Introduction

[001] The invention relates to a process for the continuous production of an aqueous hydroxy-aryl formaldehyde resin solution, comprising the steps of preparing a reaction mixture of a hydroxy-aryl compound and aqueous formaldehyde, adding a catalyst to the reaction mixture and reacting the reaction mixture in the presence of the catalyst. The invention further relates to a continuous plug flow polymerisation reactor and to the use of such reactor in a process for preparing a resin solution The invention also relates to the use of a specific continuous mixing device for continuous mixing of catalyst into a reaction mixture. The invention further also relates to the hydroxy-aryl formaldehyde resin solution and to the resin obtainable by the process, to the use thereof for the manufacture of adhesive compositions, to adhesive compositions comprising the hydroxy-aryl formaldehyde resin.

Prior art

[002] Attempts to make phenol resins continuously have a long history and thus a large number of prior art. The efforts for continuous processes began in the 20ies when V. H. Turkington made an attempt to find a continuous process for phenolic resins in US1660403. He used a simple tube reactor with a coil in a heating oil bath as heating device. This type of tube reactor is different from the invention because of the discontinuous mixing of phenol and formaldehyde and US1660403 does not use a static mixer. Although the patent does not mention any Reynolds numbers it is not likely that turbulent or quasi-turbulent stage is reached, which has been found to be necessary for the technology of this invention.

[003] US3816376 describes a continuous phenolic resin production process but teaches a cascade of at least three stirred vessels with back mixing of the solution. According to the Bodenstein theory, the back mixing reduces the Bodenstein number (see below) and widens the molecular weight distribution, which gives undesired resin molecules according to the working theory of this invention. Thus, US3816376 has a great disadvantage in the process and the object of the invention is to provide a process that does not have at least one of the mentioned disadvantages of the prior art.

[004] Another cascaded process is described in GB680245 where a predetermined portion of phenol and formaldehyde is pumped into a cascade of vessels, where a portion of formaldehyde in every vessel is applied in order to prevent gelling. By back-mixing the resin which is obtained has a wide variety of molar ratios. Also GB 783537 teaches a cascaded vessel system, where the resin which has been formed in the vessel cascade is removed from the vessel bottom or be collected in a resin collector. From this collector the resin is removed to be dehydrated Cascade technologies are not aim of the present invention. [005] US3657188 claims a static mixing device only to mix the formaldehyde and the phenol stream, whereas the static mixer is no device to carry out the reaction for resin making; the solution is pumped through heat exchanger. The first mixing can also be maintained by a batch mixer. Thus there is no similarity to our invention.

[006] Another approach is described in GB1460029, which uses an reaction vessel which includes a system of baffled walls to get a mixture through the flow of the resin solution. Formaldehyde and Phenol are mixed and heated together in a preheated vessel and then pumped through the baffled reactor, but this reactor type is basically different from the present invention.

[007] JP2005075939 mentions a manufacture approach of novolak, where the formaldehyde and phenol is mixed with a phosphoric acid solution in a continuous mixing device. Although there is no indication that this mixing device is used for the reaction, it is mentioned that both, mixing devices with mechanical moving parts and mixing devices without mechanical moving parts can be used. The latter type may include in-line mixers with 180 degree rotation, which have a reduced mixing performance as the in-line mixers mentioned in this invention. Additionally, the condensation reaction takes several hours, which is too long for such a producing method.

[008] JP5986618 shows a semi-continuous approach, where acid, phenol and formaldehyde are premixed in a batch vessel and then continuously pumped through a column which is filled with mixing device of the type as they are known from distillation and rectification. Like JP20050075939 also in-line mixing devices with 180 degree rotation are described, which are not aim of this invention. The disadvantage of this application is the insufficient mixing effect.

[009] A mechanically mixed tube is introduced by WO03037955, whereas the reaction educts are added into a multistage stirring structure to be reacted; in the second step unreacted material is removed. Multistage stirring mechanical devices are not comparable with the present invention.

[0010] US4413113 teaches a continuous feeding of phenol and paraformaldehyde, which is put into a mixing vessel by a conveyor screw. The mixing is discontinuous and the slurry is made up with water to a solution which is - after addition of catalyst - pumped into a liquid heated coil with circulation means. GB1323301 shows a very similar approach, with the difference that all ingredients are added at the same time. These approaches are not comparable to the approach in this invention.

