Expandable graphite is produced by a reaction of an acid (usually sulfuric, nitric, or organic acid) inserted into the layered crystal structure of the flake graphite. In this process, we must use some oxidants, such as H2O2, HNO3 or KMnO4; Alternatively, graphite intercalation compounds can be prepared by electrochemical methods. After the reaction is completed, it is neutralized, washed and dried to produce the final product.
The expansion characteristics of expandable graphite are different from those of other expandable agents. When exposed to high temperatures, the expandable graphite expands into a vermicular shape with very low density due to the decomposition of the compounds trapped in the interlayer lattice. The expanded graphite can form a very good insulation layer on the surface of the flame retardant material, which can effectively insulate the heat. Its own heat release rate in the fire is low, the mass loss is small, and the smoke generated is less. Therefore, in recent years, expandable graphite has been used in synthetic materials as an expandable flame retardant, and has attracted more and more attention in scientific research and industry.
The application of expandable graphite in fire retardant materials has made great progress, in addition to the traditional fire ring, smoke bars, fire doors, fire blocking, flame retardant coatings and other conventional products, other applications are becoming more and more extensive. In foam products, the flame retardant of expandable graphite has played a great role. PU foam with expandable graphite has been used in seats on planes, trains and other vehicles. Expandable graphite has also been used in wire and cable synthesized from PP, PE and other polyolefins to enhance flame retardancy. In addition, expandable graphite also has a place in areas such as asphalt coils used in roofing.
With the increasingly extensive development of expandable graphite in the direction of flame retardant, various industries have also put forward different requirements for the performance of expandable graphite, so that it is facing opportunities and challenges in the development process.
1, PU polyurethane industry
As mentioned earlier, expandable graphite is an effective flame retardant for polyurethane PU foam materials. Expansible graphite not only has flame retardant effect in PU foam, but also has smoke suppression effect, which is a halogen-free flame retardant that has been paid more and more attention by the industry. However, there are also difficulties in applying expandable graphite to PU foam.
Most of the reaction systems for PU foaming equipment are liquid, and in order to effectively add expandable graphite, the equipment must be improved. At the same time, in order to prevent friction damage to the equipment caused by the added expellable graphite during the processing process, the ash content of the expellable graphite has also become an important index affecting its application. This requires that the ash in the expandable graphite must be soft ash, which makes the selection conditions of the expandable graphite raw materials more stringent.
For some special products, the subsequent hot pressing process of PU foam also puts higher requirements on expandable graphite. For example, the Start Expansion Temperature (SET) of the expandable graphite can not be lower than a certain temperature, otherwise the expandable graphite will begin to expand in the process of hot pressing, affecting the performance of the product.
In addition, in order to make the expellable graphite well dispersed in the polyol, an important component of polyurethane, it is not easy to settle, and it is necessary to surface treat the expellable graphite, change its surface properties, or make it become a thixotropic system of the polyol/graphite, so as to solve the dispersion problem.
2. Coating industry
The fine particles of expandable graphite can be added to the ordinary coating to obtain a flame retardant anti-static coating with good performance. The large amount of light non-combustible carbon layer formed in the fire can effectively block the radiation of heat to the substrate and effectively protect the substrate. This application requires that the expandable graphite must have a finer particle size while ensuring its flame retardant effect. At the same time, for different coating formulations, the pH of the expellable graphite surface should also be adjusted appropriately.
3, engineering plastics field
At present, there have been a lot of reports on the use of expandable graphite in flame retardant polyolefin, flame retardant rubber, flame retardant styrene plastics, etc. Expandable graphite can also be added to flame retardant engineering plastics, which can effectively enhance the fire resistance and expansion performance of plastics, while not affecting the flexibility and other physical properties of the plastic itself, and does not volatilize toxic gases, acid fog, smoke, etc., in the flame, and does not increase the risk of the material. However, this also puts forward strict requirements for the initial expansion temperature of expanded graphite, such as expexpable graphite for engineering plastics such as PBT, PA, in order to make it not expand during processing, the initial expansion temperature of expexpable graphite is required to reach 280-300 ℃. All of these are the new requirements for expellable graphite put forward by relevant industries in recent years.
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