Mechanism of Expandable Graphite Fire Retardant Action in

Chapter 8

Mechanism of Expandable Graphite Fire Retardant Action in Polyurethanes 2

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G . Camino1, S. Duquesne , R. Delobel , B . E l i n g 3 , C . Lindsay3, and T. Roels 1

Downloaded by PEPPERDINE UNIV on September 26, 2017 | http://pubs.acs.org Publication Date: September 15, 2001 | doi: 10.1021/bk-2001-0797.ch008

Dip Chimica IFM, Università Di Torino, Via P. Giuria, 7, 10125 Torino, Italy GéPIFREM, ENSCL, BP 108, 59652 Villeuneuve d'Ascq, France ICI Polyurethanes, Everslaan 65, 3078 Everberg, Belgium 2

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This study deals with the effects of expandable graphite (EG) on the mechanism of degradation of polyurethane (PU). We have shown that the mechanism of expansion of the graphite is due to oxidation of the carbon layer by H S O 4, rather than its decomposition, as previously suggested in the literature. On the other hand, we have shown that the mechanism of degradation of P U is little affected by the presence of E G . However, H S O induces additional reactions which do not modify the final structure of the residue. Finally, the expansion of the P U / E G formulation generates, on the surface, an insulative layer which protects the underlying material, resulting in the fire retardant properties of interest. 2

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Polyurethanes (PU) are different from most plastic in that they represent a large family: from foam insulation to shoe soles, car seals to abrasion-resistant coatings. Rigid foam is one of the most effective practical insulation materials used in applications such as building, pipes or domestic refrigerators. The use of P U in building is restricted because, in case of fire, it acts as a fuel generator with subsequent propagation of the fire to the surrounding combustibles and

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© 2001 American Chemical Society

Nelson and Wilkie; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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91 finally the destruction of property and the loss of human lives by direct burns or inhalation of toxic smoke. In order to reduce the flammability of P U , flame retardants are added to the polymer. The most widely used F R additives in rigid foam systems are chlorinated phosphate esters. However, in case of fire, halogenated fire retarded polymers evolve more toxic combustion products than the untreated polymer (23). Moreover, generation of highly corrosive hydrogen halides occurs. Therefore, legislation tends to limit the use of halogenated fire retardant systems for environmental and safety reasons (2-4). These are the reasons why current research focuses on the development of halogen-free, non-toxic and environmentally friendly fire retardants and in particular on intumescent systems (5,6). Intumescence describes a material which forms a blown cellular charred layer upon heating. The creation of this layer limits the heat and mass transfer to and from the underlying material, thus insulating the substrate from the heat source. Intumescent fire retardant systems have been developed using expandable graphite (EG) which is capable of imparting fire retardancy to various materials when incorporated into them (7-11). In particular, it provides flame retardancy of interest in flexible polyurethane foams (12-15), acting to smother burning as well as to insulate the foam from the heat source. The purpose of this paper is to investigate the effect of E G on the degradation mechanism of P U coating. This coating may be applied on various materials, such as polymers or composites and, in particular, on rigid P U foam.

Experimental

Materials Raw materials were commercial polymeric diisocyanate diphenylmethane (PMDI) and polyester polyol from ICI. The P U coating is obtained by polycondensation of the isocyanate with the polyol without further addition of additives or catalyst apart from those present in the proprietary commercial products. The molar ratio N C O / O H has been fixed at two. The components were stirred (400 rpm) in disposable paper cups (500 ml) at room temperature for 1 min and allowed to polymerize for 24 h. Expandable graphite (Callotek 500, Graphitwerk Krophfmuhl) is a graphite intercalation compound (GIC). The first known phenomenon of intercalation explains the secret of the production of the fine Chinese porcelain, seven centuries before Christ (16). However, the first GIC was only prepared in 1841, accidentally, by Schafhautl while analyzing crystal flakes of graphite in a solution of sulfuric acid (17). The graphite structure consists of layers of


92 hexagonal carbon structures within which a chemical compound (e.g. H S 0 ) can be intercalated (Figure 1). E G is prepared either by oxidation with a chemical reagent (18) or electrochemically (19-20) in the intercalating acid (i.e. H S 0 , H N 0 , etc). The chemical reaction in the case of H S 0 is expressed by the following equation: 2

2

4

3

2

24«C + m H S 0 + 1/2 0 2

4

• C

+ 24n

4

( H S O - ) ( m - l ) H S 0 + 1/2H 0 4

2

4

2

(1 )

The result is a graphite acid salt, so named because of its ionic nature in which the positive charge of the oxidized graphite network is balanced by negatively charged acid anions (211,222) and also includes acid molecules (233) as shown in Figure 1 for H S 0 GIC. Elemental analysis shows that Callotek 500 contains 2.8 wt.-% S corresponding to 8.5 wt.-% H S 0 . The P U / E G (15 wt.-%) coating was made in the same way as the P U coating (see above), using polyol containing E G prepared by stirring both components 1 min at 4000 rpm using a Turbo Turrax apparatus. 2


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Thermogravimetry (TG) Thermogravimetric analysis has been carried out either at a heating rate of 10°C/min or in isothermal conditions under nitrogen (60 cmVmin) using a horizontal thermobalance Du Pont MOD.951 Thermogravimetric Analyzer. Samples (about 10 mg) were held in open silica pans.

Pyrolysis Unit Pyrolysis of the materials (samples of about 100-200 mg) in an open glass holder has been carried out in the pyrolysis unit shown in Figure 2, using nitrogen as a carrier gas, either in isothermal or programmed heating conditions. Aluminum foil is wrapped around the walls of a water cooled trap inside the degradation tube in order to collect the high boiling products (HBP), which are volatile at reaction temperature but nonvolatile at ambient temperature. Gaseous degradation products are trapped from the carrier gas using the gas trapping system of Figure 3 which allows direct IR analysis through the K B r windows or sampling for G C - M S through the septum.

