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Epoxy and Adhesives

What is epoxy?

Epoxy is a two-part polymer consisting of a resin component and a hardener component. When the two liquid components are mixed, the resin and hardener molecules begin to crosslink, creating an exothermic (heat-producing) reaction, causing the material to cure to a hard plastic solid. There are also epoxy resin and hardener components that are in a putty form; when combined, they cure to a hard solid that can be tooled, machined, and painted.

What is epoxy used for?

Due to its superior strength, strong adhesion to a number of different substrates, waterproof properties and excellent chemical resistance, epoxy is important in many industries, including marine, aerospace, construction, consumer, and industrial. It is used in structural adhesives, fairing compounds, repair putties, fiber-reinforced plastics (FRP), coatings, primers, paints, sealers, flooring, and more.

Examples of products made from or coated with epoxy materials include wind turbine blades, high-tech boat hulls, composite parts, aircraft, automotive parts, bicycles, snowboards, electronic insulators, generators, transformers, terrazzo flooring, and household appliances.

How does epoxy cure?

Epoxy cures through an exothermic reaction that occurs when the epoxy resin and hardener components are mixed together at an engineered ratio. In this chemical reaction, the molecules in the two components react and begin to crosslink, which generates heat. Other factors that affect the cure rate of epoxy include ambient air temperature, substrate temperature, hardener speed, the depth of the epoxy pour, and the mass of the mixed epoxy. For example, warmer temperatures make epoxy cure faster, while cooler temperatures result in a slower cure.

Some epoxy systems offer different hardener options for slower or faster reactions, or to achieve different working properties or cure rates. An epoxy will cure faster and generate more heat if it is applied thicker, or in a larger mass, due to the concentration of material and lower heat dissipation. Applying an epoxy in a thinner film, or smaller mass will slow the cure rate and generate less heat. Most ambient temperature epoxies take 5-10 days to fully through-cure.

Are there different types of epoxy? Are all epoxies the same?

All epoxies are not the same. When choosing an epoxy for your project, it's important that you know the gel time, working time, pot life, cure time, sag resistance, chemical resistance, finish clarity, and any other properties that will make the epoxy system ideal for your intended application. In general, the two main types of epoxy include one-component and two-component systems.

One-component epoxies are ready to use - there's no need to pay attention to mix ratios. The benefits include ease of application, reduced waste and accelerated productivity. Such no-mix systems are used in a number of industries, including optical, medical, electrical, aerospace, and automotive.

Two-component epoxy systems feature a separate resin component and hardener component. The resin and hardener must be mixed at the precise ratio recommended in order for the material to cure properly. Cured two-component epoxies are desired for their tensile and flexural strength, adhesion to a variety of substrates, and other properties. They're used in boat building and repair, construction, art, jewelry making, automotive parts and more. Some epoxy systems can be cast, turned, machined, or poured in deeper layers than others. Some epoxies are better for casting and some are better for coating. Some are clear, and some have an amber color, depending on the hardener used. The cure speed is also dependent on the epoxy hardener.

As alternatives to pure epoxy, and depending on the properties and performance desired, you could also consider two other types of resin - polyester resin and epoxy acrylates. These two resins cure at a faster rate,, and polyester resins can typically be used in cooler temperatures than epoxy. Acrylates cure more quickly than pure epoxy and they offer good chemical resistance.

What's the difference between epoxy and polyester resin?

Epoxy and polyester resin both cross link and generate heat throughout their cure, but are very different materials. Unsaturated polyester resins require the addition of a catalyst to control the rate of the cure at ambient temperatures, while epoxy consists of a resin and a hardener. Both are used as adhesives or for lamination with reinforcement fabrics. Polyester resin generally has decent UV resilience when compared to epoxy resin, while epoxies generally boast better adhesion, greater strength properties, greater chemical resistance, overall durability, and will not shrink once through-cured. Polyester resin has limited bond strength when applied over many substrates, while epoxy has a much higher secondary bond strength to a wider range of substrates. Polyester resin is also more cost effective, by comparison.

Is epoxy safe? Is epoxy resin toxic?

For your safety, it's important to wear a proper respirator to prevent health problems that can result from inhaling epoxy fumes or dust. Do not breathe any fumes from the resin and hardener components separately, or once they're mixed together, and do not inhale dust generated by sanding cured or partially cured epoxy.

Take extra care to avoid getting resin or hardener on your skin or in your eyes, or overexposure to epoxy. Skin contact can cause inflammation, or contact dermatitis. A more serious condition known as epoxy sensitization (allergic dermatitis) occurs when you develop allergic reactions to epoxy. These reactions include serious respiratory problems and skin irritations resembling poison ivy include red eyes, itching and swelling. It's best to avoid any exposure to epoxy because sensitization can occur after many exposures over an extended period - or after a single exposure. Cured epoxy that was properly mixed and cured is generally inert, but in most cases it should not be considered food safe or safe for food contact unless the manufacturer specifies that it is.

Is epoxy waterproof or water-resistant?

