METHOD FOR RECYCLING MELAMINE FORMALDEHYDE RESIN FROM WASTE GENERATED DURING THE MANUFACTURING AND PROCESSING OF WOOD-BASED PANELS

Description

[0001] The present invention relates to a method for recycling or digesting melamine-formaldehyde resin from waste generated during the manufacture, processing and return of wood-based panels. Description

[0002] In all industrial sectors, the question of how to recycle residual materials or products after their use is becoming increasingly common. This issue has gained significant momentum due to EU regulations mandating reuse / recycling for all products in the future. Furthermore, the sharp rise in raw material prices has made recycling increasingly attractive. Disposal costs for residual materials have also increased considerably. In addition, the desire of all producers to operate as sustainably as possible is gaining prominence. This means using raw materials efficiently, avoiding emissions, and minimizing waste.

[0003] These developments are particularly relevant for products containing oil- or gas-based raw materials. These products are mostly plastics, and these quantities are significant. While the recycling of thermoplastic materials (polyethylene, polypropylene, polyvinyl chloride) has been standard practice for decades, the concept of recycling thermosets / thermomers (phenolic resins, melamine resins, urea resins, etc.) has not yet gained widespread acceptance.

[0004] This is partly due to the difficult-to-break cross-linked structure of thermosets / thermomers, which is seen as a particular advantage of these products, as it makes them very resistant to thermal or chemical attack. It is also due to the historically very low prices of the raw materials.

[0005] Initial approaches to recycling have been developed for urea-formaldehyde adhesives, which are used, among other things, as glues in wood-based materials. After curing, these adhesives are very sensitive to moisture / water at higher temperatures. Here, the material (e.g., particleboard) can be broken down through hydrolysis. The hydrolyzed adhesive remains primarily in the resulting wood chips and serves as a base for the added adhesives in the subsequent production of a new wood-based material. However, it must be noted that this process focuses more on recovering the wood matrix than on re-recovering the adhesive components.

[0006] However, past efforts have demonstrated the feasibility of recycling thermosets. DD 155779 describes how production equipment contaminated or clogged by cured melamine resins could be cleaned. EP 0612 793 B1 and US 5,356,938 describe how cured melamine resin (fibers) can be broken down and reused. However, these only refer to residues from molded body / fiber production and the return of these products. US 2019 / 0047181 A1 describes the reprocessing of, among other things, carbon fiber-reinforced polyimide, polyurethane, epoxy resin, and vinyl ester. CN 111 675 827, CN 111 777 566, CN 111 825 626 and CN 111 875 843 describe methods for recovering melamine from tableware made from melamine-formaldehyde resin.For this process, the dishes are first crushed and then treated with an aqueous organic solvent and an acidic catalyst, such as HCl, HNO₃, H₂SO₄, AlCl₃, and similar substances, at temperatures between 80 and 150°C. The considerable amounts of organic solvents in combination with water required by these processes make them rather unfavorable from an ecological perspective. Furthermore, the presence of an acid in the aqueous phase is also problematic.

[0007] The recycling of thermosetting resins from wood-based materials, impregnating agents, or laminates has not yet been described. Melamine resin-containing waste is generated at many stages of the value chain during the production and processing of wood-based materials.

[0008] In the production and processing of wood-based panels into floorboards or furniture components, impregnated decorative papers, overlay papers, and backing papers are pressed onto the wood-based panels. These papers are impregnated with thermosetting resins (urea resin, melamine resin) in impregnation plants or impregnation channels. During plant start-up, product changeovers, operator error, or changes in production parameters due to technical reasons, large quantities of impregnating agents of substandard quality are generated. These are disposed of (e.g., through thermal recovery) and represent an average loss of 3–8%, depending on the order size, duration of the disruption, etc. Since a modern impregnation channel operates at speeds of 80 m / min and more, this results in several million square meters of rejects per year. Impregnating agents typically contain approximately...50% by weight resin is lost, resulting in significant quantities of urea / melamine resin being lost. Additionally, impregnation processes generate waste resin due to trimming of the impregnating materials and dust formation. These residues are also disposed of. Furthermore, residual resins are produced during impregnation, which are currently disposed of and can amount to considerable tonnages over the course of a year.

[0009] During further processing of the impregnating agents, for example by pressing them into wood-based panels, waste is generated at the presses by scraping devices that remove excess impregnating agent from the panels. Expired impregnating agents that can no longer be used in production are also disposed of. All these residual materials or waste have the advantage that the melamine resins are not yet fully cured, so recycling could be easier.

[0010] Highly melamine-reinforced adhesives are also frequently used in the production of wood-based panels (such as particleboard and fiberboard). The same applies to these panels as to impregnated wood-based panels: waste is generated at all stages of the value chain and must be disposed of.

