JP7779752B2 - Melamine resin foam and its manufacturing method - Google Patents

Description

The present invention relates to a melamine resin foam and a method for producing the same.

Melamine-formaldehyde reaction precursors are obtained by reacting melamine monomer with formaldehyde in the presence of a catalyst. Adding a blowing agent and heating the resulting mixture produces porous melamine resin foams. These foams are widely used in household cleaning sponges due to their excellent hardness and durability.

Melamine resin foams have been widely studied. Specifically, a melamine-formaldehyde reaction precursor-based elastic foam and a method for producing the same are known, which involve foaming an aqueous or alcoholic solution or dispersion containing a melamine-formaldehyde reaction precursor, an emulsifier, a blowing agent, a curing agent, and optionally conventional additives, by irradiating it with microwave radiation, and then curing the foam by crosslinking the precondensate (Patent Document 1).

However, when the elastic foam described in Patent Document 1 was manufactured and used as a cleaner sponge, while it had excellent hardness and durability, it tended to be somewhat lacking in resilience and feel, leaving room for improvement.

Special Publication No. 2-50943

The problem that this invention aims to solve is to provide a melamine resin foam that is fine-grained due to its small cell area and cell lattice width, has high hardness and flexibility due to its excellent breaking strength and breaking elongation, is smooth to the touch due to its suppressed foreign body sensation and noise, and can return to its original size even after washing, as well as a method for producing the same.

The present invention relates to a melamine resin foam comprising a melamine-formaldehyde reaction precursor (A) obtained by reacting a melamine monomer with formaldehyde in the presence of a catalyst, a blowing agent (B), a surfactant (C), and a curing agent (D), wherein the blowing agent (B) is a mixed solution containing a hydrofluorocarbon (b1) and a hydrofluoroether (b2) foamed.

The melamine resin foam of the present invention has a fine texture due to its small cell area and cell lattice width, excellent breaking strength and breaking elongation, high hardness and flexibility, a smooth feel due to the suppression of foreign body sensation and noise, and the ability to return to its original size even after washing.

<Melamine-formaldehyde reaction precursor>
The present invention uses a melamine-formaldehyde reaction precursor (A). Component (A) is produced by adding a basic catalyst to an aqueous solution containing melamine monomer and formaldehyde, followed by heating to add formaldehyde to three amino groups present in the melamine monomer. This precursor is also called methylol melamine, and corresponds to the precursor used as the base for the elastic foam described in Patent Document 1.

Aldehydes other than formaldehyde may be added as long as the effects of the present invention are not impaired. Examples of other aldehydes include paraformaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, crotonaldehyde, benzaldehyde, acrolein, glyoxal, furfural, salicylic aldehyde, and cinnamaldehyde.

The molar ratio of melamine monomer to aldehydes, including formaldehyde, is preferably 1:1-5, and particularly preferably 1:2-4. By blending within this range, the molecular structure of the melamine resin becomes three-dimensionally network-like, which tends to dramatically improve mechanical properties.

Examples of basic catalysts include metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide, as well as ammonia. The basic catalyst is added before the start of the methylolation reaction to adjust the pH of the aqueous solution; the pH at this time is preferably 8 or higher but lower than 12, and particularly preferably 8.5 or higher but lower than 12. Keeping the pH within this range tends to enable efficient synthesis of a melamine-formaldehyde precondensate.

The reaction temperature is preferably 80°C or higher and lower than 110°C, and particularly preferably 85°C or higher and lower than 105°C. The reaction time is preferably 10 minutes or higher and lower than 3 hours, and particularly preferably 15 minutes or higher and lower than 1 hour and 30 minutes.

After the above reaction is completed, the melamine-formaldehyde reaction precursor (A) may be subsequently subjected to a methylenation reaction. Specifically, after the synthesis of the above methylol melamine, an acidic catalyst is added and the mixture is heated to synthesize the melamine-formaldehyde reaction precursor (A1) having a methylene bond.

