PVC K value and MI

[Tom]

I appreciate the help I've been getting with PVC, and now I'm back for more.

Is there any correlation between K values and MI-V values? Even a rough correlation will help.

Specifically I'm looking for MI-V values for RPVC of K 56 to 59.

Thanks.

[Len]

MI, what the heck is MI? Depending on the dilute viscosity method your question can be answered as follows:

IV or Inherent viscosity ASTM D-1243, is related to K, see chart:

K/IV
56 / 0.66
57 / 0.68
58 / 0.70
59 / 0.72

Over the short span (above) this scale is linear.
y = 0.02x - 0.46
R^2 = 1

Len

[Skip]

Tom,

I'm not aware of MI (Melt Index) ever being used for rigid PVC compounds. In melt index testing, a given weight (volume??) of molten material at a given temp. is forced by plunger through a given orifice diameter in a set time frame ----and doing that to a rigid PVC melt probably results in shear burning. MI is typically used for polyolefins, which are fairly stable at melt temp/times of testing.

[Tom]

Amazingly there has always been a big valley between PVC and the rest of the plastics industry.

I have access to a lot of PVC Melt Index tests, all tested at condition V which is 20,000 grams at 175C (347F). This is 10x as much weight as many other polymers use. The results run from 0.1 to 10,000 grams/10 minutes.

I might be able to make my own correlation by looking up the K values of the grades listed, but I think a lot of the grades might be obsolete.

Are there some good repositories of PVC grades and K values?

Thanks.

[Skip]

Tom, I'd be interested in the types of PVC compounds you have MI data on---plasticized, or rigid??
K-value is a rating for PVC resin only, not compound. Since PVC resin alone would be difficult to melt and test via MI, a series of identical formulations, differing only in the K-value or intrinsic viscosity of the resin would be the only way I can think of to directly correlate MI to K value.

[Len]

Amazingly there has always been a big valley between PVC and the rest of the plastics industry.

I have access to a lot of PVC Melt Index tests, all tested at condition V which is 20,000 grams at 175C (347F). This is 10x as much weight as many other polymers use. The results run from 0.1 to 10,000 grams/10 minutes.

I might be able to make my own correlation by looking up the K values of the grades listed, but I think a lot of the grades might be obsolete.

Are there some good repositories of PVC grades and K values?

Thanks.

No, no, no! MI is a compound property, not a resin property! K value is a dilute solution viscosity of the resin, only! The word grades, refers to compounds; filled, clear, lead stabilized, tin stabilized, reinforced, flexible and rigid, impact modified...the list goes on! Each of these additives and their level w/in the compound affect (MI) compound viscosity.

You are only going to be able to make correlation in exactly the same formulation between what you call MI and the base resin K value. PVC compounds; being made up w/ multiple ingredients, are notorious for having widely variant flow properties w/in the same formulation. W/ regard to compound MI and it's correlated to K value of the base resin, unless all materials and component levels in the compound are carefully controlled, you're looking at a correlation not unlike the correlation between global warming and Al Gore's street sled.

I'd suggest the big valley (mental void) between PVC and the rest of the plastics industry might be in your head.

Len

[Tom]

Len,

I appreciate the insight.

Perhaps you could have left out the insult. For a "know it all", you did not even understand my reference to MI. I've been involved in plastics processing for 30 years, and I will admit there is plenty I still don't know, but I'm willing to learn.

For the others who helped,

I appreciate the insight provided without insult. I was not trying to upset anyone by stating that I thought there was some divide between PVC and the other polymers. I is just and observation.

With regards to melt viscosity, it is nice to know the viscosity of the material you are working with in order to design plastic processing equipment. It would seem that PVC is capable of being measured in a melt viscometer because I have plenty of that data. Whether degradation occured in the testing is unknown. MI is essentially a single point within that data.

I already understood that the K value is an indication of the molecular weight of the polymer. Molecular weight distribution is not indicated. I had no feel for how the flow might be affected by fillers and additives, but apparently it is significant. With just a K value to work with, which apparently has little to do with the flow properties of a compound, how does anyone design equipment?

[Louis]

Tom,

Do not mistake passion for insult.


Louis

[Skip]

Tom, perhaps the best (in my opinion) overall indicator of a PVC compound's viscosity, flowability, fusion(gellation, and processing stability is the torque rheometer (Brabender) curve.A Type 6 Mixer head with roller blades works well for PVC. After fusion, a higher or lower melt torque (in meter/grams at the given Rotor RPM, Temp. and sample wgt.) is a direct indicator of melt flow--viscosity. With powder blends, the time to fusion and peak torque at fusion gave an indication of lubricity. On a separate test with the same compound, we'd stop and pull small samples for color comparison as degradation proceeded. Many processors use the Brabender as a QC tool in their blending operation.
In my experience, we usually had a customer's given machine sitting in front of us, and had to design the PVC compound to run optimally on that machine, tweaking lube balance,stabilizer,process aids---never had the experience the other way around!!

