Investigation of factors that influence coloration in polycarbonate based compounded plastics

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.