WO2024104889A1

WO2024104889A1 - Method of recycling melamine-formaldehyde resin from wastes that occur during production and processing of woodbase material boards

Info

Publication number: WO2024104889A1
Application number: PCT/EP2023/081333
Authority: WO
Inventors: Georg Seidack; Norbert Kalwa
Original assignee: Flooring Technologies Ltd.
Priority date: 2022-11-15
Filing date: 2023-11-09
Publication date: 2024-05-23

Abstract

The present invention relates to a method of degrading melamine-formaldehyde resin from wastes that occur during production and processing of woodbase material boards, comprising the following steps: providing waste products that are obtained during production and processing of woodbase material boards and comprise polycondensed melamine-formaldehyde resin, optionally comminuting the melamine-formaldehyde resin-containing waste products, mixing the melamine-formaldehyde resin-containing waste products with water and adding at least one reducing sulfur compound selected from a disulfite, hydrogensulfite and/or dithionite, a sulfinate or sulfinate analogue, heating the aqueous mixture/slurry of melamine-formaldehyde resin-containing waste products and at least one reducing sulfur compound to temperatures between 60°C and 120°C, preferably between 80°C and 100°C, with control of the reaction until a clear solution is obtained, removing extraneous materials from the clear solution, optionally further heating the mixture of melamine-formaldehyde resin-containing waste products and at least one reducing sulfur compound that has been freed of extraneous materials at temperatures between 60°C and 120°C, preferably between 80°C and 110°C, with control of the reaction until no polycondensed melamine-formaldehyde resin is detectable (any longer) in the reaction mixture, and cooling the reaction mixture and removing the mixture of melamine derivatives obtained, especially removing melamine methylols and melamine sulfonate.

Description

Process for recycling melamine-formaldehyde resin from waste generated during the production and processing of wood-based panels

The present invention relates to a process for recycling or breaking down melamine-formaldehyde resin from waste arising during the manufacture, processing and return of wood-based panels. The present invention also relates to the use of melamine derivatives recycled from waste products containing melamine-formaldehyde resin.

Description

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 important. This topic has been given a significant boost in relevance by the EU's requirements, which stipulate that all products must be reused/recycled in the future. It is also clear that recycling is becoming more and more interesting due to the sharp rise in the prices of these raw materials. In addition, the disposal costs for waste materials have risen sharply. In addition, the aspect that all producers want to produce as ecologically as possible is becoming more and more important. This means that raw materials should be used effectively, emissions should be avoided, and as little waste should be produced as possible.

These developments are particularly relevant for products that contain raw materials based on oil or gas. These products are usually 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 idea of recycling thermosets/thermosets (phenolic resins, melamine resins, urea resins, etc.) has not yet caught on.

This is partly due to the cross-linked structure of the thermosets/thermosets, which is difficult to break open and is seen as a particular advantage of these products, as they are considered to be very resistant to thermal or chemical attack. But also to the fact that in the past the prices of the raw materials were sometimes very low.

The first approaches to recycling were developed for urea-formaldehyde glues, which are used as glue for wood materials, among other things. After curing, these glues are very sensitive to the attack of moisture/water at higher temperatures. Here, the material (e.g. chipboard) can be broken down by hydrolysis. The hydrolyzed glue essentially remains in the resulting chips and serves as the basis for the glue added when a new wood material is produced. However, it must be stated that this process is more about recovering the wood matrix than about recovering the glue components.

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/clogged by hardened melamine resins can be cleaned.

EP 0612 793 B1 describes how hardened melamine resin (fibers) can be broken down and put to use. However, it only refers to residues from molded body/fiber production and returns of these products. Hardened melamine residues/waste are processed by reductive digestion with bisulfite, hydrogen sulfite or dithionate. The vast majority of the products produced are melamine methylol sulfones. The document also describes possible uses for these sulfones. However, the sulfones can only be used in small quantities in the applications. In addition, because a large excess of reducing agent is used, a lot of waste is generated in the form of inorganic sulfur salts. These must be disposed of after separation. In addition, the reaction time is at least eight hours.

Another process is described in CN 1 11675827. There, hardened melamine resins from various sources are heated under reflux in a mixture of water and organic solvents using acids or Lewis acids and broken down. The ratio of solvent to melamine waste is at least 80 to 1. Here, too, relatively long reaction times of up to twenty-four hours are specified. The reaction is also carried out under pressure at > 100°C. In addition to the resulting melamine, large quantities of mixtures of solvents, acids and salts remain. These residues must in turn be disposed of. All of this makes the process complex, expensive and questionable from a recycling perspective.

CN 11 1 777 566 and CN 1 11 875 843 describe similar processes for the recovery of melamine from tableware made from melamine-formaldehyde resin. Crockery is first crushed and then treated with an aqueous organic solvent and an acid catalyst, such as HCl, HNO ₃ , H ₂ SO ₄ , AICI ₃ and similar, at temperatures between 80-150°C. The considerable amounts of organic solvents in combination with water that these processes require make them rather unfavourable from an ecological point of view. In addition, there is also an acid in the aqueous phase, which is also problematic.

However, the recycling of thermosetting resins from wood-based materials, impregnates 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.

In the case of the production and further processing of wood-based panels into floorboards or furniture parts, impregnated decorative papers, overlay papers and counter-papers are pressed with wood-based panels. These papers are impregnated with thermosetting resins (urea resin, melamine resin) in impregnation systems or impregnation channels. When the systems are started up, when products are changed, when operating incorrectly or when the production parameters are changed for technical reasons, large quantities of impregnated products of poor quality are produced. These are disposed of (e.g. by thermal use) 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 waste per year. Impregnates usually contain around 50% by weight of resin, which also results in significant amounts of urea/melamine resin being lost. In addition, impregnation also produces waste resins due to trimming of the impregnated material and the formation of dust. These residues are also disposed of. Residual resins are also produced during impregnation, which are now disposed of and can also represent considerable tonnages over the course of a year.

