Abstract
Pigments are mainly added to polymers to obtain a desired colour in plastic product. The added pigments may interact with the polymer leading to changes in crystallisation behaviour of the polymer. Shrinkage is a well-known phenomenon when using pigments in semicrystalline polymers. This can cause issues in obtaining correct interference fit between critical parts such as caps, closers, and valves and can create problems in automatic filling lines. It has previously been reported that the onset crystallisation temperature measured using differential scanning calorimetry (DSC) can be used to predict shrinkage in moulded parts when using a variety of neat pigments in semicrystalline polymers. However, the crystallisation and shrinkage behaviour of commercially used masterbatch pigments (mix of pigment in a carrier) is poorly understood. This study focuses on the influence of masterbatch pigments on the crystallisation, morphology, and shrinkage behaviour of polypropylene and polyethylene in order to better understand the mechanism of shrinkage and possibly overcome this problem.Chapter 4 and 5 of this thesis deals with the investigation of a detailed crystallisation kinetics study of PP and PE in the presence of a variety of commercial masterbatch pigments. The masterbatch pigments containing organic red ingredients such as quinacridone and diketopyrrolopyrrole were found to cause greater nucleation effects in both PP and PE. The commercial white masterbatch pigments containing only inorganic ingredients were found to have minor nucleation effects. A detailed investigation using DSC, LPOM, and SEM revealed that the organic based masterbatch pigments can cause greater changes in the crystallisation rate and results in reduction in spherulitic sizes. This resulted in greater shrinkages and higher crystallinity. A strong correlation was found between shrinkage and crystallinity of both PP and PE. Tensile tests revealed somewhat higher values of modulus with the strong nucleating pigments in both PP and PE.
| Date of Award | Aug 2022 |
|---|---|
| Original language | English |
| Supervisor | Edward Archer (Supervisor) & Eileen Harkin-Jones (Supervisor) |
Keywords
- shrinkage
- pigments
- crystallisation
- nucleation
- polypropylene
- polyethylene
- morphology
- crystallinity