Fire-retardant applications of polymer–carbon nanotubes composites: improved barrier effect and synergism
Fire safety regulations are becoming more and more stringent; consequently, researchers have to develop more efficient flame-retardant polymers. In order to do so, they are investigating flame-retardant nanocomposites and, more precisely, carbon nanotubes-based nanocomposites. This chapter briefly presents the fire issues and the general state of regulation. Then the fire mechanism and the ways to counter it are discussed, followed by a section specifically on carbon nanotubes’ applications. Carbon nanotubes can enhance flame-retardant properties as a unique filler of a polymer or as a synergistic agent to protect polymers in bulk, or as a filler in flame-retardant coatings which can protect many types of material.
If we just consider fire statistics: one domestic fire begins every two minutes and, unfortunately, half of the people involved are left badly injured afterwards; two out of three people who die as a result of fires, die because of smoke toxicity and not because of the flames themselves; thirty domestic fires per day, in France, are due to carelessness; one fire out of four is due to a defective electrical system; 75% of domestic fires occur at night (Comodo, France, 2009). So fire protection is a necessity and a huge challenge.
The growth of a fire can be described as follows: a fire starts, it develops quite quickly and after a few minutes, it becomes dramatically important: all vapour and heat start a general ‘flash-over’. Once it reaches that stage, there is no longer any way to stop it (see Fig. 23.1).
To counter that problem, the industry has been really active in developing solutions to prevent fires starting. ‘Fire-retardant’ solutions are materials that will delay the flash-over due to their low contribution to fire. ‘Fire-resistant’ solutions are materials that will act after flash-over and that will limit the physical propagation of fire from one area to another, and keep, for example, the integrity of walls or roofs in order to protect both people trapped in fires and the firemen from building collapse.
• Flame spread defines the size of the flames, and/or the time it takes for the flame to cover a determined distance on a sample (i.e. how long it will take the fire to grow and to reach other items).
• Smoke opacity and toxicity are important for the evacuation of people trapped in a fire; if the smoke opacity is too high, people and firemen will not be able to see anything or to find their way out. If smoke toxicity reaches too high a level, people will not be able to breathe and this can cause death or injury from smoke inhalation.
Globally, the level of flame retardancy and resistancy required is the combination of different fireproof characteristics of materials. These criteria will vary depending on the final application, and are defined by standards which are different in buildings or in transportation, for example. In transportation, the standards are not the same for all modes such as cars, trains or ships. Furthermore, the standard for the same product, in the same application, varies from one country to another. So when you develop a flame-retardant product, it is important to have an idea of the market targeted, and to have a global overview of the fireproof requirements the product will have to comply with.
As an example, in transportation, the time to escape from a burning car is really short, but in an aeroplane, this time can be longer, so it is a requirement that the smoke toxicity due to burning material used for an aeroplane must remain very low. This is not the case for the material used in a car, as people can jump out of the vehicle very quickly (Fig. 23.2).
The fire behaviour of products constitutes a major impact in many areas: building and construction, transport, electric and electronic engineering, furniture, etc. The issue of fire protection is the source of the largest number of standards, regulations or legislations at the national level as well as the international level.
Governments worldwide try to maintain and, where possible, improve, the fire safety of consumer products and public areas to help reduce fires. The role of fireretardant materials in this is well recognized, but the requirements are highly dependent on the targeted application and market, as explained previously. Indeed, to ensure the fire safety of buildings, all building products, materials and furnishings need to be tested for flammability and fire resistance before they can be used in construction. Not only the construction materials but also decorative materials such as carpets, curtains and upholstery need to comply with fire standards for use in public buildings and homes.
Building materials like concrete, steel, bricks, timber and stone will all burn once they reach a sufficient temperature to start a fire. Thanks to treatment with fire-retardant chemicals or coatings, their fire resistance and fire retardant qualities can be improved. Whether for building construction, transportation equipment, industrial processes or offshore applications, understanding of fire and certifying fire standards are absolutely essential (Magma Firestop, 2009)
• Cars are mainly covered by FMVSS302 and ISO3795 (see Fig. 23.3), the regulation is the same kind in the EU and the USA.
23.3 Descripton of test FMVSS302. (DSM research, summary of text method, available from http://www.dadc.nl/AD/summary/4-Flammability/37-summary-FMVSS.pdf, accessed 15 December 2009.)
When it comes to the development of plastic products, some standard (or not standard) tests are commonly used and well recognized by flame-retardant compound developers and researchers. These tests enable a quick and efficient evaluation of flame-retardant properties of thermoplastics. These tests are classic ones and are described below.
Laoutid et al. (2009) explain very well the principle of the main fire tests which can be performed at the laboratory scale, as discussed below.
Cone calorimetry is one of the most effective medium-sized polymer fire behaviour tests. The principle of cone calorimetry is based on the measurement of the decreasing oxygen concentration in the combustion gases of a sample subjected to a given heat flux (in general from 10 to 100 kW/m2). Figure 23.4 illustrates the experimental set-up of a cone calorimeter. Standardized in the United States (ASTM E 1354), the cone calorimeter test is also the subject of an international standard (ISO 5660). The sample (100 × 100 × 4 mm3) is placed on a load cell in order to evaluate the evolution of mass loss during the experiment. A conical radiant electrical heater uniformly irradiates the sample from above. The combustion is triggered by an electric spark. The combustion gases produced pass through the heating cone and are captured by means of an exhaust duct system with a centrifugal fan and hood. The gas flow, oxygen, CO, CO2 concentrations and smoke density are measured in the exhaust duct.
