Behind the frequent fires of electric vehicles: halogen-free flame retardant materials are the key

Electric bicycles continue to be loved by ordinary people due to their convenience, speed, and environmental protection. In the course of more than ten years of development, the number of electric bicycles has exceeded 140 million. However, with the continuous increase in the number of possessions, recent incidents of sudden fires of electric vehicles have occurred throughout the country, especially in the hot summer.

    On May 15, a fire broke out in Antai Jinyuan District on the northwest side of Sanquan Road, Wenhua Road, Zhengzhou City. More than 10 electric vehicles were burned, and the paint of two cars nearby were baked. Fortunately, no casualties were caused.

    On June 25, a community in Zhengzhou City, Henan Province caused 15 deaths and 2 injuries due to a fire caused by privately pulling wires to charge electric bicycles.

    On July 1, a fire broke out in the Xiangjushi district on Wenlao Road, Zhengzhou City, Henan Province, and dozens of battery cars and bicycles were burned.

    On August 12, a charging electric car was instantly burned into a skeleton in the family yard of Zhengfei Company in Zhengzhou, Henan.

    On October 12, Shaozhuang, Huayuan North Road, Zhengzhou, Henan, was suspected to be on fire due to charging of electric vehicles. A fire broke out in a 10-storey house under construction. Due to the dense crowds, many people ran out. The accident caused the death of 1 woman and 5 minor injuries.

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    The frequency of accidents involving electric vehicle fires in Henan is really untold. In fact, not only this year, but also a long time ago, Henan battery car accidents have occurred frequently. Since 2008, there have been 1,336 electric bicycle fires in Henan Province, resulting in 40 deaths and 35 injuries. The number of electric bicycle fires and casualties has both been on the rise. As of the end of October this year, there were 486 such fires, which has exceeded the total of last year.

    The root cause of electric vehicle fires is the material

    Relevant sources said that electric vehicle fire accidents are mainly related to the irregular production, maintenance and charging of electric vehicles.

    Some of them believe that in order to save costs, some low-quality electric vehicle chargers have insufficient wire diameters at the input and output ends, and the shell plastic material is flammable and heat-resistant. After the rechargeable battery pack is fully charged, continue to charge the rechargeable battery pack with high current. It is easy to cause line fire or battery explosion.

    In a large-scale electric bicycle fire simulation experiment in Shanghai, three battery cars were ignited after simple treatment. The plastic parts on the car emitted a lot of thick smoke at high temperature. The scene was instantly covered with black smoke. Even if people were wearing protective equipment, Can't get close. Three minutes later, the temperature in the center of the fire reached 700°C, which could reach 1000°C in an instant. The poisonous gas from the burning plastic and the explosion of the battery made people unable to escape.

    Many casualties are not directly caused by open flames, and toxic smoke is often more lethal than open flames. At present, almost all the parts used in electric bicycles are made of flammable materials. A fire will quickly form smoke. Once people inhale it, it will cause the loss of self-rescue ability.

    At present, the electric vehicle industry has been using the "General Technical Requirements for Electric Bicycles" implemented in 1999, and some technical indicators are not clearly stipulated. Because there are no fire protection indicators in the current national standards, most electric vehicles do not do well in fire protection. For example, electric vehicles use plastic shells. Once spontaneously ignited, the plastic shell will burn quickly and produce harmful gases. If the electric vehicle shell uses flame-retardant materials, and the circuit is treated with flame-retardant, even if the circuit is short-circuited and cause a fire, the flame-retardant treatment can prevent the electric vehicle from igniting.

    Halogen-free flame retardant materials are hot

    It is understood that most domestic electric vehicle materials are mainly ABS engineering plastics and polyurethane materials. These plastic materials do not have flame-retardant properties, and will produce a large amount of harmful substances when burned. This is also an important reason for the high mortality of battery car fires. So which materials can replace the existing materials for electric vehicles to achieve effective flame retardancy and not cause toxic harm to the human body?

    In the traditional process, the key raw material for flame retardant is halogen, but the halogen will emit harmful gases such as dioxins after burning, causing great pollution to the environment. Therefore, for an environmentally friendly flame-retardant electric vehicle material, the halogen-free flame retardant material becomes the key, and the halogen-free flame retardant for engineering plastics is a hot spot.

    ABS halogen-free flame retardant material

    Since most electric vehicles use ABS engineering plastics, we will start with ABS plastics. ABS is a styrene-acrylonitrile-butadiene terpolymer with excellent mechanical properties, processing properties and chemical stability. Used in high-tech fields such as electrical engineering, electronics, automobiles and aviation. However, ABS is very easy to burn, and its limiting oxygen index is only 18.0-18.8; when it burns, it releases a lot of toxic gases. Traditional halogen-containing flame retardants will produce a large number of toxic, harmful and strong irritating gases when burned, which will cause direct, potential and long-term harm to human health and the environment.

