O PAPEL vol. 79, num. 2, pp. -- - -- FEB 2018
ABSTRACT
The performance of paper in many applications, including printing,
is often determined by the nature of its surface. Although currently
in decline, except in Asia, offset printing still accounts for around
40% of all print volume globally, and continues to dominate the
high-quality end of the printing spectrum. This study focused on
offset printing and the impact that different surface properties of a
substrate can have on this process.
Offset printing of paper involves the complex interactions of
ink with a surface which is characterised by three key interrelated
properties: roughness, porosity and surface chemistry related to
surface free energy (SFE). The effects of porosity and roughness are
relatively well understood and are documented in the literature,
whereas the influence of surface chemistry (SFE) is much less clear
and has been studied to a lesser extent. Primarily this is due to the
difficulty of measuring surface energy on non-ideal substrates, where
extremes of surface features can impact the apparent surface energy
(e.g., lotus effect).
Therefore, there were two primary aims for this study. The first
was to make recommendations on the best method to calculate
surface free energy (SFE) of paper considering its non-ideal nature.
The second was to determine the relative impact of SFE on offset
printability compared to other properties such as topography
(roughness) and porosity.
The key results shown in this paper include:
A method to measure SFE on porous surfaces. The contact angle of
a liquid droplet brought in contact with a porous surface is measured
at 0.1 sec after contact in order to minimise inaccuracies caused by
drop bouncing, wetting and absorption and corrected for surface
roughness using the Wenzel model.
The observation that that surface porosity has a negligible effect
on SFE determination by contact angle.
Demonstration that isolated SFE changes caused by use of
wetting agent or corona treatment result in far smaller differences
in printability than those obtained by modifying SFE and surface
porosity simultaneously.
The chemistry of the latex polymer in the coating formulation
dominates the influence on SFE compared to pigment, with any
surface energy differences present in the pigment being almost
completely masked by latex.
It is therefore concluded that when looking to improve offset
printability the variation in surface free energy (SFE) is significantly less
important than variation in either surface roughness or surface porosity.
Keywords: Surface energy, paper coating, offset printing