PA6 and Halloysite Nanotubes Composites with Improved Hydrothermal Ageing Resistance: Role of Filler Physicochemical Properties, Functionalization and Dispersion Technique.

Polyamide 6 (PA6) suffers from fast degradation in humid conditions due to hydrolysis of amide bonds, which limits its durability. The addition of nanotubular fillers represents a viable strategy for overcoming this issue, although the additive/polymer interface at high filler content can become privileged site for moisture accumulation. As a cost-effective and versatile material, halloysite nanotubes (HNT) were investigated to prepare PA6 nanocomposites with very low loadings (1-45% w/w). The roles of the physicochemical properties of two differently sourced HNT, of filler functionalization with (3-aminopropyl)triethoxysilane and of dispersion techniques (in situ polymerization vs. melt blending) were investigated. The aspect ratio (5 vs. 15) and surface charge (-31 vs. -59 mV) of the two HNT proved crucial in determining their distribution within the polymer matrix. In situ polymerization of functionalized HNT leads to enclosed and well-penetrated filler within the polymer matrix. PA6 nanocomposites crystal growth and nucleation type were studied according to Avrami theory, as well as the formation of different crystalline structures (α and γ forms). After 1680 h of ageing, functionalized HNT reduced the diffusion of water into polymer, lowering water uptake after 600 h up to 90%, increasing the materials durability also regarding molecular weights and rheological behavior.

The Effect of Moisture on Cellulose Nanocrystals Intended as a High Gas Barrier Coating on Flexible Packaging Materials.

Cellulose nanocrystals (CNCs) exhibit outstanding gas barrier properties, which supports their use as a biobased and biodegradable barrier coating on flexible food packaging materials. As highly hydrophilic biopolymers, however, CNCs have a strong sensitivity to water that can be detrimental to applications with fresh foods and in moist conditions due to the loss of barrier properties. In this work, the oxygen and water vapor permeability of polyethylene terephthalate (PET) films coated with CNCs obtained from cotton linters were measured at varying levels of relative humidity, both in adsorption and desorption, and from these data, the diffusion and solubility coefficients were estimated. Therefore, the characterization of CNCs was aimed at understanding the fundamentals of the water-CNCs interaction and proposing counteractions. The CNCs' moisture absorption and desorption isotherms at 25 °C were collected in the range of relative humidity 0⁻97% using different techniques and analyzed through GAB (Guggenheim-Anderson-de Boer) and Oswin models. The effects of moisture on the water status, following the freezable water index, and on the crystal structure of CNCs were investigated by Differential Scanning Calorimetry and by X-ray Powder Diffraction, respectively. These findings point to the opportunity of coupling CNCs with hydrophobic layers in order to boost their capabilities as barrier packaging materials.