The phenomenon of transferring loads from adherend to adhesive joint is known as adhesion [1]. Mechanical properties of polymer control the interfacial forces necessary for good adhesion. Sustaining sufficient stresses at the interface is the basis for high adhesion [2]. There is always intermolecular diffusion taking place between the surfaces joined together by contact adhesives. In any interfusion process, the organic liquid evaporates, thus forming a strong bond between the layers; the bonding starts to take place when two properly layered bonding surfaces are brought closer. Several major parameters play a pivotal role in the auto adhesion to take place. The polymer or binding material must have diffusive properties. Diffusive property of binding material depends on solvent and the polymer used. There are only selective polymers which exhibit excellent auto adhesion properties. Intimate and close contact of the substrate is compulsory for the highest diffusion. Elastic stress and rheology of composition affect the quality of bond strength. When the adhesive is coated correctly on the material and dried in a lap of time under ambient conditions, this time is called open time. Adhesive layer alters all the physical properties in the open time due to evaporation of the solvent, interfacing of polymers, the base of polymer changes and crystallization of binding polymers [3]. When the open time has been passed, auto adhesion or good adhesive properties of the surfaces can’t be achieved.

Materials used in the formulation of adhesive were: Polychloroprene rubber (Mw. 120,000); Phenolic Resin (Mw. 700); Solvents such as Toluene, Ethyl acetate, Naphtha, DMF (Dimethylformamide); Magnesium oxide (MgO)/Zinc Oxide (ZnO); Antioxidant; Nanofillers as Multi-walled carbon nanotubes (MWCNTs; diameter 10 - 30 nm). All materials were of commercial-grade expect DMF (role) (Sigma Aldrich, 99%). Commercial grade functionalized MWCNTs were procured from Sun Nanotech^Ò with a purity of 90%. Table 2 displays the quantities of ingredients used for polychloroprene solvent-base adhesive; the amounts were optimized after multiple iterations.

Direct dissolving technique was used to carry out the experimentation in the laboratory. After successful tests at a smaller scale, the production was scaled up using an indigenously developed impeller mixing machine with controlled temperature and along with variable low rpm. All ingredients were added one-by-one in a mechanical mixer, and the temperature was gradually increased to 60˚C. The optimized adhesive was given the name “PA-30”. Dispersion of MWCNTs in DMF was performed using ultrasonic bath [38]. The dispersed solution was added to already fabricate PCA (Polychloroprene adhesive) during a continuous mechanical mixing. Mixing of adhesive is done until the pale yellow colour of the adhesive changes into greenish colour and developing a nanocomposite adhesive. The whole process took 3 to 4 hours. Schematic diagram of mixing is shown in Figure 1.

Two types of substrates 1) Canvas-to-Canvas and 2) Leather-to-Leather were used for peel testing as per ASTM standard D903 (2017). The adhesives were applied using a brush and a doctor blade and allowed to almost before joining together and gentle pressing using a roller. 180˚ angle peel testing was performed as per ASTM standard D3330, using a UTM.

Figure 2 displays the effect of MWCNTs’ concentration on peel strength of the Nano composite adhesive. There was a gradual increase in peel strength up to a certain amount of MWCNTs (0-1%) followed by a stable rather a decreasing trend. The enhancement in strength was due to compatibility of the MWCNTs with the resin and the adhesive system. In both cases i.e. canvas-to-canvas and leather-to-leather substrates, maximum peel strength was obtained by incorporating ≈ 0.8% MWCNTs.

In Figure 2, a decreasing trend in peel strength after a maximum value, was presumably due to diminution in cohesive forces between adhesive and substrate; it may also be an effect of poor wetting [6]. Another possible reason for the decrease in peel strength could be self-agglomeration of CNTs at higher contents [39].

Figure 3 displays a comparison of the amount of improvement in peel strengths of the unreinforced and reinforced adhesive, indicating a significant enhancement in peel strength on canvas-to-canvas and leather-to-leather substrates. In canvas-to-canvas substrate, peel strength improved up to 35% while on leather-to-leather substrate the rise was 21%, under optimized conditions.

The graph demonstrates that the presence of MWCNTs significantly increased the bonding capability of the adhesive. Presumably, the nanotubes caused reinforcement within the adhesive by either possible crack bridging or by crack deflection mechanism which did not allow the crack to propagate at lower loading condition [39].

The (PA-30) unreinforced adhesive formulation showed satisfactory performance in normal environmental conditions; however, it exhibited degradation issues when exposed to humidity, temperature, chemicals, seawater and mechanical loading during service.

