Process for the preparation of aminoplastic resins having very low formaldehyde emission rates

Abstract

The preparation of aminoplastic resins containing melamine, having very low rates of formaldehyde emission, and being particularly useful for the production of particle boards. In a second stage of the process for preparing the resin, additional NH.sub.2 (melamine and urea) is added to a conventional condensate solution at an increased pH between 6 and 9 so as to lower the F/NH.sub.2 molar ratio to between 0.5 and 0.9. Urea is added in a third stage to obtain an F/NH.sub.2 of between 0.30 and 0.50, with the optional addition of borax.

Description

BACKGROUND OF THE INVENTION

The present invention concerns a new process for the production of aminoplastic resins having very low formaldehyde emission rates. Its subject is more particularly a process for the production of urea-formaldehyde melamine "doped" resins which give off very little formaldehyde.

Urea-formaldehyde resins are known products which are widely used in the wood industry in particular for the production of particle boards. They are produced in a known manner by the condensation of urea and formaldehyde at pH between 4 and 7 and at a temperature near boiling point: this condensation reaction is preferably carried out in several stages.

The main drawback associated with urea-formaldehyde resins is that they give off large amounts of free formaldehyde. Attempts have been made to reduce the free formaldehyde content using various production processes: unfortunately, when particularly low formaldehyde contents are aimed at, experience shows that this objective is accompanied both by a decrease in reactivity and stability of the resins, and by deterioration of the mechanical properties of the finished boards. In order to avoid the presence of free formaldehyde, it has also been suggested that formaldehyde-free resins should be used, in particular resins based on isocyanate solutions. Unfortunately isocyanates are liable to pose toxicological problems.

Another way of limiting formaldehyde emissions consists in adding substances that fix this compound. Such substances would for instance include urea, melamine or phenol, or else high-protein flours. Unfortunately all processes that involve adding these formaldehyde-fixing additives to urea-formaldehyde resins have troublesome secondary effects, in particular, they retard the final condensation and hence the hardening of the glues.

There is therefore a need for the development of urea-formaldehyde resins having good stability and lower rates of emission of free formaldehyde, and whose utilization yields finished particle boards with good mechanical characteristics.

SUMMARY OF THE INVENTION

The present invention concerns a process for the production of urea-formaldehyde resin containing melamine characterized in that it comprises three stages, in that the melamine is added either at the first stage, or at the second, or at both first and second stages in quantities between 1 and 10 % relative to the weight of the finished liquid resin and in that at the first stage urea, formaldehyde and optionally melamine are condensed in solution at pH between 4.5 and 6 in quantities such that the F/NH.sub.2 molar ratio at this stage is between 0.9 and 1.5, preferably between 0.95 and 1.15 and in that, at the second stage, more urea and optionally melamine are added, in quantities such that the resin's F/NH.sub.2 ratio at this stage is between 0.5 and 0.9, this stage being conducted at pH between 6 and 9 and preferably between 7 and 8 and at a temperature between 70.degree. and 100.degree. C., the resulting resin being optionally subjected to vacuum distillation, then in the third stage, after cooling, and at a temperature below 50.degree. C., having a quantity of urea added such that the resin's final F/NH.sub.2 molar ratio is between 0.30 and 0.50; borax in a quantity up to a maximum of 0.5% by weight relative to the weight of finished liquid resin being optionally added.

It was found that operating under these conditions yielded urea-formaldehyde resins that were stable during storage having good viscosities enabling good adhesive-bonding. These urea-formaldehyde resins enable particle boards to be produced having lower free formaldehyde emission rates allied with satisfactory mechanical properties.

The process of the present application comprises condensing urea and formaldehyde in a first stage at pH between 4.5 and 6 in quantities such that the F/NH.sub.2 molar ratio at this stage is between 0.9 and 1.5 and preferably between 0.95 and 1.15.

In a known manner, urea and formaldehyde precondensates may be used at this stage instead of formaldehyde alone. This stage is conducted in a known manner at the reflux temperature, say at a temperature above 85.degree. C.

The second stage, object of the process of the invention, consists in adding more urea and melamine in quantities such that the resin's F/NH.sub.2 ratio at this stage is between 0.5 and 0.9. The quantity of melamine added being between 1 to 10% by weight relative to the weight of finished liquid resin: this stage is carried out at pH between 6 and 9, preferably between 7 and 8, and at a temperature between 70.degree. and 100.degree. C. This stage is then optionally followed by a phase in which the resin is concentrated by vacuum distillation: this phase allows excess water to be removed. In a third stage, urea is added after cooling, at a temperature below 40.degree. C., in a quantity such that the resin's final F/NH.sub.2 molar ratio is between 0.30 and 0.50.

