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Flame retardant properties


FIRE AND MATERIALS

Fire Mater. 25, 193–197 (2001) DOI: 10.1002 /fam.776

Flame Retardant Properties of EVA-nanocomposites and Improvements by Combination of Nano?llers with Aluminium Trihydrate
¨ nter Beyer* Gu
Kabelwerk Eupen AG, Malmedyerstrasse 9, B-4700 Eupen, Belgium E-mail: guenter beyer@eupen.com

Flame retardant nanocomposites are synthesized by melt-blending ethylene–vinyl acetate copolymers (EVA) with modi?ed layered silicates (montmorillonites). Thermogravimetric analysis performed under different atmospheres (nitrogen and air) demonstrated a clear increase in the thermal stability of the layered silicate-based nanocomposites. The use of the cone calorimeter to investigate the ?re properties of the materials indicated that the nanocomposites caused a large decrease in heat release. The char-formation is the main factor important for improvement and its function is outlined. Further improvements of the ?ame retardancy by combinations of nano?llers and traditional FRadditives on the basis of metal hydroxides were also studied. Copyright # 2002 John Wiley & Sons, Ltd.

INTRODUCTION Fire hazards are mainly the result of a combination of different factors including } } } } } } } ignitability ease of extinction ?ammability of the generated volatiles amount of the heat released on burning rate of heat release ?ame spread, smoke obscuration and smoke toxicity. The most important ?re hazards are1: heat, smoke and toxic gases. A high rate of heat release causes fast ignition and ?ame spread. Furthermore it controls the ?re intensity and is therefore much more important than ignitability, smoke toxicity or ?ame spread. The time to escape available for ?re victims is also controlled by the heat release rate. Most people die in big ?res; six times more fatalities are reported in big ?res than in all other ?res. Smoke production is a further important ?re hazard. People become disoriented in dark smoke and therefore they cannot exit unless they can see. On the other hand, the ?re ?ghters have severe problems in rescuing people in dark surroundings. The acute toxicity of ?re gases is mainly controlled by the carbon monoxide content, being responsible for over 90% of people killed by ?res.2 Each year about 5000 people are killed by ?re in Europe and more than 4000 people in the USA. Direct property losses are roughly 0.2% of the gross domestic product and the total costs of ?res are around 1% of the gross domestic product.3 Therefore it is important to develop well-designed ?ame retardant materials to decrease the ?re hazards indicated.

Polymers are used in more and more ?elds of applications and speci?c mechanical, thermal and electrical properties are required. One further important property is the ?ame retardant behaviour of the polymers, which can be ful?lled traditionally by the following routes: }Use of intrinsically ?ame retardant polymers such as PVC or ?uoropolymers. }Use of ?ame retardants such as aluminium trihydrate, magnesium hydroxide, organic brominated compounds or intumescent systems to prevent the burning of polymers such as PE, PP, PA or other polymers. In some cases these ?ame retardant systems show considerable disadvantages: }The application of aluminium trihydrate, and also magnesium hydroxide, requires a very high portion of the ?ller within the polymer matrix; ?lling levels of more than 60% weight are necessary to achieve suitable ?ame retardancy, e.g. for cables and wires. The clear disadvantages of these ?lling levels are the high density and the lack of ?exibility of the end products, the low mechanical properties and the problematic compounding and extrusion steps. }In Europe there are, at least, reservations about the general use of brominated compounds as ?ame retardants. }Intumescent systems are relatively expensive and the electrical requirements can restrict the use of these products. A new class of materials, called nanocomposites, avoids the outlined disadvantages of the traditional ?ame retardant systems. Generally the term ‘nanocomposite’ describes a two-phase material where a suitable ?ller (usually a modi?ed layered silicate) is dispersed in the polymer matrix at a nanometer (10?9 m) scale.

*

Correspondence to: Dr. G. Beyer, Kabelwerk Eupen AG, Malmedyerstrasse 9, B-4700 Eupen, Belgium Received 1 June 2001 Accepted 25 November 2001

Copyright # 2002 John Wiley & Sons, Ltd.

194 Nanocomposite properties

G. BEYER

Compared with pure polymers the corresponding nanocomposites show tremendous improvements; the content of the modi?ed layered silicates often ranges between just 2% weight and 10% weight. The following list indicates some of the most important improved properties: } Improvements in mechanical properties like tension, compression, bending and fracture } Improvements in barrier properties like permeability and solvent resistance } Improvements in optical properties } Improvements in ionic conductivity A review discusses these improvements.4 Other highly interesting properties exhibited by polymer-layered silicate nanocomposites concern their increased thermal stability and also their ability to promote ?ame retardancy at very low ?lling levels. The formation of a thermal insulation and also low permeable char to volatile combustion products caused by a ?re is responsible for these improved properties.5–8 The low ?ller contents in nanocomposites leading to the drastic improvement in thermal stability is highly attractive for industry because the end-products can be made cheaper and are easier to process.

