maintained at a given temperature to reach the state of a gel is called stability.

The molar ratio of formaldehyde/urea is determined 5 analytically by the usual methods (Kjeldahl and hydrolysis of the formaldehyde).

Example 2 The starting product used is a commercial precondensate of formaldehyde/urea with a molar ratio equal to 4.79:1 and containing 54.9% of formaldehyde and 22.9% of urea. 6000 g of this precondensate (approximately 109.8 moles of formaldehyde and 22.9 moles of urea) are acidified to pH 4.8 by the addition of 6 ml of concentrated formic acid and then the mixture is refluxed for half an hour.

Then it is neutralised to pH 7.2-7.5 by means of a 40% solution of sodium hydroxide, 1540 g of urea (25.6 moles) are added, and the mixture is taken to reflux and the pH is adjusted to 5.2-5.4 by means of formic acid. When the viscosity at 50°C of the reaction mixture (not diluted) is 12 poises, the condensation is stopped by rapid cooling, the product is neutralised by means of a 40% solution of sodium hydroxide and the solids content is brought to about 60% by the addition of water.

The physicochemical characteristics of the condensate thus obtained are as follows: Molar ratio of formaldehyde/urea 2.25:1 Solids content 60.1% Viscosity at 20°C -- 13.8 poises Reactivity at 20°C 27 minutes Stability at 50°C 15 days Stability at 40°C 60 days Stability at ambient temperature (22°C-28°C) more than 7 months 4458? - 9 Example 3 The operation is as in the preceding Example, except that in order to bring the solids content of the product to 60% a solution of about 3% of borax is used so that the content of borax in the condensate is 0.3%.

The physicochemical characteristics of the condensate are therefore the following: Molar ratio of formaldehyde/urea 2.25:1 Solids content 60.0% 0 Viscosity at 20 C 14.0 poises Reactivity at 20°C 30 minutes Stability at 50°C 50 days Stability at 40°C 102 days Stability at ambient temperature (22°C-28°C) more than 7 months Example 4 The starting substance used is a commercial precondensate of formaldehyde/urea molar ratio equal to 4.76:1 and containing 54.1% of formaldehyde and 22.8% of urea. 3000 g of this precondensate (approximately 54.1 moles of formaldehyde and 11.4 moles of urea) are acidified to pH 4.8 by means of formic acid and then the mixture is refluxed for 30 minutes.

After neutralisation by means of a 40% solution of sodium hydroxide, 760 g (approximately 12.6 moles) of urea and 475 g of water are added, the mixture is taken to reflux and the pH is adjusted to 5.2 by means of formic acid. When the viscosity at 20°C is 13 poises, it is cooled, neutralised by means of a 40% solution of sodium hydroxide and 0.3% of borax is added.

The characteristics of the condensate thus obtained are as follows: Molar ratio o,f formaldehyde/urea 2.25:1 Solids content 60.2% Viscosity at 20°C 17 poises Reactivity at 20°C 19 minutes Stability at 50°C 11 days Stability at 40°C 29 days Stability at ambient temperature (22°G-28°C) 6 months Example 5 The operation is at first as in the second paragraph of Example 4. After neutralisation, 398 g of urea (approximately 6.6 moles) are added, the mixture is taken to reflux and the pH is adjusted to 4.5 by means of 6N hydrochloric acid. o When the viscosity of the undiluted mixture at 20 C . reaches 40 poises, the mixture is cooled, neutralised by means of a 40% solution of sodium hydroxide, the solids content is brought to about 60% by the addition of water and 0.5% of borax is added.

The physicochemical characteristics of the condensate thus obtained are as follows: Molar ratio of formaldehyde/urea 2.98:1 Solids content 60.2% Viscosity at 20°C 12.3 poises Reactivity at 20°C 20 minutes Stability at 50°C 83 days Stability at 40°C 170 days Stability at ambient temperature (22°C-28 C) more than 8 months Example 6 To 100 parts of the stable condensate described in Example 3 are added 13 parts of urea and 9 parts of water. - 11 By means of the binder thus obtained, a glue (A) is prepared containing the following ingredients.

Binder 100 parts 22° Be ammonia 2 parts 70% emulsion of paraffin 7 parts Ammonium chloride ο,θ parts Water in amount sufficient to take the viscosity to a value between 150 and 180 cP at 20°C By means of this glue and a mixture of shavings of gaboon wood and hard woods single-layered particle boards are made under the following conditions: Thickness: about 18.5 mm Humidity of the shavings: 3% glue: 9% (dry/dry) baking: 5 minutes at 175°C under a maximum pressure of 25 kgfcm Density: about 0.650 The following Table tabulates the physicomeehanical characteristics (standard DIN 68-761) of the panels thus obtained, comparatively to those of analogous panels obtained by means of a glue (B) prepared like the glue (A) but replacing the 100 parts of binder by 100 parts of a conventional commercial urea-formaldehyde condensate such as is subsequently described.