The measurements of gas flow and oxygen concentration are used to calculate the quantity of heat release per unit of time and surface area: heat release rate (HRR) is expressed in kW/m2. The evolution of the HRR over time, in particular the value of its peak/maximum (pHRR or HRRmax), is usually taken into account in order to evaluate fire properties. The integration of the HRR vs. time curve gives the total heat release (TRR) expressed in kJ/m2. In addition, this test also enables characterization of the time of ignition (TOI), time of combustion or extinction (TOF), mass loss during combustion, quantities of CO and CO2, and total smoke released (TSR), as shown in Fig. 23.5.
This test is used to indicate the relative flammability of materials. It is now subjected to an international standard (ISO 4589). The value of the LOI is defined as the minimal oxygen concentration in the oxygen/nitrogen mixture that either maintains flame combustion of the material for 3 minutes or consumes a length of 5 cm of sample, with the sample placed in a vertical position (the top of the test sample is set on fire with a burner, see Fig. 23.6). The LOI is expressed in percentage (of oxygen).
As air contains 21% oxygen, materials with an LOI below 21 are classified as combustible, whereas those with an LOI above 21 are classified as self-extinguishing, because their combustion cannot be sustained at ambient temperature without an external energy contribution. The higher the LOI, the better the flame-retardant property.
The set of UL94 tests includes a range of flammability tests (small and large flame vertical tests, horizontal test for bulk and foam materials, radiant panel flame-spread test). The most commonly used test is UL94V for measuring the ignitability and flame spread of a vertical bulk material exposed to a small flame. The flame is applied to the bottom of the specimen for 10 seconds and removed. The after-flame time t1 (time required for flame extinguishing) is noted. After extinction, the flame is applied for another 10 sec. The after-flame time t2 is noted, together with the after-glow time t3 (time required for the fire glow to disappear).
During the test, the presence of burning drops, causing a piece of cotton located under the sample to ignite, must be noted. The specimen will be classified V0, V1 or V2 according to criteria listed in Table 23.1. The experimental set-up for the UL94 test is shown in Fig. 23.7.
Source: Laoutid et al. (2009).
23.7 UL94 vertical schema. Source: Laoutid et al. (2009).
In Europe, some fire disasters – like those in the Madrid discotheque, the Channel Tunnel, the Turin cinema, and King’s Cross tube station – have highlighted again the importance of safety in public places in the event of fire. The European Commission, supported by a Group of National Fire Regulators, proposed a completely new classification system, based partially on existing test methods, but partially, and critically for many construction products, on a completely new test, the so-called ‘single burning item’ (SBI) test. This test evaluates the potential contribution of a building product to the development of a fire, under a fire situation simulating a single burning item in a room corner near to that product. Heat and smoke release rate are measured during combustion. As a consequence, all national classes have been replaced by new European standard tests (ENs) ‘Euroclasses’, in the building area. However, the implementation in each Member State varies.
The Euroclasses became valid on the 1st January 2001 and after this date, every national classification for construction products was recognized on the national market for a period of time. At the end of this period, only European classification will be valid, at both the national and European level.
According to this European regulation, products will be rated from A to F as a function of the performance level observed. Euroclass A will cover products that do not contribute, or contribute only very slightly, to the development of a fire. Euroclass E will cover products that present an acceptable reaction to fire, i.e. they can resist ignition by a small flame for a short period. Euroclass F is for products that have shown no performance criteria. The Euroclass classification system will rate: flame spread (from class A to E), dripping (class D0 to D2) and smoke (class S1 to S3) (Magma, 2009) (Table 23.2).
|Main class||A1 A2 B||Non combustible (over 20 min to flash-over)|
|C||Moderately combustible (10 to 20 min to flash-over)|
|D||Moderately combustible (2 to 20 min to flash-over)|
|Sub class||S1||Low smoke production|
|S2||Medium smoke production|
|S3||High smoke production|
|Sub class||D0||No flaming droplets|
|Flaming droplet||D1||Flaming droplets which persist less than 10 s|
Source: Koxka (2009) Euroclass and reaction to fire, available from http://www.koxka.com/English/Documents/Euroclasses%20and%20reaction%20to%20fire.pdf (accessed on 24 November 2009).
To create a fire, three elements are needed: heat, combustive element and fuel. If these elements are joined together, we can observe combustion, as shown in Fig. 23.8. The source of heat increases the temperature of the product or the polymer (depending on the intensity of heat source, and of the intrinsic characteristics of the product or the polymer).This increase in temperature involves decomposition (mainly pyrolysis) and formation of organic materials with low molecular weights (often volatile and flammable, mainly free radicals) (Fig. 23.9).
When the concentration of flammable volatile species reaches a critical level in combination with oxygen, the mixture of gaseous products (i.e. fuel) catches fire. Then the flame created becomes a heat source and maintains the decomposition of the polymer, called the condensed phase.
