A method for preparing a melamine resin foam

Abstract

The application provides a preparation method of melamine resin foam and belongs to the technical field of organic polymers. Melamine, polyformaldehyde, an amino acid solution and sodium hydroxide are used as main materials of the resin, specific comonomers are added, pre-polymerization is carried out, then sodium alkyl benzene sulfonate and the amino acid solution are added, additional nanoparticles are added for re-polymerization, then a combination of sodium alkyl benzene sulfonate, n-pentane and formic acid is added as a mixed additive for microwave foaming, and finally melamine resin foam is obtained, which has good mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of organic polymer technology, and particularly relates to a method for preparing melamine resin foam. Background Technology

[0002] Melamine foam (also known as melamine polymethyl methacrylate foam) is a thermosetting elastic organic rubber-plastic foam with a fine three-dimensional network structure and high open-cell structure, produced by microwave foaming of melamine polymethyl methacrylate resin. It possesses excellent thermal insulation, stability, resistance to damp heat, sound absorption, and lightweight properties, and is widely used in architectural acoustics, professional acoustics, rail vehicles, automobile manufacturing, shipbuilding, aerospace applications, pipe insulation, high-temperature insulation, low-temperature insulation, power plants, filtration applications, large equipment manufacturing, antistatic applications, 5G base stations, white goods, supercapacitors, industrial cleaning, daily cleaning products, and furniture manufacturing.

[0003] However, melamine foam also has the following inherent drawbacks:

[0004] Melamine foam's core weaknesses lie in its high brittleness and weak tensile and compressive strength. Traditional melamine foam has low tear strength (only about 0.5-1.5 kPa) and low elastic modulus, making it prone to breakage or brittle fracture during use. For example, in automobile manufacturing, its high open-cell ratio results in a large volume, and its fragility limits its application in the trend towards lightweighting. To improve toughness, some companies have tried to introduce toughening agents such as polyvinyl alcohol (PVA), but this may sacrifice flame retardancy or increase costs.

[0005] Melamine foam is made by the condensation polymerization of melamine and formaldehyde, and it is difficult to completely remove free formaldehyde from the finished product; long-term volatilization can harm human health and the environment. Therefore, it is necessary to reduce formaldehyde release through methods such as modified resins or adsorbents. Summary of the Invention

[0006] The purpose of this invention is to address the problem of high brittleness in melamine foam by proposing a preparation method that improves its mechanical properties.

[0007] To achieve the above objectives, the present invention provides a method for preparing melamine resin foam, comprising the following steps:

[0008] S1 weighing: Calculated by mass parts, including the following components:

[0009]

[0010] S2 prepolymerization: Melamine and paraformaldehyde are added to the reactor, and the pH is adjusted to 10.5-11.5 with a strong base; the mixture is stirred to undergo a hydroxymethylation reaction; then 2,3-dimethyl-2,3-diphenylbutane, maleic anhydride, dibenzoylmethane, peroxide catalyst and amino acid solution are added and the mixture is continued to undergo a grafting reaction; the prepolymer is obtained.

[0011] S3 Repolymerization: A surfactant, the remaining amino acid solution, and nano-silica are added to the prepolymer, heated, stirred at high speed, and then reacted to obtain the repolymer.

[0012] S4 foaming: Add n-pentane, formic acid and surfactant to the polymer, stir evenly and then foam.

[0013] Preferably, it also includes the following steps:

[0014] S5 post-processing: After foaming, dry with hot air, then cut into the desired shape; to obtain the finished product.

[0015] Preferably, the surfactant in step S1 is an anionic surfactant.

[0016] Preferably, the foaming agent in step S1 is one of the following: n-butane, n-pentane, or n-hexane.

[0017] Preferably, the peroxide catalyst in step S2 is dicumyl peroxide (DCP) or benzoyl peroxide (BPO).

[0018] Preferably, in step S2, the reaction temperature of the hydroxymethylation reaction is 70-85℃; the stirring rate is ≥200rpm; and the reaction time is 1-2h.

[0019] Preferably, in step S2, the grafting reaction occurs in two stages:

[0020] The first stage of the reaction takes 20-30 minutes and the reaction temperature is 70-85℃.