During further processing of the impregnated material, for example by pressing it with wood-based panels, waste is generated by scraping devices on the presses that remove excess impregnated material from the panels, and impregnated material that has been stored and can no longer be used in production is also disposed of. All of these residual materials or waste have the advantage that the melamine resins have not yet fully hardened, so recycling could be easier. Glues that are highly reinforced with melamine are also often used in the production of wood-based panels (such as chipboards and fiberboards). The same applies to these panels as to the impregnated materials: waste is generated at all stages of the value chain and must be disposed of.

The disadvantages resulting from the current practice of waste disposal are in particular high material losses, high disposal costs and waste problems.

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.

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 creation stages of impregnation and wood-based material production and product returns. The process is intended to provide products that can be used to manufacture impregnates, coatings or wood-based materials. It should be taken into account that higher-value use should be preferred due to the recycling costs. A particular focus should be on generating as little or no waste as possible. In addition, the process should be as quick and as energy-efficient a solution as possible.

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

Accordingly, a process is provided for the degradation or decomposition or depolymerization of melamine-formaldehyde resin from waste products arising during the manufacture and processing of wood-based panels. The melamine-formaldehyde resin to be decomposed is bound in particular in the waste or waste products from paper layers and wood-based carrier panels arising in production plants for the manufacture and further processing of wood-based panels.

The present procedure comprises the following steps:

Provision of waste products arising during the production and processing of wood-based panels containing polycondensed melamine-formaldehyde resin, optionally crushing the melamine-formaldehyde resin-containing waste products, mixing the melamine-formaldehyde resin-containing waste products with water and adding at least one reducing sulphur-containing compound selected from a disulfite, hydrogen sulphite and/or dithionite, a sulphinate or sulphinate analogues,

Heating the aqueous mixture/slurry of melamine-formaldehyde resin-containing waste products and at least one reducing sulphur-containing compound to temperatures between 60°C and 120°C, preferably between 80°C and 100°C, under reaction control until a clear solution is obtained, separating foreign substances from the clear solution,

Optionally further heating the mixture of melamine-formaldehyde resin-containing waste products and at least one reducing sulphur-containing compound, which has been cleaned of foreign substances, 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, and

- Cooling the reaction mixture and separating the resulting mixture of melamine derivatives, in particular separating melamine methylols and melamine sulfonate.

A process is therefore provided in which a thermosetting resin matrix in various waste products is broken down using bisulfite/disulfite/dithionite or sulfinate. The more or less cross-linked melamine resins contained in the residues are reductively broken down using the sulfur-containing reducing agents.

The reaction products are melamine methylols and melamine sulfonate, as well as small amounts of melamine, with melamine sulfonate being the main component (> 70%). The melamine sulfonate formed has the following structure:

The impurities contained in the various residues or products can be easily separated during or after the reaction. This can be done by filtering or skimming off floating fibers, etc. This can be done during the digestion reaction (continuously) or at the end. However, it must be taken into account that the fibers, for example, can lead to an increased consumption of reducing agent. This is not necessary when processing resins, although impurities should still be separated in a cleaning step.

During the digestion process, which is carried out by boiling under reflux, the melamine resin matrix is broken up and the water-insoluble residues slowly dissolve. The auxiliary materials, which vary depending on the waste/product, are also removed. These are the auxiliary materials usually found in the resins, such as hardeners, elasticizing agents, corundum, glass beads, pigments, etc. However, other resins or glues are also removed (urea resin/glue).

The melamine derivatives released from the polycondensate precipitate when cooled and can then be filtered off and washed. This also removes the auxiliary materials, which vary depending on the waste/product. These are the auxiliary materials usually found in the resins, such as hardeners, elasticizing agents, corundum, glass beads, pigments, etc. However, other resins or glues are also removed (urea resin/glue).

The melamine derivatives can be used for a wide variety of applications after the components have been separated. After adding ammonia or other amines, the melamine sulfonates can be converted into the corresponding ammonium salts. These can be used as hardeners. However, the melamine sulfonates themselves can also be used. The melamine methylols can be used again to produce melamine resins/glues for a wide variety of applications. They can also be used as formaldehyde scavengers. in the manufacture of wood materials using urea-formaldehyde glues as a binding agent. It can also be used as a glue component to reduce swelling. In general, the components can also be used as flame retardants or a combination of flame retardants/binding agents in a wide variety of products (wood materials, laminates, etc.).

The present process thus enables an (almost) complete recycling cycle for melamine resins. This saves raw materials, avoids waste and reduces costs.

In a first aspect, the present method comprises the following steps:

Provision of waste products arising during the production and processing of wood-based panels containing polycondensed melamine-formaldehyde resin,

If necessary, 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,

Separation of non-hydrolysable foreign substances (as contaminants) 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, which has been cleaned of foreign substances, 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 melamine derivatives, in particular melamine methylols and melamine sulfonate. According to the first aspect, a process is therefore provided in which a thermosetting resin matrix in various waste products is digested with the aid of bisulfite/disulfite/dithionite.

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

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.

In particular, the waste products containing melamine-formaldehyde resin are mixed with at least one sodium disulfite or hydrogen sulfite solution with a pH value between 3.0 and 6, preferably between 3.5 and 5.5. Sodium disulfite (Na2S20s), also called sodium pyrosulfite or sodium metabisulfite, is a sodium salt of disulfurous acid, which is not stable in free form. Sodium hydrogen sulfite ( _NaHSO3 ), also called sodium bisulfite, is a sodium salt of sulfurous acid. It is not stable outside of aqueous solutions.

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.

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.

In this case, a clear solution means that no turbidity is visible (ie the solution is transparent); only the solids contained as contaminants are visible. The foreign substances mentioned are separated from the clear solution in an intermediate step. The solids contained in the reaction mixture as contaminants are separated by means of filtration, preferably hot filtration, and/or skimming. In this way, the fibers released from the paper layers or wood-based panels and floating on the surface can be skimmed off and other particles, such as corundum and/or glass beads, can be separated by hot filtration.

