The melt flow index

The influence of moisture content in polyamides

The melt flow index measurement in some polymers is remarkably influenced by both moisture content and additives. Hence the need to quantify such influence, and consequently to adopt a reliable analytical method

Authors: *Radici Group, **Dip. di Ingegneria Meccanica e Industriale, Università degli Studi di Brescia

The use of polyamides (PA) by manufacturers of items made of plastic materials is encouraged by their versatility and flexibility, good chemical resistance, stability over operating temperatures, good mechanical properties, and the possibility of extremely easy recycling of such polymers. These features allow PA to be used in various application fields such as, for instance, the manufacture of coils, commutators, switches, transformers, motor casings as well as products for domestic use. In the automotive field, PA is used in the manufacture of underhood components such as radiator parts, motor covers, expansion tanks, connectors, etc.

The wide range and the different types of PA available today on the market allow to create a sort of “identity card” of the product thus meeting the increasing need for qualification of use. The properties characterizing the polyamides and allowing to draw up their technical data sheet are contained in ISO 1874-2 [1]. In addition to the classic physical-mechanical, thermal and electric properties, those indicating the degree of flowability of molten polymer are extremely important for the plastics converters.

For this purpose, the MVR (melt volume flow rate) or MFR (melt mass flow rate) measurement according to ISO 1133 [2] is widely used today for many thermoplastic materials. In polyamides, however, and generally in all moisture sensitive products, problems arising from this kind of test may hinder its application.

The melt flow index

The melt flow index test is used all over the world in the thermoplastic polymer industry and seems likely to remain the primary instrument for the product control and quality guarantee. This kind of measurement meets the need for a fast product characterization which allows both quality control and processability evaluation through the determination of a material melt flow coefficient. However, one of the limits of this method, as indicated in ISO 1133-1 application field, is the difficulty in obtaining data characterized by an acceptable repeatability and reproducibility level for polymers sensitive to moisture inside the material, under test temperature.

Polymers that are moisture sensitive in their molten state, as for instance poly(ethylene terephthalate) (PET), polybutylene terephthalate (PBT), polyamide 6 (PA6), and polyamide 66 (PA66), are characterized through the determination of their intrinsic viscosity [ISO 1628-5 [3] and ISO 307 [4]] that reflects the molecular mass of polymeric material. In this kind of test, polymer is dissolved in special solvents (e.g. phenol/1,2-dichlorobenzene, phenol/1,1,2,2-tetrachloroethane, phenol/2,4,6-trichlorophenol, formic acid, sulphuric acid) making the intrinsic viscosity measurement a potentially dangerous and expensive procedure. In addition, the intrinsic viscosity measurement is not always representative and/or feasible for a “compound” containing reinforcement, filler or modifying agents.

As already mentioned, the melt flow index test according to ISO 1133:2005 does not adequately cover the measurement of polymers obtained from polycondensation reactions, i.e. polymers (e.g. polyamides, polyesters, etc.) with a synthesis reaction involving water formation as reaction product. In fact, if this kind of polymer having a different moisture content compared to the one determined by the chemical balance is heated to a kynetically favorable temperature – as for instance the typical processing temperatures of these polymers – different chemical reactions may occur depending on the relative moisture content, i.e.:

  • with a moisture content lower than that of chemical equilibrium: a post-polycondensation reaction with water formation and consequent molecular weight increase of starting polymer may occur; or
  • with a moisture content higher than that of chemical equilibrium: a depolymerization reaction with consequent water consumption and inevitable polymer molecular weight decrease may instead occur.

In both cases, the chemical reactions described above involve a molecular weight change and consequently a change in the viscosity of molten polymer. Since the measurements of any melt flow index (MVR or MFR) are carried out at temperatures that are kinetically favourable to a.m. reactions, the results obtained without considering the moisture content of the polymer being examined (and the possible presence of polymer additives which may interact with the macromolecules) may be not sufficiently representative and characterized by poor repeatability and reproducibility.

How to carry out the melt flow index measurement

The melt flow index method is based on the measurement of the material quantity (preheated inside a cylinder allowing the polymer to reach the fluid state at a set temperature) ejected from a piston through a nozzle having a certain length and diameter, in a given time interval. A predetermined load is normally applied to the piston. ISO 1133:2005 describes two test procedures, in particular for measurement of melt flow rate (MFR) and melt volume rate (MVR). The difference between these two kinds of measurements is that in the first case the mass of material extruded in a given time interval is measured, while in the second case the volume of material extruded in a given time interval is measured. In particular, the MFR test is expressed in g/10 minutes, while the MVR test is expressed in cm3/10 minutes. The relationship between these properties is given by the material density at the temperature and pressure of molten material and therefore by the following expression MFR=density x MVR. The higher the melt flow index, the more flowable the material (lower viscosity).

