WEST Systems Epoxy Resins
Cold Weather Bonding
Cold Weather Techniques
We know that most epoxies perform well or, at least reach a higher percentage of their potential physical properties, at temperatures of 60F and above. Some epoxy systems or resin/hardener combinations are formulated to cure in temperatures as low as 35F. However, simply using a cold weather epoxy system at this temperature does not guarantee dependable bonds. A number of other factors can significantly affect your epoxy's bonding ability in cold weather.
Epoxies can be formulated to cure under a wide range of conditions. However, each epoxy system or resin/hardener combination must be used within specific limits of application temperature, humidity, joint fit, surface preparation and cure time. They rely on a complex chemical reaction to achieve their strength and longevity, and disregarding their limitations can drastically affect the outcome of the chemical reaction and compromise the performance of epoxy bonds.
Epoxy can be used at cold temperatures, but must be handled and applied with techniques adapted to cold temperatures. This paper will discuss how epoxies work, why they perform differently under cold conditions, and what steps you can take to assure dependable bonds in cold weather.
Mixing an epoxy resin and hardener together starts a chemical reaction which produces heat. This is called an exothermic reaction. The surrounding temperature affects the rate of the reaction and the degree of cure. Warmer temperatures accelerate the reaction, while cooler temperatures retard the reaction and reduce the crosslinking activity of the epoxy molecules. More time is now required to achieve the same degree of cure, or crosslinking, as occurs in a shorter period at higher temperatures. If the temperature is too low, the epoxy may eventually harden, but may not reach a complete cure or achieve its designed physical properties. Although the partially cured epoxy may have enough strength to hold the structure together, it could fail prematurely.
Epoxy joints in various structures are subject to different types of load during their working life. Many of the joints in a boat for example may be subject to millions of small repeated (fatigue) loads. Others must resist slow stretching and deforming under steady loads (creep-rupture).
One of the dangers in using epoxy in cold weather, is that epoxy that has not cured completely will be more flexible. Testing clearly demonstrates that increased flexibility seriously reduces an adhesive's ability to resist fatigue and creep-rupture. Information about fatigue, fatigue testing, and the how flexibility affects the fatigue life of epoxy, can be found in FATIGUE ASPECTS OF EPOXY AND EPOXY/WOOD COMPOSITE MATERIALS 002-545, published by Gougeon Brothers. The notched beam test is a relatively simple test that demonstrates a flexible epoxy's inability to resist creep-rupture. Information about how you can perform this test yourself can be found in the paper NOTCHED BEAM TEST FOR CREEP-RUPTURE 002-815, published by Gougeon Brothers.
Ambient temperature has a profound affect on the working or handling properties of uncured epoxy as well as its rate and degree of cure. A change in temperature will drastically change epoxy resin's viscosity, or thickness. The viscosity of water varies little with temperature changes until it either boils or freezes. However, temperature's affect on the viscosity of epoxy is much more obvious. As the temperature drops, epoxy becomes proportionally thicker, reducing its ability to flow out. This change has three important consequences when working with epoxies in cold conditions.
First, it is more difficult to meter and mix the resin and hardener. The cold resin and hardener do not flow through the dispensing pumps easily and the thicker material clings to the surfaces of the pumps, containers and mixing tools. Colder and thicker resin and hardener take much more time and effort to blend thoroughly. The potential for inaccurate metering and incomplete mixing, compounded by a less efficient chemical reaction, greatly increases the possibility of a deficient bond.
Second, the epoxy is much harder to apply. Cold temperatures make it much more difficult to coat and wet-out surfaces with epoxy. In coating applications at low temperatures, the epoxy mixture will not flow out as easily resulting in thicker, uneven coats that require more sanding to achieve a smooth finish. In bonding applications, the thicker epoxy may not wet out or penetrate porous surfaces enough for a reliable bond.
Third, air bubbles may be introduced during mixing or application and held in suspension due to the chilled epoxy's increased viscosity. Air bubbles reduce epoxy's strength in a bond and a coating's effectiveness as a moisture barrier. In addition, air bubbles show through clear coating applications.
We've told you all of the reasons why using epoxy in cold weather is difficult and risky. However, this doesn't mean that you can never use epoxy in cold weather. With a little advanced planning and by observing the following eight precautions, most of these problems can be overcome and their consequences avoided. We've used these techniques for over 20 years, because they help assure dependable epoxy bonds in cold weather.
1. Use WEST SYSTEMS 205 Fast Hardener. 205 Hardener has been designed with a polyamine system that cures well at temperatures as low as 35F. Keep in mind the extended cure time required before removing clamps or sanding. 206 Slow Hardener and 207 Special Coating Hardener should not be used below 60F without post-curing, and 209 Tropical Hardener should not be used below 65F without post-curing. For best results, some applications such as clear coating, for which 207 Hardener is designed, should be postponed until the temperature approaches normal room temperature (72F).
2. Warm resin and hardener before using. As mentioned, the warmer the resin and hardener, the lower the viscosity. Thinner resin and hardener will flow through pumps better, cling less to containers and mixing equipment, and mix more thoroughly. The initial chemical reaction will get off to a better start and result in more crosslinking even if the mixture cools after it is applied to a cooler surface. The thinner mixture will initially flow out smoother and wet-out porous surfaces better.
