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. 2019 May 31;4(5):9569-9582.
doi: 10.1021/acsomega.9b00940.

Mechanism of Thermal Degradation-Induced Gel Formation in Polyamide 6/Ethylene Vinyl Alcohol Blend Nanocomposites Studied by Time-Resolved Rheology and Hyphenated Thermogravimetric Analyzer Fourier Transform Infrared Spectroscopy Mass Spectroscopy: Synergistic Role of Nanoparticles and Maleic-anhydride-Grafted Polypropylene

Reza Salehiyan  1 , Jayita Bandyopadhyay  1 , Suprakas Sinha Ray  1   2
Affiliations

Mechanism of Thermal Degradation-Induced Gel Formation in Polyamide 6/Ethylene Vinyl Alcohol Blend Nanocomposites Studied by Time-Resolved Rheology and Hyphenated Thermogravimetric Analyzer Fourier Transform Infrared Spectroscopy Mass Spectroscopy: Synergistic Role of Nanoparticles and Maleic-anhydride-Grafted Polypropylene

Reza Salehiyan et al. ACS Omega. .

Abstract

In this study, polyamide 6 (PA) is blended with ethylene vinyl alcohol (EVOH) to yield packaging materials with a balance of mechanical and gas barrier properties. However, the formation of gel-like structures in both polymers because of thermal degradation at high temperatures leads to a processing challenge, particularly during thin-gauge film extrusion. To address this challenge, nanoclays are introduced either directly or via a masterbatch of maleic-anhydride-grafted polypropylene to the PA/EVOH blend and time-resolved rheometry is used to study the effect of different modes of nanoclay incorporation on the kinetics of thermo-oxidative degradation of PA/EVOH blend and its nanocomposites. Time-resolved rheometry measurements allow the acquisition of accurate frequency-dependent linear viscoelastic behavior and offer insights into the rate of degradation or gel formation kinetics and cross-link density. The thermal degradation was studied by thermogravimetric analysis coupled with Fourier transform infrared spectroscopy and mass spectroscopy, allowing the prediction of the possible reactions that take place during the rheological property measurements. The results show that when nanoclays are incorporated directly, the oxidative reactions occur faster. In contrast, in the masterbatch method, oxidative degradation is hindered. The difference in the behaviors is shown to lie in the different nanoclay distributions in the blends; in the blends prepared by the masterbatch method, the nanoclays are dispersed at the interface. In conclusion, the masterbatch-containing blend nanocomposite would benefit processing and product development.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Time-resolved rheometry results for the elastic (storage) modulus (G′(t)) of the (80/20) PA/EVOH blend at different frequencies at a fixed-strain amplitude of 0.5% and temperatures of (a) 230, (b) 240, and (c) 250 °C under an air atmosphere. The inset plots show the isochronal elastic modulus (G′(ωt)) of the blends collected at different times (zero time, 300, 600, 1500, 2500, 6000, and 7200 s). PA, polyamide 6; EVOH, ethylene vinyl alcohol.
Figure 2
Figure 2
Time-resolved rheometry results for the elastic (storage) modulus (G′(t)) of the PA/EVOH/BET nanocomposite at different frequencies at a fixed-strain amplitude of 0.5% and temperatures of (a) 230, (b) 240, and (c) 250 °C under an air atmosphere. PA, polyamide 6; EVOH, ethylene vinyl alcohol; BET, organically modified montmorillonite.
Figure 3
Figure 3
Time-resolved rheometry results for the elastic (storage) modulus (G′(t)) of the PA/EVOH/MB nanocomposite at different frequencies at a fixed-strain amplitude of 0.5% and temperatures of (a) 230, (b) 240, and (c) 250 °C under an air atmosphere. PA, polyamide 6; EVOH, ethylene vinyl alcohol; MB, masterbatch.
Figure 4
Figure 4
Time-resolved rheometry results for the elastic (storage) modulus (G′(t)) of the PA/EVOH/PP-g-MA blend at different frequencies at a fixed-strain amplitude of 0.5% and temperatures of (a) 230, (b) 240, and (c) 250 °C under an air atmosphere. PA, polyamide 6; EVOH, ethylene vinyl alcohol; PP-g-MA, maleic-anhydride-grafted polypropylene.
