Study on surface characteristics of epoxy-siloxane adhesives within the framework of thermodynamic model of adhesion
DOI:
https://doi.org/10.15407/hftp17.03.320Keywords:
epoxy oligomer, hydroxyl-terminal polydimethylsiloxane, small additives, epoxy hardener, thermodynamic model of adhesion, adsorption theory, contact angle, surface tension, work of adhesion, adhesion strengthAbstract
In order to improve operational characteristics of epoxy-diane adhesives they were modified with hydroxyl-terminal polydimethylsiloxane (PDMS) grafted by means of 3-aminopropyltriethoxysilane as a cross-linker. In this way, silicon-containing epoxy oligomers (SEO) with small additives of PDMS (0.1–5.0 pbw) and three series of polymers on their basis: SEP, SEA and SEI differing in the type of applied hardener (aliphatic oligoaminoamide PO-300, modified cycloaliphatic polyamine Ancamine 2752 and aromatic isomethyltetrahydrophthalic anhydride (i-MTHPA) respectively), were synthesized. Taking into account controversial impact of the amount of introduced modifier on the strength of adhesive joints, which is also described in literature, in the work presented the dependence of energetic characteristics at the oligomer / alumina interface on the content of PDMS was thoroughly studied within the framework of thermodynamic (adsorption) model of adhesion. For that such parameters as surface energy of the substrate (?_s), surface tension of epoxy oligomers and their mixtures with hardeners (?_LG), interfacial tension of the solid surface and liquid adhesive interface (?_SL), contact angles of oligomer formulations on the surface (?), work of adhesion (WА), work of cohesion (WС) and Harkins spreading coefficient (?) were calculated. Comparison of given theoretical values with the experimental indicators of adhesion and cohesion – pull-off strength (?) and tensile strength (TS) respectively – showed some their agreement, especially in the case of ?_SL and ?_LG (unlike WА). Thus, the results of testing likewise evidenced that absolute values of adhesion were the highest for the specimens of anhydride curing (with the pull-off strength up to 38.7 MPа). The same as in the theoretical calculations, it was experimentally found that the dependence of adhesion strength on the content of PDMS for each series of adhesive joints had a maximum. It confirmed appropriateness of the application of just small additives of PDMS. The introduction of siloxane into the samples cured with amine was the most effective for enhancement of adhesion. Particularly, as compared to unmodified EA, the value of ? for SEA-0.25 increased from 6.7 MPa to 19.0 MPa, that is by 184 %. Due to the set of restrictions, inherent to the thermodynamic model of adhesion, the correlation was not always noticed (for instance, as in the case of SEP series), however, this theory can be used for initial prediction of adhesive behavior of the joints, obtained under the same conditions.
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