GB2027439A

GB2027439A - Wood Adhesive

Info

Publication number: GB2027439A
Application number: GB7926713A
Authority: GB
Original assignee: Borden Inc
Current assignee: Hexion Inc
Priority date: 1978-07-31
Filing date: 1979-07-31
Publication date: 1980-02-20
Also published as: GB2027439B; ES482972A1

Abstract

A process for preparing a wood adhesive composed of 2 to 8 mols formaldehyde, 1 to 1.6 mols melamine and 1.7 to 3.3 mols urea for each mol of phenol, which comprises condensing phenol and formaldehyde in a reactor, subsequently adding melamine to the phenol-formaldehyde condensate in the reactor and continuing the condensation of melamine at a phi of 8.5 to 9.5 and at a temperature of 85 to 100 DEG C until the contents of the reactor become clear, and adding 1.7 to 3.3 mols of urea to the reactor and condensing it with the phenol-formaldehyde- melamine condensate until a water tolerance of 70 to 180% is reached.

Description

SPECIFICATION Wood Adhesive This invention relates to a process for preparing a resin which is a condensate of phenol, formaldehyde, melamine and urea. This resin is particularly useful in making wood adhesives which are stable and wood products manufactured therewith that have superior strength properties and weatherability.

Most wood adhesives used today are based on urea formaldehyde resins which are not considered to have acceptable weatherability. Although melamine resins are increasingly used to meet higher requirements for bond strength and moisture resistance, they are not considered to be suitable for preparing weatherproof adhesive joints. Weatherproof wood joints have only been produced with phenolic resins capable of alkaline cure, but such joints are of dark colour and such phenolic resins can damage wood under the conditions of curing.

Since urea is inexpensive compared with melamine, it is desirable to use as much urea and as little melamine as possible, but it is well known that the use of urea in wood adhesives results in poor weatherability and poor durability of bonds formed with such adhesives. It has unexpectedly been discovered that, when condensed in a proper sequence, adhesives containing condensed urea will be stabie and will provide excellent weatherability to bonds formed thereby.

In the process according to the present invention, the resin is prepared by condensing phenol, formaldehyde, melamine and urea in the specified sequence. It is essential to condense formaldehyde with phenol and then condense melamine with the phenol-formaldehyde condensate before introducing urea. If this condensation sequence is not observed, a resin will not be obtained which can be used to make adhesives with weatherproof quality.On the basis of one mol of phenol, the proportions in mols of the other ingredients are as foliows: preferred broad range range phenol 1 1 formaldehyde 3-5 2-8 malamine 1.2-1.5 1-1.6 urea 1 1.8-1.9 1.4-2.3 urea II 0.65-0.75 0.2-1 Triethanolamine and caustic soda are used in the preferred embodiment to adjust the pH to the desired level. Other alkaline substances can also be used.

In preparing the condensates, formaldehyde and phenol are added to a reaction vessel and mixed thorougly. This will take the reaction between phenol and formaldehyde to the methylolation stage.

Sufficient triethanolamine is added to raise the pH to 5.6 to 6.4, preferably 5.8 to 6.2, then, with caustic soda, pH is increased to 8.5 to 9.5, preferably about 9.0. Cooling of the reactor contents is commenced and when a temperature of 400C or below is reached, melamine is added with vigorous agitation to ensure that all of the melamine is wetted. At this stage the contents are heated to 80 to 1 000C, preferably 93 to 950C, and heid at this temperature for 10 to 45 minutes, preferably about 20 minutes. Condensation of melamine is considered complete when solution becomes clear. This is followed by the addition of the first portion of urea over a period of 5 to 20 minutes, preferably 7 to 13 minutes, while holding the temperature at about 93 to 950C to obtain a polymer of desired chain length.After all the urea solubilizes, the reaction is held at 80 to 1000C, preferably 87 to 890C, until water tolerance of 70 to 180%, preferably 100 to 150% is reached. The contents at this stage are cooled to 45 to 500C and an additional 0.3 to 1 mol of urea, preferably 0.65 to 0.75 mols, is added to react with excess formaldehyde, with continued cooling to about 250C. Finally, the pH is adjusted with triethanolamine to 7.5 to 8.5, preferably 8.0 to 8.4.The condensate, in its preferred embodiment, will meet the following requirements: water dilutability 100 to 150% solids 60 to 62% Brookfield viscosity at 250C 90 to 130 cps specific gravity 1.270 to 1.285 pH 8 to 8.4 colour amber There is on the market a BASF resin known as Kauaromin 540, which is apparently covered by the Mayer U.S. Patent 3,734,918. The Mayer patent discloses in column 1 that, prior to this invention, weatherproofness could only be achieved with phenolic resins capable of alkaline cure. This patent describes the preparation of a product by condensing melamine and/or urea with formaldehyde in aqueous solution and then reacting the condensate with phenol with or without a small amount of formaldehyde and melamine.On the basis of 1 mol of melamine, 1.7 to 3 mols of formaldehyde and 0.05 to 0.2 mols of phenol are utilized. The one mol of melamine can be replaced in part by urea wherein at least 15% is melamine and less than 85% is urea, computed on molar basis. As will be shown, the BASF resin cannot provide acceptable degree of weatherability, as measured by the standard 6-cycle test under boiling and freezing conditions.

The invention will now be further described with reference to the following examples.

