Introduction to halogen-free flame retardants

Introduction to Inorganic Flame Retardants Team Leader: Zhu Hanning Team Members: Chen Zhangchao, Li Weijie, Huang Zhizhong, Xie Yongsheng, Zhu Hanning I. Overview Necessity of material flame retardancy: The time available for evacuating people and saving property in case of a fire is 15 times longer for flame-retarded samples compared to those that are not flame-retarded； The mass loss rate of the material during combustion is less than half that of the non-flame-retardant sample ； The heat release rate during material combustion is only 1/4 that of the unflame-retardant sample for the flame-retardant sample ； The amount of toxic gases generated by the combustion of the material is only 1/3 that of the unflame-retardant sample ； The amount of smoke produced during combustion is similar for both the flame-retardant and non-flame-retardant samples. Regarding the toxicity of flame retardants: the toxic gases and fumes generated during the thermal decomposition and combustion of flame retardants or flame-retardant materials, as well as the dust and vapor emitted by the flame retardants themselves ； Almost all organic flame retardants are somewhat toxic, and in particular, some halogenated phosphates can cause cancer. II. Flame-retardant mechanisms (1) Some main approaches to flame retardancy: Adding flame retardants to alter the thermal degradation pattern of polymers, thereby reducing the flammable products generated by their thermal cracking ； An external flame-retardant coating is used. To isolate oxygen from the polymer surface ； Internal barriers are used to prevent the escape of flammable gases ； Inert gases are released to dilute the combustible substances generated by the thermal pyrolysis of polymers and to disperse the heat from the flame front. (2) Gas-phase flame retardant mechanism: When heated or burned, flame retardant materials can produce radical inhibitors, thereby interrupting the chain reaction of combustion. When flame-retardant materials are heated or burned, they produce fine ions that help free radicals combine with each other to terminate the chain reaction of combustion. When combustion materials are heated or burned, they release large amounts of inert gases or high-density steam; the former can dilute oxygen and gaseous combustible products as well as lower the temperature of these combustible gases, thereby stopping the combustion. The latter covers the flammable gas, preventing it from coming into contact with air, thereby suffocating the combustion. (3) Solid-phase flame retardant mechanism: The thermal oxidizer delays or prevents the thermal decomposition that can produce flammable gases and free radicals in the solid phase. In flame-retardant materials, inorganic fillers with high specific heat capacity prevent the material from reaching its thermal decomposition temperature by storing and conducting heat. Flame retardants absorb heat during thermal decomposition, thereby slowing down or preventing an increase in the temperature of the flame-retardant material. Aluminum hydroxide and magnesium hydroxide, which are widely used in industry, belong to this category of flame retardants. When a flame retardant burns, it forms multiple carbon layers on its surface; these layers are difficult to ignite, act as a barrier to oxygen, and also prevent combustible gases from entering the combustion zone, thereby stopping the combustion. (4) Interrupting the heat exchange flame-retardant mechanism: This refers to the removal of some of the heat generated by the combustion of the flame-retardant material, which prevents the material from maintaining its thermal decomposition temperature; as a result, no combustible gases are produced any longer, and the combustion dies out on its own. For example, when a flame-retardant material is heated or burns, it can melt, and the molten material tends to drip, thereby carrying away most of the heat and reducing the amount of heat that is returned to the main material. This slows down the burning process, and eventually the combustion may be halted. Therefore, fusible materials generally have low flammability, but the hot droplets that fall can ignite other substances, increasing the fire risk. (5) Measures to improve the properties of flame-retardant plastics: Appropriately reduce the processing temperature of flame-retardant plastics ； Perform thermal stabilization treatment on the flame retardant ； Additives that possess both flame-retardant synergistic effects and plastic modification capabilities are used ； Select the correct resin substrate model ； Fully disperse the flame retardant in the substrate. III. Basic requirements for flame-retardant products: High flame-retardant efficiency, with a low amount required to achieve a certain level of flame-retardant effect ； Low toxicity or essentially non-toxic; the products generated by flame-retardant materials are also low in toxicity and have low corrosivity, making them environmentally friendly ； It has good compatibility with flame-retardant materials and does not migrate easily ； It has sufficient thermal stability (decomposition temperature of 250–400°C) ； To prevent excessive deterioration of the substrate’s properties ； Excellent stability to ultraviolet light and light ； The price is moderate. 