Review on Adhesives and Surface Treatments for Structural Applications: Recent Developments on Sustainability and Implementation for Metal and Composite Substrates

Abstract

Using adhesives for connection technology has many benefits. It is cost-efficient, fast, and allows homogeneous stress distribution between the bonded surfaces. This paper gives an overview on the current state of knowledge regarding the technologically important area of adhesive materials, as well as on emergent related technologies. It is expected to fill some of the technological gaps between the existing literature and industrial reality, by focusing at opportunities and challenges in the adhesives sector, on sustainable and eco-friendly chemistries that enable bio-derived adhesives, recycling and debonding, as well as giving a brief overview on the surface treatment approaches involved in the adhesive application process, with major focus on metal and polymer matrix composites. Finally, some thoughts on the connection between research and development (R&D) efforts, industry standards and regulatory aspects are given. It contributes to bridge the gap between industry and research institutes/academy. Examples from the aeronautics industry are often used since many technological advances in this industry are innovation precursors for other industries. This paper is mainly addressed to chemists, materials scientists, materials engineers, and decision-makers.

Keywords: adhesive; certification; surface pre-treatments; sustainability.

Figure 1

Lifecycle and routes for energy recovery, landfill disposal and recycling of plastic materials (Adapted from [82]. Reprinted from Waste Management, Vol 69, Ragaert et al., Mechanical and chemical recycling of solid plastic waste, 24–58. Copyright (2017), with permission from Elsevier).

Figure 2

PET glycolysis reaction using diethylene glycol in the presence of trans-esterification catalyst (adapted from [92]).

Figure 3

MCs morphology evolution during synthesis: (a) microemulsion before addition of the active H source, (b) shell formation after addition of the active H source, (c) free flowing powder consisting of the MCs after synthesis (scale 1:1). [107] Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer, Journal of Materials Science, “The role played by different active hydrogen sources in the microencapsulation of a commercial oligomeric diisocyanate”, by Loureiro, et al. (2020).

Figure 4

Schematic illustrations of the (a) mechanical advantage provided by surface treatments that increase the surface roughness and bonding area and (b) the effect of hydroxyl density on interfacial bonding with adhesive (represented by X). Reprinted with adjustments from [121] http://creativecommons.org/licenses/by/4.0/.

Figure 5

SEM picture after blasting an unalloyed steel (S355) with corundum of average grain size F022 (710–1000 µm). Scale bar = 500 μm. (TU Braunschweig).

Figure 6

Schematic design of a low-pressure blasting device. Adapted from [138].

Figure 7

EPMA (X-ray microanalysis; carbon content) of the aluminum die-cast surface after various pre-treatments. Adapted from [164].

Figure 8

SEM pictures of laser-treated CFRP surfaces. (a): two pulses, 600 J/cm³; (b): two pulses, 800 J/cm³; (c): 16 pulses, 600 J/cm³; (d): 16 pulses, 800 J/cm³. Scale bar = 5 μm [157]. Reprinted from Phys. Procedia, Vol 41, Kreling et al. Analytical characterization of CFRP laser treated by excimer laser radiation, 282–290. Copyright (2013), with permission from Elsevier.

Figure 9

Example of a cross-section of an anodic aluminum oxide prepared by PSA anodizing (Vrije Universiteit Brussel).

Figure 10

A life-cycle diagram, showing adhesive materials flows. Adapted from [177] “Overview of disbonding technologies for adhesive bonded joints”, A. Hutchinson, Y. Liu, et al. Journal of Adhesion, Vol 93, Pages No. 737–755, Copyright (2016), with permission from Taylor & Francis Ltd., http://www.tandfonline.com.

Figure 11

Debonding technique using polymeric nanocapsules with an adhesive thin film, which forms gas bubbles through thermal stimulus. (a) Synthesis concept of a polymeric nanocapsule containing internal vaporizing material, namely nanocapsules composed by hydrophobic methylcyclohexane (MCH)/poly(methyl methacrylate (PMMA) in the core and polyethyleneimine (PEI) in the shell. (b) Schematic representation of debonding mechanism through the formation of bubble gaps at the interface of a thin film adhesive, by heating the adhesive system [196]. Reprinted from Journal of Applied Polymer Science, Vol. 135, Issue 31, Lee et al. “Polymeric nanocapsules containing methylcyclohexane for improving thermally induced debonding of thin adhesive films”, Pages No. 10, Copyright (2020), with permission from John Wiley & Sons Inc.

Figure 12

SEM image of TEPs before (a) and after expansion (b). [206] Reprinted from International Journal of Adhesion and Adhesives, Vol 54, Banea et al., “Mechanical and thermal characterization of a structural polyurethane adhesive modified with thermally expandable particles”, Pages No 191–199, Copyright (2014), with permission from Elsevier.

Figure 13

Results from debonding evaluation test (followed induction heating at 250 °C) employing a commercial adhesive used in the automotive industry (Betamate™2098). Temperature (°C) versus time for debonding. Percentage shows the amount (wt%) of TEP in the adhesive. The more filler that is added, the faster the bond fails and the lower the failing temperature [199]. Reprinted from International Journal of Adhesion and Adhesives, Vol 59, Banea et al. “Debonding on command of adhesive joints for the automotive industry”, Pages No 14–20, Copyright (2015), with permission from Elsevier.

Figure 14

Graphite flakes unexpanded (a) and expanded (b). [177] Reprinted from “Overview of disbonding technologies for adhesive bonded joints”, A. Hutchinson, Y. Liu, et al. Journal of Adhesion, Vol 93, Pages No. 737–755, Copyright (2016), with permission from Taylor & Francis Ltd., http://www.tandfonline.com.

Figure 15

Illustration between various parameters and the final performance of a new adhesively-bonded system.

Figure 16

Required strength of the various elements of the adhesive bond system with structural adhesive bonding on aluminum (“>” means larger than, with the highest required strength on the left side, and the lowest required strength on the right side).