Abstract
In the present work, the influence of multiwalled carbon nanotubes (MWCNTs) on the flame retardancy and rheological, thermal and mechanical properties of polybutilen terephthalate (PBT) and polypropylene (PP) matrixes has been investigated. The carbon nanotube content in the thermoplastic materials was 2 and 5 wt‐%. The nanocomposites were obtained by diluting a masterbatch containing 20 wt‐% nanotubes using a twin‐screw extruder and the thermal properties were analysed by differential scanning calorimetry and thermogravimetric analysis; thermomechanical properties were determined by dynamic mechanical thermal analysis and the rheological behaviour was studied by a Thermo Haake Microcompounder. The results concerning the flame retardancy show that the MWCNTs are not equally effective as flame retardants in PP and PBT. The ignition time is increased only for PBT whereas the extinguishing time is decreased for PP and PBT. The reinforcement of the thermoplastics with multiwall carbon nanotubes is improved regarding the mechanical and thermal properties of the nanocomposites compared to pristine materials and the behaviour of thermoplastic nanocomposites regarding fire retardancy depends on the nature of the polymeric matrix.
Introduction
Carbon nanotubes (CNTs) are a fascinating reinforcement for polymers because of their unique mechanical properties and extremely large surface area per unit volume. They have been widely investigated due to unique properties of the CNT such as good electrical conductivity, thermal stability and mechanical properties using very low concentrations in polymers.1–3 Both thermoplastic and thermosetting polymers are applied as a matrix in the preparation of CNTs based nanocomposites. However, during the last years a rising importance of thermoplastics is observed (usually PMMA, PC, PP, PA, etc.) owing to an easy processability and good physical properties. In addition to their outstanding properties, the surface area per unit volume of the nanotubes is much larger than of embedded graphite fibres. For example, various studies involving single‐walled nanotubes and multiwalled nanotubes have demonstrated that CNTs can have moduli and strength levels in the range 200–1000 GPa and 200–900 MPa, respectively.4
Most composites with thermoplastic matrix are obtained in melt processing, where carbon nanotubes are mechanically dispersed in a polymer in molten state using high shear methods5 (e.g. twin screw extrusion). Melt compounding technique has been successfully used to process into nanocomposites many different kinds of engineering plastics such as polyamides, 6 , 7 polystyrene8 and polybutylene terephthalate.9 Polypropylene and polybutylene terephthalate are polymers with a wide range of commercial uses and thus, enhancing of their properties through the reinforcement with carbon nanotubes should be of significant interest.
There has been little work on the fire retardancy of nanocomposites using CNTs except by the groups of Beyer, Kashiwagi and Schartel.10–17 The main polymer that has been investigated is ethylene/vinyl acetate 10 , 11 for its applications in cable industry and the reductions in peak heat release rate are comparable to those seen with clays. The usual measure that is used to evaluate the fire retardancy of nanocomposites is the cone calorimeter,13 which measures the rate of heat release and mass loss rate, along with smoke and carbon monoxide and carbon dioxide, as a function of the applied radiant energy. The effects that one would like to see are that the time to ignition and the time to peak heat release are increased while the peak heat release rate, the total heat released and the mass loss rate are lowered. Moreover, the amount of smoke and carbon monoxide (CO) would be also very interesting to be reduced. Fire formation and development can be defined as a specific number of combustion reactions which take place in situ depending on several factors.
It is important to notice two different processes: first, material reaction to fire and second, emission of fumes from the material subjected to overheating where optical density and toxicity are factors of concern.
In this paper, the effect of the reinforcement of PP and PBT with CNTs has been shown. The effects of different weight per cent loadings of nanotubes on the thermal, rheological mechanical properties and fire retardancy analysed by mean of the ignition time and the CO/CO2 emitted during the polymeric nanocomposites combustion have been described.
Materials and methods
Materials
Masterbatch of 20 wt‐% multiwalled carbon nanotubes in polypropylene and polybutylene terephthalate were obtained from Hyperion Catalysis International, Cambridge, MA, USA. The nanotubes are vapour grown and typically consist of 8–15 graphitic layers wrapped around a hollow 5 nm core. Typical diameter range from 10 to 15 nm, while lengths are between 1 and 10 μm; a surface area of 250 m2 g−1 was determined by the BET method and were delivered in pellet form. PP (Isplen 094N2Ml) and PBT (Pocan B1505) were dried for 24 h at 70°C before use.
Extrusion of nanocomposite
Polymer composites were obtained by blending polymer matrix with the corresponding masterbatch containing carbon nanotubes in a mini extruder (MINILAB from Thermo HAAKE). Compounds were blended for 15 min at 220 and 250°C for PP and PBT respectively in counter‐rotating twin screws with a mixing speed of 150 rev min−1. Different composites formulations were obtained with filler contents ranging from 0 to 5% (w/w).
