Fire Resistance (LOI)

Fire Resistance of Plastics

Flammability of Plastics

Combustion is a complex physio-chemical process between combustible material and oxygen, accompanied by heat release and light emission. “Flammability” is referred as capability of ignition, flame spreading and heat generation.

Flammability of Plastics is a result of the chemical nature of the carbon and hydrogen atoms in polymer chain, which are readily burned to produce carbon dioxide and water vapor.

Also, presence of other atoms like halogen atoms, aromatic compounds etc. or insufficient burning conditions (heat, oxygen…) can result in additional by-products upon burning of polymers.

Overall, aromatic polymers exhibit
greater flame resistance than aliphatic polymers

Polymers’ inherent flammability can be divided into basis classes:

Inherently Flame Retardant Less Flame Retardant Quite Flammable

These polymers can see high
flame retardancy with the
addition of additives

The ability of polymer to burn depends on fire conditions as well as polymer formulations

The flammability is influenced by several factors such as:

  • Ease of ignition – how rapidly a material ignites
  • Flame spread – how rapidly fire spreads across a polymer surface
  • Fire endurance – hoe rapidly fire penetrates a wall or barrier
  • Rate of heat release – how much heat is released and how quickly
  • Ease of extinction – how rapidly the flame chemistry leads to extinction
  • Smoke evolution
  • Toxic gas generation

Hence, the study of how plastics burn has been, and continues to be, a major area of research to examine the nature of the burning phenomenon in plastics, various methods to reduce plastics flammability, and methods to test flammability.

One of the widely used methods used to assess burning capability or flammability of polymer is: “Limiting Oxygen Index or LOI”.

Check out more on Fire Resistance (LOI):

What is Limiting Oxygen Index Test?

The purpose of the Limiting Oxygen Index (LOI) test, sometimes referred to as Oxygen Index (OI) or Critical Oxygen index (COI), is to measure the relative flammability of plastics and composite materials by burning them in a controlled atmosphere consisting of a mixture of oxygen and nitrogen.

The Limiting Oxygen Index represents the minimum level of oxygen in the atmosphere that can sustain flame on a thermoplastic material.

The higher the LOI value, the higher the non-flammability

The reasons for the differences between the polymers are various, but in particular two factors may be noted:

  • The higher the hydrogen to carbon ratio in the polymer, the greater is the tendency to burning (other factors being equal).
  • Some polymers on burning emit blanketing gases that suppress burning.

The test results relate only to the behavior of the test specimens under the conditions of this test method. The results must not be used to infer the fire hazards of the material in other forms or under other fire conditions.

Applications of LOI

LOI testing tool is used:

  • As a quality control tool, during manufacturing of products and assemblies
  • To indicate the potential flammability of a material
  • And, as a semi-qualitative indicator of the effectiveness of additives during R&D

It is one of the primary characterizing tools used by plastic and electric cable industries as well as in military and transport manufacturing sectors.

How to Calculate Limiting Oxygen Index of Polymers?

The limiting oxygen index (LOI) is calculated using the formula:

Calculating LOI

Here, O2 and N2 are the minimum (oxygen and nitrogen respectively) concentrations in the inflow gases to pass the minimum burning length criterion.

The units of limiting oxygen concentration are: Percentage, %

Air contains approximately 21% oxygen and therefore any material with an LOI of less than 21% will probably support burning in an open-air situation.

The most generally used standard tests to calculate dielectric strength are:

  • ASTM D2863 - Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index)

  • ISO 4589 - Determination of burning behavior by oxygen index:

    • Part 1: Guidance – It constitutes a guidance document for the OI test.
    • Part 2: Ambient-temperature test – It describes a method for determining the minimum concentration of oxygen by percentage volume in a mixture of oxygen and nitrogen introduced at 23°C ± 2°C that will just support combustion of a material under specified test conditions.
    • Part 3: Elevated-temperature test – It describes methods of carrying out the same determination over a range of temperatures typically between 25°C and 150°C (although temperatures up to 400°C may be used). It is not applicable to materials having an OI value of less than 20, 9 at 23°C.

(ofcourse there exist several other flammability testing methods as well such as flash point determination, determination of burning rates, but they are not discussed here).

Limiting Oxygen Index – Test Apparatus

Limiting Oxygen Index - Test ApparatusThe LOI test apparatus consists of a heat-resistance glass column that allows the burning of the specimen to be observed. A slow stream of oxygen and nitrogen are pumped in at the base of the chimney where they pass through a layer of glass beads that ensures even mixing before entering the main test chamber. A small glass flame is used to ignite the upper end of the specimen, and the subsequent burning behavior is monitored.

