Melamine Cyanurate (MCA): Once A Halogen-Free Flame Retardant, Its Advantages & Disadvantages Analysis

Melamine Cyanurate (MCA): Once A Halogen-Free Flame Retardant, Its Advantages & Disadvantages Analysis

1.   Overview

Melamine cyanurate (MCA), with the molecular formula C₆H₃N₉O₃, is a compound prepared by acid-base salting reaction of melamine and cyanuric acid, MCA has the advantages of good heat resistance, water resistance, low smoke, low price, etc. Currently, melamine cyanurate has been used to improve the flame retardancy of PA6, PP and PBT. Currently, melamine cyanurate is widely used in improving the flame retardant and anti-drip properties of PA6, PP and PBT, and has achieved good results.

MCA is a large planar molecule formed by the continuous expansion of melamine and cyanuric acid through hydrogen bonding. The formation of this molecule is due to the fact that melamine and cyanuric acid molecules are triazine planar conjugated molecules, melamine has three complex groups, while cyanuric acid has three sec-amino or hydroxyl groups (reciprocal isomers), and both of them form nine hydrogen bonds in three directions, which self-assembles into a huge planar hydrogen bonding network with a regular structure, forming the MCA. The two form nine hydrogen bonds in three directions, thus self-assembling into a large planar hydrogen bonding network with a regular structure, forming the MCA molecule.

MCA is a white powder solid, odorless and tasteless, with high nitrogen content (48%±2%) and density of 1.5g/cm³. Due to its lamellar structure, it has a lubricating feeling. The compound is hydrophilic, insoluble in water, in alcohols, esters and ether solvents, solubility is very low, but can be well dispersed in oil media with high stability, 320 ℃ began to sublimate but do not decompose, decomposition temperature of about 440 ~ 450 ℃.

MCA is a nitrogen-containing fine chemical products developed abroad in the late 1970s, China began to develop MCA products at the beginning of the 1980s in the 4th m phosphorus-based positive combustibles, first used in mechanical lubricants, along with the development of the engineering plastics industry, the market demand for MCA is gradually increasing, and in the second half of 2017, there was once an oversupply of MCA in the market.

2. Synthesis method of melamine cyanurate

The production process method of MCA is mainly the aqueous phase salt formation method of melamine and cyanuric acid. Melamine and cyanuric acid are put into water according to the equimolar ratio, and the ratio of water to the total mass of melamine and cyanuric acid is 100:(8~12), heated to 85~95℃, reacted for about 1h, and then filtered to get the MCA product.

3. Characterization of melamine cyanurate

The following figure shows the infrared spectra of MCA, in which 3391 cm^-1 is the telescopic vibration absorption peak of N-H, and

3230 cm^-1 is the characteristic absorption peak of hydroxyl group, and the existence of multiple absorption peaks in the range of 3000~2500 cm- proves the existence of hydroxyl bonding peaks, which is caused by the hydrogen bonding effect in the MCA molecule, 1738 cm^-1 is the telescopic vibration absorption peak of the C=O group, 1664 cm^-1 is the telescopic vibration absorption peak of the C=N structure on the triazine ring, and 1449 cm^-1 is the telescopic vibration absorption peak of the C=N structure on the triazine ring and 1449 cm-1 for the stretching vibration absorption peak.

The heat loss curve of MCA under N₂ atmosphere is shown in Fig. The temperature of 1% decomposition is more than 320℃.

4. Flame-retardant mechanism of melamine cyanurates

MCA mainly plays the role of gas-phase flame retardant when it is flame retardant. MCA absorbs heat and sublimates and decomposes during the combustion process, taking away the heat, and at the same time, it decomposes and emits inert gases such as ammonia, which dilute the oxygen and flammable gases, so as to make the material flame retardant according to the high performance. Because of its gas-phase flame retardant mechanism, also makes MCA flame retardant nylon (no glass fiber) in the heating to 600 ℃ C, the amount of carbon residue is generally 0.

