WO2000078835A2

WO2000078835A2 - Process of preparing cyclic urea-formaldehyde prepolymer

Info

Publication number: WO2000078835A2
Application number: PCT/US2000/016842
Authority: WO
Inventors: Ted M. Mcvay; James T. Wright
Original assignee: Georgia-Pacific Resins, Inc.
Priority date: 1999-06-23
Filing date: 2000-06-20
Publication date: 2000-12-28
Also published as: WO2000078835A3; AU5748600A

Abstract

The preparation of a high parity cylic-urea-formadehyde prepolymer (cyclic urea prepolymer) is performed at a temperature of at least 85 °C and a pH between 7 and 9, whereby per mole of urea at least 4 mole formaldehyde and 0.5 - 1.2 mole of ammonia or primary amine is added. The obtained prepolymer must have a triazone content of at least 75 % as measured with C-13 NMR.

Description

PROCESS OF PRFPARΪNO CYCLIC UREA-FORM AT. nFHvnF- PRFPOLYMFR

BACKGROUND OF THF TNVFNTTON

1. Field of the Tnvention The present invention relates to a process of preparing a high purity cyclic urea- formaldehyde prepolymer (cyclic urea prepolymer) and a high purity cyclic urea prepolymer composition.

2. Description of Related Art Attempts have been made to reduce the cost of an adhesive by adding urea to thermosetting resins. However, the overall performance of the adhesive is reduced because urea-modified resins are hydrolytically unstable and are not thermally stable. It was discovered that the addition of cyclic urea prepolymers as an additive instead of urea provides a suitable alternative. Cyclic urea prepolymers are prepared by mixing formaldehyde (F), urea (U), and ammonia or primary amine and heating the mixture at an alkaline pH to an elevated temperature for a time sufficient to form a cyclic triazone/triazine polymer, the reactants being present in amounts to provide an initial F/U mole ratio between about 1.2:1 and 1.8:1 and an ammonia to urea mole ratio between about 0.05:1 and 1.2:1. Additional formaldehyde is added to the mixture to yield a cumulative F U mole ratio of between about 1.5:1 and 3.0:1. This process results in a triazone yield of about 50%. The resulting product contains a mixture of chemical structures that together are called triazone.

The mole ratio used to make the triazone determines to a large extent what triazone product or mixture of structures will be made. For example, if the most acceptable prior art mole ratios are used such as F: A:U of 4: 1 :2 then the resulting product theoretically should be mostly the cyclic urea structure with one additional mole of urea reacted onto the amine terminal group. While this product has a cyclic yield by C-13 NMR of 50% it may have certain advantages depending upon the application it is to be used in. For example, it may be desirable in fertilizers to have the terminal urea group. In addition, the pendent urea may be useful as a reactant for formaldehyde to reduce emissions of

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SUBSTTTUTE SHEET (RULE 26) formaldehyde. But, for applications where the triazone is to become part of another polymer the urea can degrade the performance of that polymer.

U.S. Patent 4,778,510 provides an example of prior art processes to prepare a cyclic urea prepolymer. This patent discloses that a narrow pH range is required to achieve the stated level of purity of 50 to 57%. Further, the reaction temperature and mole ratios of urea, formaldehyde and ammonia must be maintained in a narrow range.

The process of U.S. Patent 4,778,510 results in a 50% yield triazone cyclic urea which is believed to have a urea molecule attached to the amine nitrogen on the cyclic structure in the N - terminal position. Such a terminal urea molecule probably provides the desired fairly quick fertilizer release. However, in reactions with phenol, melamine or other thermosetting monomers, this urea hydrolyzes off of the cyclic structure, then it degrades the performance of the adhesive, and can cause a yellow coloration to occur.

