\nThis article examines three key indicators for evaluating the thermal safety of polymers<\/a>: LOI index, heat release rate (HRR) and smoke density (NBS).<\/p>\n
Indicators for evaluating the heat resistance of polymers<\/h2>\n
<\/p>\n
1. LOI index; indicates the ability of the polymer to extinguish or continue to burn<\/strong><\/h3>\nOne of the best-known criteria for measuring the heat resistance of polymers<\/strong> is the limiting oxygen index or LOI (Limited Oxygen Index). This index indicates how much oxygen a material needs to continue burning.<\/p>\nHow is LOI calculated?<\/strong><\/p>\nTo calculate LOI, the minimum concentration of oxygen (O\u2082) and the minimum concentration of nitrogen (N\u2082) 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.<\/p>\n
What does LOI mean?<\/strong><\/p>\nFor 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.<\/p>\n
2. Heat Release Rate HRR; Actual Fire Strength Index<\/strong><\/h3>\nThe heat release rate, or HRR, is a measure of how much energy a material produces per second when it burns.
\nThis index is measured in kW\/m\u00b2 and is one of the most important criteria for determining the intensity and power of a fire.<\/p>\n
What is the importance of HRR?<\/strong>
\n\u2022 If the HRR value of a polymer is high, the fire will spread quickly.
\n\u2022 The lower this value, the lower the probability of widespread and dangerous ignition.
\n\u2022 In various industries such as automotive, aerospace, cables and construction, strict standards for HRR have been defined.<\/p>\nWood is usually used as a standard reference for comparing the behavior of materials during combustion.<\/p>\n
Comparison of LOI and HRR in some polymers<\/h4>\n
<\/p>\n
![\"\"](\"http:\/\/irplastics.com\/en\/wp-content\/uploads\/2025\/12\/heat-resistance-of-polymers-300x168.webp\")
<\/p>\n
<\/p>\n
3. NBS Chamber; Measuring Smoke Production and Haze<\/h3>\n
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.<\/p>\n
How is the NBS test performed?<\/strong><\/p>\nA small sample of the material is placed in a closed chamber and examined in two states:<\/p>\n
1. Flaming \u2013 the sample is directly exposed to flame.<\/p>\n
2. Smoldering \u2013 the material is exposed to thermal radiation to simulate the pyrolysis process.<\/p>\n
In this test, the amount of light transmitted through the smoke is measured and a parameter called the specific optical density (Ds) is reported.<\/p>\n
A higher Ds number means thicker smoke, less visibility and greater risk.<\/p>\n
PVC Behavior Against Heat and Smoke<\/h4>\n
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.<\/p>\n
Common PVC test results according to ASTM E662
\n\u2022 Flameless state: about 300 Dmc
\n\u2022 Flaming state: about 600 Dmc<\/p>\n
Effect of additives on PVC performance<\/h4>\n
\u2022 Inorganic compounds such as calcium and hydrogen chloride can reduce smoke density by about 25 to 35 percent.
\n\u2022 Antimony oxide usually increases smoke.
\n\u2022 In cables, the use of molybdenum zinc or molybdenum aluminum compounds has a better performance in reducing smoke.<\/p>\n
Low-smoke and flame retardant additives for PVC<\/p>\n
To increase the heat resistance of polymers<\/strong> to heat and reduce smoke, special compounds are used, which are called FRLS (Flame Retardant Low-Smoke) additives.<\/p>\nFRLS Additive Objectives
\n\u2022 Increase LOI to 28-32%
\n\u2022 Prevent toxic gas emissions
\n\u2022 Reduce smoke and improve thermal performance<\/p>\n
Effective Compounds
\n\u2022 Aluminum Trihydrate
\n\u2022 Magnesium and Zinc Compounds
\n\u2022 Antimony
\n\u2022 Molybdates
\n\u2022 Metal Oxides such as CuO and Fe\u2082O\u2083<\/p>\n
These materials reduce smoke and harmful gases by creating a protective layer (char) or aiding more complete combustion.<\/p>\n
Summary; Why is it important to know the thermal indices of polymers?<\/h5>\n
In today’s industries, polymer selection is not based solely on price or strength. Heat resistance of polymers <\/strong>and their behavior during fire are one of the main decision criteria.<\/span><\/p>\nIndices such as LOI, HRR, and smoke density help us understand how safe a polymer is and how it reacts in fire conditions.
