Enikő Földes

Study of the effects influencing additive migration in polymers

The lifetime of polymers is affected by the consumption and physical loss of the additives used for the improvement of performance. The practice and scientific experiments revealed direct relationships between the service life of the polymer and the transport characteristics of the antioxidants applied. From an effective stabilizer low volatility, high solubility and low diffusion rate are expected. While the first parameter is determined only by the chemical nature of the additive, the solubility and the mobility depends also on the chemical and physical structure of the polymer. In our laboratories the solubility and the diffusion rate of different additives have been studied in amorphous and semicrystalline (ethylene) polymers. The effect of the nature of the small molecules, like size, shape, and hydrogen bonding, as well as the thermal history of the polymer were investigated. Quantitative relationships were established between the characteristics of the components and the transport properties.

Competitive interactions and controlled release of a natural antioxidant from halloysite nanotubes

Halloysite nanotubes used as potential carrier material for a controlled release stabilizer in polyethylene were thoroughly characterized with several techniques including the measurement of specific surface area, pore volume and surface energy. The high surface energy of the halloysite results in the strong bonding of the additive to the surface. Dissolution experiments carried out with eight different solvents for the determination of the effect of solvent characteristics on the amount of irreversibly bonded quercetin proved that adsorption and dissolution depend on competitive interactions prevailing in the system. Solvents with low polarity dissolve only surplus quercetin adsorbed in multilayers. Polyethylene is a very apolar polymer forming weak interactions with every substance; quercetin dissolves into it from the halloysite surface only above a critical surface coverage. Stabilization experiments confirmed that strong adhesion prevents dissolution and results in limited stabilization efficiency. At larger adsorbed amounts better stability and extended effect were measured indicating dissolution and controlled release.