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ISO 28343:2010 – Rubber Compounding Ingredients — Determination of Glass Transition Temperature of Process Oils by DSC
Standard test method for measuring glass transition temperature of rubber process oils using differential scanning calorimetry
Understanding Glass Transition in Rubber Process Oils
ISO 28343:2010 provides a standardized differential scanning calorimetry (DSC) method for determining the glass transition temperature (Tg) of rubber process oils, which are petroleum-derived extender oils used extensively as plasticizers and processing aids in rubber compounding. The glass transition temperature of these oils is a critical quality parameter because it directly influences the low-temperature flexibility, dynamic mechanical properties, and overall performance of finished rubber products including tires, conveyor belts, hoses, and vibration isolators. Process oils function by intercalating between polymer chains, reducing intermolecular forces and increasing free volume, thereby lowering the composite Tg and improving low-temperature flexibility. The Tg of the oil itself is determined by its molecular weight distribution and aromatic/naphthenic/paraffinic carbon composition, making it an excellent predictor of oil performance in specific polymer systems.
The glass transition temperature of a process oil determines the lowest service temperature at which the oil retains sufficient molecular mobility to effectively plasticize the rubber matrix. Oils with Tg below -60°C are preferred for cold-climate rubber products such as winter tire tread compounds, while oils with Tg above -30°C may suffice for indoor applications like rubber floor mats or gaskets.
DSC Measurement Procedure and Parameters
The standard specifies precise thermal protocols designed to eliminate thermal history effects and provide reproducible results across laboratories. Samples are first heated to 80°C and held for 5 minutes to erase any prior thermal history, then cooled at 10°C/min to at least 30°C below the expected Tg (typically -100°C for low-Tg oils), and finally heated at 20°C/min while recording heat flow. The Tg is determined as the midpoint temperature of the step-change in heat capacity during the heating scan, calculated as the temperature at which the heat capacity change reaches 50% of the total ΔCp. Sample masses of 5-15 mg are sealed in standard aluminum pans with an empty reference pan. The method requires an inert nitrogen purge at 50 mL/min to prevent oxidative degradation, which can cause exothermic artifacts that obscure the glass transition signal. Temperature calibration must be performed using at least two high-purity standards (indium: 156.6°C; zinc: 419.6°C; or mercury: -38.8°C) with calibration verification every 100 scans or weekly.
| Parameter | Specification | Rationale |
|---|---|---|
| Initial heating/erase cycle | 80°C for 5 min | Eliminates thermal history and stress relaxation effects |
| Cooling rate | 10 °C/min | Standardizes the glass formation process |
| Heating rate | 20 °C/min | Optimizes balance between sensitivity and resolution |
| Sample mass | 5-15 mg | Ensures representative bulk sampling |
| Purge gas | Nitrogen at 50 mL/min | Prevents thermo-oxidative degradation |
| Tg determination method | Midpoint (half ΔCp) | Per ISO 11357-2, best reproducibility |
Using the midpoint method for Tg determination (rather than onset or inflection point) provides the best inter-laboratory reproducibility, with typical repeatability within ±1.5°C for well-characterized naphthenic oils and ±2.0°C for aromatic oils when the standard protocol is followed precisely.
Engineering Significance of Process Oil Tg in Compound Design
The Tg of process oils directly affects rubber compound processing behavior and end-use performance. Oils with lower Tg produce softer compounds with better low-temperature flexibility but may reduce green strength during extrusion and calendering, leading to dimensional instability in uncured profiles. Paraffinic oils typically exhibit Tg values from -50°C to -70°C and are preferred for EPDM and butyl rubber compounds requiring good low-temperature properties. Naphthenic oils range from -40°C to -60°C and offer the best balance of compatibility and low-temperature performance for SBR and natural rubber compounds. Aromatic oils, with Tg from -20°C to -40°C, provide superior compatibility with high-styrene SBR and BR polymers but sacrifice low-temperature performance. In tire tread compounding, selecting an oil whose Tg complements the polymer Tg is essential for optimizing the wet grip vs. rolling resistance trade-off: lower oil Tg reduces the composite Tg, improving wet traction but potentially increasing rolling resistance.
DSC Tg measurements can be affected by oil volatility, particularly for low-viscosity, highly aromatic oils containing significant fractions of low-molecular-weight components. These volatile fractions may evaporate during the 20°C/min heating scan, causing mass loss artifacts that appear as endothermic deviations in the heat flow signal. For such oils, use high-pressure stainless steel DSC pans capable of withstanding internal pressures up to 10 MPa, or reduce the maximum temperature to 150°C while extending the isothermal hold to ensure complete thermal equilibration.