The cyclic urea prepolymer may be added into the front of a phenol-formaldehyde resin cook or added as a post blend depending upon the desired properties of the resin. For example, if the cyclic prepolymer is to be used to react with free formaldehyde to reduce volatile emissions of formaldehyde, then the cyclic urea prepolymer should be post added to the resin at the end of the resin manufacturing step. If the action of reducing the free formaldehyde needs to occur quickly, then the resin containing the post added cyclic prepolymer needs to be heated sufficiently to react with the free formaldehyde before shipping the resin. However the cyclic urea prepolymer will not be reacted into the cured structure in a controlled or optimal way and due to the lower reactivity of the cyclic amide groups may not contribute to the cured resin system in which case it may reduce the wet performance of the resin. 50% pure cyclic urea prepolymer when added to the front of a phenol-formaldehyde resin, is unlikely to react with the phenol although it will react with formaldehyde which will alter the basic F:P mole ratio and may reduce the performance of the polymer. While the N- terminal urea portion can react with formaldehyde fairly quickly, the cyclic urea secondary amide group reacts with formaldehyde only very slowly. Thus, it is better if the cyclic urea prepolymer is first reacted with formaldehyde before attempting to react the material with phenol to tie it into the overall polymer structure.

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SUBSTTTUTE SHEET (RULE 26) SUMMARY OF THE INVENTION

The present invention is directed to a high purity cyclic urea prepolymer prepared by a method comprising: mixing formaldehyde, urea, and ammonia or primary amine and heating the mixture at a pH between about 7 and about 9 at a temperature of at least about 85 °C for at least about three hours to form the cyclic urea prepolymer having a purity of at least 75%; wherein at least 4 moles of formaldehyde and about 0.5 to about 1.2 mole of ammonia or primary amine is present for each mole of urea. The minimum required reaction time can range from 4-6 hours depending upon the purity required and the pH.

The present invention is further directed to a method of preparing a high purity cyclic urea prepolymer resin comprising mixing formaldehyde, urea, and ammonia or primary amine and heating the mixture at an initial pH between about 7 and about 9 at a temperature of at least about 85°C for at least about 30 minutes; then adding a suitable acid to lower to a second pH of about 4.5 to about 6; and reacting for a time sufficient to produce the cyclic urea prepolymer resin having a purity of at least 75%, wherein at least 4 moles of formaldehyde and about 0.5 to about 1.2 mole of ammonia or primary amine is present for each mole of urea.

It was discovered that a reaction having two levels of pH, a higher pH of 7 or more, and then, after the ammonia reaction is complete, a lower pH of about 4.5 to about 6 will greatly accelerate the conversion of the cyclic product.

It was also discovered that adding the formaldehyde in to steps will also increase the purity or yield of the product.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a method for obtaining unexpectedly high yields of triazone compositions. It was discovered that the combination of pH range, time, temperature, and mole ratio of ingredients will provide a cyclic urea prepolymer having a high level of purity, that is a triazone content is at least about 75%, preferably at least about 80% and, as high as 90% or more, as measured by C_I3-NMR. U.S. Patent 4,778,510 discloses that potassium hydroxide, as a base for pH adjustment, provides the highest yields of around 50%. Surprisingly, it was found that any

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SUBSTTTUTE SHEET (RULE 26) base capable of increasing the pH to at least 7.5, that does not contribute amine protons, is adequate to achieve much higher yields of triazone if used in the first stage of the process. As cooking temperature is increased over 90°C, then the rate of reaction increases which increases the yield. Also, the higher cooking temperatures (after the initial exothermic reaction between ammonia and formaldehyde is over) reduces the amount of time required to achieve the higher purity.

In accordance with the present invention, a high purity cyclic urea prepolymer is prepared by a method comprising: mixing formaldehyde, urea, and ammonia or primary amine and heating the mixture at a pH between about 7 and about 9 at a temperature of at least about 85°C for at least about three hours to form the cyclic urea prepolymer having a purity of at least 75%; wherein at least 4 moles of formaldehyde and 1 mole of ammonia or primary amine is present for each mole of urea. The minimum required reaction time can range from 4-6 hours depending upon the purity required and the pH.

The pH is at least about 7, preferably between about 7 and about 9, more preferably between about 7.1 and about 7.5. The pH may be maintained by any suitable base which is added during the reaction. Alternatively, pH may be maintained by adding a compound at the start of the batch that will buffer the pH of the batch at the desired pH level. While any base can be used to increase the pH of the reaction mix, preferably alkali metal hydroxides are used such as potassium hydroxide, lithium hydroxide, and sodium hydroxide.

The temperature of the reaction mixture during the holding stage of the triazone should be at least 75°C, preferably at least 80°C, more preferably at least 85°C, even more preferably at least 90°C, and most preferably about 95 °C. A high yield of cyclic urea prepolymer (triazone) depends upon sufficient heat history to complete the reaction which is asymptotic or the reaction rate decreases as the purity increases.

