Lead Based Stabilisers

Primarily used extensively for PVC-U applications in Europe and the Far East viz. pipe, fittings and profile, lead stabilisers have been very cost effective heat stabilisers. For similar reasons, plus good insulation resistance, lead stabilisers have also been used in PVC-P in wire and cable across the world. Their toxicity hazards have been well known and controlled, particularly in relation to worker exposure. 

However the EU classification of lead stabilisers as reprotoxic, harmful, dangerous for the environment and danger of cumulative effects and their presence (heavy metal) causing issues in waste management strategies, resulted in their agreed phase-out in Europe (see Section 7). 

A new UK standard reducing the lead level in drinking water by 2003 (264) was an additional factor. 

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Lead chromate pigments, traditionally used for pipe and cable applications, are linked with the phase out of lead based stabilisers. Special pigment systems based on organic or organic/inorganic combinations are available (320). 

Lead stabilisers have been used in a variety of PVC as well as other polymers for many years. In some halogenated polymers, such as chlorinated PE (CPE), chlorosulphonated polyethylene (CSM), polychloroprene (CR) and epichlorohydrin (ECO), dibasic lead phthalate and dibasic lead phosphite are used to scavenge HC1 arising from crosslinking as well as from degradation. In some of these cases, the metal may participate in crosslink formation. With lead-based stabilisers, the result is typically a product with greater water and chemical resistance than if a light metal, with more soluble halide salts, were used instead. In other cases, lead stabilisers may be used solely for function in metal oxide... 

The high level of interest in reducing lead concentrations also raised some questions about the leaching of lead-based stabiliser compounds from PVC pipe. A number of European pipe manufacturers have already moved to non-lead stabiliser systems and it is likely that such moves will be completed by voluntary action before legislation is introduced [40]. 

Japan banned lead based stabilisers from water pipes as long ago as 1993, replacing them by organotin, and may soon even replace existing pipes that contain lead stabilisers, as well as encouraging the voluntary phase out of lead from sewerage pipes. 

Examination of the THF-insoluble fraction. This fraction consists of inorganic pigments and fillers such as chalk, titanium dioxide, alumina trihydrate, clay, etc. fire retardants such as antimony trioxide lead-based stabilisers such as TBLS or DBLP polymeric impact modifiers that are totally insoluble in THF, such as MBS and various acrylic rubbers and polymeric impact modifiers that are partially soluble in THF, such as nitrile rubber and some grades of ABS which, while mainly insoluble in THF, will yield a fraction consisting of SAN which is soluble in THF and will be found with the precipitated polymer. 

Various types of additives are essential to give plastics new or enhanced properties. However, the safety of flame retardants, plasticisers and stabilisers, as regards potential health hazards, is attracting attention. Brome-based flame retardants have been the subject of safety disputes for several years and several substitutes have been developed. The shift to lead-free stabilisers is making further progress. 

The characteristics of organic based heat stabilisers used in PVC are outlined and a comparison is made of the performance of an organic based stabiliser one-pack with a traditional lead stabiliser one-pack in the injection moulding of an unplasticised PVC pipe compound. Spiral flow and colour development during injection moulding are discussed and the results of customer trials on a large... 

Non-lead and heavy-metal-free (HMF) stabilised PVC compounds present a viable material alternative to vinyl lead-based systems for wire and cable applications. Trends driving the use of these non-lead and HMF materials in... 

Another approach to the rupture of thin liquid films, proposed by Tsekov and Radoev [84,85], is based on stochastic modeling of this critical transition. Autocorrelation functions for steady state [84] and for thinning [85] liquid films were obtained. A method for calculation of the lifetime At and hcr of films was introduced. It accounts for the effect of the spatial correlation of waves. The existence of non-correlated subdomains leads to decrease in At and increase in hcr as a result of the increase in the possibility for film rupture. Coupling of dynamics of surface waves and rate of drainage v leading to stabilisation of thinning films has also been accounted for [86,87]. 

Some plastics additives, such as phthalates, bisphenol-A (BPA), and nonylphenols, present as plasticisers, can evaporate indoors and can be inhaled i.e., oestrogenic butyl benzyl phthalate is found in most vinyl floor tiles, adhesives, and synthetic leathers, and bisphenol-A is a breakdown product and plasticiser of polycarbonate (PC) mainly used as glazing material. In addition to plasticisers, there are also other additives used in plastics and rubbers i.e., stabilisers used in PVC window profiles and pipes are mostly lead-based, or they can be either barium/cadmium/zinc compounds. All of these can pose serious health hazards if they are above certain concentrations in the air, through evaporation from the system. Increase of temperature indoors speeds up this evaporation process. 