1. Principles for selecting flame retardants Generally speaking, when using flame retardants with different polymers, it is necessary to consider specifically: the phase in which the flame-retardant effect occurs and the stage of the combustion process ； The flame-retardant effect must occur at the right time and in the right place ； Specific flame retardants are used according to the application requirements, in appropriate amounts and to achieve the desired level of flame resistance ； Flame retardants that have a significant impact on the properties of the flame-retardant substrate should not be used. 2. Aluminum-magnesium-based flame retardants mainly include aluminum hydroxide (aluminum oxide trihydrate) and magnesium hydroxide; both are filler-type flame retardants that are halogen-free, non-toxic, smoke-suppressing, and inexpensive. They all exert flame-retardant effects through endothermic decomposition, the generation of water vapor, and dilution. However, the amount used is very high (40–200 phr), which severely deteriorates the physical and mechanical properties of the material and causes many difficulties in its processing. To mitigate or resolve this issue, manufacturers around the world offer aluminum hydroxide and coal hydroxide with various particle sizes and particle size distributions, as well as those that have been surface-modified. These materials have better compatibility with polymers and better dispersibility in the matrix, which allows for higher addition levels and helps to improve the material’s elongation, tensile strength, and impact strength. The initial decomposition temperature of aluminum hydroxide is around 205°C, while that of magnesium hydroxide can reach 320°C; therefore, for polymers that require high processing temperatures, magnesium hydroxide is a better choice. Aluminum-magnesium-based flame retardants release large amounts of water vapor when heated. This large volume of water vapor can absorb heat and dilute the concentration of flammable gases in the polymer, while also forming a non-flammable barrier between the source of fire and the matrix material, thereby achieving the purpose of flame retardancy. At the same time, it is also a smoke suppressant, as the gas it releases is water vapor, which is harmless to living organisms and does not cause corrosion to metals; therefore, magnesium hydroxide is one of the most commonly used and effective halogen-free flame retardants. Aluminum-magnesium-based flame retardants are currently recognized as organic polymer additives in the plastics industry that possess three functions: flame retardancy, smoke suppression, and filling. It is a white powder that is non-toxic, tasteless, stable, non-volatile, has a high decomposition temperature, and does not corrode equipment; it is the preferred material for achieving halogen-free flame retardancy in organic polymers. IV. Experimental Section: As inorganic flame retardants constitute a single research topic, it is not possible to conduct experiments in a targeted and purposeful manner like with the topics of other groups. As a result, we were unable to design an ideal experimental plan for a long time; after all, the lack of experiments is a regrettable situation for any research topic. While organizing the historical section, we were inspired by the fact that the ancient Romans soaked wood in a solution of potassium aluminum sulfate, which led us to think of designing controlled experimental demonstrations; at that time, we also considered using other inorganic flame retardants to treat wood. Six flame retardants were finally selected. It is worth mentioning that when soaking wood in magnesium hydroxide or aluminum hydroxide, the initial approach was to treat it directly; however, considering the issue of sediment detachment, after repeated discussions among the team members, a suitable experimental procedure was finally determined: first soak the wooden chopsticks in sodium hydroxide solution, and after thorough soaking, then soak them separately in aluminum chloride and magnesium chloride solutions. We initially produced the product in the laboratory, and comparisons made through combustion tests showed excellent results; the wood treated with flame retardants demonstrated clear flame-retardant properties. It is worth mentioning that Yongsheng improved the experimental procedure during the tests by ensuring a concentration ratio of 1:3 between aluminum chloride and sodium hydroxide, thereby enhancing the flame-retardant characteristics of the wood. In total, we produced all six types of products in the laboratory, and conducted combustion experiments in front of the entire class; it was clearly visible that the wood chopsticks that had been treated with flame retardants were much harder to ignite than those that hadn’t been treated. V. Development Prospects: In recent years, the development of synthetic polymer materials in our country has been extremely rapid. It is estimated that by 2002, the production volume of plastics will exceed 8 million tons, and the annual consumption of plastic flame retardants will reach over 600,000 tons. If inorganic flame retardants account for 50% of this amount, and magnesium hydroxide flame retardants make up around 30% of the inorganic flame retardants, then 90,000 tons of magnesium hydroxide flame retardants will be needed each year. China’s current annual production capacity for magnesium hydroxide flame retardants is around 13,000 tons, indicating great potential for the development of this material in China. With the rapid development of China’s synthetic polymer materials industry and the continuous improvement of flame-retardant regulations, the demand for flame retardants has increased. The demand for magnesium hydroxide, an environmentally friendly inorganic flame retardant that is non-toxic and helps suppress smoke, is particularly urgent. China being a major producer of magnesium ore resources boasts unique advantages in terms of resources as well as promising market prospects. However, compared with the advanced levels abroad, magnesium hydroxide production in our country features small-scale enterprises, a limited range of products, and low technical standards; there is an urgent need to improve the overall level of the industry. Therefore, our country should improve existing production processes and achieve large-scale production, as well as boost the production and development of magnesium hydroxide flame retardants to meet the needs of its rapidly growing plastic industry, while accelerating the adjustment of the structure of products in China’s flame retardant industry. This post was last edited by zhangyong6404 on 2009-3-25 22:48.]