Spread Coating
Shahab Jafarzadeh
General manager of Danpolymer & CEO of Pakan Polymer Jam & Research and Development Deputy of SVI
Spread coating (or solvent coating) is a process by which the polymer is dissolved in a solvent which is then spread onto the textile and heated to evaporate the solvent and leave the coating. Reinforced ethylene interpolymer alloy (EIA-R) type geomembranes (e.g. XR-5) are generally produced by this manufacturing process. In the spread coating process the molten polymer is spread in a relatively thin coating over a dense fabric substrate (i.e. tightly woven fabric or non-woven fabric) using a spreading knife. The coating knife runs parallel to the weft. Penetration of the viscous polymer to the opposite side of the fabric is limited due to the dense weave of the fabric and so the material is turned over and the process repeated on the other side of the fabric. Since the polymer coating intimately encapsulates the fabric there is no tendency for delamination.
EXTRUSION COATED GEOMEMBRANES
Extrusion coated geomembranes are made by a specialized technology where fabrics or scrims are coated with polymer. It is a process by which the molten polymer is extruded as a ?at sheet ?lm and pressed into the fabric surface, adhering to and coating the surface (e.g. EIA-R geomembranes from Cooley). The extrusion coating provides a number of purported advantages over spread coating, solvent coating and lamination such as:
? Extrusion coated fabric achieves all material properties immediately upon cooling. Non-extrusion coatings may undergo changes post-production as the adhesives and solvent-borne coatings continue to cure.
? Extrusion coating provides intimate contact between membrane and fabric resulting in a stronger bond than the abrupt glue line that separates coating and fabric in the lamination process.
? Extrusion coating does not involve volatile organic constituents (VOCs) typical in solvent coating processes. Therefore, off-gassing and unpleasant odours are not an issue with extrusion coating.
? Extrusion coated fabrics have high wear and abrasion resistance unlike laminated products in which the adhesive provides a failure plane for delamination.
? Extrusion coating is monolithic with none of the micropores typically found in solvent coated fabrics as a result of the solvent evaporating during processing (Cooley, 2006).