This is followed by further heating of the reaction mixture, which has been purified of foreign substances and is made up 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 any longer.

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 (TLC), HPLC or gas chromatography. Thin layer chromatography (TLC) is preferably used, with water and/or an alcohol such as methanol, ethanol or a mixture thereof being used as a suitable mobile phase. The use of a mixture of water/ethanol (1:1) as the mobile phase is preferred.

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

The mixture of monomeric melamine derivatives comprises melamine methylols, in particular monomethylolmelamine and dimethylolmelamine, and melamine sulfonate. The proportion of melamine methylols is approximately 10-30% by weight and the proportion of melamine sulfonate is approximately 70-90% by weight.

In a final step, the mixture of monomeric melamine derivatives is separated into melamine methylols and melamine sulfonate.

The melamine methylols and melamine sulfonate obtained from the melamine-formaldehyde resin can be used for various purposes. The melamine sulfonate precipitates (after concentration of the liquid mixture) and can be separated By adding ammonia or amines, the melamine sulfonates can be converted into a soluble form.

For example, melamine derivatives recycled or obtained from waste products containing melamine-formaldehyde resin are used as hardeners for resins, flame retardants and/or for the production of melamine resin.

In one embodiment, the recycled melamine derivatives comprise a mixture of monomeric melamine methylols and melamine sulfonate, which can be separated as described.

In one embodiment, melamine sulfonate is used as a hardener for formaldehyde resins, in particular for melamine-formaldehyde resins or urea-formaldehyde resins. The melamine sulfonate is particularly suitable as a latent hardener. The solution containing melamine sulfonate is preferably used as a hardener after the melamine methylols have been separated off.

When using melamine sulfonate as a hardener, a suitable amount of melamine sulfonate in the resin to be cured is between 0.4 and 3 wt%, preferably between 0.5 and 2.0 wt%.

In a further embodiment, it is provided that the melamine methylols (separated from the mixture) are used for the production of melamine resin, in particular for the production of melamine resin for use as a binder or impregnating agent,

In this case, the recycled melamine methylols are preferably mixed with melamine and formaldehyde. In one embodiment, the melamine methylols are used in a ratio of mono- to dimethylmelamine of 1:1. Formaldehyde and melamine are added in a ratio of 1:1. The condensation that begins to form melamine-formaldehyde resin is preferably carried out to a water dilutability of 1:3.

In a further embodiment, the melamine methylols and melamine sulfonate are used as a mixture or alone as flame retardants. In the case of use as flame retardants, the ammonium salts of the melamine derivatives are preferably used. In a second aspect, the present method comprises the following steps:

Providing waste products which arise during the production and processing of wood-based panels and which contain polycondensed melamine-formaldehyde resin, optionally crushing the waste products containing melamine-formaldehyde resin, mixing the waste products containing melamine-formaldehyde resin with water and adding at least one sulfinate or sulfinate analogues (as a reducing agent), heating the aqueous mixture/slurry of waste products containing melamine-formaldehyde resin and at least one sulfinate or sulfinate analogues to temperatures between 60°C and 120°C, preferably between 80°C and 100°C, under reaction control until a clear solution is obtained,

Separation of foreign substances from the clear solution,

Optionally further heating the mixture of melamine-formaldehyde resin-containing waste products and at least one sulfinate or sulfinate analogues, which mixture has been purified of foreign substances, 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, and cooling the reaction mixture and separating off melamine sulfonate which precipitates as a salt.

According to the second aspect, the thermosetting resin matrix in the various products is broken down using a sulfinate or sulfinate analogue as a reducing agent, in particular formamidine sulfinic acid (FAS). The reaction is carried out in water as a solvent. The material to be recycled is boiled under reflux. A particularly positive aspect of this process is that, unlike in the prior art, the reaction is completed after 90 to 120 minutes. The ratio between reducing agent and melamine waste is also significantly lower. In the above-mentioned EP 0612 793 B1, the ratio is approximately 4 to 1. With FAS, the ratio can be reduced to 2 to 1.

When melamine waste is reacted with a sulfinate or sulfinate analogues, such as FAS, melamine sulfonate is the main product when FAS is used as an ammonium salt with the structure given above: The yield of melamine sulfonate depends on the melamine-formaldehyde resin-containing waste product to be digested and is between 70 and 95%, preferably between 75 and 90% based on the melamine-formaldehyde resin-containing waste product used.

The decomposition of the melamine-formaldehyde resin-containing waste products to form essentially one main product was surprising, since the use of disulfite as a reducing agent known from the prior art typically produces mixtures of melamine, melamine methylols and melamine methylol sulfonates (EP 612 793 B1).

After the reaction has been completed and the impurities have been separated as mentioned above, the reaction solution is cooled. The melamine sulfonate precipitates and can be separated from the reaction solution, washed and dried. Both the precipitated melamine sulfonate and the supernatant can be reused.

The present process according to the second aspect also does not require the use of organic solvents. Rather, aqueous solutions of sulfinates or sulfinate analogues, such as FAS, are used.

As already mentioned, a sulfinate or sulfinate analogue is used as a reducing agent. The preferred reducing agent is the sulfinate analogue formamidine sulfinic acid (FAS).

Formamidine sulfinic acid (FAS), also known as aminoiminomethane sulfonic acid, is a white to yellow odorless solid that can be produced in situ from thiourea and hydrogen peroxide. The reducing effect of FAS comes from the sulfinate ion produced during hydrolysis. FAS is now used in large quantities to decolorize recycled paper and is therefore available in large quantities.

