METHOD FOR ANALYZING THE LENGTH OF SULFUR CROSSLINKING BONDS IN A VULCANIZED RUBBER

Abstract

The present invention relates to a method for analyzing the length of sulfur crosslinking bonds in a vulcanized rubber using NMR spectrum.

Claims

1. A method of analyzing the length of sulfur crosslinking bonds in a vulcanized rubber comprising the steps of: 1) obtaining independently a .sup.13C NMR spectrum of a standard substance containing carbon atoms, and a .sup.13C NMR spectrum of the vulcanized rubber under the same condition; 2) obtaining independently FID (free induction decay) amplification values of characteristic peaks in the NMR spectrum of the standard substance and in the NMR spectrum of the vulcanized rubber; and 3) measuring the length of sulfur crosslinking bonds in the vulcanized rubber according to the following Equation:
Length of sulfur crosslinking bonds in the vulcanized rubber=2×(A/B)×(C/(D×E))  [Equation 1] in the equation 1 above, A is the number of moles of sulfur atoms in the vulcanized rubber used to obtain the NMR spectrum of the vulcanized rubber, B is the FID amplification value of the vulcanized rubber, C is the FID amplification value of the standard substance, D is the number of atoms corresponding to the characteristic peak of the standard substance in a molecule of the standard substance, and E is the number of moles of the standard substance used to obtain the NMR spectrum of the standard substance.

2. The analysis method according to claim 1, wherein the same condition refers to a condition in which, when performing NMR measurement, the number of scans, the delay time, the pulse width, the pulse power, the receiver gain, the spinning rate, and the temperature are the same.

3. The analysis method according to claim 1, wherein the standard substance is hexamethylbenzene.

4. The analysis method of claim 3, wherein the characteristic peak in the NMR spectrum of the standard substance is a peak of methyl group.

5. The analysis method according to claim 1, wherein the method further comprises a step of measuring the number of moles of crosslinking points per gram of the vulcanized rubber according to the following Equation 2:
Number of moles of crosslinking points per gram of the vulcanized rubber (mol/g)=B×((D×E)/C)/F  [Equation 2] in the equation 2 above, B is the FID amplification value of the vulcanized rubber, C is the FID amplification value of the standard substance, D is the number of atoms corresponding to the characteristic peak of the standard substance in a molecule of the standard substance, E is the number of moles of the standard substance used to obtain the NMR spectrum of the standard substance, and F is the mass of the vulcanized rubber used to obtain the NMR spectrum of the vulcanized rubber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1 schematically shows the types of sulfur crosslinking bonds in the vulcanized rubber in the present invention.

[0056] FIG. 2 shows the extraction result of the FID amplification values for the second sample in the example of the present invention.

[0057] FIG. 3 shows NMR spectra measured to verify the results analyzed in the examples of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0058] Hereinafter, preferred examples will be presented to aid in understanding of the present invention. However, the following examples are provided for illustrative purposes only, and the content of the present invention is not limited by these Examples.

[0059] In the following examples, Agilent INOVA 400 MHz SSNMR (using T3 SSNMR probe) was used for NMR spectrum, and Agilent's Vnmrj 4.2 software was used to measure the FID amplitudes values.

Preparation Example: Preparation of Samples

[0060] A rubber having a weight ratio between NR/SBR/BR of 10:58:32 was prepared, and 2 types of vulcanized rubbers were prepared by a vulcanization reaction in which the content of sulfur added was varied. The sulfur contents of each vulcanized rubber were measured by IC analysis and found to be 1.22 wt % and 1.20 wt %, respectively, and these were used as the first and second samples below.

Example 1

[0061] A .sup.13C SSNMR spectrum for 49.4 mg of hexamethylbenzene (HMB) was obtained as an external standard under the following conditions. [0062] pulse power(tpwr)=61 [0063] pulse width(pw)=90 degree pulse (3.00 usec) [0064] ax90=3800 [0065] delay time=10 sec [0066] number of scans=15000 [0067] receiver gain=60 [0068] spinning rate=3 kHz [0069] Temperature=80° C.