5. Main application fields of melamine cyanurate (MCA)

MCA is commonly used in the flame retardant of nylon, rubber, phenolic resin, epoxy resin, acrylic emulsion, PVC and other olefins, with low dosage and good flame retardant effect. At the same time, MCA is also a good lubricant additive, coating matting agent, plating grade plastic additives.

(1)   MCA flame retardant nylon

Adding 8% MCA in PA6 can make the nylon reach UL94 V-0 level, but it is not stable. Usually in 5 test samples, there are 1~2 will produce the phenomenon of dripping ignition, even if the increase in the amount of MCA can not change the situation. However, nanoscale MCA has a more stable PA6 flame retardant effect, the addition of 8% MCA can make PA6 reach UL94 V-0 grade.

The tensile strength of MCA flame retardant PA6 system is lower than that of pure PA6 system, only a slight decrease; its impact strength is about 13% lower than that of pure nylon products; however, its flame retardant performance is significantly improved, LoI value from 22.3% to 28.5%, UL94 flame retardant level from V-2 to V-0: melt flow rate increases slightly, but nanoscale MCA will lead to a decrease in melt flow rate and a decrease in fluidity. However, nanoscale MCA will lead to a decrease in melt flow rate and a decrease in fluidity.

Adding MCA into glass fiber (GF)/PA66 can make GE/PA66 composites reach V-0 level of UL 94; MCA crystals are uniformly dispersed in nylon matrix, and during combustion, MCA decomposes through its own heat absorption, generates non-combustible gas, realizes gas-phase flame retardant effect, and leaves nanoscale pores on the residual carbon surface of composite materials.

(2) MCA synergistic flame retardant PET

MCA has been applied to flame retardant PET adhesive tapes. By adding MCA into the adhesive solution of PET adhesive tapes, PET adhesive tapes can have better flame retardant properties, i.e., charring and curling and self-extinguishing when ignited.

In addition, MCA was also used to prepare MCA blended flame retardant PET, when the phosphorus content was 0.6% and the MCA addition was 5%, the limiting oxygen index of the composite material reached 28% and UL 94 V-0. This shows that the addition of MCA improves the flame retardant properties of PET.

(3) MCA flame retardant rigid polyurethane foam (RPUF/MCA)

MCA was used as a flame retardant to prepare RPUF/MCA. 30 parts of MCA made the RPUF/MCA composites reach the V-1 level of UL94, with an ultimate oxygen index of 22.0%. the addition of MCA reduced both the initial thermal decomposition temperature and the combustion smoke density of the composites, which effectively improved the composites flame retardant properties.

6. Advantages and disadvantages of MCA

1. High thermal stability: MCA can maintain stable flame retardant effect at high temperature, not easy to decompose, and can effectively reduce the combustion performance of the material.

2. Excellent flame retardant effect: MCA can form a protective layer in the material, effectively isolate the source of combustion, reduce the flame spreading speed, and improve the flame retardant grade of the material.

3. Halogen free: MCA does not contain toxic elements such as bromine and chlorine. 4.

4. Good heat aging resistance: MCA has good heat aging resistance in the material, which can prolong the service life of the material.

5. Small impact on material properties: MCA has a small impact on the physical properties and processing properties of the material, and has a wide range of applications.

However, there are some disadvantages of melamine cyanurate. Firstly, its production process may produce toxic gases, causing pollution to the environment. Second, the price of melamine cyanurate is high, which increases the production cost of the product. In addition, at high temperatures, melamine cyanurate may release toxic gases which may affect human health.

Therefore, when using melamine cyanurate, it is necessary to pay attention to its environmental and health impacts and seek more environmentally friendly and safer alternatives to promote the sustainable development of flame retardants. At the same time, with the increasing awareness of environmental protection, green and environmentally friendly halogen-free flame retardants will become the future development trend.