elasticity & extensibility of the melt

[Skip]

Viscosity----smoother flow out of die, better surface finish, faster extrusion more do-able.

Elasticity (& Extensibility)---stretching without breaking, as in blow molding, sheet'film drawdown, pipe wall thinning, etc etc.

Process aids can also improve homogeneity of the melt (at least with rigid PVC)

Downside for rigid PVC is that heat distortion (Vicat softening) is reduced a little.

[Tom]

Are not these properties improved in the view of the user?

Is higher viscosity or low viscosity better? Or is it some other dimension of viscosity?

[Len]

What Skip says regarding process aids in PVC is to the point. Melt homogeneity is a function your state of fusion, mixing of components prior to fusion, compatibility of formulation components, and the work accomplished by your extruder.

We had a rheometer that was equipped to measure melt strength. I can't recall the brand name. As the capillary stream exited the die, a pair of rollers (capstan) pulled downward on the melt strand. The force (torque) on the roller drive was recorded and higher torque indicated better melt strength. Extensibility.

Melt strength is a huge benefit in profile and extrusion calendering.

Len

[lingloke]

Dear Sirs,

Will processing aids behave the same in flexible PVC? I did come across that no significant improve in melt strength after adding PA. But if with Calcium stearate, the outcome is better. Why?

[Len]

Dear Sirs,

Will processing aids behave the same in flexible PVC? I did come across that no significant improve in melt strength after adding PA. But if with Calcium stearate, the outcome is better. Why?

Might depend on the PA being used. Or the level of usage for effective improvement in melt strength. PA's are used in f-PVC for fusion, melt strength, surface properties, etc. They work well in some alloys of NBR, ABS, and Urethane w/ f-PVC.

Len

[Skip]

In addition to Len's good comments, assuming your flex PVC contains a HIGH filler level, you might consider trying a
PVC/PVAc copolymer (i.e 85/15 PVC/PVAc). Copolymers can accept a higher filler loading than straight PVC----The highly filled vinyl homogeneous floor tiles years ago used copolymers. Just remember that copolymers are much more "zinc sensitive" than PVC when choosing a stabilizer system. Add more phosphite chelator in the stabilizer system to minimize "zinc burning".

[lingloke]

Dear Sir,

I never come across with PVAc. Please elaborate more on this. How copolymer can enhance the elasticity?
How to blend PVC and PVAc?

[Skip]

A PVC/PVAc copolymer is NOT a blend---it's made by co-polymerizing vinyl chloride and vinyl acetate in a reactor.
Ask your PVC resin supplier for his list of vinyl copolymers---whether he has an 85/15 or 80/10 vinyl chloride/vinyl aceteate copolymer. In flexible vinyls that have a high level of filler, a copolymer can handle higher levels of filler without losing properties than straight PVC.