This blog is translated from German with DeepL.
Fire protection for plastics – a requirement with an important role to play
The use of plastics often requires compliance with certain fire protection standards. Relevant standards are found primarily in electrical engineering, which are also constantly adapted to the current state of the art and have country-specific differences. Common test marks on electrical equipment are IEC, VDE, UL, DIN, CE, GS and other institutions. Often used side by side to market a device in different countries. The requirements concerning fire protection necessitate a careful selection of the plastic to be used and also have design influences.
Fire protection standards
The requirements for plastics are described in the respective standards for electrically operated devices and equipment. The measures specified relate not only to the plastic itself but also to the design of the component. On the one hand, materials should be selected that exhibit low flammability and flame spread, but on the other hand also ensure that the temperature is limited by design in the event of a malfunction. In Europe, the International Electrotechnical Commission (IEC) has established itself in this area. This is followed by the European derivatives DIN and VDE. In the USA, the Underwriter Laboratories (UL) is used, and in Canada, where the safety regulations of the Canadian Standards Association apply, the CSA is used accordingly. In principle, IEC 60947, which is authoritative in Europe, and UL 508, which is authoritative in the USA, are of central importance. For domestic installations, it is IEC 60898 and UL 1077. For domestic appliances, the more stringent IEC 60335 (with 85 sub-standards) and UL 60335 are the basis. For rail vehicles, the French standards NF 16101 or DIN EN 45545 may also be of importance.
Differences between fire protection standards
All standards pursue the same goal: “The greatest possible protection of life and property”. However, the specifications, the scope of testing and the method regarding fire behavior are different. The principles of material and component approval are accordingly opposed. UL relies on the testing of materials and documents this in the so-called yellow card. The requirements for plastics are summarized in UL 746 C (Polymeric Materials – Use in electrical equipment evaluations). The European IEC focuses on either materials or component testing, depending on the area of application.
Fire behavior testing
In all test methods, the ignition source and its effect on the test specimen are specified. The essential point of the test is the fire behavior of the material or the finished component. Different test methods measure
- the flammability,
- the flame spread (self-extinguishing),
- the contribution to heat generation and
- the evolution of fire gases and smoke.
Among the essential test methods for components is the glow wire test according to IEC 60695, in which a hot wire is pressed against the molded part for a specified period of time and it is determined whether the test specimen ignites, how long it burns and whether burning material falls off.
In materials testing, the flame test according to UL 94 is of great importance. The Bunsen burner flame acts twice briefly on the test specimen. The burning time and the falling parts on a cotton ball are determined. The result is a classification of flammability class V (vertical: UL 94 or horizontal: HB).
The flame test always depends on the wall thickness of the specimen and should be considered together with the HWI (Hot-Wire-Ignition-Test) and the HAI (Highcurrent-Arc-Ignition-Test).
Classification is in levels from 0 to 4, with 0 being the best rating. This results in UL 746C approval.
UL 746C approval can be explained simply: It is the combination of the UL 94 fire rating according to UL 94 (V0 to V2) with the HWI and HAI values.
The red fields in the table “Material requirements for E+E” correspond to the approval.
Tips for developers
- The fire behavior of engineering plastics varies greatly.
- The melting point of thermoplastics does not provide any information about the fire behavior and is therefore only relevant for production.
- The Low Oxygen Index (LOI) is relevant with regard to fire behavior.
LOI defines the necessary oxygen content in the circulating air at which the material just no longer burns. With an LOI of 15, for example, POM is considered to be readily combustible. For polyamide and PBT, this is around 24 and can be raised with a suitable flame retardant. The high-performance plastics polysulfone and polyethersulfone have an LOI of 30 to 38 – and are thus not flammable in normal atmospheres and therefore do not require additional flame retardants.
- Universal flame retardants are substances containing halogen and bromine. However, due to legal regulations, components containing these additives must be recycled separately and at high cost. For this reason, the market is tending towards halogen-free flame retardant systems, but these are no longer universally applicable and also have other effects on the properties of the material.
- During design, attention must be paid to the permissible wall thicknesses of the respective materials. Example: For a component made of a material with UL V-0 certification from 0.8mm on the Yellow Card, the minimum wall thickness must be 0.8mm for the component to be considered V-0. If all components of a product are UL V-0 certified, it can be assumed that the entire product is V-0 compliant. If, on the other hand, not all components of a product are V-0 compliant, the product may well be V-0, e.g. if only small amounts of non-V-0 compliant material are used. However, it is mandatory to prove this with a flame test in an accredited laboratory.
- Due to the EU Waste Electrical/Electronic Equipment (WEEE) regulation, as well as for health protection reasons, halogen-free flame retardants are the target. When selecting the color of the product in the design phase, attention must already be paid to the flame retardant certification with the specified colors and wall thicknesses. Depending on the flame retardant, plastics cannot be colored in all colors. Polyamides, for example, become reddish when red phosphorus is used as a flame retardant. If you are looking for lighter shades, you need plastics with other flame retardants.