Measuring and testing – With fluid pressure – Porosity or permeability
Reexamination Certificate
2001-10-19
2004-07-27
Larkin, Daniel S. (Department: 2856)
Measuring and testing
With fluid pressure
Porosity or permeability
Reexamination Certificate
active
06766682
ABSTRACT:
This invention relates generally to method and apparatus for determining gas transmission characteristics of materials, and more particularly, the gas permeability characteristics of barrier materials.
The invention is specifically applicable to and will be described with particular reference to an instrument for measuring oxygen and water vapor permeability of barrier materials for electronic displays such as organic light emitting diodes and field emission displays requiring highly sensitive permeation measurements. However, those skilled in the art, while recognizing the benefits of the invention for precisely determining gas transmission characteristics in a sensitive application, will also recognize that the invention is not necessarily limited to barrier materials but could be applied to any permeable material where the permeability of the material is desired to be determined in a short time, nor is the invention necessarily limited to detecting elemental or simple compound gases, but could have application for detecting transmission characteristics of hydrocarbons or gaseous organic compounds.
BACKGROUND
The food industry has long recognized that the shelf life of food is correlated to the solubility and diffusivity characteristics of food packaging material. It is well known that native (i.e., uncoated) plastics are very permeable to water and oxygen. Testing standards have long been established to determine permeability characteristics of packaging material. Barrier coatings have been developed to retard permeability of oxygen and water vapor through the plastics. It is to be recognized that the shelf life of food is typically measured in days. For example, 30 days is generally regarded as an acceptable shelf life for pharmaceuticals or packaged consumerable goods, such as potato chips.
In marked contrast, displays, such as television screens or flat panel displays on a laptop computer, have a significantly longer operating life. For example, display requirements typically specify that the brightness of the display will be at least 50% of its original brightness after 10,000 hours of use. However, for displays which are degraded by trace amounts of oxygen or water vapor, a serious failure rate will occur at much less than the expected 10,000 hour lifetime.
Today's display devices require fabrication using plastic substrates because plastic is lightweight, impact resistant, and can produce glass light transmission characteristics. Particularly, microelectronic devices, organic light emitting devices (OLED), and field emission display (FED) are being developed for flat display panel applications as well as electronic encapsulation packaging. Because of the high sensitivity of these materials to oxygen and water, especially formulated barrier coatings, sometimes termed engineered plastic substrates, are being developed. The barrier coatings are thin film barriers, typically consisting of metal (AL) or oxide (SiO
2
, AL
2
O
3
) having layer thickness of only about 20 to 30 nanometers and applied to plastic substrates by vacuum deposition techniques. A plurality of thin, specially formulated barrier films are applied to flexible plastic webs, films, or sheets significantly decreasing permeability of barrier coated plastics.
Uncoated plastics, whose properties are otherwise appropriate for panel displays, have permeabilities for water in the vicinity of 10 g/m
2
/day. Experimental estimates have been made which suggest that a desirable goal of about 10
−6
g/m
2
/day for water and 10
−6
cc/m
2
/day for oxygen is desired as a design permeation goal for plastic barrier materials used in panel display applications under discussion. Currently, within the trade, the best commercially available instrument for measuring gas permeabilities has a sensitivity limit of about 10
−3
g/m
2
/day for water and about 10
−3
cc/m
2
/day for oxygen. Reference can be had to an article entitled “Thin Film Technology”, pages 20-24, in the October, 2000 issue of
Vacuum Technology
&
Coating
in which target permeation rates at this level are specified with maximum sensitivity of current measuring instruments cited as being limited to the order of 0.005 cc/m
2
/day. For reference and comparison purposes, converting the sensitivity achieved by the prior art of 0.005 cc/m
2
/day to standard cc's/second/sample (i.e., 0.005 cc's×10,000/area×86,400/day) yields a published, prior art maximum sensitivity of 4,320,000 cc's/second/cm
2
. In any event, currently available instruments do not have the sensitivity needed to determine the suitability of the specially developed engineered plastic substrates for display panel applications.
An article published by
Modern Controls Inc
., entitled “Measuring Oxygen Permeability through Today's Packaging Barriers”, by Robert L. Demorest, describes the testing procedures commonly used in the packaging industry for measuring oxygen permeability, or more correctly, oxygen transmission rates. In the article, it is noted that prior to 1975, ASTM specification D-1434 was commonly used. In the ASTM D-1434 test, the plastic sample is mounted in a gas transmission cell to form a sealed semi-barrier between two chambers. One chamber contains the test gas at a specific high pressure and the other chamber, at a lower pressure, receives the permeating gas. Two procedures are set forth. In one procedure, the lower pressure chamber is maintained near atmospheric pressure and the transmission of the gas through the test specimen is indicated by a change in volume. In the second test procedure, the lower pressure chamber is initially evacuated and the transmission of the gas through the test specimen is indicated by an increase in pressure as measured by a manometer. Specifically, an initial vacuum is pulled in both chambers. The vacuum lines are closed and one of the chambers is flushed with a test gas. After some time (in the hours) has passed to achieve a steady state condition, the manometer is read over a series of time intervals to determine permeance, etc. The vacuum test is a static, not dynamic, test because the lines are closed. This gives a coarse instrument which is slow in response. Thus, the pressure test method or Dow Cell technique, is a static determination of the change of pressure in the test chamber after steady state conditions have been established from which information about the permeability of the plastic is obtained. The time of the test takes hours and the test is destructive. The sensitivity is limited. The
Modern Controls
article noted that the test was seldom capable of testing oxygen barriers below 1.00 cc/m
2
/day.
The current method in wide use today was published by ASTM as specification D-3985, in 1981. In this method, the sample to be tested is clamped between two chambers as discussed above, but, one side is exposed to a gently flowing oxygen stream while the other chamber is exposed to a nitrogen stream. As oxygen molecules permeate through the sample into the nitrogen stream, they are picked up and carried into a coulometric sensor. This sensor causes a release of 4 electrons electrochemically for every oxygen molecule which passes through it. The electrons form a current passed through a resistor creating a voltage which can be recorded. Such tests are also subject to lengthy time periods to achieve steady state conditions and the measurements are lacking in sensitivity because the permeant gas is only fractionally present in the sample.
In the literature, a paper entitled “Comparative Study of Oxygen Permeation Through Polymers and Gas Barrier Films”, 2000 Society of Vacuum Coaters, presented Apr. 15-20, 2000, at the 43rd Annual Technical Conference Proceedings, discussed four different methods which were experimented with to determine oxygen transmission rates in barrier films. The four methods investigated included Oxtran, which is the standard method used to measure oxygen with the coulometric sensor discussed above. The second method was described as a time lag method whi
Bright Clark I.
Engle Frank W.
Cole Stanley Z
Desert Cryogenics LLC
Larkin Daniel S.
LandOfFree
Precise measurement system for barrier materials does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Precise measurement system for barrier materials, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Precise measurement system for barrier materials will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3238736