Preparation and properties characterization of polyamide/clay bionanocomposites

Abstract

A novel method for polyamide/clay bio-nanocomposites fabrication was developed in an attempt to facilitate ?extensive delamination? of clay stacks in polyamide matrices. The method, the so-called ?surfactant-free organo-modification approach? did not employ any surfactants for the matrix-clay compatibilisation. The idea was to exploit the use of dimer fatty acid polyamides with protonated amine end groups as clay surface modifiers of the clay previously dispersed in a liquid medium. The clays of choice in the study were the standard smectite clays commonly used to prepare polymer-clay nanocomposites and vermiculite. They were ultimately chosen on the basis of their ability to exfoliate into nano-thick sheets. Dimer fatty acid polyamide/clay bio-nanocomposites containing either montmorillonite or vermiculite were successfully prepared using the ?surfactant-free organo-modification approach?. Bio-nanocomposites containing as much as 28 wt.% montmorillonite and 30 wt.% vermiculite were obtained. In both cases, the composites featured a mixed morphology containing some exfoliated clay sheets together with nano-sized clay tactoids. At these filler loadings, the melt viscosity, tensile strength and Young?s modulus increased. Dynamic mechanical analysis showed that the glass transition temperature of the polymer increased by as much as 5 ?C when 27.5 wt.% montmorillonite was added and 10 ?C when 30 wt.% vermiculite was added. This indicates that the high interfacial surface area, presented by the clay platelets dispersed in the matrix, significantly impaired the polymer chain mobility. A further goal of the research was to extend the application of organo-modified vermiculite to the semi-crystalline polyamide-11. In this particular case, organomodified- and unmodified vermiculite and commercial sepiolite (Pangel S9) were considered. The clays were melt-compounded into the polyamide-11 to form products that contained either no filler, i.e. neat polyamide-11 or 10 wt.% clay. The aspects that were addressed included the effect of vermiculite organomodification, the effect of the shape of the clays, the aspect ratio of the particles, and the degree of dispersion that was achieved on properties of the generated polyamide-11/bio-nanocomposites. The emphasis was given to the mechanical and flame retardant properties. Polyamide-11/clay bio-nanocomposites were successfully prepared. Tensile properties results showed improvements in tensile strength and Young?s modulus increased with the presence of the nano-fillers. Young?s modulus of the bio-nanocomposites was almost the double of that of the neat polymer. Thermo-mechanical results also showed improvements in storage modulus with the addition of all particles, especially in the temperature range corresponding to the rubbery plateau (above the glass transition temperature). Cone calorimeter test results showed that the peak heat release rate and smoke production rate values of the polyamide-11/clays significantly decreased compared with those of neat polymer. This indicates that the addition of clays not only decreased the flammability of polyamide but also effectively reduced smoke production. The key findings of the thesis are: ? The ?surfactant-free organo-modification approach? offers an alternative to conventional clay modification routes based on cationic surfactants. It provides additives suitable for the improvement of the properties of amorphous polyamide matrices. ? The solution casting route allows the preparation of amorphous polyamide/clay nanocomposites with a very high clay content, i.e. approaching 30 wt.%. ? There are at least three stiffening mechanisms operating in amorphous polyamide/clay bio-nanocomposites. The reinforcing effect of the high stiffness inorganic flakes is the primary contributor. Together with the chain confinement effect, that expresses itself in an apparent increase in the glass transition temperature, this provided an adequate rationalisation of the stiffness variation below the glass transition temperature. However, an additional stiffening effect is indicated at temperatures above the glass transition temperature. The mechanism may involve dynamic network formation based on fluctuating hydrogen bonding interactions between the matrix polymer chains and the filler particles. ? From an engineering viewpoint, the good mechanical and fire retardant properties obtained with the vermiculite samples are very encouraging considering the inexpensive nature of this filler. ? The organo-modification of vermiculite and sepiolite is not necessary for the preparation of polyamide-11/clay bio-nanocomposites with excellent mechanical and thermal properties.