[0021] The second stage reaction time is 10-15 min; the reaction temperature is 100-120℃.

[0022] Preferably, in step S3, the stirring rate is ≥2000 rpm and the time is ≥10 min.

[0023] Preferably, in step S3, the temperature of the heating reaction is 80-90℃, and the time is 60-75 min.

[0024] Preferably, in step S4, the foaming process is microwave foaming.

[0025] Preferably, the nano-silica has a D90 ≤ 20 nm and a specific surface area ≥ 200 m². 2 / g.

[0026] This invention uses an initiator (2,3-dimethyl-2,3-diphenylbutane) to simultaneously chemically graft two monomers (maleic anhydride and dibenzoylmethane). 2,3-dimethyl-2,3-diphenylbutane is a broad-spectrum free radical initiator with the advantages of a high decomposition temperature (about 150°C) and strong stability of the generated free radicals, which can reduce side reactions (such as cross-linking or degradation), thereby more efficiently initiating the grafting of two monomers simultaneously.

[0027] Maleic anhydride (MAH) is a highly polar unsaturated anhydride that can be introduced into the polymer backbone through free radical grafting to form anhydride groups, which significantly improves the tensile stress at fracture of the material.

[0028] When benzoylmethane (DBM) is grafted alone, it does not significantly improve mechanical properties. However, in this invention, its ketone structure participates in the free radical reaction, appropriately controlling the degree of crosslinking in the grafting reaction; balancing the rigidity and toughness of the material; thereby achieving the effect of improving toughness; and under the premise of equal addition amount, the mechanical improvement of the combination of benzoylmethane and maleic anhydride is better than that of maleic anhydride alone.

[0029] In this invention, a three-stage reaction is employed in the prepolymerization stage (S2) to precisely control the reaction process and prevent overpolymerization. The grafting reaction is carried out at two different temperatures: a low-temperature stage initiates the grafting of the less reactive monomer (MAH); a high-temperature stage activates the grafting of the second monomer (DBM). During this process, the continuous decomposition characteristics of a single initiator (2,3-dimethyl-2,3-diphenylbutane) are utilized, achieving a high grafting rate without additional equipment. Peroxide catalysts, such as dicumyl peroxide (DCP) or benzoyl peroxide (BPO), are used in the grafting reaction.

[0030] In this invention, the added amino acid solution generally selects glycine or lysine; it is used to regulate the reaction process, improve the resin crosslinking density and storage stability; the -NH2 of the amino acid undergoes a condensation reaction with the hydroxymethyl (-CH2OH) of the melamine-formaldehyde resin to form **-NH-CH2-O-** crosslinking bonds, constructing a three-dimensional network; thus improving mechanical properties.

[0031] Furthermore, using an amino acid solution as the reaction component allows the amino groups to undergo a condensation reaction with free formaldehyde, reducing the free formaldehyde content in the resin. Simultaneously, multi-stage hydroxymethylation and alkaline condition control help improve formaldehyde conversion rate, reducing the risk of formaldehyde release from foam products at the source.

[0032] This invention demonstrates that the addition of nano-silica significantly improves the mechanical strength of foam. The foam is prepared using a microwave foaming process, and the introduction of nanomaterials may further optimize pore structure uniformity, improving brittleness and tear strength. The performance specifications for the nano-silica are defined to ensure the effectiveness of the improved mechanical properties.

[0033] In this invention, nano-silica is mainly physically dispersed, but some chemical grafting reactions occur. Although nano-silica is added in step S3 (repolymerization stage), at which point the prepolymer has completed the hydroxymethylation reaction (S2), a large number of active groups (such as hydroxymethyl-CH2OH, maleic anhydride anhydride groups, and amino acid -NH2 / -COOH) still exist in the system; the following reactions occur:

[0034] Esterification reaction (with maleic anhydride):

[0035] SiO2-OH + maleic anhydride → ΔSiO2-O-CO-CH=CH-COOH

[0036] Condensation reaction (with hydroxymethyl or amino groups):

[0037] SiO2-OH + HO-CH2- resin → SiO2-O-CH2- resin + H2O

[0038] This invention uses a strong alkali to adjust the pH value, typically sodium hydroxide or potassium hydroxide.

