A new moisture permeability test method for the ho

New moisture permeability test method of ultra-high barrier materials

overview

the main obstacle for flexible OLEDs to enter the commercial market is that the product performance will decline under the action of moisture and O2, thus shortening its service life. Therefore, materials with higher barrier must be selected to minimize the contact between products and moisture and O2 in the air. In order to ensure that the service life of the product is greater than 10000h, 1 × The requirement of moisture permeability (WVTR) of g/(M2 · day) has become an unofficial standard in the OLED industry. Obviously, how to develop these barrier materials has become a major problem for industrial development, and when these materials are developed, which test method to choose also needs the policy support of the government is also a difficult choice. It can be predicted that the continuous improvement of the barrier performance of materials and the improvement of the mechanical properties of metals and nonmetals will promote the further improvement of the sensitivity of commercial water vapor permeability testing equipment. In order to make these test methods practically meet the test requirements of products with a variety of high barrier materials, such as OLED, the current penetration test methods must be improved or improved

the traditional moisture permeability test method

tm f-1249 standard

astm f-1249 standard has been used to test the moisture permeability of flexible barrier materials in different industries for a long time. In this method, the barrier material to be tested is used to divide the test chamber with known temperature into dry and wet sides, so that the dry side and wet side form a diffusion chamber through the test film in the middle. After passing through the test membrane, the diffused water vapor is mixed with the carrier gas on the drying side and transported to an infrared sensor regulated by pressure. The infrared sensor measures part of the infrared energy absorbed by the effluent steam and generates an electrical signal (the size of the electrical signal is directly proportional to the concentration of water vapor). Then, compare the electrical signal generated by the test sample with the electrical signal generated by the calibration film with known moisture permeability, and this ratio is used to calculate the water vapor transmission of the test sample

mocon aquatran moisture permeable instrument

2 Calcium test method

calcium test method is based on the corrosion degree of calcium film, also known as "calcium bond test". It is obtained by observing the optical changes in the process of water vapor passing through the barrier material and transforming calcium into transparent calcium salt. Because the change of calcium can be observed intuitively, this method can distinguish between massive infiltration and defective infiltration. However, this method cannot distinguish between O2 permeation and water vapor permeation

3. Radioactivity test method

the basic concept of radioactivity test method is the same as that of ASTM f-1249 standard. The difference is that this method uses tritium water as penetrant, and then measures the radioactive content through instruments

variable factors in the process of permeability test

during permeability test, due to the existence of many variable factors such as temperature control, leakage, calibration and system noise, most of the current test methods will produce unreliable test results in the low-end range of sensor sensitivity. If the variable factors cannot be measured and controlled correctly, the permeation results may be very different

1. Temperature

according to Arrhenius equation p=po · e-e/rt (where p is the target permeability, Po is the permeability constant, e is the activation energy, R is the gas constant, t is the temperature), permeability has a certain correlation with temperature, and changes with the change of temperature. According to the conventional rule of thumb, the permeability will change by 10% every time the temperature changes by 1 ℃. In a typical laboratory environment, the temperature fluctuation range can be as high as 4 ℃ within 24 hours. In other words, if the temperature is not accurately controlled in the permeability experiment, the measurement result may have an error of more than 40%. Therefore, accurate temperature control is very important to obtain reliable and accurate permeability measurement data

2. Leakage

because there is water in the air, leakage is the main obstacle to measuring water vapor permeability. No matter what method is used, only the water vapor infiltrated should be calculated, not the water leaked from the air. You should know that the seemingly small water vapor leakage of 1ppm concentration will lead to a change of 0.01g/(M2 · day) in permeability, and this order of magnitude will have a great impact on the final measurement results. In addition, because the moisture in the air is constantly changing, there is no fixed background value or leakage value that allows us to obtain a reliable permeability answer after subtracting this value. In the calcium test, because calcium cannot distinguish between O2 and water vapor, a similar problem will occur

