CN115124018A

CN115124018A - II type ammonium polyphosphate metal ion double salt, preparation method and flame-retardant paper

Info

Publication number: CN115124018A
Application number: CN202210942323.7A
Authority: CN
Inventors: 徐玲; 裘琦箐; 朱峰; 朱志强; 杨显光
Original assignee: HANGZHOU JLS FLAME RETARDANTS CHEMICAL CO Ltd; Jiangshan Jieershi Flame Retardation Material Co ltd
Current assignee: HANGZHOU JLS FLAME RETARDANTS CHEMICAL CO Ltd; Jiangshan Jieershi Flame Retardation Material Co ltd
Priority date: 2022-08-03
Filing date: 2022-08-03
Publication date: 2022-09-30
Anticipated expiration: 2042-08-03
Also published as: CN115124018B

Abstract

The invention provides a type II ammonium polyphosphate metal ion double salt, a preparation method and flame retardant paper. The preparation method of the type II ammonium polyphosphate metal ion double salt comprises the preparation of the metal polyphosphate and the preparation of the ammonium polyphosphate metal ion double salt. The preparation of the metal polyphosphate comprises the following steps: reacting phosphorus pentoxide and metal hydroxide for 0.5-10 h in a vacuum state to obtain metal polyphosphate; the preparation of the ammonium polyphosphate metal ion double salt comprises the following steps: and stirring the obtained metal polyphosphate, diammonium hydrogen phosphate and phosphorus pentoxide, and reacting for 0.5-9 h under the condition of heating and introducing ammonia step by step to obtain the ammonium polyphosphate metal ion complex salt with the crystal form of ammonium polyphosphate II.

Description

II type ammonium polyphosphate metal ion double salt, preparation method and flame-retardant paper

Technical Field

The invention relates to the technical field of modification of ammonium polyphosphate double salt, in particular to type II ammonium polyphosphate metal ion double salt, a preparation method and flame-retardant paper.

Background

Along with the requirements of environmental protection and degradability, paper products are increasingly widely used, and particularly in building decoration and some temporary buildings, except original wallpaper, temporary buildings and other paper constructions are set up in exhibition halls for the first time. But the oxygen index of the paper is only 15 percent, and the paper is extremely flammable and must be flame-retardant when used for buildings. The flame retardant of the common coating process is only on the surface of paper, the content of the whole flame retardant is low, the flame retardant grade cannot meet the flame retardant requirement of a building, or the apparent performance of the paper can be influenced by simply increasing the thickness of the coating and the content of the flame retardant of the coating.

The above problems can be solved by adding a flame retardant to the pulp. Ammonium polyphosphate (APP for short) is a flame retardant variety with high cost performance for paper, has the advantages of high phosphorus and nitrogen content, no aldehyde, no halogen, no toxicity, biodegradability and no generation of toxic substances during combustion, is widely known to have two crystal forms, namely type I and type II, which are mainly used in APP industrialization, the type I water solubility is high and easy to hydrolyze, the retention rate of the flame retardant by an internal slurry method cannot be ensured, and the type II APP water solubility is small, and is more stable in water due to the crystal form. However, ammonium polyphosphate is an inorganic high molecular compound, and compared with the traditional paper filler, the ammonium polyphosphate has the problems of higher water solubility, serious loss of the flame retardant in the paper making process, difficulty in controlling the retention amount of the flame retardant and the like.

To solve the problem of water solubility, the existing methods for improving the water solubility and viscosity of APP adopt the following three ways: surface modification treatment, microencapsulation treatment, surface activation treatment, which corresponds to methods such as silane coupling agent modification, melamine formaldehyde resin coating, organic amine modification, and the like. In which organic amine modification is used, but the retention rate is reduced.

The method comprises the following steps: and (3) coating with melamine formaldehyde resin. In patent publication No. CN1480253A, melamine-formaldehyde is treated to form a prepolymer, ammonium polyphosphate is subjected to microcapsule coating to reduce the water solubility of the prepolymer, the water solubility of APP can be reduced from 0.5g/100 ml (25 ℃) to below 0.05g/100 ml (25 ℃), but the method mainly has the problem of formaldehyde residue, and particularly can slowly release formaldehyde under the action of heat and light in the later use process.