In order to suppress or reduce fire of polymers, we need to suppress either heat, fuel or combustion, to break the fire cycle presented in Fig. 23.9. There are different steps in this cycle, either occurring in the gas or condensed phase, which are important to understand, and when it is possible to act to achieve better flame resistant (FR) properties in polymers:
• As stated above, thermally induced polymer decomposition releases very reactive free radical species such as H· or OH·, which maintain the combustion in the gas phase. Adding halogenated flame retardants to the polymer formulation can slow down radical formation. Indeed, when the material is burning, this flame retardant is decomposed and releases halogenated radical species (Cl· or Br·) which neutralize (via a chemical reaction) the radical species coming from the polymer itself. This stops the chain decomposition and therefore the combustion.
• In the gas phase it is also possible to dilute the fuel, due to the incorporation of an additive (melamine, for example) which will release inert substances during combustion. This will decrease the concentration of the reactive mixture below the limit of ignition.
• It is also possible to break the fire cycle by acting on the condensed phase by creating an endothermic reaction which will cool the polymer. In order to reach this goal, an additive, such as magnesium or aluminium hydroxides are used. These products will release molecules of water around 200–300 °C.
• In the condensed phase, another solution is to use additives which will react under high temperature to create a protective layer to insulate the underlying substrate from the heat source, as in an intumescing system (see Section 23.2.2). This ‘char’ layer limits the transfer of volatile degradation products. Furthermore, the fuel gases can be physically separated from the oxygen, which prevents the combustion process being sustained.
• A final way to limit the combustion and degradation of thermoplastics or any solid material, is to apply a flame-retardant coating to the substrate. This paint will react when submitted to high temperature and create a protective layer.
Of all the fire protection solutions, intumescence is very popular today, due to the increasingly severe legal requirements for fire protection. Intumescing systems can be used to protect polymers’ parts in bulk or to protect different types of solid material (like metallic beams in construction area) with a flame-retardant coating which becomes a protective layer during fire.
Intumescence is a specific way of fighting fire. The principle is the formation of a char layer that can protect the substrate and limit the degradation, isolating the surface from heat and preventing the release of degradation products. To realize such an effect, three components are necessary:
The intumescent mechanism is usually described as follows: first, the acid source breaks down to yield a mineral acid, then it takes part in the dehydration of the carbon source to yield the carbon char, and finally, the blowing agent decomposes to yield a gaseous product. The latter causes the char to swell and hence provides an insulating multicellular protective layer. This shield limits at the same time the heat transfer from the heat source to the substrate and the mass transfer from the substrate to the heat source, resulting in conservation of the underlying material (Jimenez et al., 2006) (Fig. 23.10).
In the past few years a new type of approach has been explored as an alternative to halogenated flame retardants which are more and more sensitive to additives due to health and environment considerations: this new approach is to enhance the flame retardent properties of polymers via ‘nano’ fillers, particularly via carbon nanotubes (CNTs).
Bourbigot and Duquesne (2006) explained very well the interest in flame-retardant nanocomposites. As a first step, it was pointed out that the first study which was made on the fire properties of nanocomposites showed that this type of material exhibits low flammability in terms of heat release rate (HRR). HRR is measured by a mass loss/cone calorimetry test (see Section 23.1.3). HRR is a major parameter in controlling flame propagation in fire.
Typically, with nanofillers, the peak of HRR is decreased by 50–70% in a cone calorimeter experiment. Indeed, this has been demonstrated by Kashiwagi et al. (2005) in a blend of poly(methyl methacrylate) (PMMA) and single-walled carbon nanotubes (SWNTs). The blend was prepared via dissolution of PMMA in dimethylformamide (DMF). Then nanotubes were dispersed by bath sonication for 24h. For only 0.2% by mass of SWNT in the final product, there was a decrease of the HRR by 25%.
Peeterbroeck et al. (2007) showed the same trend for a composite of ethyl vinyl acetate (EVA) and multi-walled carbon nanotubes (MWNTs), as shown in Fig. 23.11. For this latter case, when 3 wt% of CNT were added to EVA, the peak of HRR was decreased by 57%, and the combustion time was much longer (900s instead of 300s for virgin EVA). Dispersion of MWNT was realized by melt blending in an internal mixer. These are only two examples, many others are available in the literature. So, generally speaking, CNTS have a positive influence on the HRR of polymers, even if it is dispersed by different ways.
Another interesting phenomenon has also been observed in the case of EVA: at the end of the fire test, the formation of a cohesive char was examined for composite material. This char covered homogeneously the entire sample with no holes or cracks, which was not the case for virgin polymers.
