CASE STUDIES: Creep Buckling

CREEP BUCKLING OF PAINT CONTAINERS

PELabs was retained by a major manufacturer of one-gallon paint containers to assess the performance of various polypropylenes in resisting collapse of containers under the weight of layers of other filled containers. Containers are stacked three to five pallets high, resulting in approximate compressive loads of 163 to 276 lbs, respectively, on units in the bottom layer. A typical polypropylene (PP) container has crush resistance exceeding 500 lbs in a one-inch per minute, constant cross-head rate test, yet, for some materials, bottom layers collapse in storage within a few weeks. The three candidate materials with their respective room temperature crush resistance values are listed in Table 1.

Figure 1. Compression (decrease in height) of PP-B Container under 400lbs at Room Temperature.

Based on experience with the viscoelastic behavior of polymers, PELabs engineers chose to approach this time-dependent, buckling problem by using stress as an accelerator. Containers molded from each of the three different PP's were loaded in compression grips with a variety of dead weights. With the container loaded in compression, the decrease in height (displacement *) typically varies with time as shown in Figure 1 for a PP-B specimen loaded with 400 lbs (less than half the short-term crushing load) at room temperature. As the height change increases, there comes a time tC (=89 minutes) when the container collapses. The heavier the dead weight, the shorter the time-to-collapse. The containers buckle in the beer-can mode, as shown in the accompanying photograph (Figure 2).

Figure 2. Typical Buckling Mode of Polypropylene One-gallon Containers.

Time-to-collapse data as a function of the load for the three materials at room temperature are plotted in Figure 3. Despite having short-term collapse loads ranging from ~550 lbs to over 800 lbs, the containers collapse under 300 lb in ten minutes (PP-A) to sixteen hours (PP-B), displaying the underlying viscoelasticity of the polymers. Clearly, stacking any of these containers five pallets high (~276 lbs) could collapse some of those on the bottom layers in an unreasonably short time.

What about stacking fewer pallets, say to three or four high? The 90% confidence limits for the times-to-collapse at room temperature for the three resins are tabulated below. These were derived from a least-squares fit of the log-log data of Figure 3 and the customary assumption that the error is distributed according to the Student's t-function. The R-coefficient for the log-log fits varied from 0.83 to 0.95.

Figure 3. Log-log Plot of Collapse Time Vs. Load for Polypropylene Containers at Room Temperature

Clearly, these containers cannot be stacked more than three pallets high with any reasonable certainty, regardless of which of the three resins is used. If we require the mean time-to-collapse to exceed ninety days with ninety percent certainty, only PP-B remains as a viable candidate. However, PP-B is subject to all the usual concerns about regrind, such as reliability of supply and property variability.

Therefore, for reasons of economy, sustained availability, and lot-to-lot uniformity, PP-C is attractive. There's a sixty percent chance that its mean time-to-collapse exceeds six years stacked three high. What about the influence of a warm or hot warehouse? To answer this question, creep-buckling tests were performed at 105°F and 140°F for containers molded from PP-C. The results are displayed in Figure 4, along with the room temperature data. A statistical analysis yields a sixty-percent lower confidence limit of 110 days for the 105°F.

Figure 4. Log-log Plot of Time-to-collapse vs. Load for PP-C Containers at Three Temperatures

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