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Definition
Relevance to performance
What
determines the haze level of a clear film
Test
principles
Related
Terminology
Definition
Haze is
the scattering of light by a film that results
in a cloudy appearance or poorer clarity of
objects when viewed through the film. More
technically, haze is the percentage of light
transmitted through a film that is deflected
more than 2.5° (degrees) from the direction
of the incoming beam. This property is used to
describe transparent and translucent films,
not opaque films.
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Relevance
to
performance
Film clarity is a
highly desirable feature for most clear
packaging and label applications. It tends to
symbolize quality and contributes to the
positive visual display of the product.
Compared to other flexible plastic films,
clear OPP has one of the lowest haze values,
generally less than 3%.
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What
determines the haze level of a clear film
Haze is greatly
influenced by material selections and product
design. Resin characteristics, such as
crystallinity and molecular weight
distribution, have a key impact. Copolymers
are generally hazier than homopolymers.
Additives and coatings usually contribute to
increased haze. All other things being equal,
thicker films will be hazier than thinner
films. Additional variables, like process
temperatures in the different stages of
film-making, can further affect haze, so they
are tightly controlled.
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Test principles
As represented in
Figure 1, a unidirectional light beam is
directed onto the film specimen. After it
enters an integrating sphere, a photo detector
measures the total light transmitted by the
film and the amount of transmitted light that
is scattered more than 2.5°. Haze is the
percentage of total transmitted light that is
scattered by more than 2.5°.
Figure
1: Haze measurement
Commercial
hazemeters are typically used for this
testing, but ASTM D 1003 also allows the use
of a spectrophotometer, provided it meets the
procedure requirements. ExxonMobii uses the
SKY-Gardner XL-211 Haze-gard and Haze-gard
plus hazemeters consistent with ASTM
guidelines. The hazemeter may also be set up
to measure and display light transmission.
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Related terminology
Light
transmission |
Light
transmission is the percentage of
incident light that passes through a
film, which is represented by the
following equation.
Light Transmission
(%) = |
Tt |
x 100 |
| Ti |
|
Translucent |
Translucent
describes material that transmits light,
but also diffuses it, so that objects
cannot be seen clearly. |
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