Introduction
Butyl rubber, a copolymer of isobutylene and isoprene, is the most commonly used material for the inner tube of a bicycle tire. Its benefits include a relatively low price, long service life and minimized air leakage. For optimized properties such as maximized flexibility and minimum Rolling ResistanceThe rolling resistance is a force resisting the motion when a body is rolling across a surface. This determines the slip resistance of, e.g., car or truck tires.rolling resistance, some additives are required at a low percentage amount. In this study, used bicycle inner tubes from two different manufacturers were analyzed with TGA to identify differences.
Methods and Sample Preparation
Prior to the measurement, the samples were cut into several small pieces and placed into an open Al2O3 crucible. The samples were heated in a nitrogen atmosphere to 850°C and in an air atmosphere from 850°C to 1100°C. For the thermogravimetric investigation, the NETZSCH TG Libra® coupled to a QMS Aëolos® was employed. The measurements were carried out under the conditions detailed in table 1.
Table 1: Measurement conditions
| Sample mass | Manufacturer A (10.34 mg) | Manufacturer B (10.06 mg) |
| Crucible material | Aluminum oxide 85 μl, open | |
| Temperature program | 40°C to 850°C in nitrogen, 850°C to 1100°C in air | |
| Heating rate | 10 K/min | |
| Atmosphere | Nitrogen, air | |
| Gas flow rate | 40 ml/min | |
| QMS | 1 - 300 amu, Scan per mass: 20 ms | |
Results and Discussion
The resulting thermograms are depicted in figure 1. Under an inert atmosphere, both samples show three mass-loss steps. The first two mass-loss steps, between 200°C and 500°C, are associated with the Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. decomposition of the rubber mixture. The rubber composition was probably different in these two samples, as slightly different percentages were detected, and the mass-loss rate (DTG) peaks were shifted. The third mass-loss step was caused by the Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. decomposition of the carbonate filler. As different amounts of mass change were detected, different filler amounts were probably used.
Above 850°C, the air atmosphere caused the combustion of the residual carbon. The resulting residual mass corresponds to the Ash ContentThe ash is a measure of the mineral oxide content on a weight basis. Thermogravimetric analysis (TGA) in an oxidative atmosphere is a well-proven method to determine the inorganic residue, commonly referred to ash, in organic materials such as polymers, rubbers, etc. Therefore, the TGA measurement will identify if a material is filled and calculates the total filler content.ash content. Again, a specific difference was observed between the two samples, indicating a different amount of oxide minerals. The Ash ContentThe ash is a measure of the mineral oxide content on a weight basis. Thermogravimetric analysis (TGA) in an oxidative atmosphere is a well-proven method to determine the inorganic residue, commonly referred to ash, in organic materials such as polymers, rubbers, etc. Therefore, the TGA measurement will identify if a material is filled and calculates the total filler content.ash content of the sample from manufacturer B was about double the amount of that from manufacturer A.
The evolved gases were additionally analyzed with the quadrupole mass spectrometer (QMS) connected to the gas outlet of the thermobalance. At 218°C (214°C), both samples showed an increase in mass number 76, which can be associated with the release of CS2, a vulcanization residue; see figure 2.
The detected mass spectra at 420°C show no significant difference for either sample, with m/z 41 being the most intense fragment; see figures 2 and 3a. The measured spectra show high similarity to the main PyrolysisPyrolysis is the thermal decomposition of organic compounds in an inert atmosphere.pyrolysis product of butyl rubber 1-butene1 ; see figures 3a and 3b.
At 634°C, the mass spectrometer detected an increase in m/z 44 for the sample from manufacturer B, confirming the release of CO2 from carbonate Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. decomposition. This indicates that a higher amount of carbonat filler was used in the sample from manufacturer B.
The release of the different mass numbers can be easily compared to the TGA curve in a temperature-dependent scaling; see figure 2.
1 PyrolysisPyrolysis is the thermal decomposition of organic compounds in an inert atmosphere.Pyrolysis GC/MS Data Book of Synthetic Polymers, Tsuge Shin, Ohtani Hajime, Watanabe Chuici, Elsevier, 2011
Summary
In conclusion, TGA-MS analysis enables detailed insight into the composition of two competitive inner tubes of bicycle tires. The thermobalance determines the Thermal StabilityA material is thermally stable if it does not decompose under the influence of temperature. One way to determine the thermal stability of a substance is to use a TGA (thermogravimetric analyzer). thermal stability and allows conclusions to be drawn about the composition, such as the rubber content, filler content, carbon content and Ash ContentThe ash is a measure of the mineral oxide content on a weight basis. Thermogravimetric analysis (TGA) in an oxidative atmosphere is a well-proven method to determine the inorganic residue, commonly referred to ash, in organic materials such as polymers, rubbers, etc. Therefore, the TGA measurement will identify if a material is filled and calculates the total filler content.ash content. Even the smallest of differences could be identified. Simultaneously recorded mass spectrometer data facilitates the interpretation of the Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. decomposition processes by identifying the gases released. The use and percentage of the respective additives and fillers are decisive for the quality of the tire; e.g., calcium carbonate also has a significant reinforcing effect on both natural and synthetic rubber, and can improve consistency. It also influences the dynamic properties of rubber.