3.1 Identification of VOCs Compounds
The total ion chromatogram-mass spectrometry (TIC-MS) of the sample 1# is shown in Fig. 1, and the corresponding detected compounds have been summarized in Table 2. A total of 20 compounds could be identified, and from the point of view of chemical composition, these substances could be categorized based on different functional groups, such as alkanes, ketones, alkenes, alcohols, phenols, quinolines. Meanwhile, these 20 substances were compared with the results of raw materials (Supplementary Information (SI): Table 4: NR SPR 10#, Table 6: CIIR 1066, Table 7: CIIR 1301, and Table 8: resin SL1801) for traceability.
Table 2
The identified compounds detected from the sample 1#
| Retention time/min | Abundance | Substance | Boiling point/℃ |
Num. | (×107) |
1 | 1.44 | 126 | carbon dioxide | -78 |
2 | 1.50 | 56.6 | 3-methyl-pentane | 64 |
3 | 1.65 | 19.7 | methyl-cyclopentane | 72 |
4 | 1.84 | 128 | 2,4,4-trimethyl-1-pentene | 101 |
5 | 2.15 | 10.2 | acetone | 56 |
6 | 2.84 | 3.1 | 2-methyl-2-propanol | 83 |
7 | 3.26 | 6.2 | ethanol | 78 |
8 | 4.87 | 3.9 | octamethylcyclotetrasiloxane | 175 |
9 | 6.52 | 7.7 | 2-nonen-4-one | 201 |
10 | 6.72 | 3.9 | 1-methoxy-2-propanol | 118 |
11 | 7.25 | 5.3 | 4,4-dimethyl-heptane | 135 |
12 | 8.97 | 11.1 | 2,2,4,6,6-pentamethyl-3-heptene | 180 |
13 | 9.24 | 7.5 | 2,2,4,6,6-pentamethyl-3-heptene | 180 |
14 | 9.37 | 2.7 | (z)-5,5-dimethyl-2-hexene | 120 |
15 | 9.50 | 1.1 | 4-tert-butylcyclohexanone | 113 |
16 | 12.53 | 3.2 | —— | —— |
17 | 14.29 | 1 | butylated hydroxytoluene | 365 |
18 | 14.75 | 0.2 | benzothiazole | 231 |
19 | 14.99 | 4.9 | phenol | 182 |
20 | 15.84 | 3.8 | 1,2-dihydro-2,2,4-trimethylquinoline | 293 |
Except for the peak of air (carbon dioxide, 1.44 min), 2,4,4-trimethyl-1-pentene (1.84 min) was the most abundant substance. It was often used as organic synthesis intermediates for the production of octylphenol and nonylphenol (Chaudhuri and Sharma 1991). And it was observed in an appreciable quantity in resin SL1801 (Table 8). Therefore, it could be deduced that the source of 2,4,4-trimethyl-1-pentene was the alkylphenol resin SL1801. Similarly, 2,2,4,6,6-pentamethyl-3-heptene (8.97 min) were also detected in the resin SL1801 (Table 8). This may be introduced by the thermal decomposition of resin during processing, as the reaction scheme shown in Fig. 2 (a) (Fisher 2010). The side group will break off during the reaction and produce 2,2,4,6,6-pentamethyl-3-heptene. Besides, alkylphenol resin is easy to decompose at high temperatures resulting in the emergence of numerous small molecules, as seen in Table 8. These substances will inevitably be remained in the sample and could be detected when the material is prepared. However, other substances are more diverse, such as 2-nonen-4-one (6.52 min), (z)-5,5-dimethyl-2-hexene (9.37 min), 4-tert-butylcyclohexanone (9.50 min), etc., but their reaction pathway is not yet definite.
3-methyl-1-pentane (1.50 min) and methyl-cyclopentane (1.65 min) were also owed a high abundance in the results. As seen from Table 6 and Table 7, they were all found in the two types of chlorobutyl rubber (CIIR 1066 and 1301), which may be produced during the preparation of CIIR, as the requirement of using positively constructed alkane solvents to dissolve butyl rubber (Newman et al. 1981).
Butylated hydroxytoluene (BHT, 14.29 min), a white crystalline substance, was widely used in foods, rubber, and petroleum products (Yehye et al. 2015). Previous research has also reported that it is a potential pyrolysis product of sinapic acid in NR (Juntarachat et al. 2013). But the results of Table 6 indicated that it is more likely to be added directly to CIIR as an antioxidant (Babich 1982).
Benzothiazole (8.97 min) was a heterocyclic compound, which is often used in various products, previous studies have reported that it has been detected in NR (Hoven et al. 2003). In the rubber industry, it was used as vulcanization accelerating additives in rubber (Chung et al. 2011). In terms of structure, the decomposition of the accelerator DM is more suitable to explain the presence of benzothiazole, as shown in Fig. 2 (b).
