The term ‘Structural Adhesive’ normally describes an adhesive which can resist high loads under a variety of different pressures for long periods of time. Structural adhesives are normally high molecular weight molecules, which are required to tolerate and distribute the loads within a joint. These adhesives are designed to join two surfaces to form an integral part of a structure. The Joints will have shear strengths greater than 1 MPa typically most, ARE greater than 10 MPa.
The main reasons for using Structural Adhesive instead of mechanical fastening are:
- Mechanical fastenings can cause stress around the fitting, resulting in stress cracking and where possible failure of the substrate
- The ability to bond a wide variety of dissimilar substrates
- Even distribution of stress over the entire bonded surface
- Protection from corrosion, among others
The Structural Adhesives are known to reduce component weight, increase durability, provide better design latitude, handle high levels of stress, and require less machining. They are cheaper than other securing methods and eliminate the costs associated with metalworking and metal finishing. They are also preferred for their aesthetic advantages such as cleaner finishes without any protruding nails or weld marks.
The different base Materials for the Structural Adhesives include:
- Acrylic –a type of resin that is used in adhesives to provide substrates with better stain protection, adhesion and blocking, water resistance, better cracking and blistering resistance, and resistance to alkali cleaners.
- Urethane – This adhesive is known for its strength and versatility. It is useful for bonds that require flexibility for expansion, contraction, or other types of movement.
- Epoxy – Epoxy resins are also known as polyepoxides and belong to a class of reactive prepolymers and polymers.
- Bismaleimide – Bismaleimide (BMI) polymers are an important class of polymers for high-performance structural materials applications. … Typically, bismaleimide resins are thermally cured with a co reactant, such as a diamine, to increase the toughness of the cured system. These adhesives provide high temperature cure/service up to 315°C (600°F)
- Cyanoacrylate – These are strong fast-acting adhesives referred to as Instant Super Glue adhesives widely used in industry, cosmetic, domestic uses, etc.
- Methyl methacrylate (MMA) – MMA adhesives are basically less sensitive to differences in mix ratio and degree of mixing, thus allowing the ratio of resin to hardener to vary from 100:3 to 1:1, therefore providing more time for adhesive application and assembly of parts.
- Phenolic – Phenolics have been one of the initial adhesives used for bonding metal to metal and metal to wood. They require heat and pressure for the curing process.
- Polyimide – These are mainly used in applications where the temperature can reach 350°C (662°F) and are available as liquids or films.
- Polyester – Polyester adhesives are less expensive than epoxy and are widely used in marine and other industrial applications; however polyester is a chemically weak adhesive when compared to epoxy, and also possess a high degree of shrinkage.
- Silicone – This adhesive is not very strong; however it has very good flexibility, durability and high temperature resistance.
New primer less adhesives have been developed as a credible alternative to traditional bonding technologies for composite materials. These systems allow bonding of various materials such as sheet-molding compound (SMC) or carbon fiber-reinforced plastics (CFRP) to coated metal substrates (steel or aluminum).
The new generation of adhesive systems provides developers and design engineers new opportunities:
- Increased stability of modulus and strength values in a wide temperature range (-45 to 180 °C)
- A high level of strength and elongation, independent of the bond thickness
- Improved aging performance
- Significantly lower bonding times through optimized curing behavior – with cycle times of less than 60 seconds.
There are many reasons why Design and Production Engineers should consider Structural Adhesives and Sealants in their next design. Here are some:
- Improved Product Durability and Reliability
- Increased Product Performance
- Increased Design Flexibility
- Increased Product Quality
- Enhanced Product Aesthetics
- Improved Process Productivity and Reduced Manufacturing Costs
Methyl Methacrylate (MMA)
Methyl Methacrylate Adhesives, or MMAs, are structural adhesives that have been developed for adhering Metals, Plastics, and Composites Materials as well dissimilar materials. They are designed with toughness, flexibility and ease of use. They already have turned out to be a valuable tool for use in industries such as Aerospatiale, Transportation, Marine, Sport Goods, Building, Construction, etc.
Methyl methacrylate adhesives are acrylic adhesives that are made of a resin and hardener. Some MMAs also contain rubber and additional strengthening agents. MMAs cure quickly at room temperature, and have full bond strength soon after application. These adhesives are resistant to shear peel and impact stress.
Looking at the bonding process more technically, these adhesives work by creating an exothermic polymerization reaction. Polymerization is the process of reacting monomer molecules together, in a chemical reaction, to form polymer chains. What this means is that the adhesives create a strong bond while still being flexible.
These adhesives are able to form bonds between dissimilar materials with different flexibility, like metal and plastic. Unlike some other structural adhesives like two-part epoxies, MMAs do not require heat to cure. There are MMAs available with a range of working times to suit the specific needs.
Specific Applications include:
- Bonding Magnets in production of loudspeakers and sweep transformer core sections.
- Bonding panels of aluminum to other panels.
- For trucks and construction equipment like side and roof panels, door panels, and aerodynamic components.
- With Electric motor manufacturing like the bonding of magnets to metal housings made of steel or aluminum.
- Composite bonding of truck trailer beds using aluminum, and composite parts such as fenders, stiffeners to door panels, sliding roofs.
- Bonding hulls, deck liners, ski boats, yachts, internal fixtures, canoes and kayaks.
