The Latest Development of Expanded Graphite for Flame Retardancy
Expanded graphite is prepared by inserting an acid (usually with sulfuric acid, nitric acid or organic acid) in a layered crystal structure of flake graphite. In this process, we must use some oxidants, such as H2O2, HNO3 or KMnO4, etc .; or can be prepared by electrochemical method of intercalation of graphite compounds. After the completion of the reaction, and then neutralization, washing and drying to produce the final product.
The expansion characteristics of expanded graphite are different from other bulking agents. At high temperatures, due to the accumulation of compounds in the interlayer lattice decomposition, expansion of graphite expansion, into a very low density worm-like. Expanded graphite can form a very good insulation on the surface of flame retardant materials, can effectively heat. Its own in the fire in the heat release rate is low, the quality loss is small, resulting in less smoke. So in recent years, expanded graphite as an intumescent flame retardant used in synthetic materials, and in the field of scientific research and industry has attracted more and more attention.
Expanded graphite in the application of fire retardant materials has made great progress, in addition to the traditional fire ring, smoke, fire doors, fire blocking materials, flame retardant coatings and other conventional products, some other applications are becoming increasingly widespread. In the foam products, the expansion of graphite flame retardant has played a significant role. The addition of expanded graphite PU foam has been applied to aircraft, trains and other vehicles in the seat. Expanded graphite is also used in PP, PE and other polyolefin-synthesized wire and cable to enhance flame retardancy. In addition, in the roof of the use of asphalt membrane and other fields, the expansion of graphite also has a place.
With the expansion of graphite in the direction of the increasingly wide range of development, each industry for the performance of expanded graphite also put forward different requirements, so that the development process at the same time faced with opportunities and challenges.
1, PU polyurethane industry
As mentioned earlier, expanded graphite is an effective flame retardant for polyurethane PU foams. Expanded graphite in the PU foam not only has a flame retardant effect, as well as smoke suppression effect, is a more and more attention by the industry halogen-free flame retardant. However, it is also difficult to apply expanded graphite to PU foam.
Most PU foaming equipment is suitable for the reaction system, in order to effectively add the expansion of graphite, the equipment must be improved. At the same time in order to prevent the process, adding the expansion of graphite on the equipment friction damage, expansion of graphite ash has become an important indicator of its application. This requires that the ash in the expanded graphite must be soft ash, making the choice of expanded graphite raw materials more stringent conditions.
For some special products, PU foam follow-up hot pressing process also on the expansion of graphite put forward higher requirements. If the initial expansion temperature (Start Expansion Temperature, SET) of the expanded graphite can not be lower than a certain temperature, the expanded graphite will begin to expand during the hot pressing process, affecting the performance of the product.
In addition, in order to make the expanded graphite very well dispersed in the important constituent polyol of polyurethane, it is not easy to settle, it is necessary to subject the expanded graphite to surface treatment, change its surface properties, or make it a polyol / graphite thixotropic system, Thus solving the problem of dispersion.