Polyurethane is one of the most widely used polymer materials in the world, most of which are flexible polyurethane foam (FPUF) and rigid polyurethane foam (RPUF). The preparation of polyurethane materials is known for its low energy consumption. It has good heat insulation, insulation, high specific strength, oil resistance, cold resistance, moisture resistance and dimensional stability. It is an indispensable material for refrigeration, shockproof, gasket and insulation, but polyurethane material is flammable and soft in the air. The foam has an oxygen index of only 17 to 18, and generates a large amount of soot and highly toxic gases when burned. Therefore, research on flame retardant technology has been a hot spot in the research of polyurethane materials today. The most common method for imparting flame retardancy to PUF. The reaction type and the additive type flame retardant are added to the PUF. The reactive type flame retardant is a flame retardant element participating in the synthesis reaction of the polyurethane, and becomes a component in the main chain structure of the polyurethane material, exhibiting high flame retardancy and no precipitation appearance. The flame retardant elements are mainly halogen, nitrogen, phosphorus, antimony, etc.; the added flame retardant is a compound containing halogen, nitrogen, phosphorus, antimony, etc., and does not participate. The chemical reaction of the urethane system, but with the increase of the service life of the foam, the added flame retardant will be isolated from the product, thus reducing the flame retardant performance of the product. In the specific application, the appropriate flame retardant is selected according to the need to achieve different Flame retardant requirements.

Melamine is one of the important flame retardants in the processing of polymer materials due to its rich nitrogen source and good foaming properties. Melamine and its derivatives, which can be used alone or in combination. These flame retardants are halogen-free, low-toxic, non-corrosive, low-smoke, stable to heat and light, have good flame retardant effect and are inexpensive; their disadvantages are It is difficult to process the flame retardant plastic, has poor dispersibility in the substrate, and has strict requirements on particle size and particle size distribution. Melamine is a nitrogen-containing compound and has a synergistic effect with phosphorus-nitrogen in combination with a phosphorus-containing flame retardant, and has an excellent flame retardant effect. These flame retardants can be used in PA, PU, PO, PET, Ix3, PVC and epoxy resins as well as in textiles, coatings, wood and paper. Melamine and modified melamine, such flame retardants are widely used for their excellent thermal stability, durability and weather resistance, and excellent flame retardant effects. In this paper, the effects of melamine and modified melamine and synergistic effects with other flame retardants on the flame retardant properties, smoke suppression performance and physical and mechanical properties of polyurethane foams are reviewed.

1 Application of melamine in polyurethane foam

Melamine, also known as melamine, is a melamine, white crystalline powder, non-toxic, tasteless. Its structural formula is shown in Figure 1. It is a nitrogen-rich heterocyclic compound with remarkable thermal stability and flame retardancy. Environmentally friendly and excellent mechanical properties and resistance to aging and chemical resistance. Melamine is non-flammable, heated and sublimed, and decomposed when heated rapidly. The reason why it becomes an excellent flame retardant is mainly because the decomposition reaction occurs at 250-450 ° C, absorbs a large amount of heat, and releases ammonia to form various polycondensates; it can affect the melting behavior of the material and accelerate its carbonization into coke It is cheap, non-corrosive, non-corrosive to skin and eyes, and not carcinogenic. There are many chemical products with its main raw materials and additives, many of which have a wide range of applications. Flame retardant is one of the important auxiliaries for the processing of polymer materials. Melamine is an important flame retardant for polyurethane foams. Its unique flame retardancy and smoke suppression have attracted people's attention.

Melamine is a solid flame retardant J for molding polyurethane foam, which mainly exerts a flame retardant effect by decomposing endotherm and has been widely used in Europe. The melamine is ground into fine particles and added to the polyether polyol for use in flame retardancy of soft foam.

BASF has prepared hydrophilic and flame-retardant polyurethane soft foams using melamine as the main flame retardant. Another patent for the production of flame-retardant polyurethane foams by BASF uses expanded graphite as the main flame retardant in addition to melamine. The prepared foam is halogen-free, has good processability, thermal stability and low decolorization.