[0011] It was found that the process according to the invention can produce a hydroxy-aryl- formaldehyde solution in a continuous process without the need to use expensive equipment for processing highly viscous melts, like extruders as mentioned in WO9908856 or WO2004092239. Although it may be desired for certain applications to use an additional concentration step, the process according to the invention can provide a low solid containing liquid as well as a highly concentrated solution. Due to the well defined reaction time a well defined molecular weight distribution is obtained.

[0012] EP1785438 describes a continuous production for phenolic novolac resins which premixes the phenol, formaldehyde and acid catalyst and then the mixture is pumped through a tube reactor into the product vessel. The tube reactor has - among other possibilities like reduction of diameter and thus flow speed enhancement or variations of the length of the heating zone - also the possibility to be equipped with in-line mixers. The disadvantage of the system compared to the invention is the u-shaped tube, which support the occurrence of undesired wall effects and gel particles. Also this system is only suitable for Novolak, while the present invention can be used also for resoles.

Description of the invention

[0013] According to the invention there is provided a process for the continuous production of an aqueous hydroxy-aryl formaldehyde resin solution, comprising the steps of a. preparing a reaction mixture of a hydroxy-aryl compound and an aqueous formaldehyde, b. adding a catalyst, c. reacting the reaction mixture in the presence of the catalyst, characterised in that, in step c) the reaction takes place in a continuous plug flow of the reaction mixture, followed by d. an optional step of adding an amount of amino compound after reaction, e. an optional step of removing water to reach a higher solid content.

[0014] The catalyst can be mixed into the reaction mixture in different ways known in the art. The catalyst can be continuously added to a pre-mixture of the hydroxy-aryl compound and the aqueous formaldehyde or the catalyst can be added to the hydroxy-aryl compound followed by continuous addition of the aqueous formaldehyde. In one embodiment, in step a) the reaction mixture is prepared by mixing the hydroxy-aryl compound, preferably phenol, substituted hydroxy-aryl compounds, dihydroxy arenes, resorcinol, as a 63 - 100 wt% solution (wt % relative to the total solution weight) and the formaldehyde as a concentrated formaldehyde aqueous solution to a total solid content of 30 - 75 wt% (dry weight relative to the total weight of the reaction mixture), where after the catalyst is continuously added and finely dispersed into the reaction mixture through one or more, preferably multiple, addition points.

[0015] In an alternative embodiment, the reaction mixture is prepared by pre-mixing the hydroxy-aryl compound, preferably a phenol, substituted hydroxy-aryl compounds, dihydroxyarenes, resorcinol, as a 63 - 100 wt. % solution (wt% relative to the total solution weight) with the catalyst, where after the formaldehyde is continuously added as a concentrated formaldehyde aqueous solution to a total solid content of 30 - 75 wt% (dry weight relative to the total weight of the reaction mixture) and finely dispersed into the reaction mixture through multiple addition points. [0016] Then in step c) the reaction takes place in the formed reaction mixture in a continuous plug flow of the reaction mixture, an optional step d) adding an amount of amino compound, preferably urea or melamine, or an aldehyde, e.g. furfural after and an optional step e) for removing water to reach a higher solid content.

[0017] With the term "continuous production process" is implied that at least the reaction step is a continuous process step. The reaction mixture may be prepared separately in a batch and then added to the continuous reaction step or, more preferably, the reaction mixture is prepared in a continuous process, preferably in a prepended continuous mixing device and continuously added to the plug reactor tube reaction.

[0018] Hydroxy-aryl compounds which can be used to prepare these resins are generally aromatic hydroxyl compounds, in particular phenol and various substituted hydroxy-aryl compounds including ammo phenol, the ortho, meta and para cresols, cresylic acid, xylenol, resorcinol, catechol, hydrochinon, bisphenol A, quinol (hydroquinone), pyrogallol (pyrogallic acid), phloroglucinol, or combinations thereof. Preferably, the aromatic (di-)hydroxyl compound is a dihydroxy-arene, in particular resorcinol or hydrochinon, phenol or bisphenol A. Most preferably, the aromatic hydroxyl compound is phenol. In the preparation of a reaction mixture (step a) the hydroxy-aryl compound is added as a concentrated aqueous solution, liquid or as a solid. In case of a concentrated aqueous solution of the hydroxy-aryl compound, the concentration preferably is as high as possible, preferably at least 63 wt%, more preferably at least 80 wt%, even more preferably at least 85 wt% or even at least 90 wt% and most preferably at least 95% of its saturation value. Most preferably the phenol will be dosed as a liquid and not as solution (MP Phenol about 4O ⁰ C)

