High Temperature Resistant Adhesives Beat the Heat

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Clinton Melton
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1 High Resistant Adhesives Beat the Heat Too hot, too cold or just right? It s not just a question for Goldilocks. Engineers have to be even better judges of temperature when they select a materials system for the job at hand. And those jobs increasingly involve temperature conditions that are best described as extreme. It s no longer uncommon for aerospace, oil-and-gas, industrial and some electronics applications to require materials systems that withstand temperatures of 300 C or greater. Other applications have to contend with extreme cold, even down to cryogenic temperatures in the neighborhood of 4K. Both the high and low end of this temperature range can be a challenge for adhesives, encapsulants and sealants. Yet there are adhesive families that rise to that challenge when operating temperatures reach extreme levels. Epoxies may come to mind first because they are without equal at functioning across the temperature spectrum. But there are other adhesive families that tolerate temperature extremes. This list includes some silicones and a newly developed bismaleimide adhesive that offers even better high temperature resistance than the best epoxy. Picking the right adhesive product for extreme temperature applications may seem straightforward. After all, just about every adhesive supplier publishes temperature resistance values on their data sheets. Relying on that data, engineers will sometimes address temperature issues by simply selecting an adhesive rated for temperatures beyond their application s expected operating temperature. Unfortunately, good design practice is not that simple. resistance values on data sheets are notoriously inconsistent, in part because suppliers test adhesives so differently with some suppliers taking a far more conservative approach to reporting temperature data than others. The bottom line about this easy-toobtain data: Go ahead and use it as starting point when investigating temperature resistant adhesives, but do not consider it a reliable indication of actual continuous use temperatures. To select the right temperature resistant adhesive for a given application, you will have to dig a bit deeper than a line or two on a data sheet. Here s a closer look at some of the key factors to consider when your adhesive application has to beat the heat or cope with the cold: Get To Know The Glass Transition As polymeric materials, adhesives share a generalized thermo-mechanical response to temperature extremes. As the temperature rises past a certain point, the adhesive will begin to soften and lose some tensile strength. The adhesive will also experience a rise in its coefficient of thermal expansion (CTE). The certain point at which very high temperatures start to pose a problem differs with the individual adhesive and the application requirements. But the adhesive s glass transition temperature (T g) provides a window on where that point lies. Not to be confused with a melting point, T g is the temperature at which thermosetting amorphous polymers including most temperature resistant adhesives change from a rigid glassy state to a more pliable rubbery state. This intrinsic thermal property serves as a good indication of an adhesive product s ability to stand up to an application s temperature requirements. For example, it would not be wise to pick a structural adhesive whose T g is 100 C less than application s continuous use temperature. To do so would be inviting failure. Even in less obvious cases, picking an adhesive with too low a T g can exacerbate more subtle failure mechanisms such as creep and thermal stresses from CTE mismatch. 2

