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Basic understanding of stress cracking in HDPE products


Environment stress cracking of HDPE products is quite common because of its semi crystalline nature which gets attacked due to chemicals present in the environment. The stress cracking is more severe with detergent, bleach, motor oil and some industrial chemical packaging. ESCR problems get more pronounced in the summer, because of several factors related to increased heat and humidity.
In the 1930s Bell Laboratories devised a test for ESCR. Informally known as the Bell Lab test, this test method is today called ASTM D1693. Ten 12.5*50 mm specimens are die-cut from a 3 mm compression molded plaque. These are scribed with a 0.25 mm deep imperfection and then are bent into a “U” shape and inserted into a channel, notch-side out. The holder with the 10 specimens is inserted into a test tube filled with Igepal 630 surfactant and placed in a 50 degree C bath. As each specimen cracks, the time is plotted. F0 is the number of hours until the first one crack, F50 is when half have cracked, and F100 is when the last one breaks. The ASTM D1693 test method is generally regarded as neither precise nor reproducible. The estimate for inter-laboratory precision is 2.9 at 2 Sigma (two standard deviations). This means that a F50 value of 10 hr reported by one lab could vary (95% of the time) within a range from 3.4 to 29 hr when the same specimens were tested by another laboratory. To address the lack of precision, the concentration of the Igepal soap solution was revised to 10%, so yet another test condition was added to the ASTM method. This gives users a choice of four conditions (Two different thicknesses and use of 10% or 100% Igepal). Datasheet values typically mention the ASTM method but fail to identify the all-important test conditions, which drastically affect the time to failure. Thus a comparison of one HDPE grade ESCR to another based on datasheet values is suspect.

The bent-strip test is a constant-strain test and the initial bending stress decays with time due to relaxation. Consequently, many manufacturers of pipe, geo membranes and containers have migrated to some form of a constant-stress test whose conditions are more representative of in-service situations.
Another groundbreaking HDPE bottle application was replacement of the metal detergent can in the mid-1950s. These detergents contained surfactants that caused stress cracking and the bent-strip test was unable to predict field performance. Thus the container/bottle industry pioneered the use of testing methods based on constant stress.
Two methods are generally used to produce a sustained stress on the bottle to determine the container’s resistance to ESCR. In the constant-top-load test, a fixed weight is applied to each bottle. This test method, also known as the P&G test (named for Procter & Gamble), mimics the stress encountered from a stack of multiple cartons. Meanwhile, the ASTM D2561 test method uses a constant internal pressurization of 0.35 kg/sq.cm to produce a sustained stress. A variation of this test uses sealed bottles warmed in a 60 degree C oven. The thermal expansion of the contents produces about 0.35 kg/sq.cm in the bottle. In both methods, the bottle contains a known stress-cracking agent. Each method has its proponents, but data interpretation is a common problem.
During the summer months, stress-cracking of HDPE containers reaches its peak. This is because stress from different sources increases during the summertime. Premature cracking can be caused by top-load stress, internal pressure, molded-in stress, and tail swing. With the advent of zero-head-space cartons to save space and corrugated costs, the bottle is in a load-sharing condition with the carton sidewall or dividers, if they are used. But corrugated cardboard can lose up to 50% of its compressive strength under summer’s high humidity. Theoretically, the bottle itself also loses a bit of its compressive strength as its temperature increases. This combination often results in a creased or deformed container. Common examples are quart oil bottles cracking at a top-load-induced fold or crease in the bottom corner, or cylinder rounds cracking at the neck-body junction.
An increase in internal pressure is another source of stress. Most filling operations are done at ambient temperature, possibly even in an air-conditioned plant. The capped container is then moved to a non-air-conditioned warehouse or stored in an aluminum trailer. Enclosed trailer temperatures of 65 C have been reported during the summer. This heat will generate an internal pressure in the container of 0.3-0.35 kg/sq.cm, depending on the bottle’s contents and the amount of head space.
Constant-stress test can be another contributor to stress-cracking. HDPE is a semi-crystalline material, with no crystals in the melted state. The amount of crystalline content in the container depends on the rate of cooling. While crystallinity provides strength, it also can lead to cracking. Since blow molding has only single-side cooling, thicker sections will not cool as fast as thinner sections. Since crystallization always involves shrinkage, there is always some molded-in stress at the end of the pinch-off, where the thicker eye meets the thinner bottom corner of the container. Cosmetic surface defects, called water spots, often develop during humid months due to moisture condensation on the chilled molds. In response, molders raise the mold temperature to stop condensation, but without adding a corresponding increase in cycle time. This means the container exits the mold warmer, which results in greater crystallization and an elevated amount of molded-in shrinkage stress. Tail swing occurs when a parison does not fall straight and wanders. As molding plant temperatures soar in the summer, there is an observed drop in operator attention to such process deviations. A pinch-off eye located over on the chime of the container is more prone to cracking than a centered one.

Manufacturers can employ several remedies to correct stress-cracking problems. First, use a stronger “summer carton” in the summer months. Switching from 90 kg to 100-105 kg rated corrugated is inexpensive solution. Reducing carton stack heights in summer is another option. Consider using an internal “Z” or “H” divider during this season. Switch to a more crack-resistant HDPE grade. A slightly less crystalline HDPE will have better crack resistance, and density is a direct measure of crystallinity. A HDPE grade of 0.95 density would perform better compared to the 0.054 density grade. Consistent process cooling helps in reducing the problems. Lengthening molding cycles by a few seconds is a hard sell to management but it is the only way to compensate for the higher mold temperatures used to inhibit condensation and water spots.
(Ref: Article by Robert DeLong, Blasformen Consulting)

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