Polymer flow studies

Most polymer processing methods involve heating and cooling of the polymer melt. So far the effect of the surroundings on the melt has been assumed to be small and experience in the situations analysed has proved this to be a reasonable assumption. However, in most polymer flow studies it is preferable to consider the effect of heat transfer between the melt and its surroundings. It is not proposed to do a detailed analysis of heat transfer techniques here, since these are dealt with in many standard texts on this subject. Instead some simple methods which may be used for heat flow calculations involving plastics are demonstrated. 

TABLE 1 — ROCK AND FLUID PROPERTIES USED IN POLYMER FLOW STUDIES... 

In this figure OTS refers to octadecyltrichlorosilane and FDS to an incomplete monolayer coating of fluorinated surfactant that appeared to inhibit slip. The results were analysed in terms a slip length model. This model is widely used in polymer flow studies and assumes that slip at the wall produces a flow enhancement equivalent to displacing the solid wall by a fixed slip length, b into the slip surface. This length is also called the Navier length . The model is equivalent to... 

This example illustrates the simplified approach to film blowing. Unfortunately in practice the situation is more complex in that the film thickness is influenced by draw-down, relaxation of induced stresses/strains and melt flow phenomena such as die swell. In fact the situation is similar to that described for blow moulding (see below) and the type of analysis outlined in that section could be used to allow for the effects of die swell. However, since the most practical problems in film blowing require iterative type solutions involving melt flow characteristics, volume flow rates, swell ratios, etc the study of these is delayed until Chapter 5 where a more rigorous approach to polymer flow has been adopted. 

The nature of the liquid in contact with a surface is also very important, with respect to boundary conditions. Although slip has long been observed for highly non-Newtonian, viscoelastic liquids such as polymer flows and extrusions, many recent studies have reported slippage of Newtonian liquids under a variety of experimental conditions. This clearly indicates that care must be taken when modeling any type of micro- or nanofluidic system, no matter which liquid is employed. 

Viscosity is a measure of the resistance to flow of a material, mixture, or solution. Here we will consider the viscosity of solutions containing small, generally 1 g/100 cc (called 1% solutions) and less, amounts of polymer. The study of such dilute polymer solutions allows a determination of a relative molecular weight. The molecular weight is referred to as relative since... 

Cal ibr ation o f Instrument When 10 urn and 0.6 ym membranes were used to determine the viscosity of THF using the manufacturer s determination of M from the flow of water, the viscosities of THF were measured to be an average of 85 /, of the true value. The direct experimental P vs Q curves are shown in Figure 3. (There is, however, a systematic trend below 85 /. as membranes of even lower pore sues are used. Although this trend is puzzling it is unimportant for polymer research since most polymer solution studies need relative viscosity, specific viscosity,, measurements.)... 

Torque rheometers are multipurpose instruments well suited for formulating multicomponent polymer systems, studying flow behavior, thermal sensitivity, shear sensitivity, batch compounding, and so on. The instrument is applicable to thermoplastics, rubber (compounding, cure, scorch tests), thermoset materials, and liquid materials. 

A. Keller, A. J. Muller, and J. A. Odell, Entanglements in semi-dilute solutions as revealed by elongational flow studies, Prog. Coll. Polym. Sci., 75,179 (1987). 

Fig. 11.33 Dispersed PS 685 streaks of ligaments and droplets in the equiviscous Blend 2. PS is flowing/deforming in an affine fashion in the expected stretching and folding pattern. Experiments were conducted at 180°C and 120rpm. [Reprinted with permission from Proceedings of the Tenth Semiannual Meeting of the Polymer Mixing Study, Polymer Processing Institute, Hoboken, NJ (1995).]...

Figure 12.4 depicts schematically the experimental setup used in capillary flow studies. The primary application of the discussion that follows is in capillary viscometry, which is useful to die design. The ratio Rr/R should be greater than 10, so that the pressure drop due to the flow in the reservoir can be neglected.1 The reservoir radius cannot be too large, though, because the time required for uniform heating of the solid polymer load would be too long (see Fig. 5.6). Long heating cannot be used for sensitive polymers such as polyvinyl chloride (PVC), which readily degrade thermally.

We presented some applications of the bireftingence techniques for the characterization of orientation and stress field in both simple and complex deformations. The validity of the stress optical law in these particular flow situations was checked and assessed. Birefringence techniques appear to be very powerful for the study of polymer melt processing and for the understanding of polymer flow behaviour. 

Adsorption of albumin, y-globulin, and fibrinogen from single solutions onto several hydrophobic polymers was studied using internal reflection IR spectroscopy. The adsorption isotherms have a Langmuir-type form. The calculated rate and amount of protein adsorbed was dependent on the polymer substrate and the flow rate of the solution. Competitive adsorption experiments were also investigated to determine the specific adsorption of each I-labelled protein from a mixture of proteins. Platelet adhesion to these proteinated surfaces is discussed in relation to a model previously proposed. 

Three larger lOOg batches of polymers with the composition PHQ, TA, 4,4 -0 = 0.5/0.35/0.15 were prepared for fiber and melt flow studies. The polymerization procedure for these was as follows Temp(°C)/time(min)/pressure (mmHg) = 290/60/760 ... 

Baumgarten spun acrylic fibers and studied the effect of polymer flow rate and viscosity on fiber diameter in 1971 (Baumgarten 1971). The fibers were less than 1 pm in diameter. 

A simple picture of liquid flow of plain fluids has been developed by Eyring, using a free volume model and the theory of absolute reaction rates. Eyring s conception forms also the basis for some studies of liquid polymer flow, and a qualitative description of the ideas involved will be given in the following paragraph. 

Vallotton, R Ronti, A. Waterman-Storer, C. M. Salmon, E. D. Danuser, G Recovery, visualization, and analysis of actin and tubulin polymer flow in live cells a fluorescent speckle microscopy study. Biophys. J. 2003, 85, 1289-1306. 

Active ultrasound uses a source of sound radiation, which is appHed to a process sample, with a detector placed such that modification to the signal can be detected and related to changes in the sample. Signal attenuation, velocity measurements and wavelength selective absorption provide the means of probing the sample. This approach promises to provide both chemical and physical information but as yet has not been used extensively. A number of on-line polymer-related studies have been reported in which polymer flow behaviour, viscosity, blend characterisation, and foaming-process monitoring have been examined [19]. 

Melt fracture has been a very perplexing but fascinating problem ever since it was discovered. Another problem that seems to have the same degree of perplexity and fascination is draw resonance. Both are instabilities in polymer flows. (Draw resonance may also occur in Newtonian fluids.) Draw resonance is a periodic variation in the diameter of a spinning thread line above a critical drawdown ratio. Polypropylene and high-density polyethylene are both particularly susceptible to draw resonance. Petrie and Denn have presented a comprehensive review of the numerous theoretical and experimental studies of draw resonance conducted prior to 1976 [99]. 

Concurrent with the DSC and the TMA measurements, the water content of the polymer under study was determined thermogravimetri-cally (TG) with a Mettler TA-1 thermoanalyzer. The sample was contained in a quartz cylindrical crucible and was heated to decomposition at 10 C/min under a nitrogen flow of about 8 J2-/hr. 

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