Yoghurt is a kind of dairy product which uses fresh milk as raw material, adds beneficial bacteria (fermenting agent) to milk after pasteurization, and then cools and fills after fermentation. During the fermentation process, about 20% of sugar and protein in milk are hydrolyzed into small molecules. After fermentation, the fatty acids in milk can be increased twice as much as that in raw milk, which makes yoghurt easier to digest and absorb. Yogurt is sensitive to oxygen because of its high protein and fat content. Therefore, in order to ensure the shelf life of yogurt, the plastic yogurt cup is usually sealed with a composite film with aluminum foil as the metal material. If the composite fastness between the layers of the cover film is low, the peel strength of each single layer of the cover film is low when peeling off, and it is easy to tear off the cover film. If the separation occurs between the cover film and the cup body, but occurs between each layer of the cover film, not only brings inconvenience to consumers, but also causes the decline of corporate image. Therefore, food enterprises should test the composite fastness of the covering materials.

Figure1 Plastic yoghurt cups

Test sample

The composite film material used as yoghurt cup cover film provided by a certain brand.

Standard

The test process of the peel strength of composite film is based on the standard ISO 11339 Adhesives—T-peel test for flexible-to-flexible bonded assemblies.

Test instrument

The tensile tester used in this paper is XLW (EC) auto tensile tester independently developed and produced by Jinan Labthink Instruments Co., Ltd.

Principle

Peel strength represents the force required to peel apart the two layers of a composite film. The test instrument has two grips, the upper grip is a movable grip, it can move up and down according to the set test speed, and is equipped with a load cell, and the lower grip is a fixed grip. During the test, the two layers of film in the pre peeled section are respectively clamped on the upper and lower grip. The upper grip moves upward to peel the specimen. The load cell automatically records the force value generated in the process of specimen peeling.

Figrue2  XLW(EC)Auto tensile tester

(1) Use a sampler to cut 5 specimens with a width of 15 mm and a length of 200 mm in transverse and longitudinal directions from each sample.

(2) One end of 10 specimen shall be manually peeled off for a length of 50 mm.

(3) Clamp the two ends of the stripped end of the specimen with the upper and lower grip of the instrument, so that the long axis direction of the specimen aligns with the center line of the two grips.

(4) Set the displacement, test speed and other test parameters. Click the “start” option to start the test.

(5) The instrument automatically tests and records the force value in the process of specimen peeling, and reports the final test result after the test.

Test result

In this paper, the average value of transverse peel strength is 4.412N/15mm, and the average value of longitudinal peel strength is 5.944N/15mm.

Conclusion

XLW (EC) auto tensile tester for testing the composite fastness of yoghurt cup cover film has the advantages of simple operation, high repeatability and accuracy, the test result can reflect the real composite fastness of the cover film.

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