IE60577B1 - Process for the preparation of urea-formaldehyde resins - Google Patents

Description

This invention relates to a new process for the preparation of urea-formaldehyde resins having a low extractable formaldehyde content.

Urea-formaldehyde resins are widely used in industry as 5 binders for the production of wood-based materials, particularly chipboard, plywood, and various fibreboards. They are usually employed as colloidal aqueous.solutions having a 60-70?i solids content, the solids content being, the percentage weight left when a 2 g sample of the resin is heated at atmospheric pressure at 120°C for 3 hours.

The resins themselves are complex mixtures of various condensation products, ranging from simple methylolated ureas to fairly high molecular weight materials, and the precise properties of any particular resin will depend upon its molar ratio (i.e. the ratio of formaldehyde to urea when used in its manufacture), the type and proportion of the various linkages within the resin structure, and the proportions of high and low molecular weight material. These last two factors are dependent upon the precise manufacturing method that is used. 2o Conventional commercial resins all liberate a certain amount of formaldehyde during hot curing and after cure has taken place. The latter liberation of formaldehyde can be a serious problem since it limits the use to which, for example, particle boards may be put. A board that liberates formaldehyde above a specified level cannot bp used in unventilated areas. The measurement of the total extractable formaldehyde is therefore an important test that is carried out on particle board samples. and is usually effected using the method of the Federation Europeenne des Syndicates des Fabricants des Panneaux de Particules (FESYP) and described in British Standard 1811. In this test small rectangular blocks sawn from air-dried particle board are extracted with toluene, the toluene is washed with water, and the water measured iodometrically for formaldehyde.

A need exists for resins that have a very low content of extractable formaldehyde but which, in all their performance properties, behave like a conventional urea-formaldehyde resin.

The conventional method of manufacturing urea-formaldehyde resins comprises precondensing urea with formaldehyde in aqueous solution at the reflux at pH 7-9, and heating the mixture at pH 5-6 until the desired degree of water insolubility or viscosity is reached. The product is neutralised and may then be evaporated and blended with a further quantity of urea if desired. Such conventional resins usually have a F:U molar ratio within the range 1.3 to 2.3s1.

Resins having a F:U molar ratio as low as 1:1 have been described in British Patent Specification No. 1.420. 017.

These resins are s.aid to be suitable for binding wood-based materials and are prepared by (i) condensing formaldehyde and urea at a F:U molar ratio of at least 3:1 and at pH 3 or less, (ii) adding urea to bring the molar ratio down to 2-2.75:1, and continuing to heat at pH 3 or less (iii) optionally, adding more urea so that the final molar ratio is within the range 1-2.75:1, (iv) adjusting the pH to 5-6.5 or to 8-10, and continuing the heating until the desired product is formed. This product may then be concentrated or dried, if desired. A serious drawback with this process is that, on an industrial scale, it is difficult to obtain a consistent product and there is a serious risk of the product forming an intractable gel during the process, such formation then requiring dismantling of the bl ΰι apparatus in order for it to be cleaned. Further, when the product has been made successfully, it has poor 'wash down' properties.

This means that pipework, pumps etc. through which the resin has been passed are difficult to clean, due to the inherent stickiness of the resin.

Another process for preparing adhesive resins with low formaldehyde content is described in United States Patent No. 140 685. In this process resins having s F:U molar ratio within the range 1.0 to 1.2:1 are prepared as follows: (i) formaldehyde solution is acidified to pH 0.5-2.5, (ii) the acid solution is heated to 50-70°C, (iii) urea is added slowly until the F:U molar ratio reaches 2.9 to 3.1:1 (iv) when the mixture reaches a given viscosity it is neutralised, and (v) urea is added to give s F:U molar ratio 1.0 to 1.2:1. Like the previous process described, this process is difficult to control, especially on an industrial scale and unless careful control is maintained over the reaction the resin will gel during the condensation stage if the conditions that are described are strictly followed.

It has now been found that urea-formaldehvde resins having a low content of total extractable formaldehyde and which are suitable for bonding wood-based products, such as particle board, may be prepared with a minimal risk of forming intraclible gels if the urea is first methylolated under alkaline conditions at elevated temperature and inaf:U molar ratio within the range 2:1 to 3:1, this mixture is then acidified and condensation allowed to continue at elevated temperature. The mixture is neutralised and, in either order, it is concentrated if necessary and urea is added to adjust che Γ :U molar ratio to within the range C.S to 1.8:1. This process facilitates formation of a resin which is water soluble and, when freshly prepared, is completely transparent in appearance. This transparency is lost gradually on storing, the resin becoming slowly opalescent, so chat examination of the turbidity of the resin gives an immediate indication of its age or storage conditions.

