Table of Contents:
Introduction
Epoxy resin is a versatile and high-performance reactive resin that is used in numerous application areas such as the electrical industry, construction and the manufacture of fiber composites.
It usually consists of two components: a mixture of organic compounds containing hydroxyl groups and a hardener. Many hardeners for epoxy resins are based on amines, compounds that contain nitrogen atoms and play an important role in the chemical reaction that hardens the epoxy resin.
Amine-based hardeners react cross-linking with the epoxy groups of the resin and form a three-dimensional network of polymer chains, which leads to the curing of the material.
The amines used as hardeners can be of various types, such as aliphatic amines, which often react at room temperature, or aromatic amines, which require higher temperatures and are suitable for special applications. These reactions turn the epoxy resin into a stable and resistant material.
Factors influencing the curing process
The curing of epoxy resin is highly dependent on several key parameters, including the mixing ratio, temperature, humidity and layer thickness.
- Mixing ratio: A correct ratio of resin and hardener is crucial for complete curing. Deviations will result in the material remaining sticky or not achieving the desired mechanical properties.
- Temperature: Higher temperatures accelerate the curing process, while low temperatures slow it down. In extremely cold environments, curing may even stop completely.
- Humidity: High humidity can also slow down the process and affect the surface quality, resulting in a matt and uneven surface.
- Layer thickness: Thicker layers tend to cure more slowly, as the heat development inside the material can slow down the reaction.
Degrees of curing
The curing process of epoxy resin takes place in various stages, each of which results in different mechanical and chemical properties of the material:
- Initial hardening: In this phase, the material is no longer sticky, but still soft.
- Functional curing: The resin is sufficiently hard for many applications, but not yet fully resilient.
- Complete curing: Here the epoxy resin has reached its maximum hardness and chemical and mechanical resistance.
Mechanical properties and degree of curing
As curing progresses, the mechanical properties of the epoxy resin change significantly. A higher degree of curing tends to lead to an increase in the glass transition temperature, which increases the thermal load-bearing capacity of the material. At the same time, the chemical resistance improves, while the modulus of elasticity may decrease slightly at maximum curing.
Determination of the degree of curing with dielectric analysis
To precisely determine the degree of curing of an epoxy resin, the dielectric analysis is often used. This is an analytical method that measures the ionic conductivity present in the material during curing.
As the mobility of the charge carriers decreases as curing progresses, the curing process can be observed directly using this measured variable.
The procedure is described in ASTM E2038 and is therefore standardized.
Post-curing
An important aspect of curing epoxy resin is the process of post-curing, where the material is exposed to a higher temperature after the initial cure to complete any remaining reactions and achieve the final material strength.
The post-curing process typically improves the thermal and mechanical properties of the material, particularly in terms of strength, hardness and resistance to chemical attack.
Post-curing takes place in a controlled temperature range that varies depending on the type of epoxy resin, often between 60 and 120 °C. The duration can range from a few hours to several days, depending on the specific requirements of the application.
Post-curing is often unavoidable in the aviation industry or for highly stressed components in order to exploit the full potential of the material.
Modifications and additives
In addition to correct curing, the addition of additives can further optimize the behavior of the epoxy resin. These include
- Accelerators that increase the reaction speed and thus shorten the curing time.
- Plasticizers that make the material more flexible without significantly impairing its strength.
- Fillerswhich can improve rigidity or thermal conductivity. Such modifiers are frequently used in the automotive and electronics industries in particular to adapt the material to specific requirements.
Problems and solutions during curing
Problems during the curing process can be manifold. If the epoxy resin does not cure completely, there are several possible solutions:
- Increase the ambient temperature to speed up the curing process
- Application of a thin layer of correctly mixed epoxy resin
- In extreme cases, it may be necessary to remove the incompletely cured material and reapply it.
Sustainability and environmental aspects
In addition to disposal, the production of epoxy resin also poses challenges. In recent years, research has developed bio-based epoxy resins, some of which are based on renewable raw materials.
These sustainable alternatives reduce the fossil content in production and offer a more environmentally friendly option for use in industry. These bio-based resins often exhibit similar mechanical properties to conventional resins and are of particular interest in the construction industry and for decorative applications.
Practical aspects of processing
For many applications, a curing level of 90-95% is sufficient. In particularly demanding areas, however, complete curing may be required, which can take several days to weeks.
Post-curing at an elevated temperature can accelerate this process and further improve the mechanical and thermal properties of the material.
Comparison with other polymerization techniques
Epoxy resins are characterized by their particularly high strength and chemical resistance compared to other thermosetting systems such as polyurethanes or polyester resins.
However, these materials have different curing mechanisms and may be better or more suitable for certain applications. While epoxy resins first react by polymerizing an alcohol with epichlorohydrin to form a resin which is then cross-linked with various amines to form epoxy groups, the curing processes for polyurethanes, for example, are carried out by reactions between isocyanates and polyols, resulting in a softer and more flexible end product. This comparison can be helpful when selecting materials for specific applications.
Conclusion
Thanks to its versatile properties, such as high abrasion resistance, strength, chemical resistance and excellent adhesion, epoxy resin offers a wide range of applications in industry and trade.
However, to achieve the best possible results, the curing process must be precisely controlled. Factors such as temperature, humidity, layer thickness and the mixing ratio play a decisive role.
The desired properties and maximum performance of the epoxy resin can only be achieved by carefully considering these influencing factors and precisely determining the degree of curing, for example using dielectric analysis, and by post-curing and the use of additives.
Sustainable alternatives and a comparison with other resin systems may also offer further advantages for special applications in the future.
