Pipeline Coating and Tapes : System Composition and Function

The total tape system components consist of a primer applied directly to the pipe surface, an inner-wrap tape layer that provides a corrosion barrier and an outer-wrap tape layer that provides mechanical
protection.

1. Primer

The primer is an integral part of the cold applied tape system. It is mainly composed of butyl rubber along with tackifiers, stress corrosion inhibitors and other additives that will facilitate good adhesion to the blasted pipe surface as well as maximize bonding of the inner-wrap tape layer to the pipe. These components are typically dispersed in a high evaporating solvent matrix and sprayed at controlled thickness on the blasted pipe surface, allowed to adequately dry prior to the application of the innerwrap tape layer. Due to recent state and federal regulations mandating solvent emission controls, 100%
solids hot melt adhesive “primers” have been developed and introduced recently to the market as an alternate solution. They are also formulated with a butyl rubber and applied to the pipe surface using more specialized spray equipment.

2. Backing

The inner and outer wrap layers backings are primarily composed of Polyethylene (PE). The PE backing is formulated to the demands of the coating for a specific application on pipe, or for a specific application as a coating. The PE composition of the anti- corrosion layer product is different than the mechanical layer, due to the function of the respective layers. The anti-corrosion layer generally has
more medium and low density PE composition which provide strength and conformability, while mechanical layers require the use of select high and low density PE that provide higher mechanical strength, toughness and resistance to stress cracking. A typical formula would include low and high density PE, antioxidants, UV inhibitor and colorant.

3. Adhesive

The adhesive in both tape layers are elastomeric based. Butyl rubber (BR) is predominantly used as opposed to other elastomers. BR’s molecular structure of low unsaturation levels yields unique characteristics of chemical and moisture resistance, thermal stability, weathering and gas permeability, all essential properties required for long term in-ground performance. In addition to BR, a typical
formula would include a tackifier, oil, filler, antioxidant, biocide and a colorant. Cross-linkers may also be used in applications requiring high, long term resistance to excessive soil stress and continuous heat exposure (up to 125 °C). The adhesive works in conjunction with the primer to form a strong bond to the steel substrate,
providing the corrosion protection and long term performance that the pipe requires as well as imparting high cathodic disbonding resistance, high shear and soil stress resistance. Different adhesive formulas are used depending on the surface to be applied to. There is the steel pipe surface and the PE backing layer of the anti-corrosion layer and the backing layer of the mechanical layer. Each layer of the PE
Tape system must have a specific adhesive composition to insure achievement of a good adhesion at each interface.