The second method comprises the following steps: and (4) treating with a silane coupling agent. This is a relatively common method used to improve the water solubility of APP. Patent applications with publication numbers CN101570325A, CN101492881A, and CN102093735A mention that adding silane coupling agent to ammonium polyphosphate or modified ammonium polyphosphate such as single melamine in advance in silicate or other solvents respectively for reaction reduces water solubility and plays a certain role in migration of APP. In patent applications with publication numbers CN101235221A and CN102850824A, the water solubility of APP is reduced by hydrolyzing an aminosilane coupling agent and then performing organic coating, but the reduction is not as great as in the first method. The method adopts silicate ester hydrolysis coating, and has the defects of low existing treatment efficiency, non-uniform coating and the like.

The third method comprises the following steps: the method is mainly used for modifying APP with low degree of polymerization. In patent applications with publication numbers of CN103342833A and CN103303891A, nano/micron composition such as magnesium hydroxide, hydrotalcite and the like is adopted to improve flame retardance; the addition of chain extenders and cations to the medium is improved in patent application publication No. CN 101942227A.

In the methods, the surface modification is carried out after the crystallization II type ammonium polyphosphate is synthesized, the crystallization II type APP is mainly prepared, and the water solubility of the crystallization II type APP is lowest through the microencapsulation treatment of melamine formaldehyde resin, but the method has two problems, namely, formaldehyde is released in the subsequent use process of the paper; on the other hand, the paper making industry has the biggest problem that the concentration of paper pulp in the paper making process is quite low, the concentration of the paper pulp is about 1 percent, the flame retardant is about a few thousandths, the problem that the retention rate of ammonium polyphosphate in paper pulp is low cannot be well solved by simply physically reducing the water solubility of the flame retardant, and the subsequent wastewater treatment of the paper making industry which consumes a large amount of water is difficult.

Disclosure of Invention

The invention aims to provide a II-type ammonium polyphosphate metal ion double salt which is improved based on metal ions, has simple preparation steps, mild conditions and no special requirements on equipment, can obviously reduce water solubility and retention rate in paper pulp, a preparation method and flame retardant paper.

In order to solve the above technical problems, an embodiment of the present invention provides a type ii ammonium polyphosphate metal ion double salt, wherein the structural formula of the type ii ammonium polyphosphate metal ion double salt is as follows:

wherein M is ^z+ One or more of metal ions that are divalent or trivalent;

wherein a and n are average polymerization degrees, a is more than or equal to 2, and n is more than or equal to 100; wherein z is the valence state of the metal ion. Wherein the metal ion is located on the main chain of the double salt.

Optionally, M is one or more of zinc, magnesium, copper, aluminum, nickel, iron and calcium, preferably one or more of zinc, magnesium, aluminum and calcium, and the retention rate and the flame retardant rate of the final double salt are more excellent.

Alternatively, a may be 2-100 and n may be 200-2000.

The invention also provides a preparation method of the type II ammonium polyphosphate metal ion double salt, which is characterized by comprising the preparation of the metal polyphosphate and the preparation of the ammonium polyphosphate metal ion double salt. The method comprises the following steps:

preparing metal polyphosphate: reacting phosphorus pentoxide and metal hydroxide for 0.5-10 h in a vacuum state to obtain metal polyphosphate;

in the formula, a is any natural number of 2-100.

Preparing ammonium polyphosphate metal ion double salt: and stirring the obtained metal polyphosphate, diammonium hydrogen phosphate and phosphorus pentoxide, and reacting for 0.5-9 h under the condition of heating and introducing ammonia step by step to obtain the ammonium polyphosphate metal ion double salt with the crystal form of ammonium polyphosphate II.

In step (r), the inventors also tried to replace the metal hydroxide with a metal oxide or other metal sulfate, and finally, the ammonium polyphosphate metal ion double salt of ammonium polyphosphate form ii in the present application could not be obtained.

