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6 Film Properties (S. 109-110)
This chapter covers many of the most important properties measured by producers of blown film. These include mechanical, thermal, optical, physical, electrical, and rheological properties. The first five in this list apply primarily to the extruded film and the last one applies to the molten polymer inside the extruder and die. By obtaining the measurement values for these properties, manufacturers gain assurance that their resin or film will perform adequately, whether during manufacture (extrusion or conversion) or in final product form as used by the customer. It is vital that manufacturers document and maintain baseline data on the performance of their incoming materials and the film that they produce. This information provides the most efficient means for solving many problems that may arise in a manufacturing plant. Many extrusion performance issues are related to even slight modifications in raw material composition or processing properties.
These modifications are easily identified when prior baseline data is available. In addition, film performance deficiencies as measured by a reduction in some property, such as impact strength, may lead the technical staff to identify an undesirable drift in processing conditions. Another important reason for maintaining baseline data is that many customers require it. Tests for resin and film property values can be performed in-house, by an outside testing facility, or by a supplier. There are advantages and disadvantages to each. The most important consideration is the accuracy of the data collected. After that, consideration must be given to costs associated with the amount of data needed (equipment, consumables, training, labor, etc.) and the frequency of measurement required.
To suitably compare measurement values obtained at different times or locations, there must be assurance that the tests were performed under identical conditions. To accomplish this, a committee of experts in each particular subject area develops standardized test methods. The methods establish the exact conditions for all test parameters by all parties conducting tests to measure the property of interest. A major publisher of test methods covering polymer resins and plastic films is the American Society for Testing and Materials (ASTM), West Conshohocken, PA, USA. In this chapter, ASTM method reference numbers are included in parentheses at the beginning of each property description section.
Although publicly published test methods are invaluable for comparing measurement results, not all tests must conform to published standards. In many cases, processors, their suppliers, or their customers will design and perform customized tests in an attempt to best simulate actual product conditions. An example would be a grocery sack manufacturer that performs a routine quality check by loading a finished bag with a specified weight and then dropping the load onto the floor from a specified height to check for bag failure. Many in-house tests like this are designed and conducted regularly to best model actual use, shipping, or handling conditions. It is crucial, however, that test procedures are documented and followed identically every time so that results comparisons are valid.