Table of Contents
Introduction and meaning
Epoxy resins are among the most versatile materials in modern materials science and form a central basis for sophisticated composite systems, high-performance adhesives and protective coatings¹. Their exceptional properties result from their specific molecular structure and targeted modification possibilities, which enables a remarkable range of technical adaptations. The most important property of epoxy resins is their adhesive strength – they adhere reliably even to difficult substrates such as metals, ceramics and glass.
Basic material properties
Structure and thermal behavior
Epoxy resins are amorphous polymer materials and generally do not exhibit any crystallinity after curing. crystallinity after curing. Their epoxy-containing cross-linking structure gives them a thermoset character, meaning that they do not have a real melting point. Instead, when they reach the glass transition temperature (Tg) from a solid to a rubbery state. This transition is crucial for the mechanical and thermal performance in the area of application.
The thermal stability of epoxy resins depends largely on their cross-linking density and chemical composition². Well-formulated resin systems achieve Tg values between 120°C and 195°C, special types for high-temperature applications even up to 210°C. Above the glass transition temperature the mechanical properties begin to drop significantly, which is why the Tg is a critical parameter for the choice of application.
The cross-linking density not only determines the thermal properties, but also the mechanical performance of the cured material. A higher cross-linking density leads to stiffer but more brittle materials, while a lower cross-linking density leads to more flexible but less temperature-resistant properties. This correlation enables material scientists to tailor the properties to specific application requirements.
Curing behavior and kinetics
The curing process of epoxy resins is a complex chemical process that is influenced by various factors. Curing can take place both thermally and – with special formulations – by UV radiation. Temperature, hardener concentration, catalysts and possibly the intensity of the UV radiation determine both the speed and the completeness of the cross-linking reaction. At room temperature, curing times of several hours to several days may be required, while increased temperatures or targeted UV exposure can speed up the process considerably.
The curing kinetics typically follow an autocatalytic process in which the reaction rate first increases and then decreases again. During the gelation phase, the liquid resin transforms into a gel-like state before the final cross-linking into a solid thermoset takes place. Controlling these phases is crucial for avoiding internal stresses and achieving optimum mechanical properties.
Chemical and mechanical resistance
Epoxy resins exhibit excellent chemical resistance to a wide range of aggressive media, including dilute acids and alkalis, chlorinated hydrocarbons, mineral oils and water³. Mechanically, they are characterized by high strength and toughness, which can be precisely adjusted using nanoparticles, copolymers and flexible additives.
However, the chemical resistance varies greatly depending on the specific formulation and the media used. While epoxy resins are resistant to many organic solvents and weak acids, strong bases such as caustic soda or aggressive oxidizing agents can lead to degradation of the polymer matrix.
Water absorption also shows a wide range in the literature: while intact, well-crosslinked systems can appear almost waterproof, water can penetrate the matrix in the case of microscopic defects or incomplete curing. Typical values – depending on the resin type and degree of crosslinking – are between 1 and 4 %, which can affect the mechanical properties and the glass transition temperature.
Another important aspect is UV resistance: unmodified epoxy resins tend to yellow and become brittle when exposed to sunlight, but their resistance to UV radiation can be significantly improved by using special stabilizers and additives.
Mechanical properties in detail
The mechanical properties of epoxy resins cover a broad spectrum and depend heavily on the respective formulation, the hardener system and the degree of curing. In general, they are characterized by high strength, a high modulus of modulus of elasticity and excellent compressive and adhesive strength, which makes them ideal for structural and load-bearing applications.
The elongation at break – a measure of the flexibility of the material – can vary considerably depending on the modification. By specifically adapting the formulation, for example by adding flexibilizing additives or plasticizers, both rigid structural components and elastic sealing compounds can be created.
Notched impact strength, which is crucial for impact and shock resistance, can also be significantly improved through the use of elastomer or thermoplastic modifiers. This versatility makes epoxy resins one of the most widely applicable polymer materials in industry and research.
Main types of epoxy resins
Bisphenol A-based epoxy resins
Bisphenol A epoxy resins account for around 75% of global epoxy production and are characterized by their versatile mechanical and chemical properties. They are available as liquid or solid variants and are characterized by good adhesion, moderate flexibility and high temperature resistance. The main areas of application are laminates, adhesives and fiber composites.
Novolak epoxy resins
Novolacs are formed by the reaction of phenols with formaldehyde, followed by epichlorohydrin modification. Their high functionality (2-6 epoxy groups per molecule) results in a pronounced cross-linking density, which leads to maximum chemical and thermal resistance. They are typically used in high-temperature adhesives and anti-corrosion coatings.
Cycloaliphatic epoxy resins
Cycloaliphatic grades are produced by reacting cyclic alkenes with peracids and are characterized by their aliphatic backbone. They offer low viscosity, high weather resistance and very high glass transition temperatures. glass transition temperatures. Main applications are electronic potting compounds, radiation-cured paints and varnishes.
Diversity and modification options
The basic structure of modern epoxy resins usually consists of reaction products of bisphenol-A and epichlorohydrin. However, an enormous variety of resin variants and copolymers can be produced by modifying the initial combinations⁴. In addition to classic one- and two-component systems, nanocomposites and hybrid materials are increasingly being used to specifically optimize certain properties such as toughness or thermal stability.
Areas of application
Composite systems
Epoxy resin is used as a matrix material for fiber composites such as carbon, glass and aramid composites⁵. It offers ideal adhesion to fibers and enables high-strength, lightweight structures for aerospace, automotive and sports equipment.
