Discolouration of cable

[lingloke]

Dear Sirs,

We face discolouration and diamension problem on PVC cable extrusion.
We are using Ca/Zn based PVC compound. When compare to lead base PVC, we have to process at lower temperature and speed.
If we run at high speed, the colour tends to fade, and temperature of metering zone will increase about 10C. The thinckness of coating will be inconsistant.

Operating temp: 130-140C

How to improve in term of machine setting and formulation.

[Len]

Not really enough information to give anything but an educated guess, but w/ regard to color I'd say you are seeing the effect of non-lead stabilizers on tint strength. The lead stabilizer acts as a pigment, while the non-lead does not! So color changes that were masked while using the Lead are now more visible w/ the in Ca/Zn stabilizer. Or there is a possibility that you could be under stabilized; non-leads are not as robust in some formulations as lead stabilizers. Leads are more efficient.

As for dimensional stability, I believe you are experiencing some lubricity problems due to the switch from lead to Ca/Zn. Have you adjust your entire formulation to maintain fusion properties or did you just pull the lead and replace it w/ Ca/Zn? Non-leads can replace lead based stabilizers, but effect or change fusion characteristics; time, torque, and temperature. You may need to look at your entire formulation and especially your lubricants.

Len

[Len]

Since, you have taken the time to reply and resurrect this old post... I'll continue to expand on my original reply.

It’s difficult to say what the root cause is, based on the limited description of the process, compound formulation, and the limited observations... any of which may be unrelated to the color shift. It would also be helpful if the original poster (OP) gave some feedback based on the suggestions given thus far.

Several things stand out in the OP's condition statements;

1.) He is comparing two compounds; non-lead and lead based stabilizer, and he give no indication that the formulations were altered to compensate for the switch from lead to non-lead.
2.) He is observing a color shift or color fade in the non-lead compound. And, he is seeing a temperature increase at the metering zone compared to the lead stabilized compound during processing.

I still believe, based on the limited information given that the non-lead compound is the cause of both the color and processing issues. First, it's the only thing that's changed in his process. And secondly, i have seen these same issues in many lead to non-lead conversions, especially where the tinting strength of the lead is not considered and where the superior stabilizing properties and unique lubricating properties of the lead compared to the non-lead are not addressed.

Lead has tinting properties similar to TiO2. And unless the non-lead formulation is adjusted for the lack of tinting or lower whiteness contribution of the non-lead, then the original color match is doomed.

The processing issues are either related to these lubricity or stability differences, and can only be solved by a re-balanced formulation. Fusion studies using a Brabender or Haake can go a long way toward understanding these fusion, stability, and lubrication dynamics.

Len

[lingloke]

We have tried many ways.

Ca/Zn one pack was in powder form. We started with similar dosage as lead in the formulation (~4.5phr), then slowly increased to 5.5phr after seeing the fading issue.

We were using twin screw compounding machine, with ventilation and high speed mixer. Initially, we focused on venting system on compounding stage and "under stabilized" in the formulation. But we found no voids trapped on the pvc slab that prepared from pellets and the colour was similar to the standard. So we were very sure no venting issue. And the thermal stability has improved after adjusting the dosage.

Discoloration only happen during cable extrusion. Colour will change to lighter shade after 1-2 hours run. And we can see voids on the cross-section of cable coating.
We found that, the colour cable will be more consistent with hopper dryer (~90C) and lower extrusion rpm.
And we re-sampled the pellets to make into pvc slab, this time the slab was full with voids.
This rang a bell to me that could it be the pvc pellets pick up moisture from the environment? Why this never happen to lead base PVC compound? If this is true, what will be the solution?

[Len]

Appreciate the update, thanks. From your latest post, I assume that the voids are now the major issue, not color shift. The color shift may have been addressed by using 90C hopper dryer and slowing the extruder. My thoughts follow, I hope they are helpful.
Moisture pick up by pelletized f-PVC is not a common problem. The polymer is not sensitive to hydration during storage, unless it gets physically wet; rain, or water soaking in to the storage hopper, gaylord, or storage container, residual moisture from under water pelletizer, etc. If the pellets are wet, it’s probably not the polymer or compound absorbing moisture from the atmosphere. So, external source is responsible, in my thinking.

Dry blending could be a possible cause. Some raw materials are prone to moisture pick up and will not release this moisture if the dry blend is not sequenced (additive sequence at the mixer) properly or not taken to a high enough drop temperature.

I would think that if this is the case, the first extrusion would be where the voids appear. You do not mention voids at the pelletizing stage, but during the second stage extrusion. So, I think you’re dry blend is not to blame. Finally, I’d look into possible voids at the first stage only if you suspect the dry blend process.