In a further embodiment of the present process, the weight ratio of reducing agent to waste products containing melamine-formaldehyde resin is between 2:1 and 0.5:1, preferably between 1.5:1 and 0.8:1. The addition of FAS is preferably carried out in portions, since FAS itself is not readily soluble in warm water, and thus reasonable mixing can be ensured. In a preferred embodiment of the present process, the reaction time until the melamine-formaldehyde resin is completely degraded is between 60 and 240 minutes, preferably between 90 and 120 minutes. These relatively short reaction times were particularly surprising in view of the much longer reaction times known from the prior art. This provides a time-saving and therefore cost-effective variant for recycling melamine resin waste.

As mentioned, in a next step, the aqueous mixture / slurry of melamine-formaldehyde resin-containing waste products and at least one sulfinate is heated to temperatures between 60°C and 120°C, preferably between 80°C and 100°C, under reaction control, until a clear solution is obtained.

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.

The foreign substances mentioned are separated from the clear solution in an intermediate step. The solids contained in the reaction mixture as contaminants are separated by means of filtration, preferably hot filtration, and/or skimming. In this way, the fibers released from the paper layers or wood-based panels and floating on the surface can be skimmed off and other particles, such as corundum and/or glass beads, can be separated by hot filtration.

Optionally, the reaction mixture, which has been purified of foreign substances and is made up of waste products containing melamine-formaldehyde resin and at least one sulfinate, is further heated 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 any longer.

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. In the case of thin-layer chromatography, water and/or an alcohol such as methanol, ethanol or a mixture thereof is used as a suitable mobile phase. The use of methanol as the mobile phase is preferred. After depolymerization of the melamine-formaldehyde resin, the reaction mixture is cooled. During cooling of the reaction mixture, monomeric melamine sulfonates (C4H7N6SO3) precipitate.

In particular, when FAS is used as a reducing agent, melamine sulfonate precipitates as the ammonium salt.

If FAS is used as a reducing agent, the aqueous supernatant (after separating the precipitated melamine sulfonate) contains a solution of ammonium sulfite and sulfate. These are formed by the decomposition of the FAS to urea and sulfoxylic acid, with the urea further decomposing to ammonia and carbon dioxide. The sulfoxylic acid is oxidized to sulfite and/or sulfate. This solution (or supernatant) can be used as a hardener for melamine and urea resins. This means that the entire digestion is utilized.

The melamine sulfonate salts (precipitated and separated as solids) are used as hardeners for resins, in particular formaldehyde resins, such as melamine-formaldehyde resins and urea-formaldehyde resins. When using the melamine sulfonates as hardeners, a suitable amount of melamine sulfonates in the resin to be hardened is between 0.2 and 3% by weight, preferably between 0.5 and 2.0% by weight.

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, counter-pull impregnates, kraft paper impregnates, uncoated or coated wood-based panels, such as HDF panels, and/or laminates, such as thin laminate.

An over-aged melamine resin is a melamine-formaldehyde resin that has already at least partially condensed and is cloudy. Over-aging of a melamine resin occurs in particular when either the viscosity has increased significantly (measurement with a 4mm DIN cup in accordance with EN ISO 2431:2011: fresh resin 20-30 see. over-aged resin: > 50 see) or the resin has become cloudy.

Melamine resin dust is generated at various points during the processing of impregnated products and wood-based panels, such as at the clipper and during trimming. The resulting melamine resin dust has a grain size of > 50 pm. max. of 100-300 pm. The trimming material consists of snippets with a width of approx. 10 mm and variable lengths depending on the effectiveness of the scraper.

Overlay impregnates, decorative impregnates, counter-pull impregnates and kraft paper impregnates are based on thin paper layers that are completely or partially soaked (impregnated) with a resin, preferably melamine-formaldehyde resin.

Accordingly, both partially impregnated papers, i.e. papers 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 evenly coated with an impregnating resin. The amount of resin applied in this case is between 30 and 70% by weight, preferably between 40 and 60% by weight, particularly preferably 50% by weight, based on the paper weight of the paper. In contrast, in the case of fully impregnated paper, a resin amount of 80 - 400% by weight, preferably 90-120% by weight, particularly preferably 100-110% by weight, based on the paper weight of the paper is applied.

Overlay papers are thin papers that have typically already been impregnated with a conventional melamine resin. Overlay papers are also available in which abrasion-resistant particles, such as corundum particles, are mixed into the resin of the overlay or sprinkled onto the resin-wetted overlay to increase the abrasion resistance. Resin applications with up to 400% by weight melamine resin are common for impregnating overlay papers.

Decorative papers are special papers for the surface finishing of wood materials, which allow a wide variety of decorations. In addition to the typical prints of various wood reproductions, further prints of geometric shapes or artistic products are available. There are actually 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 impregnates is 100 to 120% by weight.

Counter-pull papers are high-quality, impregnated papers for use as counter-pull material to compensate for tensile stress on the top side of carrier boards, e.g. for one-sided surface veneers and other one-sided coatings Kraft papers are very strong and consist of cellulose fibers to which starch, alum and glue are added to achieve surface effects and increased strength. The resin application in kraft paper impregnates is between 45 and 90% by weight. These are used as the core layer of HPL (high pressure laminate) and CPL (continuously pressed laminate). While phenolic resin is used for HPL core layers, CPL is impregnated with a mixture of melamine and phenolic resin.

Impregnations can be applied in an impregnation bath, by rolling, by screen rollers, by doctoring or by spraying. In one variant, 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, it is immersed in a resin. 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 takes place to a residual moisture content of approx. 6%. The impregnate can then be pressed in a manner similar to a wood-based panel, e.g. in a short-cycle press.

Wood-based panels, such as chipboards and fiberboards such as MDF and HDF boards, are made from wood chips or wood fibers that are obtained by chipping the wood chips in a chipper or by defibrating the wood chips in a refiner. The wood fibers used in wood fiberboards have a length of between 1.5 mm and 20 mm and a thickness of between 0.05 mm and 1 mm. The size of the wood chips used in chipboards depends on whether they are used in the top or middle layer. In the middle layer, the chips start at > 0.125 mm screen width, in the top layer at > 0.8 - 1.0 mm.

