Rigid XPS foam

According to Bipro (2008), Germany’s consumption of extruded polystyrene (XPS), also known for short as extruded foam, was nearly 1.9 million m³ in 2007. More than 80% of this figure went to the building and construction sector [Bipro 2008]. Owing to its closed cell structure, rigid XPS foam absorbs hardly any water even when in direct contact with it. It is rot-proof, very resistant to compression, but not UV resistant. The upper limit of the application temperature range is about 75°C.

Rigid XPS foam is produced in a continuous extrusion process: polystyrene pellets which do not contain a blowing agent are fed into an extruder, where they are melted. A blowing agent is then added and the mixture is continuously forced through a wide-slot nozzle. This results in a homogeneous closed-cell foam. The foamed extrudate that forms downstream of the nozzle can be produced in thicknesses between 20 mm and 200 mm (up to 320 mm with multi-layer bonding). After passing through a cooling zone, the foam undergoes mechanical processing to form panels and blocks with a variety of surface and edge properties. For example, the smooth skin resulting from the extrusion process is either left on the panels or removed by mechanical means on specialised panel types for better coupling in conjunction with concrete, mortar, building adhesives etc. Panels are also manufactured with special embossed surface patterns or rills.

In the building sector, rigid XPS foam is frequently used for external insulation in heavily stressed places in contact with the soil: below the floor slab of the building, and as insulation between the basement and the soil (“perimeter insulation”). Other areas of application are insulated roof membrane assemblies (single-shell, non-ventilated flat roof) or roofs with a shallow gradient. A special feature of the insulated roof membrane assembly is that the insulation layer, unlike that of conventional warm roofs, is on top of the roofing membrane. A special application is insulation of thermal bridges, which is becoming increasingly important.

In addition to the typical insulating material products for the building sector, rigid XPS foams are also used for specialised applications. These special products account for less than 10% of the European XPS insulation market, but cover a wide range of applications. No statistical data are available for these special products (types of application, quantities used) [Anhörung 2003]. They can be divided into the following categories:

• Panels and sandwich elements (with surface coverings made of steel, aluminium or wood);

• Building material in wet areas (for covering with tiles);

• Refrigerated vehicles, refrigerated containers, cold stores;

• Pipe insulation (in view of their maximum application temperature of approx. 75°C, rigid XPS foam materials are not suitable for insulating district heating pipes).

Until about 1989/1990, rigid XPS foam was produced using the CFC R-12 as blowing agent [UBA 1989]. In 1990 German manufacturers replaced R-12 with HCFCs. They used HCFC-142b or a blend of HCFC-142b and HCFC-22 [Schwarz, Leisewitz 1999]. Due to the ban on HCFCs ten years later it became necessary to replace these CFC-substitute blowing agents as well. Up to this time, no HFCs had been used in the production of rigid XPS foam.

Even after that, some rigid XPS foam was produced without any fluorinated blowing agents at all; these products are used mainly in the building and construction sector. Other production processes use HFC-134a or HFC-152a as blowing agents [Harnisch et al 2003]. A figure of 893 t of HFC emissions from rigid XPS foam is quoted for Germany in 2007 [Schwarz 2009b]. Thus the production of rigid XPS foam insulating materials is the foam sector with the highest HFC emissions. The quantities of HFC-134a used have shown only a slight drop since 2001, whereas the use of HFC-152a was more than halved by 2007. However, this sharp drop has very little effect on climate-relevant emissions (2001: 0.8 million t CO_2eq, 2007: 0.6 million t CO_2eq), because HFC-152a has a relatively low global warming potential (140) [Schwarz 2009b].

As long ago as 1996 the biggest producers in Germany (BASF and Dow Chemical Deutschland) agreed, in a voluntary undertaking given to the Federal Environment Ministry, that by 30 June 1998 they would manufacture 80 percent of their insulation panels produced for the German market without using HCFCs and would switch their entire production to HCFC-free blowing agents by 1 January 2000. At the beginning of 1999 BASF AG switched its entire production for the building sector from HCFCs to halogen-free blowing agents (CO₂) [BASF 2010]. By contrast, Dow Chemical Deutschland did not comply with the

phase-of 2000. As an alternative, Dow Chemical uses HFC-134a as well as CO₂ [DOW 2010].

Today other manufacturers are offering, alongside HFC-free products, XPS insulating materials produced using CO₂ blended with HFC-152a or HFC-134a as blowing agents [Jackon 2010; URSA 2010].

Special products are mostly blown with blowing agents containing HFCs. Possible candidates here are HFC-152a and HFC-134a, either as single substances or in blends which may also contain CO₂ or organic blowing agents (ethanol) [Schwarz, Leisewitz 1999]:

• HFC-152a: The advantage of HFC-152a lies in its good solubility in the polystyrene melt, which results in a foam with a very fine and homogeneous cell structure. HFC-152a also offers the possibility that existing plant designed for HCFCs as blowing agent can be modified at relatively low expense to use the new blowing agent. Its technical disadvantage is its rapid diffusion out of the foam cells, which – according to various sources – results in its complete removal from the foam within a few weeks or within 2 years. As a result, HFC-152a does not permit any long-term improvement in heat insulation performance compared with XPS blown using CO₂. Since HFC-152a is flammable, its use calls for suitable operational safety measures in production. A further disadvantage is that the cost of HFC-152a is several times higher than that of CO₂.

