JEC Group have brought together the international community of composites leaders and executives in our Composites Circle as an unique networking opportunity to meet with both peers and future partners.
Fire retardation is an increasingly important part of composite construction, particularly in transportation applications. Polytetrafluoroethylene (PTFE) has long been used in applications requiring flame retardancy. With that in mind, Daikin began investigating the use of PTFE in composites.
(Published on July 2006 – JEC Magazine #26)
TODD BROWN, BUSINESS DEVELOPMENT MANAGER, DAIKIN AMERICA, INC. & CHRIS MOORE, TECHNICAL MANAGER, OPTIMER, INC.
Daikin has developed a PTFE-based fabric that can be incorporated into composite parts. This paper examines the use of this non-woven fabric and its performance in three categories: flame suppression, smoke generation and toxicity in a flaming environment (FST). The result is a wellprotected, consistent flame-retardant barrier in a composite structure which passes ASTM standards for FST.
Compatibility with composites
One of the biggest challenges was to develop a product that met current FST needs and was compatible with resins and processes used in composite applications. The PTFE-containing product must include a suitable amount of PTFE to inhibit the spread of flame and smoke, while also being porous enough to allow most resins to permeate the product. It also required enough dry integrity to allow handling of the material on the shop floor.
Initial efforts to develop such a product quickly narrowed to a lightweight fabric. PTFE films were out of the question since fluorine-containing films are widely used as release films in composites. Attempts to incorporate such a film would result in creating a barrier to resin flow. Porous films had to be made so porous that it rendered the FST properties ineffectual.
After exhaustive research, Daikin developed a fabric, referred to as Web 100-40G, which is 0.15 mm thick and comprised of two layers of non-woven material. The first ply is a standard 40 g/m2 fibreglass veil typically used in composites as a resin-rich barrier to corrosion or glass print-through. The second ply consists of a 100 g/m2 layer of PTFE fibres processed in a proprietary method which (1) allows for resin to easily wet out, and (2) provides a PTFE-rich area in the composite to reduce smoke, flame and toxicity. The PTFE by itself has little structural integrity, which necessitates the use of the fibreglass veil. The two materials are processed together to form a strong, continuous layer of PTFE, which provides consistent properties throughout the composite matrix. At 140 g/m2, it is by far the most lightweight solution to many FST applications in the fibreglass industry.
Testing Web 100-40G consisted in running multiple composite samples through three U.S. tests: 1. ASTM E162 - Flame spread (Fs) and heat evolution (Q) 2. ASTM E662 - Smoke generation 3. BSS 7239 - Toxicity
The most important number is referred to as the flame spread index (Is), which is calculated by multiplying flame spread by the heat evolution, Is = Fs x Q. These tests were chosen primarily due to the fact that they are required by the U.S. Department of Transportation (DOT) for all commercial transportation vehicles in the U.S
- Flame spread, ASTM E162 The DOT requires that flame spread index, Is, be less than 35. The threshold flame spread index (Is) of 35 is significant since more industries are demanding stringent FST requirements. In three typical constructions, each sample using Web 100-40G passed the critical flame spread test.
- Smoke generation, ASTM E662 As indicated earlier, smoke generation is a serious issue, especially in the transportation industry. The U.S. DOT requires that smoke generation during a fire be below critical standards using the ASTM E662 smoke test. ASTM E662 requires that smoke generated within 1.5 minutes Ds (1.5) be below 100, and less than 200 after 4 minutes Ds (4.0) in either the flaming or non-flaming mode.
The non-flaming E 662 test is not a difficult requirement, but the flaming mode is much more difficult to pass since an open flame source accelerates the combustion of the composite surface in combination with radiant heat.
- Toxicity, modified BSS 7239 The Boeing method for toxicity was used in testing for toxic gas. Web 100-40G was tested for all seven toxic gases listed under BSS-7239 during an ASTM E662 flaming mode smoke test. Due to the concern that PTFE may generate HF gas, seven additional tests were performed in both the flaming and non-flaming modes during ASTM E662 testing.
The results of the seven HF gas tests indicated very low emissions of HF in both the flaming and non-flaming modes. No HF tests exceeded 0.5 ppm.
In addition to the excellent FST properties, the use of Web 100- 40G also reduces the overall weight of the composite dramatically. In a current transportation application, the moulder could not meet the stringent FST and weight requirements using an ATH-filled skin coat. By using Web 100-40G, which made it possible to reduce the amount of Technofire to 0.5 mm, he was able to save 1.56 kg/m2. On just two parts of the train - a 10.7 m² interior wall surface and a 4.0 m² plenum - the moulder managed to reduce weight by 23 kg per train car. Additionally, he eliminated the use of a flame-retardant resin, thus reducing labour costs.
Web 100-40G is currently in commercial use in a monorail application in the U.S. and a composite bus application worldwide. Other applications in testing are for aerospace composites and in rail infrastructure. By replacing ATH-filled flame-retardant resins with Web 100-40G, the moulder gains flexibility, reduces weight, and saves labour costs.