Stabilisers in PVC

Although it is one of the bulk thermoplastics, PVC suffers from relatively poor stability to ageing agencies such as heat and UV light. In order to achieve its status as an important plastic it was therefore necessary to develop a specific additive group PVC stabilisers. 

These technological developments have resulted in a range of stabilisers being available that are appropriate for different grades of PVC, products and end uses. Some of the main types are  

The analyst has to bear in mind that it is common for a number of different stabilisers to be used in a PVC conpound in order achieve the optimum ageing resistance. 

Butadiene Headspace GC with mass selective detector (MSD) 

It is obvious that a large number of these stabilisers contain metal atoms and so a cost effective way of achieving an initial evaluation of the type of stabiliser present is to use a semi-quantitative XRF technique on the sample in the as received state. Care should be taken with this as some inorganic pigments/fillers (e.g., titanium dioxide) can mask certain key elements (e.g., cadmium). 

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KemI (2000) Organotin stabilisers in PVC — Assessment of risks and proposals for risk reduction measures. Stockholm, National Chemicals Inspectorate (Report No. 6/00). 

In a reactor at low pressure (0.2-0.3 MPa) and moderate temperature (375 °C) the PVC is chemically and thermally degraded. A particular feature of the process is that the chlorine in the PVC reacts in part with the fillers in PVC and is neutralised with the formation of CaCl2. In similar vein, metal stabilisers in PVC are converted into the respective metal chlorides (lead, cadmium, zinc and/or barium). At current PVC waste compositions these chlorides consist of 60% lead which can be purified and re-used. The reaction in the end results in the following solid, liquid and gaseous products. 

Progress in the field of polymer/additive analysis in the last three decades can best be illustrated by an old recipe for the direct determination of organotin stabilisers in PVC [142] ... 

While additive analysis of polyamides is usually carried out by dissolution in HFIP and hydrolysis in 6N HC1, polyphthalamides (PPAs) are quite insoluble in many solvents and very resistant to hydrolysis. The highly thermally stable PPAs can be adequately hydrolysed by means of high pressure microwave acid digestion (at 140-180 °C) in 10 mL Teflon vessels. This procedure allows simultaneous analysis of polymer composition and additives [643]. Also the polymer, oligomer and additive composition of polycarbonates can be examined after hydrolysis. However, it is necessary to optimise the reaction conditions in order to avoid degradation of bisphenol A. In the procedures for the analysis of dialkyltin stabilisers in PVC, described by Udris [644], in some instances the methods can be put on a quantitative basis, e.g. the GC determination of alcohols produced by hydrolysis of ester groups. 

Organotin stabilisers in PVC compositions were determined by means of TLC [518]. The determination of inorganic species (down to ppb range) by TLC has been reviewed on several occasions [519,520]. Instrumental methods successful in the quantitative estimation of inorganic TLC zones directly on the plate are densitometry, fluorimetry, radiometry, planimetry and visual methods. In the framework of recycling studies, in particular as to metal stabilisers in PVC, Braun and Richter... 

Braun and Richter [923] have described an application of CE in additive analysis, namely quantitative analysis of heat stabilisers in PVC, such as Irgastab 17A and 18 MOK-N, which are metal-based (in the past usually Cd, Ba and Pb, now nontoxic Ca, Zn and Sn). Quantitative metal analysis is of interest for PVC recycling purposes. Various alternative approaches are possible for such quantitative analysis, such as XRF [924], polarog-raphy [925] and AAS [923], The performance of AAS, CE and complexometric titrations in the analysis of the heavy metal content in PVC was compared [923]. For all methods investigated the metals must be separated from the polymer and transferred into an aqueous phase. 

Fractional separation of tin compounds used as stabilisers in PVC was based on substoichiometric isotope dilution analysis [446]. The tin compounds were isolated by extraction and complexed with salicylideneamino-2-thiophenol, followed by controlled addition of y-irradiated tributyl tin oxide and measurement of the y-activity. PVC containing a nominal 0.63% (Bu Sn analysed 0.614 0.016% in nine determinations. 

An overview is presented of the analytical methods employed to detect volatile organic compounds associated with solid and liquid stabilisers in PVC used in such applications as floor coverings and wall coverings and a description is given of the ways in which these methods are being utilised to improve stabiliser performance in flexible polyvinyl chloride. 

Zinc borate can be combined with exfoliated graphite (see later) to reduce smoke evolution in polyolefins. It has been suggested that zinc borate can affect the behaviour of certain calcium/zinc heat stabilisers in PVC compositions. 

NOR HALS also improve the outdoor weathering of flexible PVC and are used as light stabilisers in PVC products like patio furniture, window and door trim, fencing and pond liners. Ciba has compared the performance of NOR HALS in PVC favourably with benzophenone and cyanoacrylate stabilisers, as well as with a benzotriazole and a conventional HALS. 

Various other workers have studied the application of TLG to the determination of epoxy type stabilisers in PVC [79, 82, 83, 98-102]. 

Wolska [59] reviewed recent advances in the application of XRF spectroscopy to the determination of antimony, bromine, copper, iron, phosphorus, titanium, and zinc, in various plastics. The ED2000 high-performance EDXRF spectrometer mannfactured by Oxford Instruments can determine up to 80 elements qualitatively and up to 50 elements quantitatively between sodium and uranium in various materials, including polymers [60]. X-ray fluorescence methods are available for the determination of tin stabilisers in PVC [61, 62]. 

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