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Definition
Relevance to
package performance
What
affects the WVTR of OPP films
Test
principles
Related
Terminology
Definition
WVTR (water vapor
transmission rate) is the steady state rate at
which water vapor permeates through a film at
specified conditions of temperature and
relative humidity. Values are expressed in
g/100 in2/24 hr in US standard
units and g/m2/24 hr in metric (or
SI) units. Test conditions vary, but
ExxonMobil has standardized to 100°F
(37.8°C) and 90% RH, which is the most common
set of conditions reported in North America.
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Relevance to
package performance
A critical function
of flexible packaging is to keep dry products
dry (potato chips, pretzels, fortune
cookies...) and moist products moist (cheese,
muffins, chewing gum...). Without protective
packaging, products will quickly gain or lose
moisture until they are at equilibrium with
the environmental relative humidity. At this
point, crispy products are soggy, and chewy
products are hard and dry.
WVTR is the standard
measurement by which films are compared for
their ability to resist moisture transmission.
Lower values indicate better moisture
protection. Only values reported at the same
temperature and humidity can be compared,
because transmission rates are affected by
both of these parameters.
One of the most
valued properties of OPP is its exceptional
moisture barrier. As shown in Table 9, gauge
for gauge, OPP provides the best WVTR of all
common polymer packaging films. (For
homogeneous films like these, you can
calculate the WVTR for a particular thickness
by dividing the values in the table by the
desired gauge in mils.)
| Film
Type |
WVTR
@ 100°F (38°C), 90% RH for 1 mil
film |
| (g/100
in2/24 hr) |
(g/m2/24
hr) |
| Biax
OPP |
0.25
- 0.40 |
3.9
- 6.2 |
| HDPE
(high density polyethylene) |
0.3
- 0.5 |
4.7
- 7.8 |
| Cast
PP |
0.6
- 0.7 |
9.3
- 11 |
| Biax
PET (oriented polyester) |
1.0
- 1.3 |
16-20 |
| LDPE
(low density polyethylene) |
1.0
- 1.5 |
16
- 23 |
| EVOH*
(ethylene vinyl alcohol) |
1.4
- 8.0 |
22
- 124 |
| OPS
(oriented polystyrene) |
7
- 10 |
109
- 155 |
| Biax
Nylon 6 |
10
- 13 |
155
- 202 |
Table
9: Normalized WVTR values for common films
*Dependent
on ethylene content of the particular grade.
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CAUTION: |
In
order for film moisture barrier to
contribute its full product protection
value, package seal integrity must be
satisfactory. Poor quality seals can
negate a film's good barrier by
allowing vapor transmission through
channel leakers and imperfections.
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Within
the arena of OPP films, tensiles are generally
not critical and rarely require discussion and
specification between supplier and customer.
This is so because oriented polypropylenes
provide a dependable range of tensile values.
Other properties are usually more important to
successful performance. There are two notable
exceptions worthy of explanation.
-
Orientation
method: blown or tentered
The orientation method causes
characteristic differences in tensile
properties. Blown films are
"balanced," having similar
strength and elongation in the machine and
transverse film directions. Tentered films
(as are all ExxonMobil films) have higher
strength and lower elongation in the
transverse direction than in the machine
direction. Most OPP manufacturers produce
tenter-oriented films, which work well in
many diverse applications.
-
Modulus
and web tension
Modulus, because it is a measure of
strength characteristics in the film's
elastic region, provides valuable insight
into stiffness and how extensible the film
is under normal use tensions. When
comparing two films of identical
thickness, the one with a higher modulus
will be stiffer and stretch less under the
same tension force.
NOTE: |
High
temperature modulus testing and
empirical trials on converting
equipment have yielded an industry
rule of thumb: OPP web tensions should
be controlled to .50lbf per
inch of film width, or less, for good
registration and no permanent
deformation (elongation, neck-in,
gauge bands). From a filmmaker's
perspective, the lowest controllable
web tension is best. Thinner films and
higher converting temperatures make
this more critical.
