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Intumescent Flame-Retardant Epoxy Resin Modified by Ammonium Polyphosphate/Melamine/Sodium Alginate

Epoxy resin (EP) is a high‑performance thermosetting polymer widely used for electrical encapsulation, structural composites, adhesives, and industrial coatings because of its outstanding mechanical strength, chemical resistance, and insulating properties. However, neat epoxy resin is highly flammable; it burns vigorously with molten dripping and releases large amounts of heat and toxic fumes, severely limiting its use in fire‑safety‑critical fields. Developing halogen‑free, sustainable intumescent flame‑retardant epoxy systems has therefore become a major research hotspot for material formulators.

Traditional intumescent flame‑retardant (IFR) formulations consist of three core components: an acid source, a blowing agent, and a carbon source. Ammonium polyphosphate (APP) acts as an acid source, and melamine powder serves as a nitrogen‑rich blowing agent. At the same time, pentaerythritol and other petroleum‑based polyols are normally used as char‑forming agents. To reduce dependence on fossil resources, researchers are exploring renewable biomass alternatives. Sodium alginate‑assisted intumescent flame‑retardant epoxy resin with ammonium polyphosphate and melamine offers a promising green solution. This article reviews formulation data, fire‑test performance, thermal‑stability results, char‑layer morphology and condensed‑phase synergistic mechanism of the APP/melamine/sodium alginate IFR‑EP composite, based on published lab research findings

Basics of APP‑Melamine‑Sodium Alginate Intumescent Flame‑Retardant System

In this composite system:
  • Ammonium polyphosphate (APP): Phosphorus‑rich acid source. Upon heating, APP decomposes and releases polyphosphoric acid, which catalyzes dehydration and carbonization of polyol‑type substrates.
  • Melamine (MA): High‑nitrogen blowing agent. It decomposes and generates inert NH₃ and N₂ gases under heat, which foam and expand the forming char layer.
  • Sodium alginate (SA): Renewable biomass polysaccharide extracted from brown seaweed, used as a synergistic bio‑char source. Abundant hydroxyl groups enable efficient dehydration‑carbonization under acid catalysis, improving char‑layer density and residual‑char yield.
Neat epoxy resin (EP0) shows an LOI of merely 19.5 % and receives no UL‑94 fire rating. When adding 13 wt % APP/MA blend (12 % APP + 1 % MA, EP1), LOI rises to 26.3 % and reaches UL‑94 V‑2 grade. Adding only a small dosage of sodium alginate can further boost overall flame‑retardant performance without drastically raising total filler loading.

Formulation Screening: Optimum Sodium Alginate Loading

Laboratory‑prepared epoxy composite samples keep total APP/MA content fixed at 13 wt %, while sodium alginate dosage varies from 0.1 wt % up to 0.6 wt %:
  • SA = 0.1–0.2 wt %: LOI increases slightly; composite remains UL‑94 V‑2.
  • SA = 0.4 wt % (EP5, optimal formula): LOI reaches 27.2 %, and the epoxy composite successfully achieves UL‑94 V‑0 rating.
  • SA > 0.4 wt % (0.5 %, 0.6 %): LOI gradually drops and fire rating degrades back to non‑rated (NC). Excess sodium alginate disturbs the matched acid‑blowing‑char balance of the intumescent network.
Final optimal formulation: epoxy resin 86.6 wt %, APP 12.0 wt %, melamine 1.0 wt %, sodium alginate 0.4 wt %. Even a small addition (0.4 %) of bio‑based SA delivers critical upgrading from V‑2 to V‑0 without a high total‑additive burden.

Thermal‑Stability Analysis by Thermogravimetry

We performed thermogravimetric tests in air from 30 to 800 °C at a heating rate of 15 °C/min. Compared with EP1 (EP + APP/MA without SA), EP5 (with 0.4 % SA) delivers obvious thermal‑stability improvement:
  • Initial decomposition temperature shifts higher.
  • Char residue at 700 °C increases by 17.11 % relative to EP1.
  • Higher residual char demonstrates that sodium alginate effectively promotes condensed‑phase carbonization under APP acid catalysis, forming thermally stable carbonaceous barrier structures.

