Chinese Chemical Letters  2017, Vol. 28 Issue (5): 949-954   PDF    
Thermo-oxidative degradation of Nylon 1010 films: Colorimetric evaluation and its correlation with material properties
Li-Hai Caia,1, Zhi-Guo Qia,1, Jun Xua, Bao-Hua Guoa, Zhong-Yao Huangb     
a Key Laboratory of Advanced Materials of Ministry of Education, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China;
b Institute of Petroleum, Logistical Support Department, CMC, Beijing 102300, China
Abstract: The thermo-oxidative aging behaviors of Nylon 1010 films were studied by various analytical methods, such as measuring the chromaticity, relative viscosity, carbonyl index, UV absorbance at 280 nm and elongation at break of the aged films. The thermo-oxidative aging plots of the results obtained via these various methods at different temperatures are subjected to the time-temperature superposition analysis, which are found to be well superposed. The b* values are used as X axis and the other results, i.e., relative viscosity, carbonyl index, UV absorbance at 280 nm and elongation at break, are used as Y axis, respectively. The relationship between the b* values and the other results is obtained, from which we can derive the changes of physical and chemical properties at different b* values. Since the b* values can be quickly determined by using a portable spectrophotometer, the on-line evaluation of the thermo-oxidative aging of Nylon 1010 can be realized.
Key words: Nylon 1010     Colorimetric evaluation     Thermo-oxidative degradation     Carbonyl index     UV absorbance     Elongation at break     Activation energy    
1. Introduction

Nylon 1010, a diacid diamine type polyamide, is a biorenewable, high-performance engineering plastic processed from castor bean. Owing to the merits such as excellent mechanical properties, wear resistance, and low temperature resistance, Nylon 1010 is widely employed to manufacture bearings, gears, rollers, nuts, bolts, tubes and other components [1-4]. Note that Nylon 1010 is often used at relatively high temperatures, so the thermooxidative degradation of this thermoplastic material is liable to occur, including discoloration and deterioration of the mechanical properties. Although several groups have studied the thermooxidative aging of Nylon 6 [5-9] and Nylon 66 [10-13], to the best of our knowledge there are very few reports on that of Nylon 1010. In order to understand the actual performances of Nylon 1010, it is of great urgency to study its thermo-oxidative aging behaviors.

The thermo-oxidative degradation mechanism of Nylon 1010 should comply with the general oxidation mechanism of aliphatic polyamides because of their similar chemical structures [14]. However, the relationship between the properties and temperature or time needs to be studied, which is extremely helpful to assess the state of Nylon 1010 in use and predict its lifetime under the given storage conditions.

Many analytical methods have been used to study the changes of structures and properties of Nylon materials during the thermooxidative aging process, such as molecular weight determination [7, 9], oxygen uptake determination [15, 16], FTIR spectroscopy [17, 18], mass spectroscopy [19, 20], UV and fluorescence spectroscopy [21, 22], colorimetric evaluation [23], mechanical properties determination [18, 24], etc. Furthermore, some papers [5, 25] even focus on the correlation between the results of different analytical methods. Dong and Gijsman [5] have studied the relationshipbetween the changes of oxygen uptake and other results of different analytical methods. However, the oxygen uptake experiments should be done under some special conditions, and it is difficult to assess the state of engineering components made of Nylon 1010 conveniently. In contrast, colorimetric evaluation is quick, nondestructive and reproducible, and is widely used to establish and control the color tolerance in industry. Therefore, colorimetric evaluation is a promising technique to study the thermo-oxidative aging behaviors of polymeric films, fibers and bulks [23].

In the present work, the thermo-oxidative aging behaviors of Nylon 1010 were studied at high temperatures by a combination of several analytical methods, including relative viscosity, colorimetric evaluation, FTIR and UV spectroscopy and tensile property measurement. Based on these results, the relationship between the colorimetric changes and other properties is discussed in order to find a convenient and quick method for monitoring the performances for engineering components made of Nylon 1010.