[0039] The surfactant is anionic, typically sodium alkylbenzene sulfonate.

[0040] The beneficial technical effects of the present invention are at least as follows:

[0041] This invention demonstrates significant advantages in terms of foam uniformity, mechanical properties, and environmental friendliness through raw material modification via chemical grafting reaction, segmented process control, and microwave foaming technology. Furthermore, the application of nanomaterials and composite crosslinking agents achieves substantial improvements in mechanical properties, particularly in reducing brittleness. Detailed Implementation

[0042] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] In the description of this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the stated features. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0044] In the description of this invention, the term "for example" is used to mean "used as an example, illustration, or description." Any embodiment described as "for example" in this invention is not necessarily to be construed as being more preferred or advantageous than other embodiments. The following description is provided to enable any person skilled in the art to make and use the invention. Details are set forth in the following description for purposes of explanation. It should be understood that those skilled in the art will recognize that the invention can be made without using these specific details. In other instances, well-known structures and processes will not be described in detail to avoid obscuring the description of the invention with unnecessary detail. Therefore, the invention is not intended to be limited to the embodiments shown, but is consistent with the broadest scope of the principles and features disclosed herein.

[0045] Unless otherwise specified, the experimental methods used in the specific implementation methods are all conventional methods, and the materials and reagents used are all commercially available unless otherwise specified.

[0046] In this invention, unless otherwise specified, "%" represents a percentage by mass; the raw materials and reagents used are all commercially available products.

[0047] The nano-silica used in this invention is of the PST-QO2 type; particle size D90 = 20 nm; specific surface area = 200 m². 2 / g; pH 5-7; loose bulk density 0.1g / cm³ 3 ;Silica content ≥99.5%.

[0048] In this invention, the water used is preferably pure water (RO water).

[0049] In this invention, there are no particular limitations on the baking method and conditions.

[0050] Example 1

[0051] The preparation method of melamine resin foam includes the following steps:

[0052] S1 weighing: Calculated by mass parts, including the following components:

[0053]

[0054]

[0055] S2 prepolymerization: Melamine and paraformaldehyde are added to a sealed reactor equipped with a safety valve, maintaining a positive pressure of 0.3 MPa; the pH is adjusted to 11 with sodium hydroxide; the mixture is stirred to induce a hydroxymethylation reaction; then 2,3-dimethyl-2,3-diphenylbutane, maleic anhydride, dibenzoylmethane, dicumyl peroxide, and half of a glycine solution are added and mixing continues, with a small amount of ethanol added to assist in fusion; a grafting reaction occurs; the prepolymer is obtained.

[0056] The hydroxymethylation reaction was carried out at a temperature of 70°C and a stirring speed of 20 rpm for 1.5 h.

[0057] The first stage of the grafting reaction lasted for 20 minutes at a temperature of 85°C.

[0058] The second stage reaction time is 10 minutes; the reaction temperature is 120℃.

[0059] S3 Repolymerization: Half of the sodium alkylbenzene sulfonate, the remaining glycine solution and nano silica are added to the prepolymer, heated and stirred at high speed, and then reacted to obtain the repolymer;

[0060] The stirring speed was 2000 rpm; the stirring time was 10 min.

[0061] The temperature for the heating reaction was 90℃; the time was 60 min.

[0062] S4 foaming: Add n-pentane, formic acid and the remaining sodium alkylbenzene sulfonate to the polymer, stir evenly and then foam.

[0063] The microwave power is 500W, and the foaming time is 2 minutes.

[0064] S5 post-processing: After foaming, dry with hot air, then cut into the desired shape; to obtain the finished product.

[0065] Example 2

[0066] The preparation method of melamine resin foam includes the following steps:

[0067] S1 weighing: Calculated by mass parts, including the following components:

[0068]

[0069]

[0070] S2 prepolymerization: Melamine and paraformaldehyde are added to a sealed reactor equipped with a safety valve, maintaining a positive pressure of 0.3 MPa; the pH is adjusted to 10.5 with sodium hydroxide; the mixture is stirred to induce a hydroxymethylation reaction; then 2,3-dimethyl-2,3-diphenylbutane, maleic anhydride, dibenzoylmethane, and half a glycine solution are added and mixing continues, with a small amount of ethanol added to assist in fusion; a grafting reaction occurs; the prepolymer is obtained.

