Maya Ellison
Plastic is a common material with a variety of uses in our daily lives, from buildings to vehicles. As a by-product of petroleum, it is flammable and poses a significant fire risk. The temperature at which plastic catches fire depends on the type of plastic and the surrounding conditions. For example, the burning patterns of polyethylene (PE) and polypropylene (PP) differ due to their distinct melting points and pyrolysis points. Understanding the flashpoint of molten plastic and its ignition behaviour is crucial for fire safety, especially considering the potential fire hazards of dripping plastic.
| Characteristics | Values |
|---|---|
| Self-ignition temperature range for polyethylene and polypropylene | 350° C to 580° C |
| Decomposition temperature of PVC | 200° C |
| Decomposition temperature of PTFE | 500° C |
| Decomposition temperature of polyester | 225° C |
| Flashpoint of molten plastic | 300° C |
| Burning pattern of PE | Near-limit flame, transitional flame, and intense flame |
| Burning pattern of PP | No transitional-flame stage due to a higher melting point and lower pyrolysis point |
| Flame retardants delay ignition | A few seconds |
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What You'll Learn
- Different types of plastics have different ignition temperatures, ranging from 200° C to 580° C
- Plastic building materials can release dangerous chemicals when exposed to high temperatures
- Electrically-caused fires occur when a material is heated to its ignition temperature, followed by combustion
- Flaming combustion is when luminescent flames appear directly above the fuel material
- Flame retardants may delay plastic ignition but only for a few seconds
Different types of plastics have different ignition temperatures, ranging from 200° C to 580° C
The variation in ignition temperatures is due to the different chemical compositions and structures of various plastics. Some plastics have additives or flame retardants that can also impact their ignition temperature. It's important to note that plastics don't always require an external flame to ignite. When heated, plastics can release combustible gases that can self-ignite at temperatures slightly higher than their flame-ignition temperature.
The flammability of plastics is a significant concern, especially in the construction industry, due to the release of toxic chemicals during combustion. Burning plastics can release dangerous chemicals such as sulphur dioxide, hydrogen chloride acid, volatile organic compounds, furans, dioxins, and heavy metals. These toxic fumes have been linked to severe health issues, including respiratory and cardiovascular diseases, certain cancers, and birth defects.
Additionally, the melting and dripping of burning plastics can further complicate fire scenarios. Thermoplastics, such as polyethylene (PE), polypropylene (PP), and expanded polystyrene (EPS), can deform and melt at high temperatures, leading to new ignitions and the formation of intense plastic pool fires. This behaviour contributes to the overall intensity of the fire and the release of toxic smoke.
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Plastic building materials can release dangerous chemicals when exposed to high temperatures
Plastic is one of the most common flammable construction materials in electronic products. When plastic burns, it releases toxic chemicals, including sulphur dioxide, hydrogen chloride acid, carbon monoxide, volatile organic materials, furans, dioxins, and heavy metals. These toxins are linked to a range of severe health issues, from cardiovascular and respiratory diseases to cancers and birth defects.
The dangers of plastic fumes are well documented, and research continues to grow in this area. For example, a study of over 10,000 UK firefighters found that more than 4% had received a cancer diagnosis, with an age-specific rate up to 323% higher than the general population. Another study, following wildfires in California, found that thermally degraded plastic pipes had contaminated the water supply with dozens of dangerous chemicals, including the carcinogen benzene.
The Grenfell Tower fire of 2017 highlighted the risks of plastic building materials. Plastic insulation on the tower's exterior caught fire, releasing dangerous levels of toxic smoke. As a result, up to a dozen firefighters involved in tackling the blaze were diagnosed with terminal cancer.
The construction industry is re-evaluating its reliance on plastics due to the health risks associated with plastic fumes. Plastic building materials can melt and drip onto other combustible materials, causing fires to spread quickly and unpredictably. Additionally, flame retardants used with plastics may only delay ignition for a few seconds, and they contain chemicals that pose a threat to human health, such as endocrine disruption and thyroid dysfunction.
The published decomposition temperatures of common plastic materials vary, ranging from 200° C for PVC to 500° C for PTFE, with most others in the 300° C range. Plastic's low melting point and the toxic substances released when it burns are significant fire safety concerns.
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Electrically-caused fires occur when a material is heated to its ignition temperature, followed by combustion
Electrically-caused fires are a frequently occurring issue in electronic product safety that can have devastating consequences, as seen in the Grenfell Tower fire of 2017. Electrically-caused fires occur when a material is heated to its ignition temperature, followed by combustion. In the case of plastics, as the material is heated, it first softens, then changes colour, produces smoke, and finally boils.
Published decomposition temperatures of common plastic materials vary, with PVC having a decomposition temperature of 200° C, PTFE at 500° C, and most other plastics in the 300° C range. Polyester, for example, has a decomposition temperature of 225° C. If heating is continued beyond these decomposition temperatures, the combustible gases evolved from the heated plastic will self-ignite at a temperature slightly higher than the flame-ignition temperature.
