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Definition
Relevance to performance
What
affects the opacity of OPPalyte films
Test
principles
Related
Terminology
Definition
Opacity represents a
substrate's light blocking ability. It is
primarily used as a property of paper and
predicts the relative visibility on one side
of the paper of the images that exist on the
other side. Because white opaque films are
replacing paper in some applications,
ExxonMobil measures opacity for some white
films.
There are two common
types of opacity measurements, and ExxonMobil
uses the one called "89% reflectance
backing," also called "contrast
ratio." This value is equal to 100 times
the ratio of the diffuse reflectance of a film
sample backed by a black body (<.5%
reflectance) to the diffuse reflectance of the
same sample backed by a white body (89%
reflectance). The units are percent, and a
perfectly opaque material will have an opacity
value of 100%.
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Relevance to performance
Originally developed
for paper, this property predicts the
appearance of two-side printed paper. Higher
opacity values allow better readability on one
side, because the print showing through from
the other side is less noticeable.
Now, the same test
used for paper is being applied to opaque
polymer films that compete in paper markets,
most notably labels. Depending on the product
design and film thickness, opacity values for
OPPalyte films range from 60% to 95%.
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What affects
the opacity of OPPalyte films
Opacity, like light
transmission, is determined mostly by pigment
and cavitation characteristics, which are
controlled by proprietary resin formulations
and film-making process conditions. ExxonMobil
technology creates high opacity levels with a
minimum use of inorganic fillers, such as
calcium carbonate (CaCO3) and
titanium dioxide (TiO2).
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Test principles
TAPPI
procedure T 425 and ASTM procedure D 589
describe the same standard protocol for
determining the opacity of paper. ExxonMobil
procedure #498 follows this protocol with only
minor exceptions; the most notable being that
ExxonMobil does not precondition samples for
40 hours at standard laboratory conditions
prior to testing.
Although
opacity has to do with light-blocking ability,
the test actually measures two reflectance
values. The ratio of these values times 100 is
equal to percent opacity. The equation, with
descriptions of the variables follows.
Opacity
(89% reflectance backing) = CO.89
= 100 (R0 + RO.89)
CO.89
is the contrast ratio, which is
another name for this type of opacity
measurement.
R0
is the reflectance of the substrate when it
is backed by a black body of 0.5%
reflectance or less. Any light that passes
through a partially opaque sheet will
reflect back negligibly.
R0.89
is the reflectance of the substrate when it
is backed by a white body having a
reflectance of 89%. This value will be
higher than R0 (or equal to it,
if the sample is perfectly opaque) because
any light that passes through the substrate
will largely be reflected, and a portion of
that will transmit through the film a second
time (in the opposite direction).
Opacity measurements are made with
commercially available opacity meters that
meet criteria stated in the standard
procedures. The TAPPI Technical Services
Department provides names of test equipment
suppliers. ExxonMobil uses a Technidyne BNL-3
Opacimeter.
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Related terminology
Light
transmission |
Light transmission is
the percentage of incident light that
passes through a film. It is related to
opacity, but cannot be calculated
directly from it. In general, the lower
the light transmission, the higher the
opacity. |
Optical
density |
Optical density is a
measure of a material's light blocking
ability and is theoretically equal to log10
(100 ÷ % light transmission).
ExxonMobil uses optical density values
with metallized films. |
Oxygen
scavengers |
TAPPI stands for the
Technical Association of the Pulp and
Paper Industry. Among other things, it
provides standard procedures for testing
paper. |
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