Biodegradable melamine resin foams

Abstract

The present invention provides a process for producing a melamine-formaldehyde foam by heating and foaming an aqueous mixture M using microwave radiation, said mixture M compris- ing at least one melamine-formaldehyde precondensate having a molar ratio of mela- mine:formaldehyde ranging from 1:1.3 to 1: 2.5, at least one curative, at least one surfactant, at least one blowing agent and at least one polyalcohol, selected from the group of ethylene gly- col, diethylene glycol, glycerol, trimethylol propane, pentaeryhritol, xylitol, xylose, and sorbitol, as well as biodegradable melamine resin foam obtainable by this process.

Description

[0001] Biodegradable Melamine Resin Foams

[0002] Description

[0003] The present invention relates to a process for producing a melamine-formaldehyde foam by heating and foaming an aqueous mixture M using microwave radiation, said mixture M comprising at least one melamine-formaldehyde precondensate having a molar ratio of mela- mine:formaldehyde ranging from 1 :1.3 to 1: 2.5, at least one curative, at least one surfactant, at least one blowing agent and at least one polyalcohol, selected from the group of ethylene glycol, diethylene glycol, glycerol, trimethylol propane, pentaeryhritol, xylitol, xylose, and sorbitol, as well as biodegradable melamine resin foam obtainable by this process.

[0004] Relevant Prior Art

[0005] Melamine resin foams are used for different applications, e. g. sound absorption in room acoustics, thermal insulation in building and pipe insulation. One more important application is the use of melamine resin foams as cleaning sponge in consumer / household applications. Especially, in household applications, e.g. cleaning sponges, the biodegradation of the used materials is a strong demand.

[0006] Several methods are available to investigate biodegradation of polymers in various media, e. g. soil, sea-water, sweet-water, sludge-water. For testing the biodegradation of polymer foams used in household applications the OECD 301 F biodegradation test is one of the most important one. The OECD 301 F test investigates the biodegradation in an activated sludge for the wastewater treatment. So far, no results of biodegradation of melamine resins foams according to OECD 301 F test are described in literature.

[0007] EP 4 001 350 provides a thermoformable melamine foam and a method for preparing the same, wherein the thermoformable melamine foam comprises a condensate of a melamine- formaldehyde-based compound with a low molecular ratio M:F in the range from 1 :1.3 to 1 :1.8 to be low in formaldehyde content and shows excellent thermostability. In order to improve the heat resistance of the thermoformable melamine foam, a predetermined amount of isosorbide is added to the dispersion.

[0008] WO 2018 / 095760 A1 relates to a process for producing a melamine-formaldehyde foam by heating and foaming an aqueous mixture M using microwave radiation, said mixture M comprising at least one melamine-formaldehyde precondensate, at least one curative, at least one surfactant, at least one blowing agent and polyethylene glycol number average molecular weight Mn in the range from 500 to 10.000 g / mol as well as a melamine-formaldehyde foam obtainable by this process and its use.

[0009] WO 2014 / 170243 relates to a melamine-formaldehyde foam comprising microspheres having a core comprising at least one active and / or effective substance selected from the group consisting of foam glass, sodium sulfate, sodium lauryl sulfate, polyethylene glycols, cocoamides, fatty alcohols, quaternary ammonium salts, latent heat accumulators, flame retardants, intumescent, hydrophobicizing agents, adhesives, substances influencing the soil release behavior, formaldehyde scavenger, substances improving indoor air quality, skin care products and formulations, abrasives and mixtures thereof and having a shell comprising at least one melamine- formaldehyde resin. Sorbitol may be used as formaldehyde scavenger.

[0010] EP 0688 852 A1 relates to aqueous intumescent solutions comprising polyalcohol’s for the impregnation of porous carrier materials and for the impregnation of glass - and mineral fiber plates, - nonwovens, - matte, of plastic drainage mats, nonwovens and / or metal mats of metal fiber, of open-pored thermosetting synthetic resin foams manufactured in this way composite materials and their use in fire protection in buildings and acoustic insulation.

[0011] Cleaning implements contain an active agent, such as biocides and or glycol solvents impregnated in the erodible foam, such as melamine-formaldehyde resin foam, are known i.e. from US 2014 / 0230847 A1.

