Other Stabilisers

To reduce the formation of large crystallites in polypropylene upon heating and slow cooling with consequent loss of impact strength and transparency it is common practise to incorporate in the polymer, nucleating agents such as tertiary butyl benzoic acid or its salts or salts of aromatic sulfonates. 

To prevent the thermal decomposition of PVC with the consequent evolution of hydrogen chloride, stabilisers such as barium, cadmium, zinc, or calcium salts of fatty acids, and organotin compoimds or organophosphites are used to act as acid acceptors. 

Polydimethylsiloxones have been used as foam suppressors and antifoamimg agents 

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For this reason tribasic lead sulphate, a good heat stabiliser which gives polymer compounds with better electrical insulation properties than lead carbonate, has increased in popularity in recent years at the expense of white lead. Its weight cost is somewhat higher than that of lead carbonate but less than most other stabilisers. This material is used widely in rigid compounds, in electrical insulation compounds and in general purpose formulations. 

Kellum [115] has described a class-selective oxidation chemistry procedure for the quantitative determination of secondary antioxidants in extracts of PE and PP with great precision (better than 1 %). Diorgano sulfides and tertiary phosphites can be quantitatively oxidised with /-chloropcroxybenzoic acid to the corresponding sulfones and phosphates with no interference from other stabilisers or additives. Hindered phenols, benzophenones, triazoles, fatty acid amides, and stearate... 

The stability of many protected colloidal dispersions cannot be explained solely on the basis of electric double layer repulsion and van der Waals attraction other stabilising mechanisms must be investigated. Steric stabilisation is a name which is used (somewhat loosely) to describe several different possible stabilising mechanisms involving adsorbed macromolecules. These include the following ... 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

The first step Is to ensure that the macromolecules are bound to the surfaces. For this purpose they can be grafted chemically however this Is an expensive process, requiring the use of reactive polymers or tailor made surface active species such as block copolymers. A less expensive method Is to use macromolecules which spontaneously adsorb to the surfaces. Then experimental wisdom states that surfaces which are each completely saturated with macromolecules will repel each other (stabilisation of the suspension), while surfaces which are unsaturated will attract each other because the macromolecules will bridge them together (flocculation) (1). 

The increasing use of nonionic macromolecules as stabilisers, which has occurred since the development of the DLVO theory, has led to the awareness of other stabilising forces. The approach of particles with hydrated macromolecules adsorbed to their surfaces leads, on the interaction of these layers, to repulsion (Fig. 7.7), because of the consequent positive enthalpy change +AH which ensues. In more general terms, the approach of two particles with adsorbed stabilising chains leads... 

Other stabilised ylides that have been structurally characterised recently include (8), which also adopts a syn enolate conformation, and (9a). Carbon-13 NMR analysis of (9a) and (9b) show extensive delocalisation of the negative charge." ... 

There are other stabilisers of a similar nature, which are 4-nitrodiphenylamine, 2-nitrodiphenylamine, N-methyl-p-nitroaniline and diphenylamine. 

Pure iron(iii) oxide performs rather poorly as a WGS catalyst, due to rapid catalyst deactivation by sintering. Traditional iron catalysts typically consist of iron(iii) oxide (80-90% by mass), chromium(iii) oxide (8-10% by mass) and small amounts of other stabilisers and promoters such as copper(ii) oxide, aluminium oxide, alkali metals, zinc oxide and magnesium oxide. The small fraction of chromium(iii) oxide acts to prevent catalyst sintering, and also promotes the catalytic activity of iron. Catalyst deactivation is typically caused by poisons in the feedstock gases and by deposition of solids on the catalyst surface. 

No systematic study of the use of HALS as UV stabilisers in aromatic polyesters has been found in the literature very few articles even touch on the subject, and these are limited to PBT [157-159]. Without comparisons with other stabiliser types, it is difficult to assess the true capability of the HALS tested by these authors in PBT. It was noted by Borukaev and co-workers [159] that poly[[6[(l,l,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl]-[(2,2,6,6-tetramethyl-4-piperidyl)imino]-l,6-hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)imino]] (Chimmasorb 944 Ciba) was a better UV stabiliser than the dimethylsuccinate polymer with 4-hydroxy-2,2,6,6-tetramethyl-l-piperidineethanol (Tinuvin 622 Ciba). 

Phosphorus compounds are used quite frequently as anti-oxidants in plastics. They include phosphorodithioate esters but are mostly based on phosphites. Some quite complex molecules have been patented (12.203, 12.204) [35] (see Chapter 6.8). Such compounds may act as stabilisers and prevent deterioration of the plastic after its formation or they may function as fire retarders (see below). They are often applied in conjunction with other stabilisers. Anti-oxidants are believed to act by various mechanisms, some involving radicals. They are generally applied in 0.05-0.2% concentrations. 

Other stabilisers besides the mixed metal soap types can be assisted by using a co-stabiliser. 

Hostavin B-CAP from Clariant is a new UV stabiliser for polycarbonate and PET, based on a benzylidene-bis-malonate type UV absorber. It has good absorption in the high-energy range of the UV spectrum, and reduces yellowing. It does not interact with residual metal ions, and therefore does not affect either the colour or transparency of the polymer. Clariant recommends combining this UV absorber with other stabilisers for the long-term protection of polycarbonate and PET. 

Although the thermal conductivity of YSZ can be lowered by the addition of more Y203 [9, 15], numerous studies have shown that the 7-8 mol% YO1.5 composition exhibits the longest thermal cycle life. Both a small increase in the yttria concentration and coatings based on other stabilisers all fail at smaller numbers of thermal cycles, whether they are deposited by EBPVD or APS [16], The reason why this composition is special remains one of the major unresolved questions. 

Other stabilisers encountered in PVC, such as 2-phenylindole or urea derivatives, may be separated on silica gel layers, using chloroform [32,43]. 

By far the most important stabilisers are the hindered phenols, which are used in a wide range of polymers including the polyolefins, (e.g., PE and PP), polyamides, polycarbonate and PET. These stabilisers are effective both during processing at high temperature and for long-term use under ambient conditions. For increased effectiveness, they are usually combined with other stabilisers to attain an optimised combination of stabilisation and other properties such as discoloration. Often, the antioxidant is physically lost, primarily by extraction or volatilisation, rather than by chemical consumption [145]. The trend is therefore to use higher molecular mass antioxidants [132,139, 146]. 

ESR of paramagnetic free radicals can be used to check the efficacy of AOs and other stabilisers. ESR was used in the study of phenothiazines as antioxidants in PP aromatic secondary amines can retard polymer oxidation by reacting with alkylperoxy radicals [824]. Tkac [825] has described hydrogen and electron transfer reactions of AOs by ESR and has shown the efficiency of the ESR technique in elucidating the relationship between structure and reactivity of radicals formed from antioxidants possessing different H- and e-donor functional groups, including (hindered) phenols, amines, etc. 

Slip additives Other stabilisers Gas barrier agents... 

Reactions of lithiated dimethyl cyclohex-2-enylphosphonate (775) with aldehydes were studied and determined to proceed with a or 6 regio-selectivity. Such an unusual regioselectivity resulted from the contribution of two allylic carbanions one, stabilised by the phosphonate moiety and the other stabilised by the carboethoxy group. The products of the Horner-Wadsworth-Emmons reaction, including an analogue of some retinol metabolites (776), were formed under kinetic conditions whereas the 5-adduct (774) with benzaldehyde was obtained as a thermodynamic product (Scheme 197). °... 

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