[0019] These compounds or combinations thereof can be reacted with the various aldehydes as a class, preferably those having from 1 to about 10 carbon atoms in aliphatic or cycloaliphatic or aromatic or mixed form, to produce the condensation-type resins useful in the invention. Such aldehydes include, for example, formaldehyde, glyoxal, glutaraldehyde, acetaldehyde, propionaldehyde, crotonaldehyde, benzaldehyde, furfuraldehyde, and the like. Formaldehyde is presently preferred. The formaldehyde is added as a concentrated formaldehyde aqueous solution The term "concentrated" implies having a formaldehyde concentration of at least 40, preferably at least 45, more preferably at least 50 and most preferably at least 55 wt% in water (wt% relative to the total weight of the formaldehyde solution). By adding highly concentrated or solid hydroxy-aryl compound and concentrated formaldehyde aqueous solution, a reaction mixture is obtained having a very high concentration of hydroxy-aryl formaldehyde reactants. The total amount of hydroxy-aryl formaldehyde reactants in the reaction mixture is between 30 and 75 wt%, preferably between 45 and 70 wt%, more preferably between 50 and 65 wt% (dry solids weight relative to the total weight of the reaction mixture). [0020] In principle, the relative amount of formaldehyde and hydroxy-aryl compounds may vary between broad ranges. The molar ratio hydroxy-aryl compound to formaldehyde P/F, defined as the number of phenol-groups P divided by formaldehyde groups F in the hydroxy-aryl compound (P/F), can range between 0.1 and 10.0, preferably between 0.2 and 2.5, and most preferably between 0.33 and 1.6, for resoles preferably between 0.33 and 1 and for novolaks preferably between 1.0.and 1.6. [0021] The process can be used where the reaction conditions are not so critical, for example for preparing hydroxy-aryl formaldehyde solutions paper impregnations usually having an P/F molar ratio below 2, for example between 1.2 and 1.8. However, the process according to the invention is designed for and is preferably used in more critical conditions.

[0022] The catalyst in the reaction can be a basic or an acid catalyst for all hydroxy-aryl formaldehyde resins. However, for Novolak resins an acid catalyst is preferred and for resole formaldehyde resin a basic catalyst is preferred. It is essential for the control of the reaction that in step b) the catalyst is continuously added and finely dispersed into the reaction mixture through one or more addition points. It is highly preferred to have multiple addition points to achieve a quick homogeneous dispersion of the catalyst However, although less preferred, acceptable results can also be obtained using a single addition point in combination with appropriate mixing means, preferably a prepended static or dynamic mixing device, optionally in combination with a lower temperature. In many cases good results can be obtained by using undiluted acids like oxalic acid or undiluted bases like amines, sodium hydroxides are preferably used at concentrations of 50%. Preferred ways of adding the catalyst are described below.

[0023] A Novolak reaction mixture is preferably reacted in the reaction step in the presence of an acid catalyst, preferably a protic acid, preferably a strong acid such as Paratoluenesulfonic acid or oxalic acid. A Resole reaction mixture is preferably reacted in a reaction step in the presence of a basic catalyst, for example sodium hydroxide or amines.

[0024] The reaction step c) takes place in a continuous plug flow of the reaction mixture. The specified method of catalyst addition creates a very homogeneous reaction mixture which, under the continuous plug flow conditions with a well defined residence time, low backflow coefficient and narrow residence time distribution, can be reacted in a continuous process to a highly concentrated resin solution without substantial risk of a runaway reaction. The continuous plug flow reaction conditions can be characterized by a Bodenstein number. The Bodenstein number is a dimensionless characteristic number describing the relationship between the moles convectively supplied to the moles supplied by diffusion. Thus the Bodenstein number characterizes the back mixing within a system according to the formula: Bo = v x L / D _ax wherein v = flow velocity [m s ^"1 ], L = length of the reactor [m] and D _ax = axial dispersion coefficient, [m ² s ^"1 ]. The length of the reactor is defined as the length between the start and the stop of the reaction, typically between point of the addition of the catalyst and of the addition of the catalyst stopper. The Bodenstein number in the process according to the invention is at least 10, typically between 20 and 50. Preferably, the number is as high as possible, preferably at least 15, more preferably at least 20, even more preferably at least 30, more preferably at least 50. To achieve such high Bodenstein numbers, it is preferred that in the process the reaction mixture is continuously mixed during the reaction by in-line mixing elements in laminar or quasi-laminar plug flow. Alternatively, the reaction mixture can be mixed by turbulent plug flow. It is known in the art how to determine the Bo number for a specified process. This involves calculating theoretical velocity distribution curves for different Bo numbers for a given reactor, then measuring the velocity profiles, normalizing and fitting of the determined profiles on the calculated theoretical profiles to find the Bodenstein number.