Voids are either moisture, as you suggest, or they are from some other source, instability or volatilization during processing. I assume you are compounding and pelletizing, and then wire coating in a second step from your limited description, thus the stabilizer must protest the PVC through two heat histories to maintain color and stability on the finished cable. I still believe that the previous lead system was more robust than the non-lead. I would not rule out a stability issue w/ the non-lead as a possible cause.

However, the non-lead system may be susceptible to moisture pick up, where the lead based was not. I’m puzzled by this behavior and think the pellet is getting physically wet from some outside source, other than absorbing moisture from the atmosphere.

As a note, I hate one-pack stabilizer systems. They are difficult to tailor to specific needs; process or application wise. If you change the level, you automatically change all ingredient w/in the one-pack. Thus, adjustment of lubrication and stability are confounded.

Good luck, and please keep us aware or your progress.

Thx,
Len

[Louis]

For clarification,

You say that you saw no voids in the test run on the formulation, but then go on to say that the discoloration only happens during cable extrusion. Then, when you resampled the PVC using the same glass slide test method that you saw voids indicating the resin was wet, but this was after a few hours of running.

So,

Are you saying the resin ran well for the first few hours and did not discolour until later in the run?

And are you saying that only the resin that tests wet discolours?

[lingloke]

Yes. No problem for compounding, packing and storage. No chance that the pellets will get wet.
The compund was kept for ~ 1mth at store before test run on cable.

Initial cable run, the colour of pvc coating was as per standard. After 1-2 hours run, the melts came out from die started to be lighter and lighter. Though of overheated, I reduced temperature at metering zone. Slight improvement, but still saw voids. Then turned on hopper dryer, the colour was more stable.

Re-sampled immediately on the similar batch of compound and found that the PVC slab was full with voids. At the same time we did test on PVC lead based compound (kept at the same storage area),no voids!!

Possible that a kind of reaction happened on the compound (Ca/Zn), that casued the adsoprtion of water molecule to the compound?

[kashif_690]

Dear Sir,

I am getting a problem of moisture ingression after storage a PVC Cable compound for more than a month. we are using Ca/Zinc stabilizer BP MC 9659 and 8656 from Baerlocher.The TDS of both stabilizer mentioned that "it may get moisture in finished goods".

The fresh PVC Compound runs at high speed extrusion line with out any surface problem and no voids observed.

The moisture problem occurs usually in summer season like May - Augsut when humidity is very high. We can not store PVC Compound for more than a month otherwise during extrusion at high speed surface finished gets rough and voids appear on the insulation/sheathing or cable cross section. this reduce our productivity.

Do you think that the Lead base stabilizer is more effective than Ca/Zn and less efffected by moisture?
Please find below the recipe of Ti-1 CABLE COMPOUND:
PVC -67s 100kgs
CaCo3 62.5kgs
Ca/Zn -9659 5Kgs
DOP 30kgs
CPW 18kgs
PE-Wax 0.1

Your technical support will be highly appreciated in this regards,

Thanks
M.KASHIF

[Len]

I've been asvised and understand that calcium carbonate can pick up moisture during storage (hopper or bag). This is usually dissipated during dry blending, higher dry blending drop temperature and dwell time are needed to drive off this slight residual moisture. Its' my understanding that good dry blending eliminates this slight moisture content, initially and usually during storage.

However, it arppears that your compound is heavily loaded w/carbonate. In this case, your compound may also show a greated tendance to pick up moisture from the atmosphere, especially in time of high humidity.

Since, running "fresh" compound seems to solve the void problem, I conclude, it's either your stabilizer is hydrolizing over time. Or, I suspect the large loading of carbonate, although not usually a problem at lower loadings, is picking up moisture during storage and causing this problem.

Len

[Louis]

Is the world not switiching away from Lead?

[kashif_690]

Dera Mr. Louis & len,


Thanks for your technical assistance and detail answer. In pakistan most of PVC Compound loaded with high calcium carbonate to reduce the cost.

What i feel that the major cause of moisture absorbance is Ca/Zn stabilizer, which may get detoriate after the several time. do you think that lead stabilizer will resolve the moisture problem? or we needs to add any other ingredient to protect from moisture absorption?


Your earliest response will be highly apprecaited.
Thanks and regards,
M.KASHIF

[kashif_690]

Dear Dr. LEN,

Please find below the sequence of Mixing of PVC compound at high speed mixer:

PVC Resin + stabilizer+PE-Wax @ ambient temperature
Plastizicers (DOP+CPW) @ 65C
Calcium Carbonate @ 90C
Exhaust vent start at temperature 65C
Compound discharge in color mixer at 110C.

kindly reply reagrding the efficiency of lead stabilizer Vs ca/zn stabilizer.
I am working to switching our to Lead stabiziler to overcome moisture problem.

Regards,
M.KASHIF