**2. History and perspective of flame retardant and thermally insulative polymers**

## **2.1 Principles of polymer flammability**

Three ingredients — heat, oxygen, and fuel — are required to initiate and continue a fire [12]. Polymer starts to degrade, when it is heated by external ignition sources and reaches a characteristic temperature [39]. The surrounding oxygen amount plays key roles on polymer surface decompositions (e.g., thermo-oxidative degradation of polymers and/or thermal degradation of polymers). The amount of oxygen for polymer decomposition depends on the specific polymer used. Combustible gases as fuels may be produced at a rate dependent upon polymer decomposition rate and diffuse to the flame front [12]. After ignition and removal of the ignition source, combustion could be self-propagating if there is sufficient heat generated and polymer can absorb enough heat to sustain its decomposition processes. Polymer combustion processes could involve vapor phase and condensed phase reactions [39].

## **2.2 Theory of flame retardance**

### *2.2.1 Vapor phase flame inhibition*

The combustion process of premixed methane-oxygen flame is well investigated [40, 41]. The methane oxygen system can be used as a model for studying more complex polymer flames [41, 42]. Methane combustion is a free-radical chain reaction, which mainly consists of propagation, chain branching, and termination processes [43]. Any flame-retardant material which either decreases the concentration of these chain carrying radicals or increases the rate of termination will inhibit the flame reaction. This is thought to be mechanisms by which vapor phase flame inhibitors [39, 44].

#### *2.2.2 Condensed phase flame inhibition*

Cooling and char barrier formation are two main modes in solid phase flame inhibition [39, 44, 45]. We discuss cooling mode first. One important cooling mode used in condensed phase flame inhibition is the use of materials which decompose endothermically in the pyrolysis zone of the burning polymer [39]. For example, due to polyvinyl alcohol's ability to endothermically form water molecules, polyvinyl alcohol is less flammable than the isomeric polyethylene oxide [39, 46]. During

**3**

*Introductory Chapter: Flame Retardant and Thermally Insulating Polymers*

the spreading of a fire and thus of limited use [39, 49].

for existing and unforeseen applications.

**2.4 Critical discussion and outlook**

oxygen [40, 63].

**2.3 Thermally insulative polymers with high flame retardancy**

are desired for advanced thermal management applications [17, 50, 57].

Flame retardant and thermally insulative polymer-based composites have been developed [50, 56, 59–62]. For example, PC–PDMS copolymers have flameretardant behaviors. Chars can prevent more volatile fuel production and serve as a thermal insulator preventing the temperature from rising [59]. When a specimen of PC–PDMS was in combustion, a lot of fine bubbles and char were formed. These fine bubbles are good for thermal insulation [59]. Moreover, silica particles in situ produced by thermal decomposition of PDMS mostly stay in char layers, which improve the quantity of oxidation-resistant char coatings [59]. The resulting bubble structures and silica materials in the char layer prevented volatile and flammable fuel production, which served as an effective thermal insulator [59]. Although there are progresses on developing flame retardant and thermally insulative polymerbased materials, further understanding flame retardancy and thermal insulation mechanisms will play key roles increating next generation of thermally insulative and flame retardant polymers with outstanding performance. With unique combined properties including simple manufacturing process, low cost, excellent thermal insulation, flame retardancy, superior physical and mechanical properties, thermally insulative and flame retardant polymers will provide new opportunities

To achieve the high-performance fire retardancy of polymers, different strate-

2.By stopping the combustion through dilution of the combustible gases, or the

1.By modifying the reaction scheme of pyrolysis of polymers to produce non-combustible, and/or non-volatile products that dilute the supply of

formation of a char which suppress the oxygen supply [63, 64].

gies have been developed [63]. Followings are selected strategies.

polymer burning processes, cooling mode can be also achieved by adding thermally conductive fillers into polymers [39]. Fillers have higher thermal conductivities than that of polymers. Fillers conduct heat better than polymers. Fillers conduct heat away from hot regions more efficiently than unfilled polymers. Fillers enable polymer-filler composites difficult to burn. However, fillers at high volume fraction loadings are needed for good cooling effect, which might lead to limited use [39, 47]. Dripping is another cooling mechanism [39, 48]. Polymers that drip easily during burning processes are more difficult to burn. For example, a regular candle will not burn with no wick, due to its high dripping tendency. This is because heat is dissipated from flaming areas. However, dripping could be a hazard by resulting in

Thermally insulative polymer-based materials with high flame retardancy are attracting significant attention [50]. This is because thermally insulative materials can protect overheating damage from burn injuries and save lives [34, 51–58]. There are drawbacks for available fire-resistant polymers. Some flame retardant polymers could be expensive [57]. Some flame retardant polymers have relatively low decomposition temperatures and decompose nearby 400°C [57]. Thus, highly thermally insulative, thermally stable and flame retardant polymer-based materials

*DOI: http://dx.doi.org/10.5772/intechopen.99111*

*Introductory Chapter: Flame Retardant and Thermally Insulating Polymers DOI: http://dx.doi.org/10.5772/intechopen.99111*

*Flame Retardant and Thermally Insulating Polymers*

**2.1 Principles of polymer flammability**

**polymers**

phase reactions [39].

inhibitors [39, 44].