As Bourbigot and Duquesne (2006) explained in their work, the first point to be considered in FR nanocomposites is the nanodispersion. Indeed, Kashiwagi et al. (2005) showed, based on a complete study of PMMA–SWNT blends, that in the case where the nanotubes are relatively well dispersed (almost no agglomerates or bundles), a network layer is formed without any major cracks or openings during the burning test and this covers the entire surface of the composite. However, nanocomposites with a poor dispersion or a low concentration of nanotubes (0.2% by mass or less) form numerous black discrete islands with vigorous bubbling between the islands. This study highlighted also that the peak of heat release rate of the nanocomposite that forms the network layer is about half those which form the discrete islands. This was confirmed by Bourbigot and Duquesne with MWNTs. For their experiment, they prepared a well-dispersed CNT–polystyrene (PS) nanocomposite via melt extrusion using trialkyklimidazolium tetrafluoroborate (IM) as a compatibilizer, and the same blend for comparison, without IM. TEM (transmission electron microscopy) images combined with image analysis revealed that agglomerates (≈ 1 μm) of nanotubes dominate in the case of the PS–CNTs without IM, while well-dispersed SWNTs can be observed in the case of PS–CNT-IM blend. They evaluated the FR properties via adaptation of a cone calorimeter test. Here, they analysed the mass loss rate, which is as relevant as HRR. A reduction of mass loss rate for the PS nanocomposites is observed, compared to the virgin PS, but the mass loss rate of PS–CNT-IM is significantly lower than that of PS–CNT.
So we can conclude that the formation of a good layer to protect the underneath material is crucial to optimize the FR properties, especially a low HRR. This cohesive layer is highly dependent on the level of dispersion of CNT in the matrix. The number of agglomerate and bundles remaining after dispersion of CNT in the polymer matrix must be as low as possible, whichever process is used.
According to Bourbigot et al. (2006), the protection mechanism involves the formation of a char layer covering the entire sample surface acting as an insulating barrier and reducing volatile vapours escaping to the flame. The formation of such a layer which does not form cracks when burning is critical to obtain a low HRR from composites.
In the case of CNTs, Kashiwagi et al. (2005) explained the formation of a protective layer by the following mechanism. In the early stage of the burning/gasification test, the upper part of the sample is heated and starts melting. When the temperature of the sample becomes high enough, degradation starts to generate bubbles in the melt layer. The associated convection flow from the interior of the sample toward the sample surface pushes the CNTs up to the surface to form a protective layer. But if the dispersion of nanoparticles is not good enough, the bursting of the bubbles at the surface will induce the localized accumulation of CNTs. The final residue will be formed by numerous discrete islands instead of a homogeneous layer covering the entire sample. The quality of dispersion, the concentration of CNTs as well as the temperature and the viscosity of the melt, will influence the quality of the network layer.
Cone calorimeter tests have demonstrated that the addition of CNTs improves the flame-retardant properties of a polymer. Indeed, a significant decrease in the HRR is noted, thanks to the formation of a protective layer acting as a thermal shield. However, this layer has a secondary effect. This effect is highlighted by the influence of CNTs on the mass loss of the polymer during combustion. For example, in an EVA–CNT blend (Peeterbroeck, 2007), the presence of carbon nanotubes modified the thermal degradation of the polymer matrix (see Fig. 23.12). With CNTs, the first degradation step takes place at a higher temperature. This is probably due to a limited volatilization rate and to the production of a more thermally stable char arising from numerous cross-linking reactions.
It is assumed that the free radicals coming from the degradation of the polymer matrix are trapped in the condensed phase (involving cross-linking reactions) instead of being released in the gaseous phase. This could be due to the higher viscosity of the composites in the melt state and/or by the labyrinth effect of the interconnected fillers.
The second stage of thermo-oxidative degradation also takes place at a higher temperature in the presence of the char formed during the first degradation step which is further stabilized trough π–π interactions. Indeed, when a polymer burns, it produces gaseous products, liquid products and a solid residue, as explained in Fig. 23.13.
In the presence of CNTs, the degradation route of the polymer matrix is oriented toward the solid residue formation, which is in fact a protective layer. Then less gaseous or liquid products, which feed the flame, are produced, and the fire resistance is increased. Carbon nanotubes change the burning behaviour of the polymer to residue formation.
It can be assumed that the layer hinders diffusion of the volatile decomposition products as well as oxygen penetration, due to a tortuous pathway. This phenomenon is only obtained with nanofillers of a high aspect ratio, as is the case for CNTs.
Furthermore, the slower diffusion of free radical products would create a higher probability of collision and recombination (cross-linking), and build-up of a thermally insulative char layer on the surface of the burning material.
It has been fully demonstrated in the literature that nanocomposites containing CNTs show improved fire behaviour: a lower heat release in cone calorimetry due to the formation of a char layer which acts as a heat shield and limits volatilization. Nevertheless, if the decrease of the HRR is a good starting point to develop a FR compound, this is not enough to pass industrial standard fire tests. The simplest standard test and the best known in FR application is the UL94 test. This test measures the ignitability and self-extinguishing time of the sample (Section 23.1.3). Most of the samples of thermoplastics–CNT nanocomposites showing good performances in cone calorimeter test fail the UL94 test.
It has been explained (Bourbigot and Duquesne, 2006) that, in UL94 testing, the nanocomposite has a relatively limited contribution to the fire associated with enhanced dripping behaviour, but it is not self-extinguishing, the ignition time is shorter and it does not reduce the flammability of the materials. During a UL94 test, the formation of a char layer can be observed on the surface of the material. Nevertheless, the carbonaceous layer is not effective enough to stop the flame and the material continues to burn slowly, failing the test.
This is the reason why CNTs have been tested in combination with other additives which in turn has highlighted the synergism effect. Indeed, in the field of flame retardancy, synergistic additives are often required in order either to sharply improve the performance of the materials or to decrease the loading of FR.
