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TECHNOLOGY | NANOCOMPOSITES


Right: With graphene —and most other nano-additives — a little goes a long way


unwanted side effects, such as a reduction in recyclability. Additionally, long chain branched polymers are expensive and increase foam cost.” Polypropylene foam made the MSU way can be


re-melted and reformed. The nanoclay eliminates the need for cross-linking and means it is not necessary to add a long chain branched polymer to achieve the appropriate melt strength for foaming. It also increases stiffness. The nanocom- posite approach is also said to be less complex, since foam producers can use the nanoclay in masterbatch form.


Flame retardant gains


Nanoclays have gained a notable niche as flame retardant additives, especially in wire and cable. Günter Beyer – who became an expert in nanoclays and their fire resistance properties at Kabelwerk Eupen and now has his own consulting company, Fire and Polymers – highlights work at NIST, the National Institute of Standards and Technology in the US, investigating the background for improve- ments of flame retardancy by nanofillers. “It is historically fortunate that the first investigated system was montmorillonite, since it has always shown the best flame-retardant improvements compared to other nano-dispersible fillers like carbon nanotubes, graphene or layered double hydroxides,” he says. The flame-retardant improvements were typically measured by cone calorimeter, which showed strong reductions for the peak heat release rate (PHRR). But nanofillers did little to improve properties measured according to UL 94, or in limiting oxygen index (LOI). “What is also important is that one finds no real difference in the cone calorimeter between exfoliated and intercalated structures…This is important to the industry because it allows the application of a wide range of compounding machines from roll mills to twin-


Right: These sports shoes use graphene- enhanced


rubber soles to improve


performance and durability


screw extruders or co-kneaders,” Beyer says. “The reason for the very good flame retardancy by nanocomposites can be explained by the formation of a barrier in the case of a fire which makes it difficult for the degradation products to escape and leave the polymer,” he explains. “The reduction in PHRR is closely linked to changes in the degradation pathway which occurs in the presence of montmorillonite. Large reductions are found for PA, EVA, TPU and PS. SAN, ABS, PP and PE show moderate reductions and polymers like PMMA or PAN do not have a change in the degradation pathway and hence no significant change of PHRR.” The future of nanocomposites for flame retar- dancy is as one component of a multi-component system, according to Beyer. “It can be the combina- tion of a condensed phase flame retardant with a vapour phase flame retardant: charring by mont- morillonite and quenching of burning materials in the vapour phase,” he says. “Due to the very large interfaces between the polar nano-dispersed montmorillonite and the polymer matrix, people need to be aware of migration /absorption of polar polymer additives like antioxidants or UV-stabilisers from the polymer matrix to the montmorillonite surface; this will lead to problems both for ageing and light stability. But that can be avoided by proper additive selections.”


Graphene — the new black Away from clays, the nano-material stealing much of the limelight is graphene, first extracted by two researchers at the University of Manchester in the UK using, the story goes, little more than sticky tape to successively peel layers from a piece of graphite. The technology is rather more sophisticated today. Last year, Versarien, which has a majority share of the university’s graphene subsidiary 2-Dtech, said


30 COMPOUNDING WORLD | December 2018 www.compoundingworld.com


PHOTO: INOV-8 PHOTO: INOV-8


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