Antioxidants

The use of antioxidants in polymer processing can offer better stabilisation of the polymer during processing and also prolong its useful life in the end application.

Wells Plastics has an extensive range of products which can be used as stand alone masterbatches offering gel reductions or die lip build up control in polyethylene film extrusion, or process stabilisation in the case of polypropylene fibre production where careful control of the MFI is essential to ensure high quality trouble free production.

Long term colour and thermal stability can be enhanced by selecting the correct package of antioxidants.

In addition to stand alone masterbatches this systems can be used in multifunctional masterbatches such as in conjunction with UV Stabilisers and Processing Aids Masterbatches to offer both enhanced properties and cost effectiveness.

Application Examples:
Tapes, Fibres, Geotextiles, Films and Injection Mouldings.

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Antioxidant Masterbatches

  • Primary phenolic & secondary phosphite stabiliser packages
  • Products specifically designed for film and fibre producers
  • Process stabilisers to improve outputs
  • Finished product stabilisers to enhance lifetime performance

Polymer Oxidation

The degradation of polymers can occur during various stages of the polymer lifecycle from initial manufacture, through to fabrication and then subsequent exposure to the environment. Oxygen is the major cause of polymer degradation and its effect can be accelerated by other factors such as sunlight, heat, mechanical stress and metal ion contaminates.

Polymer degradation during thermal processing and weathering occurs through an autoxidative free radical chain reaction process. This involves the generation of free radicals, then propagation reactions leading to the formation of hydroperoxides and finally termination reactions where radicals are consumed. Hydroperoxides are inherently unstable to heat, light and metal ions, readily decomposing to yield further radicals so continuing the chain reaction.

The prior thermal-oxidative history of polymers significantly influences their photo-oxidative behaviour in service. Inhibition of this oxidative process is therefore very important and almost all synthetic polymers require stabilisation.

Types of Antioxidant Antioxidants are compounds that can interfere with the oxidative cycle thus inhibiting or slowing the oxidative degradation of polymers. There are two main classes of antioxidant depending on the way they act to interrupt the oxidative process illustrated in Figure 1. (Please use PDF document link above)

Primary Antioxidants:

These interrupt the primary oxidation cycle by removing the propagating radicals. Such compounds are also called Chain Breaking Antioxidants and examples include the hindered phenols and aromatic amines. Aromatic amines tend to discolour the end product and hence their use in plastics is limited. The phenolic antioxidants, however, are widely used in polymers. Careful selection of the phenolic antioxidant is required as the oxidation products of some phenols may discolour the polymer. The formation of chromophores is directly related to the structure of the phenol and discolouration can therefore be minimized by choosing a phenolic with a specific structure together with the use of suitable co-stabilizers.

Secondary Antioxidants:

These compounds are also called Preventative Antioxidants as they interrupt the oxidative cycle by preventing or inhibiting the formation of free radicals. The most important preventive mechanism is the hydroperoxide decomposition where the hydroperoxides are transformed into non-radical, nonreactive and thermally stable products. Phosphites or phosphonites, organic sulphur containing compounds and dithiophosphonates are widely used to achieve this, acting as peroxide decomposers.

Choosing the correct system

The choice of antioxidant varies depending on a number of factors, including the base polymer, the extrusion temperature and the performance targets of the end-use application. Efficiency of an antioxidant upon processing is dependent on its ability to reach the polymer’s attacked sites by diffusing through the viscous melt. The compatibility with and the solubility of the additive in the solid polymer, its low volatility and its resistance to extraction into the environment are also clearly important.

Synergism, where a co-operative interaction between antioxidants leads to a greater overall effect, can occur. It may arise from using two chemically similar antioxidants or when two different antioxidant functions are present in the same molecule, or when mechanistically

OutDoor Exposure

Oxidative degradation also occurs during the outdoor exposure of plastic products. The photoxidation is initiated by hydroperoxides formed during processing. Hindered phenols are relatively ineffective under photo-oxidative conditions as they are generally unstable to UV light. However a number of UV stabilisers can act as photoantioxidants and when used with hindered phenolics a synergistic effect is produced resulting in the phenols also being more effective photoantioxidants.

This information is correct to the best of our knowledge, but we would recommend that users make their own assessment to confirm that the material meets their requirements. We accept no liability for any damage, loss or injury resulting from the use of this information. Freedom from patent rights must not be assumed.

Wells Plastics Ltd

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