PROCESS FOR THE PREPARATION OF A CONDENSATION RESIN

The present invention relates to a process for the preparation of a condensation resin in an aqueous medium at elevated temperature and pressure. Processes for the preparation of condensation resins are either performed batch wise, or in recent days also (semi-) continuous. Examples of batch wise preparation are those which are performed in a stirred tank reactor, or even a combination of several of stirred tank reactors, all operated in batch mode.

A continuous process has also been considered in the art, an example whereof being presented in EP-A-355,760. In this publication a melamine- formaldehyde precondensate is prepared in a single or double screw extruder.

This has as a consequence that in the extruder (in fact a tubular reactor with dynamic elements) a lot of mixing energy is consumed, next to the fact that only viscous streams can be dealt with. The present invention recognizes that the use of an extruder is not appropriate for processes for a preparation of a condensation resin in which the process stream has a viscosity well below 50 Pa. s. The present invention also acknowledges that preparation at such low viscosities at more elevated pressures than normally attainable in an extruder are desired, as a result of which the process can be performed at a higher temperature. This all being the consequence of the fact that the resin preparation is performed in an aqueous medium. Feeding the monomeric ingredients, or monomeric precursors of the condensation resin, directly to a continuous working reactor or reactors, has the disadvantage that severe process control is needed in order to be able to feed the good proportions of the needed monomeric precursors to the reactor. For a good preparation of a desired condensation resin also the ratio of the monomeric ingredients (monomeric ratio) fed to the reactor is crucial, which needs dedicated process control and analysis.

The present invention presents a solution for the above indicated items, in that the process comprising a process step in which an aqueous master batch of at least two of the monomeric precursors of the condensation resin is prepared in a continuous loop system at a temperature below 20 ⁰ C, and a consecutive process step in which a portion of the master batch is fed continuously to one or more tubular reactors provided with static mixer elements and positioned parallel to each other, said tubular reactor(s) individually being operated at a temperature between 70 and 200 ⁰ C, and at a pressure between 0.2 and 20 MPa. As a result hereof, the process comprises

a master batch preparation in combination with one of more continuously operated tubular reactors, said reactors being parallel to each other coupled to the master batch preparation.

As another result, the process can be performed with process streams that have a significant lower viscosity than those which are suitable for an extruder-operated process. Generally, the present process can cope with viscosities up to 10 Pa. s; more preferred the viscosity of the contents of the reactor is at most 1800 mPa.s; the viscosity being determined at the local conditions in the reactor (i.e. at the local pressure and temperature conditions). Preferably, also the master batch is prepared in a continuous process. In such a system one can easily change the monomeric ratio in order to prepare condensation resins with different compositions. The alternative is the use of several master batch containers, each having their own, and from each other, differing monomeric ratios. In the preparation of the aqueous master batch(es) it should be taken care that essentially no reaction between the monomeric precursors occurs. This is to avoid that an undetermined (pre-)condensation (= polymerization) already occurs between the master batch ingredients, as a result of which the specification of the feed to the tubular reactors(s) is unknown, or at least not fully clear. As the entrance conditions of the tubular reactor(s) are than not fully known, it will also be difficult to control the degree of condensation of the stream exiting the tubular reactor(s).

For each condensation resin to be prepared in the process of the present invention, the condition under which essentially no reaction occurs between the monomeric precursors is naturally different. On the other hand, the skilled man wanting to prepare a specific condensation resin is aware of and knowledgeable with conditions under which essentially no condensation occurs. "Essentially no reaction" in the context of this invention means that at least 90 mol.- % of the monomeric precursors are present in unreacted form; preferably this amount is at least 95 mol.- %. Of course the occurrence of a condensation polymerization between the monomeric precursors in the master batch preparation can be avoided by the addition of polymerization inhibitors, but such inhibitors will be of hindrance in the consecutive process step where the condensation polymerization should take place (in the tubular reactor(s)). An alternative is the absence of condensation-accelerators or catalysts in the master batch preparation. Nevertheless the above, the temperature of the master batch shall always be a critical factor for avoiding a (start of the) condensation polymerization. This means that the temperature in the master batch preparation has to be below room temperature

