An Interpretation of the Identification Method of Styrene Elastomers
Thermoplastic elastomer TPE and styrene elastomer analysis technology
Thermoplastic elastomer TPE has high elasticity, high strength, high resilience, and the characteristics of injection molding extrusion processing. Compared with PVC, it is environmentally friendly, non-toxic and safe, has a wide range of hardness, excellent weather resistance, fatigue resistance and temperature resistance, and can be recycled. Therefore, TPE has a very wide range of uses, such as cables, shoe materials, rubber-coated materials, medical products, etc.
TPE product formulations basically contain elastomers, white oils, modified resins (PP, PE, PPO, PS), flame retardants, fillers, antioxidants, other processing aids and pigments, etc. As elastomers, commonly used styrene elastomers are SBS and SEBS. Since the raw materials produced by different manufacturers have different styrene content and molecular weight, these differences will inevitably lead to differences in product performance. How to quickly select the right elastomer when getting the sample is an important step to shorten the development cycle. The following will introduce in detail how to use microspectroscopy large-scale instrument analysis to help you quickly select the right styrene elastomer.
1. Use infrared spectroscopy to distinguish elastomer types
SBS is a styrene-butadiene-styrene block copolymer, and SEBS is the product of SBS hydrogenation. The specific structure is as follows:
In the infrared spectrum, both have peaks of styrene segments, the difference is that in SBS, 1640cm-1, 911cm-1 and 966cm-1 are characteristic peaks of butadiene, while hydrogenated SEBS has no characteristic peaks of butadiene double bonds. Due to the presence of methyl (CH3), there is a characteristic peak at 720cm-1.
Second, use NMR to distinguish elastomer types
The positions of different hydrogen have different chemical shifts. The proton peak on the double bond of butadiene in SBS is 5.0-5 ppm, while the characteristic peak of hydrogen on the double bond does not exist in the hydrogenated SEBS. The chemical shift of CH3 proton appears at 0.8-0 ppm.
III. Using NMR to calculate the content of PS segments in SBS and SEBS
6.4-7 ppm is the chemical shift of protons on the benzene ring in SBS and SEBS. After the attribution of each proton in SBS and SEBS is clear, the integrated values of various protons can be obtained from the NMR spectrum. According to the principle of nuclear magnetic resonance, the integrated area is proportional to the content of protons, so that the integrated values of various protons can be obtained. For example, if the molar fraction of PS is Mps, then Mps = Aps/(Aps + A/4), where Aps is the integral diagram of the proton area on the benzene ring, and A is the area integral diagram of the protons except the benzene ring.
According to this method, the content of polystyrene in SBS and SEBS was calculated to be 31.0% and 32.8%, respectively.
4. Calculation of polybutene-1 structural unit content in SEBS by NMR
The soft segment of polybutadiene in SBS has cis and trans-1,4-structures and 1,2-structures. The former hydrogenates into a carbon chain structure without side groups (the same as polyethylene structure), and the latter hydrogenates into a butene-1 structure. Only when the content of 1,2-structures in the soft segment is greater than 40% and it is in a random sequence with the 1,4-structure, when hydrogenated into a random copolymer of ethylene and butene-1, can the high elasticity of SEBS be guaranteed.
Since polybutene-1 contains -CH3, but there is no peak of 0.84 ppm in the NMR spectrum of SBS, it means that -CH3 is formed after the hydrogenation of the 1,2-structure, so the content of polybutene-1 structural units in SEBS can be calculated according to the ratio of the area integral of -CH3 to the integration of other protons. Let the molar fraction of polybutene-1 be M butene-1, then there are: M butene-1 = A-CH3/(A-CH3 + A/4), where: A-CH3 is the proton area integral value of -CH3, and A is the area integral value of the remaining protons.