2. Results and discussion 2.1. Colorimetric evaluation

CIE Lab is a color space which can describe all colors visible to the human eyes. The three coordinates (L*, a*, b*) are used for the calculation of the chromaticity values of the films, representing the degree of lightness, red-green and yellow-blue for a certain color, respectively [26]. Fig. 1 shows the values of L*, a* and b* as a function of aging time at different temperatures. The chromaticity value of L* decreases linearly with time, and its drop shifts toward a longer time with decreasing temperature. From the chromaticity value of a*, we can see that the films show the green phase in the initial stage, and then turn to the red phase with the increase of time. The chromaticity value of b* increases gradually and the oxidation rate decreases (auto-retardation) with the increase of time, which indicates that the yellow color deepens gradually. Comparing the three coordinates for calculating the chromaticity values, L* is affected by the surface roughness and the change of a* is complicated, so b* is the most appropriate coordinate for the colorimetric evaluation of the aged films.

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Figure 1. CIE Lab values of Nylon 1010 aged at 120, 130, 140 and 160 ℃ as a function of time (a, L*; b, a*; c, b*).

2.2. Relative viscosity

The relative viscosity of the aged films is shown in Fig. 2. The solution viscosity decreases with the increase of temperature. At a certain temperature, the solution viscosity decreases quickly in the beginning and then decreases slowly. The decrease of the solution viscosity shows that the molecular weight of the films declines and the breakage of the polymer chains occurs during the thermooxidative aging process.

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Figure 2. Solution viscosity of Nylon 1010 aged at 120, 130, 140 and 160 ℃ as a function of time.

2.3. FTIR spectroscopy

The FTIR spectra (Fig. 3) of the aged films show an increase of absorbance in the range of 1780-1700 cm-1 with the prolonged time, which results from the formation and accumulation of carbonyl groups [27]. In data processing, the carbonyl index is defined to characterize the content change of the carbonyl groups during the thermo-oxidative aging process [5]. The carbonyl index is the ratio of the integrated carbonyl groups (the bands in the range of 1780-1700 cm-1) to the reference CH2 scissoring band (the band in the range of 1468-1458 cm-1), which is not sensitive to the thermo-oxidative aging [2]. Fig. 4 shows the carbonyl index as a function of time at different temperatures. The carbonyl index vs. time plots demonstrate an auto-retardation trend when the temperature is below 120 ℃, which is similar to the chromaticity value of b*.

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Figure 3. FTIR spectra of Nylon 1010 aged at 130 ℃ for different times.

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Figure 4. Carbonyl index of Nylon 1010 aged at 120, 130, 140 and 160 ℃ as a function of time.

2.4. UV spectroscopy

The UV spectra of the aged films are found to increase in the range of 250-450 nm with the increase of time (Fig. 5). The shoulder emerging at 280 nm is ascribed to the formation of conjugated structures, which are the chromophores ofdiscoloration of the aged films [19, 21, 22, 28]. From the curves of Fig. 5, we can see that there is an absorption peak at 280 nm even for the unaged sample, which is also found on the unaged Nylon 66 fibers as reported by Karstens and Rossbach [21]. This absorption peak is attributed to the oxidation products coming from the process when preparing the film samples by melt extrusion.

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Figure 5. UV spectra of Nylon 1010 aged at 130 ℃ for different periods of time.

Fig. 6 presents the absorbance at 280 nm as a function of time and temperature. Similar to the chromaticity value of b* and the data of carbonyl index, the absorbance increases with the increase of temperature. Besides, the absorbance vs. time plots show an auto-retardation shape when the temperature is below 120 ℃.

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Figure 6. UV absorbance at 280 nm as a function of time of Nylon 1010 aged at 120, 130, 140 and 160 ℃.

