For a long time roofing membranes were made from organic felts saturated with bituminous materials such as asphalt or modified bitumen. For several years, those membranes have been improved by adding polyester or fiberglass open grids for strength and fiberglass mats for dimensional stability. See, for example, commonly assigned U.S. Pat. Nos. 4,491,617 and 4,539,254.
In this art, it is still generally believed necessary, even with the improved high strength grids, to continue to utilize during manufacture and retain in the final product an organic felt or mat of some kind, such as a polyester mat, to provide adhesion of bitumen to the reinforcing elements, to increase stiffness, to increase bulk, and to provide toughness to resist punctures. In addition, during processing in vats of high temperature bituminous material, such mats hold the grids of the high strength reinforcing elements in place and together inside the bituminous material. For example, without a polyester mat, a high-strength polyester scrim used for reinforcing comes apart in the vat of hot bituminous material during manufacture of the roofing membrane. In addition, high strength grids manufactured without a polyester mat to hold them together have a tendency to "pull-out" from the fiberglass mat and come to the surface of the roofing membrane. This occurs while the membrane is being coated with bituminous material or during the cooling stages. It is desirable, however, that the polyester grid remain embedded inside the bitumen.
As an example, one current reinforcement for roofing membranes comprises a heavy polyester, open-grid scrim (2.67.times.2.67 yarns per inch, 1000 denier, about 2.0 ounces per square yard) laminated to a lightweight polyester mat (0.5 ounce per square yard, about 17 grams per square meter). This polyester grid/mat product is led with a fiberglass mat (3.75 ounces per square yard) through a vat of bituminous material to create a roofing membrane coated on both sides with bituminous material. During processing the polyester grid is sandwiched between the two mats. One purpose of the polyester mat during this processing is to hold the polyester grid yarns against the fiberglass mat. Without such a lightweight polyester mat, the polyester grid, which is held together with a thermoplastic adhesive (without any mechanical fastening at the cross-overs of the grid), would have a tendency to come apart and/or pull out to the surface of the membrane during impregnation with hot bituminous material.
A problem with this roofing membrane, however, is delamination of the top and bottom bituminous layers in the final product. While the porosity of the lightweight polyester mat is high and saturates rapidly, the polyester mat does not allow the bituminous layers on either side of it to adhere to each other to a great degree. One method used to reduce this problem is to go to the extra step of needle-punching holes in such mats to promote saturation and adhesion, but this added manufacturing step requires additional expense.
Accordingly, a need exists to overcome the above noted drawbacks and produce better and more economical products.