[0012] CN 107 903 578 A discloses a modified melamine formaldehyde resin foam with improved toughness using diethylene glycol and propylene glycol.

[0013] CN 113 185 745 A discloses a lightweight melamine formaldehyde foam with surface treated glass fiber as inorganic filler, which has flame retardance, high temperature and aging resistance and was prepared by mixing the melamine resin in an extruder with surface treated glass fiber in the presence of diethylene glycol and polycarbonate.

[0014] US 2018 / 140158 A discloses a cleaning implement including a melamine-formaldehyde foam with improved durability, which was produced in the presence of a polyethylene glycol. Summary of the Invention

[0015] The present invention was made in view of the prior art described above, and the object of the present invention is to provide a practical method capable of preparing melamine-formaldehyde foams with increased biodegradability while maintaining sufficient flexibility and low brittleness.

[0016] Technical problem solved

[0017] To solve the problem, the present invention provides a method for producing a biodegradable melamine-formaldehyde foam obtainable by a process for producing a melamine-formaldehyde foam by heating and foaming an aqueous mixture M using microwave radiation, said mixture M comprising at least one melamine-formaldehyde precondensate having a molar ratio of mela- mine:formaldehyde ranging from 1 :1.3 to 1 : 2.5, at least one curative, at least one surfactant, at least one blowing agent and at least one polyalcohol, selected from the group of ethylene glycol, diethylene glycol, glycerol, trimethylol propane, pentaeryhritol, xylitol, xylose, and sorbitol.

[0018] Surprisingly, it was found that melamine resin foams can show an increased biodegradation if low-functional melamine resins having a molar ratio of melamine:formaldehyde ranging from 1 :1 .3 to 1 : 2.5 are co-condensated with polyalcohols .

[0019] Suitable polyalcohols comprise organic compounds with multiple hydroxyl groups (-OH)groups, preferably in the range from 2-8, more preferably in the range from 2 - 6 hydroxyl groups such as Ethylene glycol (2-carbon), Glycerol (3-carbon), Erythritol (4-carbon), Xylitol (5-carbon), Sorbitol (6-carbon). Other suitable polyalcohols are e. g. pentaerythritol, trimethylol propane.

[0020] The polyalcohol is co-condensated in the melamine-formaldehyde (MF) resin.

[0021] The above described polyalcohols - e. g. ethylene glycol - react with the methylol group of the melamine-formaldehyde (MF) resins to form ether-bridged structures with IW-cleavage.

[0022] The polyalcohol is preferably selected from polyalcohols with 2 to 6 hydroxyl groups. Most preferably the polyalcohol is selected from the group of ethylene glycol, diethylene glycol, glycerol, trimethylol propane, pentaeryhritol, xylitol, xylose, and sorbitol.

[0023] The melamine-formaldehyde precondensate has a molar ratio of melamine:formaldehyde (M / F) ranging from 1 :1.3 to 1 : 2.5, most preferably from 1 :1.5 to 1 : 1.8. Preferably the melamine-formaldehyde precondensate has a number average molecular weight Mnranging from 200 g / mol to 1000 g / mol.

[0024] Anionic, cationic and nonionic surfactants and also mixtures thereof can be used as disper- sant / emulsifier.

[0025] Useful anionic surfactants include for example diphenylene oxide sulfonates, alkane and alkylbenzenesulfonates, alkylnaphthalenesulfonates, olefinsulfonates, alkyl ether sulfonates, fatty alcohol sulfates, ether sulfates, a-sulfo fatty acid esters, acylaminoalkanesulfonates, acyl isethionates, alkyl ether carboxylates, N-acylsarcosinates, alkyl and alkylether phosphates. Useful nonionic surfactants include alkylphenol polyglycol ethers, fatty alcohol polyglycol ethers, fatty acid polyglycol ethers, fatty acid alkanolamides, ethylene oxide-propylene ox-ide block copolymers, amine oxides, glycerol fatty acid esters, sorbitan esters and alkylpoly-glycosides. Useful cationic emulsifiers include for example alkyltriammonium salts, alkylben-zyldimethylammonium salts and alkylpyridinium salts.

[0026] The dispersants / emulsifiers can be added in amounts from 0.2% to 5% by weight, based on the melamine-formaldehyde precondensate.