Optionally, in the step of preparing the metal polyphosphate, the molar ratio of the phosphorus pentoxide to the metal hydroxide is 1: 0.2-2, preferably 1: 0.3-0.8.

Optionally, in the preparation of the ammonium polyphosphate metal ion double salt, the molar ratio of the metal polyphosphate to the phosphorus pentoxide is 1: 1-30, preferably 1: 10-20; the amount of diammonium phosphate used is the sum of the moles of metal polyphosphate and phosphorus pentoxide.

Optionally, the metal hydroxide includes one or more of zinc hydroxide, magnesium hydroxide, aluminum hydroxide, copper hydroxide, iron hydroxide, calcium hydroxide.

When the metal ion is +2, taking magnesium hydroxide as an example, the structural formula of the ammonium polyphosphate metal ion complex salt is as follows:

when the metal ion is +3, taking aluminum hydroxide as an example, the structural formula of the ammonium polyphosphate metal ion double salt is as follows:

optionally, a and n are average polymerization degrees, a is greater than or equal to 2, n is greater than or equal to 100, preferably a is 2-100, and n is 200-.

Optionally, in the step of preparing the metal polyphosphate, the metal hydroxide is added into the phosphorus pentoxide under stirring, the mixture is stirred together and heated to 60-200 ℃, preferably 100-150 ℃, and the mixture is vacuumized and devolatilized to 0.005-0.08 Mpa for reaction.

Optionally, in the step of preparing the metal polyphosphate, phosphorus pentoxide is added into a reactor, the stirring speed is 20-200 r/min at 30-100 ℃, preferably 40-60 ℃, metal hydroxide is added, and the mixture is stirred for 0.5-1 h.

Optionally, in the preparation of the ammonium polyphosphate metal ion double salt: firstly adding metal polyphosphate and diammonium hydrogen phosphate, stirring, then adding phosphorus pentoxide, stirring, heating and introducing ammonia step by step, wherein the reaction is performed at a higher temperature for 1-6 hours, and then at a lower temperature for 0.5-3 hours, and the ammonia introducing amount at the lower temperature is smaller than that at the higher temperature.

Optionally, the metal polyphosphate and the diammonium hydrogen phosphate are added and stirred for 0.5 to 2 hours.

Optionally, the higher temperature is 200-350 ℃, the lower temperature is 150-250 ℃, and the ammonia introducing amount at the lower temperature is 0.5-5 m for every 100 kg of reactants ³ The ratio of the ammonia introduction amount to the reactant at a higher temperature is 0.2-2 m per 100 kg of the reactant ³ In terms of hours.

Optionally, the content of phosphorus pentoxide is more than or equal to 99.5%, the activity R value is more than or equal to 1.7, and the proportion of reducing substances is less than or equal to 0.005%.

The invention also provides an application of the ammonium polyphosphate metal ion double salt, and the ammonium polyphosphate metal ion double salt is applied to preparation of flame retardant paper by using an in-pulp method. The obtained flame-retardant paper is prepared from the following raw materials: any of the ammonium polyphosphate II-type metal ion double salt and the paper pulp formed by soaking the base paper in water and dispersing; and (3) an additive. The flame retardant can be directly added in the papermaking process by adopting an in-pulp method, which is different from a coating method, and the in-pulp method has higher retention rate (residence rate) requirement on the flame retardant and reduces the papermaking wastewater. Even if the pulp concentration is lower than 2%, for example, about 1%, although the added type II ammonium polyphosphate metal ion double salt only accounts for less than 0.5%, even less than 0.2%, for example, about 0.198%, the problem of retention rate can be still solved.