2 With a couple of important exceptions, adhesives with the best heat resistance tend to have high T g values, making them rigid across their operating temperature range. Epoxies used to be unmatched in this regard with some grades offering T g of 230 C and service temperatures up to 315 C. Master Bond s newly developed bismaleimide adhesive (see sidebar) ups the ante on heat resistance substantially with a Tg of 300 C and predicted service temperatures in excess of 340 C. The exception to this relationship between heat resistance and a high T g involves some silicones and B-stage epoxies. The unique nature of silicones molecular backbone and B-stage epoxies flexible cure state allows these adhesives to combine relatively low T g and decent heat resistance though not as good as the best epoxies or bismaleimide. Master Bond offers B-stage epoxies, for instance, that have T g 35 C and service temperature up to 260 C. Those differences between the measured T g and maximum service temperature values highlight the fact that the two are not the same thing. Plenty of successful real world applications have employed adhesives whose operating temperatures exceed T g for short periods of time or by small margins that will not cause mechanical properties to degrade enough to matter. So while T g is not a shorthand for continuous use temperature, it does serve as an indicator of good design practice in high-temperature applications. By selecting an adhesive whose T g is above the expected service temperature, you can reduce the risk of inadequate mechanical properties or thermal stresses. A Look At Cold Cases At very cold temperatures, T g does not provide the same clear window into adhesive performance as it does in high temperature applications. As temperatures dip farther and farther below the T g, adhesives become increasingly brittle and subject to low failure stresses. That reasoning would in theory seem to favor flexible adhesives with low T g values for the coldest applications. Yet in practice, the opposite is often true. Epoxy adhesives in particular do not experience a significant loss of properties even at cryogenic temperatures meaning they work best in a rigid state that extends from their T g into far colder territory. To take an example, Master Bond makes one- and two-component epoxy adhesives that provide structural bonds from cryogenic 4K to a blistering 205 C. Other high temperature epoxies likewise exhibit excellent performance at cold temperatures above cryogenic levels. PLAYING IT TOO SAFE LIMITS CHOICES Designing with a safety factor is something that is second nature to careful engineers, and it s a practice that makes sense when applied to adhesive selection. But you can play it too safe, especially when selecting adhesives for thermal resistance. By adding too much of a cushion to account for possibility of high thermal loads, you can limit your adhesive choices or even end up with the wrong product for your application. Our technical service engineers often run across cases where an engineer will determine thermal safety margins based on the temperature at which a design will theoretically fail even if that temperature is wildly unrealistic from a real world perspective. Exaggerating maximum use temperature as a safety measure usually results in the selection of adhesives with higher glass transition temperatures (T g). That s fine if you actually need all of that added temperature resistance. However, if you don t, you may end up with an adhesive that requires more difficult curing and handling methods. For example, you may find yourself moving from a product with simple room-temperature cure to a product that requires an oven cure. What s more, adhesives with a higher T g tend to be more rigid materials, which provide less give in thermal cycling applications. By opting for more temperature resistance than you actually need, you can unwittingly sacrifice a bit of thermal cycling capabilities. One often neglected factor in determining an appropriate margin of safety is the duration of exposure to elevated temperatures. Many adhesives can withstand 300 C for a few seconds. Increase that exposure to hours, days or months, and the list of suitable adhesive products shrinks to a few epoxies and bismaleimides with high T g. The message here is not to ignore safety margins, but to determine them realistically and with an eye to the length of exposure. 3

3 This ability to work in environments that mix extreme heat and cold is particularly important in aerospace applications. Adhesives with a low T g that function just fine in a purely cryogenic environment tend to exhibit large CTEs as they heat up with all the thermal stress problems that large CTEs can imply. High T g adhesives tend to have smaller, manageable CTEs across their entire operating temperature range, and may be more suitable for these mixed extreme environments. Managing Minor Trade-offs The good news about temperature resistant adhesives is that they offer a balance of properties that requires little in the way of design trade-offs. These epoxies, silicones and bismaleimides offer all the physical and mechanical properties required to address a wide range of structural bonding, encapsulation and sealing applications. resistant adhesive products are also available with added functional attributes including low-outgassing behavior, thermal conductivity, electrical conductivity, biocompatibility and more. (See Table One for a list of popular high temperature and cryogenic grades). Of course, there is no free lunch in the engineering world, and the price of enhanced temperature resistance comes in the form of more difficult and costly curing and mixing regimens. With epoxies, the grades with the very best temperature resistance are two-component systems requiring both mixing and an oven cure, possibly with fixturing. One-component systems needing an oven cure are next best in the temperature resistance department. Systems requiring room temperature cure, while easiest to use, trade off some temperature resistance. resistant products also require careful attention to the manufacturer s cure recommendations. While that advice applies to most adhesive applications, it s all the more crucial with temperature resistant products because the T g can be lowered by improper curing. Simply put, optimal properties and an optimal cure go hand in hand. So far, though, the bit of extra work required by temperature resistant products hasn t been much of an issue out in the field. These adhesives continue to grow in popularity as engineers push structural bonding and encapsulation into ever more extreme environments. A wide range of adhesive products have been formulated to withstand elevated temperatures found in industrial, oil-andgas and electronics applications. For further information on this article, for answers to any adhesives applications questions, or for information on any Master Bond products, please contact our technical experts at Tel: +1 (201)

