Process for recycling melamine-formaldehyde resin from waste resulting from the production and processing of wooden boards

Abstract

The present invention relates to a process for the degradation of melamine-formaldehyde resin from waste products generated during the manufacture and processing of wood-based panels, comprising the following steps: providing waste products generated during the manufacture and processing of wood-based panels containing polycondensed melamine-formaldehyde resin; optionally, comminuting the melamine-formaldehyde resin-containing waste products; mixing the melamine-formaldehyde resin-containing waste products with an aqueous solution of 20 to 70 wt%, preferably 40 to 60 wt%, 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; and separating impurities from the clear solution.Further heating of the mixture of melamine-formaldehyde resin-containing waste products and at least one disulfite, hydrogen sulfite and / or dithionite, purified of foreign substances, at temperatures between 60°C and 110°C, preferably between 80°C and 100°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 melamine and monomeric melamine derivatives, and separation of the mixture of melamine and monomeric melamine derivatives into melamine methylols and melamine sulfone methylols.

Description

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

[0002] In all industrial sectors, the question of how waste materials or products can be recycled at the end of their useful life is becoming increasingly relevant. This topic has gained significant relevance due to EU regulations requiring the reuse / recycling of all products in the future. It is also clear that the sharp rise in the prices of these raw materials is making recycling increasingly attractive. Furthermore, the disposal costs for waste materials have increased significantly. Furthermore, the desire for all producers to produce as ecologically as possible is becoming increasingly important. This means using raw materials effectively, avoiding emissions, and generating as little waste as possible.

[0003] These developments are particularly relevant for products containing oil- or gas-based raw materials. These products are typically made of plastics, which are also relevant in terms of quantity. While the recycling of thermoplastics (polyethylene, polypropylene, polyvinyl chloride) has been common practice for decades, the recycling concept for thermosets / thermosets (phenolic resins, melamine resins, urea resins, etc.) has not yet gained widespread acceptance.

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

[0005] Initial recycling approaches have been developed for urea-formaldehyde glues, which are used, among other things, as glues in wood-based materials. After curing, these glues are very sensitive to attack by moisture / water at elevated temperatures. In this case, the material (e.g., particleboard) can be broken down through hydrolysis. The hydrolyzed glue essentially remains in the resulting chips and serves as the basis for the added glues when a new wood-based material is manufactured. However, it must be noted that this process is more about reclaiming the wood matrix than reusing the glue components.

[0006] However, there have also been activities in the past that have shown that the recycling of thermosets should be possible. DD 155779 describes how production facilities contaminated or clogged by cured melamine resins can be cleaned. EP 0612 793 B1 describes how cured melamine resin (fibers) can be broken down and reused. However, this only refers to residues from molded article / fiber production and the return of these products. CN 111 675 827, CN 111 777 566, and CN 111 875 843 describe processes for recovering melamine from tableware made from melamine-formaldehyde resin. For this purpose, the dishes are first crushed and then treated with an aqueous organic solvent and an acidic catalyst, such as HCl, HNO 3 , H 2 SO 4 , AlCl 3 and similar, at temperatures between 80-150°C.The significant amounts of organic solvents combined with water required for these processes make them rather unfavorable from an ecological perspective. Furthermore, the aqueous phase also contains an acid, which is also problematic.

[0007] However, the recycling of thermosetting resins from wood-based materials, impregnated materials, or laminates has not yet been described. During the production and further processing of wood-based materials, waste containing melamine resin is generated at many stages of the value chain.

[0008] In the manufacture and processing of wood-based panels into floorboards or furniture parts, impregnated decorative papers, overlay papers, and counter-papers are pressed onto wood-based panels. These papers are impregnated with thermosetting resins (urea resin, melamine resin) in impregnation systems or impregnation channels. Large quantities of substandard impregnated materials are produced during system start-up, product changeovers, operating errors, or technical changes to production parameters. These are disposed of (e.g., through thermal recovery) and, depending on the order size, duration of the disruption, etc., represent an average loss of 3-8%. Since a modern impregnation channel operates at speeds of 80 m / min and more, this results in several million square meters of waste per year. Impregnated materials typically contain approx.50 wt% resin, which also results in significant losses of urea / melamine resin. In addition, resin waste is generated during impregnation due to trimming of the impregnated material and dust formation. These residues are also disposed of. Residual resins are also generated during impregnation, which are currently disposed of and can represent considerable tonnage over the year.

