Low-melting glasses: turning the fire residue into a shield

Most flame retardants act on the fire itself: they cool, dilute or poison the flame, or promote char. Low-melting glasses (LMGs) work one step later, on the quality of what remains. These inorganic glasses soften between roughly 100 and 500 °C, that is, in the temperature range where a polymer decomposes. Once molten, they wet the burning surface, bind the mineral fillers (metal hydroxides and their oxide residues, clays, silicates) as well as the char, and turn a powdery, cracked layer into a continuous, cohesive shield. The idea is not new: low-melting phosphate-sulfate, borate and sulfate glasses were already explored in the 1980s as intumescent and smoke-suppressing additives for PVC and other polymers^[1,2]. It is now coming back, driven by halogen-free formulations and by demanding applications such as cables.

Which glasses?

How do they work?

• Binding the residue. The molten glass flows between mineral particles and glues them together. In EVA/PE cable compounds where 10 wt.% of the aluminium trihydroxide was replaced by a potassium phosphate LMG, the residue became smooth, cohesive and expanded by about 300%, and the second heat release peak dropped by about two thirds at 50 kW/m²^[5].
• Sealing the barrier. A vitreous film closes the cracks through which pyrolysis gases escape and limits heat transfer to the underlying polymer. Trapped gases can also swell the viscous melt into an intumescent layer^[1,5,9].
• Reacting with the system. Phosphate glasses can react with alumina from ATH to form aluminophosphates, or with char and other phosphorus species, which reinforces the residue network^[5].
• Ceramifying at high temperature. With silicate fillers, the glassy liquid phase promotes sintering into a coherent ceramic, the basis of ceramifiable compounds that keep electrical circuits working during a fire^[8,9].

This last point is worth stressing: LMGs can improve both the reaction to fire of a material (heat release, flame spread, smoke) and the fire resistance of a component (integrity and insulation under prolonged exposure). Few additive families cover both.

Design rules

• Match the glass to the fire. The glass must soften when the polymer decomposes and the barrier is needed. In epoxy, the best results were obtained when the phase-transition range of the phosphate glass matched the combustion temperature; with 9 wt.% APP and only 1 wt.% glass, the formulation reached UL 94 V-0^[4].
• Control the viscosity. A glass that is too fluid runs off or drips; a glass that is too viscous does not cover the surface. Composition, especially the nature of the modifying cations, sets this window: in the cable study, the potassium phosphate glass clearly outperformed the sodium and the zinc-containing compositions^[5].
• Think about processing. If the glass transition is below the processing temperature, the glass behaves as a liquid filler that deforms into fibrils or droplets during extrusion. This can improve dispersion and stiffness, and strongly affects morphology^[3,6,7].
• Use them as synergists. LMGs are most effective at moderate loadings combined with hydrated minerals, phosphorus flame retardants, clays or layered double hydroxides, rather than as stand-alone flame retardants^[4,5,10].

Open questions

Three issues still limit wider use. Phosphate glasses with low softening points are often sensitive to moisture, so their long-term durability in the polymer must be demonstrated. Their effect on mechanical properties and on recyclability needs to be documented case by case. And compositions must remain free of substances of concern, which rules out the lead-based glasses historically used for low-temperature sealing. Designing the glass composition for a given polymer and fire scenario, rather than picking an off-the-shelf frit, is where the next gains will come from.

References

1. Myers R.E., Licursi E. Inorganic glass forming systems as intumescent flame retardants for organic polymers. J. Fire Sci. 3 (1985) 415–431.
2. Kroenke W.J. Low-melting sulphate glasses and glass-ceramics, and their utility as fire and smoke retarder additives for poly(vinyl chloride). J. Mater. Sci. 21 (1986) 1123–1133. doi:10.1007/BF00553241
3. Belyamani I., Otaigbe J.U., Fielding W.R. Development of new sustainable inorganic flame retardant additive system for polyamide 6,6 with improved performance. Polym. Eng. Sci. 55 (2015) 1741–1748. doi:10.1002/pen.24012
4. Liu W., Pan Y.-T., Zhang J., Zhang L., Moya J.S., Cabal B., Wang D.-Y. Low-melting phosphate glasses as flame-retardant synergists to epoxy: barrier effects vs flame retardancy. Polym. Degrad. Stab. 185 (2021) 109495. doi:10.1016/j.polymdegradstab.2021.109495
5. Alsayed D.A., Sonnier R., Otazaghine B., Jean P., Brocheton Y., Ferry L. Phosphate low-melting glasses as synergist in flame-retardant cable sheath composition: performance and mode of action. Polymers 17 (2025) 2679. doi:10.3390/polym17192679
6. Urman K., Otaigbe J.U. New phosphate glass/polymer hybrids – current status and future prospects. Prog. Polym. Sci. 32 (2007) 1462–1498.
7. Serio L., Gawne D.T., Bao Y. Effect of tin fluoride content on the structure and properties of phosphate glass–polyamide 11 hybrids. Eur. Polym. J. 99 (2018) 134–141. doi:10.1016/j.eurpolymj.2017.12.012
8. Lou F., Yan W., Guo W., et al. Preparation and properties of ceramifiable flame-retarded silicone rubber composites. J. Therm. Anal. Calorim. 130 (2017) 813–821. doi:10.1007/s10973-017-6448-4
9. Zhu H., Li J. Research on SR/frit composites: a novel low-temperature ceramifiable expandable flame-retardant material. Materials 15 (2022) 2961. doi:10.3390/ma15092961
10. Hull T.R., Witkowski A., Hollingbery L. Fire retardant action of mineral fillers. Polym. Degrad. Stab. 96 (2011) 1462–1469.