In conclusion, the metal ions of the ammonium polyphosphate II metal ion double salt are combined on the main chain, the structural formula of the ammonium polyphosphate II metal ion double salt has polyphosphate metal salt and ammonium polyphosphate ends to form the double salt of the polyphosphate metal salt and the ammonium, but the solubility of the ammonium polyphosphate II metal ion double salt is lower than 0.55g/100g, even most of the ammonium polyphosphate metal ion double salt is lower than 0.22g/100g, and the ammonium polyphosphate II metal ion double salt is more stable in water due to the crystal form; and the viscosity is within a reasonable range, such as 1.7-4 mpa.s, even 2.5-4 mpa.s, and the particle size is uniform.

According to the invention, through synthesizing the polyphosphate with a certain degree of polymerization, on one hand, the interference of the metal ions on the APP crystal form is reduced, in addition, the starting point degree of polymerization is high, crystal II type ammonium polyphosphate can be obtained, the flame retardant efficiency of the ammonium polyphosphate in paper and the water solubility of the APP are improved, the carbonization length after vertical combustion is less than 100, in the paper making process (the process of making paper from paper pulp), the APP hydrolysis is inhibited, and meanwhile, the metal ions can flocculate the ammonium polyphosphate swelled in water and combine with the paper pulp, so that the in-pulp retention rate is higher.

Drawings

FIG. 1 is an X-ray diffraction pattern of a metal ion double salt of ammonium polyphosphate form II in example 1;

FIG. 2 is an infrared spectrum of a metal ion double salt of ammonium polyphosphate form II in example 1.

Detailed Description

The following examples are given to further illustrate the embodiments of the present invention. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.

Unless otherwise stated, all% mentioned below are mass%.

Example 1

(1) Preparation of metal polyphosphates

Adding 142kg of phosphorus pentoxide into a reactor, stirring at 50 ℃ and 100 r/min, adding 29kg of magnesium hydroxide, stirring for 1h, heating to 120 ℃, simultaneously vacuumizing and devolatilizing, and reacting for 4h at the vacuum degree of 0.04Mpa to obtain metal polyphosphate 1;

(2) preparation of ammonium polyphosphate metal ion double salt

Adding 17.1kg of metal polyphosphate 1 and 132kg of diammonium hydrogen phosphate into a kneader, stirring for 0.5h, adding 134.9kg of phosphorus pentoxide, stirring, heating, introducing ammonia, and reacting at 250 ℃ for 3h, wherein the ratio of the ammonia introduction amount to the reactant is that the ammonia introduction amount is 3m per 100 kg of the reactant ³ Per hour, at 200 deg.C for 3h, ammonia per 100 kg of reactants 1.5m ³ And obtaining crystal II type ammonium polyphosphate metal ion double salt.

The X-ray diffraction pattern and the infrared spectrum of the metal ion double salt of ammonium polyphosphate in crystal form II are shown in FIGS. 1 and 2, respectively.

Example 2

(1) Preparation of metal polyphosphates

Adding 142kg of phosphorus pentoxide into a reactor, stirring at 50 ℃ and 100 r/min, adding 49.5kg of zinc hydroxide, stirring for 1h, heating to 120 ℃, simultaneously vacuumizing and devolatilizing, and reacting for 4h under the vacuum degree of 0.04Mpa to obtain metal polyphosphate 2;

(2) preparation of ammonium polyphosphate metal ion double salt

Adding 19.15kg of metal polyphosphate 2 and 132kg of diammonium hydrogen phosphate into a kneader, stirring for 0.5h, adding 134.9kg of phosphorus pentoxide, stirring, heating, introducing ammonia, and reacting at 250 ℃ for 3h, wherein the ratio of the amount of introduced ammonia to the reactant is 3m per 100 kg of reactant ³ Per hour, at 200 deg.C for 3h, ammonia per 100 kg of reactants 1.5m ³ And/h, obtaining crystal II type ammonium polyphosphate metal ion double salt.