3. Calibration

most sensors in the industrial chain have produced violent turbulence. They are relative value or concentration type, so they all need to be calibrated. Those important parameters in the typical infiltration process, such as temperature, flow control and repeatability, can only play an effective role through calibration. The specific operation steps are as follows: an electrical signal response curve is obtained through sensor calibration, and then the curve is calculated through interpolation replacement, and finally the specific amount of test material is obtained. It is worth mentioning that since these sensors usually measure only a part or a certain proportion, the calibration of the sensors must be within their working range. Generally, the ideal moisture permeability of OLED is 1 × G/(M2 · day), equivalent to the calibration level of water vapor with a content of 0.0001ppm

4. System noise

system noise includes various variables and all factors not listed above. The system noise in the penetration measurement equipment includes electronic noise, noise generated by system exhaust and carrier gas change. Almost all sensors have some electronic related noise. Therefore, while minimizing the noise, we should also carry out accurate measurement, adjust the final data and explain the causes of the noise. The noise caused by system exhaust mainly includes the absorption and adsorption of water vapor in the sensing path of gas and system components (such as valves and O-rings) outside the test sample. Similar to the effect of leakage, the water in the carrier gas must be completely removed before contacting the test sample

5. Correlation with existing test results

when the test results are related to the results obtained from the existing cumbersome experimental methods, additional verification is required. The cold mirror or hygrometer can provide a verification method for the test of barrier materials with barrier performance in the range of 0.01g/(M2 · day) and above. Although hygrometer is not the best choice for daily testing, it provides a benchmark for comparison with other testing methods. Improve the barrier performance testing technology to 1 × In the improvement process of g/(M2 · day), the main obstacle is the lack of such "benchmarking" related testing technology to verify the test results

latest improvement of penetration testing technology

based on the above considerations, American MOCON company launched its latest MOCON aquatran moisture permeable instrument. The equipment adopts the same penetration process as ASTM f-1249 standard, and uses a Coulomb absolute value sensor to replace the concentration type pulse modulated infrared sensor to measure the permeability of water vapor. It is precisely because of the application of Coulomb absolute value sensor that the test sensitivity of the moisture permeable instrument is increased by an order of magnitude, that is, from 5 × G/(M2 · day) increased to 5 × g/(m2·day)。 Although it has not reached 1 × G/(M2 · day), but the equipment has been able to accurately and reliably provide 5 × Permeability data

1. Temperature

mocon aquatran? The moisture permeable instrument has exquisite temperature control function, and the temperature of the test sample can fluctuate within the range of ± 0.1 ℃ of the set temperature, thus effectively eliminating any temperature related errors

2. Leakage

compared with other similar products, MOCON aquatran moisture permeable instrument has achieved some small improvements in the leakage problem and achieved the expected effect

3. Calibration

after MOCON AQ is changed, press the confirm key. The Coulomb electricity sensor configured by the uatran moisture permeable instrument is an absolute sensor, and its sensitivity can reach 2 × g/(m2·day)。 The sensor does not need calibration because it measures all the water vapor samples that pass through, rather than only some samples like the concentration sensor. In addition, because Coulomb technology is not affected by pressure, temperature, flow or vibration, the test results are more accurate

it is reported that the Coulomb sensor can measure the current required to complete a chemical reaction. Because these reactions occur at the electrode and must be reacted to the maximum allowed by Faraday's law, the test efficiency of Coulomb sensor is almost 100%

4. System noise

with the change of the sensor, the noise related to electronics is also eliminated, thus reducing the system noise. At present, American MOCON company is carrying out more improvements to improve the exhaust and carrier gas technology of the system, so as to further improve the sensitivity of the instrument

5. Correlation with existing test results

although in 5 × There is no "benchmarking" benchmark in the order of g/(M2 · day), but MOCON aquatran psychrometer has correlation with cold mirror, weight test method and permatran-w3/31 psychrometer in the low-end test range. In addition, the psychrometer also shows excellent linearity, and can get accurate and reliable measurement results in the low-end range. Because it has the accuracy and repeatability that other test methods do not have, even if