The choice of plastic and its implications on the fire resistance of wires and cables

Polyvinyl Chloride (PVC) has a wide range of applications ranging from consumer products such as packaging materials and credit cards to pipelines and construction materials. Compared to Polyethylene (PE) which has a similar structure, PVC is formed by replacing one of the hydrogen atoms in PE with a chloride atom.  The strong bonding between hydrogen and chloride increases the strength of PVC over PE. In addition, by releasing chloride radicals under high temperature which react to hydrogen radicals or hydroxyl radicals in a fire, PVC is also relatively fire resistant and PVC can also be made more flexible by adding plasticizers.  These properties of PVC make it very versatile in usage. In 2008, global PVC production reached 34 million tons.  According to the PVC Handbook published in 2005, about 40% of the PVC manufactured was used for insulation and jacketing for wires and cables, owing to the characteristics mentioned above. Despite its popularity, PVC emits a large amount of toxic dioxins when burnt.  The growing advocacy for a halogen free environment by environmental organizations has led to the development of Low Smoke Zero Halogen (LSZH) materials for the replacement of PVC.  However, the flame retardant performance of different materials varies greatly.  When designing wires and cables, manufacturers should therefore choose an insulation material with the appropriate fire resistant performance based on application.

Analysis of the fire resistance of materials:
UL had, in early years, conducted research on the fire resistance of materials and developed the UL 910 (NFPA262) Steiner Tunnel Test which yields a result close to a real life burning situation.  Table 1 shows the results of the fire tests jointly conducted by UL and the Building Research Establishment/ Fire Research Station.  The report was published via the Society of Plastics Industry.  Five different types of communication cables were fire-tested, namely CMX/T, CMP, CMX, IEC 332-1 (LSZH) and IEC 332-3 (LSZH) under a simulated real life environment and in the UL Steiner Tunnel.  The two test methods enable us to observe how quickly the fire spreads and how thick the smoke is when different cables are burnt.

1 CMX cable in metal conduit:
Analysis of the fire test results of 5 communication cable types by the UL 910 Steiner Tunnel

Test and a simulated real life fire
The results show that during the tests, the CMX cable having a PE insulation and PVC jacket gave out extremely thick smoke and was completely burnt. In contrast, the CMP cable with fluoro polymer insulation and jacket did not generate excessive smoke and the spread of the fire was contained within 20% of the total length of the cable. Though LSZH cables did not produce excessive smoke, the cables were completely burnt within an extremely short period of time due to their poor fire resistance performance.

                 Lastly for the CMX/T test, though the same CMX cable was used, the metal conduit served as a protector and a fire insulator resulting in very positive outcomes.In this regard, the choice of material has great implications on the fire resistance of the cables.  Taking CMX cable as an example, it uses the more fire resistant PVC as jacket but PE as insulator due to cost considerations. This combination yields the poorest overall result in the tests.

Future research direction:
To help wire and cable manufacturers design a product that not only meets safety and quality standards but is also economical to produce, UL has been investigating various fire test methods that suit the fire resistance requirements of cables using different materials.  To understand the fire resistance performance of a material, it is imperative that we also look into the characteristics of the material itself.  Hence, in addition to developing faster and more appropriate fire test methods, UL has in recent years also strengthened its research on material characteristics:

1. Whether there is a quantitative relationship between the heat released from the material under fire and the burning behaviour,
2. Implications of the composition and structure of additives on fire resistance performance, and
3. Whether material aging has an impact on fire resistance performance, etc.

In 2009, the Corporate Research Department of UL Taiwan set up an Advanced Materials Science Laboratory to further the understanding of material characteristics. The laboratory is equipped with various advanced material analysis equipment, including Field Emission Electron Microscope, Energy Dispersive X-ray Spectrometer, CT Scanner, Temperature-Controlled Tensile Tester, Calorimeter (Cone Calorimeter and Oxygen Bomb), Thermal Analysis Instruments (DMA, DSC, TGA-MS-IR) and Composition Analysis Equipment (GC/MS, ICP, FT-IR, UV).  In future, new test instrumentation and other advanced material analysis equipment will be added to further enhance research capacity. 

                In addition, the UL Taiwan Corporate Research Department is also looking into safety tests and material analysis technology for new energy sources such as Lithium batteries, solar cells and fuel cells. We are seeking cooperation with other organizations such as the Industrial Technology Research Institute, Plastics Industry Development Center and Institute of Nuclear Energy Research to develop standards and regulations that meet market requirements and at the same time addresses public safety through the provision of more detailed and accurate research results.

For more information
Manish Bhatnagar
Director - Sales and Marketing
Underwriters Laboratories
For further information, please contact:
Divya Narayan, UL India
E-mail: Divya.Narayan@in.ul.com
Tel: +91-80– 41384400