[0025] In a preferred embodiment of the invention, a reaction mixture is prepared (in step a) by reacting the formaldehyde with the hydroxy-aryl compound in the presence of an acid or a base catalyst to produce a solution of methylolated hydroxy-aryl compound in case of insulation resins or to react the educts to higher substituted polymers/oligomers. The methylolation step is preferably done at a temperature chosen above a sedimentation temperature where sedimentation of the reactants may occur, at least 30, more preferably at least 40 and most preferably at least 50 ⁰ C. Preferably the temperature is chosen below the reaction temperature to avoid excessive polymerization, above 3O ⁰ C, and below 150°C, preferably for Resols between 5O ⁰ C and 100 ⁰ C, and most preferably between 60 and 90 ⁰ C and preferably for Novolaks between 70 ⁰ C and 130°C, and most preferably between 80 and 110 ⁰ C. The pH in the methylolation step in case of Novolak is preferably adjusted between 1.0 and 14.0, in case of Novolak between 1.0 and 7.0 and in case of Resole between 7.0 and 14.0.

[0026] In the process according to the invention the pressure can rise above atmospheric pressures and the temperatures can be well above the boiling temperature of water, i.e. above 100 ⁰ C. The pressure during the condensation step is preferably above atmospheric pressure because the formaldehyde is more reactive at high temperature and high pressure conditions. In view of obtaining a higher reactivity the pressure is preferably at least 1.1 , more preferably at least 1.5, even more preferably at least 2, even more preferably at least 3 and most preferably at least 5 bar. The pressure is typically in a range between 1 and 20 bars, more preferably between 1 and 15 bars and most preferably between 1 and 10 bars. A further advantage of the high temperature during the condensation reaction is that the viscosity of the product stream is low. At low viscosity the catalyst stopper can be mixed in more quickly and more homogeneously, so a sharper cut off of the condensation reaction can be obtained resulting in better product homogeneity, a smaller molecular weight distribution and lower risk of gelation.

[0027] The viscosity of the reaction mixture at the start of the reaction step is between 1 and 100 mPas. In this description, the term viscosity implies viscosity determined according to DIN EN ISO 3219 at room temperature (20.0 +/- 0.2°C) and at a shear rate of 200/s. The viscosity of a sample was measured according to DIN EN ISO 3219 with a rotary viscosimeter (PAAR PHYSICA MCR 51 ). The system contains two static symmetric coaxial plates between which a bubble free liquid sample is applied of which viscosity has to be measured. One of the plates rotates with a defined angle velocity (rotor), while the other is static (stator). One plate is connected to a system which is able to measure the rotary moment at the point of overcoming the friction resistance of the plates with the liquid. The shear rate is 200/s.

[0028] The reaction rate depends strongly on the pH and to a lesser extent on the reaction temperature. The pH in the reaction step generally is between 1.0 and 14.0, preferably for Novolaks between 1.0 and 7.0 and in case of Resoles between 7.0 and 14.0. In view of obtaining a high reaction rate and production capacity (at lower pH) without a too high risk of a run-away reaction, the pH in the reaction step is preferably adjusted, in case of Novolaks, between 1.0 and 4 and in case of Resoles 7 and 13 The temperature during the reaction is preferably between 50 and 150 ⁰ C, more preferably between 50 and 12O ⁰ C. The residence time in the reaction step is preferably chosen between 0.5 and 300 minutes. In the process of the invention very low residence times can be achieved, preferably below 300, more preferably below 200 and even more preferably below 100 minutes

[0029] The above-described methylolation step and the subsequent reaction step are preferably combined and both performed consecutively in continuous plug flow conditions, preferably in the same tube. In this embodiment the pressure in the methylolation step and the condensation step is preferably the same. The methylolation step is not necessarily distinguished from the reaction step by a change of the pH or by a raise in temperature. The change in pH and temperature do not necessarily need to coincide; the temperature can be allowed to rise after lowering the pH.

