Science question... can you answer it?

Transform_IT · 26 Jul 2017 at 15:59

So, in Layman's terms...

I am wanting to know or have a better understanding of what melt viscosity is. It is a test method used on polymers...

Google says:

'Melt Viscosity measures the rate of extrusion of thermoplastics through an orifice at a prescribed temperature and load. It provides a means of measuring flow of a melted material which can be used to evaluate the consistency of materials, or determine the extent of degradation of the plastic as a result of molding.'

So my understanding is:

The melt viscosity is tested on the polymer to distinguish the consistency and degradation of it. The test is basically looking at how the polymer flows when it reaches its melting point. This will give a reading on how much pressure is needed to extrude the polymer from the orifice but after that I am lost.

Is this a true statement or am I missing some vital information?

Can anyone explain it in an idiots guide way for me please?

Cheers,

T_IT

exhashdotdot · 26 Jul 2017 at 16:02

Sounds pretty much exactly as is described.

It's the viscosity of a melted plastic. A ~~low~~high viscosity means it's gloopy like tar whereas a ~~high~~low viscosity means it's watery, like water. (I always get these mixed up).

The distinction between a thermoplastic and a normal plastic is necessary because most plastics burn when they're set on fire. Thermoplastics melt and flow. Hence why their viscosity might be of interest to people.

I'm assuming that you can work out what type of polymer it is, based on how viscous it is at a specific temperature. Hence it can be used for characterising a polymer.

Transform_IT · 26 Jul 2017 at 16:08

exhashdotdot said:
Sounds pretty much exactly as is described.

It's the viscosity of a melted plastic. A low viscosity means it's gloopy like tar whereas a high viscosity means it's watery, like water.

The distinction between a thermoplastic and a normal plastic is necessary because most plastics burn when they're set on fire. Thermoplastics melt and flow. Hence why their viscosity might be of interest to people.

I'm assuming that you can work out what type of polymer it is, based on how viscous it is at a specific temperature. Hence it can be used for characterising a polymer.

Brilliant, but how is the data interpreted and plotted? I know they have a graph... is it with timings or is it viscosity vs shear rate?

Shear rate is something I have trouble understanding.

touch · 26 Jul 2017 at 16:09

Transform_IT said:
The test is basically looking at how the polymer flows when it reaches its melting point.

I would say you're correct, but rather than a "melting point" like where solid ice becomes liquid water, these plastics have a transition period where they have different viscosity at different temperatures.

touch · 26 Jul 2017 at 16:09

Transform_IT said:
Brilliant, but how is the data interpreted and plotted? I know they have a graph with seconds vs ?

Temperature vs viscosity, i'd guess?

edit: the time (secconds) and load you mentioned wont be on the graph because they are only used to measure the viscosity.

exhashdotdot · 26 Jul 2017 at 16:10

Transform_IT said:
Brilliant, but how is the data interpreted and plotted? I know they have a graph with seconds vs ?

I've not really investigated the viscosity of polymers, but I'm assuming they'd be more interested in Viscosity vs Temperature than Viscosity vs Time (at constant temperature).

Transform_IT · 26 Jul 2017 at 16:21

Transform_IT said:
Brilliant, but how is the data interpreted and plotted? I know they have a graph with seconds vs ?

I know in most cases the temperature is a set parameter and is a constant figure. The figures that will change are viscosity, pressure, time, shear rate. How it is plotted... I am unsure.

rubberduck · 26 Jul 2017 at 22:40

Well a polymer melt will be non-newtonian, I suspect it will exhibit shear thinning behaviour so the harder you shear it the less viscous it becomes. The extent of this will be related to the molecular weight, chemistry and degree of branching of the polymer chain but will essentially come down to how the chains slide past each other and untangle as they are sheared. I suspect the data will be plotted using a standard viscosity as a fucntion of shear rate axis with multiple data series representing the temperatures of interest i.e. you might have series 1 be at 350 deg C, series 2 at 360 deg C, series 4 at 370 deg C and so on....

Why is this information important? I would guess it is mostly used in injection moulding whereby you have two variables to play around with when injecting your polymer melt, temperature and shear.... High temperature will make the polymer flow easier (less pressure to inject) but is more costly due to the heating duty and the risk of thermal degradation of the polymer... Lower temperature will mean more pressure is required to get the polymer in to the mould but it is more cost effective. If you go too low in temperature though you probably risk clogging injection nozzles etc. and run the risk of the polymer not behaving like a liquid (it will cool when it hits the mould also) thus creating bubbles or voids in your product.

The above is mostly educated speculation, I know a little bit about rheology but have never worked with polymer melts or injection molding.

LOAM · 26 Jul 2017 at 23:02

To me it reads that the only thing that is measured is the time taken to extrude the polymer through the orifice. The orifice, load and temp are all fixed.

aardvark · 26 Jul 2017 at 23:05

rubberduck said:
Well a polymer melt will be non-newtonian, I suspect it will exhibit shear thinning behaviour so the harder you shear it the less viscous it becomes. The extent of this will be related to the molecular weight, chemistry and degree of branching of the polymer chain but will essentially come down to how the chains slide past each other and untangle as they are sheared. I suspect the data will be plotted using a standard viscosity as a fucntion of shear rate axis with multiple data series representing the temperatures of interest i.e. you might have series 1 be at 350 deg C, series 2 at 360 deg C, series 4 at 370 deg C and so on....

Why is this information important? I would guess it is mostly used in injection moulding whereby you have two variables to play around with when injecting your polymer melt, temperature and shear.... High temperature will make the polymer flow easier (less pressure to inject) but is more costly due to the heating duty and the risk of thermal degradation of the polymer... Lower temperature will mean more pressure is required to get the polymer in to the mould but it is more cost effective. If you go too low in temperature though you probably risk clogging injection nozzles etc. and run the risk of the polymer not behaving like a liquid (it will cool when it hits the mould also) thus creating bubbles or voids in your product.

The above is mostly educated speculation, I know a little bit about rheology but have never worked with polymer melts or injection molding.

most polymers are newtonian.
but to answer the op, basically he is right.
the viscosity will change with temperature but it will be a balance between the best flow and the amount of degradation (at higher temps). you could also use the viscosity as a measure of the amount of degradation, as it will obviously change dramatically once the temp gets too high.

rubberduck · 4 Aug 2017 at 10:24

aardvark said:
most polymers are newtonian.

This is untrue, most polymer melts exhibit viscoelasticity as do aqueous solutions of polymers.

aardvark · 4 Aug 2017 at 11:54

rubberduck said:
This is untrue, most polymer melts exhibit viscoelasticity as do aqueous solutions of polymers.

fair enough - i haven't done much polymer work for at least 20 years!

Sliver · 4 Aug 2017 at 12:21

I used to work in a plastics factory. We started off by making resin, (the main ingredient in any plastic) then various additives were mixed in. Things like waxes, lead powder, powdered dyes and a few other chemicals. Once mixed, the product was fed in to extruders. The extruders melted the mix and a blade at the end of the extruder chopped up the mix in to plastic pellets, which were then packaged ready to send to the customer. The melt viscosity depends on the mix used to produce the plastic, but simply it is the temperature at which the compound can flow.