Calendering method for production of geomembrane

PVC, CSPE and scrim-reinforced geomembranes including CSPE-R and fPP-R are not produced by conventional extrusion methods and are manufactured instead by calendering. In this process the polymer resin, carbon black, ?llers and various additive are mixed either in a heated batch mixer (such as a Banbury) or a heated continuous mixer (such as a Farrel FCM). The mixture is then masticated using mixing units customary in the rubber processing industry such as a roll mill where it is homogenized. The mixed mass is then passed through a set of calender counter-rotating rollers to form the ?nal sheet. Most scrim-reinforced geomembranes (SRG) are highly ?exible liners manufactured by this type of calendering method. Typical SRG base polymers are PVC, CSPE and fPP. In the calendering process the polymer formulation is intensively mixed in a special Banbury mixer and then passed through a two-roll mill where it is ?attened. The dough-like material is then passed through a set of counter-rotating rollers which comprise the calender to produce the ?nal sheet. The sheet is made up of polymer plies laminated to the scrim support (which is an open weave fabric generally made from polyester). The adhesion of the plies is due to ‘strike-through’ through the open weave fabric. The openings in the scrim (i.e. the apertures) need to be large enough to enable good adhesion between the plies. Inadequate ply adhesion can lead to delamination which is one of the potential shortcomings of scrim-reinforced geomembranes. The scrim-reinforced geomembranes can either have one central scrim and two outer plies (hence are three-ply geomembranes) or, comprise two scrim layers on each side of a central polymer layer and with two polymer plies on each outer face (hence are ?ve-ply geomembranes). It is important that the apertures (i.e. the openings) in the scrim are suf?ciently large to enable the polymer plies to adhere to one another. This gives ‘key and lock’ bonding that is necessary to achieve the required ply adhesion and prevent delamination failure. These reinforced geomembranes are generally slightly thicker than the unit dimensions due to the additional thickness of the scrim. For instance, a 1.21 mm fPP-R geomembrane consists of thin plys of 0.5 mm fPP each side of a scrim. Reinforced geomembranes are often only available in limited widths (e.g. 2–3 m) owning to width limitations of the calendering equipment.