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What affects
the WVTR of OPP films
The most obvious
factor that impacts WVTR is thickness: if an
OPP of the same product design is twice as
thick as another, its WVTR will be half the
value. This is so because WVTR is an inherent,
bulk film property of OPP.
It is common to find
variation in the reported WVTR values for
same-gauge OPP films produced or measured by
different manufacturers. The primary factors
causing these differences are:
-
Raw
material: Homopolymer PP resin
differences in average molecular chain
length, range of chain lengths, and degree
of crystallinity can account for up to a
10% difference in WVTR. Additives and
copolymer resin layers can account for
differences of up to 30%.
-
Process: Normal
differences in process conditions between
one orienter and another account for about
5% variation in WVTR values. (WVTR is
reduced through orientation, because the
crystalline regions of the polymer matrix
are aligned. In other words, orientation
efficiently "packs" polymer
chains, so that larger spaces are
minimized. Process conditions affect this
"packing," and therefore, WVTR
values.)
-
Measurement:
The instrument manufacturer, MOCON®,states
a test precision of :t3% with their
PERMATRON-W® product line.
Therefore, trained operators using this
type of instrumentation will generate
values from .34 to .36 when testing a 1
mil OPP with a nominal WVTR of .35 g/100
in2/24 hr.
ExxonMobil performed
WVTR testing on a wide range of coex (plain,
uncoated) products and gauges produced on
different orientation lines. Graph 2 plots the
average WVTR as a function of gauge and shows
the 95% confidence range that embodies
variation from the factors described
previously. (This only involves ExxonMobil
coex films. The confidence limits would be
wider if other manufacturer's films were
included in the data, because there would be
more material and process variation.)
Graph
2: Plain OPP WVTR as a function of thickness
The
inherently good moisture barrier of OPP can be
further enhanced by additives, coatings, or
metallization. ExxonMobil produces high
barrier PVdC-coated and metallized films that
dramatically improve the WVTR of OPP and Hicor
films. The values for four of these products,
HBS-2, MET-HB, Hicor OHD, and Hicor BIHD-M,
are plotted in Graph 2.
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Test principles
ExxonMobil
uses MOCON Permatran W®
instruments for measuring WVTR. The instrument
design and the way we operate the instrument
is consistent with ASTM F 1249. ExxonMobil
standardizes its reporting to test conditions
of 100°F (38°C) and 90% RH. Conceptually, a
test cell looks like Figure 3. Dry nitrogen
gas is swept through a chamber where the test
film acts as the membrane separating this dry
gas stream from a "wet" nitrogen
stream on the other side. The partial pressure
difference creates a driving force for the
water vapor to permeate through the film to
the low pressure side. The barrier of the film
determines how much water vapor can transfer,
and this is continuously measured by an
infrared detector in the outgoing stream of
the dry side.
Figure
3: Cross-section of a WVTR test cell
The test is complete when equilibrium, or
steady state, is achieved, which is when the
infra-red sensor detects water molecules
leaving the dry chamber at a constant rate.
The amount of water vapor permeating through
the sample per unit time period is not
changing. This rate is the sample WVTR and is
recorded in units of g-H2O/100 in2/24
hr at 100°F (37.8°C), 90% RH.
This overview describes how ExxonMobil
measures WVTR and was based on articles and
product literature provided by MOCON®.
There is much more to the science of mass
transfer and to the instrumentation that
allows us to measure it. For more information,
contact MOCON in Minneapolis, MN at (612)
493-6370, or visit http://www.mocon.com.
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Related terminology
Normalized
WVTR |
Normalized WVTR is equal
to the measured WVTR multiplied by the
gauge of the sample in mils. In other
words, it is the approximated WVTR of
the material at 1 mil thick. We provided
normalized WVTR values in the first
table of this write-up in order to
compare the inherent moisture barrier
property of different classes of
materials. This technique is only
appropriate with homogeneous films and
is not appropriate for coated or
metallized films. |
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