Example 1 A polymer was prepared according to the present invention by reacting the following ingredients, measured in parts by weight. 53.92 parts of formaldehyde, as a 50% aqueous solution, and 1 0.25 parts of phenol were mixed in a reactor and triethanolamine was then added with mixing to bring the pH to about 6. To raise the pH to about 9, a sufficient amount of caustic soda was admixed. The contents of the reactor were then cooled to below 400C and melamine was added with vigorous agitation. Melamine, in an amount of 19.04 parts, was condensed with phenol-formaldehyde by heating the contents of the reactor to about 95"C. After holding the temperature at about 95oC for about 20 minutes until the solution cleared, which indicated completion of melamine condensation.

12.31 parts of urea were added with mixing over a period of about 10 minutes while the temperature was maintained at about 950C. After the urea was solubilized, it was condensed at 87 to 890C to a water tolerance of 100 to 1 50%. At this point, contents of the reactor were cooled to 45 to 500C and an additional amount of 4.48 parts of urea were added to react with the excess formaldehyde. Cooling was continued until the condensate reached a temperature of about 250C, at which point, pH was adjusted with triethanolamine to about 8.2. This resin was stable for about one mqnth.

Example 2 In this example, BASF condensate was prepared as described in Example 1 of U.S. Patent No.

3,734,918. To a reaction vessel, the following were charged: 54.22 parts by weight of a 63% solids urea-formaldehyde resin, 13.96 parts melamine, 5.92 parts of 84.55% phenol and 2.41 parts water.

The contents of the reactor were mixed thoroughly and brought to a pH of 8.5 to 8.7 with the addition of 25% caustic soda. While maintaining the pH at about 8.5, condensation was allowed to proceed at about 900C to a viscosity of 700 cps at 200C. The condensate was then cooled to about 21 OC. This resin was unacceptable for use after about one week.

The resins of Examples 1 and 2, above, were evaiuated in the laboratory under the same conditions by making particleboard samples and subjecting the samples to various tests to determine relative performance thereof. The resins were sprayed onto the chips without adding anything else to the resins. The table below summarizes important features of said tests: Table 1 6-cycle, % ret.

resin IB MOR lB MOR Ex.1 121 2648 6.1 9.5 Ex.1 122 2817 4.5 8.8 Ex. 2 129 3148 fell apart after 3 cycles Ex. 2 125 2837 fell apart after 3 cycles In the above Table 1, "IB" represents internal bond, "MOR" stands for modulus of rupture and the 6cycle test shows percent retention of the values after the 6-cycle test was concluded. Although the absolute values were not impressive due to the fact that the tests were carried out in laboratory at press times of 4 and 5 minutes, the relative values show the superiority of the resin prepared according to the present invention.

The resin of Example 1, containing 2% ammonium sulphate solids in the liquid resin, was also evaluated in a commercial operation. In a particular test, the particleboard samples were 6.35 mm (0.251 of an inch) in thickness and level of the resin was at 12% by weight. The samples exhibited a modulus of rupture of 2666 which, after the standard 6-cycle test, showed retention of 50.19/0. To the best of the Applicants knowledge, this is the only resin of any type which can provide retention after the 6-cycle test in excess of 50% at a relatively low curing temperature of about 143-1 540C.

Claims

1. A process for preparing a wood adhesive composed of 2 to 8 mols formaldehyde, 1 to 1.6 mols melamine and 1.7 to 3.3 mols urea for each mol of phenol, which comprises condensing phenol and formaldehyde in a reactor, subsequently adding melamine to the phenol-formaldehyde condensate in the reactor and continuing the condensation of malamine at a pH of 8.5 to 9.5 and at a temperature of 85 to 1000C until the contents of the reactor become clear, and adding 1.7 to 3.3 moles of urea to the reactor and condensing it with the phenol-formaldehyde-melamine condensate until a water tolerance of 70 to 180% is reached.

2. A process as claimed in claim 1 in which the condensation of phenol and formaldehyde is accomplished by mixing the two ingredients together sufficiently to reach methylolation level.

3. A process as claimed in claim 1 or claim 2 in which the urea is added in two stages: 1.4 to 2.3 mols in the first stage after the condensate of phenol-formaldehyde-melamine is prepared and 0.3 to 1 mol in the second stage after water tolerance of 70 to 180% is reached.

4. A process as claimed in claim 3 in which the condensate is cooled to a temperature of 45 to 500C before the second portion of urea is added, the condensate is cooled to about room temperature after the second portion of urea is added and the pH of the finished condensate is adjusted within the range of 7.5 to 8.5.

5. A process as claimed in any of claims 1 to 4 in which the amount of ingredients in the adhesive includes 3 to 5 mols formaldehyde; 1.2 to 1.5 mois melamine, 1.8 to 1.9 mols as the first portion of urea and 0.65 to 0.75 mol of urea as the second portion.

6. A process as claimed in any of claims 1 to 5 in which the phenol-formaldehyde condensate is cooled to a temperature of below 400C before melamine is added, melamine is condensed with the phenol-formaldehyde condensate at a temperature of about 93 to 95"C, the first portion of urea is added uniformly over a period of 5 to 20 minutes to the phenol-formaldehyde-melamine condensate maintained at a temperature of 93 to 950C, and the condensate is condensed to a water tolerance of about 100 to 150%, the finished condensate having the following specifications: water dilutability 100 to 1 50% solids 60 to 62% Brookfield viscosity at 250C 90 to 130 cps specific gravity 1.270 to 1.285 pH 8to8.4

7. A process as claimed in claim 6 in which melamine is added to the phenol-formaldehyde condensate when the condensate is at a temperature of 300C and melamine is condensed at the temperature of 93 to 950C for about 20 minutes until the condensate becomes clear.

8. A process as claimed in claim 1 substantially as herein described with reference to any of the Examples.

9. A wood adhesive when prepared by a process as claimed in any of claims 1 to 8.