PROCESS FOR THE PREPARATION OF UREA/FORMALDEHYDE CONDENSATES, AND A BINDER OBTAINED BY THE PROCESS

Abstract: Binders with a low content of free formaldehyde and with a high tackiness are obtained by condensing formaldehyde (F) and urea (U) in successive stages, under the following opera­ tional conditions: (a) reaction between fomaldehyde and urea at 80-100°C, at a pH of from 4 to 5.8, at a molar ratio F/U of from 2:1 to 3.5:1, up to a viscosity of from 400 to 2,000 cP; (b) further reaction at a pH of from 5.9 to 6.8, after the ad­ dition of urea up to a ratio F/U of from 1.5:1 to 1.9:1, up to a viscosity of from 2,000 to 3,000 cP; and (c) addition of urea to the product obtained, during cool­ ing down, at a pH of from 7.2 to 8.5, up to a ratio F/U of from 1.15 : 1 to 1.4:1.

Description: "PROCESS FOR THE PREPARATION OF UREA/FORMALDEHYDE CONDENSATES, AND A BINDER OBTAINED BY THE PROCESS The present invention relates to a process for the preparation of urea/formaldehyde condensates, having a low content of free formaldehyde and exhibiting high tack-values,such condensates being suitable for use as binders in the preparation of chip board made from wooden chips or shavings. Binders with a high tack are required for the preparation of chip board panels produced on multistage equipment and with a preceding cold pre-pressing. On such an equipment, a cold pre-pressed panel is subjected to mechanical stresses before it enters a press for final moulding under heat, and the binder must thus impart to the panel a tack sufficient to avoid scaling off of the panel while not yet heat-moulded. A low content of free formaldehyde and freeable formaldehyde allows improvement of the environmental hygiene of the panel-producing equipment, and above all permits manufactured articles to be obtained with a low content of free formaldehyde, which are thus particularly suitable for home furnishings. The present invention therefore aims to develop a process for the preparation of formaldehyde condensates with urea, exhibiting a high tack and a low content of free or freeable formaldehyde. Accordingly, the present invention provides a process for preparing condensation products of formaldehyde with urea, comprising the following sequence of operations (a) reacting formaldehyde with urea, in a molar ratio of formaldehyde/urea of from 2:1 to 3.5:1, in an aqueous solution, at a temperature of from 800 to 1000C and at a pR of from 4 to 5.8, up to a vis cosity of the solution, at 200C, of from 400 to 2,000 c.P., corresponding to a content of dry product, referred to the sum of reacted and unre acted monomers, of from 75% to 80% by weight; (b) adding urea to the reaction product until there is attained an overall molar ratio of formaldehyde/ urea of from 1.5:1 to 1.9: :1, and carrying on the reaction at a pH of from 5.9 to 6.8 and at a temperature of from 800 to 9500, up to a viscosity of the solution, at 2000, of from 2,000 to 3,000 c.P.,1corresponding to a content of dry product, referred to the sum of reacted and unreacted monomers, of from 75% to 80% by weight; and (c) adding urea to the reaction product, under cooling conditions of the solution, at a pH of from 7.2 to 8.5, in such an amount as to obtain an overall molar ratio of formaldehyde/urea of from 1.15:1 to 1.4:1. Operation (a) is preferably effected at a molar ratio of formaldehyde/urea of from 2.2:1 to 2.5:1, at a pH of from 5.3 to 5.6, and up to a viscosity of the solution of from 1,200 to 1,600 cOP. Operation (b) is preferably effected at a pH of from 6.2 to 605, and up to a viscosity of the solution of from 2,400 and 2,800 c.P. Operation (c) is carried out with simultaneous cooling of the solution to a temperature lower than the reaction temperature of the operation (b), and preferably at a temperature equal to or lower than 7O0C, and is preferably effected at a molar ratio of formaldehyde/ urea of from 1.15:1 to 1.3:1. In order to improve the preservability of the final product obtained through operation (c), it is possible to successively add to the product an alkali, so as to bring the pH value up to 7.5-9, more preferably 708-8.5. The addition of alkali may more conveniently be carried out at a temperature of from 20 to 300C. Formaldehyde and urea may be used in the various above described operational stages either in the solid