posted July 18, 2003 07:55 AM               

It is very interesting question recently i explained about this in detail in the www.polymerlinks.com discussion forum. Please click the following link it will directly take you that topic. I hope you will get the information what you needed.
http://www.polymerlinks.com/discussion_topic_list.php?id=2

With Best Regards

posted July 18, 2003 11:44 AM               

If you have a large volume of what you may believe are gels, it is quite likely that you are actually getting unmelted or cold resin spots through the die.

posted July 18, 2003 03:09 PM            

We tore down our die cleaned it. Got rid of a kenics mixer under the die. Had our screw measured and found it had about .045" wear! Now it's rebuilt.

Once and a while I still see these when we first start up. Any thoughts?

P.S. 6" barrier 30 to 1 with maddocks on end.

Tom C: I work with RM from your days at Lehigh

posted July 18, 2003 03:55 PM               

Welcome to the single screw extrusion forum.
You might like to take a look at an article by Chris Rauwendaal on "gels" http://www.plasticstechnology.com/Scripts/SP- MainPT.asp?file=d:\plasticstechnology.com\wwwroot\articles\200203ts1.html&qt=%20Gels

A couple of years ago, we experianced a severe gel problem- tried every thing to cure it (we had run the grade in question for eight years prior to this without problems). Even had the manufacturers top tech onsite for a day.

At the end of the day, after he had tried every thing we had, and the sheet still had problems, he conceded defeat and we went an found a couple of cold beers. As we drained a couple of "thought cyclinders", he remembered that their supplier of stabilizers had changed the formulation just before the batch we had problems with. The change was minor, and we seemed to be the only company have problems as a result of it.

Tom Butler at Dow had produced some papers on gels, you could try getting a copy of them.

Hi Mark, welcome to you too. Static mixers are well known gel generators, as is screw/barrel wear. You might want to look at your shut down/purge procedure to nail the start up problem.

Steve H

posted July 22, 2003 08:54 AM               

You can contact him at the SPE at Tom Butler

posted July 22, 2003 10:29 AM               

I heard a little about your problem before your post. I'm not sure of the chemistry, but it seems that TIO2 can cause reactions that can produce crosslinked material as you describe. If you ran a steady process you would probably never notice any problem. But changing to different resins disturbs the dead spots and things get pushed out (even if they should not fit).

I have seen worn barrels where very hard materials build up on the barrel and fill the gap. This could be one source.

Getting rid of a static mixer is a good idea. Theoretically they should work great, but in practice I don't like them, too many potential dead spots that don't empty when the extruder is shut down.

Barrier screws can be difficult to purge, and Maddock mixers are known to have dead spots relating to design and viscosity. Both design components can have problems when switching between resins.

Good luck. Nobody said extrusion was easy!

Tom C

posted July 22, 2003 01:33 PM            

Besides the screw rebuild, pitching the kenics, and die clean up we have done the following:

Before start up with rebuit screw we quenched extuder with process aid. Went back to prior purge compound from Union Carbide and add Ampacet antioxidant when purging.

We are still going to try higher loaded Tio2 with LDPE carrier resin. Working with supplier on that one.

posted July 22, 2003 04:14 PM               

Also, I have seen gels in blown film where they were caused by contamination- in this case there was a cellulose particle and the PE clumped around it. This can be caused by other contaminants as well, including dirt and carbon. Turned out in this case that to be some gaylord material (kraft paper, like from corrugated cardboard is all we actually could tell for sure), we could see this under a microscope (confirmed by a testing house). We were amazed at how many gels had these particles.

And in blown film if you have die lip buildup, the material can sit on the die lip for hours or days or weeks just cooking and cooking, eventually it gets attached to the bubble and looks like a gel even though it's on the surface. If this is happening, cleaning the die lips more frequently can help, so can other parameters like die gap, land length, plating condition, and plating material.

Also, a lot of resin suppliers have been cutting back on antioxidant levels in resin grades that previously had plenty. This is somewhat due to improved reactor technology that doesn't require the oxidation protection that used to be needed. That safety cushion is now gone for the blown film manufacturer, unless you specify it.

posted July 22, 2003 05:32 PM               

Purging and changeover have been involved topics in several forums in the past. You may want to search the records.

What equipment are you disconnecting from the extruder when purging?

posted July 23, 2003 03:28 PM            

The first determined the component to be LDPE with Tio2 which had crosslinked. A sample of the purge compound we used at the time was provided and determined not to have any purge components in it. They used DSC (differentential scanning calorimetry) for testing.

