Epoxy resin (e.g., E51) is a versatile polymer widely used in coatings, adhesives, electronics, and construction due to its excellent mechanical strength, adhesion, and chemical resistance. However, its low limiting oxygen index (LOI ≈19.8%) and high flammability restrict applications in fire-sensitive scenarios. Melamine powder, a nitrogen-containing heterocyclic compound, is increasingly used as a flame retardant for epoxy resin, offering low cost, low hygroscopicity, and minimal leaching.
This article details Melamine affect the properties of epoxy resin’s flame retardancy, mechanical performance, thermal behavior, and microstructure for material scientists, chemical engineers, and industry professionals.
Melamine powder interacts with epoxy resin E51 (cured with triethylenetetramine) to modify multiple performance aspects, with effects highly dependent on addition level (0–15 phr):
Limiting Oxygen Index (LOI) Improvement: Pure epoxy resin has an LOI of 19.8% (highly flammable). As melamine content increases, LOI rises linearly:
Flame-Retardant Mechanism: Melamine powder acts via gas-phase inhibition—during combustion, it decomposes to release non-flammable gases (N₂, NO₂), diluting oxygen and combustible vapors, thereby suppressing flame propagation.
Tensile Strength: Slightly decreases with increasing melamine content. Pure epoxy resin has a tensile strength of 32.8 MPa; at 15 phr melamine, it drops to 28.6 MPa (13% reduction). The decline is attributed to poor dispersion and interfacial gaps between melamine particles and the epoxy matrix.
Impact Strength: Unexpectedly increases marginally. Pure epoxy resin has an impact strength of 11.2 kJ/m²; at 15 phr melamine, it rises to 13.4 kJ/m² (19.6% improvement). This may be due to melamine particles acting as “crack arresters” in the matrix.
Melamine powder increases the epoxy resin’s T₉ (measured via DMA tanδ). Pure epoxy resin has a T₉ of 67.1℃; at 10 phr melamine, T₉ rises to 87℃; at 15 phr, it reaches 90℃. This indicates enhanced matrix rigidity and heat resistance.
Initial Decomposition Temperature: Advances slightly. Pure epoxy resin decomposes at 340℃; at 15 phr melamine, initial decomposition starts at 304℃. This is because melamine decomposes earlier, releasing flame-retardant gases.
Char Residue: Moderately increases. At 600℃, pure epoxy resin has a char residue of 5.9%; at 15 phr melamine, it reaches 14.2%. More char residue helps block heat and oxygen in the condensed phase.
Exothermic Peak Temperature: Rises from 100℃ (pure epoxy) to 112℃ (15 phr melamine), indicating slower curing and more stable crosslinking.
Pure epoxy resin (EP) has a dense, compact microstructure with no visible gaps.
Epoxy resin with 15 phr melamine (EP-15%) shows obvious melamine agglomeration. Poor dispersion leads to interfacial defects, which are the main reason for limited flame-retardant efficiency and a slight loss in tensile strength.
1. Mix melamine (0–15 phr) with epoxy resin E51, stir uniformly to disperse.
2. Add triethylenetetramine curing agent at a mass ratio of E51:curing agent = 4:1, stir well.
3. Remove bubbles in a vacuum drying oven, cast into molds, and cure at room temperature for 24 hours.
4. Key Note: Improve melamine powder dispersion by pre-treating with coupling agents (e.g., silane) or ultrasonic mixing to reduce agglomeration.
Q1: Why does melamine cause slight tensile strength loss?
A1: Poor dispersion of melamine in epoxy resin leads to agglomeration and interfacial gaps, reducing the matrix’s compactness and load-bearing capacity.
Q2: Can melamine make epoxy resin reach V-0 burning grade?
A2: No. Even at 15 phr, melamine only achieves V-1 grade. To reach V-0, combine melamine with phosphorus-based flame-retardants (e.g., ammonium polyphosphate) to achieve synergistic effects.
Q3: How to improve melamine dispersion in epoxy resin?
A3: Use silane coupling agents to modify the melamine surface, or apply ultrasonic mixing (30–60 minutes) during preparation to break agglomerates.
Q4: Does melamine affect epoxy resin’s curing speed?
A4: Slightly slows curing—exothermic peak temperature rises from 100℃ to 112℃. Extend the curing time by 2–4 hours, or increase the curing temperature to 40℃, to achieve complete crosslinking.
The core conclusion
Melamine powder effectively improves the flame-retardant performance and thermal stability of epoxy resin E51 with minimal mechanical property loss, but its dispersion quality limits its flame-retardant efficiency.
Melamine powder is a practical flame retardant for epoxy resin E51, improving LOI to 25.9% (V-1 grade), increasing T₉ to 90℃, and maintaining acceptable mechanical properties (only 13% tensile strength loss at 15 phr). Its low cost, low hygroscopicity, and minimal leaching make it suitable for industrial production.
The main limitation is poor dispersion—agglomeration limits flame-retardant efficiency and mechanical performance. Future improvements should focus on surface modification of melamine or on compounding with other flame-retardants to achieve synergistic effects. For most flame-retardant applications, 10–15 phr of melamine is a cost-effective choice, balancing performance and cost.
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