state or in solution. In operation (a) the formaldehyde may be used in the form of a pre-condensate with urea, for instance as formurea, which is a condensation product of about 5 mols of formaldehyde with one mol of urea. Each one of operations (a) and (b) may be achieved in a plurality of successive condensation operations (in general not more than 5), at pM values, with additional quantities of urea, and up to viscosity values comprised within the ranges defined above in accordance with the invention. For instance, the operations (a) and (b) may be suitably replaced by the three following operations, in which operation (a) is conducted in two successive stages (a') and (a) : (a') reaction between formaldehyde and urea, in a molar ratio of from 2.5:1 and 3.5:1, but preferably from 2.9:1 to 3.2:1, in an aqueous solution, at a temperature of from 900 to 1000G and at a pH of from 4 to 5.5, but preferably from 4o6 to 500, up to a viscosity at 200C of from 400 to 1,000 cOP., but preferably from 500 to 700 c.P., for an amount of dry product referred to the sum of reacted and unreacted monomers of from 75% to 80%. (a") addition of urea up to a molar ratio of formaldehyde/ urea of from 2:1 to 2.5:1, and continuation of the condensation at a temperature of from 900 to 10000 and at a pK of from 5.0 to 5.5, up to a viscosity at 200C of from 1,500 to 2,000 c.P., for an amount of dry product within the above indicated range; (b') neutralization of the reaction product with an al kali up to a pH of from 6 and 6.5, and continuation of the reaction, after adding urea up to a formalde hyde/urea molar ratio of from 1.5:1 to 1.9:1 at 800-950C, up to a viscosity at 200C of from 2,000 and 3,000 c.P., for an amount of dry product within the above indicated range. The above operations are followed by (c') addition of urea to the reaction product, under cooling conditions of the solution, at a pM value of from 7.2 to 8.5, up to a molar ratio of formalde hyde/urea of from 1.15:1 to 1.4:1. In each operational stage or step, the adjustment of the pH value may be obtained by the addition to the reaction mixture of buffer salts, while the neutralization operations may be carried out with alkalis such as caustic soda, sodium carbonate, calcium hydroxide, and amines. The process according to the present invention allows products to be obtained which have a low content of free and freeable formaldehyde, and which, at the same time, have a tack which, at an equal content content of formaldehyde, is not obtainable when effecting the condensation in an acid medium in the absence of above described operation (b)o The invention will be further described with reference to the following illustrative Examples. In the Examples reference will be made to the accompanying drawing, which is a graph illustrating the breaking load values (or tack) with reference to the number of determinations for each of the binders prepared according to the Examples. EXAMPLE 1 (for comparative purposes); This example concerns the preparation, through one single condensation operation in an acid medium, of a urea/formaldehyde binder having a final molar ratio of formaldehyde/urea of 1.4:1. The characterististics of the products thus obtained were determined by means of the following methods Value of the tack: 100 g of industrial wooden chips, suitably classed (with fractions comprised between 3c2 and 10 mm), were impregnated with 12 g of a 50% binder solution in a suitable mixer. With 20 g of this mix there was formed a tablet of 50 mm diameter (using a maximum load pressure of 400 kg), which was then subjected to breaking up. The broken up binded chip tablet was further crumbled and subjected to insufflation with nitrogen for 15 seconds. After such treatment there was formed a new tablet which was once again subjected to breaking up according to the same previous procedures0 These operations and measurements were repeated until a breaking load equal to or below 0o2 kg was obtained Free formaldehyde: By treatment of a sample with bisulphite in the presence of a measured excess of acid and alkalimetric titration of the unreacted acid. Freeable formaldehyde: A binder sample was heated in the presence of a catalyst consisting of NH4Cl (40 g), 32% NH40H (39 g), urea (40 g) and 266 g of H20; the formaldehyde developed was caused to be