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 a range between 5 and 25% by weight, preferably 8 to 20% by weight, particularly preferably between 10 and 15% by weight. In certain applications, the amount of binder in wood-based panels can also be over 25% by weight, for example up to 30 - 50% by weight. In the case of coated wood-based panels, the paper impregnates mentioned above are placed on the wood-based panels and pressed together. Typically, the impregnated decorative paper is first applied to the top of the wood-based panel. The decorative impregnate is then followed by at least one overlay impregnate. The counter-pull impregnate is pressed onto the underside of the wood-based panel. A typical structure of a coated HDF panel is from top to bottom: overlay impregnate, decorative impregnate, HDF carrier, counter-pull impregnate.

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

Currently known in the state of the art are, for example, flexible laminates in which the layer composite includes at least one impregnated paper layer, such as a decorative impregnate, at least one transparent paper layer (glassine), e.g. glassine treated with sulphuric acid, and/or at least one plastic film layer. Glassine is a transparent paper made from finely ground pulp, which is largely greaseproof but not water-resistant. It gets its high transparency from very sharp satin finishing. The different layers or layers can be included several times in the laminate structure, alternate or swap their positions.

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 as protective films, especially for sensitive surfaces.

The waste products containing melamine-formaldehyde resin can contain not only cellulose fibers, wood fibers and wood chips, but also inorganic abrasion-resistant particles, glass beads, color pigments, other binding agents such as urea resins and other additives. The cellulose fibers come from impregnated papers such as the decorative impregnates, overlay impregnates and counteracting impregnates described. Inorganic particles such as abrasion-resistant particles (corundum) are contained in overlay impregnates. Wood fibers come from wood-based panels (such as HDF and MDF panels) and wood chips from chipboard. Color pigments are preferably found in decorative impregnates. Other additives include flame retardants, hardeners, elasticizers and waxes. 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 during the digestion process as mentioned above.

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

While the digestion 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 indicated for the other waste products.

When using decorative impregnated material, overlay impregnated material, counteracting impregnated material, coated and uncoated wood-based panels and laminates, the waste products must be shredded. Accordingly, in one embodiment, these melamine-formaldehyde resin-containing waste products are shredded to a particle size of between 0.1 and 10 mm, preferably between 0.5 and 3 mm. The disintegration works best with very finely ground material with a particle size of less than 3 mm.

In the case of a wood-based panel coated with overlay impregnate, decorative impregnate and/or counteracting impregnate (such as wood fiber board or chipboard), it is also advantageous to initially only pre-shred the coated wood-based panel (e.g. to a size of 5 x 5 cm). The pre-shredded pieces are first treated in a steam atmosphere in order to hydrolyze the additional binding agent (preferably urea-formaldehyde resin) present in the wood-based panel.

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.

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 1/1.6 to 1/2.3, particularly preferably 1/1.6 to 1/1.8. In one embodiment of the present process, the polycondensed melamine-formaldehyde resin to be digested in the waste products is not yet completely cured; ie the polycondensate is not yet completely cross-linked. This applies in particular to overlaid resin and impregnated paper layers (which have not yet been subjected to a pressing step).

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

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

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.

Scheme I

After addition of a suitable catalyst, preferably an acid, the melamine-formaldehyde monomers undergo polycondensation, whereby the monomers are linked via ether and methylene groups and higher molecular weight precondensates and polycondensates are formed (see Scheme II).

Scheme II

Precondensates and polycondensates differ in terms of their molecular weight and solubility. The low molecular weight precondensates can still have a limited water solubility, while the higher molecular weight polycondensates are insoluble. The limited water solubility of the precondensates is due, among other things, to the fact that methylol groups are still free and the low degree of cross-linking of the mostly linear oligomers. The precondensates are therefore a polymerization intermediate.

When the polycondensates are completely cured, strong cross-linking occurs with the elimination of the remaining methylol groups, whereby tightly cross-linked plastics are formed via methylene groups (see Scheme III).

Scheme III

For synthetic resins that harden via condensation reactions, the following resin states are distinguished:

A-state: easily soluble in solvents, meltable, curable;

B-state: only partially soluble in solvent, meltable, curable;

C-state: insoluble, cured The digestion of a resin that is not yet fully cured (ie a resin in the partially cross-linked B-stage) is preferred due to the lower degree of cross-linking of the resin.

The invention is explained in more detail below using exemplary embodiments with reference to the figures.

Figure 1 Curing curves of melamine sulfonate from disulfite digestion and commercially available hardeners when used as hardener for melamine-formaldehyde resin;

Figure 2 Curing curve of melamine sulfonate from disulfite digestion and ammonium sulfate when used as hardener for urea-formaldehyde resin;

Figure 3 Curing curve of the supernatant from FAS digestion and commercially available hardeners when used as hardener for melamine-formaldehyde resin hardener

Figure 4 Curing curve of the ammonium salt of melamine sulfonate from FAS digestion and commercially available hardeners when used as hardener for melamine-formaldehyde resin

General:

In the examples, the reaction progress was monitored using thin-layer chromatography. Melamine resins/precursors were run for comparison purposes, which allowed an assessment of the degradation progress. These were melamine, monomethylolmelamine, dimethylmelamine and a melamine resin.

Example 1: Digestion of melamine residue resin waste with sodium disulfite solution

100 g of liquid melamine resin (solids content: 55% by weight, molar ratio melamine/formaldehyde: approx. 1/1.8) which had already become cloudy due to storage was transferred into a 1000 ml three-necked round-bottom flask and mixed with 300 g of a 50% sodium disulfite solution.

The mixture was boiled under reflux, whereby the turbidity dissolved after about 3 hours. The reaction progress was monitored by thin layer chromatography (Mobile phase: water/ethanol (1/1). After resin oligomers were no longer 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.

The light yellow product is a mixture of mono- and dimethylolmelamines and melamine sulfonate. The filtrate was again concentrated by half and the precipitated melamine sulfonate was also washed and dried. The yield was 95%, taking into account approx. 10% by weight of auxiliary materials in the resin (elasticizer, hardener, etc.).

