Effect of interfacial crystalline growth on autohesion of PEEK

Abstract

This work aims to clarify the role of the crystalline growth on the autohesion strength of amorphous PEEK below its melting temperature. The self-bonding strength versus temperature, pressure and time has been measured by lap shear test on 250-micron thick amorphous PEEK assembled at various conditions. The effect of the crystalline growth on the adhesion strength has been established at 155°C, 200°C and 250°C. Autohesion is temperature dependent, whereas pressure at less than 1 MPa and time up to 3 h, have less impact on the adhesion strength. Nevertheless, the evolution of the crystalline morphology with time results in increasing the interfacial strength: a gain of 40% is noticed between 1 and 3 h at 250°C to reach 0.9 MPa. The degree of crystallinity is higher at the interface than elsewhere in the material, the interface acting as nucleating agent. The evolution of the crystalline morphology at the interface with time shows the refinement of the primary lattice when the temperature is higher than or equal to the previous crystallization temperature, corresponding to the highest temperature seen by the polymeric material. The crystalline growth has an ambivalent effect on the autohesion of PEEK: it reduces the mobility of the macromolecular chains and thus their interdiffusion through the interface and at the same time, it is observed that the improvement of its crystal lattice reinforces the interfacial strength.

Experimental investigation and optimization of thermal gradients by infrared welding

“…An investigation of the adhesion regimes, adapted to infrared welding process steps, over very short times (below 2 s) is necessary to understand infrared welding and the welding time dependency of the mechanical properties. After postcuring under the same tooling and welding conditions, the average LSS value slightly increases, as expected [7], [8], up to 45.2 MPa. The influence of postcuring and the associated crystalline morphology changes should be examined more carefully through complementary test methods, as well as under varying thermal environments.…”

Ultrasonic welding of CF/PEEK composites: Influence of welding parameters on interfacial temperature profiles and mechanical properties

“…PEI is an amorphous polymer, which means that the cooling rate do not affect much its mechanical properties, in comparison with semi-crystalline ones for which a high cooling ramp inhibits the crystallization. The crystallinity affects the mechanical properties as revealed by Martineau [29]. E' is at 2.9 GPa for amorphous PEEK whereas it reaches 3.5 GPa for crystallized PEEK.…”

Laser transmission welding of PEKK: Influence of material properties and process parameters on the weld strength

“…A higher power reaching the interface would result in a higher temperature and cooling ramp. As proved on PEEK [17], the refinement of the primary lattice takes place when the temperature is higher than or equal to the previous crystallization temperature. An evolution of the lattice sizes could result in lower ultimate tensile strength.…”