REGULATING MOISTURE UPTAKE KINETICS IN LLDPE/CAO COMPOSITES BY VARYING PARTICLE SIZE DISTRIBUTION

Abstract

Purpose. The purpose of the article is to experimentally assess the suitability of calcium oxide–containing polymer composites as absorbers of atmospheric moisture and to substantiate an approach to controlling their moisture uptake kinetics.

Methodology. The composites that were researched were produced by compounding in a twin-screw extruder. Moisture absorption was evaluated using a gravimetric method in a closed volume within a glass desiccator at 23 °C, with mass changes recorded daily. Relative mass gain was used as the moisture uptake metric, reflecting the integral amount of bound moisture.

Findings. The article substantiates an approach to controlling the moisture uptake kinetics of highly filled polymer composites based on linear low-density polyethylene and calcium oxide. The motivation for the study is that silica gel remains the most widely used desiccant and provides rapid sorption with an early transition to a quasi-steady state; however, practical moisture-control tasks do not always require the fastest possible drying at the beginning of exposure. In a number of cases, prolonged moisture absorption, higher final capacity, and the ability to deliberately tailor the moisture uptake curve are more important. Given that calcium oxide is widely used as a moisture scavenger during secondary polymer recycling, this work evaluates its suitability for absorbing atmospheric moisture outside the melt when incorporated into a granulated composite and demonstrates the possibility of regulating moisture uptake kinetics through two controllable structural parameters: the composite granule size and the particle size distribution of calcium oxide in the composite.

Originality. It is shown that LLDPE/CaO composites exhibit a fundamentally different sorption behavior than silica gel: instead of a sharp initial mass increase, moisture uptake is extended over time, and increasing the CaO content leads to higher capacity and a longer time to exhaustion of the absorption capacity. Finely dispersed CaO is characterized by intensive moisture uptake at the early stage of the experiment and a smooth, monotonic approach to a level of about 32% after 90 days. For calcium oxide with fraction sizes of 200 and 320 μm, the moisture uptake curve changes to an S-shaped form with a long shallow initial section and delayed acceleration; the acceleration point coincides in time with the appearance of cracks and local damage to the granule surface, indicating an abrupt change in mass-transfer conditions and an increase in the accessible contact area between the mineral phase and water vapor.

Practical value. The obtained results confirm the suitability of LLDPE/CaO-based composites as absorbers of atmospheric moisture with a more prolonged operating mode compared to silica gel, and show that combining control of the composite granule size with control of the CaO particle size distribution provides a practical tool for tailoring moisture uptake kinetics to specific application conditions.