Mixing was done on several compounding machines. A rolling mill and internal mixer as discontinuous compounding machines were used; a BUSS kokneader (with a rotating and simultaneously oscillating screw, 11 L/D, 46 mm screw-diameter) was used as a continuous compounding machine. A processing temperature of 1608C was used for the different compounding machines. Information on the nanocomposite morphology was obtained by transmission electron microscopy (TEM) and X-ray diffraction (XRD) observation. Exfoliated silicate sheets were observed, together with small stacks of intercalated montmorillonite. This structure may be described as a semi-intercalated semi-exfoliated structure that does not change principally with the vinyl acetate content of the EVA matrix, an even larger number of stacks are observed for EVA with lower vinyl acetate contents.10 There were no great differences within the morphology of the nanocomposites related to the different compounding routes.

RESULTS Thermal stability Thermogravimetric analysis (TGA) is widely used to characterize the thermal stability of a polymer. The mass loss of the polymer due to volatilization of products generated by thermal decomposition was monitored as a function of a temperature ramp. Non-oxidative decomposition occurs when the heating of the material is done under an inert gas ?ow such as helium or nitrogen, while the use of air or oxygen allows oxidative decomposition reactions to be followed. The experimental conditions of the degradation highly in?uences the reaction mechanism of the degradation. The thermal stability of ethylene–vinyl acetate (EVA) nanocomposites was investigated;10 these are partially intercalated and partially exfoliated, independent of the EVAs used. TGA under helium (non-oxidative decomposition) and under air (oxidative decomposition) were investigated. EVA is known to decompose in two consecutive steps. The ?rst is identical in both oxidative and non-oxidative conditions. It occurs between 3508C and 4008C and is linked to the loss of acetic acid. The second step involves the thermal decomposition of the obtained unsaturated backbone either by further radical scissions (non-oxidative decomposition) or by thermal combustion (oxidative decomposition) (Fig. 1a, b). In helium the EVA-nanocomposite had a negligible reduction in thermal stability compared with the pure EVA or the EVA ?lled with Na-montmorillonite (microcomposite). In contrast, when decomposed in air, the same nanocomposite exhibited a rather large increase in thermal stability because the maximum of the second degradation peak was shifted 408C higher, while the maximum of the ?rst decomposition peak remained unchanged (Table 1). In this case the explanation for the improved thermal stability is the char formation occurring under oxidative conditions. The char acts as a physical barrier between the polymer and the
Fire Mater. 25, 193–197 (2001)

EXPERIMENTAL Materials A commercial available layered silicate based on montmorillonite modi?ed by dimethyl-distearylammo. d-Chemie/Germany was used as nium cations from Su nano?ller. Ethylene–vinyl acetate (EVA) copolymers (Exxon’s Escorene types) with different % weight vinyl acetate were used in this study. These copolymers have demonstrated their ability to promote nanocomposite formation by melt blending with nano?llers.9–11 Aluminium trihydrate (Martinal OL 104 LE) from Martinswerke GmbH, Germany was used. Processing and structure of EVA-based nanocomposites Depending on the nature of the ?ller distribution within the matrix, the morphology of the nanocomposites can evolve from the so-called intercalated structure with a regular alternation of the layered silicates and polymer monolayers to the exfoliated (delaminated) structure, where the layered silicates are randomly and homogeneously distributed within the polymer matrix. The easiest and technically most attractive way to produce these materials is by kneading the polymer in the molten state with a modi?ed layered silicate such as montmorillonite. The native Na+ interlayer cation within the silicate is replaced by a quaternary alkylammonium cation. The modi?ed ?ller is called a nano?ller and is much more compatible with the polymer matrix.
Copyright # 2002 John Wiley & Sons, Ltd.

FLAME RETARDANT NANOCOMPOSITES

195

Figure 1. TGA of EVA, EVA microcomposite with 5% weight Na-montmorillonite and EVA nanocomposite with 5% weight nanofiller under helium and air; 208C/min; EVA: Escorene UL 00328 with 28% weight vinyl acetate content.