[0009] During further processing of the impregnated materials, for example, by pressing them onto wood-based panels, scraping devices on the presses remove excess impregnated material from the panels. Over-aged impregnated materials that can no longer be used in production are also disposed of. All of these residual materials or waste have the advantage that the melamine resins are not yet fully cured, making recycling easier.

[0010] Highly melamine-reinforced glues 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 materials: 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 high material losses, high disposal costs and waste problems.

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

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

[0014] This object is achieved by a method having the features of claim 1.

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

[0016] This procedure includes the following steps: Provision of waste products containing polycondensed melamine-formaldehyde resin arising during the production and processing of wood-based panels, if applicable. Comminution of 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 contaminants) from the clear solution, Further heating the mixture of melamine-formaldehyde resin-containing waste products and at least one disulfite, which has been 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 more polycondensed melamine-formaldehyde resin is detectable in the reaction mixture, cooling the reaction mixture and separating the resulting mixture of monomeric melamine and melamine derivatives, separating the mixture of monomeric melamine and melamine derivatives into melamine, melamine methylols and melamine sulfone methylols.

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

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

[0019] The melamine derivatives released from the polycondensate precipitate upon cooling and can then be filtered and washed. This also removes the additives, which vary depending on the waste / product. These are additives typically found in resins, such as hardeners, elasticizing agents, corundum, glass beads, pigments, etc. Other resins or glues (urea resin / glue) are also removed.

[0020] The melamine derivatives can be used for a wide variety of applications after separation of the components. After addition of ammonia or other amines, the melamine methylol sulfones can be converted into the corresponding ammonium salts. These can be used as hardeners. However, the melamine methylol sulfones themselves can also be used. The melamine methylols can be reused to produce melamine resins / glues for a wide variety of applications. They can also be used as formaldehyde scavengers in the production of wood-based materials when urea-formaldehyde glues are used as binders. They can also be used as a glue component to reduce swelling. In general, the components can also be used as flame retardants or as a flame retardant / binder combination in a wide variety of products (wood-based materials, laminates, etc.).

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

[0022] In one embodiment of the present method, the melamine-formaldehyde resin-containing waste products arising during the production and processing of wood-based panels comprise overlaid melamine resin, melamine resin dust, overlay impregnates, decorative impregnates, counteracting impregnates, kraft paper impregnates, uncoated or coated wood-based panels, such as HDF panels, and / or laminates, such as thin laminate.

[0023] An over-aged melamine resin is defined as a melamine-formaldehyde resin that has already at least partially condensed and is cloudy. Over-aging in a melamine resin is particularly present when either the viscosity has increased significantly (measured with a 4mm DIN cup according to EN ISO 2431:2011: fresh resin 20-30 seconds; over-aged resin: > 50 seconds) or the resin has become cloudy.

[0024] Melamine resin dust is generated at various points during the processing of impregnated materials and wood-based panels, such as at the clipper and during trimming. The resulting melamine resin dust has a grain size of > 50 µm, with a maximum of 100-300 µm. The trimming material is in the form of chips with a width of approximately 10 mm and variable lengths depending on the effectiveness of the scraper.

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

[0026] Accordingly, both partially impregnated papers (i.e., papers impregnated on one side) and fully impregnated papers can be used. In the case of single-sided impregnated papers, 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%, particularly preferably 50 wt%, based on the paper weight. In contrast, in the case of fully impregnated paper, a resin amount of 80-400 wt%, preferably 90-120 wt%, particularly preferably 100-110 wt%, based on the paper 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 mixed into the overlay resin or sprinkled onto the resin-coated overlay to increase abrasion resistance. Resin coatings with up to 400 wt% melamine resin are used for the impregnation of overlay papers.

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

[0029] Counter-papers are high-quality, impregnated papers for use as counter-paper, 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 glue are added to create surface effects and increase strength. The resin content of kraft paper impregnated with kraft paper is 100 to 120 wt%. These are impregnated with a mixture of melamine and phenolic resin, especially when used as the core layer of CPL.