[0011] The disadvantages resulting from current waste disposal practices include, in particular, high material losses, high disposal costs, and waste problems.

[0012] As already mentioned, the EU expects a coherent recycling concept for all products. For the wood-based materials industry, this means that recycling pathways are also needed for melamine-containing products. This should only apply to products containing significant amounts of melamine.

[0013] The invention is therefore based on the technical objective of developing a process that enables the recycling of melamine using residual materials and waste from the various value chain stages of impregnation, wood-based panel production, and product returns. The process should yield products that can be used for the production of impregnating agents, coatings, or wood-based panels. It should be taken into account that, due to recycling costs, higher-value uses should be preferred.

[0014] This problem is solved by a method having the features of claim 1.

[0015] Accordingly, a process is provided for the degradation or breakdown, or depolymerization, of melamine-formaldehyde resin from waste products generated during the manufacture and processing of wood-based panels. The melamine-formaldehyde resin to be broken down is bound, in particular, to the waste or waste products from paper layers and wood-based panel substrates generated in production plants for the manufacture and further processing of wood-based panels.

[0016] The present procedure comprises the following steps: Providing waste products generated during the manufacture and processing of wood-based panels that contain polycondensed melamine-formaldehyde resin, if applicable. Crushing the melamine-formaldehyde resin-containing waste products, mixing the melamine-formaldehyde resin-containing waste products with an aqueous 20 to 70 wt%, preferably 40 to 60 wt% solution of at least one disulfite, hydrogen sulfite and / or dithionite, heating the mixture of melamine-formaldehyde resin-containing waste products and at least one disulfite, hydrogen sulfite and / or dithionite to temperatures between 60°C and 120°C, preferably between 80°C and 110°C, until a clear solution is obtained, separating non-hydrolyzable foreign substances (as impurities) from the clear solution, further heating the mixture of melamine-formaldehyde resin-containing waste products and at least one disulfite, purified of foreign substances,Hydrogen sulfite and / or dithionite at temperatures between 60°C and 120°C, preferably between 80°C and 110°C, under reaction control until no polycondensed melamine-formaldehyde resin is detectable in the reaction mixture, cooling of the reaction mixture and separation of the precipitated mixture of monomeric melamine and melamine derivatives, separation of the mixture of monomeric melamine and melamine derivatives into melamine, methylolmelamines and methylolmelamine sulfonic acids.

[0017] A process is provided in which a thermosetting resin matrix in various waste products is degraded using bisulfite / disulfite / dithionite. The more or less cross-linked melamine resins contained in the residues are reductively degraded with the aid of the sulfites. The reaction products are methylolmelamines and methylolmelamine sulfonic acids, as well as small amounts of melamine, with the methylolmelamine sulfonic acids constituting the main component (> 70%).

[0018] The contaminants or foreign substances contained in the various residues or waste products can be easily separated during or after the reaction. For example, filtration or skimming of floating fibers is suitable. This can be done continuously during the digestion reaction or at the end. This is not necessary when processing resins, although impurities should still be removed in a purification step. During reflux digestion, the melamine resin matrix is ​​broken down, and the water-insoluble polycondensates slowly dissolve or undergo stepwise hydrolysis, releasing melamine and melamine derivatives (methylolmelamines and methylolmelamine sulfonic acids).

[0019] The melamine derivatives released from the polycondensate precipitate upon cooling and can then be filtered and washed. This process also removes various additives, which differ depending on the waste / product. These additives are typically found in resins, such as hardeners, plasticizers, corundum, glass beads, pigments, etc. However, other resins or adhesives (urea-formaldehyde resin / adhesive) are also removed.

[0020] Melamine derivatives, once separated into their components, can be used for a wide variety of applications. Upon addition of ammonia or other amines, the methylolmelamine sulfonic acids can be converted into the corresponding ammonium salts. These can be used as hardeners. Alternatively, the methylolmelamine sulfonic acids themselves can be used. The methylolmelamines can then be used to produce melamine resins / adhesives for various applications. They can also be used as formaldehyde scavengers in the production of wood-based materials when urea-formaldehyde adhesives are used as binders. They can also be used as an adhesive component to reduce swelling. Generally, the components can also be used as flame retardants or in combination with flame retardants / binders in a wide range of products (wood-based materials, laminates, etc.).

[0021] The present process does not require the use of organic solvents or strong acids. Instead, it utilizes solutions of disulfites or hydrogen sulfites, which are also used as preservatives in the food industry and are therefore toxicologically and ecologically harmless. The alkali sulfates produced during the hydrolytic digestion can also be put to use.