Accordingly, this invention provides a process for the preparation of urea-formaldehyde resins which comprises (i) mixing sn aqueous formaldehyde solution containing 30-50% by weight of formaldehyde with urea at a formaldehyde:urea molar ratio of 2.0 to 3.0:1 and at pH 6-11, (ii) heating the mixture to at least 50°C, (iii) adding an acid until the mixture reaches a pH within the range 0«5-3.5, (iv) heating the mixture at a temperature between 60°C and the reflux temperature, (v) with the mixture et a temperature of et least SQ°C„ adding .0 base until the mixture reaches a pH within the range 5.5-9, and (vi) adding urea until the formaldehyde:urea molar ratio is within the range 0.0-1.0:1.

For use as a binder For particle boards or fibreboards, the txrea-iformaldehyde resin should usually have a solids content of 40 to 75% by weight. If it is necessary to evaporate the mixture in order to achieve auch a solids content, the evaporation may be carried out, under reduced pressure, either between stages (v) and (vi) or after stage (vi).

The initial mixing of formaldehyde solution, usually containing 35-40% by weight of formaldehyde, and urea usually takes place at a F:U molar ratio within the range 2.2 to 2.8:1, especially 2.3 to 2.5:1, and at pH 7-9. This mixture is then heated to at least 50°C, usually to at least 8G°C and preferably to reflux. An acid is then added to bring the pH of the mixture to within the range 0,,5 to 3.5, preferably within the range 1-2,5. The type of acid used is not critical inorganic or organic acids have both been found to be suitable. Typical acids that may be used include trichloroacetic, methane sulphonic, toluene -p-sulphonic, hydrochloric, sulphuric, sulphamic and phosphoric acids.

Heating of the acid mixture is usually effected at reflux, and usually for a period of from 1 minute to 2 hours, especially frcm 15 minutes to 1 hour. The mixture may be allowed to cool a little, but it is essential that it is at a temperature of at least 80°C whilst the base is added, if the temperature falls below 80°C there is a serious risk of an intractible gel forming, for reasons of economy, the base that is used is usually sodium hydroxide or potassium hydroxide, but any base that will raise the pH of the mixture to within the range 6.5 to 9 is satisfactory.

When the desired pH has been reached, the mixture is usually partially evaporated under reduced pressure to eliminate some of the water present, although this evaporation may take place after the further addition of urea. Evaporation is usually, effected below 50°C and preferably within the range 15-45°C.

The solids content of the product from this evaporation may be varied within fairly wide limits, depending upon the ultimate use of the final resin, but is usually within the range 5G-65SS by weight if the evaporation precedes urea addition, and 6O-7O5S by weight if it follows urea addition. Addition of urea gives the desired f:U molar ratio, preferably from 1.0 to 1.7:1, especially 1.2 to 1.5:1.

Resins made by this new process have their formaldehyde firmly bonded into the molecule, sc that both the uncured arid cured resins give low values for available formaldehyde. This may ι be shown by measuring the total free formaldehyde from the uncured resin, the liberated formaldehyde from the hot curing of particle board, and the total extractable formaldehyde from cured particle board.

A further advantage of resins made by the new process is that they have remarkably good washdown properties so that apparatus that has been used for the uncured resin may be easily cleaned with a jet of cold water, as can any spillages. This is an important consideration in an industry where large quantities of resin are handled and, if the resin residues are allowed to remain and harden, an expensive strip-down of equipment is needed for cleaning.

A further advantage of resins made by the new process is their remarkable clarity when freshly prepared. Resins having FsU molar ratios of 1.8 or less have always been opalescent, or more usually thick white liquids. To make such resins as. clear, transparent liquids allows an instant visible means of checking that the resin made by this process has been supplied and that contamination by ocher materials has not taken place.

These advantages have been achieved with no loss of strength J.η the resin. Products made using this resin, such as particleboard, have the strength which could be expected from products made with a conventional resin having the same solids content and F;U molar ratio.

Resins made by the process of this invention are particularly useful in the production of products known as medium density fibreboards, facilitating good processability and high production rates and showing good resistance to premature cure in blending operations.

These resins may be hardened by any conventional means, usually by the addition of an acid, such as formic, phosphoric, or hydrochloric acid, or by addition of an acid salt that liberates an acid when heated, such as ammonium chloride.

The process of the invention as hereinbefore defined may be 15 modified to produce resins in a solid particulate form. After step (v), instead of adding urea to dive a resin having a formaldehyde:urea molar ratio of 8.8:1 to 1.8:1, the mixture from step (v), preferably after evaporation as hereinbefore described, is reacted with urea under acid conditions to give a higher molecular weight resin which is then neutralised and dehydrated to give a solid, particulate resin.

