Abstract
Solid phase extraction (SPE) was used to separate pyrolysates of rubber blends. Then each rubber polymer in rubber blends was identified, based on interpreting infrared spectrum of separated pyrolysates. By using this method, nature–ethylene–propylene, ethylene–propylene–silicone, butyl–styrene–butadiene and ethylene–propylene–butadiene–acrylonitrile rubber blends were analysed. The analytical results show that each characteristic pyrolysate of polymer in rubber blend pyrolysate can be separated by SPE. The method for identification of rubber polymers in rubber blends by infrared coupled with SPE is flexible, rapid and low cost, compared with the method by pyrolysis gas chromatography coupled with infrared spectroscopy or mass spectroscopy.
Introduction
Since the performance of rubber products can be improved by simple blending of commercially available individual rubber in order to fulfil various needs of industry, rubber blends occupy more and more market shares.1–3 The physical and mechanical properties of rubber blends are sensitive to the types of individual polymer. Therefore, there is a need to establish a variety of analytical methods to identify polymer composition in rubber blends. There are some methods to identify polymer in individual rubber system, such as British Standard ‘Identification of rubbers by infra-red spectrometry’ and China Standard GB/T 7764-2001 ‘Rubber-identification – infra-red spectrometric method’. However, for cured rubber blends, these methods are not applicable in most cases, especially in the case that the proportion of minor component in rubber blend is less than 20% (m/m). Recently, pyrolysis gas chromatography coupled with mass spectroscopy or infrared spectroscopy has been used extensively for qualitative identification of polymer blends. 4 , 5 In this paper, solid phase extraction (SPE) was used to separate pyrolysates of rubber blend, and then infrared spectra of separated pyrolysates were identified according to China Standard GB/T 7764-2001 (Rubber-identification – Infra-red spectrometric method). By using this method, nature–ethylene–propylene, ethylene–propylene–silicone, butyl–styrene–butadiene and ethylene–propylene–butadiene–acrylonitrile rubber blends were analysed. The analytical results show that pyrolysates of similar even chemical construction such as pyrolysates of nature rubber and pyrolysate of ethylene–propylene rubber could be separated, using reversed phase SPE such as C18 and cross-linked polystyrenes sorbent. Therefore, the separations of other pyrolysates of binary rubber blends are beyond question. Compared with pyrolysis gas chromatography coupled with mass spectroscopy or infrared spectroscopy, this method is low cost, flexible and time saving. So the method will have many potential users as it will work for quality comparisons between batches of known materials. Besides, because time is the top priority for customs tasks, it is very a suitable method for customs to classify rubber samples according to their harmonised system code.
Experimental
Materials
SPE was performed on Sep-Pak C18, 1 g, 6 cc.
Nature–ethylene–propylene (90∶10, m/m), butyl–styrene–butadiene (85∶15, m/m) and ethylene–propylene–butadiene–acrylonitrile (10∶85, m/m) were delivered by Jingan District Rubber Manufacture of Fuzhou. Ethylene–propylene–silicone was made by mixing ethylene–propylene and silicone with proportion of 9∶1 (m/m).
The solvents (methanol, toluene, isopropanol and hexane) were of AR grade from Merck.
Instrument
Shimadzu FTIR8400S (scan range: 4000-400 cm−1; scan time: 32, resolution: 4 cm−1) was used in this work (Shimadzu Corporation, Kyoto, Japan).
Analytical procedures
Sample pretreatment and SPE column solvation
Rubber blend was cut into small pieces with maximum size 2 mm. Approximately 0·5 g small pieces of rubber were wrapped in filter paper. The wrapped rubbers was extracted in Soxhlet extraction apparatus for 24 h with acetone–chloroform azeotrope (Vacetone/Vchloroform = 1∶1). After extraction, the rubbers were dried in oven at 80°C to remove the solvents.
Extracted rubber of 0·2 g was placed into a pyrolysis tube. The pyrolysis tube was heated by alcohol lamp to obtain pyrolysate of rubber blends.
For each C18 SPE column, about 8 mL toluene, 8 mL isopropanol, 8 mL methanol and then 8 mL distill were used to solvate the column respectively.
