The effect of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) on the thermal and chemical stability of polypropylene (PP) when subjected to oxidation in a strong acid medium was studied. The effect of CNFs and CNTs on the crystalline morphology and the melting and crystallization temperatures was also studied. The thermal stability increased markedly; the decomposition temperature, for example, increased from 293∘C for pure PP to 312 and 320∘C for PP with CNFs and CNTs, respectively. The crystallization temperature increased perceptibly with the addition of CNTs or CNFs, from 107∘C for pure PP to 112 and 114∘C for PP with CNFs and CNTs, respectively. The oxidative degradation with nitric acid produced a reduction in molecular weight; however, this negative effect was less pronounced in the PP compositions with carbon nanoparticles. After 8 hours in nitric acid, this reduction was from 141,000 to 68,000 (for pure PP), to 75,000 (for PP-CNFs), and 79,500 (for PP-CNTs). X-ray diffraction showed that the alpha type crystallinity remains, irrespective of the nucleating agent. Finally, the intensity ratio between the (040) (at 16.7∘) and the (110) (at 13.9∘) reflections increased, which was taken as an indication of an increasing nucleating efficiency.
It is well known that polypropylene (PP) is one of the most amply used commodity plastics, mainly because of the ease of processing and the inherent wide range of properties [
This would be of utmost interest for applications such as lead-acid car battery cases and geomembranes, where the plastic, mainly PP or polyethylene (PE), is to be in contact with highly corrosive acid liquids [
In both applications the PP is in contact with highly corrosive substances which can alter its composition and structure and significantly reduce its chemical and mechanical properties. In a previous work [
In spite of the many reports on the enhancement of the PP properties, conveyed by the nucleating agents in general and by the carbon nanoparticles in particular, very little is known on the effect of these carbon nanoparticles on the chemical stability of PP compositions. The purpose of the present work is to study the relationship between the increase in the thermal and chemical resistance and the enhanced nucleation induced by CNFs, CNTs, and a commercial lithium benzoate (LiBe).
The polymer used in this study was a commercial grade isotactic Polypropylene “PP” from Total Petrochemicals, USA, with an average molecular weight Mw of 150,000 and a melt flow rate of 15 g/10 min. The carbon nanoparticles used in this study, were: carbon nanofibers, Pyrograf III (CNFs), also known as Stacked-Cup Carbon Nanotubes (SCCNT), with a nominal surface area of 20 m2/g, from Applied Sciences, Inc., USA and multi-walled carbon nanotubes (CNTs), with a nominal surface area of 200 m2/g, from Nano-Lab, Inc., USA. The commercial lithium benzoate (LiBe) nucleating agent was from Micronisers Pty Ltd., Australia. The fuming nitric acid used was from Merck, USA.
A series of PP compositions with 0.05 wt% of each of CNFs, CNTs, and LiBe were prepared via melt mixing for 10 min, at 170°C and 50 rpm, in a Brabender torque rheometer mixing chamber. Very low nucleating agent concentrations (0.05 wt%) were in order to prevent the excess of nucleating particles from interfering with the spherulitic growth, as has been observed by other authors when using high CNTs or CNFs concentrations [
Following the nitric acid treatment and the appropriate washing, all samples were first analyzed for chemical changes with respect to the time in strong acid medium, using a Perkin-Elmer “Spectrum” FTIR spectrometer with an attenuated total reflectance (ATR) attachment. The spectra were obtained at 20 scans and a resolution of 4 cm−1. Two test specimens were used for each case. The samples were then subjected to thermal analysis using a Perkin-Elmer DSC-7 in order to establish the changes of the thermodynamic variables during treatment in a strong acid medium. Each sample of approximately 8 mg was first heated from room temperature to 180°C at 10°C/min, held there for 2 min, and then cooled down to room temperature at 10°C/min, all under nitrogen atmosphere. In order to study any change in the crystalline morphology, either due to the acid treatment or to the inclusion of nanoparticles, XRD was performed in a Siemens D5000 (25 mA, 35 kV) using CuKa X-ray radiation, at 0.6 degrees/min from 1 to 35 degrees. Finally, the samples were subjected to thermal analysis using a Shimadzu TGA-50 analyzer for studies of the thermal degradation, from room temperature to 400°C, at a heating rate of 10°C/min, under air atmosphere.
A gel permeation chromatograph GPC V-2000, with a refraction index detector and “styragel” columns, was used to determine the evolution of molecular weight and molecular weight distribution with respect to the time of treatment in the strong acid medium. Samples for GPC analysis were first dissolved in trichlorobenzene and then run at 140°C, at a flow rate of 1 mL/min.
Figure
Temperatures at which there was a 10 and 50 wt% loss, due to chemical decomposition, in PP samples collected after immersion in nitric acid for 0 and 8 hours.
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0 hours in nitric acid | PP-Pure | 263 | 293 |
PP-LiBe | 268 | 304 | |
PP-CNFs | 272 | 312 | |
PP-CNTs | 277 | 320 | |
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After 8 hours in nitric acid | PP-Pure | 303 | 365 |
PP-LiBe | 316 | 379 | |
PP-CNFs | 319 | 381 | |
PP-CNTs | 321 | 387 |
TGA curves showing the weight loss with temperature, due to decomposition of pure PP and PP compositions with 0.05 wt% of nucleating agent. Samples without immersion in nitric acid. The dotted lines represent the temperatures at which pure PP weight loss reached 10 and 50 wt%.