2.5. Elongation at break

Fig. 7 shows the elongation at break of the aged films as a function of time and temperature. The elongation at break decreases with the increase of temperature, which is similar to the change of the relative viscosity. At a given temperature, the elongation at break decreases quickly in the beginning and then decreases slowly. Finally, the decreasing rate becomes quick again. The decrease of the elongation at break is mainly due to the chain scissions, and the reason is similar to the relative viscosity.

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Figure 7. Elongation at break of Nylon 1010 aged at 120, 130, 140 and 160 ℃ as a function of time.

2.6. Time-temperature superposition analysis

The time-temperature superposition analysis based on the thermo-oxidative aging plots at different temperatures is a common method adopted in the study on polymer materials [29]. In our experiments, the lowest temperature (120 ℃) is used as the reference temperature, the time axis is multiplied by a factor at any other temperature, and the shift factor is that results in the best overlap between the experimental data. In result, a shift factor aT is obtained [30]. The activation energy at a given temperature can be calculated out according to the Arrhenius equation, and the time-temperature relationship can be analyzed.

Fig. 8 summarizes the thermo-oxidative aging results obtained via various analytical methods at different temperatures by using the time-temperature superposition analysis. It can be seen from this figure that all the plots are well overlapped, which indicates that the measured properties are closely correlated to the thermooxidative aging mechanism of Nylon 1010. The double logarithmic plots of the shift factors (αT) at different temperatures and the reciprocals of temperatures are presented in Fig. 9, based on which the activation energy calculated out according to the Arrhenius equation is given in Table 1.

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Figure 8. Thermo-oxidative aging results and shift factors obtained by using the time–temperature superposition analysis (a, b*; b, relative viscosity; c, carbonyl index; d, UV absorbance at 280 nm; e, elongation at break).

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Figure 9. Arrhenius plots of shift factors determined for b*, relative viscosity, carbonyl index, UV absorbance at 280 nm and elongation at break.

Table 1
Activation energy for the aged films determined by different analytical methods.

It can be seen from Table 1 that the activation energy of b* is close to that of the UV absorbance at 280 nm, that is, ~110 kJ mol-1. This is probably because the yellowing of Nylon materials results from the generation of conjugated compounds, whose UV absorbance at 280 nm changes accordingly. Therefore, the changes of the two results derive from the same chemical reaction during the thermo-oxidative aging process.

The activation energy of the relative viscosity is close to that of the break at elongation, that is, ~100 kJ mol-1. This is probably because the changes of the two results are related to the breakage of the polymer chains. The slightly higher activation energy of the break at elongation probably originates from the increase of crystallinity during the thermo-oxidative aging process, which therefore enhances the intermolecular interactions.

The activation energy of the carbonyl index is the lowest, which is in accordance with a previous study on the thermo-oxidative aging of Nylon 6 [5]. In that work the authors attributed the reason to the generation of large quantities of gaseous products at high temperatures. The gas is derived from the reactions of chain scission and alkoxy radicals, and both reactions generate the carbonyl but gaseous product formed from alkoxy radicals does not influence the relative viscosity, which leads to the activation energy of the carbonyl index is lower than that of the relative viscosity.

2.7. Relationship between results of different analytical methods

It should be noted that the determination of b* can be realized in field, so it is used as X axis and the other results, i.e., relative viscosity, carbonyl index, UV absorbance at 280 nm and elongation at break, are used as Y axis, respectively. The plots are shown in Fig. 10, from which we expect to grasp the changes of other properties according to b*, and finally realize the on-line detection of the thermo-oxidative aging. As seen from Fig. 10, the UV absorbance at 280 nm and carbonyl index increase with the increase of b*, while the relative viscosity and elongation at break decrease with the increase of b*.

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Figure 10. Relationship between the b* values and the physical and chemical properties (a, relative viscosity; b, carbonyl index; c, UV absorbance at 280nm; d, elongation at break).