Thermal characterisation
Thermogravimetric analysis (TGA) measures the changes in weight that occur to a sample as a function of temperature. Such analysis relies on a high degree of precision in three measurements: weight, temperature and temperature change. Before starting the measurement the samples were cut into small pieces (20 mg), which were introduced to the oven where the temperature was increased from 30 to 800°C following 20°C m−1 in relation. At a temperature of 550°C an oxygen air flow was introduced to the oven disappearing all the carbon content of the composites.
A differential scanning calorimeter (DSC) was used to study the thermal transition of the PP and PBT based nanocomposites. Differential scanning calorimetry tests were carried out using a Mettler DSC 30. For PP and PBT, the temperatures were scanned between 25 and 200°C, and 25 and 280°C respectively at a heating rate of 20°C min−1.
Before the dynamic mechanical thermal analysis (DMTA) tests, 35×10×2 mm sample bars were injected using a Demag miniinjector moulding machine. Dynamic rheological measurements were performed using the dynamic mechanic thermal analyser. The measurements were carried out by subjecting the different mixture squares to a constant oscillatory shear mode (3 Hz) while the test temperature increased from 25 to 200°C.
Before the rheological measurements, the materials were dried for 3 h at 80°C in the oven. Rheological measurements were performed using an advanced microcompounder (Minilab HAAKE Rheomix CTW5). The system is based on a conical twin‐screw compounder with an integrated backflow channel. Due to the backflow channel and the integrated bypass valve, which enables the recirculation of the melt, the residence time are well defined.
The measurements were carried out at 190°C for PP, and 250°C for PBT, introducing to the extruder 5 g from each mixture and with screw frequency ranging from 10 to 360 rev min−1 in 10 steps, making 20 different measurements in each step.
Before flexural strength test, 80×10×2 mm sample bars were injected using a Demag miniinjector moulding machine. Flexural strength test was carried out using a MIDI, mechanical test machine exerting the strength in 1 mm min−1 speed.
O2 consumption and CO and CO2 generation were determined by means of suitable analysers. Therefore, for O2 determination a paramagnetic analyser was used while for gas emissions an infrared CO/CO2 generation detector was used. The combustion of the composites takes place in a tubular furnace. The polymer composites were introduced in the oven at 600°C and the combustion gasses were pushed out by a synthetic air stream to the corresponding analysers. Data of O2, CO and CO2 concentrations versus time were collected every 3 seconds in order to study the corresponding kinetics.
Results and discussion
TGA
In order to study the effect of the nanotubes on the thermal stability of the polymers, the thermogravimetric analyses of the neat polymers and the nanocomposites have been studied.
Figure 1 shows TGA plots of pure polypropylene and polypropylene reinforced with 2 and 5 wt‐% of CNTs. The reinforcement of PP with carbon nanotubes seems to have a positive effect on the thermal stability of the composites. The onset of thermal decomposition values, summarised in Table 1, increased with nanotube loading within the polymer. Compared with the pure PP, the nanocomposites showed an increase of 14°C with the addition of 2 wt‐% CNT and 26°C with the addition of 5 wt‐% in the onset of thermal degradation. The improvement in thermal stability may be attributed to good adhesion of the nanotube to the polymer matrix and also due to the thermal conductivity of the nanofillers. Figure 1 shows hence that the addition of carbon nanotubes to PP, improves the degradation temperature becoming the composite more stable for higher temperatures. It was also observed that in the case of pure PP, for temperatures higher than 600°C, a total degradation of the sample occurred, while in the case of the nanocomposites, some remaining materials were collected This remaining mass of materials that was not degraded is related with the loading percentage of carbon nanotubes in each nanocomposite.
Thermogravimetric analysis plots of pure PP and PP/CNT nanocomposites
Effect of CNT on decomposition of PP and PBT based nanocomposites
Figure 2 displays the thermal behaviour of PBT and CNT reinforced PBT nanocomposites. The thermal degradation starting temperature is 390, 393 and 392·5°C for 0, 2 and 5 wt‐% of carbon nanotubes. In the case of the PBT based nanocomposites, there is no a clear evidence in improvement of the thermal stability. The addition of carbon nanotubes to PBT hardly causes an increase in the degradation temperature. This increase is about 3°C with 2 wt‐% CNT, but as shown in Fig. 2, a higher amount of nanotubes in the polymer, such as 5 wt‐%, does not make a considerable increase in the degradation temperature. Onset temperatures of PBT and PBT/nanocomposites are also summarised in Table 1.