Examples of acidic catalysts include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, boric acid, paratoluenesulfonic acid, paratoluenesulfonic acid amide, acetic acid, formic acid, and oxalic acid. The acidic catalyst is added after the completion of the methylolation reaction or the etherification reaction and before the initiation of the methylenation reaction to adjust the pH of the aqueous solution. The pH at this time is preferably 6.0 or higher but less than 8.0, and particularly preferably 6.5 or higher but less than 7.8. Keeping the pH within this range allows for efficient synthesis of component (A1).

The reaction temperature for the methylenation reaction is preferably 50°C or higher but lower than 90°C, and particularly preferably 55°C or higher but lower than 85°C. The reaction time is preferably 10 minutes or higher but lower than 4 hours, and particularly preferably 15 minutes or higher but lower than 3 hours. The reaction can be completed by sampling a small amount of resin and gradually cooling it. The reaction can be completed when the temperature at which cloudiness is visually observable (cloud point) is between 0°C and 10°C.

After the reaction is complete, a basic catalyst can be added as needed to prevent thickening over time. In this case, the pH is preferably 8.0 or higher but less than 11.0, and particularly preferably 8.5 or higher but less than 10.5.

In the present invention, a melamine resin foam is produced by adding a blowing agent (B), a surfactant (C), and a curing agent (D) to component (A).

<Blowing Agent>
In the present invention, a foaming agent (B) is used. As the component (B), a hydrofluorocarbon (b1) and a hydrofluoroether (b2) are used. The combined use of the components (b1) and (b2) tends to make the melamine resin foam smooth to the touch and improve its recovery.

As the blowing agent (B), substances other than the above components (b1) and (b2) may be used as long as the effects of the present invention are not impaired. Other blowing agents that can be used include organic and inorganic blowing agents. Organic blowing agents include saturated hydrocarbons having approximately 5 to 18 carbon atoms, azodicarbonamide, dinitrosopentamethylenetetramine, chlorinated solvents such as methylene chloride and chloroform, and fluorinated solvents other than (b1) and (b2). Inorganic blowing agents include sodium bicarbonate and ammonium carbonate.

The blending ratio of component (B), converted to the active ingredient, is preferably 0.1 to 100 parts by weight, more preferably 0.5 to 50 parts by weight, and particularly preferably 1 to 30 parts by weight, per 100 parts by weight of component (A). Blending within this range tends to enable the efficient production of foams with excellent flexibility and elasticity.

<Surfactant>
In the present invention, a surfactant is used. By using the component (C), a uniform solution can be obtained when the components (A) to (D) are mixed. Known components (C) include anionic, cationic, or amphoteric surfactants, as well as nonionic surfactants. Among these, it is preferable to use an anionic surfactant in the present invention. Alternatively, a nonionic surfactant may be mixed with an anionic surfactant.

Anionic surfactants include carboxylate types such as commercially available soaps and polyoxyethylene alkyl ether carboxylates, sulfonate types such as linear alkylbenzene sulfonates (LAS) and branched alkylbenzene sulfonates (ABS), sulfosuccinate types such as dialkyl sulfosuccinates, sulfate ester types such as alkyl sulfate esters (AS) and polyoxyethylene alkyl ether sulfate esters (AES), as well as phosphate ester types and amino acid types.

The blending ratio of component (C), converted to the active ingredient, is preferably 0.1 to 50 parts by weight, more preferably 0.3 to 30 parts by weight, and particularly preferably 0.5 to 20 parts by weight, per 100 parts by weight of component (A). Blending within this range reduces the cell area of the foam, resulting in a finer cell structure and a uniform porous structure, which tends to suppress deviations in cell shape.

<Curing agent>
In the present invention, a curing agent (D) is used. By blending the component (D), the curing reaction of the component (A) after synthesis can be accelerated.

An acidic aqueous solution is preferably used as component (D). Examples of acidic aqueous solutions include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, boric acid, acetic acid, formic acid, and oxalic acid. The blending ratio of component (D), calculated as the active ingredient, is preferably 0.1 to 50 parts by weight, more preferably 0.3 to 30 parts by weight, and particularly preferably 0.5 to 20 parts by weight, per 100 parts by weight of component (A). Blending within this range tends to enable component (A) to cure efficiently.

