Creep-recovery behavior of novel polyurethane/carbon nanotube composites

6 October 2017
Ramin Shamsi, Reza Sheibani, and Hamed Nazarpour-Fard
Dynamic mechanical analysis and three well-known models were used to investigate the mechanical properties of samples synthesized from recycled poly-ethylene terephthalate.

Poly-ethylene terephthalate (PET) bottles (patented in 19731) account for 83–84% of the total current demand for PET resin.2 The large amount of waste PET—in the form of disposed plastic bottles—therefore presents a substantial environmental problem.3 As PET is naturally resistant to degradation, recycling of PET waste is thought to be an effective way to reduce the environmental impact of this plastic.4 For instance, recycling PET bottles leads to the conservation of fossil fuels, thereby reducing energy consumption and greenhouse effects.2 Although such low-cost physical recycling of PET is widely adopted in large-scale applications, the recovered PET tends to have reduced clarity and poorer mechanical properties.

As an alternative to physical recycling of PET, chemical recycling can be used to produce a variety of value-added products, including polyurethanes (PUs).5–7 Indeed, PUs are an important class of co-polymer in the engineering sector because they exhibit many attractive properties (e.g., adjustable mechanical strength, good chemical resistance, and potential shape-memory) and recycled PET could therefore have a large market.8 To obtain the desirable PU properties, however, it is necessary to compound the PU with an inorganic filler. For example, carbon nanotubes (CNTs) have previously been used to improve the stiffness, strength, toughness, and conductivity of PU.9, 10 Furthermore, the relaxation behavior—e.g., the creep-recovery characteristics—of PU/CNT composites have been investigated.11 Creep is an important material property that determines dimensional stability and it is essential to realizing long-term durability and reliability of polymers.12 The creep behavior of PU/CNT nanocomposites has therefore been the focus of several previous studies.13, 14 To date, however, the creep-recovery characteristics of CNT composites made with PET-recycled PU have not been examined.

In this work,15 our aim was therefore to produce a new set of PU composites from recycled PET and to study the creep-recovery characteristics of the samples. To that end, we synthesized polyester diol via glycolysis of the recycled PET waste. We used this diol to prepare our PU and multiwalled CNT (MWCNT) nanocomposites (see Table 1). In addition, we performed a dynamic mechanical analysis (DMA) to measure the creep-recovery properties of the samples at seven different stress levels (0.5, 1, 1.5, 2, 2.5, 5, and 8MPa). The full details of our synthesis method for the PU nanocomposites, as well as for our DMA measurements, have previously been provided.7, 16

The composition, physical state, and stress–strain properties of the PU and PU/CNT samples. NCO/OH: Isocyanate/hydroxyl ratio.

SampleNCO/OH (molar ratio)CNT (wt%)Physical stateYoung's modulus (MPa)Offset yield stress (MPa)
PU-11Thermoplastic30.34
PU-1.71.7Thermoset6010.7
PU-22Thermoset518.5
PUCNT-110.5Powder
PUCNT-1.71.70.5Thermoplastic373.7
PUCNT-220.5Thermoplastic81.87.4

The creep-recovery results we obtained (between 0.5 and 2.5MPa) for our PU and PU/MWCNT nanocomposites are shown as a function of time in Figure 1. We observed the lowest creep and recovery strains for the PUCNT-2 sample, followed by PU-1.7, PU-2, PUCNT-1.7, and then PU-1. This trend, however, does not follow the Young's modulus results we obtained (see Table 1). We attribute the high creep-strain of the PU-1 sample to its thermoplastic nature and thus its higher capacity for slippage and viscous flow. Moreover, we believe that the lower creep-strain of PU-1.7 compared with PU-2 was caused by the higher crosslinking degree of PU-1.7.


Experimentally measured creep and creep-recovery strain, as a function of time, for the polyurethane (PU) and PU/multiwalled carbon nanotube (CNT) nanocomposites at 30°C. Results are shown for stress levels of (a) 0.5MPa, (b) 1MPa, (c) 1.5MPa, (d) 2MPa, and (e) 2.5MPa. Sample details are given in Table 1.

