Pebax 7033

Single screw extruder? Batch drying? Continuous? Semi-continuous? What size batches and does that correlate with the time intervals for out-of-spec material? What's the downstream process like? How is the non-constant tension in the winder isolated from the die?

Small variations upstream of the die do not always show up immediately, but can hang up in the system for a while. The time interval between offsets can become larger or (less commonly) smaller when observed at the die.

John Aspen Research, -

"Turning Questions into Answers"

Opinions expressed herein are my own and may not represent those of my employer.

If the "bad" batch has been treated before extrusion exactly as a "good" batch, extrusion conditions are always identical and you are having problems only with a "bad" batch and you can reproduce this back and forth, then I believe you have to do with batch-to-batch variations and I'd contact the material supplier's technical support.

Are there any differences > Hallo each,

Thanks for the replies, John and Rolf. Just to clarify a few points:

- it is a single crew extruder, but it happily extrudes nylons and materials similar to Pebax without problem

- we have tried using the material straight from a new bag, without drying and from an opened bag, with drying and get the same issues with both arrangements

- the tubing is not wound, but cut to length, about 1 metre

- we haven't carried out mfi as the material is supplied with a certificate of conformity

- don't know about the morphology - good point

- there doesn't seem to be an extrusion batch related issue, but rather a material manufacturing batch related issue

I was rather hoping that Lawrence Alpert might be able to make some comment, too!

With regards, Bill

Rolf Wissmann wrote:

I was just fishing on this one. A flooded SSE should be more stable that a starve fed TSE. (It is flooded, right?)

Good to know. Water is not likely the issue.

I'm still interested in how you isolate the tension from the takeaway operation. Is this operation rock steady? How much variation would you need here to see the problem?

MFI would indict the whole lot, not just a temporary problem

Agreed.

I'm still trying to figure out why the problem appears and dissappears on such a short term basis. If a material lot is bad, wouldn't you be having the problem the entire time? It is conceivable that there could be an odd tail in the manufacturing lot, but I don't think that it would all be concentrated in your extruder all at once.

Let me clarify with a hypothetical example: say that a batch reactor for what ever reason produces a small amount of lower than normal MW resin in a part of the reactor. As the reactor is drained and bagged (or Gaylorded), that pocket will mix and disperse some with the normal resin. During transportation, the normal jostling will further disperse it. Passing the material through the drier will further disperse it, as will feeding it in the extruder and the extrusion itself. By the time the pocket hits the die, it should be quite disperse.

To me, the crux of your problem is: why is it a short term problem that corrects itself?

Agreed.

John Aspen Research, -

"Turning Questions into Answers"

Opinions expressed herein are my own and may not represent those of my employer.

Hey Bill, I'd like to ask you a few questions.....

Without knowing any better, it sounds as if you are having a process drift that you are not accustomed to seeing with other PA. Now Pebax can be a somewhat different animal, but it can usually be extruded with normal processing conditions and equipment. Sometimes an elongated feed section helps with a shorter transition (depending on machine size and RPM).

Can you give any specifics of your process? What size tube is this and what is your line speed, machine size, screw configuration and material moisture content?

Hi, John and Lawrence, Thanks for your replies. First, my colleague has pointed out that what I haven't made clear is that when the the outside diameter goes "out of spec", it is always a reduction in diameter - dimensional details later

- which is recovered over a period of four to five minutes. We think this indicates some sort of feed problem. Also we have seen the same issue on two different extruders, one in production and one in R&D.

The tube is 0.85mm +/- 0.03mm OD.

Both extruders are flood-fed single screw. The production machine is 25mm; 25:1 L:D ratio; 3.5:1 compression ratio plain screw. The R&D machine is 30mm; 24:1 L:D ratio; 3:1 compression ratio barrier screw.

In production the line speed is around 60m/min and in R&D 40m/min.

We don't have the moisture content, but we have dried the material according to the manufacturers instructions. In production we use a double dessicant drier and in R&D a single dessicant.

I agree about the dispersion/mixing of problem pockets of material.

Regarding the downstream, the problem has only occurred with one batch of material, and we have run three. Trouble is the problem batch is the one we have most of!

Material is Pebax 7033SA01. The batch details, from the R&D tests are:

Trials carried out on day 1: "Good" batch 192787 (made in France); material from open bag, and dried according to manufacturers instructions; screw speed 15rpm; puller speed 36m/min; head pressure 166 bar; Cp over 120 mins = 1.67 "Bad" batch 240602 (made in France); material used straight from new unopened bag without drying (as per manufacturers recommendations); screw speed 15rpm; puller speed 46m/min; head pressure 181 bar; Cp unacceptable, tube OD dropped to 0.78mm

Trials carried out on day 2: "Bad" batch 240602 (made in France); material from same bag as above, now open, dried according to manufacturers instructions; screw speed

15rpm; puller speed 45m/min; head pressure 179 bar; Cp unacceptable, tube OD dropped to 0.74mm "Good" batch 54489 (made in USA); material used straight from new unopened bag without drying (as per manufacturers recommendations); screw speed 15rpm; puller speed 36m/min; head pressure 142 bar; Cp over 120 mins = 1.43

Note:

- the extrusion screw speed is the same for all tests

- that the feed rate of the "bad" batch seems to be higher than "good", so we have to set the puller speed higher to achieve the same OD.

- recorded head melt pressure of "bad" material is higher than for "good".

- the poor performance of "bad" material is reproducible.

- the performance of "good" material is repeatable with more than one batch.

Lawrence, please could you provide any information concerning screw configuration?

Gents, your comments would be gratefully received, particularly as to any experience with the same or similar materials.

regards, Bill

L Alpert wrote:

Reduction in diameter suggest that the melt pressure to the die (forming box) is dropping, or melt temperature is decreasing, or resin viscosity is higher, or that the take-off equipment is speeding up for a few minutes.

My guess is that feeding problems cause variation in melt pressure. Are there a differences in fines (dust), "cube" shape, or surface coating of the "cubes" between good and bad lots? Something like a few drops of oil can also make life interesting.

Ernie

Since you have more output of one batch under the same conditions, you probably have a lower IV and higher MI of the "bad" lot (note the parentheses, as what you have is really "different", not so much "bad"). What you are seeing is probably normal variability in manufacturing process of the material and your extrusion. If you are going to try to lock in your process to a tight window (for instance, specifying line speed and screw RPM, ±5°, etc), you will most likely always run into problems, unless you can first identify exactly what you will need and then try to get it. Since Pebax is manufactured in a batch process, you may endanger your supply at some point (I have seen this happen too many times!) if you limit the raw material much tighter than the manufacturer. You are also using lots that are made at two different sites. Although I am sure that they have "duplicate" systems, we all know that "same as same" usually means "same with some differences".

Since you are seeing only periodic dropouts, you may want to adjust your solids conveying a little to compensate for the differences. If you want to fully understand these differences, you should run some comparison tests, such as capillary rheometer, DSC and GPC.

Pebax will run under a variety of conditions and with many feed screw designs, which you have shown using a barrier and a metering screw. It seems like you do not have far to go to sync in the supposedly "bad" lot. It may be something as simple as a rear zone change in the machine. This would least affect your extruded properties (usually a concern with balloons, not so much for catheter shafts).

If all else fails, install a transducer after the breaker and use a pressure feed back control to the extruder drive (I would recommend the Eurotherm

2704) or use a gear pump.