Bonding with WEST SYSTEM epoxy in cold temperatures
Epoxy can be used under cold weather conditions. However, a few special application techniques and simple precautions should be employed to ensure that the epoxy performs well in cold weather in the long-term.
When an epoxy resin and hardener are mixed together, a chemical reaction is started which produces heat - an 'exothermic reaction'. The ambient temperature in which an epoxy chemical reaction takes place affects the rate or speed of this reaction. Warmer temperatures accelerate, while cooler temperatures retard the reaction time.
If the reaction is too slow, even though the epoxy may harden, it may not cure completely and possibly never achieve its designed physical properties. This is where danger lies, for improperly cured epoxy may possess enough strength to hold a structure together, yet may fail after repeated loadings during normal operation.
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Temperature has a profound effect on the working properties of uncured epoxy. Ambient temperature changes will dramatically change the viscosity (thickness) of the epoxy.
When cold, the viscosity of water varies little with temperature changes until it freezes. But temperature can have an effect that is 10 times greater on epoxy molecules than on water molecules over a temperature change of 15°C. Because of this, the colder it is, the thicker the epoxy becomes, significantly reducing its flow properties.
This change has three important consequences for working with epoxy under cold conditions.
1. It is more difficult to mix the resin and hardener thoroughly
The resin flows through the dispensing pumps and out of containers with much greater difficulty and both resin and hardener are prone to clinging to the surfaces of the pumps, containers and mixing tools. Remember, because of the low temperature, the chemical reaction is much slower and compounding a less efficient exothermic reaction with the potential for incomplete and/or inaccurate mixing, is a recipe for a permanently deficient bond.
2. The mixed epoxy is much harder to apply
This is because the viscosity is similar to cold honey and is extremely difficult to coat and wet-out surfaces.
3. Air bubbles may be introduced when mixing and remain in suspension
This may happen because of the increased surface tension of the cold epoxy. This can be especially troublesome in clear-finish applications and osmosis repair work.
Here are six basic rules for using epoxy effectively in cold weather.
1. Use WEST SYSTEM 205 Fast Hardener®
WEST SYSTEM 205 Hardener has been designed with a chemically-activated polyamine system which exhibits a good cure at temperatures as low as 5°C. It exhibits a fast cure characteristic and offers less uncured exposure time thereby reducing the chances of incomplete cure due to cold temperatures.
2. Dispense resin and hardener in the proper mixing ratio
All epoxies are formulated to a specific mix ratio of resin to hardener. It is important to mix epoxy in the precise ratio recommended by the manufacturer. Increasing the amount of hardener will not accelerate cure but it will seriously compromise the ultimate strength of the cured epoxy. Please note that WEST SYSTEM Mini Pumps are designed and calibrated to dispense the correct ratio with one pump stroke of resin for every one pump stroke of hardener.
3. Warm resin and hardener before using
As discussed above, the warmer the resin and hardener, the lower the viscosity. Thinner (lower viscosity) resin and hardener will flow through dispensing pumps better, cling less to containers and mixing equipment and exhibit superior handling and wet-out characteristics.
The two epoxy components can be warmed using heat lamps or kept in a warm area until they are needed. Another simple method of warming the resin and hardener is to construct a small hot box out of rigid sheets of foil-backed insulation. Place a regular light bulb or an electric heating pad inside to maintain a temperature no greater than 30°C.
4. Stir the resin and hardener thoroughly
Use extra care when mixing the resin and hardener and mix for a longer period of time than normal. Scrape the sides and bottom of the mixing container using a mixing stick to reach the edges. Using a smaller diameter mixing pot will also improve the chemical activity because the limited surface area will contain the heat produced by the reaction.
5. Warm working surfaces
Applying warmed epoxy to a cold structure will quickly retard the molecular bonding activity of the epoxy. Ensure the structure and the surrounding area is brought up to temperature. A hull, for example, which is colder than the surrounding air may exhibit condensation and this moisture could contaminate the epoxy when it is applied. Warm the structure as much as possible.
This can be done by constructing tents around small areas and heating with portable heaters or warming the area with hot air guns or heat lamps. Small components or materials e.g. glass cloth, can be warmed before use in a hot box as described above.
6. Prepare surfaces carefully between applications
When coating under cold conditions, a thin film of epoxy does not generate much heat. The rate or speed of cure is therefore extended and some reaction with moisture in the atmosphere may occur, resulting in the formation of an amine blush on the cured surface. Immediately prior to applying subsequent coatings, wash the surface with clean water, allow it to dry thoroughly and sand.
WEST SYSTEM materials should be stored above 10°C with the container caps screwed down tightly. Storing epoxy resin in extreme cold may cause crystallisation. However, the formation of crystals does not compromise the resin. The situation can be remedied easily.
Heat water in a pot large enough to hold the epoxy resin container. Remove the lid of the resin container to avoid pressure build-up and place the container in the hot water. Be careful to ensure no water enters the resin container. Stir the epoxy with a clean stick until the liquid regains clarity and all crystals have melted. Remove from the water, replace the lid tightly and invert the container to melt any crystals which may be clinging to the top of the container. If the resin pump has crystallised, pumping warm resin through should dissolve the crystals.