The general rheological behaviour of the C -ll polyborosiloxane has been discussed in the previous section. Possible mechanisms responsible for such rheological behaviour are discussed within the following two sections. Section 5.4.3.2 presents a general flow mechanism for the C -ll polyborosiloxane with respect to the flow curve shown in Figure 105. These general flow mechanism discussions are developed further within section 5.4.3.3, which explores the flow mechanisms which may be responsible for the various unusual flow properties exhibited by the C -ll polyborosiloxane during constant ram velocity capillary extrusion.
5.4.3.2 A general flow mechanism for C-11 polyborosiloxane
The general pseudoplastic behaviour of the C -ll polyborosiloxane has been discussed previously (5.4.2). The pseudoplastic nature of the C -ll polyborosiloxane has been shown to increase rapidly at shear rates greater than 100 s'1, and to be affected significantly by shear history and temperature.
The exact molecular structure and composition of the C -ll polyborosiloxane is not available to this research programme due to the industrially sensitive proprietary nature of such information. Consequently, it is not possible to discuss quantitatively the specific molecular mechanisms which may be responsible for the rheological behaviour of the C -ll polyborosiloxane. However, general molecular mechanisms such as those presented by Dealy [26] and Cogswell [25] can be discussed briefly in a qualitative manner, with respect to the rheological properties exhibited by the C -ll polyborosiloxane.
Dealy [26] states that if a polymer is allowed to stand for some time, then the molecules will assume an equilibrium distribution of shapes and a random distribution of
orientations. He further elaborated that if the polymer is subsequently subjected to some form of deformation (for example, if it is sheared or stretched at a significant rate), then the polymer molecules will have forces applied to them that will change the shapes of the molecules away from the equilibrium distribution. An additional affect of this shear may be an alignment of the polymer molecules to some preferred flow axis. Figure 56, shows how an increase in the time between charging the C -ll polyborosiloxane to the rheometer barrel and the commencement of extrusion may result in an increase in the recorded average extrusion barrel pressure (all other experimental conditions being constant). If the C -ll polyborosiloxane is considered to contain an entanglement network of long chain molecules, of the type described by Dealy [26], then this network will be broken down during the considerable shear of the polyborosiloxane on loading into the rheometer barrel via the charging piston. Thus molecules immediately after charging to the rheometer may be considerably aligned in the flow direction resulting in decreased viscosity on subsequent flow. However, the longer the material is left in the barrel between charging to the barrel and extrusion the greater will be the extent of the re-establishment of the entanglement network of molecules such that greater resistance to flow (higher viscosity) will occur during extrusion (Figure 56). Considerable variations in the absolute value of average extrusion barrel pressures were obtained at the same ram velocity and temperature prior to the adoption of the constant barrel charging method described in section 3.2.7 (Figure 55). It is considered that these variations may be due to slight variations in charging methods used resulting in a slightly different shear history of the materials prior to extrusion. Initially the charging method and the length of time between charging the C -ll polyborosiloxane into the rheometer and the commencement of extrusion was variable (although this time was always sufficient to allow the C -ll polyborosiloxane to reach the desired temperature for the experiment). However, the capillary rheometer results presented in Figure 105, illustrating the general flow properties of the C -ll polyborosiloxane, were produced using the same predefined charging procedure as
described in section 3.2.7. This charging procedure has been shown to produce repeatable results where clear trends can be observed (Figure 105).
This general molecular theory which has been applied to the effect of shear history on the C -ll polyborosiloxane may also be extended to the more general flow properties of the polymer. As well as the shape and orientation of the molecules the interaction between the molecules must also be considered. There is considerable indirect evidence (Ferry [86]) that rheological behaviour is governed by very strong interactions between molecules. This interaction of molecules is sometimes termed an entanglement network [26]. The decrease in the viscosity with shear rate of the C -ll polyborosiloxane may be interpreted in terms of an entanglement network, where the shearing process may increase the rate of loss of existing entanglements, but not the rate of generation of new ones. Thus the number of entanglements in a given volume of material (the entanglement density ) has lower equilibrium values at progressively larger shear rates, resulting in a general decrease in the viscosity.
The effect of temperature on the flow curve of the C -ll polyborosiloxane can be seen in Figure 107. From the capillary rheometer results presented an increase in C -ll polyborosiloxane temperature results in an increase in the apparent shear viscosity of the material. This is contrary to the established theory [86] that as heat is supplied to a polymer, the molecules vibrate more rapidly with a subsequent increase in mobility (decrease in viscosity). Indeed, the increase in temperature, and subsequent decrease in polyborosiloxane viscosity is a commonly cited problem during extrusion honing [87]. It is proposed that this apparent increase in polyborosiloxane viscosity with increasing temperature is not a real effect, but an apparent effect caused by backpressure within the rheometer barrel as a result of polyborosiloxane adhesion within the capillary die. This
increased tendency to adhere to metallic surfaces at elevated temperatures has been previously observed in the industrial application of the polyborosiloxane to extrusion honing [87]. Further evidence of this phenomenon, by which C -ll polyborosiloxane adheres to the die wall, causing a restriction to flow and consequent compression and increased pressure in the extrusion barrel, may be obtained by consideration of the temperature/extrusion barrel pressure relationships illustrated in Figure 83. At low ram velocity the rheometer barrel pressure can be seen to be constant despite the increase in polyborosiloxane temperature. This may be because the C -ll polyborosiloxane adheres to the die wall but ram advancement is sufficiently slow to prevent the build up of backpressure on account of the flow of low viscosity fluid through the centre of the die cavity. However, as the ram velocity is increased the flowrate generated by the advancing ram increases to a value greater than the flow through the die (restricted by polymer adhering to the wall) and backpressure occurs, resulting in an increase in extrusion barrel pressure and an apparent increase in media viscosity. This mechanism has been discussed further with specific reference to some of the unusual flow properties measured during constant ram velocity capillary die and slit die flow in the following section.