A known method can be used to produce the foam. One example is a method in which the above components (A) to (D) are stirred to obtain a mixed solution, which is then poured into a suitable mold and foamed by heating or irradiating with microwaves, etc.

In addition, the present invention may contain various additives such as fluorescent brighteners, antioxidants, ultraviolet absorbers, heat stabilizers, pigments, preservatives, salts of inorganic acids or organic carboxylic acids, tackifiers, plasticizers, and formaldehyde scavengers.

The present invention will be explained in more detail below with reference to examples and comparative examples, but these are intended to be specific examples and are not intended to be limiting.

(Synthesis Example 1)
2,048 parts by weight of melamine monomer and 3,622 parts by weight of formaldehyde (37% aqueous solution) were charged into a three-necked round-bottom flask equipped with a stirrer, a reflux condenser, and a thermometer, and sodium hydroxide (48% aqueous solution) was added as a basic catalyst to adjust the pH to 9.5. The reaction was carried out at 95°C for 30 minutes to complete the methylolation reaction. The pH at the end was 9.0.
Thereafter, formic acid (76% aqueous solution) was added so that the pH was adjusted to 7.5, and the reaction was carried out at 75° C. for 117 minutes, and the methylenation reaction was completed when the cloud point (5° C.) was confirmed.
After completion of synthesis, the mixture was allowed to cool naturally. When the temperature had decreased to 60°C, sodium hydroxide (48% aqueous solution) was again added so that the pH was adjusted to 9.0, and excess water was removed until the solid content reached 75%. The mixture was then allowed to cool naturally again, yielding a melamine-formaldehyde reaction precursor (A) having a methylene bond (75% aqueous solution, colorless and transparent, hereinafter referred to as Synthetic Product 1).

(Synthesis Example 2)
2,048 parts by weight of melamine monomer and 3,622 parts by weight of formaldehyde (37% aqueous solution) were charged into a three-necked round-bottom flask equipped with a stirrer, a reflux condenser, and a thermometer, and sodium hydroxide (48% aqueous solution) was added as a basic catalyst so that the pH was adjusted to 12.0. The reaction was carried out at 85°C for 30 minutes, and the methylolation reaction was completed. The pH at the end was 11.0.
Thereafter, formic acid (76% aqueous solution) was added so that the pH was adjusted to 7.5, and the reaction was carried out at 75° C. for 45 minutes. The methylenation reaction was terminated when the cloud point (9° C.) was confirmed.
After completion of synthesis, the mixture was allowed to cool naturally. When the temperature had dropped to 60°C, sodium hydroxide (48% aqueous solution) was again added so that the pH was adjusted to 9.0, and excess water was removed until the solid content reached 75%. The mixture was then allowed to cool naturally again, yielding a melamine-formaldehyde reaction precursor (A) having a methylene bond (75% aqueous solution, colorless and transparent, hereinafter referred to as Synthetic Product 2).

Then, using the compound obtained in the above synthesis example, a melamine resin foam was produced by the following process.

Example 1
As component (A), 100 parts by weight of Compound 1 (75% aqueous solution),
The component (B) contains 2.7 parts by weight of hydrofluorocarbon CF ₃ (CF2) ₅ H (boiling point 71°C, surface tension 13.4 mN/m, hereinafter referred to as HFC-1) as the component (b1), and hydrofluoroether (CF ₃ ) ₃ CFCH ₂ OCFH as the component (b2). 2.7 parts by weight of HFE-2 (boiling point 76°C, surface tension 13.6 mN/m, hereinafter referred to as HFE-2), 4.5 parts by weight of n-pentane and 1.1 parts by weight of n-hexane as other blowing agents, 2.4 parts by weight of sodium dodecylbenzenesulfonate (50% aqueous solution) as component (C), 2.1 parts by weight of formic acid (88% aqueous solution) as component (D), and 0.002 parts by weight of fluorescent brightener 4-(2-benzoxazolyl)-4'-(5-methyl-2-benzoxazolyl)stilbene and 1.7 parts of sodium formate as other components were charged into a disk turbine mixer and stirred at 3000 rpm for 1 minute to obtain a mixed solution.