Rather than polymeric, our PUCNT-1 sample had a powder form because of the steric hindrance effect of the MWCNT nanoparticles and the low number of diisocyanate functional groups (see Table 1) in this particular composite. Specifically, the steric hindrance effect of the MWCNTs caused a reduction in the reactions between the isocyanate and hydroxyl (OH) functional groups. This effect also gave rise to the increased creep-strain of the PUCNT-1.7 sample. In contrast, the reinforcing effect of the MWCNTs strengthened the polymers and thus reduced the creep-strain of the PUCNT-2 sample. These effects originate from the inherently high modulus of the MWCNTs and from the good interactions between the OH groups and urethane groups on the MWCNT surfaces. The results of our measurements at 5 and 8MPa were similar to the data we obtained in the 0.5–2.5MPa range. We found, however, that the creep and recovery strains for PUCNT-2 at 5MPa were higher than those of PU-1.7. We attribute this result to the proximity of 5MPa to the 7.4MPa offset yield stress for PUCNT-2 (see Table 1).

In our study, we also fitted our experimental results with a number of well-known models (i.e., that are commonly used to model creep or recovery experimental data). First, we used the power-law model17 (a simple and common constitutive model) to examine the creep-strain–time dependency of our data. From this model we obtained several parameters, including the amplitude of the transient creep-strain (A) and the time exponent (n) of the dependency. We found that there was good agreement between the model results and the experimental data, and specifically, that the higher creep-strain samples exhibited higher values for A and lower values for n.18 We also used the Burgers model19 (a four-element model) to evaluate the creep behavior of our samples. We observed an inverse relationship between the creep-strain and the four Burgers parameters (i.e., the elastic strain, viscoelastic strain, and two viscoplastic strain parts) and, again, a good agreement between the model and experimental datasets. Lastly, we applied the Weibull distribution model20 to our recovery measurements and found a good agreement with our empirical data. The results also revealed that the increased crosslink density of PU-2 and PU-1.7 (i.e., compared with PU-1) caused a decrease in the viscoelastic strain recovery (∊VI) of the samples. The addition of MWCNTs to PU-1.7 increased the ∊VI, whereas the incorporation of MWCNTs into PU-2 had the opposite effect (i.e., evidence of the opposing steric hindrance effect of MWCNT particles).

In summary, we have synthesized novel PU nanocomposites from recycled PET waste and MWCNTs. We found that the reinforcing and steric hindrance effects of MWCNTs and the crosslinking degree of PUs are important parameters that govern the creep-recovery behavior of the samples. Our future work will be focused on other applications of these samples, and on the synthesis of nanocomposites from these PUs and alternative nanoparticles (e.g., clay, graphene, and core-shell nanoparticles).


Authors

Ramin Shamsi
Research and Development (R&D) Department, Marun Petrochemical Company

Ramin Shamsi is a senior researcher. His research fields include polyethylene terephthalate waste recycling and polyurethane synthesis.

Reza Sheibani
Research and Development (R&D) Department, Marun Petrochemical Company

Reza Sheibani is an R&D manager. His main research area is catalysts.

Hamed Nazarpour-Fard
Department of Chemistry, University of Guilan

Hamed Nazarpour-Fard is a PhD student whose research focus is nanocomposite synthesis.


References

  1. A. M. Al-Sabagh, F. Z. Yehia, Gh. Eshaq, A. M. Rabie and A. E. ElMetwally, Greener routes for recycling of polyethylene terephthalate, Egypt. J. Petrol. 25, pp. 53-64, 2016.

  2. H. Zhang and Z.-G. Wen, The consumption and recycling collection system of PET bottles: a case study of Beijing, China, Waste Manage. 34, pp. 987-998, 2014.

  3. S. Akçaözoğlu and C. Ulu, Recycling of waste PET granules as aggregate in alkali-activated blast furnace slag/metakaolin blends, Construct. Build. Mater. 58, pp. 31-37, 2014.

  4. C.-Q. Wang, H. Wang, J.-G. Fu and Y.-N. Liu, Flotation separation of waste plastics for recycling—a review, Waste Manage. 41, pp. 28-38, 2015.

