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The Constituents of PVC-U Windows
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How PVC-U Windows are Made | |
Cadmium | |
Lead | |
Organotin Stabilisers | |
Calcium Organics |
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PVC-U windows are made from sections of profile, which are cut and joined together to for the shape of the window. These profiles are produced by forcing molten PVC through a precision die, the material is cooled almost immediately before being cut to length. The flatness, squareness, and straightness of the profile is maintained by the design characteristics of the profile and the tooling. The windows are then fabricated from pieces of profile that have been cut to the correect size. These pieces of profiles are joined together using either heat-fusion welding or T-joint connectors. Steel or aluminium reinforcement sections are fixed inside the PVC-U profiles to provide additional strength, where required. |
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PVC-U is based on the polyvinyl chloride polymer, however, in order to make the material suitable for use as a window, a number of different additives and stabilisers are used. These special heat and UV stabiliser additives are an essential part of any PVC formulations destined for the manufacture of PVC-U windows.
The choice of stabiliser will largely depend on the particular end-application, and there are good technical reasons why certain stabiliser types are used for specific applications. Stabilisers often comprise a metallic component together with various organic compounds - the simple elemental form of a metal is never used. There are several types of metal salts and soaps used to stabilise PVC for window applications, and some specific examples are discussed below:
Cadmium based stabilisers are now very rarely used for PVC-U window applications in Europe, though they may be present in some older windows as cadmium based soaps were used as stabilisers. The move away from cadmium by manufacturers of window profiles has not been prompted by any major risk to people or the environment. Indeed, the 1992 EC Cadmium Directive recognised the lack of acute threat in allowing a phased reduction in the use of cadmium pigments and stabilisers, and in permitting their continued use in certain applications such as PVC-U window profiles.
The Swedish Environmental Protection Agency recently confirmed that heavy metal stabilisers are firmly bonded into the PVC material and do not leach out (19).
19. Swedish Environmental Protection Agency Press Release, 28 June 1996, "Manufacturers Liability Includes Recycling PVC Plastics", released in connection with the publication of: Hagstrom P, Oberg K. Disposal of PVC Waste, Naturvårdsverket; June 1996, Sweden.
Elemental lead is not used as a PVC additive. Special compounds known as salts and soaps are used in PVC applications to give very good processing and weatherability characteristics. The presence of lead based stabilisers in PVC products does not represent any significant risk of damage to health. The safety of lead stabilisers has been confirmed by the UK Drinking Water Inspectorate, the Swedish Environmental Protection Agency, the Swedish Water and Waste Waterworks Association, the World Health Organisation and the OECD, which have all approved the use of lead stabilisers in PVC pipes intended to carry potable (drinking) water.
In fact, an official Swedish test institute conducted tests to see how lead migration from PVC pipe would compare to the limits set down in the EU Ceramics Directive (which covers migration limits for food contact ceramics) (20). Their results showed that it is just as safe to eat meals off a lead stabilised PVC pipe as it is to eat them off a ceramic plate (21).
20. Packforsk Consultant AB, Project Nos. 215545 and 215574 for Nordic Plastic Pipe Association, November 1995.
21. Donnelly, P. Current Legislative Position of Stabilisers Used in PVC - A Risk Assessment papers and National and International Regulations ECVM/ELSA/ORTEP 1996
These stabiliser systems are now well established for use in PVC items, though their use in PVC-U window profile applications is limited. As with lead and cadmium based stabilisers, they remain firmly bound within the polymer matrix. The organotin compounds used as PVC stabilisers are safe to use (22). Indeed, certain organotin stabilisers have been approved for use in food contact packaging under EU regulations (23).
Referring back to the OSPAR Convention, a special Workshop on Plastic Additives was held in Paris in May 1997. The workshop concluded that there are several different, significant sources of certain organotin compounds found in the environment (e.g. anti-fouling paints, biocidal applications and the use of tributyl-tin) and also that natural processes acting on inorganic tin (methylation) also contributed to environmental levels (24). Octyl-tin, being only used as a plastic stabiliser is not found significantly in the environment. The environmental concentrations of organotin compounds used as stabilisers are therefore estimated as very low and should not pose an environmental risk (25).
22. 'Current legislative position of stabilisers used in PVC - A Risk Assessment' published jointly by the European Council of Vinyls Manufacturers, The European Lead Stabilisers Association and the Organotin Environmental Programme Association (1996).
23. Ibid.
24. 'Conclusions and Recommendations of the Workshop', Plastics Additives Workshop (Paris) 20-21 May 1997, Oslo and Paris Conventions for the Prevention of Marine Pollution.
25. Ibid.
Calcium Organic stabiliser systems have been used in PVC applications for over 20 years. They are generally based on metal carboxylates and will sometimes incorporate other elements to boost performance such as aluminium or magnesium.
Because of the characteristics of calcium organic stabilised materials they are widely used in many flexible and rigid PVC applications, including PVC-U window profile. This type of stabilising system can give products that have a high degree of clarity, good mechanical and electrical properties, excellent organoleptic properties and good outdoor weatherability.
In recent years there have been significant performance developments in calcium organic stabilisers, which now means they are viable technical alternatives to most other stabilising systems. These newer forms of calcium organic stabiliser are in general more complex and expensive than the traditional soaps mainly because of the specialised co-stabilisers required to meet the specific requirements of these applications. However, a number of PVC-U systems suppliers now use calcium organic stabiliser systems and the remainder are expected to follow suit over the next few years.
Some concern has been raised regarding the safety of these stabiliser systems, however, calcium organic stabiliser systems incorporating the proven range of co-stabilisers have low-toxicity and are often regarded as non-toxic. Solid calcium organic stabilisers have to meet the general requirements for dust emissions in the workplace although dust-free forms are readily available.
Any of the currently available stabiliser systems can be used in safety to produce PVC-U windows, doors or conservatories, provided compliance with the current chemical and product specific regulations is maintained. The different metallic based stabilisers are used for a variety of good technical reasons and they have provided society with very valuable products, varying from life saving medical applications to high quality, long life building products (26).
Click here to download an overview note on 'Metallic Stabilisers and PVC'.
26. Anon, (1996) "Current legislative position of stabilisers used in PVC - A Risk Assessment", published jointly by the European Council of Vinyls Manufacturers, The European Lead Stabilisers Association and the Organotin Environmental Programme Association.