[0022] In one embodiment of the present method, the melamine-formaldehyde resin-containing waste products generated during the manufacture and processing of wood-based panels include overlaid melamine resin, melamine resin dust, overlay impregnating agents, decorative impregnating agents, counter-tension impregnating agents, kraft paper impregnating agents, uncoated or coated wood-based panels, such as HDF panels, and / or laminates, such as thin laminate.

[0023] An aged melamine resin is understood to be a melamine-formaldehyde resin that has already condensed, at least partially, and exhibits turbidity. Agedness of a melamine resin is particularly evident when either the viscosity has increased significantly (measured with a 4 mm DIN cup according to EN ISO 2431:2011: fresh resin 20-30 seconds, aged resin: > 50 seconds) or the resin has become cloudy.

[0024] Melamine resin dust is generated at various points in the processing of impregnated wood-based panels, such as at the clipper and during edging. The resulting melamine resin dust has a particle size > 50 µm, with a maximum size of 100-300 µm. The edging material consists of chips approximately 10 mm wide and of varying lengths depending on the effectiveness of the scrapers.

[0025] Overlay impregnated papers, decorative impregnated papers, backing impregnated papers and kraft paper impregnated papers are based on thin layers of paper that are completely or partially saturated (impregnated) with a resin, preferably melamine-formaldehyde resin.

[0026] Accordingly, both partially impregnated (i.e., impregnated on one side) and fully impregnated papers can be used. In the case of papers impregnated on one side, only the entire surface of one side of the paper is uniformly coated with an impregnating resin. The amount of resin applied in this case is between 30 and 70 wt%, preferably between 40 and 60 wt%, and particularly preferably 50 wt%, based on the paper's weight. In contrast, in the case of fully impregnated paper, an amount of resin of 80–400 wt%, preferably 90–120 wt%, and particularly preferably 100–110 wt%, based on the paper's weight, is applied.

[0027] Overlay papers are thin papers that are typically already impregnated with a conventional melamine resin. Overlay papers are also available in which abrasion-resistant particles, such as corundum particles, are already mixed into the resin or sprinkled onto the resin-coated overlay to increase abrasion resistance. Resin coatings with up to 400% melamine resin by weight are used for impregnating overlay papers.

[0028] Decorative papers are specialty papers used for surface finishing of wood-based materials, offering a wide variety of decorative options. In addition to typical prints of various wood textures, more elaborate prints of geometric shapes or artistic designs are available. There are virtually no limitations on the choice of motif. To ensure optimal printability, the paper used must have suitable smoothness and dimensional stability, and must also be suitable for penetration of the necessary synthetic resin impregnation. The resin application rate for decorative impregnating agents is between 100 and 120% by weight.

[0029] Backing papers are high-quality, impregnated papers for use as backing material, e.g. for single-sided surface veneers and other single-sided coatings.

[0030] Kraft papers are highly durable and consist of cellulose fibers to which starch, alum, and sizing are added to achieve surface effects and increased strength. The resin content of impregnated kraft papers ranges from 100 to 120% by weight. These are primarily impregnated with a mixture of melamine and phenolic resin when used as the core layer of CPL (compressed laminate).

[0031] Impregnation can be applied, for example, in an impregnation bath, by rollers, by anilox rollers, by doctor blade application, or by spraying. In one embodiment, the paper layers are treated as follows: First, the reverse side of the paper layer (e.g., in a soaking tank) is impregnated with a resin with a solids content between 50 and 70 wt%, preferably 60 wt%. After passing through a breathing zone, immersion impregnation with a resin takes place. Excess resin is removed in a doctor blade system / squeeze roller pair, and optionally (in the case of an overlay paper layer), abrasion-resistant particles are sprinkled onto the impregnated paper layer. A drying step is then performed to a residual moisture content of approximately 6%. The impregnated paper can then be pressed with a wood-based panel, for example, in a short-cycle press.

[0032] Wood-based panels, such as particleboard and fiberboard (e.g., MDF and HDF), are manufactured from wood chips or fibers obtained by chipping wood in a chipper or by fiberizing wood chips in a refiner. The wood fibers used in fiberboard have a length between 1.5 mm and 20 mm and a thickness between 0.05 mm and 1 mm. The size of the wood chips used in particleboard depends on whether they are used in the face or core layer. In the core layer, the chips start at a particle size greater than 0.125 mm, while in the face layer, they start at 0.8–1.0 mm.

[0033] Preferred binders for wood-based panels are formaldehyde-containing adhesives, such as melamine-formaldehyde resins and urea-formaldehyde resins. The amount of binder used in wood-based panels is typically in the range of 5 to 25 wt%, preferably 8 to 20 wt%, and particularly preferably between 10 and 15 wt%. In certain applications, the amount of binder in wood-based panels can also exceed 25 wt%, for example, up to 30 to 50 wt%.

