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Title: Fundamental understanding of poly(ether ketone ketone) for high temperature laser sintering
Author: Benedetti, L.
ISNI:       0000 0004 9355 8213
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2020
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High-Temperature Laser Sintering (HT-LS) is a powder bed fusion technique employed to manufacture polymers with high service temperature, usually above 150 oC. The aerospace, automotive, and medical industries have driven the demand for processing high-performance polymers, as they could offer a lighter and cheaper alternative while maintaining the mechanical and chemical performance required to replace metallic parts in particular environments. Kepstan 6002 poly(ether ketone ketone) (PEKK) belongs to poly(aryl ether ketone)s (PAEKs) family and has a promising application in LS. The lower melting temperature united to the high glass transition temperature of PEKK (similar to PEK HP3, first commercially available HT-LS grade) enabled processing at lower temperatures whilst maintaining the high-temperature resistance of the polymer. Furthermore, the kinetics of crystallisation of Kepstan 6002 PEKK is very slow, which can assist layer adhesion during LS and improve mechanical properties in Z orientation. The present research project was developed in collaboration with Arkema. Three different grades of Kepstan 6002 PEKK were selected for initial analyses – HL1327, HL1320, and P12S959a. The powders were characterised for powder size, distribution, morphology, flow, moisture effect, and coalescence behaviour. This screening enabled the selection of HL1327 grade as the most promising for HT-LS application. The PEKK particle and powder analyses continued with an in-depth study of particle size and shape changes as a function of temperature and coalescence. The study revealed individual particle shrinkage prior to melting, followed by increased growth. The same phenomenon was observed for pairs of particles during coalescence and was attributed to the recovering of elastic deformation of the polymeric chains. The effect of intrinsic PEKK characteristics was successfully evaluated and quantified in the overall shrinkage in LS. The results identified powder properties as the main factor causing shrinkage of PEKK, as opposed to crystallisation. These results are supported by the powder characterisation developed in previous chapters. The interaction between material and process was investigated and optimised by testing different combinations of laser parameters and processing temperatures. The resulting properties were monitored regarding mechanical performance, surface topography, porosity, and crystallisation. The optimised PEKK specimens showed excellent mechanical strength (∼90 MPa) and modulus, but poor elongation, a common drawback from the LS process. The combination of fundamental material properties and process optimisation led to the development of a novel route to improve elongation and control the mechanical performance of LS PEKK. The experimental method successfully related cooling time, mechanical properties, and crystallisation of PEKK, and was able to increase elongation at break by 5.4 times. The improvement of elongation at break was attributed to the largely amorphous phase of PEKK when subjected to short cooling times. Lastly, powder recyclability was investigated from a chemical and physical perspective. PEKK can be reused following additional treatment steps, e.g., sieving. The potential for recyclability is an important remark as the material cost is significantly reduced and therefore preferred over the use of metals for high-performance applications.
Supervisor: Ghita, O. ; Evans, K. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available