MMAs require less surface preparation than other adhesives. When using MMAs to bond metal and plastics for example, you only need to use a solvent or alcohol wipe to remove any oils from the surface instead of having to clean and scoff the surface with an abrasive material.
Using methyl methacrylate adhesives allows you to work with many different materials and requires less preparation than other adhesives.
They are packaged in dual cartridges with mixing static nozzles. These products have the added benefit of good gap fill capability, and allow for easy dispensing and accurate application.
Methyl Methacrylate adhesives can replace mechanical fasteners and welding joints. They also offer advantages over other adhesive technologies. Using fasteners like bolts and screws requires more time and labor, and the bolts can corrode and loosen over time. Welding two different metals together is sometimes possible, but it is expensive and labor intensive.
Epoxy adhesives do not bond different metals well because they are more brittle and do not allow for expansion and contraction, which leads to cracks and ultimately bond failure. Polyurethane adhesives need more surface preparation and often require priming of the surface, adding an additional step.
MMAs are less brittle than epoxies and can fill larger gaps. They have higher bond strength than urethanes and better impact and fatigue resistance. They also offer good chemical and environmental resistance, and fast cure speed.
FEATURES OF MMAS INCLUDE:
- High tension strength
- No primer needed
- Excellent impact and peel strength
- Varying cure times
- High toughness and flexibility
- Cure at room temperature
- Good flexibility at low temperatures
- Minimal bond shrinkage
- Less surface preparation
- Very high resistance to fatigue
- Can bonds a wide range of materials together
- Tolerant of off ratio mixing
- Can fill large gaps
- Resistant to solvents and weathering
- Easy application
- Tolerant of oily surface and other types of surface contamination
MMAs have higher peel strength and are more temperature resistant. They develop strength faster allowing parts to be used sooner. An interesting feature is the differing processing conditions used for MMA adhesives. Besides the techniques normally used for two component adhesives application; twin Cartridges or Semi-automatic Mixing Dispensing Equipment, which mix and dispense together the Resin
and the Hardener, the two components of a MMA adhesive can each be applied separately to one of the substrates. The actual curing reaction then only takes place in the bonded joint after bringing the substrates together. This process avoids having to mix difficult mixing ratios (for example, resin: hardener 100:3) and allows use of fast curing adhesives that would have too short a work life if the resin and hardener were mixed before application.
Adhesive selection involves the following considerations:
- Substrates: What are you trying to bond? Are the surfaces the same or dissimilar, porous or smooth? Are you covering a large area? Do you have heat or solvent sensitive surfaces?
- Application restrictions: How do you intend to apply the adhesive- examples: spray, roll, heat gun, cartridge, squeeze bottle?
- Use Requirements: How does the bonded piece get used? How much strength is required? For example, bonding wood requires much more strength than decorative paper crafts do. What kind of environments might it see? Will it experience temperature extremes or water/steam?
In the vast majority of situations the general performance goal is a bonded joint stronger than the parts being bonded together. On rare occasions, however, the bond is intentionally weaker so that the joint will fail first, preventing damage to an expensive piece of structure. In all cases the bonded joint must be able to sustain all expected in-service loads and environmental conditions, which can vary considerably.
Adhesives applications also may require performance features unrelated to adhesion. For instance, in transportation applications subject to flame/smoke/toxicity regulation or in corrosive environments, adhesive flammability and/or corrosivity become significant factors.
Beyond bond performance, a number of other practical considerations influence adhesive selection:
- Mixing Method. Some two-part systems can be mixed inexpensively in a bucket with simple stirring, while others sometimes require expensive meter/mix/dispense equipment.
- Surface preparation. Some adhesives require special part cleaning and/or surface conditioning to ensure good adhesion while other adhesives are more aggressive, containing solvents or other components that dissolve contaminants and/or etch surfaces.
- Pot or work life. This is the amount of time after mixing that the adhesive is still able to adequately wet out the substrates. For small-area application, pot life of a few minutes may be acceptable. For large-area applications, pot life may have to extend to several hours.
- Curing method. Two-part adhesives (resin and catalyst) begin to react and harden after mixing, while one-part materials are air- or heat-triggered.
- Fixturing time. This is the time required for a part assembly to be clamped while the adhesive cures well enough to allow the next operation to begin, which may create a production bottleneck. The cure cycles for many adhesives may be heat-accelerated to shorten time in the fixture.
- Time to full cure. Even after the joint is strong enough to start the next manufacturing process, optimal bond strength may not be reached for an extended period of time. Product is generally not delivered to the customer unless it is at full strength, a factor in scheduling.
- Health/safety. The chemical composition of some adhesives may require that technicians wear gloves and/or a breathing apparatus. Production proceeds more quickly when materials are acceptable to assembly personnel.
- Some applications require that a bonded joint be separable for repair purposes. Adhesives can be selected that soften enough to break the bond at temperatures above the assembly’s in-service temperature range but below the temperature at which the composite sustains heat damage. Alternatively, high lap-shear/low-peel adhesives may prove suitable in some applications.
- The “total applied cost” per completed assembly includes the raw adhesive material, labor involved with application, any mixing equipment that may be required, assembly fixtures and curing ovens (if used) as well as the total production time. Each application must be analyzed to optimize fabrication costs and throughput while achieving required bond properties.
“In Conclusion, without the use and knowledge of Adhering Technology, innovative lightweight designs would barely be possible, particularly when you have to joining dissimilar and / or modern materials.”