Silicone adhesives offer flexibility and high heat resistance, making them suitable for a wide range of applications in the electrical, electronic, automotive, life science, and construction industries.
Silicone Rubber Used as an Adhesive
An adhesive is a substance that is applied to the surfaces of materials to bind them together and resist separation. Adhesives are typically organized by the method of the adhesion, divided into reactive or non-reactive adhesives, and have natural or synthetic origins. A polymer adhesive is a synthetic bonding substance made from different polymers (reactive or non-reactive) and is considered stronger, more flexible, and has a higher impact resistance than other forms of adhesives. They are used in automotive, aerospace, aviation, construction, electronics, and electrical industries. Due to the infinite possibilities of polymer types, fillers, and additives, an adhesive can be created according to specific needs of the application with great precision. To design a good adhesive connection, some factors must be taken into account: the adhesive properties and the material information. In the first, cure speed, environmental resistance, thermal resistance, and suitability for automation are important. In relation to the material information, the focus is on the properties of the material that will be bonded and its surface treatments. Some materials are inherently difficult to bond and may require surface roughening and primers to promote adhesion. These are among the most common processes used to improve the adhesion.
Chemistry of Silicone Adhesives
Silicone adhesives are supplied as either a one-part or two-part system. A one-part system cures through moisture in the air or is heated using UV or EB radiation. The second, a two-part system requires the addition of a curing agent and the mixing of the two components.
The high energy of the siloxane bonds give silicones unique high-temperature performance properties. They cure to soft thermoset elastomers with excellent property retention over a wide temperature range. The complete curing for thick sections of silicone takes a few days, but the strength may continue to increase for several weeks. This is due to the slow reaction between reactive groups on the silicone polymer and the reactive groups on the substrate surface, even though the cross-linking reaction inside the silicone occurs quickly. Silicone adhesives can be categorized according to their curing reaction. The moisture curing silicones are the most important silicone adhesives. They can also be categorized by their by-product: acetoxy silicones gives off acetic acid (but promotes corrosion), oxime silicone gives off methyl ethyl ketoxime (they have lower adhesion and slower curing reaction), and alkoxy silicones give off alcohol. The latter is the best in this group because they cure rapidly and develop good adhesion to many substrates.
Applications for Silicone Bonding
Silicone’s unique properties make it an excellent fit for many industries and applications. It bonds with commonly used substrates like metals, glass, and carbon. The life sciences industry, for example, often uses metals because of their desirable biocompatibility, durability, and resilience properties. Bonding these substrates with silicone with these same attributes extends these properties to the entire product — not just the metal — bringing these essential qualities to devices and other medical equipment.
Other industries that commonly use silicone as an adhesive include automotive, safety, and consumer product manufacturing. Many of these industries need adhesives that maintain their strength in demanding environments, like those with extreme temperatures, water submersion, vibrations, and contaminants. These industries might use silicone bonding to provide electrical and thermal insulation–for seals, gaskets, diaphragms, valves, masks, and housings.
Advantages and Disadvantages
Some of the benefits of using an liquid silicone rubber (LSR) adhesive over other materials include:
- Heat and electrical resistance: Silicone already has excellent electrical properties and can take on special formulations to become insulated with high dielectric strength, or conversely electrically conductive.
- Biocompatibility: The exceptional safety and hygiene profile of LSR allows it to be used when in contact with skin, food, and body.
- Versatility: Liquid silicone adhesives are easy to customize and formulate to meet different specifications. They can meet stringent quality standards, adhere to many different materials, and take on an array of shapes for greater design flexibility.
- Flexibility: Since LSR has a flexible, soft consistency, it can be used for applications that need malleability or movement, like thin diaphragms and pressure-sensitive components.
- Durability: LSR retains its shape and performance when compressed in a wide range of temperatures, making it an especially well-suited for push buttons and pumping applications.
While silicone offers excellent performance in many scenarios, there are also a few drawbacks to consider:
- Upfront costs: Since it must be chemically bonded to the components, it calls for advanced equipment for formulating, curing, and molding the rubber.
- Corrosion: Although some silicone adhesives release corrosive substances, special formulations are non-corrosive and chemically and thermally stable.
- Slower curing times: Silicone can take a little longer than some other materials to cure, so it may not be best for particularly fast-paced assembly.
Bonding Without Adhesives
As an alternative to using adhesives for bonding an LSR material with a plastic or metal substrate, a special injection molding process and a self-adhesive grade LSR may be the best solution. In a multi-shot injection molding process,the LSR part and thermoplastic part are molded and bonded together in the same process providing a superior bond, without secondary adhesives. When combining an LSR with a metal substrate, an overmolding process may be used. In an LSR overmolding process, a self-adhesive LSR is molded over the metal substrate within the same molding process.
At SIMTEC we help customers across industries leverage the advantages of liquid silicone rubber to select the best solution possible. We support you from start to finish.
Reach out to us for your next project and together we will find the best solution to meet your requirements