Polyether polyurethane materials are widely used in military and civilian applications. When the polyurethane material burns, the strong smoke can cause suffocation and seriously pollute the environment. On the military side, strong smoke tends to expose the launching position and launching trajectory of weapons to the enemy, and may block or affect the transmission of visible light, infrared light, laser and radar microwaves, affecting the propagation and reception of weapon guidance signals, and thus Affects weapon control and even makes the operation out of control. Zou Derong et al studied the fogging problem of polyether polyurethane elastomer for thermal insulation of solid rocket motor. When melamine was used as flame retardant and smoke suppressant for polyurethane, its smoke elimination effect was very good. The main reason is that melamine absorbs heat at high temperature and decomposes nitrogen, which promotes the formation of foamed carbon protective layer and effectively inhibits further combustion of the product. At the same time, the decomposition products of polyurethane and melamine can form a series of acids (such as HNO, HNO). The polyether polyurethane is further dehydrated and carbonized, and the decomposition products of HNO and HNO can act as a free radical terminator in the gas phase, interfere with the combustion chain, etc., and prevent the continuation of combustion.

Melamine is used as an additive flame retardant in polyurethane foams. If a good flame retardant effect is to be achieved, it often requires a large amount of addition, which will inevitably affect other properties of the polymer. Therefore, in actual production, melamine is usually used. Other flame retardants are used together.

Liu Xinmin et al. used melamine for the flame retardancy of polyurethane rigid foams. It was found that even if the amount of melamine was 50% of the amount of polyether, the flame retardant requirement could not be achieved (GB8624-88B2 grade), although the foam increased with the amount of melamine. The burning rate of plastics has decreased. Melamine is a solid flame retardant. The higher dosage makes it difficult to disperse and mix, and reduces the compressive strength of the foam. When the flame retardant effect is reached, the compressive strength can no longer meet the requirements, and the melamine flame retardant may block the foaming machine. Pipes, so melamine is not easy to use in large quantities. However, when it was used with melamine and Al(OH), DMMP, TCEP, it was found that only a small amount of the majority of the combination can meet the B2 requirements. It shows that there is a synergistic effect between melamine and other flame retardants. The synergistic effect can reduce the amount of flame retardant and achieve better results.

It has also been studied to use pulverized and surface-treated melamine dispersed in polyether polyol, and adding a composite flame retardant containing bromine, chlorine and phosphorus to obtain a flame retardant polyurethane soft foam with an oxygen index of 26, and physical properties. Unaffected to meet the flame retardancy requirements of car seats. According to GB 8332, the burning time is less than 60 s and the combustion range is less than 50 mm. When melamine is used only, even if the dosage is 50 parts, the burning time of the foam is still 61 s, and the burning range is 60 mm_8 J.

2 Application of modified melamine in polyurethane foam

Melamine is a solid flame retardant. When it is directly added to polyurethane foam, some flame retardants will migrate during use, resulting in poor durability of flame retardant. It may affect the processing performance due to poor mixing effect. Part of the physical and mechanical properties of polyurethane materials. In this context, in order to fully play the role of melamine and avoid its adverse effects, more researchers have modified it to synthesize a series of melamine derivatives.

The new melamine polyphosphate self-expanding flame retardant synthesized by Yan Junjun has the characteristics of no halogen, low smoke and low toxicity; it is compatible with polyurethane plastics and has little effect on its performance; it can be separated by heat and liberated water and Melamine, which acts as a fire retardant, can produce P-N synergistic flame retardant effect, can form a uniform and dense carbon layer, has good heat insulation, oxygen barrier, flame retardant and smoke suppressing effect, and can effectively prevent dripping To stop the spread and spread of the flame.

The melamine pyrophosphate synthesized by Kasowski MM in the United States is applied to the flame retardant of polyurethane to achieve a good flame retardant effect. The synthesis method is as follows: heating 37.9 g of melamine and 10.4 g of paraformaldehyde at 90 ° C (dissolved) In 170 grams of water) l5-30rain, and then the resulting methylol melamine solution was added to the water containing pyrophosphoric acid in 3-5 minutes to form melamine pyrophosphate. The carbamoylmethylated melamine polyol prepared from amino resin and hydroxypropyl carbamate is a powder having a hydroxyl value of 359 mgKOH/g, a functionality of about 2.3, and a melting point of 8O-85-C. The sucrose polyether polyol is mixed and used to prepare polyurethane rigid foam by water foaming, which can significantly improve the flame retardancy and thermal stability of the polyurethane.

