Posted on 13/01/2023 in Electric & Electronic Building & Construction Fire Safety 2023
Research into PIN FRs

Bernhard Schartel, Bundesanstalt für Materialforschung und –prüfung (BAM), Germany discussed durability of non-halogenated flame retardancy in E&E plastics. Stability of flame retarded polymeric materials in some applications, such as outdoor furniture, is today a “solved problem” with recognised solutions available (pinfa note: e.g. see below interview with Gabriel-Chemie). Recent publications (Tan et al. 2000 – 1 and 2000 – 2, summarised in pinfa Newsletters n°112 and 125) of accelerated ageing tests of EVA (ethylene-vinyl acetate) cables with different PIN FRs (ATH, MDH, aluminium diethylphosphinate (AlPi), intumescent) suggest that weathering for several years does not significantly reduce fire performance. Further studies by SKZ and BAM were presented (Schloch et al. 2021, not publicly available, abstract here), with different artificial accelerated weathering conditions, simulating harsh conditions and longer weathering times, covering a range of mineral and phosphorus FRs with different synergists in EVA/PE, TPU and PA66-GF. Comprehensive investigations of the ageing of the polymeric materials (all stabilised to the state of the art) followed by different fire tests demonstrates that the degradation occurs quite specifically in terms of weathering condition, polymer, and property. Accumulation of the PIN FR near the polymer surface can in fact improve fire performance with time. Massive deterioration of colour and surface properties and of mechanical properties are noted with time, but mostly no or only moderate deterioration in fire performance, except for an increase of melting-dripping in some fire tests. These effects generally are related to polymer degradation with ageing, not to PIN FRs. Covering a wide range of materials, the studies show that in general the degradation of the polymeric material occurs before the loss of flame retardancy.

José-Marie Lopez Cuesta, IMT Mines Alès, France, presented laboratory tests of HOLCIM SuperPozz (a refined mineral from coal-combustion fly ash) in PIN FR bio-based polymers PLA, PBAT, PBS (polylactic acid, poly butylene succinate, poly butylene adipate Terephthalate) with APP or MPP (ammonium- or melamine polyphosphate). SuperPozz is produced by refining fine particles from coal-burning fly ash, and can be used to replace Pozzolan, a mined mineral, to reduce the density of cement products. It is principally mullite (an aluminium silicate mineral), associated with other minerals (see Saker et al. 2018). Tests looked at replacing up to half of the PIN FR (APP or MPP) in 25% loading PIN FR in polymer compounds. Peak Heat Release Rate was significantly reduced using a combination of SuperPozz and PIN FR. Given the low density of SuperPozz, implications on polymer mechanical properties need to be investigated, and increased transport costs must be considered. The safety of the trace minerals present should be assessed. Tests have also been carried out in EVA and in polyamide 6, with good results in the polyamide.

Henri Vahabi, University of Lorraine, summarised laboratory tests looking at possibilities for PIN flame-retarding natural fibres (flax). Firstly, to develop non-halogenated flame retardant, electrically conductive natural fibres, flax fibres were soaked in acid, then for two cycles in solutions of sodium polyacrylate, polyethyleneimine (PEI), ammonium polyphosphate and poly(aniline-co-melamine). This resulted in layer-by-layer deposit of phosphorus and nitrogen compounds into the fibre, giving an electrically-conductive, self-extinguishing textile when woven (vertical flame test). The conductivity resisted 30 seconds under flame application. Secondly, to develop a bio-based phosphorus PIN flame retardant for natural fibres, phytic acid (a natural molecule, widely present in plant seeds, and non-digestible by mono-gastric animals such as humans, chickens, pigs) was applied to flax using PEI, resulting in 3.4% phosphorus w/w in the fibre. Peak heat release rate was reduced by a factor of around four, but the treated flax showed greater elongation and lower resistance to break.

Günter Beyer, Fire & Polymer, summarised some developments in PIN flame retardants, based on a literature review of 2021 – 2022. As examples, he summarised two recent studies of non-halogenated FR formulations. Paszkiewicz et al. 2021 tested halloysite nanotubes (HNT) and nano silane surface-treated ATH (ATH-sil from Nabaltec) as synergists to non surface-treated ATH (from Nabaltec) in EVA/LDPE injection moulded 0.8 mm thick samples. Halloysite is a natural mineral, a form of kaolinite, an aluminium silicate containing traces of other metals such as titanium, iron, nickel. 4 -12% of HNT and/or ATH-sil were tested with 48 – 60% standard ATH. This study concludes that these nano-synergists improve the thermal stability of the polymer compound and increase the LOI (limiting oxygen index). Heinz et al. 2021 tested maleic-acid-anhydrite grafted LLDPE as a coupling agent in EVA/LDPE blends with 30 – 60 % loading of MDH (PIN mineral FR, Huber), again injection moulded to samples. This study suggests that the coupling agent improves the dispersion of the mineral FR and the thermo-mechanical material properties.

Elke Metzsch Zilligen & Roland Klein, Fraunhofer LBF, discussed bio-sourced PIN flame retardants, including bio-based oxyimides, lignin and cellulose based, and also combination of bio-sourced polymers with PIN FRs. Partially bio-based oxyimides containing an ON(C=O)2 unit which can release flame-quenching radicals in fire and can also be reacted with sulphur or other elements. The biological polymers lignin and cellulose are widely available in wood, paper and other industry by-products and can contribute to char formation. Hydroxyl groups mean than phosphorus can be reacted into these bio-molecules to produce polymeric PIN FRs which have shown to be effective in polyamide, ABS and other polymers. Partially bio-based FRs can also be produced by using bio-based platform chemicals (such as succinic acid, itaconic acid, propane diol, butane diol) or substances which can be produced from renewable resources in future (such as acrylics or pentaerythritol) in the production of standard organo-phosphorus PIN FRs such as DOPO derivatives. Availability of lignin and cellulose are considerably greater than today’s total PIN FR production and the production of bio-based platform chemicals will grow to fulfil future requirements.

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