Gao et al. (2005) have shown a synergetic effect between nanoclays and nanotubes. In this research, clays were added to an EVA–CNT blend and fire properties were evaluated through cone calorimetry, and the carbonaceous layer was studied by X-ray diffraction analysis and scanning electron microscopy. This composite contained 2.5 phr (parts per hundred) of CNTs and 2.5 phr of clays. This material was compared to an EVA–CNT mixture (5 phr of CNTs). Composites were produced by melt extrusion of a pre-blended EVA–CNT using an internal mixer at 130 °C.
The study made by Gao et al. (2005) revealed that the protective layer (Section 23.3), contained graphitic and turbostractic carbons and the oxidation resistance of the char is a function of the degree of graphitization. It was assumed that CNTs act as the nucleation of this graphitization. This effect is enhanced when both CNTs and clays were mixed in the matrix. Here it has been demonstrated that the synergism between clays and CNT enhance char resistance.
This study shows that CNTs can have a synergistic effect with clay to enhance FR properties. Some research has also demonstrated the synergism between CNTs and standard flame-retardant polymers. For example, Ye et al. (2009) studied the synergistic effect of CNTs combined with magnesium hydroxide (MH) in EVA. They demonstrated that CNTs can considerably decrease the heat release rates and mass loss rate by about 50–60%, can prolong the combustion time to nearly double and increase the LOI values by 5%, when 2 wt% of CNTs was substituted for the MH in the EVA–MH–CNT composites. TGA data also showed that the synergistic effect of CNTs with MH increases the thermal degradation temperature and final charred residues of the EVA–MH–CNT samples. Ye et al. described the synergetic mechanism as follows:
What is also very interesting in this study is by keeping the same global loading of fillers (just replace 2 wt% of MH by CNT), the FR properties are significantly enhanced. According to this, it is legitimate to assume than CNTs would enable developers to decrease the total filler loading in the material, keeping the same level for fire behaviour. This is a crucial point as the main drawback of MH is the loading. Indeed, this product must be introduced at 50–60% to make a polymer fire-retardant. This kind of concentration significantly damages the mechanical properties of the matrices. Synergism between MH and CNT could have a positive impact on that issue.
The synergistic effect of CNTs has also been demonstrated in intumescent systems. Indeed, usually, in this type of flame-retardant solutions, synergistic agents are often used to pass specific tests required by legislation. As legislation is becoming stricter, intumescent systems have to be more efficient. The mechanical properties of the intumescent shields developed in the case of a fire are particularly important since internal pressure (due to degradation products) or the external environment can easily destroy the shield, leading to a loss of insulating properties. Previously we have shown that CNTs lead to the formation of a protective layer or at least enhance its properties (Section 23.3), carbon nanotubes are very good candidates for a synergistic approach in intumescent systems.
Bourbigot et al. (2009) recently studied nanocomposites and their ability to be fireproof, with or without conventional flame retardants. This research discussed thermoplastic polyurethane (TPU). Samples of TPU–CNT (2 wt%) were produced using an internal mixer at 180 °C under nitrogene flow to avoid oxidation of polymer. Dispersion is qualified through transmission electron microscopy. It appeared that CNTs were evenly dispersed in matrix, and many single CNTs could be observed. Then, the fire properties are evaluated.
First, a cone calorimeter test confirmed that the presence of CNTs in TPU decreased the HRR of the material by 50%. Nevertheless, LOI (limit of oxygen index) and UL94 class were not enhanced (Section 23.1.3).
The same cone calorimeter test was performed on a blend containing 30 wt% of APP in TPU (TPU–APP) and in a second test less than 1 wt% of APP was replaced by CNTs (TPU–APP–CNT). (Composites are produced by the process mentioned above: in an internal mixer at 180 °C.)
The development of an intumescent layer (TPU–APP and TPU–APP–CNT) permits a decrease by 75% of the peak of HRR (pkHRR). Note that the substitution of APP by CNTs is not beneficial in terms of pkHRR, but from the 200s, HRR of TPU–APP–CNT is close to zero compared to TPU–APP, and the total HRR is lower (29 MJ/m2 vs. 36 MJ/m2). This can be assigned to the higher quality of the char developed with CNTs, but the LOI of TPU–APP (30 vol.%) was not improved by the substitution of APP by CNTs. This led to a V0 rating at 3.2 mm in the UL94 test. TPU–APP exhibits a V2 rating without CNTs.
It is also important to note that incorporation of 30 wt% of APP provides the development of an intumescent layer when undergoing a heat flux. This intumescent char exhibits small holes at the surface but the substitution of a portion of APP by CNTs makes it more cohesive, i.e. without holes. Thus, there is a synergistic effect of CNTs with APP, leading to a compound with enhanced FR properties for a reduced total filler loading.
The synergistic approach of CNTs in FR compounds has been used in polypropylene (PP). Two compounds were prepared, both by twin-screw extrusion following specific process parameters (screw profile, temperature profile), designed to achieve a high level of dispersion of carbon nanotubes.
