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                关于包装材料的透√气性原理及影响因素
                发布时间:2020-09-03 13:37:16 点击次数:181

                食品药品安全向来是人们关注的重点,而包装作为食品药品品质的保障,包装材料的阻隔性能对食品药品的保质期,品质稳定性,以及防潮,抗氧化都有重大影响,目前,材料的透气性测试与透湿性测试已经比较普及。包装材料的透气性能研究,在医药、食品行业尤其重要,其直接影响产品的质量。单一薄膜无法满足软包装的总体需求,近年来多层复合膜○以优异的阻隔性、耐化学性等优势广泛↓应用于各领域,其中起关键作用的就是阻隔层材料。


                包装材料的透气性原理:

                气体透♀过量。在一个大气压差下,每平方★米透过面积,24h透过的气体量(标准状况■下)。

                透气系数。在单位时间内,单位压差下,透过单位面≡积、单位厚度薄膜的透气量(标准状况下)。

                气体对薄膜的透过是一个单分子扩散过程,属于质量传递过程。共包括三⊙个步骤,当在薄膜两侧存在着浓度不同的某种气体时,气体@首先溶解于薄膜之中,由于固体薄膜都有分子间隙及分子内空隙,气体在这些间隙中由高浓度向低浓度@ 进行扩散,最后在膜的另一面向外蒸发。气体对薄膜的渗透就是由无数个单分子对薄膜的渗透构成的。

                不同≡的气体(例如:氧气、氮气、二氧化碳等)对同一种包装材料的透气系数是不同的。在食品〇包装中,主要研究的〇是氧气对包装材料的透过性能,即包装材料的透氧性能。


                影响气体渗透性的因素有以下几№点:


                ①分子▃链的极性。在高聚物材料中,非极性材料有聚乙▲烯、聚丙烯、聚丁二、聚四氟乙烯等;弱极性的有聚』苯乙烯、聚异丁烯、天然橡胶等;极性聚合物有聚氯乙烯、尼龙、聚甲基丙烯『酸甲酯等;强极性的有酚醛树脂、聚酯、聚乙烯醇等。极性分︼子的相互力大,内聚能密度高,阻隔性好,扩散系数低。


                ②分子链的刚性和侧基。分子链刚性◥大、主链不灵活的材料玻璃化温度高的材料气体透过率较低。分子链侧基不╱对称,高聚物自由空间大,透过率就〒相对较高。


                ③结晶度。结晶度高,分子链排列愈紧密,气体透过结晶性物质比透过无定性物质∏需要更多的扩散活化能,因而∏阻隔性更好。一方面结晶性高聚物的透过◆率低于无◆定型高聚物,另一方面,同一↙种高聚物中结晶度高的优于结晶度低的


                ④高聚物的密度。与结晶度相⌒似,高聚物密度高、阻隔性好、渗透率低。


                ⑤取向度。通过改变高聚物的拉伸取向可显著降低气体透过率,特别对结晶高聚物,取向可使晶体按一定方向重新排列起来还可以促进结晶,使∞得渗透剂分子需经过更为曲折的路径才能透过包装材料。


                ⑥湿敏度。有些高聚物含有羟基-OHI、酰胺基-CNH-等,对水敏感,当水分子渗入,形成氢键,使高聚物膨胀、松弛,使透气性◇增加。另外一些含酯基一C-0一、氯基乕C=N的高聚物,虽然吸水,但不影响阻隔性,因为不取决于氢键。


                ⑦温度。总体上,气体或液体在高聚物中的透过率随温度升高而增加。这是因为,随着环境温度的升高,影响塑料薄膜阻隔性因素都会有相应变化,聚合物分子键的刚性下降,内聚度下降,自由体积增大。环境温度的升高还会使经过拉伸取向的聚合物分子链间的取向能降低。这些变化使薄膜的透ζ 过率随着环境温度的升高而加大。



                About the air permeability principle and influencing factors of packaging materials


                Food and drug safety has always been the focus of people’s attention, and packaging is the guarantee of food and drug quality. The barrier properties of packaging materials have a major impact on the shelf life, quality stability, moisture resistance and oxidation resistance of food and drugs. At present, the permeability of materials Testing and moisture permeability testing have become more popular. The research on the air permeability of packaging materials is particularly important in the pharmaceutical and food industries, which directly affect the quality of products. A single film cannot meet the overall demand for flexible packaging. In recent years, multi-layer composite films have been widely used in various fields with their excellent barrier properties and chemical resistance. Among them, the barrier layer material plays a key role.