[0027] Preferably the mixture M comprises a surfactant mixture comprising a mixture of 50 to 90 wt% of at least one anionic surfactant and 10 to 50 wt% of at least one nonionic surfactant, where-in the weight percentages are each based on the total weight of the surfactant mixture.

[0028] As curatives it is possible to use acidic compounds which catalyze the further condensation of the melamine resin. The amount of these curatives is generally in the range from 0.01% to 20% by weight and preferably in the range from 0.05% to 5% by weight, all based on the precondensate. Useful acidic compounds include organic and inorganic acids, for example selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, formic acid, acetic acid, oxalic acid, toluenesulfonic acids, amidosulfonic acids, acid anhydrides and mixtures thereof.

[0029] Preferably formic acid is used as curative.

[0030] Depending on the choice of melamine-formaldehyde precondensate, the mixture comprises a blowing agent. The amount of blowing agent in the mixture generally depends on the desired density for the foam. Preferably the amount in relation to the melamine-formaldehyde precondensate is chosen in an amount that the density of the foam is 6 to 12 kg / m3, more preferably 6.5 to 11 kg / m3. In principle, the process of the present invention can use both physical and chemical blowing agents. "Physical" or "chemical" blowing agents are suitable (Encyclopedia of Polymer Science and Technology, Vol. I, 3rd ed., Additives, pages 203 to 218, 2003).

[0031] Useful "physical" blowing agents include for example hydrocarbons, such as pentane, hexane, halogenated, more particularly chlorinated and / or fluorinated, hydrocarbons, for example methylene chloride, chloroform, trichloroethane, chlorofluorocarbons, hydrochlorofluorocarbons (HCFCs), alcohols, for example methanol, ethanol, n propanol or isopropanol, ethers, ketones and esters, for example methyl formicate, ethyl formicate, methyl acetate or ethyl acetate, in liquid form or air, nitrogen or carbon dioxide as gases.

[0032] Useful "chemical" blowing agents include for example isocyanates mixed with water, releasing carbon dioxide as active blowing agent. It is further possible to use carbonates and bicarbonates mixed with acids, in which case carbon dioxide is again produced. Also suitable are azo compounds, for example azodicarbonamide.

[0033] The mixture further comprises at least one blowing agent. This blowing agent is present in the mixture in an amount of 0.5% to 60% by weight, preferably 1% to 40% by weight and more preferably 1.5% to 30% by weight, based on the melamine-formaldehyde precondensate. It is preferable to add a physical blowing agent having a boiling point between 0 and 80°C. Preferably pentane is used as blowing agent.

[0034] The mixture may further comprise at least one additive selected from the group of dyes, fragrances, optical brighteners, UV and heat stabilizers, flame retardants, formaldehyde scavengers, and pigments. This additive is preferably distributed homogeneously in the foam.

[0035] The precondensate being foamed up generally by heating the suspension of the melamineformaldehyde precondensate to obtain a foamed material.

[0036] The introduction of energy is effected via electromagnetic radiation, for example via high- frequency radiation at 5 to 400 kW, preferably 5 to 200 kW and more preferably 9 to 120 kW per kilogram of the mixture used in a frequency range from 0.2 to 100 GHz, preferably 0.5 to 10 GHz. Magnetrons are a useful source of dielectric radiation, and one magnetron can be used or two or more magnetrons at the same time.

[0037] The foamed materials produced are finally dried, removing residual water and blowing agent from the foam. The process described provides blocks / slabs of foamed material, which can be cut to size in any desired shapes.

[0038] In a preferred embodiment the process comprises the steps of: a) producing an aqueous mixture M comprising

[0039] 100 parts by weight of at least one melamine-formaldehyde precondensate,

[0040] 2 to 4 parts by weight, preferably 2.2 to 3.8 parts by weight and more preferably 2.7 to

[0041] 3.3 parts of at least one curative,

[0042] 0.2 to 5 parts by weight, preferably 0.5 to 3 parts by weight and more preferably 1.25 to

[0043] 2.3 parts by weight of a surfactant mixture,

[0044] 0.1 to 5 parts by weight, preferably 0.5 to 4 parts by weight and more preferably 1 .1 to

