Paper Technology International 2020 - Journal - Page 47

PAPERTECHNOLOGYINTERNATIONAL
Figure 7: The contact angle is the angle conventionally
measured through the liquid, where a liquid-vapor interface
meets a solid surface. It quantifies the wettability of a solid
surface by a liquid. A high contact angle = low wetting, a low
contact angle = high wetting
OGR is the most common attribute for a barrier coating in
food packaging as most food will contain some level of grease, oil,
or fat. Barrier coatings work by placing an impermeable lipophobic
(fat-hating) barrier membrane between the substrate and the
packaged product.
Historically the traditional method to impart OGR properties
to paper and board was with the use of fluorochemicals which work
by reducing the surface energy of the substrate to lower than that of
the surface tension of the grease, so the grease cannot penetrate.
But these are non-degradable “forever” chemicals and are being
banned for use across the globe.
The ability of a barrier coating to maintain good OGR
properties on a crease or fold is also an important attribute.
Barrier coatings can only provide OGR to the surface of the
substrate and offer no internal protection, i.e., protection from the
migration or “wicking” of oils and fats through contact with any cut
edges inside the packaging. Here, an internal treatment to the pulp
is necessary to give wicking protection.
The industry standard of testing OGR is via 3m Kit method:
12 solutions with varying surface tensions (1 highest – 12 lowest;
higher the value the greater the OGR). This method is used on
the flat surface of the coated paper (Kit value) and on creased
and folded areas (Kit on crease value). The 3M Kit method was
developed for measuring treatment with fluorochemicals and some
manufacturers have developed internal methods of measuring the
OGR of barrier coatings.
Water vapour transmission
There are many applications where moisture control is
critical. For instance, to keep moist stuffs moist (permeability to the
environment) and dry stuffs dry (permeability from the environment).
It also applies to moisture-sensitive foods and pharmaceuticals that
are put in packaging with controlled MVTR to achieve the required
quality, safety, and shelf life.
Water vapor transmission rate, WVTR, also known as
Moisture vapor transmission rate, MVTR, is a measure of the
passage of water vapor through a barrier coating. WVTR generally
decreases with increasing thickness or coat weight of the barrier
coating and two layers of barrier coating (primer plus topcoat) are
generally more effective and efficient as micro defects or “pinholes”
in the coating layers can cause significant failure. MVTR increases
with increasing temperature and relative humidity (% RH).
The most common standard conditions for measurement are
Temperate (23 oC / 70% RH) and Tropical or “Jungle” (37 oC / 90%
RH) although others may be specified.
Oxygen transmission
Controlling the oxygen transmission is important to
preserve the quality, safety, and shelf life of sensitive foodstuffs
and pharmaceuticals by inhibiting dangerous bacterial growth and
spoiling.
Oxygen transmission rate (OTR) is the measurement of the
amount of oxygen gas that can permeate through a barrier coating
over a given period. The mode of transport is a diffusion of oxygen
from the environment to the inside of a package. Many foodstuffs
sensitive to spoiling, i.e., dairy or meats, or products with long
shelf-life expectations require a low OTR value and in some cases,
the package will have been “purged” of oxygen with an inert gas,
typically CO2 or nitrogen, before the final sealing.
OTR generally decreases with increasing thickness or coat
weight of the barrier coating and two layers of barrier coating (OTR
coating plus topcoat) or even three layers (primer plus OTR coating
plus topcoat) may be necessary to achieve very low values.
Standard test methods are available for measuring the
oxygen transmission rate of packaging materials. Completed
packages, however, involve heat seals, creases, joints, and closures
which often reduce the effective barrier of the package.
Heat sealing
Heat sealing is a method where two materials are sealed
together by heating them while applying pressure on them for some
certain time. In addition to barrier properties, heat sealability is one
of the most important properties of packaging materials. Sealability
affects the ability of the material to form hermetic seals, which is a
necessity in different packages. Tight and strong seals contribute to
the preservation of the product and, on the other hand, prevent any
premature leakages.
There are different methods in heat sealing and these are for
example hot-bar sealing, impulse heating, hot air blast heating, and
ultrasonic heating. Heat sealing can be represented by a triangle
model representing the necessary ingredients to create a seal. If one
ingredient is reduced then the other ingredients must be increased
to compensate and likewise, if one ingredient is increased then the
other ingredients could be reduced.
Several different methods can be used for heat sealing. The
suitable method depends on the material, application, packaging
process, etc.
Seal strength can be measured by tensile strength testing
instrument or by visually evaluating the fibre tear after manual
sample tear.
Figure 8: Simple triangle model of heat-sealing.
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