[0071] The hydroxymethylation reaction was carried out at a temperature of 70°C and a stirring speed of 20 rpm for 2 hours.

[0072] The first stage of the grafting reaction lasted for 20 minutes at a temperature of 85°C.

[0073] The second stage reaction time is 10 minutes; the reaction temperature is 120℃.

[0074] S3 Repolymerization: Half of the sodium alkylbenzene sulfonate, the remaining glycine solution and nano silica are added to the prepolymer, heated and stirred at high speed, and then reacted to obtain the repolymer;

[0075] The stirring speed was 2000 rpm; the stirring time was 10 min.

[0076] The temperature for the heating reaction was 90℃; the time was 60 min.

[0077] S4 foaming: Add n-pentane, formic acid and the remaining sodium alkylbenzene sulfonate to the polymer, stir evenly and then foam.

[0078] The microwave power is 500W, and the foaming time is 2 minutes.

[0079] S5 post-processing: After foaming, dry with hot air, then cut into the desired shape; to obtain the finished product.

[0080] Example 3

[0081] The preparation method of melamine resin foam includes the following steps:

[0082] S1 weighing: Calculated by mass parts, including the following components:

[0083]

[0084] S2 prepolymerization: Melamine and paraformaldehyde are added to a sealed reactor equipped with a safety valve, maintaining a positive pressure of 0.3 MPa; the pH is adjusted to 11.5 with sodium hydroxide; the mixture is stirred to induce a hydroxymethylation reaction; then 2,3-dimethyl-2,3-diphenylbutane, maleic anhydride, dibenzoylmethane, and half a solution of lysine are added and mixing continues, with a small amount of ethanol added to assist in fusion; a grafting reaction occurs; the prepolymer is obtained.

[0085] The hydroxymethylation reaction was carried out at a temperature of 85℃ and a stirring speed of 20 rpm for 1 hour.

[0086] The first stage of the grafting reaction lasted 30 minutes at a temperature of 70°C.

[0087] The second stage reaction time is 15 minutes; the reaction temperature is 100℃.

[0088] S3 Repolymerization: Half of the sodium alkylbenzene sulfonate, the remaining lysine solution and nano silica are added to the prepolymer, heated and stirred at high speed, and then reacted to obtain the repolymer;

[0089] The stirring speed was 2000 rpm; the stirring time was 10 min.

[0090] The temperature for the heating reaction was 80℃; the time was 75 minutes.

[0091] S4 foaming: Add n-pentane, formic acid and the remaining sodium alkylbenzene sulfonate to the polymer, stir evenly and then foam.

[0092] The microwave power is 500W, and the foaming time is 2 minutes.

[0093] S5 post-processing: After foaming, dry with hot air, then cut into the desired shape; to obtain the finished product.

[0094] Example 4

[0095] A method for preparing melamine resin foam differs from Example 1 in that glycine is not added in steps S2 and S3; the corresponding mass is made up with an equal amount of melamine and is added all at once in step S2.

[0096] Example 5

[0097] A method for preparing melamine resin foam differs from Example 1 in that: no nano-silica is added; the corresponding mass is made up with an equal amount of melamine and added all at once in step S2.

[0098] Example 6

[0099] A method for preparing melamine resin foam differs from Example 1 in that maleic anhydride is not added; the corresponding mass is made up with an equal amount of dibenzoylmethane.

[0100] Example 7

[0101] A method for preparing melamine resin foam differs from Example 1 in that: dibenzoylmethane is not added; the corresponding mass is made up with an equal amount of maleic anhydride.

[0102] Example 8

[0103] A method for preparing melamine resin foam differs from Example 1 in that: dibenzoylmethane and maleic anhydride are not added; the corresponding mass is made up with an equal amount of melamine and added all at once in step S2.

[0104] Example 9

[0105] A method for preparing melamine resin foam differs from Example 1 in that: dibenzoylmethane and maleic anhydride are not added; the corresponding mass is made up by replacing them with an equal amount of hydroxyethyl acrylate (HEA).

