Resin Selection for Cured-In-Place-Pipe Applications
The cured-in-place-pipe (CIPP) industry has experienced phenomenal growth since its inception nearly 40 years ago. It has become the method of choice for rehabilitation of deteriorated underground sewers and pipelines due to its low installation cost, ease and speed of installation and minimization of ancillary requirements such as traffic rerouting, security, etc.
One of the facilitators for this fast growth rate and ready market acceptance of the technique is the use of unsaturated polyester (UP) and vinylester (VE) resins. These workhorses of the thermoset composite industry have a long track record in corrosion-resistance applications. With an extensive database developed over decades of service, UP and VE resins and composites were well suited for adoption by the CIPP industry.
In addition, the physical characteristics of these resins are quite amenable to the CIPP process. Low viscosities allow them to easily flow through and wet out the felt and fiberglass tubing used to reline existing pipeline systems. The resins’ ability to cure quickly and under a variety of conditions made for easy implementation. Their ability to cure thermally, through initiation by organic peroxides, as well as by exposure to ultraviolet light widened the opportunities to design systems to fit a great diversity of field applications.
The heart of the CIPP system is the resin matrix. It causes a limp, flexible felt tube to become a rigid, structural member, allowing the free flow of liquids and effluents through it. Its main function, though, is to provide resistance to the wide variety of chemicals often encountered in municipal sewer systems. From the moderately benign soapy water, waste products and cleaning solutions from personal residences to the harsh chemicals discharged from industrial manufacturing plants, the resin itself must not degrade over the design lifetime of the CIPP system. In addition, it must display sufficient integrity to maintain long-term structural effectiveness.
In choosing a resin for a CIPP system, it is first necessary to identify the types and concentrations of the chemicals to which it is expected to be exposed. Thermal resistance is not usually an issue in underground applications but must be taken into consideration if installed near facilities that discharge liquids at above-ambient temperatures, such as discharges from pulp and paper mills. A secondary consideration is the potential for chemicals to come in contact with each other in the CIPP system. Often, municipalities have a history of material within an existing environment or they have performed analyses of their current effluents to identify types and concentrations processed within their system.
The workhorse of UP corrosion-resistance applications are isophthalic resins. The term “isophthalic” resins cover a broad range of resin types and applications in the composites industry. Corrosion-resistant resins, however, are a fairly narrow slice of polymer compositions. While it is the crosslinking through the carbon-carbon double bonds of UP resins that turns them into a 3-D, thermoset matrix, a very high crosslink density results in a brittle matrix with limited structural properties and toughness. Substitution of isophthalic acid for part of the maleic anhydride in the polymer reduces its crosslink density with little loss of resin modulus and allows an effective balance between rigidity and toughness.
The aromatic character of isophthalic acid is highly resistant to a wide range of chemicals, as well as providing good temperature resistance. The co-reactants with the acids that result in a polyester polymer are glycols or diols. These also affect the chemical resistance of a CIPP resin. Branched diols are preferred as the side groups on these molecules, which inhibit small molecule ingress into the main polymer chain and help prevent chemical attack. Ether glycols, such as diethylene glycol, are usually avoided as they exhibit somewhat of an open structure and the oxygen of the ether group can readily attract polar molecules such as water.
Corrosion-resistant isophthalic resins are usually high in molecular weight. Chain scission of the polymer backbone degrades the material and its corrosion-resistance properties, so starting with a high molecular weight means it can sustain more attacks on the backbone before it loses structural integrity. Swelling of the polymer also results in a loss of structural properties. Again, a higher molecular weight results in fewer end groups to attract attacking chemicals and also contributes to less free volume in the matrix and less ability for molecules to penetrate the network. A higher reactivity resin, meaning a higher crosslink density, also inhibits the transport of molecules through the matrix and results in a higher degree of chemical resistance. High molecular weight equates to high viscosities so CIPP resins are usually high in styrene content to lower their viscosity into the range needed for fast and efficient tube wet out of multi-layer felt tubes.
Isophthalic type resins offer a moderate degree of chemical resistance. They can readily meet the resistance requirements of ASTM F-1216 for CIPP applications. Isophthalic resins are resistant to acids, salts, dilute chemicals and offer some degree of solvent resistance so they are suitable for use in most domestic and residential piping systems. They are not recommended for alkaline and oxidizing environments and are not resistant to aromatic and chlorinated solvents.
Vinylester resins provide the next step up in chemical resistance. These resins, typically based on Bisphenol-A epoxides, contain a highly aromatic backbone, making them more resistant to chemical attack and provide greater resistance to temperature vs. isophthalics. They also contain fewer ester groups than isophthalics so they are more resistant to hydrolytic attack and have somewhat better resistance to bleaches, alkalis and solvents.
While isophthalics can handle most domestic effluents, when industrial chemicals are involved in the environment they often are more aggressive both chemically and thermally. This is where VE resins should be considered. They offer a 20 C to 50 C (68 F to 122 F) higher use temperature than isophthalic resins and are resistant to a wider range of chemicals. However, they are also not resistant or aromatic and halogenated solvents. When specifying resins for an industrial type environment, the materials the CIPP could potentially be exposed to should be reviewed thoroughly. Resins can often handle small quantities of chemicals that they normally would not resist in concentrated or long-term exposures. Also to be considered is when varied chemicals are present, there are interactions and reactions that can occur between materials that could compromise CIPP life.
The highest degree of chemical resistance in a commodity resin type is found with Bisphenol fumarate resins. As with VE resins, these contain Bisphenol-A in their backbone but these resins have a higher crosslink density so they are more resistant to some chemicals than VE and offer resistance at higher temperatures. Bisphenol fumarates find high utility in oxidizing and caustic environments. In corrosion applications, they have been used for many years in chlorine dioxide and sodium hypochlorite containments. Again, these resins are not highly resistant to aromatic and chlorinated solvents.
While the previously cited resins are used in the majority of CIPP applications, certain specialty resins occasionally find utility in specific environments. Terephthalic resins generally perform on par with isophthalics. Epoxy resins are used but their chemical resistance can vary widely, depending on the curing agents used. They often do not develop optimum properties without a post cure. Epoxy novolac vinylesters can provide better chemical and thermal resistance than Bis-A based vinylesters as they achieve a higher crosslink density. Urethane-modified vinylesters can be more resistant to some media than standard VE resins. Filled resins are becoming more popular as they can provide a more cost-effective CIPP solution but the chemical resistance of the filler must also be taken into account in any application. While many are fairly inert, materials such as calcium carbonate are readily decomposed by acids and are not a good choice in a CIPP resin system.
The CIPP industry has capitalized on the extensive history of unsaturated polyester and vinylester resins in corrosion environments to establish itself as the premier method for pipe and sewer rehabilitation. As with any corrosion application, the expected CIPP environment should be well understood before choosing the resin system. With their broad range of capabilities, unsaturated polyester and vinylester resins are well situated to participate in the continued growth rate of the CIPP industry.
Bill Carroll is a chemist associate with Reichhold Inc., headquartered in Hudson, Ohio.