V. Quantification of SBS and SEBS by NMR
The blend system of SBS and SEBS can be semi-quantified by taking advantage of the different peaks of SBS and SEBS in the NMR spectrum. Since the most and most widely used grades of SBS and SEBS produced by different manufacturers are the grades with a styrene content of 30%, it can be assumed that the grade with a styrene content of 30% used in the sample can be quantitatively calculated. The test value for the ratio of SBS to SEBS calculated by the integral area is 0.2446 (theoretical value is 0.25%).
6. Use GPC to select grades of SBS and SEBS
After determining whether SBS or SEBS is used in a sample and adding the ratio, it is necessary to face the problem of which brand to use. In simple terms, if SEBS is divided according to the molecular weight, it can be divided into low molecular weight, medium molecular weight, high molecular weight and ultra-high molecular weight. Table 1 is a comparison table of different specifications of international and domestic SEBS in the industry. You can roughly choose the brands of different manufacturers according to their use of the same molecular weight.
In addition, through the molecular weight test, the molecular weight of the selected elastomer can be well judged in what range, and then compared with the molecular weight of the measured known brand in the database, the appropriate raw material can be selected more accurately. (PS: The molecular weight test is calibrated according to the polystyrene marking, and the test values of different manufacturers may be deviated).
Comparison table of international and domestic SEBS specifications
| SEBS grade | Taiwan Oak | Kerten | Baling Petrochemical | DENASSO | Kuraray | Asahi Kasei |
|---|---|---|---|---|---|---|
| 6150 | 6150 | G-1650 | YH-501 | H 6120 | S-8004 | H1077F |
| 6151 | 6151 | G-1651 | YH-503 | H 6170 | S-8006 | H1285 |
| 6152 | 6152 | G-1652 | YH-502 | H 6110 | S-8007 | H1053 |
| 6154 | 6154 | G-1654 | YH-504 | H 6140 |
The GPC curve of Taiwan Rubber 6151 can obtain data such as number average molecular weight and weight average molecular weight from GPC data, and different instruments, different PS calibration curves, and different test conditions will lead to differences in test results.
Molecular weight of commonly used SEBS grades
| Grade | GPC (Mw) | Manufacturer | Theoretical molecular weight |
|---|---|---|---|
| YH-503 | 331,000 | Baling Petrochemical | 220,000 |
| YH-502 | 161,000 | Baling Petrochemical | 100,000 |
| YH-501 | 82,000 | Baling Petrochemical | 70,000 |
| 6154 | 175,000 | Taiwan rubber | 170,000 |
| 6153 | 108,000 | Taiwan rubber | 120,000 |
| 6151 | 284,000 | Taiwan rubber | 270,000 |
| 6150 | 116,000 | Taiwan rubber | 100,000 |
| G1650 | 99,000 | American Kraton | 100,000 |
| G1651 | 294,000 | American Kraton | 320,000 |
| G1652 | 77,000 | American Kraton | 80,000 |
In short, through the analysis of large-scale instruments such as infrared spectroscopy, nuclear magnetic resonance, and gel permeation chromatography, it can be applied to the quality control of SBS in the hydrogenation production of SEBS, and can also quickly determine which type of elastomer and relative structural information are selected for a certain product, which is convenient for material selection.
References
1. Analysis and characterization of SBS hydrogenation _SBS and SEBS structure of nickel-based catalysts. Yao Ming, Zhang Yongjian. Proceedings of the 2004 International Rubber Conference.2. Characterization of the structure and composition of hydrogenated SBS. Mei Ming, Li Lei. Elastomers, 2004-2-25, 14 (1): 20~ 24.
Introduction to Microspectrum Group Rubber and Plastics Division
Founded in 2008, Microspectrum Group (full name: Shanghai Microspectrum Chemical Technology Service Co., Ltd.) is a professional comprehensive technology service provider in the field of materials and chemistry. Its business covers materials, chemistry, environmental protection, food and biomedicine and other fields. It can provide comprehensive solutions such as formula analysis, failure analysis, product development/pre-research, product testing, and patent application for problems encountered by enterprises in Product Research & Development, quality control, and intellectual property rights.
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