Thermogravimetric analysis plots of pure PBT and PBT/CNT nanocomposites
DSC
The heat of fusion (ΔHf), melting point (Tm) and degree of crystallinity of the pure PP and PBT and the nanocomposites were determined from DSC with different weight loadings and are summarised in Table 2. The degree of crystallinity was obtained from values of the heat of fusion.
Heat of fusion, per cent of crystallinity and melting point of PP, PBT and corresponding nanocomposites
For PP, the decrease in the ΔHf with loadings of 2 wt‐% of CNT may cause the proportional reduction of the PP concentration in the composite or nucleation of the PP crystallites caused by the nanotubes. For 5 wt‐% of CNT loadings no further effect is observed. When melting temperatures of the nanocomposites based on PP and pure PP are compared, slight increases for the later samples are observed.
Rheological characterisation
Curves of viscosity versus shear rate of pure PP and PP‐based nanocomposites containing different loadings of carbon nanotubes are shown in Fig. 3. The flow behaviour of pure polypropylene is different from nanocomposites containing carbon 2 and 5 wt‐% nanotubes at 190°C. Pure PP shows a linear viscosity for different screw speeds, while the addition of carbon nanotubes to the polymer, changes the behaviour of the viscosity according to screw speed. No strain hardening behaviour exists in pure PP, when polymer molecules are orientated in the flow direction. This due to that PP does not produce any additional stress.
Effect of nanotube content on viscosity of PP based nanocomposites at 190°C
The addition of carbon nanotubes to the PP polymer causes an increase in the viscosity. An addition of 2 wt‐% makes a huge increase in the complex viscosity determined at shear rate zero, from 4000 to 70 000 cP, while the complex viscosity of the nanocomposite reinforced with 5 wt‐% increased up to 100 000 cP.
In Fig. 4 the complex viscosity curves at 250°C for the pure PBT and the nanocomposites reinforced with 2 and 5 wt‐% in CNT are reported. The rheological behaviour of the pure PBT is linear with the shear rate. This happens again because in the case of pure PBT the polymer molecules easily get the orientation of the flow and no additional stresses happened.
Effect of nanotube content on viscosity of PBT based nanocomposites at 250°C
CNT reinforced nanocomposites also show different viscosity behaviours from that shown by pure PBT. When the screw speed was increased, the viscosity of the samples was decreased. Analysing the effect the CNTs caused in PBT, Fig. 4 shows how the addition of CNTs increases the complex viscosity. When data extrapolated to shear rate zero, the viscosity of pure PBT, PBT+2 wt‐% of CNT and PBT+5 wt‐% of CNT are 7000, 30 000 and 80 000 cP respectively.
DMTA
The dynamic mechanical properties of the neat polymer matrixes and polymer nanocomposites were studied by DMTA. Figure 5 shows the temperature dependent storage modulus E′ and tan (delta) of neat PP, 2 wt‐% and 5 wt‐% CNTs containing PP.
Dependence of E′ and tan (delta) of PP/CNT on filler concentration
In all nanocomposites and for the whole range of temperatures, the incorporation of CNTs causes a measurable and gradual increase in the stiffness. The storage modulus E′ at ambient temperature for pure PBT is 1972 MPa, for PBT containing 2 wt‐% of CNT is 2083 MPa and for the nanocomposite containing PBT and 5 wt‐% of CNT is 2187 MPa. The results show an improvement of 6 and 11% in the storage modulus when the addition of CNT is about 2 and 5 wt‐% respectively. In Fig. 5, it can be observed that the storage modulus suffers an important decrease as the temperature increases. This decrease happens for pure PP and for both nanocomposites. As the DMTA experiment advances, the temperature increases and so the storage modulus decreases, reaching a null stores modulus by the time the material is completely melted, 180°C and over. Tan (delta) is the ratio of the storage modulus over the loss modulus. In the temperature range plotted in Fig. 5, no peak of tan (delta) is observed since the glass transition temperature of PP is −20°C.18
Figure 6 shows the temperature dependent storage modulus E′ and tan (delta) of neat PBT and 2 and 5 wt‐% CNTs containing PBT. The storage modulus of the nanocomposites was improved as the CNT content in the nanocomposite was increased. This improvement is very relevant when the addition of CNT is about 2 wt‐%, but there is no additional improvement due to an extra addition of CNT, such as 5 wt‐%. The storage modulus at ambient temperature for pure PP is 664 MPa, for PP containing 2 wt‐% CNT is 1768 MPa and for the one containing 5 wt‐% of CNT is 1778 MPa. This means that an increase of 2 wt‐% of CNT increases the storage modulus in 166%, while an addition of CNT up to 5% does not show an increase in the storage modulus of the nanocomposite respect to the nanocomposite containing 2 wt‐%. The position of the tan (delta) peak corresponding to the Tg of the nanocomposites does not shift significantly from pure PBT.
