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Aluminium diethyl phosphinate

In the family of dialkyl phosphinic acid salts, aluminium diethyl phosphinate has been found to be an excellent flame retardant for use in engineering plastics such as polyamides, polyesters, thermosets and elastomers. It was developed by Hoechst, later by Clariant and Ticona<ref>Weil, E.D., Levchik, S.V.: Flame retardants for plastics and textiles, p. 97 f. Carl Hanser Verlag, Munich 2009 /ref>. In 2004, Clariant opened its first commercial production line in Huerth-Knapsack near Cologne; in 2012, the second line was launched <ref>http://newsroom.clariant.com/de/clariant-opens-new-production-unit-for-halogen-free-flame-retardants-in-hurth-knapsack/?lang=en/ref>.

Nomenclature and chemical properties

Name Aluminium diethyl phosphinate Synonyms Diethyl phosphinic acid aluminium salt; Diethyl phosphinic acid, aluminium salt; Exolit OP 1240; Exolit OP 930; Exolit OP 1230; Exolit OP 935

Name	Aluminium diethyl phosphinate
Synonyms	Diethyl phosphinic acid aluminium salt; Diethyl phosphinic acid, aluminium salt; Exolit OP 1240; Exolit OP 930; Exolit OP 1230; Exolit OP 935

Molecular formula	3(C4H10O2P).Al
Molecular weight	390.3 g/mole
CAS Registry Number	225789-38-8
Physical form	White powder
Density	1.35 g/cm3, solid
Decomposition temperature	above 350°C
Solubility in water	<1000 mg/l at 25°C

Flame retardancy mode of action

Aluminium diethyl phosphinate acts as a flame retardant in the condensed phase in contributing to charring of the polymer matrix and thus protecting the substrate against heat and oxygen attack. In parallel it acts in the gas phase by radical reactions removing from the combustion zone the high energy radicals H. and OH., which determine flame propagation and heat release <ref>Braun, U.; Bahr, H.; Schartel, B.; Fire retardancy effect of aluminium phosphinate and melamine polyphosphate in glass fibre reinforced polyamide 6. e-Polymers. Vol. 10, 1, p. 443–456. 2013-08-31/ref>.

The phosphinate partly vaporises and partly decomposes to volatile diethyl phosphinic acid and an aluminium phosphate residue, which acts as a barrier for fuel and heat transport<ref>Braun, U.; Schartel, B.; Fichera, M.A.; Jäger, C. Flame retardancy mechanisms of aluminium, phosphinate in combination with melamine polyphosphate and zinc borate in glass-fibre reinforced polyamide 6,6. Polym. Degrad. Stab. 2007, 92, p. 1528-1545/ref>.

Use

Aluminium diethyl phosphinate is used as a halogenfree flame retardant for polyamides, polyesters, thermoset resins (e.g. epoxies) in electrical engineering and electronics (E&E) applications for switches, plugs, PC fans, and structural and housing components. Smartphones, washing machines, and airplane parts, among others, contain the product. Other applications include thermosetting resins as well as cable sheaths and insulation made from thermoplastic elastomers. Aluminium diethyl phosphinate can give these plastics flame retardant properties that are otherwise only achievable with expensive high-performance plastics, which are less easy to work with. Aluminium diethyl phosphinate is often used in combination with other halogen-free flame retardant additives like melamine polyphosphate or melamine cyanurate.

In glass fibre reinforced (GF) polyamide 6 and 66 formulations as well as in polyesters like PBT and PET, aluminium diethyl phosphinate shows excellent performance in the UL 94 flammability tests <ref>UL 94. Tests for Flammability of Plastic Materials for Parts in Devices and Appliances. 2013-03-28/ref> (UL 94 V0 specification is met down to 0.4 mm), as well as in the glow wire tests required for appliances<ref>http://flammschutz-online.com/web/en/news/index.htm?showid=239/ref>. Here, formulations with aluminium diethyl phosphinate meet the glow wire ignition test (GWIT) <ref>Fire hazard testing - Part 2-13: Glowing/hot-wire based test methods - Glow-wire ignition temperature (GWIT) test method for materials (IEC 60695-2-13:2010)/ref> at 775°C and the glow wire flammability test (GWFI) <ref>Fire hazard testing - Part 2-12: Glowing/hot-wire based test methods - Glow-wire flammability index (GWFI) test method for materials (IEC 60695-2-12:2010)/ref> at 960°C. Another important criterion in E&E applications is the Comparative Tracking Index (CTI)<ref>http://en.wikipedia.org/wiki/Comparative_Tracking_Index/ref> ,<ref>Method for the determination of the proof and the comparative tracking indices of solid insulating materials (IEC 60112:2003 + A1:2009)/ref> , which determines the risk of electrical tracking of insulating material that is exposed to contaminating environments and surface conditions. With formulations containing aluminium diethyl phosphinate, the highest requirement of 600 V (numeric value of the highest voltage at which an electrical insulating material withstands 50 drops of electrolytic test solution) is achieved. Further benefits of polyamides and polyesters containing aluminium diethyl phosphinate are low smoke density which makes them suitable for rolling stock applications to EN 45545<ref>EN 45545:2013. Railway applications - Fire protection on railway vehicles - Part 1: General Part 2: Requirements for fire behaviour of materials and components /ref>, as well as good light stability which is needed for outdoor applications.

The EU’s RoHS directive forced E&E manufacturers to switch to lead-free solder systems running at about 30°C higher temperatures than traditional systems, prompting the rapid growth of polymers like high temperature polyamides (HTPAs). Due to the their higher thermal stability, HTPAs can be used in the so called surface mount technology (SMT) of semiconductor components. Facing the require¬ment of lead free reflow soldering, these materials have to withstand peak temperatures of 260°C and more during the soldering process. HTPAs can only compete with more expensive materials like LCPs (liquid crystal polymers), if the flame retardant system keeps their properties on the high level required by the application<ref>Steffner K-J. Lead-free Soldering. Kunststoffe plast europe 9/2005, p. 195-198/ref> . Aluminium diethyl phosphinate has proven to be stable enough, providing very efficient flame retardancy for HTPAs.

Human health and the environment

Human health and environmental facts of aluminium diethyl phosphinate are summarized in a fact sheet <ref>http://www.pinfa.eu/uploads/Documents/Exolit_OP.pdf/ref>. Further data are given in the Arcadis Study carried out on behalf of the European Commission Health & Consumers DG, Contract number 17.020200/09/549040: “Identification and evaluation of data on flame retardants in consumer products. Final Report” of 26 April 2011, Chapter 5.23, p. 168<ref>http://ec.europa.eu/consumers/safety/news/flame_retardant_substances_study_en.pdf/ref> . Aluminium diethyl phosphinate has also been investigated in projects of the US EPA Design for Environment (DfE) programme <ref>http://www.epa.gov/dfe/pubs/projects/pcb/full_report_pcb_flame_retardants_report_draft_11_10_08_to_e.pdf/ref> and in the European FP7 research project Enfiro <ref>Enfiro: Life cycle assessment of environment-compatible flame retardants (Sep 2009 to Nov 2012) http://cordis.europa.eu/projects/226563/ref>. With the exception of being persistent and thus not readily biodegradable, aluminium diethyl phosphinate has been shown to have a favourable environmental and health profile.