(= 20 ⁰ C). More preferred the temperature in the master batch preparation does not exceed 10 ⁰ C. The master batch preparation is in the form of a continuous process and this continuous master batch process is performed in a loop system, to which system one or more of the tubular reactors are coupled in parallel. When the master batch is in the form of a dispersion/slurry, care should be taken that the homogeneity of such a dispersion/slurry is secured. The vessel in which the master batch is present can therefore be stirred. The loop can also be provided with means to maintain or secure the homogeneity. As such also static mixers can be used as such means in the loop. Each tubular reactor used in the process is, in the inside of the reactor, provided with one or more static mixer elements. Such a reactor, also named a static mixer, can be described as a pipe with immovable internal elements that achieve continuous multiple splitting and recombination, and/or turbulence of streams of material passing through and improve distributive mixing. A short compilation of various types of static mixer elements can be found for instance in: Thakur et al: Static Mixers in the Process Industries - A Review, TranslChemE, 81 (2003), 787 - 826 The tubular reactor can be filled over its full length with the static mixer elements, but also a partially filled tubular reactor (seen in the axial direction) can be applied. The unfilled part of the tubular reactor can be used for e.g. heating or cooling of the mixture.

As a result of the presence of static mixer elements in each tubular reactor a better flow profile in the process stream, with a beneficial influence on both mass and heat transfer during the transport of the process stream through the reactor is achieved. This beneficial influence is also present in comparison with the use of an empty, tubular pipe, as is disclosed in WO 2006/1 19982, for the preparation of a melamine-formaldehyde resin. The process of the present invention is suitable for the preparation of any condensation resin, in which said preparation takes place in an aqueous medium; or in other words: in all preparations where water is either a solvent or a dispersion agent. Other solvents or dispersing liquids may be present, next to water, but they are only present in a minor amount compared to water. A non-limiting list of condensation resins that can be prepared with the process of the present invention is polyamides, polyacetals, polyesters, and adhesives useful in engineered wood, such as condensation resins based on phenol, melamine (or more generally triazines), urea, and aldehydes (like (para)formaldehyde). In general, here and herein after, a condensation resin is any class of polymer formed through a condensation reaction, releasing (or condensing) a small molecule by-

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product such as water or methanol, as opposed to an addition polymer which involves the reaction of unsaturated monomers.

Condensation polymerization, a form of step-growth polymerization, is a process by which two molecules join together, with the loss of a small molecule which is often water. The type of end product resulting from a condensation polymerization is dependent on the number of functional end groups of the monomer which can react. Monomers with only one reactive group terminate a growing chain, and thus give end products with a lower molecular weight. Linear polymers are created using monomers with two reactive end groups; monomers with more than two end groups give three dimensional polymers when crosslinked.

The process of the present invention is, in each tubular reactor, performed at elevated pressure and temperature, which can be selected for the preparation of the desired resin, within the boundaries of the conditions needed for said preparation. The temperature is between 70 and 200 ⁰ C; the pressure is between 0.2 and 20 MPa. Preferably, the temperature is between 100 and 150 ⁰ C, at a pressure which is at least the corresponding vapor pressure. Of course the skilled man can decide to select his own pressure, deviating from the vapor pressure, for instance by using a pressure control. The dimensions of the tubular reactor can be chosen freely, depending on the desired throughput. Preferably the tubular reactor has a circular cross-section. The diameter of the tube will in general be at least 5 mm, and will not exceed 500 mm. The length will in general be at least 25 mm, and will not exceed 100 m. The skilled man is able to select the material of the tubular reactor and the static mixer elements, based on the materials to be processed in the reactor.

In general the process can be applied using an aqueous medium, in which the solids content of the resin is between 15 and 90 wt.%; preferably between 20 and 85 wt.%; more preferred this content is between 45 and 75 wt.%.