The role of MWCNTs was of significant consideration to overcome these ageing conditions, by increasing adhesive stiffness, thermal stability and fracture durability [9]. The reinforced Nano-adhesive (PA-N11) was formulated using MWCNTs to overcome these conditions. Due to the presence of MWCNTs, the adhesive performance was significantly higher in relatively aggressive environments. The bond strength, thermal stability and liquid barrier properties were improved due to incorporation of highly dispersed MWCNTs [40].

The shelf life of the adhesive was estimated by studying ageing under ambient environment. Peel strengths of the unreinforced and reinforced adhesives were determined after shelving at ambient condition for up to 4 months. The data were also compared with commercially available adhesives (Elephant^Ò, Dolphin^Ò, Samad ultra^Ò, Camel^Ò) of the same category, under similar conditions. Figure 4 and Figure 5 show a graphical comparison of the effect of ageing on peel strengths, tested on canvas-to-canvas and leather-to-leather substrates, respectively.

Table 3 shows the peel strengths of unreinforced and reinforced nanocomposite adhesives, and commercially available adhesives of the same category. PA-30 and commercial adhesives lost their adhesion strength to a mentionable value;

however, PA-N11 (nanocomposite adhesive) displayed significantly higher performance; the mechanical properties were enhanced with a minimum decrease in peel strength after ageing.

Table 4 shows peel strengths data of various adhesives after the bonds were exposed to of 100˚C. It is believed that the network of crosslinked phenolic resin present in adhesives is usually affected by high temperature. Thermal stability of PA-N11 was enhanced due to the presence of nanotubes which helped to resist high-temperature environment [9]. The flexibility of the adhesive system was

*Addition of 1% MWCNTs with adhesive.

*Addition of 1% MWCNTs with adhesive.

essential to operate at high-temperature environment. PA-N11 had better flexibility at high-temperature environment. PA-N11 had a minimal amount of MWCNTs, which provided a better crosslinking network to surfaces of the substrates [40]. Mismatch of properties leads to failure of adhesive bond strength even at low loads and temperature. PA-N11 showed excellent thermal stability in ageing conditions owing to the existence of the MWCNTs crosslinking network.

It is generally known that an adhesive joint is deteriorated when exposed to saltwater since the ability for the diffusion of water increases by the ingress of Cl− and Na+ in the adhesive joints, resulting in poor durability of the joint [41] [42]. The various adhesive joints were exposed to 3.5% saltwater at ambient temperature for 24 hours, and the peel strengths were determined. The testing was repeated after every month for for a duration of three months to validate the performance of the adhesives after prolonged shelving. The data is displayed in Figure 6 and Figure 7. Reinforced adhesive showed better resistance compared with unreinforced adhesives including commercial counterparts. The resistance against the corrosive nature of the solution was due to the crosslinking network of MWCNTs in the adhesive. Table 5 summarizes fall in peel strengths as a result of immersion saltwater at ambient temperature for 24 hours.

Incorporation of MWCNTs in the solvent-based adhesive successfully improves the bond strength of adhesive joint on flexible substrates. The formulations of PA-30 (unreinforced adhesive) exhibited improvement in 180˚ peel strengths on canvas and leather substrates, up to 57 N/mm and 67 N/mm, respectively. The formulation of PA-N11 (1% MWCNTs), demonstrated an enhancement in peel strength up to 77 N/mm on canvas and 81 N/mm on leather substrates respectively. The improvement in peel strength was 35% on canvas and 21% on leather substrates as compared to unreinforced adhesive. The optimized unreinforced adhesive displayed a 25% - 50% increase in peel strength compared with commercial adhesives used for investigation. The optimized reinforced adhesive showed 46% - 64% increase in peel strength compared with commercial adhesives investigated. Fall in peel strength of unreinforced adhesive after 4-month shelving at the ambient environment, was 8% - 10%, whereas the reinforced adhesives showed a decrease of 2% - 3%. Fall in peel strength of unreinforced adhesive after 24 hours’ immersion in 3.5% sodium chloride solution was 12% - 17%, whereas the reinforced adhesives showed a decrease of 4% - 6%.

The authors acknowledge the Pakistan Science Foundation financial grant to carry out this research work under Grant No.089, April 2019.

The authors declare no conflicts of interest regarding the publication of this paper.

Awais, M., Shahid, M., Saleem, M., Aneeq, F., Butt, M.S. and Umer, M.A. (2020) Development and Aging Behaviour of Solvent-Based Polychloroprene Rubber Nano-Adhesive Using Multiwall Carbon Nanotubes. Journal of Materials Science and Chemical Engineering, 8, 31-44. https://doi.org/10.4236/msce.2020.89004