Gas permeation mechanism and various test methods

Gas permeation mechanism of packaging membrane and various test methods

Abstract: This paper expounds the permeation mechanism of solution diffusion gas molecular transfer, permeation driving force, and analyzes and compares various test methods such as differential pressure method and Coulomb method

key words: dissolution diffusion transfer driving force differential pressure coulometry

with the wide application of soft packaging materials in the packaging of food, drugs, cosmetics and electronic components, the gas barrier performance of packaging film has attracted more and more attention of manufacturers. Next, I would like to share with the delegates some of the research experience of the blue light film barrier laboratory

I. gas permeation mechanism of packaging membrane

1. Dissolution diffusion mechanism

the phenomenon of gas passing through non porous membrane (including homogeneous membrane, asymmetric membrane and composite membrane) is described by dissolution diffusion mechanism

as shown in the above figure, first the gas molecules contact the membrane, and then enrich and dissolve on the membrane surface, resulting in a concentration ladder on both sides of the membrane. Under the effect of concentration difference, the working principle of gas molecule deformation measurement is: it diffuses forward in the membrane, reaches the other side of the membrane, and then analyzes from the surface. At the beginning, this process is in an unstable state, and the concentration of gas in the membrane is nonlinear; When the delay time is over, the diffusion rate of gas in the film reaches a stable state, and the concentration of gas in the film is linear along the direction of film thickness

according to Fick's first law, the diffusion flow of gas in the membrane is:

2. The migration of gas molecules in the process of permeation

the solubility of gas molecules is expressed by the solubility coefficient s

solubility coefficient indicates the solubility of polymer film to gas. The solubility varies with the boiling point of gas and the degree of liquefaction

the molecular transport capacity in the membrane is expressed by the diffusion coefficient D

diffusion coefficient indicates the transmission capacity of gas molecules in the film of thermal motion of molecular chain. As gas molecules need energy to transfer in the membrane to displace a certain volume between chains, the diffusion coefficient decreases with the increase of molecules

3 ﹐ driving force of infiltration

conditions of infiltration: C1 c2

transmission: q=p (P 1-p 2)

where: P - permeability coefficient (s · d)

p 1-p 2 - concentration differential pressure on both sides of the membrane

t - permeation time

s - membrane thickness

4. The permeability of the membrane will vary with different gases

from formula 2, it can be seen that the permeability of the gas is related to the permeability coefficient, that is, to the solubility and diffusion coefficient of the gas to the membrane

it can be seen from table 3 that for the same polyethylene material, the penetration rate of oxygen is about three times that of nitrogen, while the penetration rate of carbon dioxide is five times that of oxygen. When the use of packaging film is different, it is unscientific to judge its barrier performance and shelf life only by its oxygen permeability data

Table 3 permeability coefficient of some gases in polyethylene HD

II. Comparison of various test methods

1. Differential pressure method

test conditions: C 1 C 2 P 1-p 20

standard range: national standard iso2556, China gb1038, Japan jis-k7126, Germany DIN 53380, UK ASTM d1434

evaluation: the permeability mechanism is perfect, not limited by the types of test gases, and the most widely used standard method in the world

2. Coulometric method

test conditions: C1O2 c2o2 c1n2 C2N2 p1=p2

standard range: ASTM d3985

evaluation: there is doubt about the penetration mechanism, so it can only be used as the determination of oxygen transmission in this specific state, and has not been adopted by international standards institutions

3. ASTM d3985 composites always compete with steel or aluminum in the analysis of penetration mechanism.

due to the adsorption effect between molecules, the reverse osmosis of nitrogen molecules will inevitably affect the forward penetration rate of oxygen molecules. Therefore, there are uncertain factors in the amount of oxygen transmission under this experimental state, and the reliability of the obtained data is questionable

III. introduction to the barrier property experiment of Blu ray film

Blu ray established China's first standard laboratory dedicated to the study of the barrier property of flexible packaging film in August 2002

the laboratory is equipped with professional laboratory personnel, perfect rules and regulations, and the hardware and software meet the standards of a modern laboratory. The inspection items include the air permeability test, water vapor permeability test, tensile strength, elongation, tear, peel and heat sealing strength test of packaging materials; Puncture force test of film and sheet; Measurement of film and sheet thickness and surface friction coefficient; Heat sealing performance and dart impact performance of thin films and sheets; Sealing test of packaging, etc

the laboratory is open to the public, operates 24 hours a day, receives customers' visits, operation training, experiments, and issues inspection reports. At the same time, the polymer professional researchers in the laboratory carry out experimental research on various new materials of flexible packaging, and strive to expand the role of the laboratory in serving customers; Not only provide reliable experimental data for customers' materials, but also provide technical support for customers in product research and development, and provide all-round customer service

JIANG Yunzhong

(Jinan Languang Electromechanical Technology Development Center)

reprinted from: China plastic packaging information

Celanese materials scientists have developed various tracking technologies