Phenol (14.75 min), a colorless needle-like crystal with a characteristic odor. It comes from NR SPR10# and resin SL1801, as in line with the results of Table 4 and Table 8, respectively. In NR, it might result from the thermal degradation of lignin and fatty acid esters or microbial degradation of aromatic amino acids (Zhang et al. 2020) and in terms of resin SL180, it might stem from the decomposition products of the resin as the reaction scheme depicted in Fig. 2 (a).
From the perspective of structural similarity, it is believed that the 1,2-dihydro-2,2,4-trimethylquinoline is emerged by the decomposition of the antioxidant RD as shown in Fig. 2 (c). The antioxidant RD is a common additive that could provide better protection for rubber to resist thermal and oxidative aging.
Above all, among these detected substances, as the corresponding boiling points listed in Table 2, smaller molecular weight compounds with boiling points ranging from 56 to 200°C appear to be the domination, while some substances with higher boiling points like 1,2-dihydro-2,2,4-trimethylquinoline would also co-exist. However, due to complex reactions, although some substances are associated with raw materials, the reaction pathways are not be figured out yet.
3.2 Comparison of VOCs in resin and NR
Based on the above results, it makes sense to reduce the generation of VOCs by formulation adjustments. Therefore, NR and alkylphenol resin with more volatile substances were replaced by raw materials NR SVR 3L and resin T5600 to reduce the volatilization of VOCs. The test method is similar to the descriptions mentioned above, and the testing results of NR SVR 3L and alkylphenol resin T5600 are listed in Table 5 and Table 9, respectively.
Figure 3 shows the comparison of the abundance of NR SPR 10# and NR SVR 3L. Both types of NR mainly contain VFA and aldehydes with low molecular weight such as butyraldehyde, valeraldehyde isobutyraldehyde, and isovaleraldehyde. But it demonstrated that the abundance of TVOCs in SPR 10# was higher than that in SVR 3L. This difference was caused by the different production methods (coagulation process, drying process) and storage time in rubbers (Juntarachat et al. 2013). For example, SVR 3L is prepared from natural latex through filtration and coagulation processes, then drying and packing. SPR 10# is prepared from miscellaneous rubber containing sand and dust, which makes the NR SPR 10# have been oxidized and many impurities. The worse the grade of rubber, the more volatile substances (Hoven et al. 2003, Sakdapipanich and Insom 2006). It suggested that SVR 3L is more suitable to manufacture products with lower VOCs.
The use of tackifier resin in synthetic elastomers was for achieving the desired level of tackiness in a rubber compound (Lattimer et al. 1989). Figure 4 shows the comparison of the abundance of VOCs in resin SL1801 and resin T5600, especially for 2,4,4-trimethyl-1-pentene, 2,2,4,6,6-pentamethyl-3-heptene, due to their significant contribution to the volatilization of TVOCs in the inner liner. It can be seen that resin T5600 has a lower abundance of VOCs, which also indicated that T5600 resin was more conducive to the manufacture of products with lower VOCs.
3.3 VOCs via formulation adjustments
The above analysis resulted in the following formulation improvements as shown in Table 3. Firstly, based on the initial formulation 1#, NR SPR 10# was replaced by the NR SVR3L and was named as 2#; then based on 2#, the alkylphenol resin SL1801 was replaced by resin T5600, which were named as 3#. It should be noted that the processing of materials, preparation of vulcanizates, sample pre-treatment, and VOCs analysis for 2#, 3# were kept as the same with 1#.
Table 3
The formulation and corresponding mass (g) after raw materials replacements
Materials | 1# | 2# | 3# |
NR SPR 10# | 13 | 0 | 0 |
NR SVR 3L | 0 | 13 | 13 |
resin SL1801 | 5 | 5 | 0 |
resin T5600 | 0 | 0 | 5 |
CIIR 1066 | 58 | 58 | 58 |
CIIR 1301 | 29 | 29 | 29 |
RD | 2 | 2 | 2 |
SA | 1 | 1 | 1 |
ZnO | 3 | 3 | 3 |
DM | 1 | 1 | 1 |
Sulfur | 1 | 1 | 1 |
Environment-friendly oil | 19 | 19 | 19 |
Carbon black N660 | 58 | 58 | 58 |
Total | 191 | 191 | 191 |
Figure 5 shows the abundance of TVOCs after formulation changing according to Table 3. After changing the brand of NR, it had a positive effect on the reduction of TVOCs. And as expected, after changing the different brand of alkylphenol resin in 3#, the abundance of TVOCs continued to decrease and presented the lowest in the three samples. Compared with the initial sample 1#, TVOCs in 3# have achieved a target reduction of 20%. Above all, from the point of view of environmental protection, through the change of raw materials, TVOCs discharged in inner liner could be reduced.