Stabilisers are added to PVC to allow its processing and to improve its resistance especially in outdoor applications, weathering and heat ageing and have an important influence on the physical properties of PVC finished articles. Factors such as process technology involved, technical requirements of PVC end product, regulatory requirements and cost, influence the choice of the stabiliser used.
Different kinds of PVC Stabilisers
Calcium-based stabilisers (including Ca-Zn ones) are now largely used in wires and cables, in window and technical profiles (also foamed ones), and in any type of pipes (such as soil and sewer pipes, foam core pipes, pressure pipes, corrugated pipes, land drainage pipes and cable ducting) as well as the corresponding fittings.
Calcium-based stabilisers have also been introduced in PVC rigid calendering film production when improved organoleptics characteristics are required, for instance as alternative to tin mercaptides in transparent food packaging application or as alternative to tin carboxylates in PVC blown films shrinkable and for candy wrap. Similarly, calcium-based stabilisers are now an alternative to Liquid Mixed Metals (LMM) for several flexible applications, especially for the indoor ones, when stringent air quality requirements are in place (see “VOC improvement” under the liquid stabilisers).
ESPA members have committed to proactively use in their systems only molecules which, at the current knowledge, don’t incur into future restrictions.
The liquid mixed metal stabilisers are used in several PVC flexible applications like: calendered films; extruded profiles; injection moulded, soles, footwear; extruded hoses and plastisols (flooring, wall covering, artificial leather, coated fabrics, toys).
Liquid mixed metal stabiliser systems are based on Ba, Zn, Ca, Mg or K carboxylates. In general liquid mixed metals like Ba-Zn, Ca-Zn and Mg-Zn require the addition of co-stabilisers, antioxidants and organo-phosphites to provide optimum performance. The costabilisers are usually imparted to the liquid mixed metal stabiliser system. To adjust the viscosity different solvents are used including hydrocarbon solvents and plasticisers.
Liquid Ba-Zn stabilisers and liquid Ca-Zn stabilisers have successfully replaced cadmium-based stabilisers in any PVC semi-rigid and flexible applications.
The stabilisers are formulated to meet specific requirements such as good initial colour, long-term stability, good transparency, good printability, weatherability, ageing resistance, good compatibility with all types of PVC, with fillers, and pigments. A good stabiliser achieves the best possible compromise of all the necessary requirements, including cost.
In recent years, different technologies have been introduced to the industry to improve VOC (volatile organic compounds) emissions in flexibles articles, particularly in relation to the building and construction industry. Low emission and low odour are the key characteristics required by the industry and reductions in VOCs can be achieved using Ca/Zn solids, phenol-free liquids and pastes. The available stabilisers are suitable for all kind of PVC applications: transparent, filled, pigmented, technical and food contact. In case of pastes, the liquid carried is generally ESBO or plasticisers.
By far the largest use for tin compounds is in the stabilisation of PVC. Contrary to North America, where tin systems are used for almost all-rigid PVC applications, the main usage in Europe is for rigid, transparent applications where rigorous processing conditions require an outstanding stabilisation.
In addition to maintaining high transparency, tin stabilisers also provide a very good early colour (no yellowing) and very good colour retention (delay of yellowing). Due to their fast and complete mechanism of stabilisation, tin stabilisers are also suitable for use in opaque applications and particularly where light colours are required, or when process requirements are demanding (thick plate extrusion, furniture films). Additionally, tin stabilisers provide a very good processability with high through put and no plate out.
Moreover tin stabilisers are approved for use in food contact applications, potable water applications and some tin stabilisers are approved for use in rigid medical applications.
Examples of applications where tin stabilisers are widely used are: calendered films for pharmaceutical or food packaging, foils such as credit cards, sheets and sidings, extruded pipes and profiles, extruded blown films, injection moulding fittings and other technical articles.
Tin stabilisers can be divided into two main groups, the first containing stabilisers with tin-oxygen bonds and the second stabilisers with tin-sulphur bonds.
The first group is Tin carboxylates, which provide an excellent light- and weathering stability to PVC products and find rising use particularly in outdoor applications. Some examples are transparent panels and translucent double-wall panels for greenhouses. Specific stabilisers within this group – Octyltinmaleates – are approved for food contact, for the production of PVC blown films, like candy wrapping.
The second group is often described as Tin mercaptides. These stabilisers are highly efficient and allow the production of rigid PVC articles even under high-demanding processing conditions (calendering, extrusion and injection moulding). Tin mercaptides have a typical characteristic strong odour, detectable during processing and in many cases on PVC finished articles. They show moderate light-stability. The most powerful compounds within the mercaptide class are the mercapto-acetate (thioglycolate) ester derivates and these are the most common tin compounds applied today. The Tin mercaptides are usually mixtures of di-alkyl –and mono-alkyl tin-compounds, of which the ratio can be varied to create suitable stabilisers with best performance, mainly dependent upon process technologies and PVC end-use application. The alkyl groups are Octyls, Methyls and Butyls.
Health and environmental concerns and restrictions on specific organotins used in anti-fouling paints (TBT) generated some questions about the future of organotin compounds. Risk assessments on various organotin compounds including stabilisers have been conducted between 2002 and 2007. The final decisions resulting from these risk assessments led to some restrictions on specific applications, and clarified that there is still a future for organotin stabilisers. Most organotin stabilisers have already been successfully REACH registered.
Lead stabilisers – Phased out
Traditionally, lead-based compounds have been used in many parts of the world to stabilise rigid PVC for the construction industry use (e.g. pipes, fittings and profiles applications) and in flexible PVC, mainly for wires and cables. The list of registered substances is available below.
Types of lead stabilisers are tetra-basic lead sulphate, tri-basic lead sulphate, di-basic lead phosphite, di-basic lead phthalate, di-basic lead stearate, neutral lead stearate.
Lead-based stabilisers’ cost/performance ratio and physical properties, in terms of excellent heat and light stability, good electrical resistance, excellent short and long-term mechanical properties and low water absorption, have made them the materials of choice for many long life applications.
However, concerns have been expressed about possible adverse effects of lead on health and the environment.
Therefore, ESPA together with the VinylPlus Voluntary Commitment and EuPC (European Plastics Converters) committed to replacing lead-based stabilisers with interim targets of a 15% reduction by 2005 and a 50% reduction by 2010. Since 2000 stabilisers producers started to replace lead-based stabilisers mainly with calcium-based stabilisers.
The commitment of 100% phase-out by the end of 2015 was extended to the EU-27 in 2007 (EU-28 in 2014).
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