[0045] 3.6 parts by weight of at least one salt of an inorganic acid and / or of an organic carboxylic acid,

[0046] 1 to 40 parts by weight, preferably 10 to 35 parts by weight and more preferably 15 to 21 parts of at least one blowing agent,

[0047] 0.1 to 50 parts by weight, preferably 5 to 25 parts off at least one polyalcohol, selected from the group of ethylene glycol, diethylene glycol, glycerol, trimethylol propane, pen- taeryhritol, xylitol, xylose, and sorbitol,

[0048] 0 to 20 parts by weight of one or more additives, selected from the group of dyes, fragrances, optical brighteners, UV and heat stabilizers, flame retardants, formaldehyde scavengers, and pigments.,

[0049] 25 to 60 parts by weight, preferably 30 to 50 parts by weight and more preferably 36 to 44 parts of water, b) heating and foaming said mixture M using microwave radiation, c) annealing the foam using hot air and / or nitrogen in a temperature range from 150°C to 290°C,

[0050] The present invention is further directed to a melamine-formaldehyde foam obtainable by the processes according to the invention as described above.

[0051] The melamine-formaldehyde foam preferably comprises 1 to 30 parts by weight, more preferably 5 to 25 parts of the at least one polyalcohol per 100 parts of the dried melamine- formaldehyde foam. The at least one polyalcohol is preferable incorporated into the melamine-formaldehyde resin bridges of the open-cell structure. Incorporated means that the polyalcohol is not washed out.

[0052] The foam blocks or slabs can optionally be thermocompressed in a further process step. Thermocompression as such is known to a person skilled in the art and described for example in WO 2007 / 031944, EP-A 451 535, EP-A 111 860 and US-B 6,608,118.

[0053] The foam obtainable by the process of the present invention preferably has an open-cell structure having an open-cell content, when measured to DIN ISO 4590, of more than 50% and more particularly more than 95%. Preferably the density of the foam is 6 to 12 kg / m3, more preferably 6.5 to 11 kg / m3.

[0054] The melamine-formaldehyde foam according to the present invention can be used for acoustical and / or thermal insulation in aircraft, ship and motor vehicle construction, in mechanical engineering or in building construction or for cleaning, grinding or polishing sponges.

[0055] Examples

[0056] Hereinafter, the present invention is described in more detail and specifically with reference to the Examples, which however are not intended to limit the present invention.

[0057] Methods of measurement:

[0058] Ram pressure value [N]:

[0059] Ram pressure measurements for evaluating the mechanical quality of the melamine resin foams were all carried out as follows. A cylindrical ram having a diameter of 8 mm and a height of 10 cm was pressed into a cylindrical sample having a diameter of 11 cm and a height of 5 cm in the direction of foaming at an angle of 90% until the sample tore. The tearing force [N], hereinafter also referred to as ram pressure value, provides information as to the quality of the foam.

[0060] Shore Hardness:

[0061] The measurements were run according to ASTM D 2240. For the measurement of low density foams the scale of 000 was used (2.4 mm diameter of the sphere, spring force 1.111 N). Biodegradation:

[0062] Biodegradation was determined after 28 days in percent according to OECD 301 F: Manometric Respirometry Test. Organization for Economic Co-operation and Development Chemicals Test- ing - Guidelines. Adopted 17.07.1992.

[0063] Sludge: Klaranlage Mannheim

[0064] For the Manometric Respirometry (301 F) methods if activated sludge is used, it should be taken from a treatment plant or laboratory-scale unit receiving predominantly domestic sewage. Inocula from other sources, usually yielding lower cell densities, have been found to give higher scattering of results.

[0065] To confirm the chemical bonding of the co-condensate polyalcohol the following washing trials are performed: 10 g of the final foams are 2 x times washed out with hot (60°C) demineralized water. Afterwards the foams were dried (8h, 100°C) and weighted. Materials used:

[0066] Melamine-formaldehyde precondensates:

[0067] HF-MF: High functional MF melamine-formaldehyde precondensate HF-MF was a spray-dried melamine-formaldehyde precondensate having an average molecular weight (number average) Mn of 350 g / mol, which had a molar ratio of melamine:formaldehyde of 1 :2.8.