Accordingly, the invention also provides a modification of the process as hereinbefore described which comprises (a) carrying out steps (i) to (v) as hereinbefore described, 25 (b) reacting the mixtiire from step (v) with urea at a pH below 7 to increase the viscosity therpof, (c) adding a base until the mixture reaches a pH within the range 6.5-9, and (d) dehydrating the mixture to form a solid particulate resin.

The reaction of the mixture from step (v) with urea is usually carried out at a pH of 3 to 5 and a temperature of 30 to '60°Cs using urea in an amount sufficient to give the reaction mixture a formaldehyde:urea molar ratio of from 1.3:1 to 1.9:1. until the viscosity of the reaction mixture is 2 to 4 times the initial viscosity of the mixture. Preferably this reaction is carried out at a pH of 3.5 co 4.5 and a temperature of 35 to 50°C, using urea in an amount sufficient to give the reaction mixture a formaldehyde:urea molar ratio of 1.5:1 to 1.7:1, until the viscosity of the reaction mixture is 2.5 to 3.5 times the initial viscosity of the mixture. Preferably also, the mixture from step (v) is evaporated to a solids content of 40-75%, preferably 50-65?ό» by weight, for example by a method as hereinbefore described, before reaction with the urea.

Acids suitable for use m reducing the pH of the mixture from step (v) include those hereinbefore mentioned as suitable for use in step (iii). Any base that will raise the pH of the mixture to within the range 6.5 to 9 can be used in step (c); for reasons of economy, sodium hydroxide and potassium hydroxide are preferred. 2 The dehydration step (d) can be earned out in a conventional spray drier to form the particulate resin. Particulate resins prepared by the process described give low values for available V formaldehyde, both in the uncured and cured state. They also have excellent tack properties and are particularly suitable for use in binders for particle board, wafer board and plywood.

For use as a binder, the particulate resin may be mixed with a solid hardener therefor and optionally with additives, e.g. fillers such as wood flour and china clay, to give a storage-stable composition known as a one-shot composition which can be activated by the addition of water. Suitable solid hardeners for use m such one-shot compositions are well-known; aluminium sulphate is the most generally used material. Alternatively, the particulate resin may be dissolved or dispersed in water to reform, in effect, a liquid resin which can be mixed with a hardener therefor and optionally with additives for use as a binder. Conventional hardeners. such as the acid and salt hardeners hereinbefore mentioned, can be used.

The invention is illustrated by reference to the following Examples in which all percentages are by weight unless otherwise indicated. The solids contents are determined by heating a 2 g sample at T20°C for 3 hours under atmospheric pressure.

/ EXAMPLE 1 Aqueous formaldehyde solution (23.93 kg; 38.56% HCHO) is treated with sodium hydroxide solution (76.A ml; 10% w/v NaOHl·, and urea (7.69 kg). The mixture, which is of pH 9 and has a f:U molar ratio of 2.A0:1, is stirred and heated to reflux. It is allowed to cool slightly, to 98°C, and an aqueous solution of sulphamic acid (3A3.6 ml; 20% WH^SO^H) is added. The resultant mixture is of pH 2.0, and this is heated under reflux for 30 minutes.

Whilst maintaining the reflux, sodium hydroxide solution (509.8 ml; 10% w/v NaOH) is added to give a neutral solution (pH 7). This is allowed to cool to about AO°C» at which temperature a vacuum is applied and water is distilled from the mixture, a total of 11.3 kg water being collected. The solids content of the mixture is 59%. finally a further quantity of urea is added (5.A0 kg), giving a f:U molar ratio of 1.A1;1.

This resin is a water-soluble, colourless transparent liquid having the following properties: Solids content Viscosity at 25°C free formaldehyde 65% 0.27 Pa s 0.3% t Jj 4 EXAMPLE 2 Aqueous formaldehyde solution (75.68 g; 38.56% HCHO) V is treated with sodium hydroxide solution (0.24 ml: 10% w/v MaOH) followed by urea (24.32 g). The mixture, which is of pH j 9 and has a F:U molar ratio of 2.40:1, is stirred and heated to reflux. Methane sulphonic acid solution (1.0 ml: 20% CH^SO^H) is added, to bring the mixture to pH 2.0, and it is then heated under reflux for 30 minutes. Neutralisation with sodium hydroxide solution (20%follows and the mixture is evaporated at about 40°C to a solids content of 60%. The mixture is cooled and urea (17.08 g) is added, giving a F:ll molar ratio of 1.41:1.