                The air permeability principle of packaging materials:


                Gas transmission rate. Under an atmospheric pressure difference, per square meter of permeable area, the amount of gas permeated in 24 hours (under standard conditions).


                Permeability coefficient. The air permeability per unit area and unit thickness of the film in unit time and unit pressure difference (under standard conditions).


                The permeation of gas through the film is a single molecule diffusion process, which is a mass transfer process. It consists of three steps. When there is a certain gas with different concentrations on both sides of the film, the gas is first dissolved in the film. Because the solid film has molecular and intramolecular gaps, the gas changes from high concentration to high concentration in these gaps. Diffusion occurs at a low concentration, and finally evaporates on the other side of the membrane. The permeation of gas into the membrane is composed of countless single molecules permeating the membrane.


                Different gases (for example: oxygen, nitrogen, carbon dioxide, etc.) have different air permeability coefficients for the same packaging material. In food packaging, the main research is the oxygen permeability of packaging materials, that is, the oxygen permeability of packaging materials.


                The factors affecting gas permeability are as follows:


                ① The polarity of the molecular chain. Among high polymer materials, non-polar materials include polyethylene, polypropylene, polybutadiene, polytetrafluoroethylene, etc.; weakly polar materials include polystyrene, polyisobutylene, natural rubber, etc.; polar polymers include poly Vinyl chloride, nylon, polymethyl methacrylate, etc.; strong polarity includes phenolic resin, polyester, polyvinyl alcohol, etc. Polar molecules have high mutual force, high cohesive energy density, good barrier properties, and low diffusion coefficient.


                ②The rigidity and side groups of the molecular chain. Materials with high molecular chain rigidity and inflexible main chain materials with high glass transition temperature have lower gas permeability. The side groups of the molecular chain are asymmetric, and the free space of the polymer is large, and the transmittance is relatively high.


                ③ Crystallinity. The higher the crystallinity, the closer the molecular chain is arranged, and the more diffusion activation energy is required for gas to pass through crystalline substances than through amorphous substances, so the barrier properties are better. On the one hand, the transmittance of crystalline polymers is lower than that of amorphous polymers. On the other hand, the high crystallinity of the same polymer is better than the low crystallinity


                ④The density of the polymer. Similar to crystallinity, high polymer has high density, good barrier properties and low permeability.


                ⑤ Degree of orientation. By changing the stretching orientation of the polymer, the gas permeability can be significantly reduced. Especially for crystalline polymers, the orientation can rearrange the crystals in a certain direction and promote crystallization, so that the penetrant molecules need to go through a more tortuous path Only through packaging materials.


                ⑥ Moisture sensitivity. Some polymers contain hydroxyl -OHI, amide group -CNH-, etc., which are sensitive to water. When water molecules penetrate in, hydrogen bonds are formed, which expands and relaxes the polymer and increases air permeability. In addition, some polymers containing ester groups—C-0—and chlorine groups Pi C=N, although they absorb water, do not affect the barrier properties because they do not depend on hydrogen bonds.


                ⑦Temperature. In general, the permeability of gas or liquid in high polymers increases with increasing temperature. This is because as the ambient temperature increases, the factors affecting the barrier properties of the plastic film will change accordingly. The rigidity of polymer molecular bonds decreases, the degree of cohesion decreases, and the free volume increases. The increase in ambient temperature will also reduce the orientation energy between polymer molecular chains after stretching. These changes make the transmittance of the film increase as the ambient temperature rises.


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