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What is an adhesive and how does it work? A practical introduction to bonding technology
Are you wondering how to bond plastics? Which adhesive to use for steel and different metals? How to replace traditional joining methods such as welding, bolting, or riveting? And how to join dissimilar materials—metal to plastic, or metal to polyester/epoxy composites?
This blog is for you. We’ll publish articles regularly to answer these questions. Let’s start with the basics.
Key takeaways
✅ Adhesives join materials thanks to adhesion (bonding to the surface) and cohesion (internal strength of the cured adhesive)
✅ Adhesion is usually driven by: surface roughness, cleanliness/degreasing, and surface wettability
✅ Cohesion depends on the chemistry and curing conditions (time, temperature, humidity) as described in the TDS
Video: bonding fundamentals
In these videos we cover the fundamentals: adhesion vs cohesion, why surface preparation is critical, and how to approach bonding method selection in practice.
What is an adhesive?
The definition of an adhesive according to the European standard EN 923 is:
“A non-metallic substance capable of joining materials by bonding to their surfaces (adhesion) and reliably connecting them through internal forces (cohesion).”
Adhesion and cohesion
Adhesion—simply put—is the force created between the adhesive and the bonded material. Achieving good adhesion requires proper surface preparation.
Cohesion can be described as the internal strength between molecules within the adhesive system. In practice, it depends on suitable curing conditions. More on that later.
In short: adhesion is the bond to the substrate, while cohesion is the adhesive’s internal strength achieved through polymer network formation during curing.
What does adhesion depend on?
For most users, the main goal should be maximizing adhesion to the surface. Adhesion typically depends on:
- surface roughness,
- surface contamination,
- surface wettability.
1) Surface roughness
Roughness is a property of a solid surface and depends on the material, the type of machining, and the tool used. In bonding, increasing roughness can improve adhesion by “anchoring” the adhesive into surface irregularities. To “open” the surface (increase roughness), you can use:
- grinding,
- abrading/matting,
- sandblasting,
- shot blasting.
2) Surface contamination
A key step before bonding is cleaning and degreasing to remove dirt, grease, machining oils, and—especially for plastics—release agents used in manufacturing. For best results, use dedicated cleaners and industrial wipes that do not shed fibers.
When degreasing plastics, it’s important to use products that won’t damage the surface. One example is LOCTITE 7063, which is described as neutral for many plastics. Acetone is not recommended for this purpose.
3) Surface wettability
This parameter shows how well an adhesive can spread and contact a given substrate. The images below illustrate how a liquid droplet behaves on surfaces with different surface energies. A quick practical check is a simple water drop test—how the droplet behaves can indicate whether additional surface activation may be needed.
To increase surface energy, processes such as corona treatment, plasma treatment, etching, or priming can be used—but that’s a topic for a separate article 🙂
Good wettability of the surface
Poor wettability of the surface
What does cohesion depend on?
Cohesion is an intrinsic parameter of the adhesive and depends on its chemistry and intended application. It’s important to follow the manufacturer’s recommendations in the TDS. Key factors include respecting open time and ensuring proper curing conditions (temperature and humidity) during crosslinking.
Graphic materials were sourced from HENKEL LOCTITE training materials.
Next step: surface preparation before bonding
If you want to move from theory to practice, start with surface preparation. This is often the key stage that determines adhesion and long-term durability.
➡️ Surface preparation and cleaning – category
➡️ Industrial wipes – for degreasing and cleaning
➡️ LOCTITE 7063 – degreasing (plastic-friendly)
FAQ – bonding basics
What’s the difference between adhesion and cohesion?
Adhesion describes the bond to the substrate, while cohesion describes the internal strength of the cured adhesive.
Why is degreasing so important before bonding?
Because contamination (oils, grease, release agents) reduces surface contact and lowers adhesion.
Is acetone suitable for degreasing before bonding?
It is sometimes used in technical practice, but it can damage plastics or cause unwanted effects. For plastics, choose cleaners described as neutral for the material.
How do I avoid fibers and streaks during cleaning?
Use industrial wipes that don’t shed fibers and work “fresh”—replace wipes instead of spreading contamination.
How can I quickly assess surface wettability?
A simple water droplet test: how the droplet spreads can hint at surface energy and whether activation is needed.
Does higher roughness always improve bonding?
Often it helps by increasing contact area and mechanical anchoring, but it should be combined with proper cleaning and degreasing.
Author
Marcin Filipczyk – long-time specialist in adhesive bonding.