Adhesives
Epoxy-based adhesive systems offer strong adhesion, chemical resistance and dimensional stability for metal, ceramic and composite bonding.
Coatings
Due to their density, temperature and media resistance, epoxy resins are suitable for industrial floor coatings, corrosion protection, insulation coatings and food sealants.
Electronics
Epoxy resins are indispensable insulating materials and potting compounds in electrical engineering and electronics, for example for printed circuit boards, coils, sensors and motor housings.
Processing aspects and challenges
During processing, the pot life (processing time until gelation begins) is critical – only as much resin should be prepared as can be processed within the given time frame. The stoichiometric mixing ratio between resin and hardener is decisive for the final strength; deviations lead to sticky surfaces and poorer mechanical behavior. Important safety aspects include the handling of uncured resin, whereby suitable protective measures such as nitrile or butyl gloves and protective clothing must be observed.
Process optimization and quality control
The successful processing of epoxy resins requires precise control of various process parameters. The ambient temperature has a significant effect on both viscosity and curing speed. Low temperatures can lead to incomplete wetting and poor adhesion, while excessively high temperatures can drastically shorten the pot life and lead to thermal stresses during curing.
Humidity is an often underestimated factor that can be particularly critical for amine hardener systems. Humidity can lead to the formation of carbamates, which appear as white efflorescence on the surface and impair further processing or bonding. Controlled ambient conditions with relative humidity below 50% are therefore often required in professional applications.
Deaeration of epoxy resin formulations is another critical aspect, especially in thick-film applications or when using fillers. Trapped air bubbles can act as stress concentrators and significantly reduce the mechanical properties. Vacuum degassing systems or special stirring techniques are standard in industrial applications to ensure air bubble-free products.
Curing systems and their properties
The selection of a suitable curing system has a decisive influence on the properties of the final product. Aliphatic amines offer fast curing at room temperature, but can lead to strong exotherm and yellow discoloration. Cycloaliphatic amines cure more slowly, but offer better mechanical properties and less color development.
Anhydride hardeners require elevated temperatures for activation, but offer excellent thermal stability. thermal stability and low shrinkage. They are particularly suitable for applications where high glass transition temperatures and dimensional stability are required. Polyamide hardeners give the system flexibility and impact strength, but at the same time reduce chemical resistance and temperature stability.
Future prospects
Current research is focusing on the development of bio-based resin systems with lower toxicity and improved environmental compatibility, as epoxy resins are traditionally produced primarily from petroleum. At the same time, new nanocomposites and hybrid materials enable even more precise adjustment of material properties for specific applications.
Sustainable developments
The development of sustainable epoxy resin systems includes several promising approaches. Bio-based epoxy resins made from renewable raw materials such as vegetable oils, lignin or terpenes are already showing commercial success in less critical applications. These materials can replace up to 50 % of petrochemical base materials without significantly affecting the basic properties.
Progress is also being made in the area of recycling, which was long considered virtually impossible. Chemical recycling using processes such as solvolysis or pyrolysis is currently the subject of intensive research, as it could in principle enable the recovery of valuable organic components. However, the actual degree of recovery – especially for carbon-containing (C-C) structures – must be critically assessed, as thermal processes often result in decomposition or complete combustion. The efficiency and ecological balance of these processes should therefore be carefully examined.
Vitrimers – a new class of dynamically crosslinkable epoxy-based polymers – are a particularly innovative approach. They enable reversible bonds and therefore genuine recyclability or repairability with largely unchanged material properties.
Technological innovations
The integration of smart materials in epoxy resin formulations opens up completely new areas of application. Self-healing epoxy resins with encapsulated healing agents can automatically repair microcracks and significantly extend the service life of structural components. Shape memory epoxies enable programmable shape changes in response to external stimuli such as temperature or electric fields.
The digitalization of material development through machine learning and artificial intelligence significantly accelerates the development of tailor-made formulations. Predictive modeling enables the prediction of material properties based on molecular structure and composition, which can dramatically shorten the time from conception to market launch of new materials.
List of sources
¹ Auth, T., Böckler, M., Fendler, D., Hennig, M.: “Exposures to hydrophthalic anhydrides during activities with epoxy resins in electrical engineering.” Hazardous Substances – Air Pollution Control 70 (2010) No. 1/2.
URL:https://www.dguv.de/medien/ifa/de/pub/grl/pdf/2010_004.pdf
² Utaloff, K.: “Material properties and the thermal stability of epoxy resins.” Dissertation, University of Heidelberg, 2017.
URL:https://archiv.ub.uni-heidelberg.de/volltextserver/23420/1/Katja%20Utaloff%20Dissertation.pdf
ResinPro: “How can I protect the epoxy resin from the weather?” FAQ section.
URL:https://resinpro.de/faq/wie-kann-ich-das-epoxidharz-vor-witterungseinfl-ssen-sch-tzen/
⁴ RCT Magazine: “Epoxy resin: production & use.” 2025.
URL:https://www.rct-online.de/magazin/epoxidharz-herstellung-verwendung-einsatzbereiche/
⁵ Hübner, F.: “Modified epoxy resin formulations for the production of carbon fiber reinforced cryogenic hydrogen storage tanks in an automated lay-up process.” Dissertation, University of Bayreuth, 2024.
URL:https://epub.uni-bayreuth.de/7699/1/01_20240322_Dissertation_Hu%CC%88bner_druck_comp.pdf