[0106] Example 10

[0107] A method for preparing melamine resin foam differs from Example 1 in that: dibenzoylmethane is not added; and the corresponding mass is replaced with an equal amount of styrene (st).

[0108] Example 11

[0109] A method for preparing melamine resin foam differs from Example 1 in that: in step S2 prepolymerization, the grafting reaction time is 30 min; and the reaction temperature is 120℃.

[0110] Example 12

[0111] A method for preparing melamine resin foam differs from Example 1 in that: in step S2 prepolymerization, the grafting reaction time is 30 min; and the reaction temperature is 85℃.

[0112] Example 13

[0113] The preparation method of melamine resin foam includes the following steps:

[0114] S1 weighing: Calculated by mass parts, including the following components:

[0115]

[0116] S2 prepolymerization: Melamine and paraformaldehyde are added to a reaction vessel, and the pH is adjusted to 8 with sodium hydroxide; the mixture is stirred to induce a hydroxymethylation reaction; a prepolymer is obtained.

[0117] The hydroxymethylation reaction was carried out at a temperature of 70°C and a stirring speed of 20 rpm for 1.5 h.

[0118] S3 Repolymerization: Half of the sodium alkylbenzene sulfonate is added to the prepolymer, the mixture is heated and stirred at high speed, and then heated to react, to obtain the repolymer;

[0119] The stirring speed was 2000 rpm; the stirring time was 10 min.

[0120] The temperature for the heating reaction was 90℃; the time was 60 min.

[0121] S4 foaming: Add n-pentane, formic acid and the remaining sodium alkylbenzene sulfonate to the polymer, stir evenly and then foam.

[0122] The microwave power is 500W, and the foaming time is 2 minutes.

[0123] S5 post-processing: After foaming, dry with hot air, then cut into the desired shape; to obtain the finished product.

[0124] Performance testing

[0125] The products obtained in the above embodiments were subjected to performance testing, including the following test items:

[0126] Elastic modulus: according to the method described in GB / T 8813-2020 Determination of compressive properties of rigid foamed plastics.

[0127] Compressive strength: According to GB / T 8813-2020 Determination of compressive properties of rigid foamed plastics, select the strength at 70% compression deformation.

[0128] Tensile stress at break: according to the method described in GB / T 8813-2020 Determination of compressive properties of rigid foamed plastics.

[0129] Relative elongation (elongation at break): according to the method described in GB / T 8813-2020 Determination of compressive properties of rigid foamed plastics.

[0130] The test results are shown in Table 1.

[0131] Table 1

[0132]

[0133] As shown in Table 1, in this invention:

[0134] Without the addition of amino acids (Example 4), the flexible segments of the amino acid solution disrupt the continuity of the rigid melamine network; the flexible segments formed by amino acid grafting can inhibit crack propagation. Without amino acids, the molecular chain cross-linking density is higher, resulting in increased rigidity; and the absence of amino acids leads to a lack of energy dissipation structures in the foam, making the cell walls more prone to brittle fracture under compressive loads; furthermore, the plastic deformation capacity provided by amino acids disappears, and the material exhibits typical brittle fracture characteristics. Ultimately, although the elastic modulus is slightly improved, the compressive strength, tensile stress at fracture, and relative elongation are significantly reduced.

[0135] Without the addition of nano-silica (Example 5), the interfacial bonding between the resin matrix and the filler is insufficient, leading to reduced stress transfer efficiency. The absence of nano-silica prevents the material from hindering crack propagation through rigid particles or absorbing energy through flexible segments. Without nanoparticles, the cell wall strength is insufficient during foaming, easily resulting in large-sized pores. Ultimately, this leads to a decrease in elastic modulus, compressive strength, tensile stress at break, and relative elongation.

[0136] Examples 6, 7, 8, 9, and 10 tested the changes in different mechanical properties with different modified monomers or without modification. The results showed that only the combination of maleic anhydride and dibenzoylmethane had the best overall performance. Although substitution or omission may lead to an increase in a certain indicator, the other indicators would show a significant decrease.

