Fluidity and Viscosity

A very important property for choosing a particular thermoplastic is the melt flow index, especially in the case of polyolefins and styrene. The higher the flow rate, the lower the viscosity of the polymer and vice versa.



But ... What is viscosity in a plastic?

Viscosity is the resistance to flow that the plastic faces when it is melted, ie if the plastic is too viscous it has difficulty draining through the nozzle of the injector or die of the extruder, as opposed to a plastic of low viscosity that will drain easily.

A simple example is to compare honey with water. The honey flows with difficulty, therefore it is viscous and consequently it has low fluidity. The water flows very easily, therefore it has low viscosity and consequently high fluidity.

The melt flow index in practice

The melt flow index (MFI) is verified in a device called plastometer, common in petrochemical laboratories and plastics recyclers. The result of this analysis is used for quality control and as a guideline for the process in which the plastic will be destined, with polymers with low melt flow being used in extrusion processes, while the medium and high are directed to injection molding.

The laboratory test does not accurately reflect what occurs during processing in a plastic injector, for example. This happens because inside the plastometer channel there is not the same shear that occurs inside the barrel of the injector, just as the plastometer has no screw but a piston; and its channel is vertical, unlike the horizontal injector. Besides those that I mentioned there are infinite variables ...

What I mean by that is that there is not much difference between resins with 38, 39 and 40 MFI, for example; but there is a lot of difference between resins with a MFI between 3 and 40. In the case of recycled, the MFI is very important to verify the uniformity of the batches, because if one batch presents fluidity 10 and the other 20 it means that there were big changes In the raw material used, which may cause some changes in process parameters of the transformer.

The table below shows the relationship between the melt flow index and the different transformation processes for PE and PP. Blue cells indicate adequate fluidity for each process:

RELATIONSHIP BETWEEN MELT FLOW INDEX* AND PROCESSES OF TRANSFORMATION
MFI

Process

Blow molding Film Extrusion Thermoforming General Extrusion Rotational Molding Injection Molding
< 3,5            
3,5 a 6            
6 a 42            
> 42            

* Test method: 190°C/2.16kg. Measure unit: g/10min

Can you increase the melt flow of a plastic? And reduce?

You can do both, but there are no miracles in the world of plastic ...

Well, to increase fluidity, nucleating agents and lubricants such as molybdenum disulfide or PTFE are used. Talc, for example, works as a nucleating agent when in proportions less than 10%, slightly increasing the flowability. But when in larger proportions than this can promote opposite effect, as well as other loads (glass fiber, polybutadiene, TPU, and etc.).

How to misunderstand a melt flow index

A common misunderstanding when reading the MFI in a datasheet is not paying attention to the method that was used in the test. Let's see the example below:

Basf Terluran GP-35 melt flow index


This is a part of the technical data sheet for Terluran GP-35, Basf's famous high-flow ABS, where the results of flow tests are presented in three different test methods:

* Plastometer at 230°C and 3.8kg of load
* Plastometer at 220°C and 10kg of load
* Plastometer at 200°C and 5kg of load

The result in each of the tests was 11, 34, and 3.1, respectively.

If you use this material, you called a piggy dealer asking for a alternative resin of 11 MFI, and they sent you one of 11 MFI by the 10kg method, I'm sorry to say you screwed up...

And as in the plastics world, "there are always complications", there are two ways to do the test: measuring the melt flow rate with the result in g / 10min, and measuring the volumetric flow rate with the result in cm³ / 10min as in the case above. The difference between the results of the two forms is not very significant for most thermoplastics, since they have a density close to 1g / cm³, but we must pay attention to denser materials such as polyacetal.

Bibliography:
HARPER, Charles A.; PETRIE, Edward M. Plastics Materials and Process: A Concise Encyclopedia. Hoboken: John Wiley & Sons, Inc., 2003.
CANEVAROLO JR., Sebastião V. Ciência dos Polímeros: Um texto básico para tecnólogos e engenheiros. 2.ed. São Paulo: Artliber Editora, 2002.
WIEBECK, Hélio; HARADA, Júlio. Plásticos de Engenharia: Tecnologia e Aplicações. São Paulo: Artliber Editora, 2005.
Article posted in May 9, 2017
About the author: Daniel Tietz Roda is Plastics Technologist graduated from the FATEC/ZL and Mechanical Design Technician from ETEC Aprígio Gonzaga, in São Paulo, Brazil. Roda worked 5 years with technical assistance and development of plastics in industries and nowadays is the publisher of this website.
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