Flame-retardant plastics can save lives by delaying the spread of a fire, allowing valuable time for firefighting. But flame retardants can achieve their full effect only if the right processing aids are used in the production of the flame-retardant compounds. Evonik, a leading chemicals major, has developed – jointly with its partners – suitable formulations and has demonstrated their efficiency in standards-compliant flame-retardant tests.

Fires aren’t all alike. Strange as it may sound, there are highly problematic and less problematic fires. Highly problematic fires, particularly in buildings, are those that spread via electrical installations or via the facade. Especially cable can propagate the fire into rooms that would otherwise be well protected, for example, by a concrete wall or ceiling. Therefore, not only cables but also cable connectors or wall connections need to be based on flame retardants. Highly problematic are also such fires that spread quickly and those in which toxic gases may occur. Therefore, non-halogenated flame retardants based on nitrogen, phosphorous or metal hydroxide gain importance, said Kathrin Lehmann from  Evonik Nutrition & Care Business unit. Lehmann is head of Technical Service and Development for additives in plastics and polymer applications working in the Plastics Test Centre in Essen,Germany.

To prevent and delay fires, flame retardants are used in a variety of products such as plastics, textiles, wood products, paints, and electronic and electrical devices (E&E application). Appropriate standards define the protective action that the industry must provide for each application (see box on flame-retardant classes). Appliances are often the reason for the ignition of the fire.

Evonik has also built its own test facilities: the Evonik Technical Centre in Essen, Germany

Due to rising global safety standards and the increasing use of flammable materials, the market for flame retardants is becoming increasingly important: according to a study by the market research institute Ceresana, it is now estimated to be worth €1.9 billion ($2.23 billion). Flame retardants must be precisely adapted for the specific application, which is possible only in close collaboration with producers, processors, and users of the end products.

In cable insulations, producers incorporate the flame retardant during extrusion. Usually, they are based on metal hydroxides such as alumina hydroxide or magnesium hydroxide. However, flame retardants complicate the processing of plastics; they may also be highly corrosive or abrasive toward compounding machines. For nearly ten years, therefore, Evonik has been supplying cable compounders with Tegomer additives, based on organomodified siloxanes that serve as processing aids in the extrusion process. These have been proven to reduce power consumption and extruder head pressure because they act as a kind of internal lubricant, lowering the viscosity of the compound, which reduces friction and therefore maintenance costs.

There is still much potential in the additives for further improving the flame-retarding action of metal hydroxides. In strategic partnerships, Evonik has systematically investigated how the action of these flame retardants can be further optimised by the use of additives and how they do not only improve the compounding process. They are much more than just processing aids.

Metal hydroxides: the flame retardants of choice for HFFR cable application

Metal hydroxides are among the most commonly used flame retardants worldwide. In a fire, they produce neither toxic gases nor soot particles. The hydroxides used are mainly aluminum hydroxide (Al(OH)3 or aluminium trihydrate, ATH) and magnesium hydroxide (Mg(OH)2 or magnesium dihydrate, MDH). In the event of a fire, the metal hydroxides release water on heat uptake: the water cools down the material by evaporation. This delays the spread of the fire without generation of toxic gases.

However, to show the desired effect, the metal hydroxides must be mixed with the flammable polymer in high proportions of more than 60 per cent. ATH has the additional disadvantage for polymer processing that it releases water at temperatures as low as around 190-deg C. The melting point of the polymer, which is relevant to extrusion, lies in many cases relatively close to this value, as for EVA (ethylene vinyl acetate) or PE (polyethylene). MDH, on the other hand, releases water only above 300-degC, and is therefore used even in PP (polypropylene) or PA (polyamide).

For many years Evonik has forged strategic partnerships for flame retardants, for example, with Nabaltec, a specialist in ATH production which supported Evonik in finding reasonable results for the evaluation of flame retardants with the Cone Calorimeter. Evonik has also built its own test facilities: the Technical Centre in Essen, Germany possesses a facility that allows testing in accordance with the specifications of the UL 94 standard of the flammability of plastic materials, using the compounds produced there on twin-screw extruders. Currently also a Mass Loss Cone Calorimeter will be installed.