The objective is to find the minimum oxygen concentration in nitrogen that will result in sustained combustion for at least 3 minutes or excessive flame propagation down the specimen.

  • These methods are suitable for solid, laminated or cellular materials characterized by an apparent density 100 kg/m3 or greater.
  • The methods might also be applicable to some cellular materials having an apparent density of less than 100 kg/m3.
  • A method is provided for testing flexible sheets or film materials while supported vertically.

Composites and Factors Affecting LOI Values

LOI values for highly flammable composites, such as polyester-, vinyl ester- and epoxy-based materials, are below about 30. Composites with highly stable or aromatic polymers have much higher oxygen index values.

In general, the LOI values for polymers and polymer composites increase with their ability to yield char in a fire. This is because the formation of char occurs at the expense of combustible volatiles, which in turn increases the oxygen level required to sustain flaming combustion.

In addition to the type of polymer matrix, other factors that affect LOI value include:

  • The degree of resin cure
  • Fiber content
  • Flame retardants & other additives, and
  • The flammability of the fiber reinforcement

The LOI values can also change dramatically with temperature, usually decrease with increasing temperature.

Check Out an Interesting Video on Combustibility Tests Equipment:

Source: Devotrans

Find commercial grades matching your thermal properties target using "Property Search - Oxygen Index" filter in Omnexus Plastics Database:

Oxygen Index Values of Several Plastics

Click to find polymer you are looking for:
A-C     |      E-M     |      PA-PC     |      PE-PL     |      PM-PP     |      PS-X

