Effects of pressure on poly(ether‐ether‐ketone) (PEEK) sintering mechanisms

Abstract

This study deals with the understanding of the sintering mechanisms that occur during consolidation of an ultra-highperformance polymer: poly (ether-ether-ketone). In particular, we investigated the effects of uniaxial pressure during spark plasma sintering (SPS) processing. Glass-transition temperature (T g ) measurements under loading, stress-strain curves and scanning electron microscopy analysis allowed us to determine the role of pressure intensity and temperature of application on macromolecular chain mobility in both the bulk and the surface of the particles. First, a loss of chain mobility in the bulk of the particles was observed under high pressure. Second, it was shown that high pressure applied at low temperature leads to friction effects between the particles which enhances chain mobility in the particle shell through a local melting phenomenon. These experimental conditions favor the healing of particles and high cohesion was then observed in the resulting sintered samples. The level of friction is enhanced when pressure is applied at a low temperature since the powders are still in a glassy state. Finally, the achievement of cohesive grain boundaries was found to be related to the location and conformation of chain ends. Good healing between particles can only occur if a sufficient number of chain ends are available at the surface of the particle shell. We showed that the native powder state plays a significant role. The direct use of as-received powder leads to final material exhibiting good cohesion whereas pretreatments of the native powder are highly detrimental. It should be noted that this processing does not affect the high initial degree of crystallinity because no bulk melting is observed during consolidation by sintering.

Spark plasma sintering and mechanical properties of two grades of PEKK presenting different Tere/Iso ratios

“…These processes, referred to as polymer sintering, have already shown promising results in producing dense and homogeneous polymer parts. [13][14][15][16][17][18][19][20][21] The sintering of polymer powders differs from that of other materials, such as metals or ceramics, due to their viscous nature. 22 Polymers are indeed composed of both free macromolecular chains, that constitute the amorphous region, and of crystalline regions where the macromolecular chains rearrange themselves into crystals.…”

“…Numerous studies have demonstrated the potential of SPS technology to consolidate polymers and polymer composites. [13][14][15]17,18,20,[30][31][32][33] For instance, Sébileau et al 14 successfully consolidated homogeneous PEEK samples by SPS. They defined a Design of Experiments to assess the effect of temperature, dwell time and pressure during sintering on the density ρ ð Þ, Young's modulus (E) and Compressive yield strength (σ y ) of PEEK specimens.…”

High Pressure (HP) in Spark Plasma Sintering (SPS) Processes: Application to the Polycrystalline Diamond

“…In particularly, pressure allows the synthesis of materials, even with different thermal stabilities precursors [ 9 ]: to orientate the chemical reaction in the direction of synthesis leading to the densest phase by the Le Chatelier principle (ex: synthesis of diamond to the detriment of graphite), e.g., [ 10 , 11 ]; to initiate a new finer microstructure by driving the phase transformation in polymorphic materials (ex: Al 2 O 3 : γ → α), e.g., [ 12 , 13 ]; to improve the chemical reactivity for refractory materials sintering (borides, nitrides, carbides) to better densification compared to lower pressure processes, e.g., [ 14 , 15 ]; to allow the sintering beyond the thermal decomposition temperature by the condensation effect, i.e., pressure stabilizing structure (ex: MgB 2 ), e.g., [ 16 ]; to sinter the high-pressure stable phase in the high-pressure stability domain (ex: c-C, c-BN), e.g., [ 17 ]; to adjust the porosity, close to 0% (ex: transparent ceramics) or high porosity ( p > 50% ) (ex: bone structure mimetic), e.g., [ 18 ]; to increase the thermal stability of precursors by condensation effect by avoiding the departure of OH − , H 2 O, others volatile elements) [ 19 ]; to decrease the sintering/consolidation/densification temperature by its driving force in order to avoid grain growth (which is always activated by high temperature), e.g., [ 20 ]; to favor the structural phase existing only at lower temperature (ex for amorphous calcium phosphate), e.g., [ 21 ]; to allow the consolidation of thermally unstable materials such as organic materials (ex: polymer) [ 22 ] and to allow the consolidation of composite constituted by materials of different thermal stability (ex: polymer composites) [ 23 ]. …”

“…to allow the consolidation of thermally unstable materials such as organic materials (ex: polymer) [ 22 ] and to allow the consolidation of composite constituted by materials of different thermal stability (ex: polymer composites) [ 23 ].…”

Thermo‐oxidative degradation during sintering of polyethylene particles

“…Inspired by powder metallurgy, sintering is a well‐established procedure for producing functional polymeric parts such as separation membranes, 1 orthopedic implants, 2,3 bio‐engineered scaffolds, 4 and drug‐release media 5,6 . Also, in cases where conventional melt processing is not feasible, sintering is a viable procedure for producing polymeric parts.…”