Improved fire protection is needed for cable applications and wall connections for cable

Together with Nabaltec, Evonik has tested various formulations using ATH into which organomodified siloxanes have been mixed resulting in a better dispersion of the ATH or MDH. The additives must be adsorbed on the ATH particles in the correct way. There are two possibilities: post-treatment of the ATH particle with the organomodified siloxanes (OMS), or the use of the OMS during compounding for in-situ hydrophobisation of the metal hydroxides. The results from the formulation trials and test series were positive for both of these approaches.

To act effectively as a flame retardant, ATH must normally be mixed with a polymer in a proportion of 63 to 65 per cent to achieve the highest UL 94 flame retardancy rating of V-0. Thanks to the additives from Evonik, the same efficiency can be achieved with a lower percentage of ATH; this was previously possible only by using a higher ATH concentration.

Depending on the required flame retardancy rating, the percentage needed is 59 to 61 only. These values have been confirmed in tests, on equipment of the type used in the categorisation of materials by the flame retardancy rating.

Plastics insulation with additives from Evonik to achieve unique properties

The use of organomodified siloxanes extends the compounder’s formulation freedom allowing to use two to six per cent less metal hydroxide still achieving UL 94 V-0 classification but gaining mechanical properties. The available measurement data show that the additives enable compounders to improve their plastics insulations by one to two flame-retardant classes what is of high importance in a market where standards have become increasingly stringent over the last few years, for example, CPR legislation classes in Europe.

Moreover, with additives from Evonik, the combination of flame retardant and polymer also scores better on all other parameters than a pure ATH polymer mixture. The additive ensures that the metal hydroxide is more effectively dispersed in the melt. For example, the greater freedom of formulation allows significant reduction in stress whitening of the cable insulation when bent. Furthermore, twisting (kinking) of the cable strand, which is particularly likely when cables are drawn over long distances, as in cable ducts of multistory buildings, does not occur. Water absorption can also be reduced, which improves the quality of the insulation by reducing conductivity.

Evonik’s facility allows testing in accordance with the specifications of the UL 94 standard

In connection with flame retardancy, the additives also bring about better char formation, which is the development of a crust on the surface of the burnt material. This is an important property: in the event of a fire, this encrustation prevents the ambient heat, or even direct fire, from penetrating rapidly into the internal layers of the cable. This also allows more time for evacuation and firefighting, and extends the interval after which a short circuit (due to burning away of the insulation) could cause further problems in the building, such as the trapping of passengers in an elevator.

The reason for the high mechanical stability of the material made from the ATH-polymer mixture lies in the additives. In the event of fire, they form oxides such as SiO2, H2O, and CO2, which are non-flammable solids and gases. Due to the SiO2 formed, silicification occurs, and because the additives are distributed as fine droplets through the entire cable sheath, a substructure remains that ensures the stability of the ash.

Outlook for the use of Tegomer and Tegopren additives

Due to the many advantages that organomodified siloxanes offer for flame retardants, the tests by Evonik and its partners have aroused much interest among cable manufacturers: flame retardants based on metal hydroxides are enhanced in quality by additives that simplify and even widen the scope of their use.

Such additives allow new applications in which, for example, MDH for polypropylene (PP) requires improved flowability in injection moulding applications, which is ensured by the organomodified siloxanes.

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The UL 94 test

Various methods have been established for the classification of flame retardants and their use in polymer compounds such as cable insulation. In the UL 94 tests, test laboratories expose defined test materials to open flames. The power and duration of the flame are also specified.

There are various levels that a flame-retardant compound of a plastic can attain in the UL 94 tests:

V-2: extinction of a vertically clamped sample within 30 seconds

V-1: same as V-2; in addition, there must be no dripping of a fused polymer

V-0: same as V-1; in addition, the flame must be extinguished within ten seconds

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Versatility of application

Additives based on organomodified siloxanes are used in many industrial applications. Examples are to be found in cosmetics, coating formulations, and in the production of PU foams. Siloxanes of this type are chemical compounds containing various organic functional groups as well as dimethylsiloxy units. Examples of such groups are alkyl, polyester, polyether, acrylate, hydroxyalkyl, and aminoalkyl groups. Organically modified siloxanes may be linear (with the organic groups at the two ends), comb-like (with the organic groups at right angles to the siloxane chain), or a mixture of the two variants. The desired properties of the molecules can be attained through different combinations of these various options, as with a construction kit. Evonik possesses extensive expertise in the design of the chemical behaviour of organomodified siloxanes. The products are marketed under the Tegomer and Tegopren brand names.