The process of the present invention is very suitable for the preparation of a condensation resin, wherein said resin is prepared from an aldehyde (preferably (para-)formaldehyde), a triazine (preferably melamine), an aromatic alcohol (preferably phenol), or urea. Next to the individual ingredients, also mixtures of said ingredients can be used (like a mixture of melamine, urea and formaldehyde, resulting in a so-called MUF-resin, or a mixture of melamine, urea, phenol, and formaldehyde, resulting in a MUPF-resin). The preparation of the resin according to the present invention can also start with a so-called precondensate, which is a low-molecular precursor of the desired resin, but in which already some degree of condensation

between the constituting monomers has taken place. Preferably, the condensation resin to be prepared according to the process of the present invention is an aminoplast (a condensation resin based on a triazine or urea, and an aldehyde) or a phenolic resin (a condensation resin based on an aromatic alcohol, and an aldehyde). In the case of an aminoresin, the triazine is preferably melamine; the aldehyde is preferably (para-)formaldehyde. In the case of a phenolic resin, the aromatic alcohol is preferably phenol; the aldehyde is preferably (para-)formaldehyde,

Even more preferred is a condensation resin, based on (para-) formaldehyde, melamine and urea. In case a condensation resin is prepared having melamine and

(para-)formaldehyde as the at least two constituting ingredients of the final resin, the F/M ratio (being the molar ratio between the (para-)formaldehyde (F) and the melamine (M) in the condensation resin) is generally between 0.25 and 7.5; preferably this ratio is between 0.5 and 6.0. More preferred, this ratio is between 1.0 and 1.8. In the present process, all the melamine-formaldehyde resins as disclosed in WO 2006/1 19982 can be prepared.

The ingredients necessary for the preparation of the condensation resin are metered to the tubular reactor(s) as an aqueous master batch.

The master batch is generally fed to the tubular reactor(s) via the front end of that tubular reactor. Between the container/loop of the master batch and said reactor(s) a preheating of the master batch mixture can be performed, to at least partially preheat the mixture from the temperature used in the master batch preparation. Equipment suitable therefore are known to the skilled man.

Of course also a combination of the above can be used. Reference can be given to a situation in which part of one or more of the ingredients needed for the condensation resin is fed as a premix to the tubular reactor, and in which the remaining amount(s) are directly fed to the reactor, possibly at multiple locations along the length of the reactor. All this is within the skills of the man skilled in the art of process technology. Next to the ingredients necessary for the preparation of the condensation resin, additives can also be present in the final resin. These additives, the nature and function of which are known to the skilled man, are also fed to the tubular reactor(s), or added to the master batch feed to the reactors, and are for instance catalyst, fillers, emulgators, wetting agent, anti blocking agent, etc. The reader is referred to the literature hereon, including the teachings of the EP-A-355,760.

The product resulting from the process of the present invention is a condensation resin in an aqueous medium, at elevated temperature and pressure. This makes this product very suitable for use in an impregnation process, in which a substrate (like paper, wool, etc.) is impregnated with the resin, especially when the impregnation process is also performed under elevated pressure (which results in an improvement of the degree and/or speed of impregnation).

This effect is the more suitable, when the pressure in the impregnation process is essentially the same or lower, than the pressure in the resin preparation. Then the process for the preparation of the condensation resin and the impregnation process can be directly coupled, so as to avoid for instance storage of the resin, and reheating and repressurizing the stored resin to the impregnation conditions.

The impregnation process can be a one-step process in which only one resin is used for impregnation; or a multi-step process, in which two or more different resins are used. As an example, a first impregnation can be performed with a urea/formaldehyde based resin; and a second impregnation with a melamine/formaldehyde based resin. Both feeds can be prepared in one system according to the invention; preferably the resin feeds are prepared in two parallel positioned systems, each comprising a master batch preparation and one or more, static mixer filled, tubular reactors. Preferably, in the system wherein the preparation of the condensation resin and the impregnation process is the master and dictates the conditions of the slave (the resin preparation). In other words: the impregnation process gives a feed-back to the resin preparation, in order to have a resin preparation consonant with the needs in the impregnation process. Preferably, the impregnation process controls the resin preparation via a process control unit. Such a control unit can be a computer, which, given the desired conditions of the impregnations, sets and controls the process parameters for the resin preparation (like monomer ratio, temperature, concentration of monomers, etc.).

