Melt Flow viscosity relationship

Dutta, A, On viscosity-melt flow index relationship, RheoL Acta, 23,565-569 (1984). 

Many of the comments in the previous chapter about the selection of grade, additives and mixing before moulding apply equally in preparation for extrusion. It is important of course that the material should be appropriate for the purpose, uniform, dry, and free from contamination. It should be tested for flow and while many tests have been devised for this it is convenient to classify them as either for low or high rates of shear. The main terms used in such testing ( viscosity , shear rate , shear strain , etc.) are defined in words and expressed as formulae in ISO 472, and it is not necessary to repeat them here. Viscosity may be regarded as the resistance to flow or the internal friction in a polymer melt and often will be measured by means of a capillary rheometer, in which shear flow occurs with flow of this type—one of the most important with polymer melts—when shearing force is applied one layer of melt flows over another in a sense that could be described as the relationship between two variables—shear rate and shear stress.1 In the capillary rheometer the relationship between the measurements is true only if certain assumptions are made, the most important of which are ... 

Busse (1967) gives the following relationship between melt flow index and inherent viscosity ... 

Adequate MW is a fundamental requirement to meet desired properties of plastics. With MW differences of incoming material, the fabricated product performance can be altered. The more the difference, the more dramatic change occurs in the product. Melt flow rate (MFR) tests are used to detect degradation in products. MFR has a reciprocal relationship to melt viscosity. This relationship of MW to MFR is an inverse one as one drops, the other increases or visa-versa. 

The resistance of melt flow exhibited within a body of material identifies its viscosity. It relates to plastic melt flow which in turn relates to the processing behavior of plastic. During melt flow internal friction occurs when one layer of fluid is caused to move in relationship to another layer.487 Ordinary viscosity is the internal friction or resistance of a plastic to flow. It is the constant ratio of shearing stress to the rate of shear. Shearing is the motion of a fluid, layer by layer, like the movement of a deck of cards. 

Fillers usually enhance the viscosity of the polymer melts. The viscosity of these systems depends not only on the characteristics of the melt but also to a great extent on the nature and volume of the filler. Several empirical relationships have been proposed between the viscosity of the melt and that of the melt filled with noninteractive particles for Newtonian flow. Among them, the empirical equation of Maron and Pierce (55) stands out ... 

Fluoropolymer manufacturers and suppliers have developed time-temperature-shear-rate data for melt viscosity or melt flow rate (index) to provide an assessment of the thermal stability of these polymers. Figures 6.1 and 6.2 show the melt viscosity of a few commercial grades of polyvinylidene fluoride as a function of temperature at a fixed shear rate. The relationships between melt viscosity and shear rate, and shear stress versus shear rate, are presented in Figs. 

S-6.5. Melt flow index/rate and its relationship with melt viscosity of polyvinylidene fluoride are given in Tables 6.2 and 6.3. 

There are two main corrections that have to be applied to the information obtained from the capillary rheometer. First, there is an entrance pressure drop when the molten polymer enters the capillary, which is taken into account through the entrance or Bagley correction. This pressure drop is related to elastic deformations of the melt at the entry of the capillary [15]. Secondly, the non-Newtonian shear rate is expressed in terms of an apparent viscosity (defined in terms of a Newtonian flow). The relationship between the non-Newtonian and Newtonian shear rates, expressed as in the following equation, is known as the Rabinowitch correction [13, 16] ... 

Many processes are available for the production of plastic products and the choice depends upon the relationship between material properties, processing method and end-product properties, in addition to the choice of forming method, which all have technical and economic aspects. Three important features of plastic melts during processing are shear viscosity, melt flow index (MFI) and melt elasticity. MFI is the quantity of polymer extruded under specific load and temperature conditions in a given time. The elastic properties of the melt are a major factor in determining the residual strain and moulding defects [8]. 

There has been little or no published work on the melt rheology of PHB, presumably because of problems associated with its melt instability. However, some information on melt viscosity can be derived from melt flow index data such as that given in Fig. 9, using the empirical relationship ... 

Compositions with terephthalate levels of over 60% would not be as desirable, however, since flow was not measurable above this level (Table 1). The polyesters which were examined all can be prepared with melt flow characteristics similar to those of commercial polysulfone resins as indicated by Figure 2. While the viscosity/temperature relationships for the two types of polymers are different this should not be a serious problem in the utilization of the aromatic polyesters. 

The melt-flow index is an extremely simple test, and essentially consists of measuring the amount of polymer melt flowing through a tube of specific dimensions under a specific pressure over a specified amount of time. Experimentally, the apparatus is a small extruder. The index obtained has a sort of inverse relationship to viscosity at any given temperature, and is indirectly related to polymer molecular weight. 

Combs, R. L. and Nation, R. G., Relationships among melt flow, glass transition temperature and inherent viscosity of tfamnoplastic polyestms, J. Pofym, ScL, 30, 407-414 (1970). 

The shear stress that a molecule is exposed to at constant shear rate is directly proportional to viscosity (Newtonian flow behavior), it also holds that, at constant viscosity, shear stress is directly proportional to shear rate, which in turn is directly proportional to screw speed in first approximation. Further important relationships include the temperature-viscosity relationship and shear-thinning behavior for non-Newtonian melts. Energy input increases with increasing shear rate as well as with increasing shear stress. Because the temperature of the plastic increases due to energy input, its viscosity decreases, resulting in a more moderate increase in energy introduction [35]. 

Rheology deals with deformation and flow and examines the relationship between stress, strain and viscosity. Most theological measurements measure quantities related to simple shear such as shear viscosity and normal stress differences. Material melt flows can be split into three categories, each behaving differently under the influence of shear as shown in Figure 10.9 Dilatent (shear thickening), Newtonian and Non-Newtonian pseudoplastic (shear thinning) behaviour. 

In Fig. 5 relative viscosity vs. shear rate relationship is plotted for the capillary measurement data. It is seen that at low shear rate below which the melt flow is stable the shear thinning is enhanced for the Exact 5361/60% Dechlorane suspension. The relative viscosity was dropped in the shear rate region above which the melt flow is unstable. 

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