Table 1. Maximal temperature at the main degradation peak (DTG) measured under air ?ow at 208C/min for EVA and EVA-based nanocomposite with different nano?ller contents; EVA: Escorene UL 00328 with 28% weight vinyl acetate
Nanofiller content (weight %) Maximal temperature at the main degradation peak (C8)

was already obtained at a layered silicate level of 2.5– 5.0% weight. Flammability properties From an engineering point of view, it is important to know what hazards within a ?re must be prevented and only then strategies for measurements and improvements can be developed. Extensive research at NIST (National Institute for Standards and Technology, USA) led to the important conclusion that allows signi?cant simpli?cation of the problem for hazards in ?res: The heat release rate, in particular the peak heat release rate, is the single most important parameter in a ?re and can be viewed as the ‘driving force’ of the ?re.12 Therefore, today the universal choice of an engineering test for ?ame retardant polymers is the cone calorimeter. The measuring principle is the oxygen depletion with a relationship between the mass of oxygen consumed from the air and the amount of heat released.
Fire Mater. 25, 193–197 (2001)

0 1 2.5 5 10 15

452.0 453.4 489.2 493.5 472.0 454.0

super?cial zone where the combustion of the polymer is occurring. The results in Table 1 on the maximal temperatures of the main degradation peak for EVA nanocomposites are very informative. The optimum for thermal stabilization
Copyright # 2002 John Wiley & Sons, Ltd.

196

G. BEYER

Figure 2. Rate of heat release vs time measured with a cone calorimeter (heat flux: 35 kW/m2) for various EVA (Escorene UL 00328 with 28% vinyl acetate) based materials: (a) Pure EVA matrix and EVA matrix with 5% weight of Na-montmorillonite; (b) EVA+3% weight of nanofiller; (c) EVA+5% weight of nanofiller; (d) EVA+10% weight of nanofiller.

The cone calorimeter is standardized as ASTM E 1354 and ISO 5660. In a typical cone calorimeter experiment the polymer sample (as a plate of 100 ? 100 ? 5 mm) in aluminium dishes is exposed to a de?ned heat ?ux (mostly 35 kW/m2 or 50 kW/m2). Simultaneously the properties ‘heat release rate’, ‘peak of heat release’, ‘time to ignition’, ‘total heat released’, ‘mass loss rate’, ‘mean CO yield’, ‘mean speci?c extinction area’ etc. can be measured. The ?ame retardant properties of the EVA nanocomposites were determined using cone calorimetry under a heat ?ux of 35 kW/m2 (Fig. 2). Under such conditions, simulating a small ?re scenario, the effect of the nano?ller was observed by 3% weight. A decrease by 47% of the peak of heat release as well as a shift towards longer times were detected for a nanocomposite containing 5% weight of the nano?ller when compared with the pure matrix EVA. Increasing the ?ller content to 10% weight did not improve any further the reduction of the peak of heat release. As a decrease in the peak of heat release indicates a reduction of the burnable volatiles generated by the degradation of the polymer matrix, such a drop clearly indicates the ?ame retardant effect due to the presence of the nano?ller and its ‘molecular’ distribution throughout the matrix. The ?ame retardant properties are further improved by the fact that the peak of heat release is spread over a much longer period of time. The ?ame retardant properties are due to the formation of a char layer during the nanocomposite combustion. This char acts as an insulating and nonburning material that reduces the emission of volatile products (fuel) into the ?ame area. The silicate layers of the nano?ller play an active role in the formation of this char but also strengthen it and make it more resistant to ablation. Cone calorimeter experiments with a heat ?ux of 35 kW/m2 also showed that pure EVA was completely burned without any residue. In contrast to the previous result an early strong char formation was found for the EVA nanocomposite within an analogous cone calorimeter experiment; but now this char is stable and does not disappear by combustion (Fig. 3a, b).
Copyright # 2002 John Wiley & Sons, Ltd.

Figure 3. a. Char formation of pure EVA and EVA nanocomposite with 5% weight nanofiller by cone-calorimeter combustion after 50 s. EVA: Escorene UL 00328 with 28% weight vinyl acetate. Heat flux: 35 kW/ m2; charred polymer plates of 100 ? 100 ? 5 mm on aluminium dishes. b. Char formation of pure EVA and EVA nanocomposite with 5% weight nanofiller by cone-calorimeter combustion after 200 s. EVA: Escorene UL 00328 with 28% weight vinyl acetate. Heat flux: 35 kW/m2; charred polymer plates of 100 ? 100 ? 5 mm on aluminium dishes.