[0068] LF-MF: Low functional melamine-formaldehyde precondensate LF-MF was a spray-dried melamine-formaldehyde precondensate having an average molecular weight (number average) Mn of 320 g / mol, which had a molar ratio of melamine:formaldehyde of 1 :1.6.

[0069] SM-1 : Surfactant mixture of 80 wt.% alkanesulfonate mixture and 20 wt.-% alkyl polyethylene glycol ether mixture.

[0070] Co-condensate additives (polyalcohol, polyol):

[0071] Ethylene glycol, diethylene glycol, glycerol, trimethylol propane, pentaerythritol, xylitol, xylose, sorbitol, glucose, fructose, galactose, maltose all drawn by Sigma-Aldrich.

[0072] Comparative Example C1

[0073] 100 parts by weight of a spray-dried melamine I formaldehyde precondensate HF-MF was dissolved in 30 parts by weight of water. To this mixture was added 2.75 parts by weight of sodium formate, 3.1 parts by weight of formic acid, 1.5 parts by weight of the surfactant mixture SM-1 and 17.8 parts by weight a blowing agent mixture of 80 wt.-% of n-pentane and 20 wt.-% of isopentane. This mixture was stirred vigorously and then foamed in a mold of polypropylene by irradiation of microwave energy at 2.54 GHz. The foam was afterwards cured in an oven at 100°C for 8h and annealed at 240°C for 10 min. Comparative Examples C1.1 - C1.16

[0074] 100 parts by weight of a spray-dried melamine I formaldehyde precondensate HF-MF was dissolved in 30 parts by weight of water. To this mixture was added 2.75 parts by weight of sodium formate, 3.1 parts by weight of formic acid, 1.5 parts by weight of the surfactant mixture SM-1 and 17.8 parts by weight a blowing agent mixture of 80 wt.-% of n-pentane and 20 wt.-% of isopentane. Next, 10 to 20 wt.-%, based on the dry melamine / formaldehyde precondensate, of the polyols as listed in table 1 were added This mixture was stirred vigorously and then foamed in a mold of polypropylene by irradiation of microwave energy at 2.54 GHz. The foam was afterwards cured in an oven at 100°C for 8h and annealed at 240°C for 10 min.

[0075] Comparative Example C2:

[0076] 100 parts by weight of a spray-dried melamine I formaldehyde precondensate LF-MF was dissolved in 30 parts by weight of water. To this mixture was added 2.75 parts by weight of sodium formate, 3.1 parts by weight of formic acid, 1.5 parts by weight of the surfactant mixture SM-1 and 17.8 parts by weight a blowing agent mixture of 80 wt.-% of n-pentane and 20 wt.-% of isopentane. This mixture was stirred vigorously and then foamed in a mold of polypropylene by irradiation of microwave energy at 2.54 GHz. The foam was afterwards cured in an oven at 100°C for 8h and annealed at 240°C for 10 min.

[0077] Examples 2.1 - 2.8, 2.13 - 2.16 and Comparative Examples C2.9 - C2.12

[0078] 100 parts by weight of a spray-dried melamine I formaldehyde precondensate LF-MF was dissolved in 30 parts by weight of water. To this mixture was added 2.75 parts by weight of sodium formate, 3.1 parts by weight of formic acid, 1.5 parts by weight of the surfactant mixture SM-1 and 17.8 parts by weight a blowing agent mixture of 80 wt.-% of n-pentane and 20 wt.-% of isopentane. Next, 10 to 20 wt.-%, based on the dry melamine / formaldehyde precondensate, of the polyols as listed in table 2 were added. This mixture was stirred vigorously and then foamed in a mold of polypropylene by irradiation of microwave energy at 2.54 GHz. The foam was afterwards cured in an oven at 100°C for 8h and annealed at 240°C for 10 min.

[0079] To confirm the chemical bonding of the co-condensate polyalcohol the following washing trials are performed: 10 g of the final foams are 2 x washed out with hot (60°C) demineralized water. Afterwards the foams get dried (8h, 100°C) and weighted. The mass loss should be around 0%. No increased biodegradation is observed for reducing sugars in combination with low functional MF resin formulation, e. g. Glucose, Fructose, Galactose, Maltose und Lactose.

