Influence of pigments on the crystallisation, shrinkage, and mechanical properties of LDPE

Pigments are widely used industrially, and it is known that they can act as nucleation agents in semicrystalline polymers, leading to changes in shrinkage and warpage. Variations in nucleation behaviour between different pigments can cause dimensional problems when parts are manufactured in a range of colours. It is widely known that some organic pigments such as reds and blue results in particularly high levels of nucleation and shrinkage. Previous studies have investigated the relationship between the effect of pigments addition on nucleation and the resulting shrinkage for a range of neat pigments using DSC (Differential scanning calorimetry). However, the effect of using masterbatch pigments with semicrystalline polymers is not fully understood. Masterbatch pigments are used much more widely than neat pigments due to the handling issues associated with liquids and powders. To understand the effect of masterbatch pigments on the crystallinity and shrinkage of semicrystalline polymers, 9 masterbatch pigments from a range of suppliers were added via injection moulding to LDPE (Low density polyethylene). The samples were analysed using DSC (Differential scanning calorimetry), XRD (X-Ray diffraction), FTIR (Fourier transform infrared spectroscopy) and tensile testing. DSC testing showed that adding pigment caused an increase in the onset temperature of crystallisation, and it was found that the magnitude of this nucleation effect was correlated with increased shrinkage. A correlation was also observed between the percentage crystallinity measured via DSC, XRD and FTIR and the measured shrinkage. It was observed that the orange, purple and lavender samples showed the largest changes in crystallinity and highest shrinkage. These results will be of interest to both polymer processors and companies involved in developing new masterbatch formulations.