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Organic “High-Build” Spray-in-Place Liners – An Emerging Class of Rehabilitation Methods
The term “polymer” represents a broad class of materials that, by definition, are large covalently bonded chains formed by the combination of smaller molecules. Polymers used for waterline rehabilitation are classified as synthetic “engineered lining/adhesive.” These polymers combine a resin and a hardening agent to form a fast curing thermoset material with a crosslinked molecular composition. Depending on the type of adhesive formed, the reaction may require a curing aid such as a catalyst additive or heat.
The resin and curing agent for the three polymers discussed are typically in a liquid state prior to and when combined. Once the chemical reaction is complete, 100 percent of the resin is transferred to a solid state; hence the term, “100 percent solids.” Depending on the type and hardening agent combined with the resin, the resultant polymer may possess either rigid or elastic material properties (also called elastomeric). Polymer linings are either nonstructural (AWWA Class I) or semi-structural (AWWA Class II or III), where a differentiation is made between liners with inherent ring stiffness versus those that rely entirely on adhesion to the host pipe to be self-supporting. Semi-structural liners can be applied to the pipe wall in multiple layers called “high build,” and depending on their design and application, may be a Class II or Class III.
Developed 40 years ago, fusion-bond-epoxies were adopted by many water utilities during the 1990s, and are now well accepted and widely practiced for water quality enhancement and corrosion protection for steel, ductile iron and cast-iron pipes. Epoxies are formed by a reaction between bisphenol A (the resin) and epichlorohydrin (the hardener). These chemicals are typically combined with a catalyst (dicyandiamide, aliphatic diamines or aromatic amines) at an elevated temperature to initiate the reaction.
The setting characteristics of epoxy resin necessitate minimally a 16-hour cure period before commencement of return to service procedures, resulting in a 36-hour shutdown period during which customers must be served by temporary supply arrangements. The application of multiple coats further increase construction duration and cost, thus epoxies are typically applied as a thin layer aimed at corrosion protection, the bridging of small crack and gaps and for improving the pipe’s hydraulics (Guan 1999). For this reason, epoxies are considered to be “non-structural,” and are expected to add little to the structural strength of the host pipe. However, several vendors are currently working on high-build epoxy formulations.