3. Warm the resin and hardener with heat lamps or keep it in a warm area until you are ready to use it. You can build a small portable hot box out of rigid sheets of foil-backed insulation, with a regular light bulb or an electric heating pad inside to maintain a temperature of 70F to 90F. This method allows you to keep the warm resin and hardener close to your work and allows less time to cool off between dispensing and application.
4. Dispense resin and hardener in the proper mixing ratio only. Altering the amount of hardener will seriously compromise the epoxy's ultimate strength. WEST SYSTEMS Mini Pumps are designed and calibrated to dispense the correct ratio--one full pump stroke of hardener for every one full pump stroke of resin. If you are not able to warm the resin and hardener, do not use excessive force when dispensing. Keep steady pressure on each pump and allow each pump head to make a full stroke down and a full stroke up. Remember, the resin and hardener become thicker and more difficult to pump when they are cold.
5. West Systems Stir SticksStir the resin and hardener thoroughly. Mix the resin and hardener longer than normal and scrape the sides and bottom of the mixing container. Use a mixing stick shaped to reach the corners of the pot. For a given volume of resin and hardener, a smaller diameter mixing pot will improve the chemical activity because the limited surface area will not dissipate heat produced by the reaction.
6. Warm the bonding surface as much as possible. The epoxy will thin out as it is applied to a warm surface. It will flow out much smoother and penetrate better, resulting in a stronger bond. Warming can be done by constructing tents around small areas and heating with portable heaters, warming the area with hot air guns, hair dryers or heat lamps. Small components or materials (such as fiberglass cloth) can be warmed before use in a hot box as described in above. Avoid unvented heaters that burn fossil fuels (kerosene or fuel oil). Unburned hydrocarbons have been known to contaminate bonding surfaces, and elevated moisture and CO2 levels may inhibit epoxy's cure.
Another temperature related problem occurs throughout the year, even in warm climates, when overnight temperatures drop well below daytime temperatures. The daily variation in temperature may cause moisture contamination problems if epoxy is applied to an exposed structure or surface too early in the day. A hull, for example, that has cooled overnight may remain colder than the surrounding air until the afternoon. Water vapor can condense on the cooler surface and affect the adhesion and cure of epoxy applied over it. If the bonding area cannot be heated, allow the surface and the surrounding area to come up to air temperature before applying epoxy.
7. Prepare surfaces carefully between applications. When coating at cold temperatures, the slower cure can result in the formation of an amine blush on the surface. The blush feels like a waxy film on the surface of the cured epoxy. Just before applying subsequent coatings, wash the surface with warm water using a 3-M Scotchbrite(TM) pad. Before the water evaporates, dry the surface with plain white paper towels and sand any remaining glossy areas with medium grit sandpaper.
8. Allow additional cure time before removing clamps or stressing joints. As a general rule, double the cure time for every 18F drop in temperature. Allow extra time for pre-stressed joints and joints that will be subject to high loads.
9. Post-cure the epoxy if possible. Post-curing can help to complete the epoxy mixture's crosslinking and boost the epoxy's physical properties even after a week or two of cold temperature. Post-curing simply is the process of applying heat to complete or speed the cure after the epoxy has reached a partial cure at ambient temperature. Elevate the temperature of the epoxy and substrate gradually to avoid thermal shock. Although any temperature elevation will improve crosslinking, try to boost the temperature to room temperature or warmer. The time required depends on the hardener used, the post-cure temperature and how much further the cure has to go. Generally, higher post-cure temperatures require shorter post-cure times. Do not exceed 140F and do not remove clamps or load the joint until after the final cure. Heating a porous material may cause air within the material to expand and "out-gas". If an epoxy coating applied over the material has not gelled enough before starting the post-cure, bubbles from the out-gassing material may show up in the cured coating. Allow the epoxy to reach an partial cure before post-curing.
A variety of post-cure techniques can be used. In some cases your shop will naturally warm itself enough to complete the cure during the day, following a cold night. Outdoors, building a plastic tent to trap solar heat can easily boost the temperature enough for post-cure even during cool weather. Turning up the thermostat, using radiant heaters, electric heaters or electric blankets are the most common way to control the post-cure temperature in a shop. It is not necessary to heat the entire structure if you are working on only a small area. Tents of plastic or insulated board are very helpful for confining heat to specific areas and provide greater mobility with a limited heat source, both indoors and outdoors.
Cold Weather Storage
It is best to store WEST SYSTEMS materials above 35F with the container caps screwed down tightly to avoid moisture contamination. Resin that is subject to freeze/thaw cycles may crystallize. However, the formation of crystals does not permanently harm the epoxy, and they can be removed easily. Place the open containers in a pan of hot water and stir the epoxy with a clean stick until all crystals have melted and the liquid becomes clear. Remove from the water, replace the lids tightly and invert the container to melt any crystals which may be clinging to the top of the container. If the resin in the pump has crystallized, pumping warm resin through it should dissolve the crystals.