ASTM D1709 - Standard Test Methods for Impact Resistance of Plastic Film by the Free-Falling Dart Method:
The ASTM D1709 standard describes two methods for testing the impact resistance of plastic films using the free-falling dart method. This process aims to determine the energy that causes plastic film to fail under specific conditions of impact from a free-falling dart, expressed in terms of the weight of the dart falling from a specified height that would result in a 50% failure rate of the tested specimens.
- Test Method A uses a dart with a hemispherical head of diameter 38.10 ± 0.13 mm, dropped from a height of 0.66 ± 0.01 m. This method is suitable for films whose impact resistances require masses ranging from approximately 50 g to 6 kg to puncture them.
- Test Method B uses a dart with a hemispherical head of diameter 50.80 ± 0.13 mm, dropped from a height of 1.52 ± 0.03 m. It is applicable for a range of approximately 0.3 kg to 6 kg.
Two testing techniques are described in the standard: the staircase method and the alternative technique.
1. The staircase method involves modifying the weight of the dart after each sample test, based on the observed result (failure or non-failure) of the sample. The weight of the dart is either decreased or increased by uniform increments.
2. The alternative technique tests samples in successive groups of ten, using a specific dart weight for each group and varying the dart weight by uniform increments from one group to another.
Both methods yield equivalent results in terms of impact failure weight values obtained and the precision with which they are determined.
The ASTM D1709 standard cautions against directly comparing results obtained using different test methods or varying conditions. Factors influencing the results include dart velocity, impinging surface diameter, effective sample diameter, material construction, finish of the dart head, and film thickness.
It is also noted that the impact resistance of the plastic film, although partially dependent on thickness, does not have a simple correlation with sample thickness. Therefore, impact values cannot be normalized across a range of thicknesses without the risk of producing misleading data regarding the true impact resistance of the material.