**2.2 Theory of flame retardance**

*2.2.1 Vapor phase flame inhibition*

*2.2.2 Condensed phase flame inhibition*

mechanisms in polymers remain challenging. Advanced polymers with combined properties of flame retardancy, mechanical strength and heat insulation are needed. Such polymers will provide broader prospect in civil applications than singlefunction polymers, for example, building insulation applications [36–38].

This introductory chapter not only aims to present the current landscape flame retardant and thermally insulative polymers, but also highlights next generation of flame retardant and thermally insulative polymers for fire protections applications. This introductory chapter summarize fundamental interests and technological importance of flame retardant and thermally insulative polymers, which include principles of polymer flammability, theory of flame retardance, thermally insula-

**2. History and perspective of flame retardant and thermally insulative** 

Three ingredients — heat, oxygen, and fuel — are required to initiate and continue a fire [12]. Polymer starts to degrade, when it is heated by external ignition sources and reaches a characteristic temperature [39]. The surrounding oxygen amount plays key roles on polymer surface decompositions (e.g., thermo-oxidative degradation of polymers and/or thermal degradation of polymers). The amount of oxygen for polymer decomposition depends on the specific polymer used. Combustible gases as fuels may be produced at a rate dependent upon polymer decomposition rate and diffuse to the flame front [12]. After ignition and removal of the ignition source, combustion could be self-propagating if there is sufficient heat generated and polymer can absorb enough heat to sustain its decomposition processes. Polymer combustion processes could involve vapor phase and condensed

The combustion process of premixed methane-oxygen flame is well investigated

[40, 41]. The methane oxygen system can be used as a model for studying more complex polymer flames [41, 42]. Methane combustion is a free-radical chain reaction, which mainly consists of propagation, chain branching, and termination processes [43]. Any flame-retardant material which either decreases the concentration of these chain carrying radicals or increases the rate of termination will inhibit the flame reaction. This is thought to be mechanisms by which vapor phase flame

Cooling and char barrier formation are two main modes in solid phase flame inhibition [39, 44, 45]. We discuss cooling mode first. One important cooling mode used in condensed phase flame inhibition is the use of materials which decompose endothermically in the pyrolysis zone of the burning polymer [39]. For example, due to polyvinyl alcohol's ability to endothermically form water molecules, polyvinyl alcohol is less flammable than the isomeric polyethylene oxide [39, 46]. During

tive and fire-retardant polymers, and critical discussion and outlook.

**2**

polymer burning processes, cooling mode can be also achieved by adding thermally conductive fillers into polymers [39]. Fillers have higher thermal conductivities than that of polymers. Fillers conduct heat better than polymers. Fillers conduct heat away from hot regions more efficiently than unfilled polymers. Fillers enable polymer-filler composites difficult to burn. However, fillers at high volume fraction loadings are needed for good cooling effect, which might lead to limited use [39, 47]. Dripping is another cooling mechanism [39, 48]. Polymers that drip easily during burning processes are more difficult to burn. For example, a regular candle will not burn with no wick, due to its high dripping tendency. This is because heat is dissipated from flaming areas. However, dripping could be a hazard by resulting in the spreading of a fire and thus of limited use [39, 49].

### **2.3 Thermally insulative polymers with high flame retardancy**

Thermally insulative polymer-based materials with high flame retardancy are attracting significant attention [50]. This is because thermally insulative materials can protect overheating damage from burn injuries and save lives [34, 51–58]. There are drawbacks for available fire-resistant polymers. Some flame retardant polymers could be expensive [57]. Some flame retardant polymers have relatively low decomposition temperatures and decompose nearby 400°C [57]. Thus, highly thermally insulative, thermally stable and flame retardant polymer-based materials are desired for advanced thermal management applications [17, 50, 57].

Flame retardant and thermally insulative polymer-based composites have been developed [50, 56, 59–62]. For example, PC–PDMS copolymers have flameretardant behaviors. Chars can prevent more volatile fuel production and serve as a thermal insulator preventing the temperature from rising [59]. When a specimen of PC–PDMS was in combustion, a lot of fine bubbles and char were formed. These fine bubbles are good for thermal insulation [59]. Moreover, silica particles in situ produced by thermal decomposition of PDMS mostly stay in char layers, which improve the quantity of oxidation-resistant char coatings [59]. The resulting bubble structures and silica materials in the char layer prevented volatile and flammable fuel production, which served as an effective thermal insulator [59]. Although there are progresses on developing flame retardant and thermally insulative polymerbased materials, further understanding flame retardancy and thermal insulation mechanisms will play key roles increating next generation of thermally insulative and flame retardant polymers with outstanding performance. With unique combined properties including simple manufacturing process, low cost, excellent thermal insulation, flame retardancy, superior physical and mechanical properties, thermally insulative and flame retardant polymers will provide new opportunities for existing and unforeseen applications.

#### **2.4 Critical discussion and outlook**

To achieve the high-performance fire retardancy of polymers, different strategies have been developed [63]. Followings are selected strategies.


The different types of flame retardants in polymers based on halogens, heavy phosphorus-organic compounds and/or transition metals have shown good flame retardance performance [26]. However, toxic gases and smoke are formed during burning processes [66]. Environmental safety of flame retardants in polymers is a major issue [22, 63]. Fire-retardant polymer-based materials are desired to have high resistance to ignition, low combustion rate, retention of low flammability, acceptability in properties and appearance, no health safety and environmental issues, and little (or no) economic penalty.