The first compound contained only APP (PP–APP) and the second contained the same amount of APP plus less than 2 wt% of CNTs. The total filler loading rate for both samples remains below 20%. Once the materials were extruded, two types of samples were produced:
• 12.5 × 130 × 3.2 mm3 bars produced via injection moulding (material temperature set at 270 °C, and mould temperature set at 40 °C) to study fire resistance via the UL94 test at 3.2 mm of thickness and perform SEM (scanning electron microscopy) observations on the surface of these samples (see Fig. 23.14).
• 12.5 × 130 × 2 mm3 bars produced via compression moulding (190 °C, 150 bar) to study FR behaviour via the UL94 test at 2 mm of thickness and perform SEM observations on the surface of these samples (see Fig. 23.15).
The first analysis was the SEM observation prior burning test on injected samples. In the PP–APP blend (Fig. 23.14 (a)), we can see small particles of APP spread quite homogeneously in the matrix. In the PP–APP–CNT blend (Fig. 23.14 (b)), particles of APP seem to be bigger and their shape is highly oriented toward injection flow. This orientation is less visible in the PP–APP blend. This difference in particles’ size (i.e. dispersion) and orientation may be attributed to the increase of viscosity brought about by addition of CNTs.
The first test which was done to characterize the flame-retardant properties of these two samples was the vertical UL94 test on samples with a thickness of 3 mm. Both materials (with and without CNTs) were classified V0. As no difference in fire behaviour could be seen for such a thickness between the two samples, we chose to decrease the thickness (that is the reason why 2 mm thick samples were produced). Indeed, it is well known that the thickness is a great influence on the result of the UL94 test. The thicker the sample, the more difficult it is to ignite it during the test. Furthermore, in many applications, legislation requires UL94 V0 material for a thin sample (for example, in cables and wires, the required thickness can be 0.8 mm). Some 2 mm of thickness was thin enough to highlight a difference in behaviour between the two materials. Indeed, in this case, the PP–APP blend just reached V2 classification. Times of burning are short, but flaming droplets occurred and ignited cotton, while the PP–APP–CNT blend reached V0 (short burning times, small flames and no flaming drops). These results are already enough to show a synergistic effect of CNT when added with APP. Indeed, for the same amount of APP, the addition of a very small quantity of CNTs enhances the FR properties compared to the sample with APP only.
At the end of the test we collected the burnt samples (samples of 2 mm of thickness which showed different FR properties) and performed new SEM observations (see Fig. 23.15). We can see a structure with lots of holes in the char layer formed on PP–APP blend (Fig. 23.15 (a)), which is typical of an intumescing system. We can also see some agglomerates of needles. We can assume that these needles could be the result of some kind of ‘crystallization’ of phosphoric acid coming from the degradation of APP at high temperature.
When CNTs are added to the formulation (PP–APP–CNT blend, Fig. 23.15 (b)), the structure of the char layer is modified. The structures seems to be different (the holes are less numerous and smaller) and we could not see any needles. So it seems that CNTs really act on the structure of the char layer. Today, further investigation is needed for a better understanding of the mechanisms involved.
CNTs can be used to improve the flame-retardant properties of polymers in combination with ammonium polyphosphate in intumescent systems (to protect plastics in bulk). As they enhance the FR properties of the polymer, they can be considered synergist as FR properties are enhanced compared to a sample with the same loading of APP without this nanofiller. Another flame-retardant application using CNTs can be the development of flame-resistant coatings, as long as they show specific interaction with the binder.
As we have already demonstrated, the dispersion of nanofillers in the polymer matrix is crucial (Section 23.3.2). The better the dispersion, the better the FR properties. In many cases, poor dispersion results in agglomeration or phase separation, leading to a dramatic loss of the materials’ properties. To overcome this problem, a number of strategies have been developed with various degrees of success. They usually come at a high price, due to the necessity of modifying the surface of the filler.
In joint research, Nanocyl and Professor Dubois (University of Mons, Belgium) developed a combination of an optimized carbon nanotube which has been specifically developed for its strong interaction toward polysiloxane chains and a proprietary mixing method. The CNT used for this application was a unique multi-walled CNT which has been upgraded to strengthen the interface between the CNT surface and polydimethysiloxane (PDMS). This unique combination turns out to be the most efficient approach to impart new key properties to the silicone matrix. Viscometric, rheological and theoretical studies were performed that demonstrate the remarkable potential of dispersing a low amount of carbon nanotubes in silicone, paving the way to unexpected applications, e.g. in the field of fire endurance. These properties all rely on the nature of the nanotube–silicone interfacial interactions, which are dominated by additive CH-π interactions between the methyl groups of the polymer and the nanotube surface.
PDMS is the most common silicone elastomer owing to its ease of fabrication and advantageous chemical/physical properties, such as low surface energy, low glass transition temperature and high chain mobility. Currently, to compensate for their poor mechanical properties, silicone materials have to be reinforced by incorporation of particulate materials, silica being the most commonly used filler. However, this reinforcement still requires a relatively high mass fraction of minerals (> 10 wt%). Over the past few years, much attention has been paid to polymer nanocomposites, which represent a rational alternative to conventional filled polymers. The key point for the improvement of properties as diverse as mechanical, thermal, and barrier performance is the effective/individual dispersion of the nanofillers in the matrix. To reach this objective, the type of nanofiller, filler size and the nature of the interface formed within the matrix all have to be optimized. In this context, carbon nanotubes are of prime interest; however, they have a strong tendency to agglomerate in densely packed bundles, and their dispersion in polymers still remains a major challenge.
