Abstract:
Structured latex particles with copolymer of poly(vinyl acetate-co-VeoVa10) core and poly(glycidyl methacrylate) (PGMA) shell were synthesized by emulsion polymerization. Starved feed and semi-continuous seed emulsion polymerization were used to control the morphology of the core-shell particles. Strategies for avoiding secondary particle formation in emulsion polymerization were implemented to investigate the effect of surfactant, chain transfer agent, starved feeding, and organic phase initiator on the creation of the core-shell morphology. The results suggest that core-shell morphology of the two stage emulsion was favoured by higher concentration of emulsifier in the seed latex. The particle size distribution of core-shell latex was broader than that of the core latex, and the average particle size of the core-shell latex was larger than that of the core latex. The core-shell structure was not produced using seed emulsion with emulsifier concentration at or below the critical micelle concentration. The addition of chain transfer agent, 1-dodecanethiol (DDM), caused a slight decrease in the particle size of the seed emulsion from 198 to 140 nm with increasingly narrow particle size distribution. Secondary particle formation was suppressed by increasing seed particles numbers, by the organic-phase initiator in the second stage, and by starved-feed conditions. Investigation of the isothermal crosslinking process of the core-shell latex composites with the curing agent Anquamine 419 based on real-time Fourier transform infrared (FTIR) spectroscopy data revealed that the cure rate was sensitive to the curing temperature and decreased with increasing conversion, based on the quantitative analysis of the concentration profiles of primary amine groups. Study of the rheological behaviour associated with the crosslinking process of the coreshell latex composites and four curing agents, ethylene diamine, diethylene triamine, triethylene pentamine, and Anquamine 419A, demonstrated that these systems were reactive and very sensitive to temperature. It was found that the gelation times and gelation temperatures could be determined from the abrupt variation of viscoelastic material functions, storage modulus (Gʹ), loss modulus (Gʺ) and viscosity (n*) across the time sweep or temperature sweep, respectively. Isothermal kinetics models of the crosslinking process based on the isoconversional method predicted the relationship of curing rate, degree of conversion, reaction nature and kinetic parameters, and temperature. The temperature dependence of the rate constants was found to be well described by the Arrhenius relationship. The temperature dependencies of Gʹ, Gʺ and n* were found to be frequency independent at the gel-point, Tgel, providing a fingerprint for determining Tgel of the crosslinkable systems. The variation of G' and Gʺ with frequency followed well the power law showing Gʹ=Acwb, and Gʺ=A²ωc through the frequency sweep. Dynamic mechanical thermal analysis (DMTA) was used to explore the effect of crosslinking on viscoelastic properties of the crosslinked films that were made from mixtures of the prepared latexes and the curing agent Anquamine 419. Reducing tanδ values and broader tanδ peaks indicated formation of crosslinks. It was observed that crosslinking increased the glass transition temperatures and the storage modulus in rubbery plateau region indicating the improvement of creep and heat resistance. The performance of the crosslinked emulsions as wood adhesives was evaluated in accordance with the ISO 6238 standard by measuring the maximum shear stress that could be applied to wood joints before cohesive and/or adhesive failure occurred. Additional performance tests were conducted by soaking glued wood joints in cold water for 24 h prior to shear strength determination, and by immersion of glued joints in boiling water and determining visually the time for the joint to fail. The crosslinked adhesives showed superior performance to unmodified poly(vinyl acetate) emulsion adhesive.