Thema des Monats

Comfort is crucial: plastics provide protection against the cold

Climate change is leading to extreme weather conditions. This means hotter summers and colder winters than we are accustomed to in our part of the world. Germany has again been suffering from frosty weather during the winter of 2010 / 2011, with masses of snow and icy temperatures arriving as early as the beginning of December. It is a good thing that we have plastics, which are very effective at helping to stop our homes from cooling down.

Up to two thirds of the warmth provided by heating systems escape through the outside walls, basements and roofs of single-family homes built in the 1960s that are not insulated. About 2 000 litres of heating oil are wasted completely every year as a result. Houses need a warm jacket made from modern insulation materials in order to stop the heat loss and reduce energy consumption. Such insulation materials achieve far more than even the thickest of walls: a concrete wall that is a good metre thick does not insulate any more effectively than standard insulation material that is only two centimetres thick. What are known as “passive houses” with an insulation layer that is up to 40 centimetres thick therefore require practically no heating at all – a comfortable room temperature is reached here mainly thanks to the heat emitted by the residents.

The classic insulation materials that are used include:
• Such natural organic materials as wood fibres, cellulose, hemp, flax, coconut, rushes;
• Such synthetic organic materials as mineral wool (stone wool, fibreglass) and mineral foam (aerated concrete, pumice, calcium silicate, foam glass, expanded perlite);
• Synthetic organic materials, particularly rigid polystyrene or polyurethane foam.

Renewable, natural materials are suitable for heat insulation inside houses, but can hardly be used at all for outdoor insulation, because they absorb a great deal of water and are highly flammable. Another factor that decides whether an insulation material is suitable is minimal heat permeability. A good insulation material is very resistant to thermal energy, i.e. it conducts heat poorly. Heat conductivity is measured in watts per kelvin-metre (abbreviation: W/(mK)). In the building industry, the lambda unit (λ) is used. The lower the conductivity level, the better the insulation properties of the material:
• Wood wool: 0.09
• Calcium silicate: 0.065
• Expanded perlite: 0.05
• Coconut fibre: 0.045
• Mineral wool: 0.04
• Polystyrene (PS): 0.035
• Polyurethane (PU): 0.025
• Polyisocyanurate (PIR): 0.023

The most important fully synthetic plastic in the heat insulation field is foamed polystyrene. Styrene (C8H8) obtained from crude oil refineries and made from benzene (C6H6) and ethylene (C2H4) acts as the basic material. In the presence of catalysts or high temperatures, styrene polymerises into polystyrene: the monomers link to form macromolecule chains; the material does not rot and the water absorption capacity decreases.

Rigid expanded polystyrene foam (EPS) is in most widespread use. It is made from polystyrene granulate, into which pentane (C5H12) has been incorporated as a blowing agent. When it is heated up to more than 90°C, the pentane evaporates and foams the basic material up to 20 to 50 times its volume. Blocks, panels and other moulded parts can then be made from EPS in the course of further hot air treatment.

Extruded rigid polystyrene foam (XPS) is produced continuously in indefinite lengths. Polystyrene is melted in an extruder and is then pressed through an opening of specified shape under high pressure after carbon dioxide (CO2) has been added as a blowing agent (extrusion). The XPS material comes out of the other end of the die. The thickness of the material can be varied as required by altering the width of the extruder opening.

EPS and XPS satisfy the most exacting of heat insulation requirements, while they resist ageing and moisture and only conduct a minimum amount of heat. Both these foam materials need to be shielded from UV radiation, however, because sunlight makes them brittle and turn yellow. Their environmental performance is impaired by the use of the blowing agents. Pentane contributes to summer smog, for example, although it does not get into the stratosphere. Partially halogenated hydrochlorofluorocarbons (HCFCs), which used to play a role in the production of XPS, are no longer used in Germany.

Polystyrene already developed into the leading insulation material in the 1960s. It became generally known at the time under its trade name Styropor. The ultralight material (air content in the pores: up to 98 per cent) was invented as long ago as 1949 by Fritz Stastny (1908 – 1985), a chemist at BASF in Ludwigshafen. The company recently succeeded in continuing to develop EPS into Neopor, thus reducing the heat conductivity of the material to 0.032 W/(mK). This is achieved by adding graphite particles to the unprocessed granulate. The foam plastic panels, which have an iridescent silvery-grey colour as a result, now absorb infrared radiation as well, which reduces the transport of thermal energy even more. Neopor panels can therefore be up to 30 per cent thinner than conventional polystyrene panels and make it possible to reduce the thickness of insulation structures considerably.

Foam materials made from polyurethane (C3H8N2O) are very popular heat insulation materials in the building industry too. They are produced by the addition polymer reaction of diols / polyols with polyisocyanates (blowing agents). The reaction mixture is poured into moulds in which both the final foaming process and the curing process are carried out under the influence of pressure and heat. The blocks of foam material are, finally, cut into panels or films. Rigid polyurethane foam is used as insulation material on flat and pitched roofs and, like polystyrene, needs to be protected against UV light.