[0034] In the case of coated wood-based panels, the paper impregnating agents mentioned above are applied to the wood-based panels and pressed together. Typically, the impregnated decorative paper is first applied to the top surface of the wood-based panel. This is followed by at least one overlay impregnating agent. The backing impregnating agent is then pressed onto the underside of the wood-based panel. A typical structure of a coated HDF panel, from top to bottom, is: overlay impregnating agent, decorative impregnating agent, HDF core, backing impregnating agent.

[0035] Laminates are layered materials made of pressed paper impregnatings that are laminated onto substrate boards.

[0036] Currently known in the art are, for example, flexible laminates in which the layered structure comprises at least one impregnated paper layer, such as a decorative impregnating agent, at least one transparent paper layer (glassine), e.g., glassine treated with sulfuric acid, and / or at least one plastic film layer. Glassine is a largely greaseproof, but not wet-strength, transparent paper made from finely ground cellulose. Its high transparency is achieved through very fine calendering. The various layers can be present multiple times in the laminate structure, alternating or changing their position.

[0037] The plastic film layer used in the laminate consists of polymers, in particular polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), thermoplastic polyurethane (TPU), or polyurethane. Such plastic films are used especially as protective films on sensitive surfaces.

[0038] Melamine-formaldehyde resin-containing waste products can contain not only cellulose fibers, wood fibers, and wood chips, but also inorganic abrasion-resistant particles, glass beads, color pigments, other binders such as urea resins, and other additives. The cellulose fibers originate from impregnated papers, such as the decorative impregnating agents, overlay impregnating agents, and backing impregnating agents described above. Inorganic particles, such as abrasion-resistant particles (corundum), are contained in overlay impregnating agents. Wood fibers come from wood-based panels (such as HDF and MDF boards), and wood chips from particleboard. Color pigments are predominantly found in decorative impregnating agents. Other additives include flame retardants, hardeners, elasticizing agents, and waxes.

[0039] The solids mentioned above, which are contained in the melamine-formaldehyde resin-containing waste products, arise as foreign substances or contaminants during the pulping process and are separated as mentioned during the pulping process.

[0040] As can be seen, the melamine-formaldehyde resin-containing waste products to be processed differ in their composition, so that optional further adjustments and modifications of the present process are possible.

[0041] While the processing of melamine resin dust does not require further comminution of the waste product before mixing with at least one disulfite, hydrogen sulfite and / or dithionite solution, this is recommended for the other waste products.

[0042] In the case of using decorative impregnating agents, overlay impregnating agents, backing impregnating agents, coated and uncoated wood-based panels, and laminates, the comminution of the waste products is necessary. Accordingly, in one embodiment, these melamine-formaldehyde resin-containing waste products are comminuted to a particle size between 0.1 and 10 mm, preferably between 0.5 and 3 mm. The comminution process works best with very finely ground material with a particle size of less than 3 mm.

[0043] In the case of a wood-based panel (such as fiberboard or particleboard) coated with overlay impregnation, decorative impregnation, and / or backing impregnation, it is also advantageous to first pre-shred the coated wood-based panel (e.g., into a 5 x 5 cm format). The pre-shredded pieces are then treated in a steam atmosphere to hydrolyze the additional binder present in the wood-based panel (preferably urea-formaldehyde resin).

[0044] The residue obtained after hydrolysis (preferably from the impregnating agents and wood fibers or wood chips) is separated and further crushed and only then mixed with the at least one disulfite, hydrogen sulfite and / or dithionite solution for the hydrolytic degradation of the melamine-formaldehyde resin according to the present process.

[0045] The melamine-formaldehyde resin contained in the waste products and to be broken down has a molar ratio of melamine / formaldehyde of 1 / 1.5 to 1 / 2.5, preferably of 1 / 1.6 to 1 / 2.3, particularly preferably of 1 / 1.6 to 1 / 1.8.

[0046] In one embodiment of the present process, the polycondensed melamine-formaldehyde resin to be deconstructed in the waste products is not yet fully cured; i.e., the polycondensate is not yet fully cross-linked. This applies in particular to layered resin and impregnated paper sheets (which have not yet been subjected to a pressing step).

[0047] In another embodiment of the present process, the polycondensed melamine-formaldehyde resin to be deconstructed is completely cured in the waste products. This applies in particular to pressed uncoated and coated wood-based panels and laminates.

[0048] The different polymerization and crosslinking states of melamine-formaldehyde resin are briefly described below.

[0049] Melamine and formaldehyde initially react to form water-soluble monomer products by forming methylol groups on the amino groups of melamine (see Scheme I). These melamine-formaldehyde monomers are also called melamine methylols.

