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.
Relevance to package performance
A critical function of flexible packaging is to keep dry products dry (potato chips, pretzels, fortune cookies, etc.) and moist products moist (cheese, muffins, chewing gum, etc.). Without protective packaging, products would quickly gain or lose moisture until they reached equilibrium with the environmental relative humidity around them, at which point crispy products would be soggy, and chewy products would be hard and dry.
WVTR is the standard measurement by which films are compared for their ability to resist moisture transmission, with lower values indicating better moisture protection. Only values reported at the same temperature and humidity can be compared, because transmission rates are directly 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.)
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|
|HPDE (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|
|Table 9: Normalized WVTR values for common films|
|*Dependent on ethylene content of the particular grade.|
CAUTION: In order for a film’s 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.
What affects the WVTR of OPP films
The most obvious factor that impacts WVTR is thickness: if one OPP is twice as thick as another OPP with the same product design, the thicker film’s WVTR will be half the value of the thinner one; this is 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® claims a test precision of :t3% with their PERMATRON-W® product line, which means that trained operators using this type of instrumentation will generate values between .34 and .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 coextruded (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 coextruded films; the confidence limits would be wider if other manufacturers’ 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 strong moisture barrier of OPP can be further enhanced with 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.
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 transference 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 the point when the infrared sensor detects water molecules leaving the dry chamber at a constant rate. In other words, equilibrium is achieved when the amount of water vapor permeating through the sample per unit of time remains unchanged. 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.
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 table above in order to compare the inherent moisture barrier property of different classes of materials; remember that this technique is only appropriate with homogeneous films and not for coated or metallized films.