Dependence of E′ and tan (delta) of PBT/CNT on filler concentration
Flexural strength
In order to study the effect of the reinforcement of nanocomposites on the mechanical properties compared to those of neat polymers, for pure PP and PBT and CNT reinforced PP and PBT flexural tests have been carried out.
Table 3 shows the mechanical properties measured by flexural testing for all the nanocomposites.
Summary of mechanical properties of PP and PBT based formulations
The inclusion of CNT to both polymers, PP and PBT causes an increase in the mechanical resistance. For PP nanocomposites an addition of 2 wt‐% of CNT causes an improvement of 7% in the resistance and when the concentration of nanotubes increases up to 5 wt‐%, a 20% resistance improvement can be reached. The mechanical effect on CNT reinforced nanocomposites is more visible when the matrix is PBT; 2 and 5 wt‐% of CNT loading causes a resistance improvement of 16 and 23% respectively.
Fire retardancy
The ignition and extinguishing time of nanocomposites were determined by means of a tubular furnace and a paramagnetic O2 consumption detector and an infrared CO/CO2 generation detector. In Fig. 7, the oxygen content of the air which surrounds the burning samples (PP and PP based nanocomposites) as a function of the combustion time is shown. The initial oxygen percentage in the atmosphere is 20·95%. The ignition time for PP is 260 s and was decreased to 255 s for the 2 and 5 wt‐% reinforced nanocomposites.
Nanotube dependence of oxygen consumption in PP/CNT nanocomposites
The peak of the maximum oxygen consumption happened after around 320 s for the three samples with a small deviation. Concerning the second stage of the combustion process, the time needed to be extinguished increases as follows: PP<PP+2 wt‐% MWCNT<PP+5 wt‐% MWCNT. In the case of PP, the reinforcement of the matrix with MWCNT has a negative effect on the ignition and extinguishing time compared to that of pristine PP.
In Fig. 8, the oxygen content of the air which surrounds the burning PBT and PBT based nanocomposites as a function of the combustion time is shown. In the case of the PBT the ignition time is retarded from 267 s (pristine PBT) to 279 s (PBT+2 wt‐% MWCNT and PBT+5 wt‐% MWCNT). The peaks of the curves corresponding to the nanocomposites and the extinguishing time were increased as follows: PBT<PBT+2 wt‐% MWCNT<PBT+5 wt‐% MWCNT. For the PBT series therefore, the MWCNTs show a positive effect since they increased the ignition time of the pristine PBT and also retarded the extinguishing time of the thermoplastic matrix.
Nanotube dependence of oxygen consumption in PBT/CNT nanocomposites
In Table 4, a summary of the ignition time and the total CO and CO2 accumulated during the combustion of the burning samples is shown. The total CO and CO2 emitted is defined as the area under the curve plotting the emitted and recorded CO and CO2 percentage versus combustion time. For the PP and PBT series, the CO and CO2 recorded were increased as the MWCNT was increased in the polymer.
Ignition time and accumulated CO and CO2 after combustion of pristine thermoplastics and their corresponding nanocomposites
Conclusions
The rheological, thermal and mechanical properties of PP/CNT and PBT/CNT nanocomposites were measured. The viscosity of the nanocomposites increases with the increase in the CNT content. The rheological properties change from a linear behaviour in the case of pristine PP and PBT to exponential behaviour in the case of the 2 and 5 wt‐% nanocomposites. The thermal properties are improved with the reinforcement of the thermoplastics with carbon nanotubes as the onset of thermal decomposition values, increased with nanotube loading within the polymer. This effect is more evident in the case of PP than that of PBT. The storage moduli of the nanocomposites are improved in the PBT/CNT and PP/CNT nanocomposites, especially in the case of 2 and 5 wt‐% PBT/CNT where E′ increases in 166%. Finally, it has been also shown that the inclusion of CNT in PP and PBT causes an increase in the mechanical resistance measured by flexural tensile.
An experimental setup combining a tubular furnace and an analyser for O2, CO and CO2 designed in GAIKER shows that the ignitability and the amount of CO and CO2 emitted during the combustion of PP‐, and PBT reinforced composites with MWCNT are highly depending on the nature of the thermoplastic matrix. The ignition time is increased for PBT and the extinguishing time is decreased for PP and PBT.
Footnotes
Acknowledgements
The authors thank to the Department of Industry, Trade and Tourism of the Basque Government (SAIOTEK Program) for the financial support of this research work.