[0050] Upon addition of a suitable catalyst, preferably an acid, the melamine-formaldehyde monomers undergo polycondensation, resulting in the linking of the monomers via ether and methylene groups and the formation of higher molecular weight precondensates and polycondensates (see Scheme II).

[0051] Precondensates and polycondensates differ in their molar mass and solubility. Low-molecular-weight precondensates may exhibit limited water solubility, while higher-molecular-weight polycondensates are insoluble. The limited water solubility of precondensates is due, among other things, to the presence of free methylol groups and the low degree of cross-linking of the mostly linear oligomers. Precondensates are therefore polymerization intermediates.

[0052] When the polycondensates are fully hardened, strong cross-linking occurs with the elimination of the remaining methylol groups, resulting in the formation of tightly cross-linked plastics via methylene groups (see Scheme III).

[0053] Therefore, the following resin states are distinguished for synthetic resins that harden via condensation reactions: A-state: readily soluble in solvent, fusible, curable; B-state: only partially soluble in solvent, fusible, curable; C-state: insoluble, hardened

[0054] The extraction of a resin that is not yet fully cured (i.e., a resin in the partially cross-linked B state) is preferred due to the lower degree of cross-linking of the resin.

[0055] In one embodiment of the present process, the melamine-formaldehyde resin-containing waste products are mixed with an aqueous 20 to 70 wt%, preferably 40 to 60 wt% alkali or alkaline earth solution of at least one disulfite, hydrogen sulfite and / or dithionite.

[0056] In particular, the melamine-formaldehyde resin-containing waste products are mixed with at least one sodium disulfite or sodium hydrogen sulfite solution with a pH between 3.0 and 6, preferably between 3.5 and 5.5. Sodium disulfite (Na₂S₂O₅), also called sodium pyrosulfite or sodium metabisulfite, is a sodium salt of disulfurous acid, which is unstable in its free form. Sodium hydrogen sulfite (NaHSO₃), also called sodium bisulfite, is a sodium salt of sulfurous acid. It is unstable outside of aqueous solutions.

[0057] In a particularly preferred embodiment, the waste products containing melamine-formaldehyde resin are mixed with at least a 40-60 wt%, in particular 50 wt% sodium hydrogen sulfite (or sodium bisulfite) solution.

[0058] As mentioned above, in a next step the mixture of melamine-formaldehyde resin-containing waste products and at least one disulfite, hydrogen sulfite and / or dithionite is heated to temperatures between 60 and 120°C, preferably between 80 and 100°C, until a clear solution is obtained.

[0059] In this context, a clear solution is understood to mean that no turbidity is visible (i.e., the solution is transparent); only the solids contained as impurities are visible.

[0060] The aforementioned impurities are separated from the clear solution in an intermediate step. The separation of solids contained as contaminants in the reaction mixture is carried out by filtration, preferably hot filtration, and / or skimming. This allows the fibers released from the paper layers or wood-based panels and floating to be skimmed off, and other particles, such as corundum and / or glass beads, to be separated by hot filtration.

[0061] This is followed by further heating of the reaction mixture, purified of foreign substances, consisting of melamine-formaldehyde resin-containing waste products and at least one disulfite, hydrogen sulfite and / or dithionite, at temperatures between 60 and 120°C, preferably between 80 and 110°C, under reaction control until no melamine-formaldehyde resin oligomers are detectable in the reaction mixture.

[0062] The reaction control of the depolymerization or hydrolytic degradation of the melamine-formaldehyde resin is carried out using suitable analytical methods, such as thin-layer chromatography, HPLC or gas chromatography.

[0063] After depolymerization of the melamine-formaldehyde resin, the reaction mixture is cooled. During cooling, the product mixture of monomeric melamine derivatives precipitates.

[0064] The mixture of melamine and monomeric melamine derivatives comprises methylolmelamines, in particular monomethylolmelamine and dimethylolmelamine, and methylolmelamine sulfonic acids, in particular dimethylolmelamine sulfonic acid and monomethylolmelamine sulfonic acid. The proportion of methylolmelamines is approximately 10–30 wt% and the proportion of methylolmelamine sulfonic acids approximately 70–90 wt%.

[0065] In a final step, the mixture of melamine and monomeric melamine derivatives is separated into melamine, methylolmelamines, and methylolmelamine sulfonic acids. This separation can be achieved by adding ammonia or amines to the mixture, whereby the methylolmelamine sulfonic acids are converted into a soluble form and separated from the melamine and methylolmelamines.