[0030] Considering that the reaction is exothermal the generated heat must be transported out of reaction mixture. This is preferably done by cooling the reactor, for example using a cooling liquid circulating in a double wall around the reactor tube. In view of avoiding run-away reactions and an undefined broad molecular weight distribution, it is most important that the heat transfer is efficient and that the temperature is very homogeneous throughout the reactor cross-section. It was found that the static mixing elements in the process according to the invention provide such efficient mixing and temperature homogeneity during the reaction. It is most easy to choose the set- temperature during the reaction to be substantially isothermal because this provides an easier control of the reaction temperature conditions. Optionally, the reaction step may comprise two or more, preferably 2 to 10, preferably 2 to 6, more preferably 2 to 4 consecutive substantially isothermal sub-steps at different set temperatures.

[0031] In the process according to the invention it is preferred that the reaction rate is set and/or controlled by controlling the pH with the amount of catalyst added. For a chosen residence time and temperature conditions in the reactor, the reaction rate is adjusted by the amount of added catalyst such that at the end of the reaction step, preferably at the end of the reactor, the desired viscosity of the formaldehyde resin solution is obtained. The desired viscosity depends on the envisaged application, but is typically for Resols between 5 and 6000 mPas (determined at 20° according to DIN EN ISO 3219:10/94 ), for Novolaks preferably between 5000 and 60000mPas (at 40 ⁰ C)

[0032] At the end of the reaction step the reaction is stopped by cooling of the reaction mixture and/or by adding and mixing an additional compound (e.g. urea) or a catalyst stopper (neutralisation). The reaction mixture is cooled before, during and/or after said addition. In the process according to the invention, the reaction mixture directly obtained after the reaction step has a very high solids content of 40 to 75 wt.%, preferably 45 to 75 wt.%, more preferably 50 to 75 wt.% at most preferably 55 to 75 wt.% without an additional concentration step.

[0033] In view of the envisaged end use as adhesive composition, it is preferred to reach a molar ratio of hydroxy-aryl (P) to formaldehyde (F) compound (defined by P/F) is between 0.1 and 10.0, preferably between 0.2 and 2.5, and most preferably between 0.33 and 1.6, for resoles preferably between 0.33 and 1 and for novolaks preferably between I .O.and 1.6, with a total dry solid content preferably between 30 and 100 wt%. The hydroxy-aryl formaldehyde resin solution obtained after the reaction step (c) is preferably cooled down, optionally according to the application it can also be reacted with one or more portions of amino compound (step d) preferably urea or it can be neutralized. The compound added is chosen in view of the desired properties of the envisaged user application. For example, the additional compound can be higher aldehydes e.g. Furfural) or Urea. The post-addition of urea has the advantage of further decreasing residual free formaldehyde, which is crucial for certain applications. The amount of additional component can range between 0,1 and 30 weight percent, but preferably is between 2 and 25, more preferably between 3 and 20 weight percent of hydroxy-aryl compound is added.

[0034] To be able to use in step d) a temperature below 100 ⁰ C and atmospheric pressure where the reactivity is relatively low, urea is the most preferred post addition compound in view of its highest reactivity. Melamine can also be used, but is less reactive and less preferred. Apart from the additional amino compounds, hydroxy-functional aromatic compounds, preferably phenol can be added and preferably reacted at temperatures above 100 ⁰ C to achieve full conversion of the hydroxyfunctional aromatic compound, in reaction with the resin solution.

[0035] In the post addition step d) the amino compound can be added as a highly concentrated solution in water. However, it is preferred to add water as little as possible. The additional amino compound, preferably urea is preferably added as plastified composition to avoid too much addition of water. The plastification of the amino compound urea can take place by kneading the compound at elevated temperature where the compound is plastic, preferably close to the melting point, preferably in a single- or double screw extruder or a planet-extruder. The advantage thereof is that the decomposition of the compound is prevented. Optionally, a small amount of water is added to prevent too high temperatures and decomposition. The addition is preferably done continuously from the fore mentioned plastification extruder, but may also be done by continuous addition from a batch. [0036] The resin can further be extended by addition, preferably after step (d), of 1 to 15, preferably 1 - 12, more preferably 1 - 10 wt% (relative to the total weight of the resin solution) of extension components, preferably chosen from the group of rape seed flour, proteins, wheat flour. This is mostly done to reduce the cost of the resin solution.

