Adhesive Solutions for Lightweighting

Adhesive solutions for lightweighting have transformed the joining of lightweight similar/dissimilar substrates. AZoM spoke to Rohit Ramnath of Master Bond Inc. about lightweighting and the types of adhesive solutions that are available.

Why use structural adhesives over mechanical fastening such as rivets and welding in lightweighting applications?

Structural adhesive bonding has been transformative in joining lightweight similar/dissimilar substrates. They speed processing, reduce labor costs, fill gaps, improve aesthetics while distributing stress over the entire bonded joint, increase load bearing capacity and resist cyclic fatigue. Additionally they seal against liquid/gases, eliminate protrusions/punctures and act as insulator against galvanic corrosion in metal assemblies.

What type of concerns must design engineers address when bonding lightweight substrates?

Obtaining good adhesion to certain lightweight substrates can be challenging. A diverse range of chemistries has been developed to overcome this complication and to facilitate bonding specific hard-to-bond metallic, nonmetallic surfaces. Another predicament can occur when adhering dissimilar substrates subjected to thermal mechanical stress. This can cause delamination due to the different coefficients of expansion and contraction between the surfaces. Lightweight substrates frequently generate vibration/noise and require adhesives with sound damping qualities. Sometimes lightweight substrates are weaker and may need a reinforcement to increase bonding durability. Other considerations range from costs, aging characteristics, joint geometry, stress/strain, alignment, assembly speed/manufacturing efficiency, and aesthetic issues.

What are the advantages of using adhesives for lightweighting in the automotive industry?

Automotive manufacturing companies have embraced the use of multi-material joining with structural adhesives to lower vehicle weight, fuel consumption, and carbon dioxide emissions. This is being accomplished by using materials such as aluminum, magnesium, high strength steel, foams, plastics, carbon fiber composites, and glass fiber composites. Reduction in wall thickness of essential materials has been invaluable in this effort. Adhesives have streamlined production, enhanced design flexibility, adhered well to hard-to-weld substrates and curtailed spotwelds. This has played a key role in shortening cycle times, moderating manufacturing costs, while enhancing durability, handling, safety, abating noise/vibration and extending vehicle life spans. New innovations/improvements in steel and other alloys, development of unique sustainable materials has enabled car manufacturers to make important strides in reaching their lightweighting goals. These manifold benefits could only be mastered through the use of adhesive bonding.

Why has lightweighting gained such importance for the aerospace industry?

Lighter weight materials such as aluminum, plastics, composites including carbon reinforced parts, honeycomb has led to increased payloads per flight and reduced fuel consumption, carbon dioxide emissions. Military/commercial aircraft design engineers have made major strides in producing advanced structures noted for their high strength, corrosion resistance, resistance to temperature extremes, impact, and dynamic loads through incorporating advanced structural epoxy adhesive technology. This has enabled a significant decrease in production cost and operational expenses. Leading the way has been the use of epoxy film and preform systems. These systems provide highly reliable structural bonds between similar/dissimilar surfaces, precise positioning, unlimited working life, no waste, good storage stability, uniform bond line thickness, and fast cures at moderate temperatures. These B-staged formulations are characterized by enhanced toughness, moisture/chemical stability, can be used for joining intricate shapes and simplify manufacturing processing.

Which other industries have shown a considerable increase in the use of adhesives for lightweighting?

They range from shipbuilding to rail to wind energy to sports to electronics. As the cost of lightweighting materials decreases the range of applications should grow dramatically.

Could you give some examples of Master Bond compounds employed in lightweighting of equipment/parts?

Master Bond two component system EP21TDCHT can be used for bonding aluminum to carbon fiber reinforced composite parts. This system features superior toughness, resistance to rigorous thermal cycling, bonds well to substrates with different coefficients of thermal expansions and has dependable electrical insulation properties. It exhibits high shear/peel strength, outstanding dimensional stability and serviceability from -100°F to 350°F.  Master Bond UV15FL is deployed for adhering acrylic to glass surfaces. This one component no mix formulation has an advantageously fast cure time in seconds upon exposure to the proper commercially available UV light source. It is a cationic curing system with exceptional thermal stability and has a Tg of 115-120°C. This product has enhanced toughness and excellent light transmission characteristics.

Another widely used formulation for adhering polycarbonate to titanium is Master Bond EP29LPSP. NASA low outgassing approved EP29LPSP is serviceable from 4K to +275°F, has a Shore D hardness >65, and has excellent electrical insulation properties. This low viscosity compound has a long working life and low exotherm upon cure. Single component thermal curing Supreme 10HTCL can be used for bonding PEEK to magnesium. It possesses high peel/shear strength, superior thermal cycling ability, and withstands 1,000 hours at 85°C/85% RH. It cures in 60-75 minutes at 250°F, has a flowable viscosity for easy application and an “unlimited” working life at room temperature. Serviceability is from 4K to +400°F.

Supreme 42HT-2ND has a paste viscosity and can used for adhering piezo ceramic to stainless steel 316 surfaces. It has a long working life and excellent temperature resistance from -80°F to +425°F. This 100% reactive formula has good physical strength, withstands thermal cycling/shock and a wide range of chemicals such as fuels, oils, bases, solvents.

What are the benefits of adding heat for curing epoxies?

For many epoxies, adding heat for curing helps to increase the cross-link density, which results in increased bond strength, better toughness and impact resistance, enhanced thermal stability, chemical inertness, and dielectric characteristics. Utilization of thermal curing techniques has played an important role in optimizing cured properties & accelerating cycle times, for joining lightweight substrates and maximizing reliability. Special, one component epoxy systems are available, which require no mixing and are engineered to cure at moderate temperatures rapidly.

Choosing the right mix of materials is a delicate balancing act requiring consideration of what factors?

Assessment of customer requirements is essential to meet lightweight expectations and satisfy tough standards. Meeting the proper material mix is no doubt dependent on using the proper adhesive systems. Reduction in the weight, size, and number of components offer long term opportunities and many competitive advantages. Concerns about sustainability and cost constraints must be thrown into the equation for the immediate future. In order to overcome these apprehensions, use of cutting edge adhesive products can satisfactorily overcome these barriers for immediate use.

About Rohit Ramnath

Rohit Ramnath is a Senior Product Engineer for Master Bond Inc., a custom formulated adhesives manufacturer. He analyzes application oriented issues and provides product solutions for companies in the aerospace, electronics, medical, optical and oil/chemical industries. He graduated from Carnegie Mellon University with a Masters Degree in Chemical Engineering.