[0066] Methylolmelamines and methylolmelamine sulfonic acids, obtained from melamine-formaldehyde resin, can be used for various purposes. Methylolmelamine sulfonic acids can be used as hardeners, and methylolmelamines can be used in the production of melamine resin for use as binders or impregnating agents.

[0067] The invention will be explained in more detail below using exemplary embodiments. General:

[0068] In the exemplary embodiments, the reaction progress was monitored by thin-layer chromatography. For comparison, melamine resins / precursors were also included, allowing for an assessment of the degradation progress. These included melamine, monomethylolmelamine, dimethylmelamine, and a melamine resin. Example 1: Dissolution of melamine residue and resin waste

[0069] 100 g of a melamine resin that had already become cloudy due to storage (solid content: 55 wt%, molar ratio melamine / formaldehyde: approx. 1 / 1.8) were transferred to a 1000 ml three-necked round-bottom flask and mixed with 300 g of a 50% sodium disulfite solution.

[0070] The mixture was boiled under reflux, and the turbidity dissolved after approximately 3 hours. The reaction progress was monitored by thin-layer chromatography (elevating medium: water / ethanol (1:1)). After no resin oligomers were detectable by thin-layer chromatography (6 h), the reaction solution was cooled, the precipitated product was filtered off via a suction strainer, washed with cold water, and then dried.

[0071] The pale yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfonic acids. The filtrate was further concentrated by half, and the precipitated product was also washed and dried. Taking into account approximately 10 wt% excipients in the resin (elasticizing agents, hardeners, etc.), the yield was 95%. Example 2: Dissolution of melamine residues

[0072] Melamine resin dust, which had accumulated during impregnation of the clipper and the edging, solidified into 100 g. (Mol ratio melamine / formaldehyde: approx. 1 / 1.8) It was transferred to a 2000 ml three-necked round-bottom flask and mixed with 600 g of a 50% sodium bisulfite solution.

[0073] The mixture was boiled under reflux, during which the resin slowly dissolved. Once a clear solution was obtained, the suspended fibers (paper and cellulose) were skimmed off, then hot-filtered to remove corundum and glass beads. These were washed and reused. The reaction was then boiled again under reflux. The reaction progress was monitored by thin-layer chromatography (eluent: water / ethanol (1:1)). After no resin oligomers were detectable by thin-layer chromatography (12 h), the reaction solution was cooled, the precipitated product was filtered off using a suction strainer, washed with cold water, and then dried.

[0074] The pale yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfonic acids. Taking into account approximately 10 wt% excipients in the resin (elasticizing agents, hardeners, etc.), the yield was 91%. In a further experiment, the amount of sodium bisulfite was reduced to 150 g in 300 ml of water. No difference in yield was observed. Example 3: Dissolution of melamine residue overlay

[0075] Overlay impregnating agents that could no longer be used in production due to age were ground to dust in a laboratory mill under cooling. 100 g of this solid overlay impregnating agent (molar ratio resin: melamine / formaldehyde: approx. 1 / 1.8) were transferred to a 2000 ml three-necked round-bottom flask and mixed with 400 g of a 50% sodium disulfite solution.

[0076] The mixture was boiled under reflux. Once a clear solution was obtained, it was hot-filtered to remove paper fibers, corundum, etc. The mixture was then boiled again under reflux. The reaction progress was monitored by thin-layer chromatography (eluent: water / ethanol (1:1)). After no resin oligomers were detectable by thin-layer chromatography (after 12 h), the reaction solution was cooled, the precipitated product was filtered off using a suction strainer, washed with cold water, and then dried.

[0077] The pale yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfonic acids. Taking into account approximately 20 wt% excipients in the overlay impregnation (elasticizing agent, corundum, hardener, etc.), the yield was 86%. Example 4: Dissolution of decorative impregnating agent

[0078] Decorative impregnating agents that could no longer be used in production due to age were ground to dust in a laboratory mill under cooling. These agents had a core impregnation with urea resin (resin coating: approx. 50 wt%) and a second, covering impregnation with melamine resin (resin coating: approx. 50 wt%). 200 g of this decorative impregnating agent (molar ratio resin: melamine / formaldehyde: approx. 1 / 1.8) were transferred to a 2000 ml three-necked round-bottom flask and mixed with 400 g of a 30% sodium disulfite solution.

[0079] The mixture was boiled under reflux. Once a clear solution was obtained, it was hot-filtered to remove paper fibers, corundum, etc. The mixture was then boiled again under reflux. The reaction progress was monitored by thin-layer chromatography (eluent: water / ethanol (1:1)). After no resin oligomers were detectable by thin-layer chromatography (after 12 h), the reaction solution was cooled, the precipitated product was filtered off using a suction strainer, washed with cold water, and then dried.