This resin is a water-soluble, colourless transparent liquid having the following properties: Solids content 71% Viscosity at 25°C 0,90 Pa s EXAMPLE 3 Example 2 is repeated, replacing the methane sulphonic acid by phosphoric acid (5.0 ml; 20% H,P0/;).

The resin is a water-soluble, colourless, transparent 20 liquid having the following properties; Solids content 71% Viscosity at 25°C 1.72 Pa s F:U molar ratio 1.41.-1 EXAMPLE 4 Softwood core chips (1540 g)„ dried to a moisture content of 2%. are blended with a mixture comprising the resin described in Example 1 (173.8 g), ammonium chloride solution (12.0 g; 10% NH^Cl), a commercial wax emulsion (12.0 g; 65% wax), and water (65.0 g). Part of the resultant blend (1650 g) is placed within a wooden frame 355mm square, and consolidated at room temperature under a pressure of 791 kPa. The resultant consolidated sheet is removed from the frame and compressed at 145°C for 8 minutes under a pressure of 2.77 MPa. The resultant chipboard has a thickness of 19mm. The board is tested, with the following results: f:U molar ratio 1.41:1 Resin content of board (% solids) 8 Density (kg/m^) 067 Modulus of rupture (mPa) 21.57 Transverse tensile strength (mPa) 0.90 Total extractable formaldehyde (%) 0.013 It may therefore be seen that the total extractable formaldehyde, measured in accordance with BS 1811, in the board is extremely low. 7 EXAMPLE 5 Aqueous formaldehyde solution (75.68 g; 38.56?« HCHO) is treated with sodium hydroxide solution (0.24 ml; 10% w/v NaOH) and urea (24.32 g). This mixture,* which is of pH 9 and has a F:U ratio of 2.40:1, is heated to reflux and sulphamie acid (1.2 ml; 15% NH^SO^H) is added, giving the mixture a pH of 2.0. The mixture is -then heated under reflux for 30 minutes, and aqueous NaOH added to pH 7. After cooling to 25°C, urea (17.08 g) is aoded, and the product evaporated in vacuo below 40°C to a final weight of 81.12 g.

The product is a colourless, transparent resin having an FsU molar ratio of 1.41:1, a solids content of 68.5?«, and a viscosity at 25°C of 0.58 Pa s.

EXAMPLE 6 Aqueous formaldehyde solution (748.3 g: 38.68% HCHO) is treated with sodium hydroxide solution (1.2 ml; 20% w/v NaOH) and urea (251.7 g). The mixture, which is of pH 9 and has a F;U molar ratio of 2.30:1, is stirred and heated to reflux.

Aqueous sulphuric acid (20 ml; 15% H.,50^) is added, to bring the mixture to pH 2.0, and it is then heated under reflux for 30 minutes. Agueous sodium hydroxide (14 ml; 20% w/v NaOH) is added to bring the mixture to pH 7.4. The mixture is then evaporated in vacuo at about 40°C to a solids content of 59%. Urea (46.2 g) is added to part of the evaporated resin (190.3 g) to give a resin 8 having a F:U molar ratio of 1.30:1» This resin is a watersoluble, colourless transparent liquid having a solids content of 66% and a viscosity at 25°C of 0.55 Pa S„ v.

EXAMPLE 7 Example 1 is repeated to the stage at which water is distilled from the mixture, to a solids content of 57%. To 100 parts by weight of the resulting mixture, 25.55 parts by weight of urea is added to give a resin having a F:U molar ratio of 1.25:1.

This resin is a water-soluble colourless transparent liquid having a solids content of 66% and a viscosity at 25°C of 0.32 Pa s.

EXAMPLE 8 Example 6 is repeated to the stage where the mixture is evaporated, the evaporation being carried out to give a solids content of 50% instead of 59%. The evaporated mixture has a viscosity at 25°C of 0.08 Pa s. Urea (30 parts) is added to the evaporated mixture (200 parts) and the resulting mixture is stirred until the urea is dissolved, giving a mixture with a viscosity at 25°C of 0.07 Pa s. The f:U molar ratio of the mixture is 1.,57:1. The temperature of the mixture is adjusted, if necessary, to 35-60°C and aqueous sulphuric acid (1 part; 15?i i-LSO,) is added to bring the mixture to pH 3.9» The mixture is 2 4 stirred for 30 minutes, the temperature rising to 50°C, samples t being taken at regular intervals for viscosity determination, until the viscosity of the mixture is increased to 0.22 Pa s. Aqueous sodium hydroxide (0.4 part; 20ό NaOH) is added to bring the mixture to pH 7. The mixture is then dehydrated in a spray drier to give a resin powder having a softening point of 133°C, a F:U molar ratio of 1.56:1 and a free formaldehyde content of 0.27%.