According to the invention borax may be added at the end of the synthesis, in a quantity up to a maximum of 0.5% by weight relative to the weight of finished liquid resin: addition of borax improves the resin's stability and reactivity and also the swelling of the boards.

A second alternative of the process of the invention comprises introducing the melamine during the first condensation stage, while operating under the conditions previously set forth.

A third alternative of the process of the invention comprises introducing melamine both during the first and during the second condensation stages, respecting the pH and F/NH.sub.2 ratio conditions previously set forth.

The following examples illustrate the present invention. The quantities are expressed in parts by weight.

EXAMPLE 1

13,355 parts of a 50% solution of formaldehyde, and 6,360 parts of urea beads are placed in a reactor equipped with a condenser, a stirrer and a heating device: the reaction mixture is heated to a temperature between 85.degree. and 98.degree. C. for about 1 h 30 minutes and maintained at pH near 5 by the addition of formic acid. On completion of condensation, determined by measuring the viscosity which should be between 275 and 300 mPa s at 40.degree. C., the pH is brought to a value near 7 by addition of caustic soda.

3,825 parts of urea and 825 parts of melamine are then added to the reaction mixture: the reaction is conducted at a temperature between 75.degree. and 80.degree. C. and pH near 7.

At the end of this stage the F/NH.sub.2 molar ratio is 0.62. The reaction mixture is then concentrated under vacuum at 50.degree. C., allowing 545 parts of water to be distilled out. Then 3,430 parts of urea are added at 30.degree. C. The resulting final resin has a F/NH.sub.2 molar ratio of 0.47: its pH is 0.5. Its other characteristics are as follows:

______________________________________Viscosity (20.degree. C.) 750 mPa sSolids content % 65Dilutability with water (20.degree. C.) 0.5/1Gelling time at 100.degree. C. 5 minutes(with 5% of a 15% solution ofammonium chloride)Stability at 20.degree. C. 60 days(time after which the resin'sviscosity reached 3 Pa s)______________________________________

The resulting resin is used to produce particle boards. To this end the resin is cured with 1.5% of ammonium chloride relative to the quantity of dry resin. The boards are pressed at 180.degree. C. for 3 minutes: they have the following characteristics:

______________________________________Thickness (mm) 20Relative density (kg/m.sup.3) 615Swelling 24 hours in waterat 20.degree. C., %According to NF standard 17.5According to DIN standard 18.4Perpendicular tensile strength 4.6(daN/cm.sup.2)Perforator formaldehyde (mg/100 g 4.0dry board)______________________________________

The boards' characteristics were established according to the following standards:

______________________________________Thickness NFB 51200Relative density NFB 51222Perpendicular tensile strength NFB 51250Swelling NFB 51252 DIN 68763Formaldehyde content CEN-EN-120______________________________________

EXAMPLE 2

Example 1 is repeated but using 6% of melamine relative to the weight of liquid resin. The resulting resin has the following characteristics:

______________________________________Viscosity (20.degree. C.) 820 mPa sSolids content % 65.3Dilutability with water (20.degree. C.) 0.7/1Gelling time at 100.degree. C. 7 minutes(with 5% of a 15% NH.sub.4 Cl solution)Change in stability 65 days: (time after which viscosity reaches a viscosity of 3 Pa s)______________________________________

Boards made with this resin have the following characteristics:

______________________________________Thickness (mm) 20Relative density (kg/m.sup.3) 611Swelling (in water at 20.degree. C. during24 hours %)According to NF standard 15.9According to DIN standard 16.6Perpendicular tensile strength 4.5(daN/cm.sup.2)Perforator formaldehyde 4______________________________________

EXAMPLE 3

Example 2 was repeated without adding melamine but adding 0.1% of borax at the completion of synthesis. The resulting resin has the following characteristics:

______________________________________Specific gravity (20%) 1.277Viscosity (20.degree. C.) 590 mPa sSolids content % 65.1Dilutability with water 1.5/1pH 8.3Gelling time at 100.degree. C. 1 minute(using 5% of NH.sub.4 Cl solution) 5 secondsBoards made with this resin have the followingcharacteristics:Thickness (mm) 20.5Relative density (kg/m.sup.3) 616Swelling %NF 23.3DIN 24.5Perpendicular tensile strength 2.6(daN/cm.sub.2)Perforator formaldehyde 3.5______________________________________

EXAMPLE 4

Example 1 is repeated but using the following reagents:

______________________________________ in the first stage: 1840 parts of 50% formaldehyde 800 parts of urea the F/NH.sub.2 molar ratio is 1.15 in the second stage: 444 parts of urea 240 parts of melamine the F/NH.sub.2 molar ratio is 0.65 in the third stage: 676 parts of urea______________________________________ the resulting resin's F/NH.sub.2 molar ratio is 0.44

The resulting resin has the following characteristics:

