Elastomers flow behaviors

While polymer melts and non-cross-linked elastomers flow readily when stress is applied, structural plastics must resist irreversible deformation and behave as elastic solids when relatively small stresses are applied. These plastics are called ideal or Bingham plastics with their behavior described mathematically by... 

Because of their reversible chain extension and the subsequent formation of small crystalline domains, the functionalized polymers exhibit properties typical for thermoplastic elastomers. At low temperatures the hydrogen-bond interaction contributes to the properties comparable to covalent interactions, whereas at high temperatures these interactions disappear and the materials exhibit flow behavior typical for a low-molecular-weight polymer. DSC,72 73 light and X-ray scattering,7174 dynamical mechanical analy-... 

In our laboratory a systematic study was made with the aim of relating the composition of ABS polymers and their rheological properties. The findings enabled us to advance a hypothesis on ABS flow behavior and on the role of the elastomer phase. They also suggested a rheological criterion for polymer compatibility. Finally, on the basis of a method described previously (II), it was possible to use the rheological data to predict ABS processability. 

Before discussing the flow behavior of polymeric nanocomposites (PNCs), the nature of these materials should be outlined. As the name indicates, PNCs must contain at least two components, a polymeric matrix with dispersed nanoparticles [Utracki, 2004]. PNCs with thermoplastics, thermosets, and elastomers have been produced. The nanoparticles, by lUPAC s definition, must have at least one dimension that is not larger than 2 nm. They can be of any shape, but the most common for structural PNCs are sheets about 1 nm thick with the aspect ratio p=D/t= 20 to 6000, where D is the inscribed (or equivalent) diameter and t is the thickness of the sheet. These inorganic lamellar solids might be either natural or synthetic [Utracki et al., 2007]. 

Rubbers are fascinating materials for study (as well as having enormous importance in technological applications) because they combine characteristics typical of the three states of matter. Uncrosslinked, they resemble liquids in their flow behavior. Crosslinked, they are able to recover their original dimensions after being severely deformed (stretched, compressed or sheared) above their glass temperatures. Rubbers or elastomers however resemble solids in their resistance to flow and to maintain their shape, if crosslinked and below their glass temperature, when subjected to deformation. Finally, as mentioned below, an explanation for their response to deformation can be described in terms of the behavior of a perfect gas. 

The flow behavior of block copolymers differs from that of the parent homopolymers. Let us first examine the temperature dependence of the viscosity rj for the thermoplastic elastomers. Below the glass transition temperature of polystyrene (about 110 C) the triblock material has a viscosity intermediate between that of the parent homopolymers, as shown in Figure 4.22. This is normal and expected. However, at a temperature where flow is well developed in the polystyrene, 140 C, an inversion occurs, the block copolymer assuming the higher viscosity (Holden et a/., 1969b). The reason for this inversion lies in the difficulty of pulling styrene blocks out of their normal phase and into and through the polybutadiene phase, and vice versa. Motions of this type are required for viscous flow, and... 

First, a series of incompatible systems is discussed, including blends of different elastomers, two-component fibers and films, blends having paperlike characteristics, two-component membranes having highly ordered structures, and wood. Next, some aspects of the flow behavior of blends are considered, with emphasis on the effects of flow on morphology. Finally, the behavior of compatible blends, including isomorphic composition, is described. 

The scientific classification of materials according to their flow behavior corresponds, in a limited sense, to the classification according to their commercial application. A distinction is made here between thermo-plasts, fibers, elastomers, and thermosets. This classification naturally only applies at the application or processing temperature under consideration. 

Rabinowitsch correction n. The correction factor derived by Rabinowitsch (1929) applied to the Newtonian shear rate at the wall of a circular tube (including capillary) through which a non-Newtonian liquid is flowing, gives the true shear rate at the wall. For pseudoplastic liquids such as paints and some polymer melts the correction is always an increase. If the fluid obeys the power law it reduces to a simple correction factor (3n+ l)/4n, where n is the flow-behavior index of the liquid. Munson BR, Young DF, Okiishi TH (2005) Fundamentals of fluid mechanics. John Wiley and Sons, New York. Harper CA (ed) (2002) Handbook of plastics, elastomers and composites, 4th edn. McGraw-Hill, New York. 

