Dispersion of additive masterbatches in polyolefin plastics
There has been a growing trend in recent years for polymer product manufacturers to use natural polymer and additive masterbatches instead of premixed compounds. For both polymer converters and polymer manufacturers, masterbatching makes economic sense. For the converter, the advantage is in the ability to buy and store in bulk a small number of base polymers which may be modified according to the dictates of the order book. This prevents the need to maintain an inventory of a large number of special compounds. Masterbatch base is very often low molecular weight polyethylene or some suitable low molecular weight compound. They are usually used with a wide range of polymer compounds. Manufacturers assume good additive dispersion in the products with use of masterbatch. This study investigates the quality of dispersion in masterbatches and the extent they can be used with varying polyolefin polymers. Also investigated is how additive particles are transferred from the masterbatch to another polymer during mixing and any morphological features that might relate to the degree of dispersion. A quantitative dispersion procedure in polyolefin products is also sought. X-ray microradiography, light microscopy and ultraviolet microscopy have enabled pigment and ultraviolet absorber dispersion in masterbatches and products to be studied. Pigment dispersion in low density polyethylene masterbatch is almost invariably bad. Iron oxide particularly was found to be the most poorly dispersed compared to other inorganics such as zinc sulphide, titanium dioxide and cadmium sulphide. On the other hand, the distribution of Cyasorb 531 in LDPE masterbatch is uniform. The degree of dispersion of UV absorber in polyethylene products depends on the difference between the melt flow index between the masterbatch base and the base polymer. Simulated sunlight exposure experiments have shown that increased absorber distribution significantly increases photostability of a high MFI HDPE/LDPE UV masterbatch blend. A semi-automatic procedure for quantifying pigment dispersion in polyolefin products has been developed. It involves a motorised stage scanning of a microtomed section of a polyolefin product with measurements being made with a photometer operating in a dark-field illumination and interfaced to a microcomputer. The procedure has enabled the point of significant agglomeration as well as the effect of shear rate and temperature on degree of dispersion in extruded products to be determined.