Secondary stabilizer

Other materials that are often referred to as secondary plasticizers iaclude materials such as epoxidized soybean oil (ESBO) and epoxidized linseed oil (ELO) and similar materials. These can act as lubricants but also as secondary stabilizers to PVC due to thein epoxy content which can remove HCl from the degrading polymer. 

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. 

FIGURE 15.5 Secondary stabilization by phosphates and thio-compounds. 

How do primary and secondary stabilizers act to reduce degradation in polyvinyl chloride ... 

By definition, a generalized anomeric effect is observed at carbon of an XCY system when a molecule preferentially adopts a conformation that optimizes a secondary, stabilizing electronic interaction involving overlap between the lone pair on one heteroatom with the a orbital of the bond between the central carbon atom and the second heteroatom . Figure 5a illustrates that in XNY systems, as with anomeric carbon centres, two anomeric interactions are possible and involve either an ny-CT x ° nx-o NY overlap where nx and ny represent the p-type lone pairs on X and Y and NX and NY represent the N—X and N—Y a orbitals. In either case, the result is a net stabilization of the lone pair of electrons (Figure 5b). Except where the nitrogen is symmetrically substituted, one of these interactions will be strongest. 

In addition to their role in primary stabilization related to viscosity increase, some hydrocolloids (particularly carrageenan) are traditionally used as secondary stabilizers. Many of the primary stabilizing hydrocolloids, including locust bean gum and carboxy methyl cellulose induce precipitation of the milk proteins in the mix. This phenomenon in ice cream mix is known as wheying-off, and may be due to direct protein-polysaccharide binding and/or protein-polysaccharide incompatibility in the water phase40. The latter phenomenon may be due to decreased solvent quality due to the competition between protein and polysaccharide for solubilisation. 

An NN contact would presumably provoke elimination of N2 from the cage, and therefore a secondary stability condition is that all forced homonuclear links should be BB rather than NN. This can only be achieved by having an excess of four B atoms. A (BN)n/2 ipp isomer must have three unfavourable NN contacts. 

The inclusion of heat stabilizers is essential to protect the system against thermal decomposition at elevated temperatures during processing. For this purpose, tin carboxylate esters or liquid calcium-zinc stabilizers are preferred. Thio-tin compounds are very effective as heat stabilizers but must be regarded with caution, bearing in mind that they can lead to unpleasant and unacceptable residual odours. Secondary stabilizers that can be used include epox-idized soya bean oil. 

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. 

Secondary stability (s, stable) is obtained when terms beyond the first are repulsive, opposing the largest attractive contribution of the leading term. 

Figure 13.26 Secondary stabilization from coarse strata into finer strata. 

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. 

Using new methods (7) for following the transformation of metal soaps in the polymer, this chapter studys the synergism obtained from chloride ions between zinc and calcium stearate in the absence or presence of secondary stabilizers during PVC thermomechanical treatments in the Brabender plasticorder or in a rolling mill. 

Synergistic Mixture Zinc Stearate and Organo Compounds as Secondary Stabilizers. From the previous results, it is clear that in order to prevent PVC cross-linking and to increase the action time of stabilizer (previously defined with the Brabender plasticorder (1,3,4,6,7)), it is necessary to impede zinc chloride accumulation in the polymer. 

To study the influence of secondary stabilizers on zinc chloride formation, we have prepared PVC sheets on a rolling mill with the different binary mixtures zinc stearate and a-phenylindole, epoxidized soya-bean oil, or /3-aminocrotonate esters, and we have followed the zinc chloride formation by coulometric titration of chloride ions. In Figure 5 we have combined the percentage of zinc chloride that may be liberated with respect to the initial amount of zinc stearate in the presence of a-phenyl-indole (Curve 1 )> epoxidized soya-bean oil (Curves 2 and 3), and butane-diol- -aminocrotonate (Curve 4) during heating in a stove at 180°C. 

Figure 5. Zinc chloride formation in the sheets of rigid PVC heated in a stove at 180°C under nitrogen atmosphere ana in the presence of a secondary stabilizer. (1) a-pnenylindole, 2.13 phr (2) epoxidizea soya bean oil, 5.53 phr (3) epoxidized soya bean oil, 16 phr (4) bu-tanediol-P-aminocrotonate, 2.7 phr.

Primary stability studies are those used to directly support the expiry dating or shelf-life of the drug substance or drug product, while secondary stability studies are those that provide supporting information. These could be from lab-scale batches, development batches, or experimental batches. The contents of the stability data tables are described above in Section 13.4.2. 

In later stages, the precipitation of a salt film is a secondary stabilizing factor. This salt layer may serve as a reservoir for locally high halide concentrations and a barrier to take over a large portion of the potential drop stabilizing pitting at lower levels of the local current density. This precipitation is ruled by the product of Sand s equation (Eqs. 4 and 6). For i t < i /r, open pits are stabilized by accumulated halides, whereas for i > i Jx, a salt layer stabiKzes additionaUy. 

U626 is a rather complex molecule. Very interestingly the phosphite moieties in U626 can have also an important role in the stabilization. Phosphites are well-known stabilizers and they are called secondary antioxidants, while hindered phenol-based stabilizers are known as primary antioxidants. Phosphites have the ability to react with hydroperoxides to yield phosphates according to scheme 5.[20,38] U626 combines primary and secondary stabilizers in the same molecule. 

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. 

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