Effectiveness of organic and inorganic pigments for mass colouration of thermo-mechanical pulp used in wood–plastic composites

Abstract

At present, to provide wood–plastic composites (WPC) with colouration and UV-protection, pigments are added in powder form or pre-dispersed in a polymer matrix (masterbatch) during compounding. Usually, the wood particles are not coloured prior to compounding, although this may improve the colour stability of the WPC. In this research project, two types of pigments were used to pre-colour thermo-mechanical pulp: an aqueous inorganic pigment preparation based on iron oxide and a water-based preparation of organic and inorganic pigments. Pre-coloured fibres were mixed with high-density polyethylene and maleic-anhydride grafted-PE in a kneading mixer and injection-moulded. As a reference, uncoloured fibres were mixed with pigments. Colour stability of the WPC was determined as ΔE* after 400 h of Xenon weathering. Pre-treatment of the fibres alone was not sufficient to improve colour stability of WPC. However, the use of fibres pre-treated with inorganic iron oxide pigments in combination with a pigment masterbatch to colour the polymer matrix resulted in a significant reduction of ΔE* compared to when either pre-coloured fibres only or uncoloured fibres plus pigment were used. The mixture of organic and inorganic pigments was less effective in improving colour stability than the application of inorganic pigments only. Flexural strength and modulus of elasticity of WPC with coloured fibres were slightly reduced compared to WPC with uncoloured fibres. Water uptake was higher for WPC with organic pigments than for WPC with inorganic pigments only. Fibre pre-colouration appears useful to obtain WPC with low colour change as a result of weathering.