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Abstract
Polyurethane adhesives and coatings are used for a variety of applications. The emerging dendritic and hyper-branched (HB) polymers, having three-dimensional (3D) morphology, have opened new avenues to tailor the architecture of polyurethane adhesives and coatings. The methodology followed in this study was based on partial replacement of the polyol in the isocyanate/polyol mixture with HB or dendrimer moieties having reactive functional groups. The resulting formulations were characterized and optimized with respect to adhesion properties (shear and peel strengths) using various HB and dendritic polymers. The results have shown that the incorporation of 1 to 2% (by weight) of hyper-branched polyamidoamine (PAMAM) results in shear strength increase (46 to 78%) compared to a reference formulation (no HB), following curing at room temperature. This was attributed to an increase in cross-link density. Higher concentrations of the hyper-branched polymer resulted in reduced shear strength due to plasticization. When the polyurethane was post-cured (80 °C for 5 h) to enhance diffusion of the HB, both shear and peel strengths increased by 64% and 100%, respectively, compared to the reference formulation. The simultaneous increase of shear and peel strengths is very unique to this adhesive system. This unique phenomenon is attributed to the incorporation of the 3D HB PAMAM which imparted a 3D cross-linked network architecture, as well as an energy-absorbing structure. In the case of PAMAM dendrimer, an optimum was exhibited in the concentration range of 0.05-0.1% (by weight). When the samples were post-cured a less significant increase in shear strength was observed (12-31%), compared to the HB-containing formulation. When the concentration was increased beyond the optimum a reduction in shear strength was obtained as a result of plasticization, as in the case of HB-containing formulations. It was concluded that the 3D architecture obtained by the introduction of 3D dendritic and HB polymers should lead to new directions for enhancing simultaneously the shear and peel strengths of polyurethane adhesives and coatings.