The most common resin used in CIPP is polyester resin. It has excellent properties for many applications and is economical but other resins are used for specific applications. In particular, vinyl ester resins may be used for better resistance to chemical attack and epoxy resins for higher strengths and use in potable water applications. Fillers may be added to the resin to increase the modulus of the cured resin and to reduce costs. The common requirement is that the resin composition be designed so that the liner can be installed in the uncured and flexible state and then that the curing and hardening will be initiated while the liner is expanded against the interior of the host pipe. The flexibility in the exact composition of the resin means that installations can be tailored to specific requirements but also implies that adequate quality control measures be used to document the installation.