The purity of the product depends on the proper combination of pH, temperature, and time of reaction. For instance, The yield of triazone at 95°C, 7.5 pH, and a reaction time of 3.5 to 4 hours is about 80% whereas the yield of triazone at 90°C is slightly less at about 75 to 77% at the same pH and reaction time. If the temperature of the batch during the hold time is 80°C then the triazone yield is reduced to 60% when held at the same pH and reaction time. If the pH is about 9.5 and the cooking temperature is about

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SUBST-TUTE SHEET (RULE 26) 95°C, then the reaction time is at least 3 hours to obtain the 80% yield. If the cooking temperature is less than 95°C, then the cooking time will be longer. Cooking at a high pH (for example a pH of 9) for three (3) hours at 95°C results in a lower yield (72%) of triazone. Cooking at pH 10.2 caused a precipitate to form in the container during the hold period.

At one hour of reaction at 90°C and pH 7.5 to 9, only 40% yield of triazone is obtained, at 2 hours of reaction, 63% yield is obtained, and after three hours of reaction, 75% yield is obtained. Achieving the full 80+% purity requires at least four hours of reaction time under these conditions. Although additional reaction time would increase the yield of triazone further, such times are usually not commercially feasible.

C₁₃ NMR can be used to determine the mass balance between cyclic urea, free urea, monomeric urea substituted in various and hexamine that provides an excellent near 100% accounting of all the formaldehyde and urea used in the reaction and the conversion of this formaldehyde and urea into triazone. At least 4 moles, preferably about 4 to about 4.5 moles, of formaldehyde and 0.5 to

1.2 moles of ammonia or primary amine is present for each mole of urea. To achieve 80% or higher purity, the mole ratio is at least 4: 1 : 1 F: A:U with 29% ammonia. In contrast, a 4:1:2 F:A:U mole ratio provides a maximum purity of about 50%. A 3:1:1 F:A:U mole ratio provides a purity intermediate between the 4: 1 :2 and the 4:1: 1 mole ratios. The minimum number moles of formaldehyde required to make the triazone at the higher yield is higher than required for lower purity, but once the minimal level of formaldehyde is charged the formaldehyde content is not critical to achieve the desired levels of purity. That is levels of formaldehyde higher than the minimum do not necessarily reduce the yield of the triazone. The ammonia is present in amounts of about 0.5 to about 1.2 per 1 mole of urea, preferably about 0.8 to about 1.1. Although ammonia may be introduced into the urea and formaldehyde at any time as an aqueous solution, it is preferable to use a two-stage addition method to control the exotherm. It is preferred that all the formaldehyde, all the urea, and part, preferably half, of the ammonia solution be added initially. Then the remaining ammonia is added after the exotherm is complete. Anhydrous ammonia may also be used with equal effectiveness.

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SUBSTTTUTE SHEET (RULE 26) Table 1 Typical product analysis from C₁₃-NMR analysis

Component: F:A:U of 4:1 :1 F:A:U of 3:1 : 1 F:A:U of 3:.75:2

Urea 1.5 2.2 2.7

MSU 1.6 11.6 16.3

MDU 0.4 1.7 0

D&TSU 17.6 25.6 24.4

HMT 12.0 9.1 1.8

Triazone 79.2 60.6 56.7

Reading from right to left, the above table shows the unexpected triazone yield that was achieved when the moles of ammonia were increased and the further increase in yield of triazone when the moles of formaldehyde was also increased. These triazone yield levels were obtained after the full three hour reaction time at a pH of 7 to 9.5 and a maximum temperature of 95°C. Shorter reaction times provide lower yields of triazone. The following table demonstrates the importance of reaction time.

Table 2 Impact of reaction time on yield of triazone at 95 °C

Component: 60 Minutes 90 Minutes 1 0 Minutes

Urea 1.3 0.7 1.5

MSU 5.3 3.9 1.6

MDU 0 0 0.4

D&TSU 48.4 39.8 17.6

HMT 25.5 21.5 12.0

Triazone 45.0 55.6 79.2

A full reaction time of three hours is required to achieve a high yield of triazone. The pH was 7 to 9.5 and the mole ratio was 4:1:1. It is important that all of the parameters are maintained to achieve the high yield.

