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Nanotechnology boosts fire performance of TPU compounds

By David Reed, UT EditorLondon-Research by one of Europe’s leading specialists in the use of nanotechnology has led his company to make several hundred tonnes a year of thermoplastic polyurethane/nanoclay cable-covering compounds which offer improved fire performance. More exotic (and expensive) nanomaterials, such as carbon nanotubes, also offer improvements in fire performance of finished products, said Dr Günter Beyer of Kabelwerk Eupen AG, speaking during an on-line training course on nanocomposites in fire retardant applications.Most of his hour-long report focused on compounds of EVA (ethylene vinyl acetate) and TPUs with the various additives, which included organoclays intercalated with quaternary ammonium compounds, nanocarbon variants, and the recently developed POSS (polyhedral oligomeric silsesquioxane) materials, as well as conventional flame retardants such as aluminium trihydrate (ATH). Testing their fire performance was primarily done using the cone calorimeter which exposes the samples to high heat flux and can measure useful parameters such as the time to ignition of the samples, their rate of heat release over time as well as the amount of smoke production. Beyer’s work showed that relatively small amounts of nano-fillers, typically 5 phr (parts per hundred of resin), gave significant improvements in all of the properties examined.In final remarks, he recommended a triple approach, combining carbon nanotubes and organoclays with conventional fire retardants such as aluminium trihydrate as yielding the best performance. He largely attributed the improvements to the greater char integrity and stability generated by the compounds. This, Beyer explained, both protects the underlying sheathing materials from further heat exposure and also reduces the rate of release of flammable decomposition products from the polymeric component.The researcher also told the 30 or more seminar participants that carbon nanofibres are another class of nanocarbon with promising performance at low levels of addition. However, he cautioned, these materials have high length-to-diameter ratios-“similar to that of asbestos,”-so that users should get information on their possible health effects before beginning to use them.Organised by SpecialChem SA, the Paris-based on-line speciality chemical information resource, the seminar discussed the recent developments in the use of nanomaterials as FR agents in polymers.Participation in the seminar simply required basic computer software with an Internet link (preferred browser Internet Explorer) and, of course, a telephone. The cost was €200 (less than $250) per seminar and, after registering, participants receive the speaker’s presentation a few days in advance; they also had the chance to join in the discussion session which followed the presentation.My own experience was a little mixed, with my computer connection dropping three times during the event, which took a few minutes to re-establish and caused me to miss some of the presentation. All told, though, the concept is reasonably viable means of communicating detailed information on a focused topic-largely dependent, of course, on the skills of the presenter which, in Beyer’s case, were excellent. Participating in a web seminar is not quite as valuable as attending a presentation in person but, of course, saves the considerable amounts of time, effort and money involved in attending a conference in another location. But I digress!Beyer’s specific interests concern the improvement of the performance of materials used to cover electrical cables which, because of high fire performance demands in many applications-such as power and control cables in nuclear plants-often use highly filled compounds, such as 60 parts by weight of aluminium trihydrate in a compound with a further 40 parts by weight of polymer and other additives. For some applications the ATH level can be increased to 70 parts by weight, with only 30 parts of polymer, Beyer said, indicating that such highly filled compounds have high densities and yield end products which lack flexibility and have low mechanical performance. They can also have problematic behaviour during compounding and extrusion steps, he added.There is, thus, a need for compounds with similarly high levels of flam retardance but, if possible, with lower FR levels. Some of the nano-scale systems Beyer has been examining offer such prospects.For example, Beyer’s early research efforts showed that just 2 or 3 phr (parts per hundred resin) of an organoclay significantly increased the time to ignition and reduced the rate of heat release of the cables when tested in a cone calorimeter. They also gave lower levels of smoke, Beyer reported, attributing both sets of improvements to increased char formation and the production of a more stable char.Kabelwerk Eupen is already making several hundred tonnes a year of nanoclay/TPU compounds, Beyer said, “Not because it’s nice, but because of their performance.”In more recent work, the researcher has examined the influence of adding carbon nanotubes to the formulations – these are highly structured materials and are available in two main types: SWCNT (single-walled carbon nanotubes) and MWCNT (multi-wall carbon nanotubes); the latter typically have as many as eight concentric tubes, Beyer explained. When used in an EVA polymer, the MWCNT gave even better performance than the nanoclay/EVA compounds and, when used in conjunction with the organoclay, yet better performance resulted, according to Beyer. The peak rate of heat release (PHR) was halved and time to ignition was also extended, his results showed.Turning to the newer POSS materials, Beyer said that work reported by other researchers had shown one POSS material (octamethyl POSS) gave reductions in the PHR, while another variant (identified only by the tradename Firequench) cut the time to ignition. However, when he tried a combination of both materials in a typical ATH-filled compound, he found no improvement in the rate of heat release with 5 phr of the POSS materials, whereas 5 phr of organoclay gave a significant improvement when tested in the cone calorimeter.Beyer also reported some work on the use of thermoplastic polyurethane compounds with nanoclays as cable coverings. When tested in the vertical array specified in Underwriters’ Laboratories’ UL 1666 ‘riser’ test, the TPU alone melted and dripped in the test chamber, while a compound with 5phr of nanoclay showed no signs of this behaviour.The Belgian researcher emphasised that this test is far more severe than tests used in Europe: the latter use a maximum heat flux of 20 kW, while the UL test has a 145-kW loading applied for 30 minutes.As a general concept, Beyer suggested that the triple blends-a nanoclay with a micro-FR (such as ATH or magnesium hydroxide) and polymer-gave the best fire performance.”

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