Soft compound

Compounds based on S—EB—S usually contain polypropylene, which improves solvent resistance and processibiUty and raises upper service temperatures. Compounds intended for use in the automotive industry are able to survive 1000 hours air exposure at temperatures of 125°C with only minor changes in properties (54). Very soft compounds have been developed to replace foam mbber for interior trim parts. In this and similar appHcations, these soft compounds are usually insert molded over polypropylene or metal and then coated with flexible polyurethane paint (55). Other automotive appHcations include products intended for sound deadening, flexible air ducts, and gear shifter boots, as weU as improving the properties of sheet mol ding compounds. 

If multi-section tires or experimental and control tires are mounted on the same axle, as is often the case for tests with truck tires on commercial fleets, the inverse of the above relation should be applied because the tires are now running under an imposed common slip. For the stiffer compound the slip is now larger and for the softer it is smaller and the stiffer compound becomes poorer and the soft compound becomes better than would be the case if whole tires of the same group had been mounted on the same axle. 

If stored for too long, magnesium oxide can convert to magnesium carbonate or hydroxide with resultant loss of activity as an acid acceptor. Magnesium oxide supplied in stick form overcomes this problem. Some forms of pelletised magnesium oxides can be too hard to break down in soft compounds and problems can arise from hard undispersed ceramic -like particles. 

Welding is easy by all the processes for rigid PVCs. Sometimes it can be more difficult for flexible PVCs and even impossible by frictional techniques for very soft compounds. 

Increasingly, very soft compounds are being used in, for example,... 

During processing, the plastisol is heated slowly. When the gel point is reached, the plastic absorbs the plasticizer. However, in a very soft compound, the plastic dissolves into the plasticizer. Because each plastic particle remains a separate particle, the resultant gel has no useful physical properties. But on further heating, the plasticized plastic partially melts and flows into the plasticizer this occurs at the fusion point or over the fusion range. On cooling, the material comprises the tough rubber compound known as a flexible vinyl. 

Figure 15.6. Metabolism of bufuralol (15) produces oxidative, active metabolites (16), (17)that lead to a final inactive acid metabolite (18).Starting from a designed (hypothetical)inactive metabolite (20), a series of inactive metabolite-based soft compounds (19) were designed.

Activated soft compounds are not analogs of known drugs, but are derived from nontoxic... 

Bases containing the elements P, As, Sb, Se, and Te are soft compounds according to the HSAB concept and therefore form stable complexes with the soft transition metals and thus deactivate the catalysts. The hard oxygen and nitrogen bases hardly react with die transition metals due to the hard/soft dissymmetry. 

Vulcanization, named after Vulcan, the Roman God of Fire, describes the process by which physically soft, compounded rubber materials are converted into high-quality engineering products. The vulcanization system constitutes the fourth component in an elastomeric formulation and functions by inserting crosslinks between adjacent polymer chains in the compoimd. A typical vulcanization system in a compound consists of three components (1) activators (2) vulcanizing agents, typically sulfur and (3) accelerators. 

Phthalates are the most frequently used esters. Dibutylphthalate (DBP) tends to give soft compounds with tack dioctylphthalate (DOP) is less volatile and tends to produce harder compounds because of its higher molecular weight. Polymeric esters such as polypropylene adipate (PPA) are used when low volatility is required along with good heat resistance. 

The FMO definition also helps explain why Pearson s hard-hard and soft-soft interactions form stable complexes. Hard compounds have a large HOMO-LUMO gap, as shown in Figure 14.9 for F. Therefore, hard Lewis acid-base complexes tend to form strongly ionic compounds, such as LiF, where the interaction is dominated by electrostatic attractions. Soft compounds, on the other hand, have a small HOMO-LUMO gap, as shown in Figure 14.9 for I, so that these types of interactions form covalently bonded acid-base adducts, where the strength of the interaction is controlled primarily by the energies of the FMOs that participate in the bonding. 

Nylon A wide variety are currently on the market. The most common, nylon 6 and 6/6 can be an additive at up to 10% by weight because they impart stiffness to an otherwise soft compound. Textile nylon scrap is usually nylon 6 or 6/6. Nylon 6 castings are suitable. Nylons 11 and 12 are even more suitable, but generally not available. 

The ultrafine materials are all semi-reinforcing in character, as a consequence of their fine particle size. The uncoated and fatty-acid-coated grades exhibit very poor interaction with polymer, so the modulus (a function of polymer-filler interaction) is very low, thus soft compounds are readily obtained. The high purity ensures that these fillers are very white and heat-ageing performance is superior to naturally derived calcium carbonates. One of the outstanding features of these fillers is their good high-temperature tear resistance [60]. 

Another change that results when substituents are interchanged is the polarizability of the molecule. The polarizability of a molecule is defined as the extent to which the electron cloud of the structure can undergo distortion, and the polarizability of molecules and groups was extensively discussed in Section 1.1.12. A hard compound, in which the electron cloud is tightly held, is not very polarizable (see the discussion of hard and soft acids and bases in Section 5.6.1). Conversely, a soft compound is one in which the electron cloud is more diffuse, and therefore polarizable. Differences in polarizability influence nucleophile... 

Thicker coats are required for soft compounds (below 45 IRHD). 

The effects of mixing soft compounds using mbbers which are frozen is that small nibs of unmixed polymer which can spoil an extruded section are left in the compound. In harder compounds ca 70 MLl + 4 Mooney viscosity at 100°C) this is less of a problem, but variations in the temperature and properties of the polymer can affect the mixing cycle and lead to variability of the mixed compounds. 

Compatibility with fillers and oil Filler dispersion in soft compounds Dimensional stability and shape retention Green strength especially of heavily loaded compounds... 

Sulfur is difficult to disperse in NBR and particularly in soft compounds resulting in orange peal or dimpled surfaces or sulfur spots hence sulfur dispersions added at the beginning of the mixing cycle are recommended. It is even better, as shown in a 50 phr DIDP extended NBR in Table 2.31, to use sulfurless cures [20]. Some of the EV cure systems match the physical properties of the sulfur and semi-EV cures and exhibit improved scorch safety. 

When not used in applications that justify semibulk handling, lead-in-air requirements are usually met through the use of dispersions, such as prills, extruded strands, or pressed tablets. With direct use of lead stabilizer powder, addition is customarily scheduled with the resin, for which the stabilizer has affinity. The dispersed forms include additives to prevent agglomeration, which often in soft compounds detract... 

The performance capability of OBCs can be further enhanced by incorporation of conventional process oil, allowing greater flexibility to the compounder to tailor the performance to meet the application requirements for soft compounds. Oil incorporation offers the benefits of increased processability and reduced hardness of the OBC compounds. This paper describes the properties and processability characteristics that can be achieved by oil... 

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