Sustainable , non-halogenated flame retardants for thermoplastics
Julia Eichhorn and Dr Tobias Moss of Budenheim present sustainable additives for non-halogenated flame retardants and upcoming challenges
During the last few decades , flame retardants ( FR ) have become one of the most important additives used in polymers . They are found in many products of daily life , like household appliances , computers , consumer electronics , home furnishings and automotive interiors , and in buildings , public transport and passenger aircraft . In general , the use of FRs is mandated by regulation or industryspecific standards . Megatrends and changes in mobility behaviour by the electrification of cars and bikes have further increased demand . Whereas the original aim was to prevent fires , FRs must nowadays fulfil multiple requirements , including low smoke production , no toxicity in the smoke and the FR itself , and a low environmental impact ( Figure 1 ). The burning of a polymer is a complex phyciso-chemical process that depends on many factors . 1 To fight fire , the replenishment of at least one of three ingredients in the classic ‘ fire triangle ’ - fuel , oxygen and heat - must be prevented . Several types of FRs are available for polymers : halogenated , inorganic and organic . Each has a different
Flame retardant APP
Loading in PP (%)
Fire
Evolution of fire protection standards
Smoke Toxicity mechanism to prevent burning and each has its pros and cons . Halogenated FRs act in the gas phase by decreasing the concentration of reactive radicals and they are very effective at low loadings . However , they produce toxic and corrosive smoke and are evaluated critically by authorities due to their toxicological profiles . Metal hydroxides like aluminium trihydroxide ( ATH ) decompose when energy is consumed and release inert gases . In addition to the cooling effect , the inert gases dilute the combustible gases and thus decrease the available amount of fuel . However , using ATH often requires high loadings , which can have a negative effect on the mechanical properties . A third class of FRs is intumescent systems , which form a foamed
Environment
70s 80s 90s Today
UL-94 ( 1.6 mm )
LOI (%)
APP / Penta / Melamine Uncoated 35 V-2 29 250 Budit 667 Coated 25 V-0 33 310 Budit 669S Coated 25 V-0 32 305
Table 1 - Comparison of flame retardant properties of different intumescent systems
TG ( 2 % wt . loss ), (° C )
Figure 1 - Evolution of fire protection standards
protective layer in case of a fire ( Figure 2 ). This layer prevents the underlying polymer from heating and decomposing . Additionally , it prevents the formation of combustible gases and the diffusion of oxygen to the remaining polymer . They are based on polyphosphates and contain additional synergists , depending on the application and polymer . Melamine cyanurate ( MC ) is mainly used for unfilled polyamides . It leads to a chain scission so that the polymer is dripping away prior to decomposing and releasing combustible gases . MC is widely used as a synergist in FR mixtures like intumescent systems for polyolefins , that is , polyethylene ( PE ), polypropylene ( PP ) and their copolymers .
Intumescent FRs for polyolefins
Polyolefins can be protected by using an intumescent FRs system based on ammonium polyphosphate ( APP ). APP decomposes at temperatures above 280 ° C under the release of ammonia . The polyphosphoric acid generates a glassy surface . 2
54 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981