Depletion Analysis of Antidegradants

Total 25–200 °C 200–550 °C 550–800 °C

1 74,66 1,26 52,11 21,29 25,71 materials was found to be roughly 0,70–1,55 weight percent (%). The weight percent of polymers in the rubber compounds 1, 2, 5, and 6 was found between 52–55 wt %. Carbon black was about 12–13 wt % (rubber compound 5), 21–22 wt % (rubber compound 1 and 2), and 31–32 wt % in the rubber compound 6. The weight percent of silica and zinc oxide was found to be roughly about 14–15 wt % (rubber compound 6), 23–25 wt % (rubber compound 1 and 2), and 32–23 wt % in the rubber compound 5. The differences between the results might be due to the amount of phr the ingredients in the rubber compounds. For example, rubber compound 5 had a high amount of silica which can be seen in a high percentage mass changes in the residue. Then again, rubber compound 6 had a high amount of phr of carbon black in the recipe that can be seen in the low amount of residue, as can be seen in Table 9.

6.6 Depletion Analysis of Antidegradants

Antidegradant depletion in the rubber compounds was analyzed by GC-MS following the ISO 10638 standard. The testing results were obtained by analyzing the amount of 6PPD and TMQ antidegradants in the solvent extract of rubber compounds 1, 2, 5, and 6. Table Table 9. Decomposition of polymers and fillers of rubber compounds 1, 2, 5, and 6 at 90 °C temperature for heat aging times of 7 days.

10 illustrates the ingredients differences between the rubber compounds. The results of rubber compounds 1, and 2 are not included in this section due to the short measurement period as well as measurement error caused by the GC-MS.

Table 10. The ingredients (phr) of rubber compounds 1, 2, 5, and 6.

Ingredients 1 2 5 6

Natural rubber 33,00 33,00 33,00 33,00

Polybutadiene rubber (cis) 37,00 37,00 37,00

Polybutadiene rubber (vinyl) 37,00

Styrene-butadiene rubber (1) 41,27 41,27 41,27 41,27

Resins & Oils 1:1,6 1:1,6 1:1,6 1:1,6

Carbon black & silica 1:1,2 1:1,2 1:3,1 1:0,4 Organosilane 8 w% from phr of other ingredients stays the same

phr

After the measurement of antidegradants, the antidegradant type was identified by the mass spectrum. The characteristic peaks of each antidegradant were indicated by the mass spectra and gas chromatograms following the ISO 10638 standard. Table 11 shows the mass-to-charge ratios of 6PPD and TMQ antidegradants in the mass spectrum.

An example of the gas chromatograms and mass spectrum of 6PPD and TMQ and ti-degradants for rubber compound 5 at a temperature of 90 °C and the aging time of 7 days are shown in this section. The peak of 6PPD was found at about 20,38 min, and TMQ at 22,84 in the chromatogram, as presented in Figure 46 and Figure 47.

Table 11. The value of mass/ charge ratio (s) m/z of 6PPD, and TMQ antidegradants.

Anti-degradants

6PPD 211 268

TMQ 158 173

Characteristic mass/ charge ratio (s) m/z

Figure 46, and Figure 47 show the detector signal intensity (mV) as a function of the retention time (min). The relative concentration of each antidegradant in the mixture of the rubber compound can be identified using a gas chromatogram. Figure 46 indicates the first intensity peak of 6PPD at about 20,4 minutes.

Figure 48, and Figure 49 show the mass spectrum of the ion signal abundance (%) versus the mass-to-charge ratio of 6PPD antidegradant. The mass spectrum was used to identify each characteristic peak of 6PPD and TMQ antidegradants. The peaks of 6PPD are located in the mass spectrum of the mass-to-charge ratio of 211 and 268. The peaks of TMQ are located in the mass spectrum 158 and 173 in m/z, as displayed in Figure 49.

Figure 46. The intensity (mV) versus time (min) of gas chromatograms for 6PPD antidegradant at 90 °C and the aging time of 7 days.

Figure 47. The intensity (mV) versus time (min) of gas chromatograms for TMQ antidegradant at 90 °C and the aging time of 7 days.