Example 3

(1) Preparation of metal polyphosphates

Adding 142kg of phosphorus pentoxide into a reactor, stirring at 50 ℃ and 100 r/min, adding 37kg of calcium hydroxide, stirring for 1h, heating to 120 ℃, simultaneously vacuumizing and devolatilizing, and reacting for 4h under the vacuum degree of 0.08Mpa to obtain metal polyphosphate 3;

(2) preparation of ammonium polyphosphate metal ion double salt

Adding 17.65kg of metal polyphosphate 3 and 132kg of diammonium hydrogen phosphate into a kneader, stirring for 0.5h, adding 134.9kg of phosphorus pentoxide, stirring, heating, introducing ammonia, and reacting at 250 ℃ for 3h, wherein the ratio of the amount of introduced ammonia to the reactant is 3m per 100 kg of reactant ³ Per hour, at 200 deg.C for 3h, ammonia per 100 kg of reactants 1.5m ³ And obtaining crystal II type ammonium polyphosphate metal ion double salt.

Example 4

(1) Preparation of metal polyphosphate

Adding 142kg of phosphorus pentoxide into a reactor, stirring at 50 ℃ at the speed of 100 r/min, adding 26kg of aluminum hydroxide, stirring for 1h, heating to 120 ℃, simultaneously vacuumizing and devolatilizing, and reacting for 4h at the vacuum degree of 0.04Mpa to obtain metal polyphosphate 4;

(2) preparation of ammonium polyphosphate metal ion double salt

Adding 25.2kg of metal polyphosphate 4 and 132kg of diammonium hydrogen phosphate into a kneader, stirring for 0.5h, adding 134.9kg of phosphorus pentoxide, stirring, heating, introducing ammonia, and reacting at 250 ℃ for 3h, wherein the ratio of the amount of introduced ammonia to the reactant is 3m per 100 kg of reactant ³ Per hour, at 200 deg.C for 3h, ammonia per 100 kg of reactants 1.5m ³ And obtaining crystal II type ammonium polyphosphate metal ion double salt.

Example 5

(1) Preparation of metal polyphosphates

Metal polyphosphate 1 was prepared in the same manner as in the step (1) of example 1 under a vacuum of 0.01 MPa.

(2) Preparation of ammonium polyphosphate metal ion double salt

Adding 34.2kg of metal polyphosphate 1 and 132kg of diammonium hydrogen phosphate into a kneader, stirring for 0.5h, adding 127.8kg of phosphorus pentoxide, stirring, heating, introducing ammonia, and reacting at 250 ℃ for 3h, wherein the ratio of the ammonia introduction amount to the reactant is that the ammonia introduction amount is 3m per 100 kg of the reactant ³ Per hour, at 200 deg.C for 3h, ammonia per 100 kg of reactants 1.5m ³ And obtaining crystal II type ammonium polyphosphate metal ion double salt.

Example 6

(1) Preparation of metal polyphosphates

Adding 142kg of phosphorus pentoxide into a reactor, stirring at 50 ℃ and 100 r/min, adding 13kg of magnesium hydroxide and 16 kg of aluminum hydroxide, stirring for 1h, heating to 120 ℃, simultaneously vacuumizing and devolatilizing, and reacting for 4h at the vacuum degree of 0.005Mpa to obtain metal polyphosphate 6;

(2) preparation of ammonium polyphosphate metal ion double salt

Adding 34.2kg of metal polyphosphate 6 and 132kg of diammonium hydrogen phosphate into a kneader, stirring for 0.5h, adding 127.8kg of phosphorus pentoxide, stirring, heating, introducing ammonia, and reacting at 250 ℃ for 3h, wherein the ratio of the ammonia introduction amount to the reactant is that the ammonia introduction amount is 3m per 100 kg of the reactant ³ Per hour, at 200 deg.C for 3h, ammonia per 100 kg of reactants 1.5m ³ And obtaining crystal II type ammonium polyphosphate metal ion double salt.

Comparative example 1

The polymerization degree of the general II-type ammonium polyphosphate sold on the market is more than or equal to 1000.

Comparative example 2

Commercially available II-type ammonium polyphosphate and aluminum hydroxide are proportioned according to a mass ratio of 100:1.9 and then mixed at a high speed to obtain an ammonium polyphosphate aluminum hydroxide mixture.

Comparative example 3

The melamine formaldehyde resin microcapsule ammonium polyphosphate is sold in the market.

