Schematic illustration of PBT with FR top layer and fire test in cone calorimeter.
Source: BAM
In many industries – from cars and trains to household electronics – thermoplastic materials reinforced with glass fibres are essential. Glass-fibre-reinforced polybutylene terephthalate (PBT) composites are strong, lightweight, and easy to process, but as most of polymers, highly flammable. Conventional flame-retardant (FR) strategies rely on adding large amounts of additives to polymer material, which often reduces mechanical performance, increases density, and raises costs.
Our study presents an efficient alternative: instead of distributing flame retardants throughout the whole material, we concentrate them only in a top surface layer – the part of the specimen that flames reach first. The top layer provides effective fire protection, while preserving the mechanical integrity of the underlying composite. We tested several combinations of flame-retardant additives and compared them with traditional fully filled glass fibre reinforced PBT.
Fire tests showed that in some of the flame retardant systems, the concentrated top layer often protects better than a fully filled material, although the total FR content is reduced. When exposed to heat, the top layer forms a stable protective barrier, which slows down pyrolysis, reduces heat release, and shields underlying laminate.
Our results demonstrate that the concentration of flame retardants in the top layer can outperform traditional approaches in industry, which can improve fire safety while reducing flame retardants content, lowering costs, and maintaining material performance. The study shows that a thin, well-designed protective layer can be more effective than mixing additives in the entire material – proving that, in this case, less truly is more.
Less is More: Optimised Fire Performance in Glass Fibre‑reinforced Polybutylene Terephthalate Laminates with a Concentrated Flame‑Retardant Top Layer
Weronika Tabaka, Bernhard Schartel
Composites Part C: Open Access, 2025