[0037] In a plug flow reaction, the reaction mixture is pumped through the housing, preferably a tube, wherein the chemical reaction proceeds as the reaction mixture travels through the tube. In a preferred embodiment of the process according to the invention, the reaction step takes place in a static mixer comprising housing, preferably a tube, containing in-line mixing elements. The static mixer is preferably used in laminar or quasi-laminar flow conditions at Reynolds number below 500, and is even more preferred at Reynolds number below 300 or below 100. In a preferred embodiment the plug flow conditions are characterized by a Reynolds number less than 30, preferably less than 25 and more preferably less than 20. In an alternative embodiment of the invention, plug flow conditions and intense mixing can also be achieved without mixing elements if the reaction takes place in a tube reactor with turbulent plug flow, most preferably at a Reynolds number above 2300 Good results may be achieved in thee process according to the invention Reynolds number above 1500, more preferably above 1700, even more preferably above 2000. Preferably, the Bodenstein number is at least 10, typically between 20 and 50. Preferably, the number is as high as possible, preferably at least 15, more preferably at least 20, even more preferably at least 30, more preferably at least 50.

[0038] The in-line mixing element can in principle be any object that changes the flow direction of the reaction mixture in the tube and causes mixing. The in-line mixing elements can be glass beads, metal balls, spikes or baffles statically positioned inside the tube. The static mixer device preferably consists of a number of consecutive static mixer elements positioned in the tube (in-line) comprising one or more baffles at an angle relative to the flow direction forcing changes in the flow direction. The baffles in each subsequent mixer element have an orientation different from the orientation in the previous and/or following mixer element. Preferably, the orientation of the baffles in each consecutive mixer element is offset by 90°. The baffles can be alternating spiral parts with each spiral part offset by 90°. Preferably, the mixing elements comprise two sets of two or more, preferably to 2 - 10, baffles wherein the baffles of each set are positioned essentially plan-parallel to each other, but at an angle, preferably 90°, with the baffles of the other set and wherein each baffle of one set is positioned alternately next to a baffle of the other set in a crosswise arrangement to divide the flowing reaction mixture into separate streams flowing in different directions within the mixer element, and wherein the baffles in each subsequent mixer element are offset at an angle, preferably 90°, relative to the previous mixer element. Suitable static mixers are for example mikromakro© mixers from company Fluitec. The static mixer preferably has a double wall for temperature control with a cooling fluid. [0039] Normally, static mixers are used for mixing or blending and optionally reacting two or more fluid streams. In the process according to the invention the reaction does not involve mixing of two or more fluid streams. Instead, in the present invention the static mixers are used for creating homogeneous reaction conditions throughout the reaction mixture to maintain a good heat transition to the tube walls and achieve homogenous temperature distribution to avoid local temperature rises due to the exothermic reaction. The mixing elements maintain a high shear force avoiding wall effects whilst maintaining a plug flow at laminar flow conditions. The static mixer reactor maintains a running reaction of the hydroxy-aryl compounds and formaldehyde, wherein the reactants and reaction products are intensely mixed to maintain a well defined molecular distribution and a low risk of a runaway reaction and gelation. Another advantages that the static mixer reactor does not have moving parts (e.g. propeller mixer etc.), which makes the process more reliable

[0040] To obtain a homogeneous temperature distribution and mixing it is preferred that the reaction mixture is continuously mixed from the beginning to the end of the reaction, that is from the moment the catalyst is added to the moment that a basis added to neutralize the catalyst and stop the reaction. Therefore, it is preferred that the static mixer comprises in-line mixing elements substantially along its full length. In view of avoiding wall effects and obtaining sufficiently homogeneous temperature and molecular distribution, it is preferred that the tube of the static mixer has an inner diameter at most 10 cm, preferably at most 7 cm, more preferably at most 5 cm and most preferably at most 3 cm. because of the preferred small diameter, in large number of mixing elements are needed; preferably at least 22, more preferably at least 44, more preferably at least 66 mixing elements.

