The Mechanism of Stabilization

Organotin mercaptide stabilizers have an anti-oxidative action. This contributes to the stabilization in as much as the dehydrochlorination is much faster in the presence of air (oxygen) than in the presence of inert gas, and the loss of HCl is noticeably retarded by phenolic antioxidants. 

Organotin mercaptide stabilizers also break autoxidation chains, and, compounds of this type are even patented as antioxidants for other plastics. 

The induction period - the axial section on the time coordinate of the dehydrochlorination curve -is a common criterion of all heat stabilizers. Normally, within this period, the processing takes place. The length of the induction period may be considered simply as a measure of the heat stability of PVC. However, this does not - and this must be emphasized - allow any definitive conclusions to be drawn concerning the initial color, which is of the utmost importance from the practical standpoint. From the shape of the dehydrochlorination curve, especially from its gradient, very important conclusions can be drawn, particularly concerning the interactions of stabilizer conversion products with PVC, and also with each other. 

Just like the ability to bind HCl, this exchange reaction is a general characteristic of all efficient PVC heat stabilizers and stabilizer systems. An essential condition of this exchange reaction, is of course, that the transferred groups - in this case a mercaptocarbonic acid ester group - have a lower tendency to be eliminated than the chlorine atom. 

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However, a second mole of alcohol or hemiformal caimot be added at the ordinary pH of such solutions. The equiUbrium constant for hemiformal formation depends on the nature of the R group of the alcohol. Using nmr spectroscopy, a group of alcohols including phenol has been examined in solution with formaldehyde (15,16). The spectra indicated the degree of hemiformal formation in the order of >methanol > benzyl alcohol >phenol. Hemiformal formation provides the mechanism of stabilization methanol is much more effective than phenol in this regard. 

In conclusion, it may be said that a lot of literature has been published that favors the Frye and Horst mechanism of stabilization. Most of this is based on studies done on low-molecular weight model compound for al-lylicchlorines in PVC, i.e., 4-chloro-2-hexene. Although the large contribution of these studies toward understanding the mechanism of stabilization of PVC cannot be denied, the extrapolation of these results to the processes involved in the actual stabilization of the polymer should be done with extreme care. The polymer represents a complex mixture of macromolecules, which in the melt is not only physically a very different system compared to the low-molecular weight model compound, but invariably contains, apart from stabilizers, other additives, such as plasticizers, lubricants, processing aids, etc., that further complicate the situation. The criticism of the Frye and Horst mechanism is also based on solid experimental evidence, and hence, the controversy is still very much alive. 

The observed reversal in the thermal stability of the copolymer at a critical composition, which appears to be between 30 and 40 mol% of ethylene, may be explained on the basis of the emergence of phase-separation between the nonpolar ethylene and polar vinyl chloride blocks. Although crystallization of the ethylene blocks in the copolymer is only observed when more than 70 mol% ethylene units are present, the possibility of phase-separation occurring at lower contents of ethylene units cannot be excluded. Also, round about the critical copolymer composition, the Tg of the copolymer may be reduced to a level that would facilitate separation between the unlike phases by increased molecular mobility within the polymer matrix. As has been discussed earlier, occurrence of phase-separation in the copolymer would not only make the mechanism of stabilization due... 

The HM and LM pectins give two very different types of gels the mechanisms of stabilization of the junction zones in the two cases are described and few characteristics given. The different molecular characteristics (DE, distribution of methoxyl or acetyl substituents, neutral sugar content or rhamnose content) play an important role on the kinetic of gelation, mechanical properties of the gel formed and also on the experimental conditions to form the stronger gels. All these points were briefly discussed. 

At high bulk viscosity, lowering the surface tension is not relevant for the mechanism of stabilization of foams, but for all other mechanisms of foam stabilization a change of the surface properties is essential. A defoaming agent will change the surface properties of a foam upon activation. Most defoamers have a surface tension in the range of 20 to 30 mNm . The surface tensions of some defoamers are shown in Table 21-2. 

We have visualized the mechanism of stabilization in the following steps ... 

In piperidine the electron lone-pair can occupy either an axial or an equatorial position in 1-methylpiperidine the axial orientation (lb) is favoured by 99 1 over the equatorial (la). PE spectra and ab initio calculations on methylpiperidines indicate that axial 2-methyl substituents lower the amine lone-pair ionization potential by about 0.26 eV, while equatorial 2-methyl substituents as well as methyl groups on carbon atoms 3 and 4 lower the lone-pair IP by less than 0.1 eV63. This establishes the mechanism of stabilization of the amine radical cation as hyperconjugative electron release, which is larger for CC bonds than for CH bonds. The anti-periplanar orientation of the nitrogen lone-pair and the vicinal C—Me bond (lc) is much more favourable for this type of interaction than the synclinal orientation (Id). 

Another aspect of polysorbates is that they are inherently susceptible to oxidative degradation. Often, as raw materials, they contain sufficient quantities of peroxides to cause oxidation of protein residue side chains, especially methionine (59). The potential for oxidative damage arising from the addition of stabilizer emphasizes the point that the lowest effective concentrations of excipients should be used in formulations. For surfactants, the effective concentration for a given protein will depend on the mechanism of stabilization. It has been postulated that if the mechanism of surfactant stabilization is related to preventing surface-denaturation, the effective concentration will be around the detergent s critical micellar concentration. Conversely, if the mechanism of stabilization is associated with specific protein-detergent interactions, the effective surfactant concentration will be related to the protein concentration and the stoichiometry of the interaction (39). 

