Why epoxy resin turns yellow

More than a filler

For a while, the use of casting resins to fill gaps in cracked wood or knotholes was very popular. They were often colored to create a striking contrast to the wood. More recently, work with a considerable proportion of cast resin between embedded wooden parts has appeared again more often.

Sometimes the resin even represents the greater part. The process is also interesting for completely different approaches, such as at the Italian manufacture Riva 1920. Entrepreneur Maurizio Riva was looking for a way to sensibly use the countless pieces of wood waste that resulted from production utilize. He found it in a method that is usually used to fix smaller plants for eternity: poured in resin.

Search for the one

There are three different types of potting resins: polyester resins, polyurethane resins and epoxy resins. There are countless products of all these types on the market, including transparent versions that can be colored. But there are few that are suitable for large-area application. «Polyester resins and polyurethane resins usually have a shrinkage dimension. When they harden, they contract and can thus detach from the wood, ”says Mario Burkart from Burkart & Pfaffen Dekorationsbau GmbH in Bürglen UR. “As a rule, epoxy resins do not show any shrinkage behavior, but unlike other resins, they are usually not UV-resistant and therefore yellow. There are very few exceptions, and it is precisely these that can be used. "

The secure fixation of decay

If you ask the furniture makers which resins they use, you usually just hear: "It's resin." The actors are silent about which resins are involved and how they are to be processed. And there are reasons. “The epoxy casting resins on the market have their problems with larger dimensions. In addition to yellowing, deformations, breaks, air bubbles or a cloudy appearance also occur, especially with larger volumes to be poured, ”explains Andrea Forti, founder of the Alcarol brand from Italy.

The company researched and experimented for six years before overcoming the difficulties associated with large-scale work with resin and wood. Today, Alcarol uses its own “formulations” of the resins to embed any wood - something that is littered with drill holes or is in a state of decay due to fungi - and turns it into furniture. “The process to get there and the necessary equipment are much more complex and expensive than it looks. The risk of investing a lot of energy and money before you get useful results is high, ”warns Forti.

Individual recipes for special cases

As a carpenter, epoxy resin is best known as a high-strength two-component adhesive that reliably bonds almost all materials together. However, these resins are used in many areas. For example, to insulate electrical components or in the production of load-bearing aircraft parts or for the production of lightweight fiber composite materials. Epoxy resins play an important role there. “The resins then have to meet completely different requirements, for example they have to have short response times for large-scale production. We speak of resin systems, as further components are added in order to precisely set the desired properties, ”explains Karlhorst Klotz, editor-in-chief of the specialist journal“ Kunststoffe ”. In the trade, such systems have to react more robustly to deviations from the ideal conditions. Since wood and resin form a remarkably stable bond, epoxy-based casting resins are also used to reinforce and repair load-bearing components.

Build up in layers

Large-volume casting work with high optical requirements corresponds less to the thrust of the chemical industry. "If the resin does not have to be crystal clear, but if it is colored, then more products come into question that also work," says Mario Burkart. For all products, however, it is true that they cannot be cast in one layer, but have to be built up in several layers, the expert continues.

Obstacle course with a clear goal

Christian Petkov from Rostock, Germany, has also experimented a lot and is now also building massive table tops with wood and resin. “It was a very long and costly way to get the results. We talked to many manufacturers and they all told us that such dimensions don't work, ”explains Petkov.

They didn't want to accept that and tried out many different processes and changed the mixing ratios. This often meant that they had to dispose of the projects over and over again. Either it got way too hot too quickly in the reaction time, or it just didn't set. Also at the beginning the formwork was often leaky and the whole mass was spread over the floor. It wasn't easy because there are so many different factors to consider in order to get good results.

What to Know

Epoxy casting resins have a bulk density of a good gram per cubic centimeter and are therefore around twice as heavy as wood. A high resin content therefore makes for heavy furniture. Heat is released during the chemical reaction of the components, which is why it is important to observe the manufacturer's instructions on possible layer thicknesses, as otherwise the heat can increase enormously.

Tempo and bubbles

The more mass of epoxy resin you mix, the shorter the pot life becomes. In order to lengthen working hours, you have to reduce the mass, for example by pouring the mixture into a flat baking pan and applying it to the workpiece from there. With some products, you can avoid the formation of air bubbles by venting them in a vacuum. For others, such an approach has exactly the opposite effect. Air bubbles can then be removed by carefully running the flame of a propane gas burner over the cast surface.

Visible traces

When integrating wood in resin, you should always make a preliminary test with the original wood. “Because it can happen that the resin penetrates into the wood at the front, which then looks ugly. Especially with colored resins. If that happens, you should first seal off the wood with a primer, ”says Burkhart. With all suitable resins, the surface must be sanded and polished after casting.

That is why Burkart does not see mold making as that difficult. “The form of the formwork is not so relevant and can be made with synthetic resin-coated panels. However, the surface should be coated with an appropriate release agent. "

If you want to cast a table top with wood, you also need time and experience in Burkart's eyes.