Example 2: Digestion of melamine residues with sodium bisulfite solution

100 g of solid melamine resin dust (molar ratio of melamine to formaldehyde: approx. 1/1.8) that had accrued during impregnation on the clipper and the trimming were transferred to a 2000 ml three-necked round-bottom flask and mixed with 600 g of a 50% sodium bisulfite solution.

The mixture was boiled under reflux, during which the resin slowly dissolved. After a clear solution had formed, floating fibers (paper and cellulose) were skimmed off, then filtered hot to separate corundum and glass beads. These were washed and reused. The mixture was then boiled under reflux for a further period. The reaction progress was monitored using thin-layer chromatography (mobile phase: water/ethanol (1/1). After no more resin oligomers could be detected using 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.

The light yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfones. The yield was 91%, taking into account about 10% by weight of auxiliary materials in the resin (elasticizer, hardener, etc.). In a further experiment, the amount of sodium bisulfite was reduced to 150 g in 300 ml of water. No difference in the yield was detectable. Example 3: Digestion of melamine residue overlay with sodium disulfite solution

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: 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.

The mixture was boiled under reflux. After a clear solution had formed, it was filtered while hot to separate paper fibers, corundum, etc. The mixture was then boiled under reflux for a further period. The reaction progress was monitored using thin-layer chromatography (mobile phase: water/ethanol (1/1). After no more resin oligomers could be detected using 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.

The light yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfones. The yield was 86%, taking into account approx. 20% by weight of auxiliary materials in the overlay impregnate (elasticizer, corundum, hardener, etc.).

Example 4: Digestion of decorative impregnate with sodium disulphite solution

Decorative impregnates that could no longer be used in production due to storage were ground to dust in a laboratory mill while cooling. These were impregnates that had a core impregnation with urea resin (resin application: approx. 50% by weight) and a second covering impregnation with melamine resin (resin application: approx. 50% by weight). 200 g of this decorative impregnate were transferred in solid form (molar ratio of resin: melamine/formaldehyde: approx. 1/1.8) to a 2000 ml three-necked round-bottom flask and mixed with 400 g of a 30% sodium disulfite solution.

The mixture was boiled under reflux. After a clear solution had been obtained, it was filtered hot to separate paper fibers, corundum, etc. Then it was boiled under reflux. The reaction progress was monitored by thin layer chromatography (mobile phase: water/ethanol (1/1). After no more resin oligomers could be detected by thin layer chromatography (12 h), the reaction solution was cooled, the The precipitated product is sucked off through a suction filter, washed with cold water and then dried.

The light yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfones. The yield was 78%, taking into account approx. 10% by weight of auxiliary materials in the resin (elasticizer, hardener, etc.) and approx. 50% by weight of urea resin in the core.

Example 5: Digestion of HDF with high melamine content using sodium disulfite solution

An HDF containing approximately 50% by weight of binder, of which approximately 80% by weight is melamine resin, was ground to dust in a laboratory mill while cooling. 200 g of this dust was transferred in solid form to a 2000 ml three-necked round-bottom flask and mixed with 400 g of a 30% sodium disulfite solution.

The mixture was boiled under reflux (approx. 6 h). It was then filtered while hot to separate fibers, etc. The mixture was then boiled under reflux again. The reaction progress was monitored using thin layer chromatography (mobile phase: water/ethanol (1/1). After resin oligomers were no longer detectable using 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.

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 auxiliary materials in the resin (elasticizer, hardener, etc.).

Example 6: Digestion of thin laminate with sodium disulphite solution

A thin laminate (decor and parchment) was ground into dust in a laboratory mill under cooling. The thin laminate consisted of a decorative impregnate (paper weight: 100 g/m ² , resin application: 120%) and a parchment: paper weight: 40 g/m ² . 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 disulphite solution. The mixture was boiled under reflux. After a clear solution had formed, it was filtered hot to separate the paper fibers. The mixture was then boiled under reflux. The reaction progress was monitored by thin layer chromatography

(Mobile phase: water/ethanol (1/1). After resin oligomers were no longer 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.

The light yellow product is a mixture of mono- and dimethylolmelamines and mono- and dimethylolmelamine sulfones. The yield was 81%, taking into account approximately 10% by weight of auxiliary materials in the resin (plasticizer, hardener, etc.).

Example 7: Digestion direct coating floor construction with sodium disulphite solution

A laminate floor with an HDF substrate from a return was pre-shredded in a shredding device (chipper) to a size of approx. 5 x 5 cm. The coating of the laminate floor consisted of an overlay impregnate (paper weight: 25g/m ² , resin application: 400% melamine resin), a decorative impregnate (paper weight: 60 g/m ² . Resin application: 100%) and a countercoat (paper weight: 80 g/m ² , resin application: 120% melamine resin). The urea-formaldehyde glue of the HDF was then hydrolyzed in a cooker at increased pressure in a steam atmosphere.

The coatings were sieved from the wood fibers and ground into 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.

The mixture was boiled under reflux. After a clear solution had formed, it was filtered while hot to separate paper fibers, wood fibers, corundum, etc. The mixture was then boiled under reflux for a further period. The reaction progress was monitored using thin-layer chromatography (mobile phase: water/ethanol (1/1). After no more resin oligomers could be detected using 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. 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 auxiliary materials in the resin (plasticizer, corundum, hardener, etc.).

Example 8: Use of melamine sulfonate from digestion with sodium disulfite solution as hardener for melamine-formaldehyde resin

The melamine sulfonate from Example 1 is dissolved in warm water (50°C, 20% solution).

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 in Figure 1).

As the comparisons show, the melamine sulfonate 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 sulfonate from digestion with sodium disulfite solution as a hardener for urea-formaldehyde resins

The precipitated melamine sulfonate from Example 1 is dissolved in warm water (50°C, 20% solution).