\nAppropriate additives and formulations can also improve these properties and multiply the final safety of the product.<\/p>\n
<\/p>\n
Get in Touch with Afshin Karami’s Trading Company (Avan Zar Tejarat Pishro)<\/h6>\n
How is LOI calculated?<\/strong><\/p>\n To calculate LOI, the minimum concentration of oxygen (O\u2082) and the minimum concentration of nitrogen (N\u2082) 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.<\/p>\n What does LOI mean?<\/strong><\/p>\n 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.<\/p>\n The heat release rate, or HRR, is a measure of how much energy a material produces per second when it burns. What is the importance of HRR?<\/strong> Wood is usually used as a standard reference for comparing the behavior of materials during combustion.<\/p>\n <\/p>\n <\/p>\n 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.<\/p>\n How is the NBS test performed?<\/strong><\/p>\n A small sample of the material is placed in a closed chamber and examined in two states:<\/p>\n 1. Flaming \u2013 the sample is directly exposed to flame.<\/p>\n 2. Smoldering \u2013 the material is exposed to thermal radiation to simulate the pyrolysis process.<\/p>\n In this test, the amount of light transmitted through the smoke is measured and a parameter called the specific optical density (Ds) is reported.<\/p>\n A higher Ds number means thicker smoke, less visibility and greater risk.<\/p>\n 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.<\/p>\n Common PVC test results according to ASTM E662 \u2022 Inorganic compounds such as calcium and hydrogen chloride can reduce smoke density by about 25 to 35 percent. Low-smoke and flame retardant additives for PVC<\/p>\n To increase the heat resistance of polymers<\/strong> to heat and reduce smoke, special compounds are used, which are called FRLS (Flame Retardant Low-Smoke) additives.<\/p>\n FRLS Additive Objectives Effective Compounds These materials reduce smoke and harmful gases by creating a protective layer (char) or aiding more complete combustion.<\/p>\n In today’s industries, polymer selection is not based solely on price or strength. Heat resistance of polymers <\/strong>and their behavior during fire are one of the main decision criteria.<\/span><\/p>\n Indices such as LOI, HRR, and smoke density help us understand how safe a polymer is and how it reacts in fire conditions. <\/p>\n2. Heat Release Rate HRR; Actual Fire Strength Index<\/strong><\/h3>\n
\nThis index is measured in kW\/m\u00b2 and is one of the most important criteria for determining the intensity and power of a fire.<\/p>\n
\n\u2022 If the HRR value of a polymer is high, the fire will spread quickly.
\n\u2022 The lower this value, the lower the probability of widespread and dangerous ignition.
\n\u2022 In various industries such as automotive, aerospace, cables and construction, strict standards for HRR have been defined.<\/p>\nComparison of LOI and HRR in some polymers<\/h4>\n
<\/p>\n3. NBS Chamber; Measuring Smoke Production and Haze<\/h3>\n
PVC Behavior Against Heat and Smoke<\/h4>\n
\n\u2022 Flameless state: about 300 Dmc
\n\u2022 Flaming state: about 600 Dmc<\/p>\nEffect of additives on PVC performance<\/h4>\n
\n\u2022 Antimony oxide usually increases smoke.
\n\u2022 In cables, the use of molybdenum zinc or molybdenum aluminum compounds has a better performance in reducing smoke.<\/p>\n
\n\u2022 Increase LOI to 28-32%
\n\u2022 Prevent toxic gas emissions
\n\u2022 Reduce smoke and improve thermal performance<\/p>\n
\n\u2022 Aluminum Trihydrate
\n\u2022 Magnesium and Zinc Compounds
\n\u2022 Antimony
\n\u2022 Molybdates
\n\u2022 Metal Oxides such as CuO and Fe\u2082O\u2083<\/p>\nSummary; Why is it important to know the thermal indices of polymers?<\/h5>\n
\nAppropriate additives and formulations can also improve these properties and multiply the final safety of the product.<\/p>\nGet in Touch with Afshin Karami’s Trading Company (Avan Zar Tejarat Pishro)<\/h6>\n