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
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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughboroiugh University.