167
8.2 Thermosets
The thermal stability and flame retardancy of thermoset polymers can be improved
by addition of flame-retardant chemical elements during their synthesis. Here, the
focus is placed on two families of thermoset polymers: epoxy and poly(urethane)
(PU). Several types of flame retardant elements have been used in the synthesis of
flame retardant epoxies (Rakotomalala et al. 2010) and PUs (Kowalczyk et al. 2018).
8.2.1 Epoxy
Epoxy resins are known as versatile thermosetting resins, and are widely used in
different sectors, such as coating and paint, composite, electrical and electronic (Jin
et al. 2015) and their application continues to increase due to their excellent properties. Its global market is currently around $7 million, and it is estimated that it will
grow at a compound annual growth rate (CAGR) of 6.2% to reach around $11 million by 2022 (Alliedmarketresearch.com). Several review papers have been published on flame retardancy of epoxy resins (Rakotomalala et al. 2010; Weil and
Levchik 2004; Gérard et al. 2010; Levchik et al. 2005). In addition, a recent comprehensive survey classified epoxy compounds in terms of flame retardation
(Salmeia and Gaan 2015). The reactive approach in flame retardancy of epoxy resins is very interesting due to the presence of an oxirane ring at the end of the polymer chain and its high reactivity. There are numerous works on the flame retardancy
effect of phosphorous-containing compounds, which benefit mainly from char formation in the condensed phase (Kowalczyk et al. 2018; Levchik and Weil 2004).
Among them the effect of chemically bonded 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives to the macromolecule chains of
thermoset resins has dedicated a considerable number of research papers
(Rakotomalala et al. 2010; Salmeia and Gaan 2015). DOPO is currently the most
commonly used phosphorus flame retardant in epoxy resins (Wang 2016). Its action
is mainly in gas phase (Rakotomalala et al. 2010). DOPO is an interesting reactive
phosphorus molecule with a high possibility of chemical reaction with epoxy resins
due to its P-H bond. A wide variety of DOPO sorts and its derivatives have been
reported (Fig. 8.1) (Salmeia and Gaan 2015).
There are three main ways for the chemical addition of DOPO and its derivates
into epoxy. First, the DOPO can be modified to obtain a diphenolic-DOPO, then the
obtained DOPO reacts with a conventional epoxy resin such as diglycidyl ether of
bisphenol A (DGEBA) (Fig. 8.2) (Lin et al. 2000; Ho et al. 2009). Second, DOPO
can directly react with the epoxy and then is cured with a conventional hardner
(Fig. 8.3) (Zhang et al. 2007). Third, modified-DOPO is used as a curing agent for
epoxy resin (Fig. 8.4) (Lin et al. 2005; Huo et al. 2019; Xu et al. 2015).
A series of chemically-modified epoxy resines with DOPO and its dervaitives are
presented in Table 8.1. This table also shows the results of flammability tests
8 Flame Retardancy of Reactive and Functional Polymers
8.2 Thermosets
The thermal stability and flame retardancy of thermoset polymers can be improved
by addition of flame-retardant chemical elements during their synthesis. Here, the
focus is placed on two families of thermoset polymers: epoxy and poly(urethane)
(PU). Several types of flame retardant elements have been used in the synthesis of
flame retardant epoxies (Rakotomalala et al. 2010) and PUs (Kowalczyk et al. 2018).
8.2.1 Epoxy
Epoxy resins are known as versatile thermosetting resins, and are widely used in
different sectors, such as coating and paint, composite, electrical and electronic (Jin
et al. 2015) and their application continues to increase due to their excellent properties. Its global market is currently around $7 million, and it is estimated that it will
grow at a compound annual growth rate (CAGR) of 6.2% to reach around $11 million by 2022 (Alliedmarketresearch.com). Several review papers have been published on flame retardancy of epoxy resins (Rakotomalala et al. 2010; Weil and
Levchik 2004; Gérard et al. 2010; Levchik et al. 2005). In addition, a recent comprehensive survey classified epoxy compounds in terms of flame retardation
(Salmeia and Gaan 2015). The reactive approach in flame retardancy of epoxy resins is very interesting due to the presence of an oxirane ring at the end of the polymer chain and its high reactivity. There are numerous works on the flame retardancy
effect of phosphorous-containing compounds, which benefit mainly from char formation in the condensed phase (Kowalczyk et al. 2018; Levchik and Weil 2004).
Among them the effect of chemically bonded 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives to the macromolecule chains of
thermoset resins has dedicated a considerable number of research papers
(Rakotomalala et al. 2010; Salmeia and Gaan 2015). DOPO is currently the most
commonly used phosphorus flame retardant in epoxy resins (Wang 2016). Its action
is mainly in gas phase (Rakotomalala et al. 2010). DOPO is an interesting reactive
phosphorus molecule with a high possibility of chemical reaction with epoxy resins
due to its P-H bond. A wide variety of DOPO sorts and its derivatives have been
reported (Fig. 8.1) (Salmeia and Gaan 2015).
There are three main ways for the chemical addition of DOPO and its derivates
into epoxy. First, the DOPO can be modified to obtain a diphenolic-DOPO, then the
obtained DOPO reacts with a conventional epoxy resin such as diglycidyl ether of
bisphenol A (DGEBA) (Fig. 8.2) (Lin et al. 2000; Ho et al. 2009). Second, DOPO
can directly react with the epoxy and then is cured with a conventional hardner
(Fig. 8.3) (Zhang et al. 2007). Third, modified-DOPO is used as a curing agent for
epoxy resin (Fig. 8.4) (Lin et al. 2005; Huo et al. 2019; Xu et al. 2015).
A series of chemically-modified epoxy resines with DOPO and its dervaitives are
presented in Table 8.1. This table also shows the results of flammability tests
8 Flame Retardancy of Reactive and Functional Polymers