One Application Case of Halogen-Free Flame Retardants: Innovative Application of MCA in Nylon Materials

One Application Case of Halogen-Free Flame Retardants: Innovative Application of MCA in Nylon Materials

Melamine cyanurate (MCA), as a representative nitrogen-based halogen-free flame retardant, holds a significant position in nylon flame retardant modification due to its environmental friendliness, high efficiency, and minimal impact on material properties. This article systematically elaborates on its application value from the aspects of MCA's basic characteristics, flame retardant mechanism, application cases in nylon, and innovative technologies.

I. Basic Overview of MCA Flame Retardant
MCA is an adduct formed by melamine and cyanuric acid through hydrogen bonding. Its molecular formula is C₆H₉N₉O₃, with a molecular weight of 255.2. It is a white crystalline powder or granule, odorless, with a melting point around 350°C. It exhibits good thermal stability, showing almost no thermal loss during prolonged heating below 300°C. It begins endothermic decomposition around 350°C and further sublimates at 440-450°C. MCA is difficult to dissolve in water but soluble in organic solvents like ethanol and formaldehyde. It is weakly acidic and possesses excellent lubricity (friction coefficient only 0.1-0.15) and dispersibility. As an eco-friendly flame retardant, MCA is halogen-free, does not produce toxic hydrogen halide gases during combustion, and has low smoke density. It complies with EU ROHS and REACH directives. Its high nitrogen content (up to 59%) is the basis for its flame retardant effect, achieved through synergistic multi-mechanisms.

II. Flame Retardant Mechanism of MCA: Synergistic Multi-Pathway Action
MCA's flame retardant action is achieved through synergistic effects in the gas phase, condensed phase, and cooling:

1. Gas Phase Flame Retardancy: MCA decomposes at high temperatures, releasing non-combustible gases such as nitrogen, ammonia, and water vapor, effectively diluting the concentration of oxygen and flammable gases and inhibiting the combustion chain reaction.

2. Condensed Phase Flame Retardancy: MCA alters the thermal degradation path of nylon, promoting rapid surface dehydration and charring, forming an intumescent, foamed carbon layer. This carbon layer covers the substrate surface, providing heat insulation and oxygen barrier effects, preventing further decomposition of internal material and escape of combustibles.

3. Cooling and Melt-Dripping Effect: The decomposition process of MCA is endothermic, lowering the surface temperature of the material. Simultaneously, it improves the melt flowability of nylon, promoting melt dripping during combustion, which carries away heat and combustibles, further suppressing burning.

III. Application Cases and Performance Characteristics of MCA in Nylon Materials
MCA is particularly suitable for non-reinforced nylon 6 and nylon 66. Its typical application cases are as follows:

1. Flame Retardant Effect in Pure Nylon:

• Nylon 6 (PA6): Requires the addition of approximately 10% MCA to achieve the UL94 V-0 flame retardancy standard. For example, Mitsubishi Engineering-Plastics Co., Ltd. (Japan) developed a PA6/MCA composite material achieving V-0 rating while maintaining relatively high impact strength and flexural strength.

• Nylon 66 (PA66): Due to better synergy with MCA, only about 8% addition is needed to achieve V-0 rating. MCA promotes the formation of a denser char layer in PA66, resulting in higher flame retardant efficiency.

2. Key Data Comparison:

3. Innovative Application in Composite Materials:

• Reinforced Composites: In glass fiber (GF) reinforced nylon, the MCA addition amount needs adjustment (e.g., below 15%) because GF may cause a "candlewick effect" affecting flame retardancy. It is usually used in combination with other additives.

• Multi-Component System Composites: A research team from Sichuan University used grafted N-methylol acrylamide as a compatibilizer to flame retard a PA6/PP/wollastonite composite with 10% MCA, achieving an oxygen index of 31% and good mechanical properties (tensile strength 54.1 MPa, impact strength 59.7 J/m).

4. Breakthrough in In-situ Polymerization Technology:
Traditional physical blending methods have drawbacks like poor dispersion and easy precipitation of the flame retardant. Zhejiang Xinli New Material Co., Ltd. developed an in-situ polymerization technology. This technology combines MCA (addition amount only 1.5-3%) with nylon monomers (e.g., adipic acid, hexamethylenediamine) at the salt-forming stage and enhances thermal stability through metal ion (e.g., Fe³⁺) complexation. This technique introduces MCA into the polymer chain via chemical bonding, achieving "intrinsic flame retardancy." The resulting nylon 66 material achieves V-0 rating (LOI ≥ 37.8%), and its mechanical properties are significantly superior to blended materials (tensile strength ≥ 83.8 MPa, flexural strength ≥ 124.6 MPa).

