1. The Common Problems of Film Specimen
1.1 Specimen Homogeneity
Homogeneity is critical to the application of flexible packaging materials and it’s also one of the main causes for package breakage. If the specimen is homogeneous, the actual strength of the whole package would be close to the expected value. If the specimen is not homogenous, the position with inferior strength would be the defect of the package which would possibly cause package leakage.
Usually, the homogeneity of the specimen could be reflected by the discrete degree of the test data. The discreteness of the test data is different from the fluctuation of test data. Data fluctuation is usually related to the precision, resolution of the testing instrument and the environmental factors. However, the discreteness of test data includes not only the fluctuation of test data, but also related to the homogeneity of specimen closely.
The thickness of film is not uniform which is decided by its manufacturing craft. Compared with its length and width, the thickness of the material is a minor parameter and that is why the film is easy to be injured. Such injury would cause weakness of the strength and other properties of the film. As for composite film, the injury may occur to the outer layer, or the inner layer which is not easy to observe such as the Aluminized layer, Aluminum foil or silicon-plated layer. Since the homogeneity and injuries of the material are both the critical factors which may influence the test data, thus the specimen is usually required to be homogeneous and free of wrinkles, indentation or pinholes. However, in practice, the operator should identify the cause for the injury of the material. If the injury of the material occurs in process of transportation or caused by other external factors, the material with such injury should not be selected as test specimen. If the injury is common in the material itself, then it is unnecessary to avoid the injured part when preparing test specimen.
According to the experience, the homogeneity would directly lead to difference of test data between the specimens. It is also the main cause for high data discreteness. Therefore, the operator should try to get the specimens with good homogeneity. If such specimens are not available, the operator could adjust the specimen size or increase the quantity of specimens so that the test data would be more accurate.
1.2 Specimen Size
Specimen size is an important factor affecting the accuracy of test data. The specimen size is closely related to the homogeneity of the specimen. The operator should select proper specimen size until the data discreteness is within acceptable range. The evaluation criterion for the homogeneity of the specimen differs for different industries. However, in the same industry, there is similarity in homogeneity evaluation of the specimens. Therefore, the empirical parameters obtained from long-term tests and researches are of great referential significance, e.g. the specimen size ranges in the test standards. However, it should be noted that though all the test data of the same test item has been expressed by unit length or unit area, only the test data of specimens with same or similar size are comparable with each other. Because the homogeneity of the specimen would be influenced by changing the specimen size thus the test data would be affected especially when the specimen size is reduced.
However, the test range of testing instrument could usually be adjusted by changing the test area. Take the barrier property test for example, it has been widely used to adjust the test range of testing instrument by changing the test area. In principle, there is nothing wrong with this method except that in actual tests, test data would fluctuate when the test specimen is not homogeneous. Therefore, the operator should estimate the bias of test data caused by changing test area of the specimen in actual tests and make a correct evaluation of the properties of test specimen. It is recommended to increase the quantity of the specimens when performing tests of small size specimen in order to balance out the impact of the homogeneity.
1.3 The Quantity of Specimens
Statistically speaking, the test data would be close to the real value by testing the specimens as many as possible. Yet the testing cost will rise and the testing efficiency will be reduced as a result. Usually, the quantity of specimen stipulated in the standards could meet the testing requirements. However, the quantity of specimens should be increased if the homogeneity of the specimen is not good and the test data is incorrect obviously.
For the tests of regular specimens, it’s recommended to adopt the quantity of specimen stipulated in the standards. If possible, more specimens can be prepared for backup use. When the homogeneity of specimen is inferior, it’s advisable to increase the quantity of specimen to 2 or 3 times of the specified number in standards and then calculate the test results. The operator could adjust the quantity of specimen according to practical operation experience. For example, when the test area is extended, it would not cause data fluctuation thus the quantity of specimen could be the same as that specified in standards. If the test area is decreased, the data fluctuation would be influenced greatly and it is suggested to increase the quantity of specimen to 2 times of the specified number and recalculate the test results. Adjust the quantity of specimen to ensure the accuracy of the test data.
2. The Influence of Specimen Selection to Actual Tests
If the film specimen is tested to obtain the test data only, when the discreteness is not within the acceptable range due to the unqualified homogeneity of the specimen, the operator could increase the quantity of specimen or take other measures when preparing specimen to avoid the great variation between the test results and actual condition. However, if the film specimen test is performed in material comparison, instrument comparison, inter-lab comparison or even calibration of testing instrument, then the specimen is required to have very good homogeneity to avoid test errors. Therefore specimen selection should be taken into consideration when the common film is used in material comparison, instrument comparison, inter-lab comparison and instrument calibration, etc. All the factors including uniformity of thickness, test area and quantity of the specimen should be taken into consideration in process of evaluation of the film specimen.
3. Applications of Reference Film
The greatest difference between reference film and common film is that the reference film is featured with better homogeneity and stability. Similar to other standard materials, the composition and structure of reference films are usually simple. However, one or several kinds of properties of reference film can reach the specified uncertainty range in standards. As well the homogeneity of reference film should be examined before it is tested to ensure the accuracy of target value. Additionally the reference film could be used in data comparison and data transmission.
At present, barrier property tests including gas transmission rate test, water vapor transmission rate test and organic gas transmission rate test etc. are most common among all the test items for flexible packaging materials. It is complicated to calibrate the testing instruments for barrier property tests. Take the differential-pressure method gas permeability tester for example. The general parameters that need to be calibrated include pressure, temperature and humidity which may influence the accuracy of the test data. Theoretically speaking, it is possible to calibrate each parameter separately. However there are too many parameters which need to be calibrated and the procedures of calibration are complicated, so the calibration should not be performed frequently. In daily operation, the instruments may give abnormal test data in barrier property test which may be neglected because of frequent replacement of specimens. Therefore it is quite critical to verify the accuracy of the data obtained by the testing instruments.
It’s an effective method to verify the accuracy of the testing instrument by testing reference film with already known data. The testing principle is very similar to the blank test for some testing items. The common film is unqualified for calibration tests because of its uncertainty of homogeneity. Perform specimen test with the reference film and then compare the test data with the target value to judge the status of the instrument and decide whether it is necessary to recalibrate the instrument. With reference film, the instrument calibration could be performed quickly and accurately and improves the test efficiency.
The homogeneity of specimen should be paid more attention to in specimen selection. Though it has been recognized and popularized to complete the data comparison and instrument calibration by testing film specimen, improper film selection would also lead to disorder of data system, or invalid comparison results. Since reference film is featured with better homogeneity and stability, data comparison and instrument calibration can be guaranteed after replacing the film specimen with the reference film.