Effects of oxygen and carbon dioxide plasmas on the surface of poly(ethylene terephthalate)

Electrocharging face masks with corona discharge treatment

We detail an experimental method to electrocharge N95 facepiece respirators and face masks (FMs) made from a variety of fabrics (including non-woven polymer and knitted cloth) using corona discharge treatment (CDT). We present practical designs to construct a CDT system from commonly available parts and detail calibrations performed on different fabrics to study their electrocharging characteristics. After confirming the post-CDT structural integrity of fabrics, measurements showed that all non-woven polymer electret and only some knitted cloth fabrics are capable of charge retention. Whereas polymeric fabrics follow the well-known isothermal charging route, ion adsorption causes electrocharging in knitted cloth fabrics. Filtration tests demonstrate improved steady filtration efficiency in non-woven polymer electret filters. On the other hand, knitted cloth fabric filters capable of charge retention start with improved filtration efficiency which decays in time over up to 7 h depending on the fabric type, with filtration efficiency tracking the electric discharge. A rapid recharge for a few seconds ensures FM reuse over multiple cycles without degradation.

Effect of non-oxidative plasma treatments on the surface properties of poly(p-phenylene terephthalamide) (PPTA) and poly(p-phenylene benzobisoxazole) (PBO) fibres as measured by inverse gas chromatography

It has been shown in previous works that the interfacial adhesion in PPTA- and PBO-epoxy composites can be improved by modifying the surface properties of these high-performance fibres upon exposure to non-oxidative plasma treatments. In this work, the effects developed on both types of polymer surface were examined as a function of treatment gas nature (He or N₂) and exposure time (one or four minutes) using inverse gas chromatography at infinite dilution (IGC). From the adsorption of n-alkanes, it has been proved that non-oxidative plasma treatments led to energetically heterogeneous surfaces in the case of PPTA, and to low-energy surfaces in the case of PBO. Nevertheless, it was proved with the 1-min plasma treatments (either under helium or under nitrogen) that chemical reactivity was enhanced on the PBO surface. Such a behaviour was ascribed to the presence of low-molecular weight oxidized materials. The mechanisms involved in surface activation of PPTA were not equivalent under He or N₂ exposure. Nitrogen plasma exposure led to a PPTA surface that is chemically reactive as a result of polarity enhancement. Helium plasma-treated PPTA surface was characterized by the presence of branched arrangements that intensified the number of chemical contacts onto reactive sites. Finally, for both fibre sets, if the purpose is to enhance the chemical surface reactivity, it makes no sense to increase the plasma exposure time from 1 to 4 min.

Surface Characteristics of Activated Carbons Obtained by Pyrolysis of Plasma Pretreated PET

Activated carbon materials have been prepared by pyrolysis of plasma pretreated recycled PET. The obtained carbon materials have been texturally characterized by N2 (77 K) and CO2 (273 K) adsorption. Atomic force microscopy (AFM) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) have been used to analyze the surface of the treated precursors. Carbon materials obtained by He, N2, and CO2 plasma pretreatments (4 min) of the precursor and subsequent pyrolysis have shown a higher adsorption capacity than the corresponding chars (untreated pyrolised PET). This effect seems to be related to the elimination by the plasma treatments of low-molecular-weight products in the precursor, which are responsible for the formation of amorphous carbon deposits during the carbonization that blocks the porosity. Longer periods of treatment (15 min) do not favor the opening of the microporosity because cross-linking reactions in the precursor producing high molecular weight deposits prevail. The development of porosity is less relevant if oxygen plasma is used, as a considerable amount of oxygen functionalities are also formed. These groups can decompose during pyrolysation producing the above-mentioned amorphous carbon deposits. The textural characteristics of the carbon materials obtained after 4 min of plasma treatment on the precursor are very similar to those obtained after 4 h of CO2 (1073 K) activation of the same char. Therefore, this method can be an alternative to avoid the burnoff and high energy cost of the activation step.