21 February 2020
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Application Spotlight: What’s the Difference between Adhesives & Sealants?

A common question that many design engineers have when assembling products is, how do I decide which adhesive or sealant to use for which application?
Adhesives and sealants are often made of similar materials and have similar processing times, meaning that the differences aren’t always obvious.

Although certain chemistries work better as one or the other, many adhesive technologies can also be formulated as sealants. However, sealants don’t usually have enough adhesion ability to hold two surfaces together. They are not used as primary bonding materials and are subject to creep under load.Sealants are usually used on outside surfaces.

What are the main differences between adhesives and sealants?

The main differences between adhesives and sealants are strength, and other physical characteristics associated with strength. Generally, adhesives have higher strength and lower elongation at break than sealants: adhesives are generally over 1000 psi lap shear while sealants are less than 1000 psi lap shear.

Consequently, adhesives are more rigid and durable than sealants, since they’re designed to keep two surfaces stuck to each other over long periods of time strongly enough so they can’t be separated. They have a more highly cross-linked, more complex molecular structure than that of sealants, which aids their ability to grip and bind surfaces together. They also have greater cohesiveness, which results in higher strength values.

Adhesives are usually divided into three basic types: physically hardened, chemically cured, and pressure-sensitive. Physically hardened adhesives begin in a liquid form that hardens after application, and come in one of three types: organic solvent, water-based, or hot melt. They can have a wide range of properties and uses, and they have widely ranging chemistries. Chemically cured adhesives, both one- and two-component, are generally very strong and resistant to temperature, humidity, and many chemicals. Their chemistries include cyanoacrylates, silicones, methyl methacrylates, and urethanes. Pressure-sensitive adhesives remain viscous and don’t completely solidify, so temperature and load can affect the quality of the bond they form.

Sealants are much more flexible than adhesives, since they usually contain an elastomer, with a molecular structure that is loosely cross-linked and a generally paste-like consistency. This lets them fill gaps between the surfaces of components or substrates to form air-tight and water-tight barriers. Compared to adhesives, sealants generally have higher rates of shrinkage.

Sealants are usually divided into three types: one-component, two-component, and tapes. One-component types are the most common and can be easily applied, and chemistries include silicone, urethane, solvent-based acrylics, solvent-based butyls, water-based latex, silyl-modified polymer (SMP), and polysulfides. Two-component types, comprising an activator and a base component, require mixing equipment and applicators. Their chemistries include silicone, urethane, and polysulfides. Sealant tape types are most commonly butyl chemistries.

The Ellsworth Adhesives Europe blog is packed full of fascinating articles, useful advice and application tips and tricks, making it a valuable resource for manufacturers and engineers alike.

To learn more about the key uses of adhesives and sealants, check out the full post here.

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