Antioxidants and Light Stabilisers

Light stabilisers also prevent photodegradation. UV absorbers such as hydroxybenzophenone or hydroxyphenyl triazole types, operate by absorbing and dissipating UV radiation prior to potential degradation of the polymer. No permanent chemical change occurs, so activity is retained. Hindered amine light stabilisers (HALS) are also used but their activity is not particularly well understood (80). 

Cyanoacrylates have been proposed as a new range of UV absorbers for PVC and other plastics (307). 

There are various accelerated light stability test methods available and/or exposure in harsher climates (Florida, Arizona) to test the weathering performance. The prediction of photoageing stability based on accelerated laboratory testing is very difficult, but some attempts have been made for PVC-P (445). The most relevant point is to use as reference a formulation known to have good outdoor weathering performance. 

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Most synthetie polymers are fairly stable for long periods provided that they are (a) not heated and (b) kept away from light. But they can be attacked very slowly by oxygen, and the process of oxidation is aceelerated by either heat or hght. The topics of antioxidants and light stabilisers are related and tend to overlap, although in practice the additives are usually classified under different headings. 

Even this fails to fully characterise the final plastic material, since additives (e.g. antioxidants and light stabilisers) are mixed into the polymer resin, from which the geomembrane is extruded, either early by the resin manufacturer or during the course of geomembrane manufacture (Gugumus 1990 Zweifel 2001). Such additives are of paramount importance for the use of plastics, since they help to guarantee an appropriate service lifetime of the plastic products. 

Ammonia as a reagent gas was found to yield very simple Cl mass spectra. Finally, a recent report analysed a number of additives (antioxidants and light stabilisers) in PP compomids. Three ionisation methods (El, Cl, FI) were used, and supplemental MS-MS and atomic composition (AC-MS) results were used for chemical structure elucidation/confirmation of various ingredients. 

SEC suffers from poor resolution and low sensitivity [5], while GC is limited by the high molecular weight and polar nature of many antioxidants and light stabilisers, which are designed to be reactive and so decompose when exposed to heat [9]. HPLC the most widely used instrumental method also has limitations [10-12]. HPLC lacks a simple sensitive universal detector that is compatible with all liquid mobile phases. UV or fluorescence detectors, which are commonly used, require that additives have a chromophoric moiety, while the universal refractive index detector only functions under isocratic conditions. As a result, Vargo and Olson have coupled HPLC with mass spectrometry (MS) for this type of application by using a moving belt interface [13]. 

In addition to the tendency to colour formation due to photo-reduction already mentioned, organo-titanates can also give undesirable colour effects in the presence of phenolic functionality such as often encountered in antioxidants and light stabilisers. The ready reduction of titanium IV also causes problems in peroxide cures where deactivation of the free radicals and reduction in cure efficiency can occur. 

Munteanu, Dan, Isfan, A., Isfan, C. and Tincul, I. (1987) Analysis of Antioxidants and Light Stabilisers in Poljrmers by Modern Liquid Chromatography, Chromatographia, 23(1), 7-14, pp. 211-315. 

Phosphites Tris-(p-nonylphenyl) phosphite (X) No Widely used in conjunction with conventional stabilisers (q.v.) in PVC. Some types appear to be useful heat and light stabilisers in polyolefins. Function primarily as peroxide decomposers rather than chain-breaking antioxidants. 

David et al. [184] have shown that cool on-column injection and the use of deactivated thermally stable columns in CGC-FID and CGC-F1D-MS for quantitative determination of additives (antistatics, antifogging agents, UV and light stabilisers, antioxidants, etc.) in mixtures prevents thermal degradation of high-MW compounds. Perkins et al. [101] have reported development of an analysis method for 100 ppm polymer additives in a 500 p,L SEC fraction in DCM by means of at-column GC (total elution time 27 min repeatability 3-7 %). Requirements for the method were (i) on-line (ii) use of whole fraction (LVI) and (iii) determination of high-MW compounds (1200 Da) at low concentrations. Difficult matrix introduction (DMI) and selective extraction can be used for GC analysis of silicone oil contamination in paints and other complex analytical problems. 

F. Gugumus, The use of accelerated tests in the evaluation of antioxidants and light stabihsers, in Developments in Polymer Stabilisation, ed. G. Scott, Elsevier Applied Science, 1987, Ch. 6. 

Clariant businesses are organised into five divisions pigments and additives masterbatches textile, leather and paper chemicals functional chemicals, and finally life science and electronic chemicals. Several polymer additive products have been mentioned already in Chapter 5. They include waxes, UV and light stabilisers, antioxidants, antistatic agents, flame retardants and phosphorus flame retardants. Some additives are targeted at other industries besides plastics. The company claims to be the world leader in pigments. 

Plastics contain additives such as heat and light stabilisers, antioxidants, UV-absorbers, lubricants and plasticisers, which cannot be used in any device/item to be placed inside the body. However, these additives are absolutely necessary for the processing and... 

The term antioxidant is used to describe the inhibition of polymer peroxidation in both abiotic and biotic systems [6,8,11]. It embraces more specific technological terms such as antidegradants (thermoantioxidants), antifatigue agents (mechanooxidation inhibitors) and light stabilisers (photoantioxidants). It was seen above that peroxidation of carbon-chain polymers occurs by a free radical chain mechanism (reactions 1 and 2) and that the hydroperoxide products that are the intrinsic initiators for peroxidation dissociate to oxyl radicals under the influence of heat and light. Both hydroperoxide formation and hydroperoxide decompostion are thus catalysed by transition metal ions. 

Obviously, use of such databases often fails in case of interaction between additives. As an example we mention additive/antistat interaction in PP, as observed by Dieckmann et al. [166], In this case analysis and performance data demonstrate chemical interaction between glycerol esters and acid neutralisers. This phenomenon is pronounced when the additive is a strong base, like synthetic hydrotalcite, or a metal carboxylate. Similar problems may arise after ageing of a polymer. A common request in a technical support analytical laboratory is to analyse the additives in a sample that has prematurely failed in an exposure test, when at best an unexposed control sample is available. Under some circumstances, heat or light exposure may have transformed the additive into other products. Reaction product identification then usually requires a general library of their spectroscopic or mass spectrometric profiles. For example, Bell et al. [167] have focused attention on the degradation of light stabilisers and antioxidants... 

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