    Special phosphorus-nitrogen intumescent flame retardant is one of the most promising new high-efficiency and non-toxic flame retardant systems with engineering application prospects. Three main components are needed to produce expansion: acid source, carbon source and gas source. Due to the synergistic effect of the thermal decomposition of the acid source, the dehydration and carbonization of the carbon source, and the decomposition of the foaming source to produce gas to expand the carbon layer, a fluffy and porous carbon layer is formed on the surface of the flame-retardant polymer when it burns, which prevents heat transfer. The diffusion of oxygen and oxygen effectively delays and prevents thermal degradation of the polymer, inhibits the production of volatile combustible components, and achieves the purpose of flame retardancy.

    Polyurethane halogen-free flame retardant material

    As a conventional polymer material, polyurethane has outstanding performance, but conventional polyurethane materials are easier to burn, and flame retardant treatment of polyurethane is required in industries such as automotive interiors and home furnishings. Conventional polyurethane flame-retardant treatment method is to physically add flame-retardant fillers. Because of environmental requirements, halogen-free flame-retardant fillers are mainly used.

    At present, most of the halogen-free flame-retardant thermoplastic elastomers on the market are prepared from melamine urethane and phosphate composite flame retardants, inorganic magnesium aluminum flame retardants, etc. The former has poor processing performance and is prone to migration; while the latter At the cost of sacrificing the flexibility of the elastomer, its application in various fields is greatly restricted. The use of intumescent flame retardants to produce halogen-free flame-retardant thermoplastic elastomers can obtain excellent flame-retardant properties while basically retaining the special plasticity of thermoplastic elastomers, which has a wide range of application fields and good market prospects.

    Polypropylene halogen-free flame retardant material

    According to a market research report released by PersistenceMarketResearch, the global low-smoke halogen-free flame-retardant polypropylene market is expected to reach US$664 million in 2020. The rising demand for PP in major end applications such as construction, automobiles, and electronic products is still a key factor driving the global low-smoke, halogen-free and flame-retardant PP market. The public is paying more and more attention to halogen toxicity, fire safety issues and environmental issues, which has a positive impact on the growth of the low-smoke halogen-free flame-retardant PP market.

    The flame retardant used by Jushi Chemical's halogen-free flame-retardant PP is an intumescent halogen-free flame retardant, which functions mainly through the condensed phase. At a lower temperature, the flame retardant can produce acid that can be used as a dehydrating agent; at a slightly higher temperature, the acid promotes the dehydration reaction of PP, and the system melts in the process; the water vapor generated by dehydration is heated by PP The incombustible gas produced by the decomposition makes the system in the molten state expand and foam; at the same time, the ester group in the PP is dehydrated and carbonized to form inorganic substances and carbon residues, and the system further foams; when the reaction is close to completion, the system gels And curing, finally forming a porous foam carbon layer.

    Selective use of halogen-free flame retardant materials

    Since halogen-free flame retardant materials are so good, why not use electric vehicles? The person in charge of products in Wuxi area of an electric vehicle group said that the plastics used for electric vehicle shells generally have an ignition point of only more than 400 degrees Celsius. At present, as the profit margin of the electric vehicle industry itself is limited, manufacturers have no incentive to study the use of new non-flammable materials. Make electric car shell.

    Moreover, although some domestic companies have successfully imitated the halogen-free flame retardants used in engineering plastics, the product cost has obvious advantages, but because the synthesis process still needs to be further improved, the product still has the problem of heat-resistant instability, so domestic modification Plastic factories are often more cautious when choosing domestic substitute products. Products with high requirements and good prices still use imported flame retardants, while those with low requirements and low prices use domestic substitutes. This inevitably leads to a good resistance. The problem of high fuel material prices.

    Therefore, cost and high threshold limit electric vehicle companies to choose halogen-free flame retardant materials as substitutes for general materials. However, with the frequent occurrence of electric vehicle fires and the attention of relevant departments, this phenomenon is expected to be improved.

    On November 2, in order to strengthen the fire safety management of electric bicycles, and effectively prevent and contain electric bicycle fire accidents, the Henan Provincial Public Security Department organized police types such as public security, economic investigation, traffic, criminal investigation, and fire protection. The legal representatives and persons in charge of 77 electric bicycle manufacturers in 13 province-administered cities of the province conducted intensive interviews.

    The relevant leaders of the Henan Provincial Public Security Department put forward specific requirements for strengthening the source control of electric vehicle fire safety. One of them is that enterprises need to organize enterprise technical experts to conduct special technical diagnosis and optimization, use non-combustible or non-combustible materials on the body, and install battery leakage and overload. Protective devices can effectively improve the intrinsic safety of electric bicycles.

    However, in an environment where the industry is facing fierce competition, it is not realistic for electric vehicle manufacturers to use halogen-free flame-retardant materials in their vehicles. It is recommended to appropriately limit the use of plastic products for electric vehicle body parts and improve the combustion performance level of electric vehicles. Halogen-free flame-retardant materials are often used around the interior of the car body that is prone to electrical failure to ensure that even if an electrical failure occurs, it will not cause the electric vehicle to catch fire.