4 POPULAR MASTER BOND HIGH TEMPERATURE RESISTANT APPLICATIONS Two Component Epoxies Master Bond Grade Mix Ratio by weight Color Code Mixed Viscosity RT, cps Set-Up Time Minutes, RT Cure Schedule Temp/Time, F Service Temp Range, F Applications EP21TDCHT 100/100 B amber 100, , RT F +350 F Toughened system is well suited for bonding & sealing of dissimilar substrates. Thermal cycling. EP30HT 100/25 B clear 35,000-45, RT F Transparent system with superb physical strength, chemical resistance, and electrical insulation properties. EP42HT-2 100/40 B amber 3,000-4, RT F +435 F Biocompatible. Resists repeated chemical, ETO, radiation & steam sterilization. Low viscosity. EP46HT-1 100/35 A tan B brown 27,000-30,000 >24 hrs 2-3 hours at 250 F 300 F +550 F Features a glass transition temperature in excess of 235 C. Requires oven cure. EP /5 or 100/10 B tan 8,000-10, hrs F F +300 F Adhesive/sealant. Long working life. Requires cure at 150 F-200 F. 45HTQ 100/30 A tan B brown 100, , hrs F plus F -80 F to +450 F Downhole application: adhesives and potting; Mineral filled. Requires heat cure. Outstanding chemical resistance. EP112FLAN-1 100/80 A gray B gray 25,000-35, hrs 2-3 hours at 220 F 250 F +500 F Flexibilized, low viscosity. Thermally conductive for potting & serviceable up to 500 F. One Component Epoxies Master Bond Grade Viscosity RT, cps Color Code Storage Stability, RT Cure Schedule Temp/Time, F Service Temp Range, F Applications 10HT >250,000 gray 6 months 4K to 400 F Cryogenically serviceable. NASA low outgassing. Superb thermal cycling & high strength properties. EP17HT ,000 yellow to brown 6 months F F Snap curing, high temperature resistant epoxy. Can cure in sections up to 1/4 thick. EP19HT amber clear 4 months Ultra low viscosity, for impregnations, coatings, and laminations. EP36CLV solid soft white to amber clear 1 year F 500 F Flexible epoxy resin B-stage for casting, sealing and impregnations. For sustained service up to 500 F. 3AOHT paste off-white to light yellow 6 months F F +350 F Fast curing and thermally conductive. Good thermal cycling properties. 10HTS paste silver 3 months 4K to 400 F Low volume resistivity. Used for cryogenics, NASA Outgassing, die attach, and other circuit board applications. Mastersil ,000 black 6 months 7 77 F Up to 572 F One component, room temperature curing silicone RTV resists up to 600 F. 5

5 MEASURING GLASS TRANSITION TEMPERATURE THERMO MECHANICAL ANALYSIS CTE Vitreous T g DIFFERENTIAL SCANNING CALORIMETRY Vitreous Heat Capacity T g Rubbery Rubbery Thermoset adhesives, such as epoxies, do not melt before degrading in the way that thermoplastic materials do. Instead, these adhesives undergo a state change as they heat to a point known as the glass transition temperature (T g ). Above the T g, the adhesive remains in a rubbery, more ductile state. Below the T g, the adhesive becomes increasingly strong yet more brittle. The graphs here show how lab methods can determine T g by measuring changes either in the material s coefficient of thermal expansion (CTE) or in its heat capacity. As a rule of thumb, adhesives with the highest T g have the best heat resistance meaning that they deliver the best tensile properties at high temperatures. There are, however, some special materials with relatively low Tg and good heat resistance. The ductility exhibited by these materials makes them a good fit for thermal cycling applications. Exceptions aside, though, T g is often used as a shorthand for heat resistance in the adhesives world. 6

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