absorbed by MOl and its quantity was determined by polarography. Methtlenic formaldehyde: Calculated as the difference between total formaldehyde and the oxidizable formaldehyde. Free formaldehyde in the panels: Colorimetric proportioning of the formaldehyde on an aqueous extract of the panels (method Besgp)O 566.1 parts by weight(b.w.) of an 80% solution of formurea, 266.7 parts b.w. of a 71% urea solution, 0.5 parts b.w. of caustic soda at 30% concentration, and 0.3 parts b.w. of acetic acid at 80% concentration were loaded into a reactor, then heated up to 940C and 0.3 parts b.w. of 80% acetic acid were added thereto, thereby obtaining a solution with a pH of 5.8. This mixture was then condensed at 95 C up to a viscosity, at 20 C, of 1,200 c.P., with reference to a content of dry product, referred to the loaded monomers, of about 77%. At this point, 1o3 parts bow of 30% caustic soda were added to the mixture, which was then cooled0 The pM of the solution was equal to 7.8. Once the temperature had reached 550C, there were added 163.4 parts b.w. of an 86.5% urea solution and 1o4 parts b.w. of a 30% caustic soda for the final adjustment of the pH to a value of from 7.8 to 8.5, whereafter the cooling was continued until a temperature of 250C was reached. The binder thus obtained exhibited the following characteristics pH, at 200C = 805 Viscosity at 200C = 430 c.P. Dry content = 65.5% b.w. Maximum tack value = 1.7 kg (according to curve 1 of the accompanying drawing). Molar ratio formaldehyde/urea: first stage = 2.0 final stage = 1.40 Free formaldehyde = 002% b.w. Free able formaldehyde = 006% b.w. based on the dry product Methylenic formaldehyde = 29% based on the total for maldehyde Free foamaldehyde in the panels: 20 mg/1OO g of chip board. EXAMPLE 2 Into a reactor were loaded 521.2 parts b.w. of an 80% b.w. aqueous solution of formurea, 211.6 parts bow of a 71% urea solution, 0.5 parts b.w. of 30% caustic soda, and 0.3 parts b.w. of 80% acetic acid. The mixture was heated up to 94 C and then 0.35 parts bow of 80% acetic acid were added thereto whereby there was obtained a solution with a pH of 5.6. The mixture was condensed at 95 C up to a viscosity at 2O0C of 1,500 c.P. The content of total monomers in the solution amounted to about 77%. At this point there were added 0.43 parts bowo of 30% caustic soda and 111.8 parts bow of a 71% urea solution, whereby the pH value of the solution became equal to 6o4, and condensation was carried on at 9000 up to a viscosity at 200C of 2,500 c.PO The total content of monomers in the solution amounted to about 76%. The solution was then neutralized with 0o92 parts b.w. of 30% caustic soda, thus obtaining a pH of 8.0. The reaction mass was then cooled down. Once the temperature reached 550C, 151.4 parts bow of a 71% urea solution and 1.5 parts b.w. of 30% caustic soda were added to the reaction mixture, thus bringing the pH to a final value of from 7.8 to 8.5, whereafter the cooling was continued until a temperature of 250C was reached. The binder thus obtained exhibited the following characteristics pH, at 200C = 8.0 Viscosity at 200C = 600 c.P. Content of dry product = 64.5% b.w. Maximum tack value = 7.1 kg (according to curve 2 of the accompany ing drawling), Molar ratio formaldehyde/urea: First condensation : 2.2 Second condensation : 1.7 Final : 1.30 Free formaldehyde = 0.2% b.w. Freeable formaldehyde = 0.3% b.w. based on the dry product Methylenic formaldehyde = 41.3% based on the total formaldehyde Free formaldehyde in the = 10 mg/100 g of chip board0 panels EXAMPLE 3 Into a reactor were loaded 521.2 parts b.w. of an aqueous 80% formurea solution, 211.6 parts b.w. of a 71% urea solution, 0.49 parts b.w. of 30% caustic soda, and 0.28 parts b.w. of 80% acetic acid; thereupon there were added 0.27 parts b.w. of 80% acetic acid, thus bringing the pH value to 5.7, and then the mixture was condensed at 950C up to a viscosity at 200C of 1,400 c.P. At this point 0.46 parts b.w. of 30% caustic soda and 84.5 parts b.w. of a 71% urea solution were added to the mixture, whereby the pH value of the solution became equal to 6.3, and the condensation was then carried on at 9000 up to a viscosity at 200C of 2,500 c.PO Then there were added 0.9 parts b.w. of caustic soda, thus bringing the pK value up to 7.9, after which the mixture was cooled. Once