Silicone reactive hot melts reduce downtime and increase worker safety. They also promise resistance to UV light, high and low temperatures, and water immersion
By Steve Block
TIPS FOR SELECTING INDUSTRIAL SEALANTBy Ross Noel With so many products to choose from, making sure you select the proper sealant is especially important. Selecting the wrong industrial sealant, or applying the correct sealant in the wrong manner, can have serious consequences. Keep the eventual application of your sealant in mind, and also make sure it has the following characteristics: 1. Temperature Stability Look for stability over a wide temperature range. Once fully cured, high-quality sealants will perform across a wide range of temperatures. The best sealants can withstand temperatures ranging from -85°F to +599°F (-65°C to 315°C). Select a sealant that functions reliably at temperatures outside the performance range to accommodate unexpected temperature changes. 2. Weather Resistance Weather resistance and chemical stability are important. UV rays, radiation, and weather can cause low-quality sealants to crack and crumble and become brittle, compromising the seal over time. Look for sealants with good resistance to these and other erosive factors. Also, some organic sealants react to atmospheric pollutants or chemicals in a way similar to how iron reacts with water to produce rust. Use a sealant that does not readily degrade after prolonged contact with common elements and industrial chemicals. Bond Strength Good industrial assembly sealants provide durable adhesion to a wide variety of industrial materials including glass, ceramics, wood masonry, and many metals and plastics. A variety of factors contribute to the bond strength of individual sealants including chemical composition, cure type, and substrate penetration. Electrical Properties If electrical properties are a concern, be aware that some organic sealants are less suitable for applications where they may be exposed to electricity. In such instances, a high-quality silicone sealant may be a better option. Low Flammability In fire conditions, some sealants are more burn-resistant than others. Silicone sealants, in particular, are especially reluctant to burn and many comply with UL flammability standards. Most important, make sure you select a sealant that can perform at the highest temperature you require but also offers a secure and flexible hold at lower temperatures. When in doubt, consult with a qualified expert. Ross Noel is a senior application engineer of technical service at Dow Corning.
Just the Facts About Silicone Reactive Hot Melts Silicone reactive hot melts acquire their initial strength by rapid cooling and then chemically react to form a resilient three-dimensional network. They are able to overcome thermal expansion variation, which is a frequent cause of sealing failure. They demonstrate excellent surface wetting characteristics to a wide range of common industrial substrates. They are non-corrosive, have no unpleasant odors, are non-flammable, and do not need local ventilation to remove off-gas by-products.
| It's crucial to select a sealant that can perform at the highest temperature you require but also offers a secure and flexible hold at lower temperatures. |
Hot melt adhesives and sealants were introduced decades ago to help increase production speeds in assembly applications. Recently, hot melt technology has been extended to a wide variety of applications, including industrial maintenance, but market growth has been limited due to several key drawbacks such as sagging at elevated temperature, cold flow, or low bond strength to certain substrates. A new generation of silicone reactive hot melts has been developed to improve bonding performance, reduce downtime, and increase worker safety. Silicone reactive hot melts acquire their initial strength by rapid cooling and then chemically react to form a resilient three-dimensional network. This new generation of sealants provides excellent resistance to ultraviolet light, high and low temperatures, and water immersion to further extend service life. Benefits of silicone hot melt technology in industrial maintenance include the following: Adhesion without primer to glass, metals, paints, and plastics Low volatile organic content (VOC) of less than 10 gr/liter Minimized repair downtime Non-hazardous to work with Ability to prevent contact corrosion for bonding to metals Elastic bonding of dissimilar materials with different thermal expansion coefficients Ability to absorb and dissipate energy from vibration
| click the image to enlarge Graph 1: Mechanical properties of silicone hot melt in function of temperature |
Let's review benefits in detail below. Bonding Performance:
Because of the unique stress relaxation capability and high elongation (up to 500 percent) properties of silicone, these new reactive hot melts are ideal for bonding substrates with different thermal expansion rates. They are able to overcome thermal expansion variation, which is a frequent cause of sealing failure. The dynamic energy forces are dispersed in the resin network. Substrate Adhesion:
Due to their low surface tension, adhesives generally show poor wetting on most metal substrates and require treatments that increase surface energies. These include flame, plasma, or corona treatment prior to applying the adhesives. Another pretreatment technique employs solvent-based primers, adding labor to the total unit cost. Silicone reactive hot melts demonstrate excellent surface wetting characteristics to a wide range of common industrial substrates. Their stress relaxation capability and high elongation also makes them ideal for bonding two materials with different coefficients of thermal expansion such as plastics and metals or glass. High Temperature Performance:
With the high operating temperatures typical of industrial equipment, an adhesive with low modulus at high temperature or low shear rate can provide better elastic recovery following continuous high and low temperature fluctuations. Graph 1 shows the visco-elastic properties of silicone hot melt as a function of temperature at 1 Hz and 1 percent deformation. The material shows a very high modulus at low temperature (or high frequency) and a low modulus at high temperature (or low frequency). With a G'/G" ratio greater than 1, the material naturally absorbs and disperses energy easily. At 100°C, under a constant tensile stress of 7,000 Pa, the silicone hot melt showed 30 percent deformation at equilibrium and remained unchanged with the stress maintained during six months at 100°C. The adhesive showed 100 percent elastic recovery when the stress was released.
| click the image to enlarge Graph 2: Viscosity/temperature dependence of silicone reactive hot melt |
Further developments have improved creep resistance of silicone reactive hot melts. Newly developed materials demonstrate only 10 percent deformation under 25,000 Pa constant stress at 70°C and 100 percent elastic recovery five minutes after load removal. Reduced Downtime:
Because the material is applied hot (120°C) as a liquid and acquires instant strength by cooling down, it can reduce downtime in industrial equipment maintenance operations. Easy-to-use hot melt sealants are usually dispensed from a gun applicator. The temperature required to achieve proper application viscosity ranges greatly depending on the hot melt material selected. The most common reactive hot melt system is based on moisture-curable polyurethanes with application temperatures that vary between 120°C and 180°C. Silicone reactive hot melts are applied at 120°C. Open time the time available to correctly position parts varies from one hot melt material to another. Most hot melts offer short open times of less than one minute. Silicone reactive hot melts provide 15 minutes of open time to position for repairs. Extended open time reduces opportunities for error and potential rework. Worker Safety:
With new VOC regulations limiting the use of primers, low VOC sealants are providing safer conditions for maintenance workers, and new 100 percent silicone neutral-curing material is characterized by a low VOC (less than 10g/l). Material by-products must also be considered in industrial plant maintenance applications. Silicone reactive hot melts are non-corrosive, have no unpleasant odors, are non-flammable, and do not need local ventilation to remove off-gas by-products. In addition, the lower application temperature of 120°C for this material reduces burn hazards. Recognizing the importance of the health and safety of plant workers, these advantages add to the material's overall value. In conclusion, silicone reactive hot melt technology is expanding to provide new improvements in industrial plant maintenance applications. Worker conditions, downtime expenses, substrate adhesion, and temperature performance are several considerations that will determine the appropriate hot melt material to meet your specific application. It's always wise to consult with the sealant provider to identify the best product to meet specific plant maintenance needs. Steve Block currently serves as the global technical manager for performance adhesives and sealants at Dow Corning Corp., which offers a wide range of silicone lubricants and sealants used in industrial equipment and maintenance. He leads, manages, and directs the global development and commercialization of new products, new technologies, and improved process technology associated with performance adhesives and sealants. He has held positions in manufacturing, product development, application engineering, and strategic business development during his 21 years at Dow Corning. He has a bachelor's degree in chemical engineering and a master's degree in engineering management. Additional information is available at www.dowcorning.com.