The invention will be elucidated with the following Examples and comparative experiment, which are intended to show the benefits of the invention. The Examples and experiment were performed in one or more heated steel tubular reactor(s) with an internal diameter of 10 mm and a length of 2.0 m. The reactor(s) were provided with 96 SMXL static mixer elements of Sulzer having a diameter of 10 mm. The result of the Examples was determined with respect to the so- called water-tolerance (W.T.) of the obtained resin. This WT. is the amount of resin that can be dissolved in water at room temperature (dimension: gram/gram).

Example I

Formalin (with 30 wt. % formaldehyde (F)), melamine (M), di-ethylene glycol (DEG) and caprolactam were mixed with water to obtain as a master batch a dispersion with an F/M-molar ratio of 1.65. Table 1 gives the used recipe (in wt. %). The ingredients were mixed in a storage tank, provided with an external loop with a circulation pump. The temperature in the storage tank was 5 ⁰ C; the pressure in the tank was atmospheric.

A feed pump was placed in the loop in order to feed part of the circulating master batch to the tubular reactor. In the feed line a heat exchanger was present to preheat the feed to the tubular reactor.

Table 1

The feed to the tubular reactor was varied. The temperature of the mixture entering the tubular reactor was 120 ⁰ C. At the exit of the tubular reactor the temperature was 140 ⁰ C; thereafter the mixture was cooled via a water bath to room temperature. The pressure in the tubular reactor was set at 1 MPa. Table 2 gives the realized water tolerances (W.T.) of the produced resin, as a function of the flow through the tubular reactor.

Table 2

Example Il

Example I was repeated, but now with an F/M molar ratio of 1.4. Table 3 gives the recipe (in wt. %).

Table 3

The flow was set at 5.6 kg/h; it resulted in a water tolerance of 0.6.

Comparative experiment A

Example I was repeated, but now in absence of the static mixer elements in the tubular reactor (i.e. with the use of a non-filled tube). After 4 hours of experimentation, the tube appeared to be plugged internally due to the formation of polymer on the internal wall of the tube.

Example III

Example I was repeated, but now with 3 tubular reactors coupled parallel to each other. For this coupling, three feed pumps were placed in the external loop of the master batch preparation system. Each feed pump provided individual feed to one reactor. Every reactor had individual feed heater and temperature control, so it was possible to produce three resins that differed in WT. at different rates simultaneously. Additionally, reactor 3 had a feed point for formalin upstream the feed heater where additional formalin was added to increase the F/M-ratio. The results are presented in Table 4.

Table 4

Example IV

The resin produced in Example 1.4 was used to continuously and inline impregnate a paper sheet. For this, a master batch preparation system and a reactor were fitted in front of the resin kitchen of an existing paper impregnation line.

Resin was produced according to the recipe given in Table 1 and the process conditions of Example 1.4. The resin produced in the continuous reactor was immediately processed in the resin kitchen and fed to the paper impregnation line. The thus produced impregnated paper sheets were used to press test specimens of laminated resin paper on a wood based panel. Test results showed a resin quality equal to commercial product. In new equipment designs, the resin kitchen can even be omitted by feeding the impregnation ingredients to the reactor, creating a direct coupling from the reactor to the impregnation line, if desired

Example V

To an existing paper impregnation line, capable of core- and surface impregnation, two master batch systems and two reactors were fitted to the resin kitchens of said impregnation line. For core impregnation, a urea-formaldehyde resin was continuously produced in the first master batch /reactor system, processed and fed to the core impregnation section of said impregnation line. For surface impregnation, a melamine-formaldehyde resin was continuously produced in the second master batch/reactor system, processed and fed to the surface impregnation section of said impregnation line. The thus produced impregnated paper sheets were used to press test specimens of laminated resin paper on a wood based panel. Test results showed a resin quality equal to commercial product.