Types and characteristics of flame retardant thermoplastic resins

From the wildfires in California, we can see that the rational use of flame-retardant materials is crucial for both building materials and industrial products. Improving the flame-retardancy of materials is not only a strict requirement for industrial production standards, but also a strong guarantee for our life safety.

1. Definition of Flame Retardancy

Flame retardancy refers to the property that a material is not easy to burn and has the property of self-extinguishing even if it burns. Unlike metal materials, plastics, as organic matter, may burn under certain conditions. Products or parts used in high temperature environments or near fire sources may cause fires if the combustion spreads. Therefore, in order to improve safety, the flame retardancy of the material becomes an important consideration. In many industrial fields, such as electronics and automobile manufacturing, there are strict requirements for the flame retardancy of the plastics used.

1.1 Evaluation standards for flame retardancy

UL94 standard: The UL94 standard divides the flame retardancy level from low to high into six levels: HB, V-2, V-1, V-0, 5V-B, and 5V-A. Among them, V-2 and above are self-extinguishing, and the level division is determined by the length of time required for the flame to extinguish naturally. For example, in the selection of shell materials for some electronic equipment, it is usually required to reach the V-0 level to ensure that it can quickly extinguish itself in the event of an accidental fire and reduce the risk of fire spread.

Oxygen index: The oxygen index is an indicator specified by JIS to indicate the value of the material's combustion tendency. It indicates the oxygen concentration required for the material to continue to burn. The higher the value, the higher the flame retardancy. For example, some high-end building insulation materials will use flame retardant plastics with a higher oxygen index to meet the requirements of building fire safety.

2. Determination criteria for flame retardancy
2.1 HB grade determination
HB is determined by a horizontal combustion test. A sample of a specified size is fixed horizontally on the ground, and a flame with a length of 20mm is applied to one end for 30 seconds. Then, the burning speed of the sample after the flame is removed is used for determination. If the sample thickness is more than 3mm and the burning speed is less than 40mm/min, or the sample thickness is less than 3mm and the burning speed is less than 75mm/min, it is determined to be HB grade. Some ordinary plastic shells may use plastic materials that reach HB grade, which have the characteristics of being difficult to burn to a certain extent.

2.2 V-0/V-1/V-2 grade determination
V-0/V-1/V-2 is determined by a vertical combustion test. A flame with a length of 20mm is applied to the lower end of the sample. If the burning stops within 30 seconds, the flame is applied for another 10 seconds. The grade is determined based on the burning time of these two tests and the reaction to the surrounding environment. For example, in the packaging materials of electronic components, to achieve the V-0 grade, it is necessary to meet strict conditions such as the burning time of the sample is less than 10 seconds, the second burning time is less than 30 seconds, the total burning time of 10 times is less than 50 seconds, and the absorbent cotton placed under the sample is not ignited by dripping, and there is no burning in the fixed position.

2.3 5V-A/5V-B grade determination
5V-A/5V-B is tested under more stringent conditions. A sample of the specified size is placed vertically on the ground, and a 125mm flame is applied from the bottom. It is removed after 5 seconds, and this action is repeated 5 times. Then, the sample is placed horizontally and the same operation is repeated. The grade is determined based on the burning time of these two tests and the reaction to the surrounding environment. The casings of key components of some large electrical appliances may need to reach 5V-A or 5V-B levels to withstand more severe fire protection tests.

2.4 Oxygen Index

The oxygen index (Oxygen Index) specified by JIS refers to the minimum oxygen concentration required for plastics to maintain combustion when ignited.
The oxygen concentration in the general air is about 21%. The higher the oxygen index (OI) value, the more difficult it is for the material to burn.
[Oxygen Index (OI)]
Below 22: Flammable, easy to burn
23 to 27: Flammable, but self-extinguishing
Above 27: Difficult to burn

3. Flame retardant thermoplastic resin

3.1. Flame retardant plastics of grade V-2 and above

3.2. HB grade flame retardant thermoplastic resin