Phosphorus-based flame retardants for thermoplastics When a flame-retarded plastic material is selected for an application, the decision of the design engineer is based on the technical property profile and the price/performance ratio of the product. Looking at the large number of available flame retardant compounds on the market, it is important to understand differences in the performance of materials due to the chemical structure of the flame retardant, explains Elmar Schmitt of Clariant Produkte (Deutschland) GmbH. In addition to halogen-containing organic types, flame retardants (FR) are based on inorganic minerals and nitrogen compounds among others, as well as products now available that are based on phosphorus [1]. Either the chemical element itself is used, such as red phosphorus (P). Alternatively, a variety of inorganic or organic, solid or liquid chemicals with different physical and chemical properties can be used. Figure 1 shows some of the most important products and their chemical structures. Two facts are interesting from a chemical point of view. Firstly, the phosphorus content in these molecules varies greatly, from almost 100% (red P) to 9.5% (TCPP) and this does not always correlate to the flame retardant efficiency of the specific substance. Secondly, the phosphorus atom is found in all possible oxidation states between 0 and +5. This indicates that unlike halogen-based FRs, the flame retardant effect of the P- products cannot be described by a single mechanism. Typically, a P-based FR is designed to develop its activity in combination with the starting decomposition of the specific polymer it is used for. It can offer a partial gas phase contribution to the flame extinguishing effect, which is comparable to bromine- or chlorinecontaining FRs. However, the main feature is mostly char-forming activity, sometimes combined with foaming-up (intumescence), which then forms a protective top layer on the plastic surface [2]. The obvious advantage of such a solid-phase mechanism is that it causes less release of smoke and off-gases in a developing fire situation, thus keeping secondary fire damage as low as possible. The classes of phosphorus-based FRs shown in Figure 1 are mainly used in polyamides, polyesters, polyolefins and styrenics. The main area of application for the compounded materials is injection-moulded electrical and electronic (E & E) parts. The flame retardant classification will typically be UL 94 V-0, V-1 or V-2. Other tests, such as glow wire scenarios, are important for certain segments of this market. Besides the electrical market, a very important market is flame retardant fabrics for public buildings and public transportation seating. Figure 1: Examples of phosphorus-based flame retardants. Polyamides For decades, the glass fibre-reinforced polyamides (PA) market segment was dominated by two very different types of FRs - brominated polystyrenes (or 26 ISSN1464-391X/07 2007 Elsevier Ltd. All rights reserved.
Table 1: UL 94 classifications of polyamides containing phosphinate based FRs Figure 2: Metal salts of organic phosphinic acids chemical structure. PA 6 Exolit OP 1311 PA 66 Exolit OP 1312 HT-PA (PPA) Exolit OP 1230 UL 94 V-0 UL94 5V A 1/64 1/32 1/16 1/8 1/32 1/16 1/8 20 18 16 20 19 18 16 15 19 18 16 15 14 12 16 15 poly-bromostyrenes) and red phosphorus (for PA 66, mainly in Europe). In recent years a new flame retardant chemistry has joined them for polyamides. Products based on organic phosphinic acid salts (Figure 2) have become established not only for polyamides but also for thermoplastic polyesters, certain thermosets, and some additional niche applications. The organic phosphinates produced commercially do not show properties similar to those of inorganic salts but are hydrophobic fine powders with a very low residual solubility in water. For PA 6 and 66, synergistic blends with melamine derivatives are the products of choice. The phosphinate-based FRs are mainly used to achieve a V-0 classification according to the UL 94 test. Class 5V A can be reached as well. The required loading depends on several parameters, such as the type of PA polymer or blend, thickness of material, glass-fibre content, and flame retardant grade used. Table 1 shows typical dosages recommended for the most important PA resins (at 25 or 30% glass-fibre content) with the corresponding phosphinate or blend. In contrast to other halogen-free FRs, such as melamine cyanurate or red phosphorus, the phosphinate-based systems can be used at nearly all glass levels up to 40 or 50%, and with non-reinforced polyamides as well. Glass fibres are neither essential for the performance or cause antagonistic effects. Exolit OP 1311 and OP 1312 are synergistic blends with an optimized charforming ability. Due to the intrinsically better charring properties of high-performance polyamides, most of which are partially aromatic, the Exolit OP 1230 performs well on its own. Apart from the self-extinguishing ability of the materials, examined in small flame scenarios, other tests simulating electrical failures are crucial for the E&E market. In the hot-wire ignition (HWI) test according to UL 746 A (IEC 60 695-2-20), a glass fibre-reinforced PA 66 containing Exolit OP 1312 reaches the highest