Cellulose Nanocrystals/Polymer Nanocomposites for Application in Adhesives

Abstract

Cellulose nanocrystals (CNCs) are rod-shaped nanoparticles derived from cellulose, the most abundant polymer in the world. CNCs are as strong as Kevlar™, have a high aspect ratio (traditional nanoparticles are spherical) and thus, a higher surface area, which makes them ideal for use in nanocomposites. In addition, CNCs are considered the only safe nanomaterial according to Health Canada. In this thesis, CNCs were used to produce nanocomposites via in situ semi-batch emulsion polymerization. The target application for these nanocomposites was as pressure sensitive adhesives (PSAs). In the past, CNCs have been blended with polymers rather than added in situ. Emulsion polymerization is considered a more sustainable method to synthesize polymers compared to say, solution polymerization. However, adhesives synthesized using this method tend to have a lower shear strength due to poor gel network formation. As a result, conventional emulsion-based PSAs suffer from the inability to increase certain adhesive properties (e.g., tack and peel strength) while simultaneously increasing shear strength. In this thesis, we demonstrate how the use of CNCs via in situ emulsion polymerization overcomes this classic problem. Two polymer systems were tested: isobutyl acrylate (IBA)/n-butyl acrylate (BA)/methyl methacrylate (MMA) and 2-ethylhexyl acrylate (EHA)/BA/MMA. The use of CNC with IBA, a relatively hydrophilic monomer, rather than with EHA, a highly hydrophobic monomer, resulted in the simultaneous improvement of tack, peel strength and shear strength of the PSA films. Dynamic mechanical analysis (DMA) also indicated improved storage and loss moduli with increasing CNC content, further supporting the reinforcing effect of the CNCs within the PSA. EHA followed similar trends as IBA for conversion, particle size, viscosity, pH, glass transition temperature and gel content. On the other hand, the use of CNC with EHA yielded less improvement in adhesive properties due to poor dispersion of the CNCs because of the hydrophobic repulsion by the EHA.