The PVOH coating has improved the conductivity by one order
of the magnitude compared to corrugated board alone (Fig. 5).
Noticeably, the resistivity of PVOH-coated sample is lower than 2wt
%, 5wt% and 9wt% GNP added PVOH coating, indicating that PVOH
treatment alone would result in acceptable resistivity in all humidity
level for ESD protective packaging. Both PVOH and water
have hydrogen bonds, in which anions cannot move freely due to
the strong hydrogen bonds. However, the protons can move along
these hydrogen bonds acting as proton conductor.
Adding GNP into the PVOH coating could initially block proton's movement till a
sufficient amount of the GNP are accumulated to form a conductive
network at the percolation threshold.
It is evident that the surface resistivity of the GNP incorporated
PVOH coated samples had a sharp drop by 5 orders of magnitude
under the 9% RH and 4 orders of magnitude under 35% RH after the
GNP loading reached 9wt%, indicating a conductive network of GNP
particles is formed. The electrical percolation threshold is 13wt%
under 55% RH as opposed to 9wt% under 9%e35% RH.
The similar findings of the abrupt increase in conductivity were
also found in graphite/HDPE and graphite/carbon/epoxy composite
[4,5]. The result is also in agreement with the commercial Polycarbonate/
Carbon black composite which surface resistivity is
102e106 U/sq. when the carbon black filler is above 15wt% [17].
Considering the density of the GNP used in this study is twice that
of the density of the graphite (0.968 g/cm3) used, when using the
GNP as a filler, the amount of the volume can be reduced significantly
to reach an electrical percolation threshold.
Variations in relative humidity altered the value of surface resistivity
to a great extent and deviate the electrical percolation
threshold. Before the electrical percolation threshold, the surface
resistivity becomes significantly higher when the preconditioning
changes from 55% RH to 35%RH and 9%RH. This observation is in
agreement with Fromm and Vermillion's finding [2]. However, the
change of the surface resistivity with preconditioning becomes less
significant after the electrical percolation threshold. That phenome
could be caused by the presence of the large amount of the GNP
that change the hydrophilic nature of the PVOH.
The surface resistivity measured at 9% and at 35% RH are
consistent with the PVOH simply being more conductive at higher
RH. Once a conductive network of GNP forms, the resistivity
measured at 9% and at 35% RH is very similar. The surface resistivity
measured at 55% RH suggests that the PVOH swells so that the GNP
particles separate. In this case, the percolation threshold at 55%
may occur at a somewhat higher concentration.