Figure 5
Figure 5
Gel point (crossover) evolution of PA/EVOH blend, PA/EVOH/BET nanocomposite, PA/EVOH/MB nanocomposite, and PA/EVOH/PP-g-MA blend as a function of the temperature acquired from time-resolved rheometry (time sweep tests) measurements at a frequency of 0.1 rad/s. PA, polyamide 6; EVOH, ethylene vinyl alcohol; BET, organically modified montmorillonite; MB, masterbatch; PP-g-MA, maleic-anhydride-grafted polypropylene.
Figure 6
Figure 6
Isochronal zero-time elastic moduli G′(ωt=0) of (a) PA/EVOH, (b) PA/EVOH/BET, (c) PA/EVOH/MB, and (d) PA/EVOH/PP-g-MA samples collected at different times from time-resolved rheometry tests under an air atmosphere at 230, 240, and 250 °C. PA, polyamide 6; EVOH, ethylene vinyl alcohol; BET, organically modified montmorillonite; MB, masterbatch; PP-g-MA, maleic-anhydride-grafted polypropylene.
Figure 7
Figure 7
Three-dimensional (3D) tomogram from transmission electron microscopy images showing the nanoclay distributions in (a, a′, a″) PA/EVOH/MB and (b, b′, b″) PA/EVOH/BET nanocomposites. The silver, blue, golden, and yellow background colors represent the PA, nanoclays, EVOH and PA/EVOH blend, respectively. The cryogenically fractured scanning electron microscopy (SEM) surface images of (c) PA/EVOH, (d) PA/EVOH/BET, and (e) PA/EVOH/MB. (f) The number-averaged droplet radii (Rn) was estimated by analyzing 50–100 droplets from several SEM images captured for each sample. Copyright 2017. It is reproduced with permission from Elsevier Ltd. PA, polyamide 6; EVOH, ethylene vinyl alcohol; BET, organically modified montmorillonite; MB, masterbatch.
Figure 8
Figure 8
Isochronal zero-time elastic moduli G′(ωt=0) of the PA/EVOH blend, PA/EVOH/BET nanocomposite, PA/EVOH/MB nanocomposite, and PA/EVOH/PP-g-MA blend collected at different times from time-resolved rheometry tests under an air atmosphere at (a) 230, (b) 240, and (c) 250 °C. PA, polyamide 6; EVOH, ethylene vinyl alcohol; BET, organically modified montmorillonite; MB, masterbatch; PP-g-MA, maleic-anhydride-grafted polypropylene.
Figure 9
Figure 9
Plateau moduli (GN) of the PA/EVOH blend, PA/EVOH/BET nanocomposite, PA/EVOH/MB nanocomposite, and PA/EVOH/PP-g-MA blend as a function of time at (a) 230, (b) 240, and (c) 250 °C extracted from inset plots in Figures 1–4. PA, polyamide 6; EVOH, ethylene vinyl alcohol; BET, organically modified montmorillonite; MB, masterbatch; PP-g-MA, maleic-anhydride-grafted polypropylene.
Figure 10
Figure 10
Calculated cross-link densities of PA/EVOH blend, PA/EVOH/BET nanocomposite, PA/EVOH/MB nanocomposite, and PA/EVOH/PP-g-MA blend from the plateau moduli (GN) at 7200 s at different temperatures. PA, polyamide 6; EVOH, ethylene vinyl alcohol; BET, organically modified montmorillonite; MB, masterbatch; PP-g-MA, maleic-anhydride-grafted polypropylene.
Figure 11
Figure 11
FTIR analysis of the gas evolved during thermal degradation (studied by hyphenated TGA–FTIR–MS) of PA/EVOH blend, PA/EVOH/BET nanocomposite, PA/EVOH/MB nanocomposite, and PA/EVOH/PP-g-MA blend at isothermal temperatures of (a–d) 230 and (a′–d′) 240 °C. PA, polyamide 6; EVOH, ethylene vinyl alcohol; BET, organically modified montmorillonite; MB, masterbatch.
Figure 12
Figure 12
FTIR analysis of the gas evolved during thermal degradation (studied by hyphenated TGA–FTIR–MS) of (a) PA/EVOH blend, (b) PA/EVOH/BET nanocomposite, (c) PA/EVOH/MB nanocomposite, and (d) PA/EVOH/PP-g-MA blend. PA, polyamide 6; EVOH, ethylene vinyl alcohol; BET, organically modified montmorillonite; MB, masterbatch.
Scheme 1
Scheme 1
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Scheme 5
Scheme 5
Scheme 6
Scheme 6
Scheme 7
Scheme 7. Experimental Conditions of TGA–FTIR–GC–MS under (a) Isothermal Conditions at 230 and 240 °C and (b) Temperature Ramping from 50 to 900 °C at a Rate of 20 °C/min
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