Development of expandable graphite for fire protection

Brief introduction

Expandable graphite (eg) is formed by inserting acid into the layered crystal structure of flake graphite (usually prepared by reaction with sulfuric acid, nitric acid, or organic acid). In this process, we must use some oxidants, such as H2O2, HNO3, or KMnO4; or we can prepare graphite intercalation compounds by electrochemical method. After the reaction is completed, we can neutralize, wash and dry to make the final product.

The expansion characteristics of expandable graphite are different from other expansion agents. When subjected to high temperature, due to the decomposition of the compound absorbed in the interlayer lattice, the expandable graphite expands and becomes a worm with low density. The expanded graphite can form a very good thermal insulation layer on the surface of flame retardant materials, which can effectively insulate heat. It has a low heat release rate, small mass loss, and less flue gas in a fire. Therefore, in recent years, expandable graphite as an intumescent flame retardant has been used in synthetic materials and has attracted more and more attention in scientific research and industrial fields.

Great progress has been made in the application of expandable graphite in fire and flame retardant materials. In addition to conventional products such as traditional fire retardant rings, smoke sticks, fire doors, fire blocking materials, and flame retardant coatings, other applications are becoming more and more widely. In foam products, the flame retardancy of expandable graphite has played a great role. PU foam with expandable graphite has been applied to seats in aircraft, trains, and other transportation vehicles. Expandable graphite has also been used in wires and cables synthesized from PP, PE, and other polyolefins to enhance flame retardancy. In addition, expandable graphite also occupies a place in the fields of asphalt coiled materials used on roofs.

The new development direction of expandable graphite

With the development of expandable graphite in the direction of flame retardant, various industries also put forward different requirements for the properties of expandable graphite, which makes it face opportunities and challenges in the process of development.

PU polyurethane industry

As mentioned previously, expandable graphite is an effective flame retardant for polyurethane PU foam. Expandable graphite not only has flame retardancy and smoke suppression effect in PU foam but is also a halogen-free flame retardant which attracts more and more attention from the industry. But it is also difficult to apply expandable graphite to PU foam.

Most PU foaming equipment is suitable for liquid reaction systems. In order to effectively add expandable graphite, the equipment must be improved. At the same time, in order to prevent the friction damage of the added expandable graphite to the equipment in the processing process, the ash content of expandable graphite has also become an important index affecting its application. This requires that the ash in expandable graphite must be soft ash, which makes the selection conditions of expandable graphite raw materials more strict.

For some special products, the subsequent hot pressing process of PU foam also requires higher requirements for expandable graphite. For example, the start expansion temperature (set) of expandable graphite shall not be lower than a certain temperature, otherwise, the expandable graphite will begin to expand during hot pressing, affecting the performance of the product.

In addition, in order to make the expandable graphite well dispersed in polyols, an important component of polyurethane, and not easy to settle, it is necessary to carry out surface treatment on the expandable graphite to change its surface properties, or make it become a thixotropic system of polyols/graphite, so as to solve the dispersion problem.

Coating industry

When the fine particles of expandable graphite are added to the ordinary coating, a flame retardant and antistatic coating with good performance can be obtained. A large number of light non-combustible carbon layers formed in the fire can effectively block the radiation of heat to the substrate and effectively protect the substrate. This application requires that expandable graphite must have fine particle size while ensuring its flame retardant effect. At the same time, according to different coating formulations, the pH of the expandable graphite surface should also be adjusted appropriately.

Engineering plastics field

At present, there have been a large number of reports on the use of expandable graphite in flame retardant polyolefins, flame retardant rubber, flame retardant styrene plastics, and so on. Expandable graphite can also be added to flame retardant engineering plastics, which can effectively enhance the fire resistance and expansion performance of plastics. At the same time, it does not affect the flexibility and other physical properties of plastics, and does not emit toxic gas, acid mist, smoke, etc. in the flame, and does not increase the risk of materials. However, this also puts forward strict requirements for the initial expansion temperature of expandable graphite, such as expandable graphite used in engineering plastics such as PBT and Pa. in order to make it not expand during processing, the initial expansion temperature of expandable graphite is required to reach 280 – 300 ℃. All the above are the new requirements for expandable graphite put forward by relevant industries in recent years.

Through the concerted efforts of manufacturers and end-users, expandable graphite has made great progress in the field of flame retardant. The development of any new product of the manufacturer is derived from the problems found by the customer in use and the new requirements put forward. It is believed that with the deepening of research and the development of science and technology, we can solve various problems that exist and will arise in the future, such as changing the inherent black, obtaining a higher expansion ratio, obtaining lower initial expansion temperature, achieving homogeneous dispersion in polyols, latex and other carriers and a series of other targets. It is hoped that with the joint promotion of upstream and downstream customers, expandable graphite can play a role to the greatest extent and make more contributions to the fire and flame retardant industry.