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634 J. Zhang and S. Ciampi (a) (b) Fig. 1. glass, PMMA, and nylon gain a positive charge, whereas PDMS, PTFE, and PVC become negatively charged. Our Faraday pail data therefore suggest that solid CO2 should be placed in a triboelectric series between PMMA and PVC (Fig. 1b). Furthermore, the magnitude of charging is strongly atmosphere-dependent: data comparing tribocharging in air and argon are shown in Fig. 2a and Fig. 2b, respectively. In 2018, Grzybowski and coworkers observed the magnitude of the charges developed on polymers by contact-electrification to be dependent not only on the material, but also on the nature of the atmosphere surrounding the polymer.[15] The dielectric strength of air (3 kV mm1)[16] is six times than that of argon (0.5 kV mm1),[17] and our data are consistent with an atmosphere of high dielectric strength favouring tribocharing. For the tribocharging mechanism, guided by our recent work demonstrating that charge carriers are polymer fragments,[18] we suggest that triboelectrification between solid CO2 and polymers can be ascribed to material transferred from the polymer to CO2, and this may be of relevance to gas–solid fluidized reactors.[19] Our Faraday pail data show that for PDMS, PTFE, and PVC samples handled in argon, ,34, 52, and 90 % (respectively) of the tribocharge has dissipated 20 s after their separation from a CO2 surface (Fig. 3b). For these polymers, a faster decay is observed in air, where ,85, 91, and 96 % of the negative charges are lost over the same period of time (Fig. 3a). Charge dissipa- tion, in both air and argon, is faster for PVC than for PTFE or PDMS (Fig. 3c), which is probably linked to the lower work function of the former.[18] We hypothesized that the differences in charge magnitude observed under argon or in the presence of atmospheric water could be due to the different abilities of these media to stabilize tribocharges.[20] Under ambient conditions, polymers are known to harbour surface water layers, and charges can potentially dissipate ‘laterally’ through surface conductance. Under a dry and inert atmosphere this mechanism of charge dissipation is likely to be inoperative or less efficient. (a) 2 1 0 –1 –2 –3 –4 –5 CO2 PDMS (+) Rabbit fur Nylon Negatively charged 30° Faraday pail Wood PMMA Silk – –– +– –+ – PDMS (–) Glass Paper (a) Illustration of the experimental setup in which dry ice samples are rubbed against (slid down) a polymer surface. The amount and sign of the net charge gained by the plastic sample are measured by inserting it into a Faraday pail connected to a high-precision electrometer. (b) Relative position of various dielectric materials in a triboelectric series: materials located towards the right () tend to acquire a net negative charge on contact with materials located to their left (þ). The tentative position of CO2 in the series (red ink in (b)) is based on the Faraday pail data presented in Figs 2 and 3. (b) 1 0 –1 –2 Glass CO2 Number of contact-separation cycles PVC PDMS PTFE 0 10 20 30 40 50 60 70 (c) –1 –2 –3 2 1 0 P 0 10 20 30 40 50 60 70 Number of contact-separation cycles PV Glass PMMA Nylon PVC PTFE PDMS C Glass PMMA Nylon PVC PTFE PDMS PT FE D MS Nylon PMMA In Air In Ar Fig. 2. Conclusion The conclusions of the present work are two-fold. First, solid CO2 can be tentatively placed between PMMA and PVC in a triboelectric series. This is important because it can guide the engineering and design of materials such as to either maximize or minimize their static charging on contact with solid CO2. For example, the Australian Standard concerning the manufactur- ing of CO2 extinguishers (AS/NZS 1841.6:2007) states that ‘a discharge horn shall be constructed of electronically non- conductive material’. CO2 extinguishers are primarily used on electrical fires, or on fires involving electrical equipment, hence the need of an insulating horn, but here we have shown that not all insulators have the same tendency to build up static elec- tricity after contact with dry ice. Second, we have shown that Tribocharging of polymer samples against dry ice. (a) Faraday pail data for samples charged by cyclically sliding solid CO2 across an inclined surface. Data were acquired under ambient air: glass, nylon, and PMMA samples gained a net positive charge, whereas PVC, PTFE, and PDMS became negatively charged. (b) Tribocharging data for experiments per- formed under argon; all the polymers’ net charge is smaller in argon than in air. (c) Difference in the tribocharging – air versus argon – for samples that were subject to 40 contact–separation cycles. Charge density [nC cm–2] Charge density [nC cm–2] Charge density [nC cm–2]PDF Image | Position of Solid Carbon Dioxide in the Triboelectric Series
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