The resulting mixed solution was poured into a polyethylene container and foamed by irradiating it with microwaves at 1.32 kW for 300 seconds using a microwave irradiator (product name: YMD-12, frequency: 2450 MHz, manufactured by Yamamoto Vinita Co., Ltd.). The mixture was then demolded and heated at 200°C for an additional 30 minutes to fully cure, yielding the melamine resin foam of Example 1.

(Other Examples)
A melamine resin foam was obtained in the same manner as in Example 1 using the formulation shown in Table 1. The following products were used.
_CF3 ( _CF2 ) _5CH2CH3 (boiling point ₁₁₄ °C, surface tension 15.5mN _/ m, hereafter referred to as HFC-2)
_C4H9OCH3 (boiling point ₆₁ ° _C , surface tension 13.6mN/m, hereafter referred to as HFE-1)

The melamine resin foams obtained in the above examples were cut out from the interior of the foam, excluding the foam surface, to prepare test pieces, and their physical properties were evaluated. The results are shown in Table 2.

<Cell area>
Using a microscope (product name: VHX-700F, manufactured by Keyence Corporation), each test piece was observed at 300x magnification, and five cells were randomly selected per field of view to output images of a total of 30 cells from six fields of view. The area of each specimen was then measured, and the average value was taken as the cell area (μm ² ).

<Cell grid width>
Using a microscope (product name: VHX-700F, manufactured by Keyence Corporation), each test piece was observed at 700x magnification, and five specimen cells were randomly selected per field of view, resulting in a total of six visual fields, with images of a total of 30 specimen cells being output. The grid width of each specimen was then measured, and the average value was taken as the cell grid width (μm).

<Breaking strength and elongation>
Using a compression testing machine (product name: MCT-2150, manufactured by A&D Co., Ltd.), each test piece (60 mm × 60 mm × 60 mmt) was compressed with a Φ5 cylindrical jig at a test speed of 10 mm/min, and the strength (N) and elongation (mm) at break were measured.

<Resilience>
Each test piece (60 mm x 120 mm x 30 mm thick) was immersed in tap water and washed a stainless steel sink 100 times, then left to stand for 30 minutes to check its restorability. If it restored to its original size, it was evaluated as ◯, and if it did not, it was evaluated as ×.

<Tactile sensation>
The feel of each test piece (60 mm x 120 mm x 30 mmt) was checked by touch. If the feel was smooth, it was rated as ◯, and if there was a foreign body sensation or foreign body noise, it was rated as ×.

Claims (4)

A melamine resin foam comprising a melamine-formaldehyde reaction precursor (A), a blowing agent (B), a surfactant (C), and a curing agent (D), characterized by being produced by foaming a mixed solution containing a hydrofluorocarbon (b1) and a hydrofluoroether (b2) as the blowing agent (B). 2. The melamine resin foam according to claim 1 , wherein the melamine-formaldehyde reaction precursor (A) is a melamine-formaldehyde reaction precursor (A1) having a methylene bond. The melamine resin foam according to claim 1 or 2, characterized in that the mixed solution contains a fluorescent whitening agent. A method for producing a melamine resin foam, comprising: a melamine-formaldehyde reaction precursor (A); a blowing agent (B); a surfactant (C); and a curing agent (D), wherein the foaming agent (B) is a mixed solution containing a hydrofluorocarbon (b1) and a hydrofluoroether (b2); and wherein the melamine-formaldehyde reaction precursor (A) is obtained by adding a basic catalyst to an aqueous solution containing a melamine monomer and formaldehyde, heating the mixture to cause a reaction, and then adding an acidic catalyst and heating the mixture to cause a reaction.