The aim of this research was to investigate the self-healing potential of damaged Al joints when bonded using novel eco-epoxide adhesives derived from tannic acid (TA). Two eco-epoxy components based on TA, (A) glycidyl ether and (B) glycidyl phosphate ester of TA, were produced. The effect of the eco-epoxy components on the self-healing ability was assessed in terms of the energy dissipation recovery after partial failure in a double cantilever beam (DCB) test, which was compared to the reference epoxy (R). The self-healing process required 2 h and 2 bars in an autoclave at 180 °C. Techniques such as DSC, FTIR and DMA showed residual activity and potential self-healing capability of the used adhesives. A combination of two monitoring techniques, Digital Image Correlation (DIC) and Acoustic Emission (AE), was used to monitor the strain distribution and damage propagation in the DCB specimens. The healing index for adhesives R, B and A was found to be 8.9%, 3.0%, and 82.5% respectively. The findings of this work highlighted the potential of using bio-based epoxy adhesives in structural adhesive bonding, as well as the prospect of utilizing their self-healing ability to restore the strength of such bonded parts.

@en

The aim of this paper is to study the self-healing capability of fractured Al joints bonded with novel eco-epoxide adhesives synthesized from a bio-renewable raw material (tannic acid – TA). Two synthesized eco-epoxy components based on TA, (A) glycidyl ether and (B) glycidyl phosphate ester of TA, were used as a replacement for the toxic epoxy component based on Bisphenol A. The effect of the eco-epoxy components on the self-healing capability was measured as a recovery of shear strength in a single lap joint (SLJ) test after complete failure, which was compared to the reference epoxy (R). The self-healing procedure was performed in an autoclave at 180 °C for 2 h and 2 bars. A combination of two monitoring techniques, Digital Image Correlation (DIC) and Acoustic Emission (AE), was used to monitor the strain distribution and damage propagation in the SLJ. The measured shear stress of A and B adhesives in the SLJ had values in the range of 2.3–5.1 MPa. A fracture analysis showed complete adhesive failure for all the tested adhesives, which was not affected by the self-healing process. Out of all adhesives, only the A adhesive demonstrated the capability to heal. The recovery of the shear strength for adhesive A was higher than 50% of the virgin case. In addition, the AE analysis managed to capture a clear distinction between the signals for the virgin and the self-healed tests for adhesive A. Results obtained in this study highlighted the promising potential of using bio-based epoxy adhesives in structural adhesive bonding with the possibility of using self-healing in the recovery of the strength of such bonded joints.@en

Two synthesized eco-epoxy components based on TA: (A) glycidyl ether and (B) glycidyl phosphate ester, are used, as a replacement for the Bisphenol A (BPA) based epoxy component, for bonding aluminum (Al) and carbon fiber reinforced polymer (CFRP). Their effect on the mode I fracture toughness (GI) is evaluated by Double Cantilever Beam (DCB) testing while using Digital Image Correlation (DIC) for in-situ crack tip monitoring. Compared to the reference adhesive, an improvement of (GI) of Al (43%) and CFRP (100%) is obtained when using adhesive B. Moreover, regardless of the adherend material, a stick-slip pattern of crack growth is observed. Weak adhesion of the reference adhesive leads to an adhesive failure vs. a cohesive-adhesive failure in the case of adhesive B. On the contrary, the modification of adhesive A has an adverse effect on the GI of Al (-33%) and CFRP (-78%) as opposed to their reference counterparts.@en

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.

@en

This paper presents a new process for obtaining eco-epoxide adhesives synthesized from bio-renewable raw material (tannic acid-TA) and used for bonding lightweight materials (aluminum (Al) and carbon fiber reinforced polymer (CFRP). Two synthesized bio-epoxy components based on TA, (A) glycidyl ether and (B) glycidyl phosphate ester of TA, were used as a replacement for the toxic epoxy component based on Bisphenol A. The effect of eco-epoxy components on the interface adhesion was measured by the determination of adhesion parameter b, which was compared to the reference epoxy (REF). The increase of adhesion parameter b was 77.5% for A and 151.5% for B. The adhesion of both eco-adhesives was tested using the bell peel test (BPT) with the Al and CFRP adherends. When compared to REF, the average peel load for B was 17.6% (39.3%) and 58.3% (176.9%) higher for the Al and CFRP adherends, respectively. Complete adhesion failure of REF reflected the weak adhesion to both Al and CFRP, which was improved by the addition of eco-epoxy components A and B showing the presence of cohesive failure. The microhardness testing method of interface adhesion was proven to be a fast and reliable testing method, providing a qualitative indication in adhesive selection.

@en