  5. R. Shamsi, M. Abdouss, G. M. M. Sadeghi and F. A. Taromi, Synthesis and characterization of novel polyurethanes based on aminolysis of poly(ethylene terephthalate) wastes, and evaluation of their thermal and mechanical properties, Polym. Int'l 58, pp. 22-30, 2009.

  6. G. M. M. Sadeghi, R. Shamsi and M. Sayaf, From aminolysis product of PET waste to novel biodegradable polyurethanes, J. Polym. Environ. 19, pp. 522-534, 2011.

  7. R. Shamsi and G. M. M. Sadeghi, Novel polyester diol obtained from PET waste and its application in the synthesis of polyurethane and carbon nanotube-based composites: swelling behavior and characteristic properties, RSC Adv. 6, pp. 38399-38415, 2016.

  8. V. A. Lemos, M. S. Santos, E. S. Santos, M. J. S. Santos, W. N. L. dos Santos, A. S. Souza and D. S de Jesus, Application of polyurethane foam as a sorbent for trace metal pre-concentration—a review, Spectrochim. Acta Part B: Atomic Spectro. 62, pp. 4-12, 2007.

  9. G. Li, L. Feng, P. Tong and Z. Zhai, The properties of MWCNT/polyurethane conductive composite coating prepared by electrostatic spraying, Prog. Organic Coat. 90, pp. 284-290, 2016.

  10. J.-H. Han, H. Zhang, M.-J. Chen, G.-R. Wang and Z. Zhang, CNT buckypaper/thermoplastic polyurethane composites with enhanced stiffness, strength and toughness, Compos. Sci. Technol. 103, pp. 63-71, 2014.

  11. T. Zhai, D. Li, G. Fei and H. Xia, Piezoresistive and compression resistance relaxation behavior of water blown carbon nanotube/polyurethane composite foam, Compos. Part A: Appl. Sci. Manufac. 72, pp. 108-114, 2015.

  12. Y. Jia, K. Peng, X.-L. Gong and Z. Zhang, Creep and recovery of polypropylene/carbon nanotube composites, Int'l J. Plasticity 27, pp. 1239-1251, 2011.

  13. Z. Yao, D. Wu, C. Chen and M. Zhang, Creep behavior of polyurethane nanocomposites with carbon nanotubes, Compos. Part A: Appl. Sci. Manufac. 50, pp. 65-72, 2013.

  14. Y. Jia, Z. M. Jiang, X. L. Gong and Z. Zhang, Creep of thermoplastic polyurethane reinforced with ozone functionalized carbon nanotubes, Express Polym. Lett. 6, pp. 750-758, 2012.

  15. R. Shamsi, G. H. Asghari, G. M. M. Sadeghi and H. Narzarpour-Fard, The effect of multiwalled carbon nanotube and crosslinking degree on creep-recovery behavior of PET waste originated-polyurethanes and their nanocomposites, Polym. Compos., 2017.

  16. R. Shamsi, G. Mir Mohamad Sadeghi and G. H. Asghari, Dynamic mechanical analysis of polyurethanes and carbon nanotube based composites obtained from PET waste, Polym. Compos., 2016.

  17. W. N. Findley, J. S. Lai, K. Onaran and R. M. Christensen, Creep and relaxation of nonlinear viscoelastic materials with an introduction to linear viscoelasticity, J. Appl. Mech. 44, pp. 364, 1977.

  18. J.-L. Yang, Z. Zhang, A. K. Schlarb and K. Friedrich, On the characterization of tensile creep resistance of polyamide 66 nanocomposites. Part II: Modeling and prediction of long-term performance, Polymer 47, pp. 6745-6758, 2006.

  19. I. M. Ward and J. Sweeney, Mechanical Properties of Solid Polymers, John Wiley & Sons, 2012.

  20. K. S. Fancey, A mechanical model for creep, recovery and stress relaxation in polymeric materials, J. Mater. Sci. 40, pp. 4827-4831, 2005.

DOI:  10.2417/spepro.006965