The second lab using FTIR spectroscopy, DSC, and TGA (thermogravimetric analysis) found oxidized polymer and 6% Tio2. The oxidized polymer was primarily LDPE with a small amount of LLDPE. Remember my earlier post indicated a LLDPE carrier for the white concentrate. They summarized the large gels were caused by build up of oxidized polymer in dead zones of our extruder and suggested we add antioxidant during the shutdown process. Also recommended was the use of process aid to prevent die build up. We have for several years and continue to use process aid when extruding film.

No other contaminates such as cardboard, kraft paper, or cellulose in these gels. I also have a microscope and have been persistant looking at these "boogers". Our resin is delivered by train and loaded into silos. Our concentrates do come in plastic lined boxes however.

We have pulled the die pin more times than I can count and cleaned it as well. This does not include the two times we pulled the entire die down this year, in which the first time we dumped the kenics below the die. All metal looks great after cleaning.

One other thing I should mention is that our operators average 30 years of experience. The extruder is a 13 year old Gloucester.

Purging.

I have read every post in here on purging and have searched endlessly on the topic. Maybe not enough. We have done chemical purges, disco purges with HDPE....you name it.

Our 6" screw is about 17 feet long motored with 400hp. We dump 200 pounds of Union Carbide purge with a five gallon bucket of antioxidant raising and lowering RPM in disco fashion. It was recommended to us to do this at operating temperature. If we shut down for a few days the extruder sits cold. Before start up temperatures are brought up to 300f. Extruder begins to idle as the purge exits the die. Resin added when temps are met for production.

We don't disconnect anything from extruder when purging. It's a big rascal with the top of the die about seven feet off the floor. Trim grinder is off and that's about it.

posted July 23, 2003 10:30 PM               

My concern was that you purge the extruder with a high viscosity purge. Then the die assembly is flushed with low viscosity production resin, which usually has trouble forcing out the high viscosity purge.

That said, your not finding purge as a contaminant.

Seems that you are getting buildups of the TIO2-LDPE combination in the dead areas. While the process is running it is likely these dead spots are stable and not disturbed. When you change resins, or purge, or startup, or shutdown the dead spots are disturbed and some of the contaminating material moves through the die at various speeds.

I think you need to attack from multiple angles:

1) Less dead spots
2) Less oxidation in the resin.
3) Less reactive TIO2 (I think there are coated grades)
4) Better purging and changeover procedures.

Do you have a screen changer installed?

Tom C

posted July 24, 2003 08:42 AM            

I meant to include screen pack in last post. Currently 40,60,80,60,40.

Your multiple angles.

1. Less dead spots. We removed the kenics and fixed the screw. We also had plugs put in where the kenics fit in tube. Where else can we go without redesigning from feed throat to die. Got to consider cost factors too. Why no problems for the last twelve years?

2. Less oxidation in resin. Sounds easy...increase antioxidant.

3. Less reactive Tio2. I'm not sure if ours is coated, but we have always specified the most heat stable grade.

4. Better purging procedure. I'm open. Somone else want to chime in with what works best for them?

posted July 24, 2003 11:24 AM               

Do you think all your gels are related to the TIO2-crosslink problem?

Do you find material on the screen pack which seems related to the gels?

Do you get gels when running white? Or just clear after white?

Fractional melt materials tend to have different processing properties. They stick to metal less, have more of a plug flow through the system, and are less reactive.

posted July 24, 2003 11:46 AM            

All gels have Tio2 in them.

It looks like it may also be on the sceen pack. Maybe I shold test for crosslinking as well.

posted July 31, 2003 01:38 PM            

Received article from Tom Butler at Dow - good read. Also found good article on internet from South Africa. Written by Henk Lourens, Senior Scientist at Sasol Polymers.http://www.pisa.org.za/Henk%20Lourens%20paper.doc

If I can get details on Tio2 and can put info out I will. As we work through this problem I plan to also follow up with what ends up solving this issue.

posted July 31, 2003 03:18 PM            

Tio2 coating - proprietary
particle size - proprietary

sorry, I tried.

posted August 02, 2003 07:23 AM               

Me thinks your sales agent doesn't simply know, or it may also be that what they are selling you is off-grade TiO2, and don't want you to know it. This kind of information should be on their data sheet.

Call their Technical Manager.

You, if anyone has a right to know.

Send them a whopping bill for your lost time, sales loss's, and trouble shooting.

posted August 04, 2003 11:39 AM            

After the gels appeared we went back to 50% white for two months and then qualified a 70% Tio2 white with LDPE carrier. Same melt index resin, same grade Tio2, same supplier, only differnce is % Tio2 in masterbatch.

We run basically two materials here.
1. Straight LDPE homopolymer in either clear or white film. Gels are predominant when we run this.
2. A blend of EVA copolymer LDPE (majority) with LLDPE.