ASTM D 2561:Environmental Stress Crack Resistance of Polyethylene Blow Molded Containers "Bottle ESCR"

The test is usually carried out on 16-oz., 25-g., Boston Round bottles blow molded from polyethylene resin, although a container of any size and shape can be used. Bottle design, processing conditions and the resin used are all variables affecting the results of this test.

ASTM specifies three environments for this

test, so it is important to know which procedure was used in order to compare test results. It is also important to know that while ASTM calls for inspection to stop after 360 hours, many suppliers continue to check the bottles to 500 or 1,000 hours after the test begins. Successful results from these lengthier tests are reported as >500 or >1,000 hours.

Procedure A: A minimum of 15 containers is filled to nominal capacity with the chosen test liquid, usually a proprietary liquid product or an aqueous solution of polyoxyethylated nonylphenol (a surfactant). The containers are heat sealed and placed in an oven at 140°F.

Inspection takes place hourly for the first eight hours and at least once every 24 hours thereafter. Results are reported as the number of hours elapsed before 50 percent of the specimens failed (F50).

Procedure B: A minimum of 15 containers is filled to one-third of their overflow capacity with the same stress cracking solution used in Procedure A. The containers are heat sealed and placed in beakers vertically with the finish up. The beakers are then filled with a sufficient amount of stress cracking agent to cover the chime area of the container and placed in an oven at 140°F Inspection and reporting occurs in the same way as Procedure A.

Procedure C: A minimum of 15 containers is filled to one-fourth of their overflow capacity with the same stress cracking solution used in Procedure A, The containers are then sealed with a special cap assembly and  placed in an oven at 140°F. The bottles then are subjected to an internal pressure of five psi, introduced through the cap assembly, for the duration of the test. Inspection and reporting is carried out in the same way as Procedure A.

3. PBI 11: Recommended Practice for

Determining Top Load Stress Crack

Resistance of Blow Molded Polyolefin

Bottles

The purpose of this test is to simulate the conditions a container undergoes during shipment or storage. A minimum of 15 bottles is filled to nominal capacity and heat-sealed as in ASTM D 2561. The bottles are then placed in a jig that applies a constant top load for the duration of the test. Bottles and jig are placed in an oven at 1220°F. Inspection and reporting occurs in the same way as ASTM D 2561, Procedure A. Data from this test appears to reproduce reality more closely than ASTM D 2561 or ASTM D 1693.

All of the test procedures described generate valuable data that provides insight into the phenomenon of ESCR. However, these methods are very sensitive to testing error; the data must be used with caution. As an example, some testing errors can occur during the ASTM D 1693 "Bent Strip ESCR" test because of:

Surface imperfection on the test specimens
Defective method of fabricating the test specimens
Different rates of cooling of test specimens
Incorrect or inconsistent concentration of the reagent
Defects in the specimen holder
Temperature variations within the water bath
Inconsistent depth, length or width of the notch cut in the specimen
Inconsistent rates at which the specimens are bent
Defective condition of the die, if the specimens are cut from a sheet
Inconsistent data reporting

With such a large number of variables, it is not surprising ASTM has found the precision of D 1693, when performed at different laboratories, to be 2.9, which is expressed as two standard deviation limits on the 50-percent failure point. This number means that if one laboratory were to test a material and find that the 50-percent failure point (F50) occurred at 10 hours, other laboratories, testing the same material, could be expected to generate data that varies between 3.4 hours and 29 hours.

ASTM has determined the precision of D 2561 to be 1.8 and PBI reports the precision of PBI 11 is approximately 1.5, which means these two tests are more reproducible than D 1693. However, the ramifications of all these precision numbers are very important. Differences in ESCR performance of various products could actually be attributed to varying degrees to test error. In addition, test method guidelines permitting different surfactants and container styles add to the confusion. Additionally, some resin manufacturers may make minor modifications to the test procedures to suit their own needs.

When considering ESCR, it is important for the resin customer to compare "apples to apples. " Find out test conditions such as temperature, surfactant used, bottle type and weight, among other variables. For best comparison, the polymers in question should be subjected to side-by- side testing, using the same equipment, operator and environment. These conditions minimize much of the test variation and give the fairest comparison.

Equistar technical service representatives are available to assist you in understanding ESCR data. In certain cases, they may even be able to coordinate a test program to evaluate ESCR properties of several different resins of your choice. For further information, please your Equistar sales or technical service representative.

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