• HFC-134a: Compared with HFC-152a, this blowing agent has the advantage that it is not flammable and that it remains in the product much longer, because its diffusion from the foam takes place almost as slowly as that of HCFC-142b. The half-life of a 100-mm thick panel is 76 years for HFC-134a and 84 years for HCFC-142b.

Following the loss of 25% of the cell gas during the year of production, the annual emission rate works out at 0.66% for a panel thickness of 100 mm. One great disadvantage of HFC-134a is its high global warming potential. Unlike HFC-152a, HFC-134a does not dissolve readily in polystyrene, so the quality of the foam is limited with regard to cell structure and homogeneity. When used for rigid XPS foams, HFC-134a is therefore mostly combined with other blowing agents (HFC-152a, CO₂, ethanol). The manufacturers’ main argument is an improvement in heat insulation performance as the primary objective of using HFC-134a, which is claimed to be advantageous in the long term because of the slower diffusion of the blowing agent out of the foam.

Reduction options

Some 80% of the rigid XPS foam materials produced in Germany today are foamed with CO2

or a combination of CO2 and organic blowing agents (approx. 2 to 3% ethanol) [Bipro 2008].

In the case of XPS, unlike rigid PUR foam, the CO2 does not originate from a chemical reaction, but has to be added from outside like the other blowing agents. CO2 is relatively difficult to handle as the blowing agent in an expansion plant and requires different technology from the HCFCs formerly used, because the production process is designed differently with regard to pressure. Fairly thin panels of up to 60 or 70 mm can be produced at

involved in modifying the plants is greater. However, the cost of converting the production plant is offset by the much lower cost of CO₂ compared with fluorinated blowing agents.XPS producers work on the basis that converting a plant costs in the region of 30 to 50% of the cost of a new plant [Anhörung 2003].

Using a combination of CO₂ with 2 to 3% of an organic blowing agent (ethanol), it is possible to produce the entire product range without any sacrifice in quality. Germany’s largest manufacturer uses this approach [BASF 2010]. In addition to HFC-free XPS products, other manufacturers also offer XPS insulating materials using HFC-134a as blowing agent or CO₂ blended with HFC-152a [Dow 2010; Jackon 2010; URSA 2010]. HFC-152a, which is completely emitted during production, is employed here to create finer cell structures.

Before the introduction of the new European product standards¹

According to manufacturers, the use of HFC-134a and HFC-152a is largely confined to specialised fields of application for rigid XPS foam, such as use in refrigerated vehicles and refrigerated containers or in panels and sandwich elements [Anhörung 2003]. But some manufacturers still offer XPS panels foamed with HFC for roof insulation, perimeter insulation or similar applications [Dow 2010; Gefinex 2010; URSA 2010]. For a technical point of view, it is possible to replace HFCs with HFC-free blowing agents not only in applications in the construction sector, but also in specialised applications. With the present state of the art, restrictions apply only to the lowest thermal conductivity levels (< TCG 035) in conjunction with greater panel thicknesses of 80 mm or more [Anhörung 2003]. An alternative here is to use CO₂-foamed multi-layer panels, which thanks to multi-layer bonding technology permit a panel thickness of up to 320 mm and heat transfer coefficients (U-Wert) of 0.15 W/(m²*K) [Bipro 2008]. Another possibility for such applications is to use vacuum insulation panels made from microporous silica or other rigid foam materials (e.g. HFC-free rigid PUR foam).

– in the case of rigid XPS foam this means DIN EN 13164 [DIN 2001a], replaced by DIN EN 13164 [DIN 2009a] – the standards for building products required the calculated heat conductivity for insulating materials for the construction sector to be stated in steps of 0.005 W/(m*K). This design value was specified in the form of a thermal conductivity group (TCG) (e.g. “TCG 035”). Since then the nominal thermal conductivity figure has been required to be specified in steps of 0.001 W/(m*K) or preferably 1 mW/(m*K).

Another important aspect is the disposal of insulating materials foamed with HFC-134a. To permit environmentally sound waste management of rigid XPS foam products, they would have to be installed in a way that allows them to be removed separately and disposed of separately. Since controlled removal has not taken place in the construction sector in the past and cannot be expected in future either, it has to be assumed that the remaining HFC-134a left in the foam at the end of the period of use will undergo complete and uncontrolled release [Schwarz, Leisewitz 1999; Bipro 2008].

Conclusions

It is not necessary to use HFCs as blowing agents in rigid XPS foam for the construction sector. The entire product range can be produced with CO2 as blowing agent or using a combination of CO2 with 2 to 3% ethanol. Overall, the target should be a complete phase-out of HFCs in the production of rigid XPS foam.

All of the four manufacturers in Germany are already using CO2 for their production (at least for a large proportion of their range), so the argument of high capital cost cannot be the decisive factor here.

Rigid XPS foam for thermal conductivities of less than TCG 035, which is produced for a small market segment of specialised applications, cannot be produced using CO2 as blowing agent given the present state of the art. In this market segment one could conceivably replace rigid XPS foam with other products (such as HFC-free rigid PUR foam or vacuum insulation panels made from microporous silica).