SPE separation procedures
About 50 mg of pyrolysate was loaded onto the column. Methanol (8 mL), methanol/toluene (Vmethanol/Vtoluene = 9∶1, 8 mL), methanol/toluene (Vmethanol/Vtoluene = 7∶3, 8 mL), methanol/toluene (Vmethanol/Vtoluene = 3∶7, 8 mL), methanol/toluene (Vmethanol/Vtoluene = 1∶9, 8 mL) and toluene (8 mL) were used as eluants respectively at a flowrate of about 2 drop/min for nature–ethylene–propylene pyrolysate. After separating, six eluates were obtained. Then, the eluates were evaporated in a nitrogen stream to remove the solvents. For ethylene–propylene–butadiene–acrylonitrile and butyl–styrene–butadiene pyrolysate, methanol (8 mL), methanol/toluene (Vmethanol/Vtoluene = 9∶1, 8 mL), methanol/toluene (Vmethanol/Vtoluene = 7∶3, 8 mL), methanol/toluene/isopropanol (Vmethanol/Vtoluene/Visopropanol = 1∶1∶0·2) and toluene were used as eluants respectively. For ethylene–propylene–silicone pyrolysate, methanol (8 mL), toluene (8 mL) and hexane (8 mL) were used as eluants respectively.
Results and discussion
Nature–ethylene–propylene rubber blend
Figure 1 indicates that no characteristic peaks for ethylene–propylene pyrolysate were detected. Therefore, if only on the basis of pyrolysate infrared spectrum of nature–ethylene–propylene rubber blend, it was impossible to tell the existence of the minor component of ethylene–propylene polymer in nature–ethylene–propylene rubber blend.
Infrared spectrum of pyrolysate of vulcanizates: nature–ethylene–propylene rubber blend
Figure 2 was the same as Fig. A2 (pyrolysate of vulcanizates: isoprene rubber) in GB/T 7764-2001. And Fig. 3 was almost the same as Fig. A14 (pyrolysate of vulcanizates: ethylene–propylene rubber) except in the positions of about 1086 and 1260 cm−1 where there are two extra small peaks. These small peaks are attributed to remnant of silane reagent from SPE itself, which could be demonstrated by Fig. 4 (infrared spectra of the eluate eluted by toluene or hexane, Blank Sep-Pak C18 SPE).
Infrared spectrum of eluate eluted by methanol/toluene (Vmethanol/Vtoluene = 3∶7) (pyrolysate of vulcanizates: nature–ethylene–propylene rubber blend)
Infrared spectrum of eluate eluted by toluene (pyrolysate of vulcanizates: nature–ethylene–propylene rubber blend)
Infrared spectrum of eluate eluted by hexane (blank Sep-Pak C18 column, 1 g, 6 cc)
Above analytical results show that characteristic pyrolysates for both nature rubber and ethylene–propylene rubber can be separated by SPE column. Hence, each polymer in nature–ethylene–propylene rubber blend can be identified by pyrolysis→SPE separation→infrared spectra analysis.
Ethylene–propylene–butadiene–acrylonitrile rubber blend
Figure 5 indicates that no characteristic peaks for ethylene–propylene pyrolysate were detected. Therefore, according to GB/T 7764-2001 method, it was impossible to tell existence of the minor component of ethylene–propylene polymer in ethylene–propylene–butadiene–acrylonitrile rubber blend.
Infrared spectrum of pyrolysate of vulcanizate: ethylene–propylen–butadiene–acrylonitrile rubber blend
Figure 6 was the same as Fig. A6 (pyrolysate of vulcanizates: butadiene–acrylonitrile rubber) in GB/T 7764-2001, and Fig. 7 was the same as Fig. A14 (pyrolysate of vulcanizates: ethylene–propylene rubber) in GB/T 7764-2001.
Infrared spectrum of eluate eluted by methanol (pyrolysate of vulcanizate: ethylene–propylen–butadiene–acrylonitrile rubber blend)
Infrared spectrum of eluate eluted by methanol/toluene/isopropanol (Vmethanol/Vtoluene∶Visopropanol = 1∶1∶0·2) (pyrolysate of vulcanizate: ethylene–propylene–butadiene–acrylonitrile rubber blend)
These analytical results show that characteristic pyrolysates for both butadiene–acrylonitrile rubber and ethylene–propylene rubber can be separated by SPE column. Hence, each polymer in ethylene–propylene–butadiene–acrylonitrile rubber blend can be characterised by pyrolysis→SPE separation→infrared spectra analysis.
