Influence of Temperature Fluctuation on the Barrier Property of the Material
This article is to discuss the reasons why the temperature fluctuation could influence the barrier property of the material. KeywordF Temperature fluctuation, barrier property, permeance, high polymer
As is known to all, temperature fluctuation could cause a great change in the barrier property of the polymer. There are mainly 2 factors, structure and permeation characteristics, which influence the barrier property of the materials.
1. Structure Characteristic of the High Polymer According to its molecular arrangement, the solid high polymer could be divided into crystallized, non-crystalline and orientation type polymer. Most of the crystallized polymer is semi-crystalline with both crystallized and non-crystalline parts, in which the only difference is its degree of crystallization.
Crystallized part of the polymer is theoretically regarded as an area that cannot be permeated during the diffusion of the molecules in polymer. Most of the microcosmic diffusion models are based on the non-crystalline polymer. Among all the models that describe the diffusion of simple infiltration in the non-crystalline rubber-like polymer, “Pace” model and “Datyner” molecular model are the most popular.
The molecule of the infiltration could permeate through the matrix in ways of "vertical movement " and " horizontal movement ". "Vertical movement" refers to movement of infiltration molecule along an axis direction of the channel formed by adjacent and parallel molecule chain while the "horizontal movement" refers to movement of infiltration molecule vertical to an axis direction of the channel formed by adjacent and parallel molecule chain.
The model of free volume is also a kind of popular diffusion model. For this model, free volume of the polymer is considered to as the "empty volume" within the polymer molecule chains assuming that molecule chain of polymer and the movement of infiltration molecule are mainly determined by the available free volume within the polymer-infiltration system. The longer of the polymer molecule chain is, the more its conformation is. When the temperature is increased, conformation change of molecule chain will be accelerated as a result of thermal motion and this in turn will decrease the extent of polymerization.
For the molecule model of “Pace” and “Datyner”, it could be explained that when the temperature is increased, the path formed by the parallel molecular chains would become “wider” so that the horizontal movement of the molecule of infiltration would be boosted meanwhile the change of conformation of the molecular chains would become faster so the space between the molecular chains would be increased and the vertical movement would be accelerated.
For free volume model, when the temperature rises, the free volume for the permeation of filtration would be increased in the polymer, so the diffusion rate of the infiltration in the polymer would be improved, that is, when the temperature rises, the barrier property of the material would be reduced.
2. Characteristics of the Gas Molecular Movement In regular barrier property tests, common inorganic gases are used as infiltration material. Since gas is featured with diffusibility and compressibility, the state of certain quantity of gas can be expressed with three parameters i.e. pressure, volume and temperature. Under normal temperature and pressure, the gas molecular size is much smaller compared with the average space between molecules which could be neglected. So it could be considered as ideal gas and conforms to the equation for ideal gas.
pV nRT
Internal energy of ideal gas:
In the equation :
E —— Internal energy of ideal gas
i —— liberty index of gas molecules
n —— mol
R —— constant of mol, 8.31J/mol · K
T —— thermodynamics temperature
For specific gas, “I” and “n” are fixed value and the internal energy of ideal gas is only the function of temperature. In this function, the internal energy of ideal gas is in direct proportion to “T”. This result is approximately coincides with that of the test performed at a temperature which is close to the room temperature. Under the normal temperature and pressure, the higher the gas temperature is, the more intensive the thermal motion of gas molecule is and the greater the energy will be. When gas diffuses as infiltration material within polymer, when the temperature rises, the energy of gas molecule would be increased. As a result, the energy of gas molecule could reach the value needed to diffuse among molecular chains. In this way, diffusion coefficient of gas molecule in the polymer will be increased and the barrier property of the material will be reduced.
3. Arrhenius Relation The influence of temperature fluctuation on the permeation of inorganic gas in polymer is obvious. When the temperature rises, the value of P, D and S would be increased. Their relationship with temperature conforms to Arrhenius equation.
The influence of temperature fluctuation on P, D and S is different if the polymer and infiltration gas are different. Though there is difference existing in the corresponding value of P0, D0, S0 and EP, ED, and ¢H, they are all found to be in accordance with the Arrhenius equation. The data fitting available in Labthink i-GASTRA 7100 Gas Permeability Tester can be used to get P0, D0, S0 and EP, ED, and ¢H values of infiltration gas as well as specific specimen. Perform data fitting of the oxygen permeability of PC film@ and another unknown film AA at 30 °C, 35 °C and 40 °C , We can get the following data of fitting: For PC film, P0 9305.716509507,EP 16822.675460039 and for film A, P0742581.566783723, EP36973.239405092. From 30 °C to 40 °C, increasing extent of the oxygen permeability coefficient of film A is much larger than that of PC film.
Note:
@ For test data and data curve fitting of PC, please refer to Film Permeability under Specific Temperature-Permeability Coefficient Fitting which could be provided by Labthink.
A Film A is provided by Mecadi Lab, Germany.
For detailed test data, see section 4 of this article.
4. Permeability testing of film A under different temperature Film A is unknown in material and it is 80 μm in thickness. Labthink i-GASTRA 7100 Gas Permeability Tester is used to perform the oxygen permeability test at the temperature of 23 °C, 30 °C, 35 °C, 40 °C and 45 °C . For test data, see Table 1.
If we provide fits to experimental data under 23 °C and 30 °C in Table 1, the oxygen permeability we get at 35 °C is 31.754cm3/m2 · 24h · 0.1Mpa , oxygen permeability at 40 °C O2 is 38.744cm3/m2 · 24h · 0.1Mpa and oxygen permeability at is 45 °C 46.978cm3/m2 · 24h · 0.1Mpa. These results are very close to the real test results listed in Table 1. If we use the test data at 23 °C, 30 °C, 35 °C and 40 °C to fit the oxygen permeability at 45 °C , the result we get is 50.234 cm3/m2 · 24h · 0.1Mpa, which differs only 0.925% from the real test result. The above results prove that the more the fitting data is applied, the closer the fitting result will be to the real test result. Since the material of film A is unknown, it can be inferred there is no specific requirements for the material in the data fitting process.
Now export the fitting data of oxygen permeance of film A at the temperature between -100 °C to 150 °C( 173 ~ 423K ) in form of Excel, we could get Graph 1 .From this graph we can see that the infiltration permeance of film A begins to increase obviously between 50 and 60 °C and it is gradually accelerated as the temperature rises. The infiltration amount reaches about 400cm3/m2 · 24h · 0.1Mpa at a temperature of 100 °C with certain barrier property is maintained.
5. Conclusion
To be concluded, the temperature fluctuation has a significant influence on the barrier property of the material. The application scope of temperature should be considered as an important index for selection of packaging materials. When the temperature exceeds the specified range of the material, the performance would be affected greatly. Data Curve Fitting in Permeation (DCFP) is a ideal means by which the barrier property of the material at specific temperature could be tested scientifically and reliably.
