Novel Dry Underwater Pressure-Sensitive Adhesive

By Bismay Prakash RoutReviewed by Susha Cheriyedath, M.Sc.Jan 28 2022

In a recent study published in the journal ACS Materials Letters, researchers developed a novel, low-cost, easily synthesized dry pressure-sensitive adhesive (PSA) that can be used underwater.

Study: Simply Formulated Dry Pressure-Sensitive Adhesives for Substrate-Independent Underwater Adhesion. Image Credit: Gorodenkoff/Shutterstock.com

The prepared PSA copolymer demonstrated an optimized viscoelastic characteristic by controlling ratios between hydrogen (H)-bonding of acrylic acid (AA) and hydrophobic butyl acrylate (BA). It showed high underwater bonding strength of more than 115 kPa with a preload of 250 kPa between different types of commercial substrates viz. plastics, metals, and glass.

Underwater Applications of PSAs

Underwater and high moisture repairing works of diverse engineering material pairs, in which load-bearing is not that high, often require cost-effective strong adhesives as an alternative to welding, which is costly, energy-intensive, requires skilled workers, and limited to only metals and alloys.

The main issue with underwater adhesion is the entrapment of the hydrated layer between two substrates leads to the weakening of interfacial molecular interaction and subsequent progressive swelling. PSAs are viscoelastic materials that exhibit increased resistance to relative motion (shear stress) with an increase in strain, a glass transition temperature below room temperature, low modulus of rigidity, sufficient flowability, and resistance to creep.

Currently, catechol polymer-based adhesives have garnered a lot of attention owing to their strong metal-nonmetal interfacial interaction via both oxidative covalent cross-linking and metal coordination (non-covalent) interactions. Mussels use naturally produced catechol moieties of 3,4-dihydroxyphenylalanine to achieve self-healing strong surface adhesion in seawater.

However, it is difficult to mass-produce catechol-based polymers due to their high oxidation affinity in air. Based on similar mechanics, to facilitate both covalent and non-covalent interfacial molecular interactions, commonly used AA and BA can be copolymerized at the optimal molar composition to introduce covalent and noncovalent interactions such as metal coordination, electrostatic, hydrophobicity, H-bonding, and van der Waals interactions.

About the Study

In this study, researchers synthesized a dry PSA based on free radical copolymerization of H-bonding AA and hydrophobic BA monomers and characterized them at various BA:AA molar ratios viz. 1.0:1.0, 1.6:1.0, 2.8:1.0, and 3.6:1.0, denoted as p(BA_1.0-AA_1.0), p(BA_1.6-AA_1.0), p(BA_2.8-AA_1.0), p(BA_3.3-AA_1.0), respectively. This will be the order of PSA compositions throughout the below observations:

Observations

The molecular weights of the PSA samples were 31265, 26038, 36683, and 44879, respectively. The polydispersity indices (PDI) of all samples were close to each other and varied between 1.4 and 1.8. The moderate hydrophobicity of the BA-co-AA copolymers with a contact angle close to 80°, led to the breaking of the hydration layer in between the two substrates. The intermolecular bonding of the BA-co-AA copolymers increased with the increase in AA monomer content.

The self-standing p(BA_1.6-AA_1.0), p(BA_2.8-AA_1.0) samples demonstrated low Young’s moduli of ~0.55 MPa and more than 2000% elongation at break. The other two samples either exhibited very high viscosity or very low elongation at break (tensile strength). The four samples displayed glass transition temperatures (T_g) of 35.8, 5.2, −8.4, and −19.7 °C, respectively. The p(BA_2.8-AA_1.0) sample demonstrated the highest underwater bonding and debonding strength of 56.9 ± 5.8 kPa and 45.1 ± 7.0 J m−2, respectively, to a stainless steel substrate. Further, with an increase in the preload from 25 to 640 kPa, the bonding strength increased from 21.8 kPa to 318.6 kPa.

Additionally, the p(BA_2.8-AA_1.0) sample indicated self-healing capability. Moreover, the p(BA_2.8-AA_1.0) PSA showed a decrease in bonding strength from 143 to 112 kPa after it was immersed in water for 3 days, after which the strength almost remained constant up to 60 days. Also, owing to the suitable hydrophobicity, p(BA_2.8-AA_1.0) PSA had a low swelling ratio in water.

Conclusions

In summary, the researchers of this study synthesized a novel dry underwater adhesive by free-radical copolymerization of H-bonding AA and hydrophobic BA monomers to achieve both covalent and non-covalent interfacial molecular interaction between varieties of substrate materials.

The low moduli and high deformability of two PSA samples, i.e. p(BA_2.8-AA_1.0) and p(BA_1.6-AA_1.0) with T_g below room temperature, adequate hydrophobicity, low swelling ratio, high elongation at break (>2000%), low Young’s moduli (~0.55 MPa), and self-healing properties make them suitable self-standing PSAs for cost-effective underwater repairing.

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