High Temperature Silicone Adhesive
Several engineering projects require that materials be durable and can endure harsh environments. Silicone adhesives are lasting and able to hold up to high temperatures.
There are many types of silicone. Select the type that best meets your needs. Options include low modulus, which requires minimal force to stretch, neutral cure, which releases an odorless alcohol and acetoxy silicone sealants.
Pressure Sensitive Adhesives
Pressure sensitive adhesives (PSA) require a low amount of pressure to bond the tape with surfaces. This allows them to be used with different types of equipment and allows a wider range of applications. They do not require water, heat, or solvents to activate. They also rely on shear and peel strength rather than lap shear strength to determine their adhesion capabilities.
They are available in a wide variety of base-chemistry formulations. This determines, in part, their bonding capabilities to various substrates and their resistance to high temperature silicone adhesive chemicals and environmental conditions. Silicone PSAs typically offer the best shear and peel strength compared to rubber-based or acrylic PSAs.
These types of PSAs are able to withstand high temperatures without losing their initial tack. They are formulated to adhere to many different types of surfaces, including plastic films that may need to be treated with corona or plasma processing. They also provide good resistance to environmental factors such as UV radiation, ozone and weathering for outdoor use.
One example of a silicone high temperature PSA is the Henkel Contractors Silicone. This is a one-part, economical acetic cure that withstands up to 180°C without slumping or yellowing. It can be used for glazing, sealing and bonding applications. It is compatible with most metals, glass, woods and plastics. It also dries clear and is VOC compliant.
One Part Adhesives
One component adhesives, also called self-curing or single-component, are available in a wide variety of formulations and recipes. The different ingredients in these adhesives allow bonded joints to have differing levels of hardness, visco-plasticity and elasticized properties, as well as media-resistant characteristics. They are typically cured by heat or exposure to UV light. They are easy to use, with working times from a few minutes up to an hour depending on the recipe.
Silicone is a popular choice for many applications because it is highly flexible and resists mildew. It is often used in bathrooms and kitchens to seal sinks, showers and bathtubs because it can provide a clean and neat seal, guard against the formation of mildew residue and remain unaffected by hot or humid conditions. It is also commonly used as a weather seal for doors and windows because it provides an airtight seal, eliminates air exchange and is unaffected by extreme temperatures or the elements for very long periods of time.
Several types of one-part silicone are available that are resistant to high temperatures and can be used for formed-in-place gaskets up to 600oF. These are especially effective for automotive applications such as sealing and bonding valve covers, oil pans, water pumps and axle housings. This high temperature silicone adhesive type of silicone is also able to withstand vibration, humidity and chemicals that can corrode other adhesives and sealants.
Fast Cure Adhesives
In their zeal to lower assembly costs, engineers sometimes overemphasize cure speed. This can lead to the use of fast-curing adhesives that have physical property trade-offs that aren’t ideal for a given bonding application. It can also result in the misapplication of these adhesives, which can actually slow down production.
Cyanoacrylates are one of the most common examples. They can be very fast curing if the bonding surfaces are at optimum humidity, no air pockets are present, and they are not over-applied. However, they can take longer than an epoxy that doesn’t require a heat cycle to reach their full cure strength. If this is the case, a chemical accelerator may be needed to boost the cure speed of these products.
The same goes for other fast-cure adhesives. It’s important to understand their limitations and not use them in applications where they aren’t a good fit. This can include those requiring a thermally sensitive adhesive, and those with stringent mechanical properties that don’t match up well with other fast-cure materials.
Two-part fast epoxies are often the best choice for applications with these constraints. They have set-up times as quick as a minute, don’t need a heat cycle to initiate the cure, and offer the best balance of mechanical and physical properties in this fast-cure class. They do have the drawback of requiring special mixing and dispensing equipment, which should be factored into any capital investment decisions regarding these products.
High Temperature Adhesives
When bonding items that will be exposed to high temperatures, you need an adhesive that can handle them as well. Heat resistant silicones are able to withstand a range of temperatures, including direct exposure to flames. They can also withstand humidity and moisture, which makes them popular in home improvement projects. They can be used to seal and bond items that are prone to exposure to extreme heat, such as fireplace inserts or solar panels.
When selecting a high temperature silicone adhesive, consider the type of materials you’ll be working with and the environment in which the item will be installed. Different types of silicones are available for specific materials, and some offer different levels of elongation, flexibility and other characteristics. For example, low modulus silicones will stretch more easily, whereas high modulus silicones are stiffer. Acetoxy silicones release a vinegar-smelling odor and only stick to metals, while neutral alkoxy sealants are safe to use on a wide variety of materials.
When working with high temperature silicones, it’s important to follow the manufacturer’s instructions for application. Most of these products need to be fully cured before they can be used, and this may take up to 24 hours. During this time, it’s best to choose a workspace that can be kept clean and away from children or pets, particularly if the adhesive or sealant has an unpleasant odor.