Butyl–styrene–butadiene rubber blend
Figure 8 indicates that no characteristic peaks for styrene–butadiene pyrolysate were detected. Therefore, according to GB/T 7764-2001 method, the minor component of styrene–butadiene polymer in butyl–styrene–butadiene rubber blend cannot be detected.
Infrared spectrum of pyrolysate of vulcanizate: butyl–styrene–butadiene rubber blend
Figure 9 was the same as Fig. A4 (pyrolysate of vulcanizates: styrene–butadiene rubber), in GB/T 7764-2001 and Fig. 10 was the same as Fig. A10 (pyrolysate of vulcanizates: butyl rubber) in GB/T 7764-2001.
Infrared spectrum of eluate eluted by methanol (pyrolysate of vulcanizates: butyl–styrene–butadiene rubber blend)
Infrared spectrum of eluate eluted by methanol/toluene/isopropanol (Vmethanol/Vtoluene/Visopropanol = 1∶1∶0·2) (pyrolysate of vulcanizate: butyl–styrene–butadiene rubber blend)
The analytical results show that characteristic pyrolysates for both butyl rubber and styrene–butadiene rubber can be separated by SPE column. Hence, each polymer in butyl–styrene–butadiene rubber blend can be distinguished by pyrolysis→SPE separation→infrared spectra analysis.
Ethylene–propylene–silicone rubber blend
Figure 11 indicates that both characteristic peaks for ethylene–propylene pyrolysate and silicone pyrolysate were detected. Therefore, if the minor component of pyrolysate has a strong infrared absorbance peaks, it is possible to identify it in the vulcanizate of binary rubber blend just according to its pyrolysate infrared spectrum (GB/T 7764-2001 method).
Infrared spectrum of pyrolysate of vulcanizate: ethylene–propylene–silicone rubber blend
Figure 12 was the same as Fig. A25 (pyrolysate of vulcanizates: silicone rubber) in GB/T 7764-2001, and Fig. 13 was almost the same as Fig. A14 (pyrolysate of vulcanizates: ethylene–propylene rubber) in GB/T 7764-2001 method, except in the positions of about 1086 and 1260 cm−1 where there are two extra small peaks. These small peaks are attributed to remnant of silane reagent from SPE itself, which could be demonstrated by Fig. 4.
Infrared spectrum of eluate eluted by methanol (pyrolysate of vulcanizate: ethylene–propylene–silicone rubber blend)
Infrared spectrum of eluate eluted by hexane (pyrolysate of vulcanizate: ethylene–propylene–silicone rubber blend)
With all the above analytical results, the following conclusions could be obtained. First, most pyrolysates of binary rubber blends could be separated in order to get each polymer's characteristic pyrolysate just by using a series of mixture solvents of methanol, toluene, isopropanol and hexane with reversed SPE column. Second, if toluene or hexane is used as eluants, when interpreting the pyrolysate infrared spectrum, two small peaks at 1100 and 1256 cm−1 should be attributed to remnant of silane reagent from SPE itself. Third, positive phase SPE columns such as silica and alumina, are not able to separate pyrolysates of binary rubber blends, but C18 and various brands of polymer (cross-linked polystyrene) SPE columns are the most suitable columns. Fourth, the time to process rubber pyrolysis should not be too long, and separation of pyrolysate should be done after pyrolysis. In addition, infrared spectra of pyrolysate also need to be obtained immediately after separation.
Conclusions
GB/T 7764-2001, BS 4181-1984 and ASTM D2702-1994 are very convenient methods to identify polymer in individual rubber system. However, it is hard to use them to identify polymers of binary rubber blends especially when one of component proportion is less than 20% (m/m). From this study, we draw the conclusions that each kind of polymer's characteristic pyrolysate in rubber blend pyrolysate could be separated by using SPE, and the separated efficiency of SPE is good enough for the separation of each characteristic pyrolysate. Hence, by coupling with SPE, GB/T 7764-2001 could be extended to identify the polymers in binary rubber blends. In addition, the method for identification of polymers in rubber blends by infrared coupled with SPE is flexible, rapid and low cost, compared with the method by pyrolysis gas chromatography coupled with infrared or mass chromatography.
Footnotes
Acknowledgements
We wish to thank Zhiyuan Yining Foundation for the financial support.