Secondary stabilizing mechanism

We saw in Chapter 5 that a separate liquid phase is present in dough. This liquid phase plays a crucial role in dough expansion. The initial concentration of gas nuclei and their size distribution created during dough mixing is determined by how they are incorporated in the liquid phase and by the surface active components that adsorb at the air-aqueous interface and stabilize them. 

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The model introduced by Gan et al. (1990) shown in Figure 7.1 applies at this time. Primary and secondary stabilizing mechanisms operate. The primary film is the gluten-starch matrix. When the gas bubbles come into close contact, Gan and colleagues postulated, based on electron microscope pictures, that discontinuities may occur in the gluten-starch matrix surrounding them. Nevertheless, the bubbles may continue to expand because thin liquid lamellae remain around the gas cells. The liquid film provides the secondary mechanism preventing destabilization. 

The mechanism of secondary stabilization by antioxidants is demonstrated in Figure 15.5. TnT-nonylphenyl phosphites, derived from PCI3 and various alcohols, and thio-compounds are active as a secondary stabilizer [21], They are used to decompose peroxides into non-free-radical products, presumably by a polar mechanism. The secondary antioxidant is reacting with the hydroperoxide resulting in an oxidized antioxidant and an alcohol. The thio-compounds can react with two hydroperoxide molecules. 

To predict the transport and fate of colloids in the subsurface, it is important to understand both the mechanism of particle deposition and that of remobilization in porous media. Experiments by MacDowell-Boyer (1992) and Monte Carlo simulations of Brownian particles near the surfaces of the media indicate that the secondary stability minimum (see physical model on colloid stability. Figure 14.10) can play an important role in the deposition and reentrainment of submicron particles at ionic strengths relevant to groundwater. 

Owing to metal chlorides titration by the coulometric method, and carboxylic acid titration by the potentiometric method, it is possible to follow the metal soaps consumption during thermomechanical heat treatments. This new technique provides a better understanding of the stabilization mechanisms of PVC with the calciumr-zinc system, and offers a better explanation of synergistic effects between metal soaps and secondary stabilizers such as epoxidized soya-bean oil, a-phenylindole, and butanediol-p-aminocroto-nate. The influence of these last stabilizers on zinc chloride formation enables us to classify them into short- and longterm stabilizers. 

In the zinc-calcium formulations, metal soaps are associated with secondary stabilizers such as alkylphosphites, epoxy compounds, a-phenyl-indole, or -aminocrotonate esters that bring a synergistic effect. The first attempt to study the mechanisms behind this effect has been the reaction of all of these compounds with 4-chloro-2-hexene. 

Conformational restrictions and reduced nucleophilic reactivity of the backbone NH centers have been suggested as probable stabilizing mechanisms with the higher order structures. In recombinant human lymphotoxin, asparagine deamidation can be inhibited by secondary and tertiary structures due to reduced polypeptide flexibility [18],... 

In Figure 14.3 a secondary minimum stabilizing mechanism is shown where stability is achieved simply through an appropriate balance of attractive and... 

Concerning the enhanced steric and electrostatic stabilisation mechanism, for example, the secondary DBS layer has the -SO3- groups oriented towards the dispersion medium forming a local electric field with hydrated NH4+ ions around the particles. Consequently, the degree of stability of water-based magnetic fluids depends on the pH of the medium and the stabilization mechanism outlined earlier is specific only to water as the dispersion medium. 

Another method of curbing sedimentation is controlled flocculation. For systems where the stabilizing mechanism is electrostatic in nature, for example those stabilized by surfactants or polyelectrolytes, the energy-distance curve shows a secondary minimum at larger particle separations. This minimum can be quite deep (few tens of kT units), particularly for large (> 1 pm) and asymmetric particles. The... 

Most of the additives used to protect the polymer from degradation are eventually consumed during the stabilization processes. In addition to chemical loss, the stabilizers can also be depleted by precipitation, migration to the surface, and extraction. For all these reasons, the concentration of additives diminishes with time up to a point where no protection effect could be observed. In general, the stabilization lifetime increases with the amount of added stabilizer, up to an optimum determined by the finite solubility of the additives in bulk polymers, or by secondary reactions which may have adverse effects on the stabilization mechanisms (Figure 15.22). 

In combination with primary antioxidants, secondary antioxidants delay the consumption of the former by decomposing hydroperoxides to non-radical compounds and by preventing the formation of new radical decomposition products. Many stabilizer types supplement each other, thus protecting against too rapid consumption and increasing their effectiveness multiple times. That is why secondary antioxidants are very often called synergists or co-stabilizers . Detailed descriptions of the secondary antioxidants mechanisms can be found in [518], [519], and [523]. 

Hindered amine light stabilizers have attracted enormous scientific and commercial interest due to their efficiency as photo-antioxidants. The stabilization process in the presence of HALS involves the interference of secondary >NH or tertiary (>NCH3) groups with hydroperoxides POOH generated by oxidation chain mechanism of polymer alkyl radicals P. HALS-derived nitroxides >NO, formed through scavenging peroxyls POO by hydroxylamine species >NOH, are the key intermediates in the HALS stabilization mechanism because they are able to quench alkyl radicals arising in the initiation step of oxidation [100, 101]. The involvement of HALS in the stabilization process can be attributed to their ability to remain an... 

PVC can be protected with a variety of stabilizer systems. The choice of stabilizer depends on the probable decomposition mechanisms involved. Primary and secondary stabilizers are generally used in combination. The primary stabilizers are mainly acid acceptors such as metallic soaps. Typical examples are, metallic soaps such as cadmium, barium, and zinc stearates which also act as lubricants. These chemicals simply react with HCl and are able to reduce further degradation. The mechanism of stabilization and properties of various metal soaps are reviewed (39,40). Calcium and zinc stabilizers are low in toxicity and they are often prepared in the form of a dispersion in epoxidized oil. Barium and cadmium stabilizers are mainly solid products those which are liquid contain phosphite groups. The compatibility of PVC stabilizers containing heavy metals (41) and synergetic effects of metal stabilizers (42) are reviewed. 

One important experimental fact is that the rate of reaction of alcohols with hydro gen halides increases m the order methyl < primary < secondary < tertiary This reac tivity order parallels the carbocation stability order and is readily accommodated by the mechanism we have outlined... 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

A reasonable mechanism is shown in equation 23 methyllithium attacks the sulfur atom, giving the secondary carbanion 36 by cleavage of the four-membered ring. A rapid proton transfer produces the sulfonyl-stabilized carbanion 37 which reacts with the added electrophile to give the product (equation 23)3. 

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