[0070] The FID amplification value was obtained for the peak corresponding to the methyl group of HMB in the obtained .sup.13C SSNMR spectrum, and the result thereof is shown in Table 1 below.

[0071] Subsequently, a .sup.13C SSNMR spectrum for 54.9 mg of the first sample was obtained under the same conditions as above. In the NMR spectrum thereof, the FID amplification value for the characteristic peak of alpha-methine, which is a crosslinking point, was extracted, and the result is shown in Table 1 below.

[0072] The measured results were calculated as shown in Table 1, and the average length of sulfur crosslinking bonds the first sample was analyzed.

TABLE-US-00001 TABLE 1 Mass of the first sample 54.9 mg Sulfur content in the first sample 1.22 wt % (A) # of moles of sulfur atoms in the first 2.09 × 10.sup.−5 mol sample (B) FID amplification value of the first sample 6.09612 Mass of HMB 49.4 mg Molecular weight of HMB 162.14 g/mol (C) FID amplification value of HMB 2770.08 (D) # of methyl groups in HMB 6 (E) # of moles of HMB 0.000305 mol # of moles of crosslinking points in the first sample = B × ((D × E)/C)/(54.9 mg) = 7.33 × 10.sup.−5 mol/g Average length of sulfur crosslinking bonds in the first sample = 2 × (A/B) × (C/(D × E)) = 10.4

Example 2

[0073] A .sup.13C SSNMR spectrum for 56.7 mg of the second sample was obtained under the same conditions as in Example 1. In the NMR spectrum thereof, the FID amplification value for the characteristic peak of alpha-methine, which is a crosslinking point, was extracted, and the result is shown in FIG. 2 and Table 2 below. As for the FID amplification value for the standard substance, the value previously obtained in Example 1 was used.

[0074] The measured results were calculated as shown in Table 2, and the average length of sulfur crosslinking bonds in the second sample was analyzed.

TABLE-US-00002 TABLE 2 Mass of the second sample 56.7 mg Sulfur content in the second sample 1.20 wt % (A) # of moles of sulfur atoms in the second 2.13 × 10.sup.−5 mol sample (B) FID amplification value of the second 5.06992 sample Mass of HMB 49.4 mg Molecular weight of HMB 162.14 g/mol (C) FID amplification value of HMB 2770.08 (D) # of methyl groups in HMB 6 (E) # of moles of HMB 0.000305 mol Number of moles of crosslinking points in the second sample = B × ((D × E)/C)/(56.7 mg) = 5.90 × 10.sup.−5 mol/g Average length of sulfur crosslinking bonds in the second sample = 2 × (A/B) × (C/(D × E)) = 12.7

Experimental Example

[0075] In order to verify the results obtained in Examples 1 and 2 above, the verification was conducted by the method described in the literature (Polymer Testing 29 (2010) 953-957).

[0076] The literature relates to a method of estimating the cavity size by using toluene as a probe and measuring the degree of a chemical shift of toluene trapped in a cavity of natural rubber. The trapped toluene has a smaller difference in the chemical shift with untrapped toluene as the cavity size in natural rubber increases. Conversely, the smaller the cavity size in natural rubber, the greater the difference in the chemical shift with the untrapped toluene.

[0077] The number of moles of the crosslinking points and the average length of the sulfur crosslinking bonds in the first and second samples used in Examples 1 and 2 can correspond to the cavity size signified in the literature above. Accordingly, the NMR spectra were obtained according to the method described in the literature above, and the results are shown in FIG. 3. As shown in FIG. 3, the difference in the chemical shift of the second sample was smaller than that of the first sample, suggesting that the cavity size of the second sample is larger than that of the first sample. These results are consistent with the results showing that the number of moles of cross-linking points of the second sample is relatively smaller than that of the first sample and that the average length of sulfur crosslinking bonds in the second sample is relatively larger than that of the first sample measured in the previous examples.