Measuring and improving fire resistance in composites

FR composites primer

Composites have been used in fire resistant applications for decades. Generally, inorganic fibers (e.g., glass, carbon, basalt, ceramic) and inorganic matrix materials (e.g., ceramic/carbon, metals, polysialate/geopolymers) do not burn and most can withstand high temperatures. However, when most organic fibers and polymer matrices are exposed to high temperatures and fire, they will decompose into:

• non-flammable volatiles (e.g., CO₂, water)
• flammable volatiles (e.g., CO, methane), which react with oxygen to feed the fire
• solid carbonaceous char
• smoke that can include toxic fumes.  

A composite’s fire performance is measured by a variety of characteristics, including:

• Ignition
• Ability to self-extinguish
• Flame spread
• Burn through
• Heat release
• Smoke generation
• Smoke toxicity.

Limiting oxygen index (LOI) is also frequently cited. It measures the minimum oxygen concentration (in vol%) necessary for combustion; thus, higher LOI means higher flame resistance. The standard tests for these performance measurements vary by industry and range in test sample size from small coupons to full-scale constructions representative of in-service use. The flame heat flux used in these tests also varies.

Bio-based phenolic/furan

CW’s Feb 2019 feature article discusses recent developments in prepregs using polyfurfuryl alcohol (PFA), which is made from from agro-waste biomass (e.g., corn cobs, sugar cane bagasse). This product has a long history, with TransFuran Chemicals (TFC, Geel, Belgium) claiming more than 40 years of production and annual capacity of 40,000 metric tonnes (MT). The company’s BioRez and Furolite resins are VOC-free liquid thermosets with a PFA backbone and Tg>180°C, available in a range of viscosities and formulated for different processes including resin infusion, prepregs and snap-cure compression molding. Cure temperatures range from < 95°C for infusion resins to >120°C for prepregs. These resins have been tested in composites that meet aircraft, rail and marine fire performance requirements. For more details, see TFC’s presentation from the June 2018 meeting of E-LASS (European network for lightweight applications at sea).

 

Bio-based phthalonitrile to replace PMR-15

The US Navy is also pursuing bio-based FR composites. It is funding a Strategic Environmental Research and Development Program (SERDP) to replace the 600°F capable polyimide resin PMR-15 with a sustainable resveratrol-based phthalonitrile (PN) resin. The goal, according to Navy research chemist Dr. Matthew Laskoski, is a high-performance thermoset that does not rely on toxic methylene dianiline and can be processed below 200°C yet performs well when fire tested per ASTM E 1354-1. First generation PN composites actually achieved this but were derived from petroleum-based bisphenols and were also hard to process. Alternatively, resveratrol is already being produced commercially via fermentation of sugars with engineered yeast. Testing of second generation PN composites, fabricated by hot-pressing E-glass-reinforced prepreg, showed better ignitability, heat release and mass loss rate performance vs. state-of-the-art glass fiber/polymer composites. The resveratrol-based PN composite retained 95% of its weight at 700°C and offers an easily processed resin system with exceptional fire-resistant performance at high heat fluxes. The team has also completed award-winning development on bio-based cyanate esters.

 

Self-extinguishing, inorganic polymer composite foams

Another green development, this one in India, concentrates on fire-retardant, self-extinguishing inorganic polymer composite foams. As explained by Dr. Guruswamy Kumaraswamy, at the Academy of Scientific and Innovative Research, (AcSIR, New Delhi), “we have demonstrated a polymer foams with a very high inorganic content that do not use FR additives yet exhibit exceptional fire retardance in burn tests. They are self-extinguishing, do not drip or collapse when exposed to flame and show a 75% lower peak heat release rate than commercially available fire-retardant polyurethanes.” These hybrid foams are prepared using ice-templating with a green solvent, water, as a porogen (pore-inducing material). They also show excellent mechanical properties. “We can use ice-templating to produce hybrid foam from cheap commercial materials and our approach appears to be chemistry-independent within a wide range of materials,” says Kumaraswamy. “We’ve also shown that we can play with the crosslinking density to increase modulus by more than 100-fold, and there is no loss of elasticity with increase in modulus.” Though he has not tried harder foams, he believes it should be possible. His team’s interest has been to produce high-performing FR foam for automotive seats, but the overall approach may hold promise yet for various composite insulation foams, especially as the process has already been scaled without loss of mechanical performance or fire-retardant properties.