WO2012112043A1

WO2012112043A1 - A method for reducing the formaldehyde content of a resinous starting material

Info

Publication number: WO2012112043A1
Application number: PCT/NL2012/050085
Authority: WO
Inventors: Gerardus Wilhelmus Schuren
Original assignee: Trespa International B.V.
Priority date: 2011-02-16
Filing date: 2012-02-16
Publication date: 2012-08-23
Also published as: NL2006218C2

Abstract

The present invention relates to a method for reducing the formaldehyde content of a resinous starting material, wherein the method comprises the steps of: a) providing a phenolic formaldehyde resin as starting material having a formaldehyde content in the range of from 0.01 wt.% to 25 wt.%., and b) subjecting said phenolic formaldehyde resin as starting material to a contact process in the presence of a heterogeneous catalyst such that the formaldehyde of the resinous material thus treated is lower than the formaldehyde content of the resinous starting material, preferably in the range of from 5 wt.% to 0 method for reducing the formaldehyde content of a resinous starting material.Awt.%.

Description

A method for reducing the formaldehyde content of a resinous starting material.

The present invention relates to a method for reducing the formaldehyde content of a resinous starting material. I n addition, the present invention relates to the use of such a resinous material as well as to a panel comprising resin impregnated cellulose fibers.

US patent No. 4, 1 16,921 relates to a resin to be used in the production of moulded products. According to said document, such resins are characterized by a relatively narrow molecular weight distribution and low molecular weight, wherein the polydispersity of such resins is low. The polydispersity ranges from about 1 .5 to about 5, in particular from about 1 .7 to about 3. Said document furthermore indicates that the duration of the reaction is determined by the desired polydispersity.

I nternational application WO 01/46101 relates to so-called stable bisphenol compositions, which are used in usual lamination processes.

US patent No. 4,337,334 relates to the preparation of a phenol resin, wherein the phenol component comprises the group of high molecular weight phenolic compounds, which latter compounds are obtained as by-products in the preparation of bisphenol A.

Several publications relate to the reduction of the formaldehyde content in resins, e.g. EP 0 148 050, EP 0 480 778, US 6 1 14 491 .

A phenol resin that is mentioned in the introduction is known per se from International application WO 91/19749, which has the same inventor as the present application. According to said I nternational application, the molecular structure of the phenol resin obtained as a result of the reaction must meet a number of requirements, using a special ratio of the total number of reactive sites (A) in the phenol resin to the total number of sites (B) in the phenol resin to which formaldehyde is added, to the total number of sites (C) in the phenol resin in which two molecules of the phenolic compounds are condensed with each other through a methylene group, which ratio is as follows: (A): (B): (C)= 1 :(0.85 to 1 .0):(less than or equal to 0.05, in particular less than or equal to 0.02).

From International application WO 01/74750 there is furthermore known a mixture which is used in the preparation of phenol resins, epoxide resins or formaldehyde resins, which starting mixture contains 35 to 75 wt. % of ρ,ρ-bisphenol A, 5 to 25 wt. % of ο,ρ-bisphenol A and 20 to 50 wt.% of secondary products which are produced during the preparation of bisphenol A, wherein the sum of the proportions by weight of ρ, ρ-bisphenol A and ο,ρ-bisphenol A is 50 to 80 wt. % and wherein the sum of the proportions by weight of ρ,ρ-bisphenol A and ο,ρ-bisphenol A and the secondary products is 100 wt.%. I n particular, the mixture additionally contains 0 to 90 wt. % of phenol, with respect to the total weight of the mixture then produced. Further details with regard to the phenol resin prepared with said starting material are not provided in said document, not to mention the specific requirements that are made of a phenol resin used in the production of rigid moulded products.

US 2008/0085968 relates to molding compositions, in particular to thermoplastic molding compositions which comprise polyoxymethylene polymer, zeolitic material and thermoplastic polyurethanes. The use of zeolitic material as a constituent of a polyoxymethylene-containing molding composition is to reduce formaldehyde emission.

EP 0 619 344 relates to acetal resin composition comprising specific oxymethylene copolymers, hindered phenol type antioxidants, and ion adsorbents. The acetal resin compositions are capable of avoiding the formation of mold deposits and the smell of formaldehyde gas during mold .

J P 2008 260923 relates to a polyacetal resin composition which is excellent in the low formaldehyde emissions and thermal stability.

