Here's a technical article by our collaborator Dr. Prithu Mukhopadhyay.
Retrouvez la version francophone de cet article ici.
Polymers or plastics are inherently flammable when enough oxygen and heat are present, specifically those with high carbon content. That is why fire hazards and risks are always considered when making thermoplastics, thermosets or composite formulations.
Understanding fire risk is essential. It is the potential of the plastics to get ignited or contribute to fire growth in a particular environment. Additives which retard a flame or show the ability to slow down fire growth when ignited are called flame retardants (FRs).
Often plastic products are required to be flame retarded to comply with regulations. This is because plastic products replace traditional materials in most sectors, including medical, clothing, communication, energy, transportation, and construction. When well formulated in plastic products, FR additives diminish fire risks, reducing material damage and fatalities if a fire occurs.
The selection of FR additives to meet standards and regulations is a challenging task. That is because mechanical and/or other required properties of the polymers or parts may be affected depending on the type of FR used. In other words, a perfect FR or a combination (synergy) of FR additives for any polymeric system must have various qualities. Broadly speaking, a satisfactory FR system must 1) be added in small amounts, 2) be inexpensive, 3) be easy to mix with polymers, and 4) not exhibit a deleterious effect on processing conditions, production rate, or corroding the metal surfaces of moulding or extrusion equipment. Moreover, neither workers should be exposed to harmful fumes and volatiles during part manufacturing or consumers during part usage.
The first commercial use of FRs was in Army tents. These were halogenated hydrocarbons and waxes, which were added to polymers to effectively retard fire. As the usage of plastics expanded, the search for newer FRs grew. In the early 1970s, only about ten kilos of the car weight was due to plastics. By the 1990s, that number increased to about 100 kilos.
Similarly, polyurethane foam has increased use in automotive interiors, bedding, furniture, packaging, insulation, etc. The use of polymers has risen substantially in every electrical and electronic component. Accordingly, the insurance industry pushed for stricter regulations, which vary depending on the specific plastic products’ application.
How do FR additives work? They work in the condensed (i.e. solid phase) or the gas phase. In the condensed phase, FR additives remove thermal energy or form a char, which acts as a barrier against heat and mass transfer. This is called an intumescent system. A typical intumescent system includes an acid source, a char-forming compound, and a gas-evolving compound. Instead of forming a char, sometimes FRs work as an insulating coating, such as silicone FRs for polyolefins. Alumina trihydrate (ATH) and magnesium hydroxide (MH) also function in the condensed phase. However, these do not form char or a coating layer. Instead, they act as a heat sink while diluting the fuel, namely, the plastic. Globally, inorganic hydroxides (ATH and MH, or IH) represent over 50% of the total FR market because of their lower relative cost, toxicity, and corrosivity. With IHs, an essential factor to consider is their decomposition temperatures. ATH decomposes at 2300C while MH decomposes at 3400C, releasing water vapour to cool the heat like a heat sink. If a plastic formulation needs a higher processing temperature, MH would be preferred over ATH. Particle sizes of IH FRs are another factor to consider since this also has a considerable difference in combustion behaviour.
In the gas phase, however, polymer fragments form radicals with oxygen in a chain reaction. Less energetic species are formed when these radicals react with halogens or phosphorus, interrupting the chain reaction. Improved flame retardancy is also seen when halogenated materials are mixed with metal oxides, such as PVC or chlorinated PE, with acid-scavenging antimony oxide. This flame retardancy arises from the formation of volatile antimony species such as trihalide and oxyhalide.
To increase flame retardance and to meet requirements (UL-94) of plastics products, other factors need consideration. These include presence of other additives (including plasticizers, process aids, antioxidant) in the formulations, flow behaviour of the recipe, and processing conditions. Use of synergists play a vital role as well.
Developing a formulation requires time and expense to conduct product application-relevant tests. For a manufacturer, these activities add to the formulations cost. Consequently, small or bench-scale tests conducted in the laboratory are more suitable than large-scale tests. Limited Oxygen Index (LOI) and UL-94 tests are principally carried out. The LOI can be used to rank and compare different materials for their flammability whereas UL-94 is a fire classification test based on a pass or a fail categorization. The higher the LOI, the better the flame retardancy. However, LOI of any plastic formulation depends on the sample thickness and the test parameter. There is no correlation between LOI and UL-94.
In summary, understanding combustion in a service environment and FR chemistry are keys to successful flame retarded plastics formulations.
Prithu Mukhopadhyay, Ph.D.
[Dr. Mukhopadhyay, published by Wiley, is Editor-in-Chief of the Journal of Vinyl and Additive Technology of the Society of Plastics Engineers (SPE). He has been in the plastics field for over 35 years. As a former senior scientist at IPEX, he led the development of highly successful piping products. He actively participated in different standards development committees, including ASTM, CSA, UL, and NSF International. Dr. Mukhopadhyay is passionate about new plastics technologies and has chaired the New Technology Committee of the Society of Plastics Engineers.]