Table 1 Physicomechanical characteristics of the panels Glue A Glue B Density 0.640 0.643 Thickness (in mm) 18.6 18.6 Swelling after 24 hours immersion in water at 20°C 8% 7.8% Absorption after 24 hours immersion in water at 20 C 41.2% 42.3% _2 Bending, flexion (in kgfcm ) 165 133 Sag (in mm) 5.5 5 Resistance to transverse pulling away (in kgfcm-2) 12.5 11.2 Swelling after 3 hours in water at 70°C 28.0% 27.9% Resistance to transverse pulling away (in kgfcm-2) after 3 hours in water at 70°C 1.4 1.4 The commercial condensate used for the preparation of the glue (B) may be obtained in the following way: A mixture of 21,2 moles of formaldehyde in the form of neutralised 44% formalin and 10 moles of urea is brought to the boil and maintained there for 10 minutes. After cooling to 90°C, the mixture is acidified to pH 5.0-5.2 by means of formic acid, then condensed at the boil until there is obtained a tolerance to water at 1O°C equal to 2.3/1, i.e. when a mixture of 1 volume of resin and 2.3 volumes of water clouds at 10°C. It is neutralised by means of sodium hydroxide, then concentrated under reduced pressure so as to eliminate 22.8 moles of water; the viscosity at 20°C of the product is then 10-12 poises. After cooling to ambient temperature, 3.7 moles of urea are added.

The final characteristics of the condensate are as follows: Molar ratio of formaldehyde/urea Solids content Viscosity at 20 C Reactivity at 20°C Stability at 50°C Stability at 40°C Stability at ambignt θ temperature (22 C-28 c) 1.54:1 64.7% 7.2 poises more than 8 hours days 18 days 105 days Example 7 7.1 parts of urea and 20 parts of water are added to 100 parts of the stable condensate described in Example 3. By means of the binder thus obtained, a glue (C) was prepared containing the following ingredients: Binder 100 parts Rye flour 40 parts Ammonium chloride 0.9 parts Water in amount sufficient to regulate the viscosity o to 40 poises at 20 C.

By means of this glue, plywood is made under the following conditions: Plywood with 5 plies of gaboon 20/10 mm at 5-6% humidity _2 glue: 400 gm double face baking: 10 minutes at 11O°C under a maximum pressure _2 of 12 kgfcm The control of the plywood is effected according to the British Standard 1455 or 1956 (knife test). The grades given from 1 to 10 (10 being the best) represent an average of 3 tests. 44881 The following table tabulates the results obtained with the glue (C) compared with a glue (D) prepared like glue (C) but replacing the 100 parts of binder by 100 parts of a conventional commercial condensate described hereafter.

Table 2 Average Grade Glue C Glue D On dry plywood 7.9 7.7 After 72 hours in cold water 2.9 2.6 After 72 hours in water at 70°C 2.5 1.5 The commercial condensate used for the preparation of the glue (D) may be prepared in the following way: A mixture of 16.2 moles of formaldehyde in the form of 30% formalin and 8 moles of urea is brought to the boil and maintained there for 10 minutes. After cooling to 85°C, the reaction mixture is taken to pH 4.5 by the addition of formic acid and condensed at the boil until the tolerance of the resin to water at 10°C reaches 0.5/1. After neutralisation, and cooling, the product is concentrated under reduced pressure at 50°C until the viscosity at 20°C is about 30 poises.

The final characteristics of the condensate are as follows: Molar ratio of formaldehyde/urea 1.77:1 Solids content 64.9% Viscosity at 20°C 27 poises o Reactivity at 20 C 45 minutes Stability at 50°C 2 days Stability at 40°C 13 days Stability at ambient temperature (22°C-28°C) 70 days

Claims (14)

1. A process for the manufacture of a urea-formaldehyde condensate having a formaldehyde/urea molar ratio of 2:1 to 3.5:1 which comprises: (a) refluxing at a pH from 4 to 5.5 for a period from 10 minutes to one hour . an aqueous combination of formaldehyde and urea of which the molar ratio formaldehyde/urea is from 4:1 to 5.5:1 and the content of active materials is from 60% to 85% by weight, then (b) after neutralisation of the reaction mixture, adding urea thereto so as to obtain a formaldehyde/urea molar ratio of 2:1 to 3.5:1, (c) then refluxing the reaction mixture at a pH from 4 to 6 until the viscosity at 20°C of the reaction mixture, determined on a sample adjusted to 60% of solids content is from 8 to 20 poises, and finally (d) cooling and neutralising the product and, if its solids content exceeds 65%, bringing it to a value at most equal to 65%.

2. A process according to claim 1 in which the pH in stage (a) is 4.5 to 5.2.

3. A process according to claim 1 or 2 in which the refluxing treatment in stage (a) is for 20 to 40 minutes.

4. A process according to claim 1, 2 or 3 in which the content of active materials in stage (a) is from 70% to 85% by weight.

5. A process according to any of claims 1 to 4 in which the amount of urea added in stage (b) is such that the formaldehyde/urea molar ratio is from 2:1 to 3:1.

6. A process according to any of claims 1 to 5 in which the pH in stage (c) is from 4.5 to 5.5. 4 4 5 8 7 lb

7. A procetis according to any of claimn 1 to b in which the refluxing treatment in stage (e) is effected until the viseosity at 20°C for 60% of solids content is from 10 to 15 poises. 5

8. A process according to any of claims 1 to 7 in which the solids content of the condensate is brought to a value less than or equal to 60%.

9. A process according to any of claims 1 to 8 in which a stabilising agent is added to the condensate.

10. 10. A process according to claim 9 in which the stabilising agent is borax.

11. A process according to claim 9 or 10 in which the amount of stabilising agent is less than 1% by weight of condensate. 15

12. A process according to claim 11 in which the amount is from 0.1% to 0.5% by weight of condensate.

13. Process for the manufacture of a urea-formaldehyde condensate having a formaldehyde/urea molar ratio of 2:1 to 3.5:1 substantially as Herein described with reference to and as illustrated in any of the Examples 1-5.

14. ·. Urea/formaldehyde condensates wherever obtained according to a process as claimed in any of claims 1 to 13.