| dc.description.abstract | Consistently producing compounded plastics in the correct colour without making
adjustments of the colour formulation or the processing conditions is very challenging for
coloured plastics manufacturers. Conversely, the principal objective of the present research
was to identify the scientific and engineering factors that directly or indirectly cause deviation
and inconsistency in the output colour of compounded plastics grades and suggest viable
solutions to prevent these colour variations.
The current study mainly focused on investigating and analysing the individual and/or
combined effect of the processing conditions on the colour and appearance of resulting
compounded plastic grades. This study highlights individual and combined influences on the
output colour, of three process parameters: temperature, screw speed and feed rate. Typical
plastic grades and associated colour formulations were selected for experimentation and
analysis in consultation with the innovation team of SABIC IP at their Cobourg plant. Included
among the selection criteria was the frequency of colour variation encountered by a plastic
grade during regular production. A wide variety of research tools and techniques were
employed in this study, these include, for example, statistical methods such as Box-Behnken
design (BBD); characterization techniques such as thermogravimetric analysis (TGA); imaging
and image analysis using scanning electron microscopy (SEM); numerical analysis of the
kneading discs zone to evaluate the mixing efficiency under varying processing conditions in
a co-rotating intermeshing twin screw extruder.
Past production data of two low Chroma opaque polycarbonate (PC) plastic grades - PC1
and PC2, were statistically analysed with the aim to quantify the influence on output colour
caused by small adjustments in colour formulation made during production. This study
revealed that the output colour is quite sensitive to minute changes in the amount of white,
black, and yellow pigments in units of PPH – parts per hundred parts of polymeric resin. A
Design of Experiments (DoE) approach was applied to develop a better understanding of the
relationship between process variables and output colour. Such a relationship and optimal
processing conditions were investigated using Box-Behnken design of response surface for
three polycarbonate resin-based plastic grades: a low Chroma translucent grade (G1), a high
Chroma opaque grade (G2), and a high luminous opaque grade (G3). The obtained
experimental results verify the fitness of the statistical model employed and suggests
processing conditions that ensure consistency in output colour of the plastic grades examined. To further investigate the relationship explained by statistical analysis, a novel technique was
introduced to quantify dispersion of colour pigments in polymeric matrix under varying
processing conditions, it is based on scanning electron micrography and image analysis. A
correlation between the processing conditions and distribution graphs for pigments particle size
and inter-particle distance was established and compared with the colorimetric data. The results
obtained through these investigations could help plastics compounders achieve consistency in
plastics coloration. To visualize the flow behaviour of kneading discs zone in a co-rotating
intermeshing twin screw extruder used in experimentation, a 3D numerical analysis was carried
out using OpenFOAM® software. This study evaluates the dispersive mixing parameter λ for
a high Chroma opaque polycarbonate grade (G2) by simulating a 3-D isothermal flow pattern
in the kneading discs region of the twin screw extruder. A quasi-steady state finite element
method was implemented to avoid time dependent moving boundaries. The values of the
mixing parameter λ obtained compare the flow behaviour of the kneading discs zone under
varying processing conditions. Simulation results correlate well the input process variables
with the dispersive mixing in the zone of the kneading discs and compare well with
experimental colorimetric data.
The research work presented in this thesis significantly contributes to understanding the
influence of process variables to the extrusion process, especially of temperature, screw speed
and feed rate, on the output colour of polycarbonate resin grades. | en |
