|dc.description.abstract||Contact electrification (C.E.), a phenomenon studied for millennia, develops contact
charges on material surfaces, when two materials are contacted and then separated.
Accumulation of contact charges and their uncontrolled sudden discharges on
dielectric polymers pose major drawbacks in industries i.e. pharmaceutical,
(micro)electronics, and space, causing million-dollar losses annually. The overall
mechanism of C.E. is unclear until now, however, recent efforts have proven that
chemical bond-breakages on polymer surfaces result in mechanoions – which are
indeed the contact charges on the surfaces. These studies also showed that removing
mechanoradicals (co-formed upon bond-breaking) by molecular radical scavengers
destabilizes the mechanoions (charges) and render the doped polymer material
antistatic. This method of static charge mitigation has an advantage over the
conventional methods (e.g. doping with metals, carbon powder, conductive
polymers, or surface humidity enhancers) because it is not based on an increase in
surface conductance and smaller doping concentrations are needed to achieve antistatic behavior. However, currently used molecular radical scavenger doping is
generally not cost effective method to be upscaled for industrial use.
Lignin; however, is a “low-cost” material (the second most abundant polymer on
earth, a by-product of paper production) that can act as a radical scavenger. In this
thesis work, lignin was extracted from some examples of both hard and softwood.
Firstly, it was verified that lignin doping in low concentrations (1 – 5% w/w) reduce
the contact charge accumulation on common polymers such as on a crosslinked
elastomer polydimethylsiloxane, and on thermoplastics polypropylene,
polyethylene, polylactic acid, and polystyrene. Then, the mechanism of the observed
charge dissipation was discussed in the light of the results obtained from surface
conductance of polymers upon doping, 31P NMR and solid state 13C-NMR
spectroscopy, total phenol content, and the reacted number of radicals before and
after grinding - which was shown essential to get homogeneous doping- of lignin.
The results pointed out a mechanism involving a radical scavenging activity without
any change in the surface conductance of the material, similar to that with molecular
radicals. The understanding of lignin’s charge dissipation mechanism will be helpful
in industrial utilization of lignin as an antistatic additive and in assessment of the
limitations of this utilization.||en_US