Intensity [mV]

Time [min]

The concentration of antidegradants in the rubber compound was studied with GC-MS at different aging temperatures and periods. Figure 50 provides the mass reduction of 6PPD antidegradant versus the aging time and the storage time in the rubber compounds 5, and 6. As displayed in Figure 50 (a-d) the mass amount of 6PPD decreased measurably at 90 °C. Rising the aging temperature from 70 °C to 90 °C, the amounts of 6PPD decreased significantly. 6PPD antidegradant decreased by 6,5 mg/g, 4,4 mg/g and 2,8 mg/g at 90 °C correspondingly. A temperature rise might help the antidegradant to migrate from the middle to the outer layer of the rubber compound reacting by the oxygen simultaneously. However, Figure 50 (b and d) shows that the 6PPD decreased in the same way with an increase of storage time regardless of the aging temperature.

In addition, the 6PPD amounts in the un-aged rubber compounds 5, and 6 differ from Figure 48. The relative abundance (%) against mass/charge ratio (m/z) of 6PPD antidegradant at 90 °C and the aging time of 7 days.

m/z Abundance [%]

Figure 49. The relative abundance (%) against mass/charge ratio (m/z) of TMQ antidegradant at 90 °C and the aging time of 7 days.

Abundance [%]

m/z TMQ

each other. 6PPD mass amounts are higher at 90 °C than other heat-aging temperatures. This might be due to the measurement order.

Figure 51 shows the mass reduction of TMQ antidegradant versus the aging time and storage time in the rubber compounds 5, and 6. As displayed in Figure 51 (a-d), the mass amount of TMQ decreased measurably at 90 °C. For instance, TMQ decreased by 4,5 mg/g, and 4,6 mg/g for 7 and 14 days at 80 °C, and 70 °C. Since the aging time and aging temperature are vital parameters for the depletion of TMQ and 6PPD antidegradants in the rubber compounds. Though, the TMQ’s mass depletion of rubber compound 6 decreases against the storage time regardless of the aging temperature.

The mass reduction of rubber compound 6 does not follow the same linear line at 90 °C (see Figure 51d). TMQ’s mass reduction falling points decreasing faster at 90 °C compared to other aging temperatures. This is due to the measurement order, the aging temperatures, and the aging times. In addition, the TMQ amounts in the un-aged rubber compounds 5, and 6 differ from each other. TMQ mass amounts are higher at 90 °C than other heat-aging temperatures. The temperature increase accelerates the oxidation process of rubber compounds following the reduction of TMQ.

Figure 50. Mass (mg) reduction against (a and c) the aging time (d) at a temperature of 90 °C (circle), 80 °C (square), and 70 °C (triangle) as well as (b and d) the storage time (d) of 6PPD antidegradant for rubber compounds 5 and 6.

c) d)

a) b)

To sum up, the mass reduction of antidegradants decreased with the increase of tem-peratures. The mass depletion of both antidegradants was faster in the vulcanized rub-ber compound filled with the high amounts of silica fillers (rubrub-ber compound 5) than the high amounts of filled carbon black fillers (rubber compound 6). Due to the interaction between the antidegradants and silica, the antidegradants’ amounts decreased in the silica-filled vulcanized rubber compounds [47]. Mass reduction of both 6PPD and TMQ decreased with the increase in the aging temperatures. Generally, low-molecular-weight antidegradants migrate more quickly than high-molecular-weight antidegradants. The mass reduction of 6PPD was faster than the mass reduction of TMQ due to the fast diffusion of 6PPD. However, the mass reduction points of TMQ decreased rapidly in the vulcanized rubber compound 5 at 90 °C, as shown in Figure 51. The rapid mass reduc-tion might be due to TMQ’s extractability, volatility, impurities as well as temperature [66].

Additionally, an increase in the temperature accelerates the oxidation process in the rub-ber compound [60].

Figure 51. Mass (mg) reduction against (a and c) the aging time (d) at a temperature of 90 °C (circle), 80 °C (square), and 70 °C (triangle) as well as (b and d) storage time (d) of TMQ antidegradant for rubber compounds 5 and 6.

a) b)

c) d)