[0041] As described above the rate is highly dependent on the pH and it is essential that (in step b) a catalyst is continuously added to the flowing reaction mixture and finely dispersed into the reaction mixture through one or more addition points. The catalyst can be added in different ways, for example by one or more, preferably multiple separate supplies (for example tubes or hoses) each connected to the tube of the static mixer. Preferably, the catalyst is continuously added in a prepended continuous mixing device comprising a tube with mixing elements wherein the tube has one or more, preferably multiple addition points for finely dispersing the catalyst into the reaction mixture

[0042] The static mixing device of step b) more preferably is double walled comprising an inner tube and an outer casing wherein the inner tube comprises at least 4, preferably at least 6 static mixing elements which tube is perforated, preferably only at the position of the first or first two mixing elements, and wherein the outer casing provides a closed space over at least the perforated part of the inner tube and has an inlet opening for adding catalyst in said closed space to finely disperse droplets of the catalyst through the perforations into the inner tube. Alternatively the static mixing device can be one or more perforated tubes, which can be inserted into one or several static mixing elements, preferably into the center. [0043] For a good operation of the mixing device, the number of addition points (preferably perforations) is preferably high, the diameter of the perforations is small and the perforations preferably are arranged substantially equidistant on the surface of the tube. Preferably the tube comprises 4 - 40 perforations having a diameter between 0.1 and 1 mm, preferably between 0.1 and 0.5 mm. The number of perforations and the diameter thereof are chosen in view of the amount of catalyst that needs to be added. Almost ideal mixing is achieved when the reaction mixture in the static mixer passes 8 mixing elements. However, acceptable mixing may also be achieved with 4 or 6 mixing elements. The invention also relates more generally to the use of the above described static mixing device in a process for the continuous preparation of a resin solution, preferably a formaldehyde resin solution, most preferably a hydroxy-aryl compound resin solution described herein, for continuously dispersing a catalyst through said addition point(s) into a reaction mixture flowing through the tube.

[0044] Regarding the mixing several options are possible. One possibility among others is addition of phenol and catalyst before, then adding of aldehyde, or addition of aldehyde to phenol and then addition of calalyst. It is to be understood that the present invention may be embodied with other mixing modifications which may occur to skilled artisans without departing from spirit and scope of the present invention.

[0045] The invention also relates to a continuous plug flow reactor and to the use thereof in a process for the preparation of hydroxy-aryl formaldehyde resins, comprising a static mixer comprising a thermostated, preferably double walled, tube having an inner diameter of between 2 and 10 cm, preferably between 2 and 7 cm, more preferably between 2 and 5 cm and comprising at least 20, preferably at least 40, more preferably at least 60 and most preferably at least 80 in-line mixing elements providing a high mixing efficiency, preferably characterised by a Bodenstein number of at least 20, preferably at least 40, more preferably at least 60 and most preferably at least 80.

[0046] More in particular, the invention relates to a continuous plug flow reactor for the continuous production of a resin solution, comprising, a) an optional mixing section for preparing a reaction mixture, b) a continuous mixing device as described above for mixing catalyst into the reaction mixture, c) a continuous plug flow reactor section as described above and d) an optional catalyst stopper or additional compound section comprising a thermostated static mixer comprising a catalyst stopper or additional compound inlet and static mixing elements for mixing the catalyst stopper or a additional compound into the reaction mixture. The above-mentioned components are connected to operate as one continuous tube reactor.

[0047] The invention further relates to an hydroxy-aryl formaldehyde resin solution obtainable by the process according to the invention described above, having a viscosity for Resols between 5 and

6000 mPas (determined at 20° according to DIN EN ISO 3219:10/94 ), for Novolaks preferably between 5000 and 60000mPas (at 40 ⁰ C) and a solids content between 30-100 wt.% The invention also relates to the resin isolated from the resin solution. The hydroxy-aryl-formaldehyde resin solution which is directly obtained by the process according to the invention can be used without removal of water, i.e. additional solution concentration step, for the manufacture of an adhesive composition. The invention also relates to an adhesive composition comprising the hydroxy-aryl-formaldehyde resin and optional additional hardeners and additives.

