As a metal material, aluminum foil is non-toxic, tasteless, has excellent electrical conductivity and light-shielding properties, extremely high moisture resistance, gas barrier properties, and its barrier performance is incomparable and irreplaceable by any other polymer materials and vapor-deposited films. of. Perhaps it is precisely because aluminum foil is a metal material completely different from plastic, its performance and characteristics are not recognized by many people, and even many misunderstandings have occurred. For example, there is a view that the barrier performance of aluminum foil and plastic composite is equal to the simple superposition of the barrier performance of pure aluminum foil and the barrier performance of plastic film. This is wrong, because the prerequisite of the simple superposition principle is that the two layers of materials are each microscopically uniform. The concentration of the diffuser on the interface is uniform, and there is no diffusion along the interface direction. Aluminum foil composite materials do not meet these conditions, and the diffusion of pinholes in aluminum foil composite materials is significantly higher than other positions. There is also another view that the air permeability of the aluminum foil composite material is equal to the air permeability of the plastic material multiplied by the area ratio of the pinholes in the aluminum foil. It is also wrong, because it is not only the air permeability of the plastic at the pinholes of the aluminum foil, but also the air permeability near the pinholes of the aluminum foil. Plastic is also involved in ventilation. Some people think that if the thickness of the plastic is doubled in the aluminum foil composite material, the barrier performance is doubled. Of course, this idea is also wrong. In fact, when the pinhole size of the aluminum foil is small enough compared with the thickness of the plastic, the thickness of the plastic is increased. , Has basically no effect on the overall barrier performance of the composite material. Some people think that as long as the barrier performance of composite materials is high, the sealing performance of packages made of it must be good. In fact, as a package, the factors that affect the sealing performance include frontal penetration, side penetration and leakage. High barrier materials can only be said to have very low penetration, which does not mean that there is very low side penetration and leakage. In addition, in the packaging process of forming, filling, heat-sealing, etc., as well as the sterilization, transportation and sales of the packaged product, the aluminum foil layer may be crushed, crushed, or broken, which affects the overall sealing performance of the package.
The pinhole aluminum foil of aluminum foil is a metal foil made by rolling aluminum. The perfect aluminum foil can completely block gas, water vapor and light. In fact, due to the influence of many factors in the production of aluminum foil, aluminum foil, especially aluminum foil with a thickness of less than 20 μm, inevitably produces pinhole defects. It is precisely because of the existence of pinholes that the oxygen transmission rate and water vapor transmission rate of the aluminum foil are not zero. The size and number of aluminum foil pinholes have a decisive influence on the moisture resistance, gas barrier properties and light-shielding properties of aluminum foil and its composite materials.
The national standard GB3198-1996 “Industrial Pure Aluminum Foil” specifies specific provisions for pinholes. “Pinholes visible to the naked eye are allowed on the surface of the aluminum foil, but the pinholes must not be densely packed, and the diameter of the pinholes in the medicinal aluminum foil shall not be greater than 0.3mm. , And not more than 5/m2. The pinhole size of other industrial pure aluminum foils must not exceed 0.5mm. The number of pinholes must not be too many.”, the standard appendix B also gives the pinhole inspection methods and pinholes Number standard.
Just because pinholes are an important indicator of aluminum foil, many foreign technical documents have stipulated the pinholes of aluminum foil (see Table 1). American ASTM B-479 “Technical Standard for Annealed Aluminum Foil and Alloy Aluminum Foil for Flexible Packaging Barrier” stipulates the pinhole of aluminum foil, and also provides the relationship curve between aluminum foil thickness and water vapor transmission rate. Japanese Industrial Standard JIS Z1520-1990 “Composite Aluminum Foil Standard” also provides reference data for the water vapor transmission of composite aluminum foil.
The number of pinholes in aluminum foil is related to the thickness of aluminum foil. As the thickness of aluminum foil increases, the number of pinholes decreases rapidly. The logarithm of the number of pinholes in aluminum foil is basically proportional to its thickness. (1) When its thickness reaches 20μm, aluminum foil It can be completely pinhole-free. There is a big gap between the quality level of domestic aluminum foil and foreign countries, mainly in the number and size of pinholes. Foreign standards stipulate that there should be no pinholes when the thickness of aluminum foil is greater than 0.020mm, while the Chinese standard stipulates that there should be no pinholes when the thickness of aluminum foil is greater than 0.050mm; foreign standards stipulate that pinholes greater than 0.1mm are not allowed, and the pinhole diameter is generally 0.015mm. According to the national standard GB3198, the pinhole inspection is not a factory inspection item. According to the national standard GB3198, the maximum size of the pinhole can be up to 0.5mm. Although the quality of my country’s aluminum foil has improved in recent years, the quality of domestic aluminum foil is still unstable, and pinholes are frequent and sometimes less. Improving the quality of aluminum foil and reducing the number and size of aluminum foil pinholes are currently urgent problems that domestic aluminum foil factories need to solve.