Classically, unfilled PDMS is a very fluid liquid. Upon adding CNTs to the unfilled matrix via mechanical blending, a huge increase in viscosity is observed, which results in a PDMS matrix that has totally lost its capacity to flow. Quite interestingly, CNTs also act as rheo-thinning agents, allowing brush-paint (poorly filled) PDMS with a very low filler content, prior to the cross-linking reaction, for any orientation of the substrate.
For comparison, PDMS filled with a needle-like magnesio-silicate nanofiller (natural sepiolite clay), or a commercially available organo-modified layered alumino-silicate clay (Montmorillonite, from Southern Clay Products, Texas: Cloisite 30B®) were also prepared and analyzed. From the measurements, a viscosity value of 4980 cP can be extracted for the neat matrix. (This measurement was realized in a cone-plan viscosimeter at 25 °C.) Significant differences are found between the three types of nanofillers. A very slight increase in viscosity was observed as the Cloisite 30B® content was increased. However, the values remain very low and do not exceed 5900 cP for a solid fraction as high as 5 wt%. In the case of natural sepiolite-filled PDMS, the increase in viscosity is more pronounced than for Cloisite 30B®, with values reaching 7940 cP for 5 wt% loading. The most spectacular effect is observed with CNTs: the viscosity values strongly increase between 0.1 wt% (+ 25%), and 0.3 wt% (+ 280%), for which a value of 14000 cP was reached.
The rheological behavior of PDMS filled with CNTs has been studied in more detail by following viscosity as a function of shear rate for two nanofiller loadings (0.5 and 0.7 wt%). For the unfilled system, the viscosity is independent of shear rate over a broad range (5–200 s− 1). On the contrary, the filled systems exhibit two different regimes. As long as the shear rate is low (< 40–50 s− 1), the viscosity decreases with increasing shear rate; then, for higher shear rate values, the viscosity reaches a constant, minimum value. Clearly, the nanofilled silicone behaves like a thinning fluid.
The strength or density of filler–polymer interactions can be correlated to the amount of PDMS chains adsorbed per gram of CNT after a solvent extraction. This quantity is determined via ‘bound rubber’ tests. In this test the percentage of silicone resin attached to the CNT is measured.
Only PDMS composites filled with natural sepiolite or CNT show a significant amount of adsorbed polymer at the nanofiller surface after extraction. As is the case in the viscosity study, the more pronounced effects are observed with the carbon nanotubes. For example, 2.5 g of polymer per gram of nanofiller are adsorbed when natural sepiolite is the filler while the value increases to more than 20 g per gram of CNT as filler.
Since the nature of PDMS–CNT interactions are fundamental for the efficiency of the filler in reinforcing the polymer matrix, modelling studies (based on molecular dynamics (MD) simulations) were performed to assess at the microscopic level the adsorption properties of PDMS on the surface of carbon nanotubes. The simulations were carried out on a model system consisting of an armchair single-walled carbon nanotube that interacts with a single PDMS chain (ranging in size from 12 to 200 repeating units). As initial structures in the MD simulations, the polymer chains were built according to an elongated helical-like conformation (Khorasami et al., 2005) and brought in van der Waals contact to the nanotube. Structure obtained after a 400 ps simulation at 300 K was in the NVT canonical ensemble. It can be clearly seen that the polymer chain completely wraps around the CNT surface. A detailed analysis of the atomic spatial distribution shows that the PDMS chains organize to form a regular radial pattern from the CNT surface: (1) a first layer of methyl groups pointing towards the CNT; (2) the oxygen and silicon atoms belonging to the polymer backbone; and (3) an outer layer formed by the remaining methyl groups. The calculations show that about one methyl group out of two binds to the CNT surface (i.e. belongs to the first layer around the CNT). Adsorption energy per repeat unit converging to ~ 2.6 ± 0.1 kcal/mol in the longest chains can be extracted by averaging over the MD trajectories after equilibration. Such physical adsorption of the PDMS chains onto the CNT is mostly triggered by CH-π interactions (Nishio et al., 1998) between the PDMS methyl groups and the π-electron-rich surface of the carbon nanotube. CH-π interactions are weak hydrogen bonds; they are largely due to dispersion forces and partly to charge-transfer and electrostatic forces. The calculated adsorption energy per repeating unit corresponds to roughly three times the typical value for a CH-π interaction, which is consistent with the picture above of one methyl group per repeating unit binding to the surface. It is worth stressing that the energy destabilization induced by the change in conformation upon going from the isolated PDMS chain to the chain wrapped around the CNT is small (less than 0.6 kcal/mol per repeating unit), which, together with the presence of a high density of methyl groups amenable to CH-π bonding, explains the unique adsorption properties of PDMS on the CNT surface (Beigbeder et al., 2008).
