flame retardants | Polyamides Table 1: Flame retardants for polyamides detailing mode of operation
for polyamides and their principal mode of operation. Not only do these various additive systems work in
Source: Clariant
breakers and switches – where once again flame retardance is a requirement. New applications such as LED lighting and photovol-
taics, as well as automotive electronics, are providing the highest growth opportunities for halogen-free flame retarded polyamides. LED applications in particular also require materials with high resistance to UV radiation. Growth opportunities can be seen as well in computer server farms for cloud computing and the “Internet of Things”. Polyamides 6 and 66 are semi-crystalline polymers
known for their good mechanical properties and good thermal stability at a competitive price. But they are not inherently flame retardant. In fire situations, polyamides show a higher heat release rate than polycarbonate but less than polyester. This affects the amount of flame retardants necessary to pass industry fire test standards.
Fibres and flame retardance In many applications, polyamides are required to be reinforced with glass fibres and this negatively affects their flammability performance - as it does in fact with other polymers - because glass fibres tend to act like a wick in a candle. So while an unreinforced standard polyamide may have a UL94 V2 flammability rating, a glass reinforced compound of the same polymer is likely to not achieve any classification. Along the supply chain from polymer producer to OEM
there are numerous trends and economic drivers that have an effect on the development of new flame retard- ants and flame retarded compounds. These include: the emphasis on “green” credentials, colourability, laser markability, speed of processing and miniaturisation. Compounders and injection moulders, therefore, need to have a deep understanding of what kind of flame retardants are available for the plastics they are process- ing, how and how well they perform, what effect they have on other important properties in a particular application, and how they can be considered in terms of sustainability. Table 1 outlines the various types of flame retardant
32 COMPOUNDING WORLD | December 2015
different ways, but they also work at different addition rates and so have different effects on the properties of the polymer. Magnesium hydroxide, for example, needs to be added at a level of more than 45% by weight in a glass reinforced PA6 to obtain a V0 performance, while brominated polystyrene/antimony trioxide synergistic systems provide the same result with a loading of just over 26%. An organic phosphinate with a nitrogen-con- taining synergist comes out even better, requiring a loading of 20% in polyamide 6 and 66, and in semi- aromatic polyamides as little as 12-15%.
Reviewing the options Magnesium hydroxide has the advantage of low smoke density. Electrical properties are also good, and parts can be coloured. But the high addition rates give problems in processing, in mechanical properties and in part density. Melamine cyanurate is the standard flame retardant
for non reinforced polyamide applications. It is added at the same levels as organic phosphinate. But in glass fibre reinforced polyamides it can only achieve a V2 performance. It cannot be used in PA66 either, since it begins to degrade at temperatures above 275°C (the melting range of PA66 is around 45°C higher than that of PA6). It does have good electrical properties though, and parts can be made in any colour. So, for applica- tions where moderate flame properties are required, it can be a cost-effective solution. Red phosphorus achieves a V0 rating in polyamide 66
with glass fibres with a loading of just 7%. Since it is a flammable powder, it is commonly used a a masterbatch. Finished parts have good mechanical and electrical properties but caution is called for during processing due to phosphine emissions and in hot and humid environ- ments corrosion of copper conductors is sometimes reported. Colour options in finished parts are severely limited to dark red, brown and grey/black shades. On paper, brominated polystyrene synergized with antimony trioxide is a very good solution - parts have good mechanical properties and high glow wire ignition temperatures, and they can be coloured. But electrical properties are not the best and, for components where weight counts, the high density of the additive system works against it. A standard 30% glass reinforced polyamide 66 has a density of around 1.35 g/cm3
, but a
grade flame retarded with a Br/AtO system weighs in at just under 1.70 g/cm3
. And in consumer electronics,
where public image counts much more than it does in industrial E&E, the fact that it is a halogenated solution is an important negative. Organic phosphinates synergized with nitrogen work
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