Bridged DOPO derivatives as flame retardants for PA6
Introduction
Polyamide 6 (PA6) belongs to the class of engineering plastics, due to its excellent mechanical strength, high abrasion, chemical resistance and good processability [1], [2]. This combination of attractive properties is the basis for its applicability in the automotive, electronics and electrical industry. Although PA6 is classified as a slow-burning polymer, its flame resistance is insufficient for most of these applications. Consequently, PA6 needs to be flame retarded [3], [4]. Commonly used method to reduce the flammability of thermoplastics is to introduce a flame retardant (FR) additive into the material via melt processing. Typically, an effective flame inhibition is not the only requirement to be fulfilled by a FR additive. Among other properties, an ideal FR additive needs to have a very good compatibility with the polymer matrix i.e. to cause no deterioration of its desirable properties and to be easy to process [2], [5]. Furthermore, the environmental and health profile [6], [7] as well as the cost of the FR additive are also crucial factors for a successful commercial application [8]. Hence, the selection of suitable FR additives or the development of new FR systems is a challenging task.
For some applications, halogen containing FR additives are still commonly applied in industry for flame retarding PA6 [4]. However, some of these additives have low thermal stability, that could lead to the formation of corrosive and toxic products during their processing [9]. Additionally, some of these FRs have been also proven to be environmentally hazardous, due to their bioaccumulation in living organisms [10]. Non-halogenated additives are thus preferred for thermoplastic applications. The available non-halogenated FR systems for PA6 include metal hydroxides, red phosphorus, melamine cyanurate, melamine polyphosphate, melamine pyrophosphate, ammonium polyphosphate and metal salts of alkyl phosphinates [3], [4]. However, none of these FRs fully meet the requirements of an ideal plastic additive.
The versatility, efficiency, ease of preparation and eco-profile of the phosphorus-based compounds, has encouraged the research and development of promising phosphorus-based FR solutions for PA6, over the last two decades [11], [12], [13]. The melting of the FR in the temperature range of the thermoplastic polymer processing could be a key factor for a good additive homogenization and subsequent uniform FR behaviour of the material. Additionally, sufficient thermal stability is required from an additive to “survive” the melt processing conditions of the polymer. Finally, for an efficient gas-phase fire inhibition mechanism, it is typically required that the FR additive is characterized by a similar decomposition temperature as the thermoplastic polymer [14].
In the view of the requirements mentioned above, derivatives based on 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) could be an attractive FR alternative solution for PA6. DOPO based FRs have been shown to be effective for epoxy resins [15], [16], [17], [18], [19] and polyurethane foams [14], [20]. Good fire inhibition characteristics were observed at a phosphorus content as low as 2–3 wt.% in these formulations [14], [15], [16], [17], [18], [19], [20]. DOPO and its derivatives are known to predominantly act by a gas-phase mechanism, through the formation of PO• radicals [14], [15], [17]. This radical quenching mechanism might be assisted by a condensed-phase activity via the incorporation of specific functionalities at the DOPO moiety, such as tetra-[(acryloyloxy)ethyl] pentaerythrit [21], tris(acryloyloxy)ethyl isocyanurate [22] or pentaerythritol diphosphonate [23]. Thus, a rational design of the introduced functionalities may enable the optimization of the DOPO-based FRs for tailored applications, such as in PA6 engineering plastics.
Two bridged, thermally stable and meltable DOPO derivatives: 6-((6-oxidibenzo[c,e][1,2]oxaphosphinin-6-yl)methoxy)dibenzo[c,e][1,2]oxaphosphinine 6-oxide) (DiDopoMeO) and 6,6′-(ethane-1,2-diylbis(azanediyl))bis(6H-dibenzo[c,e][1,2]oxaphosphinine 6-oxide) (DiDopoEDA) were developed, synthesized and evaluated as FR additives in PA6 engineering plastics. These additives were incorporated into the PA6 matrix through melt extrusion and showed good melt processability. The flammability and decomposition behaviour of the formulations containing the bridged DOPO derivatives were evaluated relative to a formulation comprising the commercially available aluminium diethylphosphinate (Exolit® OP 1230). Exolit® OP 1230 is a non-melting, P-containing salt, known to induce polyamides a good fire inhibition [3].
Section snippets
Materials
Polyamide 6 (Natur A26), used in this study, was provided by EMS-CHEMIE AG. The aluminium diethylphosphinate (Exolit® OP 1230) was supplied by Clariant GmbH. DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) was purchased from Krems Chemie Chemical Services AG. Ethylenediamine, tetrachloromethane, triethylamine, paraformaldehyde, dichloromethane and toluene were purchased from Sigma Aldrich and used without any further purification. The chemical structures of the studied bridged DOPO
Thermal decompositions studies
Measuring the thermal degradation under normal pressure and a defined heating program is considered to simulate well [30] the burning conditions in the presence of a flame, such as during the UL94 vertical flammability test. Therefore, the thermal decomposition of the FR additives as well as the FR/PA6 formulations was assessed by thermogravimetric analyses. The TGA curves, showing the weight loss profile and their corresponding first order derivatives (DTG) are presented in Fig. 1a–b, and
Conclusions
Two bridged DOPO derivatives, DiDopoMeO and DiDopoEDA, were developed as flame retardants for PA6 engineering plastics. The good melt-processability of these FR additives enabled their successful incorporation in approx. 17 wt.% into the PA6 matrix through melt extrusion, as well as their subsequent processing via injection moulding. The obtained DiDopoMeO/PA6 and DiDopoEDA/PA6 formulations were evaluated against a PA6 formulation containing the commercial FR additive Exolit® OP 1230. When
Acknowledgements
The authors thank EMS-CHEMIE GmbH Co. for kindly providing the Polyamide 6 and Clariant GmbH for supplying Exolit® OP 1230. Furthermore, special thanks to Pierluigi Barbadoro for his assistance in the polymer processing, as well as to Elisabeth Michel and Shuyu Liang (all from Empa, St. Gallen) for their support in conducting various analytical measurements.
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