These novel reinforced PDMS exhibit a number of remarkable properties. For flame-retardant applications, they can be used as a protective coating. In order to have an idea of their performance, fire testing was performed. This test is an adaptation of ISO 2685 which evaluates the fire endurance of aluminium plates coated with a PDMS layer containing CNTs. The principle of the test is to measure the temperature reached by the unprotected side of the plate versus time, meanwhile the coated side is exposed to a flame of 1050 °C (60 kW m− 2). The goal is to determine the heat shield properties of the coating. The better the shield properties are, the cooler the aluminium plate is. Thanks to this barrier, the plate will remain at a temperature lower than the one which would involve a loss of mechanical properties (load bearing).
So during this test, the temperature of the unprotected plate increases to 380 °C within a few minutes, which causes the fast destruction of the plate. The fire endurance times (of the order of 80 min.) of the plates coated with the PDMS–CNT mixture are much longer compared to those coated with unfilled PDMS (around 15 min) and the maximum temperature reached in the presence of the nanofillers (290 °C) is much lower than the corresponding value for neat PDMS (405 °C). These results unambiguously demonstrate the significant improvement in fire resistance of substrates protected by CNT-filled silicone. This can be attributed to the thermal percolation phenomenon where the number of thermal conductive paths increased with nanofiller content and to a very high thermal conductivity of the carbon nanotubes. Interestingly, the control of the CNT–silicone interface permits preparation of this type of formulation for very varied use, such as a sprayable paint for protecting foams or paintable coatings for composite protection and with very different silicone types.
A similar test, UL1709, has been performed on stainless steel and in this case the temperature did not go over 400 °C (the final higher temperature is due to the fact that stainless steel has a higher thermal conductivity than aluminium). So these tests demonstrated the very good ability of this PDMS–CNT coating to protect metal substrates from fire.
Further investigations have been done to show that the PDMS–CNT coating can also be used as a flame-resistant barrier on wires and cables. Samples of electric cables were coated with the CNT–silicone dispersion (a 150 micron layer). These samples pass the UL94V0 test and the cable fire Test easily (Fig. 23.16 (a) and Fig. 23.16 (b)). The coated cable stops burning just in few seconds, as the polymer layer of the unprotected sample just burns entirely (the copper wires are fully visible at the end of the test).
Based on these impressive results, it was decided to test the performance of the PDMS–CNT coatings on foams. Indeed, it has always been very difficult to produce flame-retardant foams which pass UL94 at V0. But it appears in this case that flexible foams, both olefinic or polyurethane (PU), can be spray coated or impregnated with PDMS–CNT paint. When spray coated, only the outer foam shell will be protected but nevertheless the foam will not burn. When a flame is applied to the unprotected foam, it just burns very quickly (the sample is totally destroyed in a few seconds). The same protocol is applied to a coated sample, which does not catch fire at all.
The very good affinity between PDMS and CNT (thanks to CH-π interactions) enables development of a coating with impressive flame-retardant properties. This coating can be applied to very different types of substrates like metal, plastic (cables and wires) or foams.
Understanding the wide range of fire regulations is essential for work in the flame retardancy field. Indeed, we have seen that the structure of regulations is complex, and highly dependent on the final application (railway, cable industry, buildings) and on the country (regulations differ from France to Germany, for example). Fortunately, this is moving toward uniformization, at least in Europe, thanks to Euroclasses, applicable today in the construction field.
From a technical point of view, the understanding of fire behaviour is also essential, of course, to develop FR compounds. We have explained that the fire mechanism is an endless cycle: the heat source involves the degradation of the polymer, the degradation products may be highly reactive and constitute a fuel which burns and creates the flame, and the flame heats the polymer which continues to degrade. To counter the fire, the solution is to break this cycle. This can be done in two points, either in the condensed phase or the gaseous phase. It is important to note that the intumescent system (formation of a protective layer in the condensed phase during fire) is the most interesting, as it can handle very high temperatures. Concerning nanocomposites, carbon nanotubes can be used in the condensed phase. We have explained that adding CNTs to a polymer enhances its flame-retardant properties. More precisely, CNTs induce a significant decrease of the heat release rate (in the cone calorimeter test) due to the formation of a protective layer. This layer will limit the volatilization of degradation products and trap radical species which will induce cross-linking reactions to form a more important and cohesive solid residue at the end of the fire test. To have the best char formation, i.e. the best fire properties, it has been demonstrated that the quality of dispersion of the nanotubes in the polymer matrix is essential. With poor dispersion, the char residue will present cracks or holes in the best case, or even just discrete islands. This char layer will not be as protective.
Nevertheless, although carbon nanotubes do have a real positive influence on the fire properties of a polymer, they do not permit it to pass the fire test as a single FR additive, as in the UL94V test, for example. That is the reason why they can be used as synergistic agents in combination with ammonium polyphosphate in an intumescing system.
We have shown that adding a few percentage of this flame-retardant additive brings enhanced FR properties for a lower total filler loading, so a better retention of mechanical properties of the matrix. Such a system has been developed today in TPU and PP.
Beside this, CNT can also be used to develop flame-retardant coatings with silicone, due to their great affinity with methyl group of PDMS. This paint forms a protective layer when in contact with a flame. This layer acts as a heat shield and prevents the substrate from melting and degradation. Such paints can be applied to many types of substrates like aluminium or stainless steel parts, cables and wires, and even to foams.
So today, it is clear that CNT nanocomposites have an application in flame retardancy. Nevertheless, the chemical or physical interactions which would explain their synergistic behaviour are still not very well understood.
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