[0080] The pale yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfonic acids. Taking into account approximately 10 wt% excipients in the resin (elasticizing agents, hardeners, etc.) and approximately 50 wt% urea resin in the core, the yield was 78%. Example 5: Dissection of HDF with a high melamine content

[0081] An HDF (high-density fiberboard) containing approximately 50 wt% binder, of which approximately 80 wt% is melamine resin, was ground to dust in a laboratory mill under cooling. 200 g of this dust were transferred to a 2000 ml three-necked round-bottom flask and mixed with 400 g of a 30% sodium disulfite solution.

[0082] The mixture was boiled under reflux (approx. 6 h). It was then hot-filtered to remove fibers, etc. Afterwards, it was boiled again under reflux. The reaction progress was monitored by thin-layer chromatography (eluent: water / ethanol (1:1)). After no resin oligomers were detectable by thin-layer chromatography (12 h), the reaction solution was cooled, the precipitated product was filtered off via a suction strainer, washed with cold water, and then dried.

[0083] The pale yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfonic acids. Taking into account approximately 10 wt% of excipients in the resin (elasticizing agents, hardeners, etc.), the yield was 55%. Example 6: Digestion of thin laminate

[0084] A thin laminate (decorative layer and parchment) was ground to dust in a laboratory mill under cooling. The thin laminate consisted of a decorative impregnation (paper weight: 100 g / m², resin coating: 120%) and a parchment (paper weight: 40 g / m²). 200 g of the ground thin laminate were transferred to a 2000 ml three-necked round-bottom flask and mixed with 600 g of a 30% sodium disulfite solution.

[0085] The mixture was boiled under reflux. Once a clear solution was obtained, it was hot-filtered to remove the paper fibers. The mixture was then boiled again under reflux. The reaction progress was monitored by thin-layer chromatography (elevating medium: water / ethanol (1:1)). After no resin oligomers were detectable by thin-layer chromatography (after 12 h), the reaction solution was cooled, the precipitated product was filtered off using a suction strainer, washed with cold water, and then dried.

[0086] The pale yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfonic acids. Taking into account approximately 10 wt% excipients in the resin (elasticizing agents, hardeners, etc.), the yield was 81%. Example 7: Exposure of direct coating floor structure

[0087] A laminate floor with an HDF core, recovered from a recycling process, was pre-shredded to a size of approximately 5 x 5 cm in a shredder. The laminate floor's coating consisted of an overlay impregnating layer (paper weight < 25 g / m², resin application: 400% melamine resin), a decorative impregnating layer (paper weight < 60 g / m², resin application: 100%), and a backing layer (paper weight < 80 g / m², resin application: 120% melamine resin). Subsequently, the urea-formaldehyde adhesive of the HDF was hydrolyzed in a pressure cooker under elevated pressure in a steam atmosphere.

[0088] The coatings were sieved from the wood fibers and ground to dust in a laboratory mill under cooling. The moisture content of the coatings was approximately 20%. 200 g of the ground coating were transferred dry into a 2000 ml three-necked round-bottom flask and mixed with 400 g of a 30% sodium disulfite solution.

[0089] The mixture was boiled under reflux. Once a clear solution was obtained, it was hot-filtered to remove paper fibers, wood fibers, corundum, etc. The mixture was then boiled again under reflux. The reaction progress was monitored by thin-layer chromatography (eluent: water / ethanol (1:1)). After no resin oligomers were detectable by thin-layer chromatography (after 12 h), the reaction solution was cooled, the precipitated product was filtered off using a suction strainer, washed with cold water, and then dried.

[0090] The pale yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfonic acids. Taking into account approximately 20 wt% excipients in the resin (elasticizing agents, corundum, hardener, etc.), the yield was 65%. Example 8: Use of methylolmelaminesulfonic acid as a hardener

[0091] The pale yellow product from embodiment 1 is dissolved in warm water (50°C, 20% solution). It contains methylolmelamines and methylolmelamine sulfonic acids.

[0092] The clear solution is used as a hardener for melamine resins (BASF Kauramin 796). Its curing behavior is compared with hardeners already in use (see Table 1 and diagram of the [reference to table / diagram]). Figure 1 ). Gelling times of BASF Kauramin 796 melamine resin (solids content approx. 63%) Hardener liquid / liquid wt. % 528 1448 1770 XXL Hardener sulfonic acid solid / liquid wt. % product 0,5 06:08 03:31 03:03 16:44 0,4 24:34 1,0 04:30 03:08 02:52 08:58 0,6 16:49 1,5 04:10 02:35 02:24 08:34 0,8 10:44 2,0 03:28 01:51 02:10 06:55 1,0 8:23

[0093] As the comparisons show, the melamine sulfone hardener is classified as a latent hardener (similar to XXL). These hardeners are preferred today because their use avoids pre-curing in the process. Example 9: Use of melamine methylols for condensation

[0094] The light yellow product from embodiment 1 is added to warm water (30°C). Concentrated ammonia is then added to the slurry until a pH of 7.5 is reached. This dissolves the methylolmelamine sulfonic acid. This can be used as a hardener for melamine resins (see embodiment 7). The slurry is then filtered off. The filter containing the methylolmelamines is dried. The proportion of methylolmelamines in the recycled melamine resin ranges from 10 to 30 wt%, depending on the molar ratio (melamine to formaldehyde). In this case, it was 20 wt%.

