Flame Retardancy Testing of Polyolefin Materials

Flame Retardancy Testing of Polyolefin Materials

The flame retardancy of materials is primarily evaluated in terms of flammability, flame spreadability, and heat release. Generally, the evaluation and testing standards for flame retardant performance are largely consistent for different types of flame-retardant materials. During the combustion of polyolefin materials, due to their tendency to produce molten dripping, their flame spread performance becomes the most critical factor. Flame spreadability is typically tested by horizontal and vertical burning methods. However, for polyolefin materials, vertical burning tends to be more intense than horizontal burning, producing more flaming drips. Therefore, in the production and application of polyolefin products, end-users are also more concerned with the vertical burning performance of flame-retardant polyolefins.

Numerous international and national standards have been established for testing the performance of flame-retardant materials. This chapter will introduce several simple, convenient, and most commonly used laboratory test methods for evaluating the flame retardancy of polyolefin materials.

I. UL-94 Test
Underwriters Laboratories established the UL-94 test method to evaluate the flammability of plastic parts used in equipment and electrical appliances.
UL-94 is a testing standard that primarily focuses on the ignitability and flame spread of plastics and has been adopted as a standard test method by many countries. This test method includes five testing procedures and rating categories:

1. Horizontal Burning Test for Plastics: HB rating.

2. Vertical Burning Test for Plastics: V-0, V-1, or V-2 rating.

3. 500W (125mm) Vertical Burning Test: 5VA or 5VB rating.

4. Radiant Panel Flame Spread Test: Thin Material Vertical Burning Test VTM-0, VTM-1, or VTM-2 rating.

5. Foamed Plastic Specimen Horizontal Burning Test: HBF, HF-1, or HF-2 rating.

The following section focuses on the most commonly used Plastic Vertical Burning Test.
The UL-94 Vertical Burning Test is one of the plastic flammability test methods developed by Underwriters Laboratories, widely adopted internationally. It can indicate the ignitability, dripping, and self-extinguishing characteristics of specimens under specified test conditions. Standard specimen dimensions: 3.2mm (or 2.4mm, or 1.6mm, or 0.8mm) × 12.7mm × 127mm. Each set consists of 5 specimens. Each specimen is ignited twice, with each ignition lasting 10 seconds. After removing the ignition source, the flaming and after flame combustion times are recorded, along with whether flaming drips are produced. To achieve a V-0 rating, the flaming combustion time for each specimen after each ignition must not exceed 10 seconds. For V-1 or V-2, it must not exceed 30 seconds. For the second ignition, to achieve V-0, the flaming combustion time after removal of the flame must not exceed 30 seconds. For V-1 or V-2, it must not exceed 60 seconds. To achieve a V-0 rating, the total flaming combustion time for all 5 specimens after 10 ignitions must not exceed 50 seconds. For V-1 or V-2, it must not exceed 250 seconds. No specimen shall exhibit flaming or after flame combustion up to the holding clamp. To achieve a V-0 or V-1 rating, there must be no ignition of surgical cotton by dripping particles. Otherwise, the rating is V-2. The corresponding rating is awarded only if all five requirements meet the minimum criteria for that level.

II. Limiting Oxygen Index Test
The Limiting Oxygen Index (LOI) method is a commonly used test to evaluate the flammability of polyolefin materials. The specimen is mounted vertically in a combustion column, and a mixture of oxygen and nitrogen flows upward through it. The top of the specimen is ignited. If the specimen does not ignite within 30 seconds, the oxygen concentration is increased. After stable combustion is achieved, the ignition source is removed, and the burning time and length of the specimen are observed simultaneously. If the burning time exceeds 3 minutes or the burn length exceeds 5 cm, the test is repeated with a new specimen at a lower oxygen concentration. Ultimately, the minimum oxygen concentration (expressed as volume percent of oxygen in the mixture) that just supports steady burning, where the specimen extinguishes within 3 minutes after removal of the ignition source or the burn length does not exceed 5 cm, is determined. This value is the Oxygen Index of the material. Although the LOI test environment does not fully simulate real fire conditions, it provides specific numerical values. This is more advantageous for assessing differences in flame retardancy among samples modified with different flame-retardant formulations based on the same base resin. The Limiting Oxygen Index method is an ideal screening method for evaluating flame-retardant samples or formulations.