Finally, compared with the pure EVA matrix, the nanocomposite burned without producing burning droplets (UL 94 vertical procedure),14 a characteristic feature that further limits the propagation of a ?re. This is an important characteristic for products to be classi?ed within the new Euroclasses regulating ?ame retardancy classes in Europe. Combination of the traditional ?ame retardant ?ller aluminium trihydrate (ATH) with a nano?ller To achieve typical ?ame retardancy for cables required by the most important international cable ?re test (IEC 60332-3-24)15 a ratio of 65% weight ATH and 35% weight of a suitable polymer matrix like EVA is often used for cable outer-sheaths.13 Therefore in typical experiments the performances of the two compounds were compared. Both compounds were prepared on a BUSS extruder (46 mm screw diameter, 11 L/D). One compound was made of 65% weight ATH and 35% weight EVA with 28% vinyl acetate and a second compound was made by 60% weight ATH, 5% weight of the nano?ller and 35% weight EVA with 28% vinyl acetate. Both compounds were investigated with TGA in air and with a cone calorimeter at 50 kW/m2. TGA in air clearly showed the delay in the degradation caused by the small amount of nano?ller (Fig. 4).
Fire Mater. 25, 193–197 (2001)

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197

The great improvements in ?ame retardancy by the nano?ller also opens the possibility of decreasing the level of ATH within the EVA polymer matrix. To maintain 200 kW/m2 as a suf?cient peak heat release level, the content of ATH can be decreased from 65% weight to 45% weight by the presence of only 5% weight nano?ller within the EVA polymer matrix. The reduction in the total amount of these ?llers results in improved mechanical and rheological properties of typical EVA-based cable compounds.

CONCLUSION
Figure 4. TGA in air of a compound with 35% weight EVA and 65% weight ATH in relation to a nanocomposite compound with 35% weight EVA, 60% weight ATH and 5% weight of nanofiller. EVA: Escorene UL 00328 with 28% weight vinyl acetate. ATH: Martinal OL 104 LE.

The char of the EVA/ATH/nano?ller compound created by the cone calorimeter was very rigid and showed only a very few small cracks; but the char of the EVA/ATH compound was much less rigid (less mechanical strength) and with many big cracks. This also explains why the peak heat release rate in the case of the nanocomposite was reduced to 100 kW/m2 compared with 200 kW/m2 for the EVA/ATH compound. To obtain the same decrease for the peak heat release rate by the ?ame retardant ?ller ATH only, the content of ATH would have to be increased to 78% weight within the EVA/ATH compound.

The thermal properties of EVA are improved by very low loading levels of a suitable nano?ller within the polymer matrix. For these EVA nanocomposites TGA in air shows a delay of the degradation; the peak of heat release measured by a cone calorimeter is dramatically reduced. Char formation in case of the nanocomposites has been improved and is responsible for the better ?ame retardancy. The results are also valid for EVA nanocomposites in combinations with metal hydroxides like aluminium trihydrate and opens the possibility for new ?ame retardant compounds with reduced total ?ller contents.

Acknowledgement
The author thanks Professor Dubois of the University Mons-Belgium for the TGA measurements.

REFERENCES
1. Hirschler MM. Polymeric Materials: Science and Engineering. Vol. 83. ACS Meeting August 2000, Washington, DC. 2. Brabrauskas V. Fire Mater. 1995; 19: 205. 3. Stevens GC. Conference Flame Retardants 2000. London, UK. 4. Alexandre M, Dubois Ph. Mater. Sci. Engineer. 2000; 28: 1. 5. Beyer G. Polymer News, November 2001. 6. Le Bras M, Camino G, Bourbigot S, Delobe R (eds). Fire Retardancy of Polymers: The Use of Intumescence. Royal Society of Chemistry: Cambridge, 1998; 196. 7. Gilman JW, Kashiwagi T, Giannelis EP, Lichtenhan JD. SAMPE J. 1997; 4. 8. Lee J, Takekoshi T, Giannelis EP. Mater. Res. Soc. Symp. 1997; 457: 513. 9. Beyer G, Alexandre M, Henrist C et al. World Polymer Congress, IUPAC Macro 38th Macromolecular IUPAC Symposium, Warsaw, 2000. 10. Beyer G, Alexandre M, Henrist C et al. Macromol. Rapid Commun. 2001; 22: 643. . lhaupt R. Polymer 11. Zanetti M, Camino G, Thomann R, Mu 2001; 42: 4501. 12. Babrauskas V, Peacock RD. Fire Safety J. 1992; 18: 255. 13. Herbert MJ, Brown SC. Conference Flame Retardants Elsevier Applied Science: London, 1992; 100–119. 14. UL 94. Test for Flammability of Plastic Materials for Parts in Devices and Appliances, 1966-10-00, Underwriters Laboratories Inc. 15. IEC 60332-3-24. Tests on Electrical Cables under Fire Conditions } Part 3–24: Test for Vertical Flame Spread of Vertically-Mounted Bunched Wires or Cables; Category C, 2000-10-00. International Electrotechnical Commission.

Copyright # 2002 John Wiley & Sons, Ltd.

Fire Mater. 25, 193–197 (2001)


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