Towards a Versatile and Sustainable Platform for Tailor-Made Adhesives

The main objective of the present thesis work was to explore the possibility to design and synthesize versatile environmental-friendly adhesives and to adjust their properties to be suitable for different purposes by studying the adhesives structure-property relationships. The thesis consists of a summary part and three articles. Three different types of adhesives were developed in this study and the main results have been published in three articles.

In the first part of this study, the aim was to develop poly(styrene-co-butyl acrylate) emulsion based adhesives that simultaneously exhibit high cohesive and adhesive strength via structured core–shell particle design. The structure–property relationship was examined by various characterization methods including size-exclusion chromatography (SEC), cryogenic transmission electron microscopy (cryoTEM), differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Optimized properties were achieved by changing the core–shell ratio, degree of crosslinking of the core by using different concentrations of ethylene glycol dimethacrylate (EGDMA) and modification of the soft shell by varying the concentration of a thiol based chain transfer agent (CTA). Crosslinking of the core part enabled high cohesive strength of the adhesive, whilst the relatively soft shell facilitated high adhesion by enabling the proper wetting of the substrate. The optimal adhesive performance in terms of tack, peel strength and shear strength were achieved when the latex particles had a crosslinked core, a shell prepared without CTA and a core–shell ratio of 1:1.5. Thus, the goal of combining sufficient energy storage and energy dissipation properties in one particle through structured particle design was successful.

In the second part, a number of chain-extended moisture-curable urethane prepolymers were synthesized in order to develop isocyanate terminated urethane prepolymer formulations that would at the same time display both high adhesive strength and low viscosity. Proton nuclear magnetic resonance spectroscopy (1H-NMR), size exclusion chromatography (SEC), differential scanning calorimetry (DSC), and Brookfield viscometry were utilized for characterizing the prepared urethane prepolymers. In addition, the adhesion strength of the cured prepolymers was determined by tensile shear strength test according to the DIN 1465 standard. Especially, the role of different types of linear (butanediol, pentanediol) and branched chain extenders (di-propylene glycol (di-PPG), tri-propylene glycol (tri-PPG) and the influence of their dosage on the degree of microphase separation between hard segments (HS) and soft segments (SS) in urethane prepolymers were studied. Furthermore, the benefits of utilizing a one-step versus a two-step polymerization process were investigated. The results revealed that the extent of phase separation of different urethane prepolymers was dependent on the extent of hydrogen bonding interactions which was extensively studied by attenuated total reflectance infrared spectroscopy (ATR-FTIR). The incorporation of branched chain extenders (di-PPG and tri-PPG) did not result in notable phase separation between hard segments and soft segments, while linear chain extenders (pentanediol and butanediol) readily promoted phase separation. The degree of phase separation was particularly pronounced for butanediol, and when the linear chain extender ratio was higher than or equal to 0.74. Compared with a two-stage process, one-stage process produced more randomly distributed polymer chains with highly dispersed hard segments. Thus, urethane prepolymers exhibiting strong adhesive strength with simultaneously low viscosity were successfully developed by systematic adjustment of structural parameters.

The third research work explored the possibility of synthesizing moisture-curable silane-terminated poly(urethane-urea)s (SPURs) of low viscosity. First, NCO-terminated urethane prepolymers were prepared, followed by silane end capping. The impact of polyol molecular weight and the ratio of isocyanate to polyol (NCO/OH) on viscosity and the properties of SPUR were examined. As alternatives to the organotin catalysts traditionally used for the polyurethane synthesis and curing processes, a toxicological more favourable bismuth carboxylate catalyst was evaluated. In addition, the effect of organofunctional groups in the aminosilane structure (R1–NH–R2–Si(OR3)3), i.e., R1 (alkyl, aryl or trimethoxysilylpropyl), the spacer R2 (α or γ) and alkyl group R3 (methyl or ethyl), was examined. The chemical structures of the SPURs were investigated by nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FT-IR) and the mechanical properties were evaluated by tensile tests. The results show that silane-terminated, moisture-curable polyurethanes can be efficiently synthesized and cured with bismuth carboxylate catalyst. Thus, SPUR exhibiting low viscosity, with adequate tensile strength and elongation could be prepared using environmentally benign bismuth carboxylate catalyst having a high metal content of 19%–21%, by utilizing secondary aminosilane end-cappers and an optimal combination of the polyol molecular weight and NCO/OH ratio.