Cone Calorimeter : Fire‑Reaction Performance

The cone calorimeter is the key lab method for simulating real‑scale fire exposure. Test condition: heat flux 35 kW/m² according to ISO 5660‑1‑2002. Major performance data are summarized below:
  • Peak heat‑release rate (PHRR): EP5 reduces PHRR by 50.1 kW/m² compared with EP1, showing suppressed heat output during combustion.
  • Total heat release (THR): Relative to neat EP0, EP1 reduces THR by 8 %, while EP5 (with SA) cuts THR by 10 %.
  • Toxic‑gas suppression: Both CO and CO₂ emission‑peak values decrease significantly. CO peak drops from 0.054 g/s (neat EP) down to 0.024 g/s for EP5; CO₂‑release peak falls from 1.35 g/s down to 0.69 g/s. The time to reach gas‑emission maximum is obviously delayed, which helps reduce smoke‑toxicity hazards in fire scenarios.

Char‑Layer Morphology & Condensed‑Phase Synergistic Mechanism

Digital photos of post‑combustion residues reveal huge differences among samples:
  • Neat EP0: Almost no residual char left; sample is completely burned‑through.
  • EP1 (EP + APP/MA without SA): Intumescent char forms but contains many open holes and a loose, fragile structure. Hot gas and oxygen can easily penetrate the barrier layer.
  • EP5 (EP + APP/MA + 0.4 % SA): Produces thick, compact, continuous, multi‑layered intumescent char with few holes. This dense physical barrier blocks heat transfer, isolates oxygen and restricts volatile‑flammable‑gas escape, which is the core reason for improved flame retardancy.
FT‑IR spectra of char residues further verify the synergistic mechanism. Characteristic absorption bands for P‑O‑C and P‑O‑P prove phosphate structures originating from APP decomposition. Meanwhile, characteristic peaks for carboxylate groups confirm sodium‑alginate participation in char‑network construction. Sodium alginate acts as a condensed‑phase synergist: under polyphosphoric acid catalysis from APP, SA undergoes dehydration‑carbonization; melamine releases inert gas to expand char volume. Together they build a high‑quality protective intumescent barrier.

Key Advantages of Sodium‑Alginate‑Modified Intumescent Epoxy System

  • Low‑dosage bio‑synergist: Only 0.4 wt % sodium alginate achieves UL‑94 V‑0 upgrading; does not require large‑quantity extra filler addition.
  • Halogen‑free flame retardant: APP/melamine/SA belongs to the halogen‑free phosphorus‑nitrogen‑biomass IFR class, with low‑smoke and low‑toxicity combustion products.
  • Renewable raw‑material source: Sodium alginate derives from sea‑weed biomass, conforming to green‑chemistry and carbon‑reduction trends.
  • Condensed-phase-dominated mechanism: Improves char‑layer quality rather than merely gas‑phase inhibition, effectively lowering heat release and toxic‑gas output.
Formulators should note that excessive sodium‑alginate loading will damage intumescent balance and degrade fire‑safety performance. Precise dosage control is critical for practical epoxy‑resin formulation work.

Potential Industrial Application Scenarios

  • Fire‑safe epoxy adhesives and potting resins for electrical‑electronic components
  • Halogen‑free flame‑retardant epoxy composite substrates
  • Environment‑friendly intumescent epoxy‑based protective coatings
  • Green‑construction‑related epoxy structural materials requiring fire‑safety certification

conclusion

Biomass sodium alginate can serve as an efficient condensed-phase synergistic char source for ammonium polyphosphate/melamine intumescent flame-retardant epoxy resin. At a low addition of only 0.4 wt %, the composite achieves LOI 27.2 % and UL‑94 V‑0 rating. TG and cone‑calorimeter results confirm higher char residue, reduced peak heat‑release rate and suppressed toxic‑gas emission. Sodium alginate cooperates with APP acid‑catalysis and melamine gas‑blowing effect to generate compact continuous intumescent char‑layer, delivering obvious fire‑safety promotion.

This research provides a feasible route for developing green, halogen‑free intumescent epoxy formulations using renewable marine‑biomass resources. In real‑world formulation development, formulators need to balance flame‑retardant efficiency, processing properties and mechanical performance of final epoxy‑based products.

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