______________________________________Viscosity (20.degree. C.) 930 mPa sSolids content 65%pH 7.5Dilutability with water 0.6/1Stability at 20.degree. C. 41 days (for viscosity 20.degree. C.: 3 Pa s)______________________________________

Boards made with this resin have the following characteristics:

______________________________________Thickness (mm) 20.3Relative density 605Perpendicular tensile strength 4.3Swelling %NF 19.3DIN 20.6Perforator formaldehyde 3.5______________________________________

EXAMPLE 5

The resin is produced using the following reagents:

______________________________________1st stage 46.62% formaldehyde 2529 parts Urea 1123 parts (hence F/NH.sub.2 = 1.05)2nd stage Urea 522 parts Melamine 360 parts3rd stage Urea 1466 parts______________________________________

The resulting resin's F/NH.sub.2 ratio is 0.35. The resin has the following characteristics:

______________________________________Viscosity 20.degree. C. 520 mPa sSolids content % 66.0pH 8.3Dilutability with water 0.5/1______________________________________

The same resin having had 0.1% borax added during production has the following characteristics:

______________________________________Viscosity at 20.degree. C. 530 mPa.sSolids content 66.1pH 9.5Dilutability with water 0.5/1Stability = 20.degree. C. = 105 days (time after which theviscosity reaches 3 Pa s).______________________________________

The boards produced have the following characteristics:

______________________________________Thickness (mm) 20.4Swelling 24 h (%)NF 35.4DIN 35.9Perpendicular tensile strength 0.8Formaldehyde content 2.5______________________________________

EXAMPLE 6

Example 1 is repeated, using in the first stage:

______________________________________50% formaldehyde 1840 partsUrea 705 partsMelamine 240 parts______________________________________

This stage is conducted at an acid pH of 6. The F/NH.sub.2 molar ratio is 1.05.

Next, when the viscosity of the reaction medium (at 40.degree. C.) is 550 mPa s, 964 parts of urea are added. The F/NH.sub.2 molar ratio is 0.5; this stage is carried out at pH 6.5. Then, as the reaction reaches completion, 251 parts of urea are added. The resin has the following characteristics:

______________________________________Viscosity 20.degree. C. 315 mPa.sSolids content 64.6%pH 8.2Dilutability with water 0.75/1Gelling time at 100.degree. C. 15 minuteswith 5% NH.sub.4 Cl solutionStability at 20.degree. C. 170 days______________________________________

Boards made with this resin have the following characteristics:

______________________________________Thickness (mm) 20.3Relative density (kg/m.sup.3) 619Swelling %NF 12.8DIN 13.7Perpendicular tensile strength 4.9(daN.cm.sup.2)Perforator formaldehyde 3.5______________________________________

EXAMPLE 7

Example 1 is repeated using the following reagents:

______________________________________1689 parts of a urea-formaldehyde precondensate con- taining 57% formaldehyde and 23.6% urea, 582 parts of water 529 parts of urea______________________________________

The F/NH.sub.2 molar ratio at this stage is 1.05. After this acid condensation stage, the following are then added:

______________________________________ 464 parts of urea 240 parts of melamine______________________________________

The F/NH.sub.2 molar ratio at this stage is 0.62.

In a third stage 4 parts of borax are added, and 496 parts of urea. The resulting final resin has the following characteristics:

______________________________________F/NH.sub.2 = 0.47Viscosity at 20.degree. C. 0.74 Pa sSolids content 65.3%pH 8.3Gelling time at 100.degree. C. 13 minutesStability at 20.degree. C. 70 daysCharacteristics of the boards:Thickness (mm) 20.2Relative density (kg/m.sup.3) 610Swelling 24 h - water 20.degree. C. (%)NF 18.8DIN 19.1Perpendicular tensile strength (daN/cm.sup.2) 3.0Perforator formaldehyde (mg/100 g) 4.5______________________________________

EXAMPLE 8

Example 1 is repeated using the following reagents:

In the first stage: 2677 parts of a 47.3% solution of formaldehyde 1207 parts of urea

The F/NH.sub.2 molar ratio at this stage is 1.05.

In the second stage the following are used:

451 parts of urea 540 parts of melamine

The F/NH.sub.2 molar ratio at this stage is 0.62.

As condensation reaches completion 6 parts of borax are added and 1125 parts of urea.

The resulting resin has the following characteristics:

______________________________________F/NH.sub.2 = 0.40Viscosity at 20.degree. C. 1.09 Pa sSolids content (%) 65Specific gravity (20.degree. C.) 1.280pH 9.3______________________________________

EXAMPLE 9

The same procedure is followed as in example 2, but with the addition of 0.25% of borax.