White [23,24] observed the circulatory flow behavior using markers with flow visualization technique in the R-R handed screw and double-flighted rotors mixer (Figure 8). This makes possible measurement of the circulation time as a criteria of mixing. Since the flow visualization experiments have limitations in observing the overall flow motion in the mixer, the following experiment was carried out to determine the distribution of elastomers. 

Mechanical Properties. In addition to the hydrodynamic effects of particulate fillers on polymer flow behavior, an enhanced stiffening effect is observed in filled poljmiers. For soft polymers, such as elastomers, with diluted filler loading, effects of filler on modulus are proportional to that on viscosity and can be represented by the Einstein equation, equation 1, with viscosity terms replaced by modulus terms. However, this viscosity to modulus relationship only holds when the poljmier is incompressible, such as elastomers with Poisson s ratio of 0.5, and when the rigidity of the filler is very much greater than that of the poljmier. 

Figure 18.17 shows the flow behaviors of the elastomers, SEES, EPR, and EPDM. 

There is a large literature on the influence of additives on particle-filled elastomers and thermoplastics. Many particle-additive systems are marketed to produce polymer compounds containing small particles, which have strong interparticle forces. Small molecule additives are used in compounds with very small polar particles. Larger particles (i.e., particles greater in size than 5 pm) generally do not require associated additives because their flow behavior is dominated by hydrodynamic factors as described in Section 2.3. Some small nonpolar particles do not have suitable additives. Carbon black has relatively weak interparticle forces (Section 2.4.3), and additives have not been found to significantly modify the flow and mechanical characteristics of its compounds. 

Butadiene copolymers are mainly prepared to yield mbbers (see Styrene-butadiene rubber). Many commercially significant latex paints are based on styrene—butadiene copolymers (see Coatings Paint). In latex paint the weight ratio S B is usually 60 40 with high conversion. Most of the block copolymers prepared by anionic catalysts, eg, butyUithium, are also elastomers. However, some of these block copolymers are thermoplastic mbbers, which behave like cross-linked mbbers at room temperature but show regular thermoplastic flow at elevated temperatures (45,46). Diblock (styrene—butadiene (SB)) and triblock (styrene—butadiene—styrene (SBS)) copolymers are commercially available. Typically, they are blended with PS to achieve a desirable property, eg, improved clarity/flexibiHty (see Polymerblends) (46). These block copolymers represent a class of new and interesting polymeric materials (47,48). Of particular interest are their morphologies (49—52), solution properties (53,54), and mechanical behavior (55,56). 

The particle size of the dispersed phase depends upon the viscosity of the elastomer-monomer solution. Preferably the molecular weight of the polybutadiene elastomer should be around 2 x 10 and should have reasonable branching to reduce cold flow. Furthermore, the microstructure of the elastomer provides an important contribution toward the low-temperature impact behavior of the final product. It should also be emphasized that the use of EPDM rubber [136] or acrylate rubber [137] may provide improved weatherability. It has been observed that with an increase in agitator speed the mean diameter of the dispersed phase (D) decreases, which subsequently levels out at high shear [138-141]. However, reagglomeration may occur in the case of bulk... 

There are several possible explanations for this nonequll-Ibrlum behavior. The composition of the Neoprene may be changed by the leaching of components Into the liquid permeant, thereby opening pores to Increased capillary flow of water (vls-a-vls diffusion). Changes In composition of the elastomer and the residual permeant are discussed In later paragraphs. 

While IPNs can be and have been made extremely tough and impact resistant, many of the proposed applications involve such diverse fields and sound and vibration damping, biomedical materials, and non-linear optics. This is because the presence of crosslinks in both polymers reduces creep and flow, allowing relatively stable materials with a wide range of moduli to be prepared. Thus, those materials with leathery mechanical behavior, combinations of elastomers and plastics, are especially interesting to scientists, inventors, and engineers. 

In order to improve properties and compatibility of PP/EPDM blends, ternary blends and composites are sometimes prepared from the PP/EPDM blends. For instance, Sanchez et al. (10) prepared ternary blends of PP, high density polyethylene and EPDM with several blending ratios and investigated the melt rheological behaviors. They discussed the effect of the shear rate on the viscosity and flow curve in terms of the exponent of low power for a non-Newtonian liquid. They showed that addition of an elastomer to the polyolefin blends changes the shape of the viscosity-composition curve, and they discussed it in terms of the possible morphology of the blend. Similar works have been also reported by Ha et al. (11,12). 

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