[0095] The methylol melamines were subsequently used to produce a melamine resin, with the proportion of recycled methylol melamines being 20% ​​by weight. The ratio of melamine to formaldehyde was assumed to be 1 to 1.5.

Claims

1. Process for the degradation of melamine-formaldehyde resin from waste products generated during the manufacture and processing of wood-based panels characterized by the following steps - Providing waste products generated during the manufacture and processing of wood-based panels that contain polycondensed melamine-formaldehyde resin, - If necessary, shredding the melamine-formaldehyde-resin-containing waste products, - Mixing the melamine-formaldehyde-resin-containing waste products with an aqueous 20 to 70% by weight, preferably 40 to 60% by weight, solution of at least one disulfite, hydrogen sulfite and / or dithionite, - Heating the mixture of melamine-formaldehyde resin-containing waste products and at least one disulfite, hydrogen sulfite and / or dithionite to temperatures between 60°C and 120°C, preferably between 80°C and 100°C, until a clear solution is obtained, - Separating foreign substances from the clear solution, - Further heating of the mixture of melamine-formaldehyde resin-containing waste products purified from foreign substances and at least one disulfite, hydrogen sulfite and / or dithionite at temperatures between 60°C and 120°C, preferably between 80°C and 110°C, under reaction control until no polycondensed melamine-formaldehyde resin is detectable in the reaction mixture, - Cooling of the reaction mixture and separation of the resulting mixture of melamine and monomeric melamine derivatives, - Separating the mixture of melamine and monomeric melamine derivatives into methylolmelamines and methylolmelamine sulfonic acids.

2. Process according to claim 1, characterized in that the melamine-formaldehyde-resin-containing waste products produced during the manufacture and processing of wood-based panels comprise overstored melamine resin, melamine resin dust, overlay impregnates, decorative impregnates, uncoated or coated wood-based panels, such as HDF panels, and / or laminates, such as thin laminate.

3. Process according to one of the preceding claims, characterized in that the melamine-formaldehyde-resin-containing waste products contain cellulose fibres, wood fibres, inorganic abrasion-resistant particles, glass beads, colour pigments, further binders and other additives.

4. Process according to one of the preceding claims, characterized in that the melamine-formaldehyde resin contained in the waste products has a molar ratio of melamine / formaldehyde of 1 / 1.5 to 1 / 2.5, preferably of 1 / 1.6 to 1 / 2.3, more preferably 1 / 1.6 to 1 / 1.8.

5. Process according to one of the preceding claims, characterized in that the melamine-formaldehyde-resin-containing waste products are comminuted to a particle size of between 0.1 and 10 mm, preferably between 0.5 and 3 mm.

6. Process according to one of the preceding claims, characterized in that the melamine-formaldehyde-resin-containing waste products are mixed with an aqueous 20 to 70% by weight, preferably 40-60% by weight, alkali metal or alkaline earth metal solution of at least one disulfite, hydrogen sulfite and / or dithionite.

7. Process according to one of the preceding claims, characterized in that the melamine-formaldehyde-resin-containing waste products are mixed with at least one sodium disulfite or sodium hydrogen sulfite solution having a pH of between 3.0 and 6, preferably between 3.5 and 5.5.

8. Process according to one of the preceding claims, characterized in that the melamine-formaldehyde-resin-containing waste products are mixed with at least one 40-60% by weight, in particular 50% by weight, sodium hydrogen sulfite solution.

9. Process according to one of the preceding claims, characterized in that the separation of the foreign substances contained in the reaction mixture is carried out by means of filtration, preferably hot filtration, and / or skimming.

10. Process according to one of the preceding claims, characterized in that the reaction control of the degradation of the melamine-formaldehyde resin is carried out using suitable analytical methods, such as thin-layer chromatography, HPLC or gas chromatography.

11. Process according to one of the preceding claims, characterized in that by adding ammonia or amines to the mixture of monomeric melamine derivatives, methylolmelamine sulfonic acids are converted into a soluble form and separated from methylolmelamines and melamine.