Co-stabilisers

Mixed metal stabiliser efficiency can be improved by adding co-stabilisers. These additives typically show little or no stabihsing effect by themselves, but they increase the effectiveness of the primary stabiliser. Most, but not all, are organic. 

They include polyols (mannitol, sorbitol, glycerol or pentaerythritol), hindered phenol antioxidants, the dihydropyridines, epoxidised soya bean oil, certain other epoxy compounds such as isooctyl epoxystearate, the alkyl phosphites (triphenyl phosphite, trisnonylphenyl phosphite, tristearyl phosphite or a diphosphite), 1,3-diketones, and the ketocarboxylates. 

There are also a few inorganic co-stabilisers such as hydrotalcite, i.e., the mineral Mg6Al2(0H)i6C03.4H20, and certain sodium almninosilicates (zeolites). Nitrogenous compoimds such as phenylmea, iV,iV -diphenylthiourea, and 2-phenylindole can also have a stabilising role. 

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

The co-stabiliser does not just promote stabilisation. In a number of cases it modifies the properties of the formulation, allowing it to be tailored to a specific application. Some can impart specific benefits epoxy compounds prevent formulations containing zinc from turning black in zinc burning . They may also bring disadvantages, such as increased plate-out or higher water absorption. 

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Substituted cis-3,4-epoxyocta-7-en-l-ynes are converted into tricyclic 5,6-dihydropyran-2-ones 36 and 37 on treatment with Co2(CO)8 in benzene a Co-stabilised cyclic allene intermediate is proposed. The cyclopentanone-fused products 36 arise from tandem [5+1] -[2+2+1] cycloadditions, while the cyclobutane-fused derivatives 37 are produced through a tandem cyclocarbonylation and [2+2] cycloaddition <07JOC567>. 

Ca Zn formulations are a complex blend of calcium and zinc soaps together with acid acceptors and organic co-stabilisers. Efforts have been made to increase the acid absorption capability in these systems (46,82,137, 295). The role of the co-stabiliser is also critical in maintaining initial colour and early colour hold on processing (47, 220, 308, 406, 407). Alternative calcium carboxylates have also been evaluated (227). 

Another major development has been the replacement of zinc with specific organic co-stabilisers. Most costabilisers only function well in the presence of zinc. Zinc free stabilisers are claimed to have a better processing window than Ca Zn, although initially there were also some mistaken perceptions about zinc being a heavy metal (159). 

The performance of organotin mercaptides is not only based on the amount of tin metal content, but on the organotin species, mercaptide ligand chemistry and organic co-stabiliser (453). The mechanism of organothiotin stabilisation has been studied extensively (221,347, 348, 387, 388). 

Ba Zn based systems now have the majority market share for stabilisation of PVC-P in Europe due to extensive formulation development based on increasing the barium content and the important role of new and existing organic co-stabilisers (414). Suppliers include Akros. 

The role that acrylic processing aids play in compound processability, end product properties and manufacturing efficiency has been reviewed (79). The functions of acrylic process aids, with very high molecular weight, have also been discussed from a rheological point of view (290). Co-stabilising process aids have also been developed (335). 

Reactions alkylations, reactions with epoxides and aldehydes, conjugate additions Heterocyclic synthesis with allyl silanes Reactions with Co-stabilised cations An Allyl Dianion The Role of Tin in Anion Formation Halide Exchange with Chelation Indium Allyls Allyl Anions by Deprotonation The synthesis ofall-trans dienes The synthesis ofall-trans retinol... 

Chain breaking - donor antioxidants such as hindered phenols are ineffective under conditions of photo-oxidation due to their rapid consumption in the higher rates of initiation present compared to thermal oxidation, and to photodegredant activity of their derived molecules such as quinones and quinone methides. Although they have limited photo-antioxidant ability when used alone, they can be protected from photoljnic destruction by, for example, UV absorbers, and for this reason they can produce good synergistic results with a variety of co-stabilisers. 

Liquid stabilisers are more convenient to disperse in PVC formulations. They are usually either barium-zinc or calcium-zinc with a co-stabiliser, and are used in flexible PVC. They are usually less prone to plate-out than solid ones, and often give better clarity. Sohd grades can be dissolved conveniently in the liquid phosphite co-stabilisers that are often used in the same formulation. 

Although co-stabilisers generally have no beneficial stabilising role when used alone, some of them require only tiny, catalytic amoimts of a Group 11 metal (e g., zinc), in order to act as stabilisers. The diketones are effective when used in combination with a zinc carboxylate, and substituted uracils have some stabilising power, even without zinc. 

The antioxidant systems for polyolefin hot water pipe traditionally rely on hindered phenol antioxidants combined with phosphites or phosphonates and sulfur based co-stabilisers. The last-mentioned substances can taint the water so, for potable water systems, HALS stabilisers have replaced the sulfur compoimds. 

Phenols can sometimes be generated by a reaction between hydroxyl-containing solvents (alcohols and diols) and phosphite co-stabilisers. Concerns have been expressed about the toxicology of nonyl phenol from tris(nonylphenyl)phosphite, a liquid phosphite stabihser. 

Traditional substances used as co-stabilisers include polyols, epoxidised soya bean oil, phosphites and certain antioxidants. Arkema markets Stavinor calcium stearate co-stabilisers for use with calcimn-zinc systems. Akcros Chemicals sells P24D, which stands for pyrrolidene-2,4-dione. Siid-Chemie is now marketing a synthetic hydrotalcite, Sorbacid 911, while the PQ Corporation is selling two grades of aluminosilicate (zeolite) imder the name Advera 401. They act as scavengers for acids and metal chlorides, and enable users of tin mercaptides to use less tin, saving on material costs. They also allow stabiUsation to take place with the very low tin levels required by law in food contact applications. 

In addition to the antioxidants, other co-stabilisers, such as metal soaps as acid acceptors, are also added. Metal soaps are used to neutralize resi-... 

Barium/cadmium (Ba/Cd) and barium/cadmium/zinc (Ba/Cd/Zn) based stabilisers are general purpose stabilisers for plasticised PVC. Large quantities are consumed in leathercloth, calendered sheeting, extrusions, injection moulded footwear and flooring. They are not suitable for cable extrusion due to their inferior electrical properties. They exhibit synergism with phosphite chelators such as tris nonyl phenyl phosphite, and are often used with 3-5 phr of epoxidised vegetable oils as co-stabilisers. 

Calcium/zinc stabilisers (Ca/Zn) are chiefly used in applications such as food packaging where their non-toxic properties are of paramount importance. Although cheap, their heat stabilising efficiency is relatively poor, and they are often used as co-stabilisers with aminocrotonates and epoxidised vegetable oils, particularly in food grade extruded or calendered film. Their use in blow-moulded... 

Phosphite chelators such as tris-nonyl phenyl phosphite are used as co-stabilisers with Ba/Cd and Ca/Zn to improve heat and light stability at an addition level of about 0.5 phr. Similar co-stabilising effects are shown by the epoxidised vegetable oils such as epoxidised soya bean oil, used at 3-5 phr. Epoxies may also be used at higher levels of 10-15 phr as secondary plasticisers to improve low temperature performance, when their presence also improves compound heat stability. Epoxidised soya bean oils are regarded as non-toxic and are therefore widely used as co-stabilisers in rigid and flexible food packaging applications. 

Other co-stabilisers are esters of jS-aminocrotonic acid, usually used with Ca/Zn materials in gramophone records and food packaging applications. The use of a-phenyl-indole, with Ca/Zn stabilisers for oil-bottles, once popular in France, has declined due to its very poor light stability and poor initial colour. 

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