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Definition
Relevance
and non-relevance to performance
What
affects film yield and how well it is
controlled
Test
principles
Related
Terminology
Definition
Yield is
the measure of a film's coverage per unit
weight. Values are expressed as in2/1b
in US standard and m2/kg in metric
(or SI) units.
Unit weight is the reciprocal
of yield and is presented in units of lb/ream
or g/m2. The film industry tends to
use yield values, while the paper industry
favors unit weight. Our discussion here will
focus on yield, but the principles also hold
true for unit weight.
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Relevance and non-relevance to
performance
Films
are generally sold by the pound or kilogram;
yet film area is what determines how many
packages can be wrapped or how many labels can
be produced. Therefore, yield is a critical
property for determining the correct quantity
of film to purchase, and it impacts the
economics of the application.
OPP is a high-yield material,
which makes it very economical compared to the
alternatives. As show in Table 2,
pound-for-pound at equal gauges, OPP offers
53% more coverage than OPET and 28% more
coverage than oriented nylon.
| Film
Type |
Density |
Yield
of 1 mil Film |
Unit
Weight of 1 mil Film |
| g/cc |
in2/lb |
m2/kg |
lb/ream |
g/m2 |
| OPP |
.906 |
30,600 |
43.5 |
14.1 |
23.0 |
| LLDPE
(linear low density polyethylene) |
.92 |
30,100 |
42.7 |
14.4 |
23.4 |
| HDPE
(high density polyethylene) |
.95 |
29,200 |
41.5 |
14.8 |
24.1 |
| Hicor
OHD (oriented high density
polyethylene) |
.96 |
28,900 |
41.0 |
15.0 |
24.4 |
| Biax
Nylon |
1.16 |
23,900 |
33.9 |
18.1 |
29.5 |
| OPET
(oriented polyester) |
1.39 |
19,900 |
28.3 |
21.7 |
35.3 |
| Cellophane |
1.45 |
19,100 |
27.1 |
22.6 |
36.9 |
Table
2: Yield values of common films
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What affects film
yield and how well it is controlled
For a
solid, uncoated film, yield is determined by
resin density and average film gauge. Resin
density is altered negligibly by process
conditions; so yield is controlled to the
degree that thickness is controlled.
ExxonMobil has sophisticated on-line gauge
measurement and control systems that keep
machine and transverse direction thickness
profiles tight to target. Because yield is
such a critical property, it is also checked
routinely in the lab. For all film types, film
is rejected if it is outside the specification
tolerance limits of ±5% of target.
But most ExxonMobil film yield measurements are
well within 3% of advertised yield.
Cavitated white films (OPPalyte)
and coated films have additional factors that
effect yield. The cavitated core of OPPalyte
films reduces film density to as low as 60% of
solid OPP (as low as .53 g/cc). Therefore, per unit
thickness, OPPalyte films offer even higher
yields than solid OPP films. Yield is the controlled
property, so if cavitation density varies
slightly with process conditions, thickness (not
yield) will be impacted.
NOTE: For more information
about thickness, review the section on gauge.
ExxonMobil
coatings are applied as thin layers onto OPP
base films. These coatings are
denser
than the base films. Therefore, gauge for gauge,
yields are slightly lower for coated films.
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Test principles
To
measure yield, a precise area of film is cut
and then weighed on an analytical balance. The
area is divided by the weight, expressed as
in2/lbor m2/kg,and reported to three
significant digits. The recommended minimum
specimen size is 300 in2 (1940 cm2)
and should be obtained by folding a single
layer of film multiple times in the machine
and transverse directions.
Example: Film
is folded two times in the machine
direction and two times in the
transverse direction. Therefore,
16 layers of film will be cut at
once. The folded film is
precision cut with a 5 inch (.127 m)
diameter round die, and the measured
weight is 3.496 grams. Based on
this example, the following equations
demonstrate the yield and unit weight
calculations in US standard and metric
units.
| Yield
(in2/lb) = |
Total
specimen area |
= |
p(2.5)2
in2 × 16 × 453.59
g/lb |
= |
40,761
in2/lb |
| Weight
of specimen |
3.496
g |
It
is reported as 40,800 in2/lb.
| Yield
(m2/kg) = |
Total
specimen area |
= |
p(.0635)2
m2 × 16 × 1000 g/kg |
= |
57.976
m2/kg |
| Weight
of specimen |
3.496
g |
It
is reported as 58 m2/kg.
| Unit
weight (lb/ream) = |
1 |
= |
1
lb |
× |
432,000
in2 |
= |
10.598 |
~ |
10.6
lb/ream |
| Yield |
40,761
in2 |
ream |
| Unit
weight (g/m2) = |
1 |
= |
1
kg |
× |
1,000
g |
= |
17.249 |
~ |
10.3
g/m2 |
| Yield |
57.976
m2 |
kg |
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CAUTION: |
Although yield is
related to thickness, it is inaccurate
to use a micrometer
to assess yield. Micrometers lack the
necessary accuracy and precision, and
they only measure thickness at a small
point, whereas the previously
described method directlymeasures
average yield over a large
area. Also, thickness cannot be
converted to yield without knowing the
exact density of the film.
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Related terminology
Basis
weight |
Basis weight is a common
paper industry term for unit weight, or
weight per unit area. Units are lb/ream.
"Ten pounds of poly" means 10
lb/ream, which equals 43,200 in2/lb
(because a ream equals 432,000 in2)
or .70 mils of LDPE (at a specific
gravity of .92). |
MSI |
MSI stands for thousand
square inches. Yield can be converted
from in2/lb to MSI/lb by
dividing by 1000. Film pricing is
commonly quoted in $/lb or $/MSI, and
yield can be used to convert from one
set of units to the other. |
Ream |
Ream
is a paper industry term equivalent to a
coverage area of 3000 ft2 or
432,000 in2. |
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