Yes, epoxy is 100% waterproof if it has been mixed at the proper ratio and allowed to fully cure under the specified conditions. For this reason, epoxy makes an ideal sealer for porous wood. Its ability to repel water is also one of the main reasons epoxy resin is used as a waterproof finish coating for wood, metal and other surfaces. It prevents wood rot and metal corrosion. As a water-repellent material, epoxy is also ideal for permanently repairing leaks around the house or on a boat.

Can epoxy be removed?

It's possible to remove epoxy, but the method you use depends on whether the epoxy is mixed (resin and hardener combined) and whether it's cured or uncured. It's important to wear proper personal protection and have adequate ventilation before removing epoxy. Uncured epoxy resin is easy to remove with lacquer thinner, acetone, denatured alcohol, or white vinegar; most, but not all, uncured hardener can be removed with soap and water or denatured alcohol. Mixed but uncured epoxy can be removed with denatured alcohol or white vinegar, in most cases.

Cured epoxy must be removed mechanically. If there's only a small amount, try to remove it by sanding or scraping gently. Epoxy can be removed from some substrates by heating it with a torch or heat gun, but this should only be performed if it will not damage the substrate and if it can be performed safely. If any material remains, sand or grind the surface to remove it. Always take extra care to avoid having liquid epoxy come in contact with skin. If you get epoxy on your skin, wash the area immediately with soap and water, and rinse thoroughly. A pumice-type soap can help if the resin is sticky. Do not use any type of solvent (such as acetone or denatured alcohol) to remove epoxy from your skin.

What's the difference between epoxy and glue?

Though epoxy can be used to glue things together, not all epoxies are designed as adhesives. Epoxy is a two-component material that consists of epoxy resin and hardener. When the two components are combined, they begin to crosslink at the molecular level, producing an exothermic reaction that triggers curing at room temperature.

Glue is a one-component material that cures when exposed to air. Epoxy is a petroleum-based material, whereas glue typically refers to cyanoacrylates (such as Super Glue), resin adhesives, polyurethane wood glue and regular household glue (such as Elmer's). Cyanoacrylates (aka instant adhesives) are best for smaller-scale applications, such as general household repairs, where a fast cure time and high strength are important.

Resin adhesives (aka resinous solvent adhesives) are typically made from one of the four most common resins - polyester, polyurethane, silicone and epoxy. Although all epoxy adhesives contain resins, resin adhesives also contain other compounds designed to produce varied adhesive properties. As with epoxies, resin adhesives have two components that must be mixed and cured properly for maximum bonding strength. Resin adhesives also come in different forms, including powder, emulsion, spray and liquid.

Why does epoxy turn yellow?

Epoxy components or cured epoxy can take on a yellowed appearance if exposed to UV light, oxygen, moisture, or high temperatures. In the containers, the hardener can typically turn yellow before the resin does. Even if the plastic bottles are unopened, yellowing can still occur, especially if the bottles have not been stored properly. It's important to pay attention to where you store the components (do not expose them to UV light, heat or moisture), how long they've been sitting on the shelf (old components will yellow), keep the lids on tight during storage, and put the lids on the resin and hardener bottles immediately after dispensing.

Yellowing can occur in cured epoxy if the curing reaction was too hot or if there was any moisture on the surface when the epoxy was poured. If the epoxy cured properly and crystal clear, it can turn yellow if subjected to continuous UV exposure. Keep cured epoxy surfaces out of direct sunlight, or coat the surface with a UV stable polyurethane or varnish finish.

What safety precautions are needed when using epoxy?

Take proper safety precautions when working with epoxy resin and hardener in any form - liquid, putty, uncured, or in the form of dust generated by sanding cured or partially cured epoxy. Be sure there is adequate ventilation in your work area, and wear proper personal protective equipment (PPE) including protection for hands (disposable gloves), skin (disposable paper suit or sleeves), face (face shield), eyes (safety glasses), lungs (proper respirator) and clothing (apron, paper suit, paper sleeves).

When you are done, dispose of epoxy properly according to local, state, federal and international regulations. Once the resin and hardener components have been mixed together, work diligently to use the epoxy because any that remains in the mixing cup beyond its pot life and working time will heat up at an extremely fast rate, and could start smoking and even melt the plastic mixing cup.

What tools are used to apply epoxy?

The tools you need to apply epoxy depend on the application. For coating a surface,, use a plastic spreader, plastic squeegee, disposable chip brush, or a short nap, epoxy-safe, solvent-safe roller cover. For laminating fiberglass or carbon fiber cloth, use a disposable chip brush, a plastic epoxy spreader, or special laminating rollers. For injecting epoxy into cracks, holes, and gaps, use a pipette, a plastic syringe, or an acid brush (glue brush). For fairing applications, use a putty knife, plastic spreader, plastic squeegee, fairing batten or other contoured tool. For bonding with thickened epoxy, use a chip brush, acid brush, plastic squeegee or a notched spreader.

A digital gram scale is important to have on hand if measuring epoxy by weight. In addition, plastic mixing pots and stir sticks are needed to mix resin and hardener. Always protect yourself by wearing proper PPE (personal protective equipment), including a proper respirator, protective gloves, and protection for eyes, face, skin and clothing.