III. Cone Calorimeter Test Method
The Cone Calorimeter (CONE) test method obtains numerous combustion performance parameters through small-scale burning of material samples. It provides significant reference value for studying the flame retardant mechanisms of modified flame-retardant polyolefins. Its principle is based on oxygen consumption, i.e., the heat released during material combustion is always proportional to the amount of oxygen consumed in the combustion process. The instrument is called a Cone Calorimeter, named after the shape of the conical heater. It is a new generation apparatus for determining the combustion performance of polymeric materials, based on the principle of measuring oxygen consumption changes in the combustion system before and after testing. Various combustion parameters of combustible materials in fires can be obtained from the CONE, including Heat Release Rate (HRR), Total Heat Released (THR), Effective Heat of Combustion (EHC), smoke and toxicity parameters, and Mass Loss Rate (MLR), among others. Changes in these parameters can simulate the impact of different materials on the surrounding environment and trapped individuals in real fires.
Heat Release Rate refers to the rate of heat released per unit area of the sample, measured in kW/m². The CONE provides the dynamic change of HRR over time during the combustion process of polymer materials. The maximum value of HRR is the Peak Heat Release Rate (PHRR).
HRR is one of the most important combustion behavior parameters. Its magnitude reflects the burning intensity of the material. A higher HRR or PHRR indicates a stronger ability of the fire to radiate heat to the surroundings. On one hand, this can cause greater burn damage to trapped individuals. On the other hand, the intense and rapid heat release feeds back to the surface of the polyolefin material, accelerating the thermal decomposition rate of the base resin, thereby generating more volatile small molecule gases. These gases can lead to poisoning and suffocation of trapped individuals, and their flammability accelerates flame spread. Therefore, reducing the HRR and PHRR of polyolefin materials will be the most effective method to reduce the hazards of materials in real fires.
Total Heat Released is the total heat released per unit area of material from the start to the end of combustion, measured in MJ/m². For the effective heat of combustion of the volatile portion, a higher THR indicates that the polyolefin material releases more heat during the overall combustion process, signifying greater hazard of the polyolefin material in a fire. If HRR reflects the flashover and spread characteristics of the test sample, then THR reflects the persistence of the test sample during burning.
Effective Heat of Combustion refers to the ratio of the measured heat release to the mass loss at a given moment, i.e., the unit heat released when small molecule volatile gases produced during thermal decomposition burn, measured in MJ/kg. During combustion, when the EHC value of the test sample stabilizes, it suggests that the modified material likely employs condensed-phase flame retardancy, hindering the release and escape of small molecule flammable gases and reducing their combustion degree in the flame.
Cone calorimeters are generally equipped with smoke measurement devices. They can test parameters such as Specific Extinction Area (SEA, m²/kg, the extinction area of smoke released per unit mass of sample during combustion), Total Smoke Production (TSP, m³/m², the volume of smoke released per unit area of material), Smoke Production Rate (SPR, m²/s, the extinction area of smoke produced per unit time during combustion), and concentrations of toxic and harmful gases like carbon monoxide and carbon dioxide. Smoke and toxicity parameters are not necessarily related to HRR values. However, the massive release of smoke in real fires is a crucial factor affecting the escape of personnel and the rescue efforts of firefighters. Therefore, they are another important aspect for evaluating the hazard of materials in real fires.
Mass Loss Rate is the rate of mass change of a polymer material during combustion, measured in g/s. It reflects the thermal decomposition rate, volatilization, and combustion degree of the polymer under a certain burning intensity. A lower value indicates less generation of small molecule volatile gases due to combustion and radiant heat in a fire, thereby reducing the potential harm caused by the material.

The cone calorimetry test for polyolefin resins is conducted according to GB/T 16172-2007, using specimens with dimensions of 100mm × 100mm × 10mm.

IV. Conclusion
Flame retardancy testing for polyolefins is a rigorous "data-driven" competition: from UL-94 vertical burning to Limiting Oxygen Index, to the heat and smoke release curves from the Cone Calorimeter, each step quantifies the material's true safety rating. Selecting the right test method and understanding the combustion mechanisms behind the data are key to finding the optimal balance between efficiency and cost for halogen-free formulations, truly upgrading "flammable" polyolefins into "flame-retardant" safety barriers.