ddchem High Tg Solutions: features of our product range with high glass transition temperature
Whether you are considering an epoxy system for external house painting, the undercoating of an automobile or flooring applications, the behaviour of the epoxy system at the different temperatures needs to be taken into consideration.
The operating temperature is one of the main factors that influences the physical properties of a cured epoxy coating. One of the parameters that helps to understand the behaviour at a specific temperature is the glass transition temperature (Tg) that is indeed one of the most critical properties to consider when choosing the correct system for your application as it affects:
WHAT IS THE GLASS TRANSITION TEMPERATURE?
A glass transition temperature (Tg) is the temperature range at which an amorphous polymer (like an epoxy system) moves from a hard or glassy state to a softer, often rubbery state. In short, knowing the glass transition temperature range of a material indicates when the material changes from a flexible state (Figure 1) to a more rigid state (Figure 2), and vice versa.
Figure 1 – Rubbers normally feature a Tg lower than -50 °C, way below the typical service temperature. These materials are then always in a flexible state, that makes them perfect to be used as tyres.
Figure 2 – Polycarbonate sheets normally feature a Tg above 100°C. Because of this, they are in a glassy rigid state that makes them perfect to be used for construction materials, 3D printing etc.
EPOXY ADHESIVES AND COATINGS: WHY DOES Tg MATTER?
Epoxy coatings are normally characterized by excellent mechanical properties, high chemical resistance to many chemicals and superior adhesion to various materials. These features are normally encountered when the polymer is a rigid solid (although in some cases a degree of flexibility is required), with the glass transition temperature (Tg) above the service temperature.
Because of this, if the Tg is not high enough, the system might shift to its rubbery state when high service temperatures are reached resulting in poor system performance (Figure 3).
Figure 3 – Poor performances in terms of mechanical properties and chemical resistance might be encountered when an epoxy polymer Tg is lower than the typical service temperature.
An epoxy system in its rubbery state can present low cohesive strength, surface protection and degree of adhesion resulting in poor mechanical properties and reduced chemical resistance. For this reason, using an epoxy system with a Tg higher than the service temperature is typically the best solution to optimize product performance.
HOW DOES POST-CURING INFLUENCE THE Tg VALUE?
The cure of epoxy resins is a complex process that goes through various stages as summarized in the time-temperature transformation diagrams in Figure 4 and Figure 5. These diagrams describe changes in the state of the curing material as a function of time and temperature with the glass transition temperature Tg that can be utilized as a practical parameter for following this curing process:
• Tg(0) Upon mixing, Tg(0) is the glass transition temperature of the starting materials (part A + Part B);
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• Tg(cure) When the system is cured at ambient temperature, the components immediately after mixing are in a liquid state and the reaction takes place. The Tg rapidly increases to the gelation point until a vitrification stage is reached. In this stage the reaction rate becomes extremely low and the material appears to be in a “frozen” state where it will no longer be able to reach full cure, unless conditions are radically adjusted (e.g. provide a heat post-cure). Tg(cure) is the highest reachable Tg under these curing conditions (Figure 4);
Figure 4 – A generalized diagram representing the curing stages of an epoxy system. In this example the epoxy system is cured at ambient temperature for a certain amount of time, reaching a glass transition temperature Tg(cure). Only when the service temperature is below the Tg(cure), the system will be in its glassy state.
• Tg(max) After an epoxy system has been cured at ambient temperature, a higher Tg can be achieved only by applying a post-curing where the system is heated at a temperature higher than Tg(cure). A fully cured epoxy system can normally be achieved by post-curing the system above 100-150°C. With the correct post-curing, the system will then reach a maximum glass transition temperature Tg(max) (Figure 5).
Figure 5 - A generalized diagram representing the curing stages of an epoxy system. In this example the system is cured at ambient temperature and then post-cured at a temperature higher than its Tg(cure). The post-curing increases the Tg of the system until a Tg(max) is reached when enough heat is applied. The system will now have a higher Tg and will be in its glassy state until this new Tg(max) is reached.
It is clear that post-curing is an essential procedure when high glass transition temperatures need to be reached. An example of the entity of the post-curing effect on the glass transition temperature is reported in Table 1.
HOW DOES THE EPOXY RESIN INFLUENCE THE Tg VALUE?
The part A resin component plays a major role in determining the final Tg value of a cured epoxy system. While using standard liquid epoxy resin (BADGE) allows to reach high Tg values, using modified epoxy resins formulated with reactive diluents or filler normally results in lower Tg values (Table 2). Because the difference in the Tg, when using a pure or modified epoxy resin, can be over 30°C, when a high Tg is needed, modified epoxy resins must be carefully evaluated.
Tg OF DIFFERENT CURING AGENTS
In the following graphs, the glass transition temperature of different curing agents is reported. Since the Tg is a temperature range, the Onset (beginning of the transition), Tg (Half cP extrapolated) and End (end of the transition) are reported. All the curing agents where mixed with standard liquid epoxy resins (BADGE) and cured for 2h at 80°C + 3h at 180°C.
• Formulated cycloaliphatic polyamine and adducts
• Formulated aliphatic polyamines and adducts
• Formulated Mannich Bases