Selection of Polymer

The first decision in the design of a rubber formulation is the selection of the base polymer to be used. This choice will be determined principally by the end-product specification and/or service conditions. This will stipulate such conditions as upper and lower operating temperatures, type and level of chemical or fluid resistance, mechanical or electrical properties, etc. This task requires a comparison of the basic properties of the polymer types and grades available with a view to making a selection on a cost-effective basis. There are few areas of ambiguity about the choice of polymer to be used as most polymers have a specific profile of chemical and physical properties. Where a single polymer cannot provide the requisite properties, then a blend of two or more may often be used. The performance aspects of the polymer are discussed further in Section 7.5. 

Polymer 100 Determines the base properties of the end product, (e.g., elasticity, resistance to oxidation, fluids and chemicals low-temperature, dynamic and electrical properties) 

Curing system 5-15 Determines the rate of cure to an elastic material. Influences the mechanical properties and stability of the rubber 

Antioxidants/ antiozonants 0-7 Inhibit attack by oxygen and ozone. Improve flex fatigue and inhibit the degradation effects of metal impurities 

Coupling agents 0-2 Modify the polymer-filler interface 

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The following sections discuss the prediction of a selection of polymer properties. This listing is by no means comprehensive. The sources listed at the end of this chapter provide a much more thorough treatment. 

Table 9.2 Values for the Mark-Houwink Coefficients for a Selection of Polymer-Solvent Systems at the Temperatures Noted...

Service temperature limitations must be considered in the use of composites, not only in the selection of polymer and process, but sometimes in the selection of the reinforcement as weU. Composites cannot generally perform as weU as metals or ceramics in very high temperature appHcations, but they can be made fire resistant to meet most constmction and transportation codes. 

Figure 5.11 gives some comparative data for a selection of polymers subjected to tbe flaming condition mode. 

The proper selection of polymers for coagulation has a significant impact on organic removal. 

Selection of polymers used in the manufacture of chemical protective clothing (CPC) is a complex task. It involves evaluating breakthrough times and permeation rates in conjunction with such task requirements as tactility and resistance to cuts and abrasion. But, it involves a more basic problem — that of deciding which polymer(s), in the absence of test data, might be most likely to resist permeation by a specific chemical. These decisions are faced not only by users of CPC (e.g., industrial hygienists), but also by poljnner chemists and CPC manufacturers. 

E. Bakker, R.K. Meruva, E. Pretsch, and M.E. Meyerhoff, Selectivity of polymer membrane-based ion-selective electrodes — self-consistent model describing the potentiometric response in mixed ion solutions of different charge. Anal. Chem. 66, 3021—3030 (1994). 

Fig. 19. The sulfated glycoprotein mimics were tested for biological activity against the se-lectin family of proteins, which are involved in the inflammatory response. These results suggest that length may be a factor in the selectivity of polymers for different proteins...

Finally the synthesis of inorganic-polymer composite membranes should be mentioned. Several attempts have been made to combine the high permeability of inorganic membranes with the good selectivity of polymer membranes. Furneaux and Davidson (1987) coated a anodized alumina with polymer films. The permeability increased by a factor of 100, as compared to that in the polymer fiber, but the selectivities were low (H2/O2 = 4). Ansorge (1985) made a supported polymer film and coated this film with a thin silica layer. Surprisingly, the silica layer was found to be selective for the separation mixture He-CH4 with a separation factor of 5 towards CH4. The function of the polymer film is only to increase the permeability. No further data are given. 

The Maxwell model can also guide the selection of a proper polymer material for a selected zeolite at a given volume fraction for a target separation. For most cases, however, the Maxwell model cannot be applied to guide the selection of polymer or zeolite materials for making new mixed-matrix membranes due to the lack of permeabihty and selectivity information for most of the pure zeolite materials. In addition, although this Maxwell model is well-understood and accepted as a simple and effective tool for estimating mixed-matrix membrane properties, sometimes it needs to be modified to estimate the properties of some non-ideal mixed-matrix membranes. 

Certain SEC applications solicit specific experimental conditions. The most common reason is the limited sample solubility. In this case, special solvents or increased temperature are inavoid-able. A possibility to improve sample solubility and quality of eluent offer multicomponent solvents (Sections 16.2.2 and 16.8.2). The selectivity of polymer separation by SEC drops with the deteriorating eluent quality due to decreasing differences in the hydrodynamic volume of macromolecules with different molar masses. The system peaks appear on the chromatograms obtained with mixed eluents due to preferential solvation of sample molecules (Sections 16.3.2 and 16.3.3). The multicomponent eluents may create system peaks also as a result of the (preferential) sorption of their components within column packing [144,145]. The extent of preferential sorption is often sensitive toward pressure variations [69,70,146-149]. Even if the specific detectors are used, which do not see the eluent composition changes, it is necessary to discriminate the bulk sample solvent from the SEC separated macromolecules otherwise the determined molecular characteristics can be affected. This is especially important if the analyzed polymer contains a tail of fractions possessing lower molar masses (Sections 16.4.4 and 16.4.5). 

To a much greater extent than either metals or ceramics, the mechanical properties of polymers show a marked dependence on a nnmber of parameters, inclnding temper-atnre, strain rate, and morphology. In addition, factors snch as molecnlar weight and temperature relative to the glass transition play important roles that are not present in other types of materials. Needless to say, it is impossible to cover, even briefly, all of these effects. We concentrate here on the most important effects that can affect selection of polymers from a mechanical design point of view. 

In this example, we see how selection criteria need not be entirely quantitative. Such properties as resistance to solvents and printability are important to the selection of polymers for certain applications, yet difficnlt to quantify. 

Understanding of the mechanism of radiation degradation of polymer molecules is essential for development of improved and new industrial processes, for radiation-induced modification of polymer properties, and for selection of polymers for use in radiation environments. This means that the detailed chemical reactions resulting from absorption of radiation must be known. This fundamental understanding must enable us to relate the chemical structure of a polymer to changes in its chemical, physical and material properties. Such structure-property relationships require a great deal of research work, but they are the key to further advancement on a scientific basis. 

So far, the selection of polymers for chemical sensors based on fluorescent dyes in polymer matrices has been largely empirical, based on the accomplishment by the polymer of a number of requisites that are desirable for any support aimed to be used in optical sensing. These features are the following ... 

Table 8.4 Permeabilities and selectivities of polymers of interest in air separation...

The importance of preclinical models in the selection of polymer carriers... 

Selection of polymer type and FR additive technology can have significant impact on meeting flame resistance requirements as specified by the above standards.56 Therefore, material selection and standards are directly interrelated. A summary of key material options for FR wire and cable applications follows. 

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