**3. Polyurethane flexible foam fire retardants**

#### **3.1. Halogenated phosphorous flame retardants**

In recent years the phasing out of some types of halogenated FR (flame retardant) due to persistence at environment and bioaccumulation and toxicity has been more investigated.

TMCP (Tris (2-chloroisopropyl) phosphate) and TDCP (Tris (1, 3-dichloroisopropyl phosphate) are two well-known liquid FR which are used in polyurethane flexible foam to make fire resisted **(Figure3-4). Table (2) illustrate some important parameters of the mentioned fire retardants.** [2]

Polyurethane Flexible Foam Fire Behavior 105

Studies show that in the foams with only liquid FR (TMCP , TDCP) a very divergent combustion behaviour has been indicated. TMCP containing foams show lower TWL(total weight loss) and shorter burn time compared to TDCP containing foams.Moreover, TMCP containing foams didn't show any significant dripping and subsequent hole formation, a phenomenon seen at all levels of TDCP addition. TMCP and TDCP addition leads to decrease in the THE( total heat evolved) but an increase in the amount of smoke and carbon monoxide produced and this is why normally some amount of other FR such as melamine is added to the TMCP and TDCP containing foams to decrease total heat evolved, total smoke

Due to the above mentioned reasons it has been a driving force to move toward the halogen free FR to compensate those weakness of halogenated FR, **despite of some disadvantages that the halogen free FR have e.g. they are mostly in solid state and they show process difficulties.** There are different types of halogen-free flame retardants which are behaving with different mechanisms. First group acts according to the expansion inside the polymer and oxygen-diffusion prevention and second group does by the cooling of the ignited surface of the polymer. One important example of the above mentioned groups are leaded

EG is a graphite intercalation compound in which some oxidants, such as sulfuric acid, potassium permanganate, etc. are inserted between the carbon layers of the graphite [4].

produced and CO emission significantly[2,3].

by expandable graphite (EG) and Melamine powder respectively.

**Figure 5.** Comparison of Lattice graphite, Diamond and C60

Fig.5 illustrates the chemical structure of Graphite, diamond and C60 [1].

**3.2. Halogen-free flame retardants** 

*3.2.1. Expandable graphite* 

**Figure 3.** Chemical structure of TMCP

**Figure 4.** Chemical structure of TDCP


**Table 2.** Properties of TMCP and TDCP

Studies show that in the foams with only liquid FR (TMCP , TDCP) a very divergent combustion behaviour has been indicated. TMCP containing foams show lower TWL(total weight loss) and shorter burn time compared to TDCP containing foams.Moreover, TMCP containing foams didn't show any significant dripping and subsequent hole formation, a phenomenon seen at all levels of TDCP addition. TMCP and TDCP addition leads to decrease in the THE( total heat evolved) but an increase in the amount of smoke and carbon monoxide produced and this is why normally some amount of other FR such as melamine is added to the TMCP and TDCP containing foams to decrease total heat evolved, total smoke produced and CO emission significantly[2,3].

#### **3.2. Halogen-free flame retardants**

104 Polyurethane

**mentioned fire retardants.** [2]

**Figure 3.** Chemical structure of TMCP

**Figure 4.** Chemical structure of TDCP

**Table 2.** Properties of TMCP and TDCP

TMCP (Tris (2-chloroisopropyl) phosphate) and TDCP (Tris (1, 3-dichloroisopropyl phosphate) are two well-known liquid FR which are used in polyurethane flexible foam to make fire resisted **(Figure3-4). Table (2) illustrate some important parameters of the** 

> **Property TMCP TDCP**  CL content (%) 32.5 49 Mw(g/mol) 327.55 430.91 P content (%) 9.5 7.1 physical state at 25°C clear liquid clear liquid Water solubility(%) < 0.05 < 0.05

Due to the above mentioned reasons it has been a driving force to move toward the halogen free FR to compensate those weakness of halogenated FR, **despite of some disadvantages that the halogen free FR have e.g. they are mostly in solid state and they show process difficulties.** There are different types of halogen-free flame retardants which are behaving with different mechanisms. First group acts according to the expansion inside the polymer and oxygen-diffusion prevention and second group does by the cooling of the ignited surface of the polymer. One important example of the above mentioned groups are leaded by expandable graphite (EG) and Melamine powder respectively.

#### *3.2.1. Expandable graphite*

EG is a graphite intercalation compound in which some oxidants, such as sulfuric acid, potassium permanganate, etc. are inserted between the carbon layers of the graphite [4]. Fig.5 illustrates the chemical structure of Graphite, diamond and C60 [1].

