The first appearance of fire retardants can be traced back to the Egyptians in 450 B.C. when they used potassium aluminum sulfate on wood. The wood was soaked in this solution with the intent of reducing its flammability. From that period to the 20th century many new materials were developed before and after the first scientific experiments in 1821 to study ways in which flammability can be controlled. Much of the breakthrough in research came as a result of growing needs for new materials during and after World War II (WWII). During this period, synthetic polymers were developed and various additives used to reduce flammability of these types of materials.
Why FR resistant polymers are needed
The flammability of polymers depend on the type: thermoplastics are known to be more flammable than thermosets. The high number of products that are made from these polymers in everyday consumer goods and the high flammability when subject to certain temperatures is a major safety concern. This reality is evident in the growing use of polymers in the automobile, rail, aerospace, electronic, electrical, transportation industry and the military. The construction industry, while slow, has been experimenting with the use of high performance polymers in structural and non-structural applications.
For a fire to occur three basic elements must exist: heat, fuel and oxygen. A fire retardant can limit one or more of these three. The use of fire resistant (FR) resins and plastics is intended to increase the resistance to ignition, reduce smoke and fume emission, reduce the rate of flame speed and reduce the heat release. With numerous stringent regulations established worldwide, products must meet the required fire safety level to be used as general consumer products. Some polymers are also known to release toxic smoke that is dangerous to human health so without the use of fire resistant additives the rate of heat release and flame spread, and smoke propagation is very high.
Another important point to note is the competitively of polymers when compared to traditional materials. Without the use of FRs, many polymers are excluded from various applications aluminum, concrete and steel, which are non-flammable, are leaders. These traditional materials, however, lose considerable mechanical and physical properties when subjects to high temperatures. As a result, the need for polymers that can be used alongside these materials or separately are research areas being explored worldwide.
Different types and Uses
Fire resistant polymers can be divided into four groups: halogenated, inorganic FRs, organophosphorus FRs and nitrogen-containing FRs. For simplicity these basically fall under two major groups of FRs which are: halogenated FR's and halogen Free FR's. The halogenated FR's are generally more efficient, less expensive, easier to process and have better physical properties. Their disadvantage lies in the toxicity levels. This setback has led to their use in different industries being challenged by different parties due to health and environmental concerns. Halogen free FR's on the other hand are a "greener" type of material with less toxicity and less smoke. They also are less corrosive and as a result, easier to handle with a wider variety of tools. The main halogenated FRs used are brominated and chlorinated whereas halogen-free FRs include metal hydroxides, melamine-based and phosphorous based FR's. Halogenated epoxy vinyl esters and polyesters also exist on the market.
The thermoplastic resins that are inherently inflammable are polysulfones, polyphenylene sulfide, polyetherimide, polyetheretherketone and fluoropolymers. (PES, PPS, PEI, PEEK, PTFE) These do not need to undergo any type of modification to meet specific requirements. The use of nitrogen- containing FRs is predominant in polymers that have nitrogen.
The type of FRs used will depend on the function of the polymer. Ashtrays and cookware are some examples of the use of polymers with high fire resistance. Many components in electrical and electronic components are also made with the use for FR polymers. Other uses can include water treatment plants storage, ducting, roofing and equipment exposed to high temperatures.
Future of FR polymers
With the implementation and constant modification of fire safety laws, the use of FR polymers is expected to increase in the future. More effort is being made by manufacturers to produce products that are free from toxins. As a result, phosphorus-based fire retardants will increase in the near future as more attention shifts to non-halogenated FRs.
New developments are also on the rise to make materials that are more efficient in retarding fires. The combination of nanoclays with metal hydroxide retardants is one nanotechnology research area that promises to grow in the near future due to the efficiency of the mixture and the limited amount of nanoclay that is needed to make this possible.