Advancements in Nano-Based Adhesive Bonding

Nanostructured adhesives are a novel technology that incorporates nanoscale materials like nanoparticles or nano-fillers to enhance adhesion and other key properties.

Image Credit: noprati somchit/Shutterstock.com 4ft timber peeling machine

Studies show that nanoparticles can act as interfacial links, bonding hydrogels, or biological tissues. These adhesives have various physicochemical properties and are used in the aerospace, automotive manufacturing, and electronics industries.

Nano-based adhesive bonding involves adding nano-sized particles, such as nano-fillers and nanocrystals, to impart strength, flexibility, and longevity to modern adhesives. These nanoparticle-infused adhesive matrices offer superior attributes compared to conventional adhesives.1

Researchers have aimed to improve adhesive properties by adjusting the surface area of nanoparticles and their assembly form, enhancing cohesion and adhesion performance.2

Polymeric coatings are commonly used to protect substrates from chemical, physical, and biochemical degradation through suitable adhesives. Epoxies, polyurethanes, acrylics, and silicones are the most studied base polymers for these applications.3 These polymers are favored for their excellent processability, chemical resistance, strong adhesion, and reasonable cost. However, their long-term durability can be insufficient.

Incorporating nanofillers is effective in enhancing the performance of these base polymers. Nanofillers improve the mechanical properties of polymeric adhesives, such as crack resistance, wear resistance, and corrosion resistance.4 Various nanofillers, including nanoclays, carbon nanotubes, metal or metal oxide particles, ceramic fillers, and cellulose nanomaterials (CNMs), have been investigated for their integration into polymeric adhesive nanocomposites.

Research has shown that CNMs are particularly effective in enhancing polymeric adhesive matrices. CNMs are reusable, biodegradable, non-toxic, and require less energy during fabrication. For instance, adding 5 wt. % nano-silica to an epoxy coating reduced mass loss during abrasive tests by 30 %. Similar improvements were observed for CNM-reinforced acrylic adhesive composites with 10 wt. % fiber content.5

CNCs are particularly desirable as adhesive reinforcements due to their high crystallinity and aspect ratios. Integrating CNCs into adhesive systems increases bond strength and improves joint creep resistance and stiffness. These enhancements are vital for structural applications and are made possible by the high modulus of elasticity of the nanocrystals, which enhances the stiffness of the composite.

Nanostructured epoxy adhesives are also popular, offering enhanced characteristics such as increased mechanical strength, thermal and chemical durability, and decreased setting time. Nanoscale materials improve adherence on uneven or rough surfaces due to a higher surface area-to-volume ratio.

These nanomaterial-enriched adhesives exhibit significantly improved bonding strength, toughness, and durability. Their rapid curing ability is invaluable in industries requiring efficient production processes. Additionally, nano-structuring enhances resistance to harsh chemicals and high temperatures, making these adhesives suitable for demanding environments and optimizing them for specific industrial applications.

Many methods have been developed to create various types of nanoparticle-filled adhesives. One common approach is the intercalation method, a top-down technique that downsizes fillers to nanodimensions by exfoliating layered silicates.

Another top-down approach is the direct mixing of a matrix polymer and nanofillers, which breaks down aggregated fillers during the mixing process. The melt-compounding technique has also been efficiently used to develop filler nanocomposites. This method is cost-effective because it does not require surface modification of nanofillers and is environmentally friendly, avoiding the use of organic solvents and surfactants. It is also compatible with many industrial processes.6

In addition to these methods, adhesives can be improved by incorporating nanotubes through a dispersion method. One effective technique is using a three-roll mill, which can break down agglomerates to below 5 µm.

Ultrasonic systems (US) can also be used for dispersion, but long dispersion times and high power can shorten the nanofillers, altering their aspect ratio and affecting dispersibility.

Researchers have improved the process by successfully dispersing various nanofiller types directly into a tetra hydro-methyl phthalic anhydride hardener via ultrasonication without needing an additional solvent system.7

Nanofillers incorporated within adhesives significantly improve their mechanical and electrical properties. Epoxy resins (EP) are popular for polymer filler composites in high-end structural applications due to their outstanding performance and the demand for high-performance structural materials.

Functionalizing nanofillers with silane groups is an efficient and economical way to strengthen modern adhesives. Graphene oxide (GO) is advantageous among two-dimensional carbon nanomaterials due to its high surface area and tunable structural properties, making it suitable for use as a nanofiller in polymeric adhesive composites.

A recent study demonstrated that grafting isophorone diisocyanate (IPDI), known for its high reactivity and unique structure, onto GO improved dispersion by weakening interlayer interactions. Additionally, incorporating amino-terminated butadiene acrylonitrile (ATBN), known for its excellent toughening effects, into GO created high-performance fillers.

Compared to pure EP, the modified composite showed significant improvements: peel strength increased by 579 %, shear strength by 99 %, tensile strength by 134 %, and impact strength by 65 %, with Al elements detected in the T-peel samples.8 These results confirm the effectiveness of nanofillers and nanoparticles in enhancing modern adhesives.