The structure of tris(2,3.dibromopropyl)isomelamine is similar to that of the polyurethane structural unit, so the dispersibility in the polyurethane foam is good, and the flame retarding effect is excellent. The microencapsulation of flame retardant is also applied in polyurethane materials, such as a flame retardant which is easy to decompose, microencapsulated with a polymer to prevent volatilization, so that the base material does not fade, and at the same time, physical properties can be improved.

Some people have different factors such as different coating, dosage, particle size and synergistic effect with other flame retardants on the flame retardant properties, mechanical properties and smoke suppression properties of soft polyurethane foams. In the study of the influence, it was found that the melamine-formaldehyde resin/zinc borate double-layer capsule coated microcapsule red phosphorus has the best flame retardancy in polyurethane, the oxygen index is up to 34.5, and the melamine-formaldehyde resin capsule is coated. The decrease in mechanical properties of the microcapsule red phosphorus polyurethane foam material is small.

 It is patented that in a highly reactive polyether polyol medium, melamine, cyanoguanidine, formaldehyde monomer is dispersed or partially grafted into a highly reactive polyether polyol structure, the reaction temperature is between 40 and 160 ° C, and the reaction time is 5 After 16 hours, MPP with a particle size of 10 to 200 nm was obtained, and its viscosity was lower than 2300 mpa. s/25oC, density 1.120 to 1.126g/m1. The MPP is compounded with a highly reactive polyether polyol and an auxiliary agent, and the synthesized high resilience polyurethane foam has an oxygen index of I>28, a smoke density of ≤60%, and a rebound rate of I>60%.

A polyurea polyol emulsion ̈ can be prepared by polymerizing liquefied melamine and isocyanate in a polyether polyol, and the polyurea particles are stably dispersed in the emulsion. The high resilience polyurethane soft foam prepared by using the aminopolyurea polyol as a main raw material has obvious improvement in open porosity and resilience of the foam compared with the conventional polymer polyether polyol, even without adding a flame retardant. Excellent flame retardant effect. The aminopolyurea polyol has a foam oxygen index of 24 and the foam has self-extinguishing properties in accordance with ASTM 1692. In addition, thermogravimetric analysis shows that the melamine polyurea polyol is advantageous for improving the thermal stability of the polyurethane foam. In short, the honey aminopolyurea polyol can simultaneously introduce a molecular structure of melamine (or its derivative) and a structure with certain flame retardant properties such as polyurea molecules in the foam molecular structure, and has a good flame retardant effect and does not change with time. Evaporation and flame retardant volatilization and migration are reduced, and the prepared flame retardant has high oxygen index and does not contain halogen, phosphorus, antimony and other elements. The material has good fluidity, low acid value, less water and low smoke. Has good uniformity and mechanical strength. It can be seen that the amino polyurea polyol has great development potential.

With the continuous expansion of the application field of polyurethane flame retardant materials, its development has entered a period of rapid development. In recent years, due to environmental considerations, flame retardants are developing in the direction of high efficiency, low smoke, low toxicity and low cost, and at the same time, it is required to not significantly affect the processing properties and physical and mechanical properties of the polymer, so the performance is excellent. Halogen-free flame retardants, especially halogen-free, low-smoke, low-toxic flame retardants will become one of the hot spots in flame retardant research in recent years. The melamine flame retardant is a large series of halogen-free flame retardants. The melamine-based polymer is used for the flame retardancy of polyurethane, and its effect is more obvious, and the flame retardant is halogen-free, low-toxic, non-corrosive, low-smoke, and The heat and light are stable and the price is low, so it has good economic benefits.

At present, melamine-based flame retardants and their flame retardant technologies have been extensively researched and developed, opening a new chapter in the development of flame retardant technology. Due to the unique properties of the polymers of such flame retardants, their development potential and application prospects are enormous.