Plastic materials rated HB have pyrolytic gases that are flammable, and the heat of the flame is usually sufficient for sustained pyrolysis. After ignition, when the external heat source is removed, flaming combustion is sustained. On the other hand, materials rated V2, V1, V0, or 5V have mostly non-flammable pyrolytic gases. After ignition, when the external heat source is removed, the rate of combustion decreases until the fire goes out.
Flaming combustion, which is the more familiar combustion process, is a chemical chain reaction that occurs exclusively in the gas phase. Gases are evolved from solids or liquids by heating them to a suitable temperature. Flaming combustion is indicated by luminescent flames appearing directly above the fuel material. It is important to note that solids and liquids do not burn, only gases do.
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Flaming combustion is when luminescent flames appear directly above the fuel material
Flaming combustion is a process that involves the combustion of fuel vapours above the fuel material. When a combustible material is exposed to a constant external heat source, its surface temperature begins to rise, and the temperature inside the solid also increases over time. As the surface temperature reaches a certain level, pyrolysis begins, and fuel vapours are released and mix with the air. If the mixture exceeds the lower flammability limit, it ignites, resulting in flaming combustion. This process is characterised by the appearance of luminescent flames above the fuel material.
The initiation of flaming combustion is known as flaming ignition, which can be caused by a small pilot, such as a flame, an electric spark, or a glowing wire. During flaming combustion, the rate of heat release is typically higher compared to smouldering or pyrolysis, leading to faster fire spread and a hotter environment. The temperature profile within the fuel is highest at the exposed surface and decreases with depth.
In the context of fire, "fuels" can take the form of gases, liquids, or solids. However, during flaming combustion, the process involves a reaction between fuel vapour and oxygen from the air. The combustion of fuel vapour results in the production of light, contributing to the luminescence observed above the fuel material.
The visibility of flames during flaming combustion depends on the source fuel. For example, methanol burns cleanly and produces minimal visible red light due to its low flame temperature of around 2000 Kelvin. In contrast, heterogeneous combustion of solids, such as burning coal or charred logs, may not exhibit a visible gaseous flame but can still be considered a fire.
The various stages of flaming combustion are characterised by the total radiant flux, which includes both immediate and room-scale contributions. The radiation reaching the fuel surface is influenced by the temperature of the flame, its optical thickness, and the fraction of the flame visible from the perspective of the fuel. Smaller flames generally produce lower levels of radiation compared to larger flames, and sooty flames are optically thicker than those with low soot content.
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Flame retardants may delay plastic ignition but only for a few seconds
Plastic is a common flammable construction material, and its combustion is a serious fire hazard. Plastics have a low melting point, and when they burn, they release toxic chemicals, including sulphur dioxide, hydrogen chloride acid, and volatile organic materials, as well as furans, dioxins, and heavy metals. These toxic fumes have been linked to severe health issues, including cardiovascular and respiratory diseases, certain cancers, and birth defects.
To mitigate the fire risks associated with plastics, flame retardants are often used. Flame retardants are chemical compounds added to plastics to prevent, delay, or slow down combustion, reduce smoke formation, and prevent melt collapse. While they are effective tools to reduce fire risks, they may only delay ignition for a few seconds, and their overall benefit in improving fire safety has not been proven.
There are two main types of flame retardants: halogenated and non-halogenated. Halogenated flame retardants, including brominated and chlorinated types, are the most common and are highly effective at relatively low load levels. However, they have been linked to health concerns, such as endocrine disruption and thyroid dysfunction. Non-halogenated flame retardants, on the other hand, include intumescents (phosphorus-based) and metallic oxides. While they are increasing in demand, they require higher load levels and may need additional adjustments to maintain the mechanical properties of the plastic.
Flame retardants work by interfering with or eliminating one of the key ingredients necessary for combustion: fuel, oxygen, or an ignition source. They employ various methods to inhibit combustion, such as gaseous inhibition, solid char-formation, and endothermic cooling. Gaseous inhibition uses halogenated compounds compatible with many base polymers. Solid char-formation creates a thick layer of carbon char on the plastic's surface, insulating and shielding it from the fire. Endothermic cooling uses metallic oxides or hydrated minerals to remove heat and release water molecules, cooling the plastic and limiting reactive gas formation.
While flame retardants can provide some delay to plastic ignition, it is important to understand their limitations. Research suggests that the benefit of flame retardants in improving fire safety is not conclusive, and their effectiveness may only be temporary. Additionally, some flame retardants may pose a threat to human health, emphasizing the importance of selecting the appropriate type without compromising the material's properties.
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Frequently asked questions
The flashpoint of plastic, or the temperature at which it will ignite, is around 300°C. However, the temperature at which plastic catches fire depends on the type of plastic and the surrounding conditions. For example, the top surface temperature of a burning polyethylene (PE) pool during an experiment was found to be around 450°C.
The type of plastic is a significant factor. For instance, polypropylene (PP) has a higher melting point and lower pyrolysis point than polyethylene (PE). The surrounding conditions, such as the ground temperature, can also influence the burning dynamics and patterns of a plastic fire.
Yes, the shape and size of the plastic item can impact its flammability. Additionally, the presence of other materials or substances, such as adhesives, can influence how easily plastic ignites.