The clear solution is used as a hardener for urea resins. The curing behavior is compared with a hardener already in use (see diagram in Figure 2). As the values show, the melamine sulfone hardener is similar to ammonium chloride hardener in its hardening behavior. However, the dosages differ due to the very different molecular weights. The sulfone hardener is also significantly more effective with the urea-formaldehyde glue than with the melamine resin.

Example 10: Use of melamine methylols from digestion with sodium disulfite solution for condensation

The light yellow slurry product (before precipitation of melamine sulfonate) 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 melamine sulfonate. The melamine sulfonate can be used as a hardener for melamine resins (see Example 7). It is then filtered off with suction. The filter with the melamine methylols is dried. The proportion of melamine methylols in the recycled melamine resin is between 10 and 30% by weight, depending on the molar ratio (melamine to formaldehyde). In this case it was 20% by weight.

The melamine methylols were then used to produce a melamine resin, with the proportion of recycled melamine methylols being 20% by weight. It was assumed that the ratio of melamine to formaldehyde was 1 to 1.5.

It is assumed that the ratio of mono- to dimethylols is 1:1. The remaining mixture with formaldehyde to melamine is added in the same ratio. The formaldehyde is introduced as a 33% solution. 10% by weight of ethylene glycol is added as an elasticizing agent based on the amount of solids. The pH is adjusted to approx. 8.5 with sodium hydroxide solution. Melamine and the melamine methylols are slowly added as powder while stirring. Condensation is carried out for approx. 20 minutes at 95°C. The temperature is then reduced to 85°C. Condensation is continued for approx. 30 to 40 minutes until the water dilutability is 1:3. The mixture is then cooled to room temperature.

Example 11: Digestion of melamine residues with FAS

100 g of melamine resin dust, which had accrued in an impregnation on the clipper and on the trimming, were solidified (molar ratio melamine/formaldehyde: approx. 1/1.8) in a 1000 ml three-necked round-bottomed flask containing 600 ml of water. The aqueous solution was heated to boiling while stirring. Then 117 g of FAS (formamidine sulfinic acid) were added in four equal portions (approx. 30 g each), each at 10 min intervals.

The mixture was boiled under reflux, during which the resin and the FAS slowly dissolved. The reaction progress was monitored using thin layer chromatography (mobile phase: methanol). After a clear, yellowish solution had formed after about two hours, it was filtered hot to remove impurities. The reaction solution was then cooled, the precipitated product was filtered off with suction, washed with cold water and then dried.

According to analysis, the white product consists exclusively of melamine sulfonate as ammonium salt. The yield was approximately 90%, taking into account approximately 10% by weight of auxiliary materials in the resin (elasticizer, hardener, etc.).

Example 12: Melamine residue overlay digestion with FAS

Overlay impregnates that could no longer be used in production due to overlay were ground to dust in a laboratory mill under cooling. The overlays were made with a-cellulose paper with a grammage of 25 g/m ^2. In addition to the resin, the impregnate also contained approx. 25 g of corundum/m ^2. 100 g of this overlay impregnate (molar ratio of resin: melamine/formaldehyde: approx. 1/1.8) was transferred to a 1000 ml three-necked round-bottom flask and slurried with 600 ml of water. The solution was then heated to boiling while stirring and then mixed with four equal portions of 82 g of FAS.

The mixture was further boiled under reflux. After a clear solution had formed, it was filtered hot to separate paper fibers, corundum, etc. The mixture was then further boiled under reflux. The reaction progress was monitored using thin layer chromatography (mobile phase: methanol). After about 2 hours, the reaction solution was cooled, the precipitated product was filtered off with suction, washed with cold water and then dried. The white product is the ammonium salt of melamine sulfonate. The yield was 89%, taking into account approx. 20% by weight of auxiliary materials in the overlay impregnate (elasticizer, corundum, hardener, etc.) and the cellulose fibers.

Example 13: Decomposition of decorative impregnate with FAS

Decorative impregnates that could no longer be used in production due to storage were ground to dust in a laboratory mill under cooling. These were impregnates that had a core impregnation with urea resin (resin application: approx. 50% by weight) and a second covering impregnation with melamine resin (resin application: approx. 50% by weight, molar ratio of melamine resin:melamine/formaldehyde: approx. 1/1.8). The paper weight of the impregnates used was 80 g/m ² . 100 g of this decorative impregnate was transferred to a 1000 ml three-necked round-bottom flask and mixed with 600 ml of water. The mixture was then heated to boiling while stirring. Then 54 g of FAS were added in four equal portions.

The mixture was boiled under reflux for a further 1 hour. After a clear solution had formed, it was filtered hot to separate paper fibers, corundum, etc. It was then boiled under reflux for a further hour. The reaction progress was monitored using thin layer chromatography (mobile phase: methanol). The reaction solution was then cooled, the precipitated product (melamine sulfonate as ammonium salt) was filtered off with suction, washed with cold water and then dried.

The yield of melamine sulfonate as ammonium salt was 78%, taking into account approx. 10 wt.% auxiliary materials in the resin (elasticizer, hardener, etc.), the paper fibers and approx. 50 wt.% urea resin in the core.

Example 14: Use of the supernatant from FAS digestion as hardener

The yellow solution from the digestion in Example 11 is used as a hardener. It consists of a mixture of ammonium sulfite and ammonium sulfate. The solids content is approximately 20%.

The curing of the melamine resin Kauramin 796 is determined and compared with other commercially available hardeners (hardeners 528, 1448 and XXL) (see Table 2 and diagram in Figure 3). The supernatant of ammonium sulphite and ammonium sulphate is used in different concentrations or purification levels (FAS product purified liquid/liquid (w=15%); FAS product solid/liquid).

Table 2

As the data in Table 2 and the corresponding hardener curves in the diagram in Figure 3 show, the recyclate hardener from the supernatant (FAS product) is similarly reactive as the standard hardeners used and significantly more reactive than the latent hardener XXL.

