A method was developed to produce a polymer material called PBO — known commercially as Zylon — that’s used in bulletproof vests and other high-performance fabrics. The new approach could be useful in making PBO products that resist degradation — a problem that has plagued PBO-based materials in the past.
The traditional way to make PBO uses polyphosphoric acid (PPA) as both a catalyst for necessary chemical reactions and as a solvent. PPA is a strong, highly corrosive acid and has been pinpointed as the source of PBO degradation. Molecules of the acid become lodged in the polymer chain, leaving the fibers susceptible to degradation when exposed to light and moisture over time. That degradation has led to the recall of PBO-based body armor in the past.
The nanoparticle catalyst is capable of performing the new reactions required to make PBO without using PPA. Catalyzing the reactions with nanoparticles would also require less energy and can be performed using renewable formic acid as a hydrogen source. Until now, composite nanoparticle catalysts have largely been used to make only small organic molecules. Whether a composite catalyst, which in this case is made from particles of gold and palladium alloys, could be used to catalyze the controlled growth of polymer chains was previously unknown.
An alloy composition of close to 40 percent gold and 60 percent palladium was shown to be optimal for controlling the rate of reactions needed to make PBO. Particles around 8 nanometers in size produced a reaction speed that maximized the molecular weight of the PBO polymers. To find out if the PBO was resistant to degradation, mechanical testing was done. Those tests showed that the PBO polymers made with the nanoparticle catalyst were more resistant to degradation than commercially available Zylon — even after being boiled in water and acid for days.
Future work will focus on generating PBO polymers with higher molecular weights. The polymers generated in this work were significantly lighter than those of the commercial-grade product, which limits their initial mechanical strength; however, the work is a proof-of-concept for the idea that composite nanoparticles can produce degradation-resistant PBO.