Production equipment for membranes is a big investment. A typical reinforced geomembrane comprises one or more synthetic sheets coated or laminated to a fabric reinforcement. The interadhesive layer can be extruded on the top and underside of the fabric. Typically the amount of plastic used in coating is 200 to 500 gsm according to Davis-Standard, the machinery suppliers. A combination of two co-located extruders can extrude adhesive and bulk polymer simultaneously and the edge trim can be added back in to reduce waste and cut material use. Krauss-Maffei Berstorff has looked at the different requirements for extruders, depending on the form in which the material is supplied to the membrane manufacturer. For example, a single screw extruder is sufficient for processing precompounded TPO and PVC pellets at a rate of up to 500 kg/h, while a co-rotating twin screw extruder is needed for high output (over 1500 kg/h)and direct compounding.
TPO-based membranes are now widely used, for example in the Tempodrome in Berlin (Sika). LyondellBasell have compared it to other materials: less is needed, which can reduce transport and other logistical costs. It has good root resistance for green roofs and can be produced in light colours to reflect sunlight and cut building cooling costs. To reduce dust and dirt on the roof, a thin polypropylene film can be added by heat lamination – this was tested on a roof in Ferrara, Italy in 2009.
In 1987, a 2-component spray on membrane based on polyurea was introduced in the USA and this market is growing globally at 10%/year, mainly in the US and Asia as Europe is only now developing an interest in this technology. BASF supplies this polymeric material in the form of an isocyanate and an amine. The reaction of the 2 components is very fast, reaching green strength in seconds. It is used in landscaping with geotextiles, in containment, in truck beds and roofing and as a simple way to repair flat roofs.
Another innovation in the US that is now being offered in Europe is the polyurethane foam adhesive system from ADCO Roofing Products. It can be used to attach fleece-backed EPDM, TPO, PVC, and SBS-modified bitumen membranes. A similar 2-part polyurethane system has been in use for attaching insulation since the 1970s. According to the SPRI, 52% of membranes are now attached using adhesives, 43% by mechanical methods and 5% use ballast.
Fluoropolymers are at the top of the range of roofing membranes and are used in transparent domes and other structures: this is an area where Hightex has expertise. In a project at Green Point Stadium in Cape Town in 2009, 27,000 square metres of PTFE-coated silver glass fabric was used, while in Johannesburg 50,000 square metres of the same structure (sand-coloured) was used in the Soccer City Stadium. Hightex currently has projects in the Olympic Stadium in the Ukraine, BC Place in Vancouver and the National Stadium in Warsaw. At the company office a pneumatic ETFE membrane incorporates flexible photovoltaic panels. If the Gottlieb –Daimler Stadium in Stuttgart incorporated photovoltaics over 80% of the area, it could generate 730,000kWh/a.
Dow Elastomers has studied formulation for membranes: an aluminium trihydrate flame retardant is halogen-free and allows high production rates. The polyolefins can have high stabilizer additions to reduce the effects of weathering, without causing plate out or affecting welding.
