A study of adhesion on stainless steel in an epoxy/dicyandiamide coating system: Influence of glass transition temperature on wet adhesion
Introduction
Adhesive bonding of metallic materials is now established as an indispensable industrial technology for the construction of airplanes and cars [1,2]. Nevertheless, several technological problems of strategic importance remain unsolved in this important industrial technology. The most important problem includes “loss of adhesive strength in the presence of humidity (wet adhesion)”. This unsolved problem is actually identical in nature to the problem of the adhesion of a corrosion protective coating layer to a metallic substrate in a humid environment [3].
It is well known that stainless steel exhibits inferior adhesion to organic coatings compared to other metals such as cold rolled steels or aluminum alloys due to the CrO2 passivation layer present on its surface. Various types of pretreatment had been discussed to activate the surface of stainless steel, and several have been proposed and used in the aerospace industry (mechanical, chemical, and combination of mechanical and chemical) [4]. However, if these chemical pretreatment processes are employed, a residue (smut) deposits on the surface. At the end of the process this residue needs to be removed by another chemical treatment, such as de-smutting. For this reason, the chemical pretreatment process is not attractive environmentally, and has prompted considerable interest in a more environmentally friendly and effective organic primer replacementit.
In the last few decades, there have been several studies on organic primers (thin layer), for example the use of an aqueous solution of poly(acrylic acid) and electrolytic polymerization with triazinethiol compounds to improve adhesion in humid environments [5,6]. However, they mainly focused on the formation of strong (covalent or ionic) interactions between the stainless steel and the organic layer, and there are very few studies focusing on the physical properties of the organic primer, such as the glass transition temperature (Tg) [7].
Having examined the rate of water penetration into adhesive joints, it remains to consider the mechanism whereby it causes weakening. Water could influence the adhesive or the interface and in the case of permeable adherends changes in substrate properties could occur. Some of the processes which might occur would be reversible and so be accompanied by recovery in strength on removal of water but others would be irrecoverable. Gledhill has studied the durability of structural metallic adhesive joints, employing an epoxy adhesive, and also measured the diffusion coefficient and solubility of water in the bulk epoxy adhesive under the same environmental conditions, and he has suggested the rate of interface disbonding in these joints is controlled by the availability of water at the interface, which in turn is governed by diffusion of water through the adhesive [8]. The temperature Tg is defined as that at which there is an increase in the thermal expansion coefficient. The increase in the thermal expansion coefficient above Tg can be explained by the greater degree of freedom available to molecular segments. The larger volume between molecules gives more degree of freedom so that the same increase in temperature will give a greater increase in volume. In other words, a higher Tg restrains the mobility of polymer network and its bonding to metallic substrate can be stabilized.
In this study the influence of the Tg of the organic primer coating on the adhesion in a humid environment, especially in the critical condition (in boiling water), was investigated in an attempt to optimize the primer coating design. As a result of this investigation, a novel epoxy based primer coating was proposed, which did not lose its adhesion on stainless steel even after boiling water immersion.
Section snippets
Primer coating
Table 1 shows a brief listing of the composition of the investigated primer coatings (varnish).
Epoxy resin (diglycidyl ether of bisphenol-A type and phenol-novolak type) supplied by Mitsubishi Chemical Co. was used as the main resin. Dicyandiamide (DICY) was used as a curing agent and it was also supplied by Mitsubishi Chemical Co. In order to optimize the Tg, imidazole curing accelerators were additionally used and they were supplied from Shikoku Kasei Co. The surface of stainless steel that
Depression of Tg after immersion in boiling water
In Fig. 3, the influence of phenol novolac ratio on Tg and adhesion properties can be seen. As expected, a higher Tg can be obtained using a higher phenol novolac ratio, which introduces higher numbers of epoxy groups to increase the network density.
In Fig. 3 the depression of Tg after boiling water immersion for 7 days can be also seen, together with adhesion properties. Fig. 4 shows the TMA charts of primer B. As shown in this table, the Tg depression was not so significant for B and C
Conclusions
A novel epoxy coating was developed by controlling its Tg. This coating had a highest wet Tg greater than 100 °C, and maintained its adhesion on stainless steel even after immersion in boiling water. If this coating material was applied to conventional two components epoxy structural adhesive as a metallic primer, it improved its adhesion durability on a stainless steel sheet in water significantly even if it was not chemically modified (without pretreatment). This indicated that relatively weak
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