J P 2006 181537 relates to a formaldehyde gas treatment agent or coating material excellent in formaldehyde adsorption capability and formaldehyde selectivity. The formaldehyde gas treatment agent contains a composite metal hydroxide.

US 6,590,020 relates to thermoplastic polyoxymethylene molding materials containing from 10 to 99.98% by weight of a polyoxymethylene homo- or copolymer and may contain further conventional additives and processing assistants, such as formaldehyde scavengers, plasticizers, adhesion promoters and pigments in an amount of from 0.001 to 5% by weight.

WO 96/34041 relates to a molded article of a resin composition obtained by adding a specific amount of a solid solution of a specific magnesi um oxide and aluminum oxide to a polyoxymethylene resin. These molded articles from the polyoxymethylene resin composition can be used in gears, chassis, cams, rollers, and the like.

The present inventors found that the presence of formaldehyde in a resinous material will result in the emission of unwanted components when the resinous material has been used in the manufacturing of for example construction materials, like HPL (high pressure laminates) panels. In addition, the release of formaldehyde during its production in the factory should be kept as low as possible, due to safety and health requirements.

The object of the present invention is to provide a method for catalytically reducing the formaldehyde content of a resinous starting material such that it is possible to reduce the formaldehyde content significantly without adversely effecting the physical characteristics of the resinous material itself.

Another object of the present invention is to provide a method for catalytically reducing the formaldehyde content of a resinous starting material such that it is possible to obtain a resinous material without additional, unwanted chemicals, especially so-called formaldehyde scavengers.

Another object of the present invention is to provide a resinous material having a very low formaldehyde content, especially a formaldehyde content below the detection limit of the chemical analytical method used when filing the application.

According to the present invention, the method for reducing the formaldehyde content of a phenolic formaldehyde resins as starting material comprises the steps of:

a) providing a phenolic formaldehyde resin as resinous starting material having a formaldehyde content in the range of from 0.01 wt. % to 25 wt.%. , and

b) subjecting said phenolic formaldehyde resin as starting material to a contact process in the presence of a heterogeneous catalyst such that the formaldehyde of the resinous material thus treated is lower than the formaldehyde content of the resinous starting material, preferably in the range of from 5 wt. % to 0 wt. %.

Using such a method, it is possible to achieve one or more of the aforesaid objects. The lower limit of 0 wt. % should be interpreted as the detection limit of the at present available analytical methods. The standard analytical method for measuring the formaldehyde content used here is the EPA analytical method 8315a (Revision 1 , December 1996) .

This means that according to the present method the formaldehyde content of the phenolic formaldehyde resin as starting material can be reduced below the detection limit of the analytical apparatus. In practice, such a low content of formaldehyde does not necessarily means zero weight percent, but its content is in the range of several ppms. Due to the use of a heterogeneous catalyst in the present method for catalytically reducing the formaldehyde content of a phenolic formaldehyde resin as starting material the amount of residual catalyst in the final product will be very low. If necessary, the residual amount of heterogeneous catalyst can be reduced to a minimum by additional process steps, such as filtration.

This is in clear contrast with other methods for reducing the formaldehyde content of a resinous starting material in which methods generally formaldehyde scavengers are being used, and these scavengers must remain in the final product for performing its function, i.e. scavenging the formaldehyde content.

The present invention is not restricted to a method in which the formaldehyde content of the phenolic formaldehyde resin as starting material treated is always below an amount of 5 wt. %, but the present method shows that a reduction of the formaldehyde content in a phenolic formaldehyde resin as starting material can be obtained in an effective way by usi ng a heterogeneous catalyst.

The present method clearly shows that the formaldehyde content can be reduced without the incorporation of additional chemicals, like formaldehyde scavengers. Therefore, in a preferred embodiment the present invention clearly disclaims the use of those chemicals, e.g. urea, melamine, primary and secondary amines and other amine based modifications. Disadvantages of these scavengers are, among others, odor problems, precipitation, crystallization and reduced pot life of the resin.

The heterogeneous catalyst according to the present invention is preferably of the alkaline type. According to a preferred embodiment the heterogeneous catalyst according to the present invention consists of alkaline metal oxides and/or hydroxide or of mixtures thereof, or of alkaline solid inorganic materials, e.g. N-doped carbon nanotubes, ion exchangers).