**Figure 5.** Comparison of Lattice graphite, Diamond and C60

When EG is subjected to a heat source, it expands to hundreds of times of its initial volume and creates voluminous, stable carbonaceous layer on the surface of the materials. This layer limits **the heat transfer from the heat source to the substrate and the mass transfer from the substrate to the heat source resulting in protection of the underlying material [5, 6]. The redox process [7]** between Sulfuric acid and graphite generates the blowing gases according to the reaction:

Polyurethane Flexible Foam Fire Behavior 107

**Figure 7.** Particle size and distribution of 8% of EG (0.25mm) (SEM×200)

combustible gas evolution and burning are reduced.

generating of volatile fuel is reduced.

Melamine acts as fire retardant and smoke-suppressant according to the following

 Melamine is believed to act as a heat sink, increasing the heat capacity of the combustion system and lowering the surface temperature of the foam. Thus the rates of

 The nitrogen content of the melamine may partly end up as nitrogen gas when melamine burns, providing both a heat sink and inert diluents in the flame. The presence of melamine in the foam results in less heat generated by the flame, consequently less heat fed back to the foam and the rate of foam pyrolysis, i.e.

 Due to a chemical interaction between melamine and the evolved isocyanate fraction creating from degradation of polyurethane foam. This interaction reduces the amount

of diisocyanate the main contributor to the smoke and CO release (Fig.8).

*3.2.2. Melamine* 

combined mechanisms [8].

$$\text{C} + 2\text{H}\_2\text{SO}\_4 = \text{CO}\_2\text{\uparrow}\_+ \, 2\text{H}\_2\text{O}\_+ \uparrow\_2 \, \text{SO}\_2\text{\uparrow}\_+$$

The fire retardancy of EG is done by two steps [1]:


The more characteristic factors for EG which should be considered are:


Figure (6, 7) show particle size and distribution of two types of EG with different sizes (0.18mm, 0.25mm)

**Figure 6.** Particle size and distribution of 8% of EG (0.18mm) (SEM ×200)

**Figure 7.** Particle size and distribution of 8% of EG (0.25mm) (SEM×200)

#### *3.2.2. Melamine*

106 Polyurethane

according to the reaction:

oxidizing graphite.

Expansion volume

Strength

(0.18mm, 0.25mm)

SET (start expansion temperature)

The fire retardancy of EG is done by two steps [1]:

When EG is subjected to a heat source, it expands to hundreds of times of its initial volume and creates voluminous, stable carbonaceous layer on the surface of the materials. This layer limits **the heat transfer from the heat source to the substrate and the mass transfer from the substrate to the heat source resulting in protection of the underlying material [5, 6]. The redox process [7]** between Sulfuric acid and graphite generates the blowing gases

 The EG expands under the impact of Heat up to about 500 times of its original volume and creates a very large surface. It allows a quick oxidation of the carbon. The oxygen is taken out of the air and makes the air almost inert. This inert air extinguishes the fire. EG doesn't create flames while oxidation and will extinguish if no more heat will be applied to the glowing graphite. Therefore, no source of fire will be generated by the

Figure (6, 7) show particle size and distribution of two types of EG with different sizes

The more characteristic factors for EG which should be considered are:

**Figure 6.** Particle size and distribution of 8% of EG (0.18mm) (SEM ×200)

Melamine acts as fire retardant and smoke-suppressant according to the following combined mechanisms [8].


Polyurethane Flexible Foam Fire Behavior 109

**4. Properties of the polyurethane flexible foam with different types of** 

Comparison between halogenated flame retardants which are mainly liquid with halogen free flame retardants ( expandable graphite and melamine powder) which both are solid can

The best choice for the processing as it is clear will be the liquid grade which has a good

Expandable Graphite has a limit pot life (3-4 hours) and when it subjects with the polyol component, it attacks to the catalyst of the polyol and destroys the catalyst during the foaming process**. For the foam producing, the highly recommendation is the EG containing polyol should react with proper isocyanate before the EG pot life reaches or the EG should be injected by an individual stream and mix with the polyol stream in the mixing head instead of pre-mixing with polyol.** Otherwise the produced foams will be collapsed. The other disadvantage of this technology is related to the fact that EG is very corrosive and make the mixing head to be damaged and it is preferred to be used a hardened grade of mixing head, a

The advantages of this technology is the good homogeneity of the EG particles inside the

**fire retardants** 

 Processing Reactivity Fire properties Physical properties.

polyol.

be categorized as **four** items.

**4.1. Foaming process** 

dispersion inside the polyol and less side effects.

damaged mixing head needle picture is showed in (Fig.10)[9].

**Figure 10.** Mixing head needle corrosion by EG

**Figure 8.** Chemical reaction between melamine and diisocyanate (MDI)

Figure (9) shows particle size and distribution of melamine powder inside the flexible foam.

**Figure 9.** Particle size and distribution of 8% of melamine (SEM ×200)
