Bonding of composites - why is it worth choosing this technology?

Bonding composites – why is this technology worth choosing?

In recent years, we have seen a strong increase in the use of composites across many industries, transport and aviation. The reasons are clear: reduced weight, improved aesthetics, increased resistance to environmental conditions and wider design options. Demand for products with higher performance is growing as well—and composites often deliver exactly that. The key question is: how to join composites in a way that preserves strength and aesthetics?

Key takeaways

✅ Bonding enables joining composites without rivets or drilling, helping reduce local laminate weakening

✅ A bonded joint can distribute loads more evenly than point fasteners

✅ Adhesive selection is always a trade-off: open time, curing time, viscosity, service conditions

✅ Success depends on surface preparation and repeatable process control (cleaning, degreasing, stabilization)

 

Bonding composites – why is it worth choosing this joining method?

Besides mechanical and thermal joining methods, composites often require solutions that further optimize strength and aesthetics. Today, various structural adhesives (epoxy, acrylic, polyurethane) can be used to join composites without reinforcement such as rivets, clamps, welding or other mechanical fasteners. In many applications, bonding can be the best option for joining composites and plastics.

Advantages of bonding composites

Benefits of bonded joints in composite applications include:

• structural adhesives can work well when bonding composite materials to other plastics and to many other substrates (e.g., wood, glass, metals) without compromising functional performance,
• bonding supports a favorable mass-to-stiffness ratio—lighter designs while maintaining required stiffness,
• bonded joints can meet sealing requirements and help provide resistance to water, rain and drafts,
• aesthetics: bond lines can be visually minimal, and bonded parts can often be finish-ready,
• mechanical reinforcement (e.g., rivets) can locally weaken laminates, while bonding helps distribute loads more evenly and supports higher overall load transfer.

Limitations of bonding composites

Bonding composites also comes with constraints. Adhesive cost can be significant (sometimes higher than the composite material cost per kilogram). Bonding requires technological time in the process. In some cases, bonded parts need fixtures/tools for stabilization and must be kept in specific positions until they can move to the next operation. Some adhesives require surface preparation steps such as degreasing, abrasion and/or primers, which increases process cost. Thermal expansion differences can matter as well. Aesthetic aspects are important too—for example, controlling and removing adhesive squeeze-out may add time and cost.

 

Structural adhesives – selecting the right adhesive

For very high-strength composite bonding, methacrylate (MMA) adhesives are often a strong choice. They are versatile and can provide durable bond lines. An example is MELKIB MMA POWER 10.

What should you consider during selection?

• Required speed: what open time and curing time do you need? This is critical when implementing bonding into production and matching the current process.
• Optimal viscosity: adhesive consistency and how it will be dispensed.
• Service conditions: temperature, moisture, media exposure and dynamic loading.
• Aesthetics: squeeze-out control and finishing requirements if the joint is visible.

Mini process checklist (to keep it repeatable)

• identify the composite type (matrix + fiber),
• define joint geometry and required performance,
• plan surface preparation (cleaning/degreasing, possible abrasion and/or primer if needed),
• match adhesive timing to assembly and fixturing (open time/curing time),
• stabilize the parts until safe handling is possible,
• control squeeze-out and appearance for visible parts.

If you need help selecting the right solution for your process, use the contact page and provide: material type, joint geometry, service conditions and required assembly time.

 

Video: composite bonding & technology selection in practice

To complement the theory, here are two videos from our channel. The first supports process understanding (application and control), and the second adds practical guidance for adhesive selection and technology comparisons.

 

Fiber-reinforced plastics – what are composites?

A composite is a material created by combining two or more materials, where one acts as the binding material (matrix) and the others act as reinforcement and are introduced in fibrous, granular or layered form. The resulting structure often provides much higher mechanical, aesthetic and performance properties than each component alone.

The most common group includes fiber-reinforced plastics. Fiber composites consist of a matrix (resin/plastic) and reinforcing fibers. The matrix surrounds fibers. Both the matrix and fibers can be selected and tailored to requirements. Together, they can deliver higher strength than either component alone. The matrix may be a synthetic resin or thermoplastic, while fibers may be glass or carbon. A composite made of multiple layers of fibers and matrix is called a laminate. Fibers mainly reinforce and carry loads, while the matrix positions fibers, transfers loads between fibers and protects them against environmental effects. Fiber composites often show direction-dependent properties based on fiber orientation.

Example fiber orientation in a composite.

Matrix materials

As matrix materials, thermosets and thermoplastics are used, and less often ceramics and metals.

Thermoset matrix systems:

• unsaturated polyester (UP)
• vinyl ester resin (VE)
• epoxy resin
• phenol-formaldehyde resin (PF)
• polyurethane resin (PUR)

Thermoplastic matrix systems:

• polypropylene (PP)
• polyamide (PA)
• polyetherimide (PEI)
• polyphenylene sulfide (PPS)
• polyether ether ketone (PEEK)

Criteria for choosing the matrix

• production process (processability of thermosets/thermoplastics),
• cost,
• matrix material properties (mechanical properties, service temperature limits, resistance to media and radiation, moisture absorption),
• production properties (viscosity, processing time, curing conditions and time),
• work safety and toxicity.

Thermoset matrices typically have low viscosity before curing, longer cure times, often good bondability, and are non-melting after cure. Thermoplastic matrices often have higher processing viscosity, require shorter processing cycles, can show relatively lower matrix-fiber adhesion, may be more challenging to bond, and are meltable/shapeable after processing and can deform.

FAQ – bonding composites

What is the most common reason for weak composite bonds?

Most often: insufficient surface preparation (dust/contamination, lack of degreasing), incorrect adhesive choice for timing and service conditions, and lack of stabilization until safe handling.

Are MMA adhesives always the best choice for composites?

Not always—selection depends on the application, joint geometry, required performance and service conditions. MMA adhesives are often excellent for structural applications, but in other cases epoxy or polyurethane may be more suitable.

Why does thermal expansion matter in composite joints?

Differences in thermal expansion between joined materials influence how the joint behaves with temperature changes. When designing the process, it’s important to consider service environment so the bond line remains durable over time.