Under the conditions of the present invention, higher yields are obtained with the if the reaction time is extended. However, It is not simply holding the reaction of prior art processes for a longer time. For example, In US Patent 4,776,879 holding the reaction mixture for one hour after the exotherm generates 50% yield of triazone at a mole ratio of 4: 1 :2 F: A:U but, holding the same reaction mixture three to even five hours does not produce any further increase in the yield of the triazone.

Other conditions and circumstances can effect the yield of triazone. For example, if the percentage of methanol in the formaldehyde is doubled, then the time required to achieve the same yield of triazone will be approximately doubled. On the contrary, lower levels of methanol in the formaldehyde will, to a limited extent, increase the yield of triazone because the reaction is faster. If the formaldehyde used to make the triazone comes from a silver catalyzed formaldehyde process where the methanol level could be as high as 1.5 to 2% during times of high production levels, then the reaction mixture must be cooked for as long as six hours. If the formaldehyde comes from the iron catalyzed formaldehyde process with the extra converter then the methanol in that formaldehyde will typically be 0.2 to 0.3 % and this will impact the yield of the triazone. It is known in the art that methanol is a stabilizer for the formaldehyde polymer and will stabilize the reactions of formaldehyde with other components of resins.

Table 3 The impact of other metal hydroxide bases on the yield of triazone

Component OH 3 Hr. LiOH 3 Hr. C OH 3 Hr. NaOH 3 Hr.

Urea 2.31 1.5 4.9 0.7

MSU 4.9 1.6 23.7 2.5

MDU 0.3 0.0 4.9 0.4

D&TSU 17.5 17.6 26.5 16.3

HMT 12.4 12.0 1.4 13.1

Triazone 75.2 79.2 45.0 80.5

In each of the above examples, the corresponding carbonate salt was added to the reaction mixture to maintain the pH at the targeted level of 7.5. Monovalent catalyst have approximately equal performance within the accuracy of the analytical method. When barium hydroxide was as a base, a large amount of precipitate formed in the bottom of the storage container.

The present invention is directed to producing a triazone cyclic structure. There are several structures that are cyclic in nature and there can be substitutions on these cyclic

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SUBSTTTUTE SHEET (RULE 26) structures. In addition, if an amine is used instead of ammonia, then the various N- functional groups will be different but the same high purity will be achieved.

The high yield cyclic urea prepolymer provides a purer product since more triazone is present but also because side products are reduced. The high yield triazone is more stable than the lower purity triazones in, for example, melamine resins, dimethylol urea, etc. Evidence of improved stability is demonstrated by very high yield triazone when concentrated to 80 to 85% solids or 15 to 20% water. This triazone of the present invention is more stable than the lower yield triazone, which cannot be concentrated above 10% due to viscosity limitations The process may be changed so that the triazone is made at the end of the formaldehyde process the way concentrated urea-formaldehyde is made. The reaction time can decreased by increasing the pressure of the reaction for higher than 100 °C reaction temperatures.

In another embodiment, a high purity cyclic urea prepolymer resin is prepared in a process having two levels of pH. Formaldehyde, urea, and ammonia or primary amine are mixed and heated at an initial pH between about 7 and about 9 at a temperature of at least about 85°C for at least about 30 minutes; then a suitable acid is added to lower to a second pH of about 4.5 to about 6. The mixture is reacted for a time sufficient to produce the cyclic urea prepolymer resin having a purity of at least 75%. As above, at least 4 moles of formaldehyde and about 0.5 to about 1.2 moles of ammonia or primary amine is present for each mole of urea.

In this case, the reaction rate is significantly increased requiring half of the reaction time that was required at the higher pH levels. For example, a reaction time of 45 minutes at 90 °C and at pH 9 resulted in only 40% cyclic urea or triazone. A much faster rate of triazone formation was seen when the reaction pH was reduced to pH 5 at the same reaction time and temperature and the resulting purity was 69%. It takes approximately twice the amount of time to increase the purity from 40 to 69% using only the pH 7 to 9 range.