More preferably the heterogeneous catalyst consist of oxides and/or hydroxides of elements or mixtures of elements from the first group of the Periodic Table, also referred to as alkali metals, consisting of the elements lithium, sodium, potassium, rubidium, caesium, and francium or of elements or mixtures of elements of the second group of the Periodic Table, also referred to as earth alkali metals, consisting of the elements beryllium, magnesium, calcium, strontium, barium, and radium or of elements or mixtures of elements of the third group of the Periodic Table, also referred to as rare earth metals, consisting of the elements scandium, yttrium, lanthanum, and actinium or of elements or mixtures of elements ranging from order number 58 to 71 of the Periodic Table, also referred to as lanthanides, consisting of cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium or of elements or mixtures of elements of the Group four of the Periodic Table, consisting of titanium, zirconium, and hafnium or of elements or mixtures of elements of the Group eleven of the Periodic Table, consisting of copper, silver, and gold or of elements or mixtures of elements of the Group twelve of the Periodic Table, consisting of zinc, cadmium, and mercury or of mixtures thereof like for example spinels; either in pure form (eg. CuAI₂0₄, ZnAI₂0₄, MnAI₂0₄), as mechanical mixtures of different spinels or as mixed spinels (eg. (Cu)x(Zn(1 -x))AI₂0₄); perowskits, or layered double hydroxides (LDH).

Even more preferably the heterogeneous catalyst according to any of the preceding claims consists of oxides and/or hydroxides of lithium, sodium, potassium, beryllium, magnesium, calcium, barium, lanthanum, cerium, zinc, zirconium, and copper or mixtures thereof like for example spinels; either in pure form (eg. CuAI₂0₄, ZnAI₂0₄, MnAI₂0₄), as mechanical mixtures of different spinels or as mixed spinels (eg. (Cu)x(Zn(1 -x))AI₂0₄); perowskits, or layered double hydroxides (LDH).

The earth alkali metal is monovalent or divalent, preferably divalent.

The earth alkali metal is preferably derived from the group of oxides, hydroxides, carbonates, sulfates, and phosphates or any other inorganic earth alkali metal salt, or from the group of organic earth alkali metal salts consisting of formates, acetates, oxalates, stearates, or of mixtures thereof.

Preferably the earth alkali metal is derived from oxides, hydroxides, carbonates, sulfates, phosphates or mixtures thereof.

More preferably the earth alkali metal is derived from oxides, hydroxides, carbonates or mixtures thereof.

Another embodiment of the invention uses as a heterogeneous catalyst the alkaline metal oxides and/or hydroxides or mixtures thereof supported on a carrier material/substrate.

I n more detail the carrier material/substrate is selected from the group of inorganic matrices consisting of aluminum oxide, silicon oxide, silicon-aluminum oxide, titanium oxide, zirconium oxide, bauxite, clays, pumice, activated carbon, carbon nanotubes, molecular sieves or from the group of organic matrices consisting of polymers, and polymeric resins.

According to an embodiment of the present invention the heterogeneous catalyst is of the aluminium magnesium hydroxy carbonate type, especially comprising one or more metals chosen from the group of cesium, lithium, sodium, potassium, beryllium, magnesium, calcium, barium, lanthanum, cerium, zinc, zirconium, and copper or mixtures thereof, which are preferably derived from the group of oxides, hydroxides, carbonates, sulfates, and phosphates or any other inorganic metal salt, or from the group of organic metal salts consisting of formates, acetates, oxalates, stearates, or of mixtures thereof.

Typical surface areas are 2-400 m²/g: preferably 10-250 m²/g. The present catalyst can be used in the form of powders, flakes, spheres, pellets, rings, extrudates or in any other suitable form. The catalyst particles may be used in a wide range of dimensions, for instance, as pellets with a diameter of 1 -5 mm or as powders whose particles have a grain size of 15-35 mesh (largest diameter of approx. 13-0.5 mm), 30-80 mesh (largest diameter of approx. 0.595-0.177 mm) or 100-325 mesh (largest diameter of approx. 0.15-0.04 mm). I n general, the catalytic process proceeds faster as the catalyst particles are smaller.