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

[0032] Wood-based panels, such as particleboard and fiberboard (MDF and HDF), are made from wood chips or wood fibers obtained by chipping wood in a chipper or by defibrating wood chips in a refiner. The wood fibers used in particleboard 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 middle layer. In the middle layer, the chips start at a mesh size of > 0.125 mm, while in the face layer they range from > 0.8 to 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 typically ranges between 5 and 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 even exceed 25 wt%, for example, up to 30-50 wt%.

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

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

[0036] Currently known in the state of the art are, for example, flexible laminates in which the composite layer comprises at least one impregnated paper layer, such as a decorative impregnated paper, 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 transparent paper made from finely ground pulp that is largely greaseproof but not water-resistant. It achieves its high transparency through a very sharp calendering process. The various plies or layers can be contained multiple times in the laminate structure, alternate, or swap their positions.

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

[0038] In addition to cellulose fibers, wood fibers, and wood chips, the melamine-formaldehyde resin-containing waste products can also contain inorganic abrasion-resistant particles, glass beads, color pigments, other binding agents such as urea resins, and other additives. The cellulose fibers originate from impregnated papers, such as the decorative impregnates, overlay impregnates, and counteracting impregnates described above. Inorganic particles, such as abrasion-resistant particles (corundum), are contained in overlay impregnates. Wood fibers originate from wood-based panels (such as HDF and MDF boards), and wood chips from particleboard. Color pigments are preferably found in decorative impregnates. Other additives include flame retardants, hardeners, elasticizers, and waxes.

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

[0040] As can be seen, the melamine-formaldehyde resin-containing waste products to be digested differ in their composition, allowing for further adaptations and modifications to the present process. While the digestion of melamine resin dust does not require further comminution of the waste product prior to mixing with at least one disulfite, hydrogen sulfite, and / or dithionite solution, this is indicated for the other waste products.

[0041] However, when using decorative impregnated materials, overlay impregnated materials, counteracting impregnated materials, coated and uncoated wood-based panels, and laminates, comminution of the waste products is necessary. Accordingly, in one embodiment, these melamine-formaldehyde resin-containing waste products are crushed to a particle size between 0.1 and 10 mm, preferably between 0.5 and 3 mm. The crushing process works best with very finely ground material with a particle size of less than 3 mm.

[0042] In the case of a wood-based panel coated with an overlay impregnate, decorative impregnate, and / or counteracting impregnate (such as fiberboard or particleboard), it is also advantageous to first pre-shred the coated wood-based panel (e.g., to a size of 5 × 5 cm). The pre-shredded pieces are first treated in a steam atmosphere to hydrolyze the additional binder present in the wood-based panel (preferably urea-formaldehyde resin).

[0043] The residue obtained after hydrolysis (preferably from the impregnates and wood fibers or wood chips) is separated and further crushed and only then mixed with at least one disulfite, hydrogen sulfite and / or dithionite solution for the hydrolytic degradation of the melamine-formaldehyde resin according to the present process.

[0044] The melamine-formaldehyde resin contained in the waste products and to be digested 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.

[0045] In one embodiment of the present process, the polycondensed melamine-formaldehyde resin to be digested in the waste products is not yet fully cured; that is, the polycondensate is not yet fully crosslinked. This applies particularly to overlaid resin and impregnated paper layers (which have not yet been subjected to a pressing step).

[0046] In a further embodiment of the present process, however, the polycondensed melamine-formaldehyde resin to be digested in the waste products is fully cured. This applies in particular to pressed uncoated and coated wood-based panels and laminates.

[0047] The different polymerization and crosslinking states of melamine-formaldehyde resin are briefly described below.

[0048] Melamine and formaldehyde initially react to form water-soluble monomer products, forming methylol groups on the amino groups of melamine (see Scheme I). These melamine-formaldehyde monomers are also called melamine methylols.

[0049] After 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).

[0050] Precondensates and polycondensates differ in their molecular weight and solubility. For example, low-molecular-weight precondensates may still have 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 crosslinking of the mostly linear oligomers. The precondensates are thus a polymerization intermediate.

[0051] When the polycondensates are fully cured, strong crosslinking occurs with the elimination of the remaining methylol groups, forming tightly crosslinked plastics via methylene groups (see Scheme III).

[0052] For synthetic resins that cure via condensation reactions, the following resin states are distinguished: A-state: readily soluble in solvent, meltable, curable; B-state: only partially soluble in solvent, meltable, curable; C-state: insoluble, cured

[0053] The digestion of a resin that is not yet fully cured (i.e. a resin in the partially cross-linked B-stage) is preferred due to the lower degree of cross-linking of the resin.