These results corroborate the enhanced thermal and chemical stability of the nucleated compositions. This enhanced stability is related to the crystalline structures produced in PP, in the presence of nucleating agents. Miltner et al. and Assouline et al. [
The increase in the decomposition temperature of the carbon-nucleated compositions may also be due to the barrier effect of the carbon nanoparticles, which when well dispersed, form a barrier that obstructs the oxygen diffusion [
With respect to the decomposition of PP in nitric acid, Figure
Variation of CO and NO2 groups with the time of immersion in nitric acid, as taken from the peaks at 1555 (NO2) and 1710 (CO) cm−1 in the IR absorbance diagrams.
The decomposition first involves the nitration of the PP chain, prior to the chain scission that starts to occur as a result of the formation of carbonyl groups. This is why the carbonyl bands in the FTIR analysis appear only after 22 hours in nitric acid [
GPC results, showing the decrease in molecular weight of the pure PP samples with the time of immersion in nitric acid are presented in Figure
(a) GPC curves showing the variation of molecular weight with the time of immersion in nitric acid for pure PP and (b) showing the differences in molecular weight between the nonnucleated and nucleated PP’s, of samples collected after 8 hours in nitric acid.
Figure
Figure
TGA curves showing the weight loss with temperature due to decomposition of pure PP and PP compositions with 0.05 wt% of nucleating agent. Samples collected after 8 hours of immersion in nitric acid. The dotted lines represent the temperatures at which pure PP weight loss reached 10 and 50 wt%.
Comparing the results among samples immersed in nitric acid for 0 and 8 hours (Figures
Considering that the immersion in nitric acid tends to wash out the amorphous fraction of the PP samples, then, the immersed samples in Figure
The XRD diffractograms in Figure
XRD diffractograms of pure PP and PP compositions with 0.05 wt% of nucleating agent; (a) without immersion in nitric acid and (b) after 8 hours in nitric acid.
This indicates that the crystalline type remains the same alpha crystals. Both, pure PP and PP compositions with nucleating agent, either with 0 or with 8 hours of immersion in nitric acid, present the same peaks at 2(theta), that is, 13.9, 16.7, 18.3, 21.6, and 21.9 degrees, which correspond to the planes (110), (040), (130), (111), and (041), respectively, of this alpha morphology. These results coincide with those found for PP/SWNT and PP/MWNT, as reported by several authors [
In addition, a clear tendency for the increase of the intensity ratio between the (040) (16.7 degrees) and the (110) (13.9 degrees) reflections can be observed. This increase can be taken as an indication of an increasing nucleating efficiency, as reported elsewhere [
Figure
DSC fusion thermograms of pure PP and PP compositions with 0.05 wt% of nucleating agent, without immersion in nitric acid.
However, the fusion interval appears to be narrower for the PP compositions with nucleating agent, LiBe, CNTs, or CNFs, surely due to the greater crystal size homogeneity.
In addition, it is observed in the inserted Table that the enthalpy of fusion increases very slightly in the nucleated samples. Considering thus the very small enthalpy changes among the PP and the PP with the different nucleating agents, it can be concluded that the percent crystallinity remains approximately the same.
Figures
Variation of the crystallization (a) and melting (b) peak temperatures, as obtained from DSC, for pure PP and PP compositions with 0.05 wt% of nucleating agent, after immersion in nitric acid for different times.
Nucleating efficiency (NE) and crystallization time
The nucleating efficiency (NE) [
It is assumed that the greater nucleating efficiency of CNFs and CNTs, as observed in Figure
Finally, considering that the amount and morphology of the crystalline structure in a semicrystalline polymer greatly affects in a positive way its chemical and mechanical properties, the inclusion of carbon nanoparticles as nucleating agents in PP would ensure better chemical resistance and mechanical properties when subjected to aggressive environments along its useful life.
Carbon nanoparticles, such as CNTs and CNFs increased markedly the thermal and chemical properties of PP, acting as very efficient nucleating agents, even better than the commercial LiBe, The decomposition temperature increased from 293°C for the pure PP to 312 and 320°C, for the PP compositions with CNFs and CNTs, respectively. The oxidative degradation with nitric acid produced a reduction in molecular weight, as determined by DSC; however, this was much less pronounced in the PP compositions with carbon nanoparticles. This reduction was from 141,000 to 68,000, 75,000, and 79,500, for pure PP, PP-CNFs, and PP-CNTs, respectively.
XRD results showed that the crystalline type was not affected by the nucleating agent; it remained of the alpha type. In addition, it was found that the carbon nanoparticles are highly efficient nucleating agents for the crystallization of PP. The crystallization temperature (DSC) and the decomposition temperature (TGA) of PP, both increased with the addition of CNTs or CNFs.
In general the mechanical performance of PP composites can be enhanced by the addition of CNTs or CNFs as nucleating agents. This increased resistance of PP/carbon nanoparticles compositions to thermal and chemical attack increases the viability of using PP in such demanding applications as lead-acid car battery cases and films to be used as geomembranes.
Considering that the amount and morphology of the crystalline structure in a semicrystalline polymer greatly affects in a positive way its chemical and mechanical properties, it can be concluded that the inclusion of carbon nanoparticles as nucleating agents in PP would ensure better thermal and chemical resistance when subjected to aggressive environments.
The authors wish to thank CONACYT for its financial support to carry out this study, through Project 84424. In addition, one of the authors F. Avalos-Belmontes wishes to thank CONACYT for its support to spend a sabbatical at CIQA. Finally, the authors wish to thank M. Sánchez-Adame, A. Espinoza-Muñoz, M. C. Gonzalez-Cantu, T. Rodriguez-Hernandez, B. Huerta-Martinez, J. Rodriguez-Velazquez, J. F. Zendejo, E. Hurtado-Suarez, and P. Siller-Flores for their valuable technical and informatics support.