______________________________________Viscosity 20.degree. C. 0.60 Pa sSolids content (%) 64.3Dilutability in water at 20.degree. C. 0.7/1Gelling time at 100.degree. C. 4 minutes(with a 15% NH.sub.4 Cl solution)______________________________________

Boards, produced with an 18% higher rate of application of adhesive have the following characteristics:

______________________________________Thickness (mm) 20Relative density (kg/m.sup.3) 645Swelling (24 h - water 20.degree. C. %)NF 9.5DIN 12.2Perpendicular tensile strength (daN/cm.sup.2) 5.5Perforator formaldehyde (mg/100 g dry board): 4______________________________________

EXAMPLE 10

The same procedure is followed as in example 5, but with addition of 0.25% of borax

______________________________________Viscosity 20.degree. C. 0.66 Pa sSolids content (%) 65.5pH 9.2Dilutability with water 20.degree. C. 0.5/1______________________________________

Boards, produced with an 18% higher rate of application of adhesive have the following characteristics:

______________________________________Thickness (mm) 20.3Relative density (kg/m.sup.3) 630Swelling 24 h - water 20.degree. C. (%)NF 28.3DIN 30.0Perpendicular tensile strength (daN/cm.sup.2) 1.8Perforator formaldehyde (mg/100 g) 2.5______________________________________

EXAMPLE 11

The same procedure is followed as in example 8 but with the addition of 0.25% of borax.

______________________________________Viscosity 20.degree. C. 0.88 Pa sSolids content (%) 64.6%Specific gravity 20.degree. C. 1.280pH 9.2Dilutability in water 20.degree. C. 0.5/1______________________________________

Boards, produced with an 18% higher rate of application of adhesive, have the following characteristics:

______________________________________Thickness (mm) 20.0Relative density (kg/m.sup.3) 645Swelling 24 h - water 20.degree. C. (%)NF 11.4DIN 14.2Perpendicular tensile strength (daN/cm.sup.2) 4.8Perforator formaldehyde (mg/100 g) 3.5______________________________________

Claims

1. Process for the production of urea-formaldehyde resins containing melamine characterized in that it comprises three stages, in that the melamine is added either at the first stage, or at the second stage, or at both first and second stages in quantities between 1 and 10% relative to the weight of the finished liquid resin and in that at the first stage urea, formaldehyde and optionally melamine are condensed in solution at pH between 4.5 and 6 in quantities such that the F/NH.sub.2 molar ratio at this stage is between 0.9 and 1.5 and in that, at the second stage, more urea and optionally melamine are added in quantities such that the resin's F/NH.sub.2 ratio at this stage is between 0.5 and 0.9, this stage being conducted at pH between 6 and 9 and at a temperature between 70.degree. and 100.degree. C., the resulting resin being optionally subjected to vacuum distillation, then in a third stage, after cooling, and at a temperature below 50.degree. C., having a quantity of urea added such that the resin's final F/NH.sub.2 molar ratio is between 0.30 and 0.50; borax in a quantity up to a maximum of 0.5% by weight relative to the weight of finished liquid resin being optionally added.

2. A process according to claim 1, wherein borax is added to the finished liquid resin.

3. A process according to claim 1, wherein the F/NH.sub.2 molar ratio in the first stage is between 0.95 and 1.15.

4. A process according to claim 1, wherein the second stage is conducted at a pH between 7 and 8.

5. A process according to claim 3, wherein the second stage is conducted at a pH between 7 and 8.

6. A process according to claim 1, wherein all the melamine is added in the second stage.

7. A process according to claim 5, wherein all the melamine is added in the second stage.

8. An aminoplast resin as produced by the process of claim 1.

9. An aminoplast resin as produced by the process of claim 2.

10. An aminoplast resin as produced by the process of claim 5.

11. An aminoplast resin as produced by the process of claim 6.

12. An aminoplast resin as produced by the process of claim 7.

13. In a process for the production of particle boards comprising compressing a mass of wood particles with a liquid binder at an elevated temperature, the improvement which comprises employing as the liquid binder a solution of an aminoplast resin according to claim 8.

14. In a process for the production of particle boards comprising compressing a mass of wood particles with a liquid binder at an elevated temperature, the improvement which comprises employing as the liquid binder a solution of an aminoplast resin according to claim 10.

15. In a process for the production of particle boards comprising compressing a mass of wood particles with a liquid binder at an elevated temperature, the improvement which comprises employing as the liquid binder a solution of an aminoplast resin according to claim 12.

16. A particle board as produced by the process of claim 13.

17. A particle board as produced by the process of claim 14.

18. A particle board as produced by the process of claim 15.

19. A process according to claim 1, wherein all of the melamine is added in the first stage.

20. An aminoplast resin as produced by the process of claim 19.

21. A process according to claim 1, wherein the final liquid resin was formed from components consisting essentially of formaldehyde, urea and melamine.

22. A process according to claim 1, wherein the final F/NH.sub.2 molar ratio is not more than 0.47.