[0054] In one embodiment of the present process, the melamine-formaldehyde resin-containing waste products are mixed with an aqueous 20 to 70 wt%, preferably 40-60 wt% alkali or alkaline earth solution of at least one disulfite, hydrogen sulfite and / or dithionite.

[0055] 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 (Na 2 S 2 O 5 ), also called sodium pyrosulfite or sodium metabisulfite, is a sodium salt of disulfurous acid, which is unstable in its free form. Sodium hydrogen sulfite (NaHSO 3 ), also called sodium bisulfite, is a sodium salt of sulfurous acid. It is unstable outside of aqueous solutions.

[0056] In a particularly preferred embodiment, the melamine-formaldehyde resin-containing waste products are mixed with at least a 40-60 wt%, in particular 50 wt%, sodium hydrogen sulfite (or sodium bisulfite) solution.

[0057] 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.

[0058] In this case, a clear solution means that no turbidity is visible (i.e. the solution is transparent), only the solids contained as contaminants are visible.

[0059] The aforementioned foreign substances are separated from the clear solution in an intermediate step. The solids contained in the reaction mixture as impurities are separated by filtration, preferably hot filtration, and / or skimming. This allows the floating fibers released from the paper layers or wood-based panels to be skimmed off, and other particles, such as corundum and / or glass beads, to be removed by hot filtration.

[0060] This is followed by further heating of the reaction mixture, which has been purified from foreign substances and consists of waste products containing melamine-formaldehyde resin 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 can be detected in the reaction mixture.

[0061] 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.

[0062] After depolymerization of the melamine-formaldehyde resin, the reaction mixture is cooled. During cooling, the product mixture of monomeric melamine derivatives precipitates.

[0063] The mixture of melamine and monomeric melamine derivatives comprises melamine methylols, especially monomethylolmelamine and dimethylolmelamine, and melamine sulfone methylols, especially dimethylolmelamine sulfonic acid and monomethylolmelamine sulfonic acid. The proportion of melamine methylols is approximately 10-30 wt%, and the proportion of melamine sulfone methylols is approximately 70-90 wt%.

[0064] In a final step, the mixture of melamine and monomeric melamine derivatives is separated into melamine, melamine methylols, and melamine sulfonmethylols. Separation can be achieved by adding ammonia or amines to the mixture of melamine and monomeric melamine derivatives, converting melamine sulfonmethylols into a soluble form and separating them from melamine and melamine methylols.

[0065] The melamine methylols and melamine sulfone methylols obtained from melamine-formaldehyde resin can be used for a variety of purposes. For example, melamine sulfone methylols can be used as hardeners, and melamine methylols can be used in the production of melamine resin for use as binders or impregnating agents.

[0066] The invention is explained in more detail below using exemplary embodiments. General:

[0067] In the examples, the reaction progress was monitored using thin-layer chromatography. Melamine resins / precursors were run for comparison, allowing an assessment of the degradation progress. These were melamine, monomethylolmelamine, dimethylmelamine, and a melamine resin. Example 1: Digestion of melamine residue resin waste

[0068] 100 g of a melamine resin that had already become cloudy due to storage (solids content: 55 wt%, molar ratio melamine / formaldehyde: approx. 1 / 1.8) were transferred into a 1000 ml three-necked round-bottom flask and treated with 300 g of a 50% sodium disulfite solution.

[0069] The mixture was boiled under reflux, during which the turbidity dissipated after approximately 3 hours. The reaction progress was monitored by thin-layer chromatography (mobile phase: water / ethanol (1 / 1). After no more resin oligomers were detectable by thin-layer chromatography (6 h), the reaction solution was cooled, the precipitated product was filtered off with suction, washed with cold water, and then dried.

[0070] The light yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfones. The filtrate was again concentrated by half, and the precipitated product was also washed and dried. The yield was 95%, taking into account approximately 10% by weight of additives in the resin (elastifier, hardener, etc.). Example 2: Digestion of melamine residues

[0071] 100 g of solid melamine resin dust (molar ratio melamine / formaldehyde: approximately 1 / 1.8) obtained from impregnation on the clipper and the trimming area was transferred to a 2000 ml three-necked round-bottom flask and mixed with 600 g of a 50% sodium bisulfite solution.