It is known that, when certain metal compounds are present in air at concentrations above the levels of homeostatic regulation, they can act as a health hazard. The most common metals and metal compounds that can be found in the indoors atmosphere, are mainly those of antimony, lead, (methyl) mercury, and cadmium, which can exist in plastics as additives (mostly found and used as stabilisers). These are believed to disrupt the endocrine system by causing problems in steroid production. The fate of these metal and metal ions has been more extensively studied for the lead and lead-based compounds, however, other metals have been studied much less than others. 

Formulating lead-free flexible PVC containing Firebrake ZB (from Borax) with excellent heat stability is possible with Ca/Zn based stabilisers. An LOI of about 30 can be obtained with 30 phr of ATH and 6 phr of Firebrake ZB. These can be improved when part of the diisodecyl phthalate plasticiser is replaced by a phosphate plasticiser and calcium carbonate is present as an additional functional filler. 

Not all countries will abandon lead within the same time schedule as Europe. Almost half of China s stabiliser consumption was lead-based in 2002, and this is not expected to change very rapidly. Chemson, the leading European stabiliser manufacturer, entered into partnership with Dalian Shide, the biggest PVC company in China, in 2001 to build a plant that produces both lead and calcium-zinc stabilisers. However, China is importing some tin and calcium-zinc types. 

There has been a big effort to develop different approaches to stabilisation that make more use of organic substances and completely avoid heavy metals. Many of these products were originally aimed at the pipe industry. The recent changes in Emopean legislation relating to the permissible levels of lead in water have focused attention on PVC potable water pipes. Crompton Vinyl Additives GmbH markets heavy metal-free (organic based stabilisers)... 

Lead, barium and cadmium based stabilisers are widely used for rigid PVC [16]. However, calcium and zinc containing materials are preferable as they offer good heat and weather stability and are used in flexible PVC applications. Foamed PVC can be considered more... 

In addition to plasticisers, there are a number of different additives used for different purposes, i.e., stabilisers used in PVC window profiles and pipes are mostly lead-based, or they can be either barium/cadmium/or zinc compounds. All of these can pose a health hazard if they migrate out of the system above certain concentrations. 

SEM-EDS is often used to quickly identify any additive. An example of the value of SEM-EDS is provided by elucidation of the cause of contamination in a pigmented PVC film, where the presence of Pb points to inadequate dispersion of a Pb-based stabiliser [141], Lead stabiliser migration from a highly... 

High resolution proton and 13C NMR spectra of lead-based heat stabilisers and their reaction products, obtained using the magic angle spinning technique, support structural assignments inferred from IR spectra. These demonstrate that lead stabilisers are unique compounds, rather than the double salts of lead oxide found in textbooks. 8 refs. 

In suspension processes the fate of the continuous liquid phase and the associated control of the stabilisation and destabilisation of the system are the most important considerations. Many polymers occur in latex form, i.e. as polymer particles of diameter of the order of 1 p.m suspended in a liquid, usually aqueous, medium. Such latices are widely used to produce latex foams, elastic thread, dipped latex rubber goods, emulsion paints and paper additives. In the manufacture and use of such products it is important that premature destabilisation of the latex does not occur but that such destabilisation occurs in a controlled and appropriate manner at the relevant stage in processing. Such control of stability is based on the general precepts of colloid science. As with products from solvent processes diffusion distances for the liquid phase must be kept short furthermore, care has to be taken that the drying rates are not such that a skin of very low permeability is formed whilst there remains undesirable liquid in the mass of the polymer. For most applications it is desirable that destabilisation leads to a coherent film (or spongy mass in the case of foams) of polymers. To achieve this the of the latex compound should not be above ambient temperature so that at such temperatures intermolecular diffusion of the polymer molecules can occur. 

Heat stabilisers for PVC act by HC1 scavenging and include organotins, mixed metal salt blends, and lead compounds. The latter account for nearly 64 % of volume (in 1994), followed by barium/cadmium and organotin compounds. Cadmium-based heat stabilisers are rapidly being replaced due to environmental concerns. Barium/zinc and calcium/zinc compounds show a high growth rate. It is expected that methyltin stabilisers will soon dominate the growing PVC pipe market. 

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