Example 15: Use of melamine sulfonate from FAS digestion as hardener

The white product from the embodiment 1 1 is dissolved in warm water (50°C, 20% solution). It contains the ammonium salt of melamine sulfonate. The melamine sulfonate is used in different concentrations (sulfonate product w = 20% liquid/liquid; sulfonate product solid/liquid).

The clear solution is used as a hardener for melamine resins. The curing behavior is compared with hardeners already in use (hardener 528, 1448 and XXL) (see Table 3 and diagram in Figure 4).

Table 3 As the hardener curves show, the ammonium salt of melamine sulfonate is a rather latent hardener (similar to XXL). These hardeners are preferred today because they are used to avoid pre-curing in the process. This hardener can of course be used in combination with other hardeners to create a tailor-made

reactivity of the resins in which it is used.

Claims

Patent claims

1. Process for the degradation of melamine-formaldehyde resin from waste products arising during the manufacture and processing of wood-based panels, characterized by the following steps

- Provision of waste products containing polycondensed melamine-formaldehyde resin arising during the production and processing of wood-based panels, optional shredding of the waste products containing melamine-formaldehyde resin,

- Mixing the melamine-formaldehyde resin-containing waste products with water and adding at least one reducing sulphur-containing compound selected from a disulfite, hydrogen sulphite and/or dithionite, a sulphinate or sulphinate analogues,

- Heating the aqueous mixture / slurry of melamine-formaldehyde resin-containing waste products and at least one reducing sulphur-containing compound to temperatures between 60°C and 120°C, preferably between 80°C and 100°C, under reaction control until a clear solution is obtained,

- Separation of foreign substances from the clear solution,

- Optionally further heating the mixture of melamine-formaldehyde resin-containing waste products and at least one reducing sulphur-containing compound, which has been cleaned of foreign substances, 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, and

2. Process for the degradation of melamine-formaldehyde resin from waste products arising during the manufacture and processing of wood-based panels according to claim characterized by the following steps

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,

Separation of foreign substances from the clear solution,

Further heating of the mixture of waste products containing melamine-formaldehyde resin and at least one disulfite, hydrogen sulfite and/or dithionite, which has been cleaned of foreign substances, at temperatures between 60°C and 120°C, preferably between 80°C and 110°C, under reaction control until no polycondensed melamine-formaldehyde resin can be detected in the reaction mixture,

- Cooling the reaction mixture and separating the resulting mixture of melamine derivatives, in particular melamine methylols and melamine sulfonate.

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

4. Process 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 with a pH value between 3.0 and 6, preferably between 3.5 and 5.5.

5. A process for the degradation of melamine-formaldehyde resin from waste products arising during the manufacture and processing of wood-based panels according to claim 1, characterized by the following steps

Providing waste products containing polycondensed melamine-formaldehyde resin arising during the production and processing of wood-based panels, optionally shredding the waste products containing melamine-formaldehyde resin, mixing the waste products containing melamine-formaldehyde resin with water and adding at least one sulfinate or sulfinate analogues,

Heating the aqueous mixture/slurry of melamine-formaldehyde resin-containing waste products and at least one sulfinate or sulfinate analogues to temperatures between 60°C and 120°C, preferably between 80°C and 100°C, under reaction control until a clear solution is obtained,

- Separation of foreign substances from the clear solution, optional further heating of the mixture of melamine-formaldehyde resin-containing waste products and at least one sulfinate purified of foreign substances 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, and

- Cooling the reaction mixture and separating the melamine sulfonate which precipitates as a salt.

6. Process according to claim 5, characterized in that the at least one sulfinate or sulfinate analogue is added as a solid to the aqueous solution/slurry of waste products containing melamine-formaldehyde resin.

7. Process according to one of claims 5-6, characterized in that the at least one sulfinate can be prepared from formamidine sulfinic acid (FAS).

8. Process according to one of claims 5-7, characterized in that the ratio of reducing agent to waste products containing melamine-formaldehyde resin is between 2:1 and 0.5:1, preferably between 1.5:1 and 0.8:1.

9. Process according to one of claims 5-8, characterized in that the reaction time until complete degradation of the melamine-formaldehyde resin is between 60 and 240 minutes, preferably between 90 and 120 minutes.

10. Process according to one of claims 5-9, characterized in that the melamine sulfonate which precipitates as a salt after cooling of the reaction mixture is an ammonium salt.

11. Process according to one of claims 5-10, characterized in that the supernatant obtained after precipitation and separation of the melamine sulfonate salt contains ammonium sulfate (NH4)2SO4 and ammonium sulfite (NH4)2SO3.

12. Method according to one of the preceding claims, characterized in that the waste products containing melamine-formaldehyde resin arising during the production and processing of wood-based panels comprise 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.

13. Process according to one of the preceding claims, characterized in that the 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.

14. 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 from 1/1.6 to 1/2.3, particularly preferably 1/1.6 to 1/1.8.

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

16. Process according to one of the preceding claims, characterized in that the foreign substances contained in the reaction mixture are separated by means of filtration, preferably hot filtration, and/or skimming. 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. Use of the melamine sulfonates or melamine sulfonate salts obtained in a process according to one of claims 1-17 as hardeners for resins, in particular formaldehyde resins, such as melamine-formaldehyde resins and urea-formaldehyde resins. Use of the melamine methylols obtained in a process according to one of claims 1-17 for the production of melamine resin, in particular for use as a binder or impregnating agent. Use of the melamine methylols and melamine sulfonates obtained in a process according to one of claims 1-17 as flame retardants and/or binders. Use of the supernatant containing ammonium sulfate (NH^SC) and ammonium sulfite (NH^SOs) obtained in a process according to any one of claims 5-17 after precipitation and separation of the melamine sulfonate salt as a hardener for resins, in particular as a hardener for formaldehyde resins, such as melamine-formaldehyde resins and urea-formaldehyde resins.