Polymer Name Min Value (%) Max Value (%)
ABS - Acrylonitrile Butadiene Styrene 19.0 19.0
ABS Flame Retardant 28.0 28.0
ABS High Heat 18.0 19.0
ABS High Impact 18.0 19.0
ABS/PC Blend - Acrylonitrile Butadiene Styrene/Polycarbonate Blend 21.0 34.0
ABS/PC Blend 20% Glass Fiber 24.0 24.0
Amorphous TPI, Highest Heat, Chemical Resistant, 260C UL RTI 53.0 53.0
Amorphous TPI, Moderate Heat, Transparent 45.0 45.0
Amorphous TPI, Moderate Heat, Transparent (Food Contact Approved) 45.0 45.0
Amorphous TPI, Moderate Heat, Transparent (Mold Release grade) 45.0 45.0
Amorphous TPI, Moderate Heat, Transparent (Powder form) 45.0 45.0
ASA - Acrylonitrile Styrene Acrylate 19.0 19.0
ASA/PC Blend - Acrylonitrile Styrene Acrylate/Polycarbonate Blend 21.0 21.0
CPVC - Chlorinated Polyvinyl Chloride 70.0 80.0
ETFE - Ethylene Tetrafluoroethylene 30.0 30.0
EVA - Ethylene Vinyl Acetate 18.0 19.0
FEP - Fluorinated Ethylene Propylene 95.0 96.0
HDPE - High Density Polyethylene 17.0 18.0
HIPS - High Impact Polystyrene 17.0 18.0
HIPS Flame Retardant V0 17.0 26.0
LCP - Liquid Crystal Polymer 35.0 50.0
LCP Carbon Fiber-reinforced 33.0 37.0
LCP Glass Fiber-reinforced 37.0 51.0
LCP Mineral-filled 33.0 37.0
LDPE - Low Density Polyethylene 17.0 18.0
LLDPE - Linear Low Density Polyethylene 17.0 18.0
PA 11 - (Polyamide 11) 30% Glass fiber reinforced 22.0 22.0
PA 11, Conductive 21.0 26.0
PA 11, Flexible 21.0 26.0
PA 11, Rigid 21.0 26.0
PA 12 (Polyamide 12), Conductive 21.0 26.0
PA 12, Fiber-reinforced 21.0 26.0
PA 12, Flexible 21.0 26.0
PA 12, Rigid 21.0 26.0
PA 46 - Polyamide 46 24.0 24.0
PA 46, 30% Glass Fiber 21.0 23.0
PA 6 - Polyamide 6 23.0 26.0
PA 6-10 - Polyamide 6-10 23.0 27.0
PA 66 - Polyamide 6-6 21.0 27.0
PA 66, 30% Glass Fiber 21.0 27.0
PA 66, Impact Modified 21.0 27.0
Polyamide semi-aromatic 21.0 27.0
PAI - Polyamide-Imide 44.0 45.0
PAR - Polyarylate 26.0 30.0
PARA (Polyarylamide), 30-60% glass fiber 25.0 25.0
PBT - Polybutylene Terephthalate 20.0 24.0
PBT, 30% Glass Fiber 21.0 21.0
PC (Polycarbonate) 20-40% Glass Fiber 30.0 34.0
PC (Polycarbonate) 20-40% Glass Fiber Flame Retardant 35.0 40.0
PC - Polycarbonate, high heat 24.0 35.0
PCTFE - Polymonochlorotrifluoroethylene 90.0 95.0
PE - Polyethylene 30% Glass Fiber 17.0 19.0
PEEK - Polyetheretherketone 24.0 35.0
PEEK 30% Glass Fiber-reinforced 35.0 40.0
PEI - Polyetherimide 46.0 47.0
PEI, 30% Glass Fiber-reinforced 50.0 50.0
PEI, Mineral Filled 48.0 48.0
PEKK (Polyetherketoneketone), Low Cristallinity Grade 40.0 40.0
PESU - Polyethersulfone 34.0 38.0
PESU 10-30% glass fiber 45.0 45.0
PET - Polyethylene Terephthalate 23.0 25.0
PET, 30% Glass Fiber-reinforced 21.0 23.0
PET, 30/35% Glass Fiber-reinforced, Impact Modified 21.0 21.0
PETG - Polyethylene Terephthalate Glycol 23.0 25.0
PE-UHMW - Polyethylene Ultra-High-Molecular-Weight 17.0 18.0
PFA - Perfluoroalkoxy 95.0 96.0
PI - Polyimide 47.0 53.0
PLA - Polylactide 1.230 1.250
PMMA - Polymethylmethacrylate/Acrylic 19.0 20.0
PMMA (Acrylic) High Heat 19.0 20.0
PMMA (Acrylic) Impact Modified 19.0 20.0
PMP - Polymethylpentene 17.0 53.0
PMP 30% Glass Fiber-reinforced 17.0 18.0
PMP Mineral Filled 17.0 18.0
POM - Polyoxymethylene (Acetal) 18.0 18.0
POM (Acetal) Impact Modified 18.0 18.0
PP - Polypropylene 10-20% Glass Fiber 17.0 18.0
PP, 10-40% Mineral Filled 17.0 18.0
PP, 10-40% Talc Filled 17.0 18.0
PP, 30-40% Glass Fiber-reinforced 17.0 18.0
PP (Polypropylene) Copolymer 17.0 18.0
PP (Polypropylene) Homopolymer 17.0 18.0
PP, Impact Modified 17.0 18.0
PPE - Polyphenylene Ether 22.0 24.0
PPE, 30% Glass Fiber-reinforced 24.0 26.0
PPE, Flame Retardant 30.0 36.0
PPS - Polyphenylene Sulfide 43.0 47.0
PPS, 20-30% Glass Fiber-reinforced 43.0 49.0
PPS, 40% Glass Fiber-reinforced 43.0 49.0
PPS, Glass fiber & Mineral-filled 45.0 53.0
PPSU - Polyphenylene Sulfone 44.0 44.0
PS (Polystyrene) Crystal 17.0 18.0
PS, High Heat 17.0 18.0
PSU - Polysulfone 30.0 32.0
PSU, 30% Glass finer-reinforced 36.0 36.0
PTFE - Polytetrafluoroethylene 95.0 96.0
PTFE, 25% Glass Fiber-reinforced 95.0 96.0
PVC (Polyvinyl Chloride), 20% Glass Fiber-reinforced 40.0 45.0
PVC, Plasticized 20.0 40.0
PVC, Plasticized Filled 20.0 40.0
PVC Rigid 40.0 45.0
PVDF - Polyvinylidene Fluoride 44.0 83.0
SAN - Styrene Acrylonitrile 18.0 19.5
SAN, 20% Glass Fiber-reinforced 20.0 20.0
SRP - Self-reinforced Polyphenylene 49.0 55.0
TPI-PEEK Blend, Ultra-high heat, Chemical Resistant, High Flow, 240C UL RTI 42.0 42.0
XLPE - Crosslinked Polyethylene 17.0 18.0

Commercially Available Fire Resistant Polymer Grades

Also read about Flammability of Plastics

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