|M.Sc Student||Zrubaveli Parisa|
|Subject||Investigation of Nucleation and Formation of Cellular|
Thin-wall Foamed Polyethylene
|Department||Department of Polymer Engineering||Supervisors||Professor Emeritus Moshe Narkis|
|Professor Michael Silverstein|
|Full Thesis text|
Foamed polymers are plastics of reduced density owing to the presence of numerous gas voids throughout their volume. Such products possess unique properties compared to their solid counterparts, such as higher specific tensile strength, higher toughness and better thermal insulation properties. The concept of a foaming process is to saturate a polymer melt with a blowing gas in a stable condition. Then, generate cell nucleation by imposing a rapid instability that leads to phase separation of the initially gas-solid single phase equilibrium. As a result, cells nucleate and grow until solidification occurs.
Blowing agents which produce gas for the expansion process can be physical blowing agents (nitrogen gas or carbon dioxide) or chemical blowing agents (compounds which liberate gas under the foam processing by thermal decomposition). Nucleating agents are fine powders, or particles that remain thermally stable during the foaming process and provide solid surfaces for heterogeneous cell nucleation. Such agents increase the number of nucleation sites by reducing the nucleation free energy.
In the present study, continuous extrusion-foaming processes along with a detailed characterization technique have been developed. Rheological and mechanical properties of various compositions of polyolefin blends were characterized to determine a ternary PE blend as the basic matrix. The effect of die geometry, die temperature and pressure on foam characteristics of foamed extrudates was studied. Chemical blowing agents with different decomposition kinetics and various nucleating agents with different geometries and dimensions were examined to find proper blowing and nucleating agents for forming thin wall foamed polyethylene filament. Likewise the effect of various surfactants (antistatics, silicones) on foam characteristics was thoroughly studied. Thermodynamic processing conditions (temperature, pressure) and proper die geometry were determined for the extrusion foaming process. A suitable combination of a nucleating agent and surfactant in a selected polyolefin' blend has been developed. Such systems form micro-cells under given processing conditions.