the temperature attained 600C, there were added 178.9 parts b.w. of a 71% urea solution and 1.5 parts b.w. of 30% caustic soda, thus bringing the pH to a value of from 7.8 to 8.5, whereafter the cooling was continued until a temperature of 250C was reached. The binder obtained exhibited the following characteristics pH, at 200C = 709 Viscosity at 200C = 515 c.PO Content of dry product = 6402% 2% bew. Maximum tack value = 5.3 kg (according to curve 3 of the accompany ing drawing). Molar ratio formaldehyde/urea: First stage : 2.2 Second stage : 1.8 Final : 1.28 Free formaldehyde = 0.1% b.w. Freeable formaldehyde = 0025% b.w. based on the dry product. Methylenic formaldehyde = 35.8% based on the total formaldehyde Free formaldehyde in the)= 8 mg/100 g of chip board. panels) EXAMPLE 4 Into a reactor were loaded 539.4 parts bow of an 80% formurea solution and 171.8 parts b.w. of a 71% urea solution, the mixture was heated up to 94 C, and there were then added 0.3 parts b.w. of 80% acetic acid, thus bringing the pH of the solution to 5.1. The solution was then condensed at 950G up to a viscosity at 2000 of 1,400 c.P. At this point there were added 0.5 parts b.w. of 30% caustic soda and 109.4 parts bow of a 71% urea solution, whereby the pH value waq increased to 6.0, and the condensation was then carried on at 9000 until a viscosity at 2000 of 2,500 c.P. was attained. Thereupon there were added 0.7 parts b.w. of 30% caustic soda and then the mixture was cooled down. The pH of the solution was 8.0. Once the temperature reached 500C, 176.3 parts bows of a 71% urea solution and 1.6 parts b.w. of 30% caustic soda were added to the mixture to adjust the pH to a final value of from 7.8 to 8.5, where after the cooling was continued until a temperature of 250C was reached. The binder thus obtained exhibited the following characteristics pH, at 200C = 8.1 Viscosity at 200C = 540 cOPO Content of dry product = 6407% b.w. Maximum tack value = 305 kg (according to curve 4 of the accompany ing drawing). Molar ratio formaldehyde/urea: First stage : 2.5 Second stage : 1.9 Final : 1037 Free formaldehyde = 0.2% b.w. Freeable formaldehyde = 0.45% b.w. based on the dry product Methylenic formaldehyde = 33.5% based on the total formaldehyde Free formaldehyde in the) = 15 mg/100 g of chip board. panels)

Claims: C L A I M S : 1. A process for preparing condensation products of formaldehyde with urea, characterized by comprising the following sequence of operations (a) reacting formaldehyde with urea, in a molar ratio of formaldehyde/urea of from 2:1 to 3.5:1, in an aqueous solution, at a temperature of from 800 to 1000C and at a pH of from 4 to 5.8, up to a vis cosity of the solution, at 200 C, of from 400 to 2,000 c. P., corresponding to a content of dry product, referred to the sum of reacted and un reacted monomers, of from 75% to 80% by weight; (b) adding urea to the reaction product until there is attained an overall molar ratio of formaldehyde/ urea of from 1.5:1 to 1.9:1, and carrying on the reaction at a pH of from 509 to 6.8 and at a temperature of from 800 to 950C, up to a viscosity of the solution, at 200 C, of from 2,000 to 3,000 c.P., corresponding to a content of dry product, referred to the sum of reacted and unreacted mono mers, of from 75% to 80% by weight; and (c) adding urea to the reaction product, under cooling conditions of the solution, at a pR of from 7o2 to 8.5, in such an amount as to obtain an overall molar ratio of formaldehyde/urea of from 1.15:1 to 1.4:1. 2. A process as claimed in Claim 1, characterized in that operation (a) is effected at a molar ratio of formaldehyde/urea of from 2.2:1 to 2.5:1, at a pR of from 5.3 to 5.6, and up to a viscosity of the solution of from 1,200 to 1,600 c.PO 3. A process as claimed in Claim 1 or 2, characterized in that operation (b) is effected at a pli of from 6.2 to 6.5, and up to a viscosity of the solution of from 2,400 to 2,800 c.P. 4. A process as claimed in any of Claims 1 to 3, characterized in that operation (c) is effected at a molar ratio of formaldehyde/urea of from 1.15:1 to 1.3:1. Sc A binder for chip board obtained by the process according to any of Claims 1 to 4.

Inventors: CAMAIONI, DOMENICO, CONTI, NATALE
Applicants: MONTEDISON S.P.A.
Application year: 1981
Main classification: C08G12
IPC codes: C09J161, C08G12