[0080] With the use of the polyalcohols Ethylene glycol, diethylene glycol, glycerol, trimethylol propane, pentaerythritol, xylitol, xylose, sorbitol as co-condensate additives in the low-functional formulations (M / F 1 :1.6) biodegradability is increased without scarifying flexibility and brittleness (Table 2). Reduction of shore hardness is less than 20% and loss of ram pressure is less than 50%.

[0081] To confirm the chemical bonding of the co-condensate polyalcohol the following washing trials are performed: 10 g of the final foams are 2 x times washed out with hot (60°C) demineralized water. Afterwards the foams were dried (8h, 100°C) and weighted. Percental weight loss observed for the Comparative Examples 1.1.- 1.16 and Examples 2.1 - 2.8, 2.13 - 2.16 and Comparative Examples C2.9 - C2.12 is in the range of the percental weight loss of comparative examples C1 and C2.

[0082] Table 1: MF-foams from high-functional MF precondensate (M / F 1:2.8) Table 2: MF-foams from low-functional MF precondensate (M / F 1:1.6)

Claims

Claims1 . A process for producing a melamine-formaldehyde foam by heating and foaming an aqueous mixture M using microwave radiation, said mixture M comprising at least one melamine-formaldehyde precondensate having a molar ratio of melamine:formaldehyde ranging from 1 :1.3 to 1 : 2.5, at least one curative, at least one surfactant, at least one blowing agent and at least one polyalcohol, selected from the group of ethylene glycol, diethylene glycol, glycerol, trimethylol propane, pentaeryhritol, xylitol, xylose, and sorbitol.

2. The process according to claim 1 , wherein the polyalcohol is selected from polyalcohols with 2 to 8 hydroxyl groups.

3. The process according to any of claims 1 or 2, wherein a melamine-formaldehyde precondensate having a molar ratio of melamine:formaldehyde ranging from 1 :1.5 to 1 : 1.Sis used.

4. The process according to any of claims 1 to 3, wherein said mixture M comprises a surfactant mixture comprising a mixture of 50 to 90 wt% of at least one anionic surfactant and 10 to 50 wt% of at least one nonionic surfactant, wherein the weight percentages are each based on the total weight of the surfactant mixture.

5. The process according to any of claims 1 to 4 wherein formic acid is used as curative.

6. The process according to any of claims 1 to 5 wherein pentane is used as blowing agent.

7. The process according to any of claims 1 to 6, comprising the steps of: a) producing an aqueous mixture M comprising100 parts by weight of at least one melamine-formaldehyde precondensate having a molar ratio of melamine:formaldehyde ranging from 1 :1 .3 to 1 : 2.5, 2 to 4 parts by weight of at least one curative, 0.2 to 5 parts by weight of a surfactant mixture,0.1 to 5 parts by weight of at least one salt of an inorganic acid and / or of an organic carboxylic acid,1 to 40 parts by weight of at least one blowing agent,0.1 to 50 parts by weight of at least one polyalcohol, selected from the group of ethylene glycol, diethylene glycol, glycerol, trimethylol propane, pentaeryhritol, xylitol, xylose, and sorbitol,0 to 20 parts by weight of one or more additives, selected from the group of dyes, fragrances, optical brighteners, UV and heat stabilizers, flame retardants, formaldehyde scavengers and pigments, and25 to 60 parts by weight of water, b) heating and foaming said mixture M using microwave radiation, c) annealing the foam using hot air and / or nitrogen in a temperature range from 150°C to 290°C,8. A melamine-formaldehyde foam obtainable by the process of any of claims 1 to 7.

9. The melamine-formaldehyde according to claim 8, wherein the density is in the range from 6 to 12 kg / m3.

10. The melamine-formaldehyde foam according to claims 8 or 9, wherein the open-cell content according to DIN ISO 4590 is more than 95%.

11. The melamine-formaldehyde foam according to claims 8 to 10 having a biodegradation after 28 days according to OECD 301 F of more than 10 %.

12. The use of the melamine-formaldehyde foam according to claims 8 to 11 for acoustical and / or thermal insulation in aircraft, ship and motor vehicle construction, in mechanical engineering or in building construction or for cleaning, grinding or polishing sponges.

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