The resinous starting materials as mentioned in step a) above are phenolic formaldehyde resins, i.e. phenolic based resins. According to the present description the term phenolic formaldehyde based resins (hydroxyaromatic - aldehyde resins, phenol formaldehyde resins) means synthetic resins which are obtained by condensation of phenols and formaldehyde and optionally by modi fying the resulting condensates.

The present inventors found that the temperature in step b) is preferably in the range of 0 -1 10 °C.

I n the embodiment of a continuous process, the liquid hourly space velocity (LHSV) in step b) is in the range of 0.1 - 20, preferably in the range of 0.3 - 10, more preferably 0,5 - 5 [1/h]..

I n the embodiment for a batch process the residence time in step b) is preferably in the range of 10-500 minutes, preferably less than 200 minutes. The present inventors found in a special embodiment that the reaction time is influenced by the formaldehyde content of the phenolic formaldehyde resin as starting material and the amount of catalyst applied. Therefore, the residence time can be as long as 400 minutes when high contents of formaldehyde are present in the phenolic formaldehyde resin as starting material especially when the formaldehyde content is to be reduced as low as < 0.05 wt.%.

The present inventors found that the phenolic formaldehyde resin as starting material as used in step a) is preferably a mixture of two or more of the group of phenol and its derivates, bisphenol F and its derivatives, bisphenol A and its derivatives, triphenols, tetraphenols, chromanes, indanes, substituted or non-substituted phenols, and polyphenols.

The phenolic formaldehyde resin as starting material has preferably a weight average molecular weight (M_w) of 123-1000, preferably 300-600, and the resinous material obtained after step b) has preferably a weight average molecular weight (M_w) of 150-750, preferably 350-600.

The present inventors found that according to the method according to the present invention, the weight average molecule weight (M_w) ratio of the phenolic formaldehyde resin as starting material and the resinous material thus treated is in the range of 0.75-1 .50. Such a range is an indication that the physical properties of the resinous material have not been changed dramatically after carrying out the method according to the present invention.

The term "polydispersity" as used in the present description is a dimensionless parameter, which is known to those skilled in the art and which is defined as the quotient of the average molecular weight, M_w, and the molecular mass that comprises the largest number of molecules, M _n, viz. M_w/M_n. The ratio M_w/M_n can be considered to be the width of the molecular weight distribution obtained through a GPC method. If a phenol resin having a polydispersity outside the aforesaid range is used, an unsatisfactory impregnation behaviour will be observed, in particular in the case of heavier papers, which has an adverse effect, e.g. on the distribution of the resin in a moulded product formed of impregnation paper, and which is thus disadvantageous with regard to the mechanical properties and the hygric values thereof.

I n a preferred embodiment of the present invention the polydispersity of the phenolic formaldehyde resin as starting material is 1 .0-3.0, wherein the polydispersity of the resinous material obtained after step b) is 1 .0-3.0.

According to the present method the present inventors found that the polydispersity ratio of the phenolic formaldehyde resin as starting material and the resinous material thus treated is in the range of 0.75-1 .5.

I n the present invention step b) can be carried out as a continuous process, wherein the reactor is chosen from the group of tubular reactor, cascade reactor and/or plug flow reactor type, or a combination thereof. According to another embodiment the present method can be carried out as a batch process, e.g. in a batch reactor in which temperature and residence time can be controlled very accurately.

According to a preferred embodiment of the present invention the resinous material obtained after step b) is subjected to a step c), in which step c) comprises an additional treatment for reducing the amount of residual heterogeneous catalyst in said resinous material to a minimum, preferably neutralization, filtering, separation or a combination thereof.

Furthermore, it is possible in a preferred embodiment to add one or more additives to the resinous material before or after either of step b) or step c), in which said additives are chosen from the group of flame retardants, stabilizers, pigments, dispersion agents, antistatics, flow modification agents, solvents, curing agents. The exact moment of adding the additives depends on the chemical character of the additives. Care should be taken that the heterogeneous catalyst is not poisoned by any of the additives.

The resinous material obtained according to the method of the present invention can be used for impregnating inert parts, especially impregnation paper, in which the thus impregnated paper can be used for the assembly of panels. The process for manufacturing panels as such is known from US 4,927,572, US 4,789.604, which documents are in the name of the present applicant and can be incorporated here as reference.

The resinous material obtained according to the present invention is especially suitable for an impregnation paper having a weight of at least 40 g/m ², especially in the range of 120-400 g/m².

