Introduction
The heat resistance of polymers is one of the most important properties that must be carefully evaluated in various industries such as construction, automotive, cable manufacturing and electrical equipment. Many polymers show different behaviors against fire; some ignite quickly, some produce dense smoke and others extinguish automatically. Proper understanding of these behaviors helps manufacturers and consumers to select more suitable materials and increase the safety of processes and products.
This article examines three key indicators for evaluating the thermal safety of polymers: LOI index, heat release rate (HRR) and smoke density (NBS).
Indicators for evaluating the heat resistance of polymers
1. LOI index; indicates the ability of the polymer to extinguish or continue to burn
One of the best-known criteria for measuring the heat resistance of polymers is the limiting oxygen index or LOI (Limited Oxygen Index). This index indicates how much oxygen a material needs to continue burning.
How is LOI calculated?
To calculate LOI, the minimum concentration of oxygen (O₂) and the minimum concentration of nitrogen (N₂) required to sustain a flame are measured. The higher the LOI of a polymer, the more oxygen it needs to burn and, as a result, the harder it is to ignite.
What does LOI mean?
For example, PVC polymer typically has an LOI of around 45; this means that in normal air with 21% oxygen, it will quickly extinguish if the fire source is removed. Therefore, polymers with a high LOI are inherently self-extinguishing and safer.
2. Heat Release Rate HRR; Actual Fire Strength Index
The heat release rate, or HRR, is a measure of how much energy a material produces per second when it burns.
This index is measured in kW/m² and is one of the most important criteria for determining the intensity and power of a fire.
What is the importance of HRR?
• If the HRR value of a polymer is high, the fire will spread quickly.
• The lower this value, the lower the probability of widespread and dangerous ignition.
• In various industries such as automotive, aerospace, cables and construction, strict standards for HRR have been defined.
Wood is usually used as a standard reference for comparing the behavior of materials during combustion.
Comparison of LOI and HRR in some polymers
3. NBS Chamber; Measuring Smoke Production and Haze
In many fires, smoke is the primary cause of death and damage; not flame. For this reason, assessing the amount of smoke produced by polymers is of great importance.
How is the NBS test performed?
A small sample of the material is placed in a closed chamber and examined in two states:
1. Flaming – the sample is directly exposed to flame.
2. Smoldering – the material is exposed to thermal radiation to simulate the pyrolysis process.
In this test, the amount of light transmitted through the smoke is measured and a parameter called the specific optical density (Ds) is reported.
A higher Ds number means thicker smoke, less visibility and greater risk.
PVC Behavior Against Heat and Smoke
PVC is one of the polymers that is considered self-extinguishing compared to many polymeric materials. However, when burned, it produces black smoke and toxic gases.
Common PVC test results according to ASTM E662
• Flameless state: about 300 Dmc
• Flaming state: about 600 Dmc
Effect of additives on PVC performance
• Inorganic compounds such as calcium and hydrogen chloride can reduce smoke density by about 25 to 35 percent.
• Antimony oxide usually increases smoke.
• In cables, the use of molybdenum zinc or molybdenum aluminum compounds has a better performance in reducing smoke.
Low-smoke and flame retardant additives for PVC
To increase the heat resistance of polymers to heat and reduce smoke, special compounds are used, which are called FRLS (Flame Retardant Low-Smoke) additives.
FRLS Additive Objectives
• Increase LOI to 28-32%
• Prevent toxic gas emissions
• Reduce smoke and improve thermal performance
Effective Compounds
• Aluminum Trihydrate
• Magnesium and Zinc Compounds
• Antimony
• Molybdates
• Metal Oxides such as CuO and Fe₂O₃
These materials reduce smoke and harmful gases by creating a protective layer (char) or aiding more complete combustion.
Summary; Why is it important to know the thermal indices of polymers?
In today’s industries, polymer selection is not based solely on price or strength. Heat resistance of polymers and their behavior during fire are one of the main decision criteria.
Indices such as LOI, HRR, and smoke density help us understand how safe a polymer is and how it reacts in fire conditions.
Appropriate additives and formulations can also improve these properties and multiply the final safety of the product.
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