Calibration, Validation, and Quality Assurance Requirements
The standard requires that DSC instruments used for ISO 28343 testing be calibrated according to ISO 11357-1 using at least two reference materials that bracket the expected Tg range. For process oil testing, indium (melting point 156.60°C, ΔH 28.45 J/g) and mercury (melting point -38.83°C) are the recommended calibration standards. Baseline calibration must be performed daily using empty pans of matched mass (within ±0.05 mg). The standard specifies that the baseline drift should not exceed 0.1 mW over the temperature range of interest, and sensitivity must be verified using a sapphire specific heat capacity standard (NIST SRM 720). For validation of the complete measurement system, the standard provides a reference oil with certified Tg value that should be measured weekly and must fall within ±2.0°C of the certified value. These rigorous calibration requirements ensure that Tg measurements performed in different laboratories on different instruments produce comparable results, which is essential for international trade in process oils where Tg specifications are increasingly used in purchasing contracts and quality agreements between oil producers and rubber compound manufacturers.
A well-implemented calibration program per ISO 28343 requirements can achieve inter-laboratory reproducibility (R) of 2.5°C for naphthenic oils and 3.5°C for aromatic oils, as demonstrated in international round-robin tests involving 12 laboratories across 8 countries. This level of reproducibility enables oil suppliers to confidently certify Tg values in their product specifications.
Data Analysis and Interpretation
Correct interpretation of the DSC thermogram is essential for accurate Tg determination. The standard requires that the glass transition region be clearly identified as a step-change in heat flow (endothermic direction during heating), distinguished from other thermal events such as enthalpy relaxation (which appears as an endothermic peak superimposed on the glass transition), cold crystallization (an exothermic peak in the heating scan), or thermal degradation (a gradual exothermic baseline shift at high temperatures). For process oils that exhibit multiple Tg events due to phase separation between aromatic and saturated fractions, the standard specifies that each Tg should be reported separately with its corresponding ΔCp value, as the relative magnitude of the ΔCp correlates with the fraction of material undergoing each transition. The standard also provides guidance on reporting: the midpoint Tg should be reported to the nearest 0.1°C, accompanied by the ΔCp value (J/(g·K)), the heating rate used, and a description of any unusual features observed in the thermogram. This comprehensive reporting ensures that the end user can fully assess the quality and applicability of the oil for their specific polymer system and processing conditions.
One common error in Tg determination is confusing enthalpy relaxation peaks with the glass transition itself. Oils that have been stored at temperatures below Tg for extended periods exhibit an endothermic relaxation peak superimposed on the step-change. The standard clarifies that the Tg should be determined from the step-change baseline shift, and the relaxation peak should be noted separately as an “enthalpy relaxation” feature. Using a 20°C/min heating rate helps minimize this interference by reducing the time available for relaxation processes during the measurement.
FAQ
Q: Can ISO 28343 be used for plasticizers other than process oils?
A: Yes, the method is generally applicable to any organic plasticizer including phthalates, adipates, trimellitates, and phosphate esters used in PVC and other polymer systems. However, for highly crystalline or reactive plasticizers, modifications to the thermal program may be necessary to avoid crystallization or degradation artifacts.
A: Yes, the method is generally applicable to any organic plasticizer including phthalates, adipates, trimellitates, and phosphate esters used in PVC and other polymer systems. However, for highly crystalline or reactive plasticizers, modifications to the thermal program may be necessary to avoid crystallization or degradation artifacts.
Q: How does Tg correlate with process oil viscosity?
A: There is a strong correlation described by the Williams-Landel-Ferry (WLF) equation. Oils with lower Tg have lower viscosity at any given temperature. Specifically, at Tg, all amorphous polymers and oils have approximately the same viscosity (10¹² Pa·s), and the WLF equation predicts viscosity changes of approximately 4-5 orders of magnitude per 50°C above Tg.
A: There is a strong correlation described by the Williams-Landel-Ferry (WLF) equation. Oils with lower Tg have lower viscosity at any given temperature. Specifically, at Tg, all amorphous polymers and oils have approximately the same viscosity (10¹² Pa·s), and the WLF equation predicts viscosity changes of approximately 4-5 orders of magnitude per 50°C above Tg.
Q: What is the practical significance of oil Tg in tire manufacturing?
A: In winter tire compounds, oil Tg directly determines the lowest temperature at which the tire maintains adequate flexibility and road grip. A Tg difference of 5°C in the oil translates to approximately 3°C shift in the compound’s effective Tg, which can mean the difference between acceptable and unacceptable winter traction performance.
A: In winter tire compounds, oil Tg directly determines the lowest temperature at which the tire maintains adequate flexibility and road grip. A Tg difference of 5°C in the oil translates to approximately 3°C shift in the compound’s effective Tg, which can mean the difference between acceptable and unacceptable winter traction performance.
Q: How does oil Tg affect rubber processing?
A: Lower Tg oils reduce the compound’s melt viscosity at processing temperatures, improving flow and mold filling. However, excessively low Tg oils can cause sticking to mill rolls and poor green strength during extrusion, requiring process adjustments such as reduced mill roll temperatures or increased mixing times.
A: Lower Tg oils reduce the compound’s melt viscosity at processing temperatures, improving flow and mold filling. However, excessively low Tg oils can cause sticking to mill rolls and poor green strength during extrusion, requiring process adjustments such as reduced mill roll temperatures or increased mixing times.
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