Comparative example 4

Commercially available II-type ammonium polyphosphate and zinc oxide are proportioned according to a mass ratio of 100:1.9 and then are mixed at a high speed to obtain an ammonium polyphosphate zinc oxide mixture. At 180 deg.C under ammonia atmosphere (1 m per 100 kg of ammonia) ³ Ammonia gas) for 2 hours under stirring to obtain metal ion modified APP.

Table 1: examples and comparative examples comparison of Performance

Application examples 1 to 10

Examples 1 to 6 and comparative examples 1 to 4 correspond to application examples 1 to 6 and application comparative examples 1 to 4, respectively. All application examples were prepared in the following manner.

Use in pulp of: a certain amount of base paper is soaked in water and dispersed to form paper pulp with the concentration of about 1 percent, the product dosage in each embodiment and comparative embodiment is 20 percent of the base paper (namely 100 parts of the base paper, 20 parts of fire retardant, all parts by weight), a standard fiber fluffer is used for 10000 revolutions per minute and fluffing is carried out for 5 minutes, a proper amount of additives such as 0.2 percent polyacrylamide solution and 0.1 percent bentonite solution are added, the mixture is uniformly stirred and fluffing is continuously carried out for 2 minutes. The paper with the quantitative of about 200 g/square meter is made by a sheet making machine. Then vulcanizing at 100 ℃ and 2kg/cm by a flat vulcanizing machine ² Drying under pressure to obtain the flame retardant paper.

Table 2: application examples and application comparative examples Performance comparison

White water: the large amount of water discharged at the wire section of the paper machine during papermaking contains fine short fibers and other substances added to the pulp.

The product detection is carried out according to the embodiment and the application embodiment, and the detection indexes and the detection method are as follows:

1) pH: preparing 1% water solution, soaking in room temperature water bath for 30min, and testing with pH meter;

2) viscosity: preparing a 10% aqueous solution, soaking the solution in a room-temperature water bath for half an hour, and testing the solution by using NDJ-79;

3) solubility: according to the specification of 6.8 in HG/T2770-2020.

4) Granularity: according to the specification of 6.7.2 in HG/T2770-2020.

5) White water phosphorus content: according to the relevant provisions in HG/T2770-2020.

6) LOI (oxygen index): the method is carried out according to the regulation of GB/T2406.2-2009.

7) Carbon residue at 800 ℃: TGA/DSC produced by Mettler-Toliduo was used, and the temperature was raised to 800 ℃ under a nitrogen atmosphere of 50 ml/min and 20 ℃/min.

8) Vertical combustion: the sample size was 125mm by 13.5mm as specified in GB/T2408.

Analysis of the X-ray diffraction pattern of fig. 1 the ammonium polyphosphate metal ion double salt prepared by the process of this patent was crystalline form ii APP.

As seen from Table 1, the solubility of the metal ion double salts of ammonium polyphosphate in crystal form II prepared in the examples of the present application is less than 0.55g/100g, even less than 0.22g/100g in most cases; and the viscosity is substantially higher than 1.7mpa.s, even more so than 2.5mpa.s, but not too high, and is lower than 4 mpa.s; the pH value of the obtained crystal II type ammonium polyphosphate metal ion double salt is alkaline, and the granularity is uniform.

By combining table 1 and table 2, it is seen that although the water solubility of ammonium polyphosphate in melamine formaldehyde resin microcapsules is the lowest, the phosphorus content in the white water is as high as 137ppm, which indicates that the method of simply reducing the water solubility of the flame retardant has a certain effect, but the method is obviously not as good as the ammonium polyphosphate metal ion double salt in the examples in the application, not only the water solubility is slightly reduced, but also the phosphorus content in the white water is less than 85 ppm. The water solubility of the ammonium polyphosphate metal ion double salt is not obviously improved, but the phosphorus content in the white water is greatly reduced, so that the papermaking wastewater is in a processable range.

