Co-authors​ :

 Nuo XU , Wenxiang XU , Lingying PAN , Shaohua CHEN , Si QIU , Maxim VANDAELE , Dongxing ZHANG , Frederik DESPLENTERE (BELGIUM), David SEVENO (BELGIUM) 

Better understanding of how thermoplastics like polyether ether ketone (PEEK) spread on fiber surfaces within a composite, or themselves during fused filament fabrication is crucial for high-quality part production. So far, studies about the surface characteristics of PEEK have mainly focused on the calculation of surface energies and their components obtained from experiments performed at room temperature with probe liquids. This is supposedly due to the high temperatures (T > 300°C) required to melt PEEK and its high viscosity making it difficult to perform wetting measurements. However, it remains unknown if room temperature experiments can indeed give relevant indications of the behavior of PEEK in a molten state. In this study, viscosity and surface tension were first measured before the spreading behavior of molten PEEK on different substrates was investigated between 370 and 400 °C. Glass slides, silicon wafers, and steel blocks were chosen as substrates thanks to their homogeneity in terms of roughness and surface chemistry while supposedly having different surface energy components, at least at room temperature. The analysis of the contact angle dynamics showed that both the classical hydrodynamics approach and molecular-kinetic theory can model the experimental data satisfactorily. Temperature variation modified both surface tension and viscosity but did not appreciably affect the slip length. The viscous contribution to the activation free energy of wetting was found to be much larger than the contribution due to the solid/liquid interactions. This indicates that PEEK spreads through a mechanism controlled principally by its viscosity. During the cooling process, when the temperature reached the crystallization onset temperature, the contact angle between PEEK and the substrates demonstrated a discontinuous transition. Then, the work of adhesion between solid PEEK and the different substrates, which refers to the work required to separate their interface area, was predicted by different methods based on the results obtained from the room and high-temperature experiments. A good agreement was found when calculating the physical work of adhesion between PEEK and the substrate at high temperature in the molten state, at room temperature in a solidified state, and using a combination of both states. This is a promising result indicating that the bonding ability of PEEK during processing can be predicted by characterizing its surface properties in the solid state. This work provides a starting point and direction for understanding and clarifying the wetting behavior of molten PEEK and improving our understanding of the production of effective thermoplastic composites.