The influence of additives and additive migration on rheological properties and processing of polymer melt compounds

It is the objective of this dissertation to investigate the influence of low molecular weight additives on rheological properties and processing of polymer melt compounds. A range of thermoplastics of varying polarity including polyethylene, isotactic polypropylene, atactic polystyrene, poly (methyl methacrylate), and polyamide-12 were compared. Eighteen-carbon amphiphillic ω-compound additives including 1-octadecanol, octadecanoic acid, octadecamide, zinc stearate, and calcium stearate were used as additives. Octadecane was compared as a non-polar additive. Aliphatic carboxylic acids with different molecular weights and some aromatic carboxylic acids were also compared as additives. A series of particle fillers of varying character including carbon black, calcium carbonate, zinc oxide, and talc were compared. The primary concern of the present research involves the interactions between (i) additives/polymers, (ii) additives/fillers, and (iii) additives/metal surface. It also involves slippage phenomena induced in various polymers in rheometers by the presence of small amounts of additives, and the migration/deposition of additives onto the metal surfaces during processing. Systematic and critical experiments have been carried out. Capillary and cone-plate rheometer experiments were compared to investigate rheological properties including pressure drops, reduction in shear stress, slip velocity, and extrudate distortion. Pellet melting in filled mold experiments (compression molding), pellet melting in filled chamber experiments (internal batch mixer mixing), and continuous flow experiments (slit die extrusion) were compared as processing methods. Mold plates and specially designed slit dies constructed from a series of metals of varying distribution of electrons, surface tension, and chemical reactivity including aluminum, zinc, low carbon steel, nickel, and copper were compared to investigate the interactions between additives and metal surfaces during processing. The characterization of migration and deposition of additives on metal surfaces or particle filler surfaces includes contact angle measurements, infrared spectroscopy, and microscopy (scanning electron microscopy and transmittance electron microscopy). The mechanism of migration/deposition of additives in a polymer/additive binary system and polymer/(particle filler)/additive ternary system to metal or filler surfaces was critically investigated and explained in detail.; Larger effects in additives causing viscosity reduction were found in polyethylene and polypropylene. Little or no effects are found with the other polymers. The most effective additives were octadecanoic acid and zinc stearate. Octadecanamide and octadecanol had some effectiveness. Aliphatic fatty acids were more effective than aromatic carboxylic acids. The addition of carboxylic acids to polyolefins resulted not only in an apparent slippage flow along rheometer and die walls but the carboxylic acids were proven to migrate to the wall. Compounds with different particle fillers were also studied. The greatest effects were found in polyethylene/(carbon black) and polystyrene/(calcium carbonate) compounds. All of the polyethylene compounds exhibited slip flow but the polystyrene compounds did not. Apparently much of the octadecanoic acid was interacting with the polar particle fillers and not with the die wall.