Traditionally, halogen-based compounds are used as an economical route for good flame retardancy properties while having mechanical properties meeting application requirements. However, since the 90’s regulatory restrictions have increased regarding their use as they produce health and environment hazardous toxic dioxins and furans during their combustion. Black and toxic fumes are released adding another risk and rising concerns about people protection and safety during fire.
Therefore the market tends more and more towards halogen-free compounds. Around 70% are halogen free.
Types of halogen-free flame retardant additives
FYI, there are different types of halogen-free flame additives with different mechanisms: Phosphorus flame retardants, nitrogen based compound, metals hydroxides, intumescent systems…Metal hydroxides (Aluminium trihydroxide ATH and magnesium di-hydroxide MDH) comprise the most important segment in the market of the flame retardants.
Applications are various and not only limited to wires and cables including:
- Wires and cables
- Electrical housings and protective profiles
- Electrical mains, adaptors
- Carpet backing Seats, plastics structures in rail/road/maritime transportation
- Furniture Insulation products
- Consumer clothing
- Electronic circuitry
- Technical textiles
While being non-toxic and environmentally neutral, halogen-free compounds, especially ATH / MDH inorganic materials, require high level contents in order to be effective (high Limiting Oxygen Index LOI, UL94…), leading to poor mechanical performance, processing issues.
Why need a coupling agent when using halogen free flame retardant ?
Having a coupling agent is essential to maintain good mechanical performance while meeting flame retardancy requirements. The role of coupling agent is to increase the compatibility between the polymer matrix and fillers, increase mechanical properties, especially elongation at break, and tensile strength, decrease stress whitening.
We will only focus on the two common coupling agents, Silane-based and Maleic anhydride-based coupling agent, for most ATH and MDH in thermoplastic polyolefin resins.
Metal hydroxides are inorganic materials sold in powder form with different particle sizes. The finer it is the more effective. Therefore it is possible to reach the same flame retardant properties with a lower loading level, thus increasing mechanical properties. Of course, finer powder is more expensive. The main mechanisms for hampering combustion with metal hydroxides is the generation of endothermic reactions (heat absorbers), removing heat by using it to evaporate water in material structure and generation of water vapours that can dilute oxygen. In addition to this, metal hydroxides produce limited smoke which is a major benefit compared to other solutions.
The choice between the two is made by the material and process requirements comparison i.e. melting point, viscosity…
The drawback is that filling level of 60% or more are mandatory in order to reach industrial specification (for example for jacketing in wires and cables often LOI33)
Coupling agent functionalities and mechanisms
Polyolefin grafted maleic anhydride
Polyolefins grafted maleic anhydride, like Europlas product, are carbonated backbone i.e. polyethylene, polypropylene, ethylene vinyl acetate…grafted with reactive function of maleic anhydride. The fact that the maleic anhydride is located on the side of the backbone and not part of it (which is the case when co-polymerized) increases its accessibility to reactive functions thus its reactivity .
Coupling agent improves interaction between the inorganic part ATH/ MDH and the polymer matrix. On one side the backbone of EuP has an optimum compatibility with the polymer matrix (LDPE, EVA, PP, compound of these) thanks to miscibility between the 2 polyolefins.
On the other side, reactions between the anhydride maleic function and the hydroxide create a chemical bond. Polyolefins grafted maleic anhydride are provided in granules form. This makes the material really easy to handle. The components of HFFR formulations should be fed all together to the extruder.
Coupling agent addition level in halogen free flame additive formulations can be 3 to 6%. Depending on the formulation, only 50% of the total amount of ATH should be preferably added initially. The other 50% of ATH should be added with an additional feeder located in the middle of the screw extruder.
In this case to be effective the temperature has to be higher than 10°C.
As the ATH decomposition is between 180°C and 200°C, it is important to carefully control the processing temperature during compound.
Twin screws extruder can be used but some local temperature increase could happen due to friction. This effect is the main cause of degradation during HFFR compounding.
No traces of bubbles (ATH decomposition) should be seen in the final compound to ensure no degradation.
Silane based coupling agent
Silane coupling agents are usually alkoxysilane, with a general formula R-Si(OR)3, where R is a nonhydrolyzable organic radical and OR an alkoxy group. The organic group can be inert or exhibits vinyl and amino functionalities to produce a strong bond with the polymer matrix.
In the presence of moisture the alkoxy groups OR of the silane bind to the filler surface, releasing alcohol as a by-product. It can be used in conjunction with a catalyst to enhance its reactivity.
In most cases the silane is subjected to hydrolysis. Following hydrolysis, a reactive silanol group is formed, which can condense with other silanol groups. Stable condensation products are also formed with other oxides such as those of aluminium, zirconium, tin, titanium, and nickel.
For this reason silane coupling agents have really good water resistance compared to maleic anhydride based. Water for hydrolysis may come from several sources. It may be present on the substrate surface or it may come from the atmosphere.
Silane coupling agents are mostly provided in liquid form but can also be available in master batch form. For this reason it requires special equipment to process it and special care for storage (i.e. Leakage).
As for polyolefins coupling agents, it is advised to process halogen free flame additives/ compounds with silane coupling agents on a Buss ko-kneader to avoid degradation of the ATH. Silane can be introduced during extrusion via a liquid pump, or a “dry blend”. When a dry blend is prepared, a mix of polyolefins is done. Separately, a dry blend of alumina trihydrate, possible antioxidant, and coupling agent is made with a speed rotating blade mixer. Half of the latter is dry blended with all the polyolefin blends and fed to the upstream hopper of the extruder. The other half of the additives blend is fed to the hopper halfway down the barrel.
Preferably addition level should be 0.5 to 1.5%, which is a main difference with maleic anhydride based coupling agent. With such a low addition level of silane, development time to optimize the formulation is much longer, requiring a lot of tweaking to reach requirements therefore leading to high development cost.
Silane use is limited to large mono-product and stable production campaigns whereas maleic anhydride based coupling can be used rather for small campaigns and on lines which should produce various formulations.
Industrial requirements for halogen free flame retardant compound
Halogen free flame compounds are mostly used for low voltage application (it can also be for medium voltage) where cables can be submitted to mechanical stress, temperature variations, chemical etc.
The goal is to find the good balance between Flame retardancy properties (ATH / MDH content) and mechanical properties.
The limiting oxygen index LOI test, ASTMD D2863 or ISO 4589 is applied to quantify a material’s resistance to combustion. The objective is to assess the minimum oxygen concentration in nitrogen that will support the combustion of the material for at least 3 min. It is an easy way to assess flame retardancy properties even if it doesn’t represent all them.
UL94 vertical burning test classified the material in three categories depending on its performance regarding the individual duration of burning for each specimen, the total burning time for all specimens and the presence or absence of burning drips. Most of the halogen free flame compounds are classified V-2 due to the presence of flaming drips.
When considering halogen free flame retardant compounds it is highly recommended to utilize a coupling agent between the polymer matrix and the inorganic flame retardant agents (ATH or MDH). The coupling agent leads to an increase in the tensile strength of the composition without detrimentally affecting other properties.
Two kinds of coupling are industrially used maleic anhydride and silane based. Each of them has pros and cons. The one developing a HFFR formulation should be able to address his requirement (cost, process, application, development…) and choose the best coupling agent for their compound.
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