Effect of Aluminum Coating Thickness on Water Vapor Transmittance Rate of Aluminized Films

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Aluminum foil is a high barrier material. It can be almost completely impermeable when there is no flaw on the surface. Therefore, the film composited with aluminum foil (the thickness is usually 7 to 9 microns) has high barrier properties. However, the cost of aluminum foil is high, and it is prone to forming pinholes and difficult to recycle. The above shortcomings limit the application of aluminum-plastic composite film to a certain extent.

Aluminized films are widely used as substitutes for aluminum-plastic composite films to meet the general needs. Aluminum is coated on the surface of PET, CPP, BOPP, PE and other substrates through vacuum aluminum plating process. It can greatly improve the overall barrier property of the material and effectively reduce the cost because of the very thin aluminum coating. In addition, comparing with aluminum-plastic composite film, the flexibility and toughness of the aluminized film are improved. The advantages of aluminized film make it widely used in food, medicine, cosmetics and other products packaging. The barrier property of the aluminized film is mainly related to the properties of the aluminum coating, such as the thickness of the coating, the adhesion fastness of the coating on the surface of the substrate, the surface condition of the coating, etc. In this paper, the effect of coating thickness on the barrier property of materials is studied.

Test Samples

In this paper, two VMBOPP films with thickness of 440 and 520 were used as test samples.

Test Standard

Test methods of water vapor transmission rate for flexible plastic packaging materials include cup method, electrolytic sensor method, infrared sensor method, humidity method, etc. This paper is based on the principle of electrolytic sensor method. The test conforms to  GB/T 21529-2008 “Determination of water vapour transmission rate for plastic film and sheeting – Electrolytic detection sensor method”.

Testing Instrument

In this paper, C330H water vapor transmittance test system is used as the testing equipment developed and manufactured by Jinan Labthink Instrument Co., Ltd.

Test Principles

Electrolytic sensor method equipment is based on the quantitative relationship between electrolytic water molecule and electrolytic current. The test instrument has two chambers: the upper chamber is filled with high humidity nitrogen, and the lower chamber is filled with dry nitrogen. Water vapor permeates the sample from the high humidity side to the low humidity side. The dry nitrogen carries the water vapor to the electrolytic sensor for electrolysis analysis. The water vapor permeability of the sample can be obtained by analysis and calculation.

Figure 1 C330H Water Vapor Transmission Rate Testing System

Test process

(1) Three specimens of 108 mm *108 mm were cut from the sample of 440 Å coating thickness, and the specimens were sealed and clamped in three test chambers of the instrument.

(2) Input the sample name, thickness, test temperature, humidity and test mode in the control software of the instrument, click the start button to start the test. The Instrument is run automatically according to the set parameters, and the results are displayed after the test.

(3) Test the water vapor transmission rate of the samples with coating thickness of 520 Å following the operation steps of cutting-assembling-setting parameters-starting test.

Test results

The water vapor transmission rate of each sample is the mean of its three specimens. The water vapor transmission rate of VMBOPP samples with 440 Å thickness was 0.463 g/(m2.24h) and that of VMBOPP samples with 520Å thickness was 0.071 g/(m2.24h).

Conclusion

The thickness of aluminized layer is an important factor affecting the barrier performance of aluminized film. From the results of vapor permeability of VMBOPP samples with different coating thickness measured in this paper, the vapor transmission rate of 520 Å coating thickness samples is significantly lower than that of 440 Å coating thickness samples, it shows that the barrier performance of aluminized film can be significantly improved by a small increase in the thickness of aluminized layer. In the process of testing with C330H water vapor transmission rate testing system, human factors have little influence on the test results, the instrument is intelligent, the control of temperature and humidity is stable with little fluctuation, and the test results are accurate and reliable.