Fire Resistance (LOI)

The limiting oxygen index (LOI) test is used to measure the relative flammability of plastics and composite materials. This is done by burning them in a controlled atmosphere consisting of a mixture of oxygen and nitrogen.

LOI represents the minimum level of oxygen in the atmosphere that can sustain flame on a thermoplastic material. The SI unit of limiting oxygen concentration is percentage (%). LOI is also referred to as oxygen index (OI) or critical oxygen index (COI). The difference in non-flammability varies for different polymers. This could be due to several reasons. The two major reasons are:
 

• The higher hydrogen-to-carbon ratio in the polymer increases the tendency of a polymer to burn (other factors being equal).
• Some polymers on burning emit blanketing gases that suppress burning.

The higher the LOI value, the higher the non-flammability. The test results relate only to the behavior of the test specimens under the conditions of the test method. The results must not be used to infer the fire hazards of the material in other forms or under other fire conditions.

 

Formula

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

where,

• LOI is the limiting oxygen index
• O₂ is the minimum oxygen concentration in the inflow gases to pass the minimum burning length criterion
• N₂ is the minimum nitrogen concentration in the inflow gases to pass the minimum burning length criterion

Note: Air contains approximately 21% oxygen. Thus, any material with an LOI of less than 21% will support burning in an open-air situation.

 

Key applications

• It is used as a quality control tool, during the manufacturing of products and assemblies.
• It is used to indicate the potential flammability of a material.
• It acts as a semi-qualitative indicator of the effectiveness of additives during R&D.
• It is one of the primary characterizing tools used in the plastics industry. It is also used in the electric cable industry and transport manufacturing sectors.

Below you can find the list of flame retardant plastics, from inherently to less flammable polymers.

 

I. List of inherently flame retardant polymers
 

II. List of quite flame retarded polymers
 

III. List of less flame retarded polymers
 

These polymers can achieve high flame retardancy with the addition of additives.

Note: LOI values for highly flammable composites, such as polyester, vinyl ester, and epoxy-based materials, are below 30%. Composites with highly stable or aromatic polymers have much higher LOI values. Overall, aromatic polymers exhibit greater flame resistance than aliphatic polymers.

 

The LOI values for polymers and 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. This in turn increases the oxygen level required to sustain flaming combustion. Several factors that determine LOI values in a polymer are as follows:
 

• Degree of resin cure — A well-cured resin matrix is dense. It has a higher char yield. These factors contribute to a higher LOI value.

• Filler and fiber content — They can increase the LOI value of a polymer by providing a charring surface. This inhibits the spread of flames. Examples of fillers and fibers include glass fibers, minerals, etc.

• Flame retardants & other additives — These additives are added to make the polymer more resistant to fire. They release inert gases forming a barrier on the polymer surface. They can also interfere with the combustion process. Characterize the behavior of flame retardants by understanding LOI.

• Temperature — The LOI values can also change dramatically with temperature. They usually decrease with increasing temperature. This is because the polymers are likely to decompose at high temperatures. This leads to the release of combustible gases.

• The flammability of the fiber reinforcement — Fibers with low flammability can increase the LOI of the polymers. This is because the low flammability of fibers insulates the polymer.

Standard test methods
 

• ASTM D2863 — It determines the minimum oxygen concentration. This supports the candle-like combustion of plastics (oxygen index).

• ISO 4589 — It determines the burning behavior by oxygen index.
  • Part 1: It is referred to as the guidance. It forms a guidance document for the oxygen index (OI) test.
  • Part 2: It is the ambient-temperature test. It describes a method for determining the minimum oxygen concentration. This is done by percentage volume in a mixture of oxygen and nitrogen introduced. It is measured at 23°C ± 2°C which will support the combustion of a material under specified test conditions.
  • Part 3: It is the elevated-temperature test. It describes methods of carrying out the same determination over a range of temperatures. This ranges 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

Note: There are several other flammability testing methods. For e.g., flash point determination, determination of burning rates. But they are not discussed here.

 

Test apparatus

The objective of the test is to find the minimum oxygen concentration in nitrogen. This will result in sustained combustion for at least 3 minutes or excessive flame propagation down the specimen. The steps involved in measuring LOI are as follows:
 

• The LOI test apparatus consists of a heat-resistance glass column. It allows the burning of the specimen to be observed.
• A slow stream of oxygen and nitrogen is pumped in at the base of the chimney.
• These gases pass through a layer of glass beads that ensure even mixing before entering the main test chamber.
• A small glass flame is used to ignite the upper end of the specimen.
• The subsequent burning behavior is monitored.

These methods are suitable for solid, laminated, or cellular materials. These are characterized by an apparent density of 100 kg/m³ or greater. They 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.
 

Click to find the polymer you are looking for: