Carbon fibres mainly consist of carbon atoms and they have some outstanding properties, especially when it is integrated in a composite. Although the production of carbon fibres is complex, the final product has great quality, which is why carbon fiber materials are expensive in the market.
Carbon fibres are five times stronger than iron as well as rigid and corrosion resistant. Electric conductivity of carbon fibres and their tensile strength are high and thermal expansion coefficient is low. They are not poisonous for people in addition to not being flammable, which is one of their best features. Flammability, however is still a very big problem, when carbon fibres are put into polymer composites. There are three main polymer matrices, which can be used in polymer composites: epoxy resin, vinylester resin and polyester resin , which is rarely used with carbon fibres. Since almost every polymer has a polymer matrix which is flammable, special materials are added to the composites in order to make them non-flammable. They are called flame retardants and they can be integrated into composites in different ways. They can either be added to polymer matrix during the production process or they can be integrated in the main polymer chain. Flame retardants, which are added as protective coating or which form protective coatings during the exposure to high temperatures can also be used.
In general, flame retardants can be divided to phosphorous, nitrogen, halogen, intumescent and inorganic flame retardants. Halogen flame retardants mainly contain one of two elements (chlorine or bromine) and inorganic flame retardants are generally made of aluminium or magnesium. Inorganic flame retardants are mostly used as catalysts, so they speed up the effect of other flame retardants added to the composite. Intumescent flame retardants create a protective layer at high temperatures, which protects composite from ignition and degradation. Mainly halogen and phosphorus flame retardants are used for polymer composites with carbon fibres. Since halogen retardants appear to be harmful to people and to the environment, phospohorous flame retardants started to be used as a substitute.
Effectiveness of flame retardants can be determined by using various techniques. Most commonly the limiting oxygen index, and shows the percentage, where material still burns. In addition group of UL-94 tests can be performed or determine heat release with calorimetry or we can watch a chemical composition of product with x-ray photoelectron spectroscopy.
Because of the addition of flame retardants to composite materials, several problems emerge. If the ignition of material happens and the flame retardants are not effective, a candlewick effect could happen. This means that the polymer is burning on carbon fibres until fire is turned down artificially or the fuel runs out. Another problem occurs in the polarity of flame retardants; if flame retardant is more polar than polymer matrix, structure of composite becomes weak, which leads to bad mechanical properties.
Nowadays, epoxy resin is mostly used in polymer composites with carbon fibres. Materials from such substances can replace other, heavier materials, which is very useful in aviation and automobile industry. Firstly, halogen flame retardants were added to such composites, and because halogen flame retardants are toxic, more and more phosphorus flame retardants are used. Properties of flame retardants and composites have to be well researched, so that good flame retardancy can be reached and that is how polymer composites with carbon fibres properties are even improved.
|