Claims (19)

1. A process for the preparation of a urea-formaldehyde resin which comprises (a) mixing an aqueous formaldehyde solution containing 30-50% by weight of formaldehyde with urea at a formaldehyde: urea molar ratio of 2.0 to 3.0:1 and at pH 6-11. (n) heating the mixture to at least 50°C, (in) adding an acid until the mixture reaches a pH within the range 0.5-3.5, (iv) heating the mixture at a temperature between 80°C and the reflux temperature, (v) with the mixture at a temperature of at least 80°C, adding a base until the mixture reaches a pH within the range 6.5-9, and (v.i) adding urea until the formaldehyde:urea molar ratio is within the range 0.8-1.8:1.

2. · A process according to claim 1 in which the initial formaldehyde:urea molar ratio is within the range 2.2 to 2.8:1.

3. A process according to claim 2, in which the initial formaldehyde:urea molar ratio is within the range 2.3 to 2.5:1.

4. A process according to any of claims 1 to 3 in which the heating stage (ii) is effected to a temperature of at least 80°C.

5. A process according to any previous claim in which 2 1 acidification in stage (m) is effected to pH 1-2.5.

6. A process according to ^ny previous claim in which -acidiιication m stage (iii) is effected by addition of trichloroacetic, methane sulphonic, ioluene-p-sulphomc, hydrochloric, sulphuric, sulphamic, or phosphoric acid.

7. » A process according to any previous claim, in which the heating stage (iv) is effected at reflux temperature. 0. A process according to any previous claim in which the heating stage (iv) is effected for a period of from 15 minutes to 1 hour. A process according to any previous claim in which the base used m stage (v) is sodium hydroxide or potassium hydroxide.

8. 10. A process according to any previous claim, in which the mixture is evaporated under reduced pressur'd, either between stages (v) and (vi) or after stage (vi), until the solids content is within the range 40-75% by weight. J'S. A process according to claim io in which the evaporation takes place at a temperature within the range 35-45°C.

9. 12. & process according to claim jo or Π in which the evaporation precedes the urea addition stage (vi) and is effected to give a solids content within the range 50-65% by weight. 83» A process according to claim 10 or 11, in which the evaporation follows the urea addition stage (vi) and is effected to give a solids content within the range 60-70% by weight.

10. 14,., A process according to any previous claim, in which addition of urea in stage (vi) gives a product having a formaldehyde;urea molar ratio within the range 1.0 fco 1.7:1.

11. 15_ A process according to claim 14, in which the urea addition gives a product having a formaldehyde:urea molar ratio of 1.2 fco 1.5:1. 5 16. A process according to claim 1 s substantially as described in any of Examples 1 to 3 and 5 to 7. 57 bi A process according to claim 1, substantially as described herein.

12. 18. A modification of the process according to claim 1 which 10 comprises (a) carrying out steps (i) to (v) as defined in claim 1, (b) reacting the mixture from step (v) with urea at a pH below 7 to increase the viscosity thereof. (c) adding a base until the mixture reaches a pH within 15 the range 6.5-9, and (d) dehydrating the mixture to form a solid particulate resin.

13. 19.. A process according to claim 38, in which step (b) is carried out at a pH of 3 to 5 and a temperature of 30 fco 60°C,

14. 20 using urea in an amount sufficient to give the reaction mixture a formaldehyde:urea molar ratio of from 1.3:1 to 1.9:1, until the viscosity of the reaction mixture is 2 to 4 times the initial viscosity thereof. I t 2 3 1 20. A process according to claim 19. in which step (b) is carried out at a pH of 3.5 to 4.5 and a temperature of 35 to 50 o C ? using urea in an amount sufficient to give the reaction mixture a formaldehyde:urea molar ratio of from 1.5:1 to 5 1.7:1, until the viscosity of the reaction mixture is 2.5 to 3.5 times the initial viscosity thereof.

15. 21,. A process according to claim 1-8, 19 or 20, in which the mixture reacted with urea has a solids content of from 40 to 75% by weight. 10

16. 22. A process according to claim 18, substantially as described in Example 8.

17. 23. A binder comprising a solid particulate resin obtained by a process according to any of claims 18 co 22 and a solid hardener therefor. 15

18. 24» A binder comprising a resin obtained by a process according to any of claims 18 to 22 dissolved or dispersed in water and a hardener for the resin.

19. 25. A urea-formaldehyde resin whenever prepared by a process claimed in a preceding claim.