IV. Application Advantages and Precautions for MCA

1. Core Advantages:

Environmental Safety: Halogen-free, low toxicity, low smoke, low corrosion.

Good Comprehensive Performance: Minimal impact on nylon's electrical properties (e.g., high CTI value), mechanical properties, and colorability.

Processing Friendly: High thermal stability, suitable for injection molding (temperature 240-260°C) and extrusion processes, with low tendency for mold sticking.

2. Application Precautions:

Moisture Control: Requires vacuum drying at 110°C for 8 hours before processing to reduce moisture content below 0.1%, preventing moisture absorption from affecting performance.

Filler Influence: Avoid using fillers like calcium carbonate that may interfere with flame retardancy, control glass fiber content in GF-reinforced systems.

Synergistic Blending: MCA has synergistic effects with phosphorus-based flame retardants (e.g., red phosphorus) and phenolic compounds, which can improve flame retardant efficiency and reduce costs.

3. Main Disadvantages of Using MCA:

• Flame Retardant Efficiency Limited by Substrate and Processing Conditions: MCA's gas phase flame retardancy relies on rapid release of non-combustible gases at high temperatures. However, in components with large wall thickness or insufficient shear heat, gas diffusion is hindered, easily leading to a "core burn-through" phenomenon, making it difficult to pass the UL 94 5VA rating. Meanwhile, the melt-dripping effect can become a secondary ignition source in scenarios requiring Glow-Wire Ignition Temperature (GWIT) ≥ 775°C, necessitating additional anti-drip additives.

• Sensitivity to Reinforced Systems, Significant Glass Fiber "Candlewick Effect": When glass fiber content > 15%, MCA addition needs to be increased by 3–5 percentage points to maintain V-0, and the improvement in oxygen index drops sharply. Significant differences in flame retardancy between fiber-oriented layers and resin-rich areas can lead to "streak burn-through" defects, limiting its use in high-load structural parts.

• Narrow Processing Window, Metal Impurities Easily Catalyze Decomposition: MCA undergoes significant sublimation above 330°C, leading to die buildup and severe "white smoke" at vents. Residual transition metal ions like Fe³⁺ and Cu²⁺ in raw materials can catalyze premature decomposition, causing melt viscosity to drop by over 20%. This results in flash, silver streaks during injection molding, and flame retardant loss can reach 5–7% after 8 hours of continuous production, requiring shutdown for cleaning.

• Surface Migration and Blooming Issues: The hydrogen bonding between MCA and nylon is weak. Under long-term damp heat (85°C/85% RH) or alcohol wiping conditions, the product surface is prone to "blooming" or precipitation of MCA. This leads to decreased contact resistance and poor adhesion for printing/coating, failing to meet requirements like insulation resistance ≥ 100 MΩ after 500 hours of 85/85 testing for electronic connectors.

• Smoke Suppression Advantage Not Absolute. Smoke Toxicity Rebounds at High Additions: When MCA addition is ≥ 12%, although the maximum specific optical density of smoke (Ds,max) is still lower than halogenated systems, the release of toxic gases like CO and cyanide (CN⁻) compounds increases multiple times. The conversion rate of nitrogen elements can be as high as 6%, posing a potential threat during fire escape. Assessment requires cone calorimetry and NBS smoke chamber tests combined, not just relying on UL 94 rating.

Therefore, in nylon systems with high glass fiber reinforcement, high GWIT requirements, high damp heat resistance needs, or high appearance demands, MCA often needs to be blended with phosphorus-based, silicon-based, or char-forming synergists to balance flame retardancy, mechanical properties, and long-term reliability.

V. Conclusion and Outlook
MCA, as an efficient and eco-friendly nitrogen-based flame retardant, demonstrates significant effectiveness in flame retarding nylon (especially PA6, PA66) through multiple mechanisms including gas phase dilution, condensed phase charring, and cooling/dripping. Its application is evolving from traditional physical blending towards in-situ polymerization technology, greatly improving flame retardant efficiency and overall material performance.
In practical applications, MCA is often blended with phosphorus-nitrogen flame retardants, which can also quickly impart stable and durable halogen-free flame retardant properties to thin-walled or flexible substrates like cables and leather. The high-dispersion, anti-blooming modified MCA-B launched by Yinsu Flame Retardant Company is specifically designed for such scenarios and has been validated in batch applications for LSZH cable sheathing and artificial leather coating.
In the future, with increasingly stringent environmental regulations and growing demand for high-performance electronic/electrical appliances and automotive components, innovative applications of MCA in nylon—such as developing lower-addition, higher-synergy blended systems and optimizing in-situ polymerization processes—will further expand its market space.
Through scientific selection of addition formulations and processing techniques, MCA can help nylon materials achieve the optimal balance between safety, environmental protection, and performance, providing key material support for sustainable development.