The work was planned on the basis of a model of a dispersed solid particle onto which one type of sequences of a BG copolymer is adsorbed selectively while the other type sequence is dissolved in the dispersion medium. A sketch of this model is shown in Figure 1. The model is the result of applying the same arguments which had been advanced (12) in discussing the mechanism of stabilization of polymeric oil-in-oil emulsions by BG copolymers to the problem of stabilization of dispersions of solid particles in organic media. Previously, essentially the same arguments had led to the demonstration of micelle formation of styrene-butadiene block copolymers in organic media under certain conditions (15). 

This paper presents the results of a study of the reactions of several thermal stabilizers for poly (vinyl chloride) with an allylic chloride model and a tertiary chloride model. The findings of this study provide considerable insight into the mechanism of stabilizer action. 

Ions can selectively occupy part of an interface and so cause an electrical charge. This can stabilize a dispersion as we have seen in milk. The mechanism of stabilization is more complicated than simple electrostatic repulsion we discuss this further on. 

The mechanism of stabilization of polyesters by carbodiimides is based on their rapid reaction with carboxylic acids, generated in the hydrolysis of polyesters. Carboxylic acids are catalysts for further hydrolysis of the polyesters. 

Surface Characterization of the Stabilized Catalyst by Probe Molecule Reaction. HZSM-5 obtained from PQ Zeolite was chosen to study the mechanism of stabilization in light naphtha aromatization. The reactions of both molecules were carried out over stabilized and unstabilized HZSM-5. We assumed first order kinetics with respect to each reactant concentration and first order decay of each reaction, and calculated initial rate constants. Figure 6 shows the initial rate constants of cumene cracking and triisopropyl-benzene cracking over the stabilized and the unstabilized catalysts. 

The better understanding of the mechanisms of stability incomplex dermatological emulsions stabilized by surfactants and amphiphiles has enabled the development of a rapid microscopic method for evaluation of potential emulsifiers. The method is based on the observation that good emulsifier blends that stabilize emulsions by the formation of multilayers of stable gel phase also swell spontaneously in water at ambient temperature and this process can be observed microscopically. Mixtures that do not form gel phase or form metastable gels only after a heating and cooling cycle cannot be observed to swell spontaneously at ambient temperature. ... 

Mixed emulsifiers are commonly used in combination with electrolytes to attain oil/water interfacial tensions substantially less than 1 dyne/cm, eg. 10 1 to 10 dynes/cm (31). The stability of the resulting microemulsions is usually attributed to the formation of an interfacial film (32,33). Even though the mechanism of stabilization has not yet been resolved, the excess surfactant used in microemulsions usually assures good stability. However, due to the very low mixed emulsifier concentrations used in miniemulsions, an understanding of the interactions between mixed emulsifier molecules at oil/water interfaces should greatly facilitate the development of miniemulsion and mini-latex formulations to achieve good stability. 

As with UV light, heat tends to oxidize polymers. The symptoms are embrittlement, melt flow instability, loss of tensile properties and discolouration. The mechanism of stabilization is therefore to prevent oxidatimi or to mitigate its effects. Plastics, particularly thermoplastics, also require stabilization protection against degradation from heat during processing or in use. 

In this chapter, the theories as well as the experimental justification for the mechanism of stabilization and destabilization of colloidal dispersions are outlined. Interacting forces between colloidal particles are analyzed and an overview of experimental methods for assessing the dispersion and relevant properties is given. The stabilization and flocculation of dispersions in the presence of surfactants and polymers is discussed in the last two sections. 

The photodegradation of polymers has been studied mainly from the practical angle, since polymers have many commercial uses. Various kinds of stabilizers, such as ultraviolet absorbers and antioxidants, have been developed but the mechanism of stabilization has not yet been completely clarified from the point of view of the photophysical and photochemical processes involved in polymer systems. 

T his paper presents a review of the performance and the mechanism of stabilization of a new class of light stabilizers based on substituted piperidine derivatives. These light stabilizers have been found to have outstanding effectiveness in many polymeric substrates. In most instances, the light stability achieved with these compounds was far superior to that attained with conventional light stabilizers. Results of studies illustrating the performance of a representative member of this light stabilizer class, bis(2,2,6,6-tetramethyl-piperidinyl-4) sebacate (LS-1) in polyolefins, styrenics, and polyurethanes are presented in comparison with... 

This review analyzes the data on most important antioxidants for polyolefins, i. e. the data on phenols. To learn consistently the whole mechanism of polyolefin stabilization, it is not possible to consider only the facts about the kinetics of the process and relationships between the chemical structure of stabilizers and their observed efficiency. It is necessary to understand the mechanism of stabilizer action on the basis of knowledge of transformations which occur in the inhibited oxidation and of properties of resulting products. The product analyses were obtained above all from the study of models and from independent syntheses. The confirmation of results under real conditions cannot be always carried out consistently because of low concentrations, difficult isolation, and reactivity of transformation products. It is also difficult to analyze such reaction mixtures directly in a polymer6 ... 

The mechanism of stabilization of polymeric oil-in-oil emulsions consists of the following 1) Incompatibility of polymers causes the phenomenon of phase separation of polymers in solution, 2) this causes the force which drives the graft copolymer into the interface of polymeric oil-in-oil emulsions, and 3) this causes the formation of coalescence barriers. 

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