" The use of flame retardants in wires and cables must maintain the physical, rheological and electrical properties of the polymers to a level sufficient to ensure their safe and proper functioning. "
Taking into the account the amount of electrical and electronic equipment we use everyday, there is a jungle of cables to be found in every building, road side signalling, transport system engineering and so on!
50 years ago, cable insulation was usually made of textiles, which not only created a risk of exposed wires, but could also affect a wire’s electrical performance. Since then, strong and durable plastics have been developed and used as insulation material, which also offer vastly improved electrical properties. This has been particularly important for cables, which transfer data between interfacing devices, such as between a computer and a printer, which require particularly efficient plastics.
With the exception of chlorinated and fluorinated polymers, wire and cable insulation materials are typically good combustibles (internal and external cables for electrical and electronic appliances only).
One single set of computer cabling is estimated to result in 400g of cable per m2 of office space - a potential heat emission in a fire of 4 MJ/m2.
To decrease the fire hazard, flame retardants are used to reduce flammability. Apart from the ignition risk in the event of overheating, short circuits or electrical faults, the specific risk associated with wires and cables is their ability to spread the fire to the socket, curtains or wall, or to the equipment itself.
Material challenges in wires and cables insulation
Beyond flammability, the flame retardants used in E&E wires, cables and cords directly influence the properties listed below. The flame retardant polymer systems currently in use have been carefully developed and selected to meet a large number of technical and economic challenges, in particular, they must:
- Have outstanding electrical insulation properties
- Have excellent physical properties
- Demonstrate good resistance when used under specific conditions
- High flexibility is especially important for flexible E&E external cords. Various flexural tests are used to measure and specify these parameters in cable insulation materials which tend to be affected by the type of flame retardant used. While certain flame retardant fillers tend to reduce flexibility, melt-blendable flame retardants can positively impact polymer properties.
- Acceptable resistance to abrasion: especially important for E&E external cords, flame retardants are evaluated via specific testing procedures such as the “taber abrasion test” (ISO 9352). Flame retardant fillers will have a stronger impact on abrasion than melt-blendable flame retardants.
- Good resistance to ageing: over time, constant manipulation of the cables by plugging and unplugging and exposure to factors such as humidity or ultraviolet light can lead to brittleness and in turn lead to electrical failure. Insulation materials of internal and external cables must have good ageing properties in order to avoid this.
- Typical household chemicals: the above properties and the fire safety performance need to be maintained even when in contact with certain household chemicals. Power cords of kitchen appliances can be exposed to a significant amount of oils or acid (e.g. vinegar) over their lifetime and this should not affect their properties.
Use of Flame Retardants
The choice of flame retardants for wires and cables in E&E appliances is largely dependent on the desired physical and electrical properties of the final insulation material. These are determined by the polymer, the fire safety regulations and the applicable standards.
Another element defining suitable flame retardants is the processing temperature of the resulting material. Ensuring that the processing temperature stays below the degradation temperature of the flame retardant is essential.
PVC itself has relatively low flammability thanks to its chlorine content. Inorganic synergists such as antimony trioxide can be added to enhance fire safety performance of the PVC resin.
Phosphorous flame retardants
Non-flame retardant plasticisers, which can be considered as “solvent” for PVC, are typically added to the resin to obtain the necessary flexibility for wire and cable applications. However, the use of these conventional plasticisers can increase the flammability of PVC. In this case, phosphate based plasticisers like triaryl phosphates (TXP, TCP) can be used to obtain both flexibility and good flame retardancy. These flame-retardants-plasticisers have the added advantage of preserving the good electrical and physical properties of PVC.
Bis-aryl phosphates (BDP, CDP) provide good low temperature flexibility in thermoplastic elastomers and rubbers such as EPDM, SBR, NBR or TPU. They are also recommended for low smoke formulations.
Mineral flame retardants
For wires and cables made of polyolefins like LDPE, LLDPE or PP, mineral flame retardants are often used – mainly finely precipitated Aluminium Tri-hydroxide (ATH) and Magnesium Dihydroxide (MDH). While relatively inexpensive, they usually require high loadings of up to 60% of the weight of the final material. Zinc borate can be used in combination with ATH and MDH to enhance smoke suppression.
The thermal stability of MDH is considerably higher than that of ATH. Whereas ATH starts to release water at about 220°C, MDH remains stable up to about 340°C. ATH and MDH also provide additional advantages in terms of smoke density.
Given that smoke formation is one of the key fire safety parameters for cables, these compounds are also used as smoke-suppressants in cable insulation materials.
Bromine flame retardants
Due to their specific chemical structure, thermoplastic elastomers and rubbers such as EPDM, SBR, NBR or TPU demonstrate high elastic properties, and can be processed at high temperatures. These are, therefore, a material of choice for cables in E&E applications. Their excellent physical properties can however be affected by high flame retardant loadings. Brominated flame retardants, being particularly efficient, are often used to minimise the additive loading.
Ethane bis (pentabromophenyl) (EBP), Ethylene bis(tetrabromophthalimide) (EBTBP), Poly(pentabromobenzlyacrylate), Poly Brominated Styrene Copolymers, Poly Brominated Styrene Homopolymers and Tetrabromophthalate Ester are often applied to thermoplastic elastomers and rubbers with a typical loading of 12-15%wt, in combination with a lower loading of metal oxides synergists (4-5% wt), typically antimony or zinc.
Safer cables for higher quality energy
Franck Poutch and Skander Khelifi, CREPIM
HIPS, ABS, PC/ABS and PPE/HIPS enclosures pose a significant fire safety concern. For years electronic enclosures have been the key drivers for flame retardancy research, and there is ongoing work to develop new fire safety strategies.
Low smoke, low spread of flame and low heat release are key fire safety requirements for cables. Heat dissipation during functioning also has to be monitored. Depending on the plastic matrix, mineral hydroxides, halogenated systems and phosphorous flame retardants are widely used.
Research at the nanoscale level is significant in order to improve mechanical and thermal properties. Nanoclays are widely known to reinforce the char cohesiveness of the blend and are an alternative to current fire resistant barriers used in cables. For instance, using organically modified synthetic clay based on metallic hydroxide as synergist promotes char formation and can result in the reduction of mineral flame retardant loadings.
Regarding raw material approaches, new fluoropolymer compounds for cables have now been developed with an optimum balance of properties (scavenging acids during combustion). Likewise, modified PVC has been processed to generate less smoke and change the smoke characteristics.
Safer cables will protect people and infrastructure but will also facilitate network management with a lower occurrence of fire and its repercussions for installations.
1 G. Lougheed, C. McCartney, M. Kanabus-Kaminska, Initial investigations on plenum cable fires, National Research Council of Canada (NRCC-45133).