Glass fiber-reinforced unsaturated polyester composites are workhorse materials across the automotive, marine, construction and electrical industries. They combine high specific strength with low weight, corrosion resistance and cost-effective processing, which has made them a favored substitute for metals as manufacturers of electric and hybrid vehicles chase carbon emission targets and longer driving ranges. Yet the carbon-rich organic resin that binds these laminates together is inherently flammable. When polyester burns it can release dense smoke and toxic gases, a serious concern in passenger compartments, building panels and other enclosed spaces where fire safety regulations are strict.

Historically, the industry leaned on halogenated flame retardants such as brominated compounds, which are highly effective at quenching flames. Those chemicals have fallen out of favor, however, because combustion releases corrosive hydrogen chloride and related gases that threaten both human health and the environment, and restrictions under the European Union’s REACH framework have accelerated the shift toward halogen-free alternatives. Inorganic hydroxides and borates have emerged as leading candidates, and the Turkish team focused on two of the most widely available: aluminum hydroxide, supplied by Eti Maden with an average particle size of 12 to 18 micrometers, and zinc borate, supplied by Melos Kimya with particles averaging 3.2 micrometers.

Each additive fights fire through a distinct physical mechanism. ATH decomposes endothermically between roughly 220 and 300 degrees Celsius, absorbing heat from the flame zone while releasing water vapor and leaving behind an alumina residue that acts as a thermal barrier on the surface. Zinc borate operates over a higher temperature window, releasing crystal water between about 290 and 450 degrees Celsius and converting to boron oxide, which forms a glassy, dense layer that limits contact between the underlying polymer and oxygen. Because the two mechanisms complement one another, previous studies have reported synergistic effects when the minerals are used together, although the synergy typically becomes significant only when the combined filler content exceeds ten percent by weight.

To test these ideas in a structural laminate, the researchers produced composite panels by hand lay-up, embedding eight layers of 300 grams per square meter plain-weave E-glass fabric at a fixed 30 percent volume fraction in a commercial unsaturated polyester resin. The additives were dispersed into the resin using magnetic stirring at 650 revolutions per minute followed by 40 minutes of ultrasonication to prevent agglomeration, then cured with methyl ethyl ketone peroxide and cobalt naphthenate at room temperature before a two-hour post-cure at 50 degrees Celsius. Each mineral was tested alone at 5, 10 and 15 percent by weight, and hybrid formulations mixed the two additives in a one-to-one ratio at the same total loadings.

Mechanical testing revealed a familiar trade-off. The untreated laminate reached a flexural strength of 500.41 megapascals, and every flame-retardant formulation fell below that benchmark. The gentlest penalty came from 5 percent ATH, which retained about 96 percent of the neat flexural strength at 479.98 megapascals, the best value among all fire-retardant composites. Strength declined progressively as loading increased, dropping to 394.85 megapascals at 15 percent ATH, while zinc borate proved more damaging to bending performance, yielding values between 364.38 and 394.79 megapascals across its loading range. The 15 percent hybrid formulation managed only 302.62 megapascals, roughly 60 percent of the neat laminate, reflecting the stress concentrations that rigid inorganic particles introduce into the resin matrix and the resulting weakening of fiber-matrix load transfer under bending.

Impact resistance told a strikingly different story. In unnotched Charpy tests, the neat composite absorbed 135.41 kilojoules per square meter, but adding ATH raised that figure by roughly 43, 52 and 78 percent at 5, 10 and 15 percent loadings respectively, peaking at 241.03 kilojoules per square meter. The researchers attribute this toughening to crack deflection and tortuous fracture paths around the rigid inclusions, which dissipate energy even as the same particles create stress concentrations that erode bending strength. Zinc borate, whose glassy structure tends to facilitate crack propagation, delivered a more modest boost to 162.92 kilojoules per square meter at 15 percent, while the hybrid system reached 151.35 kilojoules per square meter, still comfortably above the untreated laminate.

The fire performance results were the centerpiece of the study. Under the UL-94 horizontal burning test, the neat laminate burned at 20.15 millimeters per minute. Fifteen percent ATH reduced that to 12.15 millimeters per minute and 15 percent zinc borate achieved 10.77, but the 15 percent hybrid formulation delivered the lowest rate of all at 10.23 millimeters per minute, a reduction of about 49 percent. To quantify the hybrid advantage, the team calculated a semi-empirical synergy coefficient comparing the measured burning rate with the average of the two single-additive systems at the same total loading. Values above one indicate performance beyond simple superposition, and the hybrids scored approximately 1.23 at 5 percent, 1.13 at 10 percent and 1.12 at 15 percent, supporting a genuine synergistic trend across the entire loading range.

Post-combustion examination of the charred residues helped explain why the combination works so well. ATH alone leaves a porous, cracked alumina layer that offers only partial protection, while zinc borate produces a glassy, dense and continuous borate phase that also promotes surface carbonization of the polymer. In the hybrid formulations, these two morphologies merge into a more compact, crack-resistant barrier that limits the transfer of heat and oxygen to the underlying material. The boron oxide layer effectively seals the surface while the endothermic water release from ATH cools the flame zone, a dual mechanism that prior studies had described and that this work confirms directly in a glass fiber-reinforced polyester system.

The authors position their findings within a deliberate low-loading strategy, keeping total inorganic content at or below 15 percent where conventional approaches often demand more than 30 percent of a single filler to achieve comparable fire performance. High filler loadings degrade resin fluidity, complicate processing and take a heavier toll on mechanical properties, so achieving a nearly 50 percent reduction in burning rate at modest loadings represents a practical formulation window for applications that require both fire safety and structural reliability. The team notes that UL-94 horizontal burning provides a screening-level indicator, and future work will incorporate limiting oxygen index and cone calorimetry to quantify heat release and smoke production. With relevance spanning automotive interior trim, structural panels, transportation components and electrical applications, the study offers a robust framework for comparing flame-retardant laminate formulations and demonstrates that ATH and zinc borate, used together in optimized proportions, can meaningfully improve fire behavior without sacrificing the mechanical integrity that makes glass fiber polyester composites so widely useful.

Subject of Research: Halogen-free flame retardancy and mechanical performance of aluminum hydroxide and zinc borate additives in glass fiber-reinforced unsaturated polyester composites

Article Title: Flame retardancy and mechanical effects of ZnB and ATH additives in glass fiber-reinforced polyester composites

Article References: Ari, B., Yilmaz, A. N. Y., & Bedeloglu, A. C. (2026). Flame retardancy and mechanical effects of ZnB and ATH additives in glass fiber-reinforced polyester composites. Journal of Materials Science: Polymers, 1(1), Article 9. https://doi.org/10.1007/s44493-026-00013-6

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00013-6

Keywords: flame retardancy, glass fiber composites, unsaturated polyester, aluminum hydroxide, zinc borate, UL-94 test, flexural strength, Charpy impact test, hybrid additives, halogen-free flame retardants, automotive composites, synergistic effect