The initial pH is at least about 7, preferably between about 7 and about 9, more preferably between about 7.1 and about 7.5. The second pH is about 4.5 to about 6, preferably about 5 to about 5.5. Any suitable acid may be used to reduce the pH level including, but not limited to sulfuric acid, hydrochloric acid, tannic acid, and acetic acid.

The temperature of the reaction mixture at both pH levels should be at least 75°C, preferably at least 80°C, more preferably at least 85°C, even more preferably at least 90°C, and most preferably about 95 °C.

Skilled practitioners recognize that the reactants are commercially available in many forms. Any form which can react with the other reactants and which does not introduce extraneous moieties deleterious to the desired reaction and reaction product can be used in the preparation of the urea-formaldehyde resin of the invention. Formaldehyde is available in many forms. Paraform (solid, polymerized formaldehyde) and formalin solutions (aqueous solutions of formaldehyde, sometimes with methanol, in 37 percent, 44 percent, or 50 percent formaldehyde concentrations) are commonly used forms. Formaldehyde also is available as a gas. Any of these forms is suitable for use in the practice of the invention. Typically, formalin solutions are preferred as the formaldehyde source. In addition, formaldehyde may be substituted in part or in whole with substituted aldehydes such as acetaldehyde and or propylaldehyde. Glyxal may also be used in place of formaldehyde as may other aldehydes not listed. It is to be recognized that the aldehyde is dissolved (solubilized) in water or other appropriate non- reactive organic of any desired or conventional nature, known in the art. Similarly, urea is available in many forms. Solid urea, such as prill, and urea solutions, typically aqueous solutions, are commonly available. Further, urea may be combined with another moiety, most typically formaldehyde and urea-formaldehyde, often in aqueous solution. Any form of urea or urea in combination with formaldehyde is suitable for use in the practice of the invention. Both urea prill and combined urea- formaldehyde products are preferred, such as Urea Formaldehyde Concentrate or UFC 85. These types of products are disclosed in, for example, U.S. patents 5,362,842 and 5,389,716.

Commercially-available aqueous formaldehyde and urea-containing solutions are preferred. Such solutions typically contain between about 10 and 35 percent formaldehyde and urea. A solution having 35% ammonia can be used providing stability and control problems can be overcome. An aqueous solution containing about 28 percent ammonia is particularly preferred. Anhydrous ammonia may also be used.

Ammonia and or late additions of urea are commonly used prior art techniques to reduce free formaldehyde levels in urea-formaldehyde polymer systems. The former technique suffers from reducing the cured polymers resistance to hydrolysis. The latter technique suffers from a tendency to produce a polymer system that releases smoke during the cure cycle. This present invention reduces or eliminates both of these problems, yet still significantly reduces free formaldehyde levels during cure and in the cured product. In substitution in part or in whole, for the ammonia, any primary amine or substituted primary amine may be used such as methyl amine, monomethanol amine, amino propanol and the like. Further, difunctional amines may be used such and ethylene diamine or any combination of organic amines provided that one primary amine group is available to form the triazone ring. The reaction rates are much faster and more straight forward. Another reactant of interest is sodium sulfamate to make the cyclic urea sulfonate.

The reactants may also include a small amount of a resin modifier such as ethylenediamine (EDA). Additional modifiers, such as melamine, ethylene ureas, and primary and secondary amines for example, dicyandiamide can also be incorporated into the resin of the invention. Concentrations of these modifiers in the reaction mixture may vary from 0.05 to 5.00%. These types of modifiers promote hydrolysis resistance, polymer flexibility and lower formaldehyde emissions.

The following examples are for purposes of illustration and are not intended to limit the scope of the claimed invention.

EXAMPLES

Example 1

A high yield of triazone is produced by charging 4 moles of formaldehyde (240 grams) into a reaction flask of appropriate size that is agitated with heating and cooling and then adding one mole (60 grams) of urea. The pH is adjusted with sodium hydroxide to 7.5 to 8. Next, one half mole of 30 % ammonium hydroxide is added to the reaction

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SUBSTTTUTE SHEET (RULE 26) flask whereupon the mixture begins a strong exotherm to approximately 70°C. The reaction mixture is cooled to 50°C and a second half mole of 30 % ammonium hydroxide is charged with the exotherm carrying the temperature to 80°C. The reaction is further heated to 95 °C and held at this temperature for four hours then cooled down for storage. This general procedure produces a triazone yield of 79 to 80% by C₁₃NMR.