The patent for the production of polyurethane was granted in 1937. The inventor was Otto Bayer (1902 – 1982), a chemist who worked at I.G. Farbeinindustrie in Leverkusen. Polyurethane started to be used as an insulating material in the 1950s, e.g. in buildings, freight containers and refrigerators. Global consumption amounted to more than 12 million tonnes in 2007 and is still growing.

The plastic polyisocyanurate is related to polyurethane but is both chemically and thermally stabler. The proportion of 4,4’ methylene diphenyl diisocyanate (MDI) is higher than in polyurethane. In addition to this, a polyester polyol is used as the reaction partner instead of a polyether polyol. The tautomer isocyanuric acid (1,3,5 triazine-2,4,6 trion) is present in polyisocyanurate as a structural element. Polyisocyanurate is used as a heat insulation material in buildings in the form of rigid foam panels. Its heat conductivity level (0.023 W/mK) is one of the lowest and thus best of the classic insulation materials.

In order to make heat insulation even more efficient, the Innovative Insulation Technology Network, which is linked to the Association for Contemporary Building in Kiel (www.arge-sh.de), is – among other things – doing research into the use of vacuum insulation panels (VIPs). Heat conductivity levels of between 0.003 and 0.008 W/mK can be achieved with VIPs. This means that a vacuum insulation panel only 20 millimetres thick replaces a polystyrene panel that is ten times as thick. VIPs have a similar structure to thermos flasks: a gas-tight enclosure, a vacuum and a supporting core. Open-pore plastic foam (lambda: 0.008 W/mK) is one suitable supporting material, for example; plastic films vacuum-metallised with aluminium are the standard material used for the enclosure. The improved insulation properties are attributable primarily to the vacuum; if it is destroyed because the enclosure has been damaged, the heat conductivity level increases drastically, i.e. the VIP loses almost all of its useful properties.

Plastics help us outdoors in winter too: thermal clothing made from synthetic textile fibres, particularly polyester, provides protection against the cold. The material does not freeze up, while it is wind- and waterproof too. And it keeps the body not only warm but also dry, because it passes on sweat and condensation so that both can evaporate. Acrylic and polyamide (nylon) are used in clothing designed to provide protection against the cold too. It is not unusual for rainsuits, gloves or boots in particular to be coated with polyvinyl chloride (PVC). It is exceptionally hard-wearing and resistant to environmental influences – neither acids nor temperatures of down to -50°C can harm it. PVC will already be celebrating its 100th birthday in the coming year: Fritz Klatte (1880 – 1934), a chemist from Griesheim Elektron, better known under its later name Hoechst AG, succeeded in polymerising vinyl chloride (C2H3Cl) in 1912. It was not possible to develop a marketable product, however, so that Griesheim Elektron gave up Klatte’s patents in 1926. In the year when the inventor died, BASF then managed to produce PVC that no longer decomposed. After 1945, PVC became the most-produced plastic in the world and proved to be extremely successful in window frames, floor coverings and – not least of all – gramophone records.

Incidentally: protection against the cold is not just an issue in winter where work clothing is concerned: anyone who works in cold storage depots or low-temperature laboratories wears special clothing in line with DIN EN 342 (“Ensembles for protection against cold”) all year round. Comparable measures are even advisable in leisure activities on occasions: to make sure they do not cool down too much, water sports enthusiasts like divers and surfers wear full-body suits made from polychloroprene (chloroprene rubber) that is better known under its brand names “Neoprene” (manufacturer: DuPont) or “Baypren” (manufacturer: Lanxess). It is produced by polymerisation of chloroprene (2-chloro-1,3-butadiene; C4H5Cl). Vulcanisation makes it resistant to chemicals and weathering. In contrast to most other unsaturated elastomers, polychloroprene cannot be vulcanised with sulphur; metal oxides (ZnO, MgO) are used instead. If the vulcanised material is, finally, foamed with the help of chemical blowing agents, it takes on the properties that make it ideal for heat insulation purposes. A rugged, waterproof material for diving suits was already available in 1839: vulcanised rubber (inventor: Charles Goodyear, 1800 – 1860). Its heat insulation properties were not important in this context, because the divers could wear warm woollen clothing inside this dry suit. In the standard wetsuit used nowadays (made from neoprene, which was invented in 1930 and has been on the market since 1954), divers do not remain dry, however, so that it is better if they wear nothing underneath. Because water gets into the suit – something that is deliberate rather than a mistake: heat is supposed to be exchanged with the environment via movement and circulation. It goes without saying that this is only a pleasure in such warmer waters as the Red Sea or the Indian Ocean. Professionals wear a special suit into which hot water is channelled for diving in colder oceans. What is more practicable is a “semi-dry” suit, which is even suitable for ice diving. Sealed cuffs make sure water is not exchanged here; like in a wetsuit, the neoprene is responsible for heat insulation. GD/MW