[0072] The mixture was boiled under reflux, during which the resin slowly dissolved. Once a clear solution had formed, floating fibers (paper and cellulose) were first skimmed off and then filtered hot to separate corundum and glass beads. These were washed and reused. The mixture was then boiled under reflux again. The reaction progress was monitored by thin-layer chromatography (mobile phase: 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 with suction, washed with cold water, and then dried.

[0073] The light yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfones. The yield was 91%, taking into account approximately 10% by weight of additives in the resin (elastifiers, hardeners, etc.). In a further experiment, the amount of sodium bisulfite was reduced to 150 g in 300 ml of water. No difference in yield was detectable. Example 3: Melamine residue digestion overlay

[0074] Overlay impregnates that could no longer be used in production due to over-storage were ground to dust in a laboratory mill under cooling. 100 g of this overlay impregnate (molar ratio resin:melamine / formaldehyde: approximately 1 / 1.8) was transferred to a 2000 ml three-neck round-bottom flask and mixed with 400 g of a 50% sodium disulfite solution.

[0075] The mixture was boiled under reflux. After a clear solution had formed, the mixture was filtered while hot to remove paper fibers, corundum, etc. The mixture was then boiled under reflux again. The reaction progress was monitored by thin-layer chromatography (mobile phase: 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 with suction, washed with cold water, and then dried.

[0076] The light yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfones. The yield was 86%, taking into account approximately 20% by weight of additives in the overlay impregnate (elastifier, corundum, hardener, etc.). Example 4: Decorative impregnated material

[0077] Decorative impregnates that could no longer be used in production due to storage were ground to dust in a laboratory mill under cooling. These impregnates had a core impregnation with urea resin (resin application: approximately 50 wt%) and a second covering impregnation with melamine resin (resin application: approximately 50 wt%). 200 g of this decorative impregnate (molar ratio resin: melamine / formaldehyde: approximately 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.

[0078] The mixture was boiled under reflux. After a clear solution had formed, the mixture was filtered while hot to remove paper fibers, corundum, etc. The mixture was then boiled under reflux again. The reaction progress was monitored by thin-layer chromatography (mobile phase: 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 with suction, washed with cold water, and then dried.

[0079] The light yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfones. The yield was 78%, taking into account approximately 10% by weight of additives in the resin (elastifiers, hardeners, etc.) and approximately 50% by weight of urea resin in the core. Example 5: HDF pulping with high melamine content

[0080] An HDF containing approximately 50 wt% binder, of which approximately 80 wt% melamine resin, was ground to dust in a laboratory mill under cooling. 200 g of this dust was transferred to a 2000 ml three-neck round-bottom flask and mixed with 400 g of a 30% sodium disulfite solution.

[0081] The mixture was boiled under reflux (approx. 6 h). Then, it was filtered while hot to remove fibers, etc. The mixture was then boiled under reflux again. The reaction progress was monitored by thin-layer chromatography (mobile phase: 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 with suction, washed with cold water, and then dried.

[0082] The light yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfones. The yield was 55%, taking into account approximately 10% by weight of additives in the resin (elastifiers, hardeners, etc.). Example 6: Digestion of thin laminate

[0083] A thin laminate (decorative and parchment) was ground to dust in a laboratory mill under cooling. The thin laminate consisted of a decorative impregnated paper (paper weight: 100 g / m², resin coverage: 120%) and a parchment paper (paper weight: 40 g / m²). 200 g of the ground thin laminate was transferred to a 2000 ml three-necked round-bottom flask and mixed with 600 g of a 30% sodium disulfite solution.

[0084] The mixture was boiled under reflux. Once a clear solution had formed, the mixture was filtered while hot to separate the paper fibers. The reaction was then continued under reflux. The reaction progress was monitored by thin-layer chromatography (mobile phase: 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 with suction, washed with cold water, and then dried.

[0085] The light yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfones. The yield, taking into account approximately 10% by weight of additives in the resin (elastifiers, hardeners, etc.), was 81%. Example 7: Direct coating floor construction

[0086] A laminate floor with an HDF core from a return was pre-shredded in a shredding device (chipper) to a size of approximately 5 × 5 cm. The laminate flooring's coating consisted of an overlay impregnated resin (paper weight: 25 g / m², resin application: 400% melamine resin), a decorative impregnated resin (paper weight: 60 g / m², resin application: 100%), and a countercoat (paper weight: 80 g / m², resin application: 120% melamine resin). The urea-formaldehyde glue of the HDF was then hydrolyzed in a cooker at elevated pressure in a steam atmosphere.

