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to improve the flame retardancy of polymers. The most commonly used
approach is the incorporation of flame retardants into the polymers as ‘additives’, generally via melting process, e.g. extrusion. The second form, the reactive method, is based on the attachment of flame retardants on to the polymer
chains during polymerization or by chemical modification of side chains in
order to obtain covalent bonds between the flame retardant molecules and the
polymer chains. The main reactive form of functional chain growth polymers is
the incorporation of flame retardant as co-monomers during copolymerization
(Joseph and Tretsiakova-Mcnally 2011). However, the post-polymerization
grafting of flame-retardant monomers is also possible (Tsafack and LevaloisGrützmacher 2006). For step-growth polymers, a reactive flame retardant can
be added during synthesis as a co-monomer or curing agent in the case of thermosets (Rakotomalala et al. 2010; Biswas and Kandola 2011). Reactive method
offers several advantages compared to the additive method, e.g. a homogenous
state of dispersion of flame retardant can inherently take place, thus leading to
no or very limited drop in mechanical properties. In addition, in the reactive
method, aging is less likely compared to the additive method, so that the migration of the flame retardant can be avoided due to covalent bonding between the
polymer chains and the flame-retardant molecules (Vahabi et al. 2015).
Once the flame retardant is chemically integrated into the polymer, it can act in
gas and/or condensed phases. Its action in gas phase (flame) can be the dilution of
combustible gases by the emission of inert gases (e.g. carbon dioxide (CO 2 ), NH 3 )
or the release of reactive species (e.g. HPO
●
, PO
●
), which can trap active radicals
(e.g. H
●
and OH
●
), being these responsible for flame propagation. In the condensed
phase, the flame retardant contributes to the formation and development of a char/
residue barrier in order to confine the emission of combustible gases, the penetration of oxygen into polymers and the diffusion of heat towards the polymers
(Camino et al. 1991). Most flame retardants used in reactive methods contain phosphorus and/or nitrogen in their structure (Wendels et al. 2017; Hirschler 2015). In
addition, some reactive halogenated flame retardants are still used in industry.
However, several halogenated flame retardants have already been banned, because
they are highly dangerous for human health and the environment (Mitchell
et al. 2014).
Here, we focus on the reactive approaches and especially on the fire reaction
of reactive polymers containing functional groups allowing the modification of
the polymer structure. Among the functional polymers, some thermoplastic and
thermoset candidates containing non-halogenated flame-retardant elements
have been considered, and special attention has been given to bio based reactive
solutions.
H. Vahabi et al.
to improve the flame retardancy of polymers. The most commonly used
approach is the incorporation of flame retardants into the polymers as ‘additives’, generally via melting process, e.g. extrusion. The second form, the reactive method, is based on the attachment of flame retardants on to the polymer
chains during polymerization or by chemical modification of side chains in
order to obtain covalent bonds between the flame retardant molecules and the
polymer chains. The main reactive form of functional chain growth polymers is
the incorporation of flame retardant as co-monomers during copolymerization
(Joseph and Tretsiakova-Mcnally 2011). However, the post-polymerization
grafting of flame-retardant monomers is also possible (Tsafack and LevaloisGrützmacher 2006). For step-growth polymers, a reactive flame retardant can
be added during synthesis as a co-monomer or curing agent in the case of thermosets (Rakotomalala et al. 2010; Biswas and Kandola 2011). Reactive method
offers several advantages compared to the additive method, e.g. a homogenous
state of dispersion of flame retardant can inherently take place, thus leading to
no or very limited drop in mechanical properties. In addition, in the reactive
method, aging is less likely compared to the additive method, so that the migration of the flame retardant can be avoided due to covalent bonding between the
polymer chains and the flame-retardant molecules (Vahabi et al. 2015).
Once the flame retardant is chemically integrated into the polymer, it can act in
gas and/or condensed phases. Its action in gas phase (flame) can be the dilution of
combustible gases by the emission of inert gases (e.g. carbon dioxide (CO 2 ), NH 3 )
or the release of reactive species (e.g. HPO
●
, PO
●
), which can trap active radicals
(e.g. H
●
and OH
●
), being these responsible for flame propagation. In the condensed
phase, the flame retardant contributes to the formation and development of a char/
residue barrier in order to confine the emission of combustible gases, the penetration of oxygen into polymers and the diffusion of heat towards the polymers
(Camino et al. 1991). Most flame retardants used in reactive methods contain phosphorus and/or nitrogen in their structure (Wendels et al. 2017; Hirschler 2015). In
addition, some reactive halogenated flame retardants are still used in industry.
However, several halogenated flame retardants have already been banned, because
they are highly dangerous for human health and the environment (Mitchell
et al. 2014).
Here, we focus on the reactive approaches and especially on the fire reaction
of reactive polymers containing functional groups allowing the modification of
the polymer structure. Among the functional polymers, some thermoplastic and
thermoset candidates containing non-halogenated flame-retardant elements
have been considered, and special attention has been given to bio based reactive
solutions.
H. Vahabi et al.