The product can be reacted with phenol by adding cyclic urea prepolymer to all the phenol normally used to make the base resin and adding NaOH (50%) to bring the pH to

10.5. The mixture is heated at 90 - 95°C for one hour or longer depending upon the pH.

The product of this step is a phenol-cyclic urea prepolymer reaction product that can be used to make the extended base resin.

Example 2

240 grams of formaldehyde is loaded into a reactor equipped with cooling coils. The contents of the kettle is cooled to 50°C. 60 grams of urea is charged into the kettle and the agitator turned on. The pH is adjusted to 7.5 with a base. 29 grams of 30% concentration ammonia (ammonium hydroxide) is charged with agitation. The temperature exotherms up to 65-70°C. The reaction mixture is cooled to 50°C and 29 grams of 30 % concentration ammonia is added and the contents of the reaction flask again exotherms to 65-70°C. The reaction container is heated to about 95°C slowly over a 10 minute period to prevent any loss of ammonia, then the pH is reduced to 5 and then maintained at 95°C for two more hours. The container is cooled to 25°C and bottled for storage and analysis. The product of this two hour reaction is a cyclic urea of over 80% purity which demonstrates the use of low pH to reduce the reaction time in half.

Example 3

The products of Examples 1 or 2 may be reacted with melamine by charging melamine into the desired amount of triazone. Where the extension with triazone is low, then formaldehyde will need to be added and the pH adjusted to 9.4 to 9.6 which is the most stable pH to minimize melamine to melamine reaction products. This reaction mixture is reacted for one hour then the normal resin procedure is followed to completion. Example 4

An F:A:U ratio of 4: 1 :2 was reacted with two moles of formaldehyde at 90°C for 30 minutes then blended cold with non-premethylolated triazone of the same F:A:U mole ratios. This cold blend was then cured at various pH values and compared to UF resin cured at the same pH as a coating on standard steel plates at 150 °C for 3 minutes. The following unexpected results were obtained.

Table IN. Minutes of coating resistance to 80°C water of Triazone blend

The UF resin did not tolerate the lower pH values. The blend of the methylolated and non-methylolated triazone unexpectedly maintains the coating integrity at low pH levels.

Example 5

180 grams of formaldehyde is loaded into a reactor equipped with cooling coils. The contents of the kettle is cooled to 50°C. 60 grams of urea is charged into the kettle and the agitator turned on. The pH is adjusted to 7.5 using sodium hydroxide and 29 grams of 30% ammonia is charged with agitation. The temperature exotherms up to 65- 70°C. The reaction mixture is cooled to 50-55°C and 29 grams of 30 % concentration ammonia is added and the contents of the reaction flask again exotherms to 65-70°C. The reaction container is heated to about 95 °C slowly over a 10 minute period to prevent any loss of ammonia. The pH is reduced to 5 and then maintained at 95 °C for one hour. Then 60 grams of formaldehyde is added to the reaction and the pH readjusted to 5 and the reaction continued for 70 minutes. The container is cooled to 25 °C, pH adjusted to 9.0 with buffer and bottled for storage and analysis. The product of this roughly two hour reaction is a cyclic urea of 85 to 90% cyclic content.

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SUBST TUTE SHEET (RULE 26) This product may also be reacted with melamine by charging melamine into the desired amount of triazone. Where the extension with triazone is low, then formaldehyde will also have to be added and the pH adjusted to 9.4 to 9.6 which is the most stable pH to minimize melamine to melamine reaction products. This reaction mixture is reacted for one hour then the normal resin procedure is followed to completion.

Example 6

A 5:1:1 F:A:U product is obtained by loading 300 grams of formaldehyde into a reactor equipped with cooling coils. The contents of the kettle are cooled to 50°C. 60 grams of urea is charged into the kettle and the agitator turned on. The pH is adjusted to 7.5 using sodium hydroxide and 29 grams of 30% ammonia is charged with agitation. The temperature exotherms up to 65-70°C. The reaction mixture is cooled to 50-55°C and 29 grams of 30 % concentration ammonia is added and the contents of the reaction flask again exotherms to 65-70°C. The reaction container is heated to about 95°C slowly over a 10 minute period to prevent any loss of ammonia. The pH is re-adjusted to 7.5 and then maintained at 95°C for 4 hours. The container is cooled to 25°C, pH adjusted to 9.0 with buffer and bottled for storage and analysis. The product of this reaction is a cyclic urea of 80-85% cyclic content. This product is less desirable because the free formaldehyde in the product is excessive compared to the 4:1:1 mole ratio triazone.

