Hydrolytic stabilisation

As discussed in Chapter 2, the chain ends of polyester molecules exert considerable influence on the thermal stability of these polymers. In particular, carboxylic acid chain ends which are present in the as-synthesised polymer and which build up with thermal degradation and hydrolytic reactions, can severely restrict the stability of the polymer. It is therefore unsurprising that a great deal of effort has been put into finding additives and processes which can control or eliminate such moieties. Additives of this type are known as end-cappers , although some are also referred to as hydrolytic stabilisers . 

Washington, DC, 9th-12th Oct. 1990, Paper 46. 012 EFFECTS OF EXTRACTING HYDROLYTIC STABILISERS ON URETHANE PERFORMANCE... 

Results of tests are reported to prove that collapsible fuel tank failures are due to diesel fuel extracting out the protective ingredients in the PU elastomer coatings, in particular, the hydrolytic stabiliser, aromatic polycarbodiimide. The elastomer s degradation was monitored by the degree of deterioration of physical properties. 

AKDs are waxy, water-insoluble solids with melting points around 50 °C, and ASAs are viscous water-insoluble liquids at room temperature. It is necessary to prepare them as stabilised emulsions by dispersion in a cationic polymer (normally cationic starch). Small amounts of retention aid and surfactants may also be present. Particle size distributions are around 1 fim, and addition levels around 0.1% (of pure AKD or ASA) by weight of dry fibre. This is an order of magnitude lower than the amount of rosin used in rosin-alum sizing (1-2%). Emulsions of AKD are more hydrolytically stable than ASA, and the latter must be emulsified on-site and used within a few hours. 

In common with other malonate derivatives, the a-hydrogen atom in the acetamidomalonate is reactive on treatment with a base the reagent forms a meso-merically stabilised carbanion (36) from which a variety of substituted acetamidomalonic esters can be made. For example, C-alkylation ensues when the anion is allowed to react with an alkyl halide the resulting product (37) is then subjected to the hydrolytic and decarboxylative sequence shown to yield a simple a-amino acid. 

In many cases, free imines are hydrolytically unstable. In general, it is difficult to form imines from carbonyl compounds and amines in aqueous solution. This is not always the case, and it is sometimes possible to form conjugated imines which are stabilised by delocalisation in aqueous conditions (Fig. 4-22). 

Few studies have been carried out on nonaqueous emulsions, but these can be useful as topical vehicles or reservoirs for the delivery of hydrolytically unstable dmgs. Systems such as castor oil or propylene glycol in silicone oil can be formulated using silicone surfactants the HLB number clearly does not help in the formulation, especially if the continuous phase has low polarity. The key to stabilisation lies in the sufficient solubility of the emulsifier in the continuous phase. 

Stabilisation or thorough purification of the gun-cotton next follows. Adsorl >< acids should not be neutralised at an early stttge of the purification process, as 1.1 x-ir hydrolytic action upon various impurities is an important factor at tilts jnncl.iiitc... 

The actual chemical nature of the support material may be and often is of direct importance to its usefulness as a support material.15 Silicas can react with small nucleophiles such as F-, OH- and CN. Thus, silica-supported fluorides are inactive, both as nucleophilic fluorinating agents and as bases. Similarly, silicas are not effective support materials for cyanides due to the formation of strong Si-CN bonds. For different reasons, an acidic clay would not be a suitable support for cyanides, due to the possible formation of toxic HCN. Charcoal is the most effective support material for stabilising Cu(I), probably due to its aromatic character.16 For many chemisorbed supported reagent catalysts, silicas are preferred since they give relatively strong surface bonds. However, Si-O-C bonds are hydrolytically vunerable and direct Si-C bonds are preferred.17... 

In an article in 1998 examining new trends in stabilising additives. Solera [55] noted that, while phosphites are effective process stabilisers in PET, they can themselves be subject to hydrolytic degradation. A new phosphorus-based stabiliser with increased hydrolytic stability was suggested in the form of 2,2, 2 -nitrilo[triethyl-tris(3,3, 5,5 -tetra- -butyl-l,T-biphenyl-2,2 -diyl)]phosphite (Irgafos 12 Ciba). 

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