Epoxy
10-85 usd/kgEpoxy is a high performance thermosetting resin, which is the product of a one-way chemical reaction. The reaction takes place between a resin and hardener, and is triggered by the mixing process, or as is the case with one-part systems, the reaction is accelerated with heat.
It is a diverse family, including adhesives, coatings, casting resins and laminating resins. On top of this, there are many different types of hardener and a wide range of additive available, which creates the opportunity to tailor the resin to a range of applications. Key properties include high strength, low shrinkage, excellent wet-out and adhesion with other materials, insulating, and resistance to solvents and chemicals.
One downside of epoxy is that it uses some nasty ingredients that are harmful to people and the environment, in particular bisphenol A (BPA) monomer. BPA is a key ingredient of epoxy resin, alongside epichlorohydrin (ECH) – most epoxy systems are derived from reacting these two ingredients. Bisphenol A (BPA) has been at the centre of numerous studies, because it poses a risk to people and the environment (it is an endocrine disruptor and oestrogen-mimicker). The concern is that small amounts of the chemicals present in food packaging materials migrate into the contents.
Even so, certain grades of epoxy are certified as food safe and considered non-toxic by organisations such as the US Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA). As a result, they can be utilised in food contact applications. Examples include coatings on steel water storage tanks and steel food packaging (tin cans), chocolate moulds and table tops.
Another downside is that as a thermosetting resin, it is not recyclable. Bio-based alternatives are emerging that attempt to reduce the negative impacts of this versatile resin by replacing a portion of the petrochemical derived ingredients with potentially renewable ones.
Sustainability concerns




Epoxies are some of the strongest and stiffest structural adhesives available. They have low surface energy, which helps them to develop very good coverage of the material being joined. They shrink very little as they cure, and so do not disturb the join, ensuring the strongest bond is achieved. Through the choice of hardeners, catalysts, fillers, and other additives, and by controlling the degree and method of curing, properties can be tailored for a range of diverse applications. These include products, furniture, automotive, transport, construction, marine and sports.
Most types of material can be bonded with epoxy. They provide a good barrier, both protecting the join and insulating the materials from one another. This is especially useful in situations where the materials may corrode or affect one another in a detrimental way, such as dissimilar metals that have different electrode potentials (galvanic corrosion).
Epoxy adhesives are either one-part (curing agent already mixed in) or two-part (curing agent added to kick start reaction). One-part, also called one-stage, epoxies are available as glue film (same or similar resin as pre-preg used for composites, but without the fibre reinforcement), or as a paste. The curing process usually takes place at around 150 degC. These adhesives provide a very strong bond, especially between metals – in many cases they provide an alternative to welding or fasteners.
Two-part systems are supplied as resin and harder, which are mixed to kick start the reaction – this is advantageous where low viscosity is required, or it is not practical to apply heat, for example. The curing process can be accelerated with heat – and while the application of heat may not be essential, it can help improve bond strength. Two-part systems are inherently very stable and used to bond, seal and encapsulate. Strength is lower than for one-part systems, and comparable with high performance acrylic structural adhesives.
Many techniques are used to ensure uniform bond line thickness, which is essential for producing a reliable bond that meets performance requirements. In the case of films, a nylon mesh may be used. And when applying paste, glass or plastic beads may be added.
Epoxy adhesives tend to perform well up to around 120 degC. Above this temperature, the chemical structure will begin to rearrange and ultimately lose strength. To improve high temperature properties, epoxy is combined with other resins, such as phenolic. Such alloys can tolerate 250 degC or more.
Cure temperature can also affect mechanical properties at elevated temperatures. Heat-curing one-stage systems tend to have better thermal resistance, up to 170 degC. When using room temperature curing systems, post-curing at elevated temperature can help to boost high temperature properties. Cure times range from a few minutes to more than 12 hours for large, critical-performance parts.
Some trade names include Araldite (Huntman), Scotch Weld (3M) and Permabond (Permabond).
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