However, if the metal ions are only on the surfaces of the ammonium polyphosphate particles, the metal ions are not located on the main chain, and the phosphorus content in the white water is not reduced, as shown in application comparative example 2 and application comparative example 4.

Although the present invention has been described with reference to the preferred embodiments, it should be understood that various changes and modifications can be made therein by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. The type II ammonium polyphosphate metal ion double salt is characterized by having a structural formula as follows:

wherein, M ^z+ One or more of metal ions that are divalent or trivalent;

wherein a and n are average polymerization degrees, a is more than or equal to 2, and n is more than or equal to 100;

wherein z is the valence state of the metal ion;

wherein the metal ions are positioned on the main chain of the type II ammonium polyphosphate metal ion double salt.

2. The ammonium polyphosphate type II metal ion double salt according to claim 1, wherein the metal ion is one or more of zinc ion, magnesium ion, copper ion, aluminum ion, iron ion and calcium ion.

3. A method for preparing the form ii ammonium polyphosphate metal ion double salt according to claim 1 or 2, comprising:

preparing metal polyphosphate: reacting phosphorus pentoxide and metal hydroxide in a vacuum state for 0.5-10 h to obtain metal polyphosphate;

preparing ammonium polyphosphate metal ion double salt: stirring the obtained metal polyphosphate, diammonium hydrogen phosphate and phosphorus pentoxide, and reacting for 0.5-9 h under the condition of heating step by step and introducing ammonia to obtain the II-type ammonium polyphosphate metal ion double salt.

4. The method for preparing the type II ammonium polyphosphate metal ion double salt as claimed in claim 3, wherein in the step of preparing the metal polyphosphate, the molar ratio of the phosphorus pentoxide to the metal hydroxide is 1: 0.2-2.

5. The method for preparing the type II ammonium polyphosphate metal ion double salt as claimed in claim 3, wherein in the step of preparing the ammonium polyphosphate metal ion double salt, the molar ratio of the metal polyphosphate to the phosphorus pentoxide is 1: 1-30, and the amount of the diammonium phosphate used is the sum of the molar numbers of the metal polyphosphate and the phosphorus pentoxide.

6. The method for preparing the ammonium polyphosphate metal ion double salt form II according to claim 4 or 5, wherein the metal hydroxide comprises one or more of zinc hydroxide, magnesium hydroxide, aluminum hydroxide, copper hydroxide, iron hydroxide and calcium hydroxide.

7. The method for preparing the type II ammonium polyphosphate metal ion double salt according to claim 4 or 5, wherein in the step of preparing the metal polyphosphate, the metal hydroxide is added into phosphorus pentoxide under stirring, the mixture is stirred together and then heated to 60-200 ℃, and simultaneously, the mixture is vacuumized and devolatilized to 0.005-0.08 Mpa for reaction.

8. The method for preparing the type II ammonium polyphosphate metal ion double salt according to claim 4 or 5, wherein in the preparation of the ammonium polyphosphate metal ion double salt: adding metal polyphosphate and diammonium hydrogen phosphate, stirring, adding phosphorus pentoxide, and stirring, wherein the step-by-step heating and ammonia introduction comprises the steps of reacting at a higher temperature for 1-6 hours, and then reacting at a lower temperature for 0.5-3 hours, wherein the ammonia introduction amount at the lower temperature is smaller than that at the higher temperature.

9. The method for preparing the type II ammonium polyphosphate metal ion double salt as claimed in claim 8, wherein the higher temperature is 200 to 350 ℃, the lower temperature is 150 to 250 ℃, and the ratio of the ammonia passing amount at the lower temperature to the reactant ammonia passing amount at the lower temperature is 0.5 to 5m per 100 kg of the reactant ammonia passing amount ³ At higher temperatures, the ratio of ammonia throughput to reactants is 0 per 100 kg of reactants.2～2m ³ In terms of hours.

10. The flame-retardant paper is characterized by being prepared from the following raw materials:

the form ii ammonium polyphosphate metal ion double salt as described in claim 1 or 2;

paper pulp formed by soaking base paper in water and dispersing;

and (3) an additive.