LDPE process question

I'm using an extrusion of LDPE, 640i. ID .032" Wall .005". I have a
process where I re-flow the extrusion, using a mandrel,FEP heatshrink
and hotair. I have noticed that my pre reflow process tensile strength
is about 2x of what my post reflow process strength is. Also, post
process elongation is about 3x what the pre process elongation is. I
believe that is telling me that the material is being annealed, and
becomes more amorphous  during the process.

The ID and OD do not change significantly during the process step. I
used a thermal couple under the FEP heatshrink to measure the actual
temperature that the material is exposed to during process, and the
temperature was 425F. I tried lowering the temperature to see if that
had an effect on the tensile strength of the post process material,
and there was no difference. Still about half of pre process material
strength.

Can someone explain what is happening? Why does the material loose
half of it's tensile strength, and can I do some post processing to
gain the tensile strength back? ?( Maybe irradiate the parts to
recross link?)

Thanks in advanced.
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During your reprocessing you seem to be losing the orientation of the
polymer in the extrusion direction.  If high stresses are induced
during the extrusion process, you could be seeing an exaggerated
effect.

Crosslinking should increase your tensile substantially (with a loss
of elongation), as well as increase resistance to abrasion.

E-beamed five sets of pre and post processed material (low tensile) to
150, 170, 190, 210 and 230kGy, The tensile testing results did not
have a substantial increase in post process tensile strength.
The extruded suggested that I should try annealing the post process
parts at 165F (74C) for 30 minutes. I tried this at both 74C and 84C
for 30 minutes, and still no increase in the tensile strength.
Does anyone have additional thoughts or ideas?

E-beamed five sets of pre and post processed material (low tensile) to
150, 170, 190, 210 and 230kGy, The tensile testing results did not
have a substantial increase in post process tensile strength.
The extruded suggested that I should try annealing the post process
parts at 165F (74C) for 30 minutes. I tried this at both 74C and 84C
for 30 minutes, and still no increase in the tensile strength.
Does anyone have additional thoughts or ideas?
---

If you're aiming to add tensile strength to the raw extrusion, you
could ask whoever is making your tubing to maximize crystallinity of
the part in the extrusion via using a slower cooling rate (wider air
gap between the die and the water bath AND use hot water), or add a
small percentage of MDPE or HDPE to the part (if you can withstand a
higher flex modulus).

As for annealing, you may want to extend the time some.  Most
annealing studies I've done has shown that 1-2 hrs is a good amount of
time, little property change after that but 30 minutes was not long
enough (most of these studies were with PA and PUR, not PE).  For the
temperature, perform a DSC if you can, and use the Tg (of slightly
above) temperature.  If a DSC is out of the question, go to the higher
temp of 84C....keep going up until you damage the part if you have to,
and test along the way (maybe in 2C-3C increments after the 84C).

What is your raw tubing tensile PSI vs. the reflowed part vs. the
resin TDS of 1650 psi??  How short are you of your goal?

It is odd that the E-beam didn't add tensile strength to the finished
component. It is a common practice to use Ebeam for heat shrink tubing
prior to expansion.