performance level category 0 (PLC 0), corresponding to a range-mean ignition time of more than 120 seconds. The required dosages are equivalent to the UL 94 5V A level. PLC 0 is achieved in the high-current arc ignition (HAI) test as well. More than 200 arcs are needed to ignite the flame-retarded PA 66 GR. The recently revised glow-wire test according to the European standard EN/IEC 60695 measures two key figures, GWFI (glow-wire flammability index) and GWIT (glow-wire ignition temperature). For electrical components used in household appliances in particular, it is often important to reach levels of 960 C (GWFI) and 775 C (GWIT). PA Figure 3: Heat release at different heat-flux settings. compounds with phosphinate-based FRs will usually pass both test levels on moulded plaques. However, the GWIT performance can be borderline in finished parts so adaptations of the formulation may be necessary. Fire test data from the cone calorimeter exhibit additional strengths of the phosphinate-based FRs [3]. The excellent char-forming behaviour leads to a low heat-release rate even at high heat-flux values. Figure 3 shows the enormous reduction of heat-release rate compared to the non-flame-retarded PA 66 compound. When exposed to a heat flux of 35 kw/m 2, the PA 66 specimen with Exolit OP 1312 does not even ignite. At 50 kw/m 2 heat flux, the time to ignition is increased by 30% from 56 to 73 seconds (not visible due to low resolution of the chart). Since the heat-release rate is the most important factor for the speed of fire propagation, these results confirm an excellent degree of protection against fire 27
spread. The property profile of PA 66 GR with the synergistic blend Exolit OP 1312 at the UL 94 V-0 level is highlighted by a relatively low loading, low material density and a high comparative tracking index. In order to determine the total performance profile of a flame-retarded PA 66, there are even more specific requirements in terms of application. For example, the phosphinatecontaining materials must be laser-markable, laser-weldable, non-corrosive to typical metals and alloys used as electrical contacts, and also stable enough under long-term heat-ageing conditions. Polyamides with high heat-deflection temperatures, such as HDT/A 280 C, are a rapidly growing group of speciality products in the E&E industry. These materials are also called high-performance polyamides (HPPA) or polyphthalamides (PPA). As flame-retarded compounds they can be used in the surface mount technology (SMT) of semiconductor components. Facing the stringent requirement of lead-free reflow soldering, these materials have to withstand peak temperatures of 260 C and more during the soldering process. HP-PAs can only compete with more expensive materials such as LCPs (liquid crystal polymers) if the FR system keeps its properties at the high level required by the application. The phosphinate Exolit OP 1230 has proven to be stable enough and provides efficient flame retardancy for HP-PAs (see Table 1). As well as good mechanical and electrical properties of the polymer compound, the processing performance is also highly important. Unlike other FR materials, a phosphinate-based compound shows no blistering effects in the solder dip test at 260 C. The product also reaches the excellent moisture sensitivity level (MSL) class 2, according to the standard IPC/Jedec J-STD-020-C [4]. As a new alternative to the existing brominated FRs used in HT-PAs, the product also features good weld-line strength of the phosphinate-containing material and good flow properties of the polymer melt in injection moulding. Polyesters For a long time, suitable phosphorus-based FRs for thermoplastic polyesters complying with the UL 94 V-0 level at low thickness were not available. The development of the phosphinate product group has closed this gap and opens up the opportunity for halogen-free flame-retarded PBT or PET. The first sets of glass fibre-reinforced and non-reinforced FR polyester compounds are now on the market. They also have interesting property profiles [5]. The synergistic formulations used in thermoplastic polyesters are different from those developed for polyamides, due to the different degradation behaviour of the polymers. Similar to the flame-retarded polyamides based on phosphinates, the new FR polyester compounds show advantages in density and electrical properties, compared to materials based on brominated FRs. In particular the CTI values that can be reached (Figure 4) allow for a broader range of applications with higher voltage requirements. On the other 28