[0087] The coatings were sieved to remove 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 was transferred dry into a 2000 ml three-necked round-bottom flask and mixed with 400 g of a 30% sodium disulfite solution.

[0088] The mixture was boiled under reflux. After a clear solution had formed, the mixture was filtered while hot to remove paper fibers, wood fibers, corundum, etc. The mixture was then further boiled under reflux. The reaction progress was monitored by thin-layer chromatography (mobile phase: 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 with suction, washed with cold water, and then dried.

[0089] The light yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfones. The yield was 65%, taking into account approximately 20% by weight of additives in the resin (elastifier, corundum, hardener, etc.). Example 8: Use of methylolmelamine sulfones as hardener

[0090] The light yellow product from Example 1 is dissolved in warm water (50°C, 20% solution). It contains melamine methylols and melamine sulfonmethylols.

[0091] The clear solution is used as a hardener for melamine resins (BASF Kauramin 796). The curing behavior is compared with hardeners already in use (see Table 1 and diagram of the Figure 1 ). Gelling times Melamine resin BASF Kauramin 796 (solids content approx. 63%) Hardener liquid / liquid wt.% 528 1448 1770 XXL Hardener Sulfone 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

[0092] As the comparisons show, the melamine sulfone hardener can be classified as a latent hardener (similar to XXL). These hardeners are preferred today because they avoid pre-curing in the process. Example 9: Use of melamine methylols for condensation

[0093] The light yellow product from Example 1 is added to warm water (30°C). Then, concentrated ammonia is added to the slurry to a pH of 7.5. This dissolves the melamine methylol sulfones. These can be used as hardeners for melamine resins (see Example 7). The slurry is then filtered off with suction. The filter containing the melamine methylols is dried. The proportion of melamine methylols in the recycled melamine resin is between 10 and 30 wt.%, depending on the molar ratio (melamine to formaldehyde). In this case, it was 20 wt.%.

[0094] The methylolmelamines were then used to produce a melamine resin, with the recycled melamine methylol content being 20% ​​by weight. The melamine to formaldehyde ratio was assumed to be 1:1.5.

Claims

1. Process for the degradation of melamine-formaldehyde resin from waste products generated during the production and processing of wood-based panels characterized bythe following steps - Providing waste products containing polycondensed melamine-formaldehyde resin arising during the production and processing of wood-based panels, - If necessary. Comminuting 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 100°C, until a clear solution is obtained, - Separating foreign substances from the clear solution, - Further heating the mixture of melamine-formaldehyde resin-containing waste products and at least one disulfite, which has been 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 the reaction mixture and separating the resulting mixture of melamine and monomeric melamine derivatives, - separating the mixture of melamine and monomeric melamine derivatives into melamine methylols and melamine sulfone methylols., 2. Method according to claim 1, characterized in that The waste products containing melamine-formaldehyde resin arising during the production and processing of wood-based panels include overlaid 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. Method according to one of the preceding claims, characterized in thatthe melamine-formaldehyde resin-containing waste products contain cellulose fibers, wood fibers, inorganic abrasion-resistant particles, glass beads, color pigments, other binding agents and other additives.

4. Method 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 to formaldehyde of 1 / 1.5 to 1 / 2.5, preferably of 1 / 1.6 to 1 / 2.3, particularly preferably 1 / 1.6 to 1 / 1.

8.

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

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

7. Method according to one of the preceding claims, characterized in that the waste products containing melamine-formaldehyde resin are mixed with at least one sodium disulfite or hydrogen sulfite solution having a pH value between 3.0 and 6, preferably between 3.5 and 5.

5.

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

9. Method according to one of the preceding claims, characterized in that the foreign substances contained in the reaction mixture are separated by filtration, preferably hot filtration, and / or skimming.

10. Method 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. Method according to one of the preceding claims, characterized in that By adding ammonia or amines to the mixture of monomeric melamine derivatives, melamine sulfonmethylols are converted into a soluble form and separated from melamine methylols and melamine.