It is seen from the relationship between b* and UV absorbance at 280 nm (Fig. 10c), the plots at different temperatures are overlapped in a fairly high degree, which indicates this relationship is not necessarily related to temperature. This is mainly because the yellowing of Nylon materials results from the generation of conjugated compounds [21, 22], and the UV absorbance at 280 nm is the reflection of the conjugated compounds generated in Nylon 1010.

The relative viscosity or elongation at break, instead, increases with the increase of temperature for a given b* (Fig. 10a and d). The reason is that the colorimetric values were determined on the sample surface and the viscosity or elongation is the characteristics of the bulk polymer. The higher the aging temperature, the greater the degreeof oxidation on the surface than the bulk of the polymer, therefore at the same b*, the decrease of the viscosity or elongation at break at the higher temperature is less than that at the lower temperature. This may be the reason why the activation energies determined from b* and UV absorbance at 280nm are slightly larger than that from the relative viscosityand elongation at break.

As to the relationship between b* and carbonyl index, the carbonyl index decreases with the increase of temperature for a given b*. Dong and Gijsman attributed this phenomenon to the rapid increase of the produced gas with the increase of temperature during the thermo-oxidative aging process [5].

3. Conclusion

The thermo-oxidative aging results were obtained via colorimetric evaluation, relative viscosity, carbonyl index, UV absorbance at 280nm and elongation at break at different temperatures by using the time-temperature superposition analysis. All the plots are well overlapped.Theactivation energies determined from b* and UV absorbance at 280nm are about 110kJmol-1, and that from the relative viscosity and the break at elongation are about 100kJmol-1. The activation energy from the carbonyl index is the lowest, which is 84kJmol-1. The relationship of the various analytical methods is studied, fromwhich we derive the changes of the physical and chemical properties at different b* values. Since the b* values can be quickly determined by using a portable spectrophotometer, the on-line evaluation of the thermo-oxidative aging of Nylon 1010 can be realized.

4. Experimental 4.1. Materials

Unstabilized and unfilled Nylon 1010 pellets were supplied by Yixing Chemicals (Yixing, China). The pellets were dried under vacuum for not less than 8h at 80 ℃ prior to processing. Nylon 1010 films were prepared from the pellets in a single screw extruder (Haake Polylab System, Rheomex 252p series) with a slit die (0.75mm × 50mm). The temperatures from the hopper to the die were 210, 230, 230 and 225 ℃, respectively. The films were stretched by using a roll drawing system just after the melt left the die. The temperature of the drawing rolls was 30 ℃. The width and thickness of the prepared films were ~20 and 0.3mm, respectively.

4.2. Thermo-oxidative aging tests

Nylon 1010 films were exposed in forced air venting ovens at temperatures between 120 ℃ and 160 ℃ for a pre-determined time. The films were regularly removed from the ovens and stored at ambient temperature in a desiccator containing silica gel before characterizations.

4.3. Characterizations and measurements

The colorimetric properties were obtained by using an SP 62 portable sphere spectrophotometer (X-Rite, USA) under illuminant D65 using 10 standard observers in terms of CIE lab values (L*, a*, b*).

The relative viscosity was determined by using a 1.1 mm Ubbelohde viscometer. Nylon 1010 (0.25 g) was dissolved in a solvent (50 mL) to form a solution. The solvent was 96 wt% aqueous solution of sulfuric acid. Both the solvent and the solution were filtered before testing. The relative viscosity was measured at 25 ℃.

Attenuated total reflection FTIR (ATR-FTIR) spectra were obtained at a resolution of 4 cm-1 at 32 scans per run, by using a Nicolet 6700 spectrometer (ThermoFisher Scientific, USA). The ATR accessory was equipped with a diamond crystal. Data were processed by a standard OMNIC software.

UV spectra were recorded on a TU-1905 UV-vis spectrophotometer (Beijing Purkinje General Instrument, China) in the range of 220-500 nm.

The elongation at break was measured by using a UTM-1432 universal mechanical tester (Chengde Jinjian Testing Instrument, China) at a crosshead speed of 50 mm min-1.

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