The present invention further relates to a panel comprising a resin impregnated cellulose fiber, wherein the resinous material is obtained according to the present method. The resinous materials are phenolic formaldehyde resins, i.e. phenolic based resins. According to a special embodiment the present panel meets the requirements of F**** (four star rating) according to JIS A 4 XXX.

I n addition, the present method relates to a resinous material obtained according tot the present method, in which resinous material the formaldehyde content is in a range of 3 wt. %-0 wt.%, preferably less than 0.05 wt. %, in which resinous material preferably no formaldehyde scavengers are present. As mentioned before, according to the present method the formaldehyde content of the phenolic formaldehyde resin as material can be reduced below the analytical detection limit of the analytical apparatus used, e.g. high performance liquid chromatography (HPLC) and detected and quantified by means of ultraviolet/visible (UV/Vis) detection . The resinous material according to the present invention, i.e. the resinous material obtained by carrying out the present method, originates from phenolic formaldehyde resins as resinous starting materials.

I n order to provide a better understanding of the invention, the present invention will now be explained by means of a number of examples, in which connection it should be noted, however, that the present invention is by no means limited to such special examples.

The method for determining the weight average molecular weight is based on GPC.

The method for determining the free formaldehyde content in a resin is based on of high performance liquid chromatography (HPLC) and detected and quantified by means of ultraviolet/visible (UV/Vis) detection . The free formaldehyde content of the samples is derivatized in the presence of 2,4-dinitrophenylhydrazine (DNPH). The new formed formaldehyde hydrazine adduct is then separated by means of high performance liquid chromatography (H PLC) and detected and quantified by means of ultraviolet/visible (UV/Vis) detection. This analytical method is a standard method for the determination of carbonyl compounds by H PLC in this technical field and can be classified as EPA analytical method 8315a (Revision 1 , December 1996), which contents are here incorporated by reference.

The present invention is further specifically described by the following examples of this invention and comparative examples. Preparation of Catalysts

Catalyst A

Catalyst A is Pural MG70 and is a commercial catalyst from Sasol. Pural MG70 is an aluminium magnesium hydroxy carbonate. The ratio MgO:AI203 is 70:30, its surface area (m2/g) is > 180. Preparation of Catalyst B

32.6 g Cs(OAc) (Sigma Aldrich, 1001096359, Lot #MKBD2689V) are dissolved in 215 g water while gently stirring. The so derived Cs(OAc) solution is completely added to 120 g of Catalyst A. The mixture is slowly rotated in flask of an evaporator for 90 minutes. Subsequently the water is removed by evaporator under gentle heating (40 °C) under reduced pressure (< 20 mbar) and afterwards dried for 6h at 100 °C. Finally the catalyst is thermally treated under N₂: 25 °C 400 °C within 30 minutes, 400 °C kept overnight. After cooling to room temperature (rt) the catalyst is ready to use.

Preparation of Catalyst C

262.4 g La(NO)₃ are dissolved in 200 ml distilled water and then 15.6 g of MgO powder are added mixture A. Mixture A is stirred for 30 minutes at rt. 24g HN0₃ (65 wt%) are dissolved in 216 g distilled water -^solution B.

206 g AI2O₃ (Puralox SCCa-5/200) and solution B are added to mixture A mixture C.

Mixture C is subsequently stirred at rt with an overhead stirrer for 75 minutes, then the water is removed by filtration. The residual is dried for 4h at 100 °C and subsequently dried for 3h at 150 °C.

Calcination of the so derived solid is carried out as follows: 25 °C 700 °C with 7 °C/min. ; 700 °C for 2h; 700 °C 850 °C with 7 °C/min. ; 850 °C. After cooling to rt the catalyst is ready to use. Preparation of Catalyst D

The same steps as mentioned for the preparation of Catalyst B are carried out, except that 120 g LaCeZr mixed oxide (Priem, SNAC-LCZ, La:Ce:Zr = 5:30:65) is used instead of Catalyst A. Preparation of Catalyst F

Preparation according to US 2004/0234448 Example 2.