Figure 4: CTI values of glass-reinforced PBT with different flame retardants (Non Hal = Celanex XFR 6840 GF 30). hand, a small loss in physical properties caused by the chemical structure of the FR system has to be taken into account. However, the phosphinate-based fire retardant improves the flowability of the PBT grades. Although they are a relatively young product group, the phosphinatecontaining FRs have been intensively studied in view of their toxicological and environmental properties [6]. To date the results are very satisfying and do not generate any concern related to their use in plastics. Moreover, phosphinatecontaining materials do not require separation under the new European Regulation on Waste Electrical and Electronic Equipment (WEEE) [7]. Polyolefins If phosphorus-containing FRs are used in polypropylene, polyethylene or polyolefin blends, they are usually part of an intumescent formulation. This term describes FRs that work exclusively by a strong char-forming mechanism out of the solid phase. The main component can be ammonium polyphosphate or another phosphate acting as an acid source during the surface pyrolysis of the polymer. Other ingredients in the formulation (synergists) support the phosphate in creating a char, through crosslinking and foaming reactions in the right sequence at the temperature level of the beginning polymer degradation. Typical loadings of these FRs for a UL 94 V-0 or 5VA classification are in the range 20-35% depending on grade, material thickness, melt-flow properties and possible other additives or fillers. Because their mechanism is completely different from brominated FRs, the flame-retarding effect of the intumescent systems is not linearly increased with the dosage. A dose of 20-25% is usually needed to raise the flame retardancy of a polypropylene from non-classifiable to V-0. The explanation is that a minimum amount of FR per polymer surface/mass is required to form the compact char layer that protects the polymer from further flame attack. On the other hand, linear results are obtained if the Limiting Oxygen Index (LOI) is chosen as a criterion for flame retardancy. The conditions are very different the flame is burning on top of the specimen, like a candle, and is not applied to the lower end of the specimen as in the UL 94 vertical test. The increase of FR dosage causes a linear rise in LOI values. In this case, the result is determined by the substitution of available fuel (polymer) by a FR system and not by the char formation at the surface. The main advantages of intumescent P- based FRs in polyolefins are: Very low smoke density in the developing phase of a fire. No corrosivity of the smoke/off gases, which is important for electronics. Low heat-release rates, therefore reduced speed of fire spread. Table 2: Physical properties of flame-retarded and neat polypropylenes PP Homopol. (Moplen HP500N*) with Exolit AP 760 PP Copolymer (Moplen EP340M*) With Exolit AP 760 FR loading [% by wt.] - 30-30 UL 94 class (1.6 mm) n.c V-0 n.c V-0 Density [kg/l] 0.90 1.02 0.905 1.03 E-Modulus [MPa] 1550 2300 1150 1300 Tensile yield stress [N/mm 2 ] 30 28 21 16 Tensile yield strain [%] 8 4 6 4 Impact str., Charpy [kj/m 2 ] 27 20 n.b. 70 Notched impact str. [kj/m 2 ] 3.0 1.9 45 7 * Moplen is a registered trademark of Basell Polyolefins 29
Low density of the compounded materials (approximately. 1.00-1.05 kg/l). No interference with HALS-type light stabilizers, which is an advantage for outdoor applications. Because they are filler-type powders, the intumescent FRs have an influence on the physical property profile of the polyolefin materials used. The rigidity of a polypropylene, for example, will be increased in terms of its tensile and impact strength as well as elongation (see Table 2). Variation of the base polymers can broaden the applicability of the intumescent FRs in injection moulding and extrusion applications. To date, intumescent systems can usually not be used in films or thin applications (approx. < 0.5 mm), or in parts with continuous water contact, such as drainage pipes, because of a slow loss of FR content over time due to residual solubility. Outdoor applications with occasional rain exposure are suitable. Typical applications are mainly moulded parts for the electrical industry or extruded profiles, such as cable ducts. Conclusions In thermoplastic engineering resins, such as polyamides and polyesters, phosphinatebased FRs are a new alternative to established products. Phosphinate-based FRs offer a well-balanced property profile with some major advantages, such as low material density, low heat-release rate and high tracking resistance. The outstanding thermal stability enables them to be applied in high-performance polyamides as well. For polyolefins, intumescent systems based on phosphates are state-of-the-art in the nonhalogen FR range. Intumescent systems have a similar set of material properties as shown for the engineering plastics. References [1] A. R. Horrocks and D. Price (Eds.): Fire Retardant Materials. Cambridge, Woodhead Publishing, 2001. [2] M. Le Bras, G. Camino, S. Bourbigot and R. Delobel (Eds.): Fire Retardancy of Polymers The Use of Intumescence. Cambridge, The Royal Society of Chemistry, 1998. [3] B. Schartel and U. Braun, Mechanisms of Phosphorus Flame Retardants. In: Flame Retardants 2006. London, Interscience, 2006, pp. 153-154 and unpublished results. [4] K.-J. Steffner, Lead-free Soldering. In: Kunststoffe plast europe 9/2005, pp. 195-198. [5] Ticona GmbH, Kelsterbach, Germany: Product literature on Celanex XFR PBT grades, 2006. [6] T. Marzi and A. Beard: The Ecological Footprint of Flame Retardants over their Life Cycle A Case Study. In: Flame Retardants 2006. London, Interscience, 2006, pp. 21-31. [7] European Union (2003): Directive 2002/96/EC of the European Parliament and of the Council on waste electrical and electronic equipment (WEEE). Office Journal of the EU, pp. L37/24-38. 30