Fourier Transformed Infra-red spectroscopy IR spectra of original and degradation products were recorded using a Perkin-Elmer FTIR 2000 spectrometer connected to a Grams Analyst 2000 Perkin Elmer data station. Solids were ground and mixed with K B r to form pellets. Liquids and gases were respectively examined between K B r discs or in the gas cell of Figure 3.



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Figure 1: Structure of Ηβ0

4

graphite acid salt.


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H 0 2

H 0 2

water cooled trap


carrier gas ·

—^ carrier gas aluminium foil

glass sample holder Figure 2: Glass pyrolysis unit

KBr windows

septum

- c a r i e r gas

U traps Figure 3: Gas trapping system.


95 Combustion behavior Minimum oxygen concentration (01) for self-sustained combustion of a vertical down burning specimen after top ignition was measured by the 01 method ( A S T M D2863/77) using a Stanton Redcroft apparatus on specimens 100 χ 1 0 x 3 mm . 3


Blowing Measurement Blowing was measured as a function of temperature at a heating rate of 20°C/min using a homemade apparatus previously described (244). Briefly, a pellet of the material (diameter: 15 mm, thickness: 2mm) on which a position transducer probe (diameter: 20 mm) is located is heated in a glass tube (diameter: 22mm). The probe displacements are recorded as a function of temperature.

Results and Discussion Decomposition of E G The thermogravimetry of the E G (Figure 4) had to be carried out step by step in the pyrolysis unit raising the temperature 10°C/min and weighing the residue. Indeed, the thermobalance could not be used because of the E G expansion (about ten times) blowing the sample away from the holding pan. The major weight loss of E G that occurs between 200°C and 350°C with a maximum rate at about 250°C is responsible of the sample expansion, which begins at 200°C. Half and total expansion is respectively reached at 260°C and 350°C. In the literature (255), it is suggested that expansion occurs via the sulfuric acid decomposition, according to: H S0 2

S0

—> S 0 + H 0

4

3

2

S 0 + 1/2 0 2

3

(2) 2

However, sulfuric acid decomposition temperature (about 340°C) (26) is higher than that of which expansion begins. Moreover, the original amount of H S 0 in the E G is on the order of 8.5 wt-%, whereas the weight loss after the expansion in Figure 4 is about 25%; this cannot be explained by absorbed water. In order to better understand the expansion mechanism, we have collected the gases evolved under isothermal conditions (T=250°C, 30 min) and analyzed them using IR spectroscopy. The gas phase of E G pyrolysis is composed of S 0 (1400-1300 c m ) , C 0 (720-635, 2400-2250 and 3700-3600 c m ) and water (1850-1350 and 4000-3600 c m ) . These gases escape through the edges of the 2

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1

1

2

1



Figure 4: TG (—) and blowing measurement (- -) ofEG.



Figure 5: IR spectrum ofgases collected from EG pyrolysis (T=250°C).


98 graphite particles, leading to the irreversible expansion. Rather than H S 0 decomposition, these data clearly show that a redox process between H S 0 and graphite (reaction 3) generates the blowing gases. 2

4

2

4

C + 2H S0 2

• C 0 + 2H 0+ 2S0

4

2

2

(3)

2

A similar redox process was previously reported in the literature for S 0 intercalated graphite as the process promoting exfoliation upon heating (277) : 3C + 4 S 0 - » S + 3 C 0 + 3 S 0 (4) 3

(S)

2

3

2

Partial oxidation of the C to C O could be ruled out by a thermal volatilization analysis experiment ( T V A (288)) in which non-condensable C O would have been easily detected. The quantity of S 0 evolved was evaluated by bubbling the carrier gas into aqueous 1% H 0 solution, resulting in the formation of H S 0 which was then titrated with NaOH. The result was confirmed by elemental analysis of original and residual E G which, however, does not rule out occurrence of reaction (3) because we did not perform analysis for elemental sulfur independently from H S 0 determination. E G evolves about 0.8 wt.-% of S 0 corresponding to only about 16 % of the weight of the H S 0 intercalated in the graphite which participates in the expansion process as shown in reaction (3). This is qualitatively confirmed by the presence of residual sulfate IR bands (1160-1060 cm" (29)) in the E G residue after the isothermal treatment, which is comparable to that of original E G . In these spectra, the IR absorptions of water (3410 and 1650 cm" ) are also visible. These data confirm that only a minor fraction of the intercalated H S 0 takes part in the expansion mechanism by the redox reaction (3). Further volatilization of H S 0 at higher temperature (>340°C) does not apparently contribute to expansion (see Figure 4) possibly because the gases evolved in the redox process (200-350°C) have pushed the crystalline graphite sheets apart freeing the intercalated H S 0 . The material generated by heating E G to expansion temperature is a puffed-up material of low density with a worm like structure.


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Degradation of polyurethane coating The weight loss of the P U coating is shown to take place in three steps, as observed from the T G curve of Figure 6. This curve shows that the P U coating decomposes to volatile products with the main step between 220 and 450°C. The residue from the first step is relatively stable and slowly further volatilizes at higher temperatures with a maximum rate at 530°C (2 step). Above 610°C (3 step), weight loss occurs at a steady rate and is still not complete at 800°C. In order to characterize the products of the degradation, we have repeated every degradation step to completion under isothermal conditions (Table 1). For each step, gases, H B P and residue have been examined using IR spectroscopy. nd

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Mechanism of Expandable Graphite Fire Retardant Action in

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