The MVR is perhaps the preferred measure of the flow behaviour of a thermoplastic polymeric material, as it is independent from its density and therefore more directly correlated with its flowability. To better explain this point, the case in which different density materials are compared, is herein considered: for example, two materials loaded with different amounts of inert fillers, having the same polymeric base. Due to density contribution (the filler density is remarkably higher than that of polymer) to the measured MFR value, the comparison of MFR data does not immediately reveal which of the two systems is actually the more flowable.

CAMPUS® [5] too, a database for plastic materials, suggests the use of the MVR instead of the MFR value.

All MFR and MVR test conditions are specified in ISO 1133, while the main material properties are contained in the polymer matrix standards (e.g. ISO 1873 for PP, ISO1874-2 for PA).

Experimental data

The results of some MVR tests on samples of different types of PA are herein reported. In particular, the following types of PA have been considered:

A pure PA66, medium viscosity (for injection moulding);

B pure PA66, high viscosity (for extrusion);

F pure PA66, medium viscosity (for compound with specific function);

C pure PA6, medium viscosity (for injection moulding);

D pure PA6, high viscosity (for extrusion);

E pure PA6, medium viscosity (for yarn extrusion).

In order to study the influence of residual moisture inside the material at the downstream end of the drying process on the measured MVR, tests have been carried out on samples with two different moisture contents.

Before carrying out MVR measurements, samples have been dried so as to allow the standard moisture content of about 3,000 ppm to reach two target values: approximately 500 ppm (0.05%) and approximately 1,000 ppm (0.1%). Both values examined result remarkably below the sale specifications usually agreed upon. The residual moisture content after drying has been determined according to ISO 15512 [6].

1 MVR of PA66 determined at 275 °C

 

2 MVR of PA6 determined at 250 °C

The diagrams (figure 1 and 2) show that the values obtained are lower or equal to 500 ppm in the first set of samples, and between 700 e 1,000 ppm in the second set. MVR tests have been carried out on latest generation equipment (Ceast plastometer model MMF 7026, see figures 3 and 4) providing a temperature variability inside the cylinder chamber of +/-0.5 °C compared to the set temperature (ISO1133-1 allows a double tolerance).

3 Plastometer
4 Test in progress: note the material flowing from the capillary

Particular attention should be paid to the manipulation of samples during all stages, i.e. from drying to testing.

Tests have been performed according to ISO 1133. The following test conditions have been applied to the different types of PA (see table 1).

 

Table 1 PA used for the determination of the MVR

SampleTemperature (°C)Load (kg)
A27502.16
B2755
F27502.16
C25001.02
D2505
E25001.02

These test conditions have been chosen in order to:

• reduce stay-time in the machine so as to minimize the degradation effects;

• allow the test to be performed within the max. time set;

• perform tests in the moisture interval typical of conforming supplies.

MVR measurement has been repeated at least four times for each material and moisture content of granules. The results obtained are shown in table 2 and in figures 1 and 2, where the MVR values depending on the moisture content of granule are reported. The MVR average value and the standard deviation are reported for each material.

 

Table 2 MVR test results

SampleMoisture content (%)MVRm (cm3/10 min)
average
MVRm (cm3/10 min)
standard deviation
A0.0433.41.21
A0.0737.81.44
B0.0324.60.61
B0.0740.71.53
F0.0440.32.14
F0.0853.50.46
C0.0417.20.14

The diagrams suggest that:

• as far as the examined moisture contents are concerned, MVR remarkably depends on the moisture level of granules in PA66 samples, while the melt flow index appears to be poorly influenced by the residual moisture content at the downstream end of the drying process in PA6 samples;

• MVR measured in PA66 samples for the different moisture contents of granules is characterized by a lower repeatability compared with the MVR measured in PA6 samples, as the wider standard deviation bands indicate.

Conclusions

As shown, MVR measurement in polyamides may result strongly influenced by the moisture content of granules, in particular in PA66. The use of equipment with a more restricted temperature control (+/-0.5 °C) of heating cylinder may not sufficiently ensure good measurement repeatability.

Bibliography

1 ISO 1874-2:2006 Plastics – Polyamide (PA) moulding and extrusion materials – Part 2: Preparation of test specimens and determination of properties

2 ISO 1133: 2005 Plastics – Determination of the melt mass flow rate (MFR) and melt volume flow rate (MVR) of thermoplastics.

3 ISO 1628-5:1998 Plastics – Determination of the viscosity of polymers in dilute solution using capillary viscometers – Part 5: Thermoplastic polyester (TP) homopolymers and copolymers

4 ISO 307:2007 Plastics – Polyamides – Determination of viscosity number

5 CAMPUS® plastics database www.CAMPUSplastics.com

LEAVE A REPLY

Please enter your comment!
Please enter your name here