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Polymers 2019, 11, x FOR PEER REVIEW 6 of 13 Polymers 2019, 11, 609 6 of 12 same sample of PA12 laser sintering powder has been deposited and measured multiple times with sample of PA12 laser sintering powder has been deposited and measured multiple times with an an application speed of 10 mm/s. application speed of 10 mm/s. (a) (b) FiFgiugruere2.2E.leEcltercotsrtoasttiactsicursfuacrefapcoetepnottieanltfioarl(fao)rvi(rag)inviarngdin(ba)nudse(bd)PAus1e2dlaPsAer1s2inltaesreinrgsipnotewrdinegrdpuorwindger ladyuerindgeploaysietriodnefpoorsirteiponeaftoerdraeppealitceadtioanppolfictahteiosnamofethpeowsadmeer spaomwpdlerwsaitmhpalevewloitchitya ovfel1o0cimtymo/fs1.0 Mmeamsu/sr.eMmeanstusrheamvenbtesehnapverbfoeremnepderafto3rm0%edrealtat3i0v%ehruelmatidvietyh.umidity. The first application yields lower potentials for both kinds of ageing states with rather steady The first application yields lower potentials for both kinds of ageing states with rather steady trends along the direction of powder deposition, whereas consecutive experiments exhibit elevated trends along the direction of powder deposition, whereas consecutive experiments exhibit elevated values and a clear maximum within the first 50 mm of the powder layer. Single particle contacting values and a clear maximum within the first 50 mm of the powder layer. Single particle contacting experiments with atomic force microscopy (AFM) have shown that the amount of charge transferred by experiments with atomic force microscopy (AFM) have shown that the amount of charge transferred contact electrification correlates well with the number of contacts for a variety of insulating materials by contact electrification correlates well with the number of contacts for a variety of insulating like polystyrene (PS) and polydimethylsiloxane (PDMS) [30–32]. Similar experiments conducted with materials like polystyrene (PS) and polydimethylsiloxane (PDMS) [30–32]. Similar experiments polymer tips contacting a plain surface on a Kelvin probe force microscopy (KPFM) system do also conducted with polymer tips contacting a plain surface on a Kelvin probe force microscopy (KPFM) show an influence of the force involved in frictional contact, with a higher force leading to increases in system do also show an influence of the force involved in frictional contact, with a higher force charge generation [33]. According to these observations, the elevated values of the surface potential at leading to increases in charge generation [33]. According to these observations, the elevated values earlier powder layer positions can be attributed to the intensified contacting conditions during the of the surface potential at earlier powder layer positions can be attributed to the intensified start of the doctor blading process. Since the powder reservoir in front of the applicator diminishes contacting conditions during the start of the doctor blading process. Since the powder reservoir in with increasing path length, both, the number of contacts and the forces acting onto single particles front of the applicator diminishes with increasing path length, both, the number of contacts and the decrease over the course of layer application. While there seems to be a quickly reached saturation forces acting onto single particles decrease over the course of layer application. While there seems to limit for the surface potential of repeatedly applied virgin powder, this indication is much less explicit be a quickly reached saturation limit for the surface potential of repeatedly applied virgin powder, for aged powder. Overall, the potentials measured for aged powders exceed those of the virgin this indication is much less explicit for aged powder. Overall, the potentials measured for aged powder considerably. powders exceed those of the virgin powder considerably. Whilst virgin powder has a peak potential of approximately 25 V, the aged material reaches values Whilst virgin powder has a peak potential of approximately 25 V, the aged material reaches up to almost 140 V in this experiment. Possible reasons for this could be the depletion or degradation values up to almost 140 V in this experiment. Possible reasons for this could be the depletion or of an antistatic agent in the powder material or a change of electrical properties due to alterations in degradation of an antistatic agent in the powder material or a change of electrical properties due to the molecular structure of the polymer by solid-state post-condensation. The main factor influencing alterations in the molecular structure of the polymer by solid-state post-condensation. The main charge build-up is the surface resistivity, since good conductivity results in a fast equilibration of factor influencing charge build-up is the surface resistivity, since good conductivity results in a fast charges [17,34]. It is known that polymer swelling and adsorption of ambient water have a strong equilibration of charges [17,34]. It is known that polymer swelling and adsorption of ambient water influence on the surface resistivity and therefore change electrical properties of polyamides [20]. have a strong influence on the surface resistivity and therefore change electrical properties of Consequently, a changed water absorption behavior due to alterations in the molecular structure could polyamides [20]. Consequently, a changed water absorption behavior due to alterations in the also explain the observed results. This is in line with the impedance spectroscopy measurements molecular structure could also explain the observed results. This is in line with the impedance shown in Table 1 that exhibit a reduced conductivity for aged powder, especially at low frequencies. spectroscopy measurements shown in Table 1 that exhibit a reduced conductivity for aged powder, especially at low frequencies. Moreover, the data displayed in Figure 2 shows that multiple applications of the same powder in close temporal proximity should be avoided when aiming for a lower influence of electrostatics during powder layer formation. This result is apparent for both, virgin and used powder and is clarified in Figure 3a by means of the maximum surface potential measured for up to 20 repetitionsPDF Image | Tribo-Charging during Powder in Selective Laser Sintering
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