Thursday, February 11, 2016

Environmental Stress Cracking Resistance (ASTM D 1693)

Specimen: LDPE measuring 1/8 x ½ x 3/2 inches, annealed in water or steam at 100 0 C for 1 hour and then conditioned at room temperature for 5 to 24 hours.


 The specimen is placed in an air circulating oven and then inserted in a test tube which is then filled with a fresh reagent [Igepal = RC6H4O(CH2CH2O)nCH2CH2OH  where R is C6H17 or higher homolog]. The tube is stoppered with an Aluminum covered cork and placed in a constant temperature bath at 50 0C. These are inspected periodically and any visible crack is considered as failure. The duration of the test is recorded along with the percentage of failure.


The cracking obtained in this test indicates what can be expected from a wide range of other stress cracking agents. Though the information cannot be translated directly into end use service prediction, but it serves to rank various types and grades of PE in categories of resistance to stress cracking. This test can also be used on high and medium density used on high and medium density material.

Melt Flow Index (ASTM D 1238)

To measure flow rate by extrusion Plastometer.


Any form which can be introduced into the cylinder bore may be used. e.g. powder, granules, strips of film. Conditioning required varies with material.


The apparatus is preheated to 190oC for PE. Material is put into the cylinder and the loaded piston (@ 43.25 psi) is put into place. After 5 minute the extrudate issuing from the orifice is cut off, flush and again one minute later. These cuts are discarded. Cuts for the test are taken at 1, 2, 3 or 6 minutes depending on the material or its flow rate. The melt index is calculated and recorded as gm/10 minute.

It is primarily useful to raw material manufacturer as a method of controlling material uniformity. The melt index value is strongly indicative of relative “flowability” of various kinds and grades of PE. The ‘property’ measured by this test is basically melt viscosity or ‘rate of shear’. In general, the higher molecular weight materials are more resistant to flow.