Examples disclosing a method for catalytically reducing the formaldehyde content of a resinous starting material Example 1A (comparative example)

A 50°C preheated 0.5 liter double-walled laboratory batch reactor equipped with an overhead stirrer and a reflux condenser is charged with 400g of a Phenol Resol type impregnation resin. 10g of the resin are taken as zero time sample. No catalyst is utilized in this experiment (blank test) and therefore this example is not according to the present invention . The temperature during the test is kept at 50 °C by a external thermostat unit. Samples of 10g resin are taken after 360 and 1440 minutes, to analyze and monitor the progress of the formaldehyde reduction. Selected samples are analyzed and monitored to detect specific changes in the composition. All samples are stored in a refrigerator to prevent further conversion until analysis.

Example 1 B

A 50°C preheated 0.5 liter double-walled laboratory batch reactor equipped with an overhead stirrer and a reflux condenser is charged with 320g of a Phenol Resol type impregnation resin. 10g of the resin are taken as zero time sample. Under moderate stirring 40 g of the heterogeneous Catalyst A are added to the preheated resin. The temperature during the test is kept at 50°C by a external thermostat unit. Samples of 10g resin are taken after different times (see table 2 for more details), to analyze and monitor the progress of the formaldehyde reduction. Selected samples are analyzed and monitored to detect specific changes in the composition. All samples are stored in a refrigerator to prevent further conversion until analysis. Example 1 C

The same steps as mentioned in Example 1 B are carried out, except that Catalyst B is utilized.

Example 1 D

The same steps as mentioned in Example 1 B are carried out, except that a Bisphenol-A mixture + Phenol Resol type impregnation resin is utilized.

Example 1 E

The same steps as mentioned in Example 1 C are carried out, except that a Bisphenol-A mixture + Phenol Resol type impregnation resin is utilized. Example 1 F

The same steps as mentioned in Example 1 B are carried out, except that Catalyst D is utilized and the Phenol Resol type impregnation resin has a lower free formaldehyde starting value.

Example 1 G

The same steps as mentioned in Example 1 F are carried out, except that Catalyst B is utilized.

Example 1 H

The same steps as mentioned in Example 1 F are carried out, except that Catalyst C is utilized. Example 1 1

The same steps as mentioned in Example 1 F are carried out, except that Catalyst F is utilized.

A formaldehyde content <0.05% means below the detection limits of the analytical method.

The results disclosed in Table demonstrate that the efficiency for removing the formaldehyde content can be increased upon introducing metals in the catalyst (compare Catalyst A and catalyst B). In addition both Catalyst A, Catalyst B, Catalyst C and Catalyst F inhibit the ageing of the resin. The most preferred Catalysts A and B showed a decrease of the free formaldehyde content, and Mw and polydispersity were similar or show a tendency to decrease.

I n addition, the results also show that a mixture of different types of phenolic formaldehyde resins having a relatively high formaldehyde content can be treated in an effective way by the method according to the present invention.

Claims

CLAI MS

1 . A method for reducing the formaldehyde content of a resinous starting material, characterized in that the method comprises the steps of:

a) providing a phenolic formaldehyde resin as starting material having a formaldehyde content in the range of from 0.01 wt.% to 25 wt.%. , and

b) subjecting said phenolic formaldehyde resin as starting material to a contact process in the presence of a heterogeneous catalyst such that the formaldehyde of the resinous material thus treated is lower than the formaldehyde content of the resinous starting material, preferably in the range of from 5 wt.% to 0 wt. %.

2. A method according to claim 1 , characterized in that, the heterogeneous catalyst is of the alkaline type.

3. A method according to any one or more of claims 1 -2, characterized in that, the heterogeneous catalyst comprises one or more of alkaline metal oxides, alkaline hydroxides and alkaline solid inorganic materials, wherein the alkali metals are preferably chosen from the First group and/or Second Group of the Periodic Table, even more preferably chosen from the group of oxides and/or hydroxides of lithium, sodium, potassium, beryllium, magnesium, calcium, barium, lanthanum, cerium, zinc, zirconium, and copper or mixtures thereof, especially magnesium oxide and/or calcium oxide.

4. A method according to anyone or more of the preceding claims, characterized in that the heterogeneous catalyst is of the aluminium magnesium hydroxy carbonate type, especially comprising one or more metals chosen from the group of cesium, lithium, sodium, potassium, beryllium, magnesium, calcium, barium, lanthanum, cerium, zinc, zirconium, and copper or mixtures thereof, which are preferably derived from the group of oxides, hydroxides, carbonates, sulfates, and phosphates or any other inorganic metal salt, or from the group of organic metal salts consisting of formates, acetates, oxalates, stearates, or of mixtures thereof.