As to the "big valley" between PVC and other plastics processing, I don't see that much of a problem as long as the processor knows his molten PVC in the machine at "x" degrees has 10,15, or 20 minutes' stability time before onset of HCl and "sinus clean-out time"!! No time for a coffee break here!!

[Tom]

The thinking and techniques in PVC seem foreign to those used in engineering polymers and polyolefins, where I spend most of my time. This is not to say that one is good and one is bad. But there are big differences. This is why PVC guys fail when presented with Nylon, and Nylon guys won't touch PVC. This does create a divide. The same goes for rubber vs. plastic.

What is the point? When I try to use engineering plastic thinking when working with PVC there is a communications problem. Should I just stay out and let you PVC guys handle it yourself, perhaps. If there are any good PVC single screw designers who can design a screw for a compound where the only specification I have been given is a K value of 56 to 59, please call.

For those who question my comments on a divide, please show me the common ground outside of it will flow out of a die.

In engineering plastics we design the equipment to fit the polymer.
We measure the shear viscosity directly. This allows for accuarate design of the equipment.
We don't usually have to worry about clearing out the plant.
Little dead spots are not a problem.
We melt the polymer.
We don't usually have to heat the polymer while blending.
We use single screws and co-rotating twin screws.

No judgements here, but I do see differences.

[Louis]

Not to digress to far from the topic, but I've cleared out a plant on more than one occasion with engineering polymers. Take Acetal, early in my career I raised the barrel temperatures on a 2 1/2" machine thinking that I could bake the screw clean as I routinely did with nylon.

I never had that problem again.

We routinely use rigid and flexible PVC in both our extrusion and molding operations and I can't recall ever having an issue. Like any resin, you need to treat it with respect.

[Tom]

Louis,

I did say usually.

I think that you are at some advantage becasue you are likely not making the PVC compounds at the same time you are extruding.

[Louis]

Louis,

I did say usually.

I think that you are at some advantage becasue you are likely not making the PVC compounds at the same time you are extruding.

Thank goodness for that!

Though I used to make color concentrates and radiopaque filled compounds, many that were PVC, at the rate of 60,000 pounds per year for medical devices. We then extruded various catheters and multi-lumen products using those compounds. I must say that is nice to have in-house capabilities like that to control quality.

[Skip]

Tom, upon further reflection, I think you do make a good point re PVC vs. "the rest of the polymer industry". I'll try to cite a few specifics I'm aware of----

1)- Most other polymers can be extruded or molded "as is" from the producer, perhaps with added color concentrate &/or filler or glass. PVC resin cannot be processed without formulating and compounding in lubricants,stabilizers(specific to process and end use), plasticisers, process aids. Color, fillers, glass also can be added as well.

2)-PVC evolved out of the rubber industry (most early producers were rubber companys--Goodyear,B.F.Goodrich,Uniroyal-U.S.Rubber,Firestone,General Tire. Rubber formulators and compounders were notorious for developing complicated formulations with a host of additives--antioxidants, curatives,lubricants,carbon blacks, etc.

3)- Most other polymers-Engineering as well as Olefins- evolved out of the petrochemical industry and other chemical companys. Most of these resins contain perhaps an antioxidant package, UV absorber, FR system, a minimal amount of total additives--and can be processed directly from the producer.

4)-Depending on how it's formulated, a given K-Value PVC could have a widely divergent set of fusion(melting),flow, and lubricity properties (all well seen in a Brabender curve). A "store-bought" compound producer (a Poly One,Georgia Gulf,ec.) might be able to furnish a single viscosity value for his compound, but not sure what it would be---don't know if can K values be determined on a compound?

5)-Yes, I'll agree there is a difference in thinking between PVC and other resin processors.There is also a difference in plant personnel thinking. I can recall several situations of plant-wide power failure, and watching the Chinese Fire Drill as everyone scurried to strip down pipe dies before onset of catastrophic degradation! As to extrusion screw designs, since the vast majority of PVC extrusion is via counter rotating twins--either conical or parallel--the main single screw development I'm aware of has been longer and longer L/D's, from 24:1 out to 30,32,36:1 L/D's, with the longer residence times allowing more mild temp./RPM conditions so as to run almost like a low shear twin. Carlon's (Lamson & Session)PVC pipe/conduit was made this way and they ran very low cost formulations.

I guess it's a question of preferring Fords or Chevys!!

[Len]

Tom,

I'm not a know it all. I understand your use of MI, and it is entirely inappropriate for describing PVC molecular weight. Dilute solution viscosities are used for this purpose in PVC resins. I tried to show a simple correlation above between various dilute solution methods.

The only exception I've seen for the use of MI in PVC is in formulation specific situations. Situations where two parties; supplier and user agree that the melt flow of a specific compound, tested under specific conditions of time and temperature, orifice size and pressure is applicable to an agreed upon assessment of quality or fitness for processing of that specific material.

My understanding of MI (melt Index) is that it's a term specific to PE resin. It's use in other resins is a miss-use of the term, and might better be called MF=Melt Flow in these other resin systems.

Sorry you were offended, none intended!
Len