Stricter Fire Standards For Appliances Require Higher-performance Grades

Thermoplastics used for unattended domestic appliances with a rated current over 0.2 A now must pass tougher flammability tests for assessing fire safety. This requires materials development work for manufacturers of polyesters and polyamides.

The Glow-Wire Ignition Temperature (GWIT) determined according to the standard IEC 60695-2-13, with its new relevance for domestic appliance applications as part of IEC 60335-1, is just one example of constantly changing flame-retardancy standards.

The main reason for these changes is that different groups are pursuing differing interests. For example, country-specific testing procedures are competing with the efforts of the European Union in harmonizing testing methods, while consumer and environment associations focus on ecological aspects. Plastics manufacturers and component producers increasingly need economic viability.

The GWIT is a measure of ignitability. To determine this temperature, three test specimens are brought into contact with a glowing wire, each for 30 s. The material passes if it does not ignite in any of the three specimen tests and/or a flame is visible for no longer than 5 s. It receives its “grade” in the form of a so-called GWIT classification, which is 25°C higher (30°C in the case of testing temperatures between 900 and 960°C) than the maximum temperature of the tip of the glow wire that did not cause ignition of the material in the three tests.

Until now, the tests have concentrated on the burning or glowing times of the specimen and its drip behavior in a fire. To this end, glowing wire tests were carried out either on the finished part according to IEC 60695-2-11, or on plaques, in line with IEC 60695-2-12. This enables determination of the Glow-Wire Flammability Index (GWFI), which is the temperature at which a flame or glowing area on the specimen is extinguished within 30 s of removal of the glowing wire. In addition, a piece of tissue paper placed underneath the specimen must not be ignited by dripped material or glowing pieces of the specimen.

The new test for appliances

Plastics for parts carrying over a 0.2-A current in unattended domestic appliances such as refrigerators, washing machines, and dishwashers now must undergo testing under IEC 60335-1 (4th edition). The test is depicted schematically in Fig. 1. A material can only be considered usable if a specimen proves that it has a GWFI of at least 850°C at a thickness conforming to the wall thickness of the application. The test procedure is dependent on whether a sheet of the material can pass the test according to IEC 60695-2-13 with a GWIT of 775°C.

If it does, it may be used without restriction for the domestic appliances listed in the standard. If not, further time-consuming tests must be carried out on the final component. In such a case, the standard first prescribes a glow-wire test at 750°C. If the part does not burn for longer than 2 s when in contact with the wire, it may be used for the intended application without any further testing. If the time is over 2 s, the components in its immediate vicinity in a subassembly must, at a minimum, be classified under the U.S. test standard UL 94 V-1. Alternatively, the finished part can undergo a needle flame test.

This stricter test method for domestic appliances affects conventional thermoplastics differently. For polycarbonate, it is inconsequential because even product grades that have not been made flame-retardant pass the glow-wire test without problems with the specified GWIT of 775°C. Conventional polyesters and polyamides, on the other hand, no longer are able to comply, which is why the market has been demanding improved grades.

New, partially halogen-free polyester and polyamide grades that are able to satisfy the stricter specifications have been developed. However, polyamides and polyesters can only establish themselves in a wide variety of applications if they also have good mechanical and electrical properties. In addition, they are only allowed to release minimal quantities of volatile components when subjected to heat, to prevent corrosion of metal contacts.

Halogen-free polyester grades meet tougher specs

Pocan DP 2004 is a non-reinforced, halogen-free polybutylene terephthalate (pbt) that is able to satisfy the criteria of IEC 60335-1. Another new material is Pocan DP 4035, a halogen-free flame-retardant polyester with 30% glass-fiber reinforcement, for electrical and electronics applications. Both polyesters (Table 1) have improved electrical properties (e.g., tracking resistance) and smoke behavior compared to traditional halogen-containing counterparts. They have, for example, high arc resistance, as determined by standard tests according to UL 746 A and ASTM D 495. In addition, these polyesters offer advantages in material recycling, particularly with respect to imminent EU regulations on electrical scrap. The latest version stipulates that parts made of halogen-free grades of a certain plastic must not require separation for recycling. This is designed to significantly simplify recycling of end-of-life appliances.

Both materials have low tendency to corrode and show little discoloration under heat, which means that components made from them preserve esthetics in long-term service. In addition, they comply with the specifications of the common environmental labels such as “Blue Angel,” “TCO ‘99,” and the “EU Environment Label.” Their applications include coil carriers, caps for electric safety switches, plug-and-socket connectors, and edge connectors.

FR polyamides offer better performance

Two fully halogen-free and three bromine-containing polyamide grades currently being launched pass the stricter tests governing household appliance applications (Table 2). Even at low wall thicknesses, they pass gwit glow-wire tests, in most cases significantly exceeding the minimum requirement of 775°C and thus providing an added safety margin. All GWFI values are above 850°C. This means that users can avoid expensive and time-consuming testing of finished components under IEC 60335-1. The GWIT results already are listed with Underwriter Laboratories.

Internally-conducted tests on grade Durethan DP2-2851/30 H1.0 (Table 2) show that even complex-shaped components made of the nylon pass the glow-wire test under IEC 60335-1, not just at 750°C, but well above 800°C.

All five grades exhibit outstanding electrical data (Table 2). For example, the tracking resistance of Durethan DP2-1851/30 and Durethan DP2-2851/30 H1.0 (heat-stabilized), in terms of CTI A (comparative tracking index), is well above 400 V, which is high for polyamides with a bromine-based flame-retardant. The two halogen-free flame-retardant grades, DP 2801 and DP 2802/30, attain peak values of 600 V.

Potential electrical/electronic applications for the materials are plugs, cable straps, cable run supports, electrical safety switches and caps, and components for satellite dishes.

However, there is a focus on white goods, in view of the stricter IEC 60335-1 specifications. Conceivable applications include housing parts for pumps, electrical switches integrated in the control panel of washing machines or dishwashers, light-bulb sockets in refrigerators, edge connectors, and plug-and-socket connectors. Recently, it has been observed in domestic appliances that some of the parts in the immediate vicinity of current-carrying components are also being made in flame-retardant plastics. One example is a nylon drip tray in a well-known brand of refrigerator.

*Michael Wagner is project manager responsible for development and market launch of flame-retardant thermoplastic polyesters and nylons for Bayer Polymers, Leverkusen, Germany. Information on the new polyester and polyamide grades can be found at www.plastics.bayer.com.