[0048] The invention is illustrated with the drawing in Figure 1 describing a continuous plug flow reactor for the manufacture of a resin solution, comprising a catalyst addition section (1 ), a continuous plug flow reactor section (2) comprising four connected static mixers, a catalyst stopper inlet section (3), a catalyst stopper or additional compound mixing and cooling section (4) and a resin solution outlet section (5). All sections (1 ) to (5) are provided with static mixing elements and are connected to operate as one continuous tube reactor, wherein the reaction mixture is continuously makes from the beginning (6) to the end (7) of the reactor. The catalyst addition section (1 ) comprises an outer tube (8) providing a closed space for feeding catalyst through the perforations (13) into the inner tube. Samples of the reaction mixture can be withdrawn (12) for measurement of initial values, for example of the pH or the viscosity. The continuous plug flow reactor section (2) in this example comprises four static mixers. The static mixers are double walled for cooling and are provided with temperature and/or pressure measuring means (c) and control means (9) to monitor and control the temperature and/or pressure conditions. In this example, the four static mixers are connected and controlled in a single cooling loop to keep substantial isothermal conditions over the full length of the reactor. The catalyst stopper and additional compound inlet section (3) comprises a sample withdrawing means (12) for measurement, in particular of the pH and the viscosity and means (10) for addition of a catalyst stopper/additional compound. In the preferred mode, a steady state operation mode is set by adjusting the amount of catalyst added (8) to get the desired viscosity of the resin solution at (12). The catalyst stopper/additional compound is mixed into the reaction mixture in section (4) and cooled by cooling water (11).

[0049] The obtained end-product, i.e. the product obtained directly condensation or, as the case may be, after post addition of additional amino compound, are characterised in that they have a very small Mw polydispersity ^ ^~~ Mn compared to conventional batch produced resin solutions (wherein Mw is the weight average molecular weight, and Mn is the number average molecular weight as determined by gel permeation chromatography (GPC)). In a PF resin, Q for that portion of the resin in the Mw range 900 and 3000 gr/mole is below 1.15, more preferably below 1.12, even more preferably below 1.10 and most preferably even below 1.08. For the portion of the resin in the Mw range 3000 and 4000, Q-values of below 1.20, preferably below 1.15 or even below 1.10 are achievable. For the portion of the resin having a Mw above 4000, preferably in the range 4000 to 10000, the Q may be somewhat higher, but still below 1.4, preferably below 1.35, more preferably below 1.30 and even more preferably below 1.25. [0050] The PF resin solution according to the invention is advantageously used as binder resin in particle boards, for example wood fiber boards or as paper impregnation resin. The invention therefore also relates to particle board comprising particles and a binder resin, or resin impregnated paper wherein the binder resin is the PF resin according to the invention. The resin has distinct advantages in the manufacture and end-quality of the particle boards because of the molecular characteristics and high reactivity of the resin. A further advantage of the present invention is that the particle board made with the PF resin solution according to the invention have, at comparable composition and production conditions, lower formaldehyde emission (measured by EN 120) compared to traditional batch prepared particle boards. Formaldehyde Emissions typically are below 6, preferably below 5.5, more preferably below 5, even more preferably below 4.5 and most preferably below 4 mg / 100 g.

[0051] The particle board product comprising PF resin according to the invention cures faster compared to boards made by traditional batch resins. The production time of a particle board according to the invention (expressed in sec/mm; time required to fully cure per mm thickness of the board) is preferably at least 5% lower, more preferably at least 10%, even more preferably at least 20% and most preferably at least 30% lower (based on production rate of 8 sec/mm and average board pressing temperature of 220 ⁰ C in a particle board press). A shorter press time also implies less energy consumption and higher production capacity. The reactivity of the PF resin can be measured by determining the B-Time. After the hot plate of the B-time-oven is tempered to 13O ⁰ C, 420μl of the sample (or 0,5g) will be placed in a hollow of the hot plate (the equipment is described in DIN 16916-02-C1 ). At the moment when the whole sample amount is placed on the plate a stop watch will be started. The sample will be stirred circularly from the rim to the centre with a glass rod. The diameter of the glass rod is 5mm, at the top 2mm. If the B-time is longer than three minutes the sample will be stirred continuously for one minute and then in intervals of one minute for 10 seconds. When the sample becomes viscous, the stirring should be done continuously The endpoint will be indicated by lifting the glass rod. When the sample should snap up rubber-like the end point is reached and the time measure is stopped.