Stabilisation-synergism

Pigment system antagonistic towards UV stabiliser Synergism between pigment system and UV stabiliser ... 

There has been a revival of interest in recent years in antimony mercaptides as alternatives to the organo-tin stabilisers. This stems from the low level of toxicity and the strong synergism with calcium stearate. However, compared to the... 

Low oxidation state complexes are usually stabilised by n-acid ligands such as CO, PR3 (phosphines), olefins etc. The use of the word acid here refers to Lewis acidity, i.e. electron acceptance. Binding to n acid ligands occurs in a synergic fashion that comprises two components ... 

The study of PVC stabilisation by stannic compounds in the presence of HC1 scavengers shows that the essential stabilisation process is nucleophilic substitution of the allylic chlorine by the thioglycolate or mercaptopropionate groups. It is claimed that their capability to form complexes with HC1 and transport this degradation catalyst to HC1 scavengers are essential for the marked synergism observed between these two types of stabilisers. 14 refs. 

Metcalf listed attempts to relate synergism to such diverse factors as stabilisation of droplet siiie, reduction [sic] of rate of knockdown, stimulation of flight activity, prevention of deterioration of the toxicant, increased penetration into the insect or formation of molecular complexes between synergist and insecticide. He added that recent investigations have made il very clear that virtually all examples of pronounced synergism arc related to interference by the synergist of the iji viYo metabolic detoxification of the insecticide . 

Removal of the latter in the cyclic regeneration mechanism of piperidine stabilisers by reaction with hindered phenol radical intermediates is also believed to be important in antagonism. During thermal oxidation the interactions were more favourable and synergism prevailed due here to the complementary behaviour of nitroxyl free radicals and antioxidant radicals in free radical scavenging processes. 

The effectiveness of one class of antioxidant may be enhanced by combining it with another type of stabiliser. Hence, combining two different peroxy radical scavengers in a lubricant gives improved oxidation stability compared with either alone. This form of additive response is called synergism or, more specifically, homosynergism because they operate by the same stabilisation mechanism. The mechanism of this synergism is outlined as Reaction sequence (4.70) [74] ... 

These compounds have been found to be more effective than the tin alkylmaleates. Here stabilisation efficiency has been found to depend upon the nature of the substituents on the aryl rings of the molecule and highly effective synergism is observed with phenyl salicylate. Initially, the chlorine radicals from the photooxidation of the polymer are believed to react with the stabiliser according to scheme 13. The stabiliser radicals will then react with the polymer chains and graft in as shown. The... 

Allied Signal Incorporated [87] have patented cinnamamides of the general structure Ph[NH-(C=0)-C(CN)=CPh2]2, and have observed that these are especially effective stabilisers for aromatic polyesters. Clariant have noted that a combination of benzylidene bis-malonates with hindered amine light stabilisers showed excellent synergism in UV-stabilising polyalkylene terephthalates [88]. 

A new stabiliser for water pipes has been made available by Clariant. It is a free radical scavenging antioxidant called Hostanox O 310 XP, and is said to have a positive synergism with the same company s UV stabiliser, Hostavin N30 . 

Powerful synergism is achieved, for example, in the melt and thermal stabilisation of PP by using combinations of hindered phenols (CB-D) and phosphites and/or phosphonites (PD) and sulfur compounds (PD) in fact phosph(on)ites are seldom used alone. The latter enhance the melt stabilizing effect of hindered phenols (Table 1) and reduce discoloration of the polymer caused by phenol transformation products. The cooperative effect of hindered phenols (PhOH) and phosphites (P) occurs through two steps, whereby phenols scavenge alkylperoxyl radicals and phosphites decompose peroxides in a nonradical reaction which leads to enhanced melt stability of the polymer (Figure 7). Further interaction between the colored transformation products of phenol and the phosphite antioxidant, or its product(s), results in noncolored products, hence... 

In terms of stabilisation against discolouration, primary phenolic antioxidants exhibit some inhibition activity whereas combinations with phosphites can display powerful synergism. This is consistent with the involvement of a free radical oxidation process in deacetylation through ketone/hydroperoxide initiation. 

Wytox 312 is a cost effective secondary liquid phosphite antioxidant that functions as a peroxide decomposer and as a processing stabiliser. When used in conjunction with hindered phenolic and thlodlproplonate antioxidants, a synergism is seen. In addition to improved antioxidant activity of these combinations, Wytox 312 also inhibits discoloration of the polymer caused by some phenolic antioxidants. Polymer applications Include polyolefins, styrenics and a variety of other polymers. 

Plastics additives may be divided into two main categories —those which modify polymer properties by physical means, such as plasticisers, lubricants, impact modifiers, fillers, pigments, etc, and additives which achieve their effect by chemical reactions such as PVC heat stabilisers, antioxidants, ultraviolet absorbers and flame retardants. In selecting them, however, care must be taken to match the additives to the polymer they are intended to protect, taking into account such factors as compatibility, efficiency and possible synergism of two or more additives in the particular polymer under consideration. 

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. 

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