5. A method according to anyone or more of the preceding claims, characterized in that the temperature in step b) is in the range of 0 -1 10 °C.

6. A method according to anyone or more of the preceding claims, characterized in that the heterogeneous catalyst is supported on a carrier, preferably selected from one or more of the group of inorganic matrices consisting of aluminum oxide, silicon oxide, silicon-aluminum oxide, titanium oxide, zirconium oxide, bauxite, clays, pumice, activated carbon, carbon nanotubes, molecular sieves or from the group of organic matrices consisting of polymers, and polymeric resins, in which the carrier preferably has surface areas in the range of 2-400 m²/g, especially in the range of 10-250 m²/g.

7. A method according to anyone or more of the preceding claims, characterized in that the residence time in step b) is in the range of 10-500 minutes, preferably less than 200 minutes, for a batch process.

8. A method according to anyone or more of the preceding claims, characterized in that the resinous starting material is a mixture of two or more of the group of phenol and its derivates, bisphenol F and its derivatives, bisphenol A and its derivatives, triphenols, tetraphenols, chromanes, indanes, substituted or non-substituted phenols, and polyphenols.

9. A method according to anyone or more of the preceding claims, characterized in that, the phenolic formaldehyde resin as starting material has a weight average molecular weight (M_w) of 123-1000, preferably 300-600.

10. A method according to anyone or more of the preceding claims, characterized in that, the resinous material obtained after step b) has a weight average molecular weight (M_w) of 150-750, preferably 350-600.

1 1 . A method according to anyone or more of the preceding claims, characterized in that, the weight average molecular weight (M_w) ratio of the phenolic formaldehyde resin as starting material and the resinous material thus treated is in the range 0.75-1 .50.

12. A method according to anyone or more of the preceding claims, characterized in that, the polydispersity of the phenolic formaldehyde resin as starting material is 1 .0-3.0.

13. A method according to anyone or more of the preceding claims, characterized in that, the polydispersity of the resinous material obtained after step b) is 1 .0-3.0.

14. A method according to anyone or more of the preceding claims, characterized in that, the polydispersity ratio of the phenolic formaldehyde resin as starting material and the resinous material thus treated is in the range of 0.75-1.5.

15. A method according to anyone or more of the preceding claims, characterized in that, the formaldehyde content of the phenolic formaldehyde resin as starting material is at most 10 wt.%, preferably at most 5 wt.%, in particular at most 2.5 wt.%, more preferably at most 0.5 wt.%.

16. A method according to anyone or more of the preceding claims, characterized in that, step b) is carried out as a continuous process, wherein the reactor is chosen from the group of tubular reactor, cascade reactor and/or plug flow reactor type, or a combination thereof.

17. A method according to anyone or more of the preceding claims, characterized in that, the resinous material obtained after step b) is subjected to a step c), in which step c) comprises an additional treatment for red ucing the amount of residual heterogeneous catalyst in said resinous material to a minimum, preferably neutralization, filtering, separation or a combination thereof.

18. A method according to anyone or more of the preceding claims, characterized in that, one or more additives are added to the resinous material before or after either of step b) or step c), in which said additives are chosen from the group of flame retardants, stabilizers, pigments, dispersion agents, antistatics, flow modification agents, solvents, curing agents.

19. The use of a resinous material according to any one or more of the claims 1 -18 for manufacturing panels, obtained by impregnating inert parts, especially impregnation paper, with said resinous material and subsequently compressing the assembly of resin impregnated inert parts under conditions of increased temperature and pressure for obtaining said panels.

20. The use of a resinous material according to claim 19, wherein the impregnation paper has a weight of at least 40 g/m², especially in the range of 120-400 g/m².

21 . A panel comprising resin impregnated cellulose fibers, characterized in that a resinous material obtained according to any one or more of the claims 1 - 18 is used.

22. A resinous material obtained according to any one or more of the claims 1 -18, characterized in that the formaldehyde content is in the range of 3 wt. %-0 wt.%, preferably less than 0.05 wt. %, in which resinous material no formaldehyde scavengers are present.

23. A resinous material according to claim 22, characterized in that no formaldehyde scavengers are present.