Glass Transition Temperature, often called Tg, is a material’s natural physical and mechanical properties, especially plastics, including epoxy resins. Many high-strength epoxies require heat to cure and form a strong bond. As the cure temperature of epoxy is customised by the adhesive manufacturers as per the industrial requirements, one question arises. Does cure temperature influence Glass Transition Temperature?

Both play a significant role in material processing and adhesive applications in various industries; hence, knowing their relation can help you optimise your process. Follow this blog to find out more about the relationship between cure temperature and glass transition temperature.

 

A] Understanding Epoxy Glass Transition Temperature

The glass transition is a mechanical property of amorphous polymers to undergo a transition from their solid brittle state to a viscous rubbery state. The temperature at which this transition takes place is known as glass transition temperature or Tg for short. The transition temperature is always lower than the melting point of the polymer.

At this temperature, the polymer chains of epoxy start to move allowing flexibility. Every polymer has a different molecular structure; hence, their Tg also varies. Depending on the Tg of an epoxy, they are used for different applications.

Certain epoxies are crafted to operate above their transition temperatures, which allows for flexibility in the bond. While other epoxies designed to provide rigidity are crafted to operate under their transition temperature.

There are several factors affecting glass transition temperature, which can be manipulated to customise the epoxy according to the application requirements. These factors can be external, such as humidity and moisture, or internal, such as cross-linking and molecular weight and structure.

B] Understanding The Curing Temperature Of Epoxy Adhesives

The curing process hardens a polymer, such as epoxy and silicone resin by creating cross-linking polymer chains. There are several methods to cure epoxy resin; however, heat-cured adhesives are the most widely used for their stronger bonds and durability.

Applying heat can sometimes accelerate the curing process and even lead to stronger bonds. Some of the desired benefits of heat-cured adhesives are their ability to resist corrosion, chemicals, and extreme temperatures.

The heat must be applied according to the adhesive manufacturer’s instructions as applying too little or too much heat can affect the bond strength, longevity, and curing time. The usual cure temperature of epoxy is around 21-24C, but that may change depending on the type of epoxy.

Epoxies with lower curing temperatures are suited for bonding components where the substrate might have a lower temperature tolerance, such as IC boards. This helps to protect the substrate components during the curing process.

C] Relationship Between Cure Temperature And Glass Transition Temperature

It is observed that the relationship between Tg and cure temperature is direct. This means that the Tg is relatively lower at a lower cure temperature and higher if the cure temperature is higher. To create strong bonds the epoxy must be heated to a temperature that is high enough to help it solidify but not so high that it solidifies instantly, not leaving any time to flow and set.

As a general rule of thumb, the highest temperature applied to the epoxy during curing should be a bit lower than the Tg. The actual Tg temperature of a polymer is not static and more of a range of temperature.

One of the most important factors that can raise the Tg is the polymer’s crosslink density. As we know heat curing epoxy can increase the degree of cross-linking; thus, raising Tg in the process. High-crosslinking is a desired property from any industrially used epoxy as it offers improved strength, durability, and resistance to a wide variety of things.

From this, we can conclude that cure temperature is one of the most prominent factors that affect the glass transition temperature. Understanding and optimising the cure temperature and glass transition temperature is extremely crucial to ensure that the cured epoxy meets the required specifications.

D] Methods For Measuring and Optimising Tg

Measuring and optimising Tg is of utmost importance for the right selection of epoxy for application and quality control. There are several methods for measuring the glass transition temperature; however, the three most commonly used methods are Differential Scanning Calorimetry (DSC), Thermal Mechanical Analysis (TMA), and Dynamic Mechanical Analysis (DMA).

Under the DSC method, heat is applied to the test sample and compared with a similarly heated reference to figure the Tg point. In the TMA method, a sensitive probe is used to measure the expansion of the polymer. The researchers simply have to note down the temperature at which the epoxy goes through the Tg stage. The DMA method is the most sensitive approach of them all, as it applies pressure to the epoxy and notes changes in accordance with the temperature.

There are several ways to optimise the Tg for industrial applications. Altering the polymer’s chemical composition, providing the curing conditions, and blending with other polymers are some methods to do so. To learn more, you can refer to our blog on how to optimize the glass transition temperature (tg) of an epoxy.

E] Optimising Cure Temperature

Most two-part epoxy resin you will find in the market will cure at room temperature. They can display great mechanical and chemical properties. However, certain applications require epoxies that can withstand extreme temperatures.

Certain epoxies that need to function well in cryogenic temperatures need to have their curing temperature has to be decreased. This helps to avoid premature cracks and adhesive failure. The optimum polymer cure temperature drops if the source material is less reactive or the NaOH concentration is lower in the mixture.

Similarly, boosting the optimal curing temperature can help to create strong interlinking bonds. These types of epoxy can be used in industries where the adhesive and the substrate are subject to constant high temperatures.

Conclusion

The cure temperature and glass transition temperature are two of the most significant properties of an epoxy. The epoxy’s glass transition temperature changes with the curing temperature. This means that for certain applications where the epoxy is required to stay solid even under extreme heat, the epoxy has to be optimised to have a higher curing temperature.

For different industrial applications, the Tg and curing temperature are different and need to be optimised accordingly. If you are looking for customised epoxy resin for your unique business requirements, feel free to contact us today!