[0137] During the prolonged high-temperature reaction of S2 prepolymer (Example 11), the high temperature accelerates the crosslinking reaction, leading to excessive crosslinking of molecular chains and the formation of a rigid network. Initially, the increased crosslinking density improves strength, but excessive crosslinking causes embrittlement of the cell walls, resulting in a decrease in long-term strength. The rigid network lacks toughness, and stress concentration triggers brittle fracture. The densification of the crosslinked network completely inhibits molecular chain slippage. Although there is a slight increase in elastic modulus and compressive strength, it leads to a significant decrease in tensile stress at break and relative elongation.

[0138] During the long-term low-temperature reaction of S2 prepolymer (Example 12), insufficient reactivity leads to low crosslinking degree and a loose linear molecular chain structure. The incompletely crosslinked resin matrix cannot effectively support the load, and the cells are prone to collapse. The number of hydrogen bonds and covalent bonds between molecular chains is insufficient, resulting in weak interfacial bonding. Flexible segments (amino acids) are not fully grafted, and the remaining linear molecular chains provide plastic deformation capacity. Although this can significantly improve the relative elongation, it leads to a severe decrease in elastic modulus, compressive strength, and tensile stress at break.

[0139] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. All equivalent implementations or modifications that do not depart from the present invention should be included within the scope of the technical solution of the present invention.

Claims

1. A method of preparing a melamine resin foam, characterized by, The method comprises the following steps: S1 weighing: the following ingredients are included by mass fraction: Melamine 100 Paraformaldehyde 30-40 Amino acid solution 5-10 Nano-silica 0.2-0.6 2,3-Dimethyl-2,3-diphenylbutane 0.1-0.2 Maleic anhydride 1.5-2.0 Dibenzoylmethane 1.0-1.5 Surfactant 0.5-2 Foaming agent 8-15 Formic acid 0.1-0.5 S2 prepolymerization: melamine, paraformaldehyde are added to a reaction kettle, and a strong base is used to adjust the pH to 10.5-11.5; stirring and mixing occur to carry out a methylolation reaction; then 2,3-dimethyl-2,3-diphenylbutane, maleic anhydride, dibenzoylmethane, peroxide catalyst and amino acid solution are added and continue to mix to carry out a grafting reaction; a prepolymer is obtained; In the step S2 prepolymerization, the grafting reaction is divided into two stages: The first stage reaction time is 20-30 min; the reaction temperature is 70-85℃; The second stage reaction time is 10-15 min; the reaction temperature is 100-120℃; S3 Repolymerization: the surfactant, the remaining amino acid solution and nano-silica are added to the prepolymer, and after warming, high-speed stirring is carried out, and then reaction is carried out to obtain a repolymer; S4 Foaming: the foaming agent, formic acid and surfactant are added to the repolymer, and after stirring uniformly, foaming is carried out.

2. The method of making melamine formaldehyde resin foam according to claim 1, characterized in that, Further comprising the following steps: S5 post-treatment: after foaming, hot air drying is carried out, and then cutting into a required shape is carried out; a finished product is obtained.

3. The method of making melamine formaldehyde resin foam according to claim 1, characterized in that, The surfactant in the step S1 is an anionic surfactant.

4. The method of making melamine formaldehyde resin foam according to claim 1, characterized in that, The foaming agent in the step S1 is one of the following: n-butane, n-pentane, n-hexane.

5. The method of making melamine formaldehyde resin foam according to claim 1, characterized in that, In the step S2, the reaction temperature of the methylolation reaction is 70-85℃; the stirring rate is ≥200 rpm; and the reaction time is 1-2 h.

6. The method of making melamine formaldehyde resin foam according to claim 1, characterized in that, In the step S3, the stirring rate is ≥2000 rpm; and the time is ≥10 min.

7. The method of making melamine formaldehyde resin foam according to claim 1, characterized in that, In the step S3, the temperature of the warming reaction is 80-90℃; and the time is 60-75 min.

8. The method of making melamine formaldehyde resin foam according to claim 1, characterized in that, In the step S4, the foaming process is microwave foaming.

9. The method of making melamine formaldehyde resin foam according to claim 1, characterized in that, The nano-silica has a D90≤20 nm; a specific surface area≥200 m 2 / g.

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