The number of commercial materials that make use of a melt flow rate (MFR) value as part of the published property profile confirms the practicality of the test. Almost all grades of material within numerous polymer groups offer this data point, and certifications will virtually always list the MFR value for every batch. But there are certain polymers where the availability of an MFR value is shifting across the base of suppliers, and there are a few polymer groups where an MFR value is seldom provided by any dealer.
The reasons for this differ. Some of it is down to convention. Amorphous PET, for instance, does offer a value that is associated with molecular weight (MW). But rather than using MFR, the makers of these materials have preferred to use intrinsic viscosity (IV). This test requires creating a dilute solution of the polymer in the proper solvent and then comparing the flow rate of this solution to the flow rate of the pure solvent through a piece of glassware of a uniform geometry. The greater the variance between these flow rates, the higher the IV of the material and the greater the average MW of the polymer.
Carrying out the IV test is more complicated because of the need for a delicate apparatus and the use of a few highly noxious chemicals. However, this test can be performed essentially at room temperature. This eliminates the need for drying the material as a preliminary step, something that is necessary when testing in the melt state.
Filled materials are another example where MFR testing is often avoided. This seems mostly true of filled and reinforced semi-crystalline resins such as nylons, polyesters, and polyphenylene sulfide (PPS). In PPS, for instance, where nearly all molding and extrusion grades of the polymer are highly filled, it is fundamentally impossible to see an MFR value on the data sheets. Unfilled materials that are used either in coatings or as the feedstock for the filled compounds are the only exceptions. The presence of the filler reduces the MFR.
The value obtained from the test is more reliant on the filler content and type and less upon the MW of the polymer. The filler’s presence can also raise the variability of the test results, rendering it difficult to derive a significant specification. However, the avoidance of the MFR test in reinforced and filled materials is far from common. Numerous suppliers of glass- and mineral-filled materials, including polycarbonates and polypropylenes, continue to use the MFR test as a quality-control metric and to offer values for their literature.
Emergence of the MFR Test
PPE/PS alloys signify a stimulating example of how opposed the industry can be about the use of this test. Anyone who partnered with GE Plastics (now Sabic Innovative Plastics) in the 1970s and 1980s will be aware that their Noryl data sheets never used to give MFR values and rather depended upon capillary rheometer data. If the technical people at GE were questioned about this, they would say that the MFR test does not offer anything regarding PPO materials. But in the 1990s, the MFR numbers started to appear on their data sheets, and currently, they are frequently found even for some filled grades. A few other suppliers have also begun doing the same while others do not give MFR values for their PPE/PS materials.
The polymer where there seems to be a universal consensus on avoiding the use of the MFR test is nylon. Irrespective of the type of nylon, MFR tests are more or less never done and values are seldom seen on the data sheet. If any measure of MW is offered, it typically comes in the form of a value called relative viscosity. This is a value founded on the same type of solvent-based test used for intrinsic viscosity, but the units wherein the results are reported are frequently different.
Issues with Nylon
Nylon poses certain special difficulties for the MFR test. As it is a hygroscopic material capable of hydrolyzing in the melt state, all samples of nylon that are examined in the melt state should be dried before testing. This is also the case with materials like polyester and polycarbonate. But nylon poses additional issues since the moisture that stays after drying has an impact on the flow of the polymer. A majority of the materials that can hydrolyze have to be dried to moisture contents of less than 0.020% (200 ppm). The viscosity of several of these materials is affected by the exact moisture content of the sample being analyzed.
However, between about 30 and 200 ppm moisture content, the MFR of these materials tends to be fundamentally constant. But unfilled nylons are said to be sufficiently dry once the moisture content is below 0.20%, or 2000 ppm. It appears that from 0 to 2000 ppm, the viscosity of nylon differs considerably. The relationship of MFR to moisture content for an unfilled nylon is almost linear and covers the whole range between 400 and 2000 ppm. Once the moisture content falls below 400 ppm, the MFR values start to drop more quickly.
Anyone who has attempted to mold nylon is aware of this. Very dry nylon possesses a higher melt viscosity and poses challenges in filling long, thin flow paths. This is the reason that dealers of nylon direct processors to “leave a little bit of moisture in the material.”
Conducting the MFR Test
It must be kept in mind that when the MFR test is carried out, the result for raw material is often compared to that of molded parts and a certain threshold in the change from pellets to parts is looked for to reveal if a good job was done in preserving the MW of the polymer. However, from the outcomes, it is evident that the result acquired from a single sample of material can differ by over 40% merely as a function of the moisture content of the sample. This makes achieving a steady result very hard and it makes the deduction of a pellet-to-part comparison very challenging.
However, there is a way to overcome this. It requires forming a calibration plot. Then, when the MFR test is conducted, the sample’s moisture content is measured at the same time and the result is then regularized to a specific moisture content such as 0.10% (1000 ppm). Thus, an MFR of 13.3 g/10 minutes measured at 1500 ppm would be stated as a normalized value of 12 g/10 minutes at 1000 ppm. A value of 10.4 g/10 minutes stated at 400 ppm would also be reported as a standardized result of 12.0 g/10 minutes at 1000 ppm.
Here, two tests that seemed to give considerably different results are exposed to signify typically the same values when the effects of moisture content are measured. For the majority of people, this is way too much effort. The alternative is to carry out the solvent-based test. Yet, for those who do not want to handle the challenges related to the solution testing, a more basic MFR testcan still be used even with nylons.
This information has been sourced, reviewed and adapted from materials provided by Dynisco.
For more information on this source, please visit Dynisco.