After I blogged my tentative design for a piston extruder system I began picking at the details to see if I could technically disqualify the design.
The first experiment that I tried was to melt about 30 cm^3 of CAPA in an open container. I was going to set up a double boiler remembered that I had bought a heat gun several years ago when I was doing some exploratory work with melting HDPE. With the heat gun and my infrared thermometer I was able to keep fairly close track of the process.
The CAPA went transparent at 60 degrees Celsius as per the spec. While it became glass-like in appearance afterwards its high viscosity kept its form as a pile of separate granules for a considerable time thereafter. After I raised the temperature of the CAPA to about 120 degrees it slowly began to slump under its own weight.
After about 20 minutes at 120 degrees the outlines of the individual granules had almost entirely disappeared and the mass of the melt resembled a big puddle of transparent candlewax. The viscosity of this mass was still enormous, however. When the container was rotated to the vertical from the horizontal the mass showed no propensity to flow at any appreciable rate.
While the melt slumped it retained perhaps 1-2% of the air in the interstices between the granules. These bubbles were clearly visible in the transparent melt.
My feeling is that if we were to attempt to suck this mass into a piston polymer pump it would be something that would have to be done very slowly and deliberately if we were to avoid shear fracturing of the amphorous mass.
I'm not sure that we're going to be able to do this.
I guess the next experiment would be to see if we could suck melt into a cylinder. Of especial interest would be to know how fast we could do that and what effect it would have on the surface of the melt.
I redid the experiment this morning with a toaster oven that I bought for working with HDPE at the same time I bought the heat gun. I warmed the sample to 130 degrees this time.
The even heat of the toaster oven made all the difference. The polymer melt, which I had taken out of the corelle cup serving as a crucible and flipped over, melted very evenly.
It took just at 20 minutes. There was one large bubble trapped in the melt where air was trapped by the sagging ingot of CAPA. I am giving it another 20 minutes of heating to see if that bubble surfaces.
I repeated the experiment with an ingot of HDPE that I had left over from those long ago experiments. At 160 degrees Celsius it melted and slumped beautifully in 20 minutes (there is a timer on the toaster oven). A few 3-4 mm bubbles were at the surface of the melt
Now THAT's interesting! While the caprolactone tended to behave a bit like freezing mercury in the crucible, viz, it had a high surface tension and kept a vaguely lens shape in the crucible right up to the moment that it hardened, the HDPE sample did not.
Even more interesting was the fact that the caprolactone did not shrink noticeably on cooling while the HDPE shrank quite significantly to the point that there was a clearly visible (~1-2 mm) shrinkage of the sample in the crucible. As a result of this upending the corelle ware cup that it had been heated in dropped it cleanly into my hand.
Both the caprolactone and the HDPE exhibited a nice, slightly waxy surface where they had been in contact with the corelle ware cup.
Apparently, pigmented HDPE often has crystalizers in it. The clear stuff used in milk jugs supposedly doesn't. Maybe this accounts for some of the differences you noticed compared to the CAPA...?
Don't quote me on this, I ain't no expert! ;)
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