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Analysis on the spontaneous combustion of soft polyurethane foam
page view：2756 time：[2020-08-26]
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Abstract: This article focuses on the production process and spontaneous combustion mechanism of flexible polyurethane foam. Through the analysis of the production process and the introduction of the spontaneous combustion mechanism, the basic requirements for safe production and avoiding spontaneous combustion accidents are put forward.
Keywords: flexible polyurethane foam, spontaneous combustion, isocyanate, water, heat release
Foam plastic is a plastic with numerous micropores inside. It can be made of polystyrene, polyvinyl chloride, polyurethane (ie, polyurethane) and other resins. It has the advantages of light weight, heat insulation, sound absorption, moisture resistance, corrosion resistance, etc. . It can be used as heat insulation, sound insulation and building materials, and is widely used in construction, ships and aircrafts. Among them, polyurethane foam is the most widely used. Because low-density flexible polyurethane foam (commonly known as sponge) is mostly exothermic during the production process. Therefore, spontaneous combustion often causes fire accidents. This kind of fire spreads fast and has certain toxicity, which makes it difficult to fight fire and often causes greater economic losses. Therefore, it is extremely important to understand the spontaneous combustion mechanism and preventive measures of polyurethane foam.
1. The spontaneous combustion mechanism of flexible foam
The formation of the spontaneous combustion mechanism of flexible polyurethane foam depends on the exothermic reaction of isocyanate with water and polyol. Under a given index, the isocyanate needed to react with polyol is fixed. Therefore, the actual variable that determines the amount of heat during foaming is the amount of water in the formula. The higher the water consumption, the more the reaction exotherm, and the initial characteristics close to the risk of spontaneous ignition are discoloration and scorching. The temperature or value is about 170-175℃. In addition to the amount of water used in the formulation and the isocyanate index, the heat of reaction is also determined by the size of the material block, the storage method for the first 24 hours, the location, and the environmental conditions. So, what are the mechanisms of the spontaneous combustion of flexible polyurethane foam?
(1) After the flexible foam is ejected from the mold, some monomers still continue to react and increase the temperature. The most important first reaction in the production of flexible polyurethane foam is the reaction of isocyanate with hydroxyl and amino groups. At this time, the starting product of the reaction is substituted carbamate, which is broken into amine and carbon dioxide. Amines then react with isocyanates to form substituted ureas, which are important base materials for polyurethane flexible foams. In the catalyst-free model system, the second step reaction of amine and isocyanate is much faster than the initial reaction between isocyanate and water. At the same time, a lot of heat and gas will be released. In general, the maturation stage in the production of flexible polyurethane foam needs to maintain a certain temperature to ensure the full progress of the polymerization reaction, so that the foam is cured and the temperature continues to rise. According to measurement, the temperature is generally about 150-170℃ when it is just out of the mold. If the operator makes a mistake, excessive polyisocyanate in the ingredients, etc., the foam will maintain a high temperature for a long time, and the spontaneous ignition point (415°C) of the foam will be lowered and cause spontaneous combustion.
(2) Flexible foam plastic absorbs oxygen and generates heat to promote its temperature rise. It is understood that the polymerization reaction of isocyanate is very important in the production of foam. It can be integrated into carbaimines to release carbon dioxide, which is easy to occur under heating. However, the formed carbaimines further react with isocyanates to form substituted ureaimines when cooled, so there are a small amount of carbaimines in many modified isocyanates. The above-mentioned reactions are all exothermic reactions. After the flexible foam is out of the mold, the temperature remains above 67°C for a long time (over 6-7 hours), and it is easy to absorb oxygen and generate heat, which promotes its internal temperature to rise. , When it reaches 193℃, the exothermic heat can reach 228KJ/kg. At this time, the temperature will rise sharply and promote its spontaneous combustion. Therefore, the sponge must be placed in a well-ventilated place for more than 24 hours before it can be stored.
(3) Excessive polyisocyanate and water increase the internal temperature of the flexible foam. Although, polyisocyanate has higher reactivity and low volatility, and has been widely used in many microcellular and other foam molded parts. However, in terms of dosage, diisocyanate is mainly used in the preparation of flexible foam. According to process requirements, the dosage of polyisocyanate should not exceed 3%-12%, and the amount of water should not exceed 4.2%. If the amount is exceeded, the volume of the foam should be added. , The stack is large, the temperature is high, and spontaneous combustion occurs when the heat accumulation conditions are met. After a fire, toxic gases such as carbon monoxide and cyanide gas will be produced.
2. Safety requirements for flexible polyurethane foam The production process and equipment of flexible polyurethane are relatively simple, and can be produced by small factories and township enterprises. However, flexible polyurethane foam is prone to spontaneous combustion. In recent years, fires in this area have occurred many times. It is particularly important to do a good job in fire safety.
(1) To control the reaction temperature. The production of flexible polyurethane foam is a typical exothermic reaction process, especially the reaction of toluene diisocyanate and water will generate a lot of heat, and the flexible polyurethane foam has a large volume. Good thermal insulation performance, easy to accumulate heat. During maturation and curing, the physical temperature can reach 200℃. At this time, carbon monoxide and alcohol low-molecular-weight substances are released and start to burn in the center.“Heartburn”。Once the soft polyurethane foam catches fire, it spreads quickly. According to experiments, 400 kilograms of foam can be burned out in 5 minutes. Therefore, in the production process, attention should be paid to temperature changes and emergency measures should be taken in time. A small amount of anti-oxidant Freon II can be added in the high temperature season to accelerate heat dissipation.
(2) Choose raw materials carefully. Different raw materials have different heat release conditions. Using high polyether (ester) containing water tends to generate heat. The production of low-density products requires a lot of water and is easy to generate heat. Therefore, it is necessary to conduct small batch tests and strengthen observations when replacing new fuels, and pay special attention to fire safety when producing low-density products.
(3) Strict process formula. In the production process, the formula is incomplete, easy to generate heat, more active catalysts are added, or the amount of polyether (ester) is insufficient, especially if the ratio of hydroxyl and isosine ester is imbalanced, or the mixing is uneven, strictly follow the process formula to prevent the ratio improper.
(4) The workshop must meet the specified requirements. The production workshop must be well ventilated to prevent direct sunlight, electrical equipment must meet explosion-proof requirements, and products and leftover materials are not allowed to accumulate in the workshop. The production workshop must be arranged separately from other buildings and a certain safety distance must be maintained.
(5) Storage safety requirements. The newly produced product still has a higher temperature, and some incompletely reacted materials continue to emit heat. They should wait until they are sufficiently cooled (generally more than 24 hours) before they can be stored. The warehouse building must meet the requirements of relevant fire safety regulations and have good ventilation. In summer, prevent heatstroke and cool down, avoid direct sunlight, leave a certain safe distance between stacks and stacks, and open the warehouse door outwards.
(6) Requirements for personnel quality. For flexible polyurethane foam production positions, operators should be trained in fire safety positions and can only start after passing the examination; they should stick to their posts during operation. Warehouse management personnel must understand the basic properties of flexible polyurethane foam and fire-fighting methods, consciously abide by the fire and electricity safety regulations, and strictly prohibit fireworks and ensure safety.
3. Concluding remarks During the production process, flexible polyurethane foams will cause fires due to heat accumulation, which will bring immeasurable losses to people's lives and property. Therefore, we must pay attention to the quality of raw materials, the proportion of ingredients, technical management, plant facilities, etc. to avoid fire accidents.
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