Table of Contents
Preamble
Elastomeric sealing rubbers, such as those used in the engine compartment of motor vehicles and in aviation applications, are exposed to permanent thermomechanical loads. Their sealing effect – the ability to prevent the ingress and escape of liquids and gases – is a key factor for the reliability and durability of the overall systems. The scientifically sound evaluation and prediction of the long-term behavior of these critical components requires precise analysis methods that can capture the complex interplay of temperature, mechanical stress and time.
Mechanisms of seal loss
Short-term effects
In the short term, the sealing effect may be impaired by thermal expansion and contraction, pressure fluctuations or installation errors. Even during the first few hours of operation, characteristic effects such as creeping, settling and initial signs of relaxation become apparent – especially at high temperatures. These initial changes can already have critical effects on the system tightness.
Long-term ageing mechanisms
In the long term, complex ageing mechanisms dominate, which impair the fundamental material properties:
Oxidative decomposition: Oxygen-induced chemical reactions lead to chain scission and cross-linking changes in the polymer network.
Plasticizer loss: Migration and evaporation of plasticizers reduces the flexibility and increases the stiffness of the material.
Mechanical cracks: Cyclic loading leads to crack initiation and propagation, which compromises structural integrity.
Irreversible deformations: Plastic deformation and compression set reduce the resilience and thus the sealing effect.
Studies show that EPDM rubbers (EPDM: ethylene propylene diene monomer), for example, can exhibit noticeable losses in sealing properties after years under practical conditions due to thermal and mechanical ageing, despite their excellent initial properties
Thermomechanical analysis as a key technology
Basic principle of TMA
Thermomechanical analysis (TMA) is a proven and scientifically based method for investigating the time- and temperature-dependent behavior of sealing materials. In TMA, a sample is subjected to a controlled, varying temperature program and a defined force. The change in length (expansion or shrinkage) of the material is measured as a function of temperature and load. In addition to thermal expansion, the creep and relaxation behavior as well as glass transition temperatures and phase transitions can be precisely determined.
Influence of the TMA on the sealing effect assessment
TMA is essential for evaluating the sealing effect in the engine compartment, as it allows the temperature and load-dependent deformation behavior of rubber gaskets to be precisely measured and quantified. This is crucial for predicting how well a gasket will perform its function in the long term under real operating conditions – such as high temperatures, changing loads and prolonged compression.
Research results show that the sealing behavior of profile and flat gaskets in the engine compartment does not depend solely on their initial geometry and elasticity, but also to a large extent on their deformation behavior under temperature, mechanical load and over time. The TMA provides the decisive material characteristics for this and thus allows a scientifically sound evaluation of how the sealing function changes or is lost under the typical stresses in the engine compartment.
Characteristic measured variables and their significance
With the TMA can be used to determine several decisive material properties for rubber seals in the automotive sector, which are relevant for the usability and service life in the engine compartment:
Coefficient of thermal expansion (CTE)
Definition and measurement: The CTE describes the relative change in length per unit of temperature and is an important parameter for assessing how much the sealing rubber changes with temperature fluctuations.
Practical significance: The TMA-based measurement shows how much the sealing material expands or contracts with the typical temperature fluctuations in the engine compartment. This is essential in order to avoid gaps forming and thus leaks during temperature cycles. Excessive expansion can lead to leaks, while insufficient expansion can lead to pressure loss.
Glass transition temperature (Tg)
Determination: The TMA enables the precise determination of the temperature at which the material changes from a hard-brittle to a rubbery-elastic state.
Critical importance: Tg indicates the temperature at which a material changes from firm-elastic to soft-elastic – important for operating limit management. Important to know when the seal could fail under operating conditions.
Creep and relaxation behavior
Characterization: TMA measurements record the time-dependent yielding of the material under constant load. These analyses show the time-dependent yielding or settling of the material under constant load and temperature.
Long-term relevance: Critical for long-term tightness, as loads act over a long period of time, particularly in the engine compartment, which can slowly deform the sealing material, reducing the sealing effect in the long term. Relaxation and creep mechanisms can change the structure over years.
Structural changes and long-term degradation
Detection: Long-term TMA tests reveal irreversible material losses such as settlement, which can occur particularly under cyclic loading.
Practical relevance: These characteristic values are important for predicting service life and maintenance intervals and are particularly relevant for engine compartment gaskets, which are temperature-sensitive.
Phase transitions and damping behavior
Detection: In addition to the glass transition, the TMA also makes other structural changes (e.g. softening, melting of phases) visible, which can abruptly change the properties of the material.
Systemic importance: These transitions are critical for understanding material behavior under extreme operating conditions.
Influence of thermal expansion on long-term tightness
The thermal expansion of gaskets is a key factor that influences the long-term sealing effect and reliability of gaskets in the engine compartment. TMA can be used to precisely quantify the expansion behavior of rubber materials.
Critical effects on the sealing function
Dimensional changes during temperature cycles: Elastomer seals expand when heated and contract when cooled. These cyclical movements lead to material fatigue, cracks or abrasion, especially in long-term use. If the thermal expansion is too great, this can lead to the formation of gaps or excessive compression – both of which promote leaks.
Influence on compression: Permanent thermal stress causes the sealing material to become softer, the so-called “compression set” increases. This means that the seal no longer fully returns to its original shape, which leads to a permanent gap and loss of performance.
Acceleration of ageing processes: Repeated thermal stress promotes the creep and relaxation behavior of the material, which has a direct negative effect on the sealing effect.
Different thermal expansion to neighboring components: If the CTEs of the gasket and flange do not match, uneven stresses occur, which can accelerate the failure of the gasket.
Practical application and laboratory implementation
TMA systems for automotive applications
TMA devices support high-precision, standard-compliant analyses (DIN, ASTM, ISO) of sealing rubber materials under various atmospheres and temperature programs – as are essential in automotive and aerospace laboratories. Special protocols for elastomers enable “realistic” test conditions, as are typical for practical use in the engine compartment.
Basis for decision-making in practice
The TMA provides objective, quantitatively reliable data that development engineers and laboratory staff can use to answer the following critical questions:
- What is the maximum expansion of the gasket in the relevant temperature range?
- How strongly are individual materials (e.g. EPDM vs. FKM vs. silicone) affected by compression set and creep?
Current research developments
Material innovations
A recent study shows that novel materials such as thermoplastic vulcanizates (TPV) often offer more stable mechanical properties and a comparable sealing effect to classic EPDM – whereby their long-term relaxation and setting behavior was also characterized by TMA (PMC Paper, 2023).
Scientific validation
A detailed dissertation examined the behavior of EPDM seals in the context of automotive seals both in the short and long term. It describes how relaxation and creep mechanisms can change the structure over years and points out the importance of TMA for the identification of such degradation phenomena.
Areas of application and industrial relevance
The TMA is used in material selection and development as well as in the quality control of rubber seals used, thus forming the basis for a reliable service life forecast in the automotive sector. Critical points can be identified at an early stage and materials can be further developed in a targeted manner.
This information is essential for:
- Material selection: Objective evaluation of different elastomers
- Quality control: monitoring of material characteristics in production
Conclusion
TMA-based measurement of thermal expansion and mechanical behavior is essential for selecting and designing gasket materials to withstand the real thermal requirements throughout the vehicle’s life.
Only through precise TMA characterization can sealing systems be developed that function reliably even after years in the engine compartment and reliably prevent leaks in the long term. Material selection and optimized gasket design based on TMA results significantly reduce the risk of failure due to thermal expansion.
Laboratory results from the TMA are the scientific basis for the sound assessment and specification of long-term tightness. The method is indispensable for the modern development and quality assurance of elastomer sealing systems in the automotive and aviation industries. automotive and aviation industries.
Bibliography
Nayak, J., Katheria, A., & Das, N.C. (2022). Research on the Material Compatibility of Elastomer Sealing O-Rings. Polymers, 14(16), 3323. https://www.mdpi.com/2073-4360/14/16/3323
Drobny, J.G. (2021). Investigation into Thermomechanical Response of Polymer Composite Materials Produced through Additive Manufacturing Technologies. Materials, 15(14), 5069. https://www.mdpi.com/1996-1944/15/14/5069
Technical standards:
- ASTM E831 (“Standard Test Method for Linear Thermal Expansion of Solid Materials by Thermomechanical Analysis”) for thermomechanical tests
- ISO standards: ISO 23529 for sealing materials