It should be understood that while the invention has been described in conjunction with specific embodiments thereof, the foregoing description and examples are intended to illustrate, but not limit the scope of the invention. Other aspects, advantages and modifications will be apparent to those skilled in the art to which the invention pertains, and these aspects and modifications are within the scope of the invention, which is limited only by the appended claims.

Claims

WHAT TS CLAIMED IS:

1. A high purity cyclic urea prepolymer prepared by a method comprising: mixing formaldehyde, urea, and ammonia or primary amine and heating the mixture at a pH between about 7 and about 9 at a temperature of at least about 85°C for at least about three hours to form the cyclic urea prepolymer having a purity of at least 75%; wherein at least 4 moles of formaldehyde and about 0.5 to about 1.2 moles of ammonia or primary amine is present for each mole of urea.

2. The prepolymer of claim 1 wherein about 0.8 to about 1.1 moles of ammonia or primary amine is present for each mole of urea.

3. The prepolymer of claim 1 wherein about 4 to about 4.5 moles of formaldehyde or is present for each mole of urea.

4. A method of preparing a high purity cyclic urea prepolymer comprising: mixing formaldehyde, urea, and ammonia or primary amine and heating the mixture at a pH between about 7 and about 9 at a temperature of at least about 85°C for at least about three hours to form the cyclic urea prepolymer having a purity of at least 75%; wherein at least 4 moles of formaldehyde and about 0.5 to about 1.2 moles of ammonia or primary amine is present for each mole of urea.

5. The method of claim 4 wherein the pH is between about 7.1 and about 7.5.

6. The method of claim 4 wherein the pH is obtained by adding an alkali metal hydroxide.

7. The method of claim 5 wherein the alkali metal hydroxide is sodium hydroxide

8. The method of claim 4 wherein initially all of the formaldehyde and urea and part of the ammonia is reacted, and then the remaining ammonia is added and reacted.

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SUBSTΓΠJTE SHEET RULE 26

9. The method of claim 4 wherein the temperature is at least about 90°C.

10. The method of claim 8 wherein about one half of the ammonia is initially reacted.

11. A high purity cyclic urea prepolymer resin prepared by a method comprising mixing formaldehyde, urea, and ammonia or primary amine and heating the mixture at an initial pH between about 7 and about 9 at a temperature of at least about 85°C for at least about 30 minutes; then adding a suitable acid to lower to a second pH of about 4.5 to about 6, and reacting for a time sufficient to produce the cyclic urea prepolymer resin having a purity of at least 75%, wherein at least 4 moles of formaldehyde and about 0.5 to about 1.2 moles of ammonia or primary amine is present for each mole of urea.

12. The prepolymer of claim 11 wherein about 0.8 to about 1.1 moles of ammonia or primary amine is present for each mole of urea.

13. The prepolymer of claim 11 wherein about 4 to 4.5 moles of formaldehyde or is present for each mole of urea.

14. A method of preparing a high purity cyclic urea prepolymer resin comprising mixing formaldehyde, urea, and ammonia or primary amine and heating the mixture at an initial pH between about 7 and about 9 at a temperature of at least about 85°C for at least about 30 minutes; then adding a suitable acid to lower to a second pH of about 4.5 to about 6; and reacting for a time sufficient to produce the cyclic urea prepolymer resin having a purity of at least 75%, wherein at least 4 moles of formaldehyde and about 0.5 to about 1.2 moles of ammonia or primary amine is present for each mole of urea.

15. The method of claim 14 wherein the initial pH is between about 7.1 and about 7.5.

16. The method of claim 14 wherein the second pH is between about 5 and about 5.5.

17. The method of claim 14 wherein the initial pH is achieved by adding an alkali metal hydroxide.

18. The method of claim 17 wherein the alkali metal hydroxide is sodium hydroxide.

19. The method of claim 14 wherein the temperature is at least about 90°C.

20. The method of claim 14 wherein the suitable acid is sulfuric acid.