Antioxidants and stabilisers

By the use of certain additives which divert or moderate the degradation reaction. A wide range of antioxidants and stabilisers function by this mechanism (see Chapter 7). 

J.A. Sidwell and E. Zondervan-vanden Beuken, UK Food Standards Agency project A3031/2,2004, Development of multi-methods for determining the migration of antioxidants and stabilisers included in Directive 2002/72/EC-Final project report. 

Crude palm oil is rich in minor components such as carotenoids, tocopherols, tocotrienols, sterols, phospholipids, triterpene alcohols, squalene, aliphatic alcohols and aliphatic hydrocarbons (Goh et al. 1985). The major components of interest are the carotenes, tocopherols, tocotrienols, sterols and squalene (Table 3.10). Carotenes and tocopherols are antioxidants and stabilise the oil against oxidation. During refining, the bleaching and steam deodorisation processes partially remove some of these valuable components. The amounts retained in the refined oils depend on the conditions of refining. 

Details A powdery solid, used as an antioxidant and stabiliser in plastics packaging materials and in rubber. 

PVC is also used in windows and rooflights. Transparent PVC is much less durable than the white pigmented form described above. As a result of photooxidation, catalysed by UV light, the impact resistance steadily decreases over a period of several years with associated discolouration. UV absorbers have some protective effect but complex synergistic combinations of antioxidants and stabilisers are used to... 

For further information on antioxidants and stabilisers, the reader is referred to Refs 1-4 and 11-14 at the end of this chapter. 

It was observed in the last section that most antioxidants and stabilisers are relatively low molar mass organic compounds which can be easily extracted by solvents or volatilised from the surface of the polymer at elevated temperature. This is a disadvantage in two ways. Firstly, the additives may migrate into the human environment, causing toxicity in foodstuffs or, as in the case of prostheses, directly in the body. Secondly, because they are lost from the polymer, their effectiveness is reduced. 

However, the mechanochemistry involved in polymer processing is now well understood and this provides a rational basis for design of antioxidants and stabilisers for products which are to be recycled. ... 

Some attention has been directed to making antioxidants and stabilisers part of the polymer molecule.Two main processes are used copolymerisa-... 

The problem of antioxidant and stabiliser loss from consumer products is exacerbated by the recycling process but can be mitigated by designing the stabiliser system to be an integral part of the polymer structime. 

Many scientific piiblications and patents have been directed in recent years to the problem of chemically attaching antioxidants and stabilisers to polymers. These can be categorised as follows ... 

Ironically, many polymers are not inherently stable, particularly under the high temperature and high shear conditions experienced in the barrel of an injection moulding machine. The polymer fabricator is not always aware of this fact since, particularly in the case of the polyolefin thermoplastics, the polymer manufacturer is normally concerned to ensure that his product will stand up to the most severe conditions that the fabricator is likely to impose upon it. To do this the manufacturer has normally introduced small amounts of antioxidants and stabilisers into his polymer which will ensure that the melt viscosity of the polymer remains sensibly constant during the pro-... 

The effect of antioxidants and stabilisers is to delay the onset of the rapid change in melt flow index which occurs... 

All antioxidants and stabilisers have some effect on UV stability and some are much more effective than others. This is particularly true of the peroxide decomposers (2b) some of which are very powerful UV stabilisers for polymers and are widely used in combination with UV screening agents in applications where very good outdoor stability is required . ... 

In the absence of antioxidants and stabilisers, the concentration of hydroperoxides increases rapidly in an autooxidising polymer by reactions (2)-(4) until the rate of decomposition of hydroperoxides is equal to the rate of their formation. The rate at which this state is reached in the environment is normally determined by the influence of external factors that promote the decomposition of hydroperoxides. 

Grassie N and Scott G (1985) Antioxidants and Stabilisers , Cambridge University Press, Chapter 5. 

This misimderstanding arises fi-om the fact that the non-biodegradability of the commercial polyolefins in the environment has little to do with the inherent biodegradability of the polymer structure but depends almost entirely on the antioxidants and stabilisers added to give them durability. However, this is not obvious to non-speciaUsts and many environmentalists have accepted the opinions of Greenpeace without question (see Ch. 12). 

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