Empa Activities 09/10 Materials meet Life
Towards bromine and formaldehyde free flame retardant system for cellulose Novel phosphorus and nitrogen based compounds such as phosphoramidates have been developed in our lab and have been found to be very effective as flame retardant additives for various polymers such as cellulose, polyamide, polyester, and polyurethane. The efficacy of these phosphoramidates as flame retardant additives is very much structure-related. The kind of alkyl ester group attached and the substitution on the nitrogen atom linked to the phosphorus atom is of significant importance.
organophosphorus compound such as phosphoramidates, which is formaldehyde free as well as quite effective on various polymers. Preliminary water toxicity studies carried out using the standardized Microtox test have shown that the EC50 of these compounds ranged from 2 to 80 mg/L, depending on the side chain composition. O R
O R1
P R
O
N R2
Sabyasachi Gaan, Patrick Rupper, Manfred Heuberger, Hansruedi Schmid, Axel Ritter, Viktoriya Salimova, Laurie Mauclaire Schoenholzer
Over the past decade, the use of halogen-based flame retardants has caused environmental concerns and raised several toxicity issues. This has further led to the ban of this type of flame retardants. Also, some of the phosphorus-based flame retardants used in fibrous polymers are based on formaldehyde, a carcinogen. Thus, there has been increased impetus for the development of suitable halogen- and formaldehyde-free flame retardants, which are not only effective but also environmentally safe. Phosphorusbased flame retardants have been considered to be a suitable alternative to these halogenated compounds. Furthermore, the development of formaldehyde-free phosphorus flame retardants is also needed. Over the past few years, our group has been actively involved in the development of a class of
FR Type
Structure
%P
LOI %
Char %
1 2 4
22.8 25.2 26.3
8.3 11.5 14.7
1 2 4
23.5 26.3 30.3
14.8 16.8 21.0
1 2 4
23.2 26.4 29.8
16.7 20.5 22.6
1 2 4
23.2 25.6 27.3
13.8 14.9 17.7
1 2 4
23.5 25.7 27.3
13.9 15.8 18.5
–
18.1
0
O
P
O
Triethyl Phosphate (TEP)
O
Diethyl Phosphoramidate (DEPA) Phosphoramidic acid hydroxyl ethyl diethyl ester (PAHEDE) Diethyl methoxy ethyl phosphoramidate (DEMEP) Diethyl ethyl phosphoramidate (DEEP) Untreated Cotton
O O
NH2
P
O
O
O OH
N H
P
O
O
O O
N H
P
O
O
O O
P O
N H
Tab.1: Flame retardant property of phosphoramidate-treated cotton cellulose.
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R = CH3, CH2 CH3 R1, R2 = Alkylgroups, H Alkylgroups = CH3, CH2 CH3, CH2CH2CH3, CH2CH2OH, CH2 CH2 OCH3, CH2CH2NHR3, NHR3 R3 = Alkyl, H, Phosphorus containing moiety
Fig.1: General structure of the phosphoramidates developed in our laboratory.
The general structure of phosphoramidates developed in our labs is shown in Figure 1. The phosphorus atom of these phosphoramidates is bonded to three oxygen and one nitrogen atom. Various phosphoramidates have been developed, depending on the substitutions on the oxygen and nitrogen atom. In one variation, the structure of the phosphoramidate molecule has been varied, based on the substituents on the oxygen atom, i.e. methyl or ethyl ester derivatives. Flame retardancy and thermal decomposition studies on methyl ester and ethyl ester phosphoramidate-treated cellulose have shown a difference in their flame retardant action. Phosphoramidates with methyl ester derivatives have shown superior flame retardant action on cellulose as compared to ethyl derivatives. Thermal studies have shown that methyl ester derivatives can catalyze the decomposition of cellulose to form char at a lower temperature and also lead to lower total heat release rates and heat of combustion. We have also studied the P-N synergism phenomenon by varying the substituents on the nitrogen atom linked to the phosphorus. In this research, various ethyl ester phosphoramidates were compared with analogous phosphate (triethylphosphate). The experimental data shows that primary phosphoramidates and secondary phosphoramidates with hydroxyl terminating alkyl substituents have the best flame retardant action (Tab. 1). Higher limiting oxygen in-