White Paper 09-4102
Non-Destructive CO2 Package Testing
Slashing CO2 Permeation Test Times
Overview
Plastic beverage bottles do a near-perfect job of holding their
liquid contents. The real challenge is holding CO2 gas, which
can travel through the bottle wall, the cap, or the seal where
cap and bottle meet. Because carbonation life is critical to the
shelf life of the product, the rate of CO2 loss is of interest to
both bottle manufacturers and bottlers.
Introduction
Until now, the methods of testing a container’s ability to retain
CO2 have been destructive Zahm-Nagle testing or Carbo QC
pressure testing, and Fourier Transform infrared (FTIR) testing,
which uses light to measure the amount of carbonation
remaining in the liquid after a period of time. Unfortunately,
these methods are labor intensive, require a great deal of
product, and take weeks or months to provide useful predictive
information on rates of CO2 loss.
There is, however, an effective, non-destructive way of
determining, in just hours after a bottle has been conditioned,
the amount of CO2 permeating out of a bottle and the
expected shelf-life of its contents. This method is less expensive,
far faster than other testing methods and equally accurate. And
it can be used for speedy determination of the amount of CO2
loss through the bottle wall, the closure, and across the seal.
Components of CO2 Loss
There are three places CO2 can go after a bottle has been filled
with carbonated liquid and sealed.
• As pressure inside the container stretches the bottle (bottle
creep), CO2 can leave the liquid and occupy the headspace
created by expansion of the bottle. This loss is limited by the
maximum expansion of the bottle and can be compensated
for in carbonating the beverage.
• CO2 molecules can be absorbed into the bottle wall itself
(sorption). Sorption is limited by the capacity of the wall
material to absorb CO2.
• CO2 can pass through (permeate) the bottle wall,
closure, and seal. Permeation is an ongoing process and
contributes to the majority of CO2 loss over time.
• Finally, CO2 can pass through (permeate) the bottle wall,
closure, and seal. Permeation is an ongoing process and
contributes to the majority of CO2 loss over time.
Isolating Permeation
Permeation is the primary determinant of shelf-life and
must be measured separately from the other two forms of
CO2 loss. This is accomplished by testing after “conditioning”
the bottle or bottles to be tested. Conditioning time is
determine by the specific type of bottle. It isolates the effect
of permeation after the bottle has been filled and sealed.
Conditioning allows bottle creep and sorption to reach their
maximum levels so they are no longer factors in further CO2
loss. At this point, the system has reached a steady state in
which CO2 is lost only through permeation for the duration
of the product’s shelf-life.
Channels of CO2 Loss by Permeation
Having eliminated, by conditioning, bottle creep and
sorption as means of CO2 loss, you can proceed to separate
the three possible channels of loss by permeation. CO2
can permeate through the bottle wall, through the closure,
or across the seal between bottle and closure. Placing a
thick layer of epoxy over the entire closure of a test bottle
effectively prevents permeation through the closure. A layer
of epoxy over the closure and the space where the closure
and bottle meet prevents permeation through both closure
and seal. By comparing loss from bottles with the two
different levels of epoxy covering and an untreated bottle
(or of a single bottle tested first without epoxy and then
with two different levels of epoxy), you can determine the
amount of CO2 loss via each of the three channels.
Minneapolis, MN 55428 USA
Phone 763.493.6370
E-Mail [email protected]
www.moconcom
Application Note 09-4102
Existing Methods for Measuring CO2 Loss Due to Permeation
Existing methods measure the amount of CO2 remaining in a
bottle over its entire shelf-life.
• The Zahm-Nagle method tests by piercing the bottle and
measuring the pressure inside.
• In Carbo QC testing, the liquid is moved to the device’s
measuring chamber for testing.
• FTIR testing is based on infrared (IR) absorption spectroscopy. An
IR beam is transmitted through the bottle, and absorption values
of the beam are measured to determine the concentration of
CO2 within the bottle.
Because the first two of these are “destructive” tests, the
testing period must begin with a large number of filled
bottles. All three of the tests must be repeated at intervals
over the life of the product—usually weeks or months—
using one or more bottles from the original batch for each
test. The testing process continues until a tested bottle
reaches an “expired” state, thereby indicating the shelf-life
of the product.
The Limitation of Traditional Testing Methods
The underlying problem with these tests is that they do
not measure the amount of CO2 lost from the bottle;
they measure the amount of CO2 remaining in the bottle.
Because only a small fraction of the CO2 inside the bottle is
lost every day, it is difficult to accurately measure that loss,
particularly early in the product’s shelf life. For example,
if a bottle loses one percent of its CO2 on the first day of
testing, and the test measurement of the CO2 remaining
in the bottle is off by one percent, this represents a 100
percent potential error in the measurement of lost CO2.
As the amount of CO2 in the bottle drops over time,
the accuracy of such testing may improve, but such
improvement comes at the end of the testing period,
which could be months in duration. As a result, attempts
to extrapolate shelf-life from measurements made early
in the test period will be unreliable due to small errors in
measurement or variations between bottles. This is why
these methods are generally applied over long periods of
testing.
A Better Way of Testing
The alternative to existing tests is to place the bottle within
a closed chamber and measure the increase in CO2 in the
chamber over time. Because you are directly measuring
lost CO2, an error of one percent in measurement is just
that, a one percent error—a huge improvement over
the 100 percent error in the example above. Using this
method, the greatest accuracy comes at the beginning
of the test period, providing numbers that can be used to
accurately extrapolate shelf-life based on just a few hours
of testing.
Effective, non-destructive, faster, less expensive.
Establish permeation through bottle wall, closure or seal, quickly.
Benefits of Direct Testing
Direct testing of CO2 loss offers several advantages over
existing indirect methods. By eliminating weeks or months
of testing, it can greatly reduce the time-to-market of a
bottle or beverage. If testing identifies problems, it allows
quick action to address them. And because it can provide
accurate measurements using a single bottle, it eliminates
the variances among samples that can throw off traditional
testing. Finally, the process eliminates most of the labor
cost involved in existing testing.
Application Note 09-4102
The MOCON Permatran-C® Model 10
Testing System
The Permatran-C Model 10 directly tests
permeation of CO2 from the bottle being
tested into a sealed capture cylinder
using infrared absorption spectroscopy.
Using data for bottle size, initial gas
volume, and expired gas volume, the
system can calculate the shelf-life of the
test bottle based on as few as three hours
of testing. An independent study found
that these results only varied by up to .09
gas volumes from corresponding results
using traditional Zahm-Nagle testing, a
very small difference.
The Permatran-C Model 10 system can
accommodate over 300 different bottle
sizes and fits in a standard laboratorygrade environmental chamber. Features
include auto calibration, quick-start
capability, and direct data export to other
systems.
Complete results of an independant study showing the correlation between the results of ZahmNagle method and the MOCON Permatran-C® Model 10
Minneapolis, MN 55428 USA
Phone 763.493.6370
E-Mail [email protected]
www.moconcom
Copyright © 2014, MOCON, Inc. All rights reserved.
MOCON and PERMATRAN-C are registered trademarks of MOCON, Inc.