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Abstract: this article deals with the definition, classification and testing of barrier property of materials. Common erroneous views and vague knowledge are also listed in each item with corresponding rectification.
Keywords: barrier property, classification, and test
For the reason that barrier material can improve the preservation effect and prolong the storage period of products, it has experienced a rapid development in recent years. Correspondingly, the wide application of barrier materials has also promoted the popularization of permeability testers. Since the conception of barrier material is rather new, a correct understanding of the barrier property as well as its correct testing is very important to actual production and selling.
1. Barrier Property of Materials
Barrier property is commonly in connection with particular permeable objects, which include common gases, water vapor, liquid and organic substance, etc. it refers to the property of material when specified permeable object transmits from one side to the other (from high density side into low density side). The whole transmitting process includes adsorption, dissolution, diffusion, and desorption. Gases or water vapor enter the surface of materials from higher density side. After diffusing inside material, it desorbs on the low-density side
The speed of the permeation process depends on two factors. One is the speed of solution in polymer materials, i.e., solubility parameter. The other is the travel rate of permeable molecules inside the base of polymer, which is indicated by diffusion coefficient.
2 Classifications and Interrelationship of Material Barrier Properties
Permeable processes of different objects through the same materials are different. According to the nature of permeable objects, barrier property of materials can be divided into barrier property of inorganic gases (i.e., the permeability we commonly say. It can be further divided into oxygen permeability, nitrogen permeability and carbon dioxide permeability, etc.), to water vapor barrier property (water vapor permeability), and to organic matter barrier property. Permeable principles of the three kinds of material are different. Moreover, owing to the difference of permeable objects, their permeable processes vary greatly.
Generally, when we say that barrier property of certain material is better, we refer to three aspects: better barrier property to inorganic gases, to water vapor and to organic matters. However, wrong ideas of generalization are common in describing barrier properties of materials. For example, materials with better gas barrier property is often considered as higher barrier materials, however, higher barrier material means that this kind of material should simultaneously possess better barrier property to gases, to water vapors and to organic matters. In fact, it is possible that this does not match with the reality in terms of its barrier property to water vapor and organic matter. EVOH has very high barrier property to gases in extremely low humidity, which will decrease significantly with the increasing of humidity. Moreover, its barrier property to water vapor is not good.
Special attention should be paid to the fact that even gas permeability (barrier property) to certain material varies in specific gases, such as oxygen and carbon dioxide. It has been proved by the test results that to one same material, there is certain proportional relation between its oxygen permeability, carbon dioxide permeability and nitrogen permeability. The reason is that permeable processes of inorganic gases are rather similar, with the dimension and shape of molecules as the main influenced factors. But, this proportional relation will change with different materials.
With the popularization of the conception of barrier property, insiders of the packaging industry have the demand to further the understanding of barrier property. A further understanding of barrier property will not only help packing manufacturers design the structure of packing materials effectively and reasonably, but help the research personnel to better their performance of relating test as well.
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