Rigid Polyurethane Foam (RPUF) is a widely used material in insulation, construction, and electronics due to its excellent thermal insulation and structural properties. However, its low limiting oxygen index (LOI ≈18%) and high flammability—accompanied by toxic fumes during combustion—pose significant safety risks. A melamine-based flame-retardant polyether polyol, a reactive flame retardant, addresses these issues by incorporating flame-retardant groups into the polymer backbone.
This article details its synthesis, key influencing factors, application performance, and practical guidelines for chemical engineers, foam manufacturers, and material formulators.
Reactive Flame Retardancy: Melamine’s nitrogen-containing heterocyclic structure is chemically bonded to the polyether chain, avoiding leaching or migration (a flaw of additive flame retardants).
Low Toxicity & Smoke: Unlike halogenated flame retardants, it releases non-toxic gases (N₂, CO₂) during combustion, reducing smoke and toxic emissions.
Cost-Effectiveness: Melamine is abundant and affordable, making the flame-retardant polyether suitable for large-scale industrial production.
RPUF’s inherent flammability (LOI ≈18%) restricts its use in fire-sensitive scenarios (e.g., building insulation, electrical enclosures).
A flame-retardant polyether derived from melamine replaces conventional polyether (e.g., 4110) to prepare flame-retardant RPUF, balancing insulation, mechanical properties, and fire safety.
Raw Materials: Melamine (analytical grade), 37% formaldehyde aqueous solution, co-initiators (triethanolamine/TEOA, sucrose, glycerol), epoxy propane (PO), catalysts (N,N-dimethylethanolamine), and potassium hydroxide (pH adjuster).
Equipment: 1L four-necked flask (for pre-reaction), 5L stainless steel high-pressure reactor (for PO polymerization), oxygen index tester, and viscometer.
Step 1: Prepare Melamine-Formaldehyde Low Condensation Resin
Add 37% formaldehyde to a four-necked flask equipped with a reflux condenser, and adjust the pH to 7–11 using 30% KOH or TEOA.
Add metered melamine, heat slowly to 70–90℃, and continue heating until the solution becomes transparent. Continue reacting for 4–6 hours, then cool to 25℃ to obtain the low condensation resin.
Step 2: Polymerize with Epoxy Propane (PO)
Add the melamine-formaldehyde resin, co-initiators (TEOA + sucrose), and catalyst to a 5L high-pressure reactor.
Purge the reactor with nitrogen to remove oxygen, heat to 70–90℃, and slowly add 30% of the total PO for pre-reaction (critical for reducing water interference).
After pressure stabilizes, heat to 100–105℃ for vacuum dehydration (1–2 hours) to eliminate residual water.
Control temperature at 105–110℃, add the remaining PO, and react until complete. Vacuum strip unreacted monomers to obtain the melamine flame-retardant polyether (brown viscous liquid).
Objective: Minimize water’s impact—water reacts with PO to form low-molecular-weight alcohols, reducing polyether functionality.
Optimal Temperature: 80℃.
At 70℃: Polyether hydroxyl value is 635 mg KOH/g, and viscosity is high (due to incomplete PO reaction).
At 80℃: Hydroxyl value rises to 787 mg KOH/g, and viscosity decreases (balanced reaction efficiency and water control).
At ≥90℃: Melamine-formaldehyde resin condenses excessively, leading to gelation and failed synthesis.
Melamine-formaldehyde resin alone produces high-viscosity polyether; co-initiators adjust functionality and processability.
TEOA: Reduces polyether viscosity significantly but slightly impairs dimensional stability.
Sucrose: Improves RPUF dimensional stability but lowers LOI.
Optimal Ratio: TEOA:Sucrose = 1:1 (mass ratio), balancing viscosity (5.05 Pa·s), LOI (22.2%), and dimensional stability (0.4% change at 80℃ for 48h).
Set to 510 mgKOH/g (adjusted from 430 mgKOH/g) to increase melamine resin content, enhancing flame retardancy without compromising processability.
Q1: Why use co-initiators instead of melamine resin alone?
A1: Melamine-formaldehyde resin alone produces ultra-high viscosity polyether (>20 Pa·s), which is difficult to mix with foaming components. Co-initiators (TEOA + sucrose) reduce viscosity to 5.05 Pa·s while adjusting functionality.
Q2: Can it replace all conventional polyethers for RPUF?
A2: Yes—under the same foaming conditions, it directly replaces polyether 4110 without modifying equipment or process parameters. For higher LOI (≥30%), add 20 parts flame retardant F for synergistic effects.
Q3: Does it affect RPUF’s thermal insulation?
A3: No. The flame-retardant polyether maintains RPUF’s density (30–35 kg/m³) and closed-cell structure, ensuring thermal conductivity remains comparable to conventional foam.
Q4: What causes gelation during synthesis?
A4: Gelation occurs at pre-reaction temperatures ≥90℃ (excessive condensation of melamine-formaldehyde resin) or insufficient dehydration (residual water reacts with PO). Control the temperature at 80℃ and ensure thorough vacuum dehydration.
Melamine flame-retardant polyether, synthesized with TEOA-sucrose co-initiators and 80℃ pre-reaction temperature, is a high-performance reactive flame retardant for rigid PU foam. It increases RPUF’s LOI by 2.8–4.1%, offers long-lasting flame retardancy (no leaching), and maintains mechanical/insulation properties. Its low toxicity, cost-effectiveness, and compatibility with existing production processes make it ideal for industrial applications.
For manufacturers, optimizing the co-initiator ratio and pre-reaction temperature is key to balancing polyether processability and foam performance. As fire safety regulations tighten, melamine flame-retardant polyether will play an indispensable role in advancing the safety and sustainability of rigid PU foam materials.
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