Electrically conductive adhesives (ECAs) are mainly used in IC/LED packaging and other electronic applications to substitute conventional and lead-free solders.

However, increasing the content of conductive fillers in ECAs significantly enhances electrical conductivity but compromises mechanical strength. Therefore, enhancing electrical conductivity without significantly compromising mechanical properties is essential.

Silver powders, known for their stable physical and chemical properties, are commonly used as conductive fillers in ECAs. A recent study used a tetra-functional polyurethane acrylate oligomer as the main component of silver-filled one-component ECAs to enhance performance.9 Experiments verified that the oligomer nanofiller significantly boosted adhesion properties.

The incorporation of silver flakes within the ECA matrix achieved a low volume resistivity of 1.02 × 10⁻⁷ Ω‧m and a high lap shear strength of 8.995 MPa, demonstrating that appropriate nanofillers can improve the mechanical and electrical properties of adhesives.

Nano-based adhesives are extensively used in the aerospace industry due to their superior tensile characteristics, such as fatigue resistance and enhanced mechanical properties. However, their performance in aircraft and space flight applications depends significantly on the type of nanoparticles or fillers used, their dimensionality, and the fabrication methods employed.

Recently, carbon fibers incorporated within adhesive epoxy matrices have created lightweight, specialized structural materials optimized for modern spacecraft. These materials have proven suitable for aerospace applications. Incorporating nanoparticles with different geometries, such as planar, tubular, and spherical, to adhesives is an innovative approach to enhancing the properties of adhesive joints in modern aircraft.10

With recent advancements in manufacturing, adhesives are continuously improving, and the incorporation of nanoparticles is imparting valuable properties. The current focus is on using environmentally friendly methods to enhance modern adhesives.

Bio-based adhesives improved with nanofillers are also becoming a promising area of study. Ongoing progress is expected in the field of nano-based adhesives.

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[1] Nanografi (2023). Nanostructured Epoxy Adhesives. [Online] Nanografi. Available at: https://nanografi.com/blog/nanostructured-epoxy-adhesives/#:~:text=Improved%20Bonding%20Strength%3A%20One%20of,leading%20to%20stronger%20adhesive%20bonds (Accessed on June 04, 2024)

[2] Baik, J., et al. (2022). Colloidal supraballs of mesoporous silica nanoparticles as bioresorbable adhesives for hydrogels. Chemistry of Materials. doi.org/10.1021/acs.chemmater.1c03072

[3] Kausar, A. (2020). High performance epoxy/ polyester-based nanocomposite coatings for multipurpose applications: a review. J. Plastic Film Sheeting. doi.org/10.1177/8756087920910481

[4] Kenig, S., et al. (2019). Nanocomposite polymer adhesives: a critical review. Rev. Adhes. Adhes. doi.org/10.7569/RAA.2019.097306

[5] Wang, L. et. al. (2023). Multifunctional polymer composite coatings and adhesives by incorporating cellulose nanomaterials. Matter. https://www.cell.com/matter/pdf/S2590-2385(22)00655-5.pdf

[6] Tanahashi, M. (2010). Development of Fabrication Methods of Filler/Polymer Nanocomposites: With Focus on Simple Melt-Compounding-Based Approach without Surface Modification of Nanofillers. Materials. doi.org/10.3390/ma3031593

[7] Zanghellini, B., et al. (2021). Rennhofer, H. Solvent-Free Ultrasonic Dispersion of Nanofillers in Epoxy Matrix. Polymers. doi.org/10.3390/polym13020308

[8] Xu, H., et al. (2023). Enhancement of mechanical properties of epoxy resin matrix adhesives by high-performance fillers. J Polym Res. doi.org/10.1007/s10965-023-03755-x

[9] Chen, Y., et al. (2023). A new one-component electrically conductive adhesive with excellent electrical conductivity and mechanical properties using tetra-functional polyurethane acrylate oligomers and silver particle fillers. International Journal of Adhesion and Adhesives. doi.org/10.1016/j.ijadhadh.2023.103388

[10] Kausar, A., et al. (2023). Leading-Edge Polymer/Carbonaceous Nano-Reinforcement Nanocomposites—Opportunities for Space Sector. Advances in Materials Science. doi.org/10.2478/adms-2023-0025

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Ibtisam graduated from the Institute of Space Technology, Islamabad with a B.S. in Aerospace Engineering. During his academic career, he has worked on several research projects and has successfully managed several co-curricular events such as the International World Space Week and the International Conference on Aerospace Engineering. Having won an English prose competition during his undergraduate degree, Ibtisam has always been keenly interested in research, writing, and editing. Soon after his graduation, he joined AzoNetwork as a freelancer to sharpen his